Passive emergency cooling for solar thermal collectors

Venergy

Flat plate collector overheating. 17 year old system has had maybe 5 or 6 loss of circulation events (avg say once every 3 years). Reasons have been: pump died, temperature sensor failed and pump didn’t come on, voltage surge fried the Resol controller, and a couple power outages. Several times have had pipe burst (pex carrier pipe underground) necessitating digging and repair. Even a relief valve blowoff on one of three 200 sqft arrays, leads to poor circulation in the other two and they eventually blow off. In all cases $500+ in glycol is lost. Put in bare finned tube radiation for passive cooling on one array and tested today under full sun by valving off at inlet. At first, the heat dump seemed to work as intended (finned tube in shade of collectors was kicking out heat), but then the thermal siphon shut down, the finned tube cooled toward ambient, and the line leaving the array got very hot (200 degF pipe surface temperature). Was well over 200 at top of collector. Before a catastrophe could happen, I opened the valve and pump flow quickly cooled the array. A bunch of other trials and data I could report, but do you think I had a vapor lock due to boiling in the flat plate? What to do now? Any help greatly appreciated.

hot_rod

there are a couple over-heat functions in the Resol controller, are you using those?

The first basically allows the tank to heat above setpoint, often that is enough

Night time cooling Runs the pump at night, after dhw loads are done and presents a cold tank every morning

You can also cover part of the collectors

Jamie Hall

Single point failures. One needs to look at — and engineer — the whole system, top to bottom, so that the consequences of any failure are clearly understood, and that any failure which creates either directly or through a chain of results a safety hazard or an expensive result is designed around.

This often results in parallel systems (dual pumps. Dual controllers. More than one pipe line. That sort of thing). It's an interesting exercise.

hot_rod

also, shat pressure are you tunning? Most solar thermal have 90 psi relief if they have a pump station. So you could run 35 psi or more to raise the boiling point

The expansion tank needs to be sized adequately and pre charged

What size tank or load do you have on that large array?

One to two gallons of tank per square foot of array, depending on the solar available in your location