Experimental Investigation of a Ground Coupled Desiccant Assisted Air Conditioning System during Winter Mode
Air conditioning, Desiccant, Humidification, Ground coupled heat pump, System evaluation
Air conditioning for residential as well as non-residential applications is increasing due to various reasons. Most commonly, income growth in developing and emerging countries, climate change and increased energy building standards are indicated as reasons causing growing demand for air conditioning. Currently, most air conditioning systems are based on conditioning supply air by cooling outside air below dew point temperature, relying on a vapor compression chiller. Due to high energy demands of such systems, alternative air conditioning concepts are investigated. The advantages of desiccant assisted systems in summer mode are well-established in literature, especially in terms of performance analyses in hot and humid regions when air has to be cooled and dehumidified. Even though winter mode is an essential part of the full year operation, especially for heating dominated regions, winter mode as well as full year operation are addressed only in few studies. A further comfort aspect is the increase of moisture level of supply air against the background of dry air conditions in air conditioned buildings without additional humidification systems during winter. Within a desiccant assisted system, moisture recovery by means of the existing hygroscopic materials is possible; the related energy demand is limited to auxiliary energies. In the framework of a research project at Hamburg University of Technology the combination of an open cycle desiccant assisted air conditioning system and borehole heat exchangers is investigated experimentally by means of a pilot plant in full year operation. During summer mode, dehumidification and cooling are separated within the process. Therefore, heat sinks working on higher temperature levels compared to conventional systems can be used. Furthermore, surface heat exchangers removing sensible loads from the conditioned space enable an energy efficient operation of the overall system; volume flow rates of supply and extract air can be reduced to minimal air exchange rate required by hygienic reasons. A shallow geothermal system with borehole heat exchanger is used as heat sink. To make use of this subsystem during full year operation and with regard to an equalized energy balance of the soil, a ground coupled heat pump is integrated for heating purposes in winter mode. The objective of this work is to analyze the potential of the geothermal system and to investigate the energy efficiency of the overall system, evaluating data measured during winter 2017 for a temperate climate region. Different reference systems are defined to evaluate the performance of the investigated system. Compared with a system relying on isothermal humidification, the electrical energy demand is reduced by up to 68% for humidification. Furthermore, combined heat and moisture recovery is investigated as well as the performance of the enthalpy wheel based on LiCl. An average moisture recovery efficiency of 0.75 is achieved. Besides the energetic system evaluation, measured data regarding the soil temperature and thermal comfort are presented.