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Author Topic: What's the best way to capture solar energy?  (Read 8227 times)

Offline Geezer

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What's the best way to capture solar energy?
« on: 27/06/2010 07:09:29 »
It seems there are many ways to capture energy from the Sun. Which method is most efficient?


 

Offline LeeE

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What's the best way to capture solar energy?
« Reply #1 on: 28/06/2010 06:27:51 »
I think it really comes down to what you want to do with the captured energy i.e. whether you want to use it immediately, or store it.

If you want to use it immediately, let's say to boil a kettle of water, then you just need an array of mirrors to collect enough energy and focus it on the kettle, and provided that the kettle isn't shiny and doesn't reflect most of the energy away again, then you should get a pretty high efficiency.

If you want to store the energy though, then you've got to transform it, either into electricity for example, or by driving a pump to raise water to a higher height, but because you're changing the 'form' of the energy it'll be less efficient because there'll be losses when the energy is transformed.
 

Offline peppercorn

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What's the best way to capture solar energy?
« Reply #2 on: 28/06/2010 09:08:24 »
When refining your question further, setup cost & maintenance should almost certainly be factored in as well.

A highly efficient utilisation of solar energy that costs a fortune to implement (relative to system lifetime and output) and has high maintenance costs is not going to compete economically with a cheaper but less efficient system.
 

Offline Geezer

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What's the best way to capture solar energy?
« Reply #3 on: 30/06/2010 05:24:05 »
Thanks everyone.

Let's ignore capital cost, maintenance etc. for now and define efficiency in terms of the utilization of Earth's surface area. I'm assuming we know how much solar radiation a given area of the Earth's surface can expect to receive on average.

What would be the most efficient method of collecting that solar energy? (We might collect the energy as heat, electrical energy, potential energy, or anything else you can think of.)
 

Offline peppercorn

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What's the best way to capture solar energy?
« Reply #4 on: 30/06/2010 11:56:24 »
What would be the most efficient method of collecting that solar energy? (We might collect the energy as heat, electrical energy, potential energy, or anything else you can think of.)

There still needs to be some definition of the system and the conditions it is to work in.  For instance, how much cloud cover is there? Is the collector exposed to cold air temps or strong winds that could strip heat from the storage reservoir?  If the heat energy is to be utilised - at what rate is energy drawn from the reservoir?

If this is purely an academic exercise, I'm picturing a large block of copper (perhaps sprayed with some absorbent black paint) with a thermosyphon water circuit inside to spread the stored heat out evenly. It might want a glass window on the top to stop re-emittance and insulation around the 'walls'.
 

Offline Geezer

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What's the best way to capture solar energy?
« Reply #5 on: 30/06/2010 22:40:08 »
What would be the most efficient method of collecting that solar energy? (We might collect the energy as heat, electrical energy, potential energy, or anything else you can think of.)

There still needs to be some definition of the system and the conditions it is to work in.  For instance, how much cloud cover is there? Is the collector exposed to cold air temps or strong winds that could strip heat from the storage reservoir?  If the heat energy is to be utilised - at what rate is energy drawn from the reservoir?

If this is purely an academic exercise, I'm picturing a large block of copper (perhaps sprayed with some absorbent black paint) with a thermosyphon water circuit inside to spread the stored heat out evenly. It might want a glass window on the top to stop re-emittance and insulation around the 'walls'.

Not intended to be academic. Let's say we had a couple of acres near Rugby UK. What might we do there?

EDIT: Added below.

At that location, there might be several options. Here are a some options, but there may be a lot more:

Photovoltaic cells
Steam generation
Heat pumps
Plant trees
Grow biomass
Wind turbines
« Last Edit: 01/07/2010 05:39:05 by Geezer »
 

Offline peppercorn

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What's the best way to capture solar energy?
« Reply #6 on: 01/07/2010 10:10:09 »
Let's say we had a couple of acres near Rugby UK. What might we do there?

Pick up a football and invent a new sport?  ;D   ... Sorry couldn't resist!  ;)


I'd go with covering your land with multi-junction photovoltaics:

http://en.wikipedia.org/wiki/Multijunction_photovoltaic_cell#Efficiency

"Spectrolab has developed a new multijunction concentrator solar cell with a sunlight-to-electricity conversion efficiency of 41.6 percent, a new world record in solar cell efficiency"

Integration with the national grid is likely to give the best overall efficiency in pure energy available. 

< Although would, for instance, charging batteries for a fleet of electric vehicles be at least as effective? >
 

Offline wolfekeeper

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What's the best way to capture solar energy?
« Reply #7 on: 01/07/2010 16:22:26 »
I think the mirror-thermal (steam cycle or similar) schemes are more efficient; with extreme care you can probably get about 90% efficiency, at least you can if you use the waste heat for cogeneration.

Of course in the real world, cost/watt is the more important metric, rather than raw efficiency.
« Last Edit: 01/07/2010 16:24:09 by wolfekeeper »
 

Offline peppercorn

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What's the best way to capture solar energy?
« Reply #8 on: 01/07/2010 16:27:10 »
I think the mirror-thermal (steam cycle or similar) schemes are more efficient; with extreme care you can probably get about 90% efficiency, at least you can if you use the waste heat for cogeneration.

Have you got any references to support that figure of ~90% ?

