Viking 29 - A Thermophotovoltaic Hybrid Vehicle Designed and Built at Western Washington University
Dr. Michael R. Seal, Director
Vehicle Research Institute
Western Washington University
Viking 29 is being built under a U.S. Department of Energy contract by the Vehicle Research Institute (VRI) at Western Washington University and JX Crystals of Issaquah, WA to demonstrate a thermophotovoltaic (TPV) generator. The 10 kW TPV generator being developed for use in a vehicle makes use of gallium antimonide (GaSb) photovoltaic (PV) cells surrounding a central emitter heated by a compressed natural gas flame to 1700 Kelvin. The infrared photons generated activate the PV cells to produce electricity which maintains a charge in the battery. Preliminary emission testing has shown that this generator is 50 times cleaner than an equivalent internal combustion engine (ICE).
Viking 29 is an experimental hybrid vehicle built to demonstrate the eight cylinder TPV generator. This TPV generator was developed under a separate contract from the US Dept. of Energy. The 10 kW TPV generator is cleaner and quieter than current generators with similar power.
Figure 1 - Viking 29 Thermophotovoltaic Electric Hybrid
As a series hybrid electric vehicle, Viking 29 will need both a generator system (the TPV) and an electric drive system. Nickel cadmium (NiCd) batteries are used to provide load leveling. See Figure 2 for the Viking 29 system configuration.
Figure 2 - Viking 29 Systems Configuration
THE TPV SYSTEM - The Viking 29 TPV generator is a device making use of gallium antimonide (GaSb) photovoltaic cells sensitive in the infrared range. These cells are arrayed around a fuel-fired infrared emitter. The configuration chosen is for a tubular silicon carbide emitter, mounted vertically above the recuperator, (see Figure 3).
GaSb cells surround the central emitter and are mounted on water-cooled heat sinks. The cells are wired in series strings of 19 cells. Axially mounted multiple quartz tubes break up the convective heat transfer loop while still allowing free passage of the active photons. A counter flow recuperator is mounted below each of the eight cylinder units. The departing exhaust gases heat the incoming air stream to 1000 K before the fuel is admitted to the air stream.
The eight cylinder unit is 432 mm long x 864 mm wide x 696 mm tall. The unit is designed to run at full power until the batteries are within 10% depth of discharge, whereupon the burner shuts off automatically. As the noise when running is similar to that of a desktop computer, the driver should not find the operation and shut-off as annoying as that of an engine driven generator in a typical series hybrid vehicle.
PV cells develop maximum efficiency at a specific point on the voltage-amperage curve. If voltage rises too high current drops precipitously and vice-versa. Typically maximum peak power trackers either buck or boost voltage from the input to output. The units made for the VRI by Xantrex not only stabilize the cell arrays at their maximum peak power but also raise the 32 V input voltage to 360 V needed to charge the battery.
Figure 3 - The TPV 8 Designed for Viking 29
The fuel tank for compressed natural gas (CNG) storage is mounted just ahead of the front suspension bay. A carbon filament wrapped polyethylene lined fuel tank rated to 220 bar provides the fuel for the TPV system.
A three-stage regulator reduces pressure to 1 bar, whereupon it is introduced into the combustors in the TPV burner unit. Air is introduced into the bottom of the recuperators by two squirrel cage fans driven by pulse width modulated brushless DC motors. As the air passes through the counter flow heat exchanger, it is heated by the departing exhaust gases to near combustion temperature. Fuel is added just prior to the combustion zone. As PV cell temperature should not exceed 900 C they are cooled with bottom to top water flow driven by an electric pump. The water is cooled by two large single core radiators, which are mounted nearly horizontal at the rear of the car. Large ducted axial flow electric fans provide airflow when the car is stationary.
ELECTRIC DRIVE SYSTEM - A 53 kW Unique Mobility motor and controller was chosen to power Viking 29, (see Figure 4). This motor offers 92% efficiency to the drive axle through most of the operating regime. The motor is mounted end on to a single dry plate clutch assembly running in ball bearings in a separate housing designed to remove all thrust loading from the electric motor. The clutch assembly is mounted end on to a transversely mounted, four speed, wide ratio transaxle mounted between the rear wheels of the vehicle. Driveshafts with inner and outer CV joints take the drive to the rear wheels.
Figure 4 - 53 kW Brushless DC Electric
Motor Body and Chassis Construction
Because the electric drive system running at 360 volts DC could provide a degree of risk in a chassis constructed of metal, a fiberglass composite monocoque structure was chosen for the vehicle. The battery boxes running along both sides of the car provide much of the torsional stiffness through the passenger bay. Many small "Allen head" screws are used to attach the tops of the boxes to the car to ensure chassis torsional stiffness. Removal of the covers for servicing is not difficult with a power screwdriver. The NiCd batteries used on the vehicle have a 10 year life expectancy and also have a very long maintenance interval. The batteries contain 10 kW hours at 360 V and power the 32 kW continuous duty, 53 kW peak power, brushless DC motor and controller.
For the prototype inner structural panels, a vacuum-bagged lay-up of biaxial roving in a vinyl-ester matrix was used to provide a high strength to weight ratio. As the outer skin requires a class A finish, surfacing matt and veil is used to provide a sandable surface before the outer skin is primed. All of the air ducts have been made structural to provide increased chassis stiffness, (see Figure 5).
Figure 5 - Viking 29 Monocoque Chassis
The chassis incorporates waffle panels to tie the inner and outer skins together. These waffle panels have been used throughout the car. The channels formed by the waffle shape are used for ducting to the cabin, heater, demister and cabin ventilation systems. Ventilation ducting for batteries, motor and motor controller is also accommodated this way. The motor and controller are water cooled and require air ducting to the radiator. Small brushless DC motors drive the fan and water pump for the electric cooling system.
The CNG fuel tank is used as a reaction bulkhead for the aluminum honeycomb deformable structure in the nose of the vehicle. Although not tested in a barrier crash, a 56 km/h barrier crash should be survivable in Viking 29. The VRI previously crash tested Viking 6, which utilized a similar configuration. The stiff battery boxes provide substantial side crash protection by forcing the impacting car to deform its nose structure.
Throughout the vehicle, carbon fiber reinforcement has been used only where space constraints do not permit enough stiffness with S glass reinforcement. In series production, the vehicle would use placed reinforcement reaction injection molding to speed cycle times and reduce costs. Semi-gull wing doors are used to provide improved access made necessary by the relatively wide and deep doorsills, which contain the batteries. The doors use a hidden hinge ball joint at the front lower corner of the door and a very small external hinge point at the upper-inner windshield corner. As the diagonal, air inflatable, passive restraint safety belt attaches to the door, it is necessary to provide a high strength locating pin where the door edge connects to a strong point in the door surround. The bumpers of the vehicle incorporate a stiff and sturdy fiberglass reaction beam and a sacrificial bumper skin that is easily and inexpensively replaced in the event of damage.
Placement of headlights has always been a problem on the Viking series of streamlined cars because the low and sloped front body section does not provide a mounting position for conventional lights that meets the minimum legal height required above the road surface. In the past, pop-up headlights were used which degrade vehicle aerodynamics. Mounted headlights behind the lower edge of the windshield to reduce drag have also been used. As the federal standard minimum height has been reduced from 610 mm to 560 mm, and as credit card size sealed beam headlights have become available, the headlights on Viking 29 will be mounted under clear acrylic shields in the front fenders. The centerline of the composite headlight meets the 560 mm minimum height with the low beams mounted above the high beams to ensure minimum dazzle on low beam as the headlights are dipped for oncoming traffic when using high beams. The rear lights are also lightweight sealed units mounted in round tunnels just above the rear bumper.
Only the driver's seat adjusts for tilt and front to rear position. The seat pivot point is well forward of the driver's H point. The H point rises as the seat is tilted forward which raises the shorter drivers eye height and simultaneously brings the driver's shorter arms nearer to the steering wheel. Instrumentation consists of a speedometer and tachometer mounted in a binnacle behind the steering wheel. The ammeter, voltmeter, and state-of-charge meter are mounted in multi-function instrument in the center console. The CNG fuel pressure gauge is also mounted in the center console.
The interior surfaces are finished in polished carbon fiber along with carpeting. The seats are finished in leather. The gearshift lever is situated on the central tunnel and is connected to the transaxle with two push pull cables. The handbrake lever is mounted under the instrument cluster and actuates the rear disc brake calipers with pull cables. The heater unit mounts ahead of the toe board and behind the fuel tank. This unit also provides hot air to the demister and face ducts through structural passages that are part of the monocoque structure.
SUSPENSION AND HANDLING
Short and long arm wishbone suspension links have been fitted at the front and rear of the vehicle. Direct acting coil-over dampening units have been utilized at all four corners. Front and rear anti-roll bars provide easy tuning of oversteer-understeer. Four non-power assist disc brakes provide short stopping distances with low pedal effort. To reduce lost motion in the brake system, the master cylinder pedal assembly is rigidly mounted to a stiff wall section provided in the unibody structure. Teflon lined, steel armored, flexible brake lines remove another source of lost motion due to swelling of flexible lines (when under pressure). Minimum loss of motion because of flexible components means a high multiplication factor can be used without bottoming the brake pedal. An adjustable brake bias bar is used to tune the front/rear braking ratio. There is a knob mounted on the dashboard that provides easy adjustment of the bias bar.
To reduce aerodynamic drag, the body shape was derived from a series of wind tunnel models developed at the VRI during the last several years. The CxA for the model chosen is .25 and the frontal area is 1.34 m2 for a CxA of .338. See Figure 5 for the power requirements of Viking 29.
As rolling drag is strongly dependent on vehicle weight, battery size becomes an important determinate. A 10 kg increase in battery mass will probably result in a 50 kg increase in vehicle weight as the supporting structure, suspension, bakes and motor must be increased to provide equivalent performance. If the battery is too small, the electric motor will not be able to draw full power at 75% depth of discharge (DoD) and performance will suffer as the motor can draw up to 53 kW from the system. As the TPV unit can only supply 10 kW, 43 kW must then come from the battery. Fortunately, the NiCd batteries used in this car maintain high power density to 90% DoD. Therefore, it was decided to use 10 kW hours of battery at a weight of 270 Kg.
Figure 6 - Viking 29 Projected Power Requirements
The TPV generator used as an auxiliary power unit (APU) in the Viking 29 automobile demonstrates that an extremely clean and quiet method now exists to continuously charge the on board batteries for a series hybrid car. Unburned HC and CO emissions are low enough to qualify as a virtual ZEV, but NOX emissions will probably require a reducing catalyst to reach virtual ZEV standards. Further work needs to be competed on the TPV 8 generator to optimize its performance. Matched emitters and improvement of view factor for the PV cells to increase power density would result in improvement to the overall system efficiency.
1.The US Department of Energy - Funding for vehicle design and construction was through a SBIR contract to JX Crystals. 2.The engineering staff and students of the Vehicle Research Institute at Western Washington University, including Nels Sorenson for detail design and fabrication of the monocoque structure and Bill Connelly, Gavin Campbell and Edward West for the TPV design and fabrication. 3.Lewis Fraas and Engineers at JX Crystals who provided the gallium antimonide PV cells and filters for the TPV unit. 4.Xantrex Inc. of Burnaby, B.C. for the design and construction of the peak power trackers.