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Thoughts On "Solar Water Heating: A Comprehensive Guide..." by Ramlow & Nusz

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desert_sasquatch
desert_sasquatch Member Posts: 118
I'm reading Solar Water Heating: A Comprehensive Guide To Solar Water And Space Heating Systems by Bob Ramlow and Benjamin Nusz. I thought I'd create a thread to ask questions that come up as I read it.

Question 1:

He mentions two kinds of Evacuated Tube Collectors: Single Tubes and Twin Tubes. I gather that the difference between the two is that the collector in single tube systems is in vacuum--at least until the connection (gasket?) between the actual copper tube and the glass leaks. Once that happens the tube is not worth all that much and should be replaced, and because of the thermal expansion/contraction that can occur at that joint, this sounded like a not-infrequent issue.

Whereas the twin tube collectors should, in theory, not have this problem. Vacuum is sealed on all sides with sturdy borosilicate glass (like "pyrex") so I guess it lacks the gasket-related issues of the single tube systems?

I've read a number of discussions on here about maintenance issues with vacuum tube collectors and while maybe it's not a dealbreaker given the ease with which one can replace a malfunctioning tube...I also don't see a real clear benefit over the seemingly-more-reliable and definitely-cheaper flat plate collectors. But I want to make sure I'm not unfairly judging the durability of vacuum tube collectors based on complaints that only pertained to single tube designs.

So what do people think, am I judging double tube evacuated tube systems unfairly? Are the (newer?) double wall evacuated tubes much more reliable? And if so, is the rest of the system rather durable as well (ie there's no problem where the tubes connect to...is that a manifold?...the top piece).
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  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
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    Hi, It may be a bit off topic, but my experience with evacuated tube failures has been freezing in the headers. This causes the copper around the top of the heat transmitting pipe to become pinched into place and no longer removable without damaging things. Servicing the equipment then becomes tough. I've heard that large hail can do damage as well to the tubes... Flat plates can freeze also, but aren't that hard to silver solder.
    Ultimately, what type of panel you choose depends on what system temperatures you need and what sort of freeze protection mechanism is being used.

    Yours, Larry
    desert_sasquatch
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    I'm not going to comment on the pros and cons of the various solar collectors out there -- I'm not sure that there is very much to say, actually, one way or the other, as each has its advantages and disadvantages.

    What I will say, however, is this: in fact, two things.

    First, for space heating -- such as your house -- direct solar heating, as in a passive or semipassive solar house -- is vastly preferable, and is easy to accomplish in a new build (it's much harder, of course, for an existing structure). The advantage is that the required storage mass is built into the structure, and the problem of getting enough collector area to power the system is eliminated almost completely.

    The required collector area can usually be found for space heating, but the storage mass for an existing building can be a real problem. This depends somewhat on where one is, of course.

    Second, for domestic hot water heat, the problem is usually the storage mass and insulation at the higher temperatures needed to keep the domestic hot water safe. Again, this is somewhat location dependent -- if one is in an area where one can count on three or more hours of sunlight every day, not so much of a problem. If one is in New England or some such climate, however, the storage mass becomes much greater.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    @Larry Weingarten not off topic at all, that's exactly the kind of thing I'm wondering about. The issue you describe sounds like it would impact both single and double tube systems in climates with freezing temperatures, which certainly would be of concern to me. Thanks!
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,250
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    I've never heard of this book and I've got quite a few.. Can I get on Amazon?  Mad Dog. 
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    for space heating -- such as your house -- direct solar heating, as in a passive or semipassive solar house -- is vastly preferable, and is easy to accomplish in a new build (it's much harder, of course, for an existing structure). The advantage is that the required storage mass is built into the structure, and the problem of getting enough collector area to power the system is eliminated almost completely.

    The required collector area can usually be found for space heating, but the storage mass for an existing building can be a real problem. This depends somewhat on where one is, of course.

    Thanks Jamie

    Yeah, storage mass is easier with a new build for sure.

