Micro cogeneration?

[Student 07]

Hi,
I was looking at micro cogenerators like Freewatt and they seem like a good idea. Why not get electricicty and heat from your fuel. It seems like it would work great when you're off the grid. And even on the grid, you're paying for fuel to produce heat, why not get electricity as a byproduct? I am sure it could be used, or like solar and wind power, be sold back to the utility.

What is holding me back is that they are very expensive, I have been told that a small microcogenerator costs around $50,000. The return on investment doesn't pencil out. You still have fuel costs. It seems like a technology that is ripe for a hack.

I don't understand why they are so expensive. Couldn't a guy just use a liquid cooled motor to run a generator, and instead of a liquid to air (radiator) heat exchanger (which blows heat outside) use a liquid to liquid heat echanger to preheat water for the house? The preheated water could be used for DHW or radiant floor heat. The alternative (which almost everybody is doing now) would be to use fuel to create heat and pay for the electricity seperately. The fuel for the motor would supply both electricity and heat.

I am thinking that a 250cc motorcycle engine would have plenty of power to run a 10kwh generator. The motor would be geared to operate at 1500rpm so it shouldn't take a lot of fuel. If the heat from the motor is kept at 190*, it shouldn't take long for it to heat a hot water tank.

I am hoping that somebody with experience or a better understanding than me can explain the pro's and con's of this kind of system. Thanks in advance for your responses.
Jeff

[AC_Hacker]

Originally Posted by Student 07

I was looking at micro cogenerators like Freewatt and they seem like a good idea.

Dear Student...

If you use the "search" button at the top of the page, you should find posts and threads that discuss this very topic.

This might move you a bit farther along in your quest.

It is a great idea, very hack-worthy. As one member of EcoRenovator pointed out, it is being used on a larger scale to heat towns and villages in Poland.

We should be able to do that, too.

BTW, I saw a very interesting Honda Motor Scooter that had a 50cc water-cooled engine... Could be interesting.

Please let us know what you come up with...

-AC_Hacker

[Student 07]

Hi AC

I did as you said and found some great information. It got me thinking about a co generator, so I did some number crunching. I am kind of confused about the pro’s and con’s to a co generator.

I have a portable generator with an air cooled 11hp Honda engine. It is rated at 8250W surge and 5500W continuous. The manual that came with it says at full load it uses 1 gallon of gasoline per hour. Gasoline would be a hassle for a co generator, so I would want to convert it to propane. We use propane to heat our house anyway.

If I could find an equivalent 11hp liquid cooled motor, it would require 1.26gl of propane per hour. Propane at our local gas station costs $2.65/ gallon. So it would cost $2.65/hr. to run it. Our highest electric bill is ~550kWh/ month. The generator would have to run 100hrs. / month to provide 100% of our electricity. 100hrs./month would be 3.33hrs./day and use 4.2gl of propane/day and cost $11.13/day or $333/month.

For heat I would use a liquid to liquid heat exchanger to keep the motor cool, instead of a radiator. So, ~5hp produces 12,000btu, an 11hp engine would produce (rounding off) about 24,000btu/hr. If my math is correct our house uses an average of 3,455,900btu/month. The co generator would have to run for 144hrs. to produce that much heat. It would require 181gl of propane/ month. It would cost about $480/month for propane to supply 100% of our heating.

Without a co generator we use 450gl of propane/yr. for heat and 4,225kWh of electricity per year. To me it looks like the co generator is an expensive way to produce heat and electricity. The initial cost is bad enough, but the operating cost isn’t any better. I am no math wizard, and I may have made a mistake in there somewhere, so please correct me if you see any mistakes. I know some things I rounded off and I probably should have used cost/ year in my examples, but the cost/hour would still be the same, and it looks expensive to me. The cost/btu or cost/Watt seems high compared to using the grid and a standard boiler.

I know that every time energy is converted a large portion of energy is lost. Maybe a more efficient engine would change things. Maybe a turbocharged 8hp motor that puts out 12hp? If I remember right, forced induction motors are more than 100% efficient. To be fair I also don’t think the generator would always be at full load, so fuel usage would be proportionately lower.
What am I missing? Thanks for the feedback.
Jeff

[Ryland]

Originally Posted by Student 07

I know that every time energy is converted a large portion of energy is lost. Maybe a more efficient engine would change things. Maybe a turbocharged 8hp motor that puts out 12hp? If I remember right, forced induction motors are more than 100% efficient. To be fair I also don’t think the generator would always be at full load, so fuel usage would be proportionately lower.
What am I missing?

Forced induction engines are not even close to 100% efficient, more like 30% while a standard gasoline engine is around 23% sometimes as high as 25% but that jump from 25% to 30% is a big jump! like a 20% increase.
You also have to remember that not all of the wasted energy is leaving the engine in the form of heat from the water jacket, around 50% of the wasted energy is leaving from the exhaust pipe in the form of heat as well, then you have the outside of the engine block, vibration (energy), the generator has losses in the form of heat as well as the energy that is extracted from the fuel and turned in to work, that energy is not turned in to heat so figure that 20% or so of your fuel is not turned to heat but is instead turned in to motion and then turned in to electricity and I would guess that with a home built generator that you could capture 50% or so of that total heat if you insulated the engine well and had a heat exchanger on the exhaust after you figure your energy that is being turned in to electricity.

