Posted by Kashyap Vyas on April 27th, 2016
Battery is indeed the missing link between the intermittent renewable energy sources such as wind and solar. The only way these sources can become primary and inexpensive is to have a robust storage system in place, in order to approach the scale and affordability. But with present storing technologies, can we actually think of a carbon free, clean energy generation? It’s frightening when you learn that all the batteries we have today on Earth, can store just 10 minutes of world electricity needs.
While Professors like Donald Sadoway have figured out an alternative “liquid-metal” battery that will dramatically improve batter efficiency and provide large scale energy storage, there are few engineers who are dusting off the old idea of storing energy from compressed air.
Compressed air has got a lot of energy; and it is for this reason that weve been using it to power devices like pneumatic presses, jackhammers, dentist drills and many more. In fact, 70% of companies in every corner of the world use compressed air for some aspect of their operations. But, using this pressurized air to store energy for electricity sounds to be a promising idea.
All you need is to store compressed air in a container, and when you need it, you simply deploy the pressurized air to the turbine to generate electricity. Eureka! You’ve just discovered a solution to the energy crisis the world is facing today. Not really.
Theres a major problem associated with the compressed air energy storage; as when you pressurize the air, its temperature increases dramatically. Imagine compressing air into an underground cave; when air is pumped in, the temperature and pressure inside the cave increases. But, when in stored condition, the heat is lost to the cave walls gradually, leading to a drop in temperature and pressure. At the end of the day, only a small fraction of energy can be harvested to produce electricity.
A schematic of an energy system that combines compressed air energy storage and thermal energy storage technologies Image: UCLA
One way to deal with this problem is to heat up the compressed air using secondary fuel during discharge, but this again comes at a cost of reducing the plant efficiency and usage of polluting fuels. The same approach is being used in a 290MW plant, which was built in 1978 in Germany. And, almost every compressed air storage plants we have today, works on the same principle. These plants are more focused on retaining the heat associated with compression.
The challenge is then to combine the compressed air and thermal energy into one advantageous system. For this, there has to be a special material that can withstand high pressure and temperature, and that too should make economic sense. The reason why energy storage is so much attractive today is because a breakthrough in cost and performance can make the electric grid cleaner and reliable. If there is a system that can cheaply store the energy for even a few hours at a time, wind and solar power plants can be turned totally dependable against fossil fuel plants.
At UCLA, Professor Pirouz Kavehpour and his team are working on a compressed air/thermal energy system that can generate 74 kW of power and can run for six hours. There are number of startups as well, joining the pursuit to deliver cleaner and cheaper energy and LightSail is one of them. Their model involves injecting water spray while compressing the air using piston, which rapidly absorbs the heat during compression and supplies it during expansion.
Two main components that make this possible are the motor-generator and compressor-expander set. During storage mode, the system draws electricity from the grid to run the motor, compress the air and generate heat. In the delivery mode, the compressor acts as an expander and motor acts as generator, providing electricity from the same system.
To store hot compressed air, they are experimenting with carbon fiber storage tanks, the primary ingredient of which (Carbon) is the 4th most abundant element in the universe, and almost 1000 times more abundant in the earth’s crust than Lithium that we use in today’s batteries.
With this arrangement, the idea of storing energy during off-peak hours and delivering the same during peak hours seems feasible and inexpensive. But, when it comes to storing pressurized fluid in a container, you ought to have a design that’s safe enough to withstand the stresses developed due to pressure forces.
And since the system is required to operate round the clock, fatigue plays an important role in degrading the system early. This thought opens up an altogether different avenue for regulatory bodies like ASME to incorporate modifications or additions in their standard codes for these special kinds of pressure vessels made up of non-conventional materials.
Compressed air systems for energy storage conditions must also be designed from scratch, as when off the grid, the expenses can be extremely large, since there will not be any other alternative way to meet the peak load demands.
There could be a great opportunity then for virtual simulation tools to develop robust finite element models right from the beginning when the design is in infantry stage. The entire idea of generating clean energy can be further augmented with a cost-effective design strategy to make the electricity supply even cheaper.
Commercializing this method will be the ultimate goal for anyone on this planet. However, Professor Kavehpour from UCLA, points out that going step by step is a good way to understand the system best, and they are going to be patient in their process, as eventually all these innovations run into the same engineering challenge: efficiency. If there is a lot of energy wastage in converting electricity to compressed air or stored water and back again, the cost will go up substantially. This is where batteries compete very well; there are some types which are more than 90% efficient in charging and discharging.
The trick then, for compressed air storage is to improve the efficiency in as many ways as possible, which invariably means making the better use of heat; and the efforts are actually happening. While CAES developers like LightSail are capturing heat by compressing air, there are other innovators who are harvesting heat from outside sources that would otherwise go to waste.
What is really exciting about compressed air however, is the ready availability of parts and infrastructure, leaving no supply chain issues.
After all, air is all about us.
About Author: Kashyap Vyas is an Engineer at Hi-Tech and holds a Master’s degree in Thermal Engineering with several research papers to his credit. He covers CAD and CAE topics for the engineering industry. His contributions are primarily focused on encouraging manufacturers and suppliers to adopt virtual product development tools to build efficient products with reduced time-to-market.