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Drainback solar water heat exchanger design options

Post in 'The Green Room' started by semipro, Feb 16, 2012.

  1. pring7

    pring7 Member

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    Eastern NC
    There are a few safeties in my system. I do have a tempering/mixing valve to prevent getting scalded from really hot water. There is also hot water dump to a heat exchanger (looks like a small radiator) and a pressure relief to doubly prevent steam and expansion in the closed loops. Later this week I can take some pictures of the system if anyone is interested in seeing it.

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  2. pdf27

    pdf27 Member

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    UK
    Few reasons I picked vertical rather than trying to optimise:
    1) It's quicker and easier to calculate - any tilt below that of the winter sun will give you better performance in winter than summer - but the reduced peak energy may more than make up for it. At 90 deg you can use the same panel in both places and guarantee you'll get more energy captured in winter than in summer. Optimisation to match your site is certainly possible, but that takes quite a bit of time and needs to be matched to your total heat/dhw load. For simply comparing solar resource, that isn't terribly helpful.
    2) Ease of installation - in rural locations space generally isn't an issue - for for suburban/urban locations it may be. Vertical panels take less floor area so may be more generally useful - for example they can be bolted straight onto walls.
    3) Lots of other solutions are also possible - if you're using it in conjunction with a Geothermal/Ground Source heat pump you could use it to preheat the soil for winter. Motorised blinds or some sort of tracker could be used to match supply to demand. You could even set it to be automatically filled from mains water and boil off excess heat as steam. Designing your installation to match supply and demand closely is probably the better bet in terms of long-run cost.

    Oh, and for what it's worth I calculated the residual cost of heating + DHW per year for that nominal Passivhaus as ~$150 using electric resistance heating. That's all from November to February, when the solar resource is relatively poor - so at that point from an energy point of view PV+net metering starts to make a lot more sense.
  3. woodgeek

    woodgeek Minister of Fire

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    I think we are all talking about slightly different issues at the same time. I believe the Gary can build an inexpensive and reliable DHW system and size it to provide as much water in the winter or summer as the OP requires. I believe that pdf27 can size a solar thermal system to provide DHW and most space heating, for any coordinates on the planet. I can see that for winter input, a vertical flush mount panel on a wall makes a lot of sense, thanks.

    But my kickoff question goes unanswered.....how big a system does semipro need to save how much money? If he was burning oil for DHW (as I am), he would be spending ~$1200+/yr to heat water and $1k solar DIY system like Gary's would look great....clearly <2 years even if the backup operating costs are included. BUt that is the wrong calculation....For <<$1k DIY our oil burner could put in a conventional elec tank, and run an annual cost of say, $600. If the elec backup on the solar HW system is 30% of the total (many are worse), then the operating cost of the solar is $200/yr. Now we're only saving $400/yr. Optimally, that system would be highly tilted, on the roof, to get lot of output in the summer, and wouldn't need to be that large to get the $400 savings.

    But Semipro has a HP DHW system. When I pushed my numbers, I figured that during a 3 mo heating season, I would run COP=1, and the rest of the time, I would run COP=3. in 1/4 of the year, I am conv elec, $600/4=$150. For the other 9 mo, 3x longer but 1/3rd the elec consumption, another $150 or a total of $300/yr. So now the question....how much does a given system save? A modest sized, highly tilted system might provide 85% of the 9 mo usage and 50% of the winter usage (70% overall as above), and would save, if I calculate it, $200, and the HP would still use $100/yr of elec for backup. By doubling the size and vertically mounting it, etc, Semi could save a little bit more ($50/yr?), for a lot more effort and visual impact, etc, so let's skip that.

