Photovoltaic Cells- What's to Stop You Doing This....

If we're going the mirror route, it's easier/cheaper to eliminate the PV cells altogether and just make steam for turbines.

Well actually nothing will stop you from using mirrors and PV arrays. You just won't get the power output you think. Gathering the light from say four mirrors to one PV element will produce less power than four PV elements the size of the mirrors. The reason for this gets quite complicated because it requires a good understanding of the quantum mechanics that makes a photo voltaic cell to work. The link provides a reasonable animation of how a PV cell works. The fundamental concept to explain this is that as more photons dislodge electrons then there will be more re-combinations created in the crystal than current into the load.

Some people have done that.

Some have used fresnel lenses in front of the cells. Cylindrical fresnel lenses can be arranged to focus the light and not require tracking.

In either case (reflective or transmissive) a dichroic coating can be used as part of the system to concentrate the UV, visible, and near IR wave bands where the PV cells are sensitive. The dichroic coating allows the useful energies to reach the cells but not the heat energy that could harm the cells.

Agree.

A friends startup venture just nose dived, after 3 years and alot of money invested.

They were looking to produce cells made up of fresnel + postage stamp sized PV collector. offering them both as individual cells and as full sized motorised sun-tracking arrays.

They were in full scale testing phase, but eventually they decided it was a dead end.

Basically in the end they simply couldn't make a worthwhile product. Issues involved were heat, and the lifespan of the PV collector.

Despite all the hype around the cells that operate at high temperatures. They couldn't get a product that would last longer than 2 months, operating at extreme temperatures (despite manufactures claims to the contrary).

Hope this helps.

Anthony

Was this perhaps defeated by large current densities within small cells?

It may have been - I can't pretend to have the knowledge on that level. But in general terms, it was killed by too much heat, issues with dealing with excess heat, but most of all with the degradation in the cell itself after only a few months.

My forte is water, I tried to sell him on the idea of desalinating water using the excess heat (membrane distillation style). But it was all too complicated for him/them - it became clear they weren't going to have the cute little "solar cube".

In the end they decided to kill the project (and move on to the next opportunity).

Cheers,

Anthony

There are people working on this, as they did at least 30 years ago, when cells were more expensive. They were added to a simple hot air collector with a cone mirror around them, so any waste was still fed into the heat loop. The primary problem is reduced output with rising temperature, so the current scheme is to cool them with water and use the heat collected as well. I don't know why satellites don't use this scheme. The ISS doesn't even seem to have accurate aiming, despite seven tons of extra structural weight for a super-rigid frame.

Energy focusing via mirrors or lenses does yield a higher energy density for PV. This practice increases the harvesting area of the cell. There has been considerable effort made in exploring this strategy already. It is not a new idea. The reason that this is not more widely done is the issue of heating. If you can keep the PV cell cool all is good. The diminishing returns of fewer PV cells versus implementing a cooling system starts to matter very quickly. Nothing is free. There are plenty of resources on the internet that explore concentrated photovoltaics in as little or as much detail as you want.

Thanks to all who replied- it looks like it might have very limited applications. Now, have I got this straight? A commercial PV array on a cloudless day in the Sahara *won't* produce any more power than it would on a cloudless day in a festival site in Somerset, their heat dissipation is designed accordingly, and any attempt to concentrate the sun in these circumstances would overheat and wreck the array. The difference is that cloudless days are much, much rarer in the latter case, and I was thinking in terms of capturing more light from an overcast sky, which I don't think is physically possible for anything but the lightest cloud cover. As an aside, what sort of stage is PV technology at at the moment? Watts per area? Cost per watt? Deterioration over time?

This is of special interest to me as I've just installed an array in my yard. Modules rated output is based on a standard irradiance of 1000 watts/sq meter, about the max you can get in June. Here in MN in Dec we'll get about 700. Mirrors could be used to increase the irradiance and get at least the rated output of the module. Now the formulas say output is proportional to irradiance, but I haven't been able to find any reference to levels above the standard. I suspect the proportionality isn't going to hold for long as temp and other complex factors decrease the efficiency of the modules.

FYI, outputs now are around 15 watts/sq ft, retail cost is about $2/watt for the modules alone. A turnkey system with tracking could be $10/watt. OEM's say output might decrease 1%/yr and modules might last 30 years.

fresnel lens over solar cell is more simple.

Forgot to mention, there are often local, utility, state and federal incentives. In my case, the utility kicks in about 25% of the system cost, and the Feds give a 30% tax credit on my net cost.

A hot topic in the UK at the moment- the GBP0.45 per unit feed-in tariff is being halved.

Hi One & All,

I seen PV arrays encapsulated in water!! When the PV arrays encapsulated in water the operating temperature of the PV arrays drops per amp. and output voltage increase. The hot water from the PV arrays is use to heat the water in the hot-water heater and cool water is feed back to the PV arrays.

Here a couple tracking web sights.

http://www.patriotsolargroup.com/horizontal_systems.html?gclid=CNTtvt6alKwCFcUaQgodCFlDmQ

http://www.icrepq.com/icrepq-08/224-figueiredo.pdf

http://www.youtube.com/watch?v=pLdMhxslbCE

Be safe and have fun!

Please let us know how the project come out!

BEN

Today, I saw an article on installing PV arrays on flat roofs. To save weight and cost in the support structures, they are now laying them almost flat, claiming that with the lower cost of PV material, this is cheaper overall.

There was an installation similar to your suggestion with tracking parabolic reflectors focussing the sunlight onto small very efficient solar cells trialled in Whitecliffs in Australia. A friend was involved in the commissioning and de-commissioning of the installation after the trial, and if I remember correctly He said that the solar cells were mounted on large heat sinks with something of the order of 100.000 litres of water per hour circulated around each heat sink to stop the solar cell and associated heat sink from melting! I believe there was a array of these units in the trial..... but I don't know how many.