Waste Heat Recovery: The Steam-Hybrid Prius

Photo of Joshua Butler, Richard Kallus, and Gwendolyn Zubatch Students: Joshua Butler, Richard Kallus, and Gwendolyn Zubatch

Sponsor: Joshua L. Butler

Date: Summer 2008

Requirements:

The primary goal of this system is to report a substantial increase in fuel economy through the vehicle's onboard fuel economy gauge during typical highway driving. Added weight lowers fuel economy and drivability, and space is limited by accessibility, so minimizing weight and volume is important to obtaining the best performance. The cost of the heat recovery system should be economically justified by fuel savings. The time commitment of an average automobile mechanic to install the system should be minimal, as should the number of irreversible modifications to the vehicle.

It is imperative that the heat recovery system be safe for both the driver and the vehicle. To that end, it must not induce excessive exhaust back-pressure that can cause engine damage and impair performance. Also, the maximum permissible steam pressure must be strictly limited to 150 psi by a safety relief valve.

Meeting these requirements and constraints has been a challenge given that many represent conflicting relationships, e.g., reducing steam pressure may negate fuel economy gains. However, these specifications provide a gauge with which to measure the viability of eventually introducing a production quality variant of this heat recovery system.

Problem:

In light of rising fuel costs, automotive fuel efficiency is an area of increasing interest to engineers. The focus of this project is the design and prototyping of a method to recover waste heat from a Toyota Prius exhaust system by producing steam that is used to generate electricity.

Solution:

The team designed and evaluated a conceptual prototype of a replacement exhaust system with heat recovery, electrical generation, and steam injection. The system consists of an improved exhaust manifold, a turbine-driven electrical generator, a catalytic converter, several heat exchangers, a water storage tank, and the original muffler. In this system, the open-loop water circuit generates steam from exhaust heat and injects it into the turbine. The recycled heat and the added mass flow of the steam produce significant increases in the turbo-generator's output.

With the Prius at idle speed with the air conditioner turned on, the steam easily reached 100 psi, thus proving the viability of steam generation from rejected exhaust heat. Dynamometer testing showed that the exhaust turbo-generator engaged at 18 mph and quickly reached over 40,000 RPMs. At 33 mph, we measured 77 watts of electrical power through an inefficient AC-DC converter and without the benefit of steam injection (the generator's direct output is expected to be far higher but was not measured). This proved the viability of an exhaust based turbo-generator in the Prius. Unfortunately, the generator's bearings seized due to extreme heat sensitivity, which prevented testing at highway speeds. Such testing, which will prove the viability of steam injection for this specific application and gauge the overall effectiveness of the system, is planned after replacement of the generator.

Although 77 watts is a minor gain (approximately 0.5 mpg), we expect far greater gains of approximately 5-8% when the turbine reaches its intended operating range, when steam injection is used, and when a more suitable generator is implemented. The next phase of the project will focus on developing a control system for the turbo-generator and is expected to be completed by December, 2008.

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