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Solar Thermal is Dead

pipe4zenpipe4zen Posts: 108Member
Interesting Solar Thermal vs, Solar PV argument. Thoughts?



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

  • jumperjumper Posts: 461Member
    try it

    Sound like a lot of theory. Years ago I built a PV water heater. My thinking was to avoid batteries and inverters. The hot water is the energy storage. I rigged up an open compressor and drove it with a DC motor directly from the PV cells.



    The real issue in America is how important it is NOT to save less than a dollar a day.
    · ·
  • Larry WeingartenLarry Weingarten Posts: 985Member ✭✭✭
    Martin Holladay admits...

    ... his title is intentionally provocative.  He's a smart and caring guy who does far more than just theorize.  Still, the argument is made with assumptions.   He assumes heat pumps are now all grown up.  The info I've seen both in terms of measured performance and field experience suggests something more like a youngster who isn't all that coordinated. 



    He assumes conventional solar design which ignores the newer "simple solar" ideas out there which have few, if any moving parts and little to corrode or even maintain.  Steve Baer summed it up nicely years ago when a black painted trash can was "raced" against a fancy solar system and won.  There is room in the marketplace for solar that is designed to catch the low grade heat.  The ideas exist. With elegant design, we can do much better.



    Mark Twain put it best: "The reports of my death are greatly exaggerated."



    Yours,  Larry
    · ·
  • Mark EathertonMark Eatherton Posts: 4,856Member ✭✭✭
    edited March 2012
    The author forgot to mention a few things...

    Like the fact that PV cells degrade over time, possibly as much as 10% per year... And that in some locales, you are not allowed to sell back more energy than you've historically used over a long period of time.



    An attention getter for sure, but if the base ASHP unit is located within the dwelling, then the energy it is using is NOT free. Someone paid for it...



    As for me, I've not seen ANY appreciable degradation of performance from the ST systems I've been watching over the years.



    And, when talking base efficiencies, PV is what, 12 to 15% efficient on a GOOD day, and ST is 60 to 80 % efficient? I just don't see how they can make the claim that PV and ASHP's make more sense.



    TIME, will tell...



    Heres a link to the article on degradation.



    http://photovoltaics.sandia.gov/docs/PDF/IEEE%20Quintana.pdf



    ME
    Post edited by Mark Eatherton on
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
    · ·
  • Kevin_in_DenverKevin_in_Denver Posts: 588Member
    Overpriced Solar Thermal is Dying for Sure

    I think we have to concede the argument that spending $10k to displace $50/month in gas for DHW is not a great investment.



    I've posted this before, but I'm trying to generate more interest in simple SHW. We haven't seen much buy-in from domestic water heater manufacturers in pursuing low cost goals, so maybe we are the guys to brainstorm and test. Here's a system that has no pump and no failure modes: http://www.youtube.com/watch?v=Q5rKLsJl3cY



    And although I don't think homemade equipment belong in a viable business plan, there is much to be learned from Gary Reysa, the retired Boeing engineer.

    http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
    · ·
  • Mark EathertonMark Eatherton Posts: 4,856Member ✭✭✭
    Solar hot water is not done based on simple economics....

    Never has been, never will be. It is done because it is the RIGHT thing to do for the environment, and the future occupants of Mother Earth..



    With gas prices continuing to go down, the economics might never make sense in our lifetime.



    When compared against other alternatives (LP, oil, electricity) the numbers do look better, but still won't pay for the installation costs within the theoretical lifetime of the system.



    Speaking of theoretical life times of system, I remember an ASHRAE number of around 25 years, and many systems that were installed back in the 70's are STILL cranking out good btu's. Maybe THOSE numbers need to be reviewed and adjusted. In any case, people rarely use an economic analysis to make their decision as to wether or not to do solar thermal.



    Food for thought.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
    · ·
  • PeterNHPeterNH Posts: 88Member
    40 year life is possible

       

    Hello Mark,

    There are two domestic hot water solar systems on my street.  They were both installed in the late 70's. They are glycol systems.  One system is not working, said to be a pump issue(.?)

    The other system functions perfectly. 34 years on.

    No doubt in my mind, the later system has paid for itself, long ago, even with inflation adjustments.



