The input data for SOLSTILL includes measured weather parameters and the measured temperature and relative humidity values of the air entering the still, as shown in Figure 5. The outputs of the simulation consist of the temperature of the water in the still, the cover temperature, the basin temperature, the temperature and relative humidity of the air
leaving the still, the temperature of the air leaving the condenser, the temperature and relative humidity of the air leaving the heat recovery (or the pre-heater), as well as the distillate production from the glass and from the condenser. Figures 6 and 7 show the measured and predicted values of the temperature and relative humidity of the air leaving the still and the pre-heater, respectively. Figure 8 shows the predicted and measured moisture content of the air leaving the still and Figure 9 shows the predicted and measured temperatures of the water in the still. Figures 10 and 11 show the predicted and measured distillate production from the glass and the condenser, respectively.

SHAPE * MERGEFORMAT

SHAPE * MERGEFORMAT

Figure 11. The predicted and measured distillate collected from the condenser of the still

in the open loop mode test.

As shown in figure 6, the simulation is able to predict both the trends in, and the actual values, for the temperature of the air leaving the still quite well. The maximum error is of the order of 5 0C and occurs in the early afternoon. The errors in the relative humidity calculation are more significant with errors ranging up to 20% RH with an average error of 10%. For most of the time, the calculated values are less than the measured values. The results in figure 8 confirm that the simulation program under-predicts the amount of water in the air leaving the still during the middle of the day. At other times, the measured and calculated values of the absolute humidity of the outlet air are very similar, which indicates that the differences observed in RH at these times are due to variations between the calculated and measured air temperature.

2. CONCLUSION

The development of SOLSTILL, a simulation program for estimating the performance of basin type solar stills was described. Models for both the standard free convection solar still and a forced convection solar still with enhanced heat recovery were included in the program. For the conventional free convection systems, the SOLSTILL program also enables simulation of more complex systems with many more parameters compared to the existing models found in the literature. A new model incorporating heat and mass transfer in forced convection solar stills with enhanced heat recovery was described in this paper. The design, fabrication and testing of an experimental system set up to validate SOLSTILL was detailed. The comparison of experimental and simulation results indicated that the program can predict distillate production at an acceptable level of accuracy.