Fig.2 shows the evaporation temperatures and COP obtained by using only the panels or the panels together with the heat exchanger arranged in parallel. The solar radiation intensity, I, was zero. The inlet water temperature at the condenser, tw1, was 20 °С, the ambient air temperature, ta, was about 8 °С, the wind velocity, V, was about 0.2 m/s

COP = 1 + —

H

(8)

Solving eqs.1,2 and 8 simultaneously, te and COP can be obtained similarly to the case that only the panels are used as the evaporator.

Time

during the measurements. The evaporation temperature at the exit of the expansion valve, te, was -13 °C ~-8 °C and COP was 2.3~3.5 for the case of using only the panels, but te was -8.5 °C ~-7 °C and COP was 3~3.6 when both evaporators were used.

Similar experiments were done in cloudy days as shown in Fig.3. The solar radiation intensity was about 120 W/m2 when the data were taken. The plots Fig.4 Relation between tW:1-te andCOp.

Fig. 5 Comparison between the experimental and analytical results of COP (single use of panels).

Fig. 6 Comparison between the experimental results for the heat pump using panels together with air-refrigerant heat exchanger and analytical results for the heat pump using only panels.

show the average values in 30 minutes. The use of the two evaporators gave higher COP in these cases also.

Fig.4 shows the variation of COP with the evaporation temperature. The best fitted curve was given by the following equation.

COP = 0.0019(tw l — te )2

— 0.231(tw, i — te) + 8.40

From experimental results, the power consumption of the compressor was given by the following equation.

H = 3.44tw1 +107 [W] (10)

Fig.5 shows the comparison between the experimental and analytical results of COP for single use of panels. F’ was assumed to be 0.9 in the analysis. The analytical results for tw, i=20 °C are specially agreed well with the experimental results. The present analysis assumed that there was no heat loss from the system. The heat loss from the condenser and compressor to the air in the room increases with increases of the water temperature. This may case a little smaller COP in the experimental

results.

Fig.7 Comparison between experimental and analytical results for heat pump using panels and air-refrigerant heat exchanger in parallel (2003 / 2 / 3).

Fig.8 Comparison between experimental and analytical results for heat pump using panels and air-refrigerant heat exchanger in parallel ( 2002 /12 / 26).

Fig.6 shows comparison of COP and te between the experimental results for the heat pump using panels together with the heat exchanger and analytical results for the heat pump using only panels. The water temperature was 30 °С. The agreement between the experimental and analytical results was good until about 3 p. m.. The solar radiation intensity, I, was larger than 400 W/m2 until that time. Since the evaporation temperature was about equal to the ambient temperature or higher than that until 15:00, it was expected that only the panels were used for evaporation. After about 15:30, the heat exchanger seemed to be used for evaporation also so that the evaporation temperature and COP become higher than the analytical results for the case of single use of the panels.

The value of K was obtained from the experiments by using only the air-refrigerant heat exchanger as the evaporator. Introducing the average evaporation temperature and COP obtained by experiments into eq.7, K=18.7 (W/K) was obtained. This value was used in the analysis. The same experimental results as shown in Fig.6 are shown in Fig.7. The analytical results in this figure were obtained by assuming that heat was absorbed by both evaporators when the evaporation temperature became less than the ambient air temperature. The agreement between the experimental and analytical results was good.

The results obtained for the water temperature of 40 °С are shown in Fig.8. The evaporation temperatures obtained by the analysis were 3 °C~4 °С higher than the ones obtained experimentally. The COP obtained by analysis was 0.2 higher approximately than the ones obtained in experiment. The heat loss from the condenser and the compressor might cause a little smaller evaporation temperature and COP in the experimental results.

5. Conclusions

The flat-plate collectors used as the evaporator of the heat pump was not fit for absorbing heat from the ambient air. Therefore, the evaporation temperature and COP became low when there was no or small solar radiation. The thermal performance of the heat pump was improved by adopting the air-refrigerant heat exchanger arranged in parallel to the flat-plate collectors. The predicted results of the evaporation temperature and COP for the heat pump with dual heat sources of solar heat and the ambient air agreed with these obtained experimentally.