Reply #86: Let's let readers form their own conclusion

[kristopher]

Here is a blast from the past:

Your original post where you quote me and respond is provided, as is my response:


Dr Greg on energy efficiency

For example, did you know that for our personal transportation fleet, about 80% of the energy in the petroleum fuel doesn't even need to be replaced as it is simply wasted as heat and serves no functional purpose?
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The above is a disingenuous half-truth. While it is true that the engine of a
automobile is about 20% efficient and 80% of the energy goes as waste heat.

However, what he is NOT saying is that the 2nd Law of Thermodynamics states
that you HAVE to have waste heat in order to get the 20% useful work.

He is attempting to imply that we could somehow get that 20% desired useful
energy without the 80% waste heat.

This shows IGNORANCE of the 2nd Law of Thermodynamics. We MUST according to
the 2nd Law of Thermodynamics have that waste heat because that is what carries
away the entropy.

The mechanical work energy of the engine doesn't carry away any entropy - so
you would have a source of entropy and no sink for the entropy. Such an engine
could not continue running in a cycle.

So that 80% waste heat DOES HAVE a purpose - it carries away the entropy and
it ALLOWS us to get that 20% useful work.

Without the 80% "waste heat" - you can NOT GET the 20% useful mechanical work.

This is high school level physics - get a high school physics text and turn to
the section of the Laws of Thermodynamics.

Dr. Greg

Leaving aside the other peculiarities of your statement, I'll focus on the main point - why we DON'T have to replace the wasted energy from an internal combustion engine when we consider how to power our personal transportation fleet.

The short version is that you only have that degree of waste heat in this application when your energy carrier is petroleum, not when the energy carrier is electricity.

To assist those not familiar with the topic (like "Dr." Greg):

Petroleum is an energy carrier that requires a chemical reaction to release the solar energy stored in the hydrocarbon bond. To harness that stored energy requires harnessing the heat released on combustion and converting it to mechanical energy via an internal combustion engine. So we have solar energy stored by living organisms that has been sequestered away for eons, then extracted, transported, refined, transported some more and then used in a contained explosion. Further, that mechanical energy must be transferred from the point of the reaction (cylinder) to the point of work (turning the wheel). All of those transactions are a debit against the original solar energy stored in the hydrocarbons.

The fuel 'tank to wheel efficiency' of most autos is actually between 12-20% with most vehicles coming in around 15%.

Alternatively, we can harvest the solar directly with PV or solar thermal, or we can harvest it indirectly by capturing the mechanical force of the wind or water currents (both motions are a product of solar input).

The output of all of those technologies is electricity. This electricity might be distributed for immediate use or it might be stored in various types of batteries (ie thermal, chemical or gravity).

Petroleum has been difficult to replace because of its high energy density. However, as noted above, because of waste from the process of harnessing the stored energy of petroleum, that degree of energy density isn't what needs to be replaced. All that we need to replace is the amount that turns the wheel and propels the auto down the road.

State of the art electric motors turn about 95% of their input electrical energy into mechanical energy. Since the shaft of the electric motor directly turns the wheel of the auto all of the mechanical output goes to pushing the car.

State of the art lithium batteries used in the electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) that all manufacturers are now turning to can store and deliver the input electric energy with efficiencies greater than 85%, almost always higher than 90% and often as high as 99%.

We derive the 'tank to wheel efficiency' for an electric vehicle by putting those numbers together:

From a low of 100%(input) X 85% (battery) X 95% (motor) = 80.75% efficiency

to a high of

1oo% of input stored in a 99% efficient battery through a 95% efficient motor = 94.05% efficiency for a loss to heat of between 6-20% depending on the technology versus a loss of 80-88% for internal combustion technology.

Battery technology is rapidly developing and technologies in the production design phase are capable of storing sufficient energy in battery packs of the weight used today to give an auto the weight of today's Volt or Leaf a range of about 800 miles.

When you hear all the talk about "energy efficiency" being a large part of the solution to our energy problems, this is the type of solution that is under discussion.

So we can waste between 80-88% of the energy in a gallon of gasoline used for our personal transportation or we can waste between 6-20% of each kilowatt of electricity we use.

88% waste or 6% waste.

"Dr." Greg, all of that is consistent with the laws of physics.



Also see:
http://www.democraticunderground.com/discuss/duboard.ph...

http://www.democraticunderground.com/discuss/duboard.ph...