Toward Higher Micro Gas Turbine Efficiency and Flexibility—Humidified Micro Gas Turbines: A Review

Micro gas turbines (mGTs) offer several advantages for small-scale combined heat and power (CHP) production compared to their main competitors, the internal combustion engines (ICEs), such as low vibration level, cleaner exhaust, and less maintenance. The major drawback is their lower electrical efficiency, which makes them economically less attractive and explains their low market penetration. Next to improving the efficiency of the components of the traditional recuperated mGT, shifting toward more innovative cycles may help enhancing the performance and the flexibility of mGTs. One interesting solution is the introduction of water in the mGT cycle—either as auto-raised steam or hot liquid—preheated with the waste heat from the exhaust gases. The so-called humidification of the mGT cycle has the potential of increasing the electrical performance and flexibility of the mGT, resulting in a higher profitability. However, despite the proven advantages of mGT humidification, only few of these engines have been experimentally tested and up to now, no cycle is commercially available. With this paper, we give a comprehensive review of the literature on research and development of humidified mGTs: we examine the effect of humidification both on the improvement of the cycle efficiency and flexibility and on the performance of the specific mGT components. Additionally, we will present the different possible layouts, both focusing on the numerical and experimental work. Finally, we pinpoint the technological challenges that need to be overcome for humidified mGTs to be viable. In conclusion, humidification of mGT cycles offers great potential for enhancing the cycle's electrical efficiency and flexibility, but further research is necessary to make the technology commercially available.