Need Opinions on My Cogen Plan

[Yianie123.]

I am considering building a co-gen unit.  I am planning on purchasing a 13hp gas engine = $350, 7kw Generator head+299, building a heat exchanger to add to the engine exhaust that is approximately 24"long, 3.5" diameter with a 1.75 pipe inside to be attached to the engine exhaust.  The 3.5 diameter tube will have 3/4" fittings on the outside for fluid to pass through and then go to the solar hot water tank that is used for domestic and hydronic water to heat the house.  I would only use the generator for emergencies.  The total cost I estimated is approximately $1000.00.  Any opinions are greatly welcomed.

[rossw]

I am considering building a co-gen unit.  I am planning on purchasing a 13hp gas engine = $350, 7kw Generator head+299, building a heat exchanger to add to the engine exhaust that is approximately 24"long, 3.5" diameter with a 1.75 pipe inside to be attached to the engine exhaust.  The 3.5 diameter tube will have 3/4" fittings on the outside for fluid to pass through and then go to the solar hot water tank that is used for domestic and hydronic water to heat the house.

I built mine - a little larger than yours, but same idea, pictures here

I think your 1.75" dia x 2' long pipe might be too short and too fat to really extract as much heat as you expect. The one shown in my photos above, is about 3' long and has 20, 1/2" copper tubes with enhanced surface area. It's quite effective but even that doesn't extract all the available heat.

 I would only use the generator for emergencies.  The total cost I estimated is approximately $1000.00.  Any opinions are greatly welcomed.

I seriously doubt your returns on investment then. If it's only for emergencies, it's a lot of effort to go to, and you'll need to do something else for your DHW and hydronics for the 95% of the time this isn't running....

[bob g]

lets take a look at what you are talking about here

at full load your cogen will burn about a gallon per hour, give or take, but lets call it 100kbtu's worth of fuel
for the sake of discussion.

about 1/3 of that fuel consumed will be available for recovery from the exhaust, or about 33kbtu's per hour of run time
at full rated load.

i agree with Ross, your heat exchanger plan will probably only recover about 5kbtu of the 33kbtu available, actually it might do
as well as 8kbtu and i bet i am so close to right on this one it isn't funny.

your target should be about 75% recovery off the exhaust, you don't want to take all the heat away as you don't want to cool the exhaust below about 212F for condensation reasons, and because overcooled exhaust robs engine horsepower due to pumping losses being much higher for cooler exhaust vs hot exhaust.

a good target might be somewhere in the neighborhood of 25kbtu's of recovered exhaust heat per hour to make your hot water, and do some space heating,,, that is at full load keep in mind.  lower load = less heat available, and it might not be linear. every engine operates differently in this regard and every heat exchanger is sized to be optimized for a rather narrow range of operation, so it could well be at half load you only get 1/4 of the heat that you would otherwise get at full load.

the reality of such a system comes down to this, it is probably best used intermittently for the production of hotwater and view the electrical power generated as the waste product that you either use directly or store for later use. this way you will keep the unit running at or near full designed loading for max heat production without focusing on the electrical output as the primary  product.

remember the engine is about 3x better at making heat than it is in producing electricity, so optimize the unit with an eye toward what it is truly good at, and you will probably be more happy.

if your concern remains higher for the production of electricity, i fear that the gasoline engine is a poor choice to start with, that is if you are planning on burning pump  gasoline and not some alternate fuel.

fwiw

bob g

[Yianie123.]

You both bring up some excellent points.  First, if I am limited with size for the heat exchanger, would a high velocity pump make a difference, or should I consider 2x or 3x the size of the heat exchanger?  Second, what alternative fuel would you reccommend?  My understanding is that diesel engines are a lot more expensive,  small engines can not take the alcohol fuel, so please give me more of your thoughts.

[rossw]

You both bring up some excellent points.  First, if I am limited with size for the heat exchanger, would a high velocity pump make a difference, or should I consider 2x or 3x the size of the heat exchanger?

The trick to getting output from a heat exchanger is to either have the hot spend a long time in the cold, or to increase the transfer rate (or both!).

Given you're making the unit yourself, "enhanced surface area" tubes isn't a viable option for you.
If you can't make the exchanger longer, the only other practical thing you can do is to make the gas go slower.
How do you make it go slower? Easy, more cross section in the tube. Problem is - more cross section means the "hot" stuff in the middle stays hot.
So... how? Well, like I did. Input gas into a plenum chamber, then split it into several thinner tubes. Each one carrys a portion of the hot gas, and because there are many in parallel, the gas flows more slowly down each, and being thinner tubes they get to impart more energy to the "cool" part.

Thats my take on it anyway.

  Second, what alternative fuel would you reccommend?  My understanding is that diesel engines are a lot more expensive,  small engines can not take the alcohol fuel, so please give me more of your thoughts.

