Adding Solar to system?

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VtRv

VtRv

Member
Jun 3, 2008
66
Morrisville, Vt
I have a Tarm Solo Plus wood boiler and 500 gallons of storage in a pressurized system. I've been thinking off adding solar to help with the DHW outside of the heating season. My current DHW is a Super Store hooked up as a zone off my system.

I'm wondering if it would make sense to get a dual coil hot water heater or what it would take for solar panels to give me enough usable heat in my 500g heat storage tank to heat the water. My storage is an old LP tank insulated and setting horizontally in my basement.

Any thoughts or advise would be appreciated.
 
I considered that but after looking at numbers & payback decided it just wasn't worth it. My ordinary 80 gallon hot water heater costs us about $25/mo in electricity. A HPWH should be quite a bit less than that again. I think a DHW solar setup here runs around $5k. Long payback time there. Even if that could be reduced by half by incorporating existing stuff like storage and DIY some of it, I was figuring 10+ years conservatively estimating.
 
I have a flat plate SHW system for spring summer and fall hot water. It was standalone system with a 80 gallon storage tank with one set of elements (which I have never hooked up). My hot water is an Amtrol (same as a super store) tank fed off a hot water zone. In the winter the SHW acts a preheater and in the spring summer and fall just valve out the Amtrol tank. There are some other details but I wouldn't recommend SHW with your system. It could be done but price of the panels and support equipment is steep compared to the alternatives which is heat pump hot water heater with a couple of PV panels hooked up to net meter (if its available from your utility).

if you just want SHW out of general principle, you need to look for evacuated tube type panel not flat plates as flat plates just don't put out hot enough water, they put out plenty of warm water but at best they put out water about 80 degrees F higher than the outdoor temp. Evac tubes can easily go 150 degrees. They are more expensive and there have been numerous issues with their longevity and many folks have had to replace them on and off warranty s the seals fail.
 
My GUESS would be that PV panels, supplying a HPHW would be better spent money.

JP

It's the way I went. Never seriously looked at solar hot water.
 
Are there any rebate or incentive programs in your area? www.dsireusa.org is a place to look

That, coupled with fuel cost, federal tax incentive, amount of dhw you use, etc. Every application is different as far as ROI.

If you can do the install your self that can make a difference also, on how it pencils out. Watch craigslist for used collectors. Good copper collectors don't wear out, and they sometimes get removed to re-roof and don't always go back up. You already have a tank, so a pump, hx and a control gets you going. You should be able to get a 50% solar fraction in your neck of the woods.

I prefer the flat plate style collectors, they last for decades and provide adequate temperature for DHW. I get 167F from mine in the summer and had 105F here this week with temperature in the teens. It has a lot to do with collector area to tank size. Typically 1.25 to 1.50 gallons of storage per collector square footage. if you have an 80 gallon tank a 4X8 or 4X10 would provide good performance.

Evac tubes have better insulation with the vacuum space, and perform better when ambient temperatures drop below 20F, providing 105F output. Evac tubes can also present over-heating issues during summer months, many are piped with dump zones.

Solar showers just "feel' better :)
 
If you really want SHW, I agree that evacuated tube will do that. No comment on cost. Deep Portage, where I volunteer, has a 300 tube system, northern MN climate and weather. With full sun, pretty much regardless of outdoor temperature, it will produce about 75000btuh of hot water per hour, that's 7500 btu's per 30 tube section. It also produces hot water in cloudy weather and even through light snow covered tubes.

The system is designed to start providing hot water beginning at 160F, and flow is regulated to maintain this minimum temperature of water output.

Lots of problems on the initial install, professionally designed but lots of mistakes. Those were all corrected this past summer, and since then, including this winter, operation has been trouble free and meeting expectations.
 
Thank you all for the input. I can see I have a lot more research to do.
 
You can get flat plate collectors that will output similar heat in cold weather to most evacuated tubes. in the shoulder seasons and summer, I'd stick with flat plates. beware the longevity and durability of lots of evacuated tubes. they are not all created equal. I've been designing, installing and servicing solar thermal systems for 15 years, and have always stuck with flat plate collectors. if I was going to go with tubes I'd use veissman. if you're using Flat plates, you have a huge array of decent, and lots of really good equipment available.
karl.
 
I bought solar vacuum tubes 2 years ago in hopes to heat my house and shop, or too at least help heat the house. I put 4 panels on the house and 3 on the shop. I did a VERY good job installing these and can comment with certainty that it cannot heat your house with forced air. On the other hand, if I had in-floor heating then I could use a lower grade heat 35-40 C. These panels will put out these temperatures all day long when the sun is out. If you use these panels properly they work great, but unfortunately they cannot do forced air. Put it this way...I am taking the panels down this spring and I am going to install a waste oil boiler instead. It really bothers me as I have 20K invested in this and it will not work with my application.

I can tell you this, it does do all my domestic hot water with ease because its low grade heat.


images
 
Just be aware in snow country you will need to brush the vac tubes after every snow. I visited the Living Campus in Ontario this week, they have some amazing test projects going on there with solar thermal and PV, ground loops, air to water, fun new products, even a boiler with a Sterling engine and salt storage.

This vac tube array has been under snow for about six weeks, little to any production. The flat plate was de-iced and working by 10:30 or so. Steeper pitch helps, but other ground mount arrays on site at 55 slope were also snowed under.

Also every tube has been replaced at least once in the 4 years it has been up.

Just mentioning that they are a bit more maintenance and more fragile, there are a top brand German tube.

