Can Carbon Dioxide Replace Steam to Generate Power?
The U.S. Department of Energy hopes to create a more efficient turbine that uses CO2 to make electricity. Engineers are looking into replacing steam with supercritical carbon dioxide, a technique that could unlock up to 50 percent greater thermal efficiency using a smaller, cheaper turbine. Much has changed in the modern electric power plant since Thomas Edison's era, but the parts that actually turn heat into electrons haven't changed since his eureka moments. Whether burning coal, concentrating sunlight or splitting atoms, most thermal power plants use the energy for the same thing: heating water into steam to drive a turbine. Steam-based generation produces 80 percent of the world's electricity. After more than a century of incremental improvements in the steam cycle, engineers have plucked most of the low-hanging fruit and are chasing diminishing returns, spending millions of dollars for every percentage point of efficiency improvement. These upgrades propagate to other steps in electricity production, allowing power plants to extract more work for a given unit of fuel. In a fossil fuel-fired generator, this means less carbon dioxide emissions for the same unit of electricity produced. For a solar thermal plant, this results in higher capacity at lower operating costs. Now engineers are looking into replacing steam with supercritical carbon dioxide, a technique that could unlock up to 50 percent greater thermal efficiency using a smaller, cheaper turbine. Last month, in a budget briefing and in two different hearings before Congress, Energy Secretary Ernest Moniz specifically mentioned the Department of Energy's supercritical carbon dioxide initiatives. The department's 2016 budget request allocates $44 million for research and development on this front, including a 10-megawatt supercritical turbine demonstration system.
A simpler, smaller, cleaner machine
The term "supercritical" describes the state of carbon dioxide above its critical temperature and pressure, 31 degrees Celsius and 73 atmospheres. Under these conditions, carbon dioxide has a density similar to its liquid state and fills containers the way it would as a gas. Coffee producers are already using supercritical carbon dioxide to extract caffeine from beans. Materials companies are also using it to make plastics and ceramics. "From a thermodynamic perspective, it's a very good process fluid," said Klaus Brun, machinery director at the Southwest Research Institute, a nonprofit research and development group. "You get a fairly efficient cycle and a reasonable firing temperature." In its supercritical state, carbon dioxide is nearly twice as dense as steam, resulting in a very high power density. Supercritical carbon dioxide is easier to compress than steam and allows a generator to extract power from a turbine at higher temperatures. The net result is a simpler turbine that can be 10 times smaller than its steam equivalent. A steam turbine usually has between 10 and 15 rotor stages. A supercritical turbine equivalent would have four. "We're looking at a turbine rotor shaft with four stages on it that's 4 inches in diameter, 4 feet long and could power 1,000 homes," said Richard Dennis, turbine technology manager at the National Energy Technology Laboratory. He noted that the idea of a supercritical carbon dioxide power cycle dates back to the 1940s, but steam cycles were already very efficient, well-understood and cheap, creating an uphill slog for a new power block to catch on. In addition, engineers were still finding ways to improve the combustion side of power production, so the need to improve the generation side of the plant wasn't as acute until recently.
Regulations could create an expanding market
Now regulations and climate concerns are forcing power producers to consider ideas like supercritical carbon dioxide. "In just the closed, indirect cycle, numbers suggest that the thermodynamic efficiency, at similar operating temperatures to a steam cycle, you would see a 3- to 4-percentage-point improvement," Dennis said. With further optimization and better materials, engineers could push performance even higher. The indirectly heated power block would essentially be a drop-in replacement for a steam power block. Such a device would be a boon for nuclear power stations, concentrating solar farms, geothermal installations, combined heat and power systems, and fossil fuel-fired power plants. Supercritical turbines would also be an attractive upgrade from steam systems aboard ships and submarines, producing the same power while occupying less space. Because they use carbon dioxide instead of water as their process fluid, these turbines would also work well in drought-stricken areas. In addition, a supercritical turbine could fit into a directly heated cycle, where a fuel like natural gas burns in the presence of pure oxygen inside the turbine, creating only water and carbon dioxide as waste. Operators could then remove water and sequester the excess carbon dioxide. "With modest success in the technology management program, these cycles could compete with combined-cycle natural gas turbines and carbon capture and storage," Dennis said. The system could also route waste heat back into the front end of the system, further increasing overall efficiency. However, the turbine is not the only component of the power block; a supercritical carbon dioxide system still needs heat exchangers, cooling systems and piping, which add to cost and complexity. And carbon dioxide can corrode materials, so engineers will have to redesign much of the plumbing and support hardware from steam systems to account for these problems. Another concern is that most of the supercritical carbon dioxide systems demonstrated to date were built at kilowatt scales, too small to offer useful lessons for a full-sized version. DOE's pilot proposal would go a long way toward demonstrating the feasibility of supercritical carbon dioxide. "Building a 10-megawatt plant would be a huge, huge step forward," Dennis said.
Supercritical Carbon Dioxide Power Cycles Starting to Hit the Market
Supercritical CO2 power cycles are gaining increasing attention in the engineering world. sCO2 is an ideal working fluid for use in power generating turbines because it offers high efficiency in a compact footprint and can be matched to many different heat sources. sCO2 power turbines could potentially replace steam cycles in a wide variety of power generation applications resulting in higher efficiencies and lower cost of electricity.
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.
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.
sCO2 power cycles are potentially applicable to a wide variety of power generation applications. Power generation facilities that currently use steam cycles, could in theory, be upgraded to sCO2 that would enable much greater efficiencies and power outputs. Nuclear power, concentrated solar thermal, fossil fuel boilers, geothermal, and shipboard propulsion systems have all been identified as favorable applications for sCO2 cycles and would replace traditional steam Brayton and Rankine cycles. Researchers believe that sCO2 power cycles could lower the cost of electricity approximately 15% over today’s steam cycle technologies. Lower installed cost for sCO2 systems are due to its smaller footprint and reduced balance of plant requirements.
The single phase nature of sCO2 allows for the design of simple, single phase, single pressure exhaust heat exchangers with low gas-side pressure drop. Due to the superior thermal stability and non-flammability of CO2, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperature (and thus higher cycle efficiency). Because sCO2 is a single-phase working fluid, it does not require the heat input for phase change from water to steam and does not create the associated thermal fatigue or corrosion associated with two-phase flow. Lower operation and maintenance costs for sCO2 are possible because plant personnel are not needed for water quality and treatment functions typically found in steam-based plants.
sCO2 can be used in either direct or indirect heating scenarios. Indirect heating would use the CO2 in a closed loop recuperated recompression Brayton or Rankine cycle. Indirect heating could replace steam boilers in coal plants, nuclear power, solar thermal, or heat recovery steam generators used in combined cycles. Indirect heating cycles offer thermal efficiencies greater than 50% and are non-condensing making them ideal for heat sources that offer constant temperatures (such as turbine exhaust).
Echogen is bringing to market their EPS100 8MW heat engine that is targeted for use in combined-cycle applications. Their system is self-contained, closed-loop, and has zero emissions and no water requirements (though water cooling is an option).
Direct heating can use oxy-fuel combustion in a recuperated Brayton cycle to replace coal and natural gas combined cycles and offers the capacity for integrated carbon capture. Direct heating is fuel flexible and is adaptable for coal, syngas or natural gas and is a water producer.
NET Power recently announced they will build their first sCO2 50MW demonstration plant that is direct heated. This power plant is fueled by natural gas and has zero emissions of any kind (no smokestack) and has integrated carbon capture. The plant will output pressurized CO2 that will be sold for use in Enhanced Oil Recovery.
The US Department of Energy is working on integrating sCO2 into nuclear power in order to significantly raise the efficiency and power output. DOE is also working on using sCO2 in solar thermal applications in the Sunshot CSP Program. The Southwest Research Institute (SWRI) and Sandia National Labs have active research programs today in the US and there are other programs in Europe, Asia and in industry.
Ongoing research programs are focused on demonstrating the commercial viability of sCO2 power cycles at increasingly larger power outputs. Rotordynamics continue to be analyzed and optimized. Heat exchanger research seeks to improve heat transfer correlations for varying geometries and improve heat exchanger durability through testing of materials, fabrication, channel geometry, fouling, corrosion and maintenance. Long-term corrosion and materials testing across all components also continue to be active areas of research.