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Old 03-30-12, 08:06 PM   #1
Student 07
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Question Solar Thermal is Dead?

Hi,
I was reading an article and found it interesting. I would like to hear some different views.

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

The article compares Solar Thermal and using Photovoltaics to operate a heat pump.

There are some drawbacks to a solar thermal system: possibility of leaks, cost of pumps & plumbing, storage of heated water, (for some people) Glycol anti freeze maintenance, (for some people) weight of system, stagnation in summer, low output in winter.

The price of PV's has come down significantly, so combining them with a very efficient heat pump is now very competitive. PV's dont leak, they are lightweight (comparatively) and need very little maintanence (cleaning). PV's can't store heat like a solar thermal system; however, excess energy can be sold to the local utility. Excess energy isn't diverted to a heat dump, it is sold. PV's don't have a problem with stagnation. PV's generally perform better under cloudy conditions. PVs can be mouted on a tracker to get even more energy. I am sure there are other benefits of a PV array over a solar thermal array.

The only drawback I can see is that there is no storage of energy (which I think is renewable energy's achilles heel)

In my opinion PVs combined with a very efficient heat pump could produce more hot water, for more months, than a solar thermal system. Without all the maintenance. I suspect that as the price for efficient heat pumps and PVs come down, this will become even better from a financial stand point.

Does anybody have experience from comparing the two systems?

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Old 03-30-12, 11:12 PM   #2
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Solar Thermal can be made by anyone that is a bit handy using parts from homedepot. PV's can not be made at home.

Solar Thermal can be relatively easily repaired compared to PV.

Homemade Solar Thermal can be very competitive performance wise compared to commercial Solar Thermal products.

Homemade Solar Thermal can be far less expensive then commercial Solar thermal and PV's.

But yes if you just want to sign a cheque and have someone install a system with full warranty a heatpump PV system could be very competitive depending on your location, requirements and the components selected.
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Old 03-31-12, 12:09 AM   #3
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With the PV and heat pump set up, the heat pump is going to be the weak link while the PV panels are going to last 25 to 100 years, while I can't think of anything in a solar thermal system that is going to last that long other then the basic piping, your pumps will wear out, your thermostats will wear out, seals will leak, hoses brake down.
Yes, you can build solar thermal panels with hard ware store parts but how long will they last? how good are they? you buy panels and they have low iron tempered glass, aluminum fins fused to copper tubing, anti-reflective coatings, it's kind of like saying that you can buy all the parts to build a car at the hard ware store.
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Old 03-31-12, 08:33 AM   #4
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While I agree the gap probably is closing on HP+PV vs SHW, I fail to see how HP+PV can provide 100% of your DHW as this article seems to insinuate. It says the SHW systems only provide ~62% of the DHW needs which is probably quite true of a standard system. But, how can a HP+PV system provide hot water during a week that is cloudy?!? It can not, and I'm not sure why this point has been entirely glossed over in the article. Perhaps because the HP can still be run on mains power and you don't need another form of backup heat? But, this would still reduce the fraction of solar provided heat...
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Old 03-31-12, 12:27 PM   #5
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I read this article when it was first posted, so it's been a while, but as I recall he does make some provisions in there for DIY solar thermal. Am I imagining this?
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Old 03-31-12, 03:46 PM   #6
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Default Solar Thermal is Dead...

I just read the article. Very interesting...

It just so happens that Charlie Stevens, who formerly was the policy analyst for the Oregon Department of Energy, and who was quoted in the article happens to be a friend of mine. I would say that in matters pertaining to energy, Charlie Stevens is by far the brightest, best informed guy I ever met. I have never heard Charlie give bad advice regarding energy. Ever.

In fact, he said the same statement to me as was quoted in the article, but I remember the whole thing, and the author of the article did not quote the whole statement in context.

Charlie was specifically talking about optimizing for Western Oregon weather patterns, and his advice is particularly well suited for the Willamette valley, where Portland (my fair city) is located.

Portland has rather mild winters, and the HDD is somewhere in the neighborhood of 4500 annually. This means that ambient air temperature is almost always very favorable to ASHP use. So in this case, an ASHP would be a very good way to go.

Winter time solar in Portland is very spotty, to say the least, but when winter weather patterns pass, solar energy is a highly reliable option.

So the take away is mild winters temperatures and very little usable winter sun.

Charlie's advice is predicated on the practice of selling the excess summer time solar energy to the grid. In effect, using the grid as a 100% efficient battery... can't beat that.

Charlie always advocated ASHP for Western Oregon, because of the favorable winter temperatures, and because GSHP installation in Portland is very expensive because of the soil formations, and the high likelihood of hitting ancient river rock cobbles.

When I told him about the Homemade Heat Pump Manifesto, he agreed with me that something like that was a real game changer, I also agreed with him that it wasn't for the typical home owner.

If someone lived in an area where soil conditions favored lower cost GSHP that could influence things heavily in favor of PV.

If someone lived in an area where winter temps were quite low and winter sun was reliable (like Colorado) and GSHP installation was prohibitive, then Build it Solar is your best friend.

So, Is solar thermal dead? The playing field is beginning to tilt in favor of PV, but the playing field is not flat. With the cost of PV coming down as it is, PV is looking better.

But there are many factors to consider.
  • Winter temperatures
  • Winter solar availability
  • Price of PV
  • Price of ASHP
  • Possibility of GSHP
  • Solar Thermal DIY
You really need to do the numbers.

And, as always, insulate, insulate, insulate.

-AC
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Old 03-31-12, 06:34 PM   #7
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I can kinda compare the two systems. I once had a solar hot water system.
It worked pretty well, but the heat exchanger & the storage tank started to leak.
I think the local water was the main problem. It tends to eat copper.
Eventually, I got tired of trying to repair it (so it wouldn't flood the basement).


I do have some PV running now, but I'm not using it to power my Sanyo Mini-splits.
Mostly, it's for a backup system (for grid failures) and 800w that's adding some heat to my hot water storage.

All our heating is done with ASHP (unless temps get below 10F for a while).

At 16 cents per KWH, it's not really worth it to install PV on the roof, to power my (2) Sanyos.
Last billing cycle cost was $62 (with the heat on 24-7 @ 20-21c) during the cold months, it was about $100 per month.

For heating and cooling I'm guessing our overall average is going to be somewhere around $50 a month. (@10kwh per day).

I could put up a PV & GTI system that could harvest 10 kwh ave per day,
and save me $50 a month (during very sunny months)..
But, I can see a lot of other places where I could save a couple bucks
per day, without a lot of expensive hardware.

Since there are only two people in this house, do we really need
three HD cable boxes? Is the DVR really necessary?
Do I really need this kind of internet speed?

Why does anyone need to be faster than 87% of the US??

We could cut out a few restaurant meals and easily save $50 a month..

Really don't need that PV just yet.. Have to wait a couple years for 25 cents per kwh..
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Old 04-01-12, 11:24 AM   #8
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Quote:
Originally Posted by Xringer View Post
For heating and cooling I'm guessing our overall average is going to be somewhere around $50 a month. (@10kwh per day).
Don't forget that a PV array on your roof would provide shading and reduce your cooling load.

-AC
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Old 04-01-12, 03:03 PM   #9
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Quote:
Originally Posted by Ryland View Post
the PV panels are going to last 25 to 100 years. .
Not at full capacity they won't. From the performance studies I've read they calculate that you lose roughly 1% performance per year due to UV damage and due to the break down of materials within the modules. Sure, they'll still be functioning after 50 years (maybe) but at only ~50% of design capacity. That's important to keep in mind.
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Old 04-02-12, 12:11 AM   #10
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...they'll still be functioning after 50 years (maybe) but at only ~50% of design capacity...
I took 100 (as in 100%) and multiplied it by .99 (as in 99% left in the second year) and multiplied THAT by .99, etc., etc. 50 times and ended up with 60.5 (as in 60.5%) by the 50th iteration.

-AC

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