Hybrid system with ground source storage of stone flavor for schools and institutions

Denmark focuses on integrating more renewable energy into the energy system. High targets for 100% renewable energy in electricity and heating by 2035 and oil-burners phased out by 2030 means that heat pumps will be the primary source of heat outside the district heating supply areas and the replacement for the 100,000 current homes with oil-burners. It is estimated from Dansk Energi that 630,000 heat pumps can be established in Danish single-family houses with accumulation possibilities in the building stock and storage tanks. In this ELFORSK project, we have studied, theoretically as well as in practice, how a marketed hybrid solar/heat pump system, HYSS can be further optimized by storing excess solar heat in a soil based heat storage and how such storage can be designed.

The aim of the project was to demonstrate how the use of stone flour-based soil heat storage can result in SCOP for both hot water and space heat production in excess of 6 - that is, a doubling of SCOP compared to traditional brine/water heat pumps and thus a 50% reduction of electricity consumption. The project has achieved a SCOP of 4.3 on a small scale, corresponding to a 32% reduction in electricity consumption compared to traditional brine/water 4 heat pumps.

The project also showed that doubling the solar collector area used, as well as an improved operation of the production circulation pump, can increase SCOP for hot water and space heating to over 6. Thus, the possibilities for 50% reduction of electricity consumption compared to traditional brine/water heat pumps is very possible.

The project has shown that heat pumps can be further optimized by using solar heating and ground source heat storage. A stone flour-based storage can be heated when there is excess production of solar heat and the stone flour-based storage will significantly improve the operating conditions for the heat pump and the electricity grid, thus contributing to less overall electricity consumption from many heat pumps in Danish homes, schools, institutions and other large buildings. We have not examined the profitability of ground source heat storage, but since they are relatively expensive to build, it is probably most relevant on a large scale, that is to say for larger projects such as schools, institutions and larger buildings.

By using hybrid systems with energy storage in the buildings body and storage tanks, compared with the ground source heat storage on the cold side, it is possible to reduce the electricity consumption to the least possible, so that the overall load of the electricity grid from heat pumps can be reduced by at least 25%.

Operating results from this project and other projects in Free Energy show savings in electricity consumption at this level compared to traditional brine/water heat pumps, and this will of course also have a large positive effect on operating costs for the approx. 4000 schools and institutions in Denmark that we estimate are relevant to the technology being investigated.

It is obvious to follow up on the results of this ELFORSK project with a new project that specifically focuses on a solution for a larger school or institution.