A system combining heat pumps and district heating would get the best from both techniques. But in Sweden, where both techniques are widely spread, there has rather been a competition between the two. Until now. In recent years the interest has increased, and there is a new focus on finding optimal combinations. The research project Heat Pumps in District Heating Systems has investigated three alternatives, as described below.
Heat pumps in the manufacturing industry
At a manufacturing industry in Gothenburg, Sweden, both heat pumps and district heating are used. The two heat pumps recover heat from process cooling for space and water heating, supplemented by district heating during winter. The system has reached a seasonal performance factor (SPF) of 5.2 for the heat pumps.
Currently the heat pumps and the district heating substation are connected in series. The return flow is first heated by the heat pump, then by district heating if needed. During summer, when the heating demand is lower, and the district heating is cheaper, the heat pumps are shut off.
The evaluation of the system shows that it would be preferable to connect the heat pump and the district heating in parallel instead of in series. That would result in a high COP for the heat pump and a low return temperature for the district heating. The drawback is that the control strategy would be more complex.
Hybrid heat pumps
A hybrid heat pump, as described here, can alternate between the heat pumping function and district heating. Since many houses in Sweden with heat pumps also have a connection to the district heating grid, this alternative is within reach in many cases. The house owner could then choose heating solution depending on e.g. cost or environmental factors. Such an installation would also be a preparation for a future smart grid.
Within the project an algorithm for choosing between heat pump and district heating was developed and tested in a multi-family house in Linköping. It considers current operating conditions, energy prices and hourly cost. The algorithm will mainly choose the heat pump during the autumn, winter and spring, while district heating dominates in the summer.
A hybrid heat pump is more expensive than a standard one, but the energy costs will be lower. Therefore, the life cycle costs were calculated. It shows that with a payback time of 5 years, a hybrid heat pump can cost up to 9 500 € more. A payback time of 10 years allows an extra investment of up to 17 000 €.
Heat Pumps for DHW production
The project has studied how a heat pump solution for production of domestic hot water (DHW) should be designed, using district heating return flow as a heat source. One challenge is the temperature variations, which affect the working conditions for the heat pump. Another aspect is that due to the temperature levels of the return flow, floor heating is required. It has also been shown that, for multi-family houses, a centrally placed heat pump is preferable compared to small heat pumps for each apartment, even though it leads to larger heat losses from the DHW circulation.
Markus Lindahl, Jessica Benson, Tommy Walfridson, RISE Research Institutes of Sweden, Sweden
This text is shortened by HPC.