SpinOffs

Why supercritical CO2?   Super critical carbon dioxide (sCO2) is one of the hottest topics in the turbomachinery world right now.  From my own experience, I remember when the subject occupied a few sleepy sessions at the ASME Turbo Expo several years back.  Today, its possible to spend the entire week at the conference and never leave a well-attended session dedicated specifically to sCO2. 

There is obviously a huge amount of interest in Supercritical Carbon Dioxide (sCO2) within the energy industry. One reason is because sCO2 Brayton power cycles operate in the same way as other Brayton cycles, but with a much higher power density. This has the potential for greatly reducing the size and cost of equipment. Additionally, efficiencies can reach as high as 40% for an sCO2 system, compared to about 33% for a typical heat recovery system. 

Bearing selection is an interesting concept. Can you really select the bearing – or does the bearing select you? It seems that we have come to understand where to use bearings by where they have been used successfully in the past. For instance, we’ve come to know the turbocharger bearing is a floating ring bearing, foil bearings are on the aircraft air-cycle machines, and the standard bearing for midrange industrial pumps is the preloaded pair of ball bearings in the back and a deep groove bearing up front.  We also know that the big industrial compressors and turbines use tilt pads, canned pumps and mag drives use carbon and ceramic sleeves, and if you look far enough back, you might find a whole family of machines using bronze sleeves with pick-up rings. Those old sleeves got us through the industrial revolution. Recently, I saw a vendor with a bearing offering that has a wet sump under the bearing and they use the thrust collar to pump oil up into the sleeves. What was old is new again, but better! 

The 2015 Paris Climate Talks (COP21) were successful in achieving consensus from 196 countries that climate change must be given significant attention. There was a particular focus on the release of carbon, in the form of carbon dioxide and carbon monoxide, as something that must be curtailed.

The contemporary gas turbine engine, developed in the 1940’s by Sir Frank Whittle (pictured), is still considered to be among the most efficient, external combustion engines. Today, there is a resurgent interest in power generation technology driven by humankind’s insatiable need for more power. A popular focus is to extend the Brayton Cycle, the thermodynamic basis for a gas turbine engine, to using CO2 in a closed Brayton Cycle. This is commonly referred to as a Supercritical CO2 (sCO2) system. As the name implies, the CO2 is at pressures above the critical point of CO2 or 1,070 psia. The highest pressure in the cycle can often be designed to be 4 times this pressure and operate at temperatures as high as 700°C at the inlet to the turbine. 

Supercritical CO2 cycles have the potential to significantly improve efficiency and reduce emissions in power generation. However, the unique fluid dynamic properties of supercritical CO2 that enable these higher efficiencies also complicate the design and layout of the system. This is particularly true with regard to the systems turbomachinery components.  This is because of the highly non-linear properties of CO2, which pose significant difficulties in modeling.  Today’s commercial software is not optimized to help designers working on these types of systems.