SOLAR ENERGY

Energy

Energy is the measure of ability of physical system to perform work when system undergoes change.

o Change in system - must be able to describe system accurately before and after to say it has changed

o Energy does not possess properties like those of matter o Energy can be measured and quantified o Energy is a scalar quantity (no sense of direction) o Unit for energy is erg, amount of energy needed to accelerate mass

of 1 gram at rate of 1 centimeter per second squared as it moves distance of 1 centimeter, i.e., 1 erg = 1 (g)(cm2/s2)

solar power

Introduction

Solar power is energy from the sun. The sun is 150 million kilometres away, but it is over 1 million degrees Celsius at the core. If we could harness it, there is enough solar power reaching the earth to provide all of our energy needs 10,000 times over.

Since almost the beginning of man, solar power has been used for drying clothes and food. It was not until 1954 however, that scientists in the United States produced electricity from the sun - to power satellites in space. They invented photoelectric (or photovoltaic) panels (or cells), which capture the sun's energy and turn it into electricity.

The Sun is an abundant energy resource, the energy from which is available on Earth in the form of electromagnetic radiation. The center of our Solar System fuels most types of our "renewable" energy resources. The simplest use of the Sun is to merely place something in the path of incident sunlight to increase its temperature — the average kinetic energy of the particles in motion in a body. With the aid of some clever engineering, together with common-sense engineering, the passive heating effect of the Sun can be used sucessfully and economically as an energy source. The amuont of energy available per location is relatively small, and this limitation is likely to remain in the future. In some applications, however, it is the most economical alternative; and, of course, solar energy will be available for another five billion years!.

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Solar energy, provided by the sun, is constantly replenished and will not produce harmful pollution unlike fossil fuels. Solar energy may be used passively, such as to heat and light buildings, or technology may be used to harness the sun's energy by collecting it and transforming it to generate electricity. Current technologies include photovoltaics, concentrating solar, solar hot water, e.t.c

Solar power can provide electricity for many applications.In very remote locations it may be the only practical solution since reliable power can be provided virtually anywhere. In addition, more and more residential and commercial customers are realizing the benefits of utilizing solar power for electricity to offset their utility-supplied energy consumption, to provide back up power or to operate independent of the utility grid. Solar power

can be a solution.

Sun- the ultimate source of energy

The sun has produced energy for billions of years.  Solar energy is the solar radiation that reaches the earth.

Solar energy can be converted directly or indirectly into other forms of energy, such as heat and electricity.  The major drawbacks (problems, or issues to overcome) of solar energy are: (1) the intermittent and variable manner in which it arrives at the earth's surface and, (2) the large area required to collect it at a useful rate.

Solar energy is used for heating water for domestic use, space heating of buildings, drying agricultural products, and generating electrical energy.

In the 1830s, the British astronomer John Herschel used a solar collector box to cook food during an expedition to Africa. Now, people are trying to use the sun's energy for lots of things.

Electric utilities are trying photovoltaics, a process by which solar energy is converted directly to electricity.  Electricity can be produced directly from solar energy using photovoltaic devices or indirectly from steam generators using solar thermal collectors to heat a working fluid.

There were 15 solar electric generating units operating in the US at the end of 2002, with more on the way.  Most of these are in California, though Nevada, Arizona, Texas, and Virginia have them, too.

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PHOTOVOLTAIC ENERGY

Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PVs) cell, commonly called a solar cell.  A photovoltaic cell is a nonmechanical device usually made from silicon alloys.

Sunlight is composed of photons, or particles of solar energy.  These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum.  When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed.  Only the absorbed photons provide energy to generate electricity.  When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material's atoms.  Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the

electrons naturally migrate to the surface. 

When the electrons leave their position, holes are formed.  When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery.  When the two surfaces are connected through an external load, electricity flows.

The photovoltaic cell is the basic building block of a PV system.  Individual cells can vary in size from about 1 cm (1/2 inch) to about 10 cm (4 inches) across.  However, one cell only produces 1 or 2 watts, which isn't enough power for most applications.  To increase power output, cells are electrically connected into a packaged weather-tight module.  Modules can be further connected to form an array.  The term array refers to the entire generating plant, whether it is made up of one or several thousand modules.  As many modules as needed can be connected to form the array size (power output) needed. 

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The performance of a photovoltaic array is dependent upon sunlight.  Climate conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a PV array and, in turn, its performance.  Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight with further research being conducted to raise this efficiency to 20 percent.

The pv cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight.  Beginning in the late 1950s, pvs were used to power U.S. space satellites.  The success of PVs in space generated commercial applications for pv technology.  The simplest photovoltaic systems power many of the small calculators and wrist watches used everyday.  More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes.

Photovoltaic conversion is useful for several reasons.  Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary.   The modular characteristic of photovoltaic energy allows arrays to be installed quickly and in any size required or allowed.

Also, the environmental impact of a photovoltaic system is minimal, requiring no water for system cooling and generating no by-products.  Photovoltaic cells, like batteries, generate direct current (DC) which is generally used for small loads (electronic equipment).  When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC.  Historically, pvs have been used at remote sites to provide electricity.  However, a market for distributed generation from PVs may be developing with the unbundling of transmission and distribution costs due to electric deregulation.  The siting of numerous small-scale generators in electric distribution feeders could improve the economics and reliability of the distribution system.

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SOLAR THERMAL HEAT

The major applications of solar thermal energy at present are heating swimming pools, heating water for domestic use, and space heating of buildings.  For these purposes, the general practice is to use flat-plate solar-energy collectors with a fixed orientation (position).

Where space heating is the main consideration, the highest efficiency with a fixed flat-plate collector is obtained if it faces approximately south and slopes at an angle to the horizon equal to the latitude plus about 15 degrees.

Solar collectors fall into two general categories:  non concentrating and concentrating. In the non concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).

In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area.  Where temperatures below about 200o F are sufficient, such as for space heating, flat-plate collectors of the non concentrating type are generally used.

There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing.

Solar space heating systems can be classified as passive or active.  In passive heating systems, the air is circulated past a solar heat surface(s) and through the building by convection (i.e. less dense warm air tends to rise while more dense cooler air moves downward)  without the use of mechanical equipment.   In active heating systems, fans and pumps are used to circulate the air or the heat absorbing fluid.

SOLAR THERMAL POWER PLANTS

Solar thermal power plants use the sun's rays to heat a fluid, from which heat transfer systems may be used to produce steam. The steam, in turn, is converted into mechanical energy in a turbine and into electricity from a conventional generator coupled to the turbine.   Solar thermal power generation is essentially the same as conventional technologies except that in conventional technologies the energy source is from the stored energy in fossil fuels released by combustion.  Solar thermal technologies use concentrator systems due to the high temperatures needed for the working fluid.  The three types of solar-thermal

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power systems in use or under development are: parabolic trough, solar dish, and solar power tower.

PARABOLIC TROUGH

The parabolic trough is the most advanced of the concentrator systems.  This technology is used in the largest grid connected solar-thermal power plants in the world.  One such complex in the U.S. uses parabolic troughs.  The Kramer Junction companies operate and maintain five 30-megawatt Solar Electric Generating Systems (SEGS). These SEGS comprise 150 to 354 megawatts of installed parabolic trough solar thermal electric generating capacity located in California's Mojave desert. The combined California facilities produce more than 99% of the commercially available solar generated electric power in the U.S.

A parabolic trough collector has a linear parabolic-shaped reflector that focuses the sun's radiation on a linear receiver located at the focus of the parabola.  The collector tracks the sun along one axis from east to west during the day to ensure that the sun is continuously focused on the receiver.  Because of its parabolic shape, a trough can focus the sun at 30 to 100 times its normal intensity (concentration ratio) on a receiver pipe located along the focal line of the trough, achieving operating temperatures over 400 degrees Celcius.

A collector field consists of a large field of single-axis tracking parabolic trough collectors.  The solar field is modular in nature and is composed of many parallel rows of solar collectors aligned on a north-south horizontal axis.  A working (heat transfer) fluid is heated as it circulates through the receivers and returns to a series of heat exchangers at a central location where the fluid is used to generate high-pressure superheated steam.  The steam is then fed to a conventional steam turbine/generator to produce electricity.  After the working fluid passes through the heat exchangers, the cooled fluid is recirculated through the solar field.  The plant is usually designed to operate at full rated power using solar energy alone, given sufficient solar energy.   However, all plants are hybrid solar/fossil plants that have a fossil-fired capability that can be used to supplement the solar output during periods of low solar energy.  The Luz plant is a natural gas hybrid.

