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Basic Cogeneration Theory and Application

Cogeneration facilities produce electrical power and utilize the high temperature exhaust of large gas turbines (or reciprocating diesel engines) to produce steam or hot water in place of conventional fired boilers. This means you get two forms of energy (electricity and steam) from a cogeneration facility, as opposed to a conventional CHP (Central Heating Plant).

Cogen Heat & Electric Generation

Cogeneration recovers the waste heat that is always produced in electricity generation and puts it to good use rather than letting it escape into the atmosphere. In conventional power generation, about two thirds of the energy input is wasted in this way. Combined Heat & Power (CHP) can recover the majority of this waste heat, creating a far better use of resources and cost savings, and resulting in energy savings of between 20 and 40%.

A CHP plant is an installation where there is simultaneous generation of usable heat and power (usually electricity) in a single process. The term CHP is synonymous with 'cogeneration' and 'total energy', which are terms often used in the United States or other Member States of the European Community. The basic elements of a CHP plant comprise one or more prime movers usually driving electrical generators, where the heat generated in the process is utilised via suitable heat recovery equipment for a variety of purposes including: industrial processes, community heating and space heating.

CHP can provide a secure and highly efficient method of generating electricity and heat at the point of use. Due to the utilisation of heat from electricity generation and the avoidance of transmission losses because electricity is generated on site, CHP typically achieves a 35 per cent reduction in primary energy usage compared with power stations and heat only boilers. This can allow the host organisation to make economic savings where there is a suitable balance between the heat and power loads. The current mix of CHP installations achieves a reduction of over 30 per cent in CO2 emissions in comparison with generation from coal-fired power stations, and over 10 per cent in comparison with gas fired combined cycle gas turbines. The newest installations achieve a reduction of over 50 per cent compared with generation from coal-fired power stations.

Integrates a heat pump and generator into a simple package delivering Air-conditioning, Space heating, Electrical power generation & Hot water.

The MIT Cogeneration Project represents a ten year, forty million dollar initiative by the Massachusetts Institute of Technology to generate its own electrical and thermal power. The new plant is projected to save the Institute millions of dollars over the life of the plant through the technology of cogeneration. Through cogeneration, we generate our electrical and thermal power simultaneously by utilizing the waste heat from a gas turbine to generate steam. This technology is approximately 18% more efficient than the technology that it replaces. MIT also feels strongly that environmental preservation is more important than ever. We have utilized that latest technology avaiable for reducing our emissions into the air of Cambridge. The new technology used in our plant will reduce emissions by 45% compared to our old technology. This reduction is the equivalent of eliminating 13,000 automobile round trips into Cambridge per day. MIT is also committed to making this new facility a resource for the entire MIT community. We are currently working on integrating our plant with academic departments in order that both the cogeneration facility and the academic community can benefit.

The Combined Heat and Power Association works to promote the wider use of combined heat and power and community heating. Clean and efficient Combined Heat and Power (CHP) is already in use on close to 1,400 locations around the UK.

Imagine your own guaranteed supply of electricity in a compact, quiet, self-contained package - it's called the HomeGen 7000. This new energy system produces more power, and is more environmentally friendly, more efficient and more reliable than ever. About the size of a refrigerator, the HomeGen 7000 provides 100% of a home's energy needs, plus runs on fuels (natural gas or propane) already delivered to your home.

Cogeneration is a power generation process that, in comparison to more traditional methods, can double efficiency and reduce carbon dioxide emissions by over two-thirds. This is because cogeneration harnesses heat that would otherwise be wasted in the fuel combustion process, and it produces two useful outputs: heat and power. The heat can be converted into a number of applications such as steam, hot water and cooling.

Cogeneration, the simultaneous production of useful thermal energy and electricity from the same fuel source, is more efficient than the sum of the separate processes - but only in the right conditions.

