Another Solar Myth Bites the Dust

HeatingHelp

Another Solar Myth Bites the Dust

It's good to bring fresh perspectives to solar water heating and set aside the forty year old ones. This solar design is a result of using “systems thinking”, understanding the properties of materials and employing elegant simplicity.

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Sol_Brother

Very interesting article. I would normally dismiss any idea of using plastic collectors or ones formed in a circle (as opposed to maximizing the solar aperture while minimizing the amount of material by using a flat pool collector — if you are going with plastic — as some designs of pool collectors claim to be freeze-tolerant as well (although I have not been interested in putting this claim to the test)) but I will reconsider my stance given your results.
I would encourage everyone also to learn about drainback systems. They eliminate the freezing and overheating problems you mention while maintaining the safety of having a separate and limited body of fluid running through your attic and on the roof. Other benefits are simplicity relative to pressurized glycol systems (though not as simple or inexpensive as your direct system), and no expected maintenance as the water in a properly designed system (with a stainless or plastic-lined reservoir) never needs to be changed.
I like the idea of Marathon tanks but am wary of them, having replaced several leakers. I suspect it is the tank-to-port connections that are the weak link but can't confirm this.
Glad you mentioned Martin Holladay's articles. It was shocking how one-sided and small-picture his thinking was on this topic. You noted the space on the roof issue. Also, in our area (central North Carolina) very few houses have basements and so very few have an adequately ventilated, tall, and freeze-protected space suitable for a heat pump water heater. In one case we replaced a perfectly good solar water heating system for a customer who was getting the roof filled with PV and whose tank was in the garage. That seemed like it would be okay even though for the cold parts of the year the heat pump would be operating in resistance heating mode — at much lower efficiency than she was promised by the PV installing company — but it turned out the vibration of the heat pump reverberated through the wall into her kitchen and there was nothing we could do to eliminate it. Plus it fails the simplicity test and is expensive to provide battery backup, as compared with a computer-type uninterruptible power supply for a solar pump.

Tom in Maine

Simple is good. Simple works in Southern California and much of the Southwest. The rest of the country would get a much smaller solar fraction in the non-summer months. And any wind will devastate the performance. And freeze protection severely complicates everything.
Having worked with SRCC and several entities over the years, any step to simplify and cut costs is desirable.
Unfortunately this system has severe limitations and although I hated when Martin wrote that article 12 years(!!) ago, there is a germ of truth in the simplicity and lack of moving parts in a PV/HP system. Most solar thermal people who remain solar installers have transitioned to PV's for the simple reason that they have little to fail.
I have always felt that potable water should be segregated from polymer collectors. A failure causes a deluge if no one is home. And there is undoubtedly more polymeric material in the water.
All this being said, this is a fun project that great for places where the sun always shines. With Caveats!

Tom in Maine

I meant NREL, not SRCC.

Mark Eatherton

Excellent article Larry.

Thank you for your research and ongoing promotion of THE most efficient system in the World, that being solar thermal.

As you know, I've been involved in solar thermal since before it was even on the DOE's radar. I have a 2 panels system on my mountain home that heats the home when we aren't there, and preheats the DHW when we are there. I keep the house at 40 degrees F when we are away, so all the radiant (walls, floors ceiling) surfaces are at at around 40 degrees F when the system fires, keeping the fluids below 70 degrees F, thereby maintaining solar efficiencies that are WAY high.

I wanted to pop in here, and (once again) talk about the potential that most hydronic snow/ice melt (SIM) contractors are overlooking that is staring them right in the face. That being the possibility of turning their SIM system into a large surface area high and ultra high efficiency solar collector.

During non SIM operations, there is a LOT of free energy falling onto this surface that could be put to use, either as a simple DHW preheat, heating pools/spas, and even space heating.

My concept is one of a two stage design. 1st stage would be a semi direct exchange that transfers the solar heated glycol into a storage tank with a heat exchanger for DHW preheat. When the delta T drops to the conventional "off' point, the 2nd stage kicks in.

2nd stage slows the circulation on the source (SIM) side, and directs this low grade heat into a water to water heat pump. These heat pumps would be running at around 200 to 300% efficient (COP of 2 or 3, depending). During the warmer summer months, the system would actually harvest what I refer to as "Ambient Energy", that being sensible energy that is not directly related to the solar gain. Remember, thermal energy flows from hot to cold. These systems make a lot of sense wherever there is a constant (annual) demand for DHW, which includes residential, commercial etc.

In commercial applications, like restaurants, laundries and other high hot water demand situations, I strongly believe that we could collect more energy for water heating, than they system would use in its SIM role. I've monitored some SIM systems during the summer months, and have seen slab temperatures approaching 140 degrees F. Granted, this is a stagnated condition, but the physics for extraction are there. Typical SIM systems have 5/8" PEX installed at 9" O.C., thereby making it very conducive to extracting all this free energy. When I was the Director of the RPA, I was at a code hearing conference, sitting next to a young lady. I asked her what her interest were in code development. She told me she worked for The Sierra Club, and that she was there to recommend that all SIM systems be outlawed due to the egregious waste of a precious natural resource. I told her of my reverse solar concept, and she said that she didn't realize that it was even a potential, got up and left the meeting without making her pitch. I guess what I am trying to say here, is that the environmental lobby has SIM in their gun sights, and that we as hydronic contractors had better be proactive in our applications, and start thinking outside the box, or the government and the enviro lobby will take that box (SIM) away.

These same slabs can also work as a very efficient heat rejection system due to night sky re-radiation. I've seen my solar thermal panels sub cool below ambient as much as 20 degrees under clear night sky conditions. I could use that in my mountain home for cooling from the radiant ceilings, if needed, without even firing a compressor.

Again, thank you for your continuing efforts to promote what the good Lord gives us for free, that we need to utilize to a greater degree, that being solar thermal. Stay well my friend.

ME

rick in Alaska

Great article Larry. how is it you can use a Sharkbite fitting on the poly pipe that is ips size? Is it a special ips fitting?
Rick

jumper

Simplest is air instead of liquid. No pump; no freeze worry. Basically a sort of green house with a fan to warm living space. Okay during sunshine.

Next step is really big greenhouse with air to water heat pump and storage. In north one cannot count on sunshine. In seventies there was a competition in Canada to build a solar heated home. Winner was giant storage for summer heat. I thought it was dumb and that winner needed plenty of supplementary heating.

Larry Weingarten

Hi @rick in Alaska , The fittings were supplied by the collector manufacturer, but I do know that poly pipe can be had in both iron pipe and copper tube sizes. I'll now have to get back up on that roof and see if the tubing is marked 🤠
For really cold climates, maybe a Malcolm Wells style underground house is one solution 😏
Yours, Larry

rick in Alaska

Yeah just wondering. The photo shows pe3408 pipe, and the picture of the pipe to buy, says poly pipe in ips size. I suppose you could get Sharkbites in ips, but have not seen it.
Also, I did not think poly pipe could handle hot temperatures. Do you know its upper limit?
Rick

Larry Weingarten

Hi, At working pressure, upper temperature limit of the pipe is 180F. Under full summer sun, no flow conditions, this reaches 170F, so it's close 😉
Yours, Larry

jumper

Even if it never snows; Mark Eatherton's thermal collector can be an economical method to heat some places. When water under driveway (or yard) reaches ~100° circulate under (not in) slab. In some regions perhaps over 1000 hours per annum. Ground will store that heat and floor will be pleasantly warm. Except for pump this low temperature method may last indefinitely.