    I have some thoughts on using big equator-facing windows to collect winter sun. (If that's not a big part of what you meant by "passive or semipassive" then my apologies). I was quite enamored of this idea when I first heard about it but when I crunched the math it just didn't seem to pencil out, largely because of cost constraints. I read an article about this a while back but alas, it seems to now be behind a paywall.

    But I'll try to make the case as best I can: The higher the R Value of a window the more expensive it'll be. And anything but a single-pane window (are those even available in the large sizes we're talking about?) has to be replaced after maybe 20 years iirc.

    Plus there's the thermal loss through the windows to contend with.

    So as an example, in Denver--a very sunny city--a 4X7 south facing window with a 3' overhang would "produce" (by letting the sun in) an average of a bit over 12 kBTU per day in January. According to this tool, anyhow https://www.susdesign.com/overhang/ .

    If we assume that the window is R-4 then that same month in a similar location each of those south facing windows will lose about 5.6 kBTU on an average day.

    Aaanyhow, long story short I did the math and it seemed like each 4X7 south facing window would save me almost $48 per year in heating expenses vs electric. It would save me less vs solar thermal...maybe $20 pear year? Maybe it would save me slightly more if I was careful to use thermal blinds, but this house will be rented out and I don't want to have to count on my renters to be careful about much. Anyhow, Over 20 years that's $400-$966 of energy savings per window. I figure each of these windows might cost $1,600 for materials and install...

    Beyond that there is the issue of temperature control. I know a lot of people like having a home whose temperature fluctuates a little bit between day and night, and they don't mind having to be diligent about doing some stuff to make sure the home doesn't get too hot or cold. I'm actually one of those people. But I know a lot of people aren't. And I'm trying to keep the appeal a little more universal.

    So I guess my take is that windows are nice. I want windows. And south facing windows cost a bit less than north-facing ones in the sense that there's more solar gain to be gotten. But especially in climates where a lot of cold will try to get into a home through the window glass, using passive heat from windows is more expensive than using solar thermal panels to heat the home. And trying to use windows exclusively for this is purpose is made even more difficult by the temperature fluctuations. Yes, there are climates where you don't have to heat up the home that much during the day to stay warm at night. I don't think I'm in that kind of climate.

    Also, my town has spacious lots, so panels can be mounted in the yard which avoids issues with having to move the installation when the roof gets replaced et.

    So that's why I'm looking into solar thermal.

    But I'm still learning things. Do you think I've missed a financial advantage of direct solar?



  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    Mad Dog_2 said:

    I've never heard of this book and I've got quite a few.. Can I get on Amazon?  Mad Dog. 

    Hah I'm now afraid I've started a thing about a book that may or may not be any good. I only started reading it because hot_rod mentioned that the author taught a course that specifically covered sandbed radiant, which is something I want to understand better.

    Still, I'm 20% in and I'm finding it a decent departure point from which to ask questions. The author may, however, be a bit overly sunny on solar thermal.
  • hot_rod
    hot_rod Member Posts: 22,483
    edited February 2023
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    Heat pipe, evac tube is a sealed copper tube, with a vacuum pulled in it. This allows the water methanol mix to boil at 110F, flash to steam and transfer to the headed pipe well.

    The U tube type has the system fluid inside the copper tube. Vacuum is in between the double wall glass.

    With heat pipe design you have two vacuum to keep "in the box" the glass tube, and in the copper heat pipe. Mother nature hates a vacuum. Out in the extreme elements, eventually mom wins :) pros and cons to both design.

    Some pics of the downsides of evac tubes, IMO. They also stagnate up around 600F, so they can cook glycol in a few days time. You need good overheat protection for them. Apricus tried a drainback evac tube, the Wombat. Never did take off.

    Use the output info , into a tau, alpha graph. The long hand way to determine performance instead of the SIM program :)
    Then with a graph of the slope and Y intercept, from the SRCC sheet, plot your intended working condition. Notice the huge performance penalty as you drive the operating temperature up. The flat plate go from 44% down to 16% at higher operating conditions.

