demonstration of solar driven membrane distillation in remote victoria

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Institute for Sustainability and Innovation, Victoria University, ... School of engineering and Science, Victoria University, .... With maximum 12 h of sunshine.
DEMONSTRATION OF SOLAR DRIVEN MEMBRANE DISTILLATION IN REMOTE VICTORIA 1

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Noel Dow , Mikel Duke Jianhua Zhang , Tom O’Rielly , Jun-De Li , Stephen Gray Eddy Ostarcevic 3 and Paul Atherton 1. Institute for Sustainability and Innovation, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia 2. School of engineering and Science, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia 3. GWMWater PO Box 481 Horsham Vic 3402, Australia ABSTRACT This paper presents results from a successful trial of membrane distillation (MD) to recover water from RO concentrate from GWMWater’s Edenhope Desalination Plant. The plant design capacity was 200 litres per day during cloudless days in summer. Much of the project was dedicated to constructing and verifying performance of a purpose built membrane MD module, followed by the integration of cutting edge evacuated solar tubes to collect the required thermal input. The module delivered high quality permeate, 55 µS/cm (~30 mg/l), suitable for subsequent blending to meet the capacity. Solar panels were capable of reaching the required temperature for a 60°C MD feed and their efficiencies were as high as 76%. The integrated solar MD system performed well with the RO concentrate fed continuously on-site. However, the use of solar energy and its variability was found to reduce performance to nearly half of the design capacity. This was at first due to cloudy and rainy weather, but was then due to gradual scale build up inside the module which reduced the system efficiency. A brief acid wash was found to restore peformance. Our first attempt to build a dedicated solar driven MD system was therefore successful and with further work, could be better optimised to meet requirements for reducing brine volumes from RO concentrates for inland desalination. INTRODUCTION With water becoming more scarce, water treatment plants treating poor quality water are now becoming accepted solutions to increase water security. For inland communities, reverse osmosis of brackish water sources is widely practiced to improve water quality. However, these inland RO plants typically recover only 70-75% of the feed water, the remaining 25-30% of the resource being discarded as saline RO

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concentrate. Furthermore, this RO concentrate must find a disposal route which is becoming an increasingly expensive practice. Increasing recovery is clearly advantageous in terms of collecting more water and reducing brine disposal volumes. However the recovery of water is limited by energy and membrane fouling. To increase recovery, a process like MD could be used as thermally driven processes do not require more energy input as salinity rises. Additionally, MD membranes are made from more durable materials suitable in very high salinity. MD is a process which was developed in the 1960’s, but is gaining recent interest due to rising water and energy concerns. MD has great potential in addressing water and energy simultaneously as it utilises heat instead of electricity to treat water. This heat can be low grade (40-100°C) and so can be easily sourced from waste heat or solar thermal collectors. Work on MD has also risen recently as a result of the availability of low cost high performance membranes produced from numerous suppliers worldwide (Zhang et. al, 2008). To demonsate MD however, purpose built systems are not readily available, especially in the Direct Contact MD mode which we have chosen to demonstrate. Much of our project was dedicated to the design, construction and validation of a module capable of housing enough membrane area to deliver water equivalent to a single dwelling's comsumption per day (~200 l). In this work, we show results for the demonstration of a purpose built MD system. The water was fed continuously on-site with RO concentrate from the GWMWater Desalination Plant at Edenhope in remote Victoria. The heat to drive the process was sourced from advanced solar evacuated tubes. This work follows from small scale testing presented at the previous OzWater conference

where we found Edenhope’s RO concentrate to be viably treated by membrane distillation at laboratory scale (Duke et al. 2009). EXPERIMENTAL AND METHODS Membrane Module Design The most challenging aspect of the project was to design the membrane module with no working designs from purpose built systems to work from. Coming previously from tests on single membrane layers, our design had to accommodate all flow entries and exits (2 x cold side and 2 x hot side) for a compact multiple flat sheet module. The design 2 allowed for any number of 0.23m sheets to be simply stacked to meet the area requirement. Membranes were sourced from Ningbo Chanqi, China and were hydrophobic MF membranes with 0.5 µm pore size Teflon active layers and polypropylene scrim backing as has been found to deliver best performance in MD (Dow et. al, 2008). Solar Collectors After a wide assessment of the various solar collectors available, we decided to use high efficiency evacuated tubes, the Skylight SLAGC 58-20, China, shown in Figure 1. Many of the existing solar hot water systems have poor efficiency at the higher temperatures required in our testing (~70°C). Evacuated tubes, however, have very high efficiencies even at these temperatures as their design is optimized to minimize thermal losses.

