To reduce greenhouse gas emissions and improve the thermal efficiency of thermal-to-electric generators, the Brayton Research Lab at Sandia National Labs investigates supercritical carbon dioxide as a working fluid in a re-compression closed Brayton cycle (RCBC) turbine generator. Their testing model uses two compressors that bring the supercritical CO2 to higher pressures before entering the turbine, increasing system efficiency.
Brayton Cycle Re-cap
A closed Brayton cycle pressurizes a working fluid (a superheated vapor or gas) in a compressor, and then heats it to high temperatures through solar, nuclear, geothermal, or other heat sources. The working fluid enters the turbine at a very high-energy state before expanding in an (ideally) isentropic process to turn the blades of the turbine. This mechanical work is converted into electricity through an alternator on an electric generator.
After exiting the turbine, the low-pressure fluid can be cooled through a heat exchanger to reduce wasted heat to the environment, and increase the efficiency by recycling the waste heat to the heat source. It then enters the compressor to continue the cycle. (Alternatively, non-closed Brayton cycles use chemical combustion to increase the fluid temperature before passing it through the turbine rather than passive heat exchangers, and the exhaust is lost to the environment. See reference video below).
The team investigates supercritical CO2 as a RCBC working fluid because of its high thermal efficiency and power density. CO2 has a critical temperature just below 88 ˚F, and a critical pressure of 1,071 psi (73 atm), allowing it to remain in a supercritical phase between modest temperatures of 400–750 °C. (See phase diagram.) In the supercritical phase, it is almost twice as dense as steam, so it can generate more power over the same pressure drop through the turbine. In turn, the turbine rotor shaft at Sandia is only 4 in. in diameter and 4 ft. in length, and uses only four stages to expand the working fluid for power generation.
The Demand for Higher Efficiency
When it comes to converting thermal energy into electricity, even slight improvements in system efficiency can reduce costs. Currently, steam generation is responsible for generating up to 80% of the world's electricity, reports Scientific American. With research and development, the Brayton research team aims to build a thermal energy-to-electricity conversion unit with a thermal efficiency 50% higher than those achieved in steam Rankine generators, based on the working fluid's density. It would be used to replace or supplement existing Rankine generators.
The teams mission is as follows: “By the end of FY 2019, Sandia National Laboratories shall develop a fully operational 550˚C (aprox. 1000 ˚F), 10 MWe R&D Demonstration s-CO2 Brayton Power Conversion System that will allow the systematic identification and retirement of technical risks and testing of components for the commercial application of this technology.” Flow rates will reach approximately 5.7 kg/s in the cycle.
Already the team has implemented their dual compressor Brayton cycle system at the National Solar Thermal Test Facility (NSTTF) to validate its performance for converting solar thermal energy to electricity. Through validation of hardware and simulation through its lab-based heat exchangers, they aim to present technology that will be useful to improve electricity generation in various types of thermal energy plants.
Backed by $44 million in federal funding in 2016, Sandia National Labs aims to develop hardware that will stand up to corrosive supercritical CO2, and operate safely in commercial and small-scale industrial applications. Already, it tests its bearings, expansion tanks, burst valves, and gas seals; stainless-steel piping to minimize corrosion; and heat-resistant turbine materials to withstand high operating temperatures. Piping is monitored through X-ray imaging, and data for pressure, temperature, mass flow, and density is processed in LabVIEW for further optimization. This hardware for testing is further discussed in a report from 2012.
Thus far, the team has achieved maximum temperatures of up to 930 ᵒF in their test assembly power cycle. Two recuperators contain a low-temperature printed-circuit heat exchanger (PCHE) and a high-temperature PCHE with controls to simulate heating sources found in power plants. The report in 2012 mentions a testing assembly gas chiller that can remove up to 540 kW excess heat from the fluid as it leaves the turbine through water cooling. Six immersion heaters in the high-temp PCHE add a total heating capacity of 780 kW to the fluid before it enters the turbine.
The team uses two turbo-alternator compressors (TACs) in its design (video below, 7:00) to compress the working fluid twice over. The team reports a cycle pressure ratio up to 1.45 across the TACs, which not far from the pressure ratio goal of 1.8. The TACs that are designed for 75,000 rpm currently achieve speeds between 50 and 60 rpm. The TACs are designed to generate 125 kWe.
Watch the video below for more information in a tour of the Sandia testing assembly.