In the recent decade, there has been a boom in the renewable energy sector, especially for solar and wind energy. This is great news but there is an underlining problem. Wind and solar energy are collectively called Variable renewable energy or VRE. As the name suggests it is not a stable and reliable source of energy which makes it difficult for the utility sector in providing stable output.
Read more: The Problem of Variable Renewable Energy
Fossil fueled power plants can produce a stable and controlled output but it produces loads of greenhouse gases and causes a lot of pollution. To solve this puzzle, modern technologies in Grid Energy Storage can be the last piece to the incomplete picture of attaining a 100% renewable energy supply.
- What are the types of Grid Energy Storage
- Gravity Battery Storage
- Mechanical Energy Storage
- Battery Energy Storage
- Some Honorary Mentions
What are the types of Grid Energy Storage?
Grid energy storage has been used in the past and is still in operation. Though more modern and advanced storage solutions are in development. I have tried to break it down by categorising the classification according to the form of the technology.
Gravity Battery Storage
The basic definition of a gravity battery is that it uses gravitational force to store electrical energy by displacing a heavy object at a certain height. It stores the extra electrical energy produced by a variable energy system during off-peak times of a day and discharges the energy to the grid when demand is high.
Pumped-Storage Hydroelectricity (PSH)
This is similar to a hydroelectricity plant where electricity is produced with the help of running water. The difference is that the source of energy of a Pumped-Storage Hydro (PSH) is a combination of two lakes/ reservoirs which are placed at different heights. This difference in height of the two water bodies creates a potential difference, the more the difference of height the more the potential energy. The water from the lower reservoir is pumped and stored on the upper reservoir and when required the water of the upper reservoir is released to electricity is generated.
Advantages of PSH:
- PSH is a mature technology and also is one of the most dominant and widely used grid energy storage technologies in the world accounting for 95% i.e., 181 GWs of storage energy
- It has a long life cycle of up to 60 years, large volume, and long discharge capabilities, which can go up to 5000MW
- It also has a very high-efficiency rate which can range from 65 o 85%
- Has a very low per kW and maintenance cost
- It has a negligible amount of self-discharge, which makes it a good choice for a grid energy storage
Disadvantages of PSH:
- PSH requires a huge initial investment in the construction of reservoirs
- Like a dam, it also causes environmental damage during the construction of reservoirs, pipelines, etc.
- The response time is not fast enough in suppressing the fluctuation of wind-powered grids
- The main limitation of a PSH is its dependence on topological orientation. It requires having two water bodies aligned to each other at a certain difference in height. Which is a limited source of requirement.
- The construction time of a typical PSH takes years and thus is not always viable
Mechanical Energy Storage
Compressed Air Energy Storage (CAES)
Compressed air plays a vital role in the industrial process in the transfer of energy and it is gaining pace in the domain of grid energy storage. After PSH, CAES is the second most common technique to store grid electricity.
There are many techniques, both theoretical and practical, in which it can attain very high efficiency but many are still under research. The most popular one that has emerged is where it utilises a huge underground salt cavern. During a low-demand period, ambient air is pumped into the basin at high pressure, and the excess heat generated from compression is released into the atmosphere. During discharge, the air is reheated and expanded to run the turbines. Here natural gas is used in the reheating process.
Advantages of CAES:
- Has a very low response time and can startup within milliseconds, thus is ideal in a situation in suppressing sudden fluctuation in demand or supply
- A low capital cost
- A high-efficiency rate which ranges from 70 to 85%
- Has a long discharge time just like a PSH system and generate energy for more than 24 hrs
- Is suitable to integrate it with renewable energy like wind and solar due to its rapid response time
Disadvantages of CAES:
- The main disadvantage of CAES is its geographical dependence just like a Pumped hydro storage system. There are limited underground salt cravens available that can be suitable and sustainable to set up a system.
- Current systems need fossil fuels to reheat the pressurised air when expanded, thus adds greenhouse gas to the atmosphere.
Though there are emerging CAES systems where the heat generated due to compression of air is stored and is used when the air is expanded. Thus eliminating the use of a fossil-fuelled heater.
Flywheel Energy Storage (FES)
The basic mechanism of a Flywheel Energy Storage system is that it uses the angular momentum of a wheel to store electric energy. The more rotations per minute (rpm) the more energy stored. When the energy stored is discharged, the speed of the flywheel decreases as there is the conversion of energy.
Advantages of FES:
- It requires very low maintenance and has a prolonged life cycle
- Is perfectly suitable to balance the grid when are sudden fluctuations in demand and supply. Is very suitable to compensate for the variability of wind energy.
