Energy Density and the Real Cost of Driving

Energy density by itself is not a sufficient measure of anything.  It is only useful in a specific context.  For example, one can refer to the energy density of battery technology.  And that is an extremely useful comparison because the weight of the battery in an electric car is critical to its success.  The General Motors EV1 was abandoned because the battery technology was too heavy.

Context is very important.  Comparing the energy density of the battery to the energy density of a fuel is completely useless.  And this is an argument that some people use, incorrectly, to defend fuel based vehicles.  Gasoline may be 80 times more energy dense than a lead acid battery, but what does that really mean?  Of course, gasoline has far greater energy density than a lead acid battery.  That’s an absurd comparison if not taken in its full context.

What is the efficiency of the energy conversion process?  Internal combustion systems generate a lot of heat which is loss and highly inefficient.  Taken with its frictional losses and parasitic loads like the alternator, water pump, etc., input to output efficiency for internal combustion vehicle systems is estimated at 40% or less.  The same comparison for an electric car can result in measured efficiency of 90%.   So the comparison of system efficiency for internal combustion engine passenger cars and pure electric passenger cars is that the electric system is more than twice as efficient.  Lithium batteries are 4 times as energy dense as lead acid and will reduce the required payload of 1800 to 2200 pounds of batteries to a much more reasonably 450 to 600  pounds.

What is the absolute value of the technology?  That’s the really important question.

One basis for comparison would be cost per transportation mile.  In both cases, the system efficiency is directly affected by the cost of input energy.   When gasoline is $2.50 a gallon, a 20 mile per gallon car costs 12.5 cents a mile.  When gasoline is $4. a gallon, a 20 mpg car costs 20 cents a mile.  Pure electric cars are impacted by the cost per kilowatt hour, but generally are documented as costing about 3 cents a mile.

A more complete comparison would incorporate the maintenance cost in addition to the energy cost per mile.  Again the electric vehicle has significant advantages.  There are no annual maintenance costs, although some value might be assigned to amortize the cost of the battery pack.

Further, one can include the purchase price of the vehicle.  A $20,000 gasoline powered car will cost about $15,000 to operate over 5 years at 12,000 miles per year.  An electric car will cost about $1800 to operate over the same period.  So if a comparable electric car cost $33,000 the total cost of ownership over 5 years would be the same.  Not surprising when you think about it.

It will be interesting to see what comes out of Detroit over the next two years.

Control, Motors and Efficiency

I was talking with some friends about control technology and made the observation that over the last decade the progress in the control field has been really amazing.  Particularly, the processor technology that is available for controlling electric motors is operating 1000 times faster than the control platforms of a decade ago.  We look at events in nanoseconds, not microseconds.

Increasing the control system’s frequency response is not signficant in itself.  But it does mean that software can be applied to problems that are more subtle in the operation of a particular system.  Observation of the phase relationship between the rotor and stator in an electric motor is now commonplace in 3 phase systems.  Algorithms for optimizing this relationship dynamically are also commonplace to adjust the power factor or reduce energy consumption in inertial loads like fans.

But this is not where the big energy gains will come from.  These improvements are smaller and more incremental.

Variable speed motors are systems that are made up of electric motors and power electronic systems.  Both are subject to losses in the form of heat.  In the motor bulk magnetizing of the stator, phase loss due to load, and copper losses due to the construction methods used are common.

Better metallurgy is needed to reduce losses associated with magnetizing the stator core.  The steel industry has attempted to address this issue, but the high cost of exotic alloy laminations prevents the advanced materials from becoming widely used.

Copper loss is improved in the segmented stator, but this manufacturing technique is most often found in more expensive servo motors, even though analysis suggests the cost is lower.  This may have to do with scale effect, since the servo motor world runs at much lower volumes than the AC motor world.

The other major dependency in the speed control is the power semiconductor.  The costs for power devices are falling and performance is improving.

So where are the big efficiency gains going to come from?

The control system strategy.  If the application is not well regulated you might be able to get a big increase in efficiency by measuring things more carefully.  In a cooling tower changing from a +/- 10 degree thermostat to a +/- 1 degree thermostat allowed me to implement a control system that reduced the energy consumption sufficiently to pay for the equipment in less than two years.

No new technology motor, nothing special about the variable frequency drive.  Just what was available at the time.  The big difference was the strategy.  Measuring what was important and organizing everything in the control system to achieve our objective.