    Peter
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  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    if you have net metering

    then PV could easily beat thermal, unless you happen to have a huge summer thermal load. PV can slurp up more and more easy summer watts, and you use them up in the winter.



    that's not an end of the road winner for the grid in the end (unless summer peak load continues to be the driver at least, as PV hits that peak perfectly), but it's an economic reality for the short to medium term at least.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • scott marklescott markle Posts: 611Member ✭✭
    edited April 2012
    compelling

    With silicon as cheap as it is this idea does apear to make some economic sense, wires are clearly easier to run than pipes etc. etc. For starters I'm skeptical about the pricing given for both systems (too low) I think BOS (ballence of system) costs are being overlooked. Are we also factoring incentives?



    Regardless of the actual local costs (the disclaimer was appreciated) there was a very important omision that Rob also points to.



    Gallons of hot water produced and Kw generated can be easiliy interchanged mathematically but they are very different in terms of how they are actualy utilized.



    A PV array in a given local can be expected to produce a certain amount of electricity just as a thermal system will produce a certain number of "gallons". Seems fair to say but really it's missing something. Though we might wish to do so, we can not consider the grid as a no loss battery. Net metering has not changed the fundamental problem of energy storage as it relates to solar energy. The thermal example is solar energy converted to useful work, the PV example assumes no loss in the process of becoming "used". It's missleading to count every electron a PV system generates as useful work. Did the local grid need or use all the power that was "returned" to it . Sure the grid can integrate some renewables but not too much, the intermittent nature of sunlight (clouds and night) is one of the realities that some of the jazzed up PV folks seem to be ignoring in the quest for an "electric utopia" http://taxshine.com/Utopia.html



    I'm not saying the idea presented here is bunk, I kind of like it, but if we took the net metering out of it and looked at PV vs. Thermal gallons to gallons I think the economics would fall apart.
    Post edited by scott markle on
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  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    I agree

    that net metering in its current form will have to change for larger scale adoption of solar.



    however if you use heat pumps tied to PV even on straight efficiency you're getting close on cost per delivered watt. plus that opens up solar to doing cooling loads, so you have more potential load to offset, especially in the summer where the need for "Heat" is really not that great once DHW is taken care of.



    I think thermal solar will be relegated to those with relatively high domestic loads only in the not too distant future here.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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  • PV vs Solar Thermal

    Why debate it?? We need combo PV-T panels and we need them now. Cooling off a PV panel produces about 10% more electricity, this alone seems like a good reason to have a combination panel, but here in northern Massachusetts our heating and hot water bills account for about 70% of our energy dollar compared to 30% for electricity. We HAVE to use these panels along with radiant heating to harvest hot water at about 80 degrees; making the panels MUCH more efficient, and if we can cover a large portion of the heating load, the payback will be much quicker.



    Thanks, Bob Gagnon
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  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    Combo PV-Termal panels

    Many of us have been waiting decades for these.  The Germans are close http://www.szna-usa.com/pvtherm.html
    · ·
  • This Looks Like The Answer

    To much greater solar efficiency, I read in Solar Today about four years ago that they were starting to use PV-T panels in England, are they available in the US yet, SWEI? Thanks for posting this.



    Thanks, Bob Gagnon
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  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    the new array on my roof

    will save me $2500 of power a year. 10% more is only another $250/year, and to get it, you're adding a speciality trade, and a mechanical system with some longevity and service issues that PV just doesn't have. And I've got a very large system (60 panels) so whatever is required to connect all those panels together on the roof is not an insignificant labor adder, I assume.



    Of course you get the hot water too, and that's great, as long as you have something to do with it. Unfortunately you still get it during the day and mostly in the summer. so it's still not a space heat solver for most people, still just DHW in most cases.



    the real question is what's the cost difference between PV and PVT? I suspect for what it will be for most people...a modest power generation boost and some thermal offset to a DHW load... it probably won't be worth it.



    But if there is a good "snap together" kind of install assembly for this maybe it won't be so bad.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    PV-Therm

    Is available in the US - click on the 'Contact' link -- and give Reinhard my regards.



    Remember that from a thermal perspective, this is basically an unglazed flat plate collector.  Still plenty of uses, but not much good for space heat other than in a few very mild climates.  If you have a use (or a dump load) for low temperature heat in the summer, the PV output can significantly exceed that of a standard system.
    · ·
  • Kevin_in_DenverKevin_in_Denver Posts: 588Member
    The Achilles Heel of PVT

    No one has figured out how to make a glazed PVT collector yet.



    So until then we're better off just putting in two separate systems with collectors that do their assigned job well. As opposed to collectors trying to do two jobs poorly and expensively.

    http://www.heatinghelp.com/forum-thread/129667/Dawn-of-a-new-age

    http://www.quora.com/Are-there-solar-panels-that-integrate-photovoltaic-and-thermal-systems
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
    · ·
  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    (mostly) agreed

    With PV module prices where they currently are, PVT really is a tough sell.  There are a few cases with limited collector space where it can make sense, especially if night sky cooling is involved.
    · ·
  • hot rodhot rod Posts: 3,746Member ✭✭✭
    you need a load

    for the thermal side of PVT. PV arrays can be fairly large so you need some place to use that thermal collection. i don't think it would make sense to cool the PV with thermal then run a dump to get rid of un-use-able thermal energy?



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    · ·
  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    dump loads

    Can make sense in some situations.  If the PV output goes up 20% (they claim as much as 30%) in the hot months, the effect on net metering or REC revenue could be quite significant, especially with TOU metering.
    · ·
  • Kevin_in_DenverKevin_in_Denver Posts: 588Member
    Stagnation Temps may damage the PV

    I've never heard of anyone trying a glazed PVT collector yet.

    Once the PV supplier hears about 350F stagnation temps, they say no thanks.



    I'm guessing that it's usually because most PV cells have little narrow conductors that would fail under that kind of thermal stress. Not to mention their efficiency drops off badly all summer because they would be much hotter than an unglazed PVT panel.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    Heat Load

        It seems that having all that extra heat to play around with would be a good thing?? Why not install PV-T on a building with a heated swimming pool, or a large domestic load like a school or skating rink, certainly finding a place that uses a lot of hot water year round shouldn't be that hard.

        If we design these hot water system correctly, like a well designed hydronic system, the maintenance should be very minimal, maintenance that would have to be performed anyway, whether your heating with a boiler or the sun.

        Most people say we can't get enough hot water from thermal collectors in the winter, are using them for domestic hot water only, once we lower our collection temps. to about 80 degrees for radiant heating, we greatly  increase the amount of hot water we can harvest from the sun, just because most people who are doing solar thermal, use it for domestic hot water only, doesn't mean its the right way.

        A 20% increase in electrical production for Rob would come out to $14,400. over 30 years, that seems like a small price to pay to hook a bunch of panels together, and installing a couple of tanks and a small radiant system. How much hot water for heating would one expect to collect over that same 30 year period, my guess is a lot.

        When comparing fuel costs to solar collection remember that when we compare the cost of other fuels we don't include all the cost associated with that fuel. How much are we going to have to spend removing all the mercury from the water that comes from burning coal and carbon based fuels, how much do we spend guarding out nuclear power plants from terrorist, and how much are we going to spend transporting all the nuclear waste, and guarding it for the next 1,000 years, solar and wind look cheap when you take all that into consideration. Solar thermal is not dead, it is alive and well at my house, and it can get a lot better.



    Thanks, Bob Gagnon

       
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    my solar house.JPG
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  • Harvey RamerHarvey Ramer Posts: 922Member ✭✭✭
    Here is some food for thought. How about using a PVT panel

    to run a compressor. The compressor uses the water from the PVT panel as a medium for heating. Then the thermal energy would be stored at a high temp for extraction during the night.
    · ·
  • sunlight33sunlight33 Posts: 42Member
    edited May 2013
    ST works in winter?

    Up here in New York, average temperature during the day drops below 40F from December to March, does it mean Solar Thermal will be completely useless during those months to produce DHW?



    According to this video: http://www.youtube.com/watch?v=aB8disMi3A8

    With outside temperature of 30F, ST produced 17,413 BUT in one day, given the current rate of LP at $2/gal and 91,333BTU/gal (95% efficient gas boiler), that's equivalent to about 40 cents worth of heat in a day!
    Post edited by sunlight33 on
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  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    Upstate New York

    I don't see shy it wouldn't work where you are, here in Massachusetts on the New Hampshire border we get temps down to below zero and Solar Thermal works great here. I get 100% of my domestic hot water and about 30% - 40% of my heating from my solar hot water panels.



