Last year I got excited about ground-source heat pumps, but my hopes were dashed when I found that installing the ground coupling on my limited lot size would cost upwards of $60k to drill multiple 300-ft-deep wells.

Recently I thought of this: what if the heat pump instead coupled to to a large water tank that was fed by a swimming-pool style solar collector (i.e. unglazed, and relatively cheap)?  In the winter, the collector's circulation pump would run in the daytime, replacing the heat sucked out of the tank by the heat pump.  In the summer, it would instead run at night (with a boost from the thermosiphon effect) to use the collectors as night-sky radiators, rejecting the heat that the heat pump would be adding due to A/C usage during the day.

Could such a system be workable?  Could it even work better than a ground-source heat pump?  Would the tank need to be insulated, or should it be buried uninsulated to allow some ground coupling to help (and what would be the cheapest way to get/build such a tank)?

Of course, I would need to figure out the sizings of the tank and collectors.  Would 1000 gallons of water and 200 sq. ft of collector area be enough?  Info for that: My house is in San Jose, CA, with relatively mild winters and only the occasional heat wave in the summer.   My current summertime A/C use is rather low (but I want to use it more, without the damn noisy air-coupling fan in the back yard); I'd want the capacity to deal with say a 1-week heatwave with nighttime temps in the 70s, daytime in the 90s (relatively low humdity).  In the winter, my gas bills show that my central furnace's usage peaks to about 3 therms per day, so that's 300K BTUs that I'd need to be able to extract daily from the tank for a 1- to 2-month stretch.  The house is 3800 sq. ft, and was built with decent insulation, but I'm looking for ways to improve that further.

Thanks!

I've learned a little more about the numerics of heat storage -- basically, the fact that a BTU is the heat to raise 1 pound of water by 1 deg. F. So it seems to work out that if I had a 200-cubic-foot tank (~1500 gallons, or ~12500 pounds), each 10 degrees of temperature change stores ~12500 BTUs, or a bit over 1 therm. If I can somehow reduce my house's heating needs to 1 therm/day max, then if, say, the tank started at 100 degrees F, that's 5 days of heating before the tank drops to 50 degrees (such that it becomes less efficient than ground-source), assuming no replacement of heat whatsoever. Not sure how much heat would be lost to the ground directly in that time -- which leads to the question of whether I should have the tank insulated or not.

On the collector side, I found a little calculator utility here:

http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

from this, it looks like if I had 400 sq. ft of the cheaper swimming-pool type collector, it could keep up with the demand fairly well, even if it doesn't maintain 100 deg. F tank temperature. e.g. if I assume 40 deg. ave daytime temp, and a cloudy day (sunshine at 1/2 intensity), then the pool collector could push the tank up to 50 deg. at a rate of 100 btu per sq. ft per hour. If I assume 4 hr. effective "full" daylight, that's (100 x 4 x 400) = 160000 btu, well over 1 therm gained for the day. To get the tank up to 60 deg, it gets 66.7 btu/sqft/hr, or 108320 btu = just about 1 therm for the day. It could push up to 70 deg at half that rate, or 1/2 therm/day.

So it seems like it can work with those numbers -- 1500 gal. tank, 400 sq. ft cheap collector. I just have to see what it would cost to build such a system. If anyone has any ideas for how to make it cheap but still reliable, I'd much appreciate it. And whether I'd need an insulated tank?

You are on the right track. For some people, on-peak electricity prices are 3x more than off-peak - so tank storage for a day or so is a good way to save money. It doesn't scale to weeks or months.

IMO, a tank is best on the interior of the house with some insulation. Bigger is usually better - 2500 gal or 5000 gal for more severe climates.

That 3 therms/day peak use by your furnace may be only ~2 therms/day delivered as heat to the house in a typical setup: An 80% efficiency furnace, with ~15% duct losses.

