- 1 Solar energy storage systems:
- 1.1 Solar energy storage through Sensible heat storage:
- 1.2 Solar energy storage through Water storage system:
- 1.3 Solar energy storage through Pebble bed :
- 1.4 Solar energy storage through Latent heat storage (Phase change energy storage):
- 1.5 Solar energy storage through Electrical storage system:
- 1.6 Solar energy storage through Chemical storage system:
- 1.7 Thermo-chemical energy storage (Reversible):
- 1.8 Pumped hydroelectric storage of solar energy:
- 1.9 Solar energy storage through Compressed air:
- 1.10 Solar energy storage in Flywheel:
In order to take full advantage of solar energy it must be stored in in some form of solar energy storage system. Unfortunately, the time when solar energy is most available, it will rarely be consumed because the solar energy is available at day time while the demand is maximum at prime time. There is also the problem of clouds with photovoltaic plants, and cloud cover for several days may result in substantially lowered electrical output compared to bright sunny cloud-free days. During such days energy previously stored during bright sunny times could be used to provide a continuous electrical output or thermal output.
Solar energy storage systems:
Solar energy storage systems are classified as shown in figure below.
Types of solar energy storage systems
One method of storing energy is by melting, storing or vaporization of materials the energy becomes available as heat, when the process is reversed.
Solar energy storage through Sensible heat storage:
Storage by causing a material to rise in temperature is called sensible heat storage. It involves a material that undergoes no change in phase. The basic equation for an energy storage unit operating over a finite temperature difference is
where ρ is the density of the storage medium.
Solar energy storage through Water storage system:
Among different methods of heat transfer, the most common for solar system is water, and the easiest way to store thermal energy is by storing the water directly in a well insulated tank.
Features of water storage are:
- It is an inexpensive, readily available and useful material to store sensible heat.
- It has high thermal storage capacity.
- Energy addition and removal from this type of storage is done by medium itself, thus eliminating any temperature drop between transport fluid and storage medium.
- Pumping cost is small.
Solar energy storage through Pebble bed :
Here, rock, gravel or crushed stone in a bin provides a large, cheap heat transfer surface. Rock is more easily contained than water. It acts as its own heat exchanger, which reduces total system cost. Rock can be easily used for thermal storage at high temperatures (above 100 OC).
Water storage cannot be used at normal temperature. To use water for storage, a very high temperature such as above 100 OC is to be maintained and high pressure storage system is required to contain steam. Hence, pebble bed storage has low cost of storage material. This type of storage system has been used in the solar houses or with hot air collector system.
Solar energy storage through Latent heat storage (Phase change energy storage):
Here, heat is stored in a material when it melts and extracted from the material when it freezes. Glauber’s salt (Na2SO4.10H2O) changes phase from solid to liquid requires lesser energy than those from liquid to gas. It decomposes at about 32OC releasing 56kCal/kg.
Solar energy storage through Electrical storage system:
Where V = volume of dielectric
E = electric field strength
Electric field strength is limited by the breakdown strength (Ebr) of the dielectric (e.g. mica).
As the conductivity of dielectric is finite, therefore losses occur in the storage battery.
- Inductors store energy at low voltage and high current. The energy is given by
H = (1/2) VuH(m^2)
Where μ = permeability of material
Hm = magnetic flux density
For H to be large, both μ and Hm should be large. Higher magnetic fields exert large forces on structure.
So the structure must be mechanically strong.
- Battery storage: Energy efficiency (ɳ) of battery storage is given as
Where I1= battery discharge current
E1=battery discharge terminal voltage
I2 = battery charging current
E2= battery charging terminal voltage
t1 = battery discharging time
t2 = battery discharging time
Cycle life of battery storage is the number of times the battery can be charged and discharged under specified conditions.
Solar energy storage through Chemical storage system:
Solar energy can be stored chemically in the form of fuel. The battery is charged photo-chemically and discharged electrically whenever needed. One of the possible method can be electrolysis of water by solar electricity, store hydrogen (H2) and oxygen (O2) and reunite in fuel cell to recapture electrical energy.
Solar energy can be converted into methane by anaerobic fermentation of algae. 1km2 of algae field can produce methane carrying 4MW of solar energy.
Thermo-chemical energy storage (Reversible):
Thermo-chemical energy storage systems are suitable for medium or high temperature applications only. The main benefit of this method is storage of energy at high density at ambient temperature and for lengthy duration without thermal losses.
Pumped hydroelectric storage of solar energy:
Electric power in excess of the immediate demand is used to pump water from a supply (e.g. like, river or reservoir) at a lower level to a reservoir at a higher level. At the peak hours when the demand of electricity is high, the water is allowed to flow back from high level to low level through a hydraulic turbine which drives an electric generator. The efficiency of pumped storage system lies in the range of 65 to 75%.
Solar energy storage through Compressed air:
Here, the extra energy is stored in the form of a compressed air volume. When energy demand is high, this air can be used to drive wind turbine to generate electric power.
Solar energy storage in Flywheel:
Flywheel is a source of storage energy in rotational kinetic energy. During off peak hours when the demand of electricity is low, electric motor can drive the flywheel and stores mechanical energy in it. The rotational energy of flywheel is used to drive generator to produce electricity.
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