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SOLAR DISH

A solar dish/engine system utilizes concentrating solar collectors that track the sun on two axes, concentrating the energy at the focal point of the dish because it is always pointed at the sun.  The solar dish's concentration ratio is much higher that the solar trough, typically over 2,000, with a working fluid temperature over 750oC.  The power-generating equipment used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote operations or, as with the solar trough, the energy may be collected from a number of installations and converted to electricity at a central point.  The engine in a solar dish/engine system converts heat to mechanical power by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce work.  The engine is coupled to an electric generator to convert the mechanical power to electric power.

SOLAR POWER TOWER

A solar power tower or central receiver generates electricity from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver).  This system uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower.  The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun.  Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs.  Power towers must be large to be economical.  This is a promising technology for large-scale grid-connected power plants.  Though power towers are in the early stages of

development compared with parabolic trough technology, a number of test facilities have been constructed around the world.

Solar One, near Barstow, California which operated from 1982 to 1988, at about 10 megawatts, was the world's largest power tower plant. In Solar One, water was converted to steam in the receiver and used directly to power a steam turbine.  The heliostat field consisted of approximately 1,800 heliostats.  The storage system stored heat from solar-produced steam in a tank filled with rocks and sand using oil as the heat-transfer fluid.  A consortium comprising

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the U.S. Department of Energy and a number of electric utilities, led by Southern California Edison, redesigned Solar One to a more advanced molten-salt technology, which started operation in 1996, Solar Two.

Measurement of Solar energy

Purpose: To measure solar irradiance -- the energy incident per second on a unit area exposed directly to the sun.

Apparatus: A solar energy calorimeter device -- an aluminum cube with one blackened surface enclosed in a Styrofoam box with a plastic window in front of the blackened surface. Thermometer, condensing lens, laboratory stand and clamps, stopwatch, vernier caliper.

Procedure: The experiment may be performed with or without the condensing lens in front of the plastic window. Using the lens concentrates the suns rays and gives a greater temperature rise per unit time.

A. Place the cube in sunlight with the plane of the absorbing surface perpendicular to the sun's rays. Monitor the cube's temperature rise with a thermometer and monitor the time with a stopwatch. Allow at least 40 minutes, or a temperature rise of 20CC, whichever comes first.

B. Remove the cube from the sun and place it in the shade. Monitor its temperature drop and the time required. Allow about one hour or a temperature drop of 20CC, whichever comes first.

Analysis:

A. Make a graph of cube temperature versus time on Linear graph paper. Denote the absorption region and the emission region. Draw a smooth curve through the data points for the absorption region and draw a smooth curve through the data points for the emission region.

B. Pick a value of T2 and T1 from the graph and the corresponding values of t2 and t1. (See figure 1) Use equation (3) in Theory section and calculate a value for solar irradiance.

C. Pick several different temperature intervals from your graph and for each calculate a value for solar irradiance. Find the average of all your results. This average is your final value for solar irradiance. This value should be somewhat below the accepted value of 1353 watts/m2 for the earth's upper atmosphere.

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Theory:

L = transmittance of glass condensing lens and/or plastic window

= (.96)2 = .92 without the lens

= (.96)4 * .95 = .81 with the lens

a = absorbance value of the blackened surface of the cube

= .97

m = mass of the cube in grams (labeled on Styrofoam box)

A = area of lens (if used) or area of plastic window in m2

R = energy per second radiated by the cube

E = solar irradiance (1353 watts/m2 is the accepted value for radiant energy reaching the earth's upper atmosphere)

c = specific heat of aluminum cube

= .214

T1,T2 = temperature in CC

t1 = time in seconds for temperature of cube to increase from T1 to T2 when in direct sun

t2 = time in seconds for temperature of cube to drop from T2 to T1 when in the shade

Note: The solar irradiance is often given in terms of hence the following conversions are useful:

1 = 4.184 = 1.327

 where 1 watt = 1

 Cube in direct sunlight:

energy received from = heat gained by + energy radiated

sun during time t1 cube during t1 by cube during t1

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(1) EALat1 = mc(T2 - T1) + Rt1

Cube in shade:

energy radiated by = heat lost by

cube during time t2 cube during t2

(2) Rt2 = mc(T2 - T1)

Eliminate R from the two equations by solving equation (2) for R and substituting it into equation (1). After a little algebra the result is as follows:

 

E =

 

Fig. 1

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Beliefs regarding solar energy

The measurements were taken from a detector 6,800 feet below a nickel mine in Ontario. Arthur McDonald at Queen's University in Kingston, Ontario, Canada was the projects director. He said that the models used for the sun are correct. McDonald says they wear disguises and are in the place they were expected to be found. The measuring instrument discovered electron neutrinos to be mixed within other types of neutrinos, which are even harder to detect. McDonald said that they may switch identities on their journey to the Earth, but once here the total is correct.

John Bahcall, a physicist at the Institute for Advanced Study at Princeton, conducted a measurement in the 1960s to determine the number of neutrinos passing through the Earth from the sun's core. His calculations were somewhat off. The first neutrino detector was located in the Homestake Gold Mine in South Dakota. In 1968, it found the inaccuracy. Bahcall remembers the embarrassment and was glad to hear of these new results.

As usual with new discoveries in science-for every problem solved another one seems to take its place. This time it is, "Why do neutrinos change their identities?" The standard model of physics says the neutrinos are without mass, but the newest results show they have at least a minute piece.

The definition of a neutrino: ghost particles traveling the universe, without an electric charge and almost no mass. They pass through objects without being harmed, but they are born in the hottest places. They have come from the Big Bang through the center of supernovae and stars. They will travel through a star's outer layers giving a unique insight into its core. They are measured with a neutrino telescope, which normally uses a large tank of water to capture even a few of the neutrino particles that are passing through. 

Photovoltaics

The sun's abundant energy can be captured and converted to forms more useful to us such as heat or electrical energy.  The conversion to heat energy is usually referred to as passive solar energy.  The process of converting solar energy to electrical energy is called photovoltaic power generation. The photovoltaic cell (or solar cell) was invented in the early 1950s, with the increase in semiconductor technology. 

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How solar cells work

Like other photodetectors, solar cells convert light energy into electrical energy, using the same basic principle as p-n diodes. Photodiodes, including solar cells, consist of a p-n junction of two semiconductor types, which allows current to flow in only one direction (see Fig. 1). The impurities or dopants create a positive charge on one side and a negative charge on the other, generating a potential across the junction. If a photodiode is operated without a bias potential--that is, without an additional external voltage--it becomes a photovoltaic device. When such a cell is exposed to light, photons bounce electrons into the conduction band, creating electron-hole pairs. The intrinsic potential separates the charges, and they accumulate on opposite sides of the cell. This in turn generates a voltage from which a current can be derived. In solar-cell modules, electrodes laid over the top of the cells collect the current and feed it out to the cell edge (see Fig. 2).

FIGURE 1.

Solar cells are p-n junctions. When they are exposed to light, electrons are bounced across the bandgap by photons into the conduction band, leaving holes behind.  Driven by the intrinsic potential of the junction, electrons and holes flow in opposite directions, generating a voltage and producing electrical energy.

FIGURE 2

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The basic structure of a solar cell is simple. The positive and negative doped semiconductors (with negative on top) are sandwiched between a back contact and front electrodes that carry the current of several cells out of the  module. In crystalline-silicon cells, the semiconductor layer is generally about 100 µm thick, while in thin-film cells it may be as thin as a few microns.

Solar energy is an extremely dilute source of power with a maximum of about 1 kW/m2. Averaged over the earth`s surface and over the whole year, day and night, solar radiation amounts to only about 340 W/m2. So, even at 100% efficiency a solar power station with 1000-MW capacity would need 3 km2 of solar cells.Unfortunately there are a number of inherent losses in solar cells that lower their efficiency in converting light to electric power. First, the solar power is spread over a considerable range of wavelengths, and only those photons with energies above the bandgap, the energy gap between the valence and conduction bands--will be absorbed in any given semiconductor. Below-bandgap photons, with typically 20% of total light power, are lost. Second, even those photons that have a short enough wavelength are not fully utilized because once an electron-hole pair is created, each carrying more than the bandgap energy, the charges lose energy to decay down to the bottom of the conduction band. This lost energy, another 30% of the incident total, goes into heat and is also unavailable for electric power. In addition, some light may be reflected from the top surface, and other light may not be absorbed, but passed through the semiconductor material. The recombination of electron-hole pairs causes further losses.

