DOE Makes Draft Plug-In Hybrid Electric Vehicle R&D Roadmap Available for Comment

28 February 2007

Preliminary schedule for PHEV work. Click to enlarge.

The DOE Office of FreedomCAR and Vehicle Technologies (FCVT) has developed a Draft Plug-In Hybrid Electric Vehicle R&D Plan to accelerate the development and deployment of technologies critical for plug-in hybrid vehicles. (Earlier post.)

This plan addresses all aspects of R&D from technology assessment through production readiness. It describes the necessary development of batteries and electric drive components, including near- and mid-term R&D activities as well as long-term fundamental research.

It also relies on analytical studies to quantify the potential national benefits of PHEVs, and the monitoring of global policy and technological developments to find opportunities for beneficial collaboration and stay aware of the latest advances from around the world.

DOE is proposing two generations of technology development actions in addition to long-term R&D. The agency expects the resulting component developments, when integrated and validated in a vehicle environment, to produce necessary data for technology transfer and production readiness decisions by industry.

FCTV is inviting interested parties to review the draft PHEV R&D Plan and comment. FCVT has targeted a release of the plan by April 20.

Battery performance requirements versus vehicle application. A PHEV battery will experience both deep discharges like an EV (i.e., the large swings in SOC shown in green) and shallow cycling necessary to maintain the battery for power-assist in charge sustaining HEV mode (in yellow). Click to enlarge.

Lithium-ion Batteries. DOE has worked on developing Li-ion battery technology for years in partnership with the auto industry in areas such as technology development, applied research, and focused fundamental research. While this work is directly applicable to the PHEV R&D activity, PHEV requirements are more complex.

Battery requirements are extremely sensitive to vehicle design (i.e., all-electric or charge-depleting range) and a single PHEV design has not been (and likely will never be) agreed upon. This means that battery development must cover a range of requirements from providing essentially the same functionality as in today’s hybrids (sharing power demands with the engine) to providing all the vehicle propulsion power as well as accessory loads (that could double the demand).

The requirements for a PHEV battery combine those of an electric vehicle (EV) which only depletes the battery during operation (i.e., “charge depleting only”) and a typical HEV in production today that maintains the battery state of charge within bounds (i.e., “charge sustaining”). In addition to the stringent duty cycle, the power-to-energy (P/E) ratio (an influential design parameter) is specific to each vehicle application.

—PHEV R&D plan

Acknowledging the uncertainties, DOE is developing near-, medium- and long-term goals for battery development.

• Near term: 10 mile all-electric range (AER) for a mid-size SUV, implying a 5-10kWh battery with approximately 40 kW peak power, costing no more than $4,000.

• Medium-term: To be established as PHEV requirements solidify.

• Long-term: 40 mile AER for a mid-size passenger car, and the same $4,000 system cost.

Spider chart comparing Li-ion and NiMH to DOE targets. Click to enlarge

While Li-ion batteries are making significant progress and offer significant advantages in higher specific energy and power than NiMH batteries, cost remains an challenge and durability with a PHEV duty cycle remains a question.

In approaching Li-ion battery development for PHEVs, DOE is using its approach applied to the development of NiMH batteries in the 1990s: highly interactive fundamental and applied R&D.

• Phase 1 has national laboratories and universities performing exploratory research on materials with long-term potential to improve Li-ion technology.

• Phase 2 has the national laboratories and industry/USABC focusing on cell development—s new, higher energy materials in appropriately sized cells/modules. This includes the Li-based cell configurations of Enerdel, CPI/LG Chem and A123 systems.

• Phase 3 has industry/USABC) design and build battery systems for evaluation in the laboratory and validation with industry (suppliers and OEMs) within their development environment to accelerate technology transfer. The latest generation of Li-ion batteries by Johnson Controls-SAFT is presently undergoing tests at ANL.

• Phase 4 concentrates on cost reduction through the refinement of the battery design and materials in concert with the processes and equipment required for low-cost volume battery manufacturing. Earlier Li battery developments by SAFT have entered this stage of development as well as ultracapacitors (by Nescap and Maxwell) and low-cost separators (by Celgard, UMT and AMS).

Battery R&D schedule. Click to enlarge.

The DOE/USABC will release a PHEV battery solicitation in Q2 FY07 and expects to begin benchmarking or proof of concept contracts by early spring 2007. Similarly, the applied and focused fundamental research activities are planning to ramp up work on higher energy battery materials and cells following approval of the 2007 DOE budget.

Power electronics and electric machines. (PEEM) The DOE notes that PHEVs do not present any additional technical barriers for electric drive components since the power requirements fall within the spectrum of previously considered hybrid and electric vehicles.

In examining the different options for a PHEV architecture (parallel power-sharing and series), DOE notes that the parallel power-sharing configuration (e.g., today’s production hybrids) with a modified control strategy to allow battery charge depletion for PHEV application is likely be the most cost-effective and have the least impact on the motor and power electronics. However, it also notes, because of cost, mass and packaging considerations, performance may be compromised.

In a series hybrid configuration such as the Volt, full-function electric traction components (more than twice the power as in current production hybrids) are required for full-time electric drive. This exacerbates electric propulsion system cost, but the smaller engine-generator system (used to extend the range) and the elimination of the mechanical drive should cost less than the conventional engine and driveline components. And from a longer term perspective, development of higher power electric drive components for PHEVs will benefit fuel cell vehicles where all traction and accessory power will be supplied electrically.

DOE’s PEEM activity is developing technology to meet the requirements of a variety of hybrid and electric propulsion (including fuel cell vehicles). The broad spectrum of applications and propulsion system configurations necessitates multiple technology development paths that cover components as well as integrated systems (such as the integrated motor-inverter design under development). Work in all areas is focused on improving performance, reducing volume or lowering cost.

DOE has four primary development goals in the PEEM area, including PHEV-specific activity:

• Motor R&D. Decreasing the cost and size of electric motors requires increasing speed (i.e., higher power from smaller machines) and/or redesigning for increased material utilization or lower cost materials.

  Ongoing FY07 PEEM R&D activities are focused on high speed 16,000 rpm permanent magnet motors that achieve field weakening within the structure of the motor and eliminate the need for a DC/DC boost converter. Motor speeds up to 20,000 rpm are being explored.

  Several motor designs with system-level savings for PHEVs are being explored. A motor concept with controllable winding configurations is being developed that enables high starting torque with considerably less power from the battery, potentially lowering battery cost and weight. A traction motor with a substantially higher CPSR than that required for an HEV or FCV would enable reductions in gearing that will provide vehicle cost and weight reductions.

• Power Electronics R&D. Reducing the cost and size of the power electronics requires addressing the (large) capacitors, waste heat (more tolerant components, reducing heat or dissipating it more efficiently) and new designs that reduce parts count by integrating functionality.

  A current source inverter (as opposed to a conventional voltage source inverter) is being designed and developed to eliminate the DC bus capacitor by using inductors. A portfolio of projects is being pursued that spans a range of cooling temperatures.

  A long term focus, possibly in conjunction with higher temperature wide bandgap semiconductor components such as SiC, is the use of high temperature, air-cooled systems. Such an approach would insure that technologies are being developed for all potential future vehicle platforms (HEV, PHEV, and FCV).

  Several efforts are being directed specifically at PHEV applications, including determining the potential to use the existing HEV inverter to fulfill the plug-in charging function on the vehicle. A bidirectional DC/DC converter is being explored to reduce cost and volume.

• Thermal control R&D. The objective is to maintain the electronic devices at operating temperatures that will ensure performance and reliability over the life of the vehicle while reducing system cost, weight, and volume.

• Integrated Systems Development. Efforts are being initiated to integrate the motor and inverter, focusing on development of a system that will accommodate the spectrum of performance requirements of internal combustion engine hybrid and fuel cell vehicles. The resulting range of requirements encompasses the needs of envisioned PHEVs.

PEEM Development targets. Click to enlarge.

DOE is also considering other vehicle efficiency technologies in the R&D plan. DOE does not consider vehicle-to-grid (V2G) power flow as a short-term enabler for PHEV technology, although it does acknowledge that V2G could have system-level benefits. With respect to PHEV-grid interaction, therefore, the DOE is focusing on the specific requirements of the interface for vehicle charging and the impact of charging on the grid and utilities.

DOE is requesting comments via email (addressed to AAT@ee.doe.gov) on this draft plan no later 28 March.

(A hat-tip to Mark!)

Resources:

• Plug-in Hybrid Electric Vehicle R&D Plan (External Draft, Feb 2007)