Water Treatment for Solar Panel Cleaning using Membrane Distillation Coral R. Taylor, P.E. and Sage R. Hiibel, Ph.D. Department of Civil and Environmental Engineering ~ University of Nevada, Reno Project Overview Bench- and Pilot-Scale DCMD Systems Nevada Solar & Water Express The NV Solar Nexus Water Team’s objective is to develop sustainable and advanced water/wastewater approaches to support the water needs of solar energy development and to use solar energy to process water. As part of this, a mobile water treatment trailer, the Nevada Solar & Water Express, is being developed that can treat water at various solar facilities. Figure 1: Treatment and reuse of water for cleaning solar panels powered by solar heat. Bench-Scale Used to evaluate treatment performance of commercially available membranes Maximum water flux and contaminant rejection Minimum fouling and scaling Experimental data for development and validation of computational DCMD model Figure 3: Schematic2 and lab setup of the bench-scale DCMD system. Pilot-Scale Used to treat solar panel wash waters and other wastewaters at solar energy facilities Novel stacked flat-sheet design Increased membrane surface area with small footprint Improved thermal profile over hollow-fiber systems Modular design for adjustable treatment throughput Figure 4: Side view and expanded gasket schematics of the small pilot-scale DCMD module. Mobile water treatment trailer for use on-site at solar energy facilities in Nevada Will include a variety of state of the art and standard water treatment technologies NEXUS technologies DCMD, solar distillation, solar water heater Traditional technologies Advanced oxidation, UV disinfection, filtration, etc. Modular workspace design for flexibility Will also serve as an educational demonstration unit to raise STEM awareness Photovoltaic panels will provide a portion of electrical needs Figure 7: Profile and plan views of the Nevada Solar & Water Express mobile water treatment trailer. Used Wash Water Membrane Distillation Gasket Feed Side Membrane Distillate Side Solar Heat Plan View Profile View Panel Cleaning Water Treated Water Distillate Feed Chiller Circulating Pumps Membrane Module Data Acquisition Computer Heated Bath Flux Balance Conductivity Meter Flow Meters Direct Contact Membrane Distillation Emerging treatment technology for brackish and some industrial wastewaters Temperature-driven membrane separation process Hot ‘dirty’ water circulated on feed side Cold ‘clean’ water circulated on distillate side Temperature difference creates partial vapor pressure difference (Dpvap) across membrane Dpvap drives transport of water vapor through microporous, hydrophobic membrane Provides high rejection of non-volatile contaminants and high water recoveries Figure 2: Schematic of DCMD1 Key Advantages Low operating pressures Low operating temperatures Waste heat or solar heat sources can be used Driving force not function of concentration Maintains efficiency at high salt concentrations Feed Side Distillate Side Membrane Preliminary Results Wide variety of membranes evaluated for water flux and hydrophobicity (Table 1) Hydrophobicity based on contact angles (Fig. 5) obtained with tensiometer (Fig. 6) 0.2 mm pore-size membranes (3M PP 0.2, PP 0.2, QM022) are more hydrophobic Presence of surfactants in feed water dramatically decreases membrane hydrophobicity (<90° in all cases, data not shown) resulting in pore flooding and high salt passage Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No. IIA-1301726. Thanks to Mark Lattin, Development Technician III at University of Nevada, Reno, and Peter Faught, Experimental Testing/Prototype Engineer at University of Nevada, Las Vegas, for assistance with trailer design and fabrication. Figure 5: Schematic of the contact angle between a wetted surface and a water drop3. Contact angles less than 90° indicate a hydrophobic surface. Table 1: Hydrophobicity of commercially available membranes tested. Membrane Contact Angle 3M ECTFE 110.3° 3M PP 0.2 121.2° PP 0.2 134.3° 3M PP 0.45 122.6° PTFE 0.45 134.2° QM011 125.5° QM022 134.9° Figure 6: Lab setup of tensiometer. Shown with tilting cradle for testing slanted solar panel surfaces. References 1Adham, S. et al. (2013). Desalination, 314. 101-108. 2Rao, G. et al. (2015). Desalination, 367. 197-205. 3www.ramehart.com/contactangle.htm