Performance characterization of M-cycle indirect evaporative cooler and heat recovery ventilator for commercial buildings – Experiments and model
Subhrajit Chakraborty, David Vernon, Apoorva Jha, Vinod Narayanan
Abstract
Evaporative cooling systems in buildings can dramatically reduce greenhouse gas emissions due to their low energy consumption, lack of ecologically harmful refrigerants and their diverse applications. Indirect evaporative cooling (IEC) uses water in the secondary air stream to cool the primary air without adding humidity to the indoor environment. The Maisotsenko Cycle (M−Cycle) design of an IEC has the potential to approach dew-point on the primary air by pre-cooling the secondary air stream before extracting heat in cross-flow with the primary air. Separately, a variety of technologies exist for energy recovery in commercial buildings to pre-condition ventilation air. In this study a M−cycle heat and mass exchanger was used as a heat recovery ventilator (HRV) to recover energy from the exhaust air stream during winter and as an IEC during summer to cool the ventilation air. Experiments were performed by operating the full-size IEC/HRV system with a maximum air flow rate of 2210 m3/hr in conjunction with indoor and outdoor environmental chambers simulating respective environments. The experimental results showed HRV heating sensible effectiveness between 67 % and 63 % and IEC cooling total effectiveness between 81 % and 71 % with increasing air flow rates. A detailed numerical model was developed to predict the performance of the IEC/HRV over a large range of operating conditions. The IEC model simulates heat and mass transfer from water evaporation in the secondary air and potentially from condensation in the primary air during summer season operation. The HRV model simulates heat transfer and potentially mass transfer from secondary air condensation during winter season operation. The detailed physics-based model was compared with experimental data, and it predicted the performance with high accuracy for both summer and winter operating conditions. The average absolute errors for supply air temperature between the experimental results and model predictions were 0.2 °C in HRV winter mode and 1.0 °C in IEC summer mode. The validated IEC model was used to further explore the possibility of combined ventilation and passive space cooling without a supplemental air-conditioner. The analysis shows that utilizing IEC for summer operation offers high cooling COP and even the potential for passive space cooling, which is not possible with conventional energy recovery ventilators (ERV).