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gate Information Service /gtz , PO Box 5180, 65726 Eschborn, GermanyPhone: +49 (0)6196 / 79-3094, Fax: +49 (0)6196 / 79-7352, Email: [email protected], Internet: http://www.gtz.de/gate/

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Electricity from sunlight - Solar Energy supply for Homes and Buildings

Electricity from sunlightSolar Energy supply for Homes and Buildings

Klaus HaarsEnergie Consult& GTZ-GATE, May 2002

Information & Knowledge Management

Technical Information

! Energy / Environment (E)" Water / Sanitation (W)" Agriculture (A)" Foodprocessing (F)" Manufacturing (M)

This modu leis available in:

! English (e)

" French (f)

" German (g)

" Spanish (s)

" Other(s): ...........................

File: E016e_SunlightElectricity.doc / .pdf

1 Introduction

At the start of the third millennium, 70% ofthe rural population in developing coun-tries is still without electricity. While everyGerman citizen consumes more than5,000 kWh per year, billions of people stillhave to rely on candles and kerosene toget some light during the night.

Modern households, companies as well aspublic services depend on electricity. Elec-tricity is a precondition for development.Far from the public mains, few optionsexist to operate even basic electrical ap-pliances.

Photovoltaics is one of the most promi-nent options. It can help to raise standardsof living and facilitates the operation ofservices and businesses.

Furthermore, in a world of ecological dam-

age and manmade climatic changes,photovoltaics is one of the very few envi-ronmentally friendly energy systems.

There are already millions of photovoltaicsystems of various sizes in operationworldwide, many of them in developingcountries. The solar industry produces 5million modules every year, and produc-tion is increasing at about 20% per year.

2 About this Guide

This guide has been designed for thosewho intend to install a stand-alone orautonomous PV system of medium size(power range of 200 - 2,000 Wp) for

households that are in need of light-ing, refrigeration and entertainment,

shops and bars that want to providesome basic services such as music,videos or cold drinks to customers,

offices that like to operate computersand communications equipment,

workshops that struggle to improvetheir work with small electric tools,

health centers that want to store vac-cines and drugs.

The guide helps you to decide whether

photovoltaics is the right choice to suityour specific requirements. It gives anintroduction to some essential technicalaspects and recommendations for theplanning process. This includes an expla-nation of a brief design procedure. It is notsuitable for designing a system on yourown since you will need a qualified sup-plier in any case. This is followed by somebasic rules and hints for running a system.Finally, you will find some sources to getfurther information.


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3 A First Checklist

The most essential question is, whether a

photovoltaic system is technically andeconomically feasible for your purpose.Sometimes it is quite obvious that solarpower would not be the best solution. Youcan save time and money if you realizethis early on. Please check the followingquestions.

! Is electricity really a high priority com-pared with other unsatisfied demands?

! Are you without a reasonably reliable

mains connection that is within reach orexpected to be in the near future?

! Can you afford and finance an invest-ment of at least 2,000 (1 ~ 1 US $)?

! Is your daily energy demand smallerthan 20 kWh?

! Do you want to operate applianceswith a power demand of less than onekilowatt only?

! Can you provide the technical andfinancial means for regular operation,maintenance and replacement?

! Are gensets, fuel supply and technicalservice difficult to obtain or expensive?

! Are significant changes in ownership,use, consumption or the financial andeconomic situation unlikely in future?

If your answers include a "No", it is un-likely that photovoltaics will be the mosteconomic choice for you. This doesntnecessarily mean that generator sets are asuitable option either.

Generator sets are the main competitor ofphotovoltaic systems in rural areas. Theyare widely distributed and well-known, butso are their drawbacks, such as highcosts, difficult and unreliable fuel supply aswell as intensive operating and mainte-nance requirements.

4 Photovoltaics

Sunshine is a form of energy. Solar cells

the technology is called photovoltaics -transform light into electricity.

Solar cells dont need sunshine, just light,but the sunlight intensity is several timeshigher when the sky is not overcast.

0

1

2

3

4

5

6

7

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

[kW

h/m2]

Hamburg Manila Ouagadougou

Figure 1: Global insolation for selected sites

The cell output is highest when the sun-

beams fall perpendicular on the solar cell.The light energy that falls on a surfaceover a period of time is called insolation.Typical mean daily insolations in develop-ing countries are 45.5 kWh/m2. Such en-ergy would drive a car for ~ 5 km.

