Solar Energy System of the Month:

Summer Sun all Winter: Seasonal Heat Reservoir
covers heating needs up to 100 %

A heat reservoir which almost completely bridges the gap between the overabundance of solar heat in summer and the need for warmth in winter was awarded the Innovation Prize Berlin/Brandenburg 1999.

The "Sorption Reservoir", an absolutely new invention, was developed over several years by the UFE SOLAR company in Eberswalde, Germany, together with the Fraunhofer Institute for Solar Energy Systems (ISE).

Innovation Prize Berlin / Brandenburg 1999

Innovation Prize Berlin / Brandenburg

Until now, the storage of summer solar energy for heating in winter was only efficient in large volume: for example, when an entire neighborhood was heated with a local solar array. In such installations, water is heated by the sun in summer and stored in underground tanks for use in winter. Only the great volumes in the reservoirs allow an efficient ratio of heat storage to heat loss.

Sorption Reservoir

The prizewinning sorption reservoir, developed by UFE SOLAR and the Fraunhofer ISE, Freiburg

The thermochemical heat storage, upon which the new "heat vault" is based, uses the heat transformation of reversible chemical reactions. The reservoir is charged with a supply of solar heat. The warmth starts a chemical reaction, the products of which are then separated inside the tank. The physical separation of the chemical components prevents the usual reverse reaction during cooling: the stored heat can be released at any chosen time.

Sorption Reservoir

The so-called sorption processes, suited for thermochemical heat storage, are low-temperature reactions: a gas is either fixed to a solid, porous substance (adsorption) or is taken up by a fluid or a solid (absorption). This reaction frees heat; adding heat reverses the process, freeing the gas (desorption). Steam is the gas best suited for heat storage, as it allows the most efficient transformation of chemical energy in the form of heat (the most efficient change in enthalpy, measured in joules).
Charging of the reservoir with the solar collectors: in a closed system, the sorption material (silicate=silicic acid) is dried with the heat provided by the solar energy collecting system. The resulting steam is condensed and stored separately from the desorbed or dried material (silica gel).

Discharging of the heat reservoir: before the reverse reaction, the water is transformed back into steam and taken up by the dry sorption material. Warmth is thus freed which can be used for space heating.
The advantages of this process lie in the substantially higher energy density of 200-300 kWh/m3 (water: 58 kWh/m3) and the low thermal energy loss: in contrast to chemically stored energy, this is only somewhat above 10%.

Sketch of a sorptive heat reservoir system
Sketch of a sorptive heat reservoir system. Graphic: modified from Fraunhofer ISE

Solar Space Heating

The solar system is used for charging the reservoir in summer (desorption) as well as discharging in winter (adsorption). The warmth freed by the adsorption is used for heat. A reservoir is a series of industrially produced, closed and evacuated containers, which are connected to the solar circulation system and the heating system. Each of these modules contains the sorption material, the heat exchangers, and a chamber for the condensate.

System concept of the sorption reservoir for solar heating
System concept of the sorption reservoir for solar heating. Graphic: modified from Fraunhofer ISE

System Simulations

Simulations programs of the Fraunhofer ISE predicted that a low-energy house located in Freiburg, Germany, could fully cover an annual heating requirement of 4000 kWh with a collector surface of 30-35 m2 and a reservoir volume of 10 - 12m3. This is an absolute breakthrough in the use of inexhaustible solar energy. The prizewinning sorption reservoir of UFE SOLAR, which stores summer's heat nearly without loss for many months, enables independence from oil, gas, and coal, and clears the way to 100% solar heating.

A prototype is being built, and the reservoir should be ready for serial production in mid-2001. The practical tests will be undertaken in the solar settlement "Uckermark". In this reference project, 5 houses will be equipped with UFE SOLAR collectors and reservoirs. Exact details as to the costs of the system are not yet available, but UFE SOLAR intends that an installed system should pay for itself within 15 years.

System concept of the sorption reservoir. Rate of coverage
System concept of the sorption reservoir. Rate of coverage. Graphic: modified from Fraunhofer ISE

On the way to 100% solar heating

The Innovation Prize inspires the technology company's work toward 100% coverage of heating needs through solar energy. "We are very happy about this recognition," declared Reinhold Weiser, business manager for UFE SOLAR. "The Innovation Prize will help to introduce the novel solar technology to a wider public audience." The company also pursues this goal with an EXPO 2000 project which is based in great part on the sorption reservoir. Hermann Scheer, winner of the Alternative Nobel Prize 1999, is perhaps the most prominent supporter of the Eberswalde company: he is the sponsor for the EXPO 2000 project.

A scientific article about the development of the sorption reservoir can be called up from UFE SOLAR's website under the URL:

Graphics and material: Fraunhofer ISE, Freiburg and UFE SOLAR, Eberswalde
Editor: Rolf Hug.
Translator: Genevieve Cory.

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