The net result is that, in general, the best laboratory cells achieve at most some 25% efficiency, while production cells barely exceed 10%. This modest efficiency, combined with the dilution of solar energy, has made solar energy considerably more expensive than fossil and nuclear power.  System costs, including the cells themselves, support and protective structures, and controls, remain in the area of $8/W, about a factor of 10 above conventional large-scale power-grid sources.

From space to earth

Despite their expense, solar cells have found wide use in applications where other power sources are impractical.  The oldest and best known of these is the supply of electrical power in space. Except for a relatively small number powered by nuclear sources, nearly all spacecraft today are powered by solar cells, and the need for such a power supply was the main impetus for government funding of solar-cell development in the 1950s and 1960s. For this use, the safety and relative light weight of solar cells overshadow their cost, which tends to be a small fraction of total spacecraft cost.

On earth, solar cells are used as an alternative to batteries in portable devices such as calculators or in remote sites where the expense of delivering electricity

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far exceeds the expense of producing it. Remote systems used for small-scale industry, water pumping, and village lighting (primarily in Third World countries) are now the main market for photovoltaics.

Solar cells are also used for powering remote telecommunication repeater stations, and there is a large potential for market growth in providing power for portable vaccine refrigerators, a vital component of the United Nations World Health Organization`s vaccination programs in poor nations. Due to their still-high costs, cells for residential power and for utility supply remain very small sectors of the market.

  

Solar Cell Efficiency

The efficiency of a solar cell is defined as the ratio of the electrical power output to the sunlight power it receives. You will be calculating the power output from your measurements in this lab.  The power of the sunlight is a little harder to determine.

The total power emitted by the sun is found using the Stefan-Boltzmann law:  I=AeT4 where I is the incoming power, is the Stefan-Boltzmann constant, A is the surface area of the sun (radius 6.96 x 109 m), e is the emissivity ("1" for a black body) and T is the mean temperature of the sun (6050 K).  This energy becomes less concentrated as it travels from the sun to the earth.  (Think of the power radiated from a spherical source.  What happens as you move farther away?)  Thus, the total energy incident on the earth's atmosphere is the total power emitted by the sun, divided by the imaginary spherical surface having the radius as the distance between the earth and the sun (1.496 x 1011 m).

The amount of energy that reaches the earth's surface is smaller than that incident upon the earth's atmosphere, due to absorption and reflection.  Location and time of the year are also important. In Saskatchewan, the total INSOLATION (incoming solar radiation) is about 5 kWh/day m2.  

The efficiency () of the photovoltaic cell is determined by:

  =  (P(cell) / (P(incoming) x Area of Cell)) x 100 % From the efficiency, you can determine what area of solar cells would be needed for a specific application.

A significant breakthrough in renewable energy, Spheral Solar Power cells produce electricity at considerably lower cost than conventional solar technology, and on a cost-par with fossil-fuel based electricity in many regions of the world. Once commercially available, Spheral Solar™ cells will make solar power

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feasible for a vast array of new applications and markets, changing the dynamics of the photovoltaic industry, forever.

Solar Radiation

Solar radiation drives atmospheric circulation. Since solar radiation represents almost all the energy available to the earth, accounting for solar radiation and how it interacts with the atmosphere and the earth's surface is fundamental to understanding the earth's energy budget.

Solar radiation reaches the earth's surface either by being transmitted directly through the atmosphere ("direct solar radiation"), or by being scattered or reflected to the surface ("diffuse sky radiation"). About 50 percent of solar (or shortwave) radiation is reflected back into space, while the remaining shortwave radiation at the top of the atmosphere is absorbed by the earth's surface and re-radiated as thermal infrared (or longwave) radiation.

The intensity of solar radiation striking a horizontal surface is measured by a pyranometer. The instrument consists of a sensor enclosed in a transparent hemisphere that records the total amount of shortwave incoming solar radiation. That is, pyranometers measure "global" or "total" radiation: the sum of direct solar and diffuse sky radiation. Incoming (or "downwelling") longwave radiation is measured with a pyrgeometer. Outgoing ("upwelling") longwave radiation is measured in various ways, such as with pyrgeometers or with sensors that measure the temperature of the surface.

The net all-wave radiation at a given point (Rn) is calculated by the equation:

where is incoming solar radiation,  is surface albedo,  is downwelling longwave radiation (thermal infrared radiation emitted from cloud bases and

atmospheric gases), and  is upwelling longwave radiation (thermal infrared radiation emanating from the earth's surface). All radiation fluxes are expressed as energy per unit area (generally watts per square meter, or W/m2). Accurate estimates of albedo are especially important as albedo places a fundamental limit on the amount of solar radiation that can be absorbed by the surface. For example, albedo strongly determines the rate of melt of sea ice. Over longer periods of time, changes in components of the radiation balance can be manifested in climate change.

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Factors Modifying the Role Solar Radiation Plays in the Earth's Energy Budget

The most important factors influencing how much shortwave radiation reaches the earth's surface and how much is absorbed are time and day of year, cloud cover, and albedo.

Time of Day and Year

The intensity of solar radiation varies significantly over the course of a year ranging from no solar radiation during the polar winter to a maximum of 350 to 400 watts per square meter (W/m2) in the summer. Over the course of a day, the sun's angle above the horizon (solar altitude) influences the intensity of solar radiation: the noon sun is more intense than the rising or setting sun. The maximum altitude of the sun depends on time of year and latitude. Of course, during the polar winter the sun is below the horizon for 24 hours, and there is no solar radiation, while at midsummer the sun changes little in altitude over the course of a day.

Cloud Cover

Clouds reflect some incoming radiation back to space, thereby reducing the amount of radiation that reaches the earth's surface. However, clouds also re-radiate infrared energy back toward the earth's surface, thereby moderating the temperature of the lower atmosphere. Globally, clouds have a cooling effect on the earth-atmosphere system, because of their high albedos. In polar regions however, clouds seem to have a net warming effect as the reduction in solar radiation is outweighed by the effect of clouds in increasing longwave radiation to the surface.

Albedo

Incoming solar radiation that strikes the earth's surface is partially reflected and partially absorbed, in proportion to surface reflectivity (albedo). Darker surfaces have a lower albedo and absorb more solar energy than do lighter surfaces. The albedo of a surface is also a function of the incidence angle of solar radiation (that is, the amount of solar energy a surface absorbs will depend on the solar altitude).

Newly fallen snow has an albedo of approximately 0.90, meaning that it reflects about 90 percent of incoming radiation. In contrast, melting snow has an average albedo of 0.50, meaning that it absorbs 50 percent and reflects 50 percent of the incoming radiation. Because a darker surface absorbs more solar radiation, snow covered by dust (dirty snow) melts faster than clean snow. The albedo of sea ice varies with ice age, but when snow covered is on the order of 0.70.

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Open water absorbs the most radiation of all arctic surfaces. With an albedo of about 0.08, it reflects only 8 percent of the incoming radiation. However, the variation of albedo with solar altitude is especially pronounced for the surfaces of oceans and lakes. The albedo of a water surface increases with decreasing solar altitude and approaches a mirror-like 100 percent near sunrise and sunset, or when the sun is low in the arctic sky.

Important changes in surface albedo can occur seasonally. Over land, heavy winter snow cover increases surface albedo considerably. In middle and high latitudes, significant increases in surface albedo accompany the winter formation of lake and sea ice.

A comment about the seasonal cycle of solar radiation

The following description of the seasonal cycle of solar radiation based on gridded global radiation fields has been drawn from the data section of the Arctic Climatology Project Arctic Meteorology and Climate Atlas.

The field of global radiation for March shows a primarily zonal pattern, that is, one in which radiation decreases with latitude. This occurs because in March, the amount of solar radiation at the top of the atmosphere decreases sharply with increasing latitude. From April through August, latitudinal variations in solar radiation at the top of the atmosphere are less pronounced, so that cloud cover plays a strong role in determining the flux reaching the surface. Consequently, radiation patterns from April through August are very asymmetric. Fluxes are lowest over the Atlantic sector, where cloud cover is greatest. Fluxes peak over central Greenland from May through August. In large part, this illustrates the tendency for the high central portions of the ice sheet to be above the bulk of cloud cover. The highest fluxes are found in June because radiation at the top of the atmosphere peaks in June. Note for June the rather high fluxes over the central Arctic Ocean. This is largely explained in that cloud cover over this region is comparatively limited. From July onwards, radiation fluxes decline. September shows a zonal pattern, which as with March, arises from the strong latitudinal variation in solar flux at the top of the atmosphere for this month.

