Energy efficient multi-effect distillation powered by a solar linear Fresnel collector

Conventional seawater desalination processes like the multi-stage flash (MSF) and multi-effect distillation (MED) are environmentally unsustainable. They consume large amounts of fossil fuels which are a major cause of climate change. Further, desalination plants discharge highly concentrated brine which can cause eutrophication and damage the marine life. Qatar, being a country that faces freshwater scarcity, is highly dependent on desalination for municipal water consumption. On average the daily production capacity of all desalination plants in Qatar is 1.5 million cubic meters per day. This incurs heavy costs on both the economy and the environment. It is expected that by 2020, desalination fuel costs will reach $2.55 billion. Conventional desalination can be made more sustainable by integrating it with solar energy. However, assessing the environmental competitiveness of this solution should be done in a systematic way and reflect the overall system performance. Simplistic models like merely calculating CO2 emissions are not enough and only allow for modest conclusions. Based on a previous literature review by the authors, it was found that the MED process with thermal vapor compression (TVC) is an excellent choice to couple with solar thermal energy that is provided from a concentrating solar collector. The authors also developed a configuration for solar-driven MED with TVC that is simpler in component choices and relies 100% on solar energy to provide the superheated steam required for the MED-TVC process. A model was developed for a 7-effect MED-TVC pilot plant and was validated with actual plant data. Current literature on desalination mainly focuses on membrane technologies and almost completely neglects thermal desalination. In the Arabian Gulf region, thermal desalination is predominant and hence it is required to assess its sustainability from a view point and further investigate how coupling renewable energy can reduce the environmental impacts. This work quantifies the environmental impacts of solar desalination in Qatar using life cycle assessment (LCA). Our work is based on the proposed MED-TVC solar-driven plant. The objective of this study is to assist decision making by providing information about the potential environmental impacts of solar desalination, propose system improvements and suggest references for comparison between different renewable energy-driven desalination processes in general. We identified five impact categories: global warming, freshwater eutrophication, water use, mineral resource scarcity and fossil resource scarcity. GaBi tool was used to carry the LCA. Ecoinvent database, GaBi databases, academic literature and expert opinions were used to construct a comprehensive life cycle inventory for the plant. ReCiPe method was used to assess potential impacts in the five categories. This method was used because it includes characterization factors unique to Qatar and also because it was widely used in the literature hence comparisons can be made. The functional unit was 1 m3 of freshwater at the plant. The results of the LCA are then computed, grouped and weighted. Comparisons with similar desalination systems are also made. The findings of this work are highly relevant to Qatar National Vision 2030 as they provide detailed findings on the environmental impacts of solar-desalination which is a promising solution for the problem of water scarcity in Qatar.

Conventional seawater desalination processes like the multi-stage flash (MSF) and multi-effect distillation (MED) are environmentally unsustainable. They consume large amounts of fossil fuels which are a major cause of climate change. Further, desalination plants discharge highly concentrated brine which can cause eutrophication and damage the marine life. Qatar, being a country that faces freshwater scarcity, is highly dependent on desalination for municipal water consumption. On average the daily production capacity of all desalination plants in Qatar is 1.5 million cubic meters per day. This incurs heavy costs on both the economy and the environment. It is expected that by 2020, desalination fuel costs will reach $2.55 billion. Conventional desalination can be made more sustainable by integrating it with solar energy. However, assessing the environmental competitiveness of this solution should be done in a systematic way and reflect the overall system performance. Simplistic models like merely calculating CO2 emissions are not enough and only allow for modest conclusions. Based on a previous literature review by the authors, it was found that the MED process with thermal vapor compression (TVC) is an excellent choice to couple with solar thermal energy that is provided from a concentrating solar collector. The authors also developed a configuration for solar-driven MED with TVC that is simpler in component choices and relies 100% on solar energy to provide the superheated steam required for the MED-TVC process. A model was developed for a 7-effect MED-TVC pilot plant and was validated with actual plant data. Current literature on desalination mainly focuses on membrane technologies and almost completely neglects thermal desalination. In the Arabian Gulf region, thermal desalination is predominant and hence it is required to assess its sustainability from a view point and further investigate how coupling renewable energy can reduce the environmental impacts. This work quantifies the environmental impacts of solar desalination in Qatar using life cycle assessment (LCA). Our work is based on the proposed MED-TVC solar-driven plant. The objective of this study is to assist decision making by providing information about the potential environmental impacts of solar desalination, propose system improvements and suggest references for comparison between different renewable energy-driven desalination processes in general. We identified five impact categories: global warming, freshwater eutrophication, water use, mineral resource scarcity and fossil resource scarcity. GaBi tool was used to carry the LCA. Ecoinvent database, GaBi databases, academic literature and expert opinions were used to construct a comprehensive life cycle inventory for the plant. ReCiPe method was used to assess potential impacts in the five categories. This method was used because it includes characterization factors unique to Qatar and also because it was widely used in the literature hence comparisons can be made. The functional unit was 1 m3 of freshwater at the plant. The results of the LCA are then computed, grouped and weighted. Comparisons with similar desalination systems are also made. The findings of this work are highly relevant to Qatar National Vision 2030 as they provide detailed findings on the environmental impacts of solar-desalination which is a promising solution for the problem of water scarcity in Qatar.

In many parts of the world, desalination is the only viable and economic solution to the problem of fresh water shortage. The current commercial desalination technologies rely on fossil fuels and are thus associated with high greenhouse gas emissions that are a major cause of climatic changes. Solar thermal-driven multi-effect distillation with thermal vapor compression is a clean alternative to conventional desalination technologies. To comprehend this process, as well as its features and limitations, extensive modeling is required. In this work, we proposed a plant design based on a solar field with a linear Fresnel collector that supplies heat to a multi-effect distillation plant with thermal vapor compression. The solar desalination plant model is implemented in the Engineering Equation Solver (EES). The system performance is investigated and a control strategy for reducing electric pumping is proposed. Results showed that 1 m2 of the solar field produces 8.5 m3 of distillate per year. The proposed control strategy resulted in a 40% reduction in electric pumping energy. Our results highlight the versatility of the linear Fresnel collector when coupled with thermal desalination.

In recent times two or more desalination processes have been combined to form integrated systems that have been widely used to resolve the limitations of individual processes as well as producing high performance systems. In this regard, a simple integrated system of the Multi Effect Distillation (MED)/Thermal Vapour Compression (TVC) and Permeate Reprocessing Reverse Osmosis (PRRO) process was developed by the same authors and confirmed its validity after a comparison study against other developed configurations. However, this design has a considerable amount of retentate flowrate and low productivity. To resolve this issue, two novel designs of MED and double reverse osmosis (RO) processes including Permeate and Retentate Reprocessing designs (PRRP and RRRO) are developed and modelled in this paper. To systematically assess the consistency of the presented designs, the performance indicators of the novel designs are compared against previous simple designs of MED and PRRO processes at a specified set of operating conditions. Results show the superiority of the integrated MED and double permeate reprocessing design. This has specifically achieved both economic and environmental advantages where total productivity is increased by around 9% and total retentate flowrate (disposed to water bodies) is reduced by 5% with a marginally reduced energy consumption.

This study presents a performance analysis of a Concentrating Solar Power (CSP) plant coupled to a Multi-Effect Distillation (MED) unit with an Air-Cooled Condenser (ACC) as the MED last condenser (configuration named as MED- ACC) to evaluate the advantage of this integration in comparison to the typical once-through condenser (configuration named as MED-OTC). The analysis has been performed for Crucero, Chile, which is located 100 km far from the coast and 1000 m above sea level. Thus, a seawater pumping system and energy recovery of the brine disposal are considered. The design of the MED-ACC plant has been carried out in terms of the ACC temperature limitations, and the simulation of both configurations has been performed on an hourly basis. Results obtained in the design show that configuration MED- ACC requires a lower number of effects than the MED-OTC configuration, which reduces the capacity and efficiency of the MED plant. A significant advantage of the former configuration is the reduction in the MED electric consumption with respect that of the latter configuration. Regarding annual performance results, it was found that the net electric output of the MED-ACC plant was 14% higher than the MED-OTC plant due to the seawater pumping consumption reduction, while the water production was 43% lower, despite the water capacity factor of both configurations remained similar.

