Why almost no one doing Solar Thermal?

[Crowz]

I never had any luck as I mentioned before with water but this new brick stuff to has a ton of promise.

Room Unit - Steffes

www.steffes.com

The wife and I are kicking around the idea to get some of these before winter gets here. If they work as claimed/we hope they do it would solve the winter season piece of the puzzle in our setup.

Anyone else using it?

 

[jrcromer]

I'm a big fan of "heat bricks with excess energy" ideas, and PV outperforms solar thermal in winter on this one.
You still need sunshine, but not as much, to fundamentally put a space heater in front of a bunch of bricks and all it "active thermal".

so do that.

but I'm specifically calling out the industry for turning a blind eye as to how to remove heat from the photovoltaic solar array during summer and do anything cost effective with it. Which is a shame.

 

[wattmatters]

Maybe... just maybe.... everone thinking about PV thermal is doing a really bad job at thinking about pragmatic solultions with actual paths to market. Heat pumps are doing amazing, yet solar thermal sits on the sidelines looking like an overpriced, never MVP.

Solar thermal residential water heating was pretty common in Australia through the 1980s-2000s. There are 100's of thousands of these systems still in use and they are still sold. Such systems have had decades to refine and improve. There's just not much performance and cost improvement left to eek out.

In the 2010s solar PV costs dropped dramatically and nowadays the solar thermal water heaters are more expensive than PV + heat pump water heaters, and significantly more expensive than PV + diversion to resistive element water heater.

Accordingly, sales of solar thermal water heaters have tanked. There are still use cases where they make sense, e.g. if for some reason you are not roof space limited but restricted on the amount of PV you are permitted to install. Else, PV is just a far more flexible use of energy capture roof space. Once a solar thermal system has completed a heat cycle, then it can do no more useful work.

In Australia grid tied PV systems can export excess production for some bill credit. The export credit per kWh is much less than the import tariff, so the incentive is there to self consume, but even if you don't then you still get some value from the excess generation (and it reduces use of coal power in the grid here).

I use a smart PV diverter with a 315 litre stainless steel resistive element hot water storage tank. The diverter adjusts power delivered to the heating element based on available excess solar PV capacity. Works especially well on cloudy days:



In our case our average daily water heating demand is 5 kWh. When water heating consumption is modest, then spending the extra on a heat pump water heater make little sense. I haven't imported any grid energy for water heating since October last year.

 

[jrcromer]

I think solar thermal makes lots of sense, where appropriate for the application. It's very efficient.
I also think PV as an inefficient heat collector has not been fully considered.

I think past designs tried to integrate solar thermal and PV and went too far.
I don't think anyone has considered more simply plumbing the racking.

 

[wattmatters]

It's very efficient.

PV + heat pump is as well.

 

[upnorthandpersonal]

PV + heat pump is as well.

And you can use the heat pump in summer when you want cooling instead of tons of hot water.

 

[OzSolar]

In response to his idea, the spreaders would be for placing under the solar panels to cool them

~15 years ago I installed a 70 kW (or thereabouts) PV system that used radiant in floor heat transfer plates on the back of PV modules to collect heat (looks for the pipes on the back of the modules) on a LEED Building.

It was supposed to be Net Positive with all sorts of interesting features including an attempt at seasonal heat storage in geothermal loops. The entire building was quite the science project but like most LEED buildings I was involved in there was very little thought put to future maintenance so the last I knew it's all but dead now.

 

[sunsurfer]

~15 years ago I installed a 70 kW (or thereabouts) PV system that used radiant in floor heat transfer plates on LEED Building.

It was supposed to be Net Positive with all sorts of interesting features including an attempt at seasonal heat storage in geothermal loops. The entire building was quite the science project but like most LEED buildings I was involved in there was very little thought put to future maintenance so the las i knew it's all but dead now.

Hey Ozman, how far are you from Ava? I'm assuming you are in Missouri(MO).

That is interesting on the LEED buildings. Good idea on paper, bad idea in practice. Its going to be interesting to see the crazy ideas that come out when panels get expensive again when silver gets pricy.

 

[OzSolar]

Hey Ozman, how far are you from Ava? I'm assuming you are in Missouri(MO).