INTRODUCTION

Energy from the Sun reaching the Earth drives almost every known physical and biological cycle in the Earth system. By making solar radiation calculations and examining radiation measurements, students can gain a better understanding of many physical cycles and concepts associated with the Earth system.

A detailed study of solar irradiance will give Earth & Space Science and Physics students a better understanding of:

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Solar radiation Electromagnetic spectrum Mathematical concepts that apply to solar radiation Climate variation due to latitude Seasonal weather changes Global energy balance Daily changes in solar radiation Changes in solar irradiance due to solar cycles Effects of solar irradiance variations on the earth system

This educational brief is designed to serve as a source of background information on solar radiation studies and as a reference for student investigations on this subject. Links to student investigations can be found at the end of this brief. Before beginning a detailed investigation of solar radiation, there are three terms that must be understood.

Irradiance - The amount of electromagnetic energy incident on a surface per unit time per unit area. In the past this quantity has often been referred to as "flux".* When measuring solar irradiance (via satellite), scientists are measuring the amount of electromagnetic energy incident on a surface perpendicular to the incoming radiation at the top of the Earth's atmosphere, not the output at the solar surface.

Solar Constant - The solar constant is the amount of energy received at the top of the Earth's atmosphere on a surface oriented perpendicular to the Sun’s rays (at the mean distance of the Earth from the Sun). The generally accepted solar constant of 1368 W/m2 is a satellite measured yearly average.

Insolation - In general, solar radiation is received at the Earth's surface. The rate at which direct solar radiation is incident upon a unit horizontal surface at any point on or above the surface of Earth. *I will refer to insolation as direct solar radiation at the Earth's surface.

The solar constant is an important value for current studies of global radiation balance & climate models. The problem that faces scientists studying Earth’s radiation budget and climate is that while satellites can “accurately” measure solar irradiance and calculate a solar constant, the surface insolation is much more difficult to assess. When the solar constant is calculated there are four major problems in trying to relate this radiation intensity to its effect on the Earth's surface or surface insolation.

First, the calculation is made for the top of the atmosphere and not for the surface of the Earth.

Second, the calculation assumes that the surface receiving the radiation is perpendicular to the radiation.

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Third, the calculation assumes that the surface receiving the radiation is at a mean Sun-Earth distance.

Fourth, the calculation assumes that radiation emission from the Sun remains constant.

Trying to relate calculations made for the top of the atmosphere to the surface is a problem because up to 70% of incoming radiation can be blocked by the atmosphere and cloud cover. In attempts to create global energy budget models, scientists must insert estimations for the amount of energy actually reaching the surface.

Assuming that the surface receiving the radiation is perpendicular to the incoming radiation is a problem because this is a rare occasion even at tropical latitudes due to the rotation of the Earth (time of day), tilt of the Earth's axis in relation to the incoming solar radiation (season), and the latitude and orientation of the surface. All of these factors change the angle of the surface receiving the radiation, which changes the intensity of the energy received.

Assuming that the radiation emission of the Sun is constant is a problem because this value fluctuates with cycles in solar activity. NASA satellites have measured incoming radiation since 1978 and have recorded changes in solar irradiance. This data can be accessed on the internet from Goddard Space Flight Center.

SOLAR RADIATION AND THE ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum consists of the entire range of frequencies and wavelengths at which electromagnetic waves can travel. The electromagnetic spectrum organizes energy types by wavelength and frequency. The peak wavelength of radiation emitted from an object is dependent upon the temperature of the object and can be calculated using the Wien Displacement Law when the temperature of the object is known. (In astronomy these are solid objects such as stars and planets.)

Wien Displacement Law:

maximum = 2897 / T

maximum = The peak wavelength of energy in micrometers T = The temperature of the object radiating energy  

Using this law, the peak wavelength of radiation emitted from an object is inversely proportional to the temperature of the object. The irradiance or radiation

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output of an object can be calculated using the Stefan-Boltzman Law when the temperature is known.

Stefan-Boltzman Law: E = T4

E = Surface Irradiance of the object

* = Emissivity of the object= Stefan-Boltzman Constant (5.67x10-8 W/m2K4 )

T = Temperature of the object  *Emissivity is the factor of how well a surface can absorb and emit energy. Emissivity numbers range from 0 to 1. Very black objects such as charcoal have an emissivity near 1 while shiny objects have an emissivity near 0. 

The Wien Displacement & Stefan-Boltzman laws strictly apply only to black bodies. Black bodies are capable of absorbing and emitting radiation at all wavelengths. Because the Sun & Earth are not perfect black bodies, applying these laws to them only allows approximate values to be obtained. The fact that the Sun is not a perfect black body is especially important when studying solar cycles. The most significant variations in solar radiation during these cycles occur in the UV & X-Ray portions of the solar spectrum. In order to compare solar emissions to black body emissions at the same temperature go to the Solar Spectrum/Black Body Graph.

SOLAR RADIATION ENTERING THE EARTH SYSTEM

In order to study the effects of solar radiation on the Earth system, it is necessary to determine the amount of energy reaching the Earth's atmosphere & surface. Once the surface irradiance of the Sun is determined the amount of energy reaching the top of the Earth's atmosphere can be calculated using the Inverse Square Law. The average amount of energy received on a surface perpendicular to incoming radiation at the top of the atmosphere is the solar constant. (*While this calculation can lead to a better student understanding of the Inverse Square Law, the accepted value is a yearly average from NASA satellite measurements.)

Solar Radiation Striking the top of the Earth's Atmosphere

The Inverse Square Law is used to calculate the decrease in radiation intensity due to an increase in distance from the radiation source.

Inverse Square Law: I = E(4 x R2)/(4 x r2)I = Irradiance at the surface of the outer sphere

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E = Irradiance at the surface of the object (Sun)

4 x R2 = surface area of the object

4 x r2 = surface area of the outer sphere

In order to calculate the solar constant the following equation is used:So = E(Sun) x (R(Sun) / r)2

So = Solar ConstantE= Surface Irradiance of the SunR= 6.96 x 105 km = Radius of the Sunr = 1.5 x 108 km =Average Sun-Earth Distance

Insolation: Solar Radiation Striking the Surface

I = S cos Z

I= InsolationS~ 1000 W/m2 (Clear day solar insolation on a surface perpendicular to incoming solar radiation. This value actually varies greatly due to atmospheric variables.)

Z = Zenith Angle (Zenith Angle is the angle from the zenith (point directly overhead) to the Sun's position in the sky. The zenith angle is dependent upon latitude, solar declination angle, and time of day.)

Z = cos-1 (sin sin + cos cos cos H)

= Latitude

H = = Hour Angle = 15o x (Time - 12) (Angle of radiation due to time of day. Time is given in solar time as the hour of the day from midnight.)

= Solar Declination Angle

Solar Declination Angles for the Northern Hemisphere Vernal Equinox Mar. 21/22 = 0o

Summer Solstice Jun. 21/22 = +23.5o

Autumnal Equinox Sept. 21/22 = 0o

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Winter Solstice Dec. 21/22 = -23.5o

Solar water heating 

Solar Energy can be used in many ways. One of the easiest forms of solar energy to collect and use is solar heat. The collected heat can be used for space heating, domestic hot water, heating pools and cooking.

Passive solar hot water systems are probably the oldest commercially available solar systems. At the turn of the century there were large numbers of solar water heating systems on roof tops, especially in Los Angeles and Florida. Very little has changed from the original concept. Put a water holding tank in a box, with glass on the side facing south and fill it with water. No moving parts, nothing to break down, free fuel and no pollution.

The passive solar water heater is known today by many names; PSWH, Batch Heater and Bread Box are the most common and then there is the very technical; Integrated Collector and Storage System (ICS).

The PSWH of today usually starts with a 40 gallon, glass lined tank. These tanks come disguised as ordinary electric water heaters, which are stripped of their appliance shell and insulation. Painted flat black, with high temperature engine or barbecue paint and they're ready for solar.

 The box should be well insulated to prevent energy loss and the amount of insulation should reflect your local climate. The typical box is constructed with 2X4s or 2X6s, using fiberglass batt insulation. The exterior siding may match that of your home, or some other material suitable for your area. The interior sheathing is often ridged insulation, preferably with a foil face facing in which works to reflect more

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energy onto the tank. Ridged insulation comes in various thicknesses which can help increase your insulation R-value.

The size of the box must be big enough for the tank, but also large enough to allow adequate solar gain. Typical glass sizing is 1 sq. ft. of glass for every 2 to 2 1/2 gallons of water. A standard size, double glazed, patio door replacement glass (34"X76") is ideal for a 40 gal. water heater.

A water heater has an inlet and outlet and how you attach your plumbing does make a difference. The cold water inlet has a dip tube which extends down nearly to the bottom of the tank, to deliver the cold water to the right place. The hot water outlet takes the hottest water from the top of the tank. If the design calls for the tank to lie on its side be sure that the cold inlet is at the bottom.

Be aware that the 40 gallon tank when filled with water will weigh over 350 lbs. Add to that the weight of the box/glass and it's time to reconsider putting this monster on your mobile home. Ground mounted PSWH are very common. As always be sure the system will receive full sunshine from 9 am to 4 pm. Remember, if your installing a solar system and you're working in the shade, there's something wrong.

If the collector will be installed on a frame roof it's best to attach in such a way as to spread the weight over a few rafters, and if possible, provide additional support with braces extending up to the rafters from interior walls. The ideal location is as close to the existing water heater as possible.

Shorter plumbing runs are not only more efficient, they decrease the winter freezing potential. The chances of freezing 40 gallons of water are minimal but frozen pipes are a reality. With the tank installed close to the water heater the freezing potential is minimized but not eliminated. All plumbing between the existing water heater and the PSWH is insulated, with more insulation on any pipes exposed to the outside.

Plumb the system by first supplying cold water to the solar tank. From there, the hot water outlet is plumbed to the cold water inlet on your existing water heater. As long as the solar water entering your water heater is above the thermostat setting, your water heater does nothing. When the temperature of the solar water entering the water heater is less than the thermostat setting, your water heater makes up the difference.

The temperature of the water from a PSWH depends on many variables. The amount of sunshine, ambient air temperature, the amount of insulation used, the temperature of the supply water as well as the hot water demand all effect outlet temperature. Under ideal weather conditions, and no hot water used since morning, the water temperature at 5 pm can exceed 180 degrees F.

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Solar water heating uses the sun's energy to heat water for many different uses. There are successful examples of it's use in homes for heating hot water, a pool and/or spa, as well as the home itself. It has also been used successfully in a variety of commercial and institutional buildings.

Solar water heating is worth considering because it offers very compelling economic, environmental, and energy sustainability benefits.

Economic Benefits

Solar water heating is generally a worthwhile economic investment whenever there is a combination of one or more of the following available to you:1) Average to above average sunshine

2) Average to above average cost of heating fuel (or electricity)

3) Average to above average need for heat

From a larger economic point of view, solar water heating system becomes very compelling when the hidden "economic and environmental costs and benefits" (externalities), as well as the subsidies we pay (for competing conventional fuels) are considered.

Environmental Benefits

Because most heating systems burn fossil fuels, which creates pollution, all the energy saved by a solar water heating eliminates the pollution that would otherwise add to our problems of poor air quality, poor health, smog and global climate change, a problem which requires immediate action.

Climate scientists are just now beginning to understand the consequences of burning fossil fuels to generate electricity and heat. They agree global warming will bring higher sea levels and an increase in the frequency and severity of damaging storms (how much and how soon they don't know yet). Many global banking and insurance companies, concerned about the long term health of their industries, recognize the long-term economic consequences and are supporting sustainable energy sources as part of the solution.

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Sustainability Benefits

World petroleum supplies, at the current rate of consumption, won't last a baby born today past his/her 40th birthday. Many believe the current rate of consumption won't stay constant, but will rise into the future, shortening that 40 year period. Solar water heating is one way to help stretch these limited supplies.

The energy used each year by an average family (four people) with an electric water heater is roughly equal to the energy used by a medium-sized automobile driven 12,000 miles a year at an average fuel efficiency of 22 miles per gallon (about 11 barrels of oil). A residential solar water heating system can drop that consumption (and your hot water bill) by 50 to 80%, reduce pollution, and give you a sound investment for the future.

THE GOOD NEWS

EconomicsSolar water heating can be a very attractive investment when it saves you more on your water heating bill than it costs you to buy the system. How good is a solar water heating investment? It all depends on how much the system costs and how much it saves on your hot water bill. To be thorough, it's also important to account for the cost of maintenance, repairs, the estimated future rate of inflation and energy costs, financing terms (if you borrow the money), and perhaps even environmental benefits. As complicated as this may sound, it's easy to make a decision once all these facts are accounted for - the best way to do that is with an economic analysis, where the solar system is compared to a conventional water heater. In some cases, financing or tax credits may also be available.

Solar also improves the overall sustainability of our economy, both now and in the future, by providing jobs, by helping to reduce the need to buy fuel from foreign sources, and by reducing the ever-increasing environmental costs, we are just now beginning to understand.

Because the economics of solar water heating depends on the particular application, professional assistance is recommended in evaluating the economics of your application. Professional assistance providers should check out our list of professional tools as well as a sample Life Cycle Cost (LCC) analysis comparing the performance, economics, and other features of water heating options (including solar).

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PerformanceIf the weather in your location is suitable and you use the average families amount of hot water each day (about 64 gallons), solar water heating is worthy of serious consideration. The table below will give you a general idea of the investment value, based on average U.S. sunshine and different backup water heater types (a backup heater is needed to heat the water when the sun doesn't shine).

Comparative Solar Water Heater Investment Value

(based on U.S. average fuel costs & climate)

Electric backup Typically a good investment

Propane or fuel oil backup Generally a good investment.

Natural gas backup

Generally not a good investment unless environmental benefits and fuel subsidies are included, and then it is about the same cost or more per year (annualized life cycle cost).

Environmental benefits

Because solar water heating can reduce the electricity or fuel used to heat water by 50% or more, the pollution caused by water heating is reduced by the same amount. The world's top scientists and government officials have, and will continue to make, new estimates of the cost of pollution As you might imagine, the cost of pollution continues to rise as our population increases and we better understand the consequences of climate change. As the cost of pollution goes up, the environmental benefits reaped from your solar water heater will become more and more valuable!

THE BAD NEWS

Potential for overheating or freezing

A solar water heating system may overheat or freeze if it is not properly selected, designed, and installed. This can cause expensive damage to the system, or worse, a safety hazard. Fortunately, these types of problems can be avoided by using professional assistance in getting your system.

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How competitive is solar water heating, costwise, against other water heaters?The answer is "it depends." An economic analysis of various water heating alternatives available in your area is recommended, and can usually be obtained at a reasonable cost. Although solar has a higher first cost than most other water heating systems, it can be very competive over the long term.

In general, solar pool heating is an excellent, low cost, high return investment for those interested in extending their pool heating season as well as saving energy during the winter months. Given the amount of heat a pool requires, it's not practical to use gas or electricity. Pool systems are widely available and becoming very popular due to their simplicity, low cost.

In terms of domestic hot water, homes with electric or propane gas water heaters generally make a good solar investment. Homes with natural gas water heating, due to the current low cost of natural gas, make it more difficult for solar to be a good investment. That said, do not be discouraged away from solar if you have natural gas, as certain applications can still be very cost effective. Subdivision homes, multi-family homes with central water heating, and other applications can benefit from "economy of scale." Users of large quantities of hot water, such as car washes and laundries, can also benefit more from solar energy, compared to those with small hot water needs, because the "first cost" of solar will be recovered more quickly.

For a subdivision example, the most common way most people get into new homes, the California Energy Commission completed a limited study of various water heating systems in 1995. The study covered just seven different types of water heating systems (two different types of solar systems), but it does include a fairly comprehensive economic analysis of each system. They used actual installed costs for one of the solar systems (the passive system) as it had recently been installed in homes in a local subdivision. The cost of the water heating system is financed along with the rest of the home in a 30 year mortgage at 8% annual interest. The graph below shows the results of the Commission's study:

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The "total annualized cost of ownership" is the cost to install, maintain, fix, replace, and operate the water heating system over a 30 year period. Note: for those familiar with economics terminology, this is the annualized value of the life cycle cost. In this case, a natural gas water heater is the least costly option, followed by a passive solar system with a natural gas backup. Remember that this is just one example, looking at just two different solar water heating systems (there are several hundred others available), a specific set of economic criteria, etc. Professional assistance providers can help you find the most appropriate system based on your own hot water use, climate, system options, and so on.

Now, what if we were to estimate the cost of subsidies and externalities, account for these "hidden costs" in the economic analysis example, and revise the previous chart accordingly? Subsidies and/or externalities are of interest to decision makers ranging from consumers to the U.S. Forest Service. Any time "hidden costs" are included they should be carefully evaluated to assure the basis behind the selection is consistent with the interests of those involved. The results are shown below:

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In this case, the externalities are limited to environmental impacts alone, and subsidies are estimated to add 10% to the cost of electricity and $0.05/therm for gas. Using these assumptions, the cost of a solar water heater with gas backup amounts to less than a dollar extra per month relative to gas alone. Again, remember that this is just one example, looking at just two different solar water heating systems, etc.

The solar systems that will be discussed in this section are not a part of a building's structure. The function of the solar energy equipment is to convert sunlight to heat that can be used for: (a) space heating; (b) space cooling; (c) domestic hot water.

CONSIDERATIONS:

Solar systems should be employed only after extensive conservation strategies have been implemented. Solar energy systems typically have a high initial cost and extremely low operating costs. To reduce the high initial costs, reduce the size of the required system by the load that the solar system will need to provide. In space heating and cooling applications, the home should be weatherized and insulated to very high standards. In water heating applications, hot water piping should be insulated and water conserving fixtures should be used.

The goal of the solar system should not be to accomplish 100% of the home's heating, cooling, or water heating needs under all conditions. The system should be sized to reflect seasonal variations in demand and in the sun's heating characteristics. Additionally, by combining systems to perform multiple functions (i.e. space heating and water heating), the solar system investment can provide a return all year.

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CommercialStatus

ImplementationIssues

Active Solar Heat

Active Solar Cooling

Passive Solar Water Heating

Active Solar Water Heating

Legend

Satisfactory

Satisfactory in most conditions

Satisfactory in Limited Conditions

Unsatisfactory or Difficult

COMMERCIAL STATUS

TECHNOLOGY

Active and passive solar space heating and water heating, are well-developed technologies. Active solar space cooling is marginally developed.

IMPLEMENTATION ISSUES

PUBLIC ACCEPTANCE

There are problem areas associated with the general public perception of solar systems: solar may be considered futuristic; some may believe new technological breakthroughs are needed to make solar viable; solar systems are considered uneconomic; and, the business instability of solar system providers during the early 1980s. A primary concern for owners of a solar system is whether it can be maintained by conventional means (the owner does not have to assume extraordinary responsibilities).

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GUIDELINES

Introduction

Solar energy can be captured for use in a home in several ways. This section will look at using solar energy to heat water and/or air. The hot water created by a solar system can be used for domestic hot water or space heating. Hot air solar systems are primarily used for space heating.

The fundamental requirement for a solar system is to have a sunny location where the solar collectors can be located.

The collectors should have full sun from 9 AM to 3 PM.

The collectors should face south at approximately the same angle as our latitude (30 degrees).

Collectors can be oriented as much as 30 degrees off of south and still function well. Similarly, the slope of the collectors can vary by plus or minus 15 degrees without significantly harming the performance of the system.

Active Solar Domestic Water Heating

The active water systems that can be used to heat domestic hot water are the same as the ones that provide space heat. A space heat application will require a larger system and additional connecting hardware to a space heat distribution system.

There are five major components in active solar water heating systems:

Collector(s) to capture solar energy.

Circulation system to move a fluid between the collectors to a storage tank

Storage tank

Backup heating system

Control system to regulate the overall system operation

There are two basic categories of active solar water heating systems - direct or open loop systems and indirect or closed loop systems.

o Direct Systems

The water that will be used as domestic hot water is circulated directly into the collectors from the storage tank (typically a hot water heater which will back up the solar heating).

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There are two types of direct systems - draindown and recirculating. In both systems, a controller will activate a pump when the temperature in the collectors is higher than the temperature in the storage tank.

The draindown system includes a valve that will purge the water in the collectors when the outdoor temperature reaches 38 degrees. When the temperature is higher than 38 degrees and the collectors are hotter than the storage tank, the valve allows the system collectors to refill and the heating operation resumes.

The recirculating system will pump heated water from the storage tank through the collectors when the temperature drops to 38 degrees.

These two systems have serious drawbacks. The draindown valves can fail in a draindown system and the result can be the expensive breakage of the solar collectors. The draindown valve will typically sit unused for a very long time and then will need to work the first time without failing. The cycling of air and water in a draindown system collectors as a result of periodically draining down (thereby emptying the collectors) can cause a buildup of mineral deposits in the collectors and reduce their efficiency. The recirculating system circulates buildup from potable water heated from the storage tank through collectors during potential freeze conditions and effectively cools the water (wasting energy).

o Indirect Systems

Systems that use antifreeze fluids need regular inspection (at least every 2 years) of the antifreeze solution to verify its viability. Oil or refrigerant circulating fluids are sealed into the system and will not require maintenance. A refrigerant system is generally more costly and must be handled with care to prevent leaking any refrigerant.

An indirect system that exhibits effectiveness, reliability, and low maintenance is the drainback system (see Figure 1 on next page).

The drainback system typically uses distilled water as the collector circulating fluid.

The collectors in this system will only have water in them when the pump is operating. This means that in case of power failure as well as each night, there will be no fluids in the collector that could possibly freeze or cool down and delay the startup of the system when the sun is shining.

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This system is very reliable and widely used. It requires that the collectors are mounted higher than the drainback tank/heat exchanger. This may be impossible to do in a situation where the collectors must be mounted on the ground.

An indirect or direct system can be used for heating swimming pools and spas. Lower cost unglazed (no glass cover) collectors are available for this purpose.

Figure. 1

DRAINBACK HOT WATER SYSTEM

The fluids that are circulated into the collectors are separated from the heated water that will be used in the home by a double-walled heat exchanger.

A heat exchanger is used to transfer the heat from the fluids circulating through the collectors to the water used in the home. The fluids that are used in the collectors can be water, oil, an antifreeze solution, or refrigerant.

The heat exchangers should be double-walled to prevent contamination of the household water.

The controller in these systems will activate the pumps to the collectors and heat exchanger when design temperature differences are reached.

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The heat exchanger may be separate from the storage tank or built into it.

o Guidelines summary for solar domestic water heating systems:

A well designed system will provide 50-80% of a home's hot water needs (less in winter, more in summer).

There should be 10-15 square feet of solar collector area for each person in the household.

The storage tank should hold 20-30 gallons per person.

There should be no shade on the collectors during the hours from 9:00 AM to 3:00 PM.

The collectors should face south and be tilted at a 30 degree angle (slight variations noted above will not significantly harm performance).

The collectors and storage tank should be in close proximity to the backup system and house distribution system to avoid excessive pipe losses. The pipes need to be well insulated.

Mixing valves or thermal shutoff devices should be employed to protect from excessively high temperatures.

Select systems that are tested and certified by the Solar Rating and Certification Corporation (SRCC).

Active Solar Space Heating

The active solar space heating system can use the same operational components as the domestic water heating systems, but ties into a heating distribution system that can use heated fluids as a heat source. The distribution system includes hydronic radiator and floor coil systems, and forced air systems.

Solar collectors are also constructed that heat air. The hot air developed in such collectors can be used directly in the home during the daytime or stored in massive materials (rock or water).

Water Heating Collectors o The tilt of space heating collectors is generally the latitude plus 15

degrees (45 degrees in Austin).

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The purpose is to align the collectors perpendicular to the sun's rays in the heating season when the optimal performance is needed.

o The number of collectors used in a space heating application is based on the heat load of the house.

Average heat load / collector rated heat output = number of collectors needed.

By basing the size of the collectors on the average heat load of the home during the heating season, the system will not provide enough heat during the colder part of the heating season. Since the heat load of the house is dependent upon the extent of its energy conserving features, the greatest energy efficiency the home can have, the smaller the solar system will have to be.

o The space heating system, like the domestic water heating system, must be backed up by an auxiliary heating system.

It is not practical to size a solar system to provide all of a home's heat requirement under the worst conditions. The system would become too large, too costly, and oversized for most of the time.

o The storage system should be sized to approximately 1.5 gallons of storage for each square foot of collector area. The fluid that is heated and stored (typically water) can be distributed into the house heating system in the following ways:

Air distribution system - The heated water in the storage tank is pumped into a coil located in the return air duct whenever the thermostat calls for heat. The controller for the solar system will allow the pumping to occur if the temperature in the solar heated water is above a minimum amount needed to make a positive contribution to heating the home. An auxiliary heater can be used in two ways. It can add heat to the solar storage tank to maintain a minimum operating temperature in the storage tank at all times. In this case, the coil from the solar system will be located at the air handler supply plenum rather than in the return air duct. The auxiliary heater can also be a conventional furnace that will operate less often due to the warm air entering the air handler from the solar coil in the return duct.

