SOLAR ENERGY STORAGE
An optimally designed solar electric system collects and converts solar energy when insolation is available during the day. Solar energy is however a time dependent and intermittent energy resource. The time when solar energy is most available rarely coincides with the demand for electrical energy.
Storage of solar energy in a solar system permits solar energy to be captured when insolation is highest and then later used when the need is greatest. Thus the addition of storage can increase the reliability of delivering power as per need. Storage also makes it possible to deliver short peaks of power for exceeding the rated power capacity of the plant.
The optimum energy of an energy storage system depends on the expected time dependence of solar radiation availability, the nature of loads, the degree of reliability needed, cost per KWH of the stored energy and the economic analysis as to how much of the total usually annual loads should be carried by solar and how much energy sources.
The energy may be stored in a variety of forms e.g. as heat, electrical, chemical, mechanical and magnetic. Energy storage may be in the form of sensible heat of solids or liquid medium, as heat of fusion in chemical systems or as chemical energy of products, energy could be converted to potential energy and stored in elevated fluids.
With Renewable energy conservation plants, energy storage is generally provided to extract Maximum energy during favourable periods and ensure continuous output from the plant.
Energy can be stored in various forms like thermal electrical, hydro, biomass, chemical reactants, compressed and mechanical potential/kinetic fuels etc.
The choice of the type of storage system is dictated by several techno- economic factor. The energy storage rating is specified in MWhr capacity and the rate of discharge in terms of MW per hour. Energy storage in electrical supply system form is opted in several special applications like peak load power supply and energy storage for renewable energy plants, smoothing the voltage and current, improve transient stability of the supply system, auxiliary and control circuits, remote installation, interrupted power supply system etc.P
Pumped- Hydro schemes
These are either underground pumped hydro or above ground pumped hydro schemes. It could utilize either a single hydro machine with one electrical machine ( the hydro machine operates in either turbine or pumping mode) or two hydro- machines and one electrical machine all three mounted on a common shaft in one axis. Each hydro machine operates only during their respective operating mode.
Layout of pumped storage plant – The layout depends upon topology of available sites, lakes, reservoirs, heads, distances etc. The high level reservoir may be built by constructing embarkment of natural lake. Pumped storage power plant may be overground or underground. The head H may be high, medium or low.
In this plant the upper reservoir is at ground and the lower reservoir and the power plant are underground.
Such a scheme is preferred for sites having large underground covers which can store the water of lower reservoir. The underground pumped hydro plant site doesn’t require the topology with high and low level reservoirs.E
Capacitors store large amount of electrical energy for long periods, using mica as dielectric material. Since the conductivity of a dielectric is never bill, there will always be leakage losses.
Capacitors are used to store electrical energy at high voltage and low current and inductors for storage at low voltage and high current.
A battery is used to store energy electrochemically and later regained as electrical energy. Cycle life of batteries is the number of times it can be charged and discharge tending to result in short cycle life.
Storage battery in which the reactant is generated by a photochemical reaction brought about by solar radiation is used for storage. The battery is charged photochemically and discharged electrically whenever needed.
In another device water is electrolysed with solar generated electrical energy. O2 and H2 stored and recombined in a fuel cell to regain electrical energy.
Solar energy could be used by the anaerobic fermentation of algae for the production of methane, which is stable at room temperature.
Thermochemical storage systems are suitable for medium or high temperature applications. For storage of high temperature heat some reversible chemical reactions are very attractive. Such devices have high energy density storage at ambient temperatures for long periods without thermal losses and potential for heat pumping.
Compressed air storage
Another alternative for possibly storing energy in future solar – electric system is to store the energy in a volume of compressed air.
A natural or artificial body of water can be used for collecting and absorbing solar radiation energy. A curved cover, made of transparent fiber glass, over the pond permits entry to solar radiation but reduces losses by radiation and convection. The energy stored in pond is of low grade ( 60 to 100°C). It might be suitable for a variety of applications such as space heating, industrial process heat and to obtain mechanical and electrical energy.
Compressed air energy storage
In compressed air energy storage technology, the excess electrical energy during low load hours is utilised to compress air by the motor compressor units. Energy is stored in the form of compressed air at a pressure around 50 bar. The compressed air is stored in large underground salt caverns or hard rock caverns, underground old mines or aquifers. During peak load hours the compressed air is released to drive air turbine. The turnaround efficiency of such system is around 65%.
Battery energy storage systems
Batteries work on direct current (DC) and need an AC/DC convertor for charging and DC/AC inverter for supplying power to AC loads. Storage batteries are well known and have a wide range of applications. While charging the batteries cell, electrical energy is converted to chemical energy and stored. The energy conversion in secondary battery is reversible. Secondary batteries store energy during charged stage and release it whenever required at required rate.
Charging capacity is expressed in Ampere Hours. The secondary cells have life of several hundreds or thousands charging and discharging cycles. The secondary batteries have high energy density, high discharge rate, flat discharge characteristic, good low temperature performance at low temperature. Battery installation is modular and required voltage and current can be achieved by suitable series or parallel combinations.
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