Thermal Energy Storage
Supercritical CO2 cycles can be used effectively for large-scale energy storage. There are many energy storage technologies based on different physical principles, that are developed or deployed to different degrees. This section will focus on the principle of Thermal Energy Storage (TES), which involves the storage of energy as heat. This method is not only a suitable application for sCO2 cycles but also has several other advantages in comparison to other energy storage systems (i.e., battery storage). The technology allows large capacities (in the range of several GWh) and power outputs (in the range of hundreds of MW) to be achieved, and its deployment can make a significant contribution to the stability of the transmission system and provide an increase in the utilization of renewable energy sources in the energy mix.
The central component of a TES system is the thermal storage tank. It usually consists of a container filled with a material, that can take various forms, that is able to store energy. The choice of the energy storage material effects the system layout and operational parameters. The tank can be charged using waste heat with a high heat potential from various industrial processes or using excess electricity from renewable sources or directly from the grid. Electricity can be transformed into heat with a very high level of efficiency. Thermal energy is stored in the tank and later on can be used in various ways. In the case of large-scale and high-temperature storage, however, the only practical way in which the stored heat can be used is through a thermodynamic cycle. The efficiency of the cycle strongly influences the overall efficiency of the storage system, the so-called Round Trip Efficiency (RTE).
Basic characteristics of TES systems
The main parameters used to evaluate energy storage technologies are capacity, charging and discharging power, storage time, and storage efficiency. Other parameters include investment and operating costs, flexibility, durability, material availability, and placement options. These parameters indicate the technical and economic feasibility of the equipment.
In TES systems, practically unlimited capacity and power can be achieved. The largest units currently in operation have a capacity of over 1 GWht and power in the range of hundreds of MWt. The storage units are modular. Charge and discharge time is determined by the capacity and power of the unit and typical TES systems have values in the range of tens to hundreds of hours. Therefore, the equipment can be operated within daily cycles. The storage time is determined by the thermal losses of the storage tank and can be improved with thermal insulation. Thermal energy can be stored for several days to weeks with a relatively small impact on the RTE. A disadvantage of TES systems are their relatively low efficiency (RTE) when used for heat-to-electricity conversion. This is mainly due to the efficiency of the discharge part of the system, the conversion thermodynamic cycle. Therefore, a suitable solution would be the connection of the heat store to a system with highly efficient conversion cycles in combination with an intensification of the operating parameters.
The technological requirements for a TES system are relatively simple. In addition, it is possible to use currently available environmentally friendly materials. For this reason, in comparison to other storage technologies, the investment costs per unit capacity and power are expected to be lower. Another advantage of TES systems is that they have a relatively small construction footprint and can be placed in almost any location.
Different configurations for TES systems
The configuration and parameters of a TES (thermal energy storage) system depend on the choice of storage material. Storage materials can be classified according to their state, i.e., solid or liquid. There are pilot units in operation that use solid storage materials such as gravel, metal and ceramic particles or blocks of different sizes, such as those used by Siemens Gamesa. The advantage of using solid storage materials is the low cost, good compatibility with other construction materials, and they can be heated to high storage temperatures. However, this concept requires a heat transfer media (such as air) to transfer or extract heat from the storage material, which increases the energy consumption of the system and limits its performance due to the size of some of the components. Thus, this technology is more suitable for large capacities but with lower power outputs, in the MW range.
TES systems can also use liquid storage materials, such as mineral oil or melted salt. In high-temperature storage applications, salt-based storage systems are highly developed and widely used, especially in combination with CSP (Concentrated Solar Power) units. Salt based systems are configured around the so-called double-tank arrangement, where cold salt is transferred into a hot tank via a heat source (heat exchanger) during the charging cycle. During the discharge cycle, the process is reversed, with hot salt transferring heat to the media in the conversion cycle. Currently, conventional steam cycles are mostly used to convert thermal energy into electricity, but there are projects to replace them with Brayton sCO2 cycles. This option is promising because the current most widely used storage material, nitrate-based salts (also known as solar salt), allow for operation at up to 565°C. However, the operational disadvantage is the lower temperature limit set by the melting point of the salts (223°C).
Another type of storage media is a phase change material. In this case the storage system primarily utilizes latent heat. Such systems have the potential for higher storage densities, that leads to smaller units and more compact designs. For higher temperatures, metallic materials can be used. Through the choice of a suitable alloy, the required operating temperature range can be set. For example, aluminium-silicon alloys have high latent heat and a suitable melting temperature of around 575°C. However, the selection of construction materials remains a challenge due to the degradation caused by corrosion and cyclic changes in the phase of the storage medium. Therefore, further material development is needed before this technology can be implemented on a larger scale.
There are also other TES-based concepts in various stages of development, that differ, for example, in the method of charging. One such example is the so-called PTES (pumped TES) concept, where heat is derived from electrical energy through the use of a compressor and compressed air storage.
CVR activities
The Řež Research Centre is developing its own concept for a TES system as part of the Efekt project, supported by the TA CR. The concept considers the use of an AlSi12 alloy, with phase transformation, as the storage material. The operating temperature is approximately 575 °C. The system is charged by converting excess electrical energy into heat using electrical resistance heaters that are directly immersed in the storage material. During the discharge cycle, heat from the reservoir is removed using a heat-carrying medium, which, at the same time, is the medium of the conversion circuit. The heat exchange elements are directly placed in the reservoir, thus the design provides a highly compact and simple system. The challenge for further development is primarily the interaction between the storage material and other components, which are exposed to thermo-mechanical stresses in addition to degradation though corrosion.
The Řež Research Centre is also engaged in technical and economic studies of other, potentially applicable TES technologies, for the possible future implementation of the technology on an industrial scale.