That is interesting on the LEED buildings. Good idea on paper, bad idea in practice. Its going to be interesting to see the crazy ideas that come out when panels get expensive again when silver gets pricy.

I am in the very SW corner of MO near Joplin.

It's been really fun to watch this thread. I cut my teeth on solar in the early 90's (true story) and thermal was pretty much all I did. It's interesting to see people's perspective on solar thermal today. Being an "old timer" I still clearly remember a time that your only option for solar was thermal or passive. PV was the stuff of dreams. It's taken me years to rewire my brain that PV made $ense. I'm shocked on a nearly daily basis by how much PV has penetrated the market in the last decade.

 

[spazatak]

I enjoyed reading this discussion because there's an ongoing debate about PV versus thermal. What I only heard once mentioned in this discussion was solar air heaters. Here's a pretty cool project where they used solar air heaters to heat the slab of the house which then acted as a battery in the evening.

Passive solar air heater.

 

[rogerdw]

I built a solar air heater several years ago and it is on its third winter now. It is basically 18 of the evacuated tubes normally used for heating water but without the copper collectors inside (didn't come with the batch of tubes I scored).

There's a thread over on thebackshed forum and there's a picture here ... https://www.thebackshed.com/forum/ViewTopic.php?TID=13675&P=8#168642

Air is drawn out of the house into a manifold which feeds into the tubes ... and the warmed air in the tubes is forced into an outlet manifold and back into the house. Simple and crude.

The concept works well enough, though my example is not big enough for our open space home. On dark cloudy wintery days it is not really effective ... but on many days where it can be cold, windy and blustery but still with some sunshine ... we get quite decent results.

It probably works best leading into and out of winter where it adds enough warmth to allow us to reduce our wood consumption by about a third. There have been times when the inside temp has got up to 28 degrees C and we've had to turn it off ... but normally it operates completely autonomously with a differential controller making all the decisions.

There's a temp sensor inside the tubes and another in the house. I ended up setting it so that if the temp inside the tubes reached just 12 degrees above inside temp, the fan would come on and blow the warmer air into the house. When the temp (inside tubes) dropped down to just 5 degrees above inside temp, the fan would turn off.

I did start a large version to fit on the roof, but in my slackness and getting sidetracked, have not completed. The new one has 60 tubes and I am quite confident it will be worthwhile completing and installing. I'm looking forward to shifting the small one down to my workshop which I believe would be a perfect fit and keep it a lot more comfortable in winter.



I had to replace the thermistor inside the tubes after summer because the temp gets into the hundreds of degrees C and destoyed it ... and when I turned it on that day, the air blowing into the room was 113 degrees. Would burn ya face off. ? It took an hour or so to drop to 50* and sat on that for several hours.

Here's some pix of the new one ... It has a manifold down the centre and the concept is the same ... draw air in one end and out the other ... but the tubes are laying flat on the roof ... parallel to the gutter

 

[Matt.B]

This is likely due to the flexibility of solar PV and their efficiency. You get electric from solar PV, which can be used to heat water if you wish...or for anything else.

 

[67King]

Anyone else an Alton Brown fan? He has this thing against tools that only have one purpose. Calls them "uni taskers." They take up far more resources (space in the kitchen) than devices that are used for multiple purposes. Well, solar thermal water heaters are a uni-tasker.

I looked briefly at this for my (planned) pool. Avid swimmer, looking for a lap pool, so about 2X the size of the average homeowner pool. 75X14 surface area. A system that would make my pool usable year-round was, off of memory, about $30K. That's more than a 16.2KW system with 30KW-Hr of battery backup.

 

[Crowz]

Anyone else an Alton Brown fan? He has this thing against tools that only have one purpose. Calls them "uni taskers." They take up far more resources (space in the kitchen) than devices that are used for multiple purposes. Well, solar thermal water heaters are a uni-tasker.

I looked briefly at this for my (planned) pool. Avid swimmer, looking for a lap pool, so about 2X the size of the average homeowner pool. 75X14 surface area. A system that would make my pool usable year-round was, off of memory, about $30K. That's more than a 16.2KW system with 30KW-Hr of battery backup.

Oh yeah, I watch good eats every during every meal I eat