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Glossary · Energy industry & energy transition

Pit Thermal Energy Storage

pit thermal energy storage thermal storage district heating seasonal storage solar thermal energy heat transition

Definition and delimitation

A pit thermal energy storage is a large-volume heat store. It is built as an artificially excavated pit in the ground, lined to be watertight and covered with insulation. The pit usually has the shape of an inverted, truncated pyramid with sloping embankments. It is generally filled with water, more rarely with a gravel-water mixture. The store absorbs heat and later releases it to a heating network or an industrial application. In technical circles it is known internationally as Pit Thermal Energy Storage (PTES). In German, the terms Erdbecken-Wärmespeicher and Erdbeckenwärmespeicher are also common.

The store contains thermal energy. It is stated in megawatt hours or gigawatt hours thermal. This must be distinguished from power: it indicates how quickly heat can be charged into or discharged from storage. A store holding many gigawatt hours cannot automatically cover high heat loads at short notice. For that, heat exchangers, pumps, pipelines and network temperatures must be designed accordingly. A pit thermal energy storage is therefore always assessed in terms of the interplay of volume, temperature range, charging and discharging power, heat losses and integration into the district heating network.

Design and mode of operation

A pit thermal energy storage consists of four parts:

  • The excavated pit. The excavated soil is often placed back as an embankment around the pit.

  • A lining made of plastic sealing membranes, usually of high-density polyethylene (HDPE) or polypropylene (PP).

  • A floating, insulated cover. It is the most demanding part in terms of construction, because otherwise a great deal of heat would be lost across the water surface.

  • Inlet and outlet diffusers at different heights. They feed water in and out so slowly that the temperature stratification is maintained.

Water is a good storage medium: it is cheap, available in large quantities and has a high specific heat capacity of around 1.16 kWh per cubic metre and kelvin. A rule-of-thumb calculation shows the order of magnitude. A pit of 60,000 cubic metres used between 10 and 85 degrees Celsius holds, by calculation, about 60,000 × 1.16 × 75 kilowatt hours, i.e. around 5 gigawatt hours of heat. Because warm water is lighter, stratification forms: hot at the top, cool at the bottom. Heat for the network flow can thus be withdrawn at the top and cold return water fed in at the bottom. The greater the difference between the upper and lower temperature, the more energy the same volume holds.

The maximum temperature is limited. The decisive factor is above all how long the plastic lining withstands the heat. Typically it is at most about 80 to 95 degrees Celsius. The stores operate without pressure and therefore do not directly serve steam networks or very hot flow lines. In networks with a high flow temperature, the stored heat may have to be reheated. Conversely, the lower part of the store can be cooled further with a heat pump, which increases the usable content.

Examples in operation

The technology was tested and further developed mainly in Denmark. Stores there include Marstal (around 75,000 m³), Dronninglund (60,000 m³), Toftlund (around 70,000 m³), Gram (around 122,000 m³) and Vojens (around 200,000 m³). The store in Dronninglund is about 16 metres deep and is charged by a solar thermal field with around 37,600 square metres of collector area. A storage efficiency of more than 90 percent was reported for its operation. In winter it also serves as a heat source for a heat pump.

Vojens shows how strongly the benefit depends on the overall system. According to the project database, the pit has a volume of around 203,000 cubic metres, a depth of 15 metres and a heat content of about 12,180 megawatt hours. The storage temperature is 40 to 80 degrees Celsius, and the specific construction costs are around 24 euros per cubic metre. There, the store is part of a system of around 70,000 square metres of solar thermal, an electric boiler, gas boilers and other generators. The database also states calculated heat losses of around 5,500 megawatt hours. Even large stores thus lose noticeable amounts of heat.

New is the use as a short-term store. In Høje Taastrup in Denmark, a pit thermal energy storage of around 70,000 cubic metres went into operation in January 2023. It is not connected to a solar thermal plant but directly to the district heating network. Its storage cycle lasts about one to two weeks. According to the International Energy Agency (IEA-ES Task 39), it holds around 3,300 megawatt hours and serves several combined heat and power plants and waste incineration plants. Pit thermal energy storages are therefore not only seasonal stores. They can also balance generation and demand on a weekly rhythm.

