Voltaire Energy
Written by Brooke Joseph, Dong Nguyen Minh Anh, Jens Thomsen and Theodore Grether-Murray
Problem
When you think of electricity, you most likely think about turning on a light switch and then there is light. Yet how can you be sure that when you turn that light switch the light bulb doesn’t just stay dark? Well because you trust your local grid system to provide power when you need it.
In most industrialized civilizations there is a steady supply of energy. When there is more demand they turn up the production of electricity, as simple as that. Yet what is going on in the background is far from simple. Especially if you take a look at transitioning from fossil fuels to renewable energies.
No one can deny that fossil fuels are a great way of producing energy if they didn’t emit CO2/Greenhouse gases. That is the big caveat of it all, they do. If we want to stay on planet Earth, which at least I do, then we need to find a way to produce energy without emitting CO2 or greenhouse gases. Producing a steady stream of renewable electricity that we can control which can be as cheap as fossil fuels or even better cheaper than fossil fuels.
Yet when looking at renewables like solar and wind they hit all of the targets except the one about consistency and controllability. We can’t just throw in more coal or turn up the gas line with renewables. This makes wind and solar intermittent, meaning that they don’t supply energy constantly.
When the wind doesn’t blow there is no power. That is just not a viable solution for the modern world.
If we want to make the transition from fossil fuels to renewable energy, then we need to have some way of storing renewable energy when we are producing more than we need and some way of feeding that back into the grid when we need more than we are currently producing.
So to be able to move away from fossil fuels and into a world where we produce zero carbon emission energy, we need to be able to store energy cheaply and efficiently.
The parameters for a good storage system
All this is to say that renewable energy storage is key to greenly electrifying our grid. So what would the ideal energy storage solution look like? In our opinion, the parameters for great energy storage would be that it’s:
- Cheap: Under 10$/kWh to be cost competitive with natural gas
- Can store energy for long periods of time: Certain countries have weather variations that affect the production of electricity from renewables. For example, Finland in the winter has about 4 hours of sun, which does not far well for solar electricity production. So to last seasonal change, we would have to store energy for long periods of time
- Uses no special materials: This is an important factor as we don’t want the stability of our energy grid to rely on materials that are hard to get and mined using unethical practices (for example copper for lithium-ion batteries). We would want the energy storage system to use abundant materials, that you could find anywhere.
- Transportable: This is not necessary but it is an added bonus since you can export excess electricity to other countries.
- Not geographically bound: If you think about it pumped hydro is the best type of renewable storage system we have, yet it’s geographically bound meaning not everyone can use it.
However, current energy storage systems are far from meeting these criteria. For instance, lithium-ion batteries require special materials that need to be mined in unethical ways. Thermal energy storage is great for industrial processes that need a lot of heat, but once you try to convert that heat into electricity, the efficiency goes down to around 40% (which is not optimal at all).
Pumped Hydro is an excellent option, but its geographic limitations make it unsuitable for widespread adoption. Green Hydrogen is costly and inefficient. Nuclear energy is highly effective but not widely accepted due to safety concerns. Other energy storage solutions are still in the research phase and may take another decade to become cost-effective.
One energy storage solution is called liquid air energy storage (LAES). This is a process where the air is cooled to cryogenic temperatures to convert it into a liquid, which is then heated to produce steam to power a turbine and generate electricity.
Liquid air energy storage can…
- Store energy for long periods of time to last seasonal change
- Can be transported to other countries that can use the energy
- Isn’t geographically bound and can be set up anywhere
- Uses no special material as you’re using the access electricity from renewables to cool air (which is everywhere)
LAES seems like one of the best storage systems out there so who’s working on it and how does it work?
Highview Power
Highview Power is a UK-based long-term energy storage company. It was started out of a project from 2005 coming from the University of Leeds. Its mission is to make intermittent energy sources like wind and solar a more viable solution.
It does this by using the energy during renewable’s peak production hours to cool down air until it is liquified. When there is a demand for energy again during off-peak hours (when the sun isn’t shining and the wind isn’t blowing), it will heat up the liquified air and drive a turbine to generate electricity.
