Heat pumps (domestic heat pumps)

Heat pumps (heat pump systems and units) can produce heat capacities at widely varying levels. This chapter looks at heat pumps that deliver up to 50 kW heating capacity (domestic heat pumps). These heat pumps are used as heating system heat pumps for heating and cooling of small buildings (detached or semi-detached houses) or for heating domestic water (domestic water heat pumps). If several houses are connected together, heating capacities of 100 kW may be necessary [Stiebel Eltron 2003]. Heat pumps with larger capacities of around 500 kW upwards are considered in Chapter 3.3.3.6.

Whereas a mere 400 heat pumps for heating purposes were installed in Germany in 1990 [Laue 2000], by 2001 the figure was as high as 10,100 [Frey 2003]. In the following five years the installation of heat pumps showed a further sharp increase, sometimes making substantial leaps. For example, 18,500 systems were installed in 2005, and in the following year this figure more than doubled to nearly 44,000 systems. According to the Federal Heat Pump Association (Bundesverband Wärmepumpen – BWP), 62,500 systems were installed in 2008 [BWP 2009]. There has thus been an almost continuous increase since the early 1990s.

The previous record sales figure of more than 20,000 around 1980 in the wake of the oil price shock has thus been more than doubled several times since 2006. By 2009 the share of heat pumps as a heating system for new buildings (detached houses) in Germany was as high as 25% [BWP 2010]. In other European countries too, the heat pump market has displayed impressive sales growth. In France, for example, some 135,000 systems were installed in 2008 [KK 2009], an increase of 180% on the year before.

In the capacity range required here, current practice is almost exclusively to use electrically powered heat pumps working on the compression principle. Heat pumps powered by gas engines are available for larger units (heating capacity >100 kW). Today lower-capacity gas heat pumps are also available, but hardly for capacities below 20 kW [ASUE 2008]. This segment continues to be dominated by electrically powered heat pumps.

The heat sources are groundwater (in rare cases surface water), ambient air or the soil. In 2008 some 48% of all newly installed heat pumps were equipped with horizontal flat-plate collectors or vertical ground probes for utilising heat from the soil (brine/water heat pump), while 44.9% derived their heat from the ambient air. Only 7.1% used groundwater as a heat source [BWP 2009].

The heat from the energy source used (ground heat) is first transferred to a secondary refrigerant circulating in the collectors/probes (dual-circuit system principle, see also Chapter 3.3.2). A few years ago, however, direct-evaporation systems entered the discussion:

copper pipes or plastic pipes. For the same capacity, these systems need roughly twice as much refrigerant as dual-circuit systems, but are more energy-efficient [Klima:aktiv 2007].

This is due to the fact that, unlike dual-circuit systems, direct-evaporation systems work without the additional heat transfer (intermediate heat exchanger) and the brine pump for the ground collectors. Where they use natural refrigerants such as propane, such systems are clearly superior from a climate protection point of view to those using HFC.

Despite these advantages, direct-evaporation systems are not undisputed. Apart from the larger quantity of refrigerant required in direct-evaporation systems, some experts take a critical view of the increased leakage risk [KKW, HEA 2003]. In spite of the theoretically superior design compared with dual-circuit systems, comparative measurements by a test laboratory in Vienna have shown that in practice it is not possible to detect any difference in the energy measurements. In some cases the brine systems actually displayed energy advantages [Anhörung 2003].

For a long time the blend R 502 was the standard refrigerant in heat pumps. Because of its CFC content it was first of all replaced by HCFC-22 [FKW 1998b]. From about 1997/98 manufacturers increasingly started to use HFCs as so-called “safety refrigerants” [Stiebel Eltron 2003]. The HFC blends used are R 407C, R 410A, R 404A and R 417A and the pure substance R 134a. The latter is frequently used in domestic water heat pumps.

