A comparative cycle and refrigerant simulation procedure applied on air-water heat pumps.

A vapor compression heat pump absorbs heat from the environment at a low temperature level and rejects heat at a high temperature level. The bigger the difference between the two temperature levels the more challenging is it to gain high energy efficiency with a basic cycle layout as found in most small capacity heat pump applications today. Many of the applicable refrigerants also reach their technical limits regarding low vapor pressure for very low source temperatures and high discharge temperatures for high sink temperatures. These issues are especially manifest for air-water heat pumps. Many alternative cycle setups and refrigerants are known to improve the energy efficiency of a vapor compression cycle and reduce discharge temperatures. However not all of them are feasible for small capacity heat pumps from a cost and complexity point of view. This paper presents a novel numerical approach to evaluate and compare different cycle layouts and different refrigerants in regard of multiple objectives. The emphasis is on the objective of energy efficiency which is defined as seasonal coefficient of performance. We discuss reasonable assumptions to allow a generic cycle comparison without restricting the evaluation to a single set of components. A special focus is on the heat exchanger sizes as a dominant influence on the cycle efficiency. Varying sizes are taken into account using a design of experiment factorization to determine a quadratic regression model for the energy efficiency. In combination with component cost correlations a constrained non-linear optimization problem is formulated and solved. Strengths and weaknesses of the proposed procedure are discussed in regard of its ability to help in systematically identifying promising combinations of cycle layout and refrigerant.