Use of solar generators in Africa for broadcasting equipment

Solar Energy, Vol. 19, pp. 201-204. Pergamon Press 1977. Printed in Great Britain

TECHNICAL NOTE Use of solar generators in Africa for broadcasting equipment? S. POLGAR TtltDiffusion de France, 21, rue Barb,s, 92120 Montrouge, France

(Received 14 October 1975; in revised [orm 24 March 1976)

l. POWERSUPPLYFOR TV RECEIVERS IN NIGER In Africa, solar cells were used for the first time in 1968 to provide power supply for the TV receivers in Niger. In that country, school television programmes are essentially devised for the schools located in regions not provided with power mains. The transmissions are received by the means of TV sets that are especially devised to operate under warm and wet weather conditions. These receivers, model CATEL CI 17, are equipped with 61-cm screens, and are completely solid-state. They can be powered by a d.c. power supply, between 30 and 36 V. Their consumption, extremely modest, ranges around 32 W. The power supply for these receivers had, at the beginning, been provided by high-capacity alkaline electrolyte cells. In order to secure a more practical and less expensive source of energy, an experimental solar cell was installed in 1968, to energize the TV receiver of a school, located in the vicinity of Niamey. Following a satisfactory operation of this experimental solar cell, a careful study was conducted, after which some twenty installations were set up, using silicon cells, manufactured by Radiotechnique in France and by Solar Power Corporation in the U.S.A. Presently, the programmes of the Nigerian school television are being broadcast for some 9000 pupils in 214 classes. The television receivers in the schools are powered in the following way: --54 classes on power mains --138 classes with alkaline electrolyte cells. --22 classes with solar cells. The extension of the transmission network is presently under way. According to the projects being prepared, several hundred classes will be equipped every year to receive the school television programmes. In 90 per cent of these classes, the TV receivers will be energized by solar cells.

models of solar cells used in Niger:

h

SxGx7xnxkxm W

in which: h = weekly length of operation, hr; S = surface of the solar cell, m; 67 = daily average solar radiation, measured during the less favourable month, in January: 5420Wh/m2; n = conversion efficiency of the solar cell at NOT (60°C); k = orientation coefficient, taking into account the daily rotation of the Sun and the variation of the declination of the Sun between the two solstices (0.8); m =efficiency of "charge--discharge" of the floating battery (0.8); W = consumption of TV receiver (32 W). Table 1 shows the result of the formula for two models of solar cells used in Niger to energize receivers operating on a normal voltage of 36 V.

30 •

.=...~e

20 °

~ -

4.94

~

'

oNiame¥

~

F~Larr)

t0 °

Doualc

2. SOLAR IRRADIATIONIN AFRICA The energy supplied by solar cells is evidently proportional to the solar irradiation on the location. It was therefore important to determine the amount of average total solar irradiation 67 received in Niamey. The following curves, coming from the French National Meteorological Services, indicate the average daily total solar radiation 67 received in Africa, throughout the year and during the month of January. It appears that in Niamey, during the less favourable month of January, solar radiation 67 is estimated at 5.52 kWh/m2. This value comes very close to the results of measures conducted in 1970 and 1971 with a KIPP pyranometer, which gave 5.42 kWh/m2. for the month of January. The knowledge of solar radiation 67 has allowed us to conduct a preliminary study to determine the amount of energy to be supplied by the installations.

.,;2

roraz covill~ 10 °

0o

10 °

20 °

Fig. 1. Average solar irradiation for January (kWh/m2).

30*

v

f'~Tarn,~n

20 °

tosser

Lamy i0o

3. CALCULATIONOF A SOLAR GENERATOR The following formula has been used to calculate the weekly length of operation of a TV receiver, ensured by the various

~

~

0°

10 °

oo NX B r a z z a v i l l e 10 °

tPresented at the I.S.E.S. International Solar Energy Congress and Exposition, Los Angeles, California (28 July-I Aug. 1975).

20 °

Fig. 2. Annual average solar irradiation (kWh/m2) 201

202

Technical Note Table 1. n(%)

Type

S(m2)

at 140"F

hr

Radiotechnique BPX 47 Solar Power Corporation SP 1002

0.47

8.5

30.6

0.45

8.5

28.9

4. STORAGE BA'IWERY

The electric energy supplied by the solar cell is obviously far from constant. It depends, among other factors, upon the time of the day, the sky condition, the season etc. In certain cases, the installation must be able to operate at night. It has been therefore necessary to include a storage battery in order to ensure a power supply for the TV receivers under any condition. According to measurements, it appeared that the weekly average solar radiation in Niger is practically constant throughout the year. The storage battery, being used at a discharging rate of 1 A/hr, corresponding to the consumption of the TV receiver, has a capacity of 40 Ah. Consequently, it is able to energize the receiver during 40 hr, should an important nebulosity occur, or during several nights. It is composed of 16 lead acid accumulators, 2 V each. An electronic charge regulator is connected between the solar cells and the storage battery. It is devised to protect the battery form any eventual overload in case of exceptional irradiation, or during school holidays, when the installations are not being used. 5. DESCRIPTION OF INSTALLATIONS As a result of the study conducted in 1972, determining the characteristics of the installations, 22 schools have been equipped. The following sketch shows the basic scheme of the installation. 50LAR CELL5 Amperhourmerer

