- New technical solutions for energy efficient buildings State of the Art Report New technologies for heat pumps Authors: Heimo Staller, Angelika Tisch, IFZ Oct. 2010
Background Heat pumps are machines which can change heat from a low level of temperature (ca. 0-10 C) to a higher one (typically 30-50 C). Heat is extracted from air, water, ground and waste heat and transferred to a heat pump cycle where it is raised in a technical process to a higher temperature level suitable for heating and domestic hot water heating. In most cases electricity is used to power this technical process. Fig. 1 Scheme of a heat pump [1] In Europe heat pumps using an electric drive are becoming more and more popular as heating systems in buildings. The main reason for this development is the low price of the energy generated by the pump and the low requirements for maintenance over the life span. As these heat pumps are driven by electrical energy, which is considered to be the most valuable energy source (from an environmental and economical point of view), a high COP (Coefficient of Performance) and a high HSPF (Heating Seasonal Performance Factor) are very important. For heat pumps COP and HSPF are very important benchmark values for the energy efficiency of the production of heating energy (for detailed information see glossary at the end of this text). For example a COP of 4 means that you will get 4 kwh of heating energy from an input of 1 kwh of electricity. So depending on the electricity costs, heat pumps can be a cost effective alternative to conventional heating systems (e. g. a price of 8 cent/kwh electricity means a price of 2 cent/kwh for heating energy when the heat pump has a COP of 4). Comparisons of the environmental impact of heat pumps and other heating systems should be done based on the primary energy demand and on CO 2 -emissions. The table below gives an overview of primary energy factors and CO 2 -coefficients of the most important energy sources as stated in the European standard EN15603, illustrating the high primary energy and CO 2 -factor of electricity. 2
Primary energy factor Coefficient of CO 2 - production Non-renewable total kg/mwh Heating oil 1,35 1,35 330 Gas 1,36 1,36 277 Biogenic fuels 0,10 1,10 20 Electricity (Electricity mix UCTE) 3,14 3,31 617 Local- und district heating 1,30 1,30 133 Tab. 1 Primary energy factors and coefficients of CO 2-production, European standard EN 15603 [2] The integration of solar heating in heat pump systems offers an excellent challenge to improve the environmental performance of heat pumps. Conventional combination systems of thermal solar plants and heat pumps provide energy for space heating and DHW (domestic hot water) separately. New systems using thermal solar plants as heat source for the heat pump are emerging on the European market. The following pages present a short yet comprehensive overview of new innovative heat pump systems in combination with thermal solar energy plants. Air/water heat pumps with solar energy plants This system uses the heat produced by a solar thermal collector as energy source for the air/water heat pump. In the brine cycle of the solar collector a heat exchanger for the air supply duct of the heat pump is integrated. If there is enough solar radiation, the thermal collectors provide usable heat. When the solar radiation is decreasing, the heat pump uses residual solar heat, which still has a higher level of temperature than ambient air. Periods of less solar radiation are bypassed with a buffer-storage. The HSPF of this kind of heat pumps is higher (from 3,3 up to 3,9) than the HSPF of conventional systems, saving around 20% of electricity and increasing gains up to 20%. Brine/water heat pumps with soil collectors, solar plants and two buffer storages This system includes one conventional buffer storage unit for usable heat and a second buffer storage unit for the surplus solar heat of the collectors. If the temperature of the second buffer storage drops 3
under a certain level, the heat pump starts working using the soil collector as energy source. Brine/water heat pumps with horizontal soil collectors, solar plants and thermal regeneration of the soil The horizontal soil collector (depth from 1-1.2 m) not only gathers heat from the soil but also provides the soil with heat surplus of the thermal solar collectors. The soil is used as heat storage all over the year enabling higher basic temperatures for the heat pump, which leads to a better HSPF. Because of ecological reasons the temperature of the soil should not exceed 25 C. The system can t be used in areas with high ground water levels. Brine/water heat pumps with solar plants, hybrid collectors, combination storage tanks and latent heat storage If there is enough solar radiation, hybrid collectors (see also State of the Art Report: Hybrid collectors) with brine and air as heat carrier are working like a conventional flat plate collector. In times of low solar radiation and in the night heat is extracted from ambient air by blowing air with ventilators through the collectors. The hybrid collectors provide the combination storage and the latent heat storage with energy, the heat pump only starts if the heat supply from hybrid collectors and combination storage is too low. The use of latent heat storage units offers the possibility to store heat in a smaller volume. For example a latent heat storage based on water/ice as PCM (Phase Change Material) of 320 litres has the capacity of conventional hot water storage with 2.500 litres. Glossary COP (Coefficient of Performance) The heat delivered by a heat pump is theoretically the sum of the heat extracted from the heat source and the energy needed to drive the cycle. The steady-state performance of an electric compression heat pump at a given set of temperature conditions is referred to as the coefficient of performance (COP). It is defined as the ratio of heat delivered by the heat pump and the electricity supplied to the compressor. [1] HSPF (Heating Seasonal Performance Factor) The operating performance for heating of an electric heat pump over the season is called the heating seasonal performance factor (HSPF). [1] Hybrid collectors 4
Hybrid collectors are solar collectors using two kinds of energy carrier medium. For example PV-panels with air as heat carrier or thermal panels with water/sole and air. Hybrid collectors have better energy efficiencies than conventional thermal and electrical collectors. PCM (Phase Change Materials) Phase Change Materials have the property to store or release energy by change of their aggregate state (e.g. from liquid to solid like water to ice). PCMs for heat storage et al. are water and paraffin. SPF (Seasonal Performance Factor) The operating performance of an electric heat pump over the season is called the seasonal performance factor (SPF). It is defined as the ratio of the heat delivered and the total energy supplied over the season. It takes into account the variable heating and/or cooling demands, the variable heat source and sink temperatures over the year, and includes the energy demand, for example, for defrosting. The SPF can be used for comparing heat pumps with conventional heating systems (e.g. boilers), with regards to primary energy saving and reduced CO 2 emissions. For evaluating electric heat pumps the power generation mix and the efficiencies of the power stations must be considered. [1] References [1] http://www.heatpumpcentre.org/en/abouthpp/sidor/default.aspx [2] European standard EN 15603, Energy performance of buildings Overall energy use and definition of energy rating 5