With increasing environmental concerns and greenhouse effects, almost every country is looking for ways to mitigate these concerns. Solar energy reaching the Earth’s surface provides an energy supply potential surpassing the power consumption of our civilization by three orders of magnitude. Even though solar energy seems to be our future, one of the current limitations in use of the solar thermal energy is that it is only available when there is sunlight, preventing solar-generated electricity from being produced during cloudy periods or overnight. Technologies that can store solar thermal energy and discharge it on demand to meet user needs will help to make solar energy plants more reliable and improve consumer confidence in them as electricity generators.

What is CSP and its current status

There are several ways in which solar energy can be used to generate electricity, and concentrated solar power (CSP) is one of those technologies that have been very successful. In a CSP power plant, solar radiation beams are concentrated onto a small area, where a heat transfer fluid (HTF) is heated up, and the heat energy converts the liquid into steam. The steam so produced is used to drive steam turbines contacted to electricity generators. By the end of March 2015, the CSP market has a total capacity of 5840 MWe worldwide, out of which 4800 MWe is under operation, and 1040 MWe is under construction. Spain is leading the race with a total operational capacity of 2405 MW and 100 MW under construction. Other countries such as India, China, Australia, and Middle East countries have also shown interest in CSP based electricity generation.

Types of CSP

CSP has been categorized into, trough, linear Fresnel, tower and parabolic dish, based on the way they focus the sun’s ray and whether the receiver is fixed or mobile. In parabolic trough and linear Fresnel systems, the mirror tracks the sun along one axis (line focus) and in tower and dish systems, the mirror tracks the sun along two axes (point focus). The receiver is fixed in linear Fresnel and tower systems while it is mobile in parabolic trough and dish systems. For each technology, various options exist for the HTF selections, TES technology and power cycles [2].

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Economic barriers in using CSP

One of the major barriers to the adoption of CSP technology is that it might increase the cost of solar generated electricity, having said that, many researchers claim that the CSP generating cost is expected to reduce by 50–60% through technical improvement and scaling up the volume production. Many developed countries have extensively funded projects to make CSP more cost effective. Some of these projects include the European DISTOR project funded by the European Commission, the SunShot initiative launched by the United States Department of Energy in 2011, and the Australian Solar Thermal Research Initiative (ASTRI) funded by the Australian Government. All of these projects are targeted at reducing the cost of solar thermal power generation within a given timeframe.

Can TES be a solution?

CSP works as a non-pressurized closed-loop thermal systems with efficiency limited by the carnot cycle. A thermal storage system (TES) offers a highly efficient heat storage method as an intermediate step for electricity generation. Depending on the size of the TES system, it can mitigate the short load fluctuations and shift or extend the energy supply. CSP with TES presents an economic advantage as it allows the supply of electricity at times of greatest need that in exchange generates high tariffs.

What is TES?

TES is a technique by which the thermal energy for CSP system is stored or can be stored by using a solid or liquid media. Sensible heat storage, latent heat storage using phase change materials (PCMs) and thermochemical storage through chemical reactions are the three concepts that are readily used to store thermal energy in any media. However, the designing of TES system for CSP plant is not so simple and involves a thorough consideration of following factors: (1) nominal temperature and specific enthalpy drop in load; (2) maximum load; (3) operational strategy and (4) integration into the power plant.

Sensible heat storage system: Technology and Materials

TES based on sensible heat, heat that can be sensed by change in temperature, stores/release the thermal energy by rising or lowing the temperature of storage medium which can either be liquid or solid. The amount of energy stored is dependent on the quantity of the storage material, the specific heat capacity of the material and the required temperature change. Most of the TES systems currently installed in CSP plants are using sensible storage materials, e.g. oil, molten salt, steam, ceramic and graphite. A new storage materials called “molten salt” has attracted considerable attention because of its high heat storage capacity and low cost. Examples of molten salt materials include Hitec/HitecXL and solar salt that has been used in commercially deployed trough and tower systems. Abengoa Solar modelled the result of a parabolic trough plant with 6 h of storage to analyse the feasibility, cost and performance of molten salt over conventional storage materials. The modelling results showed that the molten salt plant can reduce the storage cost by up to 43.2%, solar field cost by up to 14.8% and the Levelized Cost of Electricity (LCOE) by 9.8–14.5% relative to an oil plant with Therminol VP-1.

Latent heat storage: Technology

Latent heat is the characteristic amount of energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature [1]. ″Latent heat″ measures the change in internal energy that seems hidden from a thermometer so even though the energy is stored or released no change is temperature is observed. The topic of latent heat in itself is nothing very new. Water at 0 oC is a standard latent heat storage medium that has been used for many years. The idea of the use of latent heat storage device for TES is also quit old. However, the idea of integration of phase change material is relatively very new. These phase change materials (PCMs), store/release large amount of latent heat by changing itself from one phase to other (generally solid to liquid) and has attracted considerable attention for CSP applications over the past decade. The amount of energy stored is dependent on the specific heat and the phase change enthalpy. The energy converted during the phase change offers an isothermal (no visible change in temperature) heat storage/release process and a higher storage density compared to the sensible storage system in the same temperature change. It potentially enables a smaller and lower cost storage system compared to the sensible storage system. Currently, no TES based on latent heat storage system is under commercial use for CSP applications.

Thermochemical Storage: Big promises but still under R&D

As the name suggest, thermochemical storage, involves chemical reactions in which the energy changes during the transformation from reactant to product or vice-versa. The energy is stored by means of the reversible sorption process or chemical reaction.

AB + Heat ↔ A+B

In the endothermic reaction (a chemical reaction in which heat is absorbed), the chemical reactant AB absorbs the heat supplied from the solar field and dissociates into two products A and B that can be stored separately. In the reverse process (exothermic reaction), chemicals A and B are put in touch and the initial reactant AB is formed with a heat release. Thermochemical based storage systems possess high energy density with negligible heat loss, which potentially offers a long-term storage option with relatively small storage volume. The sorption process is usually able to store low and medium grade heat at a temperature of less than 400 °C, whereas, chemical reactions allow the storage of heat above 400 oC.  This storage system is still under R&D and a lot of chemical reactions, such as, metal salts with water, ammonia, methanol or methyl-ammonia and metal alloys with hydrogen, have been tested so far.
Solar energy has the major dis- advantage of supplying intermittent power for electricity generation. TES has proven to be successful in mitigation of short fluctuations and extension of electricity supply to more desirable periods, making CSP based solar power more versatile and market friendly. Over 80% of the CSP capacity under construction has TES, while less than 50% of the operational CSP capacity has TES. These numbers in itself show the potential that TES technology hold in deciding the future of concentrated solar power based electricity generation.

Further Readings:
•    Fath, H. E. S. (1998). Technical assessment of solar thermal energy storage technologies. Renewable Energy, 14(1-4), 35–40. http://doi.org/10.1016/S0960-1481(98)00044-5
•    Liu, M., Steven Tay, N. H., Bell, S., Belusko, M., Jacob, R., Will, G., … Bruno, F. (2016). Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies. Renewable and Sustainable Energy Reviews, 53, 1411–1432. http://doi.org/10.1016/j.rser.2015.09.026
•    Pintaldi, S., Perfumo, C., Sethuvenkatraman, S., White, S., & Rosengarten, G. (2015). A review of thermal energy storage technologies and control approaches for solar cooling. Renewable and Sustainable Energy Reviews, 41, 975–995. http://doi.org/10.1016/j.rser.2014.08.062

Bio:
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