A novel steady-state approach for the analysis of gas-burner supplemented direct expansion solar assisted heat pumps

•A fluid-less description of an inverse thermodynamic cycle is provided.•On this ground, a steady state model for solar assisted heat pumps is derived.•The model results able to describe the averaged behavior of the considered system.•The proposed approach appears suitable in view of design and regulation purposes.

Design and control strategy suggestions for direct expansion solar assisted heat pump (DX-SAHP) water heaters are stated as a result of a novel steady-state primary energy consumption analysis based on a model developed around the fluid-independent Carnot cycle. The study is devoted to devices committed to hot sanitary water production and supplemented by an instantaneous gas burner. The paper addresses several suggestions about the correct design and the optimal working conditions needed to minimize the use of primary energy, based on averaged working conditions.The maximization of primary energy savings has been selected as the criterion to define our concept of “optimal performance” since this approach benefits from the use of a direct language that permits a larger, also non-specialized, audience to acquire the basic concepts of optimal behavior, increasing the transfer of knowledge to actual embodiments. Apart from the particular criterion selected in this study, the focus is on the proposed steady state approach which, without any use of refrigerant fluid properties, allows us to extract general rules as a function of the main features of the plant and of its relevant interactions with the surroundings, making all the relationship between the involved variables explicit and meaningful. Results obtained using the present approach agree with data coming from an already consolidated dynamic simulator.