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Energy Storage 2020

image credit: © Malpetr | Dreamstime.com

This item is part of the Special Issue - 07/2020 - Energy Storage, click here for more

Global demand of electricity is increasing and the major portion of this demand is met by fossil fuels, which are associated with CO2 emissions. This emission is having its associated demerits on the planet; hence the environment and the future of the planet should not be compromised due to the increase of global demand of energy. To combat the emissions due to fossil fuel based energy sources, renewable energies have been identified as potential alternatives. Among various renewable energy sources available, solar energy is seen as the most promising and the largest source of heat which can meet the electricity demand for the future globally.

However, it is a well-known fact that radiation from sun is intermittent since it is not available during night time, cloudy days etc. and electricity grid cannot function properly if there is no balance of supply and demand. An imbalance in supply and demand may cause problems like voltage fluctuation etc. So energy storage technologies are required in order to facilitate the continuous supply of electricity as per the demand of locality. On the other hand, energy storage plays an important role in the balancing act and helps to create a more reliable grid system.

There are mainly two types of solar electricity systems – (i) photovoltaic and (ii) concentrating solar thermal plants. In the first case, lithium ion batteries are the best option for energy storage though other battery types are also available.

Concentrating solar power (CSP) system is also becoming popular day-by-day. CSP with energy storage has the ability to provide reliable power. Though this technology was available from long back, some issues like cost, government support etc. were the hindrance for the development of the technology. There are mainly three types of CSP system – (i) dish system, (ii) parabolic trough and (iii) solar tower. A concentrated solar power plant converts solar energy to electricity. It is based on focusing solar energy onto a small receiver / absorber using concentrators such as mirrors, reflecting surfaces etc. Light is converted to heat which, in turn, drives turbine to provide electricity. Here also, energy storage is introduced in order to get smooth power output. While a thermal energy storage system is designed, following points should be considered

  • Cost of the system
  • Operating range of temperatures
  • Maximum load to be handled i.e. maximum energy to be stored, and
  • The ease with which the system can be integrated into the power plant

While discussion on storage is done, it is often confined to battery storage but storage is a very vast area. Various thermal energy storage media are available for CSP plants like (i) sensible heat storage and (ii) latent heat storage. Sensible heat storage is achieved by raising the temperature of the mediums. Sensible heat storage may be classified as liquid media storage and solid media storage. Liquid media storages are water, oil-based liquids, molten salts (like nitrates, carbonates, chlorides) etc. or solids such as rocks, metals and others. But out of various sensible heat storage or heat transfer media available, molten salt CSPs are the most promising option from both economic and technical point of view.

Molten salt CSP plants can be designed as base load plants i.e. the plants can be used to generate electricity to constantly satisfy minimum demand. The amount of heat stored is a function of two factors: medium's heat capacity and the temperature rise of the medium. The larger the difference between the high temperature and low temperature system, the higher is the heat stored by the material. Though both oil and molten salt systems were found to be technically feasible, use of thermal oils is restricted due to its high cost, flammability, and temperature limitation (400OC) for which high thermodynamic efficiency of the power block cannot be achieved.  On the other hand, molten salt is used in CSP systems because it is liquid at atmosphere pressure; it provides an efficient, low-cost medium; its operating temperature is compatible with today’s high-pressure and high-temperature steam turbines, and it is non-flammable, nontoxic and chemically inert in nature. Molten salt storage technology can be of two types - (i) two-tank direct system, and (ii) two-tank indirect system.

In two-tank direct system, same fluid is used for heat transfer as well as storage medium of solar thermal energy. Here, two tanks are used: one is high temperature and the other is low temperature storage tank. Fluid from the high-temperature tank flows through a heat exchanger, where steam is generated for electricity production. The fluid exits the heat exchanger at a lower temperature and returns to the low-temperature tank. During the charging period, fluid from the low-temperature storage tank flows through the solar receiver/absorber to achieve high temperature by gaining solar energy, and then flows to the high-temperature storage tank for storage.

The basic difference between two-tank direct and indirect system is that different fluids are used as the heat-transfer and storage fluids in two-tank indirect systems. In this system, another heat exchanger is introduced where the storage fluid from the low-temperature tank flows and it is heated by the high-temperature heat-transfer fluid. The high-temperature storage fluid is stored in the high-temperature storage tank. The heat transfer fluid from the heat exchanger exits at a low temperature and flows to the solar receiver/absorber, where it is heated back to a high temperature. Storage fluid from the high-temperature tank flows through a heat exchanger, where steam is generated for electricity production, as in the case of two-tank direct system.

Another storage media used in CSP plants is the Latent heat storage, the principle of which is to store thermal energy via. transition of solid to liquid state of phase change materials (PCM) at constant temperature. Various storage media are available depending on the temperature required like water, paraffin, salt hydrate, 50LiOH/50LiF etc.

Cost/kW-h generated is declining for available solar thermal technologies and so, interest on the development of the most efficient energy storage technologies are coming forward like never before. Yet more investments are required in this field.

Researchers are trying hard to find the best method of energy storage among various options available since the sun doesn’t shine round the clock.

Discussions

Matt Chester's picture
Matt Chester on Jul 27, 2020

Another storage media used in CSP plants is the Latent heat storage, the principle of which is to store thermal energy via. transition of solid to liquid state of phase change materials (PCM) at constant temperature. Various storage media are available depending on the temperature required like water, paraffin, salt hydrate, 50LiOH/50LiF etc.

How does the efficiency of this strategy compare to other typical energy storage techniques? What about the cost?

Dr. Soumitra Mukhopadhyay's picture
Dr. Soumitra Mukhopadhyay on Jul 28, 2020

Thanks for your query..

Efficiency and cost part have not been observed to address anywhere as far my knowledge goes..

Dr. Soumitra Mukhopadhyay's picture

Thank Dr. Soumitra for the Post!

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