HFC emissions from domestic heat pump applications in 2007 amounted to about 8.5 t, and the total stock stood at 570 t. This means that emissions more than quadrupled compared with about 2 t in 2002 (HFC stock about 90 t) [Schwarz 2005; Schwarz 2009a]. In view of the fast-growing sales there is reason to fear that this trend will continue in the next few years.

Reduction options

Even before the use of HFCs, from about 1992/1993, manufacturers mostly replaced HCFC-22 with flammable hydrocarbons, e.g. propane (R 290) or propene (R 1270) [Stiebel Eltron 2003; Laue 1999]. Safety aspects connected with the use of flammable refrigerants were controllable [Bock 2003; Stiebel Eltron 2003; Hautec 2002].

However, the introduction of HFCs hardly brought any further advances in the development of heat pumps using hydrocarbons. No technical/energy optimisation took place [Stiebel Eltron 2003; HEA 2003].

The reasons for discontinuing development work on systems with hydrocarbons were many and varied. Manufacturers repeatedly cite the prevailing standards and the lack of binding legal regulations for the use of flammable refrigerants [Bock 2003; Stiebel Eltron 2003;

ASERCOM 2003; Carrier 2003]. Relevant standards in this sector, e.g. DIN EN 378 [DIN 2008], do not permit the use of flammable refrigerants below ground level in quantities exceeding 1 kg.

However, heat pumps – except those for very small heating capacities – normally contain more than 1 kg of refrigerant, and it is not usual to install such systems above ground level. In fact, heat pumps for heating purposes are generally located in the basement below ground

According to DIN EN 378, it is possible to use larger quantities of flammable refrigerant in heat pumps if certain safety measures are complied with (mechanically ventilated housing).

Other standards, such as DIN 7003 (draft) [DIN 1995b], also permit the use of larger quantities provided certain safety precautions are installed. However, European standards are observed primarily for product liability reasons. Future European standards could also permit the use of larger quantities.

The situation is different for domestic water heat pumps. Here refrigerant charges of less than 150 g are state of the art, and suitable units are on the market [Stiebel Eltron, 2009]. With this small refrigerant charge the use of flammable refrigerants such as propane does not cause any problems (e.g. under IEC 60335, Part 40) [Stiebel Eltron 2003; IEC 2003].

Another reason quoted by heat pump manufacturers in addition to the standards situation is the unwillingness of component manufacturers, e.g. makers of fully hermetic compressors in the USA and France, to give their products clearance for the use of flammable refrigerants [Bock 2003; Stiebel Eltron 2003; Laue 1999]. Around 1998, for example, the component supply industry abruptly stopped clearance of components for use with hydrocarbons. The reason given was the high product liability risk in the event of loss or injury [Anhörung 2003].

On behalf of the Swiss Federal Office for Energy (Schweizer Bundesamt für Energie), Wolfer et al [Wolfer et al 1999] produced a study which investigated risks, product liability and criminal law issues in connection with the use of ammonia and hydrocarbons as refrigerants.

For the study scenario “heat pump in detached house” the authors come to the conclusion that

“the risks of using ammonia or propane are acceptable for heat pump owners and are lower than for conventional gas-fired heating systems”.

Regarding the question of product liability and criminal liability of the manufacturer or fitter, the authors come to the conclusion that “the use of ammonia or hydrocarbons cannot be described as incorrect, because there are good ecological reasons for using them instead of ozone-depleting or climate-relevant substances. The state of the art must however be achieved.” The study indicates what measures need to be taken to avoid being held liable in the event of an accident.

The study – as the authors expressly point out – is based on Swiss legislation. The authors are nevertheless of the opinion that it ought to be possible to apply the conclusions to other European legal systems as well.

Kruse and Heidelck [Kruse, Heidelck 2002] point out that “while an accident caused by flammable refrigerants can never be ruled out completely, the safety risk is virtually negligible given proper installation and operation”.

