To get the right answer, you've got to ask the right question.
I think that people have been asking the wrong questions about wind
power. As Randall Parker notes
wind is not a power source which can be turned up or down according to
the desires of the users; you either grab and make use of it when it's
blowing, or you do without.
People have been asking how much other grid generation could be
replaced by wind. The answer is "as things are now, not much of
the total", but I think this is the wrong question. A better
question is, "What uses can wind power serve, and what else might we
need to make it serve them?"
Uses of wind-electric power
Offhand, I can think of three major categories of uses for
intermittent, non-schedulable electric power sources like wind (which
are not mutually exclusive):
- Produce power to allow other generation to be turned down, to
reduce fuel consumption and save wear and tear.
- Produce energy for storage.
- Produce energy for uses which currently do not use much
electricity, but can take advantage of the underutilized supply and idle
Storing electricity is one of the most expensive things which can be
done with it. One strategy for storage is to convert the
electricity into the desired product and store that instead.
Consumers and electric utilities have used this system to good effect
for years: some consumers who use electric water heaters have them
on timers which switch them off during peak periods, and some larger
users of air conditioning purchase off-peak electricity at night to make
ice which is then used for cooling when electric costs are higher.
The applicability of these possibilities to wind power is
questionable. Across the cold north, winds blow strongest in the
winter rather than during the air-conditioning season and a great many
users heat water with gas rather than electricity. After motor
fuel, the greatest consumer of fuel in the average northern household is
space heat. What could wind power do to help with that?
This, of course, means Back Of The Envelope time....
What can you do with it?
Assuming the following:
- The heating season is 180 days long;
- The average household consumes 50 million BTU of natural gas per
year for space heat, of which 80% (40 million BTU) is captured and 20%
- Another 7.5 million BTU is consumed during the heating season for
domestic hot water, also at 80% efficiency;
- The available wind power per household is either 3 kW or 5 kW,
and the capacity factor of the wind turbines is 25%, 30% or 35%;
- The efficiency of transmission and distribution is 90%, and there
are no grid-capacity limitations (cold weather and winter gales holding
little prospect for overheating of lines or transformers); and
- The excess wind energy is used in resistance heaters to displace
natural gas, with 100% efficiency for space heat and 80% for water.
At the low end, the total wind energy delivered per heating season
is 2916 kWh (9.96 million BTU); at the high end, it is 6804 kWh (23.2
million BTU). Here's a grid of the per-household heat production
for various figures:
| Rated power
| 3 kW
|| 9.96 mmBTU/yr
|| 11.9 mmBTU/yr
|| 13.9 mmBTU/yr
| 5 kW
|| 16.6 mmBTU/yr
|| 19.9 mmBTU/yr
|| 23.2 mmBTU/yr
Here's a table of the fractional heating fuel displacement possible
using wind electricity in simple (cheap) resistance heaters:
| Rated power
| 3 kW
|| 21% savings
|| 25% savings
|| 29% savings
| 5 kW
|| 35% savings
|| 42% savings
|| 49% savings
These figures are encouraging. Even a 21% reduction is quite a
bit compared to current gas needs; slashing requirements by 49% would be
phenomenal, and eliminate the prospect of gas shortages for some years
Someone's bound to ask if 5 kW of wind power is too much of a good
thing, supplying more energy during gales than could be used.
Well, maybe... but after you subtract 1 kW/household average for other
electricity, and recognize that the periods of highest wind are also the
periods of greatest heat loss through drafts, it doesn't look as if
overheating is a serious threat during the winter. The bulk of
that 49% savings would likely be realizable either in saved fuel at
generating plants or saved gas at homes and businesses.
Would we actually dump that much electricity as heat? Probably
not; it would make more sense to turn down other powerplants and use the
wind-generated electricity to run lights and motors (or charge cars)
before running it to resistance heat. But not all powerplants can
follow rapidly varying loads or compensate for fast ramps in other
capacity, like wind farms; this would require having enough generation
on-line to carry the system through the short-term lulls. If any
overage can be used to make space heat or hot water that we'd be using
anyway and avoid the need to burn fuel for that purpose, every bit of
wind power can be used productively even if it cannot be scheduled or
accurately predicted; the only abilities we need are to transmit it and
make the load follow the gusting wind.
