Understanding System Voltage (?'s)

Discussion in 'Alternative Energy' started by bayesoft, Jul 7, 2006.

  1. bayesoft

    bayesoft Member

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    What should be considered in determining whether you need a 12, 24 or 48 volt battery bank and wind generator?

    A converter changes 12v DC into 120v AC. Does this mean that a converter changes 24v DC to 240v AC etc? Is there really a need for running 240v through your house?

    Should I sell my current washer and dryer and get a GE Spacesaver combo that runs on 120v AC? I think my dryer is the only thing that would need 240v AC, but only during the winter. Is there a way to re-wire my dryer to run on 120v AC? Then again a large capacity dryer probably wouldn't do much drying at 120v huh?

    Sorry about all these questions. Just trying to fill in the blanks left over from surfing AE links.
     
  2. ET1 SS

    ET1 SS zone 5 - riverfrontage Supporter

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    Lots of things.

    How much wire do you want to run?

    How many watts do you think that you will need?




    No.

    An inverter chagnes DC to AC.

    A converter changes one level of DC, to another level of DC.

    You may or may not have some need for 240VAC through out your house. Most homes do not have 240vac through out their homes.

    Generally if you are running a air compressor or some other big loads.



    Buy or sale as you desire.

    I have owned 120vac/60hz dryers, and I have owned 240vac/50hz dryers. They can be converted back and forth.

    Once you convert a 240vac/50hz dryer, to run on 120vac/60hz power it will still operate at about the same, but it's timer will be slightly off.

    You have to change-out the pulley for the belt drive which changes both how fast the drum spins and also how fast it draws air through itself.
     

  3. Guy_Incognito

    Guy_Incognito Well-Known Member

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    The big reason that things run at higher voltages is to reduce the current draw. The reason you reduce the current is to reduce power losses in your cabling - and reduce some troubles with switching high currents.

    Power lost = Current Squared x Resistance, so halving the current by doubling the voltage (and keeping the same cable size) means a much smaller power loss.

    Seeing that 240V (in amercian houses) is used for high-current loads, odds are that your AE system will not be able to supply the power needed for any length of time. (Eg, starting a big motor - yes, running your dryer for 2 hours - no.)

    Also, apart from the convenience of having AC power to plug all your common appliances into, AC is used because of the arcing problem you get with DC voltages. Find a light or a power switch - it will be marked something like "10A 240VAC, 2A 32VDC". DC power maintains arcs very well and will quickly destroy switches not designed for DC switching. This is why DC power systems are generally limited to 32V. AC power doesn't hold an arc as well as the voltage crosses zero 100(or 120) times a second.

    (added edit)

    Generally , DC system voltage is dependant on the size of your system.
    A smaller system with smaller surge loads can get by with a 12 or 24V bank.
    As your AC surge loads increase, the DC surge loads get monstrous - for a 10A surge at 240V, that's 100A required from your 24V bank, and at least 200A from your 12V bank. Batteries "sag" a lot under loads like that and they're a lot less efficient. All your cabling power losses increase (due to those I2R losses mentioned earlier), you need heavier power electronics in
    your inverter to handle it , and so on.

    For example, the system I'm building will be 48V and have a 6kW inverter attached - the continuous current needed at 48V/6kW will be 125A.
    This is a pretty big draw on my batteries (350Ah cells - 2 hours to flat) but it's still managable.

    At 12V, it'd be 500A - which requires huge cables, plugs, fuses, power electronics and big deep cycle batteries that can handle that kind of draw efficiently without sagging too far.
     
  4. ET1 SS

    ET1 SS zone 5 - riverfrontage Supporter

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    To run DC voltage for any distance, you need much thicker wire and better insulation. Thicker wire as DC does not 'like' traveling distances and sees smaller wires as a resistance load. AC can more easily travel over smaller wire.

    But when making a do-it-yourself system, multiple alternators each making AC power are very difficult to synchronize; thus DIY systems will more likely be DC. DC power does not need to be in-phase.

    What I have seen folks do, to keep their expenses low. Has been to use automobile alternators with a full-bridge rectifier to make their output DC, and automobile generators [which make DC already], and run them each to your battery bank. Whether you use a 12VDC bank, or 24VDC, or 48VDC; is truly up to you. And it really does not effect the system's usefulness. It only depends on how you wish to wire it. In this day and time, small solid-state inverters are cheap. Wire your house with a 110VAC circuit, and plug your stuff in. Supply that circuit from a solid-state inverter, and keep in mind that the invertor has it's limit. You can never exceed it's load wattage limit.

    But in terms of generating power; if you make all your electricity as DC and charge your battery bank. It is much simpler to wire. And needs no synchronizing.

    :)
     
  5. bayesoft

    bayesoft Member

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    ET1,

    I know you really know your stuff. Myself I am a former Radioman (Surface Fleet). I cross-rated to CTO before my first re-enlistment. I took some basic electronics in Norfolk when I became a Mod-28 Teletype Tech.

    The ET's were essential in our line of work. When I cross-rated we still had ET's, although they had a different rate of CTM. Same difference, higher security clearance is all.

    I'm not really destitute, just frugal. I know my other post seemed to imply that I was in dire financial straits. Sorry if I gave that impression. I'm willing to pay for the convenience/efficiency of stepping up to a modified alternator (PMA). I'm more of a "buy the components and put it all together myself" kind of person. As long as it saves money.

    If I did resort to using car alternators and batteries to save money. Can they be mixed and matched on the same system as long as they are DC? Your post seemed to imply that as long as they are DC, it's all good (in-phase). Please excuse my ignorance if I am mistaken.

    Car batteries for instance.... I know they have different cold cranking amps depending on what's available. I assume that alternators are the same. An alternator on a four-cylinder engine would put out less power than a Semi-Truck alternator for example.

    What would happen if I mixed the two alternator outputs to the same battery bank?


    Guy,

    Arching is something I definately want to avoid in a strawbale house. I could probably put a few DC outlets inside of interior stud walls. They will be filled with fire resistant cellulose.

    Seeing as how DC appliances are more expensive than their AC counterparts. I'll probably just wire my house for 110v AC and replace my appliances with more efficient ones.

    If I need to run appliances with a larger load. I'll start up the bio-diesel generator while they are in use.
    ---------------------

    Thanks alot guys.... The info you have given me has given me some food for thought. I'll mull it all over this weekend and decide what to do.

    I like the advantages of a 48v system over a 12v or 24v (fewer components). That is a lot of batteries though if you take into consideration that smaller voltage batteries are where the amps are at ie; golf-cart batteries (2v).

    So if I went 12v batteries (cheaper). I would need a series of three (48v) times a parallel of x to increase the amps. Is that correct? Do all the batteries need to have the same amp rating? If not, is the output from the battery bank an average of the overall amps?

    Should batteries with lower amps be placed first or last in parallel? I'm thinking that lower amps should be first in parallel, ie; the lead series batteries.

    Logic on the other hand tells me that the higher amp batteries later in parallel will only carry the amps of the lowest amp battery. Ugh.... I just don't know.
     
  6. ET1 SS

    ET1 SS zone 5 - riverfrontage Supporter

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    I have known and worked with CTMs, good men.



    Frugal is good.

    Also for the 'survivalist' type of mind-set; doing it yourself means that when your setup is done, and later eventually breaks down, you are capable of fixing it without running to the store.


    Generators can be mixed and matched, since they are make DC, they can each charge a battery or bank of batteries.

    Alternators can not be mixed and matched without added 'tronics to support them. If you have ready access to cheap alternators, that can be handy also, as now you can run the power long distances easier with smaller gauge wire. But before you can really do anything else with that power, you really need to rectify it first.

    :)


    The battery for a vehicle was designed only for starting the engine.

    The generator / alternator was designed primarily only for recharging the battery.

    If a particular vehicle is such that it is going to have a lot of auxiliary electrical loads, then they would shift to an over-sized generator / alternator. But they will commonly stick with the same size battery. Since the battery's design function remains the same.


    Gawd, no.

    Just dont.

    Without a metric tonne of other junk, you can not sync two alternators. They will be out of phase with each other, and they will 'fight', over-heat and be 'bad'.

    If you have alternators [presumably because they are cheap] fine. Since they each put out AC, you can run that AC current long distances, for cheap to wherever your battery bank is going to be. You can hook-up heater coils, space heaters, light bulbs, all sorts of things to that power source. But do not inter-connect it with any other power source. It is AC.

    Two AC power sources need to be in sync with each other. 'In-phase' requires either mechanical gearing between the two units, or else 'tronics which you will have to pay for.

    It is way easier and cheaper to convert each alternator's output to DC. Once it is DC, then have a ball. Positive to positive, negative to negative and it can all go right onto the battery bank.

    Just for safety I would always include diodes. each DC positive lead should have a diode, each DC negative lead should have a diode. Why? If anything every shorts, it may try to back-feed into some other piece of equipment.



    A proper straw-bale house seals all straw so no air can access any straw. A straw-bale house should not be a fire hazard any more then any other house.

    'fire resistant' does not mean 'fire proof'!

    All wiring, AC and DC needs to be fully insulated.

    :)
     
  7. bayesoft

    bayesoft Member

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    Good stuff.... I always thought a car alternator generated DC not AC. Please explain a rectifier and where it belongs in the system power flow.

    As I understand this. A car alternator generates AC and it can be either fed right into the AC power panel to a circuit or converted to DC with a converter and fed to the battery bank.

    So if I install several car alternators in lieu of PMA's. Each one has to have a separate dedicated AC to DC converter before going to the battery bank. Once the AC is converted to DC then I can run all the cables together to the battery bank, correct?

    Here's a toughy.... How do I know how many batteries I need to hook up to the converter(s) DC output? Are the AC to DC converters rated at 12V/24v/48v output? That would make it easy.
     
  8. Jim-mi

    Jim-mi Well-Known Member

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    If your at all serious about a RE system than toss that electric clothes dryer out the window.
    Or into the ocean.......they make great fish shelters.
     
  9. Guy_Incognito

    Guy_Incognito Well-Known Member

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    "alternator" in vehicle terms is a bit of a misnomer. It's technically a generator , but in order to distingiush them from the real generators of yore (burshes,commutators, stationary fields,etc) they were called alternators because the 3 phase alternating supply generated is internally rectified to DC. All this is done before it gets to the Battery+ terminal on the back of the alternator, so you don't have to worry about it too much.

    I'll presume ET1's talking about "real" alternators, those that put out AC.

    So, in the "Car" sense - multiple car alternators can be used to charge a single bank of batteries at once. It's tricky for one good reason though - one alternator might be set for 14.1 volts, another might be set for 14.0 volts. the 14.1 volt one will carry the bulk of the load and the 14.0 volt one will idle along. There's ways around this - either run all your alternators off a single regulator, or full-field all the alternators for maximum output and cut them off one by one manually yourself as battery voltage rises.

    As for batteries, lead acid batteries can be sensitive. Don't put different capacity cells in series - the smaller cell will overcharge first and go flat first. Overcharging you could probably get away with, but undercharging is bad - if the other cells in the bank are strong enough, the smaller cell will got flat, down to zero volts and then go negative as the other cells discharge. Needless to say this is not good for a cell.

    In parallel, well, generally you can put cells in parallel with no problems. Total capacity will be the individual battery capacity all added up.

    With battery banks, it's probably more important to calculate the overall power stored in watts. It makes things much more convenient when you are trying to size a system.

    I'll demonstrate :

    1. You've a bunch of 2V 250AH golf cart cells. You put 6 of them in series to get 12V at 250AH. You have 12V X 250AH = 3000 watt-hours of energy storage. You could put them all in parallel, then you'd have 2V at (6 x 250AH) = 1500AH which still works out to be 2V X 1500AH = 3000 watt-hours. But at 2V, which is not really much use :)

    2. You've 3 12V car batteries, of 70AH capacity. You put them in series to get 48V at 70AH. You have 48V x 70AH = 3360 watt-hours or energy storage.

    Both banks have pretty much the same capacity, but you could probably purchase 3 car batteries cheaper than 6 golf cart cells.

    Just remember that lead-acid batteries don't like to be discharged too far too often. For all the examples of battery capacity above, you'd probably at least want to halve the capacity for real-world usage if you want any decent life from your batteries.

    So anyway, you've got 3000 watt-hours of storage. Now what?

    You can work out a lot of things from that. For example :

    If you have a light bulb that draws 100 watts, it will use 100Wh every hour you have it on. You can run that from your 3000 watt-hours of storage for 3000Wh / 100W = 30 hours before your batteries are flat.

    You have a 1000 watt electric kettle that you use for 2 minutes to boil water. The kettle will use 1000Wh in an hour. Multiply that by 2/60'ths of an hour to get your 2 minutes worth of usage = 33 watt-hours of energy. So even though your kettle uses a lot of power, because you only use it for 2 minutes at a time it's just like having that 100W bulb on for 20 minutes as far as your batteries are concerned.

    If you go through and work out how much your appliances draw and how long you use them for, you can figure out how many watt-hours you use daily.
    Then you can go size your battery bank / panels / generator usage etc so as not to get any nasty surprises later on down the track.

    This has turned into a bit of an essay :) but hopefully it'll help.
     
  10. bayesoft

    bayesoft Member

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    That was very helpful and enlightening.

    I exported the floorplan of my strawbale house to a jpeg file.

    My House

    Tonight I tweaked some of the electrical stuff to minimize power draws. Do you think this would be simple enough to wire? There are some situations where a room/closet has only one light and a switch. I could probably get away with making those DC circuits. I plan on using florescent bulbs for all the lights. Can those screw-in florescent lights be used in a 12v light fixture?
     
  11. Guy_Incognito

    Guy_Incognito Well-Known Member

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    You can get 12V compact fluoro's - they usually have a tiny edison screw fitting that prevents them from going in a normal light socket. You can also get special 12/24V ballasts for normal 20/36W fluoro tubes.

    Here's a random place on the internet that sells the kind of thing I'm talking about.
     
  12. ET1 SS

    ET1 SS zone 5 - riverfrontage Supporter

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    An alternator puts out AC, which then needs a rectifier to make it into pulsed DC before it can be used to charge a battery. Some automobile alternators have rectifiers that are mounted on the back end, while others the rectifier is mounted on the fire-wall or wheel well as a separate unit. I have owned vehicles that have had either configuration [the alternator had the rectifier mounted to it, or the rectifier was a separate unit].

    An automobile generator puts out true DC, but also requires an occasional re=charging of it's magnetic field since they were made using iron core electromagnets instead of true magnets. The 'A' terminal comes from the armature and carries the DC voltage, the 'F' terminal comes from the field wiring and needs a voltage applied to it to be able to maintain the magnetic field strength of the electromagnet. My willy today has a true generator and needs it's 'F' terminal re-charged every now and again.



    It is my understanding that this can only be done, assuming that each alternator is already connected to it's own rectifier [either internally or externally].

    :)
     
  13. bayesoft

    bayesoft Member

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    So I need to find what make and model vehicles have the rectifiers attached or inside the alternators.

    I would rather just pump the multi-unit AC to individual converters and then tie each DC output to the battery bank. Much safer than risking a faulty/blown rectifier. Also like you said, AC can be run for longer distances with smaller dia wire.

    Are there converters available that will take several AC inputs and convert/feed DC to a single positve and negative cable?

    If I had a hodgepodge of various AC alternators from the boneyard (various output). How will I know what converter to get? Is there a formula to determine this with the AC output variables?
     
  14. Guy_Incognito

    Guy_Incognito Well-Known Member

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    I'll agree with you there ET - mixing polyphase AC sources together is asking for trouble! DC outputs only.

    And the last car I owned that had a generator was a 1964 morris - I haven't come across a generator for many years now. And good riddance! :)

    Pretty much any modern alternator will have the rectifier internally. If you're looking for converters, er, that's a rectifier :) . A rectifier for a car alternator is usually sized to be juuuuuust big enough, because manufacturers are cheap. So if you had a high output alternator and a small rectifier, you'll let all the magic smoke out.

    Regarding where to put the rectifier, leave it in the alternator. You need to run 3 wires to your rectifier to convert the 3 phase AC to DC. The regulator needs to sense the DC voltage, so it needs to be hooked up at the rectifier as well. So you then need to run field wires back to each alternator. It all gets a little messy.

    And when I said AC travels longer over smaller wiring, I meant high-voltage AC. AC at the same voltage as DC will still have the same losses over the same bit of wire. The AC that a car alternator puts out is pretty much the same voltage as DC.

    So, as to what kind of alternators you should scrounge?

    If you're looking for longevity and ease of maintenance, get a brushless truck alternator, such as a Delco-Remy 25SI or 26SI. They're quite solid, with a proven design. Plenty of parts - either directly from Delco-Remy (or whoever owns them now) , and also from quite a few aftermarket manufacturers.

    With no brushes for the fields, they generally last forever - or at least until the bearings finally give way. They're easy to crack apart and replace bits - a 1/4" and 5/16" socket is all you need. Replacement parts are cheap. A new rectifier is only about $15 bucks. You can swap out the rectifier and diodes in a 25SI in about 15 minutes without having to unsolder anything or even have to take the alternator off the machine (if there's enough room to access the back of it.) But in my many years as an auto electrician, I've only had to change a few 25SI rectifiers - they're fairly bulletproof, even when running at full load.

    So, my vote goes for a 25SI :) Here's a picture of what they look like.
    [​IMG]
    A link to the internal construction of them.

    Hope this helps!
     
  15. bayesoft

    bayesoft Member

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    Ok Guy....

    I'm sold on the Delco 25SI 12v 100A Alternator.

    I'll call the boneyards tomorrow to see if I can scrounge some up. I can get a new one shipped from Grand Rapids for under $280. That price should give me some leverage in case the junkyard tries to overcharge me.

    With this alternator in mind. Are there any pointers you can give me for turning it into a wind generator? I can buy hubs w/blades from green grid. I'm sure the mounting bracket will have to be modified or fabricated.

    I'm not sure about the tail. I don't see it offered as a separate unit.

    One of these alternators should put out 1200WattHours if I understood the formula in your previous reply correctly. I'm assuming that this output is for idle engine speed.

    So for each alternator (12v100A) connected to the battery bank. I should have 12v100A total in series or parallel? For instance, if I had two of the above alternators running on masts (12v200A) - 2400WH. I would need two 12v100A batteries connected in parallel, or four 12v50A batteries, or eight 12v25A batteries.

    Does the batteries total amp rating have to be exact? I'm guessing it is probably better to go higher on the battery amp rating, since the rated amps of the alternators are probably idle speed.

    If the winds decide to gale, I want to have the capacity to store as much as I can before it is diverted to making hot water etc. But, If I go on the high side, and don't use the extra capacity, will that shorten the battery life?

    And lastly.... is 12v at 200A (2400WH) a decent starting point or should I aim higher? Opinions??

    You've both been a great help in understanding all of this.

    Edit:
    The Hornet puts out 67amps per unit.
     
  16. Guy_Incognito

    Guy_Incognito Well-Known Member

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    It's interesting to note that the permanent magnet alternators that they're using for hornet's look to originally have been Delco 10SI alternators.

    Ok - from the alternator specs, they have to spin at about 2000RPM to get maximum output.

    Working back from the hornet specs of 67amps at 14V that would give you about 950 watts absolute max for the hornet. Going from the 950 watts and the hornet's blade diameter, you'd have to have at least 1.5 times the swept area of the hornet blades to get the power output you need to spin your Delco 1200W alternator at full load. So the 88" blades that they sell would be a good help.