Efficiency on unglazed collectors is pretty crummy when you need it the most. Running them at 100F on a chilly 40F midwinter day results in a very lossy 60F delta-T. Pool heaters work great in summer running them at 90F on an 90F day, but you'll be lucky to average 20-25% efficiency out of 'em in winter use even in San Jose if you're figuring on a 100F peak tank temp.. See: http://www.builditsolar.com/References/Measurements/Collec1.gif

But for the amount of money you'd need for a ground source heat pump you can likely retrofit the building envelope to PassiveHouse standards and skip the heating/cooling systems pretty much entirely, controlling temps primarily with variable ventilation rate, similar to Nabih Tahan's bungalow retrofit project in Berkeley:

http://www.nabihtahanarchitect.com/

It's just not all that difficult or expensive to gol PassiveHouse style in a moderate & dry Bay Area climate, as opposed to Minnesota or Saskatchewan, or even sticky-swampy Florida. At that point a 500 gallon tank and 100 square feet of pool-panel would likely cover the bulk of your sensible cooling & space heating load with a few panel radiators (or radiant-ceiling/floor, if you like to just spend money.) Most Passivehouse projects just use a modest amount of cheap electric resistance heating though, skipping the cost & complexity of a real heating "system", since from an investment point of view there's no longer any payback in dollars or comfort with a bigger-deal system. But if you're enamored with solar, there are far cheaper means of getting stored heat out of the tank and into the space than heat pumps. In a PassiveHouse storage temps of 80F wold be sufficient to use directly with radiant heating/cooling.

Interesting! I'll check the links, and do more searching on "PassiveHouse".

I'm a bit unsure about a hydronic radiant system, as it seems like it would entail tearing into the house a good deal more. I was thinking that a heat pump solution would "tap into" the existing central-air AC/heating setup, which includes an existing coolant loop running from the attic out to the big compressor/fan unit in the back yard. Another advantage would be less sensitivity to the actual tank temperature, which would only affect efficiency, not necessarily output.

However, if I can get the demand down enough that simple electric baseboard heaters could do it, and the additional PV installation to counter their load is reasonable, then perhaps that would be preferable. It would certainly have less moving parts! Maintainability is actually probably a bigger concern for me than the initial cost. I could leave the existing AC + gas furnace as-is, to serve as the rarely-used backup.

I found another calculator that gives a rough estimate of where the heat loss is occurring:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

the results I got show the biggest contributor, by far, to be air infiltration -- even at the setting level described as "tight -- new, careful construction". So finding out how to tighten it up, then I guess adding a heat exchanger for ventilation (as in Amory Lovins' house), would do a lot. The next-biggest contributor was the windows, then the slab.

Question on the the windows -- they're a simple double-pane type already; would adding one of those "heat mirror" films on the interior help with wintertime heat loss? I know those films help to reduce summertime heat gain, so I'd probably want them on at least the western windows. But do they help significantly in the other direction? That is, in winter, if I had the film on all the windows, would that cut overnight radiative losses enough to counter the diminished solar gains during the day? Or would it make sense to just leave them off the southern windows?

My house's slab is interesting. First, it's massive -- it's an 18-inch-thick post-tension slab, and I think it's even 3 feet thick in a few places. Second, we have both a front and a back "porch" whose floor is really just an exposed section of the slab (with a thin cosmetic coating). I wonder if this is part of why the house (especially the first floor) is already so cool in the summer -- these "porch" sections may be acting as radiators overnight, and they're mostly shaded during the day in summer. This also means it's hard to stop their heat loss in winter; I suspect my slab losses might actually be greater than the window losses! I'm not sure if there's a practical way to insulate them, though. Plus we already have hardscape all the way up to the slab around maybe 70% of the perimeter.

I've also been musing about adding a greenhouse off the back of the house, though I'm having difficulty figuring out how it could aid the house heating, given that the air in there would probably be too humid (and perhaps fragrant) to just pump into the house.

Anyways, thanks again for your response! Continuing up the learning curve...

