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Solar Thermal is Dead?

Post in 'The Green Room' started by semipro, Apr 24, 2012.

  1. semipro

    semipro Minister of Fire

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    This is an interesting article where the author posits that PV has gotten so inexpensive that using solar to heat water directly no longer makes sense (in most cases).

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

    I contend that this probably doesn't hold true for the do-it-yourselfers out there but it has got me rethinking all my plans for building and installing a solar thermal system.

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  2. velvetfoot

    velvetfoot Minister of Fire

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    Could this work by heating a storage tank, as a kind of battery? Or, are they figuring the utility always being involved?
  3. Slow1

    Slow1 Minister of Fire

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    I have to say that I came to that conclusion for myself at least in the market I live in. With rebates currently biased toward the PV arrays it really seems to be better to build out the electric side - no wasted energy during peak production in the summer, fewer moving parts...
  4. jimbom

    jimbom Combustion Analyzer

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    !! $10,000 for a solar electric installation to run a heat pump to heat domestic hot water. Paging Mr. Goldberg. Mr. Goldberg, Mr. Rube Goldberg. A muse is horning in on your territory.
    Realstone and Dune like this.
  5. Slow1

    Slow1 Minister of Fire

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    Reality check though - when I last received a quote for a solar DHW system for my home, I was quoted over 14K after all rebates. Obviously I chose not to go forward with the project. However you can see why 10K doesn't seem too shocking. DIY clearly can be much lower, but if you are looking at retail cost of installed systems...

    I spend about $800/year heating my water. If I am looking for a 5 year payback then that gives me about a $4K budget if it is a 100% solution - right now I'm not finding any DHW solutions that deliver that for me so I wait... as oil goes up, so does the budget though...
  6. jimbom

    jimbom Combustion Analyzer

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    Very true. I am taking the same approach. We have natural gas and our DHW averages $20/month. We will be waiting a long time unless I can scrounge materials and DYI this.
  7. woodgeek

    woodgeek Minister of Fire

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    Absolutely. I concluded in my case that the (grid powered) HPWH would use about the same kWh as the solar thermal system would require for backup, so that the elec usage and carbon intensity of the two systems would be nearly a wash. My HPWH install was <$4k, and my solar thermal quote for a comparable output was $15k installed, or $5k after state and federal rebates.
  8. semipro

    semipro Minister of Fire

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    My plans for tapping solar have always included thermal for water or space heating. Now, with the availability of low interest loans, less expensive PV systems, tax rebates, and SREC sales I'm getting real serious about installing PV. Some quick calculations indicate a 7-10 year payoff period that I find encouraging. I'm thinking investment (and payoff) of a PV system and a plug-in electric vehicle would help set us up for retirement in 20 years.

    One potential pitfall is that my 20+ year old fiberglass shingle roof needs replacement and I'd always envisioned going with a metal replacement roof. It seems like a waste to install the panels on the existing roof only to have to remove and reinstall them when I install the new metal roof.
    To further complicate things I'd also planned to go to a "cold" roof with the addition of XPS foam under my new metal roof with sealing of ridges and soffitts.

    It seems like I"m going to have to move the roofing mods higher on my priority list if I'm going to be able to take advantage of existing PV incentives.

    Anyone have experience with installing PV and then reinstalling after a roofing change? It doesn't seem like it would be that much work to pull an existing array and then remount on metal but I could be underestimating what's involved.
  9. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    One alternative would be to work on other energy saving and efficiency projects until the roofs time has come, and then do the PV (or solar thermal).

    PV modules have dropped in price over the last couple years, but PV is still expensive and still has a poor payback period compared to a lot of other projects (at least for most people). This is how it came out for us: http://www.builditsolar.com/Projects/PV/EnphasePV/Economics.htm -- in the section titled "How Does PV Compare to Other Solar or Conservation Projects?"

    We kept track of all of our energy and carbon reduction projects and compared them to the PV project, and the section shows how they compare on saving and payback -- a short excerpt:

    "These numbers are kind of hard to believe when you first look at them. How can a handful of simple projects (the "Best 8") that cost less than $400 to do save 3 times as much energy as a state of the art, $10,000 PV system? Well, it may be hard to believe, but I can tell its true. Its also very likely that if you go find your "Best 8" projects and do them, your results will be similar.
    I'm not trying to be hard on PV -- I think its a great technology, and I love my new system, but be sure you do the other stuff too. If you do the other stuff first, you can pay for your PV system with the dollar savings from the other projects."

    The difference would not be quite so dramatic today, as PV system prices are down some but the other projects still have a far better return than PV -- adjusted to around todays prices, the other projects still pay about 10.5 times better than PV.

    If you do the other stuff first, you can pay for the PV with the savings -- the opposite statement is much less true.

    Not trying to be preachy, and you may have already done all the high payback projects, but just thought I'd mention the possibility for people who are just starting down the energy/carbon reduction road.

    Gary
  10. Slow1

    Slow1 Minister of Fire

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    I would discuss with installers. Different roof surface may change the mounting system components - depending on how long you wait between initial mount and re-mount that could be a big deal (i.e. parts availability etc). The recommendation I received here really biased toward getting the roof done prior to the initial solar PV mount.

    In my case after looking at the cost to remove and replace the panels/mounting system which worked out to be a full day of labor for a crew on each side was well over 50% of the cost of roofing that face of roof - not even taking into consideration the possibility of any damage to mounting system that might happen and require additional parts (and delays etc). So I opted to just go forward and have that side of the roof re-done with a roof which will outlast the panels (and me for that matter).
  11. woodgeek

    woodgeek Minister of Fire

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    Our local outfit does both roofing AND solar PV, so the PV mounts don't ruin the shingle warranty. If you wait a little longer, you can go for the Dow solar shingle....
  12. semipro

    semipro Minister of Fire

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    No "preachiness" taken.
    I couldn't agree more with what you say. Its just that we've done sealing, insulation, energy-efficient lighting and appliances and, I think, reached the point of diminishing returns there. Any further improvements in sealing and insulation will require major work like pulling all our cedar lap siding and installing additional foam sheathing.

    It appears that most of our energy costs are for water and space heating. Teenage sons creates some challenges there. We already use geothermal and small electric space heaters for cold-weather heating. We have a lot of windows and even with energy saving window treatments, losses there are substantial.

    Based on how may projects I have going on around the house and the amount of time I have left to invest in DIY solar, PV may provide a way for us to go solar within a reasonable time frame. Also, loans are relatively cheap and there are the tax and SREC incentives. Ultimately too I think we'd strive to install some electrical storage to allow independence from the grid if possible. We have a generator but would like to avoid the associated noise and fumes.

    I plan to read the article your reference on your site this weekend. I've spent so much time on your site already I'm surprised I haven't already seen it. Thanks again for the work you do on your site. and for your input.
  13. semipro

    semipro Minister of Fire

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    I'd probably do the installation (roof and PV) myself with family and friends. I hear what you're saying and appreciate the info. I'm now making plans to getting the roof done before PV. Thanks.
  14. semipro

    semipro Minister of Fire

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    And there's this option too. http://www.amazon.com/Unisolar-Flexible-Solar-Panel-Laminate/dp/B006EP6MCU
    Its a thin film laminate PV that installs in the valleys of a standing seam metal roof. I think efficiency is lower but installation sure looks to be a breeze and they look cleaner.
  15. velvetfoot

    velvetfoot Minister of Fire

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  16. woodgeek

    woodgeek Minister of Fire

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    Going through the solar boards, it is clear that the unisolar products are considered a neat idea, but there are documented instances of them not meeting their durability guarantee. The manufacturer is also in Chapter 11.

    In fairness, the Dow shingle product is a similar thin film tech whose long-term durability has not yet been proven in the field. Hopefully Dow's deep pockets have helped with that.
  17. begreen

    begreen Mooderator Staff Member

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    Can you provide some links to the Unisolar issue? I was considering putting this on a camper van roof.
  18. woodgeek

    woodgeek Minister of Fire

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  19. begreen

    begreen Mooderator Staff Member

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    Thanks, that was an informative read.
  20. btuser

    btuser Minister of Fire

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    Does it make sense to use PV to preheat DHW? I don't have room on the ground for solar and don't want to do a drainback/pump system on the roof. I'd also love to head into the direction of PV for the house long term. Can you just go direct with the juice to a electric HW tanks without a transformer + batteries contributing to loss?
  21. GaryGary

    GaryGary Feeling the Heat

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    I think you probably could.

    For a Solar Electric Panel water heating system:
    Here is an example
    DMSolar.com sells PV panels cheap -- just as an example, pick the DMSolar 250 Watt panel: http://www.dmsolar.com/somo25.html

    250 watts at STC conditions (this is full sun (1000 watt/sm) and with a panel surface temp of 20C (very optimisitic))
    Cost $320, or $1.28 per watt, but more $1.50 per watt with shipping.
    Claimed efic 17.8%
    Size: 65.6 inches by 39.1 inches or 17.4 sqft.
    Voltage at max power with full sun is 30.6 volts.

    If you hook 6 of these together (see below for why 6), then you have an array that:
    - Output is 6*250 = 1500 watts under ideal conditions
    - Array size is 235 inches wide by 39 inches tall, which is 104 sqft
    - total collector cost $1920 (plus shipping plus mounting rails for the 6 panels)
    - If you series connect all 6, you get 184 volts at 8.2 amps under full sun, which is the expected 1500 watts

    I guess you could connect this to a resistance heating element directly in the tank -- at full (1000 watt/sm) sun, the panel produces max power at 184 volts and 8.2 amps -- this matches a resistor of (184/8.2) = 22 oms.
    This would certainly be simple, and maybe with some creative panel selection you could match an off the shelf heating element.

    At this one sun condition, you can pick a resistance that allows the panel to be at its maximum power point, but as the sun drops down in intensity, the panel current drops down. At lower sun levels if the resistance does not change, the panel will no longer be at its max power point, but it will find some operational point -- not sure how much efficiency this costs you. Maybe someone knows how to figure this?
    The I-V curve for the panel is provided here:
    http://lib.store.yahoo.net/lib/yhst-21796510175022/DM250S2.pdf

    Compare this to the Solar Thermal Collector:
    Pick a 4 by 8 ft Heliodyne Gobi collector.
    Under the same sort of sun conditions, but using a 50F ambient temperature, this calculator:
    http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm gives:
    - efficiency 55.9%
    - Output = 177 BTU/sf, or 5664 BTU for the 32 sf collector, which is 1660 watts
    - Array size is 48 inches by 96 inches, or 32 sqft
    - total collector cost about $800 (plus shipping and some mounting hardware)


    So, for about the same power output
    - The PV system takes up (104/32) = 3 times as much roof space
    - The PV system collectors costs somewhat more than twice as much as the thermal collector

    If you are into simple DIY projects, the same 4 by 8 panel can be built for about $5 /sf, or $160 plus zero shipping.

    I picked these sizes because both of these configurations would be just large enough in a descent sun area for a family of 2 that was careful about hot water usage -- kind of a minimum size rig for a family of 2.


    -----------
    For about another $200 per PV panel, you could add Enphase micro-inverters to each panel, and with just a little bit more wiring the the whole thing becomes a grid-tied PV system. This has the advantage that the title of this thread gets into, but you are now up over $3000. In spite of the arguments Martin makes, I doubt that it comes up a net positive, but I've not worked through all the numbers.

    Gary
  22. semipro

    semipro Minister of Fire

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    I've looked into pre-heating water electrically using PV or wind and found that there are direct replacement water heater elements that run on 12 volts DC as opposed to 120 VAC.
    I don't think the heating element cares whether its powered by AC or DC.
    I wonder if its better to run PV panels in series using a higher voltage element versus running the panels in parallel and using the low voltage element?
    What's great about this concept is that it requires no inverters or batteries. Cheap hot water storage can be used instead.
  23. GaryGary

    GaryGary Feeling the Heat

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    The series arrangement would allow a smaller wire size from the PV array to the tank. But, as you say, finding an off the shelf heater whose voltage you can match up to is a big plus.

    Gary
  24. semipro

    semipro Minister of Fire

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    I was curious so I did a few calculations which basically verify Gary's.
    I can buy 4, 230 watt, 30 volt (full load) panels for about $1k plus shipping and then connect them in series to a 120 volt heating element installed in an old water heater tank located next to my GeoSpring HP water heater.
    The tank is already insulated and corrosion protected. I already have the tank and a heating element costs about $20 and screws right in.
    My 4 panels would produce about 900 watts with an equivalent of 4.5 hours full output per day (in our area). That's 4 kWh.
    This amount of power would raise the temp of 40 gal. (330lb) of water in my tank by about 40 degrees F so I'd end up with preheated water at about 90 degrees.
    Based on the prices Gary provided for thermal heating It seems that solar thermal would be less expensive for preheating water.
    However, running piping at our house to the roof would be tough and a ground level installation won't fly. I could mount 4 PVs very easily and run the wiring without much problem. I also would not need to worry about the complexities of piping, valves, freezing, weight support, etc.

    I've been a big proponent of using solar in the native form of heat but PV is really starting to look better.
  25. webbie

    webbie Seasoned Moderator Staff Member

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    I think the lack of maintenance will eventually swing the calc toward PV - I installed a lot of solar thermal systems back when, and there are often hidden costs over the years.

    Besides, right now even free PV could hardly compete with nat gas...damn stuff is almost free. But that will end.
    :)

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