Thermal storage is very important for achieving high solar shares even in hybrid solar/fossil process heat installations. Only very few applications fit so well to the solar resource that the need for storing solar heat might be negligible. In most cases several hours of full load storage operation are on the wish list of solar project engineers. Available storage technologies are discussed in detail in Chapter 11. They can be divided into sensible, latent and chemical heat storage technologies, which then have different pros and cons for different storage capacities and temperature levels. For low temperatures (up to 100°C), simple single water tanks are a cost-efficient storage solution, which are available in a large variety of sizes and design. Above 100°C, water storage needs to be pressurized, which adds substantially to the cost. This is why at a certain point other sensible storage materials like thermal oil become interesting. Sensible heat stores similar to those used in CSP plants (with a hot and a cold tank) are also an option when using pressurized water or synthetic oil as HTF.
Steam accumulators are conventional practice for short duration storage of solar steam. However, a substantial storage capacity for full load operating quickly becomes prohibitively expensive with steam accumulators. Latent (phase change) or chemical heat stores are options under development for storing heat from solar steam. A number of material combinations are available for different operating temperatures. Especially in the temperature regime below 300°C, some seem to be close to commercialization (see Tamme et al., 2008).
A favourable approach for cost-efficient system dimensioning during the present early phase of market development is the so-called fuel saver design, which limits the peak capacity of the solar system such that it just about meets the minimum continuous heat demand of the process. This will minimize shutdown periods of the solar system and also avoids the need for high storage capacities and consequently leads to minimum payback periods. Of course the flip side of this approach is that solar share is rather limited.