Solar Concentrators

For a few years I've wondered why there are not more solar concentrators in use. 

Everybody and their uncle knows you can make a solar oven with a cardboard box and some tinfoil. 



But you don't really see the reflective panels much anywhere else.  It seems there would be quite a number of uses for them. 

1) Around photovoltaic panels as a cheap upgrade.  I imagine at mid-summer in the desert the collector might end up cooking the panel, but in more marginal conditions such as northern/cloudy climate or the other three seasons wouldn't this make sense?  I know people are using this principle, such as photo below, why not more often? 



2)  Solar concentrating shutters: Shutters with a reflective surface to direct additional energy for passive solar gain and added light.  Would be ideal for retrofitting buildings with poor solar design.  Might have to design to deal with glare.  Shutters might be insulated, so they could be closed at night to keep heat in.  If you are talking about double sliding glass doors, 6 or more feet wide, and adding an additional 6 feet of collection space, plus insulation at night, that could be significant. 

3) Solar water heaters...basically same as #1, might cook the unit on hot days, but would seem worthwhile in marginal conditions.   

Basically, for any design that uses solar energy, is it worth considering adding a parabola or some weird trapezoidal collector made from aluminum/mirrors/tinfoil whatever?  Any thoughts?

As far as PV modules goes, you hit the nail on the head. Concentrators can overheat the modules and damage them. As it is PV module output falls as the temperature goes up. Using a concentrator could make that worse.

I believe that a tracker makes more sense, they have been proven to function well. A concentrator also would need tracking through the day to make it most effective. Even then with the way PV module prices have fallen a tracker is often more expensive than adding PV modules to the system.

mtnDon Miller wrote:
As far as PV modules goes, you hit the nail on the head. Concentrators can overheat the modules and damage them. As it is PV module output falls as the temperature goes up. Using a concentrator could make that worse.

Thanks mtnDon for the knowledgeable response.  Would it make sense with colder temps outside temps, assuming the reflector could be removed when it gets warm?

mtnDon Miller wrote: I believe that a tracker makes more sense, they have been proven to function well. A concentrator also would need tracking through the day to make it most effective. Even then with the way PV module prices have fallen a tracker is often more expensive than adding PV modules to the system.

I like the intelligent solar tracker included in the re-purposed fridge solar dehydrator thingy posted by Paul on this forum a while ago.  Park a solar unit of any type along a well-trodden pathway and humans adjust whenever they walk by. 

yukkuri_kame, seasonal adjustments might be worth the effort. PV modules do get very hot so there is a certain amount of heat they can withstand. The temperature related power loss is always there though. PV output is rated at 25 degrees C (77 F). That's cell temperature, not air temperature. On a cold day (in the 20's F) with bright sun the back side of the modules are warm to the touch. On a clear summer day I've seen the backside temperatures as high as 160 F. That cuts output by maybe as much as 25%. (I did the math for 130 F once and that loss was 15%... that was with an air temp of 75 F)

The electrical connections to the cells and any wiring that may come into contacts with hot surfaces has to be factored in as well. I expect over time cells will progress.

just speaking for myself, I like to keep things as simple as possible. But to be fair I should mention we are in the SW and have plenty of sun, very few cloudy days as a rule so I can afford a more relaxed attitude re my PV modules.

Research into photovoltaic cells is progressing, and a number of companies are now looking into what might be called 'hot' cells, cells that not only can withstand higher temperatures, but actually benefit from it.  I've seen some documentation about using fresnel lenses and the like to concentrate light into cells that have nanotube structures to gain a greater output.  This science is still cutting edge and probably won't be on the market for a couple or so years yet.  I have also seen info about solar cell paint (low efficency) and printable solar cells that can be printed on thin sheets of plastic for various technologies.  Just a few things to look forward to.

I've been playing with fresnel lenses. I did think of putting a solar panel behind one at first.
However, while I was adjusting things I had some plywood in about the position a panel would be to keep the sun off the focal point and it only took a few minutes for the plywood to catch fire.

Perhaps mirrors would help in a fixed panel installation to gather early and late sun but there's a lot of heat that needs to be dissipated when you concentrate.

If anyone wants to play with fresnel lenses keep your eye out for scrap projection TV's. There is a lens behind front face the same size as the screen.

There are a lot of videos on youtube of people gluing little mirrors, or mylar (seems easier), to old satellite dishes and catching stuff on fire in about 1 second.

light shelves:

OK - this is using the principle I was thinking of...

reflective solar heating:
http://mtbest.net/solar_heating.html

I get where you are coming from.  It's not cheap, but this is the best solar reflective material in the world at the moment.  Not suitible for the floor, but it would work on the ceiling.

http://www.reflectechsolar.com/

This may sound really stupid because I don't know much about solar panels as I am just learning but.... What about using the same material that is used in two way mirrors.  Flipped up the mirror side could act as a concentrator and flipped down over the panel it could potentially have a shading effect on the panel cooling it down a bit to maximize output?

