Solar Energy Storage System Types

As the demand for clean, renewable energy sources continues to grow, solar power has emerged as a leading solution for sustainable electricity generation. However, the intermittent nature of solar energy necessitates efficient storage systems to ensure a stable and reliable power supply. Solar energy storage systems are designed to capture excess energy during peak sunlight hours and release it when demand is high or solar availability is low. These systems are broadly categorized into thermal storage, electrical energy storage, mechanical energy storage, chemical storage, and hydro storage, each with distinct mechanisms and applications. Thermal storage can be further divided into sensible heat storage—utilizing mediums like water and rocks—and latent heat storage, while chemical storage includes advanced thermo-chemical storage technologies. 

Solar energy storage system can be classified as shown in Figure 1.

Figure 1. Types of Solar Energy Storage System

Thermal Storage

Energy can be stored by heating, melting or vaporization of material and then simultaneously energy can be recovered as heat when the process is reversed. Whenever any material is heated, it would develop following two types of changes;

(1) Sensible heat change

The rise in temperature of the material is there in sensible heating and as such it would not give any indication of phase change i.e. solid to liquid or liquid to vapor etc. The total energy responsible for this change can be represented as

Q_s = MC_p (T₂ - T₁)

Where

• M = mass of material to be heated

• Cp= specific heat of material

• T2 = final temperature of material (before boiling or melting)

• T1 = initial temperature of material at which heating is started.

(2) Latent heat change

This change indicates phase change i.e. conversion of solid into liquid or liquid into vapor through the action of heat. Here the energy responsible for this change can be represented as

Q_L = Q_S + M_wL

Where

• M_w = mass of matter evaporated

• L = Latent heat of vaporization.

The basic fundamental of above two heat changes can be incorporated for development a solar storage system based on thermal storage aspects. Thermal energy storage is essential for both domestic water and space heating applications and for the high temperature storage system needed for thermal power applications. Water and rock are two examples, in which solar energy can be stored on the basis of thermal storage aspects. The other materials, which can be used for these storages are iron shot, iron ( Red iron oxide or iron ore), concrete and refractory material like magnesium oxide, aluminum oxide and silicon oxide.

Figure 2. Sensible Heat Storage System

Water is most common material for storage of solar energy. The example of this approach is insulated water storage tank coupled with solar water heater, where energy is added by circulating water through collector and is removed by circulating water through load (Figure 2)

The optimum tank size for the float plate collector system is usually about 70 lit/m2. Water has the following characteristics for storage medium

• It is readily and easily available useful material.

• It has higher thermal storage capacity

• It permits energy addition and removal from the medium itself, thus eliminating any temperature drop between transport fluid and storage medium.

• It includes pumping for transportation purpose. The pumping cost applicable to water is small.

• The heat transfer can be made by natural convection as in domestic solar water heater or by forced convection with the help of pump, blower or fan etc.

• It is superior storage medium as compared to rock, gravel or crushed stone, because of its lower material cost and lower volume required per unit of energy storage.

The rock, gravel or crushed stone can also be used as energy transport mechanism with air in sensible heat storage. The rock permits large heat transfer area but its thermal capacity, however, is only about half that of water. Therefore, rock storage volume will be about 3 times the volume of a water tank that is heated over the same temperature interval. The rock has following characteristics-

• It can more easily contained than water.

• It acts as its own heat exchanger, thus reduction in total system cost.

• It permit storage at high temperature (above 100 oC or so), which cannot be used in liquid form without an expensive, pressurized storage tank.

• The heat conductivity of rock storage depend on air flow. The conductivity of the bed is low when air flow is not present.

• The cost of storage material is low.

• The heat transfer coefficient between the air and solid is high.

A packed bed storage unit is shown in Figure 3 it essentially includes a container, porous structure to support the bed and air distributions. In operation, the flow of heat is maintained in one direction through the bed and in opposite direction during removal of heat.

Figure 3. Packed Bed Storage Unit

Similarly, in the thermal storage rout, latent heat storage units are also available. Materials that undergo change of phase in a suitable temperature are adopted for this approach. Few examples are Glauber’s slat (Na2SO4.10H2O), Fe(NO3).6H2O, organic compounds, Salt Eutectics and water. The material used in latent heat storage units have following characteristics-

• The phase change take place at a high latent heat effect. It stores large quantities of heat.

• The phase change process is reversible over a very large number of cycles without serious degradation.

• It accompanied with limited supper cooling

• It has a mean for transfer heat into it and out of it.

