Startup Claims to Have Invented Isothermal Compressor
Jesse Remillard for Zondits, March 2, 2015
A startup called Carnot Compression LLC claims that it has invented a centrifugal compressor that achieves isothermal (constant temperature) compression. While the exact operation and design of the compressor have not been released, the nine-person company says it has achieved this by encapsulating air in water bubbles during the compression process. Carnot Compression also states that its compressor can provide very high compression ratios (upwards of 200) and provide dry, oil-free air.
The photo on the left by David Gunter from the Bonner County Daily Bee shows the two primary inventors, Bob Alderman and Mark Cherry, holding prototypes of their invention.
The reason that this is a bold and remarkable claim is that isothermal compression has always been perceived as unachievable on a portable scale due to heat transfer limitations. It is theoretically the most efficient way to compress a gas, so many have sought to develop the technology but no one has claimed to achieve it until now. Isothermal compression requires a lot of heat transfer during the compression process. Most compressors realize little heat transfer during the compression process unless they are utilizing multiple stages and intercoolers, and even then they still cannot be modeled as isothermal.
Figure 1 shows a pressure volume diagram of a compression process. On it are two isothermal lines showing constant temperature and a bold green line from one to the other, demonstrating an adiabatic (no heat transfer) compression process. The area under the process line is the work done to achieve the compression. It is easy to see that the work required is reduced the closer the temperature is kept to constant.
Figure 1. P-V Diagram of an Adiabatic Compression Process
Carnot Compression’s design is inspired by the Taylor compressor, which used the weight of water flowing into an underground cavern to power the compression and absorb the heat from the process. When the air reached the cavern it was fully compressed and was piped back to the surface while the river water was redirected and continued to flow downstream. Such a compressor was used at the Victoria mine in Michigan for 15 years before the mine was shut down, and at several other mines for even longer periods.
While the Taylor compressor utilized a constant supply of flowing water from a river to carry the heat of compression away from the process, it is not clear how Carnot Compression’s design will reject the heat from the process. But, Carnot Compressor claims to have demonstrated proof of concept prototypes, and is currently building a 100 cfm demonstration unit. The company is targeting a wide range of small applications from refrigeration and air conditioning to energy storage and compressed air, and is currently in negotiations with manufacturers.