Heat pumps and user practices – energy reductions or increased comfort?

This paper deals with individual air-to-air heat pumps in dwellings and summerhouses and the question of to what extent they deliver actual savings in energy consumption . Results show that 40% of the expected reduction in electricity consumption is transferred into increased comfort in the homes, including increased heating areas, keeping a higher temperature and a longer heating season and using the heat pump for air-conditioning. Data include electricity consumption in 185 households before and after installation of heat pumps together with survey results of 480 households. Furthermore 12 households are selected for in-depth analysis including technical inspection and qualitative interviewing. Especially for summerhouses results indicate that there on average is no reduction in electricity consumption, as energy efficiency is outbalanced by increased comfort. These results have to be taken into account when making long term energy planning for a sustainable energy system. INTRODUCTION The sale of air-to air heat pumps has been quite high, notably in Norway where there are sold some hundred thousand [1] but also in Sweden and France expanding sales figures of heat pumps are reported. In Sweden domestic heat pump sale rose from approximately 20000 to 80000 per year between 2006 and 2007 and in France from approximately 50000 to 70000 per year [2]. Studies from several different European countries has pointed out that there is good economical reason for the consumers to install air-to-air heat pumps [3, 4, 5]. The question of what role air-to-air heat pumps play in a future sustainable energy system have to be discussed together with other technical changes of the whole energy system including to what extent electricity is produced by renewable energy [6,7] and the energy renovation of the building stock [6]. Replacing direct electric heating with air-to-air heat pumps are, however, always more energy efficient because heat pumps can provide 2-5 times more heat than the electricity they use as driving force [3]. In a scenario for future 100% renewable energy systems in Denmark * Corresponding author * The project team included besides the authors also Troels Fjorbak Larsen, IT Energy; Erik Gudbjerg and Lisbeth Rasmussen, Lokalenergi and Preben Munter, Seas-nve. † The initial project was financed by Elforsk.dk and further analysis for this article was related to the Research Centre for Zero Energy Buildings (http://www.zeb.aau.dk). individual heat pumps are thus also included for areas not covered by district heating [9]. From a socio-technical point of view it can, however, be expected that the full technical potential for energy efficiency will not be met because of changes in user practices towards still higher expectations and norms of comfort [1] as is also known from studies of other types of households’ technologies [10]. Within a techno-economic perspective a corresponding phenomenon is known as the rebound effect focusing on how the economic gains that households get from implementing more efficient technologies will be used to increase consumption in other areas or within the same area resulting in higher standards and thus increased energy consumption. There has been a debate about the size of the rebound effect within the household sector and a recent review suggest a rebound effect on 20% meaning that 20% of the energy savings gained from efficient technologies within the household sector are transformed into increased energy consumption and thus not realised as energy savings [11, 12]. The purpose of the study presented in this paper was to analyse to what extent the potential reduction from installation of air-to-air heat pumps are realised or transformed into increased consumption. Furthermore, it was to go more into detail in explaining within which areas more precisely the increases in comfort is seen and to understand in more sociological terms why and how these changes occur. Today, 8% of houses in Denmark [13] and 84% of summerhouses are heated by direct electric heating [14]. The majority of these are not placed near city centres and thus reachable by district heating and the most relevant future heating supply for these homes is thus individual heat pumps [9]. As these houses have not installed central heating based on water-borne systems, the economically most attractive choice will most often be to install air-to-air heat pumps. Another argument for looking at air-to-air heat pumps in relation to changes in comfort norms is that these can easily be used for air-conditioning as well. Air-conditioning has until now not been normal in Danish households, however, having available technologies installed in the home might contribute to change this. In the following, we will first describe the methods of the study and then, in the main part of the paper, present findings and analysis for permanently occupied dwellings and summerhouse respectively. In the conclusion, results are discussed in relation to the implications for interaction between heating technologies and renewable energy systems.