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Heat pumps in the grid

The German federal government is aiming to increase the share of electricity from renewable sources to 35% by 2020, whereby the driving technologies are fluctuating energy generators such as wind power and photovoltaics. Active load control of heat pumps as part of an intelligent supply system can help to synchronise the energy supply and demand. This utilises the storage capability provided by heating and domestic hot water storage tanks and the building mass.

At the end of 2011, the electrical connected load of all heat pumps installed in Germany amounted to approximately 1.5 GW. In extreme cases, these heat pump systems could use 36 GWh of electrical energy per day. By way of comparison, the current capacity of the pumped storage power plants installed in Germany amounts to around 40 GWh. According to the German heat pump association (Bundesverband Wärmepumpe), the electrically connected load could increase to 4.4 GW by 2020. In 2012, the electricity generated by wind energy ranged between 0 and 24 GW, whereby there was a considerable seasonal dependence in terms of the energy fed into the grid. The same applies to photovoltaic energy, whose peak of 22 GW in the same period occurred, as would be expected, at midday. Due to their storage capacity, heat pump heating systems can only be used for buffering on a daily basis. The general contribution made by heat pumps is closely linked to the potential to shift operating times. Here the energy supply companies need to develop price concepts and set a corresponding price signal. In addition, the control strategies for the heat pump operation need to be adapted.

The tests conducted by Fraunhofer ISE have confirmed that only a few of the heat pumps investigated are fully loaded at temperatures in the design range. In addition to the design, a role is also played by the type of operation, the control and parameterisation as well as the storage capacity in the heating system and building. This raises questions concerning the comfort limits for the residents. The greater the controllable temperature range, the greater the flexibility with which heat pumps can be used as a controllable load. Another issue that has not yet been clarified is how electricity-led operation affects the efficiency of heat pump heating systems. Deviations from the exclusively heat-led operation can affect, for example, the system temperatures, storage losses or the compressor running times.

Another aspect that still needs to be clarified in this respect is the design of the communication and control capability. A practical solution which, however, can only be implemented in the long term, is the development of a new information and communications technology (ICT) infrastructure for bidirectional communication, such as part of the dissemination of smart meters. At the same time, each heat pump needs to be equipped with a corresponding communication gateway that is ideally uniform.

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