Exergetic analysis of a desiccant cooling system: Searching for performance improvement opportunities

The current increase of the energy consumption of buildings requires new approaches to solve economic, environmental and regulatory issues. Exergy methods are thermodynamic tools searching for sources of inefficiencies in energy conversion systems that the current energy techniques may not identify. Desiccant cooling systems (DCS) are equipments applied to dehumidifying and cooling air streams, which may provide reductions of primary energy demand relatively to conventional air-conditioning units. In this study, a detailed thermodynamic analysis of open-cycle DCS is presented. It aims to assess the overall energy and exergy performance of the plant and identify its most inefficient sub-components, associated to higher sources of irreversibilities. The main limitations of the energy methods are highlighted, and the opportunities given by exergy approach for improving the system performance are properly identified. As case study, using a pre-calibrated TRNSYS model, the overall energy and exergy efficiency of the plant were found as 32.2% and 11.8%, respectively, for a summer week in Mediterranean climate. The exergy efficiency defect identified the boiler (69.0%) and the chiller (12.3%) as the most inefficient components of the plant, so their replacement by high efficient systems is the most rational approach for improving its performance. As alternative heating system to the boiler, a set of different technologies and integration of renewables were proposed and evaluated applying the indicators: primary energy ratio (PER) and exergy efficiency. The heating system fuelled by wood was found as having the best primary energy performance (PER=109.6%), although the related exergy efficiency is only 11.4%. The highest exergy performance option corresponds to heat pump technology with coefficient of performance (COP)=4, having a PER of 50.6% and exergy efficiency of 28.2%. Additionally, the parametric analyses conducted for different operating conditions indicate that the overall irreversibility rate increases moderately for larger cooling effects and more significant for higher dehumidification rates.