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NREL Study Finds That A “Dynamic Plug-in” Vehicle Could Be A Promising Technology Pathway for Cost-Effective Vehicle Electrification

Brooker1
Cost-effectiveness of vehicle electrification using today’s assumptions. The EHEV pathway is the dynamic, road-charged hybrid. Source: Brooker et al. Click to enlarge.

A study by researchers at the US National Renewable Energy Laboratory has found that, under today’s battery assumptions, while plug-in hybrids and electric vehicles significantly decrease consumption, none of a variety of electrification pathways were cost-effective compared to conventional vehicles or hybrids, except for a “dynamic plug-in” hybrid that recharges while moving.

The study by Aaron Brooker, Matthew Thorton and John Rugh aimed to identify possible pathways to cost-effective vehicle electrification by evaluating a variety of scenarios and technology improvements. Using current battery performance and cost, they evaluated PHEVs and EVs across a range of scenarios and configurations—which including 10-, 20- or 40-mile all electric range, with low or high electric power, with or without battery replacement, and with or without opportunity charging. Electric vehicle’s cost-effectiveness improved with battery replacement and opportunity charging, but it was not enough to match the conventional platforms.

Among their general findings are:

  • Increasing battery power in plug-in hybrids (10-, 20- and 40-mile AER, from low to high power) had little effect on fuel consumption results because the battery power can provide most of the driving on the test cycles, so the fuel economy only differs slightly. For the electric-powered vehicles, the electricity cost is relatively low, reflecting the low cost of electricity and the high efficiency of batteries and motors. The gasoline, on the other hand, is a large expense, especially for the conventional vehicle. Even so, they found, extra battery costs in PHEVs and EVs outweighed the gasoline cost savings.

  • Battery replacement had minor overall improvements in cost-effectiveness. In these cases, they reduced the size of the battery but used it more aggressively to reduce upfront cost and weight and take advantage of lower future battery costs. The advantages, they found, were mostly balanced out by the increase in battery wear.

    Brooker2
    Battery life cycle curves showing the relationship of SOC swing to cycle life. Source: Brooker et al. Click to enlarge.

    For a smaller battery to provide the same electric range and regenerative braking, it must use a greater portion of the battery energy, and thus have greater depths of discharge. Since battery wear increases non-linearly with depth of discharge, each battery has to be larger than half of the single battery case.

    For example, in the high power PHEV10 case, a 5.9 kWh battery would last the life of the vehicle using 34% of the energy. Having one replacement, however, required more than half of a 5.9 kWh battery. It required purchasing two 3.7 kWh batteries using 54% of the energy to meet the life requirement. Although it was assumed that future batteries cost less and that there is a time value of money advantage to purchasing the second battery, these advantages did not make up for the total added cost of buying more total battery energy. The nonlinear wear trend balanced out the advantages for little overall gain.

    —Brooker et al.

  • Opportunity charging further decreased the gasoline consumption, and thus gasoline cost, of PHEVs, but at a greater increase in battery cost. Although the fuel cost went down, opportunity charging increased the use of the battery. As one example, in order to sustain the additional use and wear, the battery energy had to be increased from 5.9 kWh to 10 kWh in a PHEV10. Including the additional electricity cost, opportunity charging increased the total cost for the PHEV10 by $4,400.

  • On the other hand, opportunity charging decreased the EV cost. Opportunity charging increased the frequency of recharging, reducing the depth of discharges and the amount of wear, and thus reducing the amount that the battery has to be oversized to last the required life. Specifically, it reduced the battery size from 47 kWh to 32 kWh. The EV still exceeded the cost of all the other vehicle types.

  • Combining battery replacement and opportunity charging increased the use of the high cost battery to better leverage the investment, but little to nothing was gained by adding battery replacement to the opportunity charging cases under current battery life assumptions.

Their analysis found three potential approaches for improving the cost-effectiveness of vehicle electrification: what they called the electrified HEV (EHEV)—i.e., a vehicle that recharges while moving along the roadway—because of the resulting downsizing of the battery pack; significant battery improvements reducing the cost to the DOE target of $300/kWh (they used $700/kWh as the current level); or an improvement in battery life by a factor of 10.

