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Thread: harvesting waste heat from my water heater

  1. #16
    In the trades Dana's Avatar
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    Compost piles are aerobic and not immersed in a heat-sink (==water). Not a good analogy to what's going on in a septic tank which is anaerobic and immersed, and being fed new material & water at low temp daily. While exothermic, it's nowhere near as rapid or exothermic as what's happening in a compost pile. IIRC cess-pools use aerobic processes and put out slightly more heat, but also put out more gases. The temperatures at which this occurs is still well below that of domestic hot water, and raising it to that temperature would kill off the bacteria responsible.

    To keep the tank temps high enough in winter that the bacteria in septic tank even keeps up with the daily load sometimes requires insulating the tank in cold-water states like MN or the MI-UP. Extracting the even modest amounts of heat there would not be a good idea.

  2. #17
    General Engineering Contractor ballvalve's Avatar
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    Next time I open my tank I'll take its temperature.

  3. #18
    DIY Member Willie Two's Avatar
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    Anyone have thoughts on two water tanks in series ? Tank 1 gets cold water from the street, holds the water and brings it up to room temp ( of the surrounding room ) then it goes to a normal operating tank, flame or electric, to bring it up to hot water temp. The first tank is not connected to any heat source BTW.

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    It depends somewhat on how long the water sits and if the tank is insulated, and the temperature of the room. The bigger the temperature difference, the more heat you'll be able to transfer in any particular time. The heat will be coming out of the room, so that may be a consideration if it is space heated. What's the efficiency of that verses the WH?
    Jim DeBruycker
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  5. #20
    DIY Member Willie Two's Avatar
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    I would unwrap the tank insulation of course, my thinking is we use the shower / tub in the mornings so the incoming water pre - warming tank is capturing some of the house heat overnite so the energized HW tank won't have to work so hard when it calls to heat the water up for the "next" use. BTW , the prewarm tank I have and there was no cost for it. Installation would be done by me so that cost factor is minimal also.

  6. #21
    In the trades Dana's Avatar
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    What you're talking about is called a "tempering tank". Whether it's a net energy saver for you depends (as jadnashua suggests) depends on the efficiency & cost of your space-heating, since the energy it takes to bring the tempering tank up to room temp is drawn from the room, presenting extra load to the space heating systems. In BC the heating season is typically 8 months of the year or more, depending on location, and the incoming water is a bit on the cool-side. Tempering tanks make more sense in cooling-dominated climates.

    If you go with a tempering tank be sure to provide a means of catching the condensation that will surely be dripping from the sides at times during the spring/summer/fall.

  7. #22
    General Engineering Contractor ballvalve's Avatar
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    Seems like the only place at tempering tank makes sense is in a green house, at the hottest spot. Let it water the roses.

    Sunrunner used to sell a hot water tank in an insulated box with glass top to go on the roof and a shutter to close at night. Thats not hard to build and might give you all your hot water.

    Want TONS of free heat? Autos are a bad joke, spewing out a day of house heating heat in a few hours running. Put the tank in your trunk with a heat exchanger and run the radiator water thru it. Not too practical.

    Going off line with a tiny diesel engine that heats the house, and water and makes electricity can come close to beating the utility. Especially if you get most of the exhaust heat saved also.

  8. #23
    Retired Defense Industry Engineer jadnashua's Avatar
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    There was a company that designed a NG fired generator that was designed to use the waste heat to make domestic hot water or for space heating. Haven't heard from them for awhile. They were from somewhere around Boston. It was a nice unit. not sure if it still exists.
    Jim DeBruycker
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  9. #24
    In the trades Dana's Avatar
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    Climate Energy (the Boston area company) was acquired by ECR International a coupla years ago, but are still selling the kilowatt-Honda cogenerators bundled with a furnace or boiler under the name Freewatt. (The guy in the lab on the other side of my office wall is on season 4 with his hydronic Freewatt, and it's already paid for itself, but he lives in 20 cent electricity land- YMMV.) If they would decoupled the Honda from the rest of the package I could easily hack it into my existing system and it would be able to support something like 50-60% of the thermal load and 80-100% of my electricity (if net-metered.) The boilers they bundle it with are WAY oversized for my loads (even at minimum fire) so going that route would be pointless.

    Inverters for going off-grid with the Honda are a cost-adder, and you only get 1.2kw peak out of the sucker which isn't enough for peak power loads, but grid attached & net-metered they're pretty straightforward li'l beasties.

    In Japan there are getting onto 100,000 1-1.2kw Honda cogenerator installations, but the number of installations in the US is still quite modest.

    The only other micro-cogenerator vendor in the US is Marathon Engine, selling a ~4.7KW modulating EcoPower http://www.nist.gov/el/upload/5-2-Cocking-Ecopower.pdf

    The German utility LichtBlick is taking it up a notch installing large numbers (target of 100,000 by 2015) of 20kw VW-powered EcoBlue micro cogenerators, using larger thermal buffer tanks (500-1000 gallons) to optimize run-times. The units are controlled by the utility as means of grid-hardening their wind & solar resources. The fuel is paid for by the homeowner who is reimbursed at wholesale for the excess electricity, and the utility maintains the equipment and guarantees by contract that the buffer tank always has sufficient heat for the thermal loads. It would take a world class power-pig of a household to end up net-negative on the power produced, so they're essentially getting their electric bill zeroed out for a modest increase over what their gas bill would have been had they gone with a condensing gas boiler. At 100K units the utility get's a nuke's-worth of highly flexible peak-power generation capacity that's mostly capitalized by homeowners who also fuel that generator(!). If only the utility structures in the US were as creative, eh?

  10. #25
    General Engineering Contractor ballvalve's Avatar
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    Seems like the ROI is iffy with off the shelf units. The Marathon looks good. Heres a starting place for your own system that burns about anything with some sort of oil in it.

    http://www.lifeunplugged.net/alterna...selengine.aspx

    The REAL english Listers were a wonder.

  11. #26
    In the trades Dana's Avatar
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    If they'd sell the kilowatt Honda un-bundled the ROI of the grid-attached unit + 50-100gallons of thermal buffer (including indirect HW volume) would be pretty good.
    The size of the buffer required for keeping run times of the Marathon reasonably long is considerable, and installed system costs are well north of $20K for "typical" systems. For big not-so-efficient homes in high electricity rates it's a slam-dunk. Max thermal output on them is nearly 2x my design-condition heat load, so I'd get roughly zero out of the ~2:1 turndown ratio of modulating the beast, and would need at least 1000gallons of high-temp buffer to keep it real.

    IIRC LichtBlick is only nicking ratepayers a flat rate of €5000 (~$7000 USD at today's exchange rates) for VW EcoBlue systems, which means it's either at-cost, or at a somewhat subsidized rate. Given the high price of residential rate electricity in Germany the simple-return would be less than 5 years for most, under 2 for some. It's something of a no-brainer investment for many- roughly half the cost of a mod-con, with 5-10x the ROI. (When that deal comes to MA I won't hesitate! :-) )

    At the current cost of diesel it would take a quite substantial per-kwh rate to make running a ~15% thermally-efficient beast of an antique diesel engine to pay off, but it might if you had an endless supply of scrap fat that they paid you to take away. The soot emissions could get you into air-pollution trouble in urban/surburban areas. And extracting the last BTU out of the exhaust on a diesel (and particularly a junk-fat burner) takes more expensive materials & methods than with natural gas exhaust. If you could hit 65% efficiency as a space heater I'd be surprised, and hitting 15% as a generator for a total net of 80% system-efficiency would be the stuff of dreams. But that's the very low end of the off-the-shelf units- most hit 90%+,on average. I'll bet the noise of that Lister contraption is a lot more than that of the refrigerator-level hum of the suitcase sized kilowatt-Honda's too.

  12. #27
    General Engineering Contractor ballvalve's Avatar
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    Yes the Lister is loud, but remember that RED diesel is [variably much less] than un-leaded fuel with far more btu per pound. Or burn heating oil.

    They make quiet diesels too if one doesnt want the antique that runs for 100 years with a handful of repair parts.

    I say we put the little 'failsafe' battleship type nuclear reactors every hundred square miles. Better ROI than Joke Solyndra. Make them on a assembly line like a ford Lincoln.

    The best and most intriquing Uranium mine on earth is hanging over the south rim of the grand canyon. Now the Enviro morons want it dismantled for millions, rather than making it an incredible bit of mining history. And backup for the future. But a hideous piece of amusement park glass hanging out in Grand canyon space is a work of "awe" . Pathetic priorities in America.

    That 585 million dollar Solyndra handout could have saved 20,000 furniture jobs in North Carolina and kept them out of Vietnam - where their chairs made with stolen wood from Siberia fall apart reliably in 2 years.
    Last edited by ballvalve; 10-20-2011 at 10:56 AM.

  13. #28
    In the trades Dana's Avatar
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    The economic of new nukes (even "the little nuke that could" types) just don't cut it in the real world, just as Solyndra didn't/couldn't.

    Large in Solyndra's failure was that they didn't think the cost of silicon PV could crash as quickly as it did, and they had little hope of getting their expensive panel/cheap racking approach to market quickly & cheaply enough. The panels used to be the cost-driver of small scale PV followed in short-order by the racking systems necessary for mounting them. Solyndra had bet that getting the cost out of the racks was the quickest way to cutting the cost of the system, but that proved not to be the case, and panelized silicon-PV was destined to stay ahead of them on the cost curves.

