Micro cogeneration?

[Student 07]

Hi,
I have been trying to figure out if a co generator is cost effective from a heating standpoint. I have been trying to do some number crunching, but I am pretty much lost, so I hope somebody can help me out.

To figure it out I had to make some assumptions. To simplify things I consider the liquid to liquid heat exchanger to have two sides, the supply side and the storage side. The storage side would be an 80gl water tank, the water needs to go from 50* (Street water temp) to 120*, for a 70* temperature rise.

For the supply side, I think a larger motor would be the way to go. My thinking is that a large motor doesn’t have to work as hard as a small motor, meaning it will last longer. A large motor makes enough power at a low RPM, so the generator can be geared to still operate at full load even though the motor is operating at a low RPM. The larger motor should produce more heat: the more cylinders it has- the more water to hot cylinder surface area. The thermostat should keep the motor at a warm 195*, so the supply side of the heat exchanger (after the motor is warmed up) should be getting water at 195*.

So, here is what I came up with, please feel free to correct my math.
The following formula was suggested to me.

(Flow rate through heat exchanger)(Temp rise)(Specific heat of water)= heat out.

Or (4gl/m)(8.3lb/gl)(120*-50*)(1BTU/lb-F)(60 mn/hr)= heat output.

For me this works out to 4(GPM) x8.3x70x60 (min/hr.)= 139,440 BTU/H

Meaning that If I can keep the supply side at a steady 195*, it should be putting out 139,440 BTU/H

An 80 gallon tank has 667lbs of water, which I would need to raise 70*, so it would require 46,732 BTUs (1gl=8.345 lbsx70*[temperature rise] = requiring 584.15 BTU per gallon, 80 gl x584.15 BTU= 46,732 BTU) to heat that tank.

If I am putting out 139,440 BTU/H and to heat that tank I need 46,732 BTU, I should be able to heat that tank in roughly 20 minutes.

For a rough estimate of fuel usage: If I was to use a 4 cylinder automotive engine that normally gets 30mpg on the highway @2100RPM. If this car was driving at a speed of 60MPH for one hour it would travel 60 miles and use 2 gallons of fuel/hr. If that motor ran for 20 minutes (1/3hr) it would need 2/3rds of a gallon of gasoline to heat an 80 gallon tank. If a gallon of gasoline costs $3.50, it would cost $2.31 to heat the tank.

For the generator I would like the motor to turn around 1700RPM, so it may take even less fuel. My Portable generator need to turn at ~3500rpm to produce it's rated wattage. But that is where gearing comes in.

I need to do some more research as to which fuel source would be best. Propane would definitely be cleaner for the environment, but it contains less energy, so the fuel usage would be different. The larger motor would be able to turn an even larger generator, so the electricity would be an even larger bonus.

To me it is looking like the size of the co generator is important, the larger they are (to a point) the shorter they have to run and the more efficient they can be. That may be why AC’s friend saw the Ford V8 in the basement.

One thing I didn’t add in was the time for the motor to warm up. It may take 3-5minutes for the motor to reach 195*, but once the thermostat opens a snap switch could be used to turn on the circulation pump, or it could be wired to operate instead of the cars fan.

It is looking better now. I encourage everyone to check my math and provide feedback. Thanks
Jeff

[Student 07]

Looking back over my math I have to had made a mistake somewhere. Either I used the wrong formula or put in the wrong numbers.

Just thinking about it- a gallon of gasoline contains 114,000 BTUs so how could I come up with an output of 139,000 BTUs?

From just guessing I would think the motor would put out about 12,000 BTU/H and it would take about 4 hours to heat the 80 gallon tank.

If the motor uses 2 gallons of fuel per hour it would take 8 gallons and cost $28 ($3.50gl for gas) to heat the 80 gallon tank. As a bonus, by hooking it to a 5.5kWh generator it should produce 20.2kW in that 4 hour time, but for $28 it still is nowhere near being cost effective.

Even if I went back to the original scenario of using an 11hp motor (Equivalent of what is currently on my portable generator), which uses 1 gallon per hour of operation, it would look like this: 4 gallons for 4 hours costing $14. This cuts the operating cost in half; however, running this for 8 hours a day would be extremely expensive.

The only way that a cogenerator might be cost effective is if there is no grid electricity and you have a small house to heat. Even then there would be better alternatives. The cogenerator would have to run for long periods of time, otherwise the motor would be "short cycling" and never really warm up to where it is most efficient.

So now that I corrected my mistake, I am back to point A and the opinion that there is no benefit to using a co generator.

[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

[Ryland]

How much is it going to cost to build something like this?
You have a LP or natural gas generator costing $1,000 for a no name LP generator (short life span), then you need pumps, heat exchangers, pipe and a water storage tank, my thought on a good exhaust to water heat exchanger would be a gas water heater that uses a forced draft to cause a down draft, pulling the exhaust from the top down out the bottom, that way as the exhaust cools it passes by colder and colder water at the bottom of the water tank, the forced draft fan then pulls that exhaust out and blows it outside.
I would guess that a system could be build with new parts for $2,000 or so, used parts for half that but then questionable life span and chances of a Co leak increase, of course a system like this should always have a Co detector or two near by.
A synchronous motor/generator is also going to be best for the grid inter-tie part, a generator of that type will match the sign wave of the grid without any fancy inverter.
If you can run it off natural gas the electrical cost I think would come out to 8 cents per kwh before the savings of heat recovery, LP is about 8 times that cost around here, but if you are capturing 100% of the energy and 20% of that is being turned in to electricity then you get 12.8 cents per kwh from LP and 1.6 cents per kwh on natural gas here, where electricity is 12 cents per kwh, so I could save $155 a year, (1,500kwh a year, I think) that is a 6.5 year pay back if you can keep the system down to $1,000 and use really cheap natural gas to fuel it, with LP it would cost more to run then you can buy electricity for here.
From what I remember the cost per BTU of heating oil (will run a Diesel engine) cost of LP and the cost of electricity per BTU were all about the same, if you had a source for free veggie oil then you would be set.

[AC_Hacker]

Originally Posted by Ryland

LP is about 8 times that cost around here

Wow, pretty expensive.

I just did a calc on various fuels in Portland, Oregon (my fair city), using this handy calculator, assuming 100% efficiency (always an optimist!), for 100,000 BTU of heat:

• Natural Gas = $2
• Propane = $3.45
• Fuel Oil = $2.46
• Electricity, Resistance = $3.22
• Electricity, Heat Pump (COP = 2.5) = $1.29

Eight-to-one cost difference between LPG and Nat Gas?

That's really a lot.

-AC_Hacker

[Ryland]

Originally Posted by AC_Hacker

Eight-to-one cost difference between LPG and Nat Gas?

That figure was off the top of my head because I couldn't find the utility bill that showed up yesterday (co-home-owner cleaned for the holidays) but I'm pretty sure we pay around 70 to 73 cents per therm for natural gas and if you buy a 100 pound tank of LP it's $1.30 per pound, I might have screwed up my figures so please correct me if I'm wrong in converting a pound of LP to a therm of Natural gas, then 11.8 to 12 cents per KWH depending on time of year is pretty much dead on for electric rates tho.
My figures for how much electricity is produced by an LP generator came from Northern Tool on their 6000 watt LP generator, figuring that if you buy one that re jetting it for natural gas should be possible.

[nibs]

Interesting thread, in the 80's I considered Natural gas powered 10KW generator sets as an alternative power source to the grid, at 14 cent/KW grid cost it came close break even as I recall.
Now with a generator running on WMO or biodiesel, you might get much closer to break even. IMO it makes no sense to try and extract the waste heat from an air cooled motor when there are so many water cooled motors available. and using diesel motors makes more sense than using gas motors because of the longevity of the motors. Diesels can be made to run on a mix of gaseous/liquid fuels, so a natural gas / bio diesel set up is viable with up to 90% natural gas or propane.