13

APPLICATIONS OF SOLAR ENERGY IN TRINIDAD
O. Headley
(Department of Chemistry, UWI, St. Augustine)


INTRODUCTION

Most people are fascinated by applications of solar energy which range from the one-megawatt solar furnace at Odeillo in France capable of reaching temperatures of over 30000C to the solar powered calculator whose amorphous silicon solar cell has an output of less than 1 watt. The solar energy device with which most people are familiar is the solar water heater. This is now a common sight on Barbadian homes and is beginning to be installed by some residents of Trinidad and Tobago as the prospect of paying the full economic cost of electricity looms larger. The solar water heater is really quite an old idea, the original design having been patented by William Bailey of California in 1909.
At the University of the West Indies, most of our work has been on solar distillation for producing pure distilled water, solar crop driers and solar timber driers. More recently, we have extended this to include compound parabolic concentrating (CPC) solar collectors for alcohol distillation and adsorption refrigeration.

SOLAR DISTILLATION

Solar distillation is an old art. The first reported solar still was built by Carlos Wilson at Las Salinas in Chile in 1872 where he was purifying salt water to provide drinking water for mules working a mine (Harding, 1883). In Trinidad, we began work in this area in 1969 using a grant of $4.000 TT provided by the Ministry of Planning and Development. In this case, the objective was the provision of distilled water for school science laboratories, gas stations, kidney dialysis machines and other applications where minerals must be absent. A solar still is really a very simple device, the raw water or distilland is contained in a blackened water tray over which a sloped glass cover is positioned: a drain of plastic or stainless steel is attached to the lower end of the glass cover and the whole device is sealed to prevent loss of water vapour.
During operation, sunlight enters via the glass cover and passes through the water to be absorbed by the blackened water tray and transformed into heat. This heat warms the water and therefore increases its vapour pressure. The warm water radiates in the infrared, but since glass is opaque in the infrared, the heat is retained in the solar still which becomes warmer than its surroundings. This is the standard greenhouse effect and a simple solar still is often called a greenhouse still.
Since the upper surface of the glass is in contact with the ambient air, there is conduction across the glass and heat is lost from its upper surface by radiation to the sky and convection to the surrounding air. As the warm water vapour rises from the distilland surface, it transports heat by evaporation. This is the main mass transfer mechanism in the solar still and salts are left behind in the water tray while pure water condenses on the underside of the glass cover.
Since vapour pressure rises rapidly with temperature, high temperatures in the water tray encourage rapid evaporation. The maximum solar power available at midday is about 1000 W/m2, which is not very high when compared with the power available in fossil fuelled devices.
Between 1969 and 1970, eleven solar stills of different configurations were built and tested by the Solar Energy Project at the University of the West Indies, St. Augustine.
All the solar stills which we have used for producing distilled water give a pure product and we have fed their water directly to atomic absorption spectrophotometers and failed to detect calcium, sodium, magnesium, or iron which are quite common in ordinary tap water.

SOLAR CROP DRYING

Solar crop drying is as old as agriculture. Several products such as cereal grains have to be dried to a moisture content which allows them to be stored without spoilage. This is usually between 10 and 15% (wet basis).
Some crops may be left on the plant until they reach the desired moisture content, which gives the simplest form of solar drying. If this is not feasible, then the crop can be spread out in the sun on a hard, clean surface and left to dry. This surface is called a drying floor and it is the most widely used solar drier in the world. Rice in Guyana and cassava in Thailand, to give only two examples, are dried in large quantities using this procedure.
Were it not for certain disadvantages, the drying floor would be the only solar
drier in commercial use. These disadvantages may be summarised as follows:

(I) The crop is exposed to the elements, wandering farm animals and thieves.

(ii) Wind-blown contamination makes quality control difficult.

(iii) Considerable amounts of labour are needed to move the crop in when the weather is no longer friendly.

These disadvantages do not prevent rice farmers in Central Trinidad from using the public asphalt roads as drying floors. It is fairly easy to calculate that a black asphalt road can reach a temperature of 70
0C (1580F) in the middle of the day when the available solar energy is at a maximum. When covered with rice, the asphalt cannot reach such a high temperature but it retains heat which it releases to the rice during cloudy periods.
Where quality considerations are important, it is necessary to use some form of enclosed drier. The simplest of these are the wire basket drier and the cabinet drier.
These two simple driers have been used all over the Eastern Caribbean. The cabinet drier attains higher drying temperatures than the wire basket drier and one can begin drying in the latter and transfer the partly dried crop to the former when the moisture content reaches about 30%, wet basis. Figure 5 shows the resultant drying curve both in the exponential and linearised forms.