Combined Heat and Power (CHP) systems, or cogeneration systems, generate electrical/mechanical and thermal energy simultaneously, recovering much of the energy normally lost in separate generation. This recovered energy can be used for heating or cooling purposes, eliminating the need for a separate boiler. Significant reductions in energy, criteria pollutants, and carbon emissions can be achieved from the improved efficiency of fuel use. Generating electricity on or near the point of use also avoids transmission and distribution losses and defers expansion of the electricity transmission grid. Several recent developments make dramatic expansion of CHP a cost-effective possibility over the next decade. First, advances in technologies such as combustion turbines, steam turbines, reciprocating engines, fuel cells, and heat-recovery equipment have decreased the cost and improved the performance of CHP systems. Second, a significant portion of the nation's boiler stock will need to be replaced in the next decade, creating an opportunity to upgrade this equipment with clean and efficient CHP systems. Third, environmental policies, including addressing concerns about greenhouse gas emissions, have created pressures to find cleaner and more efficient means of using energy. Finally, electric power market restructuring is creating new opportunities for innovations in power generation and smaller-scale distributed systems such as CHP. Our integrated analysis suggests that there is enormous potential for the installation of cost-effective CHP in the industrial, district energy, and buildings sectors. The projected additional capacity by 2010 is 73 GW with corresponding energy savings of 2.6 quadrillion Btus, carbon emissions reductions of 74 million metric tons, 1.4 million tons of avoided SO2 emissions, and 0.6 million tons of avoided NOx emissions. We estimate that this new CHP would require cumulative capital investments of roughly $47 billion over ten years.

Cogeneration (sometimes called distributed power generation) is the generation, on site, of your own power and at the same time taking advantage of the exhaust heat from your gas turbine or other engine to meet on site heat needs. Heat can be used to heat buildings, heat dryers, generate steam through a HRSG (heat recovery steam generator), or to provide air conditioning through an absorption chiller. To determine whether you can save money with this approach a comparison of how electricity and heat are provided using the historic approach to the costs from a cogen approach is necessary.


Electricity and Heat from the Same Fuel

A Guide to Cogeneration - This Guide is designed to explain the
principles and applications of cogeneration and to help policymakers
and other professionals understand this technology. By the The
European Association for the Promotion of Cogeneration Cogeneration,
2001 (pdf, 51pp, 212kb)

This Guide has been produced by COGEN Europe to explain the
principles and applications of cogeneration. In compiling the Guide
COGEN Europe has drawn the experience of its membership, especially
the Cogeneration Policy Working Group. Our thanks to those
individuals who provided data, comments and their time. In addition
COGEN Europe has drawn upon some excellent documents produced by
national programmes.

The Guide aims to provide a definitive explanation of cogeneration.
It is designed to help policymakers and other professionals
understand this energy solution, which is now receiving a great deal
of positive attention, both for its energy efficiency and
environmental benefits. The guide is also targeted at the growing
number of students who are taking an interest in the subject. The
guide is divided into two parts:

*An general introduction to cogeneration and its principle 1 and benefits;
*A technical guide to the options available.

VW Diesel-powered GenSet

Mad Housers President Frank Jeffers has developed a Co-Generation plant for hot water and 12-volt electricity to make hut living a comparable alternative to low-income housing. A moderate-sized battery bank of golf cart or heavy equipment batteries will provide 5 huts with electricity for 2-3 days between charges. A 4-cylinder car engine attached to one or more alternators recharges the battery bank when needed.

Home Grown Cogen Project

Right now, this is a concept. As we firm up the design, and acquire the parts, I will document the construction of this project. Here are the preliminaries:


Cletrac <>

Bruce Gordon <>

Alan Robinson <>

David J. Hunt <>

Doug Wicks <>

Nick Pine <>

Steve Spence <>

Morgan Ford <> VW rabbit powered generator

Byron Cass <> The Engine Room

Here's my home built induction generator. I belted a three phase 10 HP motor to a spare pulley on my VW. The capacitor bank is connected "C-2C" and combines the three phases into a single phase. Belt tension is adjusted by sliding the motor back and forth on the unistrut. The weight of the car holds everything in place. 3500W is about the max output so far but I hope to improve that when I come up with a method of voltage regulation. RPM is regulated by a Ambach governor that I salvaged.

Morgan Ford <> automobile based generator systems.htm

Doug Wicks <>

The Rabbit diesel will deliver about 50 hp. That means you can bolt it to a
20 kva max. size generator.  You will need a bell housing and some machine
work for an adapter to direct drive the generator in which case you want to
find and 1800 rpm generator.  If you can modify the engine so it has a shaft
bolted to a flywheel you can belt drive a generator, and in this case you
could use a 3600 rpm generator.  What we do for these is machine a " thick
piece of steel, round so it can be bolted to the flywheel, and then we drill
out a 1 " hole in the center an weld a shaft in place long enough to
accommodate a pulley. Mount the whole thing on a skid complete with the
radiator and you should add a safety cover over the drive belts and pulleys.
We can provide you with the generator and controls. We might even have a
used generator around here someplace.   Please email me any other questions
you may have.

