Thermoeconomic optimization of a solar-assisted heat pump based on transient simulations and computer Design of Experiments

In the paper, a model for the simulation and the optimization of a novel solar trigeneration system is presented. The plant simulation model is designed to supply electricity, space heating or cooling and domestic hot water for a small residential building. The system is based on a solar field equipped with flat-plate photovoltaic/thermal collectors, coupled with a water-to-water electric heat pump/chiller. The electrical energy produced by the hybrid collectors is entirely supplied to the building. During the winter, the thermal energy available from the solar field is used as a heat source for the evaporator of the heat pump and/or to produce domestic hot water. During the summer, the heat pump operates in cooling mode, coupled with a closed circuit cooling tower, providing space cooling for the building, and the hot water produced by the collectors is only used to produce domestic hot water. For such a system, a dynamic simulation model was developed in TRNSYS environment, paying special attention to the dynamic simulation of the building, too. The system was analyzed from an energy and economic point of view, considering different time bases. In order to minimize the pay-back period, an optimum set of the main design/control parameters was obtained by means of a sensitivity analysis. Simultaneously, a computer-based Design of Experiment procedure was implemented, aiming at calculating the optimal set of design parameters, using both energy and economic objective functions. The results showed that thermal and electrical efficiencies are above 40% and 10%, respectively. The coefficient of performance of the reversible heat pump resulted above 4 for both heating and cooling modes. For the base case, a Simple Pay Back period of 5.36years was found; such index decreases to 2.33years in case a capital investment incentive of 30% is available. As expected, a decrease of the performance of the system was detected for weather conditions in which the availability of solar energy is scarce. The Design of Experiments analysis outlined that the appropriate selection of collector area is crucial in order to achieve a good profitability of the system under analysis.