    Since the tubes are a great insulation value with that vacuum, the glass never warms to shed the snow. So a good solar harvest opportunity and the collectors are iced over or covered in snow, in example below.

    Annealed (tube) glass breaks very easily if you try to scrape the clean. It breaks into shards so don't walk below one :)

    You can run a bit higher temperatures with evac tube, but at low temperature use the graphs to discover the flat plate may out perform them.

    Bob Nape used to have his data logger viewable online to see real time difference in the two type of collectors.

    Slides are from my 8 hr solar training class.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    desert_sasquatchkcopp
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    But I'm still learning things. Do you think I've missed a financial advantage of direct solar? 
    Not really. Passive solar is pretty pricey. You can insulate well, use the amount of windows you feel is appropriate aesthetically, and use electricity/another fuel to make up the rest. There are some direct solar benefits (think orientation for the windows that’ll be included either way), but a glass wall will be more expensive and thermally worse than a properly insulated wall. Solar prices have reached a level low enough that a lot of once halfway viable energy saving strategies are now way out of the money. 
    desert_sasquatch
  • hot_rod
    hot_rod Member Posts: 22,483
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    Passive building info here
    https://passivehouse.com/
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jumper
    jumper Member Posts: 2,333
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    Down the road PV is preferable if you can get a deal and know what you're doing.
    • Higher temperature facilitates thermal storage. Note that you do not need inverters for heating.
    • Cold outside temperature does not reduce performance.

    Is anyone familiar with those electric heaters that stored heat?
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,250
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    Email

    @!!!   Ask me how I 🤔 know..Mad Dog. 
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    The problem with photovoltaic as a space -- or domestic water -- heating source is in collector efficiency. Even the best PV cells can't convert more than about 30% of the incoming power, and most are more like 20% -- while even relatively simple direct heat capture designs can approach 90 to 100%. Makes a difference in how much collector area you need.

    On the other hand, electricity is, in some ways, easier to store than heat.

    If one can use the electricity in a heat pump in its 4 plus COP range, they're competitive in terms of collector area.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Larry Weingarten
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    As always @hot_rod thanks for the info. Crazy that the glass will break if you scrape it clean.

    OK, I think I'm feeling comfortable with my decision to avoid evacuated tubes for my particular situation!
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    jumper said:



    Down the road PV is preferable if you can get a deal and know what you're doing.
    • Higher temperature facilitates thermal storage. Note that you do not need inverters for heating.
    • Cold outside temperature does not reduce performance.

    Thanks for bringing this up. I had crunched the numbers before using the general quote I see for residential solar which is about $3-4 per watt. It seemed like it would be cost effective if I used grid-tie and heat pumps, but I'd like to avoid both due to my own health issues and related concerns about dirty electricity and mold. It didn't seem cost-effective if I used the panels to heat water directly via DC heating elements, so I left it at that.

    However now you have me wondering if I should have been pricing the panels much more closely to the $1 per watt price that I can get for good-quality REC Alpha panels, and counted whatever other installation costs there might be as being about equal to the installation costs for a solar thermal system.

    Price of Solar Thermal

    First for the solar thermal panels: I think I need 14 SunEarth Empire EP-40 panels to give me my 90% solar fraction. Each panel costs $1220 (I think with shipping?) so that's $17,080 for the panels. Or I could go with 14 of the cheaper SunMaxx TitanPower panels that cost $500 each but supposedly provide about the same power (though possibly with lower quality and less longevity). They would be $7,000 for 14.

    Price of PV

    Using the PV Watts calculator from nrel.gov I am told that for each kW of rated PV I should expect to get about 150 kWh per month or 5 kWh per day (that's with a 53 degree tilt angle optimized for winter because that's when I need the power most). In kBTUs that's 512 per month and 17 per day.

