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Old 06-14-14, 07:48 AM   #1
osolemio
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Default Best fluid for solar panel

Working on my project (still), now getting closer to getting the solar panels.

As they are focused and user higher than normal temperatures, temperatures could potentially reach 200 C (about 400 F) in worst case scenario. In normal operations, they shouldn't, but in case they do, what fluid is better for the purpose?

Also, in the winter, temperatures could reach down (rarely) to -30 C (about -22 F).

I realise there are many other parameters to consider, but any suggestions? I heard mention of some kind of spirit which is even less dangerous to the environment than glycol.

Most of the system is water based anyway, it is only between the solar panels and down to two heat exchangers, where the anti-freeze and above-boiling scenarios could exist.

Thank you

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Old 06-14-14, 08:31 AM   #2
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Remember that the higher the temperature of the circulating fluid, the lower the efficiency of the collector. This is not trivial.

The efficiency drops rapidly as collector temperatures go over 100 C (212 F). The lower the collector temp, the higher the collector efficiency and the fewer number of panels need to be installed.

This is why I advocate "drain back" collectors. Water (cheap) can be used as a collector fluid and problems with heat exchanger corrosion (and subsequent introduction of toxic materials) become moot.

Drain back collectors are one of the simplest type of collector, yet few consider their utility. They are also the longest lasting of collectors with some in Israel lasting more than 50 years! Those areas have cold winters and hot summers.

Drain back only pump when the collector temps will support water fluid movement; easy to do.

So my suggestion is - just water!

Alcohol based mixtures are my second choice as gycols tend to gum up at high temperatures.


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Old 06-15-14, 12:36 AM   #3
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Quote:
Originally Posted by stevehull View Post
...The efficiency drops rapidly as collector temperatures go over 100 C (212 F). The lower the collector temp, the higher the collector efficiency and the fewer number of panels need to be installed...
Steve, this is the perfect place for a diagram of what you are talking about.

Surely, as a consultant in the energy field, you could find a diagram to make your point clear.

Are you going to leave it to us amateurs to explain things properly?

-AC
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Old 06-15-14, 02:27 AM   #4
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Thank you for your fast replies!

I am aware of the efficiency curve, and indeed, I intend to keep these panels (and the fluid), really cool. And I can do so, as I have extensive heat storage facilities, over 1500 kWh of annual heat storage, as well as over 1000 USG of water storage, and other medium term heat storage possibilities.

The thermal peak power is around 10kW, although I might be able to tweak it even higher, given my extensive cold storage (into the ground).

The panels aren't suitable for drainback, as the way they are designed one cannot be assured they drain properly. I am stuck with some kind of anti-freeze, possibly IPA spirit or similar, trying not to have to use glycol for various reasons.

The curcuit for solar panels will be separate from all the other heat storage, which will all be water based, so the volume of the anti-freeze liquid isn't too much (compared to the 1000+ USG for the rest of the system).
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Old 06-15-14, 01:46 PM   #5
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It gets that cold in hong Kong?
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Old 06-15-14, 03:45 PM   #6
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Quote:
Originally Posted by AC_Hacker View Post
Steve, this is the perfect place for a diagram of what you are talking about.

Surely, as a consultant in the energy field, you could find a diagram to make your point clear.

Are you going to leave it to us amateurs to explain things properly?

-AC
AC - feel free to add a diagram. I welcome others that can put into pictures that which I write of.


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Old 06-15-14, 09:29 PM   #7
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If your solar collectors are expected to reach 200degC, you had better deal with the possibility of steam generation in your design. Especially if the collector flow may stagnate (purposely or accidentally) during full sun exposure. One high pressure event is all it takes to destroy lots of work.

Actually, a concentrated solar system using alcohol as antifreeze would have a LOWER boiling point than water alone. Like around 80-90 degC, depending on the strength of alcohol. So what you are proposing here is a high-temperature solar still. If designed properly, your setup might not even need much of a pump at high temperatures. Just design your heat exchanger to also function as a condenser.

Last edited by jeff5may; 06-16-14 at 04:05 AM.. Reason: info
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Old 06-16-14, 09:28 AM   #8
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The solar panels are not expected to reach 200 C, on the contrary, they should ideally be kept at 75 C or lower.

In case there is a fault in the thermal transfer system, or anything else that causes the temperature to runaway, the system should survive without substantial damage.

The panels themselves can sustain the 200 C stagnation temperature, although the PV part of them (PV-T focused panels) will degrade faster, and yield less, the higher the temperature is.

It is an experiment into ways of dealing with solar which is a combination of existing and new technologies. Being an experiment, it must be protected against unforeseen circumstances, whether that be freezing, boiling, overpressure and so on.

Since I cannot use the drain back principle, due to the design of the heat absorbers, I need to find the optimum liquid which ideally has a freezing point below -30C, high boiling point (ideally 200 C or more), doesn't gel, doesn't cost a fortune, isn't toxic and doesn't evaporate. I will have to look at characteristics of specific heat capacity vs pump power required as well, though I realise I cannot get everything but have to compromise.

I wasn't sufficiently specific in the first post, sorry about that, but I hope you understand the requirements now.

The project is located in Scandinavia. Had it been in Hong Kong, I wouldn't have had to care about freezing, on the other hand, I wouldn't have a high heating requirement either.
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Old 06-16-14, 07:23 PM   #9
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Those are some pretty stiff specs. What you are talking about here is hydraulic oil or brake fluid. Neither of which make extremely efficient heat transfer fluids.

By the way, here's a chart of what Steve said:

Last edited by jeff5may; 06-21-14 at 11:23 PM.. Reason: info
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Old 06-20-14, 03:39 AM   #10
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Quote:
Originally Posted by jeff5may View Post
Those are some pretty stiff specs. What you are talking about here is hydraulic oil or brake fluid. Neither of which make extremely efficient heat transfer fluids.
How about oils like this one?

http://s02.static-shell.com/content/...ansfer-oil.pdf

It seems to be made for the purpose, what's the downside? This would be a good "Checklist" for what other properties to look for, in a solar panel fluid: Heat Transfer Fluids for Solar Water Heating Systems | Department of Energy

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