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Simulation study suggests ORC waste heat recovery system could deliver potential 7% improvement in fuel consumption in a PHEV on highway

10 October 2012

A simple Organic Rankine Cycle (ORC) waste heat recovery system could recover up to 10% of engine waste heat under highway driving conditions, corresponding to a potential 7% improvement in fuel consumption in a light-duty plug-in hybrid electric vehicle (PHEV), with low penalization of the added weight to the vehicle electric range, according to a simulation study by researchers from the University of Stuttgart and the Ohio State University.

Their paper was presented at the ASME Internal Combustion Engine Division 2012 Fall Technical Conference by Marcello Canova, assistant professor at OSU; lead author was Philipp Skarke, from the University of Stuttgart Institute for Internal Combustion Engines and Automotive Engineering.

The analysis used the Ohio State University EcoCAR, a student prototype PHEV, as the basis for the preliminary fuel economy evaluation. Starting from a energy-based powertrain simulation model validated on experimental data from the PHEV, the researchers conducted a first- and second-law analysis to identify the potential for engine waste heat recovery, considering a variety of driving cycles and assuming the vehicle operating in charge-sustaining (HEV) mode.

They then designed a quasi-static thermodynamic model of an Organic Rankine Cycle (ORC) optimized to fit the prototype vehicle. Simulations evaluated the amount of energy recovered by the ORC system, considering both urban and highway driving conditions.

Internal combustion engines have been significantly improving in the past decade, after the adoption of technologies such as Gasoline Direct Injection, mild electrification, downsizing and turbocharging. This evolution has led to the point where achieving significant fuel savings through the introduction of new technologies has become difficult.

On the other hand, it is well known that in an internal combustion engine up to 35% of the energy developed by the fuel combustion is dissipated in the exhaust gases. Therefore, recovering a significant portion of the exhaust gas energy and convert it into mechanical power could be a highly effective solution to improve engine efficiency and to reduce CO2 emissions, without applying dramatic modifications to the existing engine hardware.

Among the different methods for engine waste heat recovery proposed in the literature, Organic Rankine Cycles (ORC) have the highest chance to couple effectiveness and technological readiness for the application to internal combustion engines of both conventional and hybrid vehicles.

—Skarke et al.

A number of prior studies have shown fuel efficiency improvements of up to 10% using a Rankine cycle for waste heat recovery—as long as packaging, cost and heat rejection rate can be balanced, the authors noted.

Osuphev
The OSU EcoCAR PHEV powertrain. Skarke et al.Click to enlarge.

The OSU EcoCAR prototype vehicle features an extended-range electric architecture with a downsized 1.8L ethanol (E85) engine coupled to a 82 kW motor/generator through a dual clutch system, as well as an 103 kW electric motor on the rear axle. With this architecture, the vehicle is able to operate in series and parallel mode, allowing for up to 40 miles of pure electric drive.

Osu2
Exhaust gas exergy map of EcoCAR engine. Operating points correspond to typical driving conditions. Skarke et al. Click to enlarge.

They determined that the average rate of energy lost to the exhaust when on the NEDC is about 3.9 kW and about 27 kW at constant velocity of 65 mph. However, they noted, this value does not represent the available amount of energy that can be recovered by Rankine cycle. The exergy flow associated to the exhaust gas energy is much lower, specifically about 1.5kW for the NEDC cycle and about 13 kW at highway conditions. This value represents the upper limit for waste heat recovery systems to convert thermal energy from the exhaust gases into mechanical energy.

There are a set of operating conditions where the vehicle actually can operate fairly frequently particularly on extra-urban and highway conditions where conditions for energy and availability are suitable for inserting a waste heat recovery system in the exhaust.

—Marcello Canova

Using a simplified ORC system—evaporator, condenser, pump and expander (scroll type)—they modeled the cycle using R−245fa as the working fluid. They selected R-245fa after evaluating about 50 different working fluids as offering the best compromise in terms of pressures, system costs, and environmental effects.

