Correct fluid?

[Ivanp]

We have a solar system with two evacuated tubes,which had become drained. The system has been refilled using a glycol mix normally purchased with flat panel collectors. The system works but the stored water temperature is down from 65 to 60 degrees C. Is this likely to be down to the fluid?

[Mike90250]

You need to verify the vacuum is still intact. At night, run some cold water into the tube, and let it sit a few minutes, feel the outer shell of the tube, then run the hottest water you can into the tube, and see if you feel any difference in the outer shell, if it feels warm at all, you have lost vacuum, and not much can be done about it.

Generally, the tubes are at the focal point of a trough reflector, and have a lexan cover over the system.

The fluid mix, will not make much difference, unless the tubes are clear, and the fluid was dyed black, to collect the heat.

65C [ 149F] is pretty toasty, as you get hotter, your system losses accelerate, and you reach equilibrium. What size is your storage tank, maybe adding more insulated storage will help.