One approach to reducing emissions of air pollutants and greenhouse gases, in particular carbon dioxide (CO2), is to increase the energy efficiency of existing thermal cycles and to develop advanced, higher-efficiency cycles.
Thermal efficiency is a measure of the performance of a power plant. The 2 main thermodynamic cycles used widely in the generation of electricity are the Rankine and Brayton cycles. These cycles are also referred to as power cycles, as they convert heat input into mechanical work output.
The Rankine cycle is the basis of all large steam power plants. This cycle utilizes a steam generator (or boiler) to produce high-pressure and high-temperature steam. In fossil-fuelled plants, this is accomplished by converting the chemical energy stored in fossil fuels into thermal energy and transferring it to the working fluid (e.g., water), which passes through the boiler to produce steam. In nuclear plants, the thermal energy is derived from a controlled nuclear reaction.
The steam from the boiler is expanded in a series of high- and low-pressure steam turbines which convert the energy into mechanical shaft work to drive an electric generator and to produce electricity. After the last turbine stage, the steam is routed to a condenser and the condensate is pumped back into the boiler, repeating the cycle.
Improvements in cycle efficiency are achieved by increasing the boiler pressure and temperature, by introducing reheats (where the steam is first expanded in a high-pressure turbine and then returned to the boiler to be reheated), and by introducing a regeneration step in which steam is “bled” from the turbine stages to preheat the boiler feedwater. Current state-of-the-art supercritical power plants operate at pressures and temperatures exceeding 300 bar and 625ºC.
The conventional Brayton cycle is a gas turbine open cycle in which air is compressed and burned with fuel in a combustor. The hot gases are then expanded in a turbine coupled to an electric generator. In a closed cycle, the working fluid is compressed, heated, and then expanded in a turbine. The gases are then further cooled prior to compression and recycling.
Improvements in cycle efficiency are achieved by the introduction of a reheat via a second combustion chamber, intercooling, and the introduction of a recuperator to preheat the fluid entering the combustor, thereby reducing fuel consumption.
The combined cycle power plant utilizes a gas turbine (Brayton cycle) to generate electricity while the waste heat is used in a heat recovery boiler to produce steam (Rankine cycle) in order to generate additional electricity via a steam turbine, thereby enhancing the overall efficiency of electricity generation. By combining Brayton and Rankine cycles, high-input temperatures and low-output temperatures can be achieved, resulting in high Carnot efficiency. This combined cycle efficiency is a sum of both cycle efficiencies, as they are both powered by the same fuel source.
CanmetENERGY is working on development and demonstration of near-zero emissions gas turbines and advanced high-efficiency power conversion systems based on the supercritical CO2 cycle. This work is in collaboration with Carleton University, and more recently with Sandia National Laboratories under the International Nuclear Energy Research Initiative . The supercritical CO2 has advantages as a working fluid due to its low critical pressure and temperature, its heat transfer and thermodynamic attributes, and its availability. The main advantage of this cycle over other closed gas turbine cycles is the high density of the working fluid entering the compressor. The work of the compressor is thus only a fraction of the total turbine output, leading to higher overall cycle efficiency.
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