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

PhD ThesisIt 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

Biomass-powered zero liquid discharge desalination of brackish water

Desalination is accepted as being a necessary technology to support the livelihood of communities. However, to prevent the harmful environmental impacts of brine, desalination needs to be designed with zero liquid discharge being the process rather than an afterthought. Existing approaches are often found to be inadequate and significant amounts of research into ways to prevent liquid waste are currently in place. The challenge is that the technology must be able to treat post-RO salinities (usually with high amounts of thermal energy) to be able to overcome the low heat capacities and high boiling points of saline solutions >70,000 mg/L. This research honours project investigates a proposal developed by Enerbi Pty Ltd that incorporates heat, mechanical and electrical energy into a desalination unit that is powered by Biomass and produces a Zero Liquid Discharge product. The system was modeled in Excel and ChemCad and found to successfully produce a dry product with moderate quantities of biomass. The proposal was then modelled to treat 60ML per year under various scenarios using two particular types of Biomass, Plantation Waste, and Oil Mallee crops. These scenarios included high-value agricultural and horticultural crop scenarios using desalinated water for irrigation and salinity, with salinity problems on site being amended via saline water uptake and intervention crop planting. The design was carried further to a Pilot Plant configuration specified using ‘off the shelf’ products, and the Pilot Plant design included upgrading the power configuration to allow for additional equipment. The Pilot Plant configuration was tested up to salinities of 85,000mg/L. It was found to successfully cope with this salinity, the most likely upper limit due to heat requirements of evaporation of hyper-saline solutions. A final concept 3D model was created to assist with placement and configuration

Optimization of multi-pressure himidification-dehumidification desalination using thermal vapor compression and hybridization

Conference site: http://www.ishmt2011.iitm.ac.in/Humidification-dehumidification (HD or HDH) desalination, and specifically HD driven by a thermal vapor compressor (TVC), is a thermal desalination method that has the potential to produce potable water efficiently in order to address the growing demand for water. This article presents a numerical study and optimization of two HD-TVC cycle configurations in order to determine the best achievable thermal performance. Through the use of nonlinear programming, it is found that the simplest configuration of HD-TVC has performance comparable to a traditional single-stage, single-pressure HD cycle (GOR 0.8–2.0), while the hybridized HD-TVC cycle with reverse osmosis (RO) has thermal performance that is competitive with existing large scale desalination systems (GOR 11.8–28.3).Center for Clean Water and Clean Energy at MIT and KFUPMNumerica TechnologyKing Fahd University of Petroleum and Mineral

Energy efficiency of permeate gap and novel conductive gap membrane distillation

This work presents numerical modeling results and flux experiments for a novel membrane distillation configuration called conductive gap membrane distillation (CGMD), as well as permeate gap membrane distillation (PGMD). CGMD has a conductive spacer in the gap between the membrane and condensing surface rather than more commonly used insulating materials. Flux measurements with two experimental systems are used to validate the numerical models for PGMD and CGMD. PGMD has 20% higher GOR (energy efficiency) than an air gap membrane distillation (AGMD) system of the same size, whereas CGMD can have two times higher GOR than even PGMD. Increasing gap effective thermal conductivity in CGMD has negligible benefits beyond View the MathML source under the conditions of this study. The direction of pure water flow in the gap has a significant influence on overall system energy efficiency, especially in the case of CGMD. Using a countercurrent configuration for the pure water flow in the gap relative to the cold stream leads to 40% higher GOR than flow cocurrent with the cold water stream.MIT & Masdar Institute Cooperative Program (Reference no. 02/MI/MI/CP/11/07633/ GEN/G/00

Combined Power, Cooling And Desalination Using Natural Refrigerant Powered By Low-Grade Heat Source

Contemporary facilities experience growing demand of energy-intensive products like cooling, power and fresh water, which turn in results in energy and environmental concerns. The separate production of these intensive products potentially consumes more primary energy compared with combined production (polygeneration). We therefore seek the most sustainable and suitable polygeneration technology. The driving energy source for the polygeneration system is assumed to be low-grade heat available from solar thermal, geothermal, industrial waste heat, etc. The natural refrigerant based ammonia-water absorption system has the potential for more exploration in the field of polygeneration. In the past decades, ammonia-water based Combined Cooling and Power (CCP) systems were experimentally proven with different configurations. It is of great interest to integrate fresh water generation with an existing CCP system; hence, increasing the system output degrees of freedom with higher system potential performance. So far, the majority of existing desalination plants are Multi Stage Flash (MSF) desalination type, while Multi Effect Distillation (MED) technology has prominent advantages relative to MSF with high thermal efficiency, lower number of effects, low pumping power, high heat transfer coefficient, and tube do not contaminate the distillate water. According to the literature review, none of the published research works has investigated the ammonia absorption system for simultaneous cooling, power and fresh water. In this work, a thermodynamic study was conducted for natural refrigerant polygeneration system operated by low-grade heat sources to produce power and cooling output through generated ammonia vapour, with the rejected heat effectively utilized for desalination of salt water though MED with single flash technology. The combined system is the result of integration of absorption refrigeration, Kalina power and MED with single-stage flash desalination cycles. The low-grade heat source energy generates the refrigerant vapour in the generator, which is divided into two parts for power and cooling. The split ratio is used to vary the power and cooling output, based on demand variation. The total heat rejection from the absorber by heat of absorption and condensation heat from the condenser are effectively utilized for desalination through the MED system. The thermodynamic performance of the system is evaluated at the typical operating conditions of heat source, sink and evaporator temperatures of 250°C, 60°C and -10°C respectively for unit mass flow rate of weak solution. The system generates 170 kWth of cooling, 25 kWe of power generation and 950 kWth of rejected heat available for MED flash desalination. The system performance is evaluated through the effective exergy of the combined system, MED flash system performance ratio and power-to-cooling ratio