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Solar energy => solar systems homemade and commercial => Topic started by: Artic_Knight on January 01, 2008, 11:26:45 PM
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i was hoping to brain storm a DIY project of turning solar hot water to electricity in the most effecient means possible. after all we can get mroe energy from the sun as solar heat than PV cells. i have seen the thermocouple device that moves heat when electricity is put in and it also generates electricity when heat is put in but this looks to be very expensive to produce a 1kw cell! any ideas?
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Try to use stirling engine for converting heat to electrisity. Search on Youtube for stirling, and you will see how to built that device, and proof that is stirling best choise for you. It's cheap device, wich reqire some precision in building
Good luck!
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The problem is that in order to convert heat to electricity, you have to waste some of it. That maximum efficiency is (Th - Tc) / Th, where Th is the temperature of the heated part, in Kelvins, and Tc is the temperature of the cold part that you're dumping the heat into -- usually ambient air or water.
See http://en.wikipedia.org/wiki/Carnot_cycle
Lets say it's 20C, i.e., 293K outside, and you use the sun to heat something up to 50C, or 323K. The maximum efficiency at which you can then convert that heat to electricity is (323-293)/323 = 0.093, or 9.3%. 90.7% of the energy you get from the sun is wasted!
So converting the heat from conventional solar-heated water to electricity is a bad idea.ÂÂ
Note that the numbers get a lot better if you use concentrating mirrors to heat something up to over 1000K like this: http://en.wikipedia.org/wiki/Solar_power_tower.
Cheers,
Mr. Entropy
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im sorry but the maximum effeciency of electricity from solar heated water is 9.3%? considering PV panels can tap 15-30% and they see usually one spectrum of light where as a black coated solar heat collecter collects multiple light spectrums.... that doesnt make sense! not even with the science of resistance or loss of energy!
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im sorry but the maximum effeciency of electricity from solar heated water is 9.3%? considering PV panels can tap 15-30% and they see usually one spectrum of light where as a black coated solar heat collecter collects multiple light spectrums.... that doesnt make sense! not even with the science of resistance or loss of energy!
S'true! That's why people use PV :-)
You have to look at the link.
Mr. Entropy
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im sorry but the maximum effeciency of electricity from solar heated water is 9.3%? considering PV panels can tap 15-30% and they see usually one spectrum of light where as a black coated solar heat collecter collects multiple light spectrums.... that doesnt make sense! not even with the science of resistance or loss of energy!
Heat is only a small part of the spectrum absorbed by the black panels. The rest is wasted, hence the poor performance.
Hans von Lieven
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Stick with it mate, you can radiate heat into an object all day long until it melts, but you can only use so much light from the sun for PV due to cell limits. The day PV cells are worth anything is the day they can match infra red absorbsion.
The principle is basic physics, heat absorbsion is not restrcted where solar cell tech in PV cells converting sunlight to electricity to heat is very restrictive and the reason it takes large panels just to watch a televsion.
The huge false pretence of a solar hot water system is that the solar cells create the heat, when the heat absorsion into the black construct of the panels and cells provides many times the heat of the electrical production of the panel they are on. that is why there are no solar hot water systems in the arctic, because they rely on that heat absorbtion more than the current created.
EG: on a 35 degree day, a solar panel of any kind, can not produce more electricity than a on a 40 degree day, not a bit not ever. The restriction is the limit of our current cell tech.
Go for it.
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a single lense can focus the solar-heat into an intense beam, capable of increasing your solar energy collection exponentially.
from the same 15 inches of sun-lit area you can get a LOT more heat.
one idea im working on is going to use a conical-shape, like those things you put around a dogs neck.
with mirrors, all focusing the light up onto a 2" downward facing mirror.
This technique is used in 'solar ovens'
reference: Solar Death Ray experiments, and subsequent links, including a large military project
http://www.solardeathray.com/ (http://www.solardeathray.com/)
Then have a 7.5-inch lense (with a hole cut out for the 2" mirror in the center) on top of the cone, chosen with an appropriate focal-point, such that all the light collected is directed onto a 1-2" area down the center of the cone. perhaps onto a black,heat collection rod or perhaps even small a flat 'panel', giving me a small intense solar collector.
Summertime is comming up, and we get a WHOLE lot of sun here in Kansas!!
