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Thermodynamic Analysis of Water Injection in a Micro Gas Turbine: Sankey and Grassmann Diagrams

par Montero Carrero, Marina ;De Paepe, Ward ;Bram, Svend;Contino, Francesco
Référence Energy Procedia, 105, page (1414-19)
Publication Publié, 2016

Article révisé par les pairs

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Résumé :

Despite appearing as a promising technology for decentralised Combined Heat and Power (CHP), the rather low electrical efficiency of micro Gas Turbines (mGTs) prevents them from being attractive for users with a variable heat demand. Hot water injection in mGTs, achieved by transforming the cycle into a micro Humid Air Turbine (mHAT), allows increasing the electrical efficiency of the units in moments of low heat demand— therefore decoupling heat and electricity production. In spite of previous research regarding simulations, experiments and economic assessments of mHAT technology, the detailed enthalpy and exergy flows between the components remained to be investigated. In the present paper, we introduce and compare the Sankey (enthalpy flow) and Grassmann (exergy flow) diagrams of an mGT based on the Turbec T100 and the corresponding mHAT cycle. Results show that the electrical efficiency of the T100 increases by 2.5% absolute points with water injection, while the total exergy efficiency decreases by only 4.1%. Although there is an enthalpy gain in the saturation tower, exergy actually decreases in this component due to the increase in entropy related to the evaporation of water. The benefits of water injection mostly rely on the increased heat capacity of the air-vapour mixture, the lower fuel consumption, the higher heat recovered in the recuperator, and the reduced power required by the compressor.