    It's doubtful given the current high cost of building nukes that they would become even REMOTELY competitive with grid-hardened PV/wind and natural gas fired micro-cogens in any reasonable time frame.

    And the Solyndra deal was a loan-guarantee, not a handout. In the end it won't end up costing the taxpayer the whole 585 megabucks. That loan guarantee program as a whole had already budgeted in far more than the Solyndra costs, and other US CIGS technology solar companies with very low-production-cost panels are in high double-digit expansion mode (notably NanoSolar, another beneficiary of the loan program, who maintains the bulk of their cheap printing-process manufacturing in California.) The US bankruptcy system is a slow greedy piece o' crap compared to how it works in Canada, but despite that the lawyers won't get away with vacuuming up 100% of the asset value on this one.

    But one bad bet is by no means the signature of the US solar industry as a whole. If it's not developed here we'll be buying it entirely from China (and loving it.) At the moment the solar balance of trade with China favors the US, but it's primarily due to US exports of silicon production technology. Cheap Chinese silcon PV is driving some US PV manufacturers to pick up an move production there (notably Evergreen.) There's a lot of consolidation going on in the industry as the prices race to the bottom, and not all novel approaches to cost competitiveness (like Solyndra) will succeed- count on it.

    At current pricing the lifecycle cost of power generated by silicon PV is already well below 10cents/kwh even for small (1-10KW peak) systems, and with CIGS technology in large arrays it's even less. Most analysts are calculating PV kwh will be at grid-parity with the cheapest fossil plants before 2020, maybe by 2015. There's plenty of room for PV taking decent slice of the pie by 2030 despite the fact that it's only making kwh-hay while the sun shines. With electrification of the automotive fleet and with smart grids & batteries it could run the whole show by 2050, but without the policy initiative to do that nobody is making that bet just yet.

    Nukes (even mini-nukes) have the ramp-up/ramp-down time problem, over and above the daunting initial up-front costs. Ain't nobody expecting sub-15-cent lifecycle kwh out of any newly constructed nukes, even if maintaining and extending the life of the existing fleet is reasonably competitive with the fossil grid. If you can't ramp it up fast, you can't crank it back overnight- most existing nukes are heat-dumping at night when there is insufficient load to be able to meet the morning load. In heavily nuclear France they can sell some of that nightly excess to Spain & Italy (at a financial loss) to avoid cooking all the fish in the rivers, but that wouldn't be the case in a high-nuke USA. (Maybe an electrified auto fleet could soak up some of that off-peak excess, if that became policy.)

    Even if it were burning raw crude oil at an estimated 2015 price of $100/bbl, at ~15% efficiency the dirty-little-diesel will not be competitive on a kwh cost basis with PV by 2015. And as a heating system a mini-split with an annual average COP of 2.5 with 5-cent PV power looks pretty good relative to the 60-65% heat output of that type of cogen with 100bbl oil.

    Then there's the first ~30% of end-use efficiency improvements for that are essentially cost-negative- they pay off in utility bill savings in very short years. From a grid policy point of view it's cheaper to reduce the base load by a nukes-worth of pre-1990 refrigerators with higher efficiency versions than it is to build new capacity (any source) to support the difference, and that's just the tip of the efficiency iceberg. Getting rid of the worst-efficiency incandescent lighting technology might be the frost on the tip of the efficiency iceberg. :-) That's cost-negative and very cheap up front, yet people bitch about that. Imagine the hue & cry of surrounding them with mini-nukes (of any design) post-tsunami, and present them with the bill for it? The meltdown fear may be as irrational as the efficient-lighting nay-saying, but the pain in the pocketbook would be real. The nuclear industry didn't crash in the early '80s due to all of the protesters, it crashed due to the high cost, the WPPS bond failure, and the very real prospect of much cheaper efficiency & conservation cutting into demand- there was not a profitable market for the output of those projects, and it was beginning to be obvious. Building giga-projects with decade-plus construction schedules is just too financially risky with the amount of low-hanging efficiency fruit that abounds, and those factors haven't changed much in the past 3 decades.

  14. #29
    General Engineering Contractor ballvalve's Avatar
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    I would rather not have all of my song birds piled up in huge heaps under wind farms, the dirty secret of wind power. And even at 6 cents a KW what does Canada and points north with 6 months of night do with a solar panel? Put whale blubber in their Lister or Cummins? Where did the 15% eff. figure come from? Even in Wisconsin, with the panels buried in snow, we rarely saw sun once a week. And the cost per KW is just a start with all the controls and perhaps batteries.

    And I dont want my So CAL and Nevada desert polluted with 10 billion acres of glass.

