Thermodynamic efficiency of low-carbon domestic heating systems: heat pumps and micro-cogeneration

Energy and exergy analysis is employed to compare the relative thermodynamic performance of low-carbon domestic energy systems based on air source heat pumps and micro-combined heat and power (cogeneration) units. A wide range of current units are modelled under different operating conditions representative of the United Kingdom to determine the energy and exergy flows from primary energy inputs through to low-carbon heating system and then to end use. The resulting performances are then analysed in order to provide insights regarding the relative merits of the systems under the different operating constraints that may be experienced both now and into the future. Although current mid-range systems achieve comparable performance to a condensing gas boiler, the state-of-art offers considerable improvements. Micro-combined heat and power units and air source heat pumps have the technical potential to improve the energy performance of dwellings. The relative performance and potential of the systems is dominated by the electrical characteristics: the grid electrical generation efficiency, the power-to-heat demand ratio and the availability of electrical export. For total power-to-heat demands below 1:1.5, air source heat pumps have greater improvement potential as their energy efficiency is not constrained. At higher power-to-heat ratios, micro-combined heat and power units offer the potential for higher overall efficiency and this generally occurs irrespective of whether or not the thermal energy from them is used effectively.