The control systems required to perform such load management would
be useful for other purposes as well:
- Extremely precise shedding of space heat loads in winter, and DHW
heating loads in any season;
- Ability to maintain the entire electric-heating load as "spinning
reserve" (available for redirection as fast as control systems permit)
when excess generation requires it as a power dump, with positive
consequences for grid reliability.
What happens if you combine this wind-power system with widespread
home cogeneration systems and plug-in hybrids? I'm going to re-do
the scenario from cogeneration@home
using the 3 kW/25% and 5 kW/30% wind power figures from the above list,
and with DHW heat requirements added.
If the house requires the same 4320 kWh for its own consumption and
the car consumes 2520 kWh, total electric requirements are 6840 kWh for
the season or 38 kWh/day. The 3 kW wind system at 100% capacity
supplies 2.7 kW (64.8 kWh/day) or 26.8 kWh in excess of electric needs.
Further assuming that:
- the wind supply is either at 100% or 0 (pessimal supply curve),
- the entire 26.8 kWh (41%, 91,500 BTU/day) is surplus and must go
- all the heat can be used,
- the DHW heat is supplied by a burner rather than the cogenerator,
- DHW heat losses are unchanged, and
- the new energy displaces fuel oil:
(3 kW, 25%
Fuel oil consumption in this case is reduced to less than 40% of the
original, and total petroleum consumption is cut by almost 80%.
What would happen if you could get 4.5 kW/household at 30% capacity
factor? On the days with wind the excess electricity creates
280,000 BTU/day of heat, or about 6% more than the average combined
space heat and DHW demand. It's likely that windy days are also
days of high heat demand, so I will assume that all of this heat can be
used and counted against total annual heating requirements.
(5 kW, 30%
In this case the remaining electric demand cannot be quite satisfied by
the cogenerator without discarding heat, so a small amount of
electricity is generated from coal again. Annual cost is down
slightly, carbon emissions are down more than 75% despite the
renewed reliance on coal, and petroleum consumption hits zero.
Gas consumption drops 12%. If the DHW supply was heated by
the cogenerator, coal use would be eliminated at the cost of somewhat
greater gas consumption.
Despite being unreliable and unschedulable, it appears that wind
could be used to offset fossil fuel consumption quite easily. In
the context of current systems it can be used to reduce fuel demand at
gas-turbine plants until they shut down; beyond this point it could be
used for space heat and domestic hot water, offsetting gas consumption
there as well. In a near-future system using cogeneration for all
space heat needs and grid-charged hybrid vehicles for transport, the
availability of wind could:
- eliminate petroleum consumption during the heating season
- reduce gas demand by 12%
- reduce total carbon emissions by more than 75%.
Is it worth using? Looks like it to me.
UPDATE 5/23/05: Corrected typo in third table. Had to catch that one myself. So much for the eyeballs of the web as fact-checkers. ;-)
2005-03-29: See below
(note: this topic was handled briefly in the
"Where to go from here?"
There is one universal truth about heat engines: none can
convert 100% of its input energy into work (in the physics sense).
Some fraction of the input energy is always going to be converted to
low-temperature heat which must be exhausted to the "heat sink", which
is typically the environment (see
). This exhaust heat is often called "waste heat".
One feature of waste heat is that it isn't always necessary to waste
it. If there is a need for heat at or below the exhaust
temperature of the engine, the engine's heat exhaust may be usable as a
heat source instead of burning fuel directly. This system of
generating power is called cogeneration.
Cogeneration effectively uses fuel energy two or more times:
the first time as high-grade (high-temperature, low entropy) heat as
input to the engine, and the waste heat is used a second time as
lower-grade (lower temperature, higher entropy) heat for other
purposes. These other purposes include industrial process heat,
distillation of water or alcohol, input to absorption chillers, or space
If both heat and electricity are required at the same time,
cogeneration can achieve much higher overall efficiences than the
separate generation of heat and electricity. For instance, a
cogenerator which takes 100 kWh of fuel to produce 30 kWh of electricity
and 70 kWh of heat can eliminate 70 kWh of fuel that would otherwise be
burned in a furnace to provide heat.
Very high effective efficiency
If the efficiency of cogeneration is rated in terms of additional
fuel burned versus additional energy produced, it can be extremely
high. A modern home furnace may burn natural gas and capture 95%
of the energy as space heat. A gas-fired cogenerator with an
electric efficiency of 30% and an overall efficiency of 90% will lose
10% of the total energy of the gas, but for each unit of electricity it
uses only 1.33 additional units of gas; its effective efficiency is a
whopping 75%. If the heat losses can be reduced to the same 5% as
the furnace, this figure increases to 86%! This contrasts very
favorably with centralized powerplants which may burn gas with perhaps
50% efficiency, or coal at efficiencies as low as 33%. If fuel is
being burned to produce low-temperature heat, cogeneration is a good way
of getting electricity with very little extra fuel and fuel cost.
Domestic cogeneration (also called combined
heat and power
) is one form of distributed electric
generation. Like standard furnaces and water heaters, domestic
cogenerators burn fuel and produce low-grade heat; unlike furnaces and
water heaters, they turn part of their fuel into electricity.
Some attractive technologies have impediments to their
adoption. Gas turbines are inefficient in very small sizes and
fuel cells remain too expensive for common use. A building which
needs ~390,000 BTU/hr of heat and can use or sell 60 kW of electricity
can use a Capstone ICHP system, but this is about ten times too big for
most homes. For this and other reasons small cogenerators use
internal combustion engines, either Otto (spark ignition) or Diesel
Broad applicability, no availability
Just about every home which needs winter heat and is not
all-electric is a possible site for cogeneration. Heating fuel is
delivered to most homes either by pipeline or by truck.
Home-heating oil is almost identical to number 2 diesel fuel, and
engines can be run quite well on either propane or natural gas.
Homes appear to be natural sites for cogeneration.
Yet for all the availability of fuel, small cogenerators are very
scarce. Perhaps this is due to a lack of true markets in
electricity, where homeowners with an excess could sell as easily as
they buy. Regardless of the reason, devices which look like they
ought to be common household appliances are not for sale.
In the absence of a market in small cogenerators, it is difficult to
determine what they would cost in volume production. Superficially
similar systems have per-kW prices all over the map. Examples:
- A gasoline-powered Generac portable generator rated at 7550 W
continuous retails for $1150 at Home Depot, or $152/kW (2005 dollars).
German FCEV study compared the cost of fuel cells vs. conventional
powertrains. The study estimated a cost of $2450 ($39/kW) for
gasoline up to $6170 ($89/kW) for a diesel parallel hybrid, with CNG and
conventional diesel in between. This study used 1997 dollars; 2005
figures would be about 20% higher.
California study found that a gas-fired, 500 kW reciprocating
cogenerator would cost $1075/kW installed. The page is dated 2002;
the figure would be about 8% higher in 2005 dollars.
These systems are not directly comparable. Here is a table of
costs and properties:
|| Emission controls
||No cooling jacket or exhaust heat recovery.
Engine speed too high for long life.
||Presumably includes radiator and exhaust.
||Presumably includes radiator and exhaust.
||Presumably includes radiator and exhaust.
||Presumably includes full heat recovery.
The comparisons are interesting. The 550 kW cogenerator's
price is off the chart; clearly it is a low-volume product with few
economies of scale, and is not comparable to the other examples.
The one-lung engine in the Generac and vehicle powertrains are
high-volume products. Curiously, the cost of the vehicle
powertrains all run roughly 3-5 times the retail price of the Generac,
despite their outputs being nearly ten times as great; their cost
appears to scale with cylinders rather than power. The complexity
of a 4-cylinder engine is roughly 4 times as much as a 1-cylinder
engine. Complexity and expensive components would appear to
account for the extra cost of CNG over gasoline (costly tanks,
high-pressure plumbing), and diesel over CNG (high-precision injectors
and injector pumps). It seems fairly safe to posit that cost is
roughly proportional to cylinder (parts) count.
What do we need?
What are the properties required of a domestic cogenerator?
- Long life: Should run 10 years with only normal maintenance.
- Serviceable: Should be easily refurbished or replaced as
maintenance or obsolescence requires.
- Quiet: Should be suitable for installation in a basement
utility room or crawlspace.
- Low pollution: Should restrict pollution to marginal
increments over conventional heating plants.
- Standard form factor: Should fit in the footprint of a
standard heating plant and be otherwise suitable for retrofit
There are some difficulties unique to cogenerators which must be
- Pollution regulations: Federal law imposes more stringent
pollution limits on generators connected to the grid than those which
are not. CNG-fired piston generators require catalytic NOx
reduction with ammonia injection to meet these requirements.
Anhydrous ammonia is clearly not an acceptable material for use in
domestic heating plants, for reasons of both safety and law
enforcement. NOx controls are primarily required to limit
production of photochemical smog on hot days. It may be reasonable
to give waivers to heating plants which only operate in chill or cold
- Grid regulation: The utility must be able to limit
backfeeding so that grid protection measures remain effective. If
the available technology for demand-side management is any indication,
this should not present large difficulties.
What's the 2010-model, million-selling home cogenerator likely to look
- It'll come in one or more boxes, totalling about the size of a
- It will have a one- or two-cylinder, long-stroke engine operating
at a speed well under 2000 RPM, perhaps as low as 1000 RPM. This
engine will operate on something like the Atkinson cycle, for improved
efficiency and lower exhaust noise.
- Maintenance will ordinarily be limited to annual changes of the
engine air filter and an oil reservoir/filter cartridge.
- The engine module will have a heavy, sound-absorbing
housing. This may be a light shell filled with sand or water
on-site. There may also be active noise cancellation for the
intake and exhaust.
- Gas- and propane-fired generators will use catalytic converters
for emissions control. Diesel generators (retrofitted for oil
furnaces or where propane delivery is not available) will use catalysts
to reduce gaseous emissions and filters to trap particulates.
- The treated exhaust will feed into a condensing heat
exchanger. The cooled exhaust will be cool enough to exit through
PVC pipes, like today's condensing furnaces and high-efficiency water
- Engine thermal efficiency will be on the order of 35%.
Alternator and electronic losses will reduce this somewhat. Good
design will recover this energy as space heat, e.g. by using the house
air to cool the electronic heat sinks.
- Units will probably have some stand-alone capability, so that
they can provide heat and essential electricity during winter grid
outages. This would require a small starting battery, some
additional sophistication in the starting and power conditioning
electronics, and a resistor bank (dump load) capable of handling excess
This cogenerator will have perhaps two cylinders. Its cooling
system will be roughly comparable to a vehicle's radiator. The
alternator will have fewer parts, less bulk and lower operating loads
than an automotive transmission. It will not need a separate
starter. It will have electronic controls on the order of a
vehicle. The exhaust system will be more sophisticated, including
not merely muffling, catalytic aftertreatment and possible filtration
but heat exchange and handling of condensate. The fuel supply is
either piped in or assumed to be in place, so there are no additional
expenses there. Its cooling fan will be the house air circulation
It appears that the entire cogenerator system will have roughly the
same complexity as half a 4-cylinder car engine, and ought to be able to
sell for roughly half the price: $1500 for natural gas or propane,
$2500 for oil-fired diesel. This purchases a system capable of
producing 5 to 10 kW continuous, plus 30,000 to 60,000 BTU/hr of heat
(roughly 6000 BTU/hr/kW).
What does it get the owner?
Apparently, one hell of a lot. The average gas-heated home in the
USA uses 50 million BTU of gas per heating season; at current retail
prices of $.60/therm, it costs about $300 a year. If an owner
replaces an older, 80%-efficient furnace with a cogenerator with 60%
thermal efficiency and 35% electric efficiency, gas consumption rises by
1/3 to 66.7 million BTU and annual fuel cost rises by about $100.
However, the cogenerator also produces 23.4 million BTU (6840
kilowatt-hours) of electricity; at a retail price of 8 cents/kWh, this
electricity is worth about $550.
The typical household uses about 1 kilowatt average; over a 180-day
heating season, one would expect the house to use only about 4300 kWh of
electricity. The cogenerator would produce considerably more
electricity than the household could consume, and it seems unlikely that
the utility's other customers (who might have their own cogenerators)
could be expected to consume it. What else could the cogenerator's
output help to displace?
Why not gasoline? If the household has an electric or plug-in
hybrid vehicle, the cogenerator could feed it during the heating
season. If the electricity was fed to a vehicle using 250 Wh/mile,
the excess 2520 kWh could drive the car for 10,080 miles. If the
car would otherwise have gotten 35 miles per gallon of gasoline, that is
a savings of 288 gallons worth nearly $590 at today's pump price.