Good data at www.solarishot.com on a couple tube and flat plate arrays in upstate NY. You can get into his data logger and compare about 7 years worth of dat comparing the two side by side systems. Surprisingly the plates collectors outperform the vac tubes in many months.
 

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I did quite a bit of research and found overall that flat plates were the best choice in my opinion, all things considered. For me durability weighs heavy on any purchase. If you're looking to do domestic water only and you have a wood boiler, you really only need solar in the warm months anyway. You can get a good deal on used flat plates if you search CL or classifieds. Solar domestic (warm season) provides the most bang for the buck and a reasonable ROI.
 
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I like the concept of evac tubes and they do have a high tech space age look that some affluent "greenies" seem to want (sort of like a stereotypical VT Prius owner). For those folks the image is just as important as the actual performance. Unfortunately, the high tech nature of the tubes and expensive manufacturing techniques needed to build them meant the majority of the tubes are built offshore and more than few of the designs just haven't had long term durability. I have encountered many folks who have had to replace multiple tubes generally with a new upgraded design as the original design proved to be less than robust. Generally the distributor folds up shop and the owner is left holding the bag. With a IF gun or camera its easy to spot bad tubes but by eye its not.

Flat plates are relatively boring and far less flashy then evac tubes. The design is quite mature and not that difficult to build, although there was a run back in the eighties when incentives were high and there were some real cheap panels put on the market that failed rapidly and gave the industry a black eye.

If you want to support a hearth.com member, Tom In Maine sells low cost flat plate collectors and would be a good resource. Of course he also sells Heat Pump Hot Water heaters so he will probably convince you to go that route.
 
I was thinking about this topic a couple of years ago and I did some extensive modeling for how I might integrate solar hot water heating into a wood boiler 500+ gallon holding tank. I used solar insolation data for my location (which is probably a bit cloudier though not colder than your location), and modeled it with a variety of different water temperatures, outdoor temperatures, patterns of sunny and cloudy days, etc. The conclusion that the data showed me was that I would need a ridiculous number of 4' x 8' panels (four or more) and, during the coldest times of the year when I needed the heat the most, I would get very little heat, and the usable temperature of the heat would be very low. Then, in the summer, I would have way too much hot water during the time when I would likely be running a dehumidifier in my basement anyways.

The equation changes somewhat if you have a really big family that uses a lot of hot water, or are a "commercial" user (if I owned a laundromat I would consider solar hot water). Then, I think the systems can be cost-effective. The system at Deep Portage that jebatty references sounds like it probably uses a lot of heat and hot water, so it becomes more practical in this case.

If you have solar PV, it is really much more cost-effective to go with a HPWH, and you get the dehumidification of the basement in the summer with that as well.
 
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Any solar system is adversely impacted by snow on the panels, whether PV, flat plate hot water, or evacuated tube hot water. If snow is a factor, major consideration should be given to a ground mount system that allows for easily brushing the snow off the panels. An extended handle floor broom works well. Avoid any solar hot water system that does not use a heat exchanger.

At Deep Portage it would have been desirable to have installed flat plate and evacuated tube systems installed side-by-side for a performance comparison. That may happen in the future, but at this point the evacuated 300 tube system is performing very well, both in summer and during a very cold winter, but no comparison is available. In fact, the system at times performs too well and heat dissipation is required to keep from over-heating the 1000 gallon hot water storage tank. Over-heating is most likely to occur on very sunny days, sometimes several days long, and low facility hot water usage. Over-heating is defined as temperature at top of storage tank > 160F.

The system as originally installed had serious installation issues, those were corrected summer 2014, and so far there has been no quality or durability issues. Minor issues remain with programming the control system to eliminate the potential for tube overheating.

Deep Portage installed this system as an educational learning tool for the benefit of the public and to help eliminate LP fossil fuel usage by the LP hot water heater. My posts on this forum and in other venues are part of the educational effort. LP usage reduction has been dramatic.

The graph shows performance on a sunny day, high facility usage. The system provided solar heated water from about 9:00am to 5:00pm. Solar provided hot water is about 75,000 btuh.
 

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jebatty said:
Any solar system is adversely impacted by snow on the panels, whether PV, flat plate hot water, or evacuated tube hot water. If snow is a factor, major consideration should be given to a ground mount system that allows for easily brushing the snow off the panels. An extended handle floor broom works well. Avoid any solar hot water system that does not use a heat exchanger.

At Deep Portage it would have been desirable to have installed flat plate and evacuated tube systems installed side-by-side for a performance comparison. That may happen in the future, but at this point the evacuated 300 tube system is performing very well, both in summer and during a very cold winter, but no comparison is available. In fact, the system at times performs too well and heat dissipation is required to keep from over-heating the 1000 gallon hot water storage tank. Over-heating is most likely to occur on very sunny days, sometimes several days long, and low facility hot water usage. Over-heating is defined as temperature at top of storage tank > 160F.

The system as originally installed had serious installation issues, those were corrected summer 2014, and so far there has been no quality or durability issues. Minor issues remain with programming the control system to eliminate the potential for tube overheating.

Deep Portage installed this system as an educational learning tool for the benefit of the public and to help eliminate LP fossil fuel usage by the LP hot water heater. My posts on this forum and in other venues are part of the educational effort. LP usage reduction has been dramatic.

The graph shows performance on a sunny day, high facility usage. The system provided solar heated water from about 9:00am to 5:00pm. Solar provided hot water is about 75,000 btuh.
Click to expand...

this is why proper design and application of any technology is necessary. And I'm not saying that just because that's what I do. and the technologies are region specific. Don't let people from the deep south design your solar thermal system for way northern climates.

karl
 
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