Steam Injection Experiments in a Microturbine - a Thermodynamic Performance Analysis

This paper reports on a series of steam injection experiments on a Turbec T100 microturbine. Combined Heat and Power (CHP) systems, such as the considered T100 microturbine, use one single primary fuel to simultaneously produce electric and thermal power. In doing so, they realize significant energy savings when compared to conventional schemes of separated production. However, a reduction in the demand for heat (like e.g. in summertime) will force this type of units to shutdown. This significantly reduces the amount of operating hours and has a severe negative impact on the net present value of such CHP investment projects.
The aim of this paper is to investigate and demonstrate the effects of steam injection in the compressor outlet of a microturbine operating under reduced heat demand conditions, this to keep the unit running. The necessary steam can be auto-raised with heat available in the turbine exhaust downstream of the recuperator. Such an injection will keep the unit running and thus avoid a forced shutdown. Furthermore it is expected that the electric efficiency will rise and that the power production will become more economically viable as a result of the increase in operating hours.
This paper reports on the influence of steam injection on the electrical efficiency and shaft speed of a T100 unit. ASPEN® simulations of the behaviour of the CHP unit are also presented. These simulations predicted a 2.2% rise in electric efficiency at nominal electrical output when 5% of the mass flow rate of air is replaced by steam.
The steam injection experiments resulted in stable runs of the unit, a predicted reduction in shaft speed and increased electrical efficiency. Validation of the ASPEN® simulations against the experimental data revealed the necessity for a more accurate determination of the air mass flow rate and more precise compressor characteristics.