Steam-cycle generation is limited by carnot efficiency law.  Photocells are not.
Plus each stage of any type of solar furnace represents an added loss.
sun to mirrors to furnace to water to turbine to electric - doesn't sound that efficient to me.
« Last Edit: 01/07/2010 16:34:37 by peppercorn »
 

Offline wolfekeeper

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What's the best way to capture solar energy?
« Reply #9 on: 01/07/2010 18:22:40 »
You need to make the distinction between turning solar energy into electricity and using solar energy. The question was how efficiently it could be used.

You're only limited by the Carnot cycle for making electricity or mechanical power; dumping the waste heat into your hot water systems or something similar can be done very efficiently.
« Last Edit: 01/07/2010 18:50:06 by wolfekeeper »
 

Offline wolfekeeper

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Offline peppercorn

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What's the best way to capture solar energy?
« Reply #11 on: 01/07/2010 19:40:38 »
You need to make the distinction between turning solar energy into electricity and using solar energy. The question was how efficiently it could be used.

You're only limited by the Carnot cycle for making electricity or mechanical power; dumping the waste heat into your hot water systems or something similar can be done very efficiently.


Good point, well made!    >:(

It's true - the question says nothing about the value (economic or otherwise) of the utilisation of the solar energy.

I would however question whether solar could actually reach near 90% in terms of the 2 acres multiplied by the roughly 1MWh/m˛ that falls on this hypothetical ground in a year due to the necessary spacing of the focusing mirrors.
« Last Edit: 01/07/2010 19:43:03 by peppercorn »
 

Offline wolfekeeper

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What's the best way to capture solar energy?
« Reply #12 on: 01/07/2010 20:23:01 »
Probably the solar thermal would win fairly easily even if you ask the same question per square metre of land, or even if you ask the question in economic terms right now- the costs of solar panels are coming down due to mass production, but the cost payback on solar thermal is still much better than photovoltaics.
 

Offline tommya300

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What's the best way to capture solar energy?
« Reply #13 on: 02/07/2010 07:05:14 »
.
I suspect scientific efficiency and cost affective should walk hand in hand.
For a simple system it might cost more than $30,000.00 to install in a personal home solar electric system. An average monthly elecric bill, lets say, without external assistance, a modest $200.00 a month bill, 2400.00 a year, in about 12.5 years you might break even. If nothing breaks in the mean time.

A dime on a dollar bet says that it is rare not to have anything break down in 12.5 years. You will need the sun 24/7 to take advantage of the efficiency unless you can store it such as in batteries, not encluded in the cost here, that also have a life span.
Maybe pumping it into the Power Grid using more complicated electronics to maintain freq. Phase and magnitude of the matching electrical scheme. Just enough to make the electric utility meter, stop or run in reverse.
Maintenance is a real issue that has not been addressed as far as I know of.
.
« Last Edit: 02/07/2010 07:07:41 by tommya300 »
 

Offline Geezer

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What's the best way to capture solar energy?
« Reply #14 on: 02/07/2010 07:58:01 »

Pick up a football and invent a new sport?  ;D   ... Sorry couldn't resist!  ;)

No prob. I happened to pick Rugby because my Mum loved the joke about the gent who said,

"I'm a little stiff from rugby."

I must have inherited her sense of humour, because it always cracks me up  ;D

Come to think of it, I might use it for my epitaph.
 

Offline peppercorn

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What's the best way to capture solar energy?
« Reply #15 on: 02/07/2010 15:25:58 »
Probably the solar thermal would win fairly easily even if you ask the same question per square metre of land, or even if you ask the question in economic terms right now- the costs of solar panels are coming down due to mass production, but the cost payback on solar thermal is still much better than photovoltaics.

But remember the imaginary site is only 2 acres - a substantial part of it would be lost to the turbine building and water heat exchangers.

I'm beginning to wonder if this question has any real validity without some sort of definition of what constitutes useful work.  Heating a constant supply of water by 1degC all year round may be the most efficient means of carrying the sun's heat away from our two acres, but it's no use to anyone!
 

Offline Mazurka

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What's the best way to capture solar energy?
« Reply #16 on: 02/07/2010 16:32:45 »
I think the mirror-thermal (steam cycle or similar) schemes are more efficient; with extreme care you can probably get about 90% efficiency, at least you can if you use the waste heat for cogeneration.

Have you got any references to support that figure of ~90% ?

Steam-cycle generation is limited by carnot efficiency law.  Photocells are not.
Plus each stage of any type of solar furnace represents an added loss.
sun to mirrors to furnace to water to turbine to electric - doesn't sound that efficient to me.


You might like to read through this http://www.cockeyed.com/incredible/solardish/dish01.shtml about the creation of home made solar reflector system (admitedly without power generation).

Anyway, - would it be cheating to answer the question by pointing people towards http://gcep.stanford.edu/pdfs/assessments/solar_assessment.pdf
Which suggests that concentrated soalr power would be around 20-30% efficient depending on the  cleanliness of mirrors etc.  However, it has a big advanatge of being able to store energy to be released at night or could be used as a combined heat and power plant, which would drive up efficeincy (depending on how you were measuring it...)
 

Offline Geezer

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What's the best way to capture solar energy?
« Reply #17 on: 02/07/2010 19:20:37 »
Thanks Mazurka!

No, it would not be cheating. The question probably has no single answer. People have brought up many excellent points.
 

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What's the best way to capture solar energy?
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