Dynamic Modelling and Control System Optimization of a Reversible Air-to-Water Heat Pump with Heat Recovery for Domestic Hot Water Production

Even if the energy demand for space heating/cooling of near zero energy buildings (nZEBs) is continuously decreasing due to the improvement of the insulation level of building envelope components, the energy requirement for domestic hot water (DHW) production cannot be similarly reduced: for this reason, the weight of DHW energy consumptions on the overall building energy performance is becoming more and more significant for nZEBs. A reversible heat pump with recovery of the condensation heat (HPHR) is one way to obtain significant energy savings and respond to this increasing influence of DHW production with respect to the energy demand in residential buildings, since this kind of device is able to simultaneously satisfy the energy needs for DHW production and space cooling during the summer season. In order to improve the energy efficiency of a HPHR, the heat recovery operating mode should be maximized during the cooling season: for this reason, a detailed analysis of the heat pump control system is needed. Heat pump performance strongly depends on the values of control parameters, which are influenced by the system working conditions, such as DHW draw-off profile, building heating/cooling load and thermal storage size. In this paper, a detailed analysis of the annual energy performance of a HPHR system is obtained by means of TRNSYS 17: several simulations are carried out by varying the control algorithm, in order to achieve the best seasonal performance factor of the system. The results reported present a series of rules for the best setting of the heat pump control system parameters to HVAC designers and heat pump manufacturers and highlight how significant energy savings can be achieved with the adoption of a HPHR with respect to traditional systems based on a gas boiler or a conventional heat pump without heat recovery.