    On an average day in January I'll need maybe 192 kBTU for heating and DHW. Eyeballing the BEopt graph I think I might need to average 160 kBTU per winter day to achieve the 90% year-round solar fraction I think I'll get from solar thermal. So I'll need roughly 9.4 kW of rated PV to manage this. Plus the DC heating element costs $31 per 1.2 kW which is...chump change, in this equation. It looks like I'd need about $9,500 worth of solar panels + heating elements to do this. So that's cheaper than SunEarth and although it's a bit more spendy than SunMaxx...I do suspect I'll have fewer maintenance issues with PV.

    But my concern is that I'm underestimating the other costs of installing a solar system. This website has a nice graph of the costs of a PV system. According to them, the cost of the panels has definitely come down, but the cost of overhead, installation, and hardware have not, to the point where they expect installation to cost as much as the panels themselves. And they expect overhead to cost as much as everything else combined.

    So I guess my question for yall is whether installation of PV on the ground will be more expensive than it would be for the solar hydronic system. I don't know why it would be but perhaps there's a certain level of expertise required when working with electricity, even when that electricity is DC and runs straight from the panels to the heating elements with maybe just a breaker or something in the middle? Or maybe I was just underestimating the cost of installing a solar thermal panel?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
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    Hello @desert_sasquatch , I'd like to throw this into the mix, to help confuse things ... of course. ;)https://www.larryweingarten.com/blog/another-solar-myth-bites-the-dust

    Yours, Larry
    desert_sasquatchGGrossratio
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    A PV system costs about $3-3.5/watt installed by a professional, without tax credits. Solar thermal is harder to know, as so few people install them. Additionally - if you use a heat pump water heater, you can use a smaller array for DHW. An air-to-water heat pump can do central and DHW too.
  • jumper
    jumper Member Posts: 2,333
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    Energy efficiency is over rated. It's $$$$ you're after.
    DHW doesn't cost that much unless you shower too much.
    Heating is a different story. Desert nights can cost mucho $$$.
    Solar energy can save $$$ only with storage.
    High temperature is necessary to store heat.
    Plus high temperature is more pleasant. People complain when system blows 80° air.
    When you pay a professional you pay $$$ for stuff additional to PV panels and installer's work.
    So look for somebody's defunct solar electricity set up. Hopefully the PVs work.
    Don't need hight tech expensive electric stuff like inverters and interconnects.
    I don't know if those electric ceramic heat storage heaters from yesteryear are still available.
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    High temperature -- or mass, @jumper . There is a very good reason that pueblos and the old haciendas of the southwest are built the way they are. Those old boys weren't fools, and they really had it figured out.

    No moving parts. No collectors. No widgets. Just time tested architecture, and it works.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    Thanks @Larry Weingarten that's a great read. Interesting strategy, though it sounds like in my sometimes snowy climate it might not work too well. What do they think about the longevity of plastic solar panels? Will the glazing protect it?
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    A PV system costs about $3-3.5/watt installed by a professional, without tax credits. Solar thermal is harder to know, as so few people install them. Additionally - if you use a heat pump water heater, you can use a smaller array for DHW. An air-to-water heat pump can do central and DHW too.

    Yeah, agreed that when air source heat pumps are used they appear to be the cheapest option. I'm leery of them growing mold similar to AC units but I guess time will tell--they're too new for me to crowd-source that info from the mold-sensitive crowd. Also they'd require inverters and some way to keep that AC current steady, probably using grid-tie. I could, of course, just power the whole thing off the grid and forget about PV but eh, there's cost savings there and I like the independence and environmentally warm feeling. But I know that the grid-tied PV + heat pumps will be the thing a lot of people go for. However for me I'd insist on using dirty electricity filters which at this point make the whole thing uneconomical.

    Cost of PV

    My question about the cost of PV though was more like "can it be done cheaper if they're just heating up hot water with a simple DC heating element?" Because then there's no grid tie, there's no inverters, there's no high voltage, just two wires connecting a panel or a short string of panels to a heating element.

    Here's the infographic I linked to before from solar.com:



    I'm going off of this but obviously they could be wrong, and I'm open to hearing it.

