The basic idea behind the TOPHAT gas turbine is the introduction of quasi-isothermal air compression by the evaporation of water in the compressor. An old idea, but never properly realized since the water must be offered in the form of extremely fine droplets if it is to evaporate during the residence time of 1-10 milliseconds. KEMA invented and patented a technology for an adequate supply of droplets, which are small enough (～2 μm) for evaporation and air cooling to take place in the compressor. This technology is called the SwirlFlash technology and is based on the supply of hot pressurized water to nozzles, situated directly in front of (or in) the compressor. As a result of evaporation cooling the compressor discharge temperature is reduced to such an extent that effective regeneration of heat from the turbine exhaust is possible. The Rankine cycle can thus be avoided, creating a very compact and flexible Brayton cycle with high efficiency and low capital cost. Laboratory tests at KEMA and tests at the Royal Naval Institute with a small 400 kWe gas turbine proved that the evaporation cooling took place as predicted. Based on these experimental results, detailed calculations have been made for an optimized TOPHAT configuration. It is concluded that an efficiency of over 60% (LHV basis) can be achieved, even for gas turbines as small as 1 MWe. A benchmark with six other cycles, all calculated for the same conditions, indicates that the TOPHAT cycle outperforms even a modern combined cycle while the capital costs are significantly lower. An additional advantage is that the TOPHAT turbine needs no cooling water.
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