Another Solar Myth Bites the Dust

I know this is from a while back but it's new to me. Thanks a ton for putting this article together and sharing, Larry! I find this fascinating at many different levels. Super cool about about all the simplifications you get by foregoing some performance… no freeze or overheat protection needed, pex sufficient, single bolt install, and on and on. I love this kind of thing where what almost feels like a science fair project ends up punching way above its weight class.

It was nuts to me that pool collectors were getting water above 110 during the week of winter solstice. I would love to cross reference the weather history at that location with your charts, since they show outside air temperature on roof but not the general outdoor temps. This got me thinking because as I've thumbed through SRCC materials on collectors, I've been thinking in terms of the outdoor temp, which could result in underestimating pool collector performance for some situations. Some speculation about why that difference might play favorably in this particular arrangement:

1. Metal roof will presumably absorb and emit or reflect some sunshine to the underside of the panels, maybe creating a slightly warmed air layer?
2. The round shape may lend itself to this, eg compared to solid rectangular panels that end up shading the entire part of the roof that they occupy.
3. Maybe the spiral shape also helps with this? Presumably cold water comes in at the outside and then gets warmed up as it approaches the center. This would lead to the warmest ambient air pocket being located near the outlet where the water has become warmest, and at the center of the coil where it is also the "most sheltered" from the wind.
4. The acrylic disk may be helping with this some, even though it's just sitting on top and would also reduce direct solar transmittance. I'd love to see an SRCC datasheet comparing the versions with and without the disk.

All these would be only small effects, but with pool collectors ambient air temp has such a huge impact on performance. So those effects are still interesting for applications like this one where the job is just slightly out of reach for conventional pool collector arrangements.

To get SRCC listing the manufacturer has to send the collector to one of the numerous certified labs. Some of the labs, in Florida for example, install the collector outdoors and monitor it for 30 days. The lab in Canada we use actually test them inside under lamps.

With the SRCC data you can predict performance, temperature, etc.

But keep in mind all the numbers are an instantaneous snapshot. Any change in wind, temperature, clouds, fluid temperature etc changes everything.

Here is an example how you could use the test data from the SRCC certification to predict stagnation conditions, for example. This could be a empty DB collector, or filled with glycol.

This is a basic flat plate type collector.

A clear example why solar glycol takes such a beating in closed loop ST glycol systems. Most glycol is rated around 325° intermittent condition.

There were, maybe still are, some higher temperature glycols developed for evac tube systems, but controversial if they were still considered non-toxic, GRAS.

Hi, There are plenty of ways to look at this. Zak, the owner used to use propane, tank-type water heaters. The view from just the monetary perspective is that its giving a return of 25% per year and he has better hot water service. For example five people in a row can shower. That wasn't possible before. It was dealing with Martin Holladay's assertion (myth) that solar thermal is dead that created the title. I do know there are precise ways to obtain solar fraction and I took a shortcut there, which I tried to explain.

Yours, Larry

Good question, I had it as well, so after five years, I collected water from the bottom of the tank and had it tested for legionella. All clear. 👍

Yours, Larry