In spite of the difficulties mentioned, heat pumps with capacities of up to 20 kW (for room heating and water heating) with hydrocarbons as refrigerant are still available on the market [Buderus 2009]. Systems with two compressors can even achieve capacities of over 25 kW [Dimplex 2009]. The problem of flammability of propane, for example, is overcome by locating the heat pump including evaporator out of doors (air/water heat pump), which means

considerably reduced [Acalor 2009; Dimplex 2009, Klima:aktiv 2007]. Installations below ground level are fitted with a gas warning device. If refrigerant escapes, the warning device starts a fan which sends the gas outside, thereby preventing the formation of a potentially explosive air/propane mixture [Acalor 2009a]. Some systems are designed as direct-evaporation systems, enabling them to achieve high energy efficiency and correspondingly high coefficients of performance [Klima:aktiv 2007].

As well as hydrocarbons as refrigerants, the use of CO₂ as a refrigerant in household heat pump applications, especially domestic water heat pumps, is possible and already established.

Domestic water heat pumps using CO₂ have already been available on the Japanese market for some years now [Kruse, Heidelck 2002], and CO₂ heat pumps for heating and hot-water are now available in Germany as well [KK 2008a]. A CO₂-air/water heat pump with a heating capacity of up to 9 kW has been available on the German market since 2008 for heating and hot water in new and existing buildings [Kaut, Sanyo 2008]. For the minimum-energy and passive house sector, a 2 kW heat pump using CO₂ as refrigerant is due to be launched shortly. This meets the entire heat requirements of a detached house with a floor area of 145 m² (hot water and heating), while achieving a much better annual coefficient of performance than a system using R 134a [Kosowski et al 2008]. In spring 2009 a pilot heat pump system with CO₂ ground heat pipe and CO₂ as refrigerant started operating at the Research Centre for Refrigeration and Heat Pumps (Forschungs-zentrum für Kältetechnik und Wärmepumpen GmbH) [FKW 2009].

Although ammonia has not so far been used in household heat pumps operating on the compressor principle and has been rated as being of no practical relevance [Stiebel Eltron 2003; Laue 1999], a study within the EU SHERPHA project demonstrated that a heat pump with ammonia is not only basically possible, but in terms of efficiency is capable of surpassing conventional systems with HFC [Palm 2008]. The study makes it clear that the limiting factor here is not so much thermodynamic or technical considerations, but rather the fact that there are hardly any components on the market that are designed for ammonia in this capacity range (around 10 kW). For example, only one manufacturer offers a semi-hermetic compressor suitable for this refrigerant. This “separating hood compressor”, in which the motor winding is not in contact with the refrigerant, is used in industrial processes where aggressive gases have to be compressed. This design has hardly been used to date in refrigeration engineering, however. Existing open compressors are hardly suitable for household heat pumps because of the risk of leaks and the toxicity of ammonia. Other components such as heat exchangers and throttle valves are also either not available at all or only to a limited extent. It has nevertheless proved possible to design a prototype heat pump using ammonia which has a refrigerant charge of only 100 g and achieves coefficients of performance in excess of 4 [Palm 2008].

The situation is different for absorption heat pumps, which are marketed with ammonia (the substance couple ammonia/water). The fundamentals of the absorption principle are described in Chapter 3.1.2. Today absorption heat pumps are only available for heating capacities of between 16 and 40 kW, which means they can only be considered for multi-family houses or

pump in the range from 4 to 10 kW is scheduled for the end of 2011, and this will be suitable for single-family houses [FAZ 2010]. Absorption heat pumps have few moving parts and are therefore very quiet [ASUE 2002].

Another alternative to heat pumps working on the compression principle is adsorption heat pumps. Chapter 3.1.2 describes the fundamentals of the adsorption principle with solid sorption materials. Since 2010 one manufacturer has been offering a gas-powered adsorption heat pump using the system water (refrigerant) and zeolite (sorbent) which has a heating capacity of up to 10 kW [Vaillant 2010]. Another manufacturer has already successfully field-tested zeolite heat pumps and plans to launch them at the end of 2012 [FAZ 2010]. There are also plans to develop units with greater heating capacities.

