Thermal (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time for heating and cooling applications and power generation. TES systems can be used particularly in buildings and industrial processes.
TES encompasses a variety of technologies that store available heat energy using different approaches in insulated repositories. A TES system normally consists of a storage medium in a reservoir/tank, a packaged chiller or built-up refrigeration system, piping, pump(s), and controls. Based on the range of operating temperature, TES can be classified into two groups: low-temperature TES (consisting of aquiferous low-temperature TES and cryogenic energy storage) and high- temperature TES (including latent (fusion) heat TES, sensible heat TES and concrete thermal storage). Aquiferous low-temperature TES normally uses water cooled/iced and reheating processes, which is more suitable for peak shaving and industrial cooling loads. Cryogenic energy storage employs a cryogen (such as liquid nitrogen or liquid air) to achieve the electrical and thermal energy conversion. For instance, Liquid Air Energy Storage (LAES) is attracting attention due to the high expansion ratio from the liquid state to the gaseous state and the high power densities of liquid air compared to that of gaseous state of the air. Latent heat TES employs Phase Change Materials (PCMs) as the storage media and uses the energy absorption or emission in a liquid-solid transition of these PCMs at the constant temperature. Concrete thermal storage utilizes concrete or castable ceramics to store heat energy, normally supported by synthetic oil as a heat transfer fluid. The above TES technologies have different features with various applications. For instance, latent heat storage can provide a relatively high storage density with a small dimension reservoir, thus the use of this technology in buildings receives attention. In addition, cryogenic energy storage is expected to be used for future grid power management. The below automation is a high-temperature thermal storage with a conventional thermal power plant.
The TES system can store large quantities of energy without any major hazards and its daily self-discharge loss is small (∼0.05–1%); the reservoir offers good energy density and specific energy (80–500 W h/L, 80–250 W h/kg) and the system is economically viable with relatively low capital cost (3–60 $/kW h). However, the cycle efficiency of TES systems is normally low (∼30–60%). TES has been used in a wide spectrum of applications, such as load shifting and electricity generation for heat engine cycles.
With the particular focus on using TES for power system and grid applications, there are many active research projects worldwide and, in addition, numerous demonstration projects are built, under construction or planned. The UK based company Highview Power Storage designed and assembled a pilot LAES facility (300 kW/2.5 MW h storage capacity) which has been in operation at Scottish and Southern Energy’s 80 MW biomass plant since 2010. In February 2014, this firm has been awarded £8 million funding from the UK government for a 5 MW/15 MW h demonstration LAES project; the designed LAES system will be alongside one landfill gas generation plant in the UK. A TES system in an office building was built by a joint U.S. and China demonstration project in Beijing, which can reduce peak electric energy consumption of 6100 kW h per month. A new central energy plant including an ice-based TES system is being built in South Florida. The completely built plant will have a total capacity of 11,500 tons of chilled water with 68,000 ton-hour of TES. A 15 MW commercial power plant, named “Solar Tres Power Tower”, is being built in Spain by Torresol Energy, and it uses molten salt as the working fluid to store heat energy. A wind power generation system combined with a sensible heat storage facility had been proposed (see the below figure). The electrical energy from wind power is used to heat a bulk storage material; the heat energy is recovered to produce water vapor which in turn drives a turbo-alternator to generate electricity. A UKERC funded project, “the future role of TES in the UK energy system”, has investigated the potential for, and limitations of, the role of TES in the transition to a sustainable low carbon energy supply system; the project has also studied the suitability of TES in managing energy generation and distribution systems with large-scale penetration.
Fig. A sensible heat storage system for wind power generation
