Desiccant-based AHU interacting with a CPVT collector: Simulation of energy and environmental performance

Desiccant-based Air Handling Units (AHU) provide significant technical and energy/environmental advantages with respect to conventional systems, especially when the regeneration of the desiccant material is obtained by means of a renewable energy source, such as solar energy. Such thermal energy may be provided by CPVT (Concentrating Photovoltaic/Thermal) collectors, simultaneously producing also electricity, which are considered one of the most promising solar technologies. In fact, CPVT thermal energy can drive (integrated by a natural-gas fired boiler) a desiccant-based AHU, since the silica-gel wheel, used for the dehumidification and included in that system, must be continuously regenerated. Here, the regeneration temperature of the wheel (40-70°C, depending on the dehumidification required) is compatible with the CPVT outlet temperature (80-100°C). Simultaneously, the electricity produced by CPVT collectors can feed the auxiliary devices of the plant. Furthermore, the heat supplied from the solar collector can be used for the pre and post heating of the process air during winter operation. A test facility in which a silica-gel desiccant wheel is included in a hybrid AHU has been located in Benevento (Southern Italy).In this paper, a desiccant-based AHU has been coupled with a novel CPVT, consisting of a parabolic trough concentrator and a linear triangular receiver. In order to analyze the system, a TRNSYS project based on models available in literature (some of them were calibrated and validated with experimental tests) has also been developed. Electricity produced by the CPVT collector is used to power the auxiliaries of the AHU, the chiller and also to balance the electric load of users, while thermal energy is used to heat the regeneration air flow during the summer period and the process air in the winter. Electricity in excess is sold to the grid, whereas the thermal energy in excess is used for production of domestic hot water (DHW). Eventual integrations of electricity and thermal energy are provided by the electric grid and by a gas-fired bolier, respectively. Energy and environmental performance of the overall system in terms of Primary Energy Saving and emission reduction with respect to a reference case are evaluated. The heat provided by the CPVT covers about 60% of thermal energy required by regeneration air and 30% of process air in winter operating mode. On an annual basis, the analyzed system obtains a Primary Energy Saving between 81% and 89%, depending on the DHW required.