Integrated thermodynamic and electrical modelling of a variable-speed reversible heat pump: analysis of the cooling mode operation

Abstract

Variable-speed reversible heat pumps are increasingly adopted for space heating and cooling in residential and tertiary sectors. It is recognized that these systems will play a key role in medium- and small-scale 4th generation district heating and cooling networks. Indeed, with the high penetration of intermittent renewable energy sources, such technology could help reduce issues related to the surplus of electricity from renewable energy sources by converting it into heat or cold. To reliably assess the benefits achievable in these emerging applications, it is necessary to develop models which can simulate not only the steady-state operation but also the dynamic response of the system with various boundary conditions (e.g., users’ demand, environmental conditions, etc.). However, heat pump operation is usually assessed by highly simplified models, which consider a Coefficient of Performance value available from the producer at fixed rating condition (eventually corrected to account for off-design operation), being the electrical modelling and embedded controls usually omitted. This paper aims at filling this knowledge gap. Starting with a reversible heat pump operating in cooling mode, this paper developed an integrated thermodynamic and electrical modeling, while considering details on refrigerant circuits, electrical motor, and embedded control. Results showed that the model can simulate the transient and steady-state operation of variable-speed chillers coupled to an induction motor, and the chilled water temperature set-point was duly maintained. Results give a full insight into the electrical and thermodynamic aspects while accounting for the effects of the inertia of the hydronic circuit on the COP of an air-cooled. Results present a faster 2% settling time for low values of inertia that serves the hydraulic system with a high overshot for water temperature.