Two models have been designed: one system includes evacuated tube solar collector and the other includes double-glazed flat plate solar collector. Each model is simulated with different insulation thickness and size of overhang and wing walls. The simulation has been performed to select the optimum collector area and slope, generator temperature so that the maximum possible solar heat is gained, and to select the suitable insulation thickness and overhang and wing walls so that minimum possible cooling load is obtained. As seen from figure 2, Nicosia needs air conditioning for 9 months from March to November. Figure 3 shows cooling load of the building in Assab and from the figure it can be seen that air conditioning is needed all year round.

For Assab the maximum solar fraction is obtained at the optimum generator hot water inlet temperature of 86°C for the system with evacuated tube solar collector. The corresponding figure for double-glazed flat plate solar collector is 85°C [10]. The cooling water temperature is assumed low enough so that the system works at low generator temperature. Figure 4 shows the generator hot water temperature for Nicosia and the result shows that the optimum temperatures are 86°C and 83°C respectively. The solar fraction is defined as the percentage of the solar energy supplied to the absorption chiller over the total energy supplied (from solar energy and auxiliary heater) to the chiller. When the hot water temperature increases the additional energy is delivered from the heater, which reduces the solar fraction.

Solar heat gain from the collector varies with slope of the collector and latitude. It is proportional to the angle between the collector surface and the incident ray from the sun. For Assab the maximum is obtained at a collector slope of 13° for both types of collectors

[10]

. For Nicosia a maximum heat is gained when the evacuated tube collector slope is 25° and the double-glazed flat plate collector slope is 24° (Figure 5).

The effect of both collector area and storage tank volume on solar fraction (SLFR) and system efficiency (SYEFF) for evacuated tube and double-glazed collectors are shown in
figures 6 and 7 respectively. System efficiency is defined as the percentage of the incident solar energy converted into cooling effect. For the case of Assab it is given in [10]. As seen from the figures the solar fraction always increases with solar collector size and storage tank volume. In the case of collector area the increase is higher at the smaller collector area, but decreases its increment as the area increases. For the case of storage tank the variation is high at high volume.

Figures 6-10 show the effect of collector area and storage tank volume on solar fraction and system efficiency for the insulated building.

The effect of solar collector area on solar fraction and system efficiency for the 2.0m3 hot water storage tank is shown for Nicosia (Figure 8) and for Assab (Figure 9). From the figure it can be seen that the solar fraction of Assab is smaller than that of Nicosia, but the system efficiency is higher. In all cases solar fraction increases with collector area. The maximum system efficiency of the evacuated tube solar collector for both locations is obtained when the collector area is 20m2. For Nicosia the maximum system efficiency is obtained when the double-glazed collector is 20 m2 while for Assab when it is 30 m2.

The effect of thermal storage tank to the solar fraction and system efficiency is very small comparing to that of the solar collector (Figure 10).

20 10 0

3

storage tank volume (m3)

Figure 10. Effect of storage tank on solar fraction and system efficiency (Nicosia).

Figure 11 shows the effect of insulation thickness on cooling load for both locations. The highest cooling load is obtained when the building is not insulated and the lowest is obtained when the insulation thickness is 0.20 m. The maximum cooling load for Assab is 265 MJ while for Nicosia it is 78.6MJ. The reduction of cooling load in the first 0.05m insulation thickness is 34% for Assab and 25% for Nicosia. For the second 0.05m insulation thickness the reduction drops to 6.2% for Assab and 2% for Nicosia. As the thickness increases the effect of reduction decreases and is insignificant for insulation thickness greater than 0.2m.

Overhang and wing wall reduce cooling load since the direct solar radiation is prevented from entering the building. As the size increases the reduction also increases (Figure 12). But the effect is very small when compared with the effect of insulation. In the first 0.5 m overhang and wing wall addition the reduction is 4% for Assab and 5% for Nicosia. When another 0.5 m is added the reduction is 2.8% and 2.6% respectively.

Figure 13 shows that solar fraction increases with insulation. This is due to the fact that energy demand decreases with insulation. But the system efficiency more or less remains the same.

Conclusion

A TRNSYS simulation model of solar-assisted air conditioning system has been developed to simulate long-term performance of buildings in Assab, Eritrea and Nicosia, Cyprus. Two collectors types: evacuated tube collector and double­glazed flat plate collectors are used for comparison purposes. The model was used to perform a parametric study of the system to investigate the effect of collector area, storage tank size and generator temperature on solar fraction and over all system efficiency. The model was also used to analyze the possibility of reducing cooling by means of addition of insulation, overhang and wing walls to the buildings.

In both locations the highest system efficiency was obtained from the system, which used an evacuated solar collector. For Assab it is 1.85 times greater than that of the system, which used double-glazed collector and for Nicosia the figure is 1.72.

The lowest cooling load is obtained for both locations when the insulation thickness is 0.20 m and the size of the overhung is 1.5 m. Further reduction could be obtained without economic advantage. Reduction of cooling load is not enough when optimum condition is needed. In addition to the cooling load reduction analysis of the overall cost of the system is important. But for economic comparison the cost of equipment, building materials and fuel oil used in the systems are not the same in all locations.