Solar thermal stagnation is the equivalent of leaving a pan of water on a cooker hob with no off switch.

It occurs when a panel collects more energy than it can handle. This is usually because the storage tank has reached its maximum temperature or the pump has failed. With nowhere for the heat to go, the collector “stagnates,” and temperatures inside can skyrocket.

How Stagnation Happens

• The sun heats the collector.
• The collector transfers the heat to the hot water tank until it reaches the correct temperature.
• As the hot water tank is no longer taking heat from the panel the circulating pump stops.
• The circulating pump has stopped but the sun is still providing heat to the collector. The energy provided from the sun to the collector is now too much to transfer away.
• The solar fluid reaches it’s boiling point and turns to a very high pressure vapour (vapor in the US).
• The vapour travels around the solar thermal system damaging seals and the expansion vessel diaphragm.
• Each time the solar fluid turns to vapour it degrades. This reduces its ability to withstand cold temperatures. It also makes it acidic which increases corrosion.

The Consequences

Your solar thermal system can handle this occasionally. However frequent stagnation can lead to:

• Fluid Degradation: The glycol (antifreeze) can “crack” or turn acidic at high temperatures. The fluid becomes a thick, sludge-like substance that ruins the system’s efficiency.
• Pressure Spikes: As liquid turns to steam, volume increases dramatically, putting stress on pipes, seals, and the expansion tank.
• Component Wear: High heat can damage sensors and the insulation around the pipes.

How can I avoid solar stagnation?

Keep the system pressure at the correct level

Maintaining the correct system pressure increases the boiling point of the fluid. It is also easier for the pump to circulate the fluid around the panel. This helps to transfer the heat more effectively.

The pressure should be between 1.5 and 2.If the pressure drops, a technician must refill the system with glycol. A solar filling machine like this one in the link is used to refill the system.

Install a solar hot water tank

Solar hot water tanks have a portion at the bottom that is not heated by other means. This is called the “dedicated solar volume”.

A correctly sized dedicated solar volume will give the solar thermal somewhere to send the solar energy. This is even if the tank has already been heated by a boiler.

Correct size of hot water tank

The hot water tank should be sized for the amount of hot water the property needs. The solar thermal collectors should then be sized for the size of the hot water tank. An undersized hot water tank will heat up too quickly causing stagnation on sunny days.

Correct shape of hot water tank

A tall thin hot water tank allow more layers of different temperature water to develop (known as stratification).

The tank heats from the top first as hot water rises in the column of water in the cylinder. A tall thin cylinder allows more more stratification to happen increasing the usable volume of the tank.

Heating the tank at the right times

After low system pressure the biggest cause of stagnation is heating the hot water tank at the wrong time. The following facts are very important:

• Solar thermal only heats the tank during daylight hours
• Solar thermal can only heat the tank to the temperature set on the hot water thermostat.

The hot water tank should be as cold as possible every morning. This allows the maximum amount of solar energy to be added during the day while the sun is shining.

The hot water program should be set for an hour or two at the end of the day.

If the program is on in the morning the tank will already be hot when the solar starts to work. This reduces the amount of water available to heat to the maximum temp.

Summary

Why does stagnation happen?

• Solar stagnation happens when the solar fluid isn’t circulating through the collector.
• During stagnation the overheated fluid in the solar collector boils.
• Stagnation damages the fluid, causes pressure spikes and higher component wear.

How do I avoid stagnation?

• Make sure the system pressure is correct.
• Set the hot water program so the hot water tank is cold before the solar starts to work.
• Size the solar thermal system correctly for you needs.

If you found this confusing:

If you found this confusing. Please let us know in the comments section so we can clear things up for you. We love feedback and we are trying to make the best possible website. If you don’t understand then others don’t as well. 

If you found this useful:

If you found this useful maybe check some of our other pages.

• How Solar Thermal Heats Your Water Efficiently
• Solar thermal flow rate
• Solar Thermal Hot Water
• What Are The Components of a Solar Thermal System?
• What is solar thermal stagnation?
• What parts are in a solar thermal system.
• Hello deary