Solar voltaic vs solar thermal

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Dune

Dune

Minister of Fire
Jan 14, 2008
3,145
Commonwealth Of Massachussetts
www.irondesign-ne.com
Getting ready to pull the trigger on a solar installation.
I tell the sales rep I want solar hot water as well as electric.
Instant resistance.
Sales guy is telling me that making hot water with an electric heat pump is more efficient than solar thermal.

Now to me this doesn't add up no matter how one arranges the numbers.

Since I have a water back in my stove, I especially don't care about diminished performance in winter months.
I would likely just drain the solar hot water system down in at the start of the heating season anyway.

Not a fly by night company, in business for 30 years.

It seems to me that the big push is solar voltaic in spite of the customer's best interest.
 
I happen to have both and installed both and I am telling you the sales guy is right on. I have lived with SHW for 15 years in a cold climate and the fundamental issue is its lot easier to store electrons through net metering than storing a winters worth of hot water from the summer. A standard flat plate SHW collector is limited to a 80 degree temperature rise at best over ambient air conditions. Lets assume you can live with 140 degree hot water storage temp. You are in great shape if its 60 degrees out but not so good when its 20 F. Sure you can go with evacuated tube collectors for a big premium and get higher output temperatures but you need a lot of them and they are notorious for loosing their vacuum over the years. Most of the legit evap tube companies went bankrupt long ago due to guarantee claims and the last time I looked the only sellers were Chinese firms selling on Ebay. The other big issue is when you want hot water the most in the winter you struggle to have it and in the summer when you don't need it you have more than you can use and have to intentionally dump heat so the glycol doesn't overheat. Front end cost is also something to consider. Flat plate SHW collectors are usually copper fins on copper pipes attached to copper headers. They are expensive plus you need a solar storage tank, pump and controls. Given that you are installing PV, its a lot easier to just install a few extra panels to compensate for the load from Heat Pump Hot Water heater which you probably can get a rebate from the power company for installing.

I use my SHW system for primary hot water from about April to the end of October, its great as I don't need to run a boiler to heat hot water and I use it as source of preheated warm water for winter use. It works but given the up front cost and operating limitations I would not recommend it to anyone for year round domestic hot water although folks with warm season hot water demand like pool owners may have a use for it,.
 
peakbagger said:
I happen to have both and installed both and I am telling you the sales guy is right on. I have lived with SHW for 15 years in a cold climate and the fundamental issue is its lot easier to store electrons through net metering than storing a winters worth of hot water from the summer. A standard flat plate SHW collector is limited to a 80 degree temperature rise at best over ambient air conditions. Lets assume you can live with 140 degree hot water storage temp. You are in great shape if its 60 degrees out but not so good when its 20 F. Sure you can go with evacuated tube collectors for a big premium and get higher output temperatures but you need a lot of them and they are notorious for loosing their vacuum over the years. Most of the legit evap tube companies went bankrupt long ago due to guarantee claims and the last time I looked the only sellers were Chinese firms selling on Ebay. The other big issue is when you want hot water the most in the winter you struggle to have it and in the summer when you don't need it you have more than you can use and have to intentionally dump heat so the glycol doesn't overheat. Front end cost is also something to consider. Flat plate SHW collectors are usually copper fins on copper pipes attached to copper headers. They are expensive plus you need a solar storage tank, pump and controls. Given that you are installing PV, its a lot easier to just install a few extra panels to compensate for the load from Heat Pump Hot Water heater which you probably can get a rebate from the power company for installing.

I use my SHW system for primary hot water from about April to the end of October, its great as I don't need to run a boiler to heat hot water and I use it as source of preheated warm water for winter use. It works but given the up front cost and operating limitations I would not recommend it to anyone for year round domestic hot water although folks with warm season hot water demand like pool owners may have a use for it,.
Click to expand...
Thanks but I am only planning on using it for 6 months per year, then draining it down.
I already have a 120 gallon tank with pump and control for the wood stove water back anyways.
 
