Is your wind farm forced to stop generation because the grid is not able to cart away the electricity it produces? Here is a possible solution.
Dr Satyanarayanan Seshadri, who teaches Applied Mechanics at IIT-Madras, has developed a compressed air storage system that can chip in and help.
How it works seems easy, in principle. Run the wind turbines and produce electricity, use the electricity to compress air and store the compressed air in a container. Later, when necessary, let the compressed air hit a turbine-generator to make electricity back. This way, compressed air becomes a store of electricity.
Sounds simple, but then, if it was that simple, such systems would already exist, right? While compressed air as a means of storage is not unknown, very few systems exist in the world, and they are big.
‘Compressed Air Energy Storage’, or CAES, literature cites the 290 MW plant in Huntorf, Germany, as the first ever to be built in the 1970s. The next one was a 110 MW plant in Alabama, US. Both these are large and use abandoned mines to store compressed air.
But Seshadri’s system would be 30-50 kWhr, capable of delivering 5 kW for eight hours — compact enough to plug into a wind farm or, for that matter, near a factory that might need energy storage.
The problem with compressed storage is, when you compress air (or, anything) it generates heat. It happens, for instance, in air-conditioners — that is why you have the ‘outdoor units’ to dissipate the heat. Even in smaller CAES systems, such as Seshadri’s, it can get as hot as 685 degrees Celsius — it creates challenges (and costs) for engineering the storage vessels. And, when the air is released, the sharp drop in pressure creates extremely low temperatures and you have to handle that too.
You could use the heat or the cool, but you must first find an application for it close by, and it wouldn’t be a ‘storage’ system anymore.
Seshadri’s design, for which he is seeking a patent, solves this problem by using the ultra-low temperatures at the end of the cycle to cool the air that is to be compressed at the beginning of the cycle. In other words, the “expanded air” coming out after turning the turbines — at the end of the cycle — is used to cool a mixture of glycol and water to a temperature of about minus 20 degrees C. This mixture is used to cool the “admission air”.
In search of funds
This has two advantages. First, since the input air is already pretty cold, the compressed air is not too hot. Second, because cool air is denser, you get to store more air. The professor calls this ‘compressed air recuperated energy storage’, or CARES.
Seshadri’s team is seeking funding for developing the prototype. “The proposed technology will be ubiquitously used in process industries for demand management and supporting peak loads,” he says.
The life of the system is far longer than, say, a lithium ion battery. Furthermore, the system is capable of giving back over 70 per cent of the energy used to compress the air, he says.
Economic viability
Asked for a comment on such a system, Dr Rahul Walawalkar, Executive Director, India Energy Storage Alliance, said that in principle, compressed air storage could be a good fit for storing energy from wind and solar. However, “Most of the projects just focus on being a technology demonstration without understanding the basic economic case,” he observed.
Seshadri insists that his machine is economically viable. “For potential commercialisation of this technology, we are in collaboration with our industry partner, Aspiration Energy, who is exposed to process industries in the industrial heating segment,” he says.
