Saturday, August 14, 2010

12 VDC Heaters

I've been looking for a way to come up with a renewable energy solution for some time--and I have not been terribly impressed with the options.  Photovoltaics are still not cheap enough to pay for themselves over any economically sensible time.  Our wind speeds aren't high enough here (or in many other parts of the U.S.) for most wind generators to pay for themselves in any economically sensible amount of time.

Part of the problem is that someone needs to come up with a wind turbine that can produce power at very low speeds.  If you can produce electricity at 5 mph, your generator will be running almost continuously in places like this (and probably where you live, too).  The problem is that the power that you get out of wind isn't linear based on wind speed.  The power is the cube of the increase in wind speed.  As a result, if you want to use low speed wind generators, they either have to be incredibly inefficient, or incredibly cheap.

Windtronics has an elegantly clever idea that Honeywell is marketing--a gearless wind turbine where the generator is the housing!  Brilliant design, and one that might actually be useful in places that have consistent but low speed winds.  But it's an expensive unit, and the last I checked, not actually shipping yet.  Worse, the price tag (because someone has to pay for the development costs) means that it still not economically sensible.  (Note: factoring in big tax credits by your state and local government doesn't make it economically sensible; it means that taxpayers are subsidizing your "Look how much I love Mother Earth" posing.)

Part of what makes all alternative energy systems spendy isn't just the cost of the wind generator, or the photovoltaic panels--it is what you do with the electricity now that you have it.  Because both wind and sun are intermittent, you have to store the electricity in batteries (for use when the sun or the wind goes away).  Or you use a grid-tie inverter to feed excess electricity back to your utility company when you are producing excess power.  This runs your electric meter backward.

Both of these are good things, no question about it--but there's a pretty sizable capital investment in either the battery backup or the grid-tie.  Battery backup has additional problems: regular maintenance; storage space; occasional replacement of batteries.  Suddenly, whatever merit the PV panels or wind generators had has become a lot more expensive--and that much harder to justify as economically sensible.

Let's simply the model.  What if you didn't need the battery backup or the grid-tie?  My oldest sister was talking to some people who build small wind generators from surplus, and apparently pretty cheaply.  (How cheap?  Still waiting to get an email back.)  They produce 12 VDC output (as do most PV or wind generators).  Instead of all the complexity of trying to make the generators replace the grid--identify something useful that you can do with 12 VDC.  That means that you need some system where 12 VDC supplements an existing system--but does not replace it.  More importantly, it needs to supplement an existing system of your house without requiring any changes to your existing house systems.

Lights?  Nope.  Even if you changed everything over from 110 VAC to 12 VDC, the power from wind generators is necessarily intermittent--and you don't want intermittent lighting!  Refrigerator?  Same problem.  It is too expensive to use an inverter that switches back and forth.

But what about heating?  At least where we live, our strongest and most consistent winds are in winter--when we get some real howling storms through here.  My guess is that many parts of America have that same experience.  Take the 12 VDC output from the generator, and feed a couple of 12 VDC electric heaters to warm your house.  It isn't going to replace your current forced air furnace--but if that heater warms your house even five degrees, that reduces the amount of LPG or natural gas your forced air furnace uses, and the amount of electricity to run the fan on the furnace.

The electric heater does not have to be terribly efficient, as long as the electricity from the wind generator is free, and as long as the wind generator isn't hideously expensive.  Where I live, even a five degree gain in overall house temperature would probably knock about $10 a month off our our gas bill, and perhaps $3 a month off the electric bill.  As long as the purchase costs of the wind generator and the heater is low enough, this can make economic sense.

I am still waiting for word on the cost of the surplus wind generators.  Any suggestions on inexpensive 12 VDC space heaters?

