Benoit Sicre, Andreas BUhring, Matthias Vetter

Fraunhofer-Institut fur Solare Energiesysteme ISE

Heidenhofstr. 2, 79110 Freiburg, Germany

Tel.: +49 (0) 761/4588-5291, Fax: +49 (0) 761/4588-9000

Benoit. Sicre@ise. fraunhofer. de

The expected high fuel conversion factor of residential combined heat and power plants (rCHP) means that they present a more environmentally friendly option for heating and simultaneous electricity generation than separate systems with decentralised heat generation, e. g. with a gas-fuelled boiler, and electricity from central power stations.

As the investment costs for rCHP plants are still high, measures such as investment subsidies and/or higher tariffs for the exported electricity are needed to support the market introduction, as are the longest possible operating times. The consequence is that simultaneous investment in a thermal solar system becomes less attractive, as the heat that it provides shortens the operation time for the rCHP, so that the price paid for electric power increases. Provided that investments have been made in both a thermal solar system and a rCHP, an operation mode which is strongly determined by the electricity generation profile can dramatically reduce the heating energy yield from the solar system.

Within a joint project, Fraunhofer ISE has investigated the possible displacement mechanisms concerning thermal applications of solar energy, and has identified ways of financially supporting decentralised rCHP plants without hindering the thermal use of solar energy. The most important results of this work will be presented in this paper.

Introduction

rCHP is an emerging technology with a high potential to provide energy efficiency and environmental benefits. The concurrent production of electricity and heat from a single fuel source can reduce primary energy consumption and associated greenhouse gas emissions. The distributed generation nature of the technology also has the potential to increase electrical transmission efficiency. Since the plant can partly meet the power demand of the household [Sicre 2004], rCHP can alleviate utility peak demand problems and the need for grid reinforcement caused by the general rise in electricity demand.

At present, most rCHP plants operate with gas-fuelled combustion engines. Extensive research efforts are currently directed toward the development of fuel-cell heating systems. In addition, it is expected that Stirling engines and other systems will soon enter the market.

Business prospects for rCHP are very good since it is a mass market product. In the recent years, the market for home heating systems (especially retrofit of central heating systems or equipment) amounted to approx. 5 million units across Europe including
approx. 900,000 units in Germany. A large part of this amount is expected to be small — scale appliances with an electric power output below 10 kW. Moreover, all over Europe, energy distribution and supply companies have expressed interest in rCHP [Bruch et al. 2003], as a way to gain new customers or keep previously bound customers. Furthermore, rCHP can be used not only in new buildings but also in old buildings that have been retrofitted according to current building thermal standards.

As the investment costs for rCHP plants are still high, measures such as investment subsidies and/or higher tariffs for the exported electricity are needed to support the market introduction, as are the longest possible operating times. The consequence is that simulta­neous investment in a thermal solar system becomes less attractive, as the heat that it provides shortens the operation time for the rCHP, so that the price paid for electricity increases. Measures to reduce the heating demand of buildings (thermal insulation, etc.) also become less interesting, as the decreased heating demand also reduces the rCHP operating time, so that the amortisation period is lengthened or amortisation may even become impossible. If investments have been made in both a thermal solar system and a rCHP, an operation mode which is determined by the electricity generation profile can throttle the performance of the solar system.

This complex has been addressed within a joint project that was initiated by the German Federal Ministry for the Environment and co-ordinated by DLR [Krewitt et al. 2004]. Within this project, Fraunhofer ISE has investigated the possible displacement mechanisms concerning thermal applications of solar energy, and has identified ways of financially supporting decentralised CHP plants without hindering the thermal use of solar energy. The most important results of this work will be presented in this paper.

Simulations were used to calculate the yields of the rCHP plant and the solar collector, if present, for different types of rCHP (low-temperature fuel-cell heating in different power ranges, high-temperature fuel-cell heating, Stirling engines), with and without a solar collector, in different types of residential buildings (from the existing building stock, low — energy house, passive house). An approach was investigated, in which the bonus (which in Germany is currently 5.11 cents per exported kilowatt-hour) is reduced to zero during periods when the potential solar gains are high, but is then increased during other periods of the year to a value which means that on average, the system operators do not suffer a financial disadvantage.