Project Readiness Package Rev 27 July 2011

Project Summary

Harvesting energy from renewable sources (solar, biomass, etc.) offers a method of providing power at remote locations using local resources. Photovoltaic systems are convenient in that they convert solar energy directly to electricity, but have the disadvantage that they do not operate at night or on cloudy days. Stirling cycle engines, coupled with an electrical generator, have been investigated as systems that can use focused sunlight as a power source as an alternative to photovoltaic devices. Overall system efficiency can actually outperform silicon based photovoltaics in many cases. In addition, although mechanically more complex than photovoltaics, Stirling cycle generators can use any heat source to produce electricity, such as solar radiation, geothermal or waste heat sources, or even simple combustion of waste biomass. This offers a degree of flexibility not matched by simple photovoltaic systems. We wish to demonstrate a small portable Stirling cycle electrical generator system that can be used to power small portable electronics using solar energy or something like a fire as a heat source.

Administrative Information:

Project Name: Solar Stirling Generator
Project Number: P12472
Project Track: n/a
Project Family: n/a
Parent Roadmap: n/a
Planning Term: Fall 2011
Start Term: Winter 2011
End Term: Spring 2011 / Faculty: Alan Raisanen (RIT)
Industry Guide: N/A
Project Customer: RIT
Project Sponsor: RIT
Project Budget: TBD

Project Context:

This project will involve design and construction of a prototype Stirling cycle electrical generator. Stirling engines are hot gas engines that use an external heat source to extract mechanical energy. A Stirling has a hot zone which is heated by a fire, hot water, sunlight, or other convenient means, and a cold zone which is cooled by some type of heatsink. Mechanical energy is extracted during the transfer of thermal energy from the hot zone to the cold zone with a pair of pistons. The Stirling thermodynamic cycle is illustrated below (figure from http://outreach.phas.ubc.ca).

(a)  The right piston is at top dead center, and the gas in the engine absorbs heat QH from the hot side at temperature TH at left. The gas expands isothermally, forcing the left piston downwards.

(b)  The right piston moves downward as the left piston reaches bottom dead center and reverses motion. Gas is transferred from the left cylinder to the right cylinder under constant volume conditions. Much of the heat contained in the gas is transferred to the regenerator, a wire mesh or other porous heat reservoir material at the center of the fluid path.

(c)  The left piston reaches top dead center, while the right piston moves upwards, compressing the gas in the system at temperature TC. A quantity of heat QC is removed from the gas to the cold reservoir at temperature TC to maintain a constant gas temperature as it is compressed.

(d)  The left piston moves back down as the right piston moves up, transferring gas from the right cylinder to the left cylinder at constant volume. Heat is re-absorbed from the regenerator mesh as it passes through the center of the system.

The working fluid in the Stirling engine is re-used for each stroke, so items like intake and exhaust valves found in internal combustion engines and steam engines are not necessary. Some mechanism must be implemented to keep the cylinders in synchronization, and is usually implemented with a crankshaft, cam, or gearing arrangement. Various configurations are possible with two or more cylinders, and many successful and compact engines have been implemented placing both pistons in the same cylinder. Stirling engines are popular among hobbyists, with many fully documented designs available on the internet.

The engine implemented by the student MSD team must be mated to some type of electrical generator to supply power to a portable electronic device, ideally through a UBS plug used to charge cellular phones and similar devices. Power conditioning and filtering of some type will be required, as the output of the generator will likely be highly variable. The generator could be a commercial device intended for the purpose, a repurposed DC motor of some type, or the students may choose to implement their own using magnets embedded in a flywheel with matching wire coils. Direct coupling of the generator to the engine, a belt drive, or a set of gears are all potential methods of connecting the generator to the engine.

The Stirling engine should be configured to operate from solar energy, with some type of collection system. A parabolic mirror or mirror array is a convenient method of concentrating solar energy. Other heat sources may also be tested, such as a fire or gas burner.

Top: solar-powered Stirling engine. Bottom: schematic Stirling engine illustrating Stirling cycle.

