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Mechanical Energy Storage Methods in Renewable Energy Plant

There are three methods is to be used for storing the energy and they are:

1. Pumped Storage

2. Compressed Air Storage

3. Flywheel Storage

1. Pumped Storage method in Renewable Plants

Pumped storage is the most successful, economical and widely used energy-storage technology presently available to electrical utilities for load levelling (peak shaving). It could also be used for storing electrical energy produced from solar and wind energy. Electrical power in excess of immediate demand is used to pump water from a supply (lake, river or reservoir) at a lower level to a reservoir at a higher level. During peak demand period when the demand exceeds the normal generating capacity, water is allowed to flow backwards through a hydraulic turbine, which drives an electric generator and produces power to meet additional demand.

In most pumped storage plants, the turbine generator system is reversible and can serve to pump water as well. In the pumping mode, the generator works as motor and draws electrical power from the electrical network. The turbine then operates as a pump driven by the motor. Start up of the turbine-generator or reversal from motor-pump to turbine-generator requires only a few minutes. The overall energy recovery efficiency of pumped storage, that is, the recovered electrical energy as a percentage of electrical energy used to pump water, is about 70 per cent.

There are relatively few suitable sites where there is water supply at a lower level and a reservoir can be constructed at a higher level. However, the use of natural or excavated underground caverns as lower reservoirs, now being developed, should greatly increase the number of possible sites.

2. Compressed Air Storage method in Renewable Energy plant

In a compressed-air storage system, excess electrical energy is used to compress air, which is stored in a reservoir to be used later in the combustor of a gas turbine to generate electric power. In a gas turbine, roughly 60% of the power output is consumed in compressing air for combustion of the gas. Compressed air can also be used to produce mechanical output through an air turbine.

Electric power in excess of immediate demand is supplied to the motor/ generator unit which drives the compressor. The compressed air at about 70 atm, is stored in a suitable reservoir. The air is heated during compassion and may have to be cooled prior to storage to prevent damage to the reservoir walls. When additional power is needed to meet the demand, the compressed air is released and heated in a combustor using gas or oil fuel. The hot compressed air is then expanded in a gas turbine connected to the motor/generator unit which now acts as generator. The overall recovery efficiency is 65 to 70 per cent. A clutch is used for coupling and decoupling the motor/generator unit with a compressor/turbine.

Compressed-air storage reservoirs would probably be too large and too expensive for above-the-surface construction; hence underground, reservoirs preferably the naturally existing ones, are being considered. Among the possibilities are natural caverns, deep aquifers, depleted gas or oil reservoirs mined-out rocks or salt caverns, and abandoned mines. A commercial installation is in operation near Bremen, Germany.

Storing the Energy is an essential requirement for all facets of our life and it has been recognized as a basic human need. It is the key to accelerating economic growth, generation of employment, elimination of poverty and growth of human development especially in rural areas. In contrast to fossil fuel and nuclear-fuel based energy, the initial input power of a renewable energy source is outside our control. The use of renewable energy supplies constitutes a diversion of a continuing natural flow of energy; there are problems in matching supply and demand In a given time domain, i.e., matching the rate at which energy is used. The mismatch varies with time on scales of months {e.g.. house heating in temperate climates}, days (e.g., artificial lighting) and even seconds (e.g., starting motors).

The means of storing energy in a readily recoverable form when the supply exceeds the demand for use at other times is known as energy storage. Storage of primary fuels (e.g., coal, oil and gas) is also a form of energy storage, but the term ‘energy storage’ generally applies to secondary energy rather than to primary energy.

The performance of various energy-storage mechanisms can be measured and compared in (i) MJS (ii) MJm³ (iii) MJKg ^-1. The first is a very important and deciding factor but is difficult to estimate. The second is important where space is a prime consideration (e.g., indoor applications). The third is considered when weight is vital (e.g., in aircrafts).

NECESSITY OF RENEWABLE ENERGY STORAGE

1. The effective utilization of intermittent and variable energy sources such as sunlight, wind, etc. often requires energy storage

2. In some circumstances, electrical energy may be generated ether on land or at sea, at a location that is too distant from a consumption centre for conventional transmission lines to be used for example Ocean Thermal Energy Conversion. Means must then be found for both storing the energy and transporting it economically to a load centre.

3. Electrically propelled vehicles, which are expected to come into increasing use, also require some form of energy storage. Since the vehicle must carry its energy supply, the storage system must be readily transportable.

4. Energy storage is also required for ‘Load levelling’ in an electric utility to reduce the overall cost of generating electrical power. More efficient plants may be operated continuously at a rated power level, and the excess power during off-peak period is stored for use when the demand exceeds the base load. Thus, the use of less efficient units to meet the additional demand for power during peak load periods is eliminated.

ENERGY STORAGE METHODS IN RENEWABLE ENERGY:

Energy can be stored in various forms and the storage methods are classified on the basis of the form in which it is stored. Some of the important energy storage methods are the following:

1. Mechanical energy storage

(a) Pumped storage

(b) Compressed air storage

2. Chemical energy storage

(a) Batteries storage

(b) Hydrogen storage

3. Electromagnetic energy storage

4. Electrostatic energy storage

5. Thermal (heat) energy storage

(a) Sensible heat storage

(b) Latent heat storage

6. Biological storage

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