# Seasonal storage

A seasonal storage or seasonal heat storage or season storage ( English seasonal thermal energy storage (STES) ) is a long-term storage of thermal energy of seasonal heat storage heating , often for a solar installation . This means that the share of renewable energy can be increased significantly.

## General

Seasonal heat accumulators are so large that they can store heat from the warm season and thus heat or even cool well-insulated houses in the cold season to a large extent or even completely with stored energy. Mostly, a solar energy surplus from summer in winter is used for heating purposes and thus several weeks to months can be bridged depending on the weather conditions and the state of charge. Storage tanks used for heating purposes are charged in sunny weather and during summer, and discharged in bad weather and during the cold season. In the future they will play an important role in the solar infrastructure. In the case of very high degrees of solar coverage, the costs for storage for even higher values ​​rise disproportionately, since the storage size can then be used less and less optimally. The dimensioning of seasonal storage tanks ranges from a supply for a single-family house to large storage tanks for supplying residential areas with a local heating network or district heating network .

## Storage types

Different storage types:

• Container heat storage
• Earth basin heat storage, for example as gravel / water heat storage
• Geothermal heat storage, up to 100 meters deep
• Aquifer heat storage

### Container heat storage

The most common type for single and multi-family houses are hot water tanks as stratified charge storage tanks , this technology is often used in so-called solar houses . In conjunction with a large solar collector surface, high levels of solar coverage can be achieved. Due to their size, these water tanks can only be installed in new buildings , or underground tanks are used outside of buildings. In colder climates, storage tanks within the building envelope have the advantage that all heat losses from the storage tank benefit the building. The losses can be calculated using the thermal resistance of the storage tank . Depending on the size of the house, the general conditions and the goal, the storage tanks in single-family houses have around 10 to 50 cubic meters and are insulated with 20 to 40 cm thick thermal insulation. If a building is to have a larger staircase, the long-term storage can often be integrated into the staircase without a large loss of space.

Volume to surface ratio

In general, the following applies to bodies ( A / V ratio ): If you double the edge length of a cuboid , its area quadruples (in general: surface ; i.e. the interface between cold and warm); its volume but ver eight fanned herself. Large bodies therefore have a more favorable ratio of volume to surface (for heat storage) :

${\ displaystyle V_ {1} = a \ times b \ times c}$ (at the cube = ) ${\ displaystyle a \ times a \ times a}$

${\ displaystyle A_ {1} = 2 \ times a \ times b + 2 \ times b \ times c + 2 \ times a \ times c}$ (Cube: ) ${\ displaystyle A = 6 \ times a \ times a}$

${\ displaystyle V_ {2} = 2a \ times 2b \ times 2c = 2 \ times 2 \ times 2 \ times a \ times b \ times c = 8 \ times a \ times b \ times c}$

This also applies to cylinders : if you double its diameter and its height, its volume increases eightfold. Even if you double the diameter of a sphere , its volume increases eightfold. A sphere has the largest ratio of volume to surface of all geometric bodies .

### Earth basin heat storage

These are also available without significant insulation to the subsurface, only sealed with a stable film. Either with a multi-level floating insulation at the top, whereby rainwater is pumped out, or it is encased on all sides with concrete. The basin shape can be an inverted truncated cone , truncated pyramid or cuboid . Depending on the size, this rather larger storage type can supply complete solar settlements with heat. They are built either as water reservoirs or as gravel water reservoirs. The gravel water storage tank has a smaller storage capacity with the same dimensions, in contrast to pure water storage tanks, because water has a higher heat storage capacity than gravel. The temperature exchange takes place via integrated well systems or indirectly via heat exchangers .

### Geothermal heat storage

The subsurface, such as layers of rock or the earth, is heated through boreholes. The heat can be recovered via the boreholes using a heat pump as required. The holes are either vertical or inclined downwards. Not every surface is equally suitable for this and there are also completely unsuitable local conditions.

### Aquifer heat storage

Under favorable hydrogeological site conditions a so-called aquifer, so one can aquifers are used for heat storage. Sometimes two wells are used for this, which, depending on the season, either supply or store heat or cold. These two wells are offset at a certain distance.

### More types

Alternatively, a solid foundation can be used as a storage mass through thermal component activation . This is heated to around 30 ° C.

## Seasonal compensation in the power grid

In order to compensate for a so-called dark doldrums in the power grid, it would be possible to use power-to-gas to convert excess energy generated from renewable sources into hydrogen and store it. This reserve could then serve as energy storage for many purposes in the colder half of the year; this would require an expansion of the hydrogen economy and an expansion of sector coupling for improved, more closely networked conversion of energy between the areas of electricity grid, households, mobility and industry. Short-term fluctuations, on the other hand, can be compensated for with battery storage power plants and pumped storage power plants.

The power-to-gas approach for seasonal storage, in conjunction with photovoltaics , is already available for heating buildings. Surplus from the warm season of the year is used in winter with the help of a fuel cell .

## literature

• Storage processes in the vicinity of vertical geothermal probes of heat pumps. In: Heating, ventilation / air conditioning building technology (HLH) No. 1/2015, pp. 19–23
• Chapter 8.3. Seasonal heat storage. In: Solar heat for large buildings and residential areas , Fraunhofer IRB , Stuttgart, ISBN 978-3-8167-8752-5 , pp. 93–94
• Chapter 13.2.1.3. Seasonal heat storage for large solar systems. In: M. Sterner, I. Stadler (Ed.): Energy storage - Demand, Technology, Integration , 2nd edition 2017, ISBN 978-3-662-48892-8 , pp. 740-744; in the first edition of the book on pp. 677–680
• Silke Köhler, Frank Kabus, Ernst Huenges: Heat on demand: Seasonal storage of thermal energy. In: T. Bührke, R. Wengenmayr (Ed.): Renewable Energy: Concepts for the Energiewende , 3rd edition, Wiley-VCH Verlag GmbH & Co. KGaA, ISBN 978-3-527-41108-5 , p. 133 -139
• Jens-Peter Meyer: Seasonal storage: Priority for the sun. In: Sonne Wind & Wärme No. 4/2018, pp. 69–71
• M. Schmuck: Economic feasibility of seasonal heat storage , expert Verlag, 2017, ISBN 978-3-8169-3398-4
• H. Weik: Expert Praxislexikon: Solar energy and solar technologies , 2nd revised edition from 2006, expert Verlag, ISBN 978-3-8169-2538-5 :
• Aquifer storage , p. 16
• Basin heat storage , p. 98
• Long-term storage , pp. 175–176
• TZS: Seasonal heat storage. In: Solare Wärme: Das Solarthermie-Jahrbuch 2019 , publisher: Solar Promotion GmbH, from February 27, 2019, p. 112