Operational simulation and an economical modelling study on utilizing waste heat energy in a desalination plant and an absorption chiller

Abstract

It is well established that a large proportion of the global emission of greenhouse gases are produced by electricity power stations and that a power plant typically emits about two thirds of its input energy as waste heat into the atmosphere. As such there is a lot of potential for additional applications that utilize this waste heat energy. Utilizing this waste heat energy in a desalination plant to produce low-cost potable water is the key to overcoming three problems at once, namely the water shortage in and and semi-arid areas, the continuing increase in oil prices by being more efficient and global warming. In all waste heat recovery or alternative energy systems based on natural phenomena (solar or wind) a major difficulty is decoupling supply from demand as thermal storage is neither efficient nor practical in many cases. A significant difficulty of gas turbine based power generation systems is the derating caused by high ambient temperatures; typically a 1% change in ambient temperature produces a similar reduction in efficiency. Therefore, by also utilizing this waste heat energy in an absorption chiller to pre-cool the gas turbine's compressor inlet-air, the effect of ambient temperature fluctuations on the gas turbine's performance would be eliminated. The combined cycle described in this study was designed in an attempt to address these issues. A gas turbine based combined heat and power plant was combined further with an absorption refrigeration unit and an MED desalination plant. The absorption unit stabilizes the operation of the gas turbine, reducing the sensitivity to changes in ambient temperature and the desalination plant acts as an energy utilization device that produces a usable product (40,000m3/day of potable water) that is easily stored and distributed as required. The simulation was performed using IPSEpro on the basis of real data obtained from an existing power plant and commercially available plants. The performance of the sub-plants was investigated using energy and exergy analyses, in design and off-design conditions using real weather data obtained from the Presidency of Meteorology and Environment in Saudi Arabia. Two different desalination technologies and two different coupling techniques were examined in four proposed plants. An optimal plant design was chosen from a comparison between all proposed plants' energy and exergy analysis results. The chosen plant was then optimized and simulated in design and off-design conditions. The initial results indicated that the simulated combined power plant's carbon footprint was reduced by 36.8% and its energy utilization factor was improved by 30.97%. This approach also stabilized the effect of ambient temperature fluctuations on the gas turbine's performance. After optimization, the carbon footprint was further reduced by 31.17% and the energy utilization factor was further improved by 6.11%. The energy destroyed through the exhaust stack was reduced by 78% and the proposed plant's overall exergetic efficiency was improved to 49.64%. Furthermore, the desalination plant's concentration factor was reduced by 0.45 and an additional product of a hot water stream at a temperature of 75°C was gained. An economic study was performed that indicated that the optimized plant is economically viable. As part of this analysis, a number of sensitivity studies defined the minimum selling prices of the plant's products and indicated the influence of fuel price, interest rates, capacity factors and project lifetime on the viability of the plant. The results also indicated that the proposed plant is a good investment, offering competitive energy and potable water prices, in regard to the location indicated by this study.