[AC_Hacker]

Originally Posted by Ryland

Forced induction engines are not even close to 100% efficient, more like 30% while a standard gasoline engine is around 23% sometimes as high as 25% but that jump from 25% to 30% is a big jump! like a 20% increase.

Student,

I might not be quite as generous as Ryland is, in his estimation of the efficiency of the engine, but pretty close. Otherwise, I agree with Ryland's analysis, including the exhaust heat recovery. The exhaust heat recovery, if it includes condensing the water vapor, will be boosted an extra 15%.

Here's a link you both might enjoy:

Forsiden - Green Carbon

-AC_Hacker

[Student 07]

Hi,

When I last posted I didn't want to get into the engine mods too much because I thought it would be more appropriate for the eco modder forum. But, in hindsight I probably should have been more specific.

I am referring to volumetric efficiency. A normal automobile engine flows only about 80% of the calculated amount of charge. With certain modifications it is possible to have an engine with a volumetric efficiency exceeding 100% at a certain speed. These modifications are usually performed on a racing engine which doesn't require the wide power band that a normal automobile engine does.

I am paraphrasing from a book published by HP books called Turbocharging written by Hugh MacInnes (page #6).

It is possible to optimize an engine’s performance for a generator, since a generator usually stays at a certain RPM. It would be possible to tune the intake and exhaust, port size, valve size etc., to get maximum power at that specific RPM.

If you're talking about the efficiency of converting the energy in fuel to heat, I would agree with AC Hacker that you are generous. For example: a 5hp motor running for one hour would require one gallon of gasoline (114,000btu), but only produce 12,000btu (at the radiator). >10%

If you're talking about the engines ability to convert fuel energy to power you are also correct. One gallon of gasoline contains 36.6kW of energy, a 11hp motor using one gallon per hour, would produce 8.2kW for the hour.

As you say the majority of the energy (heat) is going through the exhaust (a turbocharger recovers some of this energy), there are also numerous other residual losses.

I have had turbocharged vehicles before and it is possible to get plenty of power out of them . For the generator, it might be a little counterintuitive, but a larger motor may be better. It would have more than enough hp so it could be geared to operate at a lower speed, which should make everything last longer. A large motor operating at a low speed shouldn't use more fuel than a small motor operating at a high speed.

Engine modifications aside: I don't see a co generator being cost effective, unless the motors are much more efficient or it uses a much cheaper fuel source. I don't think a generator would ever be competitive with grid electricity, so the electricity that a co generator produces has to be thought of as a "bonus".

Maybe it is more effective as a heat source. I may need to go back and check my math for the heat output. ~5hp =12,000btu, but over time it may be cumulative and the btu/h may be much higher. More research,
Thanks for the feedback everyone.

[AC_Hacker]

To help refine your thinking a bit, here is a very recent report of cogen systems that are in use and under test:

REPORT_HERE

Here is an extract from the report:

Abstract: Combined heat and power (CHP) systems in both power stations and large plants are becoming one of the most important tools for reducing energy requirements and consequently the overall carbon footprint of fundamental industrial activities. While power stations employ topping cycles where the heat rejected from the cycle is supplied to domestic and industrial consumers, the plants that produce surplus heat can utilise bottoming cycles to generate electrical power. Traditionally the waste heat available at high temperatures was used to generate electrical power, whereas energy at lower temperatures was either released to the environment or used for commercial or domestic heating. However the introduction of new engines, such as the ones using the organic Rankine cycle, capable of employing condensing temperatures very close to the ambient temperature, has made the generation of electrical power at low temperatures also convenient. On the other hand, district heating is becoming more and more significant since it has been extended to include cooling in the warm months and underground storage of thermal energy to cope with variable demand. These developments imply that electric power generation and district heating/cooling may become alternative and not complementary solutions for waste energy of industrial plants. Therefore the overall energy management requires the introduction of an optimisation algorithm to select the best strategy. In this paper we propose an algorithm for the minimisation of a suitable cost function, for any given variable heat demand from commercial and domestic users, with respect to all independent variables, i.e., temperatures and flowrates of warm fluid streams leaving the plants and volume and nature of underground storage. The results of the preliminary process integration analysis based on pinch technology are used in this algorithm to provide bounds on the values of temperatures.

[ThomSjay]

An option, that A/C Hacker alluded to, is to reclaim the heat out of the exhaust. I haven't thought to deeply on this but there must be a way.....HRV style?

[Ryland]

Originally Posted by ThomSjay

An option, that A/C Hacker alluded to, is to reclaim the heat out of the exhaust. I haven't thought to deeply on this but there must be a way.....HRV style?

You have to capture the heat from the exhaust otherwise 50% or more of your heat is escaping and if you have a catalytic converter on the engine then using a air to water heat exchanger should work.
But it really comes down to figuring out if it's worth the hardware cost and I don't think it is.

[AC_Hacker]

Originally Posted by Ryland

...But it really comes down to figuring out if it's worth the hardware cost and I don't think it is.

High efficiency gas furnaces are able to convert over 95% of the energy available in gas into useful heat.

Furnaces that don't use condensing techniques run around 15% lower efficiency.

In the world of furnaces, it seems to be a no-brainer that higher efficiency furnaces are worth the extra price... unless you are a landlord and you pass the inefficiency costs off to the tenants.

So, how exactly did you came to the conclusion that hardware costs will not be offset by energy efficiency?

Are you imagining a very specific kind of CHP device?

-AC_Hacker