Kindest regards,
Doug Wicks.

Look us up on our website at .

'81-'83  rabbit diesel 1.6L 38 kW (52hp) @ 4800 rpm (torque 71ft-lb@2000)

so I am hoping I have enough HP to run the A/C Compressor, If my electrical
load is not high.

at work we have a big a/c unit for our floor, and the plate on the
compressor motor says 10hp. Hmm, should be able to hook this up to the engine as

cool engine swap/spec page.
Steve Spence
Renewable Energy Pages

Nick Pine <>

I don't know the exact figures, but you can go by gas consumption.
Multiply gph by 120K Btu per hour. You should be able to get at least 60kW (~200K
Btu/h) of heat along with the 20 kW of electricity.


From: Steve Spence <>

| I have the engine. I am missing a few pieces in order for it to run, so I
| have to make another trip to the junkyard. I still don't have a gen head.
| Looking for a 20kw unit, and will belt drive instead of direct. bell
| machine work does not appeal to me right now. while I'm looking for the
| head, I am starting on the heat exchanger for the water loop for the
| this has 2 separate circuits, 1 for domestic, 1 for baseboard heat. I'm
| still evaluating using exhaust heat for absorption cooling.
| --
| --
| Steve Spence
| Renewable Energy Pages

Well Steve I was lucky enough to get a whole engine out of the junkyard.  I
know it was running there, and was retired because  they sold a front axle
out of it.  It presently is mounted to an engine stand and waits for me to
decide on how much I want to rebuild it (just for fun .. I think)
    I did also get everything out of the car...  ie trans-axle etc.
    My present plans are to keep it at about the same stage it is in
(exposed end of crankshaft), add the flywheel back on it (was a standard)
then either make a plate to mount here for a Lovejoy (like a Lovejoy.. but
probably not a Lovejoy) joint into a 1800rpm 3phase motor.   I had hoped to
get a 20-25 hp unit, but my brother just told me last night that he has a
15hp for me @ his house.  I expect I'll go that route for a test anyway.
    Since I'm connected to the grid, and expect to be running my meter
backwards (under 30kva level of concern) I expect that operating speed will
be around 1860rpm   (1800rpm motor w/3.3% slip (1740rpm))
    I wanted to shy away from belts if I could because I am removing some of
the flywheel weight that the engine would normally have, and I wanted to use
the motor's armature to add to this flywheel weight. (the more... but not
too much... weight that is here will help buffer the rpm hunting if the unit
is run on generator mode instead of grid feeding mode)
    One of the other reasons I am drifting towards a direct couple is to
avoid placing any side stresses on the components, and avoid the losses from
the belt.
    Since I do plan on cogen use.. I will not be converting the motor into a
generator, but instead use the grid to excite the units' fields.   One
thing I will be doing is setting up the correct capacitors into the design
so that at a flip of a switch I'll have a self exciting generator.
    I also plan on having a mechanical governor installed for trottle
control.  I can't forsee finding anything electronic that is quite
affordable to my (wife's) pockets.
    Apparently you have been thinking on this for some time as well.  PLEASE
keep in-touch ! I'd  love to find out what you find out on your unit.  I
will keep you updated on my project (just don;t hold your breath...  MUCH
else has priority).

David J. Hunt <>

Doug Wicks <> seems to think it will handle the horses. I
will try that first. If it can't then direct drive is the next approach.
however, that means a different generator, and a machined bellhousing. the
belt drive unit was a 3600 rpm unit, the direct drive needs to be a 1800.
hmmm.... maybe I need to rethink this approach? I have a 5kw unit here for
testing purposes.

