Quote:
Originally Posted by stevehull
In terms of viscosity, this Shell fluid almost looks like sludge at 0 C (32 F). Also the heat capacity and thermal conductivity are low (compared to water).
I would design this system to prevent freezing (-30 C, as you specified) using an alcohol/water mixture. Minimize the distillation issue with a low percentage glycol that shifts the fraction (distillation) point higher in temp.
Am pulling physical chemistry labs out of my brain from more decades ago that I prefer to mention.
The major issue is freezing. Due to the low operating temp, an organic fluid appears way to viscous in the range of temps you operate in.
You also have huge summer insolation rates and you may want to put in a back-up circulation pump to prevent summer heat stagnation and a vapor expansive calamity (if you use alcohols).
VERY interesting problem.
Steve
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At temperatures below ~25C (77F), there wouldn't need to be any pumping or heat transfer, just the ability not to break anything, as freezing of water would do. Sludge is fine at 0C, as long as it becomes less viscous at normal temperatures again.
When the circuit is quite small, how would that affect it for having less heat capacity? Isn't it a question of pumping slightly faster, or is the difference too big between water/glycol and these types of oils? All it needs to do is go through the panels then just inside the roof to a heat exchanger - from there, it's all water in the rest of the system.
If I did use a glycol/water liquid at something like -30C freezing point, I could design a reverse cycle into the system for those very few days it should get below -30C, or even close to it - just to be safe. I know it's counter-productive, but it wouldn't take much to cycle the liquid a few times during a cold winter night - just to keep it from freezing up.
As for the lower boiling point of either water or water/glycol - what would happen if it really stagnates and boils? I would imagine it would blow the overpressure valve eventually, unless I have a disproportionally huge expansion tank.
I do have a DC direct drive pump built into the design, both on the primary and secondary side of the heat exchanger. If power is available from the PV panels, and the temperature exceeds a set limit, these pumps will start, regardless of the controller and the rest of the system.