What Matters: April 2007
Powering Up With Better Hybrids
By Rit Booth '74
Hybrid car. Photo: ©iStockphoto.com/Ian Francis.
Mention hybrid today and people think of cars with electric motors that save energy. But I believe other types of hybrid systems can save much more energy and that their potential is being overlooked.
I am specifically interested in flywheel-mechanical hybrids, which like all hybrids (if we don't include mules), combine two different energy storage devices: one reversible and one nonreversible.
Reversible energy storage devices are capable of recovering energy that a machine would otherwise dissipate as heat, which is obviously an advantage and can help reduce energy consumption. The percentage of energy that is recoverable ranges from very little to more than 80 percent depending on the machine and how it is used. Non-reversible energy storage devices cannot recovery energy.
A locked-torque test stand utilized for torque feedback algorithm validation.
Unfortunately, all existing reversible energy storage devices have low energy density compared to the most common non-reversible energy storage device, a hydrocarbon fuel tank. Low energy density means that a machine relying solely on any of the existing reversible energy storage devices (electrochemical batteries, flywheels, pressurized gas) must be charged much more often than a hydrocarbon-fueled machine must be refueled. By combining the two types of storage devices, energy may be recovered while retaining infrequent fueling intervals.
I am a proponent of flywheel-mechanical hybrid power systems which utilize a flywheel as the energy storage device and a mechanical continuously variable transmission (CVT) to transfer recovered energy between the machinery and the flywheel.
Flywheels, in both mechanical and electrical systems, were considered a viable energy storage alternative to electrochemical batteries leading up to the California Zero Emissions Vehicle (ZEV) mandate of 1990. Although ZEV was subsequently repealed, flywheel research has led to commercially available flywheel electrical energy back-up systems.
As the prevailing paradigm shifted from ZEV to hybrids, electric hybrid power systems utilizing electrochemical batteries, such as in the Toyota Prius, became dominant and are now under consideration for a variety of other machines in addition to automobiles.
While I believe the broadly defined hybrid power system concept can be a very significant tool for reducing worldwide consumption of energy, I also believe that the potential for flywheel-mechanical hybrids to save substantially more energy than electric hybrids in a variety of applications is being overlooked.
Electric vs. flywheel
There are a number of basic problems with electric hybrids, particularly hybrids utilizing electrochemical batteries, that are not widely reported. These problems are not inherent in the hybrid concept as illustrated using flywheel-mechanical hybrids as an example.
- To recycle recovered mechanical energy, an electric hybrid converts the form of the recovered energy four times (mechanical to electrical, electrical to chemical, chemical to electrical, and electrical to mechanical) resulting in low overall recovery efficiency. A flywheel-mechanical hybrid does not convert the form of the recovered mechanical energy.
- The cost per unit of power (HP, kW, etc.) is relatively high for the dynamotor1 and power electronics comprising the energy recovery subsystem of an electric hybrid. Practical electric hybrids, then, will be sized to recover energy at a rate (i.e., a power level) much lower than the maximum rate at which recoverable energy can be generated. This means energy will be wasted that could otherwise be recovered. Flywheel-mechanical hybrids may be designed to economically recover energy at the maximum rate generated.
- Although electrochemical batteries have approximately the same energy density as other reversible energy storage devices, electrochemical batteries have relatively low power density. Therefore, the limitation on energy recovery is not only the cost of the dynamotor and power electronics but also the percentage of the machinery mass and volume that may be dedicated to the electrochemical batteries. Compared to electrochemical batteries, flywheel batteries have equal energy density and much higher power density.
- Electrochemical batteries can only be charged and recharged a limited number of times, resulting in the life of the batteries being shorter than the life of the machinery. An energy storage flywheel may be engineered to have the same life as the machine into which it is incorporated.
- Ecological disposal of some electrochemical battery materials is costly. The materials in a flywheel-mechanical are commonly recycled.
Due to these problems, machinery with electric hybrid power systems cannot be justified by financial payback alone. Although subjective factors currently make ownership of hybrid automobiles attractive, these same factors have less weight in the purchase and implementation of commercial and industrial vehicles and machinery. Extension of the hybrid concept beyond niche automobiles will require either huge technological breakthroughs in dynamotors, power electronics, and electrical batteries or another paradigm shift, such as to the flywheel-mechanical hybrid
Development of flywheel-mechanical hybrid power systems requires straightforward engineering problems to be solved, not huge technological breakthroughs. Development is required in three main areas: CVT control, sealing of the flywheel vacuum enclosure, and dynamics of flywheel-machinery interaction.
CVT control is the focus of my research. When flywheels were originally being considered for zero emissions vehicles, a variety of control methods were proposed for flywheel-mechanical energy systems, but all were based on the operator directly changing the CVT ratio to change the machinery speed2. With the CVT ratio not exactly equal to the speed ratio between the flywheel and the machinery, slip must occur somewhere resulting in low efficiency and wear.
I have been granted a patent3 for a unique method to control power transfer between machinery and a flywheel utilizing a CVT for highly efficient recovery of surplus mechanical energy. This patent describes a closed loop control system in which the CVT ratio is continuously determined by torque feedback. This system both regulates torque to equal operator input and, on average, maintains the CVT at the synchronous ratio. Engineering development is required to implement the patented algorithms into commercially available transmissions.
Flywheel-mechanical hybrids will utilize high-strength metal flywheels rather than composite flywheels to allow operation within speed ratings for commercially available shaft seals and fixed speed reduction gearing. Assuring the durability of these components in a unique new application will, of course, require development.
Concerns about flywheel-mechanical hybrid power systems relate to the dynamics of the flywheel in a vehicle, whether affecting the handling of an automobile due to gyroscopic effect or the durability of the flywheel bearings in an off-road commercial vehicle. These seem to me like engineering challenges that do not require huge technological breakthroughs comparable with those required to solve the problems with electric hybrids. However, solutions to vehicle dynamics problems could wait as flywheel-mechanical hybrid power systems are being developed and implemented in numerous potential stationary applications.
A tremendous amount of money is spent worldwide on electric hybrid research, but the huge technological breakthroughs that will be required before electric hybrids have any significant impact on energy consumption makes the size of these expenditures questionable. Research dollars also need to be invested in exploring alternative hybrids, including flywheel-mechanical hybrids.
Notes
1A dynamotor may act as either a motor or a generator.
2A typical CVT ratio control system is described in U.S. patent number 3,672,244; "a foot pedal…connected to said infinitely variable transmission means to decrease the ratio of the transmission upon depression of the foot pedal and increase the ratio upon release of the foot pedal."
3U.S. Patent 6,120,411, Control Methodology for Inertial Energy Storage Devices, Richard A. Booth, Jr. '74, September 9, 2000.
About the Author
Richard (Rit) Booth '74 is currently the president of the MIT Club of Wisconsin. During his undergraduate years, when not studying mechanical engineering, he often rode bicycles or motorcycles. Upon graduation, he moved to Wisconsin and eventually found his way to Harley-Davidson where he was instrumental in the iconic company's technical about-face. (See Harley-Davidson Evolution Motorcycles, MBI Publishing Co., 2001). He has also contributed to another Wisconsin two-wheeled legend, TREK, and served as adjunct instructor at the University of Wisconsin–Milwaukee. He's now a successful masters bicycle racer. If anyone knows about energy conservation, it is masters bicycle racers! He can be reached at ritbooth@alum.mit.edu.
What Matters is a guest opinion column written by a different MIT alumnus or alumna. The views expressed are entirely those of the author and do not necessarily represent the views of the Alumni Association or MIT. Interested in writing a column? Email whatmatters@mit.edu.
