The eventual shift from our dependence on fossil fuels to sustainable, alternative energies has been exemplified by personal transportation. We rely on the high energy density of petroleum products to power our vehicles, but the rise of hybrid and electric vehicles in recent years has shown hints of the future to come. However, the shift has been gradual, mostly due to increased costs for only limited efficiency gains.
Hybrid electric vehicles (HEV) combine the flexibility of the ICE with the efficiency of the electric motor. Regenerative braking, recapturing the energy normally lost during deceleration, adds to the vehicle's range and efficiency, but current battery technology's power density is a critical limiting factor in HEV appeal. Vehicular kinetic energy can be dissipated through rapid braking at a rate that far exceeds a battery’s ability to charge. In the case of typical lithium ion batteries (the current highest energy density storage device that can be used safely), about 60% of a vehicle’s kinetic energy is still lost during braking.
The INSTAR (Inertial Storage and Recovery) system helps solve this fundamental gap in current battery technology by turning a HEV into a Triple Hybrid Electric Vehicle. Using a mechanical flywheel energy storage system (FES) to supplement the chemical batteries, INSTAR temporarily captures the excess energy produced during regenerative braking before releasing it to the chemical batteries at a slower, acceptable rate. Instead of capturing only 40% of the available energy, more than 75% can be recaptured, leading to overall efficiency gains of 5-10% for only minimal added weight and cost.
The advantage of a flywheel is that it can absorb and discharge energy much more quickly than any chemical battery. INSTAR combines the FES with a custom-designed DC brushless motor to efficiently store the excess braking energy in the mechanical flywheel before being returned through the same DC brushless motor as electricity to the chemical batteries. Due to the low duty cycle of the FES, storing the braking energy for only minutes at a time, INSTAR has been designed from the very beginning to keep costs low. Softer vacuums and mechanical bearings can be used, avoiding expensive seals and magnetic bearings, while still maintaining high storage efficiency. The DC brushless motor is a unique, variable-wound motor allowing energy recovery from the FES through its entire RPM range, even when the back EMF is low at slow speeds.
The INSTAR system works for all HEV and fully-electric vehicles, weighs 35kg, and is only slightly larger than a football. The system is modular, housing the FES and DC motor within one simple containment system, allowing for ease of construction, maintenance, and installation. Gyroscopic effects are neglected by the vertical orientation and purely electrical inputs and outputs mean placement is non-crucial. A prototype INSTAR system will be installed on a modified electric go-kart before being scaled up to a hybrid, 100 mpg Smart Car with in-hub electric motors. These two demonstration platforms will showcase the INSTAR system and provide a glimpse of the future to come.