It is generally accepted that cost-effective storage is the key to a high penetration of renewable energy in electricity grids. Much work has been done on the development of storage technology to meet these goals, but systems are still confined to several hours of operation. If longer storage periods are required then a change in technology is required. High temperature latent heat storage (LHES) using metallic phase-change materials (PCM) offers promise in this field.
The current storage capability is set at hours but the ultimate goal is days. In spite of gains available through geographic diversity, the possibility of wide-scale low-resource availability exists, and long-term energy storage is required to counter this.
Existing storage methods rely on chemical process or mechanical means, such as potential energy or pressure. Newer methods are looking at using the latent heat in bulk phase-change effects in thermal storage systems. The application of phase-change materials (PCM) for solar thermal-energy storage capacities has received considerable attention in recent years due to their large storage capacity and the isothermal nature of the storage process.
Thermal energy storage (TES) has gained traction over the years as a way to store energy from renewable sources. In particular, it has gained more attention in the application of concentrated solar power (CSP) plants. Thermal storage of energy has been in use for a long time, but most systems have operated in the medium to low temperature range and have relied on sensible heat storage for operation. Ice storage and hot water storage systems used in building HVAC systems are examples of this. CSP and other systems have used sensible heat storage capacity to provide reserves for electricity generation. Newer systems are making use of the latent heat of PCMs as heat storage.
Sensible heat storage
Sensible heat storage makes use of the fact that thermal energy can be stored in a material by changing its temperature. Sensible heat thermal energy storage (STES) uses either a solid or liquid as storage medium by increasing or decreasing the temperature as more thermal energy is stored. The amount of energy that can be stored depends on the specific heat of the storage medium, which is generally expressed as the as the amount of heat required to raise a unit mass of the material by 1°C. Adding or extracting heat energy results in a change of temperature. Materials used for sensible heat storage will depend on the temperature of the heat source and typical materials are water, oil, and molten salt. Sensible heat storage systems are currently used with many CSP plants.
Latent heat storage
Latent heat thermal energy storage (LTES or LHTES), stores energy by converting storage material from one state to another (typically a solid to liquid) .
Materials used in LTES are commonly referred to as phase-change materials (PCMs). Fig. 1 shows the operating cycle of a PCM heat storage system.
As heat is added the temperature increases until the phase-change point is reached, where the temperature remains constant until all the material has melted. Significant heat is added at a constant temperature during this stage.
Latent heat storage has two main advantages:
Comparison between latent and sensible heat storage shows that using latent heat storage, storage densities typically five to ten times higher can be reached . Latent heat storage systems operating in the low to medium temperature ranges have been in use for many years. These systems operate using organic materials, such as waxes, and are typically found in building heating and cooling operations.
New developments into higher temperature ranges are making use of metallic materials and inorganic salts. These systems operate in the temperature range of >500°C and are associated with CSP systems. Development of the linear Fresnel reflector system produces systems with temperatures within this range and this type of storage is being investigated as a possible component of LFR systems.
The primary difference between a sensible heat storage and a latent heat storage is that the PCM storage is static and the heat transfer fluid moves through the PCM store. The heat transfer fluid is separated from the PCM material and is transported through the storage by a variety of heat pipe designs.
Fig. 2 shows a typical PCM-based solar power system. Most systems use the stored heat to develop steam for power turbines. Heat is transferred from the solar collector to the PCM storage unit via a heat transfer medium (HTF) which is usually thermal oil or molten salt, although high temperature systems have been developed that use molten metal alloys as the heat transfer medium .The HTF passes through vaned pipes in the PCM store, in both the heat transfer and heat recovery mode (Fig. 3). The PCM material is retained at the phase-change temperature.
Silicon based LHES systems
Silicon, one of the most abundant materials on the planet, has a very high latent heat capacity of 1414,3 kJ/kg and high melting temperature of 1414°C, making it ideal for the storage of large amounts of energy. The use of silicon as a material for LHES is being researched by a number of institutions. One of the challenges facing silicon based LHES is the recovery of the stored thermal energy.
Very high temperature systems face problems with transferring heat from the material to a transfer medium. Recent developments of high temperature LHES systems propose direct generation of electricity and heat using thermal photovoltaic systems. These cells can produce 100 times more electric power per unit area than conventional solar cells. They are key to the system – not least because they can work at extreme temperatures, unlike other generators .
The Amadeus project, at the Technical University of Madrid (UPM), is conducting research on the use of high temperature silicon-based LHES systems. The Horizon 2020 research project will work on the storage of energy at temperatures higher than 1000°C using molten silicon-based alloys.
The direct storage of solar energy in thermal solar power plants, or the integration of both electric power storage and co-generation in the housing sector and urban areas, are examples of the potential applications of the devices to be developed by the project .
According to the researchers, isolated molten silicon can store more than 1 MWh of energy per cubic metre (m3), over ten times the capacity of current systems which use molten salts. At such high temperatures, silicon shines intensely in the same way that the sun does, thus photovoltaic cells, thermo-photovoltaic cells in this case, can be used to convert this incandescent radiation into electricity.
A South Australian company, 1414 Degrees, has developed technology to store electricity as thermal energy by heating and melting containers full of silicon at a cost estimated to be up to ten times cheaper than lithium batteries. The process generates large amounts of clean useable heat, which can easily be utilised for district heating or industrial purposes. Apparently, the company has built a full prototype of its patented thermal energy storage system, ready for commercialisation .
The company completed its first trials in September 2016 with a small prototype test system using about 300 kg of silicon to store about 150 kW of energy, dramatically improving the efficiency of wind and solar farms and is expected to launch the first commercial machine this year. The company has two target markets: industry, with a device capable of storing 10 MWh of energy; and wind farms, large solar arrays or gas-fired power stations with a 200 MWh device.
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