# using natural gas to generate electricity

Discussion in 'Hobby Electronics' started by Mr Mac, Apr 2, 2011.

1. ### Mr MacGuest

Here is an idea, get a regular (say) Holden V6 engine, convert it to natural
gas, and connect it to a 100KW electricity generator. You should be able to
find something from an army disposal sale. Total cost, say \$2000 for the
engine and \$3000 for the generator for a total of \$5000 plus a bit of elbow
grease.
Make sure the genny can supply a peak of 100KW, but typically put out say
20KW of constant reliable power.

your "electricity company" and make money!

Let's do the math to see if it is viable:

According to Origin Energy, gas is 1.1021 cents per Megajoule (in Melbourne
metropolitan area, Australia)

So One Watt (W) is 1 Joule per second, 1W=1J/s. So a kW=1000W=1000J/s. There
are 60*60 seconds in an hour, so a Kilowatt hour is
3600*1000 J=3.6*10^6 J. One mega-joule is 1 million Joules, 1MJ=10^6 J, so 1
kWh=3.6 MJ.

So with gas, 3.6MJ, or 1KWh will cost 3.3 cents.

But 1KWh of electricity from that greedy electricity company costs around
\$0.18 to \$0.20 so what is your break even point?

Now lets say you sell electricity to your neighbours for 18.3 cents per KWh,
so you are competitive.... your running profit per KWh is 18.3 - 3.3 = 15
cents.

How long will it take to get that \$5000 investment back?

Well you will need to sell 5000/0.15 = 33333KWh to break even. Now how much
does the average house use????? That is a big question. My use is around
10KWh per day but the figure seems to vary widely from house to house.

http://www.abcdiamond.com/australia/average-household-electricity-consumption/

Now I will use an average of 10KWh per day per house. Say you have 10 houses
connected to your little scheme, just to keep the numbers round. The 10
houses shouldn't overload the 20KW generator, with 100KW peak capacity, so I
think that is a realistic number anyway.

So at 100KWh generated per day, it will take 333.3 days, or just under a
year, to get your money back.

Will the engine run for a year, non stop? I would say yes. Plenty of taxis
clock up millions of km.... and apparently running an engine on natural gas
is good for it.

From Wikipedia:
Due to the absence of any lead or benzene content in CNG, the lead fouling
of spark plugs is eliminated. CNG-powered vehicles have lower maintenance
costs when compared with other fuel-powered vehicles. CNG fuel systems are
sealed, which prevents any spill or evaporation losses. Another practical
advantage observed is the increased life of lubricating oils, as CNG does
not contaminate and dilute the crankcase oil. CNG mixes easily and evenly in
air being a gaseous fuel. CNG is less likely to auto-ignite on hot surfaces,
since it has a high auto-ignition temperature (540 °C) and a narrow range
(5%-15%) of flammability.[6]

CNG emits significantly less pollutants such as carbon dioxide (CO2),
hydrocarbons (UHC), carbon monoxide (CO), nitrogen oxides (NOx), sulfur
oxides (SOx) and particulate matter (PM), compared to petrol. For example,
an engine running on petrol for 100 km emits 22,000 grams of CO2, while
covering the same distance on CNG emits only 16,275 grams of CO2. [CNG is
essentially methane, i.e. CH4 with a calorific value of 900 Kj/mol. This
burns with Oxygen to produce 1 mol of CO2 and 2 mol of H2O. By comparison,
petrol can be regarded as essentially Benzene or similar, C6H6 with a
calorific value of about 3,300 Kj/mol and this burns to produce 6 mol of CO2
and 3 mol of H2O. From this it can be seen that per mol of CO2 produced, CNG
releases over 1.6 times as much energy as that released from petrol (or for
the same amount of energy, CNG produces nearly 40% less CO2).] The
corresponding figures are 78 and 25.8 grams respectively, for nitrogen
oxides. Carbon monoxide emissions are reduced even further. Due to lower
carbon dioxide and nitrogen oxides emissions, switching to CNG can help
mitigate greenhouse gas emissions.[6] The ability of CNG to reduce
greenhouse gas emissions over the entire fuel lifecycle will depend on the
source of the natural gas and the fuel it is replacing. The lifecycle
greenhouse gas emissions for CNG compressed from California's pipeline
natural gas is given a value of 67.70 grams of CO2-equivalent per megajoule
(gCO2e/MJ) by the California Air Resources Board (ARB), approximately 28%
lower than the average gasoline fuel in that market (95.86 gCO2e/MJ). CNG
produced from landfill biogas was found by ARB to have the lowest greenhouse
gas emissions of any fuel analyzed, with a value of 11.26 gCO2e/MJ (over 88%
lower than conventional gasoline) in the low-carbon fuel standard that went
into effect on January 12, 2010.[7]

BUT, Back to the story,.... how long will that Holden engine really last?
Hard to say, without actually giving it a go, but let's do some more
educated guesses.

A regular Holden Commodore is pretty shagged with 300 000 km on the
odometer. So lets use that as a base. How many hours of operation would that
be?

Now if we use the engine as a generator, it's fair to say that we are
treating the engine nicely. There are no cold starts, the acceleration is
smooth, there is no stop-start traffic like you have on a real road, so you
could maybe assume that if it was compared to a car driven carefully and
nicely on the road, and you treated it right, it could make the 1 million km
mark without having to be re-built. OK OK, I know what you're thinking,
overly optimistic, let's make it more like 500 000 km. How many ***hours***
of operation is that. Well assuming 60 km per hour average, that's 8333
hours of operation.

Now we broke even after 333 days of operation, which is 7992 hours, so
8333-7992 = 341 glorious hours of selling electricity at 100% profit.

341/24 = 14.2 days, and with 100KWh sold per day, thats 14.2x100x0.183=
\$259.86 dollars of cold hard profit, before the engine gets changed over.

OK probably not worth the trouble. Never mind, carry on, nothing to see
here.

2. ### Sylvia ElseGuest

Whose wiring were you proposing to use to do this?
No it won't. You're running a heat engine. You cannot extract all the
energy from the gas. With your converted petrol engine, you cannot even
extract most of it. See

http://en.wikipedia.org/wiki/Carnot_cycle

3. ### Mr MacGuest

I assumed that the power generation from gas to electricity was 100%
efficient. Obviously that's not the case, but it is conceivable that the
waste heat generated by the engine could be used to heat up a large water
container, which could then be piped to the customers, and still be put to
good use.

Also, while the engine would have to be replaced yearly, I'd say the
generator would last a lot longer. No brushes, so just bearings every few
years.

4. ### Sylvia ElseGuest

Then you need a means of capturing the heat, pipework to deliver the hot
water, and meters to measure it.

You also need a way of billing customers, obtaining their payments,
dealing with people who don't pay, and so on.

It all adds to the cost.

Did you check the capacity of the gas main? Is it capable of delivering
the extra gas? If not, then it will have to be enlarged, and someone
will have to pay for that.

Kreed mentions noise. Yes, there will need to be sound insulation. More
cost.

The harsh reality is that owning and operating a generator is not a
licence to print money.

Sylvia.

5. ### terrycGuest

Now apply 5% efficency of conversion and see what you get.

Hint, go read the wikipedia article on the solar panel, especially the
bits on price parity.

6. ### Mr MacGuest

5% sounds a bit low.

The Holden Omega V6 engine uses 10.9 litres per 100km, and is rated at 190KW
at 6700RPM. I'll assume that it needs 20KW to cruise at 100km/h. (if you
have some real data, I would love to see it)

What is the real efficiency:

Well petrol has an energy of 32MJ per litre which is 8.89KWh per litre.
Multiplied by 10.9 (litres in one hour of the engine outputting 20KW) gives
96.89KWh

So we burn close to 100KWh of petrol to get 20KWh of real work..... or
around 20% efficient.

So yes, not very good figures. But heat is the biggest by-product. If we can
use the heat to warm up our houses, and swimming pools, and hot water
systems, we can internalise most of the losses.

Say you have a group of 10 to 20 apartments in a cold climate, where the
heat is not wasted, but put to good use. It's conceivable that you will get
close to the system working out.