• The phase change occurs close to its actual melting temperature.

• The material used for this is essentially harmless i.e. nontoxic, non-inflammable, non-combustible and non-corrosive.

• The preparation of phase changing material for use is relatively simple.

Electrical Energy Storage

Capacitors and inductors are used for storing electrical energy. The capacitors can store large amount of electrical energy at high voltage and low current for long period, however, inductors store electrical energy at low voltage and high current.

Theoretically total energy stored in capacitors is given as

H_cap = ½ VϵE²

Where

• V = is the volume of the dielectric

• ε = is a constant and

• E = is the electric field strength

The electric energy store in capacitor depends upon the break down strength of dielectric. Best example of dielectric material is mica. Presently the electrical energy stored in capacitor is not economical, because it can store energy upto 12 hours duration. Further, most of capacitor is costly. Since the conductivity of dielectric is nil, therefore there will always be losses due to leakage. As a result of it the storing period is limited in capacitor.

The energy stored in an indicator is given as

H_ind= ½ VμH²_m

Where µ is permeability of the material and Hm is the magnetic flux density. Therefore, in order to have more electric energy storage (Hind), both µ and Hm should be large. Further for Hm to be more, high magnetic field are necessary, which required strong supporting structure. In addition to this, at the time of releasing of electrical current from the inductor, the circuits carrying large current should be open. The opening of circuit carrying large currents is a big problem in present setup.

Electrical energy can also be stored in the primary cell, which comprises two electrodes (anode and cathode) and an electrolyte (an ionic conduction). The primary cell is very simple device for storing electrical energy. Since it has no moving parts, therefore, it works at high efficiency and always the output is in the form of electrical energy. However, these cells are expensive, because of their very low storing capability for a given volume of cell. Examples are mercury, silver-zinc cell, mercury-carbon, lead acid, nickel-iron and nickel-cadmium batteries.

Chemical Energy Storage

The chemical energy storage of solar energy consists of a battery, which is charged photo chemically and discharged electrically whenever needed. The photochemical reaction takes place in the presence of solar radiation. Some of the reactions in type of storage are:

2NOCL+Photons→2NO+Cl₂

AgCl(s) + Photons → Ags + 12Cl₂

NO₂ + Photons → NO + 12Cl₂

H₂O + 12O2H2O2

It has been observed that all in above reactions, the rate of reverse reaction is slowed down, when one of its products is precipitate, also it have side reaction and back reaction at faster rate. These are the reason why chemical storage of solar energy is not getting popular.

Hydrogen energy can be generated through electrolyzation of water in presence of solar radiation. It is also possible to electrolyze water with solar generated electrical energy, store oxygen and hydrogen and recombine in a fuel cell to regain electrical energy.

2H₂O ↔ 2H₂ + O₂

Further, solar energy can also be used for anaerobic fermentation of solar algae into combustible fuel gas i. e. methane (CH4). The methane is an excellent fuel for meeting out much energy related activities; it is lighter than air and can be easily stored at room temperature. The stored energy of CH4 can be released into thermal energy whenever it burns in presence of oxygen.

Stored energy of CH₄ + O₂ → Thermal energy (20 MJ m-3)

It has been observed that solar energy can be converted into methane gas with 2% efficiency. Thus, one square kilometer of algae filed could produce an amount of methane storing 4 MW of converted solar energy.

Photosynthesis is also a method of solar energy storage in chemical form.

6CO₂ + 6H₂O → 6C₆H₁₂O₆ + 6O₂

During this reaction, in presence of chlorophyll and solar radiation, atmospheric CO₂ and H₂O are combined and produce carbohydrates. These carbohydrates can further release stored energy in thermal energy, at high temperature, whenever needed

Hydro Storage

Hydro storage is a reasonable efficient method of storing solar energy by storage water into an elevated reservoir during period of solar energy radiation and recovering the stored energy by running water through a turbine when energy is needed (Figure 4).

Figure 4. Hydro Storage System

Pumped water at elevated will have potential energy

$$U_{g}=mgh$$

Where 

• m is mass of water and 

• h is height of tank.

If this potential energy is converted into kinetic energy to run a turbine, then Kinetic Energy (K.E.) of water during running $=\frac{1}{2}mv^{2}$

Therefore,

$$mgh=\frac{1}{2}mv^{2}$$

$$V=\sqrt{2gh}$$

$$a^2 + b^2 = c^2$$

This velocity will be used to design turbine coupled with generator.