.

Fig. 30 Energy balance of two heat pump systems for space heating and domestic water heating (balance year: 2009).
© FhG-ISE
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Efficiency versus effectiveness

When assessing heat pumps, the focus is normally on the seasonal performance factor. However, is this characteristic value always the right parameter for assessing heat pump systems? The efficiency of the heat generation does not indicate whether the heat was appropriately generated and how much electrical energy was used. This is where the term effectiveness helps, which encompasses the actual goal, namely saving heating and primary energy.

To clarify this aspect, two systems from the “WP Effizienz” monitoring project will be presented, which are demonstrated in Fig. 30. The first example shows a ground source heat pump system that achieved an annual performance factor of 4.2. It provided a specific heat energy of 100 kW/m² p.a. for space heating in the building and 21 kW/m² p.a. for heating domestic water. This results in a specific primary energy consumption of 74 kWh/m² p.a. The second example shows an air-water heat pump in a better-insulated building; the heat energy consumption was only around half with 47 kWh/m² p.a. Owing to the heat source used, the efficiency of the heat pump with 3.3 is considerably lower (although above average in the context of the measured air source heat pump systems). However, there was a roughly 30% lower specific primary energy consumption (57 kWh/m² p.a.) for covering the energy requirement for space heating and domestic water heating. It can therefore be determined that in the first example an efficient heat pump is working in a – from an energy viewpoint – less effective overall system, whereas in the second example there is a more effective overall system despite the lower efficiency of the heat pump.

This observation is also relevant when evaluating heat pump systems in increasingly energy-efficient newbuild schemes that have surface heating systems such as underfloor and wall heating. The large difference in heat sink temperatures during the heat pumps’ space heating and domestic water heating operation means that the ratio of the space heating to the hot water requirement is increasingly affecting the efficiency of the heat pumps. A heat pump in a building built to the passive house standard works for almost half of its operating time to produce hot water, i.e. at an operating point that only allows low efficiency. That has a negative effect on the annual performance factor. However, the heat pump system requires less electrical energy than a building equipped with the same heat pump, space heating and domestic hot water system but which has greater heating energy requirements as a result of its lower insulation standard.

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