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Figure 2

SPACE HEATING SYSTEM

Hydronic system with radiators - The heated water is circulated in series with a boiler into radiators located in the living spaces. Modern baseboard radiators operate effectively at 140 degrees. Solar heating systems can very often reach that temperature. Using the solar system's heated water as the source of water for the boiler will reduce the boiler's energy use particularly if it senses the incoming temperature and will not operate when that temperature is above the required distribution temperature.

Hydronic system with in-slab heat - The solar heated water is pumped through distribution piping located in the floor of the home. Lower temperatures are used in this type of system (the slab is not heated above 80 degrees in most cases). The auxiliary heat can be connected in series with the solar system's heated output water or it can be connected to the solar tank to provide a minimum temperature.

In the Austin area, most homes use an air distribution system that can provide air conditioning as well as heating. The hydronic systems are much less common but are considered highly effective in terms of comfort, efficiency, and health impact (no blowing air to stir up dust). The air distribution method described above can work quite well with a conventional gas water heater as a backup.

Air Collectors for Heating

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Appear similar to a water collector.

Usually a black metal absorber in an insulated box with a glazed cover (glass or plastic).

Air from inside the house is drawn by a fan into a series of channels in a space behind the absorber where it is heated by the hot absorber plate. The heated air then enters the home directly or enters a storage medium (such as rocks) so the heat will be available during the night.

A simple controller is used to turn on the fan(s) in this type of system. The controller uses sensors in the collector to activate the system when it is hotter in the collector than in the house interior or storage medium.

Air collectors can be mounted vertically on the south wall of a building if used for space heating only. In that location, properly designed overhang will prevent them from heating up in the summer.

For a year-round application of air heating collectors, it is necessary to use an air-to-water heat exchanger. This is not a very efficient system for heating water compared to fluid circulating collectors, since heat (and thus efficiency) is lost at each transfer point.

Air collectors are more practical in climates with longer and colder winters than in Austin. The investment in storage systems for air collectors is substantial in time, money, and materials. The use of air collectors to put heat into the house directly can be readily achieved with properly oriented windows in our area. Daytime temperatures in the winter can be relatively high; the additional hot air from an air collector can overheat a home that does not have extra thermal mass to absorb the heat.

Active Solar Space Cooling

Solar space cooling is quite costly to implement. If the solar system is used for space cooling only, installed costs can run $4,000-$8,000 per ton. It is best to use a solar system that serves more than just the cooling needs of a house to maximize the return on investment and not leave the system idle when cooling is not required. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system.

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Figure 3SCHEMATIC OF SOLAR ABSORPTION COOLING SYSTEM

T = system flow sequence

The technologies that are being developed for gas cooling systems are the same ones being developed for active solar space cooling systems. Desiccant cooling systems and advanced absorption systems are the primary technologies that are used. High temperature liquid collectors are typically used in these systems.

o Desiccant system

A moisture absorbing material (desiccant) is located in the air stream going into the living space. As the air passes through the desiccant, which is usually located on a wheel that slowly rotates into the air stream, moisture is removed from the air, dropping the humidity level in the air stream to the point that an evaporative cooler can then cool the air. The desiccant is dried by the heat generated by the solar collectors as it rotates out of the air stream.

o Absorption air conditioning

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Heat from solar collectors separates a low boiling refrigerant in a generator which receives the pressurized refrigerant from an absorber. Solar heat can also be used in the evaporation stage of the cycle.

Passive Solar Water Heating

A passive solar water heating system uses natural convection or household water pressure to circulate water through a solar collector to a storage tank or to the point of use. Active systems employ pumps and controllers to regulate and circulate water. Although passive system are generally less efficient than active systems, the passive approach is simple and economical.

Passive water heating systems must follow the same parameters for installations as active systems - south facing unshaded location with the collector tilted at the angle of our latitude. Since the storage tank and collector are combined or in very close proximity, roof structural capacities must accommodate the extra weight of a passive system which can be 300 pounds or more.

There are two types of passive water heaters : batch and thermosyphon o Batch System

The batch system is the simplest of all solar water heating systems.

Figure 3SCHEMATIC FOR GROUND-MOUNT BATCH DOMESTIC WATER

SYSTEM

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It consists of one or more metal water tanks painted with a heat absorbing black coating and placed in an insulating box or container with a glass or plastic cover that admits sunlight to strike the tank directly. The batch system's storage tank is the collector as well. These systems will use the existing house pressure to move water through the system. Each time a hot water tap is opened, heated water from the batch system tank is removed and replaced by incoming cold water.

The piping that connects to and from the batch heater needs to be highly insulated. On a cold night when no one is drawing hot water, the water in the pipes is standing still and vulnerable to freezing. In many applications, insulated polybutylene piping is used because the pipe can expand if frozen. The water in the batch heater itself will not freeze because there is adequate mass to keep it from freezing.

Since the tank that is storing the heated water is sitting outside, there will be heat loss from the tank during the night. This can be minimized by an insulating cover placed on the heater in the evening.

The most effective use of a batch water heater is to use hot water predominantly in the afternoon and evenings when the temperature in the tank will be highest.

Manufactured batch heaters have a "selective surface" coating on the tanks that will absorb heat most readily yet permits very little heat loss. This feature is very valuable in these type of systems as it helps insulate the tank.

o Thermosyphon Systems

Figure 4THERMOSYPHON SYSTEM

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The thermosyphon system uses a flat plate collector and a separate storage tank that must be located higher than the collector.

The collector is similar to those used in active systems.

The storage tank, located above the collector receives heated water coming from the top of the collector into the top of the storage tank. Colder water from the bottom of the storage tank will be drawn into the lower entry of the solar collector to replace the heated water that was thermosyphoned upward. The storage tank may or may not use a heat exchanger. The thermosyphon system is more costly and complex than the batch system. In our area, it is best to use an indirect system (one that employs a heat exchanger). In that case, antifreeze can be used in the system eliminating freeze ups.

o Sizing

The sizing of a batch system and thermosyphon system are both based on a usage figure of 20 gallons of hot water per person per day. For example, if the storage tank in these systems is 40 gallons, that would equal the requirement for two people. The collector area in the thermosyphon system should equal approximately 20 square feet per person.

The system is not sized for 100% of the energy requirement. A backup source is needed.

Solar cooking

The Solar Box Cooker is one way of using sunshine to cook food.Solar cooking uses sunshine for cooking.   We can cook food without electricity, gas,  firewood or any fuel that costs money or needs to be gathered.  Sunshine is free and available to everybody in many parts of the world all year round.  

A solar cooker or a solar oven concentrates the solar energy and heat to focus it on a small area.  While solar powered cooking takes longer than conventional cooking, the food tastes different, never gets burnt, and does not need to be watched so it is possible to do other chores while the sun is cooking the food.  Solar powered cooking does not pollute the atmosphere or generate carbon dioxide gas that is generated by combustion.

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Solar desalination

INTRODUCTION

Desalination of sea and brackish water has become a necessity in many arid and semiarid regions. Due to the fast growing population and a correspondingly high water demand in these regions the few water sources often get brackish or contaminated.

Waste and contamination of water sources are increasingly creating serious problems also in the northern hemisphere, where water supply never used to be a problem.

In most cases, the water supply in dry regions is realized using desalination plants with daily fresh water outputs between 1000 and 100,000 m3 (RO, MSF).

The water supply system using big plants running with fossile energies, however, is not always an economic and ecologic solution /1, 2/:

Isolated regions or islands have to be supplied by truck, ship or even aeroplane. This results in high costs.

The generation of high purity distilled water that can be used also for medical or industrial applications is expensive.

The use of fossile energies for desalination leads to an environmental load.

Due to the fact that big regions rely on the function of one big plant breakdowns are of concern for many people.

The presented technology is able to supply clean water with plant capacities up to 20 m3/d by avoiding the mentioned disadvantages of conventional technologies.

The membrane distillation (MD)-plant can be driven by solar heat or waste heat and is therefore ideal to deliver water at remote areas with poor infrastructure (Fig. 1):

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DESCRlPTION OF THE MEMBRANE DISTILLATION (MD)-PROCESS

It is possible to concentrate aqueous solutions of non-volatile dissolved substances by microporous membranes impermeable for water but permeable for water vapour. Driving force for this "membrane distillation" is a vapour pressure difference on both sides of the membrane due to a corresponding temperature gradient across the membrane.

