If you have a raging torrent running past your door it is easy to see how to pull energy out of it, even an inefficient marine style propeller on a shaft dropped into the current will rotate, and drive a dynamo, even an inefficient one, and produce a useful surplus of power.
Want more power, build a mk2 version 5 times the size, no problem because the river is so big and running so fast.
If however you have exactly the same amount of water flowing past your door every second, but it is flowing in a huge wide channel so slowly it is imperceptible to the naked eye apart from the odd small eddy midstream, it is very very difficult to extract any useful energy from it, in theory you could build a truly vast and precisely balanced and gimballed waterwheel, with a highly efficient chan drive to a highly efficient rare earth precision machined dynamo, but a few thousand tons of waterwheel made to those tolerances would cost millions, the ROI (return on investment) period is longer than the life of the machine.
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A mate of mine is going on about Stirling engines at the moment, they seem great, they are hyped up to hell and back, but the reality is there are only slightly more practical and efficient that using microwaves to transmit power, eg a tesla radiant energy device.
If you have a satellite in orbit with acres of photovoltaic cells collecting pure sunlight in gigawatt quantities then you can't run cables down to a base station, so beaming that power down to a base station by MASER makes sense... the trouble is that this application is then used to justify replacement of the domestic / workshop mains extension cable, and it doesn't, but the proponents always move one set of goalposts while forbidding you from moving the other set in sync.
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The challenge is building something that can pull a useful amount of energy from something that is not itself very dense in energy, we are talking light winds, solar on your roof, slow flowing currents, or low temperate differentials such as you can get from ground heat or combustion exhaust gases, and then adding them up LEGO style to get your desired power output.
For this "lego brick" technique to work you need a few things.
1/ Your lego power source needs to be compact, so no good having an exhaust heat reclamation unit that is as big as the engine itself.
2/ Your lego power source needs to be reasonably efficient, otherwise even if your exhaust heat reclamation unit can be made small enough, after one or two stages there simply isn't enough energy remaining in the exhaust gas to power the next stage.
3/ Your lego power source needs to be cheap, it needs to pay for its own manufacture in real energy terms in a fairly short period, even if it has a ten year working life and zero maintenance, it still needs to pay for itself in the first two years at the outside.
4/ Your lego power source needs to be mass producable, lego bricks could be carved by hand by craftsmen out of ivory, but you'd have to wait five years to have enough to build anything useful.
So basically is has to be small, like a lego brick, it has to be efficient, it has to be CHEAP, and is has to lend itself to mass production.....
So, if it is in any way mechanical in nature, it has to have as few parts and possible, and as few of them moving as possible, and to use as few exotic materials as possible.
It could be electronic, because electronics CAN be cheap, electronics can also be hideously expensive, so you have to be careful.
I could be plastic or ceramic, both are fairly cheap, at least while hydrocarbons and energy generally is cheap and plentiful.
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These are REAL challenges, you can get electrcicty of out a copper nail and zinc nail and an orange, but not much, remember the first red LED watches, no enough battery power to run the display unless it was momentary, button only, LCD display made the electronic watch come of age.
This is important, a NEW TECHNOLOGY made something possible within existing engineering confines, before this NEW TECHNOLOGY the only way to do it was have the EXISTING TECHNOLOGY dictate the mode of operation, eg only tell the time when you press the button to illuminate the display.
The quartz crystal wasn't new, the superhet circuit wasn't new, and button batteries weren't new, mains powered quartz clocks with mains powered red LED displays were old hat. SO if you want it in a wristwatch, something has to go, and everything has to fit in the design variable that can't be changed, battery capacity.
NB, doubling, trebling, quadrupling or even ten or 100x battery capacity STILL DOES NOT MAKE THE LED WATCH A PRACTICAL IDEA.... this is important, this is not "it sorta works", this is "no fucking way, not even close"
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We have all grown up in an era where high energy was easy and cheap to get, whether it is dozens of amperes of electrical current, or the high energy of a gas flame, or the high energy of liquid fuel, we are used to high energy, we are used to the raging torrent outside our door.
Because we are used to this, the one thing we have grown up not merely not caring about, but not even seeing, is the inefficiency of everything we use.
We don't even see the heat draining away through house walls, just turn the multi kilowatt heating system up a notch, the 300 watts of tungsten filament bulbs in the same room aren't even percieved as a heat source, not even a background heat.
The consequence of this is unless we are presented with something which is high energy, we don't even know where to begin when it comes to extracting useful work out of it. We make very expensive purified potable water, and then we use to to carry vast quantities of energy rich shit away down the sewers, where we spend even more energy destroying it as an energy source.
We spend vast quatities of energy making petrol, then we burn it in inefficient engines, run it through inefficient transmissions, waste significant amounts to run air conditioning systems, waste still more of exhaust gas treatments, waste still more or rotten illumination systems, and then waste every last bit that remains and kinetic energy in the vehicles momentum by turning it into heat in the brakes.
A slow flow small car only fuel pump dispenses fuel at a rate equivalent to between a five and ten megawatt power station.
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The basic Lister CS 6/1 start-o-matic came with a 2.5 KVA alternator.
That is not enough to run one domestic dwelling, it is enough to run one efficient industrial operation, eg a farm, it is enough to run perhaps ten domestic dwellings....
If it is used in one domestic dwelling then you need to be thinking instead of running at an average load of 250 VA over 24 hours I'll run the genny at 2500 for 3 hours a day and store the excess until required, eg battery banks.
UNTIL AND UNLESS you have got THAT down pat then there is no hope whatsoever for pulling useful amounts of power from the relatively low density energy sources around you.
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Solar powered clocks and solar powered mobile phone chargers and wind powered radios and so on are great.
But they are utterly pointless if you are addicted to devices that require absolutely huge quantities of power by comparison, either through inefficiency, you have a poorly insulated house, so you need 5 KW of fire to keep it warm, or through laziness, so you have a 2 kilowatt vacuum cleaner, and a 4 kilowatt electric kettle.
3 HORSEPOWER for a vacuum cleaner, to do the same job as a BRUSH.
6 HORSEPOWER to boil water to make a coffee, when you COULD do the same job with a single candle flame, if you wanted to...
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2 vacuum cleaners or 1 cup of coffee, just as well nature is not as inefficient, or we'd all die of energy loss in our sleep.