    But it seems like nowadays if you get PV installed by a solar installer half the cost is going to overhead--not to the actual labor to install them, not to the rack, not to the panels, not to the inverters, just to the people who make the website and knock on doors and use fancy software to design a system so it fits on my roof. Seems like maybe by hiring a local carpenter and electrician I should be able to skip all that. Plus I'm skipping the inverters. So then what's the overall cost per watt? $1.50 with $0.50 of that being the install cost?

    And what's the install cost for solar thermal? Should I assume that, whatever it is, it's about the same? (Unless it's one of Larry's PEX rolls :) But this wouldn't be).
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    The additional cost of building a passive house from scratch vs. the same usable space in a conventional house turned out to be about 5% for the two I posted above, and almost all of that was in riding herd on the builders to make sure they didn't cut corners. The additional running cost of a passive house is, oof course, zero. Depreication is the same as any other house (the two posted above are both about 40 years old with no problems.

    For what it's worth, a college friend of mine lives in a hacienda near Los Ojos, NM. Built around... not sure. late 1700s maybe? Works pretty well still... some upgrades (indoor plumbing, nice kitchen, electric lights... that sort of thing),
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    desert_sasquatch
  • jumper
    jumper Member Posts: 2,333
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    High temperature -- or mass, @jumper . There is a very good reason that pueblos and the old haciendas of the southwest are built the way they are. Those old boys weren't fools, and they really had it figured out.

    No moving parts. No collectors. No widgets. Just time tested architecture, and it works.

    Like that. My experiences with both high mass mud buildings in southwest and old stone ones in Italy is that they're pleasantly cooling in summer. Is that because they're cold in winter?

  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    Not really. Up there in the high desert there's always sunshine during the day, and that keeps the adobe pretty warm. They do have a fireplace, though -- but don't use it that much.

    European castles did tend to get a wee bit chilly -- more in Scotland where my family's from originally. In fact, downright cold. The fancier places have fireplaces you can walk into...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,483
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    A PV system costs about $3-3.5/watt installed by a professional, without tax credits. Solar thermal is harder to know, as so few people install them. Additionally - if you use a heat pump water heater, you can use a smaller array for DHW. An air-to-water heat pump can do central and DHW too.

    Yeah, agreed that when air source heat pumps are used they appear to be the cheapest option. I'm leery of them growing mold similar to AC units but I guess time will tell--they're too new for me to crowd-source that info from the mold-sensitive crowd. Also they'd require inverters and some way to keep that AC current steady, probably using grid-tie. I could, of course, just power the whole thing off the grid and forget about PV but eh, there's cost savings there and I like the independence and environmentally warm feeling. But I know that the grid-tied PV + heat pumps will be the thing a lot of people go for. However for me I'd insist on using dirty electricity filters which at this point make the whole thing uneconomical.

    Cost of PV

    My question about the cost of PV though was more like "can it be done cheaper if they're just heating up hot water with a simple DC heating element?" Because then there's no grid tie, there's no inverters, there's no high voltage, just two wires connecting a panel or a short string of panels to a heating element.

    Here's the infographic I linked to before from solar.com:



    I'm going off of this but obviously they could be wrong, and I'm open to hearing it.

    But it seems like nowadays if you get PV installed by a solar installer half the cost is going to overhead--not to the actual labor to install them, not to the rack, not to the panels, not to the inverters, just to the people who make the website and knock on doors and use fancy software to design a system so it fits on my roof. Seems like maybe by hiring a local carpenter and electrician I should be able to skip all that. Plus I'm skipping the inverters. So then what's the overall cost per watt? $1.50 with $0.50 of that being the install cost?

