The Ergosphere
Thursday, March 31, 2005


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):
  1. Produce power to allow other generation to be turned down, to reduce fuel consumption and save wear and tear.
  2. Produce energy for storage.
  3. Produce energy for uses which currently do not use much electricity, but can take advantage of the underutilized supply and idle grid capacity.
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:
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:

Capacity factor
Rated power 25% 30% 35%
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:

Capacity factor
Rated power 25% 30% 35%
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 to come.
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:
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:


Δ consumption  Cost/unit Δ cost
CO2 emission
 per unit
Old emission New emission Δ emission
(3 kW, 25%
0 2916 kWh +2916 kWh


As electricity 0 1710 kWh +1710 kWh $0.05/kWh +$85.50 0

As heat 0 41.2 mmBTU
(41.2 therms)
+41.2 therms $0.02/kWh
(off peak)
+$24.12 0

4320 kWh 0 -4320 kWh $0.08/kWh -$345.60 3.4 lb/kWh 7.34 tons 0 -7.34 tons
Natural gas 575 therms 575 therms 0 $0.60/therm 0 11.52 lb/therm 3.31 tons 3.31 tons 0
Gasoline 288 gallons 0 -288 gallons $2.00/gallon -$576.00 19.4 lb/gallon 2.79 tons
0 -2.79 tons
Fuel oil 0 43.6 gallons +43.6 gallons $2.00/gallon +$87.20 19.4 lb/gallon 0 0.42 tons +0.42 tons


13.44 tons
3.73 tons
-9.71 tons
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.

Δ consumption  Cost/unit Δ cost
CO2 emission
 per unit
Old emission New emission Δ emission
(5 kW, 30%
0 5832  kWh +5832 kWh

0 0 0 0
As electricity 0 2052 kWh +2052 kWh $0.05/kWh +$102.60 0 0 0 0
As heat 0 15.1 mmBTU
(151 therms)
+151 therms $0.02/kWh
(off peak)
+$75.60 0 0 0 0
4320 kWh 150 kWh -4170 kWh $0.08/kWh -$336.60 3.4 lb/kWh 7.34 tons .26 tons -7.09 tons
Natural gas 575 therms 505 therms -70 therms $0.60/therm -$42.00 11.52 lb/therm 3.31 tons 2.91 tons -.40 tons
Gasoline 288 gallons 0 -288 gallons $2.00/gallon -$576.00 19.4 lb/gallon 2.79 tons 0 -2.79 tons

13.45 tons
3.17 tons
-10.28 tons

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:

  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. ;-) 
Thursday, March 17, 2005


(UPDATED 2005-03-29:  See below.)
(note:  this topic was handled briefly in the appendix of "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 Entropy Blues).  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 heat.
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 cycle.

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:
These systems are not directly comparable.  Here is a table of costs and properties: 

System Size (kW) Engine cyl. Engine speed,
Fuel Emission controls 2005 $ 2005 $/kW Notes
Generac 7.55 1 3600 Gasoline None 1150 158 No cooling jacket or exhaust heat recovery.
Inadequate quieting.
Engine speed too high for long life.
Vehicle 62.8 4? 2000-3000 Gasoline Catalyst 2940 47 Presumably includes radiator and exhaust.
Vehicle 68.5 4? 2000-3000 Diesel (not stated) 5100 77 Presumably includes radiator and exhaust.
Vehicle 59.6 4? 2000-3000 CNG Catalyst 3430 58 Presumably includes radiator and exhaust.
550 (not stated) (not stated) NG Catalyst w.
ammonia NOx
639,000 1150 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?
There are some difficulties unique to cogenerators which must be solved:

The product

What's the 2010-model, million-selling home cogenerator likely to look like?
  1. It'll come in one or more boxes, totalling about the size of a conventional furnace.
  2. 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.
  3. Maintenance will ordinarily be limited to annual changes of the engine air filter and an oil reservoir/filter cartridge.
  4. 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.
  5. 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.
  6. 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 heaters.
  7. 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.
  8. 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 power.
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 blower.
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.

Final accounting

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 CnH2n.


Δ consumption  Cost/unit Δ cost
CO2 emission
 per unit
Old emission New emission Δ emission
4320 kWh 0 -4320 kWh $0.08/kWh -$345.60 3.4 lb/kWh 7.34 tons 0 -7.34 tons
4320 kWh
-4320 kWh
0.78 lb/kWh
1.69 tons
-1.69 tons
Natural gas
500 therms 500 therms 0 $0.60/therm 0 11.52 lb/therm 2.88 tons 2.88 tons 0
Natural gas
500 therms
667 therms
+167 therms
$0.60/therm +$100
11.52 lb/therm 2.88 tons 3.84 tons
+0.96 tons
Gasoline 288 gallons 0 -288 gallons $2.00/gallon -$576.00 19.4 lb/gallon 2.79 tons 0 -2.79 tons
Fuel oil
electric only)

+114 gallons $2.00/gallon +$228.00 19.4 lb/gallon 0 1.10 tons +1.10 tons


13.01 tons
3.98 tons
-9.03 tons


7.36 tons
3.84 tons
-3.52 tons

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.
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.)
UPDATE 2005-03-29:  Tom DC/VA thinks that my estimate for cogenerator cost is low, by a factor of two to four.  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 that 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 later. 
Monday, March 14, 2005

Today's chuckle

Over at The Panda's Thumb, a Fred Reed piece is fisked.  In the comments it comes into hypergolic contact with a cite of The Crackpot Index. Hilarity ensues
Friday, March 04, 2005

Read the whole thing

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.
"Where is that solar technology going to come from? [It will come] not just from improving solar cells, but from something totally new...."
We need to start working seriously on alternatives to fossil fuels.
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. 
Wednesday, March 02, 2005

Checking the shelf

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:  Altair Nanomaterials 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  Their 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:
Battery capacity,
weight, kg
 Electric range, mi
(100% discharge)
Ni-MH 611.11 61.11 16.8   Commuter, 75 kW 7.5 126      $4583 30   
  Sport, 112 kW 11.2   188      $6844 44.8 
Li-ion 694.44 92.59 6.38   Commuter, 75 kW 10      63.8 $6944 40    
  Sport, 112 kW 14.93 94.3 $10370 59.7 
Salient points:

What can we expect in the future?

What are the prospects for plug-in hybrid vehicles?
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. 
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