Design and Performance Evaluation of a Trigeneration System Incorporating Hydraulic Storage and an Inverted Brayton Cycle

Abstract

To bring the economic benefit of trigeneration to small-scale users without incorporating expensive components, an inverted Brayton cycle (IBC) is employed, which makes use of the expander section already present in a microturbine. An air accumulator provides pressurized air, which is passed through the expander section of the same microturbine used to charge the accumulator. The air passing through the IBC is cooled due to expansion, simultaneously providing power and cooling the flow. As the microturbine is indirectly fired, the flow passing through the engine or IBC can be directly vented into the household—eliminating the need for additional heat exchangers. The size of the cycle studied is on the order of 10 kW(e), suitable for a domestic household; however, the system is easily scaled for larger commercial applications. The majority of the components in the system being studied are “off the shelf” products. A feasibility study was conducted to ensure that the proposed system is economically competitive with systems currently used, such as individual generation provided by an air conditioner (A/C), a high efficiency natural gas (NG) furnace, and grid power. Simulations were run for a full year based on the actual external temperature and the electrical and thermal loads for a single family detached dwelling located in Winnipeg, Canada. Performance data were generated using MATLAB™ while the economic performance was determined with time-based simulations conducted using SIMULINK™. The system is designed to allow energy islanding by providing for all household energy needs throughout the year; however, integration with a power grid is optional. It was found that the operating costs for the proposed trigeneration system in an energy islanding mode of operation were equivalent to or less than individual generation (A/C unit, NG furnace, and grid power) during heating modes of operation and were more expensive for cooling modes of operation. The yearly energy cost for the trigeneration system exceeded the individual generation costs by 30–40%; however, there remains much room for improvement to the trigeneration concept. All economic data were based on fair market energy prices as found in central Canada.