An 18" slab exposed to the outdoors is a huge heat sink in wintertime, and where sun-esposed, adding to the cooling load in summer. The R value of a foot of concrete is still shy of R1, so this is a significant heat loss that may even exceed that of your windows (which are ~R2). Consider putting a layer of rigid XPS insulation on top and some sort of finish decking- 2-3" of rigid foam would be ideal if you can do it without running into issues at the door thresholds. The "porch" and slab edges also need to be insulated with at least 2" (R10) of XPS (pink or blue, depending on manufacturer, at box stores) around the perimeter to at least 18" below grade, which would "earth couple" the building to the thermal mass of the earth (which will run at about the average annual rather than annual seasonal air temp under the slab, which is something around 60F in your area.)

There's no credible evidence that low-E films will save energy in the winter, but they WILL increase wintertime comfort while standing next to a window. Windows that get direct sun in summer/fall may experience early failure with reflective films placed on the interior. Insulating shades/blinds are a better bet for reducing wintertime heat loss, and exterior roll-down reflective shades are better at rejecting summertime solar gain than window films (but at a much higher up-front cost.)

There are insulation companies that specialize in air-sealing, as well as a lot of obvious & less obvious DIY stuff that can be done. Until you get the infiltration down below 1/3-1/2 air exchanges per hour at 50pascals pressure (an industry standard test pressure/methodology) you won't absolutely NEED active ventilation (preferably with heat-recovery, HRV or ERV style), but any time you're under ~2 ACH/50 a ventilation system makes a noticeable difference in how fast cooking odors, etc dissipate. Air sealing contractors typically pre & post test the ACH/50 numbers and guarantee some minimum fractional reduction, but it pays to do the dead obvious big-hole stuff like dryer vents, kitchen vents, fireplaces & furnace flue, mail slot stuff yourself, as well as any recessed lighting & plumbing chases penetrating into ventilated attic spaces, etc.

If your house is stick built 2x6 & batts, it may be worthwhile to "dense-pack" blown cellulose into the cavities (compressing the batts) to reduce air infiltration. If you're looking for a more serious upgrade, stripping the siding and adding layers of rigid-foam (to almost any arbitrary R) can be a reasonable retrofit, as compared to going geothermal or massive-solar. That would be the time to consider adding high-performance windows as well. If subsidized heavily (as it might be in CA) this can be a very reasonable way to go. A friend of mine is going that route on a ~3800s.f. 3-family rental property he owns in central MA, since there's a 75% rebate up to $40K available through state subsidy for a "deep energy retrofit" under this program: https://www.powerofaction.com/der/ Click on some of the links to see what types of retrofits they're doing to buildings.

There are even system-products specifically designed to make rigid-foam retrofit easy, eg: http://www.quadlock.com/retrofit_insulation/ But using standard 4x8' rigid board to arbitrary depths can also work reasonably. In coastal CA adding 3" of iso on the exterior would more than double the clear-wall R value of typical 2x6/batt construction, and taping the seams/foaming the edges makes it air-tight as well. I'd think you could get to an air-tight R30 on the exterior, R50 blown-cellulose in the attic, and air-seal it to 0.5ACH/50 for under the $60K price tag of a geo system likely even with windows included. Depending on location & size it might be worthwhile judiciously reducing glazed area as well. Done right you should be able to reduce the space heating/cooling needs to under 25% of the current load, at which point resistance electric heating uses no more than geo on your existing setup, and the size & complexity of the solar required to go net-zero-energy (annual basis) might actually fit on the house, even if heat losses out of the slab makes PassiveHouse levels of total energy used impossible. (I think you'd need at least R10 or so under the slab to make PassiveHouse, even with major retrofits elsewhere, and that's clearly not gonna happen, eh? :-) But R10-R15 around the slab edges might, which would be huge.) Once installed, the reliability of insulation is far greater than that of the pumps compressors & blowers of mechanical systems for heating & cooling the house.

Thanks again for all the info!

One point of possible confusion here -- that $60k number is not the value I'm considering spending; it's the value that's so outrageously high that a full-fledged geothermal solution is off the table! Most of that cost is in the drilling for the geo heat loops, at least for the capacity guesstimated for a house of my size by the guy I talked to. The heat pump itself averages just $2500 per ton of capacity; since a "ton" is 288k BTU for 24 hours, a 1-ton unit could supply almost the full 3 therms of daily winter peak capacity for that I'd need, even without improving efficiency. But anyways...

