Modeling of CO₂ heat pump water heater for energy management

Abstract

Energy management is systematic activity to improve energy performances of target system, and it has already been introduced in the industrial field. Energy management system is expected to solve operational planning problem and report or suggest opportunities for performance improvement. In recent years, CO₂ heat pump water heater is attracting attention as one of the high-efficiency devices to meet domestic hot water demand. Energy management in residential and commercial sector is one way to keep the system operation efficient. However, few studies have been reported based on the thermodynamically-sound model of CO₂ heat pump for energy management. Since the heat pump model based on Japanese energy conservation law (ECL) is designed for calculating daily performance, it is not appropriate for deriving an operational planning in shorter period, such as several tens of minutes’ intervals. In this paper, we will propose a heat pump model suitable for solving day-ahead operational planning problems in energy management in a time scale of several minutes. The CO₂ heat pump model is simply composed of Lorentz efficiency and theoretical maximum coefficient of performance (COP) for trans-critical heat pump cycle. The Lorentz efficiency is identified by a series of experimental data for typical residential hot water demand with variation of ambient temperature, inlet/outlet water temperature at gas-cooler of the heat pump unit. Comparing with the base model derived by ECL model, we confirmed that the proposed model is preferable with the viewpoint of its simplicity and robust performance for wide temperature range.