1

Energy Cost Savings and Greenhouse Gas Reduction– A Comparison Between Solar Water Heating Systems and PV Solar Electric Systems

Abstract .......................................................................................................................................... 2Introduction .................................................................................................................................... 3Residential PV Electric System .................................................................................................. 5

Grid-tied System and Net-Metering ....................................................................................... 5Advantages of a Grid-tied PV System Without Batteries ................................................... 5Size of the PV System and How it Impacts the Homeowner’s Electricity Bill ................. 5Electricity Requirements for a Home .................................................................................... 6Factors to Consider for PV Sizing ......................................................................................... 7

System efficiency ................................................................................................................. 7Cell efficiency and roof foot-print ....................................................................................... 7Number of PV panels .......................................................................................................... 7

Cost of a Grid-tied PV System ............................................................................................... 8Reasons for Installing a PV System .................................................................................... 10

Environmental benefits ...................................................................................................... 11Residential Solar Water Heating (SWH) System .................................................................. 13

Sizing of a SWH System ....................................................................................................... 13Collector area for SHW systems ...................................................................................... 13Storage tank volume for SWH systems .......................................................................... 14

Pricing of SWH Systems ....................................................................................................... 15Environmental Impact of SWH Systems - Reduction of Greenhouse Gas (GHG) Emissions ................................................................................................................................ 17

Standby heat loss for tank-based hot water system ..................................................... 18Conclusions ................................................................................................................................. 20APPENDIX - I: Effect of the Sun on System Performance .......................................................... 22APPENDIX - II: Grid-Tied PV System ........................................................................................ 27APPENDIX - III: Solar Water Heating (SWH) System ............................................................... 31References ..................................................................................................................................... 38

2

Abstract The purpose of this paper is to study two solar technologies, the PV solar electric

system and the solar water heating system (also referred to as solar hot water system),

in detail, with respect to residential applications. We evaluate both these systems for

their potential economic and environmental benefits. In particular, we discuss the cost

effectiveness of both the PV solar system and the solar water heating system, and their

ability to reduce greenhouse gas emissions.

Many countries, such as Japan, Germany and the United States, indicate that solar

energy (via PV solar electric systems) is increasing its share in the energy

infrastructure, in a significant manner. However, the solar hot water system has

received much less attention from these governments (as well as from the media or

press) in recent years. The authors of this paper believe that there are significant merits

for solar hot water systems as far as economic (less expensive, quicker pay back

period) and environmental benefits (greenhouse gas emission reduction) are

concerned. A technical comparison of active solar hot water systems versus the

grid-tied PV solar electric system is provided for household use in terms of their ability

to reduce homeowner costs and their ability to prevent greenhouse gas generation.

Although these systems qualify for subsidies or tax rebates, we don’t include these cost

components in our calculation in order to eliminate the effects of policies promoting

adoption of these technologies. On the basis of this study, we suggest a review of state

and national subsidy policies for widely divergent systems such as PV solar electric

systems and tank-based hot water systems.

Methodology: We investigated the topic of this work with the help of several books and

research over the Internet. We used some available calculators from the Internet for

deriving some results. We also contacted some vendors for some of the data that we

used. Please see the References for a complete list.

3

Introduction

The technology behind a PV1 solar electric and a solar water heating system has

reached a stage where these systems not only bring significant benefits to home energy

costs but also enormous environmental benefits. With rising energy costs and

environmental concerns, both systems are understandably becoming popular. Since

electricity generation from a PV solar system or a solar water heating system is a clean

process (there is no greenhouse gas emission2 into the air), homeowners are buying

these systems in increasing numbers. However, both systems require upfront

investments, and it is in the interest of the homeowners that they understand the

financial and ecological implications of such investments.

As a first step toward installing a solar system, the homeowner must determine

how much energy the house consumes on average. In order to determine this, the

homeowner must consider two main components of energy requirement: The first

component is the non-electric energy3 required for heating the house, the hot water, the

clothes dryer and the stove/oven. The other component is the electric energy required

for powering the refrigerator, the lights, the computer, the TV, stereo equipment, etc.

Both the solar water heating (SWH) system and the PV solar electric system provide an

alternative energy source for these components in the home; SWH systems use the

freely available energy from the sun for domestic heating needs (in particular, water),

the PV solar electric system provides the necessary electricity for home appliances and

is generated by using PV solar cells.

PV electricity has already become the energy choice of thousands of users in

many markets such as in the American Southwest, Japan and Germany [4], [5].

1 A photovoltaic cell (PV cell) is a specialized semiconductor device that converts visible light into direct current (DC). Photovoltaic cells are an integral part of solar electric energy systems, which are becoming increasingly important as alternative sources of utility power [1]. For extensive technical details, see [2]. 2 Most common are carbon dioxide, methane, nitrous oxides and ozone. In the United States, greenhouse gas emissions come mostly from energy use [3]. 3 Electricity-based water heating is also common in some U.S. locations.

4

Whereas the transition to a solar-based system is happening most often where three

factors combine––ample sunshine, expensive grid-based electricity and available

government-based incentives––the other main reasons for adopting solar solutions at

residential sites are [4]:

• Low impact living––devices running with electricity provided by a solar electric

system or households using water heated by a solar water heating (SWH)

system have less overall impact on fossil fuel.