7. ### Sylvia ElseGuest

That's certainly the correct result, but I think the technical/economics
arguments still need to be expressed, because otherwise people, even
those who would be willing to examine and understand the arguments, can
be left with the impression that the whole thing is a scam by which
people are sold overpriced electricity.

Mind you, I was watching Energy Australia do some substation work near
my home the other day. They'd closed one side of the road, and had two
people controlling traffic through the one remaining lane. I have to
wonder whether that's really the cheapest way of doing that - portable
automatic traffic lights are available.

Sylvia.

8. ### Phil AllisonGuest

"Mr Mac"
** Completely nuts.

If you have gas available - then so do all your neighbours.

They will already be using it DIRECTLY it for hot water, cooking and home
heating - which are the 3 major consumers of energy in a home. Only their
fridges, CFL lights and small appliances use electricity - plus air con if
they have any.

Gas homes generally have rather small electric bills.

BTW:

Gas appliances are rather inefficient as much waste heat energy goes up
flues etc.

..... Phil

9. ### terrycGuest

Sure it wasn't NY USA you are thinking of? same steam heating and
exhaust vents releasing steam.

10. ### SwannyGuest

That's a bit like buying a PC and calling yourself an ISP.

11. ### SwannyGuest

And whre is the generator infrastructure located? Certainly can't be on
someone's residential block. The location would need special zoning.

12. ### Dave GoldfinchGuest

I had a similar idea on a much smaller scale ie convert one of those
small el cheapo generators from Bunnings to run on natural gas and use
it for backup during power outages.

Suspect this may not be possible as they are 2 strokes so presumably
lubrication would be a problem?

Anyway, as I would have to pay someone to do the conversion, I imagine
this would be a really bad idea financially.

Dave Goldfinch

13. ### Mr MacGuest

That's a very interesting building. I had no idea what cogeneration was.
This is more or less where my "invention" was heading in my head.

The electricity factories burn coal, and are probably around 50% efficient
at best (I bet its more like 40). The waste heat is dumped into lakes and
ponds. Google Hazelwood pondage in Melbourne, I waterski there during
winter! The lake is heated by the adjacent power factory, and is too hot to
use in summer.

So when we heat our houses with electricity, after conversion from coal,
then transmission losses, we are probably only using 20% of the original
energy found in the original coal. In many ways we are much less efficient
at heating our houses than the poorest houses in China, where they use coal
directly in the home. That's a pretty sad fact.

The issue of noise and pollution is nothing. My math shows that a single V6
engine is enough for a group of 10 to 20 houses. A suitably built and
suitably placed generator will be less noisy than passing cars on the roads
outside. Cars are very quiet these days. All I hear from the road outside is
tyre and wind noise, I only hear engine noise from older cars.

14. ### atec77Guest

A small turbine genny set is viable and atm actually affordable

15. ### Mr MacGuest

Where did you find a turbine genny that was affordable?

16. ### atec77Guest

China of course , you don't get those emails I think

17. ### terrycGuest

Then you are just idling the engine. Ideally you set the engine revs at
the maximum efficency, then gear the generator to run at the required
cycles, also chosing the right size (power) generator. We are not
talking about sing the car alternator here.

18. ### GrantGuest

Has anyone considering this done the math on cost of gas through the
25% efficiency of the motor and compared that to electricity charges?

If you could a) suck enough gas to run a motor, b) easily generate own
power, c) make an ROI, don't you think Honda and other generator makers
would be tapping this market to sell generator sets running off gas?

Do you recall any motors running off propane bottles? Where they're
used? Why?

Grant.

19. ### terrycGuest

Large scale, plenty of underground coal mines look at it and quite a few
have high enough gas concentrations in their exhaust vents to do so.
A few rubbish pits have now also been tapped for gas powered generation.
At one stage, it home-bloke wanted to used LPG, you purchased a petrol
powered generator and had it converted to lpg. It cost about \$400

Now, you can buy a range of lpg powered generators. Do a web search.

Why? Petrol is explosive, hard to store, convenience, etc.

Note, i'm not the person promoting the enginer conversion.

20. ### Jasen BettsGuest

a: standby generators, other standby uses
b: some forklifts
a: the shelf-life of petrol is a few months unless kept in a sealed container.
b: not sure, perhaps it's cheaper?