Solar hybrid air-conditioning design for buildings in hot and humid climates - Solar Energy Sciences and Engineering Applications - page 490

Solar desiccant cooling systems can enhance indoor air quality owing to the provi-
sion of full outdoor air. However, primary energy consumption is higher than in other
solar air-conditioning systems, such as solar absorption or adsorption refrigeration sys-
tems and conventional vapour compression refrigeration systems, in which the return
air scheme can be used at the air handling unit. Adoption of the return air design for
solar desiccant cooling depends heavily on the climatic conditions, particularly solar
irradiation, air temperature and humidity during summer. In hot and humid climates
the cooling performance of solar desiccant cooling systems using outdoor air is better
than that using return air, since a larger amount of conditioned air is involved (Fong
and Chow, 2007). Auxiliary heating is therefore involved to achieve a satisfactory
cooling performance.
15.2.3 A hybrid approach to system design
In general, the building cooling load can be divided into zone cooling load (mainly
sensible load) and ventilation load (mainly latent load). If the refrigeration cycle is used
to handle the former, and the hygroscopic nature of desiccant cooling used to tackle the
latter, the cooling load can be effectively handled in such a load-sharing approach. Due
to the separate handling of the cooling load, individual controls for zone temperature
and zone relatively humidity become more practical. The hybrid approach of system
design can be the basic SHAC or SHAC enhanced by high temperature cooling. The
details of these two alternatives are described as follows.
15.2.3.1 Principal SHAC
Figure 15.2.5 presents the schematic diagram of the principal SHAC system fully driven
by solar-thermal energy. In the configuration of the desiccant cooling unit the two
evaporative coolers can be omitted and a cooling coil adopted at the supply air stream
instead. In this case the absorption/adsorption chiller generates chilled water to the
cooling coil located at the downstream of the thermal wheel. As both the heat-driven
chiller and desiccant cooling are involved, there would be two sets of regenerative
Figure 15.2.5 SHAC system using absorption chiller (New abbreviation: DW: desiccant wheel; EAF:
exhaust air fan; OAF: outdoor air fan; SAF: supply air fan; and TW: thermal wheel).