    And what's the install cost for solar thermal? Should I assume that, whatever it is, it's about the same? (Unless it's one of Larry's PEX rolls :) But this wouldn't be).
    I don’t see how using PV at 15% efficiency makes sense compared to thermal running 45% or better to heat thermal storage

    Notice in the thermal panel test and output sheets, they use Kw as the measurement in addition to btu, so easy to compare PV to ST

    Drainback ST uses plain water, no glycol, and is overheat protected by draining down. Two of the bigger challenges with large thermal arrays go away with drainback systems.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    don’t see how using PV at 15% efficiency makes sense compared to thermal running 45% or better to heat thermal storage
    True but who actually cares about solar efficiency? It’s not efficiency people care about - it’s often cost! 
  • hot_rod
    hot_rod Member Posts: 22,483
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    don’t see how using PV at 15% efficiency makes sense compared to thermal running 45% or better to heat thermal storage
    True but who actually cares about solar efficiency? It’s not efficiency people care about - it’s often cost! 

    The OP in this thread🥸
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    @Larry Weingarten You mentioned in your article that you generally see solar thermal as requiring a yearly checkup. Is the same true of photovoltaic? Or do you view photovoltaic as a lower-maintenance system?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
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    Hi @desert_sasquatch , Dust, etc. on the PVs can reduce their efficiency, so perhaps a yearly cleaning would be good if rain doesn't do it. Connections to batteries and other high amperage connections should be checked for cleanliness and tightness, probably yearly at least. The equipment itself isn't something you want to install and forget. I'd want to know the battery condition at least daily. If there is an inverter/charger, I'd like to monitor it to understand what the norms are, so I know if/when things go sideways.

    About solar thermal and yearly service, it's also simple stuff, like keeping collectors clean, checking anode in the tank, (if there is one) along with any relief valve. We may be talking about an hour per year for the basics. But also know that sunlight, birds and weather will try to mess with any sensors and associated wiring up on the roof, so that should be looked at periodically as should any pumps and their controls.

    I think it's pretty much a wash about how much maintenance either system will need. Solar thermal often goes for years without a look from the owners. Grid tied PV does also, but if you're off grid, the state of charge in your batteries lets you know if you want to burn or conserve power. Frequent monitoring becomes important then. Have I just muddied the waters more? >:)

    Yours, Larry
    desert_sasquatch
  • hot_rod
    hot_rod Member Posts: 22,483
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    With PV installations, the charge controllers or inverters that may be installed read out a lot of data about battery condition, amount of panel output, condition of the load, etc etc

    Whether you grid tie or have a battery bank, you need a charge controller

    My system is grid tied, but I can also pull 2000W off the Sunny Boy controller if the grid goes down. It does require a manual switch to use that mode

    Many smaller PV arrays use micro inverters now. They look like a small bandaid box, mount on the racking or back of panel. So basically you convert the panel output to 240v then wire them into a disconnect or breaker

    You can also add a readout inside, connect via an ap. My last shop had a small array I installed myself with Enphase micro inverters.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    hot_rod said:


    I don’t see how using PV at 15% efficiency makes sense compared to thermal running 45% or better to heat thermal storage

    Notice in the thermal panel test and output sheets, they use Kw as the measurement in addition to btu, so easy to compare PV to ST

    Drainback ST uses plain water, no glycol, and is overheat protected by draining down. Two of the bigger challenges with large thermal arrays go away with drainback systems.

    Oh sorry, I think I only mentioned solar efficiency earlier--possibly in another thread?--in support of my assertion that the T*Sol software was estimating the panel's efficiency lower than I would have expected, and that it was probably because it was forcing the panels to operate at the DHW temperature at a minimum rather than allowing them to operate at the lower space heating temperature for that portion of the heating load. Where I'll build I won't be too concerned with 15% PV solar efficiency--I've got the space in the yard to put the extra panels, I think.

    Plus, since I'm hoping that a high yearly solar fraction will actually be the most cost-effective of all heating options, and since I don't want to do grid-tie, the effectiveness of winter solar power is of greater concern, since that's where most of my heating needs are. With that much output I'm confident that I'll be dumping heat in the summer and good parts of the spring and fall. So it's winter solar harvesting that's the limiting factor on how many panels I'd want. If that makes sense.

