Solar Panel heat dump options

mundogas

Hi,

Looking for some guidance on installing a heat dump on an evacuated tube solar system. 40 tubes and 300 litre tank. Only two of us in the house currently and I'm concerned about stagnation in the solar system, especially when we are away during the summer.

House is near passive so I am guessing that the heat dump may have to go on an external wall to avoid overheating. Build is timber frame with cellulose insulation and block other leaf.

Area has fairly hard water so currently pump will stop when tank reaches 65 degrees. I have already seen max of 183 degrees on the roof after pump has stopped, but that was before we moved in when there was little demand on the DHW.

I have turned the controller stagnation setting on but not sure this will be enough. From what I have read holiday mode is not very useful for evacuated tube systems as they don't dump heat at night.

Any other options I am missing here or recommendations for external heat dump.

Thanks in advance

yllw.ldbttr

sounds like we have similar enough problems?

http://www.boards.ie/vbulletin/showthread.php?t=2057410074

mundogas

Yep does indeed sound similar, I am thinking that a water softener might be an idea for the hard water? From what I have read people have increased the storage tank temperature from 60 degrees to 85 or so when scaling is not an issue. At least the pump could run for much longer before being locked out due to high tank temperature. The mixing valve on the outlet of the tank should maintain the temperature and safe levels at the taps.

A radiator to dump heat would be best, but I think it would need to be outside in a near passive build so its more work than I would like.

Other thing I'm not sure about is how long until the glycol starts to break down. If I could get two or three years out of it then change the fluid I would be happy with that.

Just not sure what to do but waiting for the pump to stop and the temperature at the panel to skyrocket doesn't feel like the best approach.......

quentingargan

One problem with stagnation like this is that it rapidly degrades whatever glycol is in your manifold at about 115 degrees. So every time the system stagnates, that bit of glycol is degraded. Sooner or later this will cause problems for you.

There is a freeze setting in the controller which can run the pump when the manifold falls to 4C. You should assume the system has no antifreeze and use this setting.

But it would be better to put in a heat dump - either cooling the cylinder when it gets to 60C by running water through the top coil, or by having a dump radiator.

There is a sort-of fix on some controllers where you can keep the pipework running hotter when the cylinder gets to 60 by running the pump for a few seconds every time the panel gets to about 110C. Then shut everything down if the cylinder gets to 75C. That reduces the stagnation on flatplates by keeping everything running hotter and radiating heat, but is less effective with tubes.

mundogas

Hi quentingargan,

Thanks for the reply. I hadn't realised that it was 115 degrees that the glycol started to degrade, was hoping it was going to be higher than that at about 170.

As you have said anything other than a heat dump is a sort-of fix. I have decided to bite the bullet, 3 way diverter valve and solar heat dissipater are on order. I will dump the heat to the outside of the house to avoid it overheating.

Hopefully that will have me sorted!

exaisle

Some solar panels come with a drainback system which doesnt require the use of glycol. The water in the panel is drained out of the panel to a buffer tank if conditions are unsuitable...too much solar gain or too low external temperature...not much use in your case now i'm afraid...

@quentingargan - Quentin, am I correct in thinking that when glycol degrades, it also becomes acidic?

freddyuk

Solar antifreeze fluid does not have to degrade at the first stagnation situation. If you do have a stagnation event the system may be gummed up causing lots of potentially expensive damage.
I guess most installers use cheapest Glycol mix available. Whereas you can protect your expensive investment with a fluid that is more robust and less toxic. If you do overheat the system it remains effective at high temperatures and even at excessive temperatures protects the metal components form the effects of acidic corrosion.
Here is the data sheet with product name removed so you know it does exist:

Properties:
********has been especially formulated to exploit the advantages Ethylene Glycol has over Propylene Glycol. Specifically;
• More efficient heat transfer.
• Easier to pump, especially at low temperatures. • Less volume for the same freeze protection.
• Cheaper per litre.
Although based on Ethylene Glycol, ******** has been tested and classified as Non-Toxic 1 by an EPA 2 certified laboratory. Previously Propylene Glycol was the only non-toxic glycol available. That is no longer the case. 3
******** in alliance with **********. have developed and patented the ****TM additive. ****TM prevents Ethylene Glycol from being metabolised (during digestion) into toxic by- products, which cause kidney failure, blindness and death.4 Tests carried out on ******** confirmed the toxicity was “so low that it was impossible to determine an LD50 5 value”.
The ***** additive has very little effect on heat transfer or antifreeze performance. In fact similar volumes of ****** or Ethylene Glycol are required to achieve a specific freeze protection.
******** is able to operate efficiently at ≤170°C for prolonged periods and withstand temperatures of 200°C for short periods. It contains reversibly evaporisable inhibitors that are capable of withstanding stagnation temperatures of 300°C whilst protecting all metals used in Solar systems.
********* also contains a high temperature rated polyacrylate, formulated to prevent sludge precipitation and scale formation. Thus promoting long operational life and high thermal efficiency.
******* mixtures are readily biodegradable (90% over ten days) and will not remain in the environment or bio-accumulate.






So there is an answer which is freely available to any installer. A simple and cheap by pass system is still highly recommended on any system to prevent stressing the components.

quentingargan

Thats an interesting solution Freddy, which I haven't seen on the Irish market, though I have been out of the solar thermal business for a while. I think it would be safe to name the product here - of course you can find it by googling some of your text, but unless you're selling it yourself, I think it would be useful to name it here - maybe check with a mod first....