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The Solar Sunflower

arstechnica.com

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Re: The Solar Sunflower

#41

Post by brrt »

artartart wrote: post #38

The sub-title claims 80% efficiency. Where did they get that from? 80% is close to the theoretical maximum, so I think the figure is misleading and can discredit Arstechnica and more importantly an otherwise admirable endeavor.

That's possible because they count both photovoltaic and thermal conversion efficiency. I'm not entirely clear on whether they want to use the heat as heat or always for electricity generation. They claim GaAs solar cells get 40% efficiency, which I'm ready to take at face value. 60% of the energy is then converted to heat. For a total of 80% conversion efficiency we'd need only 66% efficiency for the thermal-to-electricity conversion, which is well within theoretical reach, given they also claim >1000C achieved temperatures.

Re: The Solar Sunflower

#42

Post by PhasmaFelis »

bitsoda wrote: post #33

Ack, those circles waste so much space. They should put a in an array of hexagonal reflectors instead.

I would venture to guess that it's a manufacturing issue. The mirrors are parabolic, and there probably hasn't been much call for hexagonal parabolic mirrors before now. Perhaps they'll develop that infrastructure if it takes off.

Re: The Solar Sunflower

#43

Post by danmaz74 »

detaro wrote: post #39
danmaz74 wrote: post #25

Earlier quoted context omitted.

> Maybe the efficiency goes down to much and limits the range of regions where it is viable? Actually, efficiency for PV would go up (the colder the cell, the higher the efficiency). > Maybe there are regulatory issues with water + power systems so close together (driving up installation and maintenance prices). Maybe, that would be interesting to learn :)

I don't know if cells would be colder, since the heat collection parts works at liquid temperatures of 80°C or if they don't use water directly even over 100°C. The heat collecting system wants to be as hot as possible, the solar cells as cold as possible. Also, the cells decrease efficiency of the heat collection part.

If you get the heat from the cells, they need to become colder for sure.

Re: The Solar Sunflower

#44

Post by danmaz74 »

hacker234 wrote: post #31
danmaz74 wrote: post #7

While reading the article, I was wondering why combined PV/water heating conventional panels aren't taking off on rooftop installations. Looks like something that would make a lot of sense.

I don't have a link, but combined Solar Electric / Hot Water panels are relatively common Australia. My neighbor has them on his new house. I'll go ask him for details.

Thanks, that's interesting to know. Never saw any promotion about them here in Italy.

Re: The Solar Sunflower

#45

Post by makomk »

brrt wrote: post #41
artartart wrote: post #38

The sub-title claims 80% efficiency. Where did they get that from? 80% is close to the theoretical maximum, so I think the figure is misleading and can discredit Arstechnica and more importantly an otherwise admirable endeavor.

That's possible because they count both photovoltaic and thermal conversion efficiency. I'm not entirely clear on whether they want to use the heat as heat or always for electricity generation. They claim GaAs solar cells get 40% efficiency, which I'm ready to take at face value. 60% of the energy is then converted to heat. For a total of 80% conversion efficiency we'd need only 66% efficiency for the thermal-to-elec…

They can't convert the heat to electricity because they can't run the solar cells that hot - max operating temperature is 105C, and I'm not sure how great the efficiency and lifespan would be at that temperature. The 80% efficiency assumes they use the heat for heating, which might be a bit difficult in practice given the times of year and locations where this could actually be operated.

Re: The Solar Sunflower

#46

Post by brrt »

makomk wrote: post #45
brrt wrote: post #41

Earlier quoted context omitted.

That's possible because they count both photovoltaic and thermal conversion efficiency. I'm not entirely clear on whether they want to use the heat as heat or always for electricity generation. They claim GaAs solar cells get 40% efficiency, which I'm ready to take at face value. 60% of the energy is then converted to heat. For a total of 80% conversion efficiency we'd need only 66% efficiency for the thermal-to-elec…

They can't convert the heat to electricity because they can't run the solar cells that hot - max operating temperature is 105C, and I'm not sure how great the efficiency and lifespan would be at that temperature. The 80% efficiency assumes they use the heat for heating, which might be a bit difficult in practice given the times of year and locations where this could actually be operated.

Oh. Well, if that's so, I'm not sure why they've chosen to use photovoltaics at all, because solar thermal should give you a much better cost-performance ratio, and still give you plenty waste heat to use as heat.

Re: The Solar Sunflower

#47

Post by thrownaway2424 »

jtolds wrote: post #35
thrownaway2424 wrote: post #9

Earlier quoted context omitted.

Agreed. I saw a demonstration of a unit very similar to the one in the article, that had vacuum-focused thin film reflectors and a sodium-cooled sterling engine. That was in 1997.

This sort of attitude really bums me out. "Tried it!" With the current rate of technological advance, a ton of stuff that's failed (and even a lot that's succeeded) should be tried again. Frequently some incremental but necessary technological improvement that didn't exist 10 years ago exists now and it might just change the game. Maybe that's happening here? Perhaps incremental improvement doesn't deserve the word "…

People who are trying things that already failed should mention why it failed previously and why it will not fail this time. At least journalists who write about such things should be held to that standard.
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