Insolation data are available for manysites, usually as mean daily figures permonth. Sometimes other units of meas-urement are applied:

1 kWh/m2 = 1 MJ/m2 / 3.6 = 1 Langley / 86

= 1 cal/cm2

/ 86 = 1 BTU/ft2

/ 317

5 Systems and Components

A typical solar system for power supply tobuildings consists of a solar generator forpower generation, a battery to store thisenergy, a charge controller to operate thebattery in a suitable way, the appliancesand sometimes a power conditioning unitsuch as an inverter.

Besides these technical components, theusers always represent an important in-


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teractive part of the system which shouldnot be forgotten.

module

charge controller

appliances

battery

inverter

Figure 2: Block diagram of a PV system

5.1 The generator

The heart of a photovoltaic system is thegenerator itself. The generator is the con-nection of any number of solar modules(also called panels). A standard moduleconsists of 36 solar cells. Modules have alifetime of more than 20 years.

Retail prices for modules are currently(2001) 5 to 8 per Wp. In other words, a50 Wp module will cost about 250 to 400.

In the field, modules with mono-crystallineand poly-crystalline cells are applied al-

most exclusively.Solar modules are rated in peak watts[Wp] according to their output under opti-mal outdoor conditions. Thus, a 50 Wpmodule can be expected to supply 50 W ofpower in full sunshine. The performance isreduced by high temperatures.

The modules are connected in seriesand/or in parallel depending on the systemrequirements. A serial connection in-creases the voltage, a parallel connection

increases the current.

ExampleA 24 V system with a nominal power ratingof 300 Wp can be combined from two 50

Wp modules connected in series (2 12 V= 24 V) and three parallel strings with two

modules (3 250 W = 300 W).

5.2 The battery

Batteries are applied to store energy for

times of little or no sunlight.

They are the most sensitive component ofany photovoltaic system and require a

certain amount of care. They are not easy-going like batteries in cars.

Only lead-acid batteries are used in thefield. They come in different types:

Automotive batteries (regular type,used in cars, also called SLI batteries)# very short service life, high mainte-nance, only shallow charging$ very low price, good availability

Automotive batteries (heavy- dutytype, used in trucks and buses)# short service life, medium mainte-nance, shallow discharging$ low price, good availability

Solar batteries (different designs)# medium discharging, limited availabil-ity, medium price$ better service life, limited mainte-nance

High-quality batteries (diff. designs)

# expensive (4-6autom. battery price)

$ high service life, low maintenance,rather deep discharging

Regular batteries have to be filled up withdistilled water in certain intervals. A feware maintenance-free. Only those of thegel-type are suitable in solar systems.

Automotive batteries are unsuitable if anyalternative exists. Heavy-duty ones maybe used in smaller systems with severebudget constraints or if others are notavailable. High-quality, deep-cycle batter-

ies are optimal for solar applications. Solarbatteries are a compromise between qual-ity and costs.

The service life of batteries, which is ex-pressed in cycles of charging and dis-charging periods, increases with

the quality of the battery,

the quality of the charge controller,

low temperature,

a low depth of discharge and careful operation and maintenance.


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The depth of discharge (DoD) is thatportion of the battery capacity which is

removed during discharging. The capacityis indicated in ampere hours (Ah) or some-times in days of autonomy. This is thenumber of days a fully charged battery cansatisfy the energy demand.

Example

A 12 V, 60 Ah battery can store 12 V 60 Ah = 720 Wh. At a 75% depth of dis-charge, 540 Wh have been removed andthe remaining capacity is 180 Wh / 12 V =15 Ah. If the daily energy demand is 270

Wh, the battery has 2 days of autonomy.

As a rule of thumb, reducing the DoD byhalf will double the number of cycles.

The common 12 V battery includes sixcells of 2 V in one housing.

As with modules, batteries are connectedin series for higher voltage levels or inparallel to increase the capacity. Batteriesthat are connected should be identical in

type, capacity, age and electrolyte density.Parallel connection should be avoided ifpossible.

5.3 The controller

Batteries have to be protected againstovercharging and deep discharging by acharge controller. Charge controllers arealso referred to as charge regulators andbattery control units (BCU).

During charging, the battery voltage in-

creases. At a certain voltage, the batterystarts gassing. Gassing, the productionof hydrogen and oxygen, reduces the life

expectancy as well as the required main-tenance periods and should be avoided.

A controller avoids excessive gassing bymaintaining the voltage at the end-of-charge level of about 14.0 V, dependingon the battery type and temperature.

The end-of-charge level may be adaptedto the battery temperature by means of atemperature sensor.

In contrast to charging, the voltage of abattery being discharged declines. In adeep-discharged state, chemical proc-esses occur which will reduce the battery

lifetime quickly. To protect the battery, theloads have to be cut off at a certain volt-age. High-quality batteries are less sensi-tive to deep discharging.

For automotive batteries, manufacturersrecommend discharging of only 30%,while 80% is common practice.

Individual batteries will develop differentlyafter some time of operation. Worn-outcells have to be identified and removed inorder to protect those that are still in goodcondition.

Battery type Max. dis-charge

Cut-offvoltage

Automotive battery 30 - 50% 11.8-11.5 V

Heavy-duty battery 50 - 70% 11.5-11.2 V

Solar battery 60 - 80% 11.4-11.0 V

High-quality battery 80% 11.0-10.8 V

Table 2: Discharge conditions for batteries

A new generation of controllers adapt thecharging process and the load manage-ment to the state of charge and the ageingprocess of the batteries. System efficiencyand battery lifetime benefit from this, butsuch controllers are sophisticated andcurrently hardly available in developingcountries.

Charge controllers may provide a lot ofadditional helpful functions and features,

but this makes them more expensive andreliability may suffer as a result.

Characteristics of battery typesFigures depend on type and operating conditions

automot. solar quality

Energy efficiency 70% 75% 80%

Self-discharge/mo. 20% 4% 2.5%

Costs per kWh 70 120 300

Maintenance 4x/a 2x/a 1x/a

Table 1: Battery characteristics


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Controllers are designed for maximuminput and output currents. Safety meas-

ures may include protection against shortcircuit, overvoltage, overloads, reversepolarity, wrong connection and environ-mental hazards. Controllers should indi-cate the state of charge and a load cut-off.

5.4 The inverter

Solar modules and batteries operate withDC. The technical electricity sector, how-ever, is based on AC. Many electrical ap-pliances, devices and accessories are onlyavailable for AC. An inverter transforms

low-voltage DC supplied by a solar systeminto high-voltage AC. The input of an in-verter is designed for 12 V (24 V, 48 V,etc.), depending on the type. At the outputit produces 110 or 240 V AC. Inverters aredesigned for stand-alone as well as forgrid-connected systems.

Inverters are available with a wide varietyof different qualities and features. Not sur-prisingly, low-price inverters are generallyof poor quality and not very suitable for

solar operation.Inverters are designed for a maximumload. They provide a certain overloadcapacity for a short-term demand arisinge.g. from the starting current of a motor.

High-quality inverters have an efficiency ofmore than 90% between 10% and 100%rated power.

Inverters produce a sinusoidal currentlike the mains, a modified square or asquare wave current. Square wave invert-ers are not suitable. Sinusoidal invertersare optimal. Modified square inverters areapprox. 40% cheaper, but electronic andmotor-equipped appliances (TVs, com-puters, pumps, fans, refrigerators, etc.)may run poorly or may even be damaged.

Safety measures include protectionagainst short circuit, overload, high inputand output voltage, high temperature andreverse polarity.

5.5 Wiring

Many system failures are caused by un-

suitable or improperly installed cables,connectors, switches or sockets. Theircurrent, voltage and power ratings shouldbe in line with the system characteristics.

Unlike AC, in DC systems switching thepoles (+) and (-) will cause problems. Theapplication of special DC fittings is rec-ommended. Otherwise, high-quality ACitems should be applied. The current rat-ings of AC-switches can be applied onlyfor 12 V/ 24 V DC systems. In high-voltage

DC systems, they have to be higher.The current drawn by an appliance or cir-cuit can be calculated by dividing thepower rating by the system voltage.

ExampleA 1 kW inverter in a 24 V system will drawa current of 42 A (1,000 W / 24 V). Addsafety margins and take overload capacityinto account.

Like other electric components, accesso-

ries (particularly cables) cause losses.Since energy is expensive, these shouldbe minimized. The power loss within acopper wired cable depends on the currentI, the cable length L (back and forth) andthe cross-section A of the wire as follows:

P [W] = 0.018 I2 [A2] L [m] / A [mm

2]

ExampleFor a generator current of 5 A and a cableof 1.5 mm

2with a length of 10 m (one-

way) the power loss will be0.018 (5 A)

2 (210 m) / 1.5 mm

2= 6 W

In a 12 V system, this would be 10% of thewhole generator power.

Losses can be reduced by increasing thecross-section of wires or by higher volt-age levels. Keep the losses in the genera-tor circuit as well as in the load circuitsbelow 5% (better: 3%) and in the control-ler-battery circuit below 0.5%. The neces-

sary cross-section of the cable isA [mm] = 0.018 I L / U / 0.05 (=5%)


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Use appropriate outdoor cables to connectthe modules, protect them against sunlight

and damage (e.g. by rodents) and lay ca-bles in conduits where appropriate.

5.6 Appliances

5.6.1 Lightin g

Bulbs have an efficiency of only 5%.Halogen lamps are only slightly moreefficient. They are, therefore, not suitablefor solar systems.

Fluorescent lamps of the tube or com-pact type are 5 times more efficient andcan last 5,000 hours and more. They needa good and efficient electronic ballast foroperation. Fluorescent lamps with elec-tronic ballasts cost about 10 for AC and20 for DC.

5.6.2 Au dio and video devices

Most appliances such as TVs, receivers orvideo recorders operate on AC. There area few exceptions includ-ing some DC TVs. ColorTVs consume more thanblack & white ones.There are considerabledifferences between in-dividual brands.

TVs should not be oper-ated in stand-by. Control-

lers and inverters maydisturb operation of TVs

and radios.

Transistor radios and cassette players are

operated by batteries at a voltage below12 V. Since batteries are expensive, suchappliances should also be connected tothe solar system through a voltage con-verter. For mobile operation, rechargeableNiCad batteries can be applied that arecharged with a 12 V NiCad charger.

5.6.3 Water pum ps

Some tens of thousands of pumps pow-ered by solar generators are currently inuse worldwide. A variety of pumps for low

water demand are available on the market.These include single-stage centrifugalpumps for lifting heads


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types are more efficient. Special types areavailable for vaccine storage.

Starting at low voltage levels (


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Supply (=insolation) and demand are bothvariable throughout the year. Matching

both variables is only possible to a certainextent. With every solar system you willhave times when there is energy in excessand times when it is scarce.

This is no different from generator sets.Due to periods of maintenance, scarcefuel supply, breakdowns, etc., there arealso shut-down times.

With proper sizing and a battery auton-omy of 3 days, you can limit energy short-ages to about 1%; that corresponds to 4

days in a year.The final sizing should be done by yoursupplier, who will use a computer simula-tion program in most cases. With your ownsizing, you can check the reliability of theresults and get an idea of the size andcosts of the system.

(Use the design table on the next page. Allshaded cells that show numbers includeformulas. Do not overwrite them! Markthose cells and press F9 for calculation. If

you have a German-based PC operatingsystem, please use the notation 0,85 in-stead of 0.85.)

6.4.1 Calculat ing the demand

List all appliances you need with their re-spective power requirements. Decide howmany hours they will be operated. Calcu-late the demand ELOAD.

6.4.2 Sizing the generator

Collect insolation records from the nearest

meteorological station(s). Insolation levelsare generally not subject to much localvariation.

Next, choose a design insolation whichis the worst insolation during those timesof the year that the building is in full-timeoperation.

Tilting the generator can enhance this fig-ure. In tropical regions the effect is mostlynegligible. At higher latitudes, increase thedesign insolation by the degree of latitude

in %.

ExampleAt Dubai (25 N), the minimum insolation

of 3.7 kWh/(m2 a) in December can beincreased by 25% to 4.6 kWh/(m2a).

The generator has to cover the energyconsumption (ELOAD) plus the systemlosses. The system losses are calculatedby the efficiencies () of the components.

EGEN ~ ELOAD / (0.85 BATINV)

The battery efficiency BAT depends on thebattery type [& Table 1]. INV is the mean

operating efficiency of the inverter (not themaximum or rated efficiency) and is appli-cable in AC systems only. It may be asgood as 0.9 (high-quality, small powervariations) or as poor as 0.5 (low-quality,continuous low partial load).

ExampleAn AC system with a solar battery, a goodinverter and a demand of 4.0 kWh/d willrequire an energy supply of about

6.9 kWh/d [6.9=4.0/(0.850.800.85)].

Now the required generator power caneasily be determined:

PGEN = EGEN / GDESIGN

ExampleThe appropriate generator power for a sitein Dubai with the design insolation of 4.6

kWh/(m2d), and for the DC system abovewith 6.9 kWh/d is 1.5 kWp (=6.9/4.6)

6.4.3 Sizing the battery

Battery sizing has to take into considera-tion that only part of the rated capacity isavailable. This is the allowable depth ofdischarge (DoD).


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CALCULATION OF DAILY ENERGY DEMAND

Appliance Power Daily use Energy demand

W

h 0 Wh

W h 0 Wh

W h 0 Wh

W h 0 Wh

W h 0 Wh

W h 0 Wh

W h 0 Wh

W h 0 Wh

ELOAD = 0 Wh

BAT INV /0.85 = 0,0

=

Insolation EGEN = 0 WhMonth kWh/m2 /

Jan MIN 0 kWh/m2 + kWh/m2 = 0 kWh/m2

Feb Tilt Correction =

Mar Generator size - PGEN = 0 W

Apr

MayJun ELOAD = 0 WhJul /

Aug INV = 0Sep Oct Autonomy = d

Nov /

Dec UBAT = V

/

Depth of discharge =Priority Loads =

No. Power Total Power Battery size - CBAT = 0 Ah

W = 0 W

W = 0 W

W = 0 W

W = 0 W

W = 0 W Inverter size

W = 0 W W = PINV


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The battery capacity is calculated as

CBAT[Ah] = ELOAD [kWh/d] / INV / DoD /

UBAT [V] Aut [d] / 1000

ExampleThe capacity for a 60 V (UBAT) solar bat-tery with a DoD of 60%=0.6, an energy de-

mand of 4.6 kWh/d, a good inverter (INV=0.85) and an Autonomy Aut of 3 days is338Ah=4.6kWh/d/0.85/0.8/60V3d /1000

6.4.4 Sizing the inverter

The inverter can be sized to operate all

loads at the same time. It is very unlikely,however, that all loads are in operation atthe same time. Generally it is adequate ifthe inverter output matches the sum of thepower ratings of all high-priority loads.

PINV = PLOAD, prior.

Furthermore, the overload capacity of theinverter should be sufficient to start everysingle appliance (motors, etc.).

6.4.5 Check the cos ts

With the design figures, you can make aninitial cost estimate. The system cost [I]including transport and installation can beroughly estimated as I[]=

(6PGEN[W]+0.15CBAT[Wh]+PINV[W])1.4

Is this in line with your budget? If not, try toreduce the energy demand and go back toChapter 6.4.1.

6.5 Select a supplier

You will find suppliers of solar systems in

nearly every country of the world. The jobof a supplier should not end with the deliv-ery of the system. Check for installation,training and after-sales services.

The experience of suppliers varies widely.Try to get an impression of the quality oftheir workmanship and services. Ask forreferences and check them. Visit the com-panies' headquarters.

Contact several suppliers. Evaluate thequotations. Do not compare just the

price, also consider the technical quality,services and the suppliers' experience.

7 Running a System

Solar systems are not difficult to operateand maintain, nor are they dangerous.Nevertheless, certain aspects should beborne in mind.

7.1 Installation

7.1.1 Moun ting the generator

Maximize the output of the generator by

choosing an optimal orientation. As a ruleof thumb, you should incline the module atthe degree of latitude towards the equator.

ExampleIn Karachi, Pakistan, 24 north of theequator, the right tilt angle is 24 south.

To allow rain water to run off, the tilt angleshould be at least 15. Close to the equa-tor, turn the module in that direction toface the sun at noon during the month with

the lowest insolation.

ExampleIn Nairobi, Kenya, 1 south of the equator,with the lowest insolation in July, turn themodule at 15 to the north.

An area of approximately 10 m2 is requiredper kWp.

Output will obviously be lower if the gen-erator is in the shade of buildings or trees.Even if just a small part is shaded, thewhole generator will be affected. There-fore, avoid any shading especially aroundnoon.

An installation on the ground is simplerand allows for regular cleaning and con-trol. If theft and vandalism are a problem,the generator should be installed on theroof or on long poles. Space should be leftfor air circulation on the underside to re-duce the temperatures.

The modules have to be fixed securely.The mounting structure should be made


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of weather-proofed materials such as gal-vanized steel. Stainless steel or aluminum

are also suitable, but expensive.7.1.2 Electr ic instal lat ion

A qualified electrician with experience insolar systems should do the wiring andinstallation of the electric components.

Significant aspects to be considered ininstallation are:

! Keep all cable lengths to a minimum.

! Protect cables and componentsagainst weather, rodents and damage.

! Put the battery in a cool and well venti-lated place not far from the generator.

! Place a fuse or circuit breaker in thebattery circuit.

! Put inverters and controllers in a coolplace.

! Make sure that cables and accessoriesare suitable for your systems.

! All connections should be securely

fixed and protected.! Terminals and cables should be clearly

marked (AC / DC, + / -).

! Do not mix up the polarities.

! Earth the array and all major appli-ances.

7.2 Operation

Clearly define responsibilities for opera-tion and maintenance. Operators and us-

ers should be adequately informed andtrained.

Take care to provide for good load man-agement to balance supply and consump-tion. Reduce the consumption if the bat-tery is in a low state of charge. Make surethat users feel responsible and dont con-sume energy carelessly. The consumptioncan easily be controlled by a conventionalkWh-meter.

Indicators for the state of charge found on

most controllers are not very precise.

Check the state of charge (SOC) quar-terly by measuring the battery voltage

and/or the electrolyte density (ED). Meas-uring the electrolyte density is more reli-able. The measurement has to be made ina state of inactivity (no charging and dis-charging for at least 1 hour).

Batteries degrade in the long run. Seri-ously degraded batteries have to be re-moved to protect the other ones. Theirdifferent voltage and electrolyte densitycan identify worn-out cells. Deviations ofmore than 0.2 V or 0.02 kg/l are critical.

7.3 Maintenance

Monthly! Check the electrolyte level of all bat-

tery cells; add pure distilled water ifnecessary.

! Clean modules with water or soft cloth(in times of no rainfall).

! Check fluorescent tubes and removeworn-out tubes.

Quarterly

! Measure the electrolyte density and/orthe battery voltage and remove worn-out batteries.

! Clean and grease battery poles.

! Check all electrical connections fortightness and corrosion.

! Check fuses and protective devices.

! Check the mounting structure of thegenerator; tighten bolts if necessary.

! Check appliances for proper operation.

! Check stock of spare parts.

SOC [%] U [V] ED [kg/l]

100 12.6 1.23

80 12.4 1.20

50 12.2 1.16

20 11.8 1.13

Mean figuresthat depend onbattery type,age and tem-perature

Table 5: Battery indicators for state of charge


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8 Further Information

This guide is a summary of a brochure by

the same title available from GATE/GTZ.

For more technical details, refer toSolar Electricity (S. Roberts; 1991; Pren-tice Hall Int., UK; ISBN 0-13-826314-0)

8.1 Useful links

http://www.crest.org/cgi-bin/gem/gem.plGateway to sustainable energy informationon the web, extended link list

http://www.solarenergy.org

Non-profit organization providing assis-tance for use of renewable energies

http://www.self.orgNon-profit organization promoting solarrural electrification in developing countries

Supplier addresses

http://energy.sourceguide.com/index.shtmlRenew. energy suppliers & organizations

http://www.jxj.com/suppands/renenerg/Database of renewable energy suppliers

Solar databasehttp://eosweb.larc.nasa.gov/sse/Surface meteorology & solar energy data

http://www.eng.uml.edu/Dept/Energy/solbase.htmlSolar radiation data for cities worldwide

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