Feasibility and Theoretical Design of a Micro Gas Turbine for Domestic Combined Heat and Power

Abstract

Domestic Combined Heat and Power (DCHP) is the simultaneous production of heat and power in the home. With fewer moving components Micro Gas Turbines (MGTs) could provide a simple low-cost alternative to Stirling-based DCHP units. MGTs are attractive due to large power densities but require advanced technological strategies to address high speed bearing platforms, compact recuperators and micro impeller optimization. The research here aims to establish and assess the feasibility of a MGT within a commercial context suitable for DCHP. A continuous heat-led DCHP operating strategy using thermal storage to smooth heat demand fluctuations is proposed to utilise the preferred gas turbine operating characteristic whilst maximising electrical export. A 1 kW recuperated MGT with 15% electrical efficiency would deliver the required performance without extending beyond current technological limits provided a wide compressor operating range would be possible. Resolving the fundamental gas turbine system calculations with low component efficiencies and high recuperator pressure drops suggested a slight sub-optimum pressure ratio would significantly reduce shaft speed. Using 1D meanline and 3D CFD techniques a compressor impeller optimization study was performed which varied blade backsweep, shaft speed, and blade height at a constant pressure ratio. Limiting maximum shaft speed to 220,000 rev/min produced low pressure ratio, low diffusion impellers with a wide operating range. The two best performing impellers were selected for an off-design study to determine the MGT performance envelope to estimate potential DCHP cost and COz savings. Compared to a standard condensing boiler (with grid installation) cost and CO2 savings were 10.7% and 6.3% respectively for average UK annual power demands of 17.4 MWht and 6.1 MWhe. Following a speculative low cost recuperator conceptual design study, a coiled pipe-in-pipe recuperator was selected and optimised using CFD. Unit size was larger than anticipated but could be reduced by increasing pressure ratio and/or introducing turbulence promoters to improve overall heat exchange effectiveness.