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Extra suns on PV with water cooling project.
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Author:  njh [ Jan 4th, '07, 19:58 ]
Post subject:  Extra suns on PV with water cooling project.
PV solar panels work by converting photons into electrons. The more photons, the more electrons. The limiting factors are a) removal of the waste heat to prevent the panel from cooking above 85C and b) the current rating of the interconnections.

The interconnections appear to be seriously over engineered on typical panels (probably for mechanical reasons) so the most important step is to limit the temperature rise in strong sunlight.

The idea is to use a pond of water sitting on the surface, replaced slowly with a pump, to remove the extra heat from the panel.

Take three equal panels (or perform experiment repeatedly on a clear day) and arrange them a) with the optimal angle (= perpendicular to the sun) b) flat and c) flat with bisecting mirror and cooling pond.

(a) is the standard arrangement. (b) is the baseline for a flat panel. (c) is hopefully more than (a) and does not show any signs of breaking down or cooking.

For (c)
A temperature sensing device such as a thermocouple or temperature sensing chip is glued to the back.

A small pool of water is formed on the solar panel's surface, perhaps using plasticine to extend the edges. Water is added slowly from a tap. A mirror is placed behind the panel at an angle which directs the sunlight evenly onto the panel. The mirror must be flat to avoid hot spots. A piece of plywood with alfoil, or better, metalised mylar (like helium ballons) glued on with grease would work.

A third mirror could be placed behind this, also directing onto the panel for 3 suns concentration.

Theory: the (c) panel will produce roughly twice as much power as the (b), and hopefully considerably more than (a). At 38S mid summer (b) will have a penalty of cos(16degrees)*the power arriving on (a), or about 96% of (a). The mirror and water will also waste some light.

Assuming 15% efficient panels and 1kW/m^2 solar constant and a doubling of total power the (c) water would need to absorb 1000*0.85 = 850W of heat to stay at the same temperature as (a). 850W / (2.2MJ/kg) means that evaporating 0.4g/s (or about 1.5l/hour) of water would be sufficient to achieve this in the case of water failure. So for 4 hour's safety we would like to keep 6L/m^2, or 6mm layer over the panels at all times, the theory being that after 4 hours the sun will have dropped off enough to be within limits of the panel's engineering and can cool off passively.

If instead we are using 40C water as a coolant and want to maintain 70C on the panel surface (to allow for heat transmission through the glass) we have 30kcal/kg of water. Thus, to remove 2kW of heat we would need a flow rate of 2kW / (30kcal/kg) = 16g/s or about 1l/min. Well within the limits of a small pump.

A third mirror may be tried, possibly with a linkage to maintain the required angle to the sun (a stepper motor could adjust the angle each hour).

If it works you will be the proud owner of a fusion power station with cooling pond!
Author:  TimC [ Jan 4th, '07, 20:13 ]
Post subject: 
Another two possibilities, 1) Water cooling and no mirror AND 2) Using water as a lens running down a piece of glass in front of the solar panel... It may not work though... OR a third, Using fresnel lenses to focus light on the panel.
Author:  christopher [ Jan 5th, '07, 00:15 ]
Post subject: 
A fresnel lense would make things really interesting! Without tracking capability.... ooooh, hard to imagine!

Water cooling with no mirror, we have done this on our panels by the pump when cleaning them, and there is always a boost in production. With out house system, if a light rain hits the panels in full sun, up, up, up goes the amperage.

I like the idea of the heated water, too.

I really look forward to seeing more here.

Good luck!

C
Author:  njh [ Jan 5th, '07, 06:21 ]
Post subject: 
I'd worry about hot spots with a fresnel lens.
Author:  christopher [ Jan 5th, '07, 07:10 ]
Post subject: 
Me, too! Hence the "interesting"!
Author:  TimC [ Jan 5th, '07, 12:28 ]
Post subject: 
A Fresnel lense arrangement wouldn't work for large PV modules as it will only focus on a small area. It might work better for the smaller Jaycar solar panels.
Author:  njh [ Jan 5th, '07, 13:01 ]
Post subject: 
You can make the area in focus as big as you like.
Author:  TimC [ Jan 5th, '07, 18:29 ]
Post subject: 
Practically speaking the lens will need to be fairly close to the module. Therefore depending on the focal length of the lens it will difficult for a large cell.
Author:  christopher [ Jan 5th, '07, 21:29 ]
Post subject: 
You could have a bigger fresnel lense, too.
Author:  njh [ Jan 6th, '07, 06:36 ]
Post subject: 
Another advantage of the extra mirror approach over the fresnel lens approach is you will get direct sun + reflection. A fresnel lens may completly block the sun in the case that it is not aimed.
Author:  TimC [ Jan 6th, '07, 21:05 ]
Post subject: 
True and more so with the mirror light will be further reflected back to the panel by the water, essentially light trapped at the panel surface...

So do you think If the mirror was mounted 2-3 inches behind the panel, allowing gap for air flow behind the panel, that would work best...?
Author:  RupertofOZ [ Jan 6th, '07, 21:39 ]
Post subject:  Re: Extra suns on PV with water cooling project.
Couple of links for you two to comment on...

First is a "bifacial" panel

Bifacial Panel

Second mirror reflector (pdf)

Ridge Concentrator
Author:  TimC [ Jan 6th, '07, 21:46 ]
Post subject: 
Thanks ROZ... the second one has got me thinking... Linear actuators are expensive though but makes a lot of sense how they do it...
Author:  RupertofOZ [ Jan 6th, '07, 21:47 ]
Post subject: 
They sell small ones on eBay

Caravan Bifacial Panel
Author:  njh [ Jan 7th, '07, 00:57 ]
Post subject: 
re mirror reflector... hehe, he's even in melbourne. I'd go with 3 and trickle cooling still, as 3 suns would make the water reasonably hot for DHW production. If the back surface is well bonded to the front, thermally, you could instead cool the back and avoid any issues with blocking the light.

re linear actuators, yes they are very expensive, but rotary ones aren't. So we take a length of all-thread and a nut, and a $20 cordless drill and we put the all-thread in the cordless drill and weld the nut to the assembly. When the drill turns it winds itself in and out.

Using a cordless drill has a number of advantages - they are readily available, they have a reusable speed controller built in (the old B&D ones I'm using had a simple 555 circuit with a resistor array to provide the motor control), they have a gearbox, they have a chuck for easy attachment, they have built-in battery backup, you can test them by hand before wiring up a controller.

I'm using them for electrically actuated gate valves - a $5 gate + $20 cordless drill is a lot cheaper than the commercial ones, heck it's almost as cheap as a 24V solenoid valve! There is some subtlety in knowing what position the valve is in.

Remember, we're not after speed, rather, torque (or force, as it's known in linear circles... :).
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