High Storage Capacity

First pilot projects [3], [5] in Germany reached storage capacities of about 130 kWh/m3. That means such a TES can store about 2-3 times more thermal energy than hot water storages. If higher temperature around 200 °C would be available, Zeolite storages could reach capacities of about 250 kWh/m3. However it has to be stated that a sorption TES consist of more components that the storage container itself, e. g. heat exchangers. In closed systems the desorbed water has to be kept inside the system, which reduces the storage capacity as well [2], [3]. Open systems need in most cases a humidifier for discharging. These additional components are usually not taken into account, which makes it difficult to compare different TES technologies.

Heating and Air-Conditioning

Open and closed sorption storages are able to provide thermal energy for heating purposes as well as for air conditioning of buildings. For this application cold can be delivered from the evaporator of a closed system. Using an open system the air dried by adsorption will be humidified, which leads to low temperatures (“desiccant cooling”).

The achievable Coefficients of Performance COPth are usually between 0.3 and 0.8. The open Zeolite system in Munich [5] reached a value COPth = 0.87 at a charging temperature of 80 °C in the experiments. The storage capacity was in the range of 100 kWh/m3.

In the case of liquid absorption systems only air conditioning is possible. Such an open storage system for solar air conditioning was installed in Singapore by the company L-DCS [8]. In the current demonstration project L-DCS Technology supplied a liquid desiccant air de-humidification system (11,000 m3/h) for a factory unit in Singapore, owned by JTC Corporation. A 550 m2 flat plate solar collector array drives the desiccant regeneration and 12 m3 desiccant energy storage covers the difference between the energy need for air conditioning by absorption and the solar energy supply for regeneration.

3. Economical Limits

The invest costs of sorption storage systems are quiet high. Therefore the number of operating hours, hence storage cycles, and the amount of thermal energy provided from the storage per time has to be high as well. This makes the operation as a seasonal storage system, used for 1 cycle per year, from the economical point of view impossible.

However a higher number of storage cycles by applying the system for heating and cooling purposes can lead to economical advantages. Furthermore the higher prices for air conditioning compared to plain heating could shorten the pay back time for such installations. The Zeolite storage in Munich reached a 50 % reduction of the payback time from around 14 years (for heating only) to about 7 years (for heating in winter and air conditioning in summer) [5].

4. Conclusion

Sorption storages are due to their thermodynamic possibilities — high storage capacity and variable temperatures — very interesting for solar thermal applications. How ever for economical reasons seasonal storage of solar heat by such systems is not an option. Most of the installed demonstration plants are looking for many storage cycles per year. This can be achieved by using the system for heating and cooling or by using smaller sorption systems in heat pump applications

[3].

With respect to solar applications sorption systems for solar air conditioning are most suitable, because for a high solar fraction the integrated storage effect is crucial.

The paper concludes with the remark that even the high storage capacities and the possibility of providing heat and cold of sorption storages does not solve all solar thermal storage problems. It is still necessary to find an appropriate application and to carefully check the relevant boundary conditions.

5. Literature

[1] R. Sizmann, Speicherung thermischer Energie — Eine Ubersicht, BMFT Statusseminar “Thermische Energiespeicherung” Stuttgart, 1989.

[2] D. Jaehnig, Thermo-Chemical Storage for Solar Space Heating in a Single-Family House, Proceedings of the International Conference on Thermal Energy Storage, Ecostock 2006, Stockton, New Jersey, USA, May 31 — July 2 2006.

[3] Thomas Nunez, A Small Capacity Adsorption System in a Heating and Cooling Application: The German Field-Test in the MODESTORE Project, International Conference Solar Air-Conditioning, Kloster Banz, Bad Staffelstein, Germany, October 6th/7th, 2005

[4] ZeoTech GmbH, Internet: http://www. zeo-tech. de/

[5] A. Hauer, Thermal Energy Storage with Zeolite for Heating and Cooling, Proceedings of the 7th International Sorption Heat Pump Conference ISHPC ’02, Shanghai, China, 24.-27. September 2002.

[6] H. Kerskes, K. Sommer, H. Muller-Steinhagen, An Effective Application of an Open Adsorption, Process for Solar Thermal Heat Storage, Proceedings of the EuroSun 2006, Glasgow, UK, June 27-30, 2006. [21] [22]