The entire world today is talking about switching to renewable forms of energy, which are inexhaustible, environment friendly, ample in quantity and freely available from mother nature. Conventional, or non-renewable forms of energy are losing their appeal day by day, primarily because they are killing our planet, and also because they are expensive, such as coal, fossil fuels, natural gas, oils etc. There has been a radical change in the energy generation systems since the dawn of the 21st century. We are visibly shifting to renewable forms of energy, true, but how efficient is this prospect? Renewable energy sources include the solar irradiation from the sun, wind energy, hydro-power, biomass and biofuels, and so on. These resources are, unfortunately, not available all the time, i.e. they lack persistency and definiteness. The sun doesn’t shine equally at all times of the day, or on cloudy days. The wind velocity and air mass fluctuate indefinitely. And most importantly, we as consumers do not require the same amount of energy all the time. In electrical power networks, for instance, there are peak hours and off-peak hours. It is an irony that our peak hours and the hours of peak generation of power do not coincide. For example, in our country, we get maximum sunlight from 9am to 3pm approximately. But the peak demand for electricity rises from 5pm to 11pm usually. Even if we shifted to a fully solar photovoltaic system of power generation, could we make the best use of this technology if we didn’t have means to store the solar energy? This is exactly why the concept of energy storage is so vital in this age of renewable energy. The idea is pretty simple: it is the hoarding of excess, unused energy for a time when energy will not be available, i.e. what we exactly do when the city corporation announces a power outage for the next day. In honor of EEE Day, this article is dedicated to the electrical energy storage systems only.
Electrical energy is the one ultimate form of energy that can meet all our daily requirements — light, cool air, chilled water, tap-water, cooking and what not. But when it comes to storage, electrical energy must be converted into some other form of energy, because it is not possible to store raw electricity. Energy storage systems fall under three major classifications, viz. Mechanical, Electrical, and Electrochemical. The following is a concise concept of each type of energy storage system, of which the first four are mechanical, the next two are electrical and the last one is electrochemical in nature.
Pumped hydro storage (PHS) is known to all of us as hydro-electricity, thanks to our very own hydro-electric powerplant at Kaptai. This system utilizes the potential energy of water raised to very high altitudes by constructing dams. The water falls from above with a high velocity, hits the blades of a turbine, which rotates and runs an electric generator.
Compressed Air Energy Storage (CAES) is a fairly unknown theme to many of us. Here, air is compressed and stored at a cavern deep below the ground. As and when necessary, this pressurized air is released, which shoots upwards and rotates a turbine which in turn, moves a generator.
Flywheel Energy Storage (FES) helps electricity to be be stored as kinetic energy. A flywheel is often termed as a mechanical battery. It is a mass that can rotate about its axis. Electricity is supplied to a motor, which rotates a flywheel upto its rated speed. The flywheel maintains this speed by dint of its inertia, and supplies the rotational energy to a generator when needed.
Hydraulic Accumulators stores energy in the form of pressure. In a cylinder or accumulator with a sliding wall, a fluid is pumped continuously. As the amount of the fluid increases, the sliding wall moves upwards to make room for the incoming fluid, upto a maximum level. The sliding wall is actually a heavy mass or a spring, that gains energy as it moves. Whenever energy is to be extracted, the cylinder is emptied through a fluid outlet at a high pressure, that drives the output. In this system, energy can be stored upto a certain limit for as long as needed.
Ultracapacitors/Supercapacitors/Capacitors store electricity in the form of static charges. A capacitor is a simple device, having two conducting plates separated by a dielectric medium. The two plates store charges of opposite polarity. Since unlike charges attract, the plates are attracted to each other, but cannot travel due to the dielectric, or insulating, medium in between. These devices differ from normal capacitors in their high charge storing capabilities.
Superconducting magnetic energy storage (SMES) is a fairly new concept of energy storage. Superconductivity is the absence of resistance offered by a material below a critical temperature. So, if a material is cooled well enough, it becomes superconducting and carries a huge amount of current. A current carrying conductor has a magnetic field induced around it. A current carrying superconductor can continue to conduct current and thereby induce a huge magnetic field even if the source is disconnected from it. Thus, electrical energy can be stored in the form of magnetic energy.
Batteries can store electrical energy in the form of chemical energy, and is quite a common storage device. There are several types of batteries used for energy storage, such as, Lead acid batteries, Sodium Sulphur batteries, Lithium ion batteries, etc. This is probably the only energy storage system that we all use every day, in our phones, IPS, UPS, remotes, clocks, etc. Flow batteries are a relatively newer innovation. These batteries have separate tanks to contain the electrolytes, i.e. the reaction chamber and the electrolyte storage are distinct.
These systems, if adopted, can usher new days in renewable energy technology, as it will open the doors to 24-hour renewable power, despite the intermittent nature of renewable resources. It is a necessity for every person to know about these systems and understand their pros and cons in order to implement them. The world is progressing towards zero-energy buildings, microgrids and distributed generation technologies. Amidst these revolutionary technologies, the concept of energy storage is of utmost importance, because it can build a strong foundation for the foregoing technologies.
In the context of Bangladesh, we are in desperate need of a suitable energy system. Blessed with the tropical climate, we enjoy (or maybe suffer from!) plenty of sunlight for most of the year. We have the opportunity to avail a huge amount of solar energy that can power our entire country only if we are smart enough to collect it. If we are successful in this feat, then it will magnificently change the lot of our country for the better. And to do so, energy storage systems must be talked about and implemented on a large scale. If given the chance, which method of energy storage would you choose for Bangladesh, and why? Do give it a thought, because if you don’t, then who will?