• Clean and renewable energy––solar power is extremely environmentally friendly.

In fact, this is the reason why solar power was tapped in the first place.

Everyday, our planet is getting more and more polluted and anything that we can

do to prevent this from happening is a step towards having a better environment

not only for us but also for the generations to come.

• Low cost––solar module costs per installed watt have been declining for the last

decade at 5 to 6% per year because of technological advances, scale of

production and experimental learning.

• Local generation of electricity––shifting away from large centralized energy

production toward smaller, distributed energy generators, primarily because end

users will increasingly have cost effective options to avoid the embedded costs of

the existing energy infrastructure.

The purpose of this paper is to evaluate these technologies in detail and study

their potential economic and environmental benefits. In particular, we discuss the cost

effectiveness of both PV solar electric systems and solar water heating systems and

their ability to reduce greenhouse gas emissions [6]. Before we go into these systems

in detail, we provide a comprehensive study on solar radiation to understand how the

sun impacts the functioning of these systems. We also describe, in the context of the

PV solar electric system, what net-metering is and how this concept is playing a

significant role in popularizing the PV solar electric system.

5

Residential PV Electric System

A homeowner uses a residential PV power system [7], [8] to generate some or all

of his/her daily electrical energy demand. Usually, the PV panels, the main

subcomponent of the PV system are installed on the roof and generate electricity when

sunlight falls on these panels. The PV system may or may not be connected to the

electrical utility but may include battery backup or a generator.

Grid-tied System and Net-Metering

The most common residential PV electric system is a grid-tied system. A detailed

discussion about grid-tied PV systems is provided latter in this article (see APPENDIX –

II). U.S. law (applicable for residential homes in cities, though not applicable in rural

areas) states that utilities must allow homeowners to tie their PV solar electric systems

to the grid.

Net-metering is a significant step to encourage PV system installation.

Net-metering allows a homeowner to sell, via a grid-tied system, extra electricity

generated at the home, if any, back to the utility. This way, a homeowner can offset

part of the initial investment into a PV electric system with the help of net billing, i.e. the

cost of electricity for total units consumed minus total units sold.

Advantages of a Grid-tied PV System Without Batteries

A grid-tied system with batteries is useful for storing electrical energy when the

homeowner produces more electricity than he or she needs. However, at the present

time, battery-based systems are expensive and may increase the overall cost of a PV

system significantly. Also, for a grid-tied system without batteries, the homeowner does

not have to worry about the maintenance or replacement of batteries.

Size of the PV System and How it Impacts the Homeowner’s Electricity Bill

6

Since the cost of electricity varies depending on utility pricing tiers, the

homeowner’s chosen size of a PV system will directly impact the electricity bill. Note

that the size of the system is an important decision to make. The homeowner must

consider several factors such as upfront cost, type of PV panel and its efficiency,

wattage and voltage, etc.

The homeowner may size the PV system in such a way that all the electricity that

the home requires is available from the PV system. For a grid-tied system without a

battery, this implies that the PV system should be large enough to take advantage of

energy banking. In this approach, the PV system first generates excess electricity

during the day. This excess electricity is then fed to the utility grid and brought back

(preferably, at a lower rate) at night when the PV system is not producing electricity.

If the homeowner considers a PV system of smaller size than that mentioned

above, then a part (or even all) of the day-time electricity needed may come from the

PV system. In this case, the entire night-time electricity comes from the utility.

The homeowner must carry out a tradeoff analysis that is based on the upfront

cost of the system and how much electricity the PV system can produce during the day.

Electricity Requirements for a Home

The electricity usage (in kWh) for every home is different, and this depends on

different factors such as how the home is built, each family’s lifestyle, and appliances

that are inside the home. In order to design a PV system for a particular home, the

homeowner must consider the present and future requirements. There are two ways in

which the homeowner can evaluate his present consumption of electricity:

• Calculate the electricity consumption for each electric item in your home. This

requires the maximum wattage usage for the specific device and how long that

device will be used each day. Time is typically shown in hours.

7

• For grid-tied homes, the information (kWh consumed) in monthly electricity bills

(e.g. the last 12 monthly bills) can provide a clear indication about the

homeowner’s present electricity consumption.

Since the purpose of a PV electric system is to provide electricity for a very long

period, the homeowner must include changes in future electricity demands, e.g. addition

of a large-screen Plasma Television, in his or her calculations.

Factors to Consider for PV Sizing Please see [9], [10] and [11] for further details.

System efficiency When the homeowner builds a PV system, the size of the PV system must be

large enough to take care of all the losses before the electricity can actually reach the

wall socket. For example, the losses come from the PV panels, inverters, wires and

cables, site location, aging and shading. For a detailed calculation, see [12].

Cell efficiency and roof foot-print Although, for a similarly sized home in a specific area, you need to have a larger

area on the roof to install solar panels to meet a higher demand of solar energy, the roof

size (measured in square feet) directly depends on cell efficiencies, see [13].

Number of PV panels

In order to calculate the number of PV panels, the homeowner must evaluate the

requirement based on a typical off-the-shelf solar cell. The number of PV panels

required will be based on the following:

• Energy requirement and the homeowner’s decision on the size

• System (in)efficiencies

• Light intensity available at the site

• Characteristics of the solar cell (e.g. STC wattage)

See [14] on how a calculation is made on the number of PV panels.

