Energy requirement of water production in dual-purpose plants

Abstract

In dual-purpose plants (DPPs) producing electricity and water, a realistic estimate for the energy requirement of water production is important since the cost of energy can be combined with equipment, operation and maintenance costs to arrive at the total cost of water production. In a multi-stage flash desalination system, the kind of technology currently being used in most of the Arab world, the desalination process requires both thermal and electrical energy input, and the cost of total energy requirement accounts for nearly two-thirds of the water production cost [1]. For a given fuel input, the production of water in a DPP is associated with a reduction in electricity production; the quantum of this reduction is governed mainly by the thermodynamics and design of the DPP. It is important to estimate this loss of electricity in water production to arrive at the thermal energy cost of water production. This paper presents a brief description of different concepts that can be applied to estimate the electricity loss for water production (ELWP) in a cogeneration system through a case study of a power station in Kuwait. The ELWP has been combined with the actual auxiliary consumption of desalination units to estimate the total equivalent electricity for water (TEEW) in kWh for every cubic meter of distilled water produced. The results of the analysis have been applied to two leading power stations in Kuwait. Their values have been found to be in the range of 16–18 kWh/m 3 for one of the power station's and over 22 kWh/m 3 for the other station. They are high compared with around 6 kWh/m 3 of the reverse osmosis system [2]. Finally, for the countries in the Arabian Gulf region, it is recommended that they make a serious effort to develop alternate techniques such as reverse osmosis for seawater desalination.