Experimental and simulative analysis of a microtrigeneration system based on an air handling unit with desiccant wheel

During last years, air conditioning demand is spreading, both in the commercial (shops, warehouses, offices, schools…) and in the residential sector. This caused a sensible increase in primary energy consumption in these sectors, especially in industrialized countries, where people spend the major part of the day in confined environments, therefore it is very important to guarantee a high Indoor Air Quality and thermal comfort. Therefore, it is very important to investigate the possibility of efficiently attaining improved indoor environmental quality, without increasing energy consumption or even reducing it. Moreover, the demand for summer cooling in domestic and commercial sectors is usually satisfied by electrically driven units; this involves high electric peak loads and black-outs. Furthermore, during last years, great attention was focused on the transition from centralized to decentralized energy “production” systems, to reduce T&D energy losses: a miniaturization process is in progress. The boost towards the reduction of electrical loads for air conditioning and the decentralization of energy conversion devices is determining increasing interest in small scale trigeneration systems fuelled by natural gas (“gas cooling”), able to shift energy demand in summer from electricity to gas, at the same time allowing the exploitation of natural gas surplus during the warm season. Finally, the increasingly need to drastically reduce the use of HCFC and HFC refrigerants, because of their direct greenhouse contribution and ozone depletion potential, should be taken into account. A particularly interesting technology that meets these requirements is represented by desiccant-based dehumidification systems, eventually integrated with conventional air conditioning devices. In the most common configuration, these systems use a desiccant wheel, that consists of a rotor, filled with a desiccant material (i.e. silica gel), in which humid air is dehumidified by the desiccant material, to balance latent loads of the ambient. To guarantee continuous operation, the wheel has to be regenerated by a hot air stream. During last years, thanks to its benefits, this technology is spreading in residential and tertiary sectors and office buildings; this is not the case for European countries, in particular in Mediterranean areas, like Italy, where this technique, that allow to separately control temperature and humidity, is still rarely implemented, due to some obstacles such as high investment costs, low familiarity, lack of knowledge about performances and cost/benefit ratio and high thermal energy requirements to regenerate the desiccant material. As regards the last topic, it is possible to use a “free” thermal energy source to regenerate the desiccant material, in particular waste heat recovered from a microcogenerator (MCHP – Micro Combined Heating and Power), eventually integrated with a conventional fossil fuelled heating system (e.g. a boiler). In this case it is possible to design a microtrigeneration system (MCCHP – Micro Combined Cooling, Heating and Power), that allows to significantly increase the operating hours of the MCHP, therefore improving the energy, environmental and economic performances of the whole system. Moreover, it is also possible to regenerate the desiccant wheel by means of solar energy; in particular, the use of solar energy for space cooling requirements (“solar cooling”) is highly desirable, because its availability coincides with the need for cooling, therefore the summer peak demand of electricity due to extensive use of electric air conditioners, that matches with the peak solar irradiance, can be lowered. The aim of this thesis, starting from experimental tests carried out at Universita degli Studi del Sannio, in Benevento, is to demonstrate the technical feasibility of a MCCHP system, consisting of a hybrid desiccant-based Air Handling Unit (AHU) and a microcogenerator, and to evaluate its energy, environmental and economic performances compared to a conventional system based on cooling dehumidification and separate electric, thermal and cooling “production”. Moreover, experimental data have been used to calibrate and validate models of the main components and energy conversion devices, in order to analyze the effect of the various operating parameters, namely, regeneration temperature, outdoor air temperature and humidity ratio, etc. Finally, these models have been used to design a solar collectors system, that provides a part of the required regeneration thermal energy. The solar assisted desiccant cooling system has been simulated by means of the TRNSYS software, in order to evaluate its operational data and performance parameters.