Summary of effects of waste heat recovery on different cycles, Skarke et al.
  FUDS NEDC US06 CONST65
Power from ORC 0.38 kW 0.40 kW 1.46 kW 1.90 kW
Δ in electric range -1.9 km -2.1 km -0.8 km -0.7 km
ηBASIC 27% 27.2% 31.4% 33.9%
ηORC 26.6% 28.5% 32.5% 36.2%
Δ in fuel consumption -1.1% -4.8% -3.3% -6.9%

They found that an ORC system could recover approximately 1.7% of the fuel energy on the NEDC cycle. In this case, the low engine speed and torque reduce the energy and availability in the exhaust gas, resulting in very low energy conversion by the ORC. At steady-state highway conditions (65 mph), the simulation results show that the ORC is able to recover approximately 2.4% of the fuel energy for storage as electric energy in the battery.

They also found that the effect of the additional weight of the ORC system on the electric range is marginal, particularly at high speed.

The simulation results show that the ORC is able to produce up to 1.9 kW of electric power on highway driving conditions, which corresponds to a potential 7% improvement in vehicle fuel consumption, when the recovered energy is then utilized in the vehicle. The effects of the added weight have also been explored, showing very minor penalties on the fuel economy and all-electric range at urban conditions.

In summary, the simulation results presented in this study show that the application of ORC systems for engine waste heat recovery represent an effective way to achieve significant fuel economy improvement at highway operating conditions for hybrid and plug-in hybrid electric vehicles. As part of the future work, the benefits of the ORC will be evaluated accounting for the engine warm-up transient and the thermal dynamics of the aftertreatment system.

—Skarke et al.

Resources

  • Philipp Skarke, Shawn Midlam-Mohler, Marcello Canova (2012) Waste Heat Recovery From Internal Combustion Engines: Feasibility Study On An Organic Rankine Cycle With Application To The Ohio State EcoCAR PHEV. (ASME ICEF2012-92018)

October 10, 2012 in Fuel Efficiency, Hybrids, Plug-ins, Waste Heat Recovery | Permalink | Comments (9) | TrackBack (0)

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Comments

Seven percent.  A hybrid driving a lifetime 150,000 miles at 50 MPG would burn 3000 gallons of fuel; 7% is 210 gallons.  This would have to cost less than 210 gallons of fuel to pay off in 150k miles, and would probably need to pay off in the first 5 years (~65,000 miles) to be attractive to consumers.

The prospects for commercial vehicles seem a lot better.

I'd say it was worst than that, they're talking about PHEVs here: In a Plug-in HEV a good portion of those 150,000 miles would be done in electric mode so the savings in fuel would be less.

Given an average trip distance of 16 miles, and plugging in only at home, even an all electric range of just 20 miles could cut the savings in half.

If cost matters.

I agree with E-P: might make financial sense for long haul trucks, etc., which spend most of their time cruising at speed and rack up so many miles the investment would amortize reasonably. Adding this to a plugin hybrid is crazy; just how complicated and expensive do I want my daily driver to be?

Military vehicles, often using $100/gal fuel, would benefit even more? Diesel locomotives, heavy machinery, ships are other potential users.

Hunh. All good points about the diminishing returns. Don't you think that points to simple themocouple devices as the best way to recover some heat energy without too much cost?

Thermocouple devices are simple but what evidence do you have that they would be cheaper? And last I heard they were less efficient - about 5% instead of the 10% mentioned here.

Thermocouples would certainly be simpler and more reliable.

I suspect that any viable bottoming-cycle engine suitable for a PHEV requires integration with other vehicle systems, meaning much greater out-of-the-box thinking.

Heat is energy.

Converting heat to electricity has been done many different ways for a long time. An on-board light weight solid states (improved higher efficiency) converter to convert excess heat to electricity would be ideal for small hybrid vehicles.

Large vehicles such as trucks, buses, locomotives, ships etc could use heavier more efficient converters similar to the one described above.

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