Steve Spence
Renewable Energy Pages


Sylvan Butler <> wrote in message
> Steve Spence ( on Mon, 11 Oct 1999 13:15:19 GMT
> >Looking for a 20kw unit, and will belt drive instead of direct. bell
> >machine work does not appeal to me right now. while I'm looking for the
> Belt drive that many horsepower?  Ouch!
> How are you going to handle it?
> sdb
> --
> Do NOT send me unsolicited commercial e-mail (UCE)!
> Watch out for munged e-mail address.
> User should be sylvan and host is

> Steve, Volkswagen makes diesel-powered gensets, as well as diesel
> engines with sae standard bellhousings. If this is the same engine as the
> car
> diesel (and it appears to be), then the parts to adapt are (probably)
> available.
> Will dig thru my literature at work tomorrow and see if I can come up with
> contact
> point for you. Of course, this substitutes money for machining..but as far
> as I can
> tell, a belt drive handling 20+ hp isn't too cheap, either.
> Alan
Alan Robinson <>

Hi Steve,
    I checked your link you sent me.  Nice !  A place to compend all of this data type stuff would be great.  I usually cut and paste stuff into a html file myself...   but it is chaotically arranged.. like me :=>
    I am not against the belt drive method you know.  I was planning on making a similar bolt-up device as dwiks suggested.   The question I have is this....  why do you need a machined bell-housing?
    Do you have a welder?
    How do you plan on mounting the Engine?

ps...  since I'm in EST time, what time zone are you in ?  makes a difference if we both end up all night
Hi Again,
    Steve the reason that I'm asking that, is that I do (have a welder).  I was planning on taking two lengths of steel and make a skid to mount the engine too. Then, once that is done, I expect to go farther down the skid beams and mount a platform that will mount the engine in-line w/the crank shaft.
    The second platform is the hard one I think, but I'll design it to be held in-place until it is welded up.
    I am sure that TRUE lineup will be near impossible, so that is why I'm trying to use a joint with a 1-2 or 1-5 degree operating range.
    I figure that this way, with a motor/mounted to the skid, and the engine/mounted to the skid, I would not need any fancy bell-housing.  The skid will need to be substantial I know, but hey, I'm not going to move it every day !!!
    This arrangement is similar to what you are doing w/the pulley system I think.
    I mean.. you will be mounting the engine and then making a mount for the motor near-by....
    I'm doing the same, just not side-by-side... and I plan on really anchoring the motor to the skid-fame as well as the engine (of course there will be some trusses built in as well)
any return ideas?

David J. Hunt <>

Hi Dave.
If as Sylvan alludes to, 20kw is to much for a belt, then direct drive is
the way to go. experimentation will decide. Doug suggested the bell housing
approach. I have a small arc welder available to me (neighbor), am looking
to buy one of my own, or build one like you can see on my website. I was
planning on 2 I- beams, with motor mounts welded on, and a flat plate for
the genny. I'm in Trenton NJ.

Steve Spence
Renewable Energy Pages



Belt drives for 20+ HP are not uncommon in the Marine and commercial
fishing environment.  You basically use more belts in parallel and
multi-belt shives.
I have a 10 Kw belt driven unit running at my cabin that uses 4 belts,
off a
4-108 Perkins diesel engine.  I have had it at 110% load for 5 minutes
numerous times with no slippage or noticeable problems.

If you don't want to do belts, just mount the gen-end inline and use a
coupling between the Gen-end shaft and the output shaft of the engine.
 These types of units are very easy to fabricate and operate.  My
primary units are a identical pair of 20Kw Lugger diesel gensets that
are single bearing, direct flywheel connected, gen-ends.  They were
both rebuilt a year ago, after 40,000 hours each, and I shouldn't have
to do that again in my lifetime. 

Bruce (semi retired powder man & ex FCC Field Inspector for Southeastern

  Bruce Gordon * Debora Gordon R.N.         Bruce's Trading Post
        P.O. Box EXI                        Excursion Inlet South
     Juneau, Alaska 99850               Excursion Inlet, Alaska 99850

    University of Alaska           Remote Weather Data Collection Site               WDT-59   KWO-70   KNEG-586                  Wards Cove Packing Co.             AL7AQ@KL7HFI * KL7WJ@KL7HFI
       ****  Have bandwidth and connection   Will communicate  ****


Ok, Steve (and others)....didn't find any parts breakdowns in the info I
have..but VW diesel industrial engines are definitely available with SAE
standard bellhousings (which would mate to a sae standard gen end from
Marathon, Newage, Mecce Alte, etc...). The two sources I have for VW
gensets/industrial engines are:
Northwest Power Systems
1-800-922-4214 (fax 1-360-653-2076)
 (a big company, handles Volvo/Penta, Cummins, Lombardini, etc.)

