The performance optimization of DX-PVT heat pump system for residential heating

The performance of a solar-assisted DX-PVT heat pump (direct expansion photovoltaic-thermal) is intimately relevant to the system configuration. Inappropriate system configuration will undermine the system performance. This work assessed the effect of the equipment area of PVT modules on the PVT heat pump system driven by novel direct-expansion roll-bond PVT modules, and aimed to propose optimal system configuration. An analytical model was developed for the PVT heat pump system and validated by the experimental results. Simulations were conducted to reveal the effect of A/Vth, a proposed parameter to describe the system configuration, namely the ratio of the modules’ area to the theoretical displacement of the compressor. The results showed that the larger A/Vth lead to higher COP but lower HGF (heat gain factor). With A/Vth increasing from 0.768 to 2.303 m2 h/m3, the COP increases by 61.3% (from 3.1 to 5.0) but the HGF decreases by 39.6% (from 0.91 to 0.55). Higher levels of solar irradiation strengthens the positive effect of the increasing A/Vth on thermodynamics performance and weakens the negative effect of that on the heat collection performance. The enhancement in COP is 42.8%/70.9% with A/Vth increasing from 0.768 to 2.303 m2 h/m3 when solar irradiation is 500 W/m2/800 W/m2, while the decrease in HGF is 49.2%/39.2%, respectively. A configuration optimization was also conducted to balance the thermodynamics and heat collection performance. The A/Vth varying from 1.535 to 1.919 m2 h/m3 was suggested as the optimized configuration when solar irradiation is 500 W/m2. After optimization, the IPLV and HGF can exceed that of air-source heat pump and flat-plate collectors by 40%, showing considerable performance advantages.