Electric Vehicles and Electric Motors

A friend of mine finally got delivery of a Tesla Roadster.  This prompted discussion of the drive train and the fact that Tesla has had to go from two speed transmissions which were failing to a transmissionless drive train.  The ultimate mechatronic challenge, the electric car, is also a challenger in terms of the precise  application of electric motor technology.

But it has to be said that the motor and drive solution for the electric car is not where the problem has to be solved.  Any motor can be made to run an electric car.  What is critical is how you apply it.  The starting conditions require high torque at low speed and the running conditions require low torque at high speed.  So, typically, what looks like a small 5 to 15 horsepower running requirement at full speed, becomes a 150 horsepower starting requirement depending on how quickly you would like to start.  If you want to keep up with a Corvette, it uses 450 HP to start.

And this produces a lot of confusion.  Why not use at 2 speed transmission to help the situation.  Fine, but the ones that are available can’t handle the dynamic response of the electric motor.

Can electronics help this situation?  Interestingly, yes.  There is a control algorithm generally called vector control which allows you to manage the rotor torque and stator torque separately.  By varying the phase angle between the two, like advancing and retarding the timing of a mechanical distributor cap on an internal combustion engine, you get different speed torque curves out of the motor.  COOL!  Is there any downside to this?

Yes.  You need more current to produce more torque.  That doesn’t change.  So you have to be able to supply the current, and you have to be able to manage the heat.  The heat is transitory since you only need the high current during starting, but it is best to have sophisticated software running to keep track of the RMS temperature of the motor.  Lower operating temperatures mean longer life and reduced risk of demagnetizing the motor.

So, yes, you can run an electric car with a garden variety AC motor, and with good electronics, you can make it run fairly efficiently.  With higher efficiency motors, the benefit is increased driving range from a given power source.  High efficiency motors are frequently smaller and lighter weigh, but a weight savings in the motor of 50 or even 100 pounds is not that big a factor in the driving range when the curb weight of the vehicle is 3000 pounds.

Basically, its F=ma.  If you can reduce the mass of the vehicle, you reduce the battery payload required to power the car.  Aluminum space frames, like on the Prowler, have been studied by the car industry and can reduce curb weight by 400 pounds and reduce cost by 10% at the same time.  We need to bring all the mechatronic leverage to the situation that we can, if we are going to make electric cars that make sense.  Before its too late for Detroit.

Super Size my Motor?

There is an interesting problem with applying electric motors that is a constant source of difficulty, the nature of peak power versus continuous power.  The problem is that few systems operate at a statistical average power demand.  Frequently, this causes equipment designers to oversize the motor for the application.  At the same time, however, this can put the motor in a very low efficiency operating range.

So what’s the right solution?  Right sizing.  Yes, just like Goldilocks and the Three Bears, not too big, not too small, but just right.

There are some great DOE publications on motor sixing that can be very helpful on the AC motor side, so make sure to give those a look.  But the implications of how to deal with varying loads are complex, each requirement having its own unique conditions that need to be considered.  Is an underpowered application actually safer?  Sometimes, yes.  I recently noticed that a particular orbital sander had been designed so that if the unit became momentarily overloaded, it stalled.  Perfectly safe.  In fact, this design is to be preferred because it prevents accidentally damaging a work piece by burying the sander in the wood and removing too much material.  Who’d have thought of it?  Certainly not Tool Time Tim.  More Power!

In fact, most of us view more as better.  More power means more production.  Or does it.  In an increasingly energy conscious community, more power means more cost.  And that’s really what its all about.  The value of the motor is not just in the purchase price, but also in the operating cost.  Especially if the motor is expected to run for 8 years, 24/7.  (That’s what the life expectancy of large AC motors is)

There’s another trick to the power requirement problem.  How much time is spent at full load and how much time is spent at average power, or, what is the duty cycle?  If the system is starting and stopping frequently it puts different constraints on the motor.  If the system is typically starting only once an hour, then we can consider the thermal duty cycle of the motor.  The momentary peak power requirement is insignificant and the vendor can usually tell from their modeling and testing of their products how much impact the peak current will have on the motor’s average temperature.

After all, its Thermodynamics 101 in the final analysis.  Every energy transformation produces heat as a byproduct.  How much heat a given system can tolerate is the key to its operating life.  In electric motors, the key values are the insulation system’s temperature rating, usually in the range of 150 to 180 C and in the case of steppers, brushless dc and permanent magnet dc motors, the magnet’s ability to resist high temperature and high coercive magnetic fields that can be generated in the motor.  Both sets of limits are generally well considered by suppliers when electrically controller motors are shipped as motor/drive combinations.  This can get a little tricky when pairing motors from one vendor with controls from another vendor.

