Catch up on stories from the past week (and beyond) at the Slashdot story archive

 



Forgot your password?
typodupeerror

Comment: Re:Gamechanger (Score 1) 506

by amorsen (#49602415) Attached to: Tesla Announces Home Battery System

Plus those installations can provide a shedload of REACTIVE power, very, very useful for grid stabilization.

They can, but are they? I have only seen residential solar which reacted to grid overload/underload situations (i.e. situations which should never occur in an ideal world), not any which reacted to constant requirements for reactive power. Do you know of any which take part in the standard grid stabilization in normal use, outside of grid emergencies?

Comment: Re:Gamechanger (Score 1) 506

by amorsen (#49597421) Attached to: Tesla Announces Home Battery System

When electricity is cheap, it is because the marginal cost of producing it is low. The marginal cost is low because it does not take very much extra fuel to produce it. In other words, when electricity is cheap, its production is also less environmentally harmful. (This only holds as long as the power stations are unchanged of course.)

The Economist regularly gets this wrong by saying that electric cars are polluting more if they charge at night rather than during the day. They base this on the average pollution per kWh being higher at night. However, the average pollution does not matter. It is the marginal pollution which matters, and that is very low at night. This is really the kind of thing that economists should be specializing in getting right; I do not understand how you can be an economist and get it wrong.

Comment: Re:Show me the math on the Tesla. (Score 1) 279

by amorsen (#49589397) Attached to: New Study Suggests Flying Is Greener Than Driving

Coal plants are certainly capable of throttling their output and using less coal. But if what you say was true, my point would only be reinforced: Marginal CO2 emissions by using an extra kWh at night would be zero, because the grid would otherwise have to stabilize by dumping electricity in resistors.

Comment: Re:Masstransit is more energy efficient than perso (Score 1) 279

by amorsen (#49589299) Attached to: New Study Suggests Flying Is Greener Than Driving

DSB (Danish Railways) has a table on http://www.dsb.dk/om-dsb/dsbs-... saying that their long-distance trains do 33g CO2 per person km. Regional trains are considerably worse. Modern cars should hopefully do better than 133g per km.

Urban trains do better because people are standing up, which significantly lowers the train weight per person.

Now, Denmark is admittedly a bit of a developing country when it comes to trains. Obviously a pure electrified system running on hydro power would do a lot better. DSB's long distance trains use 0.12kWh per person km. A Tesla uses about 0.35kWh per km, which comes to 0.09kWh per person km with 4 people.

Comment: Re:Show me the math on the Tesla. (Score 2) 279

by amorsen (#49586837) Attached to: New Study Suggests Flying Is Greener Than Driving

That would be pointless because practically no one uses oil to produce electricity. Electric cars tend to charge at night where the coal plants are running at very low power and low efficiency. An idling coal plant has a very high average pollution per kWh produced but a very low marginal pollution per extra kWh.

Of course if it is a windy night the coal plants might just give up and shut down overnight, and then you really get your zero emissions.

Comment: Re:Masstransit is more energy efficient than perso (Score 1) 279

by amorsen (#49586765) Attached to: New Study Suggests Flying Is Greener Than Driving

Most other forms of transport have no chance against a decent modern passenger car with 4+ passengers. Most passenger cars are comparatively light, well below 500kg per passenger, which is very hard to beat. They do that because there is hardly any wasted space. A bus needs a walkway and it has to be tall enough to stand in, and trains are just horrendously heavy. While rubber-on-tarmac is a bit wasteful compared to metal-on-metal, it is not that bad, and the lower weight helps a lot.

Electric trains can sometimes play in that efficiency range, but it is tempting to make them faster and then the savings mostly evaporate. Cars are useful despite their lower speeds because of the time saved by almost going door-to-door.

"Pascal is Pascal is Pascal is dog meat." -- M. Devine and P. Larson, Computer Science 340

Working...