CHARGE-REGULATOR

--illllllllllllllllll

BATTERY

TELEVISION RECEIVER

COUNTER

Fig. 3. Outline of the installation. Each installation of solar generators includes: --6 silicon solar panels (Radiotechnique, France BPX 47), or --38 silicon solar panels (Solar Power Corporation, U.S.A. SP 1002) --16 lead acid accumulators for storage battery (CIPEL France, VL 203). - - I charge regulator. - - I galvanized steel frame for the solar panels. In some classrooms, a time counter, integrated in the receiver, and an A/hr m are added to the above mentioned equipment and make it possible to exert a control of the installation. The solar cells are installed either on the roof of the schools or on a stand, in order to receive the maximum irradiation. In that case, they are directed to the South. The tilting is such that, at

Fig. 4. Solar cells and television antenna. midday, at the equinox time, the rays of the Sun fall at right angle upon the solar cells. According to our observation certain panels of solar cells are fixed on a wood frame, directly set on the roof of the school. Thus, no air circulation is possible, and the reflection of the corrugated sheet metal roof submits the cells to an enormous heat. At times, the temperature may reach 176°F (80°C). The excessive heating of the cell causes a voltage drop, and consequently reduces the energetic output of the installation. Moreover, the expansion/contraction of various components of the panel leads to warping which allows moisture and dust infiltrations, and diminishes the reliability and life duration of the equipments. In the future, the solar cells will be preferably mounted on a frame fixed on a stand; should the panels be installed on a roof, they would be set as far as possible from the corrugated sheet metal roof, allowing air circulation between them and the roof. The storage battery and the charge regulator will be installed in the classroom, not far from the receiver. 6. MAINTENANCEOF INSTALLATIONS The installation of solar cells does not require the creation of a specialized maintenance service. Only three or four surveys a year appear desirable. The technician in charge of the maintenance of the reception equipments in the schools must clean the surface of the cells from sand (brought by sand winds and tornados), and carefully check the fastenings and the connections. In certain cases, after cleaning, the power output of the generator may increase by 15 per cent. Once a year, it is also necessary to check the level of the electrolyte in the batteries. 7. MEASUREMENTS In order to control the normal operation of equipments, the following measures are regularly conducted on certain testinstallations: 1. Energy supplied by the cell, indicated by the A/hm. 2. Duration of use of the receiver, indicated by the time counter fixed at the rear of the set. 3. Available voltage and density of the electrolyte of the storage battery.

Technical Note

203

Fig. 5. Television receivers and accumulators in the classroom. The measures that were conducted from the start in equipments using "Radiotechnique BPX 47" solar cells confirm the provisions of the preliminary study. The average output of the cell is 1.1 kW per week. This allows the operation of the TV receiver during 31hr per week. The variations that may be encountered throughout the seasons or from one year to another do not exceed -+5 per cent, as shown by Fig. 6. 8. ALTERNATIONOF THE CHARACTERISTICSOF SOLAR CELLS The solar cells installed in Niger are submitted to very drastic heat conditions. Thus, the temperature of the panels, during a certain period of the year, varies between 46°F (80C) in the morning and 176°F (80°C) at midday. The expansion and contraction of various components may, in the long run, alter the tightness of the cells. Violent storms, sand winds, the ambiant moisture, which exceeds 90 per cent during the rain season, may also accelerate the process of alteration and the premature ageing of the cells. For a better understanding of the phenomenon related to the ageing of the cells, and to determine the necessary checkings, it appears advisable to examine the equivalent circuit of a solar cell. Resistance Ro is normally very high. In case of moisture infiltrations, parallel resistance may show a severe drop caused by the leaks between silicon cells and the connections of the printed circuit. Resistance R, is very low; it represents the resistance of the interconnection wiring. Should the solar cell be altered, the

series resistance increases. Figure 8 shows the current voltage characteristics of a solar cell. The dotted line curves show the characteristics of a degraded solar cell. The measure of voltage Vm makes it possible to detect defective modules. The process is as follows: 1. The storage battery is disconnected and replaced by a fixed resistance, in order to eliminate any uncertainty related to the state of charge of the battery. 2. The irradiation of the site and the temperature of the cell are measured with an instrument composed of a standard cell and an integrated thermocouple. 3. Then, the measure of voltage Vm and of the current I~ of the cell, is made. The result is compared with the normal characteristics of the cells, provided by the manufacturer. Should any diverging figures appear, the technician will locate and identify the defective module with the same method, and replace it by a new element. A portable set, including all the necessary instruments is presently being devised, in order to facilitate the work of the technicians. 9. RELIABILITY OF INSTALLATIONS It should be noted that the reliability of the installations, in spite of severe conditions of operation, is excellent. On a total of 42 "radiotechnique BPX 47" modules, in use for the past 4yr, only one has been replaced in order and to be checked in a

Ah

4000.

