CHP retrofit for a large Med-Tvc Desalination Plant: High efficiency assessment for different design options under the current legislative EU framework

Abstract

Integrated power plants in "dual purpose" configuration may represent a viable option for energy cost abatement of desalted water produced by MED-TVC (Multiple Effect Distillation along with Thermal Vapour Compression). In this paper an existing large MED-TVC plant with a 36,000 m(3)/day capacity is studied: a plant retrofit is designed, based on a steam power plant with condensation and steam extraction used to drive the steam ejector. As the plant operates in CHP (Combined Heat and Power) mode, the possibility to assess the integrated "CHP + MED-TVC" as high efficiency cogeneration according to Directive 2004/8/EC is discussed. Based on a model developed in Engineering Equation Solver, a sensitivity analysis is performed: the influence of several design and operation parameters on the fraction of energy outputs assessed as "from efficient cogeneration" is investigated. This fraction was found to increase almost linearly with the number of MED units supplied with steam. Also, when all the MED units are supplied and the steam extraction pressure is decreased from 4.89 MPa down to 0.29 MPa, the CHP fraction increased from similar to 54% to similar to 77%. The assessment of the plant as "high efficiency CHP" was found highly dependent on the specific fuel adopted