The project was carried out on the Altiplano in northern Argentina. This region is situated at an average altitude of 3.600 meters. The extreme dryness and strong solar radiation cause daily ambient temperatures to vary over 30°C. In winter, the temperatures at night drop to -2O °C, while daytime solar energy levels amount up to 8 kWh/m2. At 2200 kWh per square meter this region has one of the highest amounts of annual solar insolation en­ergy in the world. Figure 2 shows the temperature and the insolation over one year. An important consideration for the installation of solar air systems is the by 30% reduced den­sity of air at altitudes of 3600 m.

2. Heating and hot water production

The two heating systems described here were installed in two kindergartens. The areas of the buildings were 100 and 120 m2 respectively. Both buildings were built in the traditional way using a clay construction. The roof is made of corrugated metal sheets with a false ceiling. The false ceiling gives only a rudimentary protection against heat loss. Figure 2 shows the simulated natural room temperature in the kindergartens. As one can see, room temperatures can fall to a minimum value of zero degrees in wintertime.

With the help of the building simulation program LACASA, which runs under MALAB/SIMULINK, calculations were made to determine the energy demand. By using a local annual weather data set, an annual heat demand of 115 kWh per square meter was

calculated. This figure assumes a room temperature of 18°C with an average annual am­bient temperature of 9.5°C. The daily heat demand was found to be around 40 kWh per day in yearly average. Figure 2 shows the simulated energy demand for two houses with different floor base. The maximum heat demand occurs in wintertime with 0,7kWh/m2d. The solar air heater consists of a single glazed collector with the air flowing over the ab­sorber. On the underside the absorber is insulated only by an air-layer. The modular con­struction of this system allows the connection in series and parallel of as many modules as necessary. One module has an aperture area of 2.33 m2. Figure 4 shows a cut through this simple collector construction. The total surface areas of the installed collectors are 18 and 29 m2 respectively. The

Figure 5 shows a picture of an installed collector and the characteristic efficiency curve. The curve shows, that 40% efficiency can be obtained, when the difference between the average collector temperature and the ambient temperature is 40°C. The curve is 5 to 10 percent better than similar known constructions. One reason for this can be seen in the lower convective losses due to the reduced air density in the high mountain region.

The air current moving through the collector is directly controlled by the amount of solar radiation. This is managed by directly connecting the ventilator to a PV panel with 70W. Further on this ensures that the unit operates independent of an electrical power source. With solar radiation levels at 1000 W/m2, an air current of 0.4 m3/s is achieved. To provide long lifetime, brushless ventilators are used. These can be connected in parallel to adapt to the collector area.

The design of the unit must take account of the heating energy demand as well as the en­ergy demand for the daily warming of around 300 litres of hot water. On a typical winter day, insolation values of 5kWh/ m2 can be expected on site. This means that on a collector surface of 29 m2, a total of 145 kWh is irradiated. Assuming an average collector efficiency of 40%, a daily heating supply of 58 kWh is available for use. Figure 6 shows the gained daily energy per square meter for different orientations of a solar air heater at a geographi­cal latitude of 21° south. As can be seen in the figure 6 the best orientation of the collector can improve the yearly energy gain by almost 100 percent.

As the first results were very promising, the collector developed during the project was adopted by a local Argentinean company which was also involved in the project. The solar air heater can also be used for other purposes as for example, the drying of fruit, vegeta­bles and tobacco, as well as being used in the mines for drying dissolved minerals. One module is sold for 350 Pesos, which equals about 45 Euro per square meter.

The air heated in the solar heater flows through a valve, into an air-water heat exchanger and then through the ventilator which presses the air though an insulated chimney into the pebble bed. The exit on of the pebble bed is connected to the collector opening, thus clos­ing the circuit (see Fig. 7). The valve prevents automatically the discharge of the storage during times without insolation.

The air-water heat exchanger is designed so that it is able to extract up to 6 kW from the air current. This is sufficient to heat 300 litres of hot water each day to a temperature of 60°C. The water tank is fixed under the roof above the heat exchanger, so that the trans­port of water takes place through natural convection. In order to reduce costs, a normal car radiator was installed. The indoor installation ensures, that the the water circuit never can freeze and so damage the installation. Measurements have shown satisfying results, since hot water with up to 60°C was produced. For each 100 liters of warm water about 2 m2 of collector area should be added.

The thermal storage for the energy carried in the air comprises of a pebble bed with a vol­ume of 11 m3. The equivalent diameter of the stones is around d= 0.15m. This presents a compromise between pressure loss and heat transfer. The thermal storage system is inte­grated in the house beneath the floor and heat is transferred by radiation and convection to the room. Additional air vents can boost the heating power through direct convection into the room. When the pebble bed storage is heated up to dT= 40°C, the energy content is around 110kWh.

Since the target region of the system is a high mountain region, the by 30 percent reduced air density had to be taken into account for the system design. While the lower density has advantages since it reduces heat losses through natural convection it rises pressure losses of the air system on the other hand. On 3700 m the convective heat losses are low­ered by 26 percent while the required electric power to achieve the same massflow as on sea level rises by 20 percent. So special emphasis has to be put on an aerodynamically optimised system since the power for driving the massflow comes from PV-modules and so is an important cost factor.

Figure 8 shows the simulated room temperature of a building with 100 m2 equipped with a solar hot air system. In this case a collector area of 28 m2 is used, the storage has a vol­ume of 16m3. Compared with the unheated building, the room temperature on yearly aver­age is 12°C higher. Instead of falling down to 4°C the minimum average temperature lies at 13°C in June.