I have a solar concentrator idea that I am pretty sure will work.  (An astronomer has ok'ed the equatorial mount idea). Anyway, parabolic dishes have  problems and this first one is a huge one. When you move the dish, you have to move the target too because the dish has to point straight at the sun and straight at the target.  And when you make a really big powerful parabolic dish, you cannot get to the darn target without setting yourself on fire!
And tracking is a nightmare because the sun (even though its motion is regular) traces a path that moves a little  up and down in the sky every day.
So my solution is to cut the dish in half, and then mount the 2 halves on a frame that is lined up with a line through the north and south pole.  Then tracking is a lot easier. Just rotate the thing slowly. And seasonal adjustment is done every night or morning.
So, I have not made this yet and hopefully someone else will make it first. It is a free idea.
Here is a video of the idea (set up for near the equator)
http://www.youtube.com/watch?v=S3MbVqExTOQ
And here is a video of a model set up to show how it would work in different places on Earth. (From about 50 degrees north to about 50 degrees south)
http://www.youtube.com/watch?v=j7pm-o3rPRU
And here is one to show how it would do Fresnel lens mount
http://www.youtube.com/watch?v=E21yqyp6xTE
Thanks
Brian

hmmmm, what about using a trough instead of the dish,,,

From Understanging solar concentrators "The theoretical annual efficiency of the three principal concentrating collectors utilized in the United States is 80 percent for the dish, 60 percent for the central receiver, and 43 percent for the parabolic trough on an annual basis."
80% as opposed to 43% is a huge difference and I think it is worth the effort.
Greater temperature is also possible with the dish.

Are there any temperature vs efficiency charts for PV's and are there PV's designed to be efficient at higher temperatures.  The idea is to maximize energy capture by circulating a cooling fluid, water for example, that keeps the cells at or about optimum temperature but collects that otherwise wasted heat for CHP purposes.  Even if somewhat more expensive the fewer cells needed reduce costs and you have the added benefit of heating your home in cold climates.

mekennedy1313 wrote:
Are there any temperature vs efficiency charts for PV's and are there PV's designed to be efficient at higher temperatures.  The idea is to maximize energy capture by circulating a cooling fluid, water for example, that keeps the cells at or about optimum temperature but collects that otherwise wasted heat for CHP purposes.  Even if somewhat more expensive the fewer cells needed reduce costs and you have the added benefit of heating your home in cold climates.

PV's get less efficient with increasing temperature, due to fundamental physics. I think there are some photocatalytic systems (solar to chemical: hydrogen and carbon monoxide generation seem to be the two main tracks of research) that remain efficient at fairly high temperatures.

Moderately-high temperatures can also degrade the cells gradually, causing permanent changes that degrade efficiency but don't destroy the cells in one fell swoop.

A cooling fluid is a great idea. I think some PV arrays produce significant voltages, in which case a non-conductive fluid would be in order, in case a leak should develop. This might be a pre-warmer for a solar thermal collector, so that the temperature of water leaving the system of panels can be a higher temperature than you'd prefer for the PV arrays.

In hot climates, it's common to design PV collectors with a gap above the roof, and a slant, such that the thermal draft raws (relatively) cool air over the roof from dawn to well after dusk. This helps keep the cells cool, but has an added benefit in making the building below easier to keep cool.

mtnDon Miller wrote: A concentrator also would need tracking through the day to make it most effective. Even then with the way PV module prices have fallen a tracker is often more expensive than adding PV modules to the system.

It's even worse than that: a typical concentrator makes a tracker absolutely necessary. The better the concentrator works, the greater the need for precision alignment. A slight misalignment will mean part of the cell goes completely dark, which can mean a disproportionate drop in efficiency.

I've seen a split parabolic that was made with a satelite dish.
The cooking surface mount went through the center bisection and the parabolic was able to rotate up and down as well as side to side to keep the solar concentration on the cooking vessel.

We have a solar concentrator. It is this amazing white stuff. A new layer forms almost every night to keep a clean reflective surface. It is called snow.

We built our cottage such that the winter sunlight is reflected off of the bowl of snow in front of the cottage and into the house. This warms the house such that we use less than 3/4 cord of wood a year to heat the cottage.

Even without the wood the cottage never dips below the 40°F range despite our very cold northern Vermont winters (-45°F) when the cottage was unoccupied during construction.

The key is storing all the energy. Our small cottage contains about 100,000 lbs of masonry which soaks up the heat and holds it.

See:
http://SugarMtnFarm.com/home/cottage

you guys should look at a concept from idea lab...  I googled it and came up with : soliant-energy.com  - but that page wasn't loading for me.

The premise as I remember it  is simple - it's a lot easier and cheaper to reflect extra light with mirrors toward the a small solar cell than it is to build an larger solar cell. 

Yes, the heat increases, but that's part of what you have to deal with in all cells.

The idea lab folks also had a really sweet *and simple* gimbling mechanism to track the sun.

I don't know if they are still pursuing it - alot of the idea lab's alt. energy PR at the moment is about a solar heated steam turbine tower.  ~maybe the concentrated lihgt (and heat) was causing problems like speculated above.