How to periodically shut down a recirculating hot water solar thermal system

I am posed with a scenario where a hot water solar thermal system has been setup. The system comprises of a cold water tank, heating arrays and then a hot water tank. The system is closed loop and water always keeps recirculating. The system uses CPC Evacuated vacuum tubes in the link below:

http://andyschroder.com/CPCEvacuatedTube/TechnicalSpecifications/

As a part of the system's primary loop, water is drawn from the cold water tank, passed through the array and it arrives into the hot water tank.

Now the secondary loop is used to consume the energy and cool down the water in this process. In the secondary loop, Water is drawn from the hot water tank, passed via a heat exchanger and it arrives back into cold water water tank.

If the heat demand from the secondary loop is consistant and higher then the supply, everything works in harmony and the hot water tank temperature stays in control.

The problems comes in when the secondary loop no longer requires heat energy(due to a downstream low demand). Since the primary loops keeps generating energy, water in the hot water tank exceeds 95C and drain procedures are activated to prevent steam generation/excessive pressures.

The problem is exagerated by the fact that the downstream system's demand pattern is very inconsistent and the demand maybe zero for several days frequently(so I know the thermal system has been over designed but downsizing permanently is not an option)

Question: Considering the situation above, what is the safest way to stop power generation from the arrays.

My understanding is that just stopping the recirculation would cause hot water residing inside the vacuum tubes to heat up beyond boiling point and pop PRVs on the array. Over some time this would also mean the copper tubes inside the vacuum tubes would scale and deteriorate aswell. Is this thought correct?

A solution that came to my mind was to stop the recirculation when the hot water tank has very high temperature and use compressed air to purge the water stagnant inside the array? Is this a good idea?

If air purging could be practical, I wanted to understand if there is a type of valve that would allow water to go though while preventing compressed air to go through? If such a mechanical valve exists, water could be purged through different parallel arrays without investing into automatic valves per array.

Looking forward to hearing your thoughts on how would you tackle this problem.

• water in the hot water tank exceeds 95C and drain procedures are activated to prevent steam generation/excessive pressures. To my understanding the water in the tank will never boil, that would mean that the incoming water already exceeds 100 °C and thus the water would evaporate in the feeding pipe. I guess the water is drained to prevent 95 °C water being fed to the solar panel and thus being heated too much there. Is the drained water replaced with cold water? This is just for my understanding, solar mike is on point with his answer if you want to reduce drained water amounts. Feb 24, 2020 at 7:46
• Can you post some actual numbers? What is the collector area, what is the flow rate in the primary loop. What does the thermal load vs time curve look like. Is this industrial, residential, what are you actually doing? Feb 24, 2020 at 11:47
• Is there an expansion vessel present in the system? How is overpressure protection achieved? Are there air vent at all high points, automatic or manual?
– mart
Feb 24, 2020 at 12:05