Simulations discussed above were repeated for Melbourne and Darwin climates. Mean monthly climate profiles for each of the three cities [8] are illustrated in Figure 6. Melbourne is a warm temperate climate, similar to Sydney, but with a colder winter and colder nights. Darwin is a tropical climate with wet and dry season differences dramatically influencing cloud cover.

The impact of air flowrate on the frequency of high zone temperature events for the Melbourne and Darwin climates is illustrated in Figures 7 and 8 respectively

Figure 7 suggests that zone temperatures can be maintained below 26°C for all but 20 hours per year without the use of any backup fossil fuel heat source. It appears that the stand-alone solar desiccant cooling process can potentially provide an acceptable comfort airconditioning solution in the Melbourne climate.

The viability of the two processes (solar cooled and two stage evaporative cooled) in Melbourne is further illustrated in Figure 9, by plotting the simulated temperature and humidity in the occupied space at each half hour interval where temperature exceeds 22°C.

In Melbourne, the less complex evaporative cooling process achieves much of the same benefit as the solar desiccant cooling process. However, the solar desiccant step reduces the number of hours where temperature is above traditional airconditioning set-point temperatures (~23°C) and reduces humidity levels in the occupied space by around 5.5%

In contrast to the Melbourne climate, Figure 8 suggests that high temperature events can not be adequately prevented in Darwin by the stand-alone solar desiccant cooling process. Evaporative cooling appears to provide only limited assistance to the solar desiccant cooling process in the tropical Darwin climate. This is understandable because outdoor air starts off significantly warmer and more humid. Consequently, evaporative cooling is less able to achieve low temperatures consistent with desirable indoor air conditions.

Fig. 10: Influence of (i) target zone temperature and (ii) solar collector area, on the number of hours that the target temperature is exceeded in Darwin, with fixed desiccant cooled air flow of 3.71 airchanges per hour.

It is apparent that the addition of extra collector area will not easily produce acceptable comfort conditions in Darwin using the solar desiccant cooling process as described in Figure 1.

Alternative cycles/ component arrangements may need to be considered for tropical climates.

5. Conclusions

The hour by hour performance of a standalone, once-through desiccant cooling system for airconditioning a commercial office space, was examined using the TRNSYS computer simulation software. The study particularly focuses on the potential for designing and operating a desiccant cooling system without any thermal backup provided to mitigate for intermittent solar availability.

The study investigated the impact of manipulating (i) indirect evaporative cooler effectiveness, (ii) desiccant cooled air flow to the office space, and (iii) solar collector area, on the comfort conditions experienced in the office space. Differences between the performance of the solar desiccant cooling system in (i) the warm temperate climates of Melbourne and Sydney and (ii) the tropical climate of Darwin were also investigated.

When low humidity air is available, the effectiveness of the indirect evaporative cooler heat exchanger was shown to have a marked impact on the achievable temperature drop. Increasing collector area and air flowrate to the occupied space, were both shown to reduce the frequency of high temperature events in the occupied space. In the warm temperate climate of Melbourne (and to a lesser extent Sydney), high ventilation rates enabled comfort conditions to be maintained at or near acceptable levels in the occupied space, with-out the use of a backup thermal source. During extreme weather conditions, evaporative cooling appears to be the dominant mechanism for cooling the occupied space.

This synergy between evaporative cooling and solar desiccant cooling, observed in the warm

temperate climates, was not evident in the tropical Darwin climate. Further research is required to

model alternative cycles with more promise in tropical climates.

Nomenclature

П Solar collector efficiency

T Collector fluid inlet temperature (°С)

Tamb Ambient temperature (°С)

G Solar insolation (W/m2)

References

[1] White S. D., Kohlenbach P., and Bongs C., “Desiccant cooling system modelling and optimisation”, International Sorption Heat Pump Conference, Seoul, Korea, September 2008, in press

[2] Lam J. C., Hui S. C.M., and Chan A. L.S., “Regression analysis of high rise fully air-conditioned office buildings”, Energy and Buildings, 26, 1997, 189-197

[3] Beccali, M., Butera, F., Guanella, R., and Adhikari, R., “Simplified models for the performance evaluation of desiccant wheel dehumidification”, International Journal of Energy Research, 27, 2003, 17-19.

[4] TRNSYS v16.x TESS Libraries Version 2.0, Thermal Energy Systems Specialists, LLC, Madison, WI

[5] LTS-Collector Catalogue 2002, Institute fur Solartechnik SPF Rapperswil. BFE Bundersamt fur Energie, Bern, Switzerland

[6] H.-M. Henning, Solar Airconditioning and Refrigeration, Task 38 of the IEA Solar Heating and Cooling Programme, presentation to Sustainability Victoria, May, (2007)

[7] 2008 ASHrAe Handbook, HVAC Systems and Equipment, pgs 40.2-3

[8] Australian Bureau of Meteorology, http://www. bom. gov. au/climate/averages/