Renewable Energy Storage Methods | Energy Storage Requirements

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).


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 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

(c) Flywheel storage

01 - Renewable Energy Storage Methods - Pumped Hydro Storage system

2. Chemical energy storage

(a) Batteries storage

(b) Hydrogen storage

(c) Reversible chemical reactions storage

3. Electromagnetic energy storage

01 - Renewable Energy Storage Methods - Electromagnetic Storage system

4. Electrostatic energy storage

5. Thermal (heat) energy storage

(a) Sensible heat storage

(b) Latent heat storage

6. Biological storage

Air-Conditioner | Air-Conditioning System | Window Air Conditioner


This is also called as window type room air-conditioner because it can be accommodated in window of walls. This unit is suitable for cooling the rooms in hot summer.

The window air-conditioner works on the vapour compression refrigeration cycle.



The basic components of a window air-conditioner are compressor, condenser, a capillary tube and an evaporator, in addition filters, accumulators, motors, fans and electrical controls. This unit consists of

1. A cooling system to cool and dehumidify the air which includes a condenser, a compressor and a refrigerant coil.

2. A filter to remove any impurities in the air. The filter is finished of with mesh, glass wool or fibre.

3. A fan and adjustable grills to circulate the air.

4. Controls to regulate equipment operation.

Working Principle

The high-pressure refrigerant which comes out of the compressor is condensed in the condenser using ambient air a coolant. The liquid refrigerant is expanded in a capillary tube and then enters the evaporator. Here, it evaporates and roots the refrigeration effect. Then fan blows the fresh air over the evaporator coil, which cools the air. The cool air is supplied to the room. Meanwhile, refrigerant from the evaporator enters the compressor to be compressed and the cycle repeats itself. The return air from the room is exhausted by the evaporator fan be blowing air over the condenser coil, where it picks up heat from the refrigerant.


The compressor used in a window air-conditioner is hermetic type, which is sealed in a casing with a motor. The condenser coil is a continuous copper coil with aluminium fins. The capillary tube is 40cm long, usually with 0.75mm diameter. Evaporator coil is also made of copper with aluminium fins. Dampers and louvers are provided for intake and exhaust on both sides of the air-conditioner casing.

A cone ton window air-conditioner circulated 4m³/min of air in the room. Some fraction of air is fresh, whereas some faction of air is recirculated. Filters are used to filter air entering the air-conditioning unit. The filter is plastic foam or metallic mat type. The controls used by the unit are

a) Thermostat Control: This is used to set the room air temperature. Usually it is a bimetallic strip.

b) Master Control: It is an electrical switch used to stop or start the compressor motor.

The installation of the unit is done either in the wall or by using external brackets or locating it inside the room with exhaust flush to the window. In either case, the condenser should be facing outside and the evaporator should be facing inside. It is advisable to install the unit for a minimum sun exposure as sun’s rays can diminish the efficiency of the condenser. The air louver inlets should not be blocked and provision for draining of the condensate must be provided. Maintenance of the window air-conditioner should include interrupted cleaning, replacing filters and air seals and lubrication of fans.