i learned of a commercial solar water purifier with many units in service in the Middle East. with very efficient performance figures
now these are high capacity systems with stationary installs, like a water plant. (while, as we know, we look to build a portable low capacity very low cost unit). MAGE unit water purification capacities span 1 cubic meter per day (1000 litres) to 50 (50,000 litres)
here’s a 930 litre per day install in Oman, by the sea, operates 24 hours a day with stored sunlight energy
i draw information from Dr.-Ing. Hendrik Müller-Holst’s presentation ‘Solar thermal desalination for decentralized production of pure drinking water – A technological overview MAGE Water Management GmbH, Presenter: Rudi Gleich, Almeco-TiNOX GmbH, at a US conference july 2010. this presentation includes a very good review of at least 6 solar water purification technologies
first piece of good news – MAGE’s solar purifier uses a selective absorber of the kind we talked about, with high sun-light emissivity and a low infrared-emissivity, made by Almeco-TiNOX. it absorbs most sunlight – ’95 % of incident light’ and radiates very little in the infra-red – ‘4 %’, (as opposed to almost 50 % for a non selective black surface absorber), so that around ‘90% of the solar energy can be used as heat’. 90% sunlight utilization! in a commercially available selective absorber material. good!
it desalinates water ‘by multiple evaporation of salty waters and
consecutive condensation of the generated humidity‘ the author likens the product’s working method to the natural sun-fired-rain making process
it ‘recovers energy by a sophisticated arrangement of the condensation-
recall, it ideally takes 5 kWh of energy to boil 7 litres of 25 deg C water into steam. and the bulk of the energy went into transitioning from 100 degree boiling water to steam (2260 KJ for 1 litre), more than 5 times the energy taken to get from 25 deg C to 100 deg C water (315 KJ for 1 litre). when the steam condenses back to water, it returns the 2260 KJ. as we’d talked, if this heat can be captured and reused, we could process significantly more than 7 litres of water with the 5 kWh we’d captured from the sun (assuming a moderate latitude location, a bright day, and a 1 square meter surface area ideal absorber pointed straight at the sun)
how much more water can we process with this energy? depends on how much of it we capture and reuse. from their numbers, MAGE’s done a great job capturing and reusing this heat. hence the ‘sophisticated arrangement of the condensation-evaporation unit’ – thermally intertwined to capture as much of the heat as possible – a design idea we’ve talked about with respect to the quencher’s ‘engine’
it ‘requires a process energy of 80°C (175 °F)‘
not 100 deg C!
i boiled some water in a vessel on the stove and measured its surface temperature using an accurate infra-red non contact thermometer. at a surface temperature of 80 deg C, much of the surface of the water was gently turbulent with masses of small quietly exploding bubbles giving up their tiny pockets of steam. water was turning into steam at 80 deg C, a little of it at a time. hence the ‘by multiple evaporation of salty waters and
consecutive condensation of the generated humidity’. i e evaporating water at the lower temperature of 80 deg C, a little bit at a time, repeatedly. it will still take 2575 KJ of energy to convert 1 liter of 25 deg C water into steam, but they’ve pulled it off at 80 deg C
additional energy is saved by not having to heat the water all the way up to 100 deg C, and instead, topping off at 80. how much energy saved for steaming 1 litre of water at 80 deg C rather than 100 deg C? recall it takes 4.2 KJ for a 1 deg C increase in temperature in 1 litre of water. so 84 KJ saved with the 20 degree reduction. a small amount, compared with the 2260 KJ of the heat available for recapture as steam condenses back to 1 litre of water. but every little bit adds to the efficiency of the unit
as the MAGE units’ numbers tell
unit produces 25 to 35 litres of water per square meter of selective absorber collector area – a very impressive number
recall we’d calculated 1 sq meter suntracking collector collects 5 KWh per day with which we could brute-force-100-deg-C evaporate at most 7 litres of water, without reusing any of the energy released when steam turned back to water
they are purifying 25-35 litres per day – up to 5 times more than we’d first calculated, for the same energy! great news! and it does not seem like their absorbers even track the sun!
reuse of energy shed by steam when it condenses back to water is essential
other features MAGE points out are that the quality of the purified water is excellent, unit is designed for 25 year life time, unit requires minimal maintenance, water does not require chemical treatment prior to processing, and that product has passed European quality tests