8

Cost of a Grid-tied PV System

Research shows that the cost of generating electricity from a PV system is in the range of $8 to $10 per system wattage and decreasing every year [15]4. Note that this

range doesn’t take into account any government rebate (subsidy) or tax credit5.

Specifying a PV system in system wattage is a standardized measure that doesn’t

include the local solar condition. A homeowner, however, must take the local solar

condition into account before he/she installs the system.

Note. The total PV system cost includes the cost of PV panels, Balance of

System (BOS) and labor cost. The cost of the solar module represents 40-50% of the

total installed cost of a PV electric system [16]. The inverter is another expensive

component. Also, the homeowner should plan for replacement of any failed component

that may be necessary during the lifetime of the system. For example, most of the PV

components are warranted for a very long period (~25 years). However, the inverters

are warranted for only five years. So, the homeowner must factor in the price of such

replacement should it be necessary.

Table 1

4 The main reason for this is that the solar module costs per installed watt have

been declining for the last decade at 5 to 6% per year because of technological advances, scale of production and experimental learning. This trend will continue and will decline even more as global solar production continues its historical growth rate of 29% annually. This is further evident by reference [4] which shows the projected cost of $ per watt of installed PV for three phases of PV growth: Rapid-growth (during the year of 2005-2020), Displacement phase (during the year of 2020-2040) and Dominant phase (2040 and beyond).

5 Usually, the subsidy is set at a level such that the net electricity cost to the customer is competitive (with conventional electricity) prompting rapid growth of the PV market and supply-chain development by manufacturers, integrators and installers [4].

9

Available Energy from a 6kW PV System Over a Single Year

Los Angeles 6 * 1800 = 10,800 kWh per year

Boston 6 * 1200 = 7,200 kWh per year

Table 2

Cost for PV Solar Electric System at Two Different Locations

Location Typical cost of a PV solar electric

system that produces same kWh.

Los Angeles $54.000

Boston $81,000

Note. If we incorporate the total price of a 6 kW system as $54,000 (assuming the

maximum price of $9 per Watt), the cost per electricity generated (in kWh) is shown in

Table 3.

Table 3 Price Per Unit of Electricity Generated by the SAME System in Two Different Locations

Location Cost of electricity over a period of 25

years

Los Angeles $0.19 per kWh

Boston $0.27 per kWh

10

Reasons for Installing a PV System

The main attraction for the homeowner in investing in a PV system is that the PV

system can generate electricity at a fixed cost over a period of 20-30 years, reducing

the homeowner’s electricity bill. This is the period for which a typical PV electrical

system is warranted. Compare it with the cost of electricity6 for the last 46 years (1960-

2006) [17] that continuously faces the threats of volatile prices, adequate access to fuel

supplies and insecurity arising from potential nuclear threats. For relevant details, see

[18]. [19].

The prices listed in Table 3 show how a homeowner benefits from purchasing

electricity for a fixed price over the life-time of a system. Other reasons are: Savings,

Payback Period. Installing a PV system reduces the electricity cost every year, thereby

recouping the homeowner’s investment during the lifetime of the PV system.

Table 4 Annual Savings in Electricity Cost (Note. No rebate or tax reduction.) Electricity requirements for the home

10,000 kWh per year

Location Los Angeles Boston Electricity rates Tier 1: $0.129

Tier 2: $0.137 Tier 3: $0.204

Tier 1: $0.110 Tier 2: $0.120 Tier 3: $0.190

6 Although the retail prices for electricity are at their lowest level, e.g. 10.4 cents

per kWh in the United States, 21 cents in Japan and 20 cents in Germany, the author in [4] argues that the current electricity prices are not FULLY LOADED. i.e.don’t include the cost of subsidies and pollution control. External costs of the generating technologies include direct costs of pollution, health care, deaths, property damage, other environmental damage, security costs (protecting fuel supplies, protecting nuclear facilities. If these factors are included, the retail electricity prices (that the consumer pays) would be substantially higher. For example, these factors can increase the natural gas-based electricity by 30-90% and coal-based electricity by 55-400%.

11

Tier 4: $0.241 Tier 5: $0.280

Tier 4: $0.210 Tier 5: $0.240

First year utility energy (2008) cost

$2,600 $3,000

Annual savings in Electricity cost

$2,200 $1,800

The payback period for these two cities is shown in Table 5. Table 5 Payback Period with 10% Annual Price Increase of Electricity (Note: No rebate or tax reduction.)

Environmental benefits

Since electricity generation from a PV solar system is a clean process (there is

no greenhouse gas emission into the air), a homeowner can get significant

environmental benefits. A PV solar system can reduce greenhouse gases that

contribute to global warming. Most electric utilities generate electricity by burning coal

or natural gas. PV solar vendors indicate that a typical 6 kW PV solar system can

reduce greenhouse gas (or carbon-dioxide) emission by almost 126,800 lb over the

system’s lifetime in California (82,475 lb in Boston) [20], [21].

Table 6

CO2 Reduction PER DOLLAR OF INVESTMENT Using a PV Solar System

Annual price increase of electricity

10%

Location Los Angeles Boston YEARS, Payback period

16

19

12

Reduction in CO2

California Boston

2.4 lb 1.5 lb

13

Residential Solar Water Heating (SWH) System

Please see APPENDIX – III for details of a Solar Water Heating System. Two

main components of a SWH system are:

• Panels (or collectors) that absorb the energy directly from the sun

• Storage tank

Sizing of a SWH System

To determine the optimum size for a domestic solar water heater, the

homeowner has to go on the basis of the number of members in the household, and the

number of appliances such as dishwashers, washing machines, etc. which would be

using the hot water system.

As a general rule (presumably, for cost effectiveness), when sizing a solar

heating or cooling system, the system should cater to 90%–100% of the household's

energy needs during the season when consumption is lowest. The installed capacity

will thus be able to provide enough hot water during the greater part of the year, but not

during the coldest months of the year.

Essentially, the size of the system will depend on the following factors:

! Orientation, shading, etc. of the roof of your house.

! Access to direct sunlight and its intensity in your location.

! Specific needs of your household (number of persons, appliances used, etc.), in

order to assess the right system size.

Collector area for SHW systems

The following are general guidelines commonly used by contractors when

installing the SWH systems:

14

! 20 sq. ft or 2 m2 (approximately) of solar collector area for households with up to

two members

! Add approximately 8 sq. ft (0.7 m2) in sunny locations, and approximately 12 to

14 sq. ft (1.1 – 1.3 m2) in cold places per additional member. This basically

depends on solar insolation, for example, a southern United States location such

as Los Angeles will have a favourable insolation value (as far as low collector

size is concerned) compared to Boston, located further north. Please see

APPENDIX – I: Effects of the Sun on System Performance.

Storage tank volume for SWH systems

Storage tank capacity is usually in the region of 1.5 gallons (5.6 litres) per sq. ft

(0.092 m2) of collector area. In very warm and sunny climates, it’s quite common to go

up to 2 gallons (7.5 litres) of storage per sq. ft of collector area.

For a two-member household, it is generally adequate to use a small 50 to 60-

gallon storage tank and an 80-gallon storage tank for households of three to four

people. Larger households would need correspondingly higher storage tank

dimensions.

In solar room heaters, unglazed solar panels measuring 12 to 14 sq. ft (1.1 to 1.3

m2) are often used. Radiators, radiant floors and other systems that use heated water

may also be powered by solar energy, and so too can central heating be accomplished.

These may need correspondingly larger areas of collection areas to be effective.

Mounting

The most common systems for mounting solar collectors are (i) on the roof, or on

the (ii) ground. Roof mounted collectors are the least expensive and by far the most

15

common, and are held in place using brackets. Ground mounted panels are positioned

at a slight incline above the ground, supported by four or more posts fixed in the ground.

Tilt, orientation and shading (please see APPENDIX – I: Effects of the Sun on System Performance) are the factors that determine the efficacy of the system. It is

important to place your collectors at the sunniest spot on the building, and to avoid any

shade falling on them between 9.a.m. and 3 p.m. Having collectors facing up to 300 E

or W of true south, with a tilt of the site’s latitude + 150 degrees will normally yield

optimal results. Any shortfall arising out of a lack in proper tilt, orientation, or even

shading can usually be overcome by increasing the area of collection.

Pricing of SWH Systems

Residential SWH systems are priced competitively, usually in the region of

$2,500-$4,000 per home, for an output of 80 to 100 gallons of hot water per day. For

houses requiring more than 100 gallons of hot water a day and using larger storage

tanks, or more than two collectors - the prices will be much higher.

The final cost will depend on certain basic factors:

! The size and type of system and functionality you choose – for instance, a simple

water heating system would be much cheaper than one that includes an air

heating system. Solar space heating systems are even more expensive.

! Climate (including the solar insolation factor) is an important factor – for instance,

Los Angeles residents will have to spend far less for an SWH system than their

Boston counterparts.

! The surroundings and orientation of your house (the output from the SWH

depends on the shading)

! The energy consumption levels of your household.

16

! The government rebates, tax incentives and subsidies which may vary from one

State to another. In our paper, we don’t take this factor into consideration in our

calculation.

! Labour cost

Table 7 Typical Cost for a SWH System Location Typical cost of a SWH system that

produces 80-100 gallons of hot water

(including labor cost)

Los Angeles $5,172

Boston $7,200

Note. In order to get an equivalent amount of hot water in Boston, the homeowner

needs a larger system. This is specifically for a different insolation value in Boston. The

higher cost reflects that.

Assuming that the system saves you about $40 a month, the system has a payback for

these two cities as follows:

Table 8 Payback Period (Note. No rebate or tax reduction.)

The references [22] and [23] provide you a list of manufacturers for residential SWH systems in the U.S.

Location Los Angeles Boston YEARS, Payback period

11

15

17

Environmental Impact of SWH Systems - Reduction of Greenhouse Gas (GHG) Emissions

Since sustained use of renewable energy can significantly reduce carbon

emissions over time - when you use a SWH system - you can get substantial

environmental benefits. If you’re replacing your current hot water system, then the

amount of reduction (in carbon emissions) will depend on how you heat your water at

the moment. If you are replacing an electricity-based hot-water system, the reduction

will be significant. Even for a (current) gas-based hot water system, the homeowner will

experience a noticeable reduction in carbon emissions (also termed as greenhouse gas

or GHG emission) into the atmosphere [24].

