Is it Time to Revive Recirculation Freeze Protection?

[Kevin_in_Denver]

Here's a simple and cheap solar hot water system design to give you guys food for thought.

The Problem: Even small SDHW systems cost too much ($6-$8k on average)
Since gas heated water is still very cheap, the average family only saves $30/month with solar

My Opinion: The US SDHW industry is in deep trouble now, and will remain so until $2000 systems are available.

A Solution?: Bring back Recirculation for Freeze Protection (RFP) in all but the coldest climates in the US

History: RFP is still used in many non-freezing climates, so it's still a free option on most solar thermal controllers. It's reliability took a hit in the 80s when many systems froze because power failures often accompanied freezing weather. (Freezing rain will often take down power lines) Because of this, most installers eschewed this method for most climates. Historically, it can't be used with flat plate collectors because in the winter it takes too much energy to prevent the collectors from freezing. (They are pointed at the night sky which is usually colder than -100F on a clear night)

Why This Solution Can Work: A. If Evacuated Tube (ET) collectors are used, the heat required for freeze protection is almost negligible, and B. In case of a power failure, the system will self-freeze-protect by thermosiphoning. C. The outdoor piping is freeze and stagnation tolerant hose.


LowCostETsolar#5preferred.jpg

Please criticize it.

Yes, the ET collector header can still freeze and break, but the probability is too low to worry about. (Drainback systems have a few failure modes including freezing, but we still use them because the Mean Time Between Failures (MTBF) is long enough.)

[russ]

Hi Kevin,

Agreed that the systems seem to costly which restricts their adoption.

1) From what posters comment the evacuated tube systems have serious freezing problems. This would have to be the heat pipe type system - the water tube would not be possible.

2) In many locations you do not want tap water recirculating through the collector - if your water is hard that would be a quick problem - here we have between 550 and 600 ppm hardness - very hard

3) The connecting line needs to be other than hose - something adequate for panel stagnation temps

4) The thermosiphon action you mention for freeze protection - that is just reverse flow back to the header? If so the loss every night would be quite high.

[Kevin_in_Denver]

Failure Mode Analysis

Russ,

The thermosiphon flow only occurs during rare power outages. The normal mode of freeze protection is pumped flow when the solar panel temperature goes down to 40F.

"A. If Evacuated Tube (ET) collectors are used, the heat required for freeze protection is almost negligible, and B. In case of a power failure, the system will self-freeze-protect by thermosiphoning. C. The outdoor piping is freeze and stagnation tolerant hose." PTFE hose, for one, can handle all the extremes and doesn't burst if it should happen to freeze - http://www.flexfithose.com/

The Chinese ET heat pipes had freezing problems in the past, but the copper powder and tapered tip of the heat pipe now prevents freeze bursting.

This paper proved that the heat used for for freeze protection is a small fraction of the collected energy: http://www.thermomax.com/Downloads/Reciculation.pdf

Lime scale is definitely a problem to consider. Under normal operation, the collector water temperature is less than 280F, which is the point that calcium sulfate really starts to plate out.

[russ]

1) So you mean any time the temp drops below 40° F the motor starts?

2) PTFE hose - with SS braid?

3) I am certain some of the guys will be happy to hear the copper stopped heat pipes from freezing - this is exactly the opposite of what is normally reported here.

4) No problems with hardness causing scaling below 280° F? That is not correct.

[LucMan]

B. In case of a power failure, the system will self-freeze-protect by thermosiphoning. C. The outdoor piping is freeze and stagnation tolerant hose." PTFE hose, for one, can handle all the extremes and doesn't burst if it should happen to freeze - http://www.flexfithose.com/

T.

The manifold however is not freeze tolerant.
It's pretty hard to offer a guaranty on this system.

[Kevin_in_Denver]

1) So you mean any time the temp drops below 40° F the motor starts? Not the outdoor temperature, just the pipe temperature or the collector header temperature. Since they are both well- insulated, the pump only kicks on for a short time.

2) PTFE hose - with SS braid? yes, it seems to be fairly common

3) I am certain some of the guys will be happy to hear the copper stopped heat pipes from freezing - this is exactly the opposite of what is normally reported here.

4) No problems with hardness causing scaling below 280° F? That is not correct.