Stripping the stucco siding and adding more external insulation would also seem to be off the table -- I did a rough measurement of the siding area, and it's somewhere between 3500 and 4000 square feet. This is a relatively new house, by the way -- about 7.5 years old (I'm the first owner). Hard to see such a project not costing several tens of thousands of dollars (I haven't been able to find any mention of CA rebates for retrofits). Plus, according to that heat-loss calculator, the walls (assuming R20) are only the #4 energy loss, after air exchange, slab, and windows.

I also don't really want to be tearing up the house too much for this.

By the way, here's my overall "master plan":

Phase I: Efficiency (reduce heating/cooling needs, and electricity usage)

Phase II: Replace my direct fossil-fuel usage (NG and gasoline) with electricity usage: find a way to stop using the gas furnace; switch to solar DHW with electric tankless as a backup; switch the cooktop from gas to induction; and use an electric car for commuting (I'm on the list for the LEAF, but the range may actually be just short of my needs, so I'll probably have to wait a few more years).

Phase III: Add enough solar PV to the roof to counter my full yearly electricity demand.

I figure the budget for Phase I and II (not counting the car) could be up to around $10-15k. As a point of comparison, my gas bills for the last 6 years total about $3700. Extending that out to a 20 year "payback period" yields about $12k. However, I'm not going to be fully eliminating the gas usage (I don't see replacing the gas dryer, and I'm not yet ready to give up the backyard BBQ ); plus there's the fact that Phase II will add to my electricity usage.  But it may be a good guess that I'd need to stay below $15k in order to keep the embodied carbon emissions in all this work to a level below the status-quo NG burning.

So here's how I see Phase I working out, in order of priority:

1. Air exchange

This looks like my #1 loss by a good margin. I'll have to look into what the "insulation companies" are in my area. One question: when you say ".. do the dead obvious big-hole stuff like dryer vents, ..." do you mean make sure there's no leakage around the edges of the "big holes," or do you mean doing something about the holes themselves?

By the way, we do already have a current problem of cooking smells lingering, in the 2nd floor central area around the top of the stairs. So adding heat-exchanged ventilation would also be a livability improvement.

Question: what's a typical electricity usage for such ventilation fans running 24/7? If it's 100 watts, that would be 72kwh/mo; is that high?

BTW, I'm also kicking around the idea of adding a greenhouse, attached to the house behind the garage (this is on the south side of the house). Perhaps floor area 150 sq. ft, total glazing maybe 300-400 sq. ft. Of course it probably wouldn't fit within the $15k budget, but I want to be able to grow a lot of my own vegetables anyways. If I have an air-exchange system, and since the greenhouse would need to be vented anyways, I wonder how much the winter heating would be helped if I included air from the greenhouse as one of the "internal" sources to be expelled? I should be able to get a duct from the GH into the main attic area where it could connect to the exchanger.

Though is humidity a problem? Actually, that would apply to venting air from a bathroom while someone is taking a shower, too -- would there be a condensation problem in the heat exchanger? Or do heat exchangers typically take that into account already?

Another idea: the western wall of the house (slightly south-facing) has a large, uninterrupted area that could fit a good-sized thermal air panel. If I added such a thing, perhaps I could make the output of that into yet another source for the heat exchanger? Of course, I'd need to be able to shut that one off at night.


2. Slab

It'll be hard to do much here, at least externally. I have concrete walk/patio/driveway right up against the slab for about 140 of the 200 total feet of perimeter (I wish I knew about insulating the slab 4 years ago!). Though the north-side "porch" area (which hardly gets any sun, even in summer) is part of the remaining open perimeter, so I could tackle that, at least. For the porch surfaces, adding 2" to 3" would raise them up too high relative to the doors, though having a nice tile for the surface would be an improvement.

