3.1. Solar desiccant cooling parametric analysis

A number of simulations were performed, for the Sydney climate, with heat provided from an unboosted flat-plate solar heat source. Based on simulations performed at constant desiccant regeneration temperature, a desiccant cooled air flowrate of 3.71 airchanges per hour was initially selected. Three solar collector sizes were examined ranging from 500m2 (0.167m2 of collector/m2 of conditioned floor space) to 1500m2 (0.5m2 of collector/m2 of conditioned floor space). The lowest (500m2) collector area simulated is similar to that suggested by Henning [6] being approximately 3 m2 of collector/kW of cooling.

Figure 4 illustrates the impact of target zone temperature and collector area on the number of hours per year that the zone temperature exceeds the target. While Figure 4 confirms that increasing collector area successfully reduces the frequency of high temperature events, the number of high temperature events is still higher than that normally expected for satisfactory comfort conditions. From a capital cost perspective, it is also unattractive to use large collector areas.

In an attempt to further reduce the frequency of high temperature events, the impact of increasing air flowrate was examined at a constant collector area of 500m2 (0.167m2 of collector/m2 of conditioned floor space). Results of these simulations are presented in Figure 5.

Figure 5 highlights a decrease in the frequency of high temperature events when supply airflow is increased. However, the concomitant increase in regeneration airflow will cause the regeneration air temperature to decrease. Further examination of the 6.18 airchanges per hour solar desiccant cooling simulation results found that the average temperature of the regeneration air during solar cooling was 40.7°C, which is at the lower limit of the range where the psychrometric model of Beccali et. al. [3] is applicable.

The use of high air flowrates also cools down the solar heated hot water faster. Consequently, the pump supplying hot water to the heating coil is stopped earlier in the afternoon and there are fewer hours of active solar cooling. It would appear that the reduction in frequency of high temperature events, at higher air flowrates, is more to do with an increase in the amount of cooling through conventional evaporative airconditioning than from increased performance of the desiccant cooling process itself.

To test the relative contribution of the desiccant dehumidification step, compared to the evaporative cooling effect, an additional set of simulations was performed using a standalone two stage indirect and direct evaporative cooling process (without dehumidification). Results of the evaporative cooling only simulations are also presented in Figure 5. Comparison of results with and without the solar desiccant dehumidification step suggest that the solar desiccant dehumidification step is effective at reducing the number of hours at which zone temperature exceeds 23°C, but conventional evaporative cooling is the dominant mechanism for eliminating the small number of highest temperature events.

Closer examination of the highest temperature events found that the pump supplying hot water to the heating coil was generally switched off during these periods, either because the hot water tank had never reached temperature that day, or because the temperature in the hot water tank had dropped below the thermostat set point in the afternoon. This result suggests that it may also be desirable to explore the potential of altering the control strategy and/or temperature set points, with the aim of keeping the hot water supply pump running for longer.