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Ford study shows Rankine waste heat recovery system on a light-duty vehicle could almost meet full vehicle accessory load on highway cycle

An organic Rankine cycle (ORC) waste heat recovery system using R245fa as the working fluid could nearly supply the full vehicle accessory electrical load in a light duty vehicle (Ford Escape) on the EPA highway cycle in both a conventional and hybrid powertrain configuration, according to a modeling study presented by Ford Motor Company at last week’s Directions in Engine-efficiency and Emissions Research Conference (DEER), in Detroit.

However, in city driving, the ORC system would supply about half of the accessory load requirement for a conventional engine, and only about one-third of the load requirement for a hybrid powertrain, due to the frequent engine off condition, according to the results presented by Quazi Hussain.

The Rankine cycle is widely used used commercially to generate power in stationary power plants, and is under consideration as a potential waste heat recovery system for use in both light-duty (BMW, earlier post; Honda, earlier post) and heavy-duty (earlier post) applications.

A generic Rankine cycle system consists of a pump, evaporator, expander, and condenser. The working fluid is pumped to the evaporator where it is vaporized by heat. The vapor flows to the expander where it can be used to generate electricity, then comes back to the condenser, where it cools back to a liquid to begin the cycle again.

Hussain and colleague David Brigham developed a transient numerical model capable of capturing the main effects of this cycle using different designs under different conditions.

Inputs into the model include exhaust mass flow rate and temperature downstream of the catalytic converter, ORC component size and geometry. Outputs include mechanical power, electric power, and system backpressure. The Ford team used an organic fluid rather than ater as the working fluid because of a number of advantages, including a very low freezing point.

The evaporator was a shell-and-tube type design with exhaust gas passing through the tubes. High side and low side pressures were assumed to be fixed; what varies is the mass flow rate of the working fluid depending on the amount of heat that is available.

They defined the Power Factor (PF) as the power generated by the ORC system divided by the vehicle accessory load need; i.e., a PF of 1 would mean that the power generated by the ORC waste heat recovery system fully met the accessory load demand, with a concomitant improvement in fuel economy.

Using the simplest ORC design (exhaust gas alone as the source of thermal input—i.e., no use of coolant fluid for preheating due to increased system complexity to support that), Hussain and Brigham found that for a 2.5-liter conventional engine in the non-hybrid Escape, the PF was 0.9 on the highway cycle and 0.513 on the city cycle. For the Atkinson-cycle 2.5L engine used in the Escape Hybrid, the PF was 0.98 on the highway, and about 0.32 in the city. In general, the power output during EPA Highway drive cycle is much higher than EPA City due to higher exhaust mass flow rate and temperature.

The ORC for a light-duty vehicle can generate enough electricity to partially offset and in some case almost fully offset accessory load on the vehicle. That obviously depends on the type of vehicle and the drive cycle. On the EPA highway drive cycle, it comes very close to meeting the electric load; on EPA city it is less, about one-third to one-half. For the hybrid it’s a little worse because the engine turns off, there is no exhaust gas. On the highway the hybrid is a little better because it is a little heavier vehicle, there is a little higher engine work, higher exhaust energy.

—Quazi Hussain

The Ford team also found that, contrary to their concerns, the backpressure with ORC waste heat recovery under this particular design was a little lower than the baseline system. The reason, they concluded it that in this particular design, the exhaust gases cool tremendously, resulting in a pressure drop. This lower backpressure will further boost the fuel economy gained by the electricity produced by the Rankine bottoming cycle.

Comments

dursun

so what model year do we see this on?

Nick Lyons

Hmmmm. For the sake of simplicity, perhaps you could replace a turbocharger (e.g. Ford's Ecoboost models) with an electric supercharger run off the energy generated your Rankine waste heat recovery system. You'd have hotter exhaust to work with for better thermodynamic efficiency, and if the Rankine cycle really did reduce back pressure, overall efficiency might well be higher. During part-load operation (highway cruising), the extra energy would mostly run accessory loads. This whole concept requires that most or all accessory loads be electrified, of course.