If an acceptable method for plugging in while traveling along the roadway can be devised, it may provide a cost-effective pathway to vehicle electrification. This approach benefits from the low electric fuel cost of a large battery without the high cost, cycling wear, weight, and efficiency loss. Even with assuming a $1,000 price for the connection device, the cost to the consumer was still lower than for today’s conventional and hybrid vehicles. This pathway requires infrastructure, but only along a small fraction of heavily traveled roadways to gain the same gasoline saving benefits as battery PHEVs.

—Brooker et al.

The fraction of infrastructure is small because most travel occurs on just a few roads. The interstate, for example, makes up 1% of the miles of roadway but carries 22% of the vehicle miles traveled. Their scenario assumed that 50% of the distance driven is connected dynamically. It also assumed an additional $1,000 cost to the consumer for the dynamic connection, the same fuel cost per mile as an HEV when not connected dynamically, and the charge depleting fuel cost per mile of a PHEV when connected.

A paper on their work, which will be presented at the SAE World Congress in Detroit in April, details the assumptions and approach for the study.

Resources

Comments

jfinlayson

Sounds good. Personally, I'm fine with driving on electrified roadways. But I shudder to think what an outcry there will be from folks who already fear that their cell phones are causing cancer and other health woes. The EMF emanating from a cell phone is tiny compared to what will be coursing through your body as you cruise these future highways.

Expect to see gasoline-burning cars completely wrapped in tin-foil.

HarveyD

Assumptions used set the results for each scenario.

Current battery, liquid fuel, electricity, repairs, insurance, registration, and HEV/PHEV/BEV purchase cost apply to yesterday's cases only.

Future cases are full of unknowns. One can only assume or guess what the relative procurement, durability and operational cost will be. Forecasting is not or rarely a precision task.

For example if you assume that a light weight rust proof BEV could last 20+ years and 300,000+ miles instead of 10 years and 150,000 miles for an equivalent ICE monster and that gas will go up to $10/gallon, the results would be very different.

However, one can assume that a common sense light weight PHEV with smaller modular plug-in batteries (up to 4 x 5 Kwh options) and a small very light (10 to 20 Kwh) on-board flex-fuel genset may be the ideal configuration for the current decade for many users.

fred schumacher

There's another way to approach this problem. First, through morphology; second, through a propulsion mode I propose calling the Battery Buffered Vehicle (BBV).

Nearly 90% of our driving is done alone and on a regular circuit. To accomplish this task, we presently use multi-purpose vehicles that seat a minimum of five, and are therefore traveling extremely lightly loaded. A much smaller vehicle, appropriate for the primary load demand, would decrease power need by at least a factor of four.

Present vehicles' engines are sized for necessary power requirement at ultimate demand and are run very lightly loaded most of the time, with high power demand being generally of very short duration. This is wasteful from a weight, volume, cost, and fuel efficiency standpoint.

Hybrids, combining IC and battery stored electric power have taken many forms, but what hasn't been tried is the battery buffered form, where a small genset runs mostly continuously providing power to a small battery bank which buffers power demand and flow. The genset is sized based on steady state power demand, with short duration extra power required for acceleration and hill climbing being drawn out of the battery pack.

The genset is optimized to operate at single speed. It can be very small, less than 30 kW, and be a single cylinder, long stroke, true Atkinson cycle in a compact, sound-deadened enclosed box. Drive could be by hub motors, thus minimizing internal mechanical requirements and allowing for a smaller external package and lighter weight and reduced wind resistance. The battery pack would be small, one-fourth the size of the Volt's, thus reducing the single greatest cost of BEVs.

Morphological changes and BBV operation combined could reduce fuel requirement for the daily commute by a factor of six or more, making biomass an achievable source of sustainable, long term fuel supply. I would argue that the small BBV would be a much less expensive solution to the problem of private transportation than the integration of electrical power cables along our highways for on the fly recharge and the building of nuclear power plants to supply them.

SJC

The Honda hybrid with heat recovery is a way to charge on the road. If they get 5 kW from the heat recovery it can extend range and use the engine less.

mahonj

I do not think that people are that rational about the cars they buy and run.

They buy from the heart (and groin) - and do not really think about the 10 year cost of running a car.

Fred - is your BBV not a parallel PHEV?

But there is a definite trend to partial electrification of cars in all forms, from BMW's "efficient dynamics" and the equivalents in VW and Volvo, to HEVs and PHEVs (coming soon).

In Europe fuel consumption is going down rapidly due to the EU's 120 gms CO2 ruling.

Whether or not you believe in AGW, there are great benefits to be had from more efficient vehicles, and it doesn't really matter how this is done as long as it is done.

The market does not provide a useful way to drive this - it is too unpredictable - oil spiked to $147 in 2008, and then went down to <$35 within a year - no corporation can plan based on that.

The EU's CO2 limits, on the other hand provide a clear signal that you MUST decrease fuel consumption if you want to sell in the EU. It takes steady engineering work to do this and the corporations need to be able to rely on the rules.

Lad

Here's my plan: Buy a leaf,then build an associated low polluting ICE extender, lightweight, genset that the Leaf can pull in a bumper trailer (one wheel), change the electronic so the batteries can be charged while underway.

The definite advantage is not carrying the extra weight until you need to extend the range of the car, i.e., long distance trip. that would be about 40 times a year for me.

Tim Duncan

Not sure how far ahead this research is looking, but the Dynamic Charging assumptions are rather appalling to me. I don't see this happening anywhere now or even being talked about by people that pay for infrastructure. Since roads are publically funded (we just assume they are there for our use) and the Dynamic Charging concept came out of the study as a low cost solution, I can only guess that the study made this same assumption about these special lanes? If reasonable guesstimates were included to accomplish inductively coupling on 50% of our major highway miles, I think results would have been much different. If this is true, it really makes me mad that it would be described in GCC as a possible efficient concept. 50% would be a massive taxpayer burden, for a very few folks that could afford to buy special cars to make use of it. Nonstarter, government is already over involved in private sector, they should not be building special energy delivery systems for special people and places. I It is an interesting technology, but not very realistic without massive government intervention to get thru the chicken and egg phase. By the time this could be done, battery and other technology will most likely have improved enough to make it obsolete. Concerned citizens should just say NO.

fred schumacher

BBV is a serial hybrid concept. The IC engine is not directly connected to the wheels. It is possible to make IC engines with efficiencies equal to fuel cells. Wartsila marine diesel engines already achieve over 50% thermal efficiency, but that style of engine cannot react to the varying power demand over very short time frames that automobiles require. Buffering the output through a battery bank eliminates this problem.

Highway pavement based inductive coupling at a road density high enough to function would be massively expensive in a country that shows it has no interest in large scale investment in new public infrastructure. The money is simply not there. Look at Congress' inability to handle even the smallest of infrastructure investments.

Lucas

Pie in the Sky ...

I proposed a drive train similar to Fred's first post over five years ago. I posted it everywhere and sent copies to all the big auto manufactures. GM had the courtesy to tell me they did not accept out of company suggestions.

I have since designed a lightweight, crash proof body. I haven't wasted my time sending it to anybody.

shopa

The less an electric vehicle weighs, the better its range.

Because batteries have much less energy per pound than gasoline, electric vehicles cannot be as heavy as gasoline powered cars.

Lighter cars do not do well in collisions. They have less steel to absorb the collision energy. The collision death rate of micro vehicles is double that of mid size vehicles.

I have invented a way to make vehicles lighter and safer at the same time. Please help me promote this patent pending invention.

