Thermo-economic analysis of solar heating and cooling systems

Abstract

During last years, air conditioning demand has spread, both in the commercial and the residential sector. This caused a sensible increase in primary energy consumption. The innovative desiccant-based air handling units are a very interesting solution for air conditioning of buildings as they operate with low temperature thermal energy, such as waste heat recovered from a microcogenerator or especially solar energy. Solar radiation is widely available in summer and simultaneously there are the greatest demands for cooling, but solar energy can be used advantageously also to balance the winter thermal load. In the most common configuration, desiccant cooling system are equipped with a desiccant wheel, in which moist air is dehumidified by the adsorbent material and then cooled down by the evaporation of water and/or through an electric chiller (hybrid plants). A hybrid air conditioning device with silica-gel desiccant wheel is installed in the laboratories of the Universita degli Studi del Sannio (Benevento, Italy), and currently operate coupled with a microcogenerator and a natural gas boiler. The system in its actual configuration can operate only in cooling mode. In this thesis the operation of the experimental air handling unit, considering the coupling of the innovative air handling unit with different solar collectors types (flat plate, evacuated tube, concentrated photovoltaic and thermal collectors), is analyzed through dynamic simulations performed with the commercial software TRNSYS 17. In order to allow the system operation even in winter mode, suitable modifications have been identified and implemented in the model. Furthermore the influence of climatic conditions on the performance of the system is assessed, and also modifications to the air handling unit layout are considered. Finally, for completeness, the system constituted by the desiccant-based air handling unit and the microcogenerator is studied. The innovative air conditioning systems were used to balance the sensible and latent load of a 63.5 m2 university classroom. Moreover, during the intermediate period, in the weekends and whenever there is a surplus of thermal energy the possibility of exploiting this energy in excess for further heating purpose is considered. TRNSYS models were used to simulate the operation of the innovative system and that of a conventional system in order to evaluate operational data and performance parameters. Primary energy saving, equivalent CO2 avoided emission and simple pay back period were evaluated. The performed analyzes showed that the desiccant-based air handling unit fed with a renewable energy source such as solar energy can advantageously replace a conventional air conditioning systems equipped with electrically-driven vapor compression cooling units if only energy and environmental performance are taken into account. However economic feasibility is still hard to obtain. For the considered application economic benefits do not occur together with the advantages mentioned above, although one considers incentives for the production of thermal energy from a renewable source. Only if the use of solar thermal is maximized considering other low temperature heating purposes (for example domestic hot water or swimming pools heating) in addition to the air conditioning economic advantages can take place.