Dr. Philip Few

Domestic Combined Heat and Power incorporating a Heat Pump

Introduction
The environmental and economic advantages of combined heat and power (CHP) have been demonstrated. This has been recognised by the UK Government, which views co-generation as a major policy mechanism for reducing greenhouse gases. Over the previous two decades, small-scale CHP installations (of less than 50kWe) have become common in municipal and commercial applications. Industry has also increased its use of CHP. The greatest use of energy within the UK, and hence the greatest source of greenhouse gases, is from domestic use. Therefore, a major reduction in greenhouse gases could be achieved by the application of CHP to the domestic sector.

Problems of UK Domestic CHP
The residential heat and electrical power needs of many cities throughout the world are met by co-generation, particularly in Eastern Europe. This is made possible by the architectural characteristics of those cities, which are largely composed of high density, high-rise flats. The supply of heat through district heating schemes is simple and cost effective. In comparison, the low density, suburban nature of British cities renders the installation of heat distribution systems uneconomic.
Preliminary modelling and experimental work have demonstrated that simply scaling down a conventional CHP plant to meet domestic base loads (less than 0.5kWe) was inappropriate, Due to heat transfer limitations and by the nature of the domestic energy requirements.
If a CHP plant is designed around an engine capable of meeting the domestic electrical base load, the thermal output of the plant will only satisfy a fraction of thermal demand of the dwelling, thus reducing the economic and environmental advantage. Using a larger engine would incur part load efficiency penalties, negating its advantage over utility energy supplied. The exportation of surplus electrical generation would maintain high engine efficiency whilst meeting thermal demand. Although surplus electricity is successfully exported by small domestic generators in a few high profile examples, it is contractually complex. Additionally, this approach does not allow for the non-concurrent nature of domestic energy demands. For example, heating demand is high, early on a winter morning, whilst electrical demand is low. By relying on electricity export to maintain engine efficiency, there is reduced operational flexibility and an increase in institutional barriers to domestic CHP installation. A CHP concept that overcomes these issues without resorting to electricity export is discussed.

The CHP/ Heat Pump (CHP/HP) Concept
The concept of heat pump incorporation into a domestic scale CHP plant a CHP/HP plant is briefly explaned
With electrical demand greater than the plant rating, the CHP/HP plant acts as a conventional CHP plant, with all electrical power being delivered to the dwelling with heat recovery from the exhaust gas.
With a high thermal demand and no electrical demand, the CHP/HP plant acts as a gas engine driven heat pump, with the electrical output being used to drive a vapour compression heat pump. Thermal delivery would be from both exhaust gas recovery and heat pump.
When the electrical demand for the dwelling is less than the plant rating, surplus electrical generation is used by the heat pump to augment the thermal delivery. This allows the CHP/HP plant to satisfy extremely low electrical base loads and high thermal demands whilst avoiding part load efficiency penalties and without resorting to export of electricity.
The primary aim of the research was to develop and assess the CHP/HP concept in comparison with conventional CHP.

Economic and Environmental Operation
Analysis of the extended computer simulation of both domestic CHP and CHP/HP showed that unit maintenance cost must be less than 2.7p/kWh of electrical output for any form of domestic co-generation to be viable under present economic conditions. However, it was demonstrated that in order to maximise economic and environmental performance, all forms of domestic co-generation should have a unit maintenance cost lower than 1.8p/kWh. The analysis also demonstrated a CHP/HP plant is less sensitive to high fuel unit costs than conventional CHP. The maximum fuel unit cost that would allow for economic domestic CHP operation is 1.6p/kWh, but heat pump incorporation extends this to 2.1p/kWh (with a target maintenance cost of 1.5p./kWh). The use of extended modelling allowed for the economic/ environmental optimisation of the heat pump specification.

Heat pump incorporation was found to have significant environmental benefits. CHP/HP reduced carbon dioxide emissions by approximately 40% over that of conventional CHP for all economic scenarios. In cases where marginal economic advantage was apparent, CHP/HP still yielded a significantly enhanced environmental advantage over CHP.
Conclusions and Future Work
This research has shown that incorporating a heat pump into a domestic scale CHP plant solves the problems associated with a domestic installation, without resorting to electricity export. It was experimentally demonstrated that a CHP/HP plant can satisfy a very low electrical base load economically. Heat pump incorporation extends the limits of economic domestic co-generation and maximises economic and environmental savings.
Future work by developing a second-generation CHP/HP plant for domestic installation using fuel cells is envisaged in which heat pump incorporation will be essential to maintain thermal delivery.

Fuel Supply
Air In
Air Out
Electrical Delivery to Heat Pump
Air Out
Exhaust Gas
Flow to Hot Water System
Exhaust Gas
Heat Exchanger

Heat Pump
System
Engine/ Generator
Electrical Delivery to Dwelling
Heat Pump
Heat Exchanger
Return from Hot Water System
Flow to Hot Water System
Electrical Delivery to Dwelling
Heat Pump
Heat Exchanger
Exhaust Gas
Heat Exchanger
Exhaust Gas
Fuel Supply
Heat Pump
System
Engine/ Generator