The distillation is performed at ambient pressure and at a maximum temperature of 80°C (175°"F). Operating costs are extremly low because the process can be driven by low temperature heat sources eg. solar heat or waste heat from diesel engines /3/.

The system is employing spiral wound desalination modules. Inside the distillation modules a thin microporous hydrophobic PTFE-membrane is used with pore diameters between 0.051 and 0.2 . This material shows the surprising property of allowing easy passage of water vapour, but of completely blocking the flow of liquid water. The high surface tension of water prevents the passage of liquid water through the sub-micron pores up to a pressure of typically 0.5 MPa (72.5 psi).

In the process one surface (hot side) of the flat sheet membrane is in contact with the process solution while the opposite surface (cold side) is in contact with distillate. Thus the diffusion gap between evaporating and condensing surfaces is

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reduced to the thickness of the membrane that is only about 30 m. With an actual pore fraction of 80% high specific evaporation rates are possible.

The recovery of the heat of condensation is done by utilizing the heat of condensation to preheat the feedwater.

DESIGN OF THE DESALINATION MODULE

Fig. 2 shows schematically the principle of a membrane distillation module with integrated recovery for the heat of condensation.

Cold feedwater (temperature Tl) enters the module and is progressively heated by the hot condenser sheet, so that it emerges from flow channel 1 (heat recovery channel) on a significantly higher temperature level (temperature T2).

Before the feedwater reenters the module into flow channel 2, the temperature has to be elevated from T2 to T3 using an external heat source. The distillation takes place from flow channel 2 across the membrane into flow channel 3. The feed water is gradually loosing heat and is getting concentrated. The temperature difference between flow channel 2 and 3 is the driving force for the process and is maintained along the whole channel length. The concentrate emerges therefore with a higher temperature than the incoming feed. The distillate is collected in flow channel 3 and emerges from the module almost at ambient temperature (between Tl and T4). The spiral wound design of the module (Fig. 3) allows high recovery rates of latent heat, eliminates the need for thermal insulation and mechanical support and performs as a compact and resistant unit.

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Experimental desalination modules for membrane distillation have been developed by several companies. Due to high manufacturing costs, poor distillate output and material failures a functional, economic and reliable module system has not been available so far /4, 6/.

When we started a R&D-program for membrane distillation modules in 1989, materials technology was more advanced and highly resistant materials were available at more reasonable prices.

The whole module construction has been optimized in terms of

distillate output pressure losses material stability manufacturing technique

Failures became less probable due to a new construction of the flow channels for feed water and distillate, that keeps pressure losses at a minimum /5/.

Module performance is constantly tested until the present day.

In case there is a high amount of waste energy available, it is possible to achieve high specific membrane flow rates by using a 2-channel module with a

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countercurrent flow of distillate and concentrate. High heat transfer rates result in a 3 to 5 times higher distillate output compared to modules with integrated recovery of the heat of condensation. These modules have been developed recently and will be available soon.

SYSTEM ADVANTAGES

efficient and compact spiral-wound membrane distillation modules recovery of the heat of condensation is integrated in the module design chemical pretreatment of feed water is not required - low system pressure insensitive to dry-running and fouling - neglectible scaling due to process temperatures below 80°C (176°F).

Because of low process temperatures solar energy and waste energy can be used to run the plant /3, 4/. The operational efficiency and the long term behaviour of the process for the seawater desalination has been proven in pilot installations on the Canary islands and on the island of Ibiza.

TECHNICAL DATA OF A PROTOTYPE PLANT

In order to test the module performance and to optimize the operation under real conditions a prototype plant has been tested on the island of Ibiza/Spain since May 1993 (Fig. 4, 5).

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Technical Data of the MD-prototype plant in Ibiza/Spain

DESALINATION UNIT

plant dimensions 2.5 x 0.8 x 1.5 m (8.3 x 2.6 x 5 ft)

number of modules 4

feed water flow 0.8 - 1.7 m3/h (3.5 - 7.5 gpm)

distillate conductivity <10 S

distillate flow 40-85 I/h (0.17 - 0.40 gpm)

brine temperatures 60 - 80 "C (140 - 175°F)

energy consumption 150 - 200 kWhlm3

(570 - 750 Wh/gal)

energy source solar heat, waste heat

SOLAR UNIT

vacuum flat plate collector area 51m2

storage collector area 45m2

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PLANT OPERATION

In order to keep heat losses for plant start up to a minimum a 24-h-operation is necessary. This also improves the plant utilisation and reduces material stress through stop and go operation. The energy to run the plant during the times without solar radiation is delivered from a storage collector with a water capacity of 10 m3.

PLANT DESIGN AND DATA FOR A SINGLE MODULE UNIT

To show the simple design of the membrane distillation systems a flow diagram (Fig. 7) for a single module unit is added. Instead of the solar collector the unit can also be combined with a waste heat source.

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Numerical codings of Fig. 7:

1. Filter2. Magnetic Valve

3. Pressure Reducing Valve

4. Flow Meter

5. Pressure Indicator

6. Heat Exchanger

7. Conductivity Meter

8. Temperature Indicator

9. Expansion Container

10. Safety Valve

11. Solar Pump

12. Degasifier

13. Insulation

Components of the desalination circuit:

Desalination module Heat exchanger Feed, brine and distillate pipes Pressure adapting valve Metal frame and housing Sandfilter Flow-meter Feed pressure indicator Conductivity meter Temperature indicator Control board

The desalination plant is a compact unit with outer dimensions of about (1.O x I.5 x 0.8m) (3.3 x 5 x 2.6 ft).

Module design:

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Spiral wound module cylinder: dimensions: height 500 mm (20 inch), diameter 460 mm (18 inch)

... membrane area: 10m 2 (108 ft2) ... condenser area: 10m2 (108 ft2)

Operational data

Prepositions:

- the feed water entry pressure is between 0.2 and 0.4 bar (2.9 - 5.8 psi) - feed water is available in big quantities - concentrate discharge possible - no oil or surfactants in the feed water

Plant data:

feed flow 300500 I/h(l.I - 2 gpm)

distillate flow 15-25 l/h(0.05 - 0.09 gpm)

distillate quality <10S/cm

feed water temperatures IO-40 °C (50-100°F)

brine temperatures 60-80°C( 140 - 175°F)

temperature level of heat source 70-90°C (l60 - 195°F)

average input of thermal energy 150-200 Wh/l(570-750 Wh/gal)

electric energy demand none (except for the magnetic valve)

Fig. 8 shows a data evaluation sheet for a solar driven single module plant. The upper diagram displays the development of the temperatures Tl - T4 (corresponding to Fig. 2 and Fig. 3) during a sunny day. In the diagram below the corresponding distillate production is displayed. Because of the high heat capacity of solar collector, piping and the module the start of the distillate production is delayed. For that reason 24 h-operation at fixed flow rates is in most cases the more economic alternative.

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Summary

Small simple desalination plants operating independent from the electric grid are either not available or not economic at all. The process of membrane distillation allows the effective use of low temperature heat sources like solar energy or waste energy from engines for small to medium scale desalination.

Although the process of membrane distillation is known since over 30 years cost-effective desalination modules have not been available so far.

In order to achieve an effective membrane distillation process spiral wound modules have been developed and optimized during a 6-year R&D program. The modules are designed as compact units with integrated recovery of the heat of condensation, allowing a highly efficient use of low temperature heat sources.

In case there is a high amount of free waste energy available the heat recovery is not needed and flow rates 3-5 times higher can be obtained. Desalination modules without integrated recovery of latent heat have been developed, too and will be available soon.

Individual plant sizes are available with product water output ranging from 0.l (25 gal) to 5m3 (1300 gal) per day.

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Solar refrigeration

Solar refrigerator (2m2)

Adsorption pair: SilicaGel + water. Tested in 2000-2001.

Solar refrigerator (1m2) and Solar cold store (20m2)Adsorption pair: zeolite NaX + water. Tested in the early eighties.

References

http://www.epsea.org/wtr.htmlhttp://www.epsea.org/stills.htmlhttp://www.solar4power.comhttp://www.eia.doe.gov

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/1/ E. Delyannis. V. Belessiotis Solar desalination, is it effective? Desalination and Water Reuse Vol. 414

/2/ J. Manwell, J. McGowan, Recent renewable energy driven desalination system research and development in North America Desalination, 94 (I994) p. 229-241

/3/ K. Schneider, T. van Gassel, Membrandestillation Chem.-lng.-Tech. 66 (1984) Nr. 7, S. 514-521

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