cooling excess solar panel heat transfer fluid

[OMidKnight]

Hello everyone

I may be bark raving mad or in cuckoo land - however the problem is

Excess heat from solar panel thermal systems. When these overheat either through excess sun or stagnation conditions domestic water temperatures of 100C plus could result. The temperature of the excessive heat transfer antifreeze from the solar panels can be between 80C and 150C.

Is there anyway that this excessive temperature can be cooled to reduce the return temperature of the heat transfer fluid? Ideally ( cuckoo land? ) the excessive heat transfer fluid temperatures would generate the desired cooling effect with little 230 volt help.

Thank you for any help. It really is a big problem and dangerous one to have uncontrolled excessive temperatures in solar water heating systems. Any help is appreciated.

Rob


Last edit - Thank you for your suggestions so far and I hope font is larger now.

[Brian_UK]

Hi OMK and welcome to the forum.

If you could edit your post to use a slightly larger font it would be easier to read for all of us.

As far as your question goes, so thoughts....

Do the solar panels not have some form of automatic blind over them to reduce heat input when it is not needed?

Could a fanned heat exchanger be put in circuit so that waste heat could be discharged like a car radiator.

Whether a peltier device could take advantage of the excess heat to drive the fan is another matter.

[The MG Pony]

Brian's idea, is a good sound one. Just blow off the extra heat into the air, or sell it off if feasible.

[momo]

During the season where excess heating occurs or low usage of water: 25-50% cover the solar collectors with safe objects firmly held (wind danger). I suggest aluminium laminate as used for office seperations (light and weather proof)...

Check the vertical inclination of the panels: you can reduce the collection of heat by changing the vertical (azimuth) angle away from the calculated ideal, The peak heating season (solar height) can therefore be avoided. Otherwise change the E/W orientation: more towards the W for evening peak water use, E for daytime peak use.

BY any install standards air venting and overpressure vents MUST be used on solar panel piping, especially with closed circuit systems in case the ballast ball cannot handle the excess expansion, or boiling .

Good to hear of excess heat production /energy saving for a change.

[ehowley]

Worked on a viessman system recently with a similar situation. Engineer added a cast iron radiator and a circulator off the collector loop to come on when heat got excessive. Rad was outside on the roof near the collectors.

[Peter_1]

Bypass ground loop and try without pump thermosiphon would normally work with these high DT's.µ
Or increase your boiler capacity.

[mohamed khamis]

Bypass ground loop and try without pump thermosiphon would normally work with these high DT's.µ
Or increase your boiler capacity.

Hi Peter_1

I think the Bypass ground loop is not suitable in this condition because the heating load is peak (at overheating ) and when u bypass the water over the solar panels the discharge water from the panels will be heated more and even u mix the bypassed (unheated ) water with the passed heated water the net product is an undesirable heated water.

In a theory,

Let us assume the design normal conditions the water inlet temperature is of 30C and it is required to heat to 80 C and the available heat input from the solar panels is 10 kW at normal conditions (not overheating) so the required water mass flow rate is:

mw = 10 kW / 4.187*(80-30) = 0.048 kg/s

During harsh conditions, if the water exit from the heating system at temperature of 100 C this means the solar heat input becomes 14 kW.

So if we make bypass (0.2 of the water will bypass the heater) and the remainder will be entered the heater so the net product of the water at temperature is of:

Now 0.8 of the water at 30C will be entered the heater with 14 kw heating capacity;

Two = (14 /4.187*0.8*0.048)+30 = 117C

so the mixed water (0.8 of the water at 117C and 0.2 of the water at 30C)

0.2*30 + 0.8*117 = 99.6 ~100C (the avoidable one)

So i think the bypass by this method is not favorable, the bypass is good solution for the chillers because the cooling load drops so the water should listen to this drop to balance the comfort conditions.

So my suggestion for "OMidKnight" is to redesign his system by increase boiler water capacity as u mentioned
corresponding to the maximum collected solar heat input ( 14 kW as in the previous example) and make bypass for return that means he makes the pipe has inverted C shape (upside down) with solenoid valve and thermostat in the discharge water, if the temperature drops under the design value of 80C the solenoid valve opens and recirculated the water to enter again the pipe without passing into the heater. I think this will protect his system.

Cheers

[mohamed khamis]

Could a fanned heat exchanger be put in circuit so that waste heat could be discharged like a car radiator.

Whether a peltier device could take advantage of the excess heat to drive the fan is another matter.

Hi Brian_UK

Caring should be taken when u install peltier device in this system because u will select the thermoelectric modules based on the hot-side temperature and cold-side temperature which mainly depends on the thermal resistance of the air.

As i understand the peltier device will work only when the heat input is peak to reduce the water temperature to the required value by the fanned heat exchanger. However, at the off-peak conditions the fan will be off this will inevitably reduce the air thermal resistance in the cold-side of the thermoelectric modules which may overheat the modules and burn it out. So it is better to select the thermoelectric modules with natural air thermal resistance and this in turn to burden on the capital cost of the system installations. The compromise should be done in this design between the capital and operating cost.

I suggest to store the overheating power in a storage system (wax molds) and exploit this power in off peak conditions.

Cheers