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# "fairly" simple solar thermal calculation

Member Posts: 20,725
Most thermal collectors sold in the US are SRCC listed. Rebate programs may require those listing numbers.

ASHRAE 93-77 is the standard labs test to, see slide that shows the collector efficiency math. Some labs still test collectors outdoors, others under heat type lights, indoors.
SRCC accredited labs are capable of testing to the standard.

At the SRCC site find the test info from the panel you are considering. Attached is a common 6.5X4' panel.

The right hand box shows performance at different conditions and also different temperatures (Ti-Ta) is temperature inlet and temperature ambient. Low temperature applications like radiant would be C, DHW in a cold climate would be D
Look for the slope and Y incept data at the bottom right. On a graph, or graph paper plot that slope.

Next use the inlet fluid parameter formula determine performance at different conditions.
inlet fluid - ambient temperature at collector, divided by the solar radiation at the collector. 317 is a bright sunny day, 200 btu/ sq ft is a reasonable number to cover sunny and cloudy days average. If you have actual data at you location use it.

Keep in mind solar radiation is an instantaneous number, it can change second by second.
We are dealing with mother nature here, not a NG flame.

The Idronics page 12 shows how to compare efficiency of a collector. Two examples, one operating at 160F inlet water temperature , compared to one operating at 95F. The collector driven at high temperature is around 16% efficient. The lower temperature, perhaps attached to a radiant concrete slab load at 95F, runs around 43%. (fig. 15)

You often here solar thermal claiming 50% efficiency, which at full sun. low temperature is possible. Solar PV at 16% or so.
A low temperature pool collector on an 80F day could reach high 80%, as there is very little loss to the ambient air. (fig. 16)

So the conversion efficiency favors the solar thermal systems when comparing to PV.

However most folks have a use for PV, electricity every day. That is not the case with ST, so that calculation gets more complicated when deciding between the two technologies. Not to mention the storage differences between PV and ST.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream

• Member Posts: 1,492
Yeap.. decisions, decisions...And how about the combinations system of both PVT(Photovotage and thermal) ...How about the PVT calculations and in any giving situation to decide on either which one of them is going to be the determing factor/leader...PV or thermal ?...Any SRCC ratings lately ?...Simulation programs? do not see it on T-sol or T-PV or them having a PVT program from Valetine.
I have seen some new very promissing new panels at the last ISH in Frankfurt but will not spec or suggest anything until the SRCC put the blessing on these collectors..
• Member Posts: 20,725
PolySun is another simulation program, similar to T Sol. They have an entry lever version. Dan Gretsch out of North Carolina used to sell and support PolySun. I don’t see his solar company around anymore.
Both of these really require a class or two to learn how to properly use them. Incorrect data entry makes the results pretty useless if you are trying to get accurate or realistic simulations.

RET Screen and F Chart are still available for something less fancy. But also less \$\$.

Still some pockets of ST around, Sun Earth and AET seem to keep plugging away. Sun Earth has a good position in Hawaii. ST has been required on all new residential building since 2010. The do a lot of commercial stuff and build big pump package skids.

Sun Wind Energy magazine out of Germany was a great trade magazine, it’s more of an online newsletter now.

I think Germany still hosts a big solar expo every year or two. Used to be in Munich, called InterSolar
Caleffi had a booth at that show for a few years, I’d like to attend again, some year. It was a huge show back in the 2005 - 2008 years.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,355
edited April 17
It's unfortunate that the most solar is available when you need to heat the least. I don't need to heat my radiators at noon in the middle of July, I do need to heat them middle of January at midnight however. And while yes PV has the same limitations of only working when the sun is shining, you can drive an AC compressor with the PV panels in the middle of July and use batteries to store the excess year around or push it to the grid.
• Member Posts: 22,077
I still like passive solar heating for space heating demands... since I know it works and works well. Domestic hot water is a good deal more problematic, as one does run into efficiency and storage issues -- but with care it can be made to work. Photovoltaic is improving, both in the efficiency of the photovoltaic cells themselves and in the cost; the big cost there if one is trying to be self-sufficient is adequate battery storage capacity. A not inconsiderable cost, however, for the self sufficient scenario, is the cost and reliability of high power stable frequency true sine wave inverters.

As I have said before, though -- KISS. Keep It Simple, Stupid.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,592
Do we know if unglazed collectors can handle glycol? My ideal case for solar thermal (in general, not for me) might be 1. DHW and pool heating in summer and 2. preheating water in winter for a liquid-to-air/water heat pump. Basically install the simplest, cheapest thermal panel and operate it on the most efficient end of the graph.
• Member Posts: 1,355
edited April 17
As I have said before, though -- KISS. Keep It Simple, Stupid.
Is this simple enough? I'm in the midst of building my long awaited deck(permits pulled, lumber/hardware purchased) and am fantasizing about a hot tub on it. This to use up some of the excessive energy my PV panels are producing. I'm sure I could figure out some way of using solar thermal to heat it tho. LOL
• Member Posts: 20,725

Do we know if unglazed collectors can handle glycol? My ideal case for solar thermal (in general, not for me) might be 1. DHW and pool heating in summer and 2. preheating water in winter for a liquid-to-air/water heat pump. Basically install the simplest, cheapest thermal panel and operate it on the most efficient end of the graph.

An unglazed collector would not be very useful in the winter months, probably around 70F ambient or lower, efficiency plunges. Just plug the numbers into the formula, the fluid temperature you want and the ambient, plot it on the unglazed collector line.

But glazed collectors do get used for pool collectors, if there is a winter load as you suggest. On another post I shower the Weller School in Fairbanks. The ST just warms the earth around the GEO pipes in summer months when there is no DHW or heating load.

I did put some pool collectors on a roof at a YMCA camp for the DHW and pool. A Canadian brand, basically you clip black garden hose on the roof
It is open just for 3 summer months. DHW was priority, then it could add to the pool. Although by July pools are too hot just from the sun shining on the, in SW Missouri. Probably more need to use the collectors to cool the pool.
Pool collectors basically extend the pool season a few weeks on either end. Open sooner, run later in the fall.

In essence GEO is transferring solar energy. By running through ST you get the efficiency gain. At the cost of running a couple pumps.

Pool collectors are typically EPDM or some composite. So glycol should be acceptable.

Solar Roll which turned into Radiant Roll was a mat of small diameter EPDM tube.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,592
An unglazed collector would not be very useful in the winter months, probably around 70F ambient or lower, efficiency plunges. Just plug the numbers into the formula, the fluid temperature you want and the ambient, plot it on the unglazed collector line.
I’d be thinking 30-40F inlet fluid, keeping the GSHP COP a touch higher at the lower end of the COP vs fluid temp range.
• Member Posts: 20,725
the colder the inlet fluid, the less loss to the sky and the higher the efficiency

Similar to a mod con boiler😗
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream