From: sherwood@fenris.space.ualberta.ca (Sherwood Botsford)
Newsgroups: sci.energy
Subject: Re: House Sized constant duty (co)-generator
Date: 24 Jul 92 18:51:40 GMT
Organization: University Of Alberta, Edmonton Canada

Mike Smith writes
> All the talk of co-generation with solar has got me started
> thinking about home sized power generation...
> 
> I use about 10kW-h/day now.  That is about 416 watts of constant
> consumption.  Call it 1 hp at the generator engine.
> 
> It seems really odd to me that I should be able to power my whole
> house from a 1 hp generator (if it was well designed and efficient)
> by having power buffered in a battery bank.
> 
> What is wrong with this picture?  A 1 hp high efficiency high
> reliability natural gas generator produces a constant 500 watts
> or so.  Any excess above immediate needs is sent to a battery bank.
> Any needs above 500w is met by a good (ala Trace) inverter.  The
> excess heat is used in domestic water heating and space heating/cooling.
> 
> How small could one of these be made?  It seems to me that a 1 hp
> gas turbine should be about the size of a quart jar!  Add a generator
> about the size of a car alternator and you have a whole system that 
> would fit in a small lead and foam lined concrete box under the water 
> heater tank.
> 

There's nothing wrong with the idea.  It's just expensive to implement and  
expensive to run.

  Lets suppose that your peak power consumption is 8 kw.  (The stove, dryer and  
the kids hair dryer all kick in at once.) 
  So your inverter has to be sized for this peak demand, even though you may  
not use it very often.
  alternately, you have a clever relay bank that shuts certain circuits off if  
the voltage drops below a set level.

  You also need a bunch of batteries.  A typical car battery has 100 amp hours  
at the rated voltage.  So that's 1200 watt hours, or 1.2 KwHr.  Since you don't  
want to discharge them flat, you are looking at about 20 batteries.  Pricy.   
And you have to worry about replacing them periodically, and you have problems   
keeping them all charged.  (If one battery gets a bad cell, then suddenly it's  
two volts below the others, and the others try to charge it.  You can get some  
awesome currents this way, and may blow up a battery.)

  Batteries have to be kept from freezing, so you have an inside room, probably  
with stringent fire code requriements.  Don't forget outside ventilation.

  Some mechanism to keep the batteries from overcharging.

  Now the little engine.  1 Hp gas engines are not terribly efficient.  Neither  
are small generators.  I would be surprised if you got over 20% of the thermal  
energy of the gas out the plug at the other end.  After all, the ginormous  
thermal plants only run about 40% conversion.
  Most small engines are not designed to run for weeks at a time.

Ok, still considering it?

  You were going to buy fuel for the engine anyway.  Consider converting your  
house to propane and 12 volts throughout.

  A friend of mine has a remote cabin near La Ronge Sask.  He commutes by boat  
in the summer, skidoo in the winter, and hovercraft during freezeup and break  
up.
  He's changed his lighting and stove over to propane, with a few 12 volt  
convenience lights in closets and such.
  The water tank is in the attic, and is filled by a gas pump running from the  
lake.  That hose doesn't have a check valve, so the hose self siphons when he  
shuts the pump off.  From the attic, everything is gravity feed.  The shower is  
feeble.
  Most of his electric conveniences are ones designed for use in RV's.  Thus he  
has an automotive stereo unit and so on.
  Heating is done with wood.
  This reduces his electrical usage to a level that can be powered with a  
single truck sized battery, which he recharges with a small honda generator.

  I asked him how much propane he went through.  He keeps two 100 lb cylinders,  
with a pressure operated switch.  He says he has to take one in every couple  
months.