Other means of heat storage

Phase-change storage

When more heat should be stored in a small volume than is possible with sensible heat storage, phase-change materials offer one solution. The change of phase can be a melting or a vaporization process. Melting processes have energy densities of 100 kWh/m2 compared to 25 kWh/m2 for sensible heat storage. Vaporization processes are combined with a sorption process. Energy has to be withdrawn at a low temperature when charging and be delivered at a high temperature when discharging the storage. Energy densities of 300 kWh/m2 can be achieved.

Reversible chemical reaction storage

The physical principle of the sorption process is illustrated in Figure 13.1.5. The basic principle is: AB + heat ^ A + B. Using heat, a compound AB is broken into components A and B, which can be stored separately. Bringing A and B together, AB is formed and heat is released.

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.5 An example of a reversible chemical storage process

Non-reversible chemical reaction storage

This means of energy storage is superior to all the above means. The classic example is petroleum. Solar energy has been stored in plant material, which decomposed into oil, and now millions of years later, with no energy loss during the storage, 10,000 kWh/m3 can be released by simply burning it -an incredible energy density. The only small problem is that the rate of use exceeds the rate of geological production, given the millions of years' time span!

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