**4. Storage technologies for bridging power**

When there is error in unit commitment and contingency reserves, these applications provide load forecast, contingency reserves and additional reserves, and require rapid response and discharge time. Lead acid, Nickel metal hydride, lithium ion and Nickel cadmium batteries can provide frequency regulation, on the other hand can limit battery life [8].

### **4.1 Storage technologies for energy management**

These applications require continuous discharge rating for several hours; following technologies are used in the technologies.

## **4.2 Thermal energy storage**

In this storage system energy is not stored and directly used, so some times this can be ignored as electricity storage technology, but can be equivalent to electricity storage in some applications. In solar thermal generator, solar power is store in molten salt or another medium is used to generate electricity [8].

## **4.3 Pumped hydro energy based storage**

In this method of energy storage, potential energy of water is being stored by pumping water from lower reservoir to the higher level at low price or during off peak time and same stored water is used in the peak hours by operating turbine to generate electrical power. To transfer water from lower reservoir to higher reservoir wind and solar based pumped storage hydroelectric storage system are shown in **Figures 1** and **2**. Electricity can be generated at low price using this technique, but it requires appropriate geography and availability of sufficient amount of water. Currently this storage system is working as large scale energy storage, along with improving the daily capacity factor of power plants. According to Electric power Research Institute about 99% of the bulk energy storage around the world (127GW) is done using pumped hydro energy storage system [42]. Large PHES system has

**Figure 1.** *Wind power based pumped hydro energy storage system [7].*

*The Role of Energy Storage with Renewable Electricity Generation DOI: http://dx.doi.org/10.5772/intechopen.96114*

**Figure 2.** *Solar PV based pumped hydro energy storage system [7].*

capacity of more than 10 MW, small PHES system has capacity from KW to 10 MW, and micro PHES system has capacity of 100KW and the Plants having capacity less than 5KW are known as Pico pumped hydro energy storage system. As installation of renewable energy is increasing, PHES is becoming an important part of energy generation system due to their efficiency in range of 70% to 80. In different geographically areas wind base pumped hydro energy storage systems are considered as most economical and technical competitive [43, 44].

As solar energy is available to us during day time about 6–8 hours in a day, hence to get the proper and maximum benefit of this solar energy solar based pumped hydro energy storage system PHES system is made; using that method the required energy is transmitted through the grid system and remaining excessive energy is used to store the water in upper reservoir from lower reservoir during off peak period, and by releasing that water during peak hours electrical power can be cost effectively generated. It has large capacity, long service and low unit cost, but due to geographical conditions impact this technology is somehow redistricted.

#### **4.4 Compressed Air Energy Storage (CAES)**

Air is compressed and energy is stored in an airtight underground storage cavern, at the time of energy requirement that compressed air is taken from vessel, heated and expanded in high pressure gas turbine, where that air is mixed with fuel and combustion takes place and exhaust is expanded through low pressure turbine to environment through chimney. CAES has compression ratio of 0.6–0.8 and heat rate of 4000–4300 BTU/kWh [8]. This technology has huge capacity, as well as operation time, and service lifetime, along with that it can supply combine heat, cold and electrical power. But, due to their low capacity and complexity, and location dependent this technology is also redistricted.

#### **5. Heat storage**

Latent heat and sensible heat storage are two basic types or technologies used in heat storage. In sensible heat storage, water (heat storage medium) is heated to increase the temperature; latent heat storage uses the regenerative material achieving phase change heat storage, which is implemented on the solar thermal power generation. During rainy season, electricity can be produced using photo thermal system; in this way output is able to adjust according to requirement. Thermal storage system have efficiency about 95%–97%, cost is low as compare to that of large scale battery storage about (1/30th).
