Answered by the PUSH-IT team:
Underground thermal energy (heat) storage is a technology that stores energy as heat beneath the earth’s surface. Heat from power plants, factories, and renewable energy sources (e.g., solar, geothermal) can be collected in summer. The heat is then stored underground until it is needed in winter, for example, to supply local or district heat networks. This can help reduce the use of fossil fuels such as coal, oil, and gas. The hotter the temperature, the more energy stored. In PUSH-IT, scientists are testing three ways to store heat up to 90°C. These are mine thermal energy storage (MTES), borehole thermal energy storage (BTES) and aquifer thermal energy storage (ATES)

Answered by Dr. Martin Bloemendal, Principal Investigator for PUSH-IT, Delft:
We need many novel technologies in the energy transition. Due to intermittency of available renewable energy and varying demand heat storage is one of the technologies we need in our energy system.

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:
The subsurface is natural – but we make wells to access the subsurface. The fluids used in the subsurface are those naturally there, and we heat them up or cool them down to store energy.

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:
This depends on the energy system used. At present the maximum designed temperature is 90 °C, but it can be stored as low as 20 °C. At higher temperatures we can store more energy in a smaller space, but the engineering is more challenging.

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:
We aim to store seasonally, i.e. produce the heat in the winter and extract in the summer. So the subsurface is slowly heated over around 3 – 6 months.

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:
Usually not, but the above ground energy system can do this, i.e. by using a heat pump or further locally heating water for hot water.

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

We use a heat exchanger. This is a piece of equipment where two closed pipes are close to each other allowing energy to be exchanged, while the water does not mix. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University: 

This depends – in general the storage systems can work with any energy that is available – this makes it very flexible. Some care needs to be taken on making sure the temperature levels from production and use are compatible, i.e. as close as possible.  

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

Typically, water is pumped around the system – this usually uses very little energy. Heat pumps are used when the temperature of storage and needed temperature are not the same. By making these temperatures close together, the energy used by the heat pump can be optimised. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

The groundwater that is found in the subsurface is used in open systems and in closed systems clean water is added at the start and can be used for a long time. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

As we use the subsurface, there is not a fixed capacity, however there is a range of capacities where a certain system cannot perform well. After this capacity is reached no more energy can be stored. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

Typically, not at the temperatures we are considering storing. To generate electricity efficiently, this should be >120°C. We are focusing on storing heat for heating purposes. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:  

Yes – absolutely. This is the aim of the technology to support these generation technologies. In addition, other sources, such as waste heat, can be used. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:

This depends on the exact design of the energy system. By increasing temperatures (as we are focusing on) fewer modifications to existing heating systems will be needed.  

Answered by Dr. Wen Liu, energy systems modeller, Utrecht University:  

In a district heating system with sustainable heat supply technologies, for example, geothermal heat and heat from solar collectors, underground thermal storage can help to increase the share of sustainable heat supply. Underground thermal energy has much higher thermal capacity than sand batteries. However, most sustainable district heating system with underground thermal storage are more expensive than fossil based heating or solar collector for individual house (see Figure. 10 in the paper Seasonal Thermal Energy Storage: A techno-economic literature review.). 

Answered by Veronika Slavíková, Vít Peřestý, Lucie Janků and Josef Vlček, RINGEN, Litoměřice, Czech Republic:   

Thermal Energy Storage systems are custom-made for every site,site; there is no rule-of-thumb to say which is the best. Generally, mine thermal energy storage is the most suitable for areas with the presence of flooded underground mines, aquifer thermal energy storage is suitable in areas with sedimentary rocks (sandstones etc.) and borehole thermal energy storage is suitable especially in crystalline rocks (granite, gneiss, etc.). The aim of the PUSH-IT project is to gather information from six test sites located across Europe over the period of several years and constrain the uncertainties in the determination of these parameters. 

In terms of costs, the largest expense is the drilling work itself. borehole thermal energy storage technology is the most expensive, because it requires tens of boreholes for reasonable effectivity. mine thermal energy storage and aquifer thermal energy storage technologies are already efficient with only two wells. Once the systems are up and running, the only operational cost is running the pumps for heat extraction, which is not particularly costly. The return on investment for these systems cannot be estimated at present. It depends heavily on the development of fossil fuel prices. 

Each technology has different risks and advantages. The biggest challenge for the borehole thermal energy storage system is drilling the boreholes as close as possible to each other without intersecting any of the boreholes. The main advantage is its flexibility (can be implemented almost in any rock environment) and closed system operation (no issues related to mineral precipitation, no water exchange between the boreholes and rock environment). 

aquifer thermal energy storage, on the other hand, requires specific geological conditions for implementation, because it is an open system. The target horizon has to be porous enough, insulated from above and located significantly deep below any drinking water sources to avoid any potential contamination. The advantage is that aquifer thermal energy storage has a high efficiency already in the first year of operation (can yield more heat and higher temperature compared to borehole thermal energy storage). 

The major technical challenge for mine thermal energy storage is precise targeting of the borehole into the flooded shaft. Potential leak of stored water into another shaft is also a risk. Benefits are similar to aquifer thermal energy storage, however an additional benefit is that old mining areas are usually less sensitive to environmental issues. 

Answered by Dr. Martin Bloemendal, Principal Investigator for PUSH-IT, Delft: 

aquifer thermal energy storage is commonly applied in aquifers at 50-300m but in principle can be applied at any depth, but deeper is usually not financially efficient. The main limitations relate to permeability of the aquifer, and the presence of a sealing overburden. There are no safety risks related to aquifer thermal energy storage. Only when drilling the borehole may collapse, but that is quite exceptional to happen. Once installed there are no safety risks. 

Answered by Thomas Olver, Project and Research Coordinator, Geothermal Engineering Ltd, Cornwall, UK:  

Challenges may vary between specific mines, dependent on what materials were extracted, to what extent, the age and current condition the mine and local geology, among many other factors. 

At the United Downs site in Cornwall, broadly, challenges included gaining an understanding of the circulation and flow of water within the workings, so that stored heat is not lost during operation, overcoming issues related to water quality in mine workings to minimise environmental risk and the risk of scaling and corrosion of underground thermal energy storage technologies installed, the development of a sound understanding of present day mine structures and condition as this has likely changed since plans were drawn, and maintaining hydrological conditions in the mine to ensure integrity of the subsurface voids are maintained. 

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University: 

Yes! Underground storage means we can disconnect supply and demand, meaning that we can more easily use locally produced energy and reduce the need for fossil fuel imports. 

Answered by Andrés González Quirós, Senior Geothermal Scientist, British Geological Survey: 

Because of the variety of technologies included in underground thermal energy storage there is also a difference on how these different aspects are considered. Systems that use and exchange not only heat but also other substance (in general water) with the subsurface generally require more comprehensive management and monitoring programs to avoid any environmental impacts or any affections to other users of the subsurface. The legal requirements vary not only by technology but also depending on the country or even region or city. Competent authorities try to adapt the legislation and monitoring requirements to their jurisdictions and have powers to enforce the law and apply sanctions. AI will probably have an increasing role to facilitate operational oversight in the future, but its current use is not widespread.  

Answered by Phil Vardon, Professor of Energy Geo-Mechanics at Delft University:  

Large systems are more efficient, therefore district heating with storage will increase sustainability. 

Answered by Elke Mugova and Stefan Klein, Fraunhofer, Bochum mine thermal energy storage site, Germany

In the case of mine thermal energy storage, the amount of energy underground heat storage can provide depends on the size of the flooded mine and the available excess heat. For example, the pilot project in Bochum is designed to store about 12.5 GWh per year, enough to supply thousands of homes. These systems are built to last at least 30 years, with most components designed for 30–50 years. They offer a constant heat source that can help balance supply fluctuations, but during a full electricity blackout, heat can be stored underground but cannot be pumped in or out. There is plenty of surplus heat available, especially from industrial cooling processes, which makes storing it underground a practical and efficient alternative to wasting it.

Answered by the PUSH-IT team

Heat is carried by water (in the case of BTES when also cooling is supplied sometimes added with an antifreeze such as glycol) through insulated pipes. Pumps circulate this fluid between the underground storage site and the heating network. For local distribution, this is very efficient. Sending heat over long distances is possible but less practical because it needs bigger pipes, more insulation, and extra pumping energy, which adds cost and heat loss, but this is done in conditions where there are for example large industrial areas with a lot of waste heat, to cities several 10’s of KM away.

Answered by Elke Mugova and Stefan Klein, Fraunhofer, Bochum mine thermal energy storage site, Germany

To keep heat from escaping in underground storage, the system must be well sealed and not connected to a mine dewatering/pumping system. In the case of mine thermal energy storage, even in ideal conditions, about 20% of the heat slowly transfers into the surrounding rock over time, based on models from other projects. This is monitored using sensors in boreholes or shafts, such as fibre optics. Mines can also store heat and cold in separate depth layers, with

large distances between those reservoirs, which reduces losses compared to conventional storage systems like pit thermal energy storage (PTES).

Answered by Elke Mugova and Stefan Klein, Fraunhofer, Bochum mine thermal energy storage site, Germany

In the case of underground thermal energy storage (UTES) systems that use flooded mine spaces, they can vary greatly in size. For example, the pilot project in Bochum uses about 10,000 m³, but future systems could be much larger, which improves economic feasibility. These mines are typically 100 to 1,000 meters deep, with low operating pressures (under 10 bar) in shallow mines. Natural geothermal heat has only a small influence in shallow depth; however, at 1,000 meters depth, temperatures are around 30 °C. During operation, heat spreads only a few meters into the surrounding rock. Conditions also depend on the type of mine, such as coal (Bochum) or metal ore (Cornwall). To keep the system sealed, conventional cementing methods are used in boreholes to prevent leaks.

Answered by Elke Mugova and Stefan Klein, Fraunhofer, Bochum mine thermal energy storage site, Germany

Pipes and materials used in underground thermal energy storage must withstand contact with mine water, so stainless steel (such as 316L AISI) or composite materials are recommended for the sections running from the pump to the surface. Drilling is done with a heavy rig, typically around 30 tons, supported by a mobile crane, and for shallow depths, like the 120 m borehole in Bochum, directional drilling wasn’t needed. For deeper or more complex sites, directional drilling may be required to accurately reach the mine workings.