Solar hybrid air-conditioning design for buildings in hot and humid climates - Solar Energy Sciences and Engineering Applications - page 493

determine the annual total energy consumption of system operation, as well as other
kinds of performance indicators, such as solar fraction (SF) and coefficient of perfor-
mance (COP). In order to have more accurate evaluation of responses and operation
of the solar air-conditioning system arising from changing boundary conditions, the
simulation time step is set at 6 minutes, so that a total of 87,600 simulations are run
for a year-round study. As electrical energy and thermal energy are involved in the
various solar air-conditioning systems, they would be converted to primary energy
consumption (E p ) for comparison purposes. The conversion of electrical energy to
primary energy takes into account the local fuel mix.
15.3.1 Principal solar-thermal air-conditioning systems
The basic design of solar air-conditioning is in accordance with the solar-thermal
approach, including the solar absorption refrigeration system, the solar adsorption
refrigeration system and the solar desiccant cooling system, as described in Section
15.2.2. From the previous study (Fong et al., 2010a), the cooling and energy perfor-
mances of these solar-thermal air-conditioning systems for office building application
are consolidated in Table 15.3.1. Their results are also contrasted with those of the
conventional AC systems using air-cooled vapour compression chillers (ACVCC) and
water-cooled vapour compression chillers (WCVCC). Although the common solar-
thermal collectors include the flat-plate collectors, evacuated tubes and parabolic
concentrators, the last type is the least effective, as found in the aforementioned study.
In principle, parabolic concentrators can harness thermal energy at higher temperature;
however, in a fixed space for collector accommodation and the changing incidence of
solar irradiation, they have the lowest overall solar-thermal gain in a year. As such,
only the flat-plate collectors and the evacuated tubes are involved in the study.
Compared to the conventional air-conditioning system in the return air scheme,
the solar absorption refrigeration system with evacuated tubes has the best energy
performance, with 35.1% and 33.6% less year-round primary energy consumption
than the ACVCC and the WCVCC respectively. The solar adsorption refrigeration
system, which has about 20% more energy consumption, does not have better energy
performance than the conventional system here. Nevertheless, its application potential
still exists in the hybrid design of solar air-conditioning systems which are discussed
in Section 15.3.3. For the same kind of solar air-conditioning system, evacuated tubes
can have better energy performance than the flat-plate collectors in the hot and humid
climate from a year-round perspective.
Solar desiccant cooling has relatively high year-round primary energy consump-
tion, since it has to handle the extra ventilation load due to the inherent nature of
full outdoor air design. Accordingly, the total cooling capacity of desiccant cooling
system is much higher than that of the other systems. In addition, the parasitic energy
consumption is comparatively high, particularly the supply air fan and exhaust air
fan. Compared to conventional AC systems in the outdoor air scheme, solar desiccant
cooling systems with evacuated tubes can have 4.9% and 1.5% less primary energy
consumption than ACVCC and WCVCC respectively. The feature of solar desiccant
cooling in effect more than satisfies the required cooling load. It is able to supply
the outdoor air amount far above the minimum requirement of the functional area,