If the homeowner replaces an electric water heater system that consumes 6,000

kWh per year for 80 gallons of water, the greenhouse gas emission reduction estimate

is 52,375 lb over a period of 25 years (assuming that the electricity, initially, used to

come from a gas-fired electric power station) [25]. If the homeowner replaces a gas-

based water heater system with a solar hot water system, greenhouse gas emissions

can be reduced up to 8,760 lb (over the lifetime of a system, that is, 25 years) [24].

Figure 1 A Graphical Representation of CO2 Produced by Two Types of Hot Water Systems

18

Table 9

CO2 Reduction PER DOLLAR OF INVESTMENT in the Case of Solar Hot Water

System that Replaces (or adds to) an Existing System

California Boston

Electric water

heater (replaced by

solar water heater)

10.0 lb 7.0 lb

Gas-based water

heater (replaced by

solar water heater)

1.3 lb 0.9 lb

Standby heat loss for tank-based hot water system We re-estimate the above data considering the fact that, in actual working

conditions, a water heater is used not only to heat the water but also to maintain the hot

water in the storage tank over a long period of time. The latter fact is known as standby

heat loss and this makes tank-based water heaters 40% to 60% energy efficient [31].

Table 10 Greenhouse Gas Reduction for Tank-based Storage Hot Water System

California Boston

Electric water

heater (replaced by

14.0 lb 9.8 lb

19

solar water heater)

Gas-based water

heater (replaced by

solar water heater)

1.8 lb 1.3 lb

20

Conclusions

This paper provides a technical comparison of an active solar hot water system

versus a grid-tied PV solar electric system in terms of their ability to reduce homeowner

costs and their ability to prevent greenhouse gas generation. These systems however,

don’t assume any subsidies or tax rebates, thereby eliminating the effects of policies

promoting adoption of these technologies. A few results are summarized for the best-

case location (such as Los Angeles, California) and less ideal conditions (such as

Boston, Massachusetts) as follows:

1) Greenhouse gas emission reduction PER DOLLAR OF INVESTMENT. Please

see Table 11:

Table 11

PV Solar Electric System Solar Hot Water System

Los Angeles 2.4 lb 14.0 lb (Replacing an electric-based system)

1.8 lb (Replacing a gas-fired system)

Boston 1.5 lb 9.8 lb (Replacing an electric based system)

1.3 lb (Replacing a gas-fired system)

2) Payback period. Please see Table 12:

Table 12

PV Solar Electric System Solar Hot Water System

Los Angeles 16 years 11 years

Boston 19 years 15 years

21

3) System cost. Please see Table 13:

Table 13

Location Typical cost of a PV

system that provides

same kWh.

(Note. This definition of a

PV system refers to

different kW systems at

two different places.)

Typical cost of a SWH

system that produces 80-100

gallons of hot water

(including labor cost)

Los Angeles $54,000 $5,172

Boston $81,000 $7,200

We conclude that there are significant merits even for the solar hot water system,

when compared to the PV electric system, as far as economic and environmental

benefits are concerned.

22

APPENDIX-I:EffectoftheSunonSystemPerformance

The performance of either system (SHW or PV Solar Electric) depends on the

solar energy at a particular location. In order to evaluate the performance, the

homeowner must consider the following factors:

LatitudeofaLocation

Residences receive different amounts of sunlight depending on the latitude of

their location. The sunlight received at a location near the equator is more than the

sunlight received at a location further from the equator. This necessitates (See Figure 2

in conjunction with Table A) a larger collector area or PV solar panels with a larger

foot-print area.

Figure 2: The dependence of surface area (for SWH collector or PV solar panel) on the latitude.

Table A. Demonstrates the impact of latitude on foot-print for two U.S. cities (for example). Location On the Map Latitude Observation

Los Angeles

34 Degrees The foot print (area) for collector as well as PV solar panels (with

23

Boston

42 Degrees the same conditions such as efficiency) should be larger in Boston.

Earth-SunGeometry-SeasonalVariation

Tilting of the earth (see Figure 3) creates the winter and the summer as the Earth

rotates around the Sun. In the Northern Hemisphere, the Earth is tilted during the winter

in such a way that the Southern Hemisphere receives more sunlight. During the

summer, the Northern Hemisphere gets more sunlight because of the angle of its

inclination with respect to the Sun.

Figure 3: Seasonal variation of solar radiation due to Earth's rotation.

EffectofSphericalSurfaceandDailyVariation

Solar radiation is not distributed uniformly over the Earth’s surface because of its

spherical surface. At each instant, the Sun’s radiation reaches half of the Earth’s

Surface. At a particular location, maximum radiation comes during the local noon (called

24

solar noon, when the Sun is halfway between the sunrise and the sunset) and less

during other times of the day.