Snake River Industrial Power
1-208-362-1444 (fax 1-208-362-9802)
 (fairly small, almost exclusively VW)

Both of them have been quite responsive in my dealings with them, but Snake
River will probably be easier to get an answer from someone who actually
works on them (rather than just selling them).

On the idea of belt drive - IF you use the best (5vx) belts, and your
smaller pullley is 8" diameter or greater, one belt is good for just over
20hp (with no margin). But keep in mind that for good surge/transient
response, you're going to need to plan for closer to 2 hp per kw, even tho
it doesn't take that much steady-state. And you may not like to run the belt
at 100% load factor.  Maybe a triple-sheave drive?  HTH,

Alan Robinson <>

For the work and money you are spending to set up a generator for CoGen why not
do it with the grid. The grid will be up far more than it is down and the power
and heat will be of use over a longer period. On top of which, with a small
generator set at 60 cycles it will supply the reactive power to excite the
induction motor and cause it to generate even when the grid is down. I bet you
could do the same thing with an inverter and a couple of batteries. You can
even make the induction motor self excite and generate independent of the grid,
you just need a good governor and a diversion regulator with some heating
elements to keep the load constant.  Think out of the box

BTW there is already a company making a generator using a VW turbo diesel and a
Newage generator end. It's an 1800 rpm direct drive unit.

Before you size the generator, get a set of performance curves from VW, you
will find that the lowest fuel consumption at power occurs at about 2200 rpm.
If you plan to run the engine at 3600 rpm only plan on using about 75%  of the
available power to drive the generator at the rated output You will need some
surge load capability. A generator that delivers 20 kw will try to deliver 25
kw to the load unless there is a breaker to cut it off. If the engine is only
able to make 20 kw of power, the generator will drag the engine speed way down
and could stall it.

You can tie an asynchronous generator to the grid very easily. Call your power
company. Mine requires a disconnect switch, an overload protection device
(breaker) and an over / under voltage relay and an over / under frequency
relay. The relays are about $350 total at Grainger's. I know it works since I
have seen two home built cogen setups one used a VW diesel and the other a
Mazda diesel. 

Cletrac <>


Similar thread on heating water from exhaust of a generator:

Condensing the moisture from the exhaust can mean saving
more of the heat... A Honda EB2500 generator has a 2.9 gallon
gas tank that lasts about 8 hours at 2500 W, so it burns about
0.36 gallons of gas per hour with a heating value of about 43.5K
Btu/h (12.75 kW) and a 4.1 heat power to electrical power ratio.
Why waste all that heat?

We might remove the muffler from an EB2500, weld a 3/4"
galvanized nipple onto the end of the $17 5" curved steel pipe
(Honda part # 18330-2B2-000) that attached the muffler to the
engine, screw on a $3 adapter and a 2' flexible stainless steel
natural gas supply hose (about $8 from Home Depot), and
attach the other end of the hose to a brass bulkhead fitting that
enters a 2' wide x 8' long x 4' deep plywood tank lined with a
single piece of EPDM rubber.

The 98 pound generator could sit on top of the tank, with some
sidewalls for noise reduction and fume removal as the 100 cfm
engine air intake depressurizes the enclosure and Grainger's
36 watt 4C688 560 cfm fan with a 149 F temperature rating
cools the engine fins.

The other end of the bulkhead fitting might be attached to a few
3/4" copper pipes with lengthwise sawcuts that run the length
of the tank a couple of inches underwater. The tank might have
a plywood cover with some galvanized metal below, with a 3"
galvanized bulkhead fitting on the top of the cover that routes
the exhaust to a chimney. A couple of float valves could keep
the tank topped up, and a couple of 300 F thermostats could
stop the generator if the exhaust overheated. The house might
also contain a Nighthawk CO detector.

Over 8 hours the gasoline would make about 2.9gx120KBtu/g
= 348K Btu of heat, which could warm the 4,000 pounds of tank
water from 70 to 155 F. This dirty water might preheat water
for showers via a soft copper tubing heat exchanger near the
top of the tank.



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