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Dune said:
Thanks but I am only planning on using it for 6 months per year, then draining it down.
I already have a 120 gallon tank with pump and control for the wood stove water back anyways.
Click to expand...
What storage tank do you have? I'm looking at doing something very similar this fall. My house has a non functional SHW device on the roof, but the previous owners bypassed it. We need a new hot water heater regardless as ours is a resistance unit from 1975. My plan is to put in a Kitchen Queen 380 or another cook stove with a UL listing and DHW capacity, a new rooftop solar collector, and a propane on demand water heater. I'm limited on space, but we currently have a 50 gallon HW tank, and a 50 gallon HPWH might be a tight fit and also on my radar. I'm pretty much sold on having SHW April - October and using the cook stove in the cold months. We might never use any propane with SHW and the cook stove. The HPWH seems less appealing since it takes up a lot of space and then we still need another tank for the cook stove.

I attached some photos of the utility room and the room above it. The room above the utility room is like an attic with plumbing access for the bathroom.

Sorry for thread jacking, but there's not a lot of information out there!
 

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Sounds like you didn't want an opinion but confirmation on a special case. Good luck finding a legit company to sell you what you want as most of the resellers don't even stock SHW equipment. Probably better off looking around on Craigslist for a good used flat panel set that folks have taken down and want to get rid of. Just make sure you confirm its a copper plate absorber and it will hold pressure as overheated antifreeze can turn acid and eat the panels from the inside out. (which on occasion happens with abandoned systems. IMO steer clear of used evacuated tube systems and given your summer only operation you really don't need them as 80 F temp rise should be adequate.

BTW I use a zone of my boiler system in the winter to get a final high temp (180 F) and use the lukewarm water from the SHW panels to blend it with the really hot water I generate from the boiler. Worse case is my SHW tank has a heater element that I can use. ​
 
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The other aspect I thought of is the solar dealer probably doesn't even have the trades to support a SHW installation. PV needs electricians and SHW needed plumbers.
 
peakbagger said:
Sounds like you didn't want an opinion but confirmation on a special case. Good luck finding a legit company to sell you what you want. Probably better off looking around on Craigslist for a good used flat panel set that folks have taken down and want to get rid of. Just make sure you confirm its copper plate absorber and it will hold pressure as overheated antifreeze can turn acid and eat the panels from the inside out. IMO steer clear use used evacuated tube systems and given your summer only operation you really don't need them as 80 F temp rise should be adequate
Click to expand...

I was actually basing a lot of my ideas around reading about your setup. You haven't used fossils fuels for hot water or heat for five years and I like that. My house already has the plumbing in place, but perhaps that won't offset the cost enough. You are right, there are few suppliers of these kinds of systems. I was going to see what the local solar contractor would offer and see what I could do elsewhere and shop around, but there isn't much shopping around to do. Should I just go with a 40-60 gallon storage tank and just use the cook stove and the on demand heater? My wife and I like the idea of on demand hot water vs the HPWH after talking about it more. The HPWH does have rebates, but my wife really does take a ton of baths. After reviewing all of my usage data from Emera I can really see where the power goes. Sometimes she takes two baths a day and maybe even a shower, and this will never change. This makes me think the on demand water heater will be the most efficient since the HPWH will be running in resistance mode most of the time. I would really like to make some moves to reduce our electricity usage soon.
 
Yeah, I assumed the plumber would be the one installing the SHW collector.
 
my last place had a solar thermal air heater. It was state of the art in 1978 when the house was built. Fortunately for me, the previous owners had gone away on vacation and had a pipe freeze, so they had to redo the basement. They removed the concrete storage tank, but left me with all of the ducting.

When I was tearing it out, I did notice that there were areas where I could see how the system was connected, and after talking to some of the owners, they had all had issues with their systems overheating the ducts, etc. It was really odd how they had it set up and tied into the furnace, but it was cool to look into more.

I thought about doing a Solar thermal water heater on our place, but after looking at the costing, it made more sense for me to put pv on the roof and do an electric on demand water heater... 4 years with that system and its been great!
 
Dobish said:
my last place had a solar thermal air heater. It was state of the art in 1978 when the house was built. Fortunately for me, the previous owners had gone away on vacation and had a pipe freeze, so they had to redo the basement. They removed the concrete storage tank, but left me with all of the ducting.

When I was tearing it out, I did notice that there were areas where I could see how the system was connected, and after talking to some of the owners, they had all had issues with their systems overheating the ducts, etc. It was really odd how they had it set up and tied into the furnace, but it was cool to look into more.

I thought about doing a Solar thermal water heater on our place, but after looking at the costing, it made more sense for me to put pv on the roof and do an electric on demand water heater... 4 years with that system and its been great!
Click to expand...

Would you suggest electric on demand over propane on demand?
 
SpaceBus said:
Would you suggest electric on demand over propane on demand?
Click to expand...
we went with the electric because our location did not allow for any venting.
 
Thanks for the various perspectives folks.
I will let the installers place the largest electric system possible then I will tie in a used collector to my existing tank. There is zero doubt in my mind that this is the most effective plan, since I already have the tank, pump and control and draining it for the winter avoids needing antifreeze.
 
peakbagger said:
Sounds like you didn't want an opinion but confirmation on a special case. Good luck finding a legit company to sell you what you want as most of the resellers don't even stock SHW equipment. Probably better off looking around on Craigslist for a good used flat panel set that folks have taken down and want to get rid of. Just make sure you confirm its a copper plate absorber and it will hold pressure as overheated antifreeze can turn acid and eat the panels from the inside out. (which on occasion happens with abandoned systems. IMO steer clear of used evacuated tube systems and given your summer only operation you really don't need them as 80 F temp rise should be adequate.

BTW I use a zone of my boiler system in the winter to get a final high temp (180 F) and use the lukewarm water from the SHW panels to blend it with the really hot water I generate from the boiler. Worse case is my SHW tank has a heater element that I can use. ​
Click to expand...
peakbagger said:
The other aspect I thought of is the solar dealer probably doesn't even have the trades to support a SHW installation. PV needs electricians and SHW needed plumbers.
Click to expand...
These guys did solar thermal for 30 years but haven't done one in years now.

For me though, buying a used collector is a lot cheaper than a heat pump and I can install it myself.
 
SpaceBus said:
What storage tank do you have?
Click to expand...
I have a 120 gallon SuperStor with an internal heat exchanger.

I think 80 is minimal and 150 is more ideal unless you have a backup.

By the way, the folks in the boiler room have the info you are looking for.
 
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Dune said:
I have a 120 gallon SuperStor with an internal heat exchanger.

I think 80 is minimal and 150 is more ideal unless you have a backup.
Click to expand...

What is your stove setup? I like the idea of using the indirect heater instead of a range boiler.
 
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I have spent most of my life building and selling solar thermal heating systems. Have also sold heat pump water heaters.
A lot of claims are made that solar electric with a heat pump is more efficient.
I think solar thermal has better efficiency.
That being said, solar electric with a heat pump is probably more reliable.
But, you can deal with service on a solar thermal system yourself, or at least, a basic plumber can.

While I am on the subject, vacuum tube systems usually perform no better than flat plate collectors. Probably a bit less efficiently for DHW.
And flat plates are repairable. Vacuum tubes are not. There are enough studies, from disinterested third parties that corroborate this. Google it. There are systems online that show side by side tests.

Contractors prefer solar electric hp systems because they only get a phone call when the heat pump dies or an inverter lets go. How often is that?
We will do a solar thermal system for our new shop radiant flloor. It is less expensive and we can keep it simple. Most thermal systems tend to get complicated and expensive with off the shelf things like expensive pump stations.
 
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PV direct heating of water is by far more efficient and useful than solar thermal, either by heat pump or resistance. There are plenty of informative youtube vids out there comparing and contrasting the two.
 
I do not want to start a flame war on this issue, but solar thermal can run 20-80% conversion efficiency.
PV's operate up to maybe 20%.
That is based on surface area that intercepts sunlight.
Actual cost might make PVs more affordable. Depends on a lot of factors.
Someone can do a DIY solar thermal system quite affordably. (www.builditsolar.com)

As I said earlier, solar electric systems are simpler with less maintenance required. That is assuming that controllers (and heat pumps when used)
last or at least are not orphaned (that is, lose their warranty when the company disappears.)
 
Hi Tom, I really would like solar DHW to work for me, but in my winter cloudy Central NY climate, it just doesn't pencil out. In the winter, when you want sunshine the most, we get very little sunshine - I would have to way oversize the system to get anything useful for DHW and perhaps for space heating (radiant floors) as well. Then, in the summer, I would have way too many BTUs when I didn't really need them. The cost would be high, the complexity would be higher than doing just solar PV, and I had a feeling that the first time it needed maintenance or repairs, I would just take it off-line. I literally had a big spreadsheet modeling various cloudiness scenarios with reduced solar insolation, and the BTU numbers just weren't there.

