“GE has announced it is working on a way to use CO2 pollution to make new types of solar batteries that could each power up to 100,000 homes. CO2 is the main contributor to climate change, and is released into the atmosphere when coal is processed at power plants. Currently environmental procedures mean that some CO2 from these plants is captured and stored, so it’s not released back into the atmosphere. But the question has always been: What do you do with the stored gas?” (1)
Figure #1: Comparison of 10 MWe Turbines (2)
What are the Benefits of Supercritical CO2? With the transition from steam generation to using Supercritical CO2 as a working fluid, we seen large gains in energy efficiency conversion, coupled with significant size (footprint) reduction of turbomachines. Other benefits include sequestering CO2 from the environment and reducing GHG emissions. Also, this system can be utilized to capture energy from other heat sources including waste heat streams and co-generation applications.
Figure 2: Relative size comparison of steam, helium and supercritical CO2 turbomachinery for Generation IV Nuclear Reactors (3)
What is Supercritical CO2? “[…] Supercritical CO2 is a fluid state of carbon dioxide where it is held above its critical pressure and critical temperature which causes the gas to go beyond liquid or gas into a phase where it acts as both simultaneously. Many fluids can achieve supercritical states and supercritical steam has been used in power generation for decades. Supercritical CO2 has many unique properties that allow it to dissolve materials like a liquid but also flow like a gas. sCO2 is non-toxic and non-flammable and is used as an environmentally-friendly solvent for decaffeinating coffee and dry-cleaning clothes.
Figure 3: CO2 phase diagram illustrating supercritical region. (4)
The use of sCO2 in power turbines has been an active area of research for a number of years, and now multiple companies are bringing early stage commercial products to market. The attraction to using sCO2 in turbines is based on its favorable thermal stability compared to steam which allows for much higher power outputs in a much smaller package than comparable steam cycles. CO2 reaches its supercritical state at moderate conditions and has excellent fluid density and stability while being less corrosive than steam. The challenges in using sCO2 are tied to identifying the best materials that can handle the elevated temperatures and pressures, manufacturing turbo machinery, valves, seals, and of course, costs. […] ” (2)
How will this work?
“[…] The design has two main parts. The first one collects heat energy from the sun and stores it in a liquid of molten salt. “This is the hot side of the solution,” Sanborn says. The other component uses surplus electricity from the grid to cool a pool of liquid CO2 so that it becomes dry ice.
During power generation, the salt releases the heat to expand the cold CO2 into a supercritical fluid, a state of matter where it no longer has specific liquid and gas phases. It allows engineers to make the system more efficient.
The supercritical fluid will flow into an innovative CO2 turbine called the sunrotor, which is based on a GE steam turbine design. Although the turbine can fit on an office shelf (see image above) it can generate as much as 100 megawatts of “fast electricity” per installed unit—enough to power 100,000 U.S. homes.
Sanborn believes that a large-scale deployment of the design would be able to store “significant amounts” of power —— and deliver it back to the grid when needed. “We’re not talking about three car batteries here,” he says. “The result is a high-efficiency, high-performance renewable energy system that will reduce the use of fossil fuels for power generation.”
He says the system could be easily connected to a solar power system or a typical gas turbine. The tanks and generators could fit on trailers. His goal is to bring the cost to $100 per megawatt-hour, way down from the $250 it costs to produce the same amount in a gas-fired power plant. “It is so cheap because you are not making the energy, you are taking the energy from the sun or the turbine exhaust, storing it and transferring it,” says Sanborn.
The process is also highly efficient, Sanborn says, yielding as much as 68 percent of the stored energy back to the grid. The most efficient gas power plants yield 61 percent. The team is now building a conceptual design, which Sanborn believes could take five to 10 years to get from concept to market. […]” (5)
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