The Ergosphere
Friday, August 19, 2005

A bite-sized cogeneration example

Continuing a thought started elsewhere, I thought I'd blog something about cogeneration and plug-in hybrids.  (Note that this is a quickie and not thoroughly cross-checked, there could be errors in the calculations - caveat reader.)

The plug-in hybrid concept is touted for its potential to eliminate petroleum, but there are valid questions about how quickly we could increase generation to supply them.  This led me to ask, what if we used petroleum to supply the energy?  Better yet, if it was used in cogenerators?

Assume for the moment that refined petroleum (sulfur and heavy metals reduced to acceptable levels) costs $12/GJ (crude currently runs about $11/GJ).  Company X switches its process heat system from natural gas at $10.00/MMBTU ($9.48/GJ) and 10% losses to a cogenerator system co-firing natural gas and fuel oil at 50% efficiency and 10% losses.  The company buys 1.25 GJ of oil to supplement each GJ of gas they formerly used, sells the electricity for $0.08/kWh ($22.22/GJ), loses 10% and uses 40%; their fuel costs go from $10.54/GJ delivered to the process to $24.48, offset by $27.78 in electricity sales.  The gross cost falls from $10.54 to $4.98/GJ of process heat; the cost of heat is cut in half.

The 2.25 GJ of fuel produces 1.125 GJ of electricity.  This travels over the grid, losing perhaps 10% en route to the customer; 1.01 GJ is delivered.  It is purchased by a GO-HEV owner whose vehicle uses 350 WH/mile (1.26 MJ/mile) at the charger.  The vehicle achieves 794 miles per GJ at the wall, or 357 miles per GJ of fuel delivered to the cogenerator; if the owner pays $0.12/kWh, the per-mile cost for electricity is 4.2 cents.

Last, assume that the electricity delivered to the GO-HEV replaces gasoline at the rate of 1/35 gallon per mile.

A barrel of crude oil is 42 gallons.  If we assume that the refined fuel oil has the same 6.1 GJ/bbl energy value as crude (probably not too far off), here's what the cogenerating system would accomplish with each barrel of fuel oil: Total fuel savings:  70 gallons of fuel (each barrel saves more than a barrel).
Gross cost savings and increased profits:  $223.71/bbl.

We have a number of policies regarding electric generation (existing regulations appears to discourage cogeneration), vehicles (air-quality policy encourages the small improvements produced by ethanol but not the much larger improvements possible with partial ZEV operation) all of which stand in the way of accomplishing this.  If we fail to examine these policies and make appropriate changes ASAP, we are fools.

(Posting this entry was an exercise in hair-tearing; see Stupid Blogspot tricks.)

UPDATE 2005-Aug-29:  I think I might have the figures correct this time.  As confirmation, here is a before-and-after table of energy expenditures and disposition for comparison purposes (not including losses in electric transmission or profits to the provider of electric transmission, which could account for the stray twenty bucks between this table and the list above and which I would have put in if I could think of a good way to represent it):

  System     Fuel     bbl (equiv.)     Fuel cost, $     Heat, GJ     VMT     Net fuel cost, $  
  Industry     Nat. gas   1.0  57.83  5.49    57.83 
  Vehicle     Gasoline   2.67  296.91    3921  296.91 
3.67  354.74  5.49  3921  354.74 
Cogen   Nat. gas   1.0  57.83      57.83 
Cogen   Fuel oil   1.25  91.50      91.50 
2.25  149.33  5.49  3921  149.33 
(If you made an egregious error in your comment, your solution is to click the delete icon and enter a corrected version.  Blogger provides no facility for a blog owner to edit comments; it's delete or leave as-is.)
I have a couple questions about the increased demand for electricity.

Eyeballing this, I gather there'll be crudely somewhere around a 50% (plus or minus a lot) increase in overall demand for electricity (assuming transportation and electricity each are roughly equal consumption of energy and half of the transportation consumption is for small automobiles).

Second as you mention, it appears with a little cleverness or market incentives, the overall demand curve for electricity will flatten out.

Due either of these have some implication for electricity production or transmission? Obviously, we'll need to build up the infrastructure, but will this encourage any sort of new technologies or practices in the electricity industry?
Electricity infrastructure is not what's limiting plug-in hybrid adoption, and lack of demand for electricity is certainly not limiting co-generation.

Europe's got plenty of incentives for co-generation and sky high gasoline taxes.

The real limiting factors are to be found in the nexus of investment cost / performance / convenience.

Co-generation is used where there is a large constant demand for heat, be it in oil refineries or in oil sands production or for ethanol production from sugar cane.

When the demand for heat is not constant or large, the investment cost for the generating unit beomes quite substantial, and/or the efficiency goes down, and/or "convenience" becomes an issue.

For example, the most efficient way to heat homes is to have large blocks of flat supplied with district heat from a co-generator, with the electrical output used to drive heat pumps.

That way each kWh of natural gas can provide 3 kWh of heat (even better, if cooking, drying, hot water and the like are met that way, a surprisingly large fraction of electricity consumption is for resistance heating).

Not only that, if it's tightly clustered flats, the requirement in kWh of heat per m2 will go down massively as well.

So that in sum total, the requirement for nat gas can easily be brought down by a factor ten when compared to natural gas heated detached family homes providing the same floor space, and that without insulating any more.

But many people want to live in detached homes with a large garden. Co-generation then becomes astronomically expensive per kWh of natural gas saved, not least because the small units available for family home sized demand have quite low efficiency for electricity generation (Whispergen being the worst I've come across).

We've discussed batteries to death, yes, I know, my main problem with goingreen is that I am not in London. But my real main problem is that I just cannot buy a suitable electric vehicle at a vaguely reasonable price.

If it was such a good deal, why on earth is nobody offering one?
OK, that's not an electric car, but a micro CHP plant.

I couldn't find a price, but I am aware of the options available in Germany:

5 kW electric, 12 kW thermal, electric efficiency 28%, 10% losses, remaining 62% thermal.

Cost: €18,000

They calculate:

Energy costs, 102,500 kWh of natural gas at 2.5 cents per kWh, ~€2,500 per year

Other costs ~€1,000 per year

Value of heat (62,500 kWh) at 3.7 cents per kWh ~€2,300

Value of electricity (most sold to the grid, as heat and electricity demand don't coincide) 27,500 kWh at 3 cents per kWh ~€800

Subsidies ~€1,900

Total surplus, ex capital cost, including subsidies: ~€1,300

Payback time at 6% interest: 30 years.

62,500 kWh of heat is a lot. The average living space per capita in Germany is 40 square metres, in the US it's about 70. For a 3 person household in the US that's about 200 square metres. New housing in Germany requires about 100 kWh/m2 of heat, assuming that applied to the US as well, we'd be talking 20,000 kWh for an average three family home, so the Dachs would be heavily oversized.

Are there smaller units?

Well, yes, eg Whispergen, with its grand electric efficiency of 10% ...

To cut a long story short, co-generation can be a great money maker, but there's a wide range of profitability depending principally on how big and steady the heat demand is, and those opportunities for co-gen not in use tend to, as you'd expect, to heavily cluster towards the uneconomic end of that spectrum.
Getting back to substantive responses:

Karl:  Yes, total energy demand would rise substantially.  (If most of the energy was sent through high-energy but lossy batteries like zinc-air, it might rise by almost 80%.)

The effect on generating capacity depends a lot on what kind of batteries are used and how much capacity is available.  Vehicles such as GO-HEVs which use small packs will need them to be fully charged every night, so the demand level for each night will follow the previous day's driving.  This requires dispatchable capacity, though DSM can allow changes in power level to be done over hours rather than minutes.  This plays well to base-load powerplants (coal or nuclear).

If the batteries are something like externally-fuelled zinc-air cells, the energy supply stored outside the vehicle can be much larger than what's on board.  This would allow demand to be tracked (or scheduled) days in advance, and resources like wind and solar could supply a lot of the energy.  Shortfalls would appear over days and could be made up over days; you could wait for the next front to come by and let the wind farms top up your energy storage.

Even short-range GO-HEV systems which have vehicles connected to the grid for much of the day could allow DSM to eliminate a great deal of simple-cycle gas turbine capacity (fast load-following) in favor of combined-cycle gas turbines.

The changes in the system appear to be flattening of the daily load curve, a shift from peaking plants to more efficient and cheaper base-load plants, and in some scenarios (lotsa storage) you get the ability to make full use of very large amounts of non-schedulable generation.  Transmission requirements from conventional plants would stay flat, transmission requirements from wind farms would increase.
Heiko writes:

"Co-generation is used where there is a large constant demand for heat..."

It appears that your addition of "large" and "constant" to the requirements is just that, yours.  The Climate Energy unit is designed for a relatively small (13,800 BTU/hr) heat demand (with supplemental heat available as required) and cycles as appropriate.

"When the demand for heat is not constant or large, the investment cost for the generating unit beomes quite substantial"

USD 8000 total, USD 4000 premium over a conventional unit.  I personally think this is awfully steep considering that it's more than the cost of the typical automobile drivetrain, but that may reflect low volume and stiff emissions limits on grid-connected stationary generators.

"That way each kWh of natural gas can provide 3 kWh of heat"

You're not going to get that kind of performance.  Even with a solid-oxide fuel cell at 60% efficiency, you're only going to get 40% waste heat + 3x60% heat-pump output = 2.2 CoP.  With an internal combustion engine at 30% efficiency, you'll peak out at maybe 1.7-1.8.

What the cogenerator/heat pump system can do is absorb a huge amount of intermittent electric supply, such as wind, solar or wave power.  The cogenerators can be run just enough to make up the difference between the electric supply and the heat demand.  Even 30% capacity factor on the other generation displaces a lot of fuel.

"Not only that, if it's tightly clustered flats, the requirement in kWh of heat per m2 will go down massively as well."

I'll let you do the redevelopment of areas of single-family homes to apartment and condominium blocks.  I hope you have a lot of money to spend for zoning changes and legal disputes, plus the loss on the property values...

"the requirement for nat gas can easily be brought down by a factor ten"

Gas requirements could be rendered negligible if you made people live in little concrete-skinned, spray-foam domes.  Unfortunately for your dream world, this would never happen easily; people will not abandon their space and creature comforts unless they are forced by necessity or at gunpoint.

You're a Brit, maybe you'd leave your home at the command of an energy czar.  Americans would shoot the people who tried to force them, and would come up with alternatives if conventional energy supplies ran short.

"But many people want to live in detached homes with a large garden. Co-generation then becomes astronomically expensive per kWh of natural gas saved..."

The Climate Energy unit costs about $4000 more than a conventional heating plant and generates 1 kW when in operation.  A $4000 premium amortized at 7% over 15 years is roughly $440/year.  If it has an annual duty cycle of 45% (including water heating), it would generate 3942 kWh (worth about $670 at Californian rates of 17¢/kWh, perhaps more in a place like New York City) while the added fuel cost at $1.00/therm is about $135.  Looks like the owner could clear $100/year before taxes, and given the US deductibility of mortgage interest it would be about $178 after taxes.  If much of the electricity was used in an EV or GO-HEV its value (avoided cost of gasoline) would be upwards of 20¢/kWh.

The economics in your Energienetz example are determined by the sale price of the electricity, which is terribly low.  Isn't the German government mandating much larger payments for wind and solar power?  Regardless, if that electricity was used to power vehicles the avoided cost of fuel would be much greater than in the US.  I'll let you work the example because I don't know the fuel costs.
Large and constant (and low temperature) aren't absolute requirements, but they rather improve the economics.

I agree that the factor 3 for the cogen/heat pump combination is optimistic.

3 cents per kWh is wholesale. The government mandates an awful lot more than that and there's some other subsidies.

The economics could brighten with a smaller unit like the Honda at the price you cite, because it's so much easier to use a large fraction of that electricity rather than export it to the grid.

Energienetz states that the Honda unit isn't available in Germany yet and being tested.

If you look at the specifications, you'll notice something else, the combined efficiency is 85% for cogen, but efficiency is 95% for heat only operation.

11/17 is 65%, so I presume that the electrical efficiency is 20%.

That depresses the nat gas savings terribly.

Take 100 kWh of nat gas and put it through the co-gen unit, you get 20 kWh of electricity and 65 kWh of heat.

Produce the heat instead via the high efficiency condensing furnace (that comes with the unit anyway for peak loads) and 65 kWh of heat require 68.4 kWh of nat gas.

Generate the 20 kWh of electricity in a combined cycle plant at 60% efficiency and you need 33.3 kWh of nat gas.

Add the two together and you get 101.75 kWh instead of 100 kWh, ie using co-gen instead of separate generation results in a saving of under 2% with the Honda, which is pretty poor.
Post a Comment

<< Home
Talk largely about energy and work, but also politics and other random thoughts

Mail Engineer-Poet

(If you're mailing a question, is it already in the FAQ?)

Important links

The Reference Library

Blogchild of

Armed and Dangerous

Blogparent of


The best prospect for our energy future:
Flibe Energy

January 1990 / February 2004 / March 2004 / June 2004 / July 2004 / August 2004 / September 2004 / October 2004 / November 2004 / December 2004 / January 2005 / February 2005 / March 2005 / April 2005 / May 2005 / June 2005 / July 2005 / August 2005 / September 2005 / October 2005 / November 2005 / December 2005 / January 2006 / February 2006 / March 2006 / April 2006 / May 2006 / June 2006 / July 2006 / August 2006 / September 2006 / October 2006 / November 2006 / December 2006 / January 2007 / February 2007 / March 2007 / April 2007 / December 2007 / January 2008 / May 2008 / June 2008 / August 2008 / October 2008 / November 2008 / December 2008 / February 2009 / March 2009 / April 2009 / May 2009 / June 2009 / July 2009 / August 2009 / September 2009 / October 2009 / November 2009 / December 2009 / January 2010 / April 2010 / May 2010 / June 2010 / July 2010 / August 2010 / September 2010 / October 2010 / November 2010 / December 2010 / January 2011 / February 2011 / March 2011 / April 2011 / May 2011 / July 2011 / August 2011 / September 2011 / October 2011 / April 2013 / November 2013 / December 2013 / January 2014 / February 2014 / March 2014 / April 2014 / July 2014 / August 2014 / September 2014 / October 2014 / November 2014 / February 2015 / April 2015 / October 2015 / March 2016 / April 2016 / May 2016 / June 2016 / July 2016 / November 2016 / December 2016 / February 2017 / May 2017 / June 2017 / September 2017 / October 2017 / November 2017 / March 2018 / May 2018 / June 2018 / October 2018 / December 2018 / January 2019 / March 2019 / June 2019 / October 2019 / November 2019 / March 2020 / June 2020 / December 2020 / March 2021 / April 2021 / May 2021 / July 2021 / January 2022 / February 2022 /

Powered by Blogger

RSS feed

Visits since 2006/05/11: