05-04-2016, 10:40 PM
Interesting start up company. They are looking to store energy using compressed air. They have a system that increases significantly the efficiency in compressing air - and subsequently expanding the air to retrieve the energy. Interestingly, this idea is NOT new. In fact, I considered it independently as a means to capture heat of compression that is normally lost. It's very simple. When air is compressed, then the mechanical energy used to compress the air is transferred to the air molecules. This results in a rise in temperature. Well, the increase in temperature also increases the pressure (all else equal). So, one ends up having to use more work to compress the air when this heat of compression is not efficiently removed. The solution here is to use a fine water spray to moderate this rise in temperature. The water is heated in the process, and the hot water is separated and stored. When the air is expanded, then the hot water is again sprayed into the air stream. Well, expanding the air cools it, and the water spray heats it up. So, it works to increase the volume of air by increasing it's temperature - meaning, more work can be derived from the expanding air through this process. Basically, it significantly increases the efficiency of the process. Overall efficiency demonstrated is about 70% of the electricity used to power the motors that drive the compressors is recovered from the generators driven by the expanders. Much of the loss is in the motor and generator, so it can't get much better. Alternatively, the system could use the heat of compression for useful applications (like water heating in buildings or space heating), and when the heat is used in such applications, then the expanding air might be used in other ways such as air conditioning - configurations I have discussed elsewhere. The system as I understand it uses piston compressors that operate also as expanders. In many ways, the system is similar to the Terrajoule system. However, compressed air requires much higher pressures to achieve useful energy storage capacity. They are talking about carbon fiber tanks for very high pressure. Well, that's a problem for me. Personally, I am more impressed with the Terrajoule system. Still, this is interesting. What I like about Terrajoule is the elegant simplicity. By contrast, this system seems more complicated and costly.