    Honestly I prefer the idea of solar thermal. I like something that's simple--simple enough that I can explain how it works rather than saying it's a magic panel that turns sunlight into lightning in a jar--and I like that a plumber should be able to repair it. But if I can get the same benefit for a slightly lower price... Well, this week I'm excited about solar PV. Next week maybe I'll figure out why it's not so cool :)
  • solsean
    solsean Member Posts: 18
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    I have been involved with lots of projects that went for the big solar heating fractions and very few of them are operational. Drainbacks help avoid fluid stagnation but the collectors are going to expand and contract significantly. Seen more than a few with the absorber plates loose from the riser tubes. The best results I have seen for large solar heating fractions have come from "seasonal storage" tanks with 10:1 storage:glazing or even bigger. In terms of text for solar thermal, Planning & Installing Solar Thermal Systems by Earthscan Publishing is the best I have found.
    desert_sasquatch
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    Got to admit that I find the whole solar heating solar hot water solar electricity thing fascinating. In a clinical sort of way. If there ever was an area where thinking outside the box was of value, that's it. Unfortunately, the whole topic is beset by a combination of fads and snake oil...

    Did anyone happen to look at the two houses I posted a few comments back in this thread? Any questions or thoughts on them?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
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    Hi @Jamie Hall , I couldn't get into the Christian Science Monitor article, but the other one looks like a classic saltbox design, with relatively low exterior surface area, for low heat loss. I did very much the same with my place. :) A question though, is do you ever have to deal with overheating or sun damage of the things/surfaces inside?

    Yours, Larry
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    Hi @Jamie Hall , I couldn't get into the Christian Science Monitor article, but the other one looks like a classic saltbox design, with relatively low exterior surface area, for low heat loss. I did very much the same with my place. :) A question though, is do you ever have to deal with overheating or sun damage of the things/surfaces inside?

    Yours, Larry

    Yes. Overheating can be a real problem. The house from the Christian Science article -- I'm so sorry you couldn't get into it; I'll see if I can find another source -- suffered from that. In fact, we had to do some rework on the original design to reduce the problem, and eventually had to resort to temperature actuated vents such as are used on greenhouses! It isn't truly passive either, as there is a low velocity fan and a heat recovery ventilator incorporated, for indoor air quality control.

    The other house is, basically, a salt box as you said. What's interesting is that there is enough glass area there that, in combination with good insulation and some real attention to detail on air sealing, it does manage to heat itself. There was some tendency for the living/dining area (first floor, south wall) to overheat, but not often. We didn't see much in the way of sun damage, but that may have been as much the choice of furnishings and the nature of the floors (hardwood with shellac finish mostly, but ceramic tile in the kitchen and bath areas) as anything else. It could get bright in there... !
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Larry Weingarten
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    I might add a comment here, though. Neither house is quite as ordinary as it seems. The house in Shrewbury was somewhat experimental -- and turned out to be serous overkill. It was also a saltbox, and the entire south wall was glass (triple) as well as the south slope of the roof. The roof -- not the attic floor -- was insulated, and there was water heat storage across the attic floor -- the effect being that the upstairs ceilings were radiant heated. Both it and the little house I built were slab on grade. The slabs were thick -- mine was almost two feet -- concrete with north to south ductwork all through them (concrete blocks laid on their sides) with plenums along the north and south walls. Gravity air flow through them. The house I built also had a concrete and brick wall in the center, running east west, for about half the length of the house (also two feet thick) with the stairs running around the north side. There were other details... too numerous to mention here!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
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    Hi, Is this the Shrewsbury house? If so, Bill Shurcliff thought highly of it!

    Yours, Larry
  • Jamie Hall
    Jamie Hall Member Posts: 23,655
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    Tha's it! Thank you! Bill was a great guy.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Larry Weingarten
  • TonKa
    TonKa Member Posts: 104
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    FWIW, this company makes solar hot water collectors which mount under a PV panel:


    It is claimed that the hybrid setup can actually improve PV efficiency slightly because the PV panel will run at a lower temperature.

    I have zero experience with them. They're just one of many websites I stumbled across which looked interesting enough to save.