The Fraunhofer Institute for Solar Energy Systems (Freiburg/Breisgau, Germany) and the company UFE Solar GmbH (Berlin, Germany) started with preliminary tests and first investigations concerning seasonal heat storage based on the adsorption process in 1995. Research work was continued with financial aid of the European Commission and the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) in the project HYDES (High Energy Density Sorption Heat Storage for Solar Space Heating). In this project, the principle technical feasibility of the sorption storage system was proved. The experience gained during this project will be of use in the frame of the new project MODESTORE which is also funded by the European Commission and on the Austrian national level by BMVIT. The work in this project started in April 2003 and will be continued for three years.

Basic Principles of an Adsorption Heat Storage System

In sensible and latent heat storage devices heat is stored together with its
corresponding amount of entropy. In these so-called direct heat storage media, heat

— i. e. energy — is transferred directly to the storage medium. The achievable energy density is limited by the entropy storage capacity of the material. Otherwise the adsorption process is a reversible physico-chemical reaction suitable to store heat in an indirect way. This kind of thermal storage allows to separate energy and entropy flow. The storage capacity is not limited to the maximum of entropy intake. The energy density can be much higher if entropy is not stored directly in the medium. Therefore a heat source and a heat sink is involved both during the charging and discharging process to withdraw or collect the necessary entropy. The storage works like a heat transformer on the principle of a chemical heat pump. During adsorption of water vapour, a phase chance takes place between vapour and liquid phase on the surface of the in this project used silica gel. The released adsorption enthalpy consists of the evaporation enthalpy of the working fluid and the binding energy of the adsorption pair.

The working principle involves several different phases illustrated in figure 1 and described below:

1. Charging process (desorption, drying of silicagel): heat from a high temperature source is fed into the device, heats the silica gel and vapour is desorbed from the porous solid. The desorbed vapour is led to the condenser and condensed at a lower temperature level. The heat of condensation has to be withdrawn to the environment.

2. Storage period: the dry adsorbent is separated from the liquid working fluid (the connecting valve is closed). As long as these components stay separate heat storage without losses is possible if the sensible heat involved is neglected.

3.

Discharging process (adsorption, loading of silica gel with water vapour): the valve between the evaporator and the adsorber is opened. The liquid working fluid evaporates in the evaporator taking up heat at a low temperature level. The vapour is adsorbed and releases the adsorption heat at a higher temperature level. This is the useful heat.

Figure 1: The working principle of an adsorption heat storage.

There are several quantities and process parameters important when the potential energy density of a sorption pair for heat storage applications is evaluated. The main ones are:

1. Temperature lift: it depends on the current loading level of the sorbent and is a material property.

2. Adsorption enthalpy: it consists of the evaporation enthalpy of the working fluid and the binding energy of the adsorption pair. A high specific evaporation enthalpy is a must for high energy densities, therefore water is one of the primary candidates.

3. Sensible heat and process management: an intelligent system design and process management along with good insulation is essential.

4. Energy density: the energy per unit volume is the quantity of primary interest. It is the product of specific energy (energy per mass of sorbent) and the bulk density ps.

After due consideration, the process of thermo-chemical heat storage with the adsorption pair silica gel and water was selected. Silica gel is a very porous and vitreous substance. The material is made up mainly of SiO2 and is extracted from aqueous silicic acid. The equipment installed in the laboratory of AEE-INTEC in Gleisdorf/Austria is filled with commercial silica gel GRACE 127 B. This silica gel consists of spherical particles with a diameter of two to three millimetres. Its bulk density is 790 kg/m3, the interior surface is 650 m2/g. The high energy density, the quantity of primary interest, is achieved by a high evaporation enthalpy, the polarity of water and the large interior surface of silica gel. Additional components like heat exchangers reduce the energy density if the whole system is considered. The system is evacuated to enable water vapour transport without use of mechanical energy. The vapour pressure add up to 10 to 50 mbar in the system.