In Germany, large pit thermal energy storages are still rare. Trade media report first plants, for example in Meldorf and in Bracht in Hesse, and plans for considerably larger stores in cities with district heating. Concrete project details should be checked with the operator in each individual case.

Difference from other heat stores

The pit thermal energy storage differs from the classic tank store made of steel or concrete mainly in size and method of construction. Tank stores are usually smaller, more expensive per cubic metre and tend to serve daily or weekly balancing. Pit stores, by contrast, reach volumes of several tens of thousands to several hundred thousand cubic metres. However, they need a lot of land, suitable soils and careful sealing against soil and groundwater.

The pit store must also be distinguished from borehole and aquifer stores. In a borehole store, heat is conducted into the rock through boreholes. The aquifer thermal energy storage uses natural water-bearing layers in the subsurface. The pit thermal energy storage, by contrast, is an artificial water reservoir with a fixed structural envelope. The IEA working group on large thermal storages treats tank, pit, borehole and aquifer stores as the four main technologies for district heating.

Role in the heating network and the power system

A pit thermal energy storage is often combined with large-scale solar thermal energy. The plants deliver a lot of heat in summer, while buildings need the most in winter. A store can close part of this gap. However, "seasonal" does not mean lossless. How much heat is lost depends on the ratio of surface area to volume, on the insulation of the cover, on the temperature level and on the storage duration. Large pits have an advantage because their volume grows faster than their surface area.

In addition, the store can absorb heat from other sources, such as data centres, waste incineration, industrial processes or wastewater treatment plants. It can decouple the operation of combined heat and power plants from heat demand and relieve peak boilers. How well this works depends on the network structure. A network with high flow temperatures places different demands than a modern low-temperature network.

For the power system, the pit thermal energy storage is important indirectly. With a store, a large-scale heat pump or an electric boiler does not have to run exactly when the network needs heat. They can orient themselves more strongly to electricity prices, the situation in the power grid or the supply of wind and solar power. This creates flexibility on the heat side. There is a limit, however: a pit thermal energy storage does not store electricity. It stores heat that may have been generated using electricity. Depending on its seasonal performance factor, a heat pump turns one kilowatt hour of electricity into several kilowatt hours of heat. An electrode boiler produces only about one kilowatt hour of heat per kilowatt hour of electricity. The store does not return electricity without further conversion. Unlike a battery storage or a pumped storage plant, it is therefore not an electricity store. It can shift load peaks, but replaces neither power grids nor firm capacity.

Site, law and economic viability

The site is a decisive factor. A pit thermal energy storage needs a lot of land close to the heating network or to an economically developable pipeline route. Soil and groundwater must be suitable. The groundwater must not be influenced in an uncontrolled way, either thermally or in terms of substances. The lining must withstand temperature changes, water chemistry and mechanical stress for decades. On covers with a layer of water, the area can hardly be used in any other way. Near settlements, the store therefore competes with housing, agriculture, commerce or nature conservation. Which permits are required, for example under building, water or nature conservation law, depends on the federal state and on the individual case. This must be checked for each site.

Per cubic metre, pit thermal energy storages are comparatively inexpensive. Their economic value, however, only arises within the system: through saved fuel, less peak boiler capacity, better utilisation of heat sources, lower CO₂ costs and more room for manoeuvre in operation. These benefits often accrue to different actors. The example of Høje Taastrup shows how this can be organised: there, several generators and network operators share the benefit of the store. Whether a store is worthwhile therefore also depends on how costs, risks and benefits are distributed, and on which system boundary is considered.

Assessment

A pit thermal energy storage is not a universal energy store. It is a large, unpressurised heat store for temperatures up to about 90 degrees Celsius. Its strengths are low specific costs at very large volume. It can be used over weeks or months. Its significance for the heat transition arises from the interplay of heat source, network, temperature requirement, land and operating strategy. If this is planned in, it can reduce fossil heat generation and enable electrical flexibility. If only the pit is considered, its benefit remains invisible.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:47

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