    Thanks, Bob Gagnon
    · ·
  • sunlight33sunlight33 Posts: 42Member
    ST for heating

    Most ST systems are for producing DHW, what system did you use to produce heating as well?
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    My Solar House

    Most systems produce DHW, that's why they don't collect much energy in the winter, they're waiting around for water temps in the collector at about 130 degrees, but I harvest energy at about 75 degrees, and simply collect much more energy at the lower temps. I use the low temp solar energy for radiant heating. My collectors are tilted to the low winter sun, which also prevents snow from building up, and I have a HUGE storage tank with really big heat exchangers to make hay when the sun is shining. Trying to collect 130 degree water in the winter here is difficult, but collecting lots of 75 degree energy is a piece of cake.



    Thanks, Bob Gagnon LEED AP
    JPG
    JPG
    my solar house.JPG
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  • Jamie HallJamie Hall Posts: 3,939Member ✭✭✭
    This is one subforum...

    where I usually just lurk (yeah, I know, not like me).  But I would like to second Bob's thoughts and comments.  Specifically, my late father-in-law made a fairly decent living doing engineering for solar heated houses in southern and middle New England, and it turns out that they work remarkably well.  As Bob noted, harvesting 75 degree F heat is not that hard to do; even getting up to 100 or so isn't that bad -- and at that point, with a decent amount of storage, a well-built house can make it through all but the worst winter conditions at normal interior temperatures (his most difficult problems were not the dead of winter, but, say February/March getting enough solar, and September/October, dumping heat).



    He wasn't too enthusiastic about DHW solar, and I don't blame him -- there you are working with much higher temperatures, and you have storage and insulation and re-radiation issues.



    And he didn't do anything with PV -- it wasn't available when he was active.



    Anyone interested in his work and thoughts, I'll be happy to correspond with...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    · ·
  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    Active solar thermal

    of mostly modern design has been with us since at least 1939 http://mit.edu/solardecathlon/solar1.html



    The true cost of energy has been carefully hidden from Americans for a long time.  If our defense budget only covered defense...
    · ·
  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    this is still ignoring

    the fact that right now I am gobbling now massive amounts of solar energy that just isn't present in the winter.... way more than 10 or 20% more, more like double over the course of a day.... and banking it for later.



    And I will use every single KWH I generate, no matter when I generate it. No "use it or lose it loss".



    and I will double its effectiveness by using that KWH in a heat pump.



    Solar thermal cannot compete for space heating. It can't even play the game for space cooling. That's just a fact.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    edited May 2013
    Thanks, Jamie

    You are one of the few who can understand, and agree with me, regarding solar space heating. I am not a fan of solar DHW either, the pay off is way too long, but if you combine domestic in the summer with space heating in the winter, then I think we're onto something. My Dad was a Milkman but he too understood that we have to heat our homes with solar hot water, and not just our domestic, back in the early 70". But he knew nothing about low temperature efficiencies and Radiant Heating, if he were alive today he would be very excited about the possibilities of using this missing piece of the puzzle. I would be happy to see some of what your Father did, and his conclusions, could you post them here so everyone can see?



    SWEI, you're right about the true cost of conventional fuels, I wonder what it cost to have our Military guard the waste from nuclear power plants for a thousand years, or to have fighter jets circling the plants for weeks after 9-11, I recently read that our govt pays the cost of insuring nuclear power plants, add that to the threat of release of radiation, nuclear seems pretty expensive. The same could be said for fossil fuels, how much wold oil and gas really cost if our govt didn't give the oil companies billions in subsidies, and what is going to be the cost of removing all the Mercury from the lakes, rivers and oceans?

    Thanks, Bob Gagnon
    Post edited by Bob Gagnon plumbing and heating on
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  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    Hot Rod

    I'm not sure you would need to dump if you have a big storage tank, I don't have to. Remember you have much greater heat losses through the tank and piping at higher temps. I have 120 evac tubes and a large box collector, tilted at about 65 degrees toward the low winter sun, so collection is automatically limited in the summer when the sun is directly overhead. I put a large coil in the tank to dump but its just been sitting there for the last decade.



