In the previous paper, White et. al. [1] used the default effectiveness factors provided in the TRNSYS type 757 indirect evaporative cooler model, being in the range 0.3 to 0.5 depending on the primary air dry bulb temperature and the secondary air wet bulb temperature. However,

ASHRAE [7] suggests that indirect heat exchangers can achieve a 60 to 80% approach to the secondary air wet-bulb temperature at a secondary to primary air flow ratio down to 0.6:1.0.

The potential benefit of increasing the effectiveness of the indirect evaporative cooler was first investigated in a series of stand-alone, steady-state simulations of (i) a two stage indirect/direct evaporative cooling process and (ii) a single stage direct evaporative cooling process. The performance of the two stage indirect/ direct process was evaluated by how much colder supply air could be made, compared with that from a single stage direct evaporative cooling process. The results of this analysis are presented in Figure 2 for a range of inlet air conditions.

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At high primary air humidity, the inclusion of an indirect evaporative cooling step has limited impact on the achievable supply air temperature compared with a simple direct evaporative cooler, irrespective of the indirect evaporative cooler effectiveness. However, at low primary air humidity, the inclusion of an indirect evaporative cooling step significantly reduces the supply air temperature compared with a direct evaporative cooler, and increasing the indirect evaporative cooler effectiveness from 50% to 70% almost doubles the impact of the indirect evaporative cooling step.

Given that the heart of the desiccant cooling process is the reduction of primary air humidity, the use of a two stage indirect/ direct evaporative cooling process appears to be sensible. The additional temperature reduction can potentially expand the number of hours over which backup cooling is not required, and increase the thermal cooling from the same quantity of dehumidified air (with resulting increase in coefficient of performance (COP)).

The impact of increasing the effectiveness of the indirect evaporative cooler was further investigated in TRNSYS simulations of the desiccant cooled building described in Section 2, but with a fixed desiccant regeneration temperature of 60°C. The resulting predictions of thermal conditions in the occupied space, for each of the two assumed values of indirect evaporative cooler effectiveness factor, are illustrated in Figure 3.

Figure 3 suggests that, under the stated assumptions, a target zone temperature of 26°C is exceeded for less than 20 hours, with air change rates of 3.71hr-1 and 4.53hr-1 for 70% effectiveness and default TRNSYS model 757 effectiveness values respectively.

The increased indirect heat exchanger effectiveness, and consequent decrease in required air flow, leads to reduced fan parasitic energy consumption and reduced equipment size and cost. The reduced flow also reduces the amount of regeneration air to be heated with an expected improvement in the coefficient of performance.