.

Fig. 1 Heat pump test rig
© FhG-ISE

Fig. 2 Coefficient of performance and heating capacity of a brine-water heat pump in accordance with the brine input temperature and the heating circuit (HC) output temperature.
© FhG-ISE

Fig. 3 Monitoring projects from the Fraunhofer Institute for Solar Energy Systems
© FhG-ISE

Fig. 4 System boundaries for determining the seasonal performance factor
© FhG-ISE

Fig. 9 Storage and distribution system in a logistics centre.
© Bosch Thermotechnik GmbH, Wetzlar

Fig. 10 Heat pumps in an apartment building in Augsburg.
© Bundesverband Wärmepumpe e. V., Berlin
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Performance of heat pumps

Research institutes, manufacturers and operators are measuring the efficiency of heat pump systems during actual operation. Although the same characteristic values are being used, the results can only be compared with one other to a limited extent since the balance boundaries and analysis methods often differ considerably.

In the European project “SEasonal PErformance factor and MOnitoring for heat pump systems in the building sector (SEPEMO-Build)”, installation and analysis methods for field measurements for heat pump systems are being developed that can be used as guidelines. In order to assess the efficiency of heat pumps, i.e. to determine their cost-benefit ratio, the coefficient of performance and the seasonal performance factor are the decisive characteristic values.

  • The coefficient of performance (COP) is determined in the steady state, i.e. under constant operating conditions. It indicates the ratio of the heating capacity to the electrical power consumption electrical power of the heat pump.
  • The seasonal performance factor (SPF) describes the ratio of the provided thermal energy to the consumed electrical energy over a longer period of time (e.g. one year).

The coefficient of performance is determined on test rigs with defined boundary conditions. For example, the B0/W35 operating point in accordance with EN 14511 is used as the rated standard operating point for brine-water heat pumps. This describes the operation with a brine temperature of 0 °C/-3 °C (input / output) and a heating circuit temperature of 35 °C/30 °C (output / input). However, the coefficient of performance is still to some extent specified according to the previously applicable standard, EN 255, whereby the brine-water heat pumps are measured with the same brine temperatures but with heating circuit temperatures of 35 °C/25 °C. When calculating the coefficient of performance in accordance with the standards, not only is the electrical power consumed by the compressor taken into account but also the electrical power consumed by the source pump and heating circuit pump in order to overcome internal pressure losses.

The coefficient of performance for heat pumps can in principle be determined for any possible steady operation conditions. Since the coefficient of performance considerably depends on the operating conditions, in particular the temperatures, it should only ever be specified and considered in relation to the operation conditions.

Dependence of the heating capacity and coefficient of performance on the temperature

The anticlockwise Carnot cycle provides an ideal reference cycle for comparing heat pump processes. With the Carcycle, the efficiency is only dependent on the upper temperature TU and the lower temperature TL between which the cycle runs (COP_Carnot = To /( To – TL ); T in K). Even if the coefficient of performance for a heat pump is considerably lower, its temperature dependence is still largely comparable with the reference cycle. The respective evaporation and condensation temperatures are therefore decisive for the efficiency of heat pumps. By way of example, Fig. 2 shows the coefficient of performance for a brine-water heat pump relative to the brine input temperature and the heating circuit (HC) output temperature. The coefficient of performance increases with a reduction in the temperature difference, i.e. with an increasing source temperature or a reduction of the sink temperature.

An increase in the evaporation temperature or a reduction in the condensation temperature causes the coefficient of performance to improve by between 1.5 and 4 % per kelvin during normal heat pump operation (operating points with frosting of air-cooled evaporators are not taken into account here). The amount by which the efficiency changes partly depends on the thermodynamic interrelationships: a temperature change of 1 K has a greater effect with a small temperature lift than with a larger temperature lift, and a change in the evaporation temperature alters the efficiency to a greater extent than the same change to the condensation temperature. However, process-based aspects also have an influence. The heating and cooling capacity of – non capacity controlled – heat pumps is also strongly dependent on the temperature, and increases with a greater evaporation temperature and lowers with a greater condensation temperature (Fig. 2).

Balance boundaries in heat pump systems

When determining the seasonal performance factor for a heat pump system, different system boundaries can be defined. By way of example, Fig. 4 depicts three possible system boundaries for a heat pump system that, with the help of a horizontal ground heat exchanger, uses the ground as a heat source and provides heat for space and domestic water heating: The “narrowest” system boundary (HP) only includes the energy required by the heat pump unit (compressor, control system and, if required, an oil sump heating system for the compressor). If the heat source circuit’s ventilator, brine or well pump is also included in the balancing scope with supplementary electrical heating when installled this is described as a heat pump system (HPS). When balancing both the HP and the HPS, the thermal energy is determined directly behind the heat pump and/or the electrical back-up heater. When considering the efficiency of the entire heat pump heating system (HPHS), only the effective energy – i.e. behind the storage systems – is taken into account. In this case the charge pumps are also incorporated into the calculation as loads.

Parameters influencing the seasonal performance factor of heat pumps

Because the temperature has a considerable impact on the efficiency, the annual performance factor for a heat pump is substantially determined by the temperature level on the heat source and heat sink side. There are a diverse range of factors that influence the operating temperatures, whereby it is not just the field of application of the heat pump that plays an important role but also the planning, installation, commissioning and operating phases.

  • The field of application of a heat pump is limited to a certain extent by the choice of heat pump technology (e.g. the type of heat source). In addition there are boundary conditions and limits in terms of the required sink temperatures: for example there are relevant differences between their use in old buildings that have not been refurbished and in new buildings. In new buildings with underfloor heating, the heating operation differs considerably from the operation for domestic hot water heating.
  • Through their choice and size of heating system, design engineers determine the required heating circuit temperatures within the framework provided by the heating requirements and the spatial conditions.
  • Careful installation, professional commissioning and controlled operation help to maintain the planned operating temperatures and adapt to any deviating requirements in practice. For example, a non-adjusted heating curve could mean that the system is operated with heating circuit temperatures that are higher than required. An unfavourable positioning of the storage temperature sensors can cause the storage tanks to be incorrectly charged, particularly with combined storage tanks: the heat pump then generates more energy at the high domestic hot water temperature level than is required. Not completely closing 3-way vents and missing check valves can cause undesired discharging of the domestic hot water storage tank.

In addition to aspects that influence the operating temperature, the auxiliary energy also, of course, has to be taken into account.

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