    Thanks, Bob Gagnon
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    Collector tilt

    NRT_Rob, what is you collector tilt? If it is tilted at 45 degrees or less its great for domestic, but not so good for space heating, and you also probably have snow sticking on your panels too, I don't have that with a steep tilt. A collector tilted about 65 degrees will collect the maximum amount of energy in the winter, and less in the summer.



    Thanks, Gagnon
    · ·
  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    not that it really matters

    I don't know why you're asking that bob. I"m PV. I don't have to worry about whether I'm pitched for winter or summer... I'm pitched for yearly ideal, about 23 degrees true south.



    What I'm saying is right now, at this very moment, and over the whole last month, I am absolutely cranking out solar power. I have no heating or cooling demands during this period. if I were thermal, GOBS AND GOBS of power would be utterly wasted. But my panels generate it and send it back to the grid where I can make use of my credits later, when I do have heating demands. I won't bother saying "heating and cooling" because when I have cooling I also have solar so that is offset in real time.



    I don't lose anything from my tank. I have no tank to get hot and stop my collection. I don't have to have a load in real time to use my power. I can double my collection power by using heat pump technology. I can cool with my solar energy.



    PV wins. Thermal is dead.



    I know how much my array cost and how much it collects. Do you know how much your system would cost for someone to install, and how much energy benefit you're getting? we could settle this with math, if so.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • SWEISWEI Posts: 4,773Member ✭✭✭✭
    Far from Dead

    just needs to be applied properly.  Unless the heat demand is year-round, ROI will suffer.  PV has the advantage there for sure.
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    edited May 2013
    It doesn't cost much more

    To add a few more collectors and some radiant wall panels, to an existing solar domestic hot water system. Whats it going to cost you to put in more PV and install and maintain a heat pump? A zero energy home builder here tried that and it didn't work, it used too much electricity even though the roof was covered in PV panels. He ended up using wood to heat his house. The reason we have to heat our houses with solar is that in Massachusetts heating is 52% of our energy dollar, domestic hot water is 17%, together its more than double your electric bill. I get a lot of heating from my solar, all of my heating load in the spring and fall, and a good amount in the winter.  Monday night is supposed to be 39 degrees out, we are still heating here. How much are you going to spend on heating and hot water in the next 40 years? How much is your oil or propane bill going to be 10 or 20 years from now, I'll still be collecting solar energy and heating 100% of my domestic hot water, and a good portion of my space heating. Solar is alive and well, but it's for people who can think outside the box, and are willing to go against the flow.



    Tilt does matter with PV. If your collectors were tilted towards the low winter sun, you would collect more energy then, and less, then you are now collecting in the summer.



    Thanks, Bob Gagnon
    Post edited by Bob Gagnon plumbing and heating on
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  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    but with PV

    I don't care WHEN I collect it. We have net metering. I just want best yearly collection. And we are, in fact, net zero... in fact I generate more electricity than I can use (darn it!).



    I am going to spend zero dollars on oil or propane or electricity. For the next 30 years. That's why I can nail down how much energy I'm using... I measure my energy usage and all my usage is electric.



    The entire point I'm making is that solar thermal only looks better in the instantaneous collection efficiency numbers. In terms of yearly energy you can put into use, I think almost any residential analysis will come down solidly in favor of PV these days. Because the efficiency difference cannot erase the massive amount of time most of us spend with less load than you have collection ability, and net metering solves that issue completely.



    the real question most people care about though is cost for useful energy gained. $3.50/watt installed and each watt will generate about 1.3 KWH/year here in the northeast. Add a heat pump and double some, potentially very large, percentage of those watts to apply to heating and/or allow for cooling offset. In my case it's probably about 70% of my total usage is "doubled" in this way.



    So what's a regular solar thermal array sized to do more than DHW cost these days, installed, before credits?

    How many useful yearly BTUs would you expect to get from it?
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    edited May 2013
    Why not both?