SolarInsolationMaps

These quantitative maps (from NREL) indicate solar radiation incidence on the

surface of the earth. You need this data for your location to determine how much

sunlight is shining down on your location (i.e. your roof). The quantitative values are

expressed in kWh/m2/day and represent the solar energy that strikes a square meter of

the earth's surface in a single day. The conversions in different units are based on

surface area as follows:

1 kWh/m2/day = 317.1 btu/ft2/day = 3.6MJ/m2/day

Figure 4 and Figure 5 show the solar insolation map for the United States in the

month of June and December. Note that NREL provided data [26] is only useful if the

site is completely free of shade-producing objects such as trees, buildings, etc. Figure 4: Solar insolation map for the United States in the month of June.

25

Figure 5: Solar insolation map for the United States in the month of December.

LightIntensity

The solar radiation reaching the SWH collector or a PV solar panel will play a

critical role in the power output. The following factors influence the amount of sunlight

reaching SHW collectors or PV panels:

1. A cloudy day may impact the sunlight reaching the panel affecting power output.

2. As mentioned, solar insolation data for a particular location is not sufficient to find

the correct intensity reaching a particular home. In order to find the correct light

intensity, the home owner also needs to know the specific sun/shade analysis of

a given site. Solarpathfinder [27] is a typical tool to carry out such analysis in a

fast and accurate manner.

3. Height of the sun in the sky - The height of the sun varies with the seasons.

When the sun is very high in the sky (summer), its rays travel through the

atmosphere more quickly over a shorter distance than when it's low in the sky

26

(winter). See Figure 6 for how the homeowner should adjust (either manually or

automatically with a tracking motor) the tilt angle for better reception of solar

radiation during different seasons and thereby getting better light intensity for

improved system performance.

4. Number of daylight hours - The difference in the number of hours of sunlight

between the seasons plays a substantial role in determining the power output.

Figure 6: Tilt the solar collector or PV solar panel for the best possible solar intensity during different seasons of the year.

.

27

APPENDIX-II:Grid-TiedPVSystem

The most common PV electric system is a grid-tied system. In fact, it is a U.S.

law (applicable for residences in the cities, but not applicable for rural areas) that states

that utilities must allow homeowners to tie their PV solar electric system to the grid. The

homeowner, on the other hand, must adhere to the utility’s rules. The utility provides

suggestions on how the homeowner should proceed (and meet certain standards) for

doing this. In particular, the homeowner should sign an interconnection agreement with

the utility.

Although there are three kinds of grid-type systems available, we only discuss

the basic type (those that have no batteries) because of its commercial prospects; lower

costs make it appealing. Here are the three types of Grid-tied PV Electric systems:

1) Basic grid-tied system

2) Grid-tied system with batteries (for storing in the day time for using electricity at

night. The homeowner can only store power when more electricity is produced

than needed by the homeowner.

3) Grid-tied system with batteries and back-up generator (needing power during grid

outages)

MainComponentsofGrid-tiedSystem

The homeowner must build a custom PV system (depending on individual

electricity requirement) from a wide variety of off-the-shelf components including PV

panels, and other Balance of System (BOS) components (such as inverters, DC

Disconnect etc) as mentioned below:

28

Figure 7: A typical grid-tied system

PV Solar Cell/Panels (also called Array): This is the main component that produces

electricity from solar radiation. Whereas a solar cell is a single PV unit, a solar panel

consists of multiple PV solar cells; where cells are connected to each other to form a

solar module that has a higher voltage and capacity. Note that the homeowner can

choose one of many types of solar cells available in the market, such as, silicon based

PV cells, Concentrator PV cells, PV roof shingles, or PV roof tile. Its power output

depends on the following factors:

• Peak power of panel

Definition: A panel's power is expressed in peak watts, the number of watts it will

produce in optimal conditions, i.e. at noon in direct sunlight in cold weather.

Maximum sun intensity is 1,000 W/m2.

• Light intensity (solar insolation). Also, an individual homeowner must consider

shading that may impact total intensity reaching the solar panel.

29

• Number of hours the panel is exposed to the Sun

• Angle of exposure to the Sun

PV Combiner: Since each panel has two wires, the total number of wires of a PC panel

array can be many. A PV combiner is an electrical element that reduces the number of

wire-pairs.

DC Disconnect: This is a safety device that interrupts the flow of electricity from the

array of solar panels. Its main purpose is to disconnect power to your controller,

inverters, and all your DC loads.

Inverters: These devices convert DC (Direct Current) electricity into AC (Alternating

Current) electricity power that is required for lighting bulbs or powering electronics and

other appliances. Inverters (that meet stringent quality and safety standards set by the

utility) are also needed in a grid-tied solar system because the homeowner needs it to

feed the utility with the AC electricity generated by the PV solar system.

Bi-Directional Electric Meter: These kWh meters can measure both delivered energy

and received energy in a grid-tied system and record them in separate registers. In a

net-metering arrangement, such a bi-directional kWh meter is very useful to measure

AC electricity, both coming from and going to, the electric utility grid.

Mounts and Supports: Properly designed mounts and clamps are essential for

providing structural stability to withstand weather wind loads, especially for the roof.

Wires and Cables: The wires and cables with proper sizes are important for building an

efficient PV solar system. The most important parameter is the American Wire Gauge

(or AWG ) that represents wire cross-sections measured in millimeters. The home

owner must pay close attention when selecting wires and cables because under-sizing

wires may cause heat (and fire) due to resistance. Note. A larger diameter wire size is

capable of carrying more current.