The final question: Where do you get the increased 1/3 in
natural gas consumption? Everything comes at a price, and in areas
where electricity is generated by burning coal the likelihood is that
some of it would come from petroleum, as fuel oil instead of
gasoline. If about 1/4 of the cogenerators switched to fuel oil,
the average household would burn 16.7 million BTU of oil and 50 million
BTU of gas. At 19,110 BTU/lb this would be 847 pounds of fuel, or
114 gallons. This displacement would not occur in areas where
electricity is generated with natural gas.
One unresolved issue is how much fuel is displaced at the electric
powerplant. If the house draws power from a coal-fired powerplant
with a heat-rate of 10200 BTU/kWh, the saved 4320 kWh would displace 44
million BTU of coal, or about 2 tons. The savings in CO2
emissions would be 7.3 tons. A gas-fired powerplant with a heat
rate of 6800 BTU/kWh would reduce its emissions by a mere 1.69 tons.
Last, I'm going to assume that heating oil costs, and is taxed,
the same as motor fuel so that there is no tax subsidy of electric
vehicles via oil-burning cogenerators. Last, for simplicity's sake,
I'm going to assume that the chemical formula of both gasoline and fuel
oil is Cn
- The cogenerator can reduce electricity expenditures on the order
of $350 per year.
- In combination with an electric or plug-in hybrid automobile,
motor fuel costs could be cut by as much as $576/year, offset by $228 in
- It appears that winter carbon emissions of the average household
could be reduced by more than 2/3.
- In areas where the marginal electric plant is a gas-fired
combined cycle plant with a heat rate of 6800 BTU/kWh, cost is cut by
over $800/year and the net carbon emission is cut by almost half.
- Raw rate of return on the cogenerator, assuming the vehicle is already
owned, is 33% to 50% per year.
This analysis does not include other improvements which could easily be levered
in. For instance, any building with central air already has most of the
hardware required to heat with a heat pump; using a heat pump, external
electricity can displace fossil fuel. Wind power is widely
available in the winter across much of the nation and could displace fuel for
lighting, heating and transportation at the same time. The potential
savings in gas consumption is immense, and cogenerators could easily
compensate for variations in wind intensity on a minute-by-minute basis.
Other issues and savings
The widespread use of cogeneration would improve many other things as well.
- Load on the electric transmission and distribution system would be slashed
during the winter, along with its losses.
- Vulnerability of the electric distribution system to disruption by e.g.
ice storms would be greatly reduced.
- Manageability of the electric grid would be greatly improved, as utilities could
request electricity to be converted to heat in dump loads or fed to the grid with a response time of seconds.
- Households would have a supply of electricity during winter grid outages as
long as heating fuel held out. In combination with a plug-in hybrid car,
the house would effectively have a complete uninterruptible power supply.
- In areas where electricity is gas-fired, net gas consumption would be reduced
and natural gas prices would fall. This would reduce total costs in a way
not captured by the above accounting.
Some people have been asking if we can afford to adopt measures like cogeneration.
Unless I'm terribly wrong, it looks like we can't afford not to.
(As always, corrections cheerfully accepted. I'll credit them to the originator.)
: Tom DC/VA thinks that
my estimate for cogenerator cost is low, by a factor of
. I can't see
how a mass-produced piston cogenerator in the 5-10 kW range could cost more than a 60 kW
automotive powerplant; the engine itself would have about half the parts count, and the
cooling system is roughly the same complexity but can be made more rugged (cheaper materials,
lower warranty costs) because it does not have vehicular weight constraints. The only
things that might do that are emissions regulations much more stringent than vehicles (already
noted), or difficulty in muffling.
Randall Parker writes
wind's lack of schedulability is a problem. I agree, but I don't think that this is a
reason to write it off; its value should be measured by what it can save. More on this
Over at The Panda's Thumb
, a Fred Reed piece
. In the comments it comes into hypergolic contact with a cite of The Crackpot Index
. Hilarity ensues
Over at Muck and Mystery
, back40 has harsh words
for a post over at WorldChanging
. He rips author Alex Steffen a new one:
The math spoken of is miser-math, or as the poet said "here's one for you, nineteen for me", since there is no intention at all of an equitable sharing. The overwhelming majority of the loot will be reserved for official use. Only the crumbs left over are subject to equitable sharing.
That's a mighty bleak view. He goes on:
No, we owe our kids increased well being rather than table scraps, leavings that we did not consume. Our task is to improve the world so that they can have a better start and reach a higher percentage of their potential than we did.