    So now Gary's $1k DIY system has a 5 yr payback (at zero cost of labor), and my $6k after rebate commercial system has a 30 yr payback (relative to a HPWH), or an apparent 6 yr payback versus staying with oil (not a valid calc IMO). To me, for a DIY project, a 5 yr payback is getting less attractive, unless it is very easy, or has other benefits, like comfort, resale value, etc.
  4. pdf27

    pdf27 Member

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    Agreed. It comes down to appropriate technology for the situation - it is exceptionally rare for one technology to fit everybody. A heat pump with a COP of 3 and electricity at $0.10/kWh is an EXTREMELY cheap way to heat water, when you've already paid for the heat pump so aren't factoring in the capital cost. For comparison, I use mains natural gas and am paying the equivalent of $0.10/kWh after heating efficiencies are factored in.

    I also threw some numbers for a system like that into the big spreadsheet of things. For Allentown, PA, a roof mounted (45deg) south facing system. Assuming $0.10/kWh and $600 annual hot water bill from resistance heating gives 16.5 kWh daily demand. A 60 x 58mm tube system (the type I have data for) will produce ~4800 kWh/year or 81% of annual demand - a saving of $480/year. Assuming you're borrowing the money to do this at 6% interest, and want it to pay for itself over 10 years, that puts the value at $3600. That's about what it would cost me to install an identical system over here using professional components but doing the work myself.

    Assuming you have a COP of 1 during December, January and February and 3 the rest of the year changes the equation though. The cash payback goes down to $220/year and the amount you can afford to spend on the system drops to $1600. At that point, you either need to get grants to get the cost down that low or you're forced to do a completely DIY system like Gary's.

    The equation changes again for highly insulated new build - for the Passivhaus example I was using at the start you can look at the three options from scratch.
    Option 1: Pure resistance electric heating. Annual cost $890/year.
    Option 2: Heat pump with COP of 3 for heating, 1 for hot water. Annual cost $670/year.
    Option 3: Solar panels with electric resistance heater for backup. Annual cost $230/year.

    For this scenario, it isn't inconceivable that the up-front costs of Options 2 and 3 are similar - and the savings from the solar system if rolled into a 30 year, 5% mortgage would justify spending nearly $7,000 more - not counting the reduced maintenence the solar system would probably need. Pure resistance heating is harder to argue against, simply because of the radically lower capital cost.
  5. woodgeek

    woodgeek Minister of Fire

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    If Semipro is still reading....the upshot is that small solar DHW system on your roof will save you 70% of what you are currently paying for DHW. IF you want space heating, you will need to go a good deal bigger on the panels and storage and plan on mounting the panels vertically on a south wall, if you have the space/inclination. 'Payback' on both ideas is not free money 'great', but ok if you go DIY and keep costs down as suggested by your early thread ideas. BUT the 'optimal' space heating system looks a bit different from the DHW system.
  6. GaryGary

    GaryGary Feeling the Heat

    Joined:
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    SW Montana
    Hi Wood,
    One of the advantages of the DIY solar thrmal approach is that its cheap to add more collector and more tank size -- you can inexpensively size the system to get a higher solar fraction than would be economically sensible for a commercial system with $30 a sqft collectors. My system provides nearly all of our hot water. We have a small electric backup tank that has additional insulation over the regular insulation. I've tracked the electricity usage on it, and its averaging about 0.2 KWH a day -- about $8 a year here -- I've not done this for a full year, so I'm not 100% sure about the number, but I'm confident that the yearly total is small.

    This, of course, depends on sun -- especially sun through the winter. I took a look at the NREL Redbook summaries for Billings, MT (close to me) and Roanoke, VA (maybe close to SemiPro) -- the VA location actually gets a bit more sun than we do --even in most winter months. VA winter temperatures are milder than MT, so that would help the collector efficiency some. So, I think that SemiPro could beat the 70% number by just sizing the system a bit larger than the conventional guidelines.
    Redbook: http://rredc.nrel.gov/solar/pubs/redbook/

    I agree that already having the HPWH certainly reduces the payback for any added approach.

    The carbon numbers are also important, and solar is good for carbon.

    Gary

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