Concerning the carnot cycle,another way to look at it is:
the maximum efficiency = 1 - (Tc/Th)
which is still 0.0927 or <10%
UNLESS - you include a work factor for the heat while you are transfering it, such as in a rankin cycle.
the equation would then look like
efficiency = Work / Heat Input. Which (at least theoretically) can be up to 100% efficient heat transfer.
Sterling engines are theoretically 100% efficient. - provided that ALL of your heat goes into the hot side of the engine.
However, In practice, we note that sterling's are not actually 100% efficient, because there is some heat output on the 'cold' side. meaning that not all of the heat was transfered into 'work'.
these guys here are doing some pretty fun stuff with it.
http://www.ergenics.com/ (http://www.ergenics.com/)
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Evacuated tube solar collectors have the best heat absorption ( >90%) because they don't have convection losses. If you really want to do this I recommend either some sort of magnification or evacuated tubes.
Then there is your choice in heat engine. Solar steam would be nice if there were off the shelf steam engines. Also piston team engines require lubrication which then in a closed cycle can foul your boiler. Doable but not the easiest way to go.
Stirling engines work great at high temperature differences, but at lower temperature differences they are a bit more lackluster. Also the machining and R&D gets expensive because again they are not off the shelf parts.
I think the best option right now is an organic Rankine (steam) cycle using refrigerant. Scroll compressors can be used as expanders with minor modifications. Then you just need a feed pump which can be any pump that provides the pressure and flow with seals that withstand refrigerant. Then for the best efficiency you need some regenerative heat exchangers, but again off the shelf parts.
Would be doing it myself, but R&D costs still make it cheaper to buy my electricity. Don't forget you will be buying each part you use at least twice since initial testing will probably poke some holes in your design, also you will need to figure some sort of grid tie or battery tie system.
Good luck! Post results if you try it!
Edit: Bad grammar
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The best thing would be solar cells, that could convert low infrared emissions to electricity.
The PN-layerjust must be selected this way, that it is the most optimized for the infrared
wavelength..We have to find the right materials to do this or use
millions of small diodes which have noise at this temperature to rectify the noise
on a silicon chip.
Lookup what Paul is doing with his diode arrays.
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It's hard to interlink in this forum - SMF should have a pugin for linking to thread's you've recently looked at or posted to - like a sidebar.
The idea behind a sterling involves moving parts and gases - along with temperatures. moving parts and devices aren't solid state, imo, the real unique device will be a solid state thermo electric device. Paul's diode array's are interesting but all diodes have a voltage drop across them, save for maybe the most expensive kind ( correct me if I'm wrong ).
A thermojuntion system or thermopile, http://www.xyroth-enterprises.co.uk/thermser.htm (http://www.xyroth-enterprises.co.uk/thermser.htm) from the thermo-electric series, may be an interesting addition to a solar or diode array.
Thermopiles are almost a waste of time unless you can create a thermonuclear junction, where a controlled ammout of heat is produced at the point of two metals.
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Hi,
I remember a device that was originally designed for cancer cure - I don`t remember the name - it was a big ring made of annealed copper, one end of the ring was heated, the other end was cooled, the two ends were connected through a siliconated junction. It was a kind of a thermoelement that transformed heat into strong, pulsating magnetic fields. I thing these strong magnetic fields could be picked up by some coils. Anyone remember the name of that device? it was also supposed to generate antigravity, the roumors said it was suppressed.
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im sorry but the maximum effeciency of electricity from solar heated water is 9.3%? considering PV panels can tap 15-30% and they see usually one spectrum of light where as a black coated solar heat collecter collects multiple light spectrums.... that doesnt make sense! not even with the science of resistance or loss of energy!
Using the solar heated water method, must be cheaper than purchasing PV panels though. I remember B&Q were selling PV panels a few years back. Everyone was saying "look at the f***ing price of those things !".
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I looked into getting solar panels installed on my house and hooking it into the grid to cut my electric bill. The cost came out to be approximately $10 USD / watt.
That would be over $10,000.00 for 1 Kilowatt of power. It would have taken over 27 years to get my money back and break even; and the warranty on the panels was for 25 years!
I refused to spend $80,000.00 just to cover most of my electric bill. What a joke the present technology is.
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yup, unconcentraited PV is a waste of money. How is solar heat only 9% efficient, black absorbs all light. A PV cell becomes more efficient if light is consentraited on it, usually it's about 15% efficient, but if the light is concentrated it's about 37% efficient. Solar Thermal build cost is or will soon be as cheap as coal and nuclear, coal and nuclear being $1300-$1800 per kW average yearly output.
The utilities pay almost 4 times as much for solar energy. This is because it outputs more when peak power demand is, like on sunny hot days when the air conditioners are on, that's when the grid pays more for electricity.
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I would have to completely disagree with only a 9% conversion ... that crap is for accademics caught in books and the four walled institutes .. no offence. But lets just take 9% .. it is still a fraction of the cost to use solar heat conversion then it is to buy the PV panels from the thieves.
For the last few months I have been converting to solar heating. It has taken some time as the off the shelf products do not transfer heat very well. I live in central Canada. On a -25 celcius day I have 234 celcius on the inside of the evacuated heat tube and 134 celcius on the heat exchanger exposed to the -25 degrees. It takes under 2 minutes from a dead start exposure to reach over 100 degrees.Depending on the load that I put on I can raise or lower the heat exchanger. A 1200 gallon water tank will be acting as a thermal mass.
I have University prof's come down and see and they can't believe what I am doing! I will post some pics and videos much later as I need to finish my stirling engine.
I will be using a stirling engine to convert excess heat to electricity. However it will be a sealed unit. The sun has around 1300 plus watts per square meter. In a sealed stirling unit one can use various gases that will get you a much higher rate of energy transfer.
Here is an older pic from an unrefined exchanger.
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The sun is good for heat - very good, and one of the primary uses of electricity in a home is the hot water heater. Supplementing a good home with a solar heat exchanger can be effective at lowering the cost / or should I say / decreasing the net use of electricity.
A solar heat exchanger with a solar powered pump may be the first step in switching to solar. As far as powering your air conditioner though - it's going to require some serious wattage.
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I would not say serious wattage ... an average central air uses around 4800 watts ... what is needed though is a good way to transfer the suns energy to electricity.
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@nickle,
I look forward to reading your posts on your home setup. It would be great as well when you have it all running if you could make instructions on how we can duplicate your setup. It is amazing you get such heat differences when starting at -25C!
Mark
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wow after reading some stuff I guess 10% is a reasonable number for solar thermal efficiency. The problem is light diffusion even on a clear day is 10%. Consentraitors can't make use of diffused light reflecting off the coulds where PV can. Also I read the heat collector diffuses IR 8%, and the black is 94% absorbive. When you add all this up, plus the few percent lost in the mirror it comes down to about 10%, but maybe 15%. Assuming the turbine is 50% efficient. Though, that being said solar thermal is still way cheaper then regular PV, and cheaper then wind, infact it's still near coal's build cost. The industry is saying Solar thermal is $3000 per kW capacity, but I would guess that they averagly run at 10%-30% capacity??? Anyways with my figures it's still much cheaper then this, they use expensive mirrors or lenses, and it's still a new technology.
http://www.theoildrum.com/node/3791
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On an overcast - cloudy day in -15 c the solar tube reaches and maintains 38 to 42 c throughout the day. The tubes are very efficient in collecting the sun's energy of both visable and non-visable light and consentrating it on the inner surfaces. With light being diffused by the atmosphere does not affect the collector that greatly to make it unusable in the practicle sense.
$3000.00 per kw is very high for solar thermal ... those folks should go back and do some first grade math. Each panel of mine has a cost of 750.00 with mods and produces well over 800 watts consistantly on a cloudy day. Sunny day's are up around 1200 watts. Storing that energy in a thermal tank to run through the night will be done by a 1200 gallon water tank in ground and insualted.
The sun in free energy and so is the wind ... just harvest it.
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On an overcast - cloudy day in -15 c the solar tube reaches and maintains 38 to 42 c throughout the day. The tubes are very efficient in collecting the sun's energy of both visable and non-visable light and consentrating it on the inner surfaces. With light being diffused by the atmosphere does not affect the collector that greatly to make it unusable in the practicle sense.
$3000.00 per kw is very high for solar thermal ... those folks should go back and do some first grade math. Each panel of mine has a cost of 750.00 with mods and produces well over 800 watts consistantly on a cloudy day. Sunny day's are up around 1200 watts. Storing that energy in a thermal tank to run through the night will be done by a 1200 gallon water tank in ground and insualted.
The sun in free energy and so is the wind ... just harvest it.
well you doing just open vaccum tubes right? They will make use of the diffused light, concentraitors can't make much use of the diffused light though. But the diffused light energy is probably much less then the direct light energy, what I want to find out is by how much.