System Setup The simplified flow diagram of the process is shown in Figure 2. The system’s integration of the solar collectors is shown with heat exhausted by an air-cooler from Alfa Laval. To increase energy efficiency, a heat exchanger was included to couple the heat passed to the cold side back to the hot side feed prior to recycling. For MD, recycle loops on both hot and cold sides are needed to achieve the desired recovery as a single pass generally achieves recoveries not exceeding 5% by thermodynamic constraints. Hot loop surge tank

Concentrate

Solar field Energy recovery heat exchanger

MD module

Feed Air cooler

Permeate product

Cold loop surge tank

Figure 2: Simplified flow diagram of the solar powered MD system The tests were first carried out in the laboratory to verify performance using an electric heater. The benchmark performance was carried out using the parameters shown in Table 1. Feed water was dechlorinated tap water made up with reagent grade NaCl. Table 1: Module benchmarking test parameters for quality assurance testing PARAMETER Feed water NaCl concentration Hot side feed temperature Cold side feed temperature

Figure 1: Picture of a solar evacuated tube panel used in this project The basic arrangement of these panels is the tube collectors are inserted into the header tank on top and the entire volume of tubes and tank filled with a static volume of water. The process fluid is passed through a pipe inside the tank to collect the heat without contacting the static volume. This volume served as a convenient storage medium for solar thermal energy.

VALUE 10 g/l 60°C 25°C

Prior to sending on site to Edenhope, the module was quality tested for performance and leaktightness. The skid mounted system was assembled with off the shelf pumps (Davey), valves and meters from local suppliers. The complete setup installed at the Edenhope WTP is shown in Figure 3.

Cooler

terms of efficiency can then be determined. In general, the energy collection efficiency of the panels was around 70%. Temperature in the static tank was around 65°C, but at the required process flows was around 50°C. This performance indicated how the panels would be arranged to achieve the optimal flow rate through each of the 8 panels. The panels were therefore verified to have the capacity to deliver hot water at the required temperature range for the MD system.

Solar field

MD plant

Figure 3: Picture of MD demonstration plant at Edenhope WTP

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Table 2: Average performance data of 2.3m solar panel at various flows passed through to remove heat

RESULTS AND DISCUSSION

Membrane flux (l/m2/h) or permeate flow (l/h)

Benchmark Performance The performance of the membrane module under conditions prescribed in Table 1 is shown in Figure 4. Stable performance was achieved with the 6layer system, delivering an average flow of 10 l/h. 12.00 10.00 8.00 6.00 Permeate flow

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Flux 2.00 0.00 0.00

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Avg insolation 2 (W/m ) 635 740 740 826

Panel flow (l/min) 1.1 2.0 3.1 4.0

Outlet temp (°C) 49 48 47 42

Heat removed (kW) 0.7 1.0 1.2 0.9

Eff. (%) 65 68 76 67

Figure 5 (end of paper) shows the results of the solar panel performance for a single day. As can be seen from the insolation data, cloudy periods broke up continuous energy input in the middle of the day. Continuous cloud period at about 2:30pm significally dropped the incoming energy, and as a result, caused the solar header tank temperature to slowly decline. From these tests it is clear the solar energy income is greatly influenced by cloud cover, and would therefore influence the MD system.

Run time (h)

Figure 4: Performance of membrane module under benchmark conditions. 2

At this scale, using 6 membrane layers (1.4m ), water with a quality of about 30 mg/l was produced in the permeate. With maximum 12 h of sunshine per day, the MD system production is 120 l/day. The high quality permeate can then be mixed with 80 l/day of the bore water source of 1,200 mg/l to a potable standard of 500 mg/l making the system capacity of 200 l/day which is our target system capacity. Our estimations of the required solar area for this module design, including a heat recovery exchanger, indicated the need for 8 solar 2 panels (equivalent of 18 m solar collection area). Solar Collector Performance The performance of the collectors is shown in Table 2. Average insolation was measured by a pyranometer and the average performance in

Integrated Solar MD System at Edenhope The key feature of this project was the demonstration of the MD system on-site to treat RO concentrate from GWMWater’s Edenhope Water Treatment Plant (WTP). The RO concentrate was found to contain approximately 3,300 mg/L TDS with potential scalant levels at: calcium 200 mg/L, magnesium 100 mg/L, sulphate 200 mg/L and carbonate/bicarbonate 350 mg/L. It also contained Nalco Permatreat PC-191T antiscalant added during the normal operation of the WTP. Before commencing testing on the Edenhope WTP brine, a run was conducted on 10 g/l NaCl. The first test of the fully integrated MD system with solar energy and air cooler was found to operate effectively and produced high quality distillate (EC