Dave R

The real question is: What kind of load do the accessories present in kW?

If I had to guess, it's well under 1kW unless you're running the AC - in which case the average use of AC is still around 500W if a well designed unit unless it's very hot out.

At 60 mph accessory draw is probably only about 5% total power consumption.

Nick Lyons

@Dave R: According to U.S.D.O.E (via Wikipedia:)

http://en.wikipedia.org/wiki/Automobile_accessory_power

...only 2% of energy goes to powering accessories. However, only 20% gets to the wheels. Now 20% of that 2% will go to drivetrain losses, leaving 1.6% to the wheels. If you can increase the energy to the wheels from 20% to 21.6%, that implys an 8% increase in fuel efficiency, so maybe this is worth doing.

Arnold

Surely this type of refinement is necessary if gains from the 3/4 of energy wasted to heat in conventional engines is ever to be realised. Nick's comment is a further extension.
No reason why the heater conditioner of BEV could not adapt a similar system as long as waste heat is available.

2 points to consider ar the bloomin obvious , what about the other 10 -20 KW lost? seeing a small 5% of potential.

That leads into the statement (in keeping with Auto industry practice of sitting on thumbs and doing as little as humanly possible) relating to a 'least complex, simple system'.
"no use of coolant fluid for preheating due to increased system complexity to support that")

That may please the finance dept for a day or two but the aim should be to

1: make a complex efficient system for cheaply through innovative cost effective design (genius). That would be necessary if this is going to see day anyway.
If successful, dividends from high volume low cost and an expanded market should keep them happy for many months!

2:Complete re design of ICE cooling system (oil and water including gearboxes) to channel heat into the recovery system.

Priorities are so different today compared to even 10 years ago when the above may have seemed a big ask. Today's R&D and manufacturing tools should be well able to perform the impossible as a matter of course.

Thomas Pedersen

Nick,

A turbine in the exhaust converts the thermal energy it extracts from the exhaust gas with 100% efficiency - minus friction losses in the bearings. A Rankine cycle of this kind would be happy to convert around 40%.

The back-pressure benefit comes irrespective of what means are used to cool the exhaust gas (including a turbine, although it happily eats the benefit itself).

Using an electrical compressor without turbine is ill-advised during continuous load, since the pressure in the cylinder when the exhaust valves open will be very high - without recovering this pressure.

For intermittent use of an electrical compressor, however, there is some merit to the idea of letting a Rankine cycle utilize the heat in the flue gas.

Bill

Interesting, I've always thought the future was a mainly Battery driven (24-36Kwh) car with a relatively small hydrocarbon fueled gen-set (20-30 Kw) output but recently I was thinking on the opposite end might be a car with a very efficient diesel engine, stop/start hybrid, flywheel regenerative braking, and a 2-3Kwh Scib type battery plug-in that would handle all the normal parasitic losses- of course the thought of what do you do if for some reason you can't recharge came to mind- some combination of solar cells, energy recovery shock absorbers, and having the flywheels generate some electricity, all seemed like good possibilities but energy recovery from the exhaust might be a more reliable addition.

ToppaTom

Good science - maybe worth the complexity, maybe not; but the waste (heat) from an ICE must be reduced by some - probably MANY methods.

Engineer-Poet

Given that the exhaust carries roughly as much energy as goes out the crankshaft, the efficiency of this ORC engine must be rather low. The NIST site will generate properties tables for R245fa up to about 226 C, but I don't have the time or inclination to try to extract enough data from it to calculate the efficiency of a candidate cycle. (OTOH, if someone wants to pay me...)

There's no reason to avoid a turbocharger with this thing. The turbine can capture the pressure drop, and the ORC boiler downstream of the catalytic converter can capture the heat. Using the heat from the engine cooling jacket and engine oil would boost output further.

ToppaTom

I suspect this system is much less cost effective than a mild hybrid in city driving.

For highway cruising it may be cost effective, even with a Diesel, or whatever, that can accelerate and climb long hills adequately, yet cruise with good efficiency.

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