- It has zero greenhouse gas emission and has a very fast response to discharge time which makes it a good replacement choice over natural gas-powered peaker plants
- Can have a higher efficiency of over 90% in case of a shorter stand by period and also depending on the components used
- Is not affected by temperature changes
- Is not subject to catch fire due to chemical reaction like batteries
Disadvantages of FES:
- Flywheels lose a lot of their energy due to friction between mechanical parts and also to the change in orientation of the Earth’s axis. It can lose almost half of the energy stored within a couple of hours.
- Can only be used as a short term energy reserve and discharge
- Requires a much larger area due to dependency of the angular mass of the wheel unlike a battery array setup
- Has a limitation in its storage capacity
The Flywheel funded by NASA, G2 Flywheel for spacecraft energy storage was built on a vacuum enclosure and the wheel was mounted on magnetic bearings. The system had a very high round-off efficiency and was designed to run at 60,000 rpm with a storage capacity of 1kw. Such type of magnetic flywheel is in the works, this could also increase the life span of the system considerable as there is no wear and tear.
Battery Energy Storage
Li-ion Battery Storage
Battery Storage technology is the second-most used electrical storage technology, with a total worldwide capacity of 365 GWh. There many types of batteries used worldwide but Li-ion batteries are gaining momentum at a very high speed. Due to Tesla’s push of battery storage tech, in recent years, the deployment of Li-ion batteries used for grid energy storage has spiked.
Advantages of Li-ion Battery Storage:
- Li-ion batteries have a very low self-discharge rate and do not exceed more than 8% per month
- They can be stacked up to increase their capacity and are not dependent on any topological conditions
- Has a high energy capacity, with the largest installed by Tesla in Australia with a total capacity of 100MW
- Low maintenance, have a high life cycle, has no toxic emissions, and low environmental impact
- Can attain a high efficiency of over 90%
- Is very responsive and can discharge within milliseconds
- Due to its high energy-to-weight ratio, it is ideal for Electric Vehicles
Disadvantages of Li-ion Battery Storage:
- One of the biggest disadvantages of a Li-ion battery is its cost. It is still quite expensive to make and also increases the cost per kW when discharged to the grid.
- It requires extra protection to prevent fire or sudden rise in temperature
- Li-ions can be fatal when caught fire. As Li-ions are made by connecting small cylinders of cell stacked up together it causes a chain reaction which called a thermal runaway.
- Li-ions are extremely sensitive to high temperatures and can burst into flames causing unprecedented damage
- Is subject to ageing if not used for a long period of time
Vanadium Redox Flow Battery Storage
Redox Flow Batteries (REDuction-OXidation) is a type of rechargeable battery that uses two Vanadium-based liquid electrolytes for both cathode and anode. These electrolytes are stored in external containers and are pumped into a tank which is separated by a membrane. Ions are exchanged through the membrane to maintain chemical equilibrium, thus generating a flow of current. The exchange of ions is reversed when the system is charged to store energy from the grid.
Advantages of Vanadium Redox Flow Battery Storage:
- The main advantage over a solid-state battery is that the power and the energy capacity can be designed separately. The power of the battery can be adjusted depending on the size of the electrodes and the energy capacity can be varied by the concentration of the electrolytes.
- As compared to a Li-ion battery, it has a longer life span with more cycles. A typical calendar life falls between 10 to 20 years with an estimated 10,000 cycles and above
- Is insensitive to temperature, which helps to have a longer life span, and also there is no need to spent extra money on safety precautions
Disadvantages of Vanadium Redox Flow Battery Storage:
- The electrolyte Vanadium is hazardous in nature and is considered toxic. Direct exposure can lead to permanent health problems and even death. So it has to be handled with high precaution.
- A redox flow battery has very low energy to volume ratio. This means for the same weight a Li-ion battery can store more energy.
- The aqueous nature of the electrolyte makes it difficult for it to be transported
Some Honorary Mentions
Here I will be mentioning only upcoming technologies which fall under the above classification but are under research and development. I will be updating the list as the story unfolds.
A UK-based company called Graviticity is in the works by collaborating with the collaboration of the Dutch winch manufacturer Huisman Equipment BV. They plan to use old mineshafts where they will lift (charge) and release (discharge) heavyweights to generate and store surplus electric energy produced during off-peak periods.
Hydrogen Energy Storage (HES)
HES is a type of chemical energy storage which uses similar technology as that of Compress Air Energy Storage. Hydrogen is generated by the process of electrolysis, where direct electric current is passed on the water to decompose it into hydrogen and oxygen. It uses a salt cavern just like a CAES to store the extracted Hydrogen. The stored hydrogen is then extracted to release energy by re-electrification. It is done either by mixing it with natural gas to reduce emissions or is used fuel to generate electricity. HES is still in a very early stage and has a low round trip efficiency of about 50%, but is gaining pace to replace the natural gas.
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