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Wanted: Link Between EVs & the Grid

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Wilco1
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Re: EVs and the grid
Wilco1   9/13/2013 5:17:49 AM
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You're right that engine efficiency varies with rpm and torque. However due to 6-speed transmissions (and CVT on hybrids) one can keep an engine very close to or at the optimum.

If you look at the graph in the same article, the 225 g/kWh line (ie. 36% efficiency) spans from 1200rpm to 3300rpm and a factor of 2 in torque (y-axis). That is more than the range one would need for typical driving. So engines are far more efficient than you think - the only case where an engine goes down to 20% efficiency is if you rev it to the maximum rpm without any load (the idling case is handled with start/stop). Hardly a realistic scenario.

New engines like HCCI will come to the market in the next few years, further increasing efficiency. So unless that 48% for a fuel cell with reformer can be improved as well, it seems unlikely it is going to be able to compete with future engines. And yes you have to compare fuel cells with engines running at their highest efficiency as that is how most hybrids work.

Bert22306
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Re: EVs and the grid
Bert22306   9/12/2013 3:48:12 PM
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But once again, I am talking about reality, not propaganda.

Here is a quote from a Wikipedia article, to show you what I'm getting at:

"The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 rpm. Beyond this speed, efficiency begins to decline due to air pumping losses within the engine. Modern turbo-diesel engines are using electronically controlled, common-rail fuel injection, that increases the efficiency up to 50% with the help of geometrically variable turbo-charging system; this also increases the engines' torque at low engine speeds (1200-1800RPM)."

And:

"The gas turbine is most efficient at maximum power output in the same way reciprocating engines are most efficient at maximum load. The difference is that at lower rotational speed the pressure of the compressed air drops and thus thermal and fuel efficiency drop dramatically. Efficiency declines steadily with reduced power output and is very poor in the low power range - the same is true in reciprocating engines, the friction losses at 3000 RPM are almost the same whether the engine is under 10% load or not having any useful output on the driveshaft."

So, the efficiency of modern internal combustion engines in actual operation in cars, *especially* in non-hybrid cars, is way lower than the efficiency at ideal operating modes. If instead you use something with an electric drivetrain and fuel cells, that efficiency does not drop dramatically at lighter loads. Quite the contrary.

So, the 48 percent efficiency of the fuel cell care with H2 reformer in no way can be compared to the peak-optimistic-never-actually-operates-there 40 or 50 percent figure cited for diesels. In diesel hybrids, during city driving only, MAYBE.

Wilco1
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Re: EVs and the grid
Wilco1   9/12/2013 6:50:34 AM
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Modern engines are far more efficient than you think. The petrol engine in the Prius is 37% efficient, and a modern diesel can be 42.5% efficient. These are actual engines on the market, not laboratory setups: http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption (also check out the amazing 51.7% efficiency of the enormous diesel engine used in Emma Maersk).

I agree 100% electric is the best solution. But I believe the roadmap for automotive fuel cells will be bumpy: at one side they are being squeezed by ever more efficient engines (coupled with thermogenerators to harvest electricity from the waste heat). On the other hand batteries continue their slow but steady improvement in capacity/cost and soon provide a range and recharging time that is good enough for 99% of people.

I'm sure fuel cells will find a niche - my guess is CHP for homes: cost/size/weight are no big issues, they can run on relatively clean methane, need no start-stop cycling, and the waste heat is used for hot water and central heating, so you can get 80+% efficiency.

 

 

 

Bert22306
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Re: EVs and the grid
Bert22306   9/11/2013 9:19:27 PM
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Wilco, yes, I looked into the overall efficiency of a fuel cell car and on-board reformer. For moderate levels of power output, the combined efficiency of the entire drivetrain, including also the electric motors, came out to around 48-50 percent.

So you might say, not that great. But it is that great. Because this compares to the actual efficiency of an internal combustion engine, in real-world driving, which is only about 18 to 20something percent. (The 30 percent figure often cited is, like so many such, ONLY under absolute ideal conditions. The inescapable limit being, the difference in Kelvin between the combustion peaks and the exhaust temperature.) So this fuel cell approach easily doubles the efficiency, and consequently creates a lot less CO2.

At high output levels, fuel cells lose some efficiency. But a hybrid-sized battery should take care of that. High output is used rarely in cars, e.g. in passing or moving away from a stoplight. The battery would provide that spurt of energy.

Car companies are showing renewed interest in fuel cells, as also reported by EE Times recently. My bet is, because battery storage by itself is just too compromised of a solution. Fuel cells capable of using less than pristine H2 are also being developed, with there again some loss of efficiency, but overall remains somewhere above 48 percent.

I would agree that your LPG or similar generator solution is the best in the near term, but my hope is for an uncompromised "true electric." With very few moving parts, and not depending on the Carnot cycle in any way.

Wilco1
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Re: EVs and the grid
Wilco1   9/11/2013 8:52:28 PM
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Bert, those are very interesting calculations from the other perspective. However note the US residential electricity consumption is only 36% of the total, so doubling it means 36% extra generation. Obviously if your electrify a big gas guzzler it wouldn't be as efficient as a purpose built 100mpge EV, but we can safely assume doubling efficiency to 56mpge will be feasible. Assuming that, the total US electricity generation would go up by just 31%. Which seems quite feasible, right?

