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re: co-generation is probably overkill for a backup generator
28 dec 2001
bughunter  writes:

>i'll repeat one of my points, that is that the application of a simple
>backup generator is to provide electrical power in the event of a utility
>power outage which may occur on the order of once or twice yearly and last
>from a few hours to a few days...

that's one application.

>under those circumstances, an improvement in efficiency by even 75% is
>not worth the capital expenditure necessary to implement the co-generation
>solution that you have described here.

true, but having bought a generator, why not operate it more often, vs
"under those circumstances"? you paid for it, and you are losing interest
on alternative investments, so why not work it harder to earn its keep? 

>> the enclosure might be as simple as a 2'x4'x6' tall box made with block
>> walls laid up dry in the basement, and plastic film over that. the genny
>> might sit on top of 2 55 gallon water drums. the box wouldn't be airtight.
>> the genny would draw in room air through a small hole in the box for
>> combustion, and all the exhaust air would end up outdoors, so the box
>> has a slight negative pressure compared to the room, and exhaust gases
>> don't end up in the room. block walls can soundproof well.

btw, it wouldn't be hard to connect a manometer across the box and turn off
the genny if there is insufficient vacuum or any detectable co in the room.

>...the laying of concrete block is a task that is a whole lot of
>demanding physical labor.

about 30 pounds per block, with no mortar. surface bonding is another
alternative, or tension applied from top to bottom...

>block walls are a reasonably good sound barrier, but how do you lay
>a block ceiling?

not hard, with a 2' span.

>or a block door?

i'd use some other form of sound insulation there. 

>neither is it intunitive how one would mount a generator atop two
>55 gallon drums without having it vibrate across the floor.

the drums weigh about 500 pounds each, when full, so they are unlikely
to vibrate across the floor. you have to use more intunition about the
generator platform. plywood with a few 2x4s, wonderboard, and so on.

>the dimsnsions of this enclosure seems inadeauate for service access
>or even to hold the 55 gallon drums.

why argue with people who cannot spell? most 55 gallon drums measure about
23" in diameter and 35" tall, max, so 2 vertical drums will fit in a 2x4x6'
tall enclosure, leaving a 2x4x3' space at the top, large enough for honda's
$1,069 68 pound 17.3"x15.7"x18.9" eb3000c generator. it might slide out.

>many small practical  "complexities" such as these start to add up to a fair
>number of design and implementation "challenges" that must be overcome
>before a usable prototype comes into exhistance.

exhistance is full of challenges.

> would need a fair amount of practical skills and labor
>to implement your design.

true. then again, you might have friends, or build more than one.

>> i'd run it into a stainless steel gas flex hose and a spiraled soft
>> copper pipe in a 55 gallon drum full of water to cool it to 212 f
>> before piping it outside...

with a small hole at the bottom outside the drum for possible condensation.

>it might be a bit tricky to adapt flexible gas piping to  the exhaust port.

that is one of the tricky bits, with heat, vibration, and toxic gases.
i looked into it with a different honda. it looked easy to remove the
muffler, weld a pipe nipple with a standard thread to the unflanged end
of the honda pipe ($20 from honda) that connects the manifold to the
muffler, and add the ss flex hose ($8 from home depot) via a $3 adapter,
and connect the end of the hose to the soft copper tubing with a
compression fitting. 

>i doubt that you could use standard fittings. something would have to be

just one weld.

>i would also be concerned that a long length of pipe might cause engine
>problems such as back-pressure and the burning of the valves.

we might measure the back pressure with the muffler installed and make
the heat exchanger back pressure (with enough parallel paths) the same.

>> that honda makes 50k btu/h of heat, not counting the electrical output,
>> about 300k btu over 6 hours, enough to heat 300k/150 = 2k pounds of
>> water 150 f, about 4 drumfuls. we might use 2 plastic drums inside the
>> enclosure and 2 more outside, connected by 8 short heater hoses pushed
>> into holes in the sides near the tops and bottoms.
>pushing heater hoses into plastic drums is asking for a leak.

steve baer does it with hot water inside houses. bulkhead fittings would
add labor and about $100 to the cost of materials. 

>insulation adds yet another small "complexity". radiating the heat into the
>space assumes that it's in a living space that "wants" heat. i'd think that
>your generator enclosure, even if it was in the house, would be in an
>unoccupied place like the basement. ideally,you would want to move the heat
>to the living space.

basement ceilings are rarely insulated, but a grate might help.

>i'd approach some of the implementation details differently. first, i'd
use a larger generator to more adequately meet my peak electrical needs.

that ups the cost and makes it less transportable.

>i'd also select a liquid cooled engine rather than an air cooled unit to
>simplify some of the heat exchange/recovery mechanism.

that would help, at extra expense, but it would still lose exhaust heat.

>i do have lots of unused basement space, so perhaps i'm a bit more
>undecided than i was before this thread.

hey, progress :-)


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