UPDATE: Of course, if you take this bare bones direct to application approach, maybe even photovoltaics start to make sense.  Sun Electronics is offering 190 watt blems for $338.20.  (That's less than $2/watt--very impressive pricing.)  I'm not sure how much loss there will be from running a hundred feet of 12 VDC wiring to a heater--but the goal is to provide heat.  All the loss that takes place within the house turns into resistance heating, right?  If you managed to get 140 watts of the output of this panel released inside the house as heat, and you managed to get it to happen for 3-4 hours a day during winter, that's 420-460 watts that don't have to be produced with a forced air furnace.

Another attractive aspect to this bare bones approach is that you don't need to hire an electrician to do something like this (unlike anything that involves connecting to the grid, or your house wiring).  Any reasonably competent handyman can install one of these panels outside, and run wiring into the house.  Making it look pretty might be a bit challenging, of course.

Here's a 1 Kw wind generator for $429.  That's at 22.5 mph, of course, which we don't get very regularly up here.  At more typical speeds for our location (5-8 mph), I suspect it will be more like 150 watts--but perhaps for a big chunk of the day.  Again: this probably doesn't make economic sense, once you spend several hundred dollars on the grid-tie inverter, and a couple hundred for an electrician to install it, and couple hundred for a tower.  But if you can get something like this directly running a DC heater, the capital cost comes way down--and even a continuous input of 120 watts of heat for ten hours a day into the house in winter makes sense.

UPDATE: This is discouraging.  A couple of readers tell me that gearboxes on wind generators are actually not life of the unit--and are effectively common maintenance items, as are various generator parts.  There's a reason, I guess, that all this alternative energy stuff needs gobs of subsidies: it probably doesn't make any sense, except as a fashion statement.   Eternity Road has a depressing example of what happens when environmental activists decide to help the downtrodden with their alternative energy needs.


  1. I wouldn't buy a 12V DC heater, at least not if you're somewhat serious about payback and redundancy in case of emergency. That's pretty inefficient.

    We have a pellet stove that can burn either corn or wood pellets. I use mostly corn since that's pretty cheap around here.

    If you search, there are many pellet stoves that can run off a car battery for several days or more while putting out 50+K BTU. They draw 30 watts, or about a taillight's worth, using that power to drive a couple of fans and some electronics and an auger every now and then. And pellet stoves are cheaper than most other forms of heat.

    Personally, I have a battery inverter and a my choice of boat battery, ATV battery, or a few cars as my backup in case I lose power and want to keep warm.

  2. Wind is attractive because once you have capitalized the generator and the heater--it's all free. I don't mind if the system is inefficient--as long as the capital costs are low.

    The problem with most alternative energy systems is that the capital costs are substantial enough that you may never amortize the capital, or it might take 20-25 years to do so. If $500 of capital investment gives something that saves $100 a year in LPG and electricity for the forced air furnace, with no supplies costs, that's a five year payback.

  3. "...once you have capitalized the generator and the heater--it's all free."

    I beg to differ. I used wind turbines for 10 years aboard my sail boat and they do suffer wear and tear especially in constant wind environments such as the trade wind areas.

  4. I tend to agree. A friend has a wind turbine he's used for a few years now and he complains about the maintenance, as well as the fact that he has to pay careful attention to when he runs it, especially if the wind is gusty and gets to too high a speed when he has to shut it down. It's not something you can run unattended, and even a 2KW system is over $2K.

    The problem seems mainly to be the gear box for him and most others. In fact, many folks view the gearboxes as consumables that have to be factored into the cost of the system, and it's not a small cost to replace one of those.

  5. I would have thought gear boxes were life of unit sort of things. This is discouraging. There really is not only no free lunch, there's not even a cheap lunch.

  6. Bah, Blogger ate my first attempt at this.

    Long cable runs are the bane of low voltage systems. That's what killed Edison vs. Westinghouse/Tesla.

    I can still imagine a system where a windmill, a belt drive, an old car alternator, voltage regulator, and Battery were used to run a household's LED-based lighting system. But I think the house should be built with low voltage lighting in mind.

    I also wonder why windmills aren't built with featherable props like an airplane so they don't overspeed.