Diesel isn't (IMO) a suitable fuel for a stationary engine, where you'll be collecting rainwater for drinking.
I chose to convert my engine to propane. It's clean, it's relatively easy to get and store. I'm not allergic to it. Engines seem to love it - plugs last for ages, exhaust doesn't get all sooty, oil stays pretty good for a long time. (I have an extra large oil system on mine - up from the normal 4 litres to 10, and after a thousand hours running, the oil is still reasonably light in colour, still slippery, doesn't smell, and oil analysis shows very little contamination. Viscocity is down a little but thats not a surprise)

[Yianie123.]

Again, you bring up some interesting points.  Did you convert your engine to propane and if so where did you get the kit?  My understanding is that the kit to convert is hundreds of dollars.   If you purchased a ready made propane engine, where did you get it from.  Additional help is greatly appreciated.

[rossw]

Again, you bring up some interesting points.  Did you convert your engine to propane and if so where did you get the kit?  My understanding is that the kit to convert is hundreds of dollars.   If you purchased a ready made propane engine, where did you get it from.  Additional help is greatly appreciated.

Did you look at my link?

It was a standard 4-cylinder, multi-point fuel-injected car engine. We removed all the EFI crap, comon rail, computer etc etc.
I added a gas mixer (looks like a donught with holes on the inside) right before the throttle butterfly, and a safety valve on the 100 KPA high-pressure side of the gas line, and a pressure regulator, and a fuel control valve (for automatic control of air/fuel ratio).

It wasn't a kit, I purchased individual parts as I needed them.

[Bruce S]

Yannie123;
 If I my add a few items here.
First I must state up front that I cannot hold a candle or even the wick  to the amount work towards this area that Rossw and bob g have done.

However the smaller engines not being usable for alternatives is not exactly correct. A correctly sized engine may be exactly where you should be heading first.
A smaller engine running at near full load when needed will be more efficient than a larger one running at say 1/2 load.
 
Rossw's setup is a thing of beauty, but I could make out the total plumbing of the vapor based fuel either.
RossW: I looked at all those great pictures of your system and couldn't quite make the total setup, but it sure looked like the high quality kits that were sold here in the USA by IMP. After reading your last post it is clearer/easier to understand.
AND I agree, once you get all the EPA mandated stuff off them they sure run a lot better.

Gas based engines lend themselves to be converted or retrofitted for multi-fuels nicely, much easier than diesels.
I have a near continuous supply of used oil, so  I work in that direction, but do have to becarefull NOT to cool the engine and exhaust too much otherwise the engine works harder to keep itself in the optimum heat range.

While still a bit controversial in due respect to production, even ethanol and methanol can also be used in these systems. 

IF I may ask RossW and bob g : are you found that allowing for back pressure to help recirculate some of the un-burned fuel? or since running at optimum speed range your not needing to?

Cheers;
Bruce S
 

[rossw]

A correctly sized engine may be exactly where you should be heading first.
A smaller engine running at near full load when needed will be more efficient than a larger one running at say 1/2 load.

Absolutely correct there - and if I had my time again, I'd certainly use a smaller engine. I used the one I did for a plethora of reasons, including:
 * readily available
 * modest price from the car wreckers
 * very common motor, spare parts should be available for a loooong time
 * a "tried and true" engine that I was familiar with
 * plenty of power even at the relatively low RPM I'm running it

That said, several of the problems I DID/DO have are:
 * commercial automotive gas mixers didn't like such a small engine with such relatively low airflow
 * such a large engine relative to the power I draw from it, is very "twitchy" when unloaded

 
Rossw's setup is a thing of beauty, but I could make out the total plumbing of the vapor based fuel either.

Sorry, I'll go see if I've got some more detailed or relevant pics. In short, I have a 2.75 ton vapour-takeoff pressure vessel outside, that sits around 200 PSI, that feeds a pre-regulator to take it to about 15 PSI that feeds the engine. That comes to a tap and connection point on the wall near the genset, with a flexible hose to the engine itself. At the "front" of the engine mounted on top is the safety valve and it's electronics (that monitors spark and turns off gas if the spark stops), that feeds the "gas converter" (in a car it would take liquid propane, and be plumbed in to the hot water to prevent it icing). Out of the converter to the servo-controlled fuel valve and then to the mixer by the throttle body.

RossW: I looked at all those great pictures of your system and couldn't quite make the total setup, but it sure looked like the high quality kits that were sold here in the USA by IMP. After reading your last post it is clearer/easier to understand.

It's likely the same bits, just sourced one component at a time.

IF I may ask RossW and bob g : are you found that allowing for back pressure to help recirculate some of the un-burned fuel? or since running at optimum speed range your not needing to?