One possibility, though, might be to attack it from the other side, adding insulation between the slab and the interior flooring. This may be possible, budget- and "tearing up"-wise, because we're already planning to at some point replace our downstairs carpeting with hardwood floors. The insulation would be an incremental cost relative to that -- except again we have to deal with the doors, including the interior doors; I'll have to see about that. But I wonder -- might such an interior approach be more effective than perimeter insulation anyways?

There's also the garage. I'm a bit confused as to whether to count that as "outside" or "inside," for heating purposes. The ventilation system doesn't connect to it, of course, but it's built as part of the house, sharing the slab, with part of the 2nd story over it. There's insulation on all 3 exterior walls, and also between the garage and the house, but I'm not sure if there's any between the ceiling and the floor above (I do know there's a double layer of drywall on the ceiling, for fire protection). I wonder if adding insulation to the garage door would do anything? It would be pretty easy to glue on some of those rigid-foam panels on the inside, though not on 100% of the area.


3. Windows

Thanks for the info about the films! Sounds like adding some better-insulating curtains would probably be the best approach; though I worry about giving up the brightness that we can currently get by opening up the white blinds slightly. I'll also look into external shades for some of the windows.

4. Take stock

After all this, I'll have to see how far down the heating/cooling demand goes. Then I can tell what heating/cooling sources I can use -- small heat pump? Baseboard electric heating? Or leave the existing furnace/AC? Couple of points: one thing I dislike about the current central-air is the "step" nature of it -- either it's blowing full-blast, generating breezes that can be disturbing, or not at all. One reason I'm attracted to heat pumps is the notion that they can come with variable-speed fans, so that you can have them on a much lower speeds for longer times, to get the same effect. It should be more efficient to push air at slower speeds, too. And less air-blowing noise. Another point -- If I use baseboard electric heaters, might there be some danger of fire, or children hurting themselves? I once left a plastic trash can too near such a heater one day, and came back to find the can half-melted! Lastly, if I include a "basement" in the greenhouse, I could use it to house a large insulated DHW tank (with any escaping heat being "used" by the greenhouse), or I could seal it and use the whole thing as a water tank (possibly up to 400 cubic ft), either for my idea of doing the heat pump + collectors thing, or even just to store collected rainwater for irrigation.

In a house this new for SURE you won't be doing full-rip retrofit. But retrofit air-sealing the house and duct sealing (to CA Title 24 2008 specification) are still likely to prove cost effective in the near or intermediate term. (IIRC Title 24 wasn't in effect until 2006- well after your place was built.)

Plugging the big holes first means using better-than average backflow preventers on dryer vents & flue dampers, etc. If you have a wood burning fireplace, a top-sealing flue damper in place of the typically leaky fire-box located versions or a "chimney balloon" that gets installed seasonally, etc. Dryer vent designs like these tend to be very tight compared to louvered backdrafters: http://www.amazon.com/HEARTLAND-21000-Dryer-Vent-Closure/dp/B00009W3I4 If you have a very long dryer exhaust run these might be a bit too high-impedance, since they're back-resistance is like adding another 10' of vent pipe, but if your dry vents pretty much directly out the side with only a few feet of flex-hose or under 10' of straight vent pipe, these are about as good as it gets.

An attached garage should be considered semi-conditioned space, and even the shared wall with the house is insulated. Hopefully the garage doors are also insulated type, with good weather stripping. Insulated garage doors are pretty common- if your's has a vinyl or pvc look to it, it might be- drill a 1/16" hole in the center of a panel- see if it isn't foam inside. They're typically R8-R10, but R12s are available too. You can also drill a 1/4" hole through the sheet rock in the garage ceiling an poke a coat hanger or crochet hook or something in there to determine if it has been insulated from the bonus room. (Seal the hole with caulk when you're done.) If not, blowing some cellulose in there isn't a huge DIY project, and would be worthwhile if the garage seems to run more than 10-15F hotter or colder than conditioned space seasonally.

