harvesting waste heat from my water heater

Discussion in 'Water Heater Forum, Tanks' started by Ian Miller, Sep 21, 2011.

  1. ballvalve

    ballvalve General Engineering Contractor

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    Looks like Evergreen should have gotten Solyndras 5 mil in order to stay here.

    Lets watch Dr. Strangelove and be reminded of our German ex-pats contribution to the rocket race after the war.

    And thank Hitler for driving so many Genius Jews here before the war.
  2. Runs with bison

    Runs with bison Member

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    Didn't see any serious replies since the end of page 1, but I would have three concerns:
    1. Condensation in the flue--if it works you will get a lot of this. This is generally not a good thing since the condensate has to go somewhere and it is moderately corrosive with all that CO2 dissolved in it (carbonic acid--although I'm accustomed to dealing with extremely corrosive HCl condensation in high dollar chemical recovery furnaces.) A condensing system typically has stainless parts to account for the carbonic acid.
    2. Potential for draft problems. Depends on how efficient the system is in removing heat and the overall dynamics of the home/outdoor temps/HVAC. Cooler air equals less draw (lower differential pressure to draw flue gas out.) I've never had combustion draft problems in any home--except in fireplaces with short chimneys.
    3. Cost vs. benefit. For my home and already efficient uses, a drain water heat recovery system is marginal at today's gas rates (I'm still curious enough that I'll probably install one some day since I've worked on similar equipment industrially.) Without doing a detailed heat transfer and mechanical design I anticipate a flue gas recovery system is going to be even lower efficiency and require a lot more copper/stainless for the same duty...even though I have ~20 feet of vertical stack to play with. Gas side heat transfer coefficients with natural draft are typically very poor even with partial condensation (the higher percentage non-condensibles are a killer IIRC) so it takes a fair bit of area to accomplish much. And that gets pricey with copper or stainless.

    Note: a key consideration for me in a drainwater heat recovery system was pressure drop on the supply water side--this can be substantial for the area required. To do this efficiently can require parallel flow paths such as dual or quad coils.

    It is more efficient to design a proper exchanger in the tank (a condensing unit with induced draft) with proper material and layout than to do an economizer on the flue gas. But condensing in the tank is also pricey. So while I would gladly take a condensing unit offered to me for free, a drainwater heat recovery system appears to be better bang for the buck with today's pricing if I must pay for it myself.
  3. here is a better idea

    you guys are a hoot....... that coil idea has been out
    for a long time but has nver taken off because there is
    not enough energy worth re-capturing......

    just put a water heater blanket on the heater and be happy....


    here is another great idea ....

    if you really want to save all the energy
    you can, why not just take the chimmney off the water heater
    and let all the heat just flow into the home....
    just think of all the energy you would save doing this......
    :p
    you could also do this with your dryer vent...
    ..
    :D if you start to get a headache, step outside
  4. Dana

    Dana In the trades

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    Sadly, people DO vent the HW heaters & dryers to the interior. While in some areas venting an ELECTRIC dryer to the interior can save a modest amount of energy with low risk to indoor air quality, it would create a mold hazard in much of North America. Venting of any gas-burner (even your kitchen range) into conditioned space adds moisture, suspended particulates, and some even nastier stuff.

    Going with power-drafted rather than atmospheric drafted hot water & space heating equipment prevents backdrafting, and lowers the air-infiltration cutting heating/cooling losses for the house independently of the equipment's own efficiency ratings. Any open flue depressurized the house- where the air gets in is usually random. It's generally better from both indoor air-quality and energy use points of view to build the house as air-tight as possible, and control the ventilation rather than just hoping the "natural" air exchanges of the air leaks & stack drives are providing the right amount of ventilation to the right spaces.
  5. People are dumb as rocks.... it is funny that we go out all the time
    and repair FVIR water heaters with lint caked up around them for this
    exact same reason.... the dryer is venting into the laundry room.....
  6. Runs with bison

    Runs with bison Member

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    Someone needs smarter clients. I've found that the IQ of the clients often corresponds to that of the contractor.

    Heck, I've seen this several times, always in the homes of folks that didn't have any common sense...and I was just passing through, stuck my nose in to see what was up. Amazing how much lint can collect in a given space if not properly ducted to the outdoors.
  7. mtcummins

    mtcummins In the Trades

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    So this is sort of related to this thread, since some of you very technical people started talking about PV etc.

    Have any of you guys looked into this? http://solarroadways.com/main.html

    Look through it before making the obvious objections (glass road surface will break, will be slick in snow, etc), as they have addressed reasonably well almost every one of those on the site. I've been following this project for a few years now, and seen just about every objection that people could bring up, and they're all generally addressed.

    It would take some serious shifts to make it work, but it seems like with some more development it could be really viable. Decentralized is the way to go, in today's climate...
  8. Dana

    Dana In the trades

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    It'll take LOT more development to turn this into a viable product on the large scale- I have a hard time imagining how they're going to be able to cost-reduce the solar roadway panels to make them competitive with conventional PV. While it's long been observed that the sunshine falling on all of the paved roadway in N.America could power the entire transportation sector (at 15% collector efficiency), that's not to say that building the PV into the very road bed is a cost-effective way to do just that. Servicing defective panels could be seriously expensive & awkward, and carrying the current at low-voltage along those miles of pavement runs into conductor capacity issues too. The PV content would be the least of their cost issues, but if they can make it cost-competitive it solves any real-estate issues of a PV powered grid, eh?
  9. mtcummins

    mtcummins In the Trades

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    Yeah, its def not quite ready for the large scale, I'll give you that. They're still working on getting the top glass surface figured out... its a complicated piece of glass to get proper traction, strength, and transparency for the PV. They have some people working on it who say its possible, just will take some work/time/money.

    Everything else actually works out pretty well on the cost scale, if you start to think about the cost of asphalt roads these days. Then factor in the utility line upgrades that have to be done in most of the country anyway (all the power lines around here don't have insulation on them anymore... they're in desperate need of replacement), the reduction in snow removal costs, etc, the numbers start to make more sense.

    I'm not EE, so I can't intelligently comment on the conducting capacities and such. Their concept is that the roads are not primarily a transmittal method, but a localized production method. Therefore they wouldn't be transmitting power over particularly long distances and the loads on them would be relatively low compared to today's setup. I think realistically, you'd have to still have some auxiliary power production for major metropolitan areas, but this could make a huge dent in the need for centralized production plants and massive transmission stations to power all the far out areas. Those areas should be serviced just fine from the roads alone.

    Its pretty fascinating... I'd encourage you to keep looking into it, and I'd like to hear more of your input. The guy doing it is an engineer (electrical, i believe), and has thought through all the transmission issues and such pretty extensively. I don't claim to understand it all, but it seems like he has it pretty well figured out. He's also got a lot of cost justification stuff on there to try to show that it wouldn't be cost prohibitive. I think a lot of it is pretty reasonable, some of it might be a little optimistic, but if any of his numbers are even remotely close, the cost factor isn't that big of an issue.

    I think the biggest issues would come in the infrastructure changes... houses required to change to DC, or adapt DC to AC to be able to tie in, if you're wirelessly powering cars, the equipment necessary for that, etc. It is all doable, but not quick or easy.
  10. ballvalve

    ballvalve General Engineering Contractor

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    A rental I have has a "80%" efficient power vent furnace [using 20% of the heat value in the gas?]

    I could cook an egg on the exhaust pipe. pathetic heat exchanger [Took the entire rig apart for a bench rebuild]

    A 40 foot run of 4" exhaust pipe IN the house, like the schoolhouses in old western America, would give me a 90% furnace. Or concoct a method to put your water preheat coil into that pipe, and accomodate any condensation.

    Panels in the road? Hello? Here are some of the Mega solar farms going up: http://en.wikipedia.org/wiki/List_of_photovoltaic_power_stations

    Here is a monster being built WITHOUT any sibsidies, and I think they are using US panels.

    http://en.wikipedia.org/wiki/Topaz_Solar_Farm
    Last edited: Dec 14, 2011
  11. mtcummins

    mtcummins In the Trades

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    That's nice and all, but solar farms don't address 80% of the things that the Solar Roadway does, and probably costs 80% as much. Not to mention they're 10 times more hideous than a traditional power plant...
  12. ballvalve

    ballvalve General Engineering Contractor

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    I want to see that snow plow for the panels.
  13. Runs with bison

    Runs with bison Member

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    Location:
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    I observed (and anticipated) the semi-condensing behavior you are talking about with my old 81%. The exhaust pipe runs through a long vertical return chase so I suspected it was gaining some efficiency from that. That and the horizontal probably total about 25-30 feet before the attic space and roof. I had noticed some indication of prior condensation on the pipe at the elbow that predates my ownership of the home, but never have seen it actually occur.

    At any rate, with the same set points, and doing careful correction for water heater use and daily temps and daily gas use over the whole winter last year the reduction in gas use with the new 95% unit was about 6% short. A fraction of this might be due to better distribution in the corners of the home and the addition of a register in one of the cooler rooms, but I suspect that the old unit was outperforming its specs due to the heat exchanger type configuration of the vent.

    I grew up with wood heat, with a substantial run of chimney pipe in the home and a pot belly stove that I would stoke when I got up to go to the bathroom and of course when I got up in the morning. We used a simple box fan behind the stove to distribute the heat through the open floor plan downstairs, and the wide open stairway fed warm air to the bedrooms above. Anyway, the chimney pipe exited the home through a window space before turning vertical and that was critical. I've seen that elbow glow cherry red from a creosote fire that kicked off when we burned too much black oak one winter--we added a cleanout to the elbow after that. I also split much of the wood, and was glad to be done splitting that cross-grained black oak anyway.
    Last edited: Dec 15, 2011
  14. mtcummins

    mtcummins In the Trades

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    This is exactly why I said that you need to read the site before making objections. I won't respond with why (if you care you can read the site, if not, no sweat off my back), but this objection is irrelevant if you read the site.
  15. Dana

    Dana In the trades

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    I make a living as an electrical engineer, and I just can't see how it could be both ruggedized sufficiently at a low enough cost to ever make it economically, given the competition.

    The amount of available suitable rooftop in N. America is huge, and although it's more expensive per lifecycle megawatt-hour, than ground mounted solar farms, it's at most 2x the cost, and it's already located a suitable load (or near one), reducing the amount of additional grid infrastructure required to make it useful. Remote solar and wind farms run into this as well- if not sited near adequate pre-existing grid resources the cost of the project explodes. (Boone Pickens had to cancel a billion dollar wind project in TX when neither the utilities nor the gummint wanted to pick up the tab for the new transmission lines it would take to get his terawatt-wind power to the load.)
  16. mtcummins

    mtcummins In the Trades

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    I have no problem with solar roofs, I think more people should do that if they're not going to use them for other cool things. I'd rather see green roofs and usable spaces in dense urban areas than solar, but if the roof isn't being used for anything (or just for equipment, etc), then I'm all for rooftop solar. Problem is, this doesn't generate nearly enough energy for the building below it, even if every building had solar roofs. I think the two systems in combination could be a good mix.

    The costs are not much different in production of a road panel vs a regular solar panel, other than the glass. Obviously that is much more. But when you factor asphalt at up to $1000/ton, utilities that need to be replaced (and would be integrated into the panels), the reduced costs of road maintenance, street lighting, etc, it starts to make more sense financially. Just plopping down panels somewhere to produce power has no offsetting cost benefit, replacing a very expensive road system with a power producing road should be less expensive than maintaining the roads and service lines and building solar farms, at least in the long term.

    Then there's all the safety increases, the reduced insurance rates, the reduction in accidents/lost lives, etc. There's a lot to consider.

    I was skeptical when I first heard about it too. Keep researching it, it makes more and more sense, and you'll see more and more how much they've thought through the good and bad, the cost structure, etc.
  17. Dana

    Dana In the trades

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    Other analysts have come to different conclusions about the fraction of the total could be provided by rooftop panels. For power-pig residences there may be insufficient roof space to meet the daylight-hours load of the building but on power-sippers it can. On large, flat low intensity commercial buildings (warehouses, some offices/R& D space) there is a surplus, but not heavy data centers or aluminum smelters.

    The PV in the roadway panel costs less than the ruggedized electronics necessary to run it. The notion that the production cost deltas between rooftop & roadway panels are primarily the glass just isn't so. The PV part is the same, sure, but not so much the rest of the package, and the rest is quite substantial. But I'll pick over their documentation some more as time allows.