Well, the length of my exhaust is a little longer than it would have been in the car - but it's also twice the diameter it would have been, so that helps keep the back pressure to "about nominal".
Because I have an oxygen sensor mounted in the exhaust manifold, and use that to control the air/fuel ratio, the exhaust is pretty clean. Running constant RPM but variable load and adjusting AFR seems to keep the engine pretty happy.

Like I said before, oil change after the equivalent of 50,000 miles and it's still in pretty good nick, I change the spark plugs after what is probably the equivalent of 500,000 miles - but I wonder why I bother - they come out looking much like the new ones go in   (only a slight exaggeration).

[bob g]

because our forum focuses on cogeneration, our members are always keenly alert looking for all sorts of possibles

this one came up today, and i thought it might be useful for the OP on this topic

http://cgi.ebay.com/10-HP-DIESEL-GENERATOR-KIT-5-6-6-5-Kw-/190482130234?pt=BI_Generators&hash=item2c599ea53a

while it is a 50hz genset, it can be turned up for 60hz operation and the excitation capacitor changed to bring the voltage back into
spec.

the engines are made by changfa, a company with a good reputation, this one is also electric start to boot.

i can't imagine getting more for the money, might be a good start for a micro cogen if one is set on using an aircooled diesel.

fwiw

bob g

[Bruce S]

because our forum focuses on cogeneration, our members are always keenly alert looking for all sorts of possibles

this one came up today, and i thought it might be useful for the OP on this topic

http://cgi.ebay.com/10-HP-DIESEL-GENERATOR-KIT-5-6-6-5-Kw-/190482130234?pt=BI_Generators&hash=item2c599ea53a

while it is a 50hz genset, it can be turned up for 60hz operation and the excitation capacitor changed to bring the voltage back into
spec.

the engines are made by changfa, a company with a good reputation, this one is also electric start to boot.

i can't imagine getting more for the money, might be a good start for a micro cogen if one is set on using an aircooled diesel.

fwiw

bob g

bob g;
 That's a pretty nice little unit! If Santa didn't have 1/2 dozen grand-kids he'd be looking into grabbing one of those too 
If nothing else it would work nicely on a 3-wheeler 

Thanks for the link!!
Bruce S

[Yianie123.]

Its looks great, but why would the government purchase 50hz units?  I am a little skepticle.  Has anyone had experience with this unit and if so how was it?  I am seriously considering this purchase, but dont like the idea of having to modify it.  Any opinions?

[bob g]

its not really a modification rather it is an option as i see it

the unit is likely marketed for either 50 or 60hz operation, with only one part needing swapped
and the governor set to either 3000rpm or 3600rpm

the engine is designed to run at either rpm, and do so just fine

the alternator will care the less which speed you spin it, however

the output voltage is dependent on the capacitors rating,
the lower mfd will allow the unit to spin up to 3600rpm and produce 60hz while keeping the voltage down
to around 120/240vac

i got a report that the engine is a true changfa brand engine, if this is true it is among the best of the chinese aircooled engine's.

i wouldn't be afraid of the unit based on having to change the capacitor and rpm, other factors notwithstanding.

it really depends on what  you want to do with this thing.

bob g

[TomW]

There are many "governments" not all of them use 60 HZ in their country.

Just my first thought on that.

Tom

[DanG]

Having a contract 'canceled' might be evidence there is a weak link in their kit...

EDIT: Canceled may be too strong a word? Auction wording is "Ended"... with 200 units in que for assembly?

[Bytesmiths]

Do you live near water?

If so, you might consider a generator designed for marine applications. These are often advertised on bulletin boards at marinas, or in boating classified newspapers.

Rather than a radiator, decent ship generators typically have a water jacket and a salt-resistant heat exchanger. The heat exchanger then takes in seawater to dissipate the excess engine heat. Because of the heat exchanger, you can run floor head through it, or even potable water!

I've got two of these that I picked up for $800 each, waiting for a project... :-)

In any event, if you are serious about harvesting excess internal combustion engine heat, you're better getting it from a water jacket than from the exhaust. Water-to-water heat transfers are much more efficient than air-to-water.

[Yianie123.]

Well, I did go back to eBay to look at the generator kit.  I did speak to the person on the site and it is exactly what you have mention.  A modification.  I did take the next step and purchased their 6.5kw diesel generator with muffler in a nice square box for $850.  It comes with wheels to move the generator and outlets/breakers.  I am usually a DIY er but on rare occasions like this one, I just write the check.  I think I did pretty good because it is a diesel.  The 2.5 gal tank is supposed to last 8hrs at 1/2 load.  Additional comments are welcomed.

[WindriderNM]

The 10 kw is re-listed on feebay but now at $695 I guess they decided it was wort more. they now have flat rate shipping.