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.The glass top should lie flush on the box with a tight fit to keep the heatin so that water vapor forms on the glass.Volunteers In Technical Assistance (VITA) has a booklet by W.R.Breslintitled Solar Still (available through PACT Publishing) which has instruc-tions for building a solar heated distillation system.The P41 electrolyzerThe P41 is a simple electrolyzer specifically designed to provide maxi-mum output with the smallest footprint possible with off the shelf compo-nents.As with every part of this project, the important factors were readyavailability of components at low cost, and ease of working with the com-ponents, so that anyone with average skills could build a device that wouldbe comparable with commercial equipment.For this electrolyzer, the primary goal was to produce enough hydrogento run a real time stationary fuel cell system within the smallest possiblespace without the gases intermingling.86 ElectrolyzersDesign for renewable energy power sourcesAn important design consideration was to maximize the efficient use ofenergy input from renewable energy power sources.Most commercialelectrolyzers are designed to run off utility power grids with a rectified DCsource.The electrode materials used in these electrolyzers reflect thatparticular type and quality of power source.Making intermittent power efficientDC power from renewable sources such as photovoltaic panels is deliv-ered intermittently.Depending upon where you are, there are very sunnyclear days, but there are usually more days when the sun is behind theclouds for five minutes then bursts out in full sun only to be behind anoth-er cloud in another five minutes.This varying amount of sunlight will causethe disassociation of hydrogen and oxygen from water to proceed at afaster or slower pace according to the unsteady flow of DC power from thePV panels.This causes sharp spikes and troughs in gas production.Thisis not a problem per se, but it led me to think of nano-nuances in regardto renewable energy delivery systems.A minimum voltage of about 1.23 V at about 77°F will disassociate waterinto hydrogen and oxygen, but this process absorbs heat.At a voltage of87about 1.49 at around 77°F, no heat is absorbed.Higher voltages than 1.49 Electrolyzersrelease heat during the disassociation process.Elevated operating tem-peratures increase the efficiency of the electrolytic process because lesselectrical input is needed.Electrode materialsExperiments with a variety of materials led to the conclusion that monelis a perfect choice for electrodes in an electrolyzer designed for intermittentpower supplies.It has an electrical resistance of about 42 micro ohms-cen-timeters at 20°C.This is much higher than, for instance, nickel, which has aresistance of 11.With higher resistance, more heat is generated as the cur-rent passes through the electrodes and electrolyte solution.Thermal flywheelFor conventional electrolyzer design, monel would not be considered tobe as efficient as other materials the heat generated would be consid-ered wasted energy.However, for intermittent renewable energy systems,monel is a good choice because thermal input dissipates at a much slow-er rate than electrical input.Thus, the thermal energy is still feeding theprocess when, for instance, the sun is momentarily obscured by a cloud.A thermal flywheel is created that retains the energy gained from thephotons as heat, which lowers the threshold for the electrolytic reaction in88terms of how much voltage and current is needed.When the sun is behind Electrolyzersa cloud, less electricity is generated, but the heat retained in the elec-trolyzer requires less electricity to continue the reaction at a more activelevel than without the additional heat.SupercapacitorsAnother approach to consistent power delivery from renewable sourcesis to consider the use of supercapacitors placed between the renewablepower source and the electrolyzer.These may or may not be viable, andwe have not yet researched this possibility.Supercapacitors are basicallya cross between capacitor and battery technology.They use electrodes,and a liquid or organic electrolyte, but they store energy by static chargerather than by electrochemical means.They can be cycled millions oftimes, and have a recharge time of seconds.Supercapacitors might alsobe viable to enhance peak load performance on the fuel cell end.Micro and macro electrode surface considerationsThe next design consideration was surface area.The greater the surfacearea of an electrode, the greater the gas production will be for a givenspace.89 ElectrolyzersRaney metal surfacesAt the present time, Raney metal structures expose the greatest amountof surface per area.(Nano-tube structures will soon surpass Raney effi-ciencies, but right now these nano-tube electrodes are very experimental.)However, as stated earlier, Raney structures have the nasty habit of act-ing as extremely fine porous filters that gum up.This would reduce theefficiency of an electrolyzer incrementally over time.Also, the microporous Raney surfaces seem to retain gas for longer periods of time inpockets, which blocks the reaction and renders a portion of the electrodesurface useless.This creates a higher active threshold, which is accept-able for industrial systems that run off consistently supplied grid power,but is not particularly suitable for intermittent renewable power supplies.Another disadvantage is that Raney surfaces are more expensive thanother alternatives [ Pobierz caÅ‚ość w formacie PDF ]