Consider also that I really need the heat in the winter for my radiant floors and higher DHW needs (heating colder incoming water), and in the summer really wouldn't need it too much. In fact, in the summer, I will be running the dehumidifier in the basement and a heat pump water heater could take most of this dehumidification load as well while heating my hot water.

Unless I home-built everything (something I have little time to do), I would still be in for $2-3k for the minimum number of collectors, pipes, pumps, controls, etc. I have the solar PV already, it's net metered (unlike in Maine), and I can get a HPWH for $1200 or so (and I'll need a backup water heater anyways), so it just comes down to what is the best way to spend my money.

Sure, it would work if you had more sunshine in the winter than we do here in Central NY. If I lived in New Mexico or even closer to NY City, the economics might change a lot.

The good news is that if I get the tinkering bug, I can still add it later. I have a 180 gallon storage tank for my ground-sourced heat pump (radiant floors) with a heat exchanger at the bottom and top of the tank. The top heat exchanger will serve as a cold water inlet to the DHW tank, and the bottom could someday be a heat exchanger for a solar DHW system. I planned for it, but just have no current intention of adding it later. We'll see how the GSHP works, but so far that just seems so ridiculously efficient at heating water that I'll probably just let that do much of the DHW pre-heating work as well.
 
tom in maine said:
I do not want to start a flame war on this issue, but solar thermal can run 20-80% conversion efficiency.
PV's operate up to maybe 20%.
That is based on surface area that intercepts sunlight.
Actual cost might make PVs more affordable. Depends on a lot of factors.
Someone can do a DIY solar thermal system quite affordably. (www.builditsolar.com)

As I said earlier, solar electric systems are simpler with less maintenance required. That is assuming that controllers (and heat pumps when used)
last or at least are not orphaned (that is, lose their warranty when the company disappears.)
Click to expand...

Under ideal circumstances, I think you might be correct. Under sub-optimal though, the PV will out perform solar thermal. At least that's been my experience.
 
I just called our local solar installers. They said they haven’t done a hot water instal in over a year. His take was system maintenance would increase break even point beyond the warrantied life of the system. The just don’t have good enough sun to put in a decent sized pv array. I asked if they ever did off grid DC direct to water heater installs and he said no. Sent me to a DIY site. Really seams to me that removing the the inverter and and extras to tie it to the grid would make PV hot water systems much cheaper. Down side is that you, if/when your water is hot you can’t use the PV power. A small trade off for a very simple system.

By my napkin math a 1kw PV system would never heat 40 gallons of water in a single day (I have four kids under 9, 40 gallons of hot water barely does laundry and dishes each day). My very hypothetical plan would be an 80 gallon heat pump water heater in series after a 40 gallon standard electric unit that is equipped with DC elements. Seems really reasonable. I don’t understand why this isn’t being sold as a viable option. Heck add a third DC element to the tank of a heat pump water heater and charge me an extra 350$, then add on the PV panels.

I am seriously investigating this option.

Evan
 
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By my napkin math a 1kw PV system would never heat 40 gallons of water in a single day

Are you sure about that?

From watching our bills since it was put in 7 years ago, & comparing to when it isn't used, our electric tank water heater uses in the area of $30/mo of electricity.

That's around $1/day - at 0.18/kwh, that's around 5 kwh/day. Which should be within the capabilities of a 1kw PV system? Maybe not every day - but certainly on clear ones?
 
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maple1 said:
By my napkin math a 1kw PV system would never heat 40 gallons of water in a single day

Are you sure about that?

From watching our bills since it was put in 7 years ago, & comparing to when it isn't used, our electric tank water heater uses in the area of $30/mo of electricity.

That's around $1/day - at 0.18/kwh, that's around 5 kwh/day. Which should be within the capabilities of a 1kw PV system? Maybe not every day - but certainly on clear ones?
Click to expand...


Nope not all;) I was pretty conservative on the output as we get morning and late afternoon shade. It seams plausible that you could could use all the PV power from a small array just to heat water most of the time. So my never statement is probably inaccurate.
 
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