Gas-cooled solar receivers for concentrating solar power plants are capable of providing high temperature, pressurized gas for electrical power generation via a Brayton cycle. This can be accomplished by expanding hot, pressurized gas directly through a turbine, or through using a heat exchanger to indirectly heat pressurized air. Gas-cooled receivers can be divided into two basic technologies. In tube based solar receivers, thermal energy is transferred to air through convection with the heated tube wall. This limits receiver efficiency since the tube wall needs to be substantially hotter than the gas inside due to the relatively poor gas heat transfer coefficient. In volumetric receivers, solar energy is absorbed within a volume, rather than on a surface. The absorption volume can be filled with ceramic foam, wires, or particles to act as the absorbing medium. In a small particle heat exchange receiver, for example, sub-micron sized particles absorb solar radiation, and transfer this energy as heat to a surrounding fluid. This effectively eliminates any thermal resistance, allowing for higher receiver efficiencies. However, mechanical considerations limit the size of volumetric, pressurized gas-cooled receivers. In order to solve this problem, several thermodynamic cycles have been investigated, each of which is motivated by key physical considerations in volumetric receivers. The cyclic efficiencies are determined by a new MATLAB code based on previous Brayton cycle modeling conducted by Sandia National Laboratories. The modeling accounts for pressure drops and temperature losses in various components, and parameters such as the turbine inlet temperature and pressure ratio are easily modified to run parametric cases. The performance of a gas-cooled solar receiver is largely a function of its ability to provide process gas at a consistent temperature or pressure, regardless of variations in solar flux, which can vary due to cloud transients or apparent sun motion throughout the day. Consistent output can be ensured by combusting fuel within the cycle, effectively making a solar/fossil fuel hybrid system. Several schemes for hybridization with natural gas are considered here, including externally fired concepts and combined receiver/combustor units. Because the efficiency of hybridized cycles is a function of the solar thermal input, the part load behavior of the recuperated cycle is examined in depth. Finally, a brief report of economic costs inherent to solar powered gas turbine engines is given. Possibilities for the future of solar power gas turbine power plants are discussed, with key issues regarding thermal storage techniques.

The environmental and economic impact of a municipal wastewater treatment plant can be reduced through a novel application of CHP technology. By operating a natural-gas fired prime mover to generate electricity and utilizing the rejected heat in the sludge drying process, energy cost savings and both site- and grid-wide emissions reductions can be realized. The design case is the Almeda Sims Wastewater Treatment Plant for the City of Houston. This plant treats the waste, and then dewaters the sludge that results from the process with a large natural-gas fired dryer. The plant handles about 40 tons of sludge per day. The proposed CHP plant would use a natural-gas fired turbine to generate base-load electricity for the facility, and utilize the heat from the turbine exhaust to reduce the natural-gas requirement at the burner tip of the dryers. Utilization of emission-reducing technology on the turbine exhaust and the reduction in natural-gas consumption at the dryer burner tip, as well as reduced electrical draw from the grid, results in a significant reduction in environmental pollutants. Modeling of the proposed system shows a potential for a CHP system efficiency of approximately 76%, compared to 30–35% efficiency of grid-wide power generation equipment. The result is an economically viable green energy solution for any municipality with significant wastewater processing needs.

A natural gas fired combined cycle power plant with indirectly-fired heating for additional work output is investigated in the current work. The mass flow rate of coal for the indirect firing mode in circulating fluidized bed combustor is estimated based on fixed natural gas input to the topping combustor. The effects of pressure ratio, gas turbine inlet temperature, inlet temperature to the topping combustor on the exergetic performance of the combined cycle configuration are analysed. The use of coal in indirect-firing mode reduces with increase in turbine inlet temperature due to increase in the use of natural gas. The exergetic efficiency increases with pressure ratio up to the optimum pressure and it also increase with gas turbine inlet temperature. The exergy destruction is highest for the circulating fluidized bed combustor (CFBC) followed by the topping combustor. The analyses show that the indirectly fired mode of the combined cycle offers better performance but with higher exergy destruction and the opportunity for additional net work output by using solid fuels (coal in this case) in existing natural gas based power plant is realized.