Hybrid solar desalination systems driven by parabolic trough and parabolic dish CSP technologies: Technology categorization, thermodynamic performance and economical assessment

Performance analysis of hybrid system of multi effect distillation and reverse osmosis for seawater desalination via modelling and simulation

Improved approach to steady state simulation of multi-effect distillation plants

Potential of solar desalination in Israel and in its close vicinity

This chapter addresses the description and thermodynamic analysis for the integration of desalination plants into the power cycle described in Chap. 4. The systems chosen for this study combine a Concentrating Solar Power plant using parabolic-trough collector technology for electricity generation with various desalination plants, giving rise to what is known as a parabolic-trough concentrating solar power and desalination (PT-CSP + D) plant. The description of the PT-CSP plant, based on the Andasol-1 (Blanco-Marigorta et al., 2011) commercial plant, is detailed in Chap. 4, showing all the model equations. The desalination technologies selected to combine with the PT-CSP plant were multi-effect distillation (MED) and reverse osmosis (RO), as discussed in Chap. 1. On one hand, the simultaneous production of water and electricity using an RO plant connected to a CSP plant seems the simpler option. On the other hand, the integration of a low-temperature MED (LT-MED) plant is an interesting alternative because it allows replacement of the conventional power-cycle condenser by using exhaust steam as the thermal energy source for the desalination plant. However, to satisfy demand, while providing a certain performance, the LT-MED plant inlet temperature should be around 70 °C (corresponding to 0.031 bar absolute), meaning that the steam does not completely expand through the turbine and therefore the power-cycle efficiency is low compared with a stand-alone electricity-generating plant. This is the reason why another alternative to the MED plant, MED with thermal vapour compression (TVC), is considered. In this case, the steam expands completely in the turbine until it reaches the permitted value for the condenser conditions. However, part of the steam circulating through the turbine is extracted and used as high-pressure steam; this, together with the low-pressure steam coming from one of the MED effects, generates the inlet steam required in the first stage of the desalination plant. Moreover, in this study, a new concept of CSP + MED plants is evaluated (which, until now, has not been studied in published works), a thermally fed LT-MED plant with steam coming from a thermocompressor (LT-MED + TVC). In this case, the low-pressure steam (the entrained vapour) used by the thermocompressor comes from the exhaust steam of a PT-CSP plant instead of one of the MED effects. In each of the systems studied, desalinated water production is evaluated as well as the power and efficiency of the dual thermal solar power and desalinated water cycle.

In recent years, the use of solar energy has been growing exponentially and applied in a wider range of applications; one of the important applications for using solar energy is water desalination. The current work investigates the proof of concept experimental setup for a novel solar multi-effect distillation (MED) using alternate storage tanks. The experimental setup consists of a MED unit, two thermal storage tanks, and a solar collector. One storage tank is used as a charging tank while the other tank is used as a discharging tank. This unique dual-tank system stores the thermal energy to be used later in the MED unit, which improves the control of the water mass flow rate and water temperature throughout the MED process. The peak temperature achieved every day in the charging tank determines the MED production capacity. This system is designed for the tanks to alternate roles every 24 hours. The testing of this design was carried out during May 2019 in Saudi Arabia. The experimental results prove the novel concept design to work efficiently providing an average production rate of about 21 kg/day with total solar collector area of 2.7 m2 and average daily performance ratio of 2.5.

Analysis of environmental effect of hybrid solar-assisted desalination cycle in Sirdarya Thermal Power Plant, Uzbekistan

A novel multi-effect methanol distillation process

Minimisation of Energy Consumption via Optimisation of a Simple Hybrid System of Multi Effect Distillation and Permeate Reprocessing Reverse Osmosis Processes for Seawater Desalination

The Multi-effect distillation (MED) associated with thermal vapor compression (TVC) process has recently been applied for a number of desalination plants in the GCC countries and emerged as a strong competitor to the multistage flash distillation (MSF) process. The MED/TVC desalination process is characterized by low power consumption compared to the MSF process. It is currently operated at low TBT of 65°C to avoid scale formation compared to MSF technology which operate at top brine temperature of 110°C, this lead to lower tendency to scale formation and less fouling risk. The use of more efficient pretreatment to MED/TVC desalination plants such as nano-filtration or high performance additive antiscalant would allow operating MED at higher TBT which allow increasing the performance ratio. A simulation steady-state program has been established to analyze the thermodynamic behavior of parallel feed flow MED/TVC unit at a wide range of TBT from 65°C up to 125°C and different number of effects with different condition of motive steam pressure on the performance ratio, and specific heat transfer and therefore the best design of MED for the future different design options were considered to optimize the operational of MED system to enhance the economics and performance of MED technology.