One main advantage of Higview power is that it can store energy for longer durations of time. Ranging from hours to weeks. This gives them a major advantage in competing against lithium-ion batteries and makes them able to compete with pumped-hydro. Which can store energy indefinitely.
An important extra fact that might have a significant impact on the future implications of LAES systems like the ones Highview is producing. Their liquified air can be used as a cooling agent in for example data centres. And for those of you that didn’t know data centres consume about 1% of the world’s power supply. Where most of the power is used to cool the machinery.
Here are two examples of projects they are working on one being in Manchester UK and the other being in Vermont USA.
The project in Vermont was announced in 2020 and is expected to be finished in 2023. The project is being built in collaboration with Encore Renewable Energy. With a system storage capacity of 250MWh, it is one of the world's largest projects in this category. It is designed to help stabilize the energy grid in the region. The power plants life expectancy is about 30–40 years.
The project in Manchester is being built in partnership with Viridor a UK-based waste management company. The project was completed in 2018 and has a capacity of 25MWh of power storage and is rated at 5MW of power delivery. It was the first commercial-scale cryogenic energy storage project in the world and was awarded the 2019 Ashden Award for energy innovation.
Of course, many more projects are underway including projects in Spain, Australia and Chile.
How does the system work? (technical details)
Let's break this system down into its fundamental components and steps. The process is as follows.
- Compression: The first step in the process of LAES is to compress the air. Electrically-driven compressors are used to compress ambient air to remove any pollutants. This is done by increasing the pressure and temperature.
- Cooling: After the air has been compressed, it is then cooled in a heat exchanger, which will remove the heat generated during the compression process. This causes the air to condense into a liquid state.
- Storage: Once the air has been liquefied, it must be stored in specially insulated tanks at a temperature of around -196°C. The insulated tanks are designed to keep the liquid air in its cryogenic (liquefied) state until it is needed.
- Expansion: When electricity is needed, the liquid air is pumped from the storage tanks and allowed to warm up. The warming process causes the liquid air to rapidly expand, turning back into a gas and generating a high-pressure stream of air.
- Turbine: The high-pressure stream of air drives a turbine, which in turn drives a generator to produce electricity.
Components of the system
Compressors (turboexpander-compressors)
The compressors that are used in LAES systems are usually either centrifugal compressors or reciprocating compressors. Centrifugal compressors use a rotating impeller to increase the pressure of the air while reciprocating compressors use a piston or a diaphragm to compress the air.
When picking between different types of compressors there are a few factors to consider. Things like the size of the LAES system, the required flow rate of the air, and the efficiency of the compressor are all things that should be taken into account when trying to pick which type of compressor is desired.
Generally speaking, centrifugal compressors are typically more efficient than reciprocating compressors when we look are large-scale LAES systems, but they are found to be less efficient for smaller systems.
Heat exchanger
A heat exchanger is a device used to transfer heat between two fluids that are at different temperatures, without direct contact between the fluids. Heat exchangers are widely used in various industries, including power generation, HVAC systems, chemical processing, and refrigeration.
There are several types of heat exchangers, including plate-fin, shell-and-tube, and spiral heat exchangers.
- Plate-Fin Heat Exchanger: A plate-fin heat exchanger consists of multiple layers of flat, corrugated plates separated by fins. The fluids flow in alternating channels, allowing efficient heat transfer between them. The advantages of plate-fin heat exchangers include compact size, high surface area, and good thermal performance.
- Shell-and-Tube Heat Exchanger: A shell-and-tube heat exchanger consists of a bundle of tubes enclosed within a cylindrical shell. One fluid flows through the tubes (the tube-side fluid), while the other flows around the outside of the tubes (the shell-side fluid). Shell-and-tube heat exchangers are versatile, reliable, and suitable for handling high-pressure and high-temperature applications.
