.

Fig. 11 Greenhouse gas emissions (without taking refrigerant losses into account) as well as the primary energy requirement for heat pumps and gas-fired condensing boilers.
© FhG-ISE

Fig. 12 HP storage system with measurement technology.
© Vaillant Deutschland GmbH & Co. KG, Remscheid

Fig. 13 Underfloor heating in a construction project in Duisburg.
© Thomas Lienemeyer, Mülheim a. d. Ruhr

Fig. 14 Installation of ground source pumps in a new-build scheme in Mülheim a. d. Ruhr.
© Thomas Lienemeyer, Mülheim a. d. Ruhr

Fig. 15 Coefficients of performance (COP) of heat pumps at the rated standard operating point as per EN 14511
© FhG-ISE
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Assessment of heat pumps

The coefficients of performance of heat pumps enable different heat pumps from various manufacturers to be compared with one another; of course under the assumption that the coefficients of performance have been determined under the same boundary conditions. Likewise, a comparison of the results from different field tests is only possible to a limited extent if they have not used precisely the same balance boundaries and analysis methods. In addition to the issue as to where the system boundaries were defined, there are also other aspects that are relevant. For example, when calculating the seasonal performance factor it makes a difference whether unused heating energy is taken into account that was produced in summer as a result of the system or as a result of faulty operation. It is also only possible to compare the same balancing periods with one another (e.g. one year).

In classifying the seasonal performance factor information, not only do the balance boundaries and the balance periods need to be specified but also the type of heating source, the application area (e.g. building standard, heating systems, ratio of the heating requirement to the domestic hot water requirement) and the operating temperatures. Quite often only the supply temperatures are specified as operating temperatures in the heating circuit. However, these are not the only ones that are decisive for the condensation temperature. The return temperature also has an impact.

This can be clearly seen when observing the course of temperatures in a condenser. For example, there is a greater condensation temperature at 35/30 than at 35/25. This is also shown by measurements made by the NTB Buchs testing centre in Switzerland. Standard-based measurements according to EN 14511 and EN 255 were conducted for 13 air-water heat pumps and 19 brine-water heat pumps, whereby the COP for air source heat pumps with a heating circuit temperature of 35/30 was on average 7 % lower than with 35/25, and brine source heat pumps were on average 6 % lower. This therefore shows that the supply and return temperatures (or alternatively one temperature and the spread) should always be specified. If it is easier to only use one operating temperature in the heating circuit when comparing several systems, it is better to use the mean heating circuit temperature than the heating circuit supply temperature.

Assessment of heat pumps in comparison with other heat generators

Different characteristic values can be taken into consideration when directly comparing heat pumps with other energy generators, e.g. gas-fired condensing boilers. The following section describes the primary energy requirement needed for space heating and domestic hot water heating and the resulting greenhouse gas emissions. Fig. 11 compares the primary energy requirement for heat pumps and gas-fired condensing boilers for different annual performance factors and annual utilisation rates. The spectrum of annual performance factors is based on the “WP Effizienz” (HP efficiency) and “WP im Bestand” (HP in existing buildings) monitoring projects. The data for the condensing boilers was determined using 60 gasfired condensing boilers as part of the “Felduntersuchung: Betriebsverhalten von Heizungsanlagen mit Gas-Brennwertkesseln” (Field test: Operating behaviour of heating systems with gas-fired condensing boilers) project. The underlying specific primary energy factors for electrical energy (2.35) and natural gas (1.12) reflect the respective final energy generated in Germany in 2010. The comparison shows that, with correct configuration and operation, heat pumps can save considerable primary energy compared with gas-fired condensing boilers. For example, ground source heat pumps with a annual performance factor of 3.9 (mean value in the “WP Effizienz” project) save 49 % of the primary energy in comparison with gas-fired condensing boilers with an annual utilisation rate of 96 %. And air source heat pumps with an annual performance factor of 2.9 (mean value in the “WP Effizienz” project) even save 32 %. However, this also shows that – depending on the efficiency of the reference systems – only very slight or no savings can be made with heat pumps with very low annual performance factors.

The greenhouse gases generated by the final energy requirements for heat pumps and condensing boilers are depicted in Fig. 11. In comparison with condensing boilers, heat pumps show a similar potential for saving greenhouse gas emission when generating heat as they do with primary energy. For example, ground source and air source heat pumps with respective annual performance factors of 3.9 and 2.9 achieve reductions in greenhouse gas emissions during operation that amount to 43 % and 23 % respectively. However, greenhouse gas emissions that are caused by refrigerant losses were not taken into account in these values.

Refrigerant emissions can occur along the entire process chain, ranging from the manufacture and application (i.e. in this case during the heat pump operation) to the disposal. The contribution made by the refrigerant to greenhouse gas emissions as a result of heating with a heat pump therefore depends on the respective refrigerant losses and the type of refrigerant used. The German Federal Environment Agency specifies a value of 2.5 % of the refrigerant charge per year for refrigerant losses from heat pumps used for heating in Germany. This value represents a mean value and covers both “gradual” emissions as well as emissions caused during servicing and accidents, and is based on an average service life expectancy of 15 years. The disposal is based on an average recycling rate of 70 %.

How this effects greenhouse gas emissions shall be shown more specifically using the example of a standard air source heat pump for two different refrigerants. It is assumed that a building has a 160-m² living space with a specific annual heating requirement of 70 kWh/m² p.a. for space heating and 17 kWh/m² p.a. for domestic hot water heating. The heat pump contains 3 kg of refrigerant and the heating capacity amounts to 7.5 kW with an annual performance factor of 2.9.

If the R407C refrigerant is deployed, which is the refrigerant most widely used in new heat pumps deployed for heating (global warming potential (GWP) value = 1.774), the greenhouse gas emissions caused by the refrigerant losses increase by 9 %. They increase by 19 % if the rarely applied R404a refrigerant is used. R404a has the highest GWP value (3.922) of all hydrofluorocarbons deployed in heat pumps used for heating.

This shows that, when the assumed refrigerant losses are also taken into consideration, heat pumps only have 8 % to 16 % less greenhouse gas emissions than gas-fired condensing boilers.

Current coefficients of performance of heat pumps

The quality label for heat pumps awarded by the European Heat Pump Association (EHPA) certifies the quality of heat pumps based on technical, design and servicespecific quality guidelines, whereby heat pumps used for heating purposes have to achieve sufficient minimum values for the coefficients of performance (measured according to EN 14511). Many of the devices available on the market achieve these threshold values (Fig. 15).

Another reference value for classifying the efficiency of heat pumps is provided by the threshold values for the annual performance factor, which are used for subsidising heat pumps. The current funding guidelines from the German Federal Office of Economics and Export Control (BAFA) stipulate that only heat pumps used in existing buildings (constructed before 2009) may be subsidised. The calculated annual performance factors must achieve values of 3.8 for brine-water heat pumps and 3.5 for air-water heat pumps.

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