    If Canada captured all its water power, it could supply America day and night. And about 40% less water over Niagara falls would still give the tourists a thrill. And keep it from moving toward the west as fast. Water power loves rain and snow and clouds, 24/7.

    And the clowns in the east and even in the northeast are blasting out old, viable hydro dams for the ten Berkely grads that care, and for the Salmon that gave up that river 80 years ago.

    As to small nuclear, it can be built failsafe and in small manageble units. Units small enough and flexible enough not to break up in a 8.0 quake. Costs would plummet just as with solar panels on a production line. The japanese are guilty of criminal design in those idiotic storage pools and placement of gen sets.

    Yosemite national park generated ALL of its carbon free power for 60 years until some enviro boss decided it wasnt in tune with the natural enviroment. Small. invisible. efficient. Redwood penstock. THEN, they left the power house in, because a new set of nuts decided it was historical. It was a run of river scheme with no impediments to any sort of fish, of which there are none in that area anyway.

    Now, Yosemite has hundreds of 40' long gensets for when the power is out predictably days and weeks at a time since the new lines run through mountains even a fox cannot traverse. A dedicated Muslim could take out the parks power for a month with a backpack acetylene torch.

    Solyndra could have moved into a shuttered woodworking plant for FREE, wired and ready to go. But flush with cash built a new, now empty cave.

  15. #30
    In the trades Dana's Avatar
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    Nobody is suggesting putting all the eggs in a single basket, and that one solution works everywhere- it's clearly not the case.

    Most of the population of Canada is well below the 50th parallel though, and solar PV is a highly subsidized & growing part of the grid in Ontario & Quebec, but those provinces also have extensive & cheap hydroelectric power as well. Much of Canada is on the natural gas grid too, making the mini & micro cogeneration a worthy approach to distributed power ala LichtBlick. I suspect the feed-in tariff for micro-PV in Ontario is even a bit TOO generous, given the current cost (IIRC they're paying ~65cents/kwh for ground mounted <10KW PV these days. that's a heluva subsidy, given that it's nearly an order of magnitude higher than the residential-retail price.)

    Even in Wisconsin the snow doesn't dwell on PV panels for weeks on end- the mounting angles are above the avalanche threshold- it'll all slough, and the solar heating of the exposed section helps it along in short order after then initial sough. "Buried in snow" could only happen if the implementation were optimally bad. It's more of a problem with evacuated-tube solar thermal panels, where the insulation of the evacuated glass drastically reduces the rate of the solar-melt.

    For reference:

    It takes merely ~7% of the total existing residential & commercial roof area in the US to provide 100% of the electric power used, assuming 15% efficient panels. That consumes 0% extra real estate. You don't need to glaze Nevada & SoCal to get there. Only if you feel a need to maintain a remote centralized-production grid model would the mega projects in the desert make sense, but PV is infinitely scalable (down to a few hundred peak watts).

    By comparison some of the bio-diesel folks got all excited earlier this year when somebody figured out that 17% of the oil imports could be offset with an algae-farm "only" the size of South Carolina. I only wish they were joking, but they seemed to be thinking that was good news.

    The mini-nuke is still a non-starter, even if some of the reasons aren't purely rational, and it's doubtful that even if they were mass-produced they could hit grid-parity with cheap fossil plants. A lot of really smart people have looked at that very carefully, and nobody is holding out even fantasy-numbers that low. The opportunity-window for the mini-nukes was already starting to be closed 40 years ago, and there's no making up for a 4 decade hiatus in product development. Standardized nuke design construction is on the fast track in China, but even there costs are high (not that hard numbers are easy to come by.)

    The plant design in Fukushima was done primarily by US contractors (General Electric), and even when some of the risks were pointed out by GE engineers as early as 1975 that was soft-pedaled by the suits, causing some engineers to quit in protest. If there was criminal negligence involved, it can't all be laid at the feet of the plant operators in Japan.

    The bird death issues with wind power have been highly overstated, with the worst data based primarily on 30-year old projects in CA. In Europe that issue has been studied to death, and it appears that the bird deaths from collisions with windmill blades are fewer per mill and much fewer per megawatt-hour output when the size of the tower and mill are larger. In Scandanavia and the Netherlands siting of wind farms (they call them wind-parks there) for optimal bird population health has more to do with keeping nesting areas from being disturbed by construction & development than the bird-blender aspects of megawatt-class turbines.

    I have no knowledge of any particular real-estate options Solyndra had as of 2008, but ready adaptability (and expandability) of an existing woodworking plant into a PV panel manufacturing facility isn't necessarily a slam-dunk, even if it might have been cheaper than new construction. To be sure the powers that be at Solyndra weren't the best at interpreting the crystal ball, nor were they dumb-lucky.

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