Parallel turbochargers for small-scale power generation

Abstract

For countries with insufficient power supply and failing power grid infrastructure, it is usually also expensive to import microturbines. Local manufacturing or local assembling in these countries is therefore an attractive solution. Off-the-shelf turbochargers used in the motor vehicle industry are readily available at relatively low costs because of large-scale manufacturing in this sector. A turbocharger can be developed into a microturbine for power generation; however, coupling a turbocharger shaft directly to a high-speed generator or gearbox can be challenging and complex due to the high speeds and high temperatures involved, as well as the limited space available on the shaft for such a conversion. The novelty of this work was to investigate the effect of mounting two off-the-shelf turbochargers in parallel, where the second turbocharger’s compressor wheel is replaced with a generator. This allows for a higher pressure ratio over the power turbine. A parallel configuration may also have an advantage over a series configuration in terms of cost since the power turbine can be smaller in size. Two different parallel configurations were modelled, namely, a low-temperature turbine (LTT) and a high-temperature turbine (HTT), where the only difference was the position of the power turbine in parallel with the main shaft. Both configurations were modelled at steady state up to compressor pressure ratios of 2, with and without a recuperator, and with and without pressure losses. It was found that the recuperated LTT configuration outperformed the recuperated HTT configuration in terms of efficiency when pressure losses were introduced. Results showed that the optimum LTT combination generated up to 4.06 kW of power at 87 kPa ambient pressure, with a thermal efficiency of 13.26 %, when coupled with a 90 % effective recuperator. For the same case, the HTT configuration produced a maximum efficiency of 8.66 % with a power output of 5 kW.