Another thing you might consider at some point is swapping out the existing furnace for a Freewatt cogenerator, which will cut down your power bill during the heating season by using the waste heat of a ng-fired kilowatt generator to support the heat load whenever the temps drop below ~40-45F outdoors, net-metering the output. See: http://www.freewatt.com/products.asp?id=169&name=WAir (The installed cost will likely be over $10K, but you'll reduce your annual power bill by a few thousand kilowatt hours. The value of that will vary with the particulars of power rates and net-metering considerations of the utility, but in my 15-22cent/kwh neighborhood full-retail net metering it's a deal. My biz-partner installed the hydronic version in his leaky 1840s antique, and between the fuel savings of the condensing boiler and the net-metering of 20cent electricity the ~$18-20K he paid will be returned tax-free in under 5 years. The warm air version is ~5-8K cheaper than the hydronic version, but use the same Honda cogen. The furnace it's married to has an ECM (high efficiency, variablespeed) blower- it's not bad. It's essentiall a 3-stage system: cogen only, low-fire, high fire on the furnace. The hydronic version also uses the cogen & boiler to heat domestic hot water, which is another significant efficiency boost.

At your wintertime temps you can get good performance out of air-source heat pumps, at a fraction of the cost of geo. If your heat load is low enough you might be able to do it all with a high efficiency mini-split (as is becoming common in less-cold parts of Asia.)

Active HRV don't need to run a 100% duty cycle. Some people put them on dehumidistat control, where they run only when indoor humidity starts to rise, others use timers in conjunction with occupancy sensors which run a minimum duty cycle in background that is increased when people are home. In CA absolute humidity levels (measured as dew-point) are low enough that there is no advantage to going with an ERV (which has a humidity exchange, along with sensible-temp exchange). HRVs are already designed to deal with condensation issues (which are very small in your climate compared to colder climates- the dew point of 30% RH 68F interior air is ~ 37F. Iif the incoming air is above that the condensation potential is miniscule, but the further below that the more significant it becomes. (In my neighborhood the average temperature, not the average low, is ~19F in January, and we're pretty warm compared to ND or MN.) The dew point of steamy shower air is higher of course, but dealing with condensation is a standard feature.

Perimeter insulation on the interior usually leaves a gap or very low R spot in the insulation where the slab insulation meets the wall. If there is a stem-wall & footing supporting the walls (likely) a retrofit on the interior side of the stemwall/footing could get complicated. But even R2.5 (1/2" of XPS) between a wood floor & the slab would make a comfort difference, as well as some heat-loss. If there is currently a sub-floor of OSB or plywood under the carpet, pulling that up and putting in XPS sheets between some sleepers to nail the hardwood floors might even preserve the same floor level. (XPS would also protect the wood from ground moisture potentially getting in through the slab, if the slab doesn't have a poly vapor retarder/capillary break under it.)

DIY thermal air panels are usually quite cost-effective, and with your typical mid-winter sunny-day temps the performance would be quite good compared to colder climates. If it's an active system with a small fan it's pretty easy to put it under thermostat control as well as differential temp control. The simplest controls for these tend to be cheap snap-disk thermostats mounted on the solar absorber in series with the blower, but using a cheap electric-baseboard wall-mounted thermostat in series with the snap-disc can keep it from overheating the place in the shoulder seasons. Low-speed low noise fans/blower are preferable. Commercial versions are available too (eg, SolarSheat, CanSolair, SunMate), but building-integrated DIY versions aren't difficult to design & implement either.

Cheap electric baseboards can indeed present a fire hazard if you're not careful or they're undersized for the load (==very high duty cycle). Low voltage DC radiant mat under floors can work fine, but not so well on an uninsulated slab (or even an insulated slab.) Oil-filled electric baseboards are more expensive, but more comfortable, and harder to set fires with. If you end up insulating under your hardwood, a low-voltage radiant under the hardwoods would make for a nice comfort-boost, if more expensive to operate than gas-fired heat.

oh thanks a lot..you have explained each and everything in a very good manner..its very beneficial to everyone..

rainwater tanks

I already know about the Solar energy system which you can apply to your house. But with the wind energy system, I don't have any information about it. Does anyone have information about how we can apply wind energy in our house?

