9.1. Options for energy storage
Because solar energy supply is variable in time, energy storage is an important issue. Energy storage is used to collect the energy generated by the solar conversion systems (thermal or photovoltaic) in order to release it later on demand. This can be a situation when sufficient power is produced during the day, and stored energy is used during the night. Also, when insolation conditions are ideal, the solar system may produce enough power for the target application, but on dull days, direct energy supply from collectors is diminished, and the energy from the storage is used to compensate the deficit. Energy storage devices help to smooth out differences and minor fluctuations in energy supply caused by shading, passing clouds, etc. Development of efficient and cost-effective energy storage is considered the main bottleneck of the universal development of solar systems.
There are quite a few different technology options for energy storage, which are briefly outlined below:
- Grid. For grid connected solar systems, the most natural and cost-effective way would be to store energy in the grid. The main idea here is that the DC power from a solar facility (array or farm) is converted to AC power and is fed to the grid and further on is used for on-site or off-site applications. This way, the grid acts as the medium that collects energy from different power-making facilities (renewable or non-renewable) and redistributes it as necessary. Since a grid does not really represent a separate system that is part of a solar plant, it will not be discussed further in this lesson.
- Fluid. Fluid-based storage is typically used with solar thermal systems. Unlike grid, which stores electrical energy, fluids store thermal energy. Fluids, such as water, oil, molten salt or others act as a medium for absorbing heat. The main idea is that the solar radiation heats the heat-transfer fluid which is accumulated in the tank. The tank is insulated, so the hot fluid keeps its temperature for a substantial period of time. When needed, the heated fluid is used in a heat-exchanger to produce steam for the electric generator. This type of thermal energy storage was discussed in more detail in Lesson 8.
- Battery. A battery is an electrochemical device that stores chemical energy in internal components and releases energy as electricity, which is generated through electrochemical reactions. Batteries are reversible, i.e., can be charged and discharged, and the parameters of these processes are regulated to avoid damage by overcharging or over-discharging. Battery life is expressed in number of charge-discharge cycles. There are many different types of batteries, some of which will be discussed further in this lesson.
- Hydrogen. The idea behind hydrogen storage is that electricity generated by solar PV systems can be used to electrolyze water - to split it to hydrogen and oxygen. Further, hydrogen gas is collected and can be used as a fuel. One of the highly efficient devices "converting" hydrogen back to electricity is H2/O2 fuel cell, which has zero carbon footprint during operation.
- Compressed air. In this case, the electrical energy produced by a PV solar system is used to run compressors to compress massive amounts of air and store it in underground, above-ground, or underwater containers. Later on, when energy is needed, the air is decompressed and is supplied to a turbine to generate electricity. Compressed air energy systems (CAES) promise high efficiencies, although this technology is not yet widely implemented.
- Pumped storage hydropower. The available energy can be used to pump water into an elevated reservoir for storage. When power is needed, the water can be discharged under gravity to run a turbine, which is connected to a generator to produce electricity. The same as compressed air systems, the pumped storage technology has high energy return on investment, although it may require special topographical conditions and water availability in order to be used.
All of the above options for energy storage should be employed with understanding the facility needs and capacity. What energy storage is efficient for small residential systems may be insufficient or too costly when scaled up to the utility-size systems. Determining capacity of energy storage for a particular solar project is an important technical and economic issue. For example, if the capacity of the storage is too large compared to the energy produced by the solar conversion facility, the total system cost will be unnecessarily increased. On the contrary, if the capacity of the storage is too small, that leads to energy dumping and overall unsatisfactory plant performance.
In the following sections, we will discuss different energy storage options that can be possibly applied to utility scale solar systems.