Thermal energy storage boosts solarthermal power plant efficiency
The availability of energy storage systems is essential for increased market penetration of solarthermal power plants. This kind of power plant uses concentrated solar radiation as the heat source to drive turbines (see Figure 1). It offers the option of directly integrating backup energy sources to compensate for fluctuations in solar insolation. This reduces the requirements placed on the electricity network and external backup capacity. Today's solarthermal power plants mostly use natural gas during low solar insolation. However, thermal energy storage systems should be integrated in future plants to achieve better matching between the availability of solar energy and demand for electricity. The main technical requirement for these storage systems is that they must work with the maximum operating temperature of about 390°C and the electrical output of solarthermal power plants, which is normally in the range between 50 and 80MW.
Two different types of storage systems are currently under investigation. Buffer storage is used to provide energy within a short reaction time. The other type, medium-term storage, provides heat over several hours. These two types of storage complement one another. Buffer storage systems have a small capacity but are able to reach a high output within a short time period. In contrast, medium-term storage systems demand a longer reaction time but show lower-capacity specific costs. Steam accumulators1 (also called Ruths storage systems) represent an option for buffer storage. In these systems, pressurized water is used as storage medium. During the charging cycle, steam is fed into the water volume where energy is directly transferred by condensation. The steam accumulator is depressurized during the discharge process and provides saturated steam. Water is used both as a working medium and storage medium, so steam accumulators show very short reaction times. The storage capacity depends on the temperature variations of the liquid volume.2 During the discharge cycle, the decrease of the water temperature also causes a decrease in pressure of the steam provided. This means that constant-pressure operation using steam accumulators is not possible. The costs for the pressure vessel limit the size of steam accumulator systems. The correlation between saturation temperature and saturation pressure is logarithmic. Thus, a high operating pressure requires a more significant pressure decrease of the steam delivered during the discharge process.
Steam accumulators can only provide saturated steam. This, combined with cost aspects, means that alternative storage systems are necessary to cover the complete temperature range relevant for solarthermal power plants. Experience with storage systems in the temperature range up to 400°C is limited, and most systems use a liquid-storage medium like thermal oil or molten salt. Both of these solutions have high specific costs. There are also environmental aspects for the thermal oil approach, while the molten salt approach has the risk of freezing at temperatures between 140 and 230°C. A promising candidate material for sensible heat storage is concrete with an embedded heat exchanger.3,4 To optimize costs, the embedded heat exchanger design must be adapted to the demands of the power cycle and consider the heat transport properties of the concrete. The German Aerospace Center (DLR) has demonstrated the feasibility of solid-media storage systems operated by parabolic trough collectors at Almeria, Spain. For large-scale installations, the specific investment costs related to stored heat are estimated at between 20 and 30€/kWh.
Sensible heat storage in concrete is an attractive option for processes using single-phase working fluids like thermal oil or air. However, the application of steam as a working medium requires the availability of isothermal storage if charging/discharging should take place at constant pressure. An obvious solution is the application of phase-change materials (PCMs) using the latent heat released or absorbed during a change of state of aggregation. Because of the temperature range resulting from the operating pressure of solarthermal power plants (20–100bar), nitrate salts with melting points between 220 and 320°C can be applied as PCMs.
Nitrate salts show low thermal conductivities of less than 1W/mK. For this reason, a concept that can compensate for the poor heat-transport properties of the storage material is a requirement for the implementation of commercial PCM storage systems. Basically, there are two approaches. One option is that the average distance for heat transfer within the material can be reduced by increasing the heat transfer area of the heat exchanger. The other approach is to increase the effective heat conductivity of the storage material by adding a material that exhibits good heat-transport properties.
In order to accelerate the development of commercial PCM storage systems, we work on both of these approaches in parallel. By adding expanded graphite, a composite material can be created that shows thermal heat conductivities allowing sufficient power densities in thermal storage systems. The average distance for heat transfer within the PCM is reduced by encapsulating the storage material (see Figure 3) or by integrating finlike structures into the storage volume. Fins made of graphite foil have proved to be a promising technique to reach high-volume specific power densities (see Figure 4). The recently developed storage systems are also attractive for applications outside the solar field. Moreover, with the increasing costs of fossil fuels, this technology represents an additional option for improved energy management in the process heat industry. The results of our research activities are currently being transferred into this area.