Scaling problems occur mainly on the hottest surfaces. For example, in an electric water heater, the scale happens on the heating element. That element is much hotter than than any surface inside a solar collector. The solubility of calcium sulfate takes a big dive at 280F. But field experience is definitely needed.
I've tested the thermosiphon "backup" freeze protection through 2 winters and it works well. I've also intentionally stagnated the system repeatedly to try to break it. Next I'll look for scale inside the header.

[inetdog]

1)
3) I am certain some of the guys will be happy to hear the copper stopped heat pipes from freezing - this is exactly the opposite of what is normally reported here.

From what I have read, the purpose of the copper is not to prevent freezing, but to cause the freezing, when it finally happens, to occur as a slush rather than a hard plug of ice, so that the expansion of the water as it changes phase to ice does not burst the tube. Along the same lines, the tapered tip is supposed to allow the expansion to happen by letting the plug of ice slip up the tube to a wider section as it freezes. Whether either of these mechanisms is as effective as claimed in preventing damage is the open question!

[Naptown]

The thing I see with this is there is really no reduction in costs other than the elimination of glycol which is a very small part of the equation when trying to bring down the costs of systems. Yes there would be a performance increase as straight water as in a direct exchange tank would increase efficiency at least when it is actively collecting heat. How much the cycling freeze protection would take away is another matter and I don't know how much that would affect the overall performance.

[Kevin_in_Denver]

From what I have read, the purpose of the copper is not to prevent freezing, but to cause the freezing, when it finally happens, to occur as a slush rather than a hard plug of ice, so that the expansion of the water as it changes phase to ice does not burst the tube. Along the same lines, the tapered tip is supposed to allow the expansion to happen by letting the plug of ice slip up the tube to a wider section as it freezes. Whether either of these mechanisms is as effective as claimed in preventing damage is the open question!

The copper powder isn't an antifreeze. It sinks to the bottom. Ice needs an impurity like this to start forming. So the purpose of the powder is to help start the ice plug at the bottom of the tube. The tapered tip helps this out also, by having less water mass at the tip, it should freeze first. The ice plug may also slip up the tube as it expands, like inetdog says. What we're trying to do is reverse the normal freezing phenomenon, which is an ice plug forming at the top of the liquid. Ice normally forms at the top because water below 39F gets buoyant. Once this plug forms and grows downward, the incompressible water then splits the pipe.

I've frozen many of these tubes many times, and they don't burst, or even expand measurably.

[Kevin_in_Denver]

The thing I see with this is there is really no reduction in costs other than the elimination of glycol which is a very small part of the equation when trying to bring down the costs of systems. Yes there would be a performance increase as straight water as in a direct exchange tank would increase efficiency at least when it is actively collecting heat. How much the cycling freeze protection would take away is another matter and I don't know how much that would affect the overall performance.

It eliminates the glycol, a heat exchanger, air elimination devices, and the labor of filling and purging.
Solar preheat tanks with a heat exchanger start at $1300, but this system can use a standard electric tank that costs $270 for 50 gallons at HD. With this type of collector, you can even eliminate the preheat tank completely, and just use the homeowner's existing tank, a la Butler Sun

Here's the ACEEE presentation that has a cost breakdown: http://www.aceee.org/files/pdf/confe...kson-Final.pdf

[Naptown]

Too bad that any installer that values his or her business will run away from this.
As a pro i look at potential failures and rate. A risky setup is something that I would run from not gravitate toward. The risk of failure is too great to be discounted. this could mean the difference between profit and bankruptcy and the stigma associated with that.

[russ]

Basically 'shade tree mechanics' - while the system can work it is not really something for public consumption as Rich noted.

[Kevin_in_Denver]

Trying not to reinvent the past

Too bad that any installer that values his or her business will run away from this.
As a pro i look at potential failures and rate. A risky setup is something that I would run from not gravitate toward. The risk of failure is too great to be discounted. this could mean the difference between profit and bankruptcy and the stigma associated with that.

I can certainly relate to this thinking. I was hired by Solaron in 1982 to engineer, develop, and implement drainback because draindown was a complete failure.

However, I think a cheaper direct system can work with a bit of good product development, which addresses all possible failure modes and snuffs them out with technology or brute force FMEA.

Many thousands of people died while boilers were developed from about 1830-1960. Compared to that, developing a reliable direct system should be easy.

And I think the residential solar hot water industry will die unless costs are reduced somehow. This system may not be the way, but as I said above, it's food for thought.

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead