“Steamback” Technique

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Traditional solar thermal systems are known for being susceptible to failure when hot water is not being used (vacations) and during power failures on sunny days.  In both cases too much heat builds up in the collectors and can cause damage to system components.

Solar is Hot engineers our systems to overcome these conditions, utilizing the “steamback” or “stagnation” technique.  In steamback systems, excess solar fluid expansion is provided such that collectors can safely go dormant during periods of low water use or power failures by actually pushing fluid out of the collectors and replacing it with a higher pressure steam.  The result is extremely robust and highly-productive solar thermal systems.

Steamback means that when the collectors overheat, a very small amount of fluid is intentionally boiled to steam to increase collector piping pressure, and as a result drain fluid out of the collectors and compress it into the remaining system piping and expansion tank.

Technically, 5 individual phases of a stagnation event are commonly discussed, listed below:

Phase 1 – Expansion of Liquid

Phase 2 – Pushing of Liquid out of the collector

Phase 3 – Emptying of collector by boiling

Phase 4 – Emptying of collector by superheated steam

Phase 5 – Refilling of collector

There is a fair amount of misunderstanding in the solar community regarding stagnation.  Before moving to this technique, Solar is Hot conducted many system tests to fully understand the process.

In early 2014 we instrumented a system which we converted to be stagnation-capable, then shut the circulation pump off to simulate a “catastrophic event”.  Everything seemed to work well and the solar thermal system recovered gracefully with no damage.  What we did not know from this test is whether or not we actually did evacuate fluid from the collectors, since all of the piping is sealed.

So, we planned another similar test.  This time we placed a scale under the expansion tank.  Since we knew the amount of fluid which would evacuate the collectors, and the weight of that water/glycol mix, we had a rough idea of how much the expansion tank weight should increase during a stagnation event.

As expected, when system pressure increased, so did expansion tank weight.  When system pressure later dropped, meaning the collectors filled with fluid again because of cooling, so did expansion tank weight.  At this point we knew we nailed it.  We had created a solar thermal system with enough expansion capacity and correct piping such that “catastrophic” events were no longer catastrophic.  Results in the graph below:

Steamback Confirmation Graph by Tank Weight

As a result of this profound success, we attempt to incorporate the stagnation technique to all applicable projects.  The results are undeniable.

An added benefit of making systems steamback capable is energy performance.  A system which can safely shut down at high temperature and go dormant is one which can also operate relatively safely at higher temperatures.  This is important because higher temperature differentials are important for maximum performance.

Today’s best solar thermal collectors are extremely efficient and can quickly overheat unless supplied with a constant fluid flow.  This also makes steamback capability attractive.

Interested in more?  Check out a video showing performance graphs for an actual “steamback” system in a 4-unit apartment building in NY.

Learn more about solar is Hot with this video:

Study a few “steamback” events more closely with our experimental interactive graphs

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