How do we store electricity?

Electricity has been known to mankind for more than a century now. Ever since, there have been a lot of changes in the industry, with rapid developments in the 20^th century. What differentiates electricity from other form of energy is its characteristic of not being able to be store and accessed when wanted, in large quantities. This fact made the electricity industry a unique one, with many controls and reserve capacity required to maintain the system stability whilst meeting the ever changing and fluctuating demand. There are many forms of electricity storage but these units mostly contain small amounts of energy and are most suitable for electronics purposes rather than providing for electricity needs. In this article, a number of possible electricity storage options for will be presented and critically analysed followed by a review on which option is currently the best choice to be taken forward. This will be followed by a review of possible options for the electricity industry in Malaysia, given the existing generation mix.

The basic concept of conservation of energy is that energy can neither be created nor destroyed. Hence, energy changes its form from a range of possibilities, i.e. kinetic, mechanical, chemical and electrical energy, just to name a few. The problem with storing energy in different forms is the conversion losses that occur and these are simply known as the efficiency of the devices. So, what makes a particularly storage technology appealing? As always, many factors play their role and some are values more than others. A few characteristics that are vital include rating, size, weight and costs. Ultimately, it needs to be able to meet the particular demand need. For normal electronics devices, batteries are used to give out stable output of a known amount of electricity. However, for electricity storage used to serve electrical loads in a district or country, high discharge of energy is needed at times and at other times, a stable discharge of low levels of electricity might be needed. Therefore, it is vital to choose a storage that has the capabilities to do so. Figure 1 shows and compares the rated power and discharge time of major storage devices in a graph. The intersection of these devices is clear and easy comparison can be made. Many are concentrated in the 0.1-10MW rated power, but with long discharge times. There seem to be limited devices that can discharge rapidly, and even for those that do, their ratings seem to be low.

Figure 1 : Electricity storage ratings [1]

In order to evaluate the different storage technologies, the characteristics of major storage devices need to be known. The advantages and disadvantages of different storage technologies are compared in Figure 2, coupled with the suitable applications. Our focus here is to evaluate electrical storage technologies that can provide electricity when needed at the volume required. For this, the response time needs to be quick and the rated power should be comparable to the size of the demand. This in turn depends mainly on the size of region and the electricity load that we will be considering having storage as a support for the network needs. We will focus on two major types of storage here:

1. Pumped hydro
2. Flywheels

KEY:

Figure 2: Comparison of different storage technologies and its applications in either power or energy related usage [1]

For electricity storage, the major technology that is being used in many parts of the world is pumped hydro storage. This storage is based on having two dams, both placed at a certain height difference. When electricity demand is high, prices are high as well. This is when the water is released from the higher-levelled dam. When electricity prices are lower, water is pumped back to the higher dam and this usually happens at night. The advantage of pumped hydro is the fast response and the high power ratings that can be provided. This seems like a promising technology for most nations. However, hydro pump storages need large areas and hence affect the ecosystem. Apart from that, the building of these dams and operating systems incur high costs. The question that needs to be answered is that ‘Is it a justifiable storage mechanism to be used?’ Well, the answer depends on many things and from which perspective are we looking at this.

1. From a reliability point of view, this is a good system as it provides fast response when needed.
2. From a power plant operator’s/investor point of view, as long as they are making money (i.e. the prices between the different times of the day are far apart), they are satisfied and are willing to make that investment.
3. For the government’s point of view, for stability of the grid, this is good. From a ‘green energy’ view, this method depends on what source of energy is used in the system to pump the water upstream. If it is from a renewable source, it adds benefit to the overall system. However, if gas or coal plants are used, it might reduce its ‘green’ credibility.

Having looked at pumped storage which uses potential energy, we now look at flywheels, which use kinetic energy as a means of storage. Flywheels are not new as they have been used in many other applications such as in transportation. However, only recently this technology has been explored to provide energy storage. Its operating mechanism is simple. By accelerating the flywheel at a very high speed, we can maintain the energy in the system as rotational energy or better known as kinetic energy. Surplus energy in the system is used to charge the flywheel by accelerating the flywheel. When we need energy from the storage, the rotation speed is slowed down. Flywheels have many benefits compared to batteries due to the absence of chemicals. One such benefit is the longer life time of these devices as they are not affected by the charging and discharging cycles. They are also useful in providing fast response needed by the grid operator to maintain the system stability. However, flywheels tend to be big in size and the constant rotating motion can be noisy. This might hinder their usage in certain applications. The upfront cost of this technology is still high and hence, the cost needs to be weighed against the life expectancy and alternative storage options.

Storage is important in ensuring good reliability of supply. Countries such as the UK and US have reliability of supply of the magnitude of 99.99% and 99.97% respectively.  These figures are extremely good due to the proper management of the system. When wind and other intermittent sources of energy are introduced into the electricity network, supplies are not easy to estimate. In this scenario, good storage for charging and discharging of energy as and when needed plays an extremely vital role in maintaining the reliability of supply. Malaysia has seen increase in supply reliability. However, as consumers, we can see that electricity interruption does occur every now and then, albeit less frequent than before. Malaysia, blessed with sunshine all year round should take advantage of extracting solar power. Its intermittency of supply can be balanced out using storage devices as outlined above. What is necessary for the government to do is set realistic targets and ensure enough funding is channelled to induce more electricity generation using solar panels, coupled with suitable storage technologies. This will not only facilitate our nations move towards renewable but also increase our expertise in dealing with intermittent electricity generation.

[1] http://www.electricitystorage.org/site/technologies/technology_comparisons/

Filed under: Energy, Power