An Improved Cost Apportionment for Desalination Combined with Power Plant: An Exergetic Analyses

In this paper an improved method for fuel cost apportionment of a combined power cum a desalination plant is presented. The conventional analysis for cogeneration systems has been, hitherto, the energetic (enthalpy) method which is useful for efficiency evaluation purposes, but it may not be fully accurate for capturing the “quality of fuel energy” consumed by processes in producing two or more useful effects, e.g., electricity and water. An exergy destruction procedure is proposed for the cost apportionment of fuel energy consumed where the available work potential of expanding stream can be fully accounted for cost distribution of the designed processes. For example, the turbines, that is used for power generation, exploits mainly the sensible-energy changes of high enthalpy steam undergoing expansion at high pressures and temperatures, whilst the thermally-activated desalination processes, such as the multi-effect distillation (MED), needed only the high latent-heat of bled-steam but at low pressures that has negligible work potential if the steam were to be used in the turbines. From this analysis, the incurred exergy destruction by the desalination processes is only 2%-7% of the total destruction available to the plant with bled-steam up to 50% of the total flow. We examined the ratio of exergy destruction consumed by the water to power production and, such a ratio is used as a basis for the fuel-cost determination in the cogeneration plant. It captures not only the realistic exergetic value of bled-steam of MED desalination, but it exposes the major shortcomings of the conventional enthalpy changes where a disproportion share of the input fuel cost, up to 32% of the total fuel input, may have been erroneously apportioned and giving an unfair valuation of the operational water cost.