# Thread: for you guys who are good with the math

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## for you guys who are good with the math

Here's a link to some seventies technology.

http://www.motherearthnews.com/Renew...gy-System.aspx

I like something simple and mechanical that I can understand and I am impressed at the idea of using the engine heat to heat water for the house.

This has appeal but I have a suspicion that the energy units are created at a higher price than the article suggests.

Also I kinda doubt that his 12 volt alternator could keep the batteries properly charged in the length of time he specified, but I can't do the math. .

What say you math wizards?

Would his system be a good thing to do?

Could solar panels be added to his system?

2. ## Re: for you guys who are good with the math

Wow, that is ancient.

While it would probably work, I would hope there are better ways to do it now

I am betting that the old Pinto engine he used caused as much pollution as a dozen or so modern generators. And you could probably get as much heating without running any engine with modern solar hot water panels.

He seems to be talking about a pretty small battery bank, looks like something around the size of 4 car batteries, so it would probably recharge it - but I have my doubts about battery life or even how long he could run his high amperage automotive bulbs.

3. BB.
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## Re: for you guys who are good with the math

Some numbers from the article:

~1 gallon of fuel per run (~\$1 per gallon or a bit less)
35 amps @ 14 volts for 1 hour
~2,000 watts for 15 minutes

35 amps * 14 volt * 1 hour = 490 Watt*Hours of "12 volt" power
2,000 watts * 15 min/60minperhour=500 watt*Hours of 240 VAC power

490WH + 500WH = 990 Watt*Hours of electricity per 1 gallon of fuel (Pinto Generator)

Honda eu2000i, running at 1/4 power (1,600 watt * 1/4 = 400 watts) will run for ~15 hours on 1.1 gallons of fuel:

400W * 15H / 1.1 Gallons = 5,454 Watt*Hours per gallon of fuel (Honda Inverter Generator)

Price of electricity:

Pinto: \$2 per gallon of fuel / 0.99 kWhrs = \$2.02 per kWhr
Honda: \$2 per gallon of fuel / 5.454 kWhrs = \$0.37 per kWhr

The Honda weighs a bit under 50lbs... The Pinto setup--500 lbs+ ?

If you want, you can use the "fuel cost calculator" to compare what 1 gallon of fossil fuel gives in heat vs wood, oil, natural gas, etc... There are no efficiency numbers given for the fuel to water heat transfer (and storage tank).

I could take guesses--but I am not sure they would be of much help (other than a Pinto engine is probably not as efficient as a standard oil or natural gas furnace).

You could try and sub 4,000 watts for the well pump (instead of 2kW)--but that will still not bring the overall electrical conversion efficiency to anywhere near the Honda generator.

Is this what you were looking for?

-Bill
Last edited by BB.; February 6th, 2009 at 20:35 PST.

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## Re: for you guys who are good with the math

Thank you, Bill
I felt sure the new ways are better ways.
I appreciate the math!

5. BB.
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## Re: for you guys who are good with the math

Note, the Honda eu2000i, in my very humble opinion, is one of the best "small" generators out there--both from fuel use and noise (and not bad reliability either).

When sizing a generator, most of the "cheap noisemakers" fuel flow drops from 100% to ~50% when the loading is changed from 100% to 50%, and below 50%, the fuel flow does not drop much more...

The Honda fuel flow continues to drop even down to 25% power (or a bit less?) because it has an Inverter which allows the engine to run at the most fuel efficient speed--and when the load is light, the motor slows way down--saving fuel and noise.

In the end--sizing the generator to your loads, and monitoring fuel used and electricity generated--will help keep your costs much lower.

Many people end up with a much larger generator than they really need (unless they have A/C and/or a well pump). And even then, perhaps you run a eu2000i for the critical/lower power loads, and use the "5kW noisemaker" for 15 minutes a day to pump your well water.

-Bill

6. ## Re: for you guys who are good with the math

Bill,

You're forgetting one fundamental fact here in your calculation,,,, that is the BTU value of the Pinto engine as it goes up in flames when you accidently bump into it!

Might heat the house for a few hours!

T

7. ## Re: for you guys who are good with the math

M.E.N. - more amusing than practical...

However, getting out of the Honda air-cooled realm, recovering heat from the need to burn fossil fuel occasionally is certainly a good idea, if you need heat. Generally if you have to run the generator the sun has not been shining enough, so any thermal panels you have were not doing much good either at that point.

It is with this in mind that I have a liquid-cooled diesel (burns about 0.07-0.08 gal per KWH) and a liquid cooled gas (or propane if I get that carburetor - old Kohler, don't know the consumption rate as yet) in my generator collection. If I need to burn fossil fuel (or very perhaps if I ever tinker with it enough, wood gas) I might just as well get power and heat, rather than blowing hot air out of the generator shed. In summer, if needed, and once the DHW store is hot, they can run to a remote-mounted radiator. In winter, I want that heat inside the building, thankyouverymuch.

More complicated that just firing up an air-cooled set, to be sure. But also more efficient...
Last edited by Ecnerwal; February 7th, 2009 at 6:51 PST.

8. ## Re: for you guys who are good with the math

One other factor to consider especially with the diesel. If your ambient is cold, the chances of getting much useable heat our of the diesel into any DHW is pretty slim. Assuming that you are running the genny nearly fully loaded so it generates maximum heat, if it is cold, there may not be enough left to vent to something else. This is a problem for all internal combustion engines, but especially true for diesels. Diesels start out fairly (in a relative sense!) thermally efficient, but that efficiency is VERY dependent on proper engine temperature. If you start taking too much heat away, and the engine begins to run very inefficiently. It might even take a typical diesel ~1 hour to get up to proper temperature if it is very cold. My old 8V detroits would NEVER get warm, even with the Radiator completely covered in the dead of winter.

While I am a great fan of co-gen stuff, and not wasting heat for example, but my hunch is that the amount of headache involved in a small scale system out weighs the savings. Instead of doing a coolant DHW, perhaps one might be better to vent the radiator fan into a useable duct for something. If the case is a BIG set up that runs many hours/day, then surely it makes sense,,, small scale maybe not.

Tony

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