Annual comparative performance of direct expansion solar-assisted and air-source heat pumps for residential water heating

Abstract

In this paper, an annual comparative analysis for the coefficient of performance (COP) and the energy consumption are presented for two direct expansion heat-pump water heating systems: solar-assisted heat pump, and air-source heat pump. Two main research questions are addressed: (1) How does each system's performance differ from the other system under the same conditions? and (2) How does the climate affect the performance of an individual system to help the designer choose the more energy efficient water heater for a specific location? To make those water heaters comparable, all design parameters for both heat pumps are assumed to be the same with identical components. A sensitivity analysis is also carried out for various factors such as irradiance, ambient temperature, collector/evaporator area, wind speed, and compressor rotational speed considering three different hot water tank temperatures. The results for both heat pump water heating systems show that the climate (different months) plays the key role on the annual performance. Changes in irradiance and ambient temperature as well as the wind speed throughout the year can increase the COP by up to 33 % (from 2.1 to 2.8) under certain conditions for the case study with moderate weather. Therefore, the benefits from the local weather should be investigated before choosing the design variables and proper source (solar thermal, air) for the heat pump. Nearly the same results were obtained for both type of water heaters with slightly better performance of air heat pump in colder months of the year (December to February).

Keywords

• Heat pump
• Water heating
• Energy consumption
• Direct expansion