Top 10 Mechatronic Challenges

I recently wrote on the mechatronic challenge of wind power.  Converting wind into mechanical power that can be harnessed for man’s use has been going on since the 9th Century according to Persian historians.  Certainly wind powered grinding of grains has been around in Europe for several centuries and, lest we forget, wind power pumping of water in the United States.  So there is some irony to the cultural “buzz” about wind power at home and abroad, as if the technology were entirely new.  There’s a lot of history, we’re just updating the technology to produce energy in the age of electricity.

Water has been used for power generation as well.  Following a similar path, we learned during the early part of the industrial revolution how to locate manufacturing plants near waterways so we could convert water flow into mechanical power using the water wheel.  This is, in fact the root of all modern motion control.  All the belts and pulleys, cams, gear reduction systems follow from the work done in mechanical engineering from this period of time.  All of the electronic analogs of the mechanical behaviors found in mechanical systems are the functions which we refer to in mechatronics today.

Wind power and water power gave way in the 1800’s to steam power as the improved steam engine of Watt became the standard of energy efficiency, or should I say “cost effectiveness”.  Because the absolute value of technology is in its cost effectiveness.

Still, wind energy poses a huge technology challenge, as witnessed by the number of vairations that exist and new versions that are emerging.  And hopefully improvements will continue to come from the creativity and  imagination of engineers and inventors all over the world.

But what are the other big mechatronic challenges that come to mind?

Transportation certainly ranks in the top 10.  We have seen hydraulic, pneumatic and electric vehicle solutions touted for a variety of uses, personal transport, delivery vehicles etc.  Ballard Energy and General Motors have both been building hybrid and pure electric buses for city transportation systems for several years with some success.  Interestingly, the electric bus is easier to engineer, which seems unreasonable, but the bus has more interior space to put things like batteries and a methanol converter for generating hydrogen for fuel cells.

But there is a great lesson in what appears to be an almost chaotic string of choices in the transportation arena.  One solution will not work for all requirements.  There are many people for whom a 40 mile per day drive cycle is perfect.  The NEV, Neighborhhod Electric Vehicle, is a golf cart type solution that is rated for street usage, and because of its relatively simple performance requirements, is relatively easy to achieve and lowe cost.  As we categorize cars with greater range, the problems get more difficult, and because of the storage limitations of batteries, have only been achievable as hybrids.  But with some hybrid designs reaching 50 and 60 mpg (estimated), these vehicles may be great solutions for other users.  Although, we must consider their cost effectiveness.  If they cannot be introduced at prices well below $50,000 the absolute value of the technology is not very good.

So forget the 15 second soundbyte that will solve the world’s problems.  It doesn’t presently exist.

I would like to hear from any readers about their picks for the Top Ten Mechatronic Challenge.

Energy Efficiency

Energy efficiency is another “hot” topic. And like many topics in technology, its very prone to misunderstanding and misrepresentation.

For example, a recent prominent engineering magazine published a cover article on the subject of electric cars, one of my favorite topics. And the cover had a really cool graphic comparing the energy density of lead acid batteries to the newer lithium ion batteries which have 4 time better storage capability, and then comparing that to gasoline’s energy density which is eighty three times more power per pound than lead acid technology. Read more

Clarity

A national talk-show host I listen to comments “I would rather have clarity than agreement”. I think that is a great platform for discussion. And I approach the blog with the same goal. This posting is an attempt to clarify my previous entry.

I got into the big debate on automobile technology in the 1980’s. It started with a duty cycle chart of engine horsepower and led me to join Unique Mobility in the late 80’s to try and help bring hybrid technology to the California low emission initiative. We were not successful. Primarily because none of the Big 3 automotive companies would agree to supply a vehicle platform for the drivetrain we developed. We did succeed in building a drivetrain for BMW’s EV-1 and EV-2 which were very successful steps along the way.

But the point of my earlier blog was simply to comment on “automobile technology” as the greatest mechatronic challenge of all. You can start with a simple F=ma approach and deal with how much mechanical power must be produced to move the vehicle, which the Big 3 have been messing around with for years. We have cars made out of plastic to reduce vehicle weight (the “m”) in an effort to get lower power solutions which mean more miles per gallon, you know the rest. Read more