What is chemical energy storage?
An example of chemical energy storage is the common battery. By using the liquid inside it to store electricity it can then release it as required. Large batteries can act as chemical energy storage for industry and could make future energy generation solutions more efficient and profitable. This will be achieved by storing energy generated when demand on the grid is low and releasing this as required to help meet peak demand.
How does the battery chemical energy storage process actually work?
Batteries are portable devices that can be used in many different areas. The way a battery works is very simple and based on three components:
- The anode (the negative pole),
- The cathode (the positive pole),
- The electrolyte (the liquid chemical that produces the flow of energy).
The anode and the cathode are also known as ‘terminals’ and are made of a metal, which are then separated by the electrolyte.
Converting stored energy to electricity
If you take a device like a light bulb or a simple electrical circuit and connect it to the battery terminals, the chemical on the anode causes a release of electrons to the negative pole and ions in the electrolyte. This is the chemical oxidation reaction.
On the positive pole the cathode accepts the flow of electrons, which completes the circuit for the flow of electrons. These two reactions happen simultaneously: the ions transport current through the electrolyte while the electrons flow in the external circuit. This then generates the electric current.
Storing electrical energy in a chemical store
The process for battery energy storage works in reverse, transforming electrical energy into chemical energy. When excess electricity is produced in the grid, it can be channelled into a battery system, and then be stored in the chemical system.
The mobile phone and electric car both take advantage of a rechargeable battery system. The hope is that in the future this process could be ‘up-scaled’: when electricity is produced by intermittent renewable sources like wind or a solar PV it can be stored in big industrial sized battery systems.
What types of batteries can be used for mass energy storage?
There are a number of different battery solutions that are currently being used in industry and under consideration for mass scale national grid use. This section briefly considers each type for these ambitious future requirements.
Lithium-ion batteries are the fastest growing battery type in the consumer market today. They have many uses, including powering laptops, mobile phones and hybrid vehicles due to the high amount of energy they can store. They also have high energy-efficiency, operate well under a wide range of temperatures, can be recycled and also have a low level of self-discharge.
However to be used as a grid storage solution this type of battery will require some refinement. They will need to operate with improved lifespan (number of charging and discharging cycles that can be achieved) and improved safety. Most importantly the cost to produce the lithium-ion batteries needs to come down – a storage solution these days needs to be cost effective.
Lithium-ion polymer batteries
Like the lithium-ion battery, the lithium-ion polymer batteries not only have a high-energy output, but also have a good safety record and a longer life span. However these are also uneconomical to produce, so the production costs would need to come down to make these viable as the mass-produced storage solution.
Lead-acid batteries can be designed to power large applications and are relatively cheap, safe, and reliable. They are already being used in large storage and uninterrupted power supply solutions (e.g. emergency lighting and powering back-up generators), which means they can be increased further in size to power grids. They can also be easily recycled and an infrastructure around this process already exists.
The problem they have is that they are rather large, heavy and immobile. They also have poor cold temperature performance and a short life cycle.
There are several characteristics of a flow battery system that will enable them to provide very high power and very high capacity on a grid type system. For example, unlike a conventional battery system, the energy output is independent of the energy storage capacity. While output depends on the fuel cell stack, the energy storage depends on the size of the electrolyte tanks and these are independent from one another. This operating capability is very useful when large current flows need to be transported to a national electricity grid system.
Energy output ratio to weight can be up to three times better than lead-acid batteries, but they do have lower energy efficiency.
At the moment, there are only experimental flow battery schemes in operation and, since they haven’t been around as long as the lithium ion battery, it is taking longer for electricity distribution industries to adopt them.
A sodium–sulphur battery is a type of molten-salt battery constructed from liquid sodium and sulphur. The sodium-sulphur battery has a very high energy and power density as a result of sodium being a highly reactive alkali metal. This type of battery has a high energy density, high efficiency of charge/discharge (89–92%) and long cycle life, and is fabricated from inexpensive materials.
However, they operate at a temperature of about 300-3500C, and therefore they require energy to keep them operational. And, due to the highly corrosive nature of sodium polysulphides, such cells must be kept stationary. Therefore they are ideal for for energy arbitrage, which is when the grid system fluctuates between peak demand and supply, so the battery can help manage the load.
Could chemical energy storage be a commercially viable solution?
We are still far from producing batteries that are a viable and cost-effective solution to managing the variation in grid systems. It would be incredibly expensive to make batteries capable of storing excess energy on the grid. So if this energy storage solution was implemented it may significantly increase the cost of electricity to consumers, which would be highly unpopular in the current economic climate.
People use electricity around the clock, so electricity transmission and storage must be kept in mind as key players. Smart grids will require advanced utility-scale batteries to store electricity so it can be delivered when needed.
How does chemical energy storage compare to other storage technologies?
In the UK, the DECC has been running a programme funded by public money that looks at various energy-storage solutions that could be used in the national grid. At the moment there is no obvious solution as everything from compressed air storage to molten salt and battery power is being considered.
The UK does have pumped storage (hydroelectric), but not on a scale seen in countries like Norway and Canada, which make use of their natural topography.