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Grid Flexible Control of Heat Pumps

The European energy system is changing. Renewable energy sources are providing ever larger shares of the energy used, and this is of significance also for the other parts of the system. In this transformation, heat pumps can be an asset providing demand response to the power system. The project Flexible Heat and Power (FHP) has investigated the possibilities.

Introducing elements of demand response means that the power from intermittent sources such as wind and sun can be used more efficiently. The effects include peak shaving, balancing of consumption, and avoiding curtailment of power production. In FHP, heat pumps are clustered, and their power consumption collectively controlled in order to enhance the demand response potential.

Several steps are needed to make such heat pump clustering efficient and functional. The thermal flexibility of the buildings must be calculated, showing how much power the heat pump will need for heating. When this is done for more buildings in an area, the aggregated information can be used for, e.g., grid handling or balancing. Next, information on their required contribution is fed back to each individual heat pump.

In order to do this, it must be possible to take external control over the heat pump. In FHP, both direct and indirect external control has been tested, in a laboratory setting. It shows that direct control gives the best accuracy, but does not work on all heat pumps, while indirect control works on most heat pumps.

Indirect control is mainly done in two ways: manipulating the temperature sensor or adjusting the heating curve. In the FHP tests the first alternative was used. The basis for the manipulation is that the heat pump adjusts its heat production according to the outdoor temperature. In the tests, the outdoor temperature sensor was replaced with an adjustable precision potentiometer signalling fake outdoor temperatures. In this way it is possible to get the desired heat production from the heat pump.

Direct control means that the compressor in the heat pump is controlled directly; the ordinary internal control is bypassed. This type of control is quicker and has a higher accuracy than indirect control. The main drawback is that the heat pump has to be prepared for external control, a functionality that no product on the market has today. For the tests a computer program was used to set the compressor frequency, and an eight-hour test cycle was set up.

The overall conclusion from these tests is that external control of heat pumps is most efficiently done through direct control. Thus, a standardised way to externally control the heat pump needs to be part of the manufacturers’ standard protocols in the future. If already installed, heat pumps should be controlled externally. Manipulation of the outdoors temperature sensor could be done to almost all heat pumps. A more cost-efficient alternative would be to manipulate the heat curve.

 

Markus Lindahl, Sweden (RISE Research Institutes of Sweden)

The text has been shortened by the HPC team

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