- Spiral Heat Exchanger: A spiral heat exchanger features two spiral channels that are superimposed and separated by a thin metal sheet. The fluids flow through the spiral channels, creating a highly efficient and compact heat transfer configuration. Spiral heat exchangers are known for their self-cleaning capabilities, high heat recovery, and low fouling tendency.
Air liquefier (turbo-expander air liquefier)
The first stage of cooling usually involves the use of a pre-cooler, which cools the compressed air using a heat exchanger and a refrigerant such as water. The pre-cooled air is then passed through a series of expanders, which use a refrigerant such as helium or nitrogen to cool the air further.
The final stage of cooling typically involves the use of a cryogenic heat exchanger, which uses a very cold refrigerant such as liquid nitrogen or argon to cool the air to its liquefaction temperature. Once the air has been liquefied, it is stored in insulated tanks until it is needed to generate electricity.
The air liquefier is a critical component of the LAES system because it determines the efficiency of the system and the amount of energy that can be stored. The efficiency of the air liquefier is influenced by several factors, including the design of the heat exchangers, the choice of refrigerants, and the control system that manages the cooling process.
In recent years, researchers have developed new technologies for air liquefaction that have the potential to improve efficiency and reduce the cost of LAES systems. These include the use of advanced materials in heat exchangers and the development of new refrigerants that are more efficient and environmentally friendly.
Storage Tanks
A well-insulated tank is used in this system to store the liquid air. This is so that the heat does not leave the system.
Turbine
The turbine currently used for this process is a gas turbine which is similar to a steam turbine. It converts the gas created by the waste heat combined with the liquid air into inertial energy, which then is converted into electricity.
Different types of LAES systems
There are currently 3 main types of LAES systems.
- Diabatic storage
- Adiabatic Storage
- Isothermal storage system-LAES tank is maintained at a constant temperature during the charging and discharging processes.
The diabatic storage method involves using cold liquid air to cool a heat exchanger, which is used to heat the air again when electricity is needed. The heated air is then used to drive a turbine to generate electricity. In this system, the heat during compression is dissipated as waste which hurts the efficiency of the system
The adiabatic storage system uses compressed air to cool the incoming air, which is then liquefied and stored in a tank. In this system, the heat of compression is stored in a storage tank as waste heat. The heat is then used during the expansion process.
In the isothermal storage system, the LAES tank is maintained at a constant temperature during the charging and discharging processes.
Cost analysis
We can separate the main costs into two: capital costs (CAPX) and operating costs (OPX). Capital cost being how much money it takes to build the plant, and operating costs being how much money it takes to maintain the plant.
The levelized cost of energy of natural gas is 2 cents per kWh. So to be cost-competitive our energy storage system has to be under or equal to that. So what is the cost of liquid air energy storage from what Highview has done?
The capital cost of a 5 to 15-megawatt-hour pilot plant done in Manchester was about 10.7 million dollars. To calculate the levelized cost of energy we take the OPX + CAPX / the amount of energy produced during the systems lifecycle.
If the CAPX for this system is 10.7 million and let’s say the OPX is 1 million per year which is a conservative number for a system of this size. The system lasts 30 years.
CAPX + OPX = 10.7 million + (30 x 1 million) = 40.7 million
If the system produces 5 MW per hour than the total amount of energy that will be produced by the system over it’s lifecycle is:
30 years x 365 days x 24 hours x 5 megawatts (5000 kw) = 1 314 000 000 kWh of electricity produced during it’s lifecycle
Levelized cost of energy: 40.7 million / 1.314 billion = 0.031$/kWh
The current LAES system is about 3 cents per kWh based on the pilot in Manchester. This also adds up with their 50 mWh which costs 100 million in CAPX.
So currently Highview powers process is 1.1 cents more expensive. This means the current process is not cost-competitive with natural gas. Me and my teammates set out to solve this.
VoltAire’s solution
From talking to people at high view we figured out that the biggest part of the operating cost is maintaining the gas turbine. This is because the fins of the turbine need constant lubrification, and if one thing breaks it stops the whole process making the system lose a ton of money. On top of this, the gas turbine is the least efficient part of the process as it’s around 25 to 35% efficient.
Though the gas turbine doesn’t have a huge capital cost, over 30 years of maintenance the gas turbine becomes the costliest part of the process. To be more specific the typical maintenance cost for gas turbines is $73/MWh. If we take the 50 MWh plant that is in the works at Highview. The maintenance cost would equal to…
50 x 73 x 24 x 365 x 30 = 959 220 000 in operating costs!!! That’s almost a billion dollars from maintenance over 30 years.
To reduce the cost we would need to use an energy conversion system that could convert thermal energy into electrical energy for less capital cost and almost no operating cost. It would also have to have the same or a higher efficiency than a gas turbine.
What if I told you a system like this existed. It’s called a free-piston magnetic sterling engine. Instead of having 800 different moving pieces working in tandem, as is the case for gas turbines with their blades. A free piston magnetic sterling engine only has one moving piece.
How does a Free Piston Magnetic Stirling Engine work?
A free-piston magnetic Stirling engine is a type of Stirling engine that uses a linear motor to drive a piston instead of a conventional crankshaft mechanism.
The engine consists of a cylinder filled with a working gas, such as helium or hydrogen, and two pistons, one on each end of the cylinder. In Voltaire's case, we would use ambient air as the working gas. The pistons are connected to a linear motor, which uses the interaction of magnetic fields to create motion.
When the engine is started, the gas is heated at one end of the cylinder by an external heat source and is cooled on the other end. The pressure caused by the heat differences in the two metal plates causes the piston to move in a linear motion, that linear motion is converted to electricity with the permanent magnet (linear alternator).
This is a great video explaining how it works:
NASA has developed a free-piston Stirling engine for space applications, which is smaller than what may be needed for certain terrestrial solutions. However, scaling up the engine could potentially reduce operating costs, as the NASA engine has demonstrated high reliability and can operate for up to 14 years without maintenance.
As you can see above this is a mock-up of our prototype for what Voltaire’s liquid air system would look like. We would use an adiabatic system, where ambient air is compressed and liquified using the excess electricity from solar panels during peak hours. The waste heat from the system would be stored. The temperature difference between the waste heat and the liquid air would run the stirling engine which than would produce electricity for the grid.
The Stirling engine is 20% more efficient than traditional gas turbines and has almost no operating cost. This makes it so that Voltaire’s system is cheaper than natural gas, being under 1 cent per kWh.
Conclusion
In conclusion, at Voltaire, we strongly advocate for a sustainable future that is powered by renewable energy sources. We recognize the environmental challenges posed by traditional energy sources, and we are committed to playing our part in promoting clean energy solutions to power our society. By embracing renewable energy technologies, we can create a cleaner, healthier, and more sustainable future for generations to come.
Sources:
https://highviewpower.com — Highview power’s website
https://www.theguardian.com/environment/2019/oct/21/uk-firm-highview-power-announces-plans-for-first-liquid-to-gas-cryogenic-battery — Article by the guardian about Highview power
https://www.energy-storage.news/highview-power-unveils-plan-for-first-500mwh-liquid-air-storage-project-in-latin-america/ — Article by Energy Storage about Highview power
https://www.ft.com/content/c217830f-0fe2-4192-91fd-f5b83976d62b — Articel by the Financial Times about Highview power
https://www.ldescouncil.com/resources/highview-power-developing-2-gwh-of-liquid-air-long-duration-energy-storage-projects-in-spain/ — Article by the LDES (Long duration energy storage council about highview power
https://ease-storage.eu/news/highview-power-launches-worlds-first-grid-scale-liquid-air-energy-storage-laes-plant-in-the-uk/ — Article by EASE (European Association for Storage of Energy about Highview power
https://www.power-technology.com/news/highview-power-cryogenic-energy-facility/ — Article by Power Technology about Highview power
https://www.dw.com/en/data-centers-energy-consumption-steady-despite-big-growth-because-of-increasing-efficiency/a-60444548#:~:text=Data centers need electricity to,use 1%25 of global electricity. — Article about how much energy data centres consume from DW