  7. Explain how featherable props work. Windmills have some mechanism for preventing overspeeding, but I'm guessing that it is based on a gearbox. Is that why they don't last?

  8. As Mauser said, low voltages get killed by long runs. The system you're talking about is a simple resistively loaded system, controlled by the equations P=I*V=R*I^2.

    Consider your case: 100' of run at 12V. Assume you go big with 10 gauge copper, giving you a cost of ~ $150 in wire and a resistance of 0.12 Ohms.

    Assume you're putting out that 500W at the output of the windmill at 12V. That means you've got a current of I=P/V=41.7A. The power you're burning in your wire as heat is Pwire=R*I^2=205W!

    Now assume you run that same system at 48V, where a lot of windmills work. That system wastes 13W in the wiring resistance.

    Which one would you want to use? :-)

    As to the props, feathering changes the angle of attack and it works reasonably well, but it's a moderately low speed response mechanism. Simply put, the feedback runs pretty slowly, making the control loop pretty unstable for quickly changing conditions and it puts a ton of stress on the components before it can recover.

    In contrast, you can use electrical braking to dump power (generally into a resistor bank) which is much faster and makes the system more stable under gusty conditions. But this puts stress on the gear box since the system has to handle the additional input power and torque.

    The best method is actually to use both solutions. You use a proportional-integral control loop, with the integral path controlling the low speed "feathering" path, and the faster proportional term getting the electrical braking control.

  9. STATEMENT:...WIND ENERGY....Seems to be a reason that installations of Wind Generators(Wind Mills)are referred to as WIND FARMS. When your Ag crop is ripe you harvest, and as so, when the wind blows you harvest your wind power.

    Ag Farmers know that they can store their product for a limited amount of time before it is spoiled and of little use to them. Whereas Wind Farmers have yet to figure out a sensible method of storage for their Product. Sure they can Bank it in Storage Batteries at the loss of a percentage of what they harvested. They can Bank it in the form of Hydrogen Gas, again with the loss of Product harvested, or one day they will get smart and use it close to the production field and apply that Product (Energy) to Manufacture of Hard Products. ie: Rock Crushing, Metal Recycling, Plastic Recycling, ect. Creed of the Wind Farmers should be "USE IT OR LOSE IT".

    So what is a small producer to do with all of his FREE (Free with a big price) Energy. If you live in an area with harsh winters you can bank the energy in the form of HEAT and withdraw that heat from your Bank as you need it. If you are a small producer, then a system of power production would dictate an Alternator supplying AC to the remote Rectifier for use as DC in your resistance heating circuit for your Bank.

    By now you are asking yourself 'What is this Jerk talking about?'...Well this Jerk majored in EE and Math in the days before they offered courses in Computer Sciences. So to work in Data Processing you first got your EE on your skin and your minor would be Math (still an Art back then). There was nothing like being a new Systems A/E back in the late 60's as to now be be honored as Jerk in this NEW AGE of Progress.

    P.S....I know about Wind Farms, used to work for a Bank that pushed the paper for the Wind Farm Developments....It was not about Energy Production, It was about TAX CREDITS and still is. Oh, the War stories I could and can tell about the development of Wind Energy in California.

  10. When I was in high school, I learned that the biggest crop in Utah was lawn grass. For a long time, I've been wondering what I could do to cash in on this potential energy source. Moonshining, perhaps?

    One day, a couple of years ago, I had a fantastic idea: I could extract the carbon-14 from the grass, and then use it in a nuclear reactor. If carbon-14 is fissible enough, it should be in grass--hence, it should be easier to obtain than thorium or uranium...right?

    Turns out that fissibility is the least of my worries: Carbon-14 is a trace isotope, at about one part per trillion. Literally. It would seem I'd have to process one trillion ounces of carbon, to get one ounce of carbon-14.

    Yep, that's going to work!

  11. Ran across this article yesterday:

    "Extreme DIY: Building a homemade nuclear reactor in NYC"