Thermochemical heat storage

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Thermochemical heat storage systems store heat through endothermic reactions and release it again through exothermic reactions .

An example of a thermochemical heat storage is the sorption storage: A tank contains granules made of silica gel , which are hygroscopic and highly porous and therefore have a large internal surface (one gram has an internal surface of around 600 m²). Silica gels have the property of attracting water vapor and attaching it to their surface ( adsorption ), releasing heat. Conversely, thermal energy must be used to dry silica gels ( desorption ) .

The silica gel is in granulate form in a boiler with a heat exchanger . The silica gel is dried with energy expenditure in summer and it is under high internal tension . In winter it is ventilated a little and gradually brought up to the water vapor partial pressure of the surroundings. The resulting heat is used.

The advantage of thermochemical heat storage systems compared to conventional heat storage systems in the form of a water tank is their higher storage density of 200 to 300 kilowatt hours per cubic meter compared to only around 60 kWh / m³ for water. In addition, the energy can be stored for years without loss.

In addition to silica gels, metal hydrides or zeolites can also be used as heat stores, but these require higher operating temperatures.

Another method is the solar thermal reduction of metal oxides to metal and oxygen. The metal, such as zinc, can usually be stored and transported without any problems. In a second step, the metal reacts in an exothermic reaction at approx. 350 ° C with water to form the metal oxide, releasing hydrogen, which in turn can be burned to generate energy. One process under development is the Solzinc process .

facts and figures

Heat capacity:

  • approx. 250 kWh / m³ (practically achievable in pilot plants: approx. 135 kWh / m³)

Working temperature:

  • Metal hydrides: 280-500 ° C
  • Silica gel: approx. 40-100 ° C
  • Zeolites: approx. 130-300 ° C

Applications

Most applications for thermochemical heat storage are still in development. However, a wide range of areas of application can be identified:

  • Soda locomotive : a fire-free steam storage locomotive developed in 1883, which did not catch on.
  • Seasonal storage of solar heat : Summer heat is captured using solar thermal energy and used for heating rooms and hot water in winter.
  • Load balancing in district heating networks : In times of low load - at night and on weekends - the storage tank absorbs district heating , at peak load times it supplies the heat instead of the district heating network.
  • Dehumidification in indoor swimming pools : the storage tank absorbs moisture from the indoor air and simultaneously heats the air; Excess heat from a CHP regenerates the storage tank.
  • Energy-saving drying in dishwashers : While the rinse water is heating up, the storage tank is heated and releases moisture . When the dishes are dried later, the storage unit absorbs moisture again, releases heat, accelerates the drying process and reduces the energy required for this.
  • Cold for air conditioning : the sorption storage unit adsorbs water under negative pressure ; the remaining water cools down due to the enthalpy of evaporation removed . The sorption heat can also be used.
  • Adsorption heat pump : The storage tank serves as a short-term buffer for solar heat and is regenerated using a natural gas burner.
  • Self-cooling beer keg : The heat storage unit extracts heat from the beer at the point of use. The regeneration takes place by heating up in the brewery
  • Solar refrigerator : Similar principle to the self-cooling beer barrel. The heat accumulator is regenerated by solar thermal energy and then connected to a refrigerator. In this way, drugs can be cooled in remote areas of the Third World , for example .

The investments required for thermochemical heat storage are currently (as of 2007) still high. Accordingly, more research is being carried out into storage systems that store heat for hours rather than months. The aim is to use the memory more frequently and thus to multiply the operating cost advantage of the individual storage process.

See also

literature

  • Chapter 10.6 Thermochemical energy storage. In: M. Sterner, I. Stadler: Energy Storage - Demand, Technology, Integration , Springer-Vieweg, 2nd edition 2017, ISBN 978-3-662-48892-8 , pp. 610–616; in the first edition of the book pp. 565–571
  • Chapter 9.2.4 Thermochemical heat storage. In: M. Schmidt: On the way to the zero emission building , Springer-Vieweg, Wiesbaden 2013, ISBN 978-3-8348-1746-4 , pp. 322–323
  • Chapter 6. Sorption Heat Storage. In: Solar Energy Storage , Elsevier Academic Press, 2015, ISBN 978-0-12-409540-3 , pp. 135-154
  • Chapter 4.5.3 Thermochemical storage materials. In: Wärmespeicher , 5th revised edition, ISBN 978-3-8167-8366-4 , pp. 56–58
  • thermochemical heat storage. In: H. Weik: Expert Praxislexikon: Solar energy and solar technologies , 2nd revised edition from 2006, expert Verlag, ISBN 978-3-8169-2538-5 , p. 326

Web links

Individual evidence

  1. ↑ SOLZINC: PSI technology for an EU pilot project, by Ulrich Frommherz, Stefan Kräupl, Robert Palumbo, Aldo Steinfeld, Christian Wieckert (PDF; 743 kB) www.pre.ethz.ch. Retrieved on June 22, 2009.  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. @1@ 2Template: Dead Link / www.pre.ethz.ch  
  2. ↑ Zeolite gas heat pump  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (accessed January 17, 2013; PDF; 1.7 MB)@1@ 2Template: Dead Link / www.erdgas.info