Yes I like fuel cells, but it is a shame they cannot handle anything but hydrogen effectively - the input needs to to excruciatingly pure. Reforming is pretty inefficient even on a large scale. And while batteries are expensive, what about the cost of a reformer and fuel cell? Will it last 100000 miles? EV's are on the market today and moving towards mainstream. I don't want to be pessimistic, but it seems fuel cell cars are still 10 years away.

My feeling is that the best solution to the range anxiety of EVs is a small efficient generator module using LPG or biofuel.

 

David Ashton
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Re: EVs and the grid
David Ashton   9/11/2013 8:23:51 PM
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@Wilco1....you know your stuff, thanks for that.  Couple of points....
  • This is obviously not feasible right now....however there are not too many EVs right now.  We *should* get more energy efficient in our homes (though that is debatable, our appetite for energy seems to outstrip this), and solar panel efficiency should rise.   
  • The average home can't make enough for the home use AND EV charging with current efficiencies.  However there are vast roofs on shopping malls, industrial premises, etc which are often unused
  • Every little bit helps - especially if it can help reduce or even out base load generation requirement - which is where most of the fossil fuel usage is.
  • We shouldn't be scared to start something here because it looks like we'll never be able to fully meet our requirements....

Thanks again, interesting discussion....

Wilco1
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Re: EVs and the grid
Wilco1   9/11/2013 7:56:49 PM
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To get an idea whether roof-top solar panels could provide enough energy, let's take the annual electricity consumption per household: 4648kWh in the UK. If you added an EV as per my previous calculations, it would need another 3400kWh for average UK mileage. You would need 11kWp of panels or about 70m^2 of south facing roof (250W panels are about 1.6m^2). That's more than double the size of a typical UK roof.

The US consumption is 11698kWh and assuming 2 EVs per household at 8400 miles per year you'd need between 10kWp in LA to 17kWp in Seattle, or 64m^2 to 109m^2. Despite more sun, consumption is still too large, so panels won't fit on an average US home.

So the conclusion is that with current efficiency and usage solar panels need far more than your roof area to supply all your electricity. If overall efficiency doubles in the next 10 years or if solar films become cheap enough to use on every wall and window then it might become feasible.

Bert22306
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Re: EVs and the grid
Bert22306   9/11/2013 7:40:34 PM
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You are correct that I had mixed quantities. So I did my own comparison of energy usage for the average home vs for the average car (note: not van or SUV, but 28 mpg car).

According to

http://www.eia.gov/tools/faqs/faq.cfm?id=97&t=3

the average US home uses 940 KWh per month. And if you assume 28 mpg cars, and 1250 miles per month average driving distance per car (all fairly optimistic values for the US), that means that the average car needs 44 gallons of gasoline per month. Okay, that's realistic, and btw, a whole lot of what people call "car" these days won't come close to that economical, and that's just one car in the household.

Gasoline contains 10 KWh per liter of energy, or 37.85 KWh per gallon. So that means that this car is using 1665 KWh per month, compared with 960 KWh for the average home. Which is more than 1.7 times as much energy. One car in the household. Households have 2 or more cars, perhaps not all driven as much as 1250 mi/mo, so to say that we need to double the amount of energy delivered by the grid, to individual homes, seems hardly an overstatement.

Electric cars are more efficient, no doubt, but if there's a major shift to electric cars, we're not talking about everyone in a Nissan Leaf, either. That figure of 44 gal/month is pretty conservative for most households in the US, I'll wager. I'm sure the readers can decide on their own whether that's reasonable or not. So my contention is, the extra amount to electricity to be generated by the grid, if we all start driving battery-powered electrics, is substantial. Not negligible.

I think a good bet for the not-too-distant future is H2 reformers on board, creating H2 from some sort of hydrocarbon fuel (could be biofuel),  feeding the H2 directly to a fuel cell. Avoid the large storage battery (perhaps a hybrid-size battery is good enough), avoid distributing H2 or storing H2 in a tank, avoid all the energy starvation batteries create. No apoligies, and a real electric drivetrain.

Wilco1
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Re: EVs and the grid
Wilco1   9/11/2013 7:21:50 PM
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Bert, the Ford Focus has just a 23kWh battery so no surprise it's range is much smaller too!

Tesla claims 300 miles, the EPA achieved 265 miles and the European test achieved 310 miles. People have done drives over 420 miles on a single charge (lots of links if you Google it), so it is certainly feasible to go well beyond the advertised range. Alternatively if you use it on a track day your range will be a lot less. It all depends on how you drive it, but my 250 mile estimate for a real-world range is reasonable.

betajet
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Need about one carport's worth of solar panels
betajet   9/11/2013 7:13:14 PM
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IIRC, at a recent Embedded Systems Conference the CTO of Tesla said in the keynote address that you need about one carport's worth of solar panels to provide all the usual charging needs of a Tesla car.

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