Development and investigation of CO₂ heat pump for domestic buildings

Abstract

The residential building sector in the UK, is responsible for 25% of energy consumption and 17% of CO2 emissions, with space heating accounting for 65% of this. More than 80% of existing dwellings use gas boilers for space heating and domestic hot water and to decarbonise this energy input, heat pumps are considered to be a key technology. Despite this potential, heat pumps have so far failed to gain wide market penetration in the UK due to high capital and installation costs, inability to provide high enough temperatures to be used with existing radiators in retrofit applications and requirement for thermal energy storage. This project makes a contribution to addressing this challenge by investigating the development of a CO2 high-temperature heat pump and its integration with thermal energy storage. to satisfy space heating requirements for existing and new dwellings, facilitate the use of low tariff electricity and provide demand services to the grid. The investigations involved: i) dynamic simulations of 2 and 3 bedroom semi-detached dwellings to establish space and domestic hot water energy demand; ii) extensive experimental investigations on a CO2 heat pump developed at Brunel to establish operating characteristics; iii) simulation of the heat pump to enable design optimisation; iv) investigations on the performance and integration of the heat pump with a Phase Change Material (PCM) thermal energy storage system. The work has demonstrated that: i) The CO2 heat pump can provide significant flexibility in the provision of different water delivery temperatures from 40 oC to 80 oC to satisfy both domestic hot water and space heating demand and the requirement of different types of heat emitter in existing and new dwellings; ii) Using the performance characteristics of the heat pump, the optimum hot water storage tank size for the 4 bedroom domestic dwelling was determined to be between 200 and 300 litres: iii) Using current domestic electricity and gas prices and CO2 emission factors, the annual running cost of the heat pump was found to be approximately double that of the gas boiler due to the large difference between gas and electricity prices, but offering 40% reduction in CO2 emissions; iv) Heat pump design and optimisation work using a simulation model developed for this purpose is expected to lead to an increase in increase the seasonal COP of the heat pump and its cost effectiveness over gas boilers; v) Integration of the heat pump with a PCM storage tank designed and using Rubitherm RT70HC PCM has shown that the heat pump can charge the storage tank effectively, leading to a 50% reduction in the storage volume required for the same thermal energy storage capacity compared to hot water storage.