Entry 19 - more about solar electricity, sea-water and all that
gerald hantusch posted on October 19, 2010 | 4163 views



- a draft-note aimed at any future “Philosophy of Engineering”.


In the previous entry to this blog, I coupled together three intellectual concepts not normally treated in one lump : solar energy, sea-water, and “height above sea-level”. In illustration, I postulated that steam-driven water-pumps, using steam generated by solar concentrators, could be used to raise sea-water to fill cliff-top reservoirs, and that the exhaust-steam could make a sweet-water condensate, for use in irrigation. I pointed to the two shores of the Red Sea as a likely location for large-scale plants implementing this tripod-concept, because on both sides of the that sea, agriculture without itrrigation is impracticable, and there are no easy natural sources of irrigation water. The entry attracted a single comment, by Kenny, admitting that the Red Sea makes a plausible location, but still so obsessed with the dry, dry Sahara that the “energy-storage battery function” of a seawater reservoir was not given any attention.


I now remind readers of a fundamental factor in “steam-engine technology” : the heat-energy used to evaporate water into steam is normally completely wasted, and only some necessary additional super-heat energy is turned into either mechanical energy or electricity. In the days of James Watt, even in well-watered England, they invented the “condenser” to make it unnecessary to keep supplying fresh “feed-water” from a source, since water from available supplies costs money. Now – seawater by a sea-shore costs only what it takes to pipe it to the boiler (or the boilers) and there are no fuel-costs in solar-concentrator boilers, only the original cost of construction, plus run-time maintainance. If the de-pressurised steam is always condensed and piped away for use as irrigation water, while the boiler is continually supplied with additional sea-water, the irrigation water is essentially free, or at least low-priced. The most expensive and head-achy part of the equation is piping away some enriched sea-water to get rid of salt-solute. Essentially – the boiler has to continually circulate the sea-water – plain incoming, salt-enriched out-going, with perhaps a heat-exchanger up-front, directly coupled to the condenser.


When the concentrated solar energy is employed to harvest electricity and irrigation water simultaneously, we have the beginnings of “ener-agriculture”, harvesting energy in two forms – directly, but also indirectly to create growing-conditions that produce food-grain, silage or grass in the way Nature always produced it before, from the seeds of plants that only need moist soil and sunlight to flourish.


I leave it as an “exercise for the reader” to calculate, for a given megawatt of concentrated solar energy, the percentages of electricity-yield, of irrigation-water yield, and off efficiency-loss.


To raise one kilo-gramme of sea-water up through one meter of height, even a modern Hercules has to do work against good old Isaac Newton's “gravitation” . If subsequently – having raised volumes calculated in acre-feet or hectar-meters, tons or tonnes of the heavy “foaming brine”, into a holding reservoir, said modern Hercules wants to turn it into electricity by means of a hydro-turbine, he will have to doff his hat to the late Mr. Joule – who could brew a mean pint of beer in his spare time – but who first calculated and then measured the “mechanical equivalent of heat”. That dreaded physical constant still governs how much heat you need to have available and expend, to generate one horse-power or one kilowatt of energy, take it or leave it. You can easily do worse than Joule says you can, but you can't do better. If you want to argue, the late Joule will call up his later follower, Kelvin, and between them, they will win the argument by throwing systems of thermodynamics at you. The only serious comment you can make is – “ … but the solar energy costs me nothing !” They will come back at you and say : “. . . but did you thank the Good Lord for all that sunshine ?”


It could never have escaped readers consciousnesses (sic !) that all that super-heated steam could have been straight-coupled to a steam-turbine to yield electricity directly. My answer is – been there, done that, etc. The problem of “intermittent sunshine” remains, and there is another problem : steam is a vapour, therefore packs little weight, and to turn a steam-turbine efficiently you need the steam to move fast, i.e. be highly pressurised, with all that implies in costly materials like stainless turbine blades that don't easily ablate, lubricating oils that can run hot, and steam-pipes that don't burst or fatigue . . . to create available harvestable kinetic energy for equal amounts of money spent, water always has it over turbine steam. Once you, or the Good Lord, have provided a fine head of hydraulic pressure, you are in business. Sea water costs nothing, pumping it up-hill implies accurately calculable costs, and hydraulic turbines always cost less for equivalent watts-kilowatts or megawatts of output.


The bottom line remains the same : once you have built your reservoir or holding tank up on your convenient cliff=top plateau, it will stay there as long as the Pyramids will stay at Gizeh, with only earth-quakes to fear : terrorist bombs are not very effective against massive earth walls. You then have a high-capacity storage battery that you can recharge over and over. Also, if you really want to, you can use your “sea-water-pump” output-water directly to drive another hydro-turbine at ground-level. More fool you : nothing stops you using your reservoir-fed turbine in sunshine and daylight. O.K. - instead of feeding pump-water into the pipe that take is uphill, you can even feed it directly into the same turbine. Now, finecky readers, are you satisfied ?


What I am pleased to call “human ecologistics” is a philosophic “discipline” that has a strong “engineering dimension”. When Adam and Eve and their descendants were turned out of the original Garden of Eden, the Good Lord, in essential terms, told them : “Consider the whole Earth and the whole Universe to be the only bit of cultivable ground I leave you with. I already showed you how to be a gardener, so now, if you want to feed you and yours, work up a sweat and get on with it.”


This present generation of Adam-Eve descendants has the opportunity – through that part of their progeny that has learned “Engineering”, to use sunshine to evaporate sea-water on any convenient piece of seashore-with-nearby-cliffs, to use it to drive pumps, to pump watrer uphill, to generate energy that does not call for bread-fed muscle power, and to use the depressurised re-condensed steam as a convenient substitute for “rain-water” . The Good Lord may have been wroth, but he did not take Adam's sinfully acquired intelligence, fruit of the Tree of Knowledge, back away from his former favourite gardener. Perhaps even the Good Lord will be a little bit pleased if Adam-descended engineers make sweet-water available for irrigation where the same Lord has omitted to make rain fall !


Ener-agriculture and ecologistics can become a global system under which the general human population better feeds itself, while sustainably controlling the rest of this Earth's bio-sphere, and so create a steady-state civilisation that ultimately turns this and other planets and other presently hellish habitats into other Gardens of Eden, as the Good Lord in his divine wisdom perhaps intended in the first place . . .


Le Chaim” and “Shalom Aleikum” to all Readers !





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