SOLAR WATER HEATING

As mentioned earlier, this is one the most mature technologies for solar energy utilisation. In the mid-1970s, we built and installed four solar water heater Systems of 4 to 5m
2 solar collector area and 250 to 300 1 storage.
The solar water heater normally consists of three parts. These are the solar collector, the storage tank and the connecting pipes (Figure 6). The collector consists of a flat metal plate, usually made of copper, with pipes bonded or soldered to it; some manufacturers form the collector plate and water tubes as a single unit to reduce costs and enhance reliability. The ends of the water tubes connect up to two headers, one for admitting cold water and the other for collecting hot water. The plate-and-tube assembly, also called a fin and tube assembly, is placed in an insulated box and glazed with glass or transparent plastic. This is then connected to an insulated storage tank with insulated pipes .If the tank is placed above the collector, there is no need to use a pump since the hot water will enter the tank by thermosiphon action because hot water is lighter than cold water.
If for aesthetic or other reasons the tank has to be placed down in the house while the solar collector is on the roof, then a pump has to be used to circulate the hot water from the collector to the tank. Such a pump may be powered by a photovoltaic (PV) panel which makes electricity from sunlight or it may be run from the electric mains. In the latter case it needs control circuitry to ensure that it only pumps when the collector is hotter than the tank. A PV pump is self-regulating since the more sunlight there is the faster it runs and it cannot run at night.
The solar water heater industry in Barbados now has a turnover of about $2 million US per year and there is some indication that manufacturing in Trinidad is now economically feasible. Certainly if the Trinidad and Tobago Electricity Commission raises the cost of electricity to its economic production cost, solar water heaters will be competitive with traditional electric water heaters. At current electricity costs, most calculations show a payback period of 3 to 4 years for a 150 to 250 1 family-sized solar water heater. With a projected life span of 10 to 15 years, this is a very attractive proposition as long as the homeowner can finance the initial capital cost of $4,000 to $6,000 TT. In Barbados, solar water heaters may be bought in departmental stores which arrange the financing and the government allows the cost of the solar water heater to be claimed against income taxes.

SOLAR TIMBER DRIERS

For the manufacture of furniture it is necessary to use properly dried timber. In areas of high relative humidity, such as the Caribbean Islands, drying to a moisture content of 12 to 15% is adequate for the manufacture of stable furniture. For dry areas such as the southwestern USA, the recommended level is 6%.
Here in Trinidad we have used a greenhouse solar drier with a wood-waste furnace as an auxilliary heat source for drying local timber. This drier has 30 m
2 of solar collector area and a timber capacity of 6000 board ft. (18 m3). Air circulation is by means of four 1/4 hp fans (746 watts total) which suck air from the space between the black absorber plate and the transparent roof cover, into the plenum chamber and then blow it into the timber stacks. Hot air from the wood-waste furnace is sucked into the plenum chamber by the same fans when the furnace is in operation. This drier is therefore an example of a mixed mode drier, and a 17.6m2 unit with a kerosine burner has been used for drying coconuts, rice paddy, bagasse and cassava.
Timber species such as teak, mahogany and cedar which have been dried in the timber drier can be reduced from an initial moisture content of 50 to 60% (wet basis) to 16% (wet basis) in three weeks. This period may be reduced to 12 days if the auxillary furnace is used for 6-8 hours per day. Two other timber driers of the greenhouse type are currently being considered. One of 200 m2 and 70m3 timber capacity for a sawmill in South Trinidad and another of 20 m2 collector area and 9m3 timber capacity for a saw-mill in Belize. Construction work actually started on the 200 m2 unit but has been halted while funding is being renegotiated.

COMPOUND PARABOLIC CENTRATING (CPC) SOLAR COLLECTORS

As mentioned in the Introduction, concentrating solar collectors may be used to attain very high temperatures. In fact at the theoretical maximum concentration ratio of about 40000 to 1, the receiver should attain the surface temperature of the sun which is about 55000C. Trombe's furnace at Odeillo has attained temperatures of 38000C in a small area about 2cm in diameter at the center of the focal spot and > 20000C is attained across a spot of 30 to 35 cm in diameter (Trombe et al -, 1973). This sort of performance not only requires a large mirror (Trombe's parabolic mirror is 40 m high and 54m wide) but also a solar energy input which is high in direct radiation and low in diffuse radiation since the reflector does not focus the diffuse radiation. Here in Trinidad and Tobago, our sunlight tends to be high in diffuse radiation for the greater part of the year, so we have to use collectors of low concentrating ratio which are not too sensitive to diffuse radiation.
The CPC or Winston concentrator (Winston and Hinterberger, 1975) does not really focus light, it reflects it from two parabolic surfaces so that it falls on a receiver at the base of the system. The concentration ratio is defined as C, where

C = Aperture area
Receiver area

We have operated two CPCs with C=3.9 and 5.0 which have attained maximum temperatures of 1200 and 1600C with efficiencies of 59% and 54% respectively. At the moment we are constructing two arrays of CPCs on the roof of the Natural Sciences Building, UWT, St. Augustine. The smaller array of 4.4 m2 aperture area has a designed power output of 2400W while the larger array of aperture area 9.8m2 has a designed power output of 5000W. These arrays are for powering a charcoal-methanol adsorption refrigerator and a 5cm diameter alcohol fractionating column. With temperatures in the range of 120 to 1500C ,the CPC may be used as a source for low grade industrial pro-cess heat. This is of considerable interest to sunny islands without indigenous sources of fossil fuels.

CONCLUSION

Solar energy units have been used in the Caribbean territories for a wide variety of purposes. The technology is now sufficiently mature for it to be used in areas such as crop drying, timber drying and water heating where it has proved to be economical. To ensure that this technology is disseminated as rapidly as possible, the European Development Fund sponsored a Seminar Workshop in Antigua in January1989 at which the Caribbean Solar Energy Society (CSES) was inaugurated. Readers who are interested in obtaining more details on solar energy, the CSES or the International Solar Energy Society (ISES) may contact Dr. Headley at the UWI., St. Augustine, Trinidad.


REFERENCES

Harding, J., 1883, Apparatus for solar distillation In: Proc. Inst. Civil Eng., Vol 73, pp.284-288.

Trombe, F. et al., 1973, Thousand K~ solar furnace, built by the National Centre of Scientific Research, in Odeillo, France. Solar Energy, Vol.15 pp.57-61.




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