Major Qualifying Project: Green Stirling Engine Power Plant
Fall 2014-Spring 2015 - Worcester Polytechnic Institute
Advisor: John Sullivan
Team: N. Costa, B. Donnan, D. Ephraim, E. Graff, A. Haveles, A. Larsen, M. Mikhail, M. Rolón
Abstract
The Green Stirling Engine Power Plant project utilized a Stirling engine as an environmentally responsible means of electrical power generation. A primary goal of the project was to minimize material cost. The team's efforts centered on A) collecting solar energy through a parabolic mirror to transmit that energy to B) an adapted two-cylinder compressor engine, which served as a repurposed Stirling engine and C) convert the engine’s power to electricity through a DC generator and battery storage. To realize these efforts the available solar energy was calculated and a prototype parabolic mirror was designed, constructed and tested. The engine was modified to serve as an Alpha-Stirling design, which was mounted in a test apparatus containing the DC generator, battery storage, and a charge controller. It was necessary to design numerous engine components including the cylinder heating and cooling heads, a mass transfer tube, a crankshaft, and flywheel. The DC generator and charging system was tested and shown to produce the required electrical outputs. Overcharging the battery was prevented through the use of the charge controller and a large resistive diversion load.
Executive Summary
Powering our future will require replacing many of our current technologies, transforming the largest energy industries, and adopting stricter environmental policies for regulating carbon emissions. Engineers are providing economically and environmentally sustainable solutions to the energy problem that involve using renewable sources of energy, such as solar energy, for means of power generation. The purpose of this project was to develop a power plant module that was low-cost, consumer-scale, and environmentally friendly. The objective was to modify a two-cylinder auto air compression engine into an alpha-configured Stirling engine that operates using solar energy as the input thermal source. To generate electricity, the rotational motion of the crankshaft is transferred to the shaft of a permanent magnet electric generator by a simple pulley transmission.
A Stirling engine can be classified as a closed-cycle regenerative thermodynamic system that operates by cyclic compression and expansion of a working fluid at different temperatures. There are three primary configurations for a Stirling engine: the alpha, the beta, and gamma configuration. The alpha-configured Stirling engine consists of two cylinders – a hot cylinder and a cold cylinder – connected by a regenerator or mass transfer tube. Each of the cylinders contains a power piston connected to the crankshaft; the movement of the hot cylinder piston leads the cold cylinder piston by a 90-degree phase shift. The hot cylinder remains in contact with an external heat source and is maintained at high temperature. The cold cylinder, on the other hand, is maintained at low temperatures by rejecting heat through a heat exchanger. The working fluid undergoes expansion in the hot cylinder and is subsequently compressed in the cold cylinder. The work required to compress the fluid in the cold cylinder is less than that produced by expansion in the hot cylinder, resulting in a positive net work output. The Stirling cycle will continue as long as the temperature differential between the cylinders creates sufficient energy to overcome the internal friction of the engine. For the purposes of this project, an alpha-type Stirling engine was chosen using solar energy as the heating source.
To investigate the feasibility of green power sources for the Stirling engine, a parabolic dish was constructed using a wooden frame, a Mylar emergency blanket for a reflector, and caulking sealant. A vacuum could be drawn through a ball valve, which stretched the Mylar inwards to form a parabolic reflecting surface. The parabolic Mylar surface serves as a solar collector and is designed to concentrate solar energy at a specific focal point dictated by the curvature of the mirror, and thus, the strength of the vacuum. With the focus adjusted by altering the internal pressure, incoming solar energy was directed onto a black aluminum target plate and its temperature measured via thermocouple. The temperature was then used to calculate the total power reflected by the dish.
The maximum temperatures and power extrapolations obtained from the solar test prompted the design of a motorized, automated mount capable of both elevation and azimuth adjustments over time. This allows for easy maintenance of the reflector as it would no longer need to be aimed by human operators. The dish mount was constructed from wood and could pivot about its center on four wheels to satisfy tracking capabilities.
Extensive modifications and improvements were performed on the existing engine. New cylinder heads, mass transfer tube, and flywheel were designed and manufactured. In addition, the crankshaft was ground down to install a needle bearing to reduce internal friction. New gaskets were fabricated from temperature resistant materials. A valve was also added to enable pressurization of the working fluid. Although the internal friction was reduced by nearly 90 percent, the engine was not able to sustain its thermodynamic cycle.
The engine was connected to an electrical power generation system by a V-belt drive transmission. The electrical infrastructure consisted of a Windzilla permanent magnet DC generator for power generation, a Xantrex C35 charge controller for regulating voltage and current, a Scorpion 12V absorbed glass mat battery for power storage, and a 1kW 1Ohm resistor as a diversion load. The engine and all electrical components were housed in a custom built steel frame.
By the conclusion of the project, a solar collection system and electric power generation system were designed, fabricated, and tested. The engine modifications to reduce internal friction and dead volume were successful; however, these improvements were not enough to sustain cyclic operation of the engine.
A Stirling engine can be classified as a closed-cycle regenerative thermodynamic system that operates by cyclic compression and expansion of a working fluid at different temperatures. There are three primary configurations for a Stirling engine: the alpha, the beta, and gamma configuration. The alpha-configured Stirling engine consists of two cylinders – a hot cylinder and a cold cylinder – connected by a regenerator or mass transfer tube. Each of the cylinders contains a power piston connected to the crankshaft; the movement of the hot cylinder piston leads the cold cylinder piston by a 90-degree phase shift. The hot cylinder remains in contact with an external heat source and is maintained at high temperature. The cold cylinder, on the other hand, is maintained at low temperatures by rejecting heat through a heat exchanger. The working fluid undergoes expansion in the hot cylinder and is subsequently compressed in the cold cylinder. The work required to compress the fluid in the cold cylinder is less than that produced by expansion in the hot cylinder, resulting in a positive net work output. The Stirling cycle will continue as long as the temperature differential between the cylinders creates sufficient energy to overcome the internal friction of the engine. For the purposes of this project, an alpha-type Stirling engine was chosen using solar energy as the heating source.
To investigate the feasibility of green power sources for the Stirling engine, a parabolic dish was constructed using a wooden frame, a Mylar emergency blanket for a reflector, and caulking sealant. A vacuum could be drawn through a ball valve, which stretched the Mylar inwards to form a parabolic reflecting surface. The parabolic Mylar surface serves as a solar collector and is designed to concentrate solar energy at a specific focal point dictated by the curvature of the mirror, and thus, the strength of the vacuum. With the focus adjusted by altering the internal pressure, incoming solar energy was directed onto a black aluminum target plate and its temperature measured via thermocouple. The temperature was then used to calculate the total power reflected by the dish.
The maximum temperatures and power extrapolations obtained from the solar test prompted the design of a motorized, automated mount capable of both elevation and azimuth adjustments over time. This allows for easy maintenance of the reflector as it would no longer need to be aimed by human operators. The dish mount was constructed from wood and could pivot about its center on four wheels to satisfy tracking capabilities.
Extensive modifications and improvements were performed on the existing engine. New cylinder heads, mass transfer tube, and flywheel were designed and manufactured. In addition, the crankshaft was ground down to install a needle bearing to reduce internal friction. New gaskets were fabricated from temperature resistant materials. A valve was also added to enable pressurization of the working fluid. Although the internal friction was reduced by nearly 90 percent, the engine