    It'll be a very long time before the costs of the roadway system would make it more attractive than spending the same money on filling available rooftops. And in most commercial & residential buildings spending half to 3/4 of the money on efficiency measures would have a better lifecycle ROI (and bigger offsetting of grid-power) than rooftop PV. Rooftop PV is rapidly reaching grid-parity on lifecycle kwh-costs (it's already there in more-expensive electricity markets- cheaper than state-of-the-art coal.) But it's still more expensive than upgrading efficiency at the the load/building. Reducing heating & air-conditioning power use has as much to do with improving the building envelope as it does with the efficiency of the equipment, but it sometimes takes a sharp accounting-pencil to figure out which improvments are most cost-effective, and it's an iterative process. With lighting equipment there is great efficiency in the "off" switch too- occupancy-sensor switches & dimmers are far cheaper than the PV that it takes to make up the difference in power use most of the time.
  18. mtcummins

    mtcummins In the Trades

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    Other than this other required equipment (which I know nothing about so can't comment on), I agree 100%. I'm big on efficient building practice, energy use reduction, etc. PV will not likely be a particularly viable solution w/o combining it with many conservation measures. I currently have a very tightly sealed house (lots of closed cell foam, etc), and hydronic radiant floors run off a 94% efficient water heater (I think thats the percentage, its right in that range). I run all CFL or LED lighting, Energy Star rated appliances in almost every case, etc. This is certainly the first step.

    I currently do not have any PV, but will keep considering its feasibility over time. Right now its just not cost efficient, as I have an urban house w/ a flat roof containing a roof deck/green roof, and a single pitched roof on the other part of the house, that doesn't aim in a particularly good direction. If this had a good southern exposure, I'd probably already have panels on that roof. With its orientation, I'll need much higher PV efficiency rates to make it worth considering.

    I'm also trying to acquire the lot next to me, on which I'd build a garage. If that happens, the garage roof might become a potential PV location, as its orientation could be tailored to it much better. We'll see.

    The solar roads have cool applications, a ton of features that could nearly be worth the cost just for the benefits offered, let alone the PV. I'm not saying that money should go to one or the other... I think that private money going to rooftop solar is a great idea, and continuing subsidies from gov't/utility companies makes sense. I think that could be paired with a solar roadway program, but only install the solar roads in areas where the roads are in need of replacement anyway. Ideally, in areas where the utility lines are also in need of replacement, so that you can knock out all of that in one shot, and balance out the costs to a large degree.

    I think one of the biggest challenges is that its all DC, so you'd either have to convert it at the service line to each house (electric company equipment), or inside the house (homeowner expense).

    Would be interesting to see a flop in the whole Edison/Tesla war after all these years...
    Last edited: Dec 22, 2011
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The hassle with DC supplies is they are much more adversely affected by line losses than a/c lines. Now, they both lose energy, so generating it closer to the point of use always has advantages in total losses, but often the NIMBY (not in my back yard) effects can be onerous. I'd consider it if my condo association would let me, but like you, my roofline runs N-S, not ideal at all.
  20. Dana

    Dana In the trades

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    A 50% efficiency inverter would probably be bad enough to be illegal as a grid-tie (but might be wicked-cheap.) Most current grid-tie PV inverters are over 90% efficient across more than 3/4 their operating range, and some are over 95% even at half power. At minimal loading the efficiencies can drop to under 80%, but 50% efficiency would only occur at 1% of power or less (50% of nearly zero is even closer to zero, but in practice it's not big loss.) The ~80% efficiency knee typically only occurs when the solar input is less than ~15% of peak, so on cloudy days you might get less than 80% efficiency out of the reduced available resource, but when it's actually sunny you'll be doing much better. In terms of the total annual efficiency, the average conversion efficiency is fairly close to the peak efficiency point if the inverter is sized correctly to the PV array.

    Of course, if you're trying to convert 0.6V DC single cell output to 240AC efficiencies could be pretty low... ;-) Most panelized goods on rooftop arrays are running 12-20V out of the panel and operated in series to provide 24-48V to the inverter, for lower fractional rectifier losses, etc. Utility type arrays run even higher VDC.

    Up converting to high voltage AC and back down is less than a 5% loss, but the simplicity & ease of transforming up to truly high voltage/low current and back down for power transmission is where AC really shines. It's a more complicated picture for high voltage DC transmission lines.

    I'm wondering how roadbed & parking lot PV handles the intermittent shading factor of parked or moving cars. Seems like it would have to have a lot of switching grid and DC-DCconverters local to sub-sections of the panel to keep it all happy, which is another layer of complexity. This is sometimes done on a panel-by-panel basis for rooftop arrays with unusual shading factors to avoid the situation of shade falling on a single panel from taking a whole row or column off line, but it comes with a conversion loss and and uptick in complexity (and probably a hit in long term reliability) too.

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