Thermal energy storage for fully autarkic renewable district heating and cooling networks

Dr. M.Bloemendal^1,2, Prof. Dr. P.J.Vardon¹

¹Delft University of Technology
²KWR water research institute

As a result of climate change and insulation measures (to accommodate low temperature renewable heat sources), the cooling demand of buildings increases considerably in climates where heating is normally dominant. It is at least remarkable that cooling is hardly mentioned or even considered in the heat transition. However, it is fairly straightforward to also provide sustainable cooling with renewable heating systems that use a heat pump. When such systems also use the subsurface as a source for environmental heat, the cooling capacity stored in winter (while extracting energy for heating) is available for cooling. Due to the typical temperatures in which the thermal energy can be stored in the subsurface, so-called passive free cooling can be used where only circulation of water is needed without a heat pump. This is hyper cost-efficient cooling, usually with a COP ranging from 20-30!

In individual buildings, such heating and cooling systems are often supplied by aquifer or borehole thermal energy storage (ATES/BTES) systems. Similarly, this can also be done in district heating networks where seasonal heat storage is utilised. This could be done with a single delivery set for the underfloor heating and cooling in each building.

There are two key issues for the heating and cooling of buildings to ensure comfort and reliability: energy supply and the required temperature level. These are quite different in character to the supply of electricity. Both heating and cooling are useful – just at different times of the year – and can be considered positive and negative energy flows to a building. The temperature of supply strongly impacts the needed internal design and changing temperatures, e.g. via a heat pump, which uses a considerable amount of energy.

Thermal energy storage systems are as just that: storage systems. The energy that they supply must come from elsewhere. For buildings or collections of buildings which have balanced heating and cooling demands, energy is moved in time and space, and no additional supply is needed. However, buildings in general do not have an even heating and cooling demand. When there is a structural imbalance between heating and cooling, additional sources can be utilised, e.g. cold water from a nearby surface water could be stored during the summer for cooling purposes next winter.

Seasonal storage also adds the possibility for large scale systems to effectively harvest local additional heat or cooling capacity. Such systems, where district heating and cooling networks supply to 50-500 individual houses or equivalent other buildings, can benefit greatly from the buffering capacity the underground can provide. Several of such systems have been installed in in the Netherland, e.g. the Houthavens in Amsterdam, in Dutch: https://warmtenetwerk.nl/warmteproject/koude-en-warmte-houthavens-3/.

For heating, there remain relatively few buildings which can be directly heated at temperatures which are typically stored in the subsurface (ATES systems are usually limited in the Netherlands to 25°C). These are very new high-standard buildings, and the energy transition needs to address all  buildings. Therefore a heat pump is usually an essential yet costly feature of such systems, And has other disadvantages such as increasing electrical demand which increases costs and puts strain on the electricity network. The efficiency of thermal energy storage systems can be improved by  utilising external higher temperature heat sources to improve the heat pump coefficient of performance (reducing the external electricity input), similarly as was done at the greenhouse of Koppert-Cress https://www.egec.org/wp-content/uploads/2023/02/Bloemendal-Transition-of-ATES-to-HT-ATES-117_ExtAbstract.pdf. However, by doing this, the cooling capacity generated by using the heat pump may be reduced or even eliminated. In the latter case, the cooling capacity should also be harvested from external sources, as was done at the project in Amsterdam mentioned above. In that case, the BTES/ATES systems need to consider layouts which take into account supply and return temperatures from buildings at different times during the year. For example, for ATES systems, an additional well is needed and should be laid-out similarly to  the ATES-TRIPLET system https://www.tudelft.nl/2020/citg/geo-engineering/nwo-ates-triplet-project-granted. This means that by sourcing the right amount of thermal energy at the right temperature, and being able to store it seasonally, the amount of external energy can be reduced, even to zero – creating fully autarkic district heating and cooling networks!

All in all, seasonal underground thermal energy storage is key in enabling renewable district heating and cooling grids. It provides the key buffer between heating and cooling supply and demand – even when heating and cooling energy demands are not balanced. Common practices exist, but also some research challenges lie ahead of us to further optimise these technologies and make them suitable in wider areas of the world.