but it does not seem like an ideal world…

in an ideal lossless world, 5 KWh or 18 MJ of energy from the sun would have turned 1 litre of 25 deg C water into steam

but there are losses

let’s list some loss mechanisms and build a loss chain

sunlight strikes our black absorber. most of it gets absorbed, but a  portion get reflected, even off the best black body emulator

the absorber gets hot. it now re-radiates energy at a longer wavelength, most of which is lost back to the atmosphere (some, very little, of it may end up also heating the water)

as the energy absorber transfers the heat to the water, some of it heats up things other than water; for example the sides of the heat transfer unit – the unit that takes the heat captured from sunlight and injects it into water. and other such parts of the machine

the water, as it heats up, radiates away some heat. and heats up other parts of the machine it comes in contact with

if water is allowed to boil uncontrollably, further energy is lost, as kinetic energy, as the water roils, and bubbles seethe and burst

all these losses combine to steal away energy that, in an ideal world, could have been used to convert water into steam

the trick of course is to minimize losses

for example, as steam condenses (in a condenser) and gives up heat, if we use this heat to preheat incoming water, we’ve captured and re-used some of the energy that would have been other wise lost

we’ll certainly need some inspired engineering to keep these losses to the very minimum, if we want to keep the absorber’s dimensions as small as possible, a desirable target, in the interests of portability and reduced material cost. remember, at best, we can convert no more than 7 litres of water per day, in good weather, using an absorber of 1 square meter surface area, always pointed at the sun

and ‘always pointed at the sun’ means we’ll have a single axis tracker, tracking the sun daily from east to west. a single axis tracker that relies on human supplied mechanical energy, and possible gravity, to operate trouble free for at least 50 years. like a very well crafted mechanical clock, for example


About Vishwa Narayan

who can live without water? drinking water for all is my goal, through Theertham, a not-for-profit organization i founded and registered in Texas. you can reach me at drvishwanarayan@yahoo.com
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