Grantee Research Project Results
Final Report: Low cost desalination process for rural and coastal communities
EPA Grant Number: SU836130Title: Low cost desalination process for rural and coastal communities
Investigators: Gude, Veera Gnaneswar
Institution: Mississippi State University
EPA Project Officer: Page, Angela
Phase: I
Project Period: September 1, 2015 through August 31, 2016
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2015) RFA Text | Recipients Lists
Research Category: P3 Awards , Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities , P3 Challenge Area - Chemical Safety
Objective:
The need for freshwater can never be overstressed. Global agencies (including WHO, UNDP, UNICEF etc.) expect that 24 of the least developed countries, many of them along coastal areas without access to water and electricity, need to more than double their current efforts to reach the Millennium Development Goals (MDGs) for basic health, sanitation, and welfare. Desalination of available brackish or seawater sources is an ideal option for freshwater production. However, existing desalination technologies are energy-intensive and cost-prohibitive. Low temperature desalination using waste heat sources or solar collectors is an attractive option. Because the energy demands can be provided with low cost and minimum environmental pollution. The main objective of this design project is to investigate the technical feasibility of a multi-effect low temperature desalination process with higher thermodynamic efficiency and low environmental impact.
We have developed a novel low temperature thermal desalination system which operates under natural vacuum using low grade/waste heat sources such as process waste heat and low grade flat plate solar collectors. The process operates under near vacuum conditions created by natural gravity and barometric head. Two 33 ft. saline and freshwater columns together construct the low temperature desalination process to automatically draw the feed and produce freshwater by use of waste heat in the evaporator. Thus the process operates continuously without any external mechanical energy for pumping. Current research is based on the proof-of-concept demonstration study experience and our current knowledge to expand the use of energy recovery (by multi-effect design) in the desalination process to increase the output rates and to reduce the process/energy footprint and costs. Also, a new integrated process configuration will be developed under this research for combined freshwater and energy production for use in small communities and remote areas without access to grid. The research will be conducted through three experimental tasks to demonstrate the low temperature desalination under natural vacuum: the tasks are: 1) Design and development of novel multi-effect desalination unit powered by solar collectors; 2) Demonstrate the new desalination process (multi-stage experiments at different feed rates, brine withdrawal rates, and evaporation temperatures using solar collectors); and 3) Demonstrate the process with solar/PV thermal collectors for combined energy-water production.
Summary/Accomplishments (Outputs/Outcomes):
We have started our P3 research project with an expression of interest and design brainstorming session in August. Several iterations of design concepts were done before finalizing the current design. The goal of this activity was to identify and delineate the design details for the three-stage desalination unit. The three stage design includes heating and condensing surfaces in successive stages for energy recovery and product recovery enhancement.
Design and manufacturing of a three stage desalination unit was completed. A second unit is under construction for simultaneous evaluation of heat sources derived from solar collectors and photovoltaic thermal collectors.
The solar collectors were installed for the demonstration solar energy driven desalination process. Commercially available solar collectors (from Heliatos® ) were purchased and connected in series to provide heat source to the desalination unit.
Two photovoltaic modules capable of producing approximately 250 W (together) of electricity were purchased from a commercial vendor. These were modified to include cooling water circulation system to extract the heat accumulated by the photovoltaic module surface. The heat extracted from the photovoltaic modules will be used for desalination purpose. This task has been completed and the modified modules are available for demonstration. Fig. S1 shows the experimental setup for low temperature desalination process.
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Experimental details and results
e have conducted a series of experiments with different hot water source temperatures at 55°C, 65°C, and 75°C respectively. Evaporation and condenser areas were 0.31 m2 respectively for all the stages. A coil-type heat exchanger was used to supply heat in the first stage. Representative temperature profiles are shown in Figure S2. The experiments were run for a total period of 18 hours which includes 10 hours of heating and 8 hours of cooling (no heat supply) periods. The freshwater production rates increased with increase in the source water temperatures. These were 160, 230, and 260 mL/hr at 55°C, 65°C, and 75°C respectively. The total output from the three stage unit will 30 L/d-m2. The energy efficiency of the three stage desalination unit over the heating and non-heating periods has exceeded 72%. Efficiency can be further increased with proper insulation and further fine-tuning of the process. The productivity of the unit is at least 2.5 higher than the solar still productivity per given area and it is well comparable with other thermal technologies. Further controlling the vacuum conditions have the potential to almost double the current productivity.
Experimental studies will be continued to evaluate solar collectors and photovoltaic thermal collectors as heat sources for desalination. Electricity production with and without the modification to the photovoltaic thermal collectors will be evaluated. Through experimental studies, critical factors related to the design of large scale production units such as evaporator and condensing surface areas, heat exchanger surface areas and flow rates will be determined. Saline water and brine flow rates will be optimized to enhance energy recovery within the system.
[INSERT FIGURE S2]
Broader Impacts
Coastal counties in the U.S. are home for more than 53% of the nation’s population and this situation is very much similar to that at the global level. Desalination of brackish and sea water sources is increasing globally due to this high potable water demand. Developing energy-efficient low temperature desalination processes will conserve high quality water/energy sources for other beneficial uses while eliminating environmental pollution. New, low cost heat transfer materials (carbon doped copper-nickel alloys/titanium sheets) development for enhanced heat transfer is the innovative aspect of this project, followed by techno-economic demonstration with low cost construction materials such as plastic shells. The proposed desalination system does not contribute to greenhouse gas emissions either directly or indirectly and is a sustainable process. This research and design project will provide opportunities for graduate/undergraduate students to gain hands on experience in design, fabrication, and pilot scale testing, and modeling of the desalination processes. This research-design project can have far-reaching benefits by providing safe potable water in an affordable and sustainable manner.
Conclusions:
A three-stage low temperature desalination process was demonstrated using a synthetic waste heat source as a driving force for thermal desalination. Results from the process indicate that about 30 L/d/m2 of freshwater can be produced using a source temperature of 60°C in three stages with an acceptable thermal energy efficiency. Due to wet and cloudy weather, solar energy could not evaluated as a heat source in the desalination process. The project will continue experimental studies to demonstrate and to optimize the process conditions for the desalination process powered by solar collectors and photovoltaic thermal collectors.
Because solar energy is intermittent, it is appropriate to utilize a thermal energy storage system to accumulate the unused thermal energy during the sunlight hours for efficient operation in non-sunlight hours when ambient conditions are more favorable for evaporation-condensation process. Therefore, we propose to use a sensible thermal energy storage unit (essentially a hot water tank) integrated with solar and photovoltaic thermal collectors to maximize the solar energy utilization for desalination.
The design to operate under vacuum conditions presents several challenges such as the strength of the materials that could be available at low cost and the energy recovery and reuse scheme in successive stages for enhanced freshwater output. Optimization of heat transfer surfaces and water and vapor transport flows need to be studied to minimize pressure losses (if any) within the system. Finally techno-economics of the new solar powered low temperature desalination system, payback periods for cost, energy, and environmental emissions should be evaluated.
Journal Articles:
No journal articles submitted with this report: View all 3 publications for this projectSupplemental Keywords:
solar energy, low temperature desalination, natural vacuum desalination, photovoltaic thermal collectors, clean energy and water, sustainability, resource-efficient technologyThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.