    The 25SI is a self-exciting alternator, so basically no wires are needed except for the 12V battery lead. It will kick in at about 1000 rpm, so gear accordingly - I'm unsure of what RPM those blades on that site will spin at normally. They will then 'hold in' until just a few hundred rpm before dropping out again. You can wire a indicator light to them to make them kick in earlier if you want, but with a small (0.25A or so) drain on your system when it's not windy. Kicking in later allows the rotors to get up to speed a bit, but it really depends on how much wind you have in your region. You could always wire it via a switch, (just like the battery light in your car) and only turn it on when you want low-speed cut in and off when there's no wind so there's no power draw.

    So with all that, if your 1200W alternator can run at full capacity for an hour, yes, you'll get 1200Whr out of it. That's actually quite a bit really. For a real world example, in my grid-connected house with a heap of things humming away 24/7, I use 24kWh a day. That's what that single alternator could put out if it ran continuously. And I've got a heck of a lot of stuff here :)

    A note with the delco alternators - once your battery starts to hit full charge, they will throttle back the current with the regulator. This could cause your rotor speed to go right up in windy weather. If you don't want it to do that and you want to have a load dump down the line somewhere for heating water etc, there's only one wire you have to move inside the alternator to bypass the regulator and make it run flat out

    As for the batteries, more capacity is better. The only difference with having more battery capacity in your setup would be that it would take longer to charge up fully (and discharge, of course). Basically with lead-acid batteries, the further you run them flat before discharge, the shorter their life. A car battery might only last 20 full cycles down to dead flat before a cell fails - deep cycle batteries, maybe 100 cycles. All reputable manufacturers of deep cycle batteries will have graphs showing how long a battery will last with respect to % discharge. But for maximum battery life, no less than 25% of the battery capacity should be used. So, bascially the more storage capacity you can get, the longer they will last. It's an important consideration when sizing your battery bank - it's all too easy to go "yeah, I'll get 500Ah of storage at 12V, that's 6000Wh". It's actually only going to be 1500Wh if you want your batteries to last.

    Man, my replies seem to be really wordy. I've really got to lay off the coffee :p
     
  17. bayesoft

    bayesoft Member

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    Just a quick thought.... I can't seem to find a battery regulator that can handle 100Amps. The biggest I can find is rated for 60Amps.
     
  18. Guy_Incognito

    Guy_Incognito Well-Known Member

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    You can make your own with an adjustable voltage sensing relay and a power relay rated to whatever current you want your load dump to run at. Plenty about if you google for "voltage sensing relay".

    Basically, you set the relay to 14.8V (or whatever you want the max float voltage to be) and then the connect the normally open to drive your big load dump relay and load dump. The hysteresis (gap between on and off in the relay voltage settings) will cycle your load dump around 14.8V-14.something volts. So basically, the battery voltage will rise up to a point, the relay will cut in, your big load will take all the alternator output until the wind drops off again or the battery voltage drifts/falls enough for the relay to drop out again. Probably better to have slightly more than the rated output of the alternator (eg, 110A for a 100A alternator). You can normally check the relay datasheet to get an idea of how close the cut-in/out voltage can be set on it. You don't want it too close as you'll wear out your big relay too much.

    You might find certain brands of automotive dual battery relays will do the job if you can adjust the cut in / out voltage on them - they're basically the same thing.
     
  19. JAK

    JAK Well-Known Member

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    If you were off grid and your only AC load was a combination washer/dryer like this one:
    http://www.splendide.com/wdc6200cee.htm

    How important is it that you get exactly 60hz? If you rewired it to run the electronics directly off the battery, but had it automated somehow to run off an AC generator from a wind turbine or a solar steam engine could the washer and dryer motors and heating coils work if they were connected to a power source that wasn't steady but varied within limits of voltage, current, and frequency?

    I think this would be toughest to do with wind power, since it varies alot. On the other hand a solar steam engine would probably have to have 300 square feet of collector to produce the requires 1.5 kwh for a load, due to a combines collector and thermodynamic efficiency of perhaps 5% at best. Still, even if you came home to a load half done that would not be so bad as long as you did not burn out a motor, or burn down the house.

    How would standard AC motors respond to a fluxuating power source?
     
  20. ET1 SS

    ET1 SS zone 5 - riverfrontage Supporter

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    By getting hot. Maybe real hot.