30

Monitor: These devices can monitor the performance of the solar system continuously.

It is equipped with an LCD screen, a keyboard with large buttons and an on-screen

menu.

31

APPENDIX-III:SolarWaterHeating(SWH)System

The main component that the SWH uses to tap solar energy consists of panels

(or collectors) that absorb the energy directly from the Sun. The other important

component is the storage tank. Solar energy tapped by means of solar collectors is

ideal for heating water in the bath and in the kitchens, although its use can also be

extended to heating water in swimming pools and to both heating and cooling of indoor

spaces. SWH systems can be used in any climate, and can meet 50 to 60% of the

household’s requirements of water-heating even in cloudy weather. In places with very

sunny climate, it could even take care of up to 80% of the domestic needs.

Note. Small systems where the collectors measure around 3m2 do not have a storage

tank, but larger heaters have a well insulated storage tank located indoors,

DifferentTypesofSWHSystems

There are two types of SWH systems:

• Active Systems: These have electronic parts such as pumps and controllers for

circulating water or other heat transfer fluids through the collectors.

• Passive Systems: These systems are based on gravity and depend on the

natural tendency of water to circulate as it is heated.

An active SWH system basically consists of a collector, storage system for the

solar energy heated water, a pump and controls that direct the pump when to go on and

off. Most of these systems have a backup system for days when solar power is not

adequate. Active systems are more popular than passive ones as they provide the user

more control over the water heating. An active SWH system uses solar flat plate

32

collectors for heating the water. These are comprised of numerous parallel copper risers

that are put together as a panel and are very effective in the heating process.

ActiveSWHsystems

There are basically two types of active SWH systems such as direct and indirect

circulation types that pertain to the manner in which the water is heated in the solar

water heater [28]:

Indirect circulation systems: These are used in places having very cold

climates, where temperatures may drop below zero in winter months. They come fitted

with pumps for injecting heat exchangers and anti-freeze fluids.

Direct circulation systems: These are used in places having warm and

temperate climates, so they do not necessitate the use of anti-freeze fluids and heat

exchangers.

In active solar heating systems, pumps are used to circulate water or antifreeze

between the solar collector and the storage tank. The connections between the solar

collector and the indoor storage tank are established by means of inlet and outlet pipes.

The choice of pump for this function will depend on (a) the size of the system and (b)

the distance and height between the collector and the storage tank. If you’re using a DC

pump, you will need a DC power source such as a photovoltaic panel. AC pumps can

be run by plugging them into a regular wall outlet like any other appliance. A reliable

make of pump can last as long as 20 years even with daily use.

DirectCirculationActiveSWHSystems

The direct circulation system essentially uses solar energy to heat water directly

without any other medium. It also directly circulates the water from the water tank to the

solar heated water storage system and thereafter into the home. Such systems utilize

33

pumps to circulate pressurized potable water through the collectors and into the home.

These kinds of active solar water heater systems are ideal for regions that are not too

cold and do not have freezing temperatures for long spans of time. It is also essential

that the water available in the area not be hard or acidic, as this can hinder the

efficiency of the solar water heater. A direct circulation system may use a drain-down or

a drain-back system in case of freezing temperatures

In case of the drain-down systems, to avoid freezing, the drain-down collector

isolates the solar energy heated water storage system and the water is drained out of

the collector. This may lead to loss of water and if the valves don’t function optimally

there may be damage to them. Drain-back systems, on the other hand, drain the water

whenever the pump is switched off. This is found to be a more effective system and

minimizes damage.

OpenLoopActiveSolarWaterHeaterSystems

The open loop systems are direct circulation systems and hence fall under this

basic category (i.e. under Direct Circulation Active Solar Water Heater Systems). For

details, see [29].

In the open loop active solar water heater system, the water moves in from the

water tank, is heated, and the solar energy heated water is supplied to the home. These

systems use a circulating pump to circulate the water through the solar collectors to the

storage tank. A differential thermostat controller is used to activate the pump. Such a

controller is able to sense the heat present in the solar collectors. A solar storage tank

feeds into the existing system and provides preheated solar water.

34

Figure 8: Open loop active solar hot water system

In areas with freezing temperature the open loop active solar water heater

systems use an automatic drain-down system in order to protect the solar collectors and

feed lines. Such systems are referred to as ‘open loop’ as they are not separate from

the hot water system and utilize the same water. Open loop active solar water systems

are cheaper than other active solar water systems because they do not utilize heat

exchangers or anti-freeze. They are also found to be more effective in heating water as

the water is heated directly from the solar collectors. This speeds up the process as

there is not an intermediate phase before heating of the water.

The open loop active solar water heater systems are recommended in areas

where the winters are moderate and not prone to freezing temperatures. In areas with

freezing day and night temperatures the valves of the open loop active solar water

heater system may be damaged and could also damage the solar collectors. An open

35

loop active solar water heater system is configured according to several factors

including the energy needs of the household and availability of solar energy.

Indirect-CirculationActiveSWHSystems

In indirect-circulation systems the heat transfer fluids are pumped through the

collectors. The water is heated by heat transference from heat exchangers to the

potable water. Sometimes indirect-circulation systems may also offer ‘overheat

protection’. This is basically to protect the collector and glycol fluid from becoming

overheated when the water to be heated is less and the solar radiation is very high.

Such overheat protection mechanism may include a thermostatically controlled auxiliary

heating element in the solar energy heated water storage tank which turns on and off

when the water has reached the desired temperature.

There are two basic indirect systems for active solar water heater systems [30].

Drain-back systems: These systems use pumps to circulate water through the

collectors. When the pump is switched off, the water in the collector simply drains back

into the reservoir tank and avoids the water from freezing in the tank. This is a good

choice in areas that are very cold. When installing drain-back systems, special attention

needs to be paid to the piping. In order to ensure that all the water in the collector is

drained out, it is crucial that the piping slopes downward. The purpose remains the

same, the difference being that it is used in an indirect circulation system that transfers

heat from the heat exchangers to heat the water.

Antifreeze: Indirect circulation systems use antifreeze. The heat transfer

liquids are a glycol-water mixture that is created depending on the minimum

temperature that is expected in the area.

36

Figure 9: Indirect-circulation closed loop SWH system [5]

ClosedLoopActiveSolarWaterHeaterSystem

An active closed loop solar water heater system heats the water indirectly

through the heat transfer from the heat exchangers. The active closed loop solar water

heater has antifreeze that is circulated in the collector loop and has a heat exchange

coil that ensures that the antifreeze and water do not mix even if there is a leak, hence

the name ‘closed loop’. This system is found to be the best choice for regions with a

freezing climate. Closed loop systems usually have a back up heating system which is

initiated only when the temperature falls below the thermostat setting.

Table 14 A Comparative Table of Active Solar Water Heater Systems Direct Circulation Active Solar Water Heater Systems

Indirect Circulation Active Solar Water Heater Systems

Circulate pressurized potable water. Use heat transfer fluids, which are pumped though the collectors.

May use a drain-down or drain-back Use antifreeze and have a drain back

37

system. The drain-back system is found to be more effective.

system.

38

References [1] http://searchcio-midmarket.techtarget.com/sDefinition/0,,sid183_gci1028949,00.html [2] Solar cells :An Introduction to Crystalline Photovoltaic Technology, Jeffrey A. Mazer, Kluwer Academic Publishers, Boston, 1997 [3] http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html [4] Solar Revolution: The Economic Transformation of the global energy industry, Travis Bradford, MIT Press, Boston, Massachusetts, 2006 [5] Sustainable Solar Housing, Edited by S. Robert Hastings and Maria Wall. Earthscan, Sterling, London, 2007 [6] Solar Electric Power Generation - Photovoltaic Energy Systems Modeling of Optical and Thermal Performance, Electrical Yield, Energy Balance, Effect on Reduction of Greenhouse Gas Emissions, Krauter, Stefan C.W. Springer, Berlin, 2006 [7] Solar Electricity Second Edition, Edited by Tomas Markvart, University of Southampton, UK, 2000 [8] http://www.abcsolar.com/pdf/2001-09-04_500-01-020.pdf [9] Solar Technologies for Buildings, Ursula Eicker, John Wiley and Sons, NJ, 2003 [10] Solar Cells and their Applications, Edited by Larry, D. Partain, Wiley, NY, 1995 [11] Solar cells for Photovoltaic generation of electricity: materials, devices and applications, Marshall Sittig, Park Ridge Corporation, 1979 [12] http://rredc.nrel.gov/solar/codes_algs/PVWATTS/system.html [13] http://www.eere.energy.gov/consumer/your_home/electricity/index.cfm/mytopic=10840 [14] http://www.phys.ufl.edu/~liz/power.html [15] http://www.aurora-energy.com/FAQ.html [16] http://www.solarbuzz.com/StatsCosts.htm [17] http://www.eia.doe.gov/bookshelf/brochures/rep/ [18] The Solar Economy: Renewable Energy for a Sustainable Future, Hermann Scheer, Earthscan/James & James, 2004 [19] Renewable energy : a global review of technologies, policies and markets / edited by Dirk AÈmann, Ulrich Laumanns and Dieter Uh. Earthscan, London, Sterling, VA, 2006 [20] http://www.sunpowercorp.com/For-Homes/~/media/Downloads/for_homes/SPWRCho_SS.ashx [21] http://www.infinitepower.org/calc_carbon.html [22] http://www.eia.doe.gov/cneaf/solar.renewables/page/solarreport/contacts_stpcm.html [23] http://www.house-energy.com/Solar/Solar-Panels-Manufacturers.htm#collectors [24] http://www.mrsolar.com.au/shw.htm [25] http://www.infinitepower.org/calc_carbon.htm [26] http://www.nrel.gov/gis/solar.html [27] http://www.solarpathfinder.com/ [28] http://www1.eere.energy.gov/solar/sh_basics_water.html

39

[29] http://www.authorstream.com/Presentation/Octavio-26382-Solar-Hot-Water-Household-First-StepConservation-Efficiency-Applications-Similar-Techno-as-Entertainment-ppt-powerpoint [30] http://www1.eere.energy.gov/solar/sh_basics_water.html [31] http://www.tanklesswaterheaters.ca/