My website is www.safersmallcars.com

danm

I haven't checked your post yet shopa but just want to thank others for their intelligent posts!
-
Fred, you site the Volt and it actually will try to do what you suggest, except for grossly overweight. You are correct that vehicles need to "morph" downward in size/weight. GM is still stuck in the 20th century idea of a "car". It will probably take India or China to create your vision, then it will be imported to U.S. and sell wildly.
-
Instead of a trailer carrying an extra power source, how about a space within the vehicle already prepared for a small ICE or FC ? You just drop it in and hook up the exhaust pipe and fuel line and GO !

Giant

Take the concept of opportunity charging a little further to having a grid like that used for electric trolley cars. The battery can then be quite small!

HarveyD

Could a very light weight quick plug-in genset, in the back or front trunk, be a practical solution?

However, if it can be made very light (50 lbs or so), one may elect to leave it in the car on a permanent basis.

SJC

The trailer made for the Tzero had steering that made it easy to park and back up. If the trailer could hold items like a trunk for a trip maybe, but not for commuting, the parking would be worse than for an SUV.

Imagine a Tesla with trailer, it takes all the sport out of a sports car. Interesting idea, but don't take anything away from a customer nor add anything that they do not want.

ai_vin

If the generator is lite enough you don't need a trailer. You could use a hitch mounted cargo carrier/tailgate box; http://www.stowaway2.com/

Bryan

I've read much regarding the limited technical capacity for utilizing lithium-ion batteries (having to oversize a battery to avoid deep discharge). Is there a new technology on the horizon that can overcome this seemingly cost ineffective characteristic of lithium-ion batteries? Maybe with lithium-air?

ai_vin

BTW there's no reason the generator carrying tailgate box has to be 'box shaped.' It could be an aerodynamic wedge (what ecomodders; http://ecomodder.com/ call a "boattail"): Such an add-on can improve your fuel economy by 15%

For a sports car like the Tesla it would be like fitting it with a Le Mans style "long tail."
http://www.automobilesreview.com/uploads/2009/07/Porsche-917-L-Long-Tail-1971.jpg

Roger Pham

The problem with tailgate box is more weight on the real wheels, shifting the center of gravity rearwardly and creating dangerous oversteering tendency when turning sharply at high speeds or slippery surface. This will cause the vehicle to spin-in and out of control, similar to the problem with the rear-engine Chevy Corvair years ago.

The best position is in the front below the hood. Increase weight on the front wheel will cause understeer tendency, which would be safer, and more traction on the driving wheels, ideal for driving in sands or snow.

Just imagine building the Chevy Volt without the engine and the generator, but leaving an empty bay below the hood for a removable genset of about 20-30 kW. Then, the engine-less Volt can be sold for thousands of dollars cheaper for those city dweller with short travel distances and charging jacks at work or at curbsides. The genset would be sold seperately for the suburban dweller who must travel longer distances.

sulleny

Roger, I suspect this is precisely what GM will do to create a vertical BEV "runabout." They may even OEM the drivetrain and electronics for a subsidiary to market as a runabout. What all BEV makers have to be ware of is confusing the public with an AV without range extension.

The key is to position the vehicle as a short range commuter or town type vehicle. This will become a new category of transport product - especially for two car owners. One full range serial/parallel hybrid, one short range BEV. Until batteries improve which is happening rapidly.

SJC

The commuter market is big out here in the suburbs of southern California. Many are buying used Civics and other small cars, fixing them and running the wheels off them. This can not go on forever and the first car maker to offer a better alternative wins.

ai_vin

@Roger
That's a hypothetical, worst case, possibility that apparently hasn't been encountered by a lot of users.

SJC

I would like to think that we could all do buried inductive charging that could steer the car as well, but that would require more investment than we can muster.

ai_vin

Here's an idea for you; http://www.christian-foerg.de/portal/project14_mov.php

Now, while I think the design of their vehicles is too far out, I like the idea of a linear motor in the pavement that both propels and recharges your batteries.

SJC

MagLev trains turn on the power before the train gets to that section of track and turns it off after the train passes to save power. You would not need to do that with inductive roads because traffic is usually there except for some hours late at night and in the early morning.

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