The differences in the energy efficiency of heat pumps depending on their power source and heat source and their impact on CO₂ emission savings cannot be dealt with in this report on fluorinated greenhouse gases, as a wide range of factors need to be considered. Examples of relevant overviews include Laue [Laue 2000], Leven et al [Leven et al 2001] and ASUE [ASUE 2002]. In a comparative study of electric heat pumps and conventional and renewable heating technologies, the Federal Environment Agency has already determined and assessed the environmental balance of this type of system [UBA 2008]. Although today heat pumps are regarded as a mature technology, ongoing improvements mean that one can expect further increases in the efficiency of these applications in the future [Löffler 2008].

It must be stressed that in addition to the refrigerant issue, the question of energy efficiency has gained considerably in importance. Since the heat pump’s successful entry into the market, manufacturers of heat pumps and components have succeeded in making improvements in this field. Manufacturers claim that thanks to the input of development work, today’s heat pumps with HFC refrigerant charges achieve better coefficients of performance than units which were built in the mid 1990s and which used hydrocarbons (mostly propane). On the one hand this is not surprising, because any further development also results, among other things, in improved energy efficiency. On the other hand, manufacturer’s information is frequently at variance with the actual figures measured for systems already installed. First-time purchases of heat pumps are supported, as in the case of solar collector and biomass systems, by the Federal Environment Ministry (BMU) under the

“Guidelines on support for measures for the use of renewable energy sources in the heating market”, in cooperation with the Federal Office of Economics and Export Control (Bundesamt für Wirtschaft und Ausfuhrkontrolle – BAFA), in the context of the German government’s market incentives programme [BAFA 2009]. The same applies to heat pumps powered by gas engines. The main criterion for a system’s eligibility for support is its annual coefficient of performance, which is to be determined in accordance with VDI Guideline 4650. However, the latest findings of a field study conducted by the Fraunhofer Institute for Solar Energy Systems indicate that the annual coefficients of performance calculated before installation are rarely achieved in practice [Morhart 2009]. It remains to be seen whether the revised version of the Guideline will bring any improvement in advance calculation of the annual coefficient of performance. Another point that needs to be improved is the fact that the refrigerant used

in helping natural refrigerants (e.g. propane, CO₂), which are of virtually no climate relevance compared with HFCs, to finally make a breakthrough in the field of heat pump applications and replace climate-relevant refrigerants. This would seem to be an urgent necessity, especially in view of rising sales figures and the associated growth of HFC emissions.

Conclusions

For low-capacity heat pumps (up to 20 kW) and correspondingly small refrigerant charges, eco-friendlier alternatives are available today in the form of hydrocarbons (propane) and CO2. This means that in this application it is already possible to avoid the use of HFCs entirely. This applies in particular to domestic water heat pumps. Units for this field are available at slightly higher prices (additional cost of housing ventilation comes to about 1.5% of total costs).

Many experts regard hydrocarbons as refrigerants with very good thermodynamic properties combined with acceptable risks for use in heat pumps. Many manufacturers have nevertheless stopped further technical development work on these heat pumps, including energy optimisation, and are now concentrating entirely on HFCs. One possible way of making the use of propane more attractive for manufacturers would be to ensure that heat pumps with natural refrigerants receive greater support than HFC systems under the German government’s market incentives programme.

In the capacity range from about 16 to 40 kW (e.g. for multi-family houses and commercial purposes), not only compressor heat pumps are available, but also absorption heat pumps which do not contain any halogenated refrigerants. Development work has already started on absorption heat pumps for single-family houses (capacity range from 4 to 10 kW). The start of series production is planned for 2011 [FAZ 2010].

Adsorption heat pumps having a heating capacity of up to 10 kW (for single-family houses) and using water as refrigerant are already available on the German market. Work is in progress on the development of heat pumps with greater heating capacities.