I'm a bit skeptical of rooftop wind energy in general, mainly because houses aren't built with that in mind, and likely won't be strong enough to do well under the twisting forces that a significant wind turbine would generate. And any turbine that won't cause a problem there will probably be too low of a capacity to be worth the installation. Personally, I also don't like the idea of putting bird blenders up around my house.

It can probably make economic sense for those with large lots (an acre or more), mounted on independent towers. But even then you probably won't get the efficiencies that the utility-scale turbines can get by dint of being hundreds of feet tall.

Posted By dcmeserve on 30 Aug 2010 02:44 PM
I'm a bit skeptical of rooftop wind energy in general, mainly because houses aren't built with that in mind, and likely won't be strong enough to do well under the twisting forces that a significant wind turbine would generate. And any turbine that won't cause a problem there will probably be too low of a capacity to be worth the installation. Personally, I also don't like the idea of putting bird blenders up around my house.

It can probably make economic sense for those with large lots (an acre or more), mounted on independent towers. But even then you probably won't get the efficiencies that the utility-scale turbines can get by dint of being hundreds of feet tall.

Dynamic loading of the building structure is the least of it- most houses aren't nearly tall enough (or located high enough) to have a sufficient wind resource at rooftop level to be worth pursuing.  If your home is a stone hut on the top of a windswept mountain ridge, maybe there's enough wind at rooftop level, but for the rest of us, not so much.

At seashore locations there can be, but the cost of a proper pole-mount to get it even higher above the ground usually pays for itself many times over in increased output. 

Micro-wind mounted to the house is usually just a house-ornament.  Photovoltaic $olar is almost always a better investment.

Moving from rooftop height to say 60 feet is only a couple of mph difference unless you have obstructions (like trees). Building a separate tower isn't cost effective for a small turbine - you will spend less money and get more output by buying a slightly larger turbine. The real issues are things like site selection, intermittent output, rotor size and initial costs. I agree that solar is more cost effective, less intermittent and quieter for most residential applications.

Windspeed at 60' above the deck compared to a roof ridge 20' up is only a couple mph if your windspeed at the ground is under 10mph- it's typically a scaling-factor curve. At 60' up windspeed is about a 1.5x factor over that measured near the ground, while a 20' rooftop is about a 1.2x factor, which makes the difference in windspeed at 60' vs. 20' about 1.25x. Big whup, right?











Well yes, in fact it IS!

Since power output increases by the cube of the windspeed, a 1.25x windspeed multiplier results in nearly a 2x increase in power. Any turbine worth putting on your roof at will usually be WELL worth putting a bit higher.

The cost of the mount is usually a fraction of what it costs to double the output with a bigger turbine.

That is unless you're talking about some tiny sub-500W roof ornament, of course, in which case the visual statement was already being made. If you're talking about real power, you really have to do the wind survey and financial analysis to see what makes sense. (And what makes sense for 99%+ of households is to spend the money on better end-use efficiency measures rather than on teeny-weeny-windmills or PV. You can usually buy a heluva lot of Energy Star appliances, smart powerstrips, occupancy sensors for lighting & ventilation, high efficiency lights etc to SAVE 1000kwh/year for the amount of PV or micro-turbine that would actually PRODUCE 1000kwh/year.

Nope - from 20' to 60' is typically a ~60% increase in generated power (what you really care about), not 100%. And at the size that you would consider putting on your house, you will spend far more on a tower than you would buying a second turbine.

1.25³ =1.95 (nearly 2x, a 95% increase in power) not 1.5.

But I'm not sure you'd ever put anything bigger than a 500watter on a house (for a lot of reasons), so you have a point.  OTOH it's simply not worth putting up anything ANYWHERE that small (at any height) grid-attached, in terms of getting an ROI. (Off grid, different circumstance, different story.)

10 mph has little value as a sample point. Use 3 ^ (3/7) = 1.6 where the first 3 is the increase in height, the second 3 is increase in power and 7 is from the wind power profile law. 100' (5x) will get you to 100% but you could buy two more roof turbines for that.

I agree - consider small scale wind only for some special offgrid/low sun/windy situations.