    Why does it have to be one or the other. We will soon have combination panels that harvest electricity an lots of 75 degree water, and super-insulated houses will stay warm for days, we will use our houses for storage. There are simple solutions to all the problems we now have with solar hot water. The cost of solar panels are getting less expensive too, as people buy more of them, you paid $3;50 watt a few years ago maybe? It's down to about $2.00 a watt now and the DOE expects it to be about a dollar a watt by 2020, solar thermal and pv will be more competitive with conventional fuels, as prices drop.

    Thanks, Bob Gagnon
    Post edited by Bob Gagnon plumbing and heating on
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  • Jamie HallJamie Hall Posts: 3,939Member ✭✭✭
    See what I can do...

    I've never posted a picture in all the years I've been on here!  Have to figure out how...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    · ·
  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    it's "one"

    because one makes a lot more sense than the other. solar thermal has poorer economics for space heat. Unless you can do it damn near free.



    if you can get solar PV in for $2/watt PRE TAX CREDIT, then the discussion is completely over (though here in maine it's not that cheap). taking up roof space for hot water panels past the DHW demand is then seriously a waste of time and money and at $2/watt I'm not even sure solar thermal for DHW makes much sense when you can do an electric water heater tied to PV. at $3.50/watt PV was already compelling for space heat. Heck, at $2/watt, and 0.15/kwh electricity, you're below a ten year payback on PV with no credits at all. If that's after credit then it's pretty much the same as my cost last year.



    Seriously... name an installed price and a yearly expected BTU delivery for a solar thermal system. anything you want to run through polysun or any of the solar calculators out there. we can compare this very easily. I just can't do solar thermal collection calcs myself at this time.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
    · ·
  • Bob Gagnon plumbing and heatingBob Gagnon plumbing and heating Posts: 1,288Member
    Rob

    It's easy to make excuses, to not do something, whats hard to do is spend your own money to see what really works. You mention tank losses as a big problem with solar. First you have similar losses with an electric water tank, but secondly the solar storage tank should be within the buildings thermal envelope to minimize those losses and those losses will contribute to the heating load for about 6 months out of the year where I live, so for six months I'm not really losing anything. Since I don't run my boiler at all, this works out great for me as it keeps my basement from freezing. Its not to complicated to add extra insulation to a tank to cut losses during the summer months, I have 8" of foam on my tank to do that. It cost a few bucks for the foam, but it will last indefinitely, lengthening the amount of cloudy days my tank will stay hot. You say we have to use solar thermal right away but that's what we have storage tanks for.

    Thanks, Bob Gagnon
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  • NRT_RobNRT_Rob Posts: 1,009Member ✭✭
    I have no idea

    what you are talking about with your "excuses" comment, but I'll assume you aren't trying to insult me, since I did, in fact, spend my own money to see what works. I'm net zero, remember?



    If you have a storage tank that can allow you to shift your solar collection above and beyond your DHW needs from April through october into your heating season, then great. I'd love to see the specs on that tank.



    Until then, I have about five or six months of solid, high output solar collection that you don't get to utilize like I do, because I went PV. If you want to wave your hands about collection efficiency, I'll simply point this out... http://solarelectricityhandbook.com/solar-irradiance.html

    go ahead and see what you've got, month by month.



    I just ran this for Augusta Maine. I have heating demands from November into April. But let's be nice: I'll let you use every KWH available from October through May (I have a very tight shop, most homes need more heat than i do) and just assume you don't lose any at all, you use it all. In Augusta, that's an average of 2.54 KWH/m2/day or about 210 KWH/m2 for the seven months. you're about 60% efficient and so that's 126 KWH/m2. You don't get to use any solar in any other month for space heat, only domestic, which we can agree is a tiny load compared to space heat, right?



    I'm PV. Let's say I'm 10% efficient. so I only get 21 KWH for the same seven months you do... but, I can drive a heat pump which doubles it to 42 KWH/m2 applied to my heating. However, the OTHER five months a year give me about 723KW/m2 gross... ten percent and doubled gives me another 144KW/m2.



    So I can run 186KW/m2 for heating with solar. with solar thermal you can only do your 126. I'm 50% higher by area, and that's still give YOU may in this comparison.... realistically we're being nice. Plus I can apply it to cooling, DHW and power and not just DHW or space heat, if my loads are too low.



    this is by area though, the real question is cost. So, how much area does your system cover with panel, and how much would it cost retail before credits? We can do this with math Bob. it works.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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