But Steffen disagrees rather vehemently with such outcomes:
Planet's shrinking, clock's ticking: what to do?
There are four usual answers.
I'm ashamed to say, some people still think the first and worst is an option, that we ought to "let nature take care of the problem." ....
The die-off plan isn't discussed much in liberal polite company. That it's ever discussed at all -- at the tail end of a century that saw the Nazis, the killing fields of Cambodia and ethnic genocides from Armenia to Rwanda to Bosnia -- is disgusting. It rings like jackboots on cobblestones to imply that a large number of one's fellow beings shouldn't be here, or may not be able to survive.
The idea of drastic forced reductions in population is horrible. People are rightly appalled to hear about forced sterilizations, or the abuses of China's one-child policy. But the idea of die-off, of reducing the world's population through the simple expedient of letting millions and millions of people starve and murder each other when we might have saved them: that's both idiotic and evil.
It's idiotic because dying and desperate people are no respecters of nature, the future or their legacies – they're desperate, and they will use any and all resources and tools at their disposal to survive....
And it's evil, simply and totally wrong, to do next to nothing to avoid the death, destruction and pain already unfolding around us in magnitudes that dwarf the Holocaust.
Steffen goes on:
And are we really willing to cut back enough to live lives that are truly sustainable? I certainly don't see any evidence that we are.... And if we aren’t willing to live within those limits, why do we think everyone else will be?
This is a story without clear good guys and bad guys. The First and Third worlds now live around the corner from each other, mutually dependent everywhere.
He's got a warning, and a prescription:
Unfortunately, the model we used to get rich is no longer replicable. For everyone on Earth to follow the Western model of development, we'd need somewhere between five and ten additional planets'-worth of resources and waste sinks, which is somewhere between five and ten more than we've got. As 2002 Jo'berg Memo puts it, "[T]here is no escape from the conclusion that the world's growing population cannot attain a Western standard of living by following conventional paths to development. The resources required are too vast, too expensive, and too damaging to local and global ecosystems."
What we need, then, is a new model. We need a new model which allows unprecedented prosperity on a sustainable basis. We need a new model which will let everyone on the planet get rich and stay rich, while healing the planet's ecosystems. We need to create what some Brits call "one-planet livelihoods" which are so prosperous, so dynamic, so enticing that the alternative of chasing the old model of green follows gold seems simply moronic.
Designing a system which would lead to that kind of sustainable prosperity would already present an epic challenge. But we're not done yet. For that system also needs to work in the real world. It must be rugged and shock-proof.
What did back40 have to say about the same subject?
Looting the earth is not our best option since we are quite capable of producing our own loot.
Funny, I thought that's what Steffen was saying, with the caveat that it is EXTREMELY
important that we start doing it RIGHT NOW
Even Randall Parker seems to agree
. He quotes nobelist Richard Smalley approvingly, and winds up with an opinion of his own:
"It would be very nice to have an alternative to fossil fuels—an alternative to nuclear fission—that would be capable of providing energy for what will probably be 10-15 billion people in the middle of this next century. I believe that if this alternative exists, it has to be solar. Right now we do not even have a solar technology that is even laughably close to being able to handle—for example—80% of all the world's energy production. If you don't do 80%, you're not touching the problem. And if you don't provide energy technology that is economically cheaper than the alternatives, it won't be adopted at all.
We need to start working seriously on alternatives to fossil fuels.
"Where is that solar technology going to come from? [It will come] not just from improving solar cells, but from something totally new...."
Just maybe, if we would take the time to read the whole thing, we'd find out that we're all reading from the same book. A little understanding instead of flaming is all it may take to get to the same page.
One of the things I like to do every so often is look at various
commercial offerings and announcements and see what they imply for
certain trends. Something I haven't looked at in a while (and not
blogged about here) is advances in battery technology and the
implications for the best immediate prospect for slashing oil
consumption: plug-in hybrids.
Batteries have been the weak link of electric vehicles for well over
a century, so any development is of great interest. One bit of
recent news was very exciting:
announced a new anode material for lithium-ion
(Li-ion) batteries which triples their current capacity and drastically
shortens the necessary charging time. The implications for EV and
HEV use is obvious: more power and better regenerative braking
from the same battery pack.
According to certain authorities, the average commuter travels 22
miles a day or less; this means that a car which can travel 22 miles on
electric power alone can eliminate these people's fuel requirements for
work travel, and cut total fuel needs by as much as 80%. Longer
all-electric range translates to less fuel use.
According to EPRI
, a compact electric vehicle would
require about 250 watt-hours per mile of range (it is not clear if this
is measured at the charger input, the charger output, or between the
batteries and the motor). Others differ; AC Propulsion
claims 205 Wh/mile
for their tzero (presumably as output from the batteries), while Commuter Cars
says a Tango would
need about 180 Wh/mile. For a slightly larger vehicle, the EPRI
figure 250 Wh/mile seems to be reasonable for a BOTE analysis.
Range is the other figure. 30 miles is well over the average
commute, and would certainly capture the 80% reduction in fuel
requirements projected by analyses which find 22 miles is
sufficient. To obtain 30 miles range at 250 Wh/mile and 80%
discharge, a battery would require a capacity of 30 * 0.250 / .8 =
9.375 KWh; call it 10 KWh even, for simplicity's sake.
The last element is power. To meet consumer demands, a car
will probably need at least 100 horsepower, perhaps 150
horsepower. This means that the battery must be able to supply 75
to 112 kilowatts of power for acceleration.
For batteries, I like to look at batteryspace.com
best Li-ion offering at the moment is a pack of 50 cells in the 18650
configuration (18 mm diameter by 65 mm long), which store 2000
milliamp-hours (2 amp-hours) at 3.6 volts nominal; for this they're
asking roughly $5.00/cell. The 50-pack is specified at 81 ounces,
or roughly 45.6 grams/cell. The specifications say that they are
limited to a 2.5 C (5 amp) discharge rate. Suppose that Altair's
electrode technology can triple this to 7.5 C; at that rate, a 10 kWh
battery would be able to supply 75 kWpeak
, nearly as much as
a typical NiMH battery.
For NiMH, the cost leader is a 10-pack of C cells, 4500 mAH at 1.2
volts nominal for $3.30/cell. Assuming a 10 C discharge rate, a
10 kWh pack would be able to supply 100 kWpeak
I chose to assume two different configurations: a commuter car
with 75 kW (100 HP) of power, and a sport model with 112 kW (150 HP) of
power, with 30 miles minimum all-electric range at 100% discharge. My
calculations came out like this:
| Electric range, mi
|| Commuter, 75 kW
| Sport, 112 kW
|| Commuter, 75 kW
| Sport, 112 kW
- Li-ion batteries are getting very close to NiMH in cost per unit
energy. (This is new in my experience.)
- The commuter configuration with the NiMH battery sits right
at the "sweet spot"; it has neither excess power nor excess capacity
for the range requirement. This is partly due to the cells
chosen; some NiMH cells have much higher discharge rates (up to 20 C)
and could provide very high performance for similar weight and only
slightly greater cost.
- Cars using Li-ion batteries are power-limited and require greater
battery capacity to meet performance specifications. This adds
- The Li-ion batteries make up for this with a substantially
greater all-electric range.
- The Li-ion batteries are also substantially lighter, by as much
as an adult passenger's worth for the sport configuration.
- Either Li-ion car would probably be able to run entirely on
electricity for a large majority of most user's driving.
What can we expect in the future?
- Cost of Li-ion cells will continue to fall. If they follow
the standard experience curve of 20% cost reduction for every 2x
increase in cumulative production, an 8x production increase will see
the cost of the commuter battery close to $3500.
- Cost of NiMH will also fall, but probably not as fast.
- Li-ion will probably be the cost leader for both energy/$ and
power/$ in a few years.
What are the prospects for plug-in hybrid vehicles?
- Cost of a Li-ion battery will approximate the cost of a gasoline
drivetrain when it hits the $2000-3000 range.
- This requires about a 2.5-4x decrease in cost, depending on
- We can expect this at a 16x to 64x increase in cumulative
- Use of cells for traction batteries will consume far greater
volume than portable electronic gear, and would increase production
much more rapidly than the current trend.
California once tried to force battery technology with the ZEV mandate.
Unfortunately, the initiative was ahead of the technology; it was
too much, too soon, and the few ZEV's which hit the roads cost up to $1
million apiece. But times have changed, and the technology is
ripening. If California tries again with a PIH mandate, the cost
curve is ready to meet us.