Customer Needs Assessment and Engineering Specifications:


Project Interfaces:

This project is a stand-alone task sponsored by RIT. This project will very likely lead to follow-on projects, so thorough documentation of the design and build process is crucial to the success of follow-on MSD teams.

Staffing Requirements:

The solar Stirling system’s various components interact very strongly. Although different engineers will retain responsibility for various subsystems, strong inter-team communication will be essential to ensure that all subsystems interact reliably. For instance, the electrical engineer will need to work closely with the mechanical engineers to verify that the mechanical load (torque) required by the generator at a given power (watts) will not overload the ability of the Stirling engine to maintain the desired rpm, and the overall thermal throughput of the system, defined by materials properties such as thermal conductivity, can support the required power level at the desired operating speed. None of these systems operates independently of the others.

Position Title / Position Description
Lead Engineer (ME/IE) / The Lead Engineer is a mechanical or industrial engineer responsible for maintaining project schedule, coordinating project tasks, and systems integration.
The lead engineer should have strong leadership ability and communications skills. The lead engineer will be responsible for establishing realistic compromise device architecture and engineering parameters to meet desired performance objectives. Basic familiarity with mechanical engineering concepts is required. The lead engineer should have taken the DPM course.
Mechanical Engineer I (ME) / The mechanical engineer I will be responsible for designing and specifying the mechanical components used in the Stirling engine focused on the piston / linkage /heat transfer sections. Basic familiarity with machine components such as seals, bearings, and linkages is a must. A working knowledge of heat transfer concepts will also be necessary. Fabrication experience using manual machine tools, welding, and/or CNC processes will also be helpful.
Mechanical Engineer II(ME) / The mechanical engineer II will be responsible for designing and specifying the mechanical components used in the Stirling engine with a focus on the electrical generator, mechanical linkages, and solar collection. Basic familiarity with machine components such as seals, bearings, and linkages is a must. A working knowledge of heat transfer concepts and electrical generators will also be necessary. Fabrication experience using manual machine tools, welding, and/or CNC processes will also be helpful.
Thermal Engineer (ME) / The Thermal engineer will retain overall responsibility for modeling and designing an efficient thermal process for the Stirling engine. The size of the solar collection mirror, heat flow through the engine, and modeling of temperature throughout the system will be critical responsibilities for this engineer. Strong knowledge of heat transfer concepts is essential, as well as a working knowledge of materials properties.
Electrical Engineer (EE) / The electrical engineer will be responsible for implementation of the electrical power generation system including filtering, rectification (if necessary), storage, and monitoring. Any electrical components necessary for a self-start feature will also be supported by the electrical engineer. Strong analog electronics knowledge will be essential.

Project Deliverables:

(a)  Solar collector

(b)  Stirling engine

(c)  Electrical generator

(d)  Electrical output in USB format

Item (a) does not count against the total 20 lb system weight specification. Students are free to use whatever size mirror required, within reason, so system electrical output should not be constrained by the available solar power input.

Project Constraints:

Usage of commercially available off-the-shelf components is encouraged wherever applicable. Budget will be available for purchase of these items or parts for their fabrication, within reason. Target system budget is $500. Implementation or modification of Stirling designs found in the open literature is also encouraged, rather than a complete “clean sheet of paper” design.

Required Faculty / Environment / Equipment:

Describe resources necessary to support successful Development, Implementation and Utilization of the project. This would include specific faculty expertise for consulting, required laboratory space and equipment, outside services, customer facilities, etc. Indicate if required resources are available.

Category / Source / Description / Resource Available (mark with X)
Faculty / Dr. Alan Raisanen / Overall project guidance / X
ME Faculty / Thermal and mechanical modeling / X
Environment / Test area / Setup of mechanical and electrical tests / X
Equipment / Machine Shop / Fabrication / Welding of mechanical components / X
Materials / metal / steel, aluminum, etc for structural materials / Purchase
Other / Metrology tools / Metrology instruments to verify system measurements / X (borrow)

Page 1 of 5