3000.

2000.

1000

f

,

Fig. 6. Energy supplied by the equipments.

SE Vol. 19, No. 2-~J

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J

204

Technical Note $9768 Cost per hr: ~ $0.48 (2.05 FF).

Rs

Current 5ource

o

Fig. 7. Equivalent circuit.

I

llc c

Vco 0

Vm3

V

Vm 2 Vm~I

Fig. 8. Current-voltage characteristics of a solar cell.

laboratory. We do not have, at present, sufficient information concerning the behaviour of the 570 "Solar Power Corporation SP 1002" modules, which have only been installed at the beginning of 1975. 10. COST OF INSTALLATIONS The costs of the two autonomous sources of energy presently in use for the operation of TV receivers in isolated regions, i.e. the solar cell and the chemical battery, can only be compared on the basis of the cost per hr of operation. For example, the cost of an installation of a solar generator composed of 6 "Radiotechnique BPX 47A" new modules, with frame, storage battery and charge regulator, amounted, in June 1975, to $3100 (13,000 FF) free of tax, c.i.f, in Niamey. For the evaluation of the cost per hr of operation, we have arbitrarily based the calculation on a life duration of 10 yr. In fact, it appears possible to extend the life of the generator beyond 10 yr, provided the replacement of 2-3 per cent of the modules and the storage battery are replaced. Only long-term experience may confirm this assumption. According to the method of calculation indicated above, the equipment will have, in 10 yr, supplied power to operate a TV receiver during a total of 25,500 hr, at the rate of 49 hr a week. $310o Cost per hr: 25-~-~$0.12 (0.51 FF). The price of a alkaline electrolyte cell amounted, in June 1975, to $976 (4,100 FF) free of tax, c.i.f, in Niamey. It ensures the operation of the receiver during 2000 hr.

The comparison between the two sources of energy shows that in the case of a highly isolated country such as Niger, the resort to solar generators is, from now on the most advisable from the economical point of view. It appears extremely difficult to anticipate the evolution of the cost of solar cells in the future. According to various documents published by specialists or by the manufacturers, the present price could be divided by 10 or by I00 in the forthcoming years? On the other hand, since the cost of the chemical battery is expected to follow the general price increase of raw materials, the comparison will prove to be more and more favourable to solar cells. It. EXTENSION OF USE OF SOLAR CELLS IN AFRICA The preliminary study of an installation requires the knowledge of the isolation on the chosen site, as well as its annual curve. The measures of insolation, classically conducted through the means of thermopiles, give only an imprecise figure, since the regions of the solar spectrum measured by the thermopile and by the solar cells are quite different. Moreover, they do not take into account the variations of temperature which also exert an influence on the energetic output of the solar cell. A wide-scale isolation measurement campaign has recently been undertaken in African countries likely to resort to solar energy. Experimental stations, composed of one panel, a load resistance and an A/h m have been installed in various regions of Togo, Gabon, Ivory Coast and Zaire. With the help of the measurements coming from such surveys, we could evaluate the necessary equipments to energize TV translators and receivers to be installed in these countries. In certain countries, the "primary" network, generally composed of high-power TV transmitters doe not cover the whole of the territory. The coverage of the shadow zones by the means of low-power translators energized by solar cells is presently under study. Standard translator equipments should be modified, in order to eliminate unessential parts, such as power mains input, signal lamps, ventilators, stand-by systems, etc. to reduce the power consumption. 12. CONCLUSION Considering the satisfactory operation of solar generators in Niger, it is confirmed that a power supply for TV receivers, translators, microwave beams, radio beacons, etc. may be ensured in an economical way through the use of solar energy, particularly abundant in that country. In other lands, the competitivity of solar cells compared with other sources of energy, depends on the isolation at the site where they are installed. The survey, which is presently being conducted in several countries, will provide in 1977 the necessary informations. The various manufacturers have put on the market, in the past years, several models of solar cells. These are submitted to various modifications to improve the output, the tightness and the reliability of the modules. In order to ensure the safety of operation of our future installations, every time a new product, corresponding to our needs, is available, it should be tested in a laboratory, to check the characteristics advertised by the manufacturer. It should also undergo a test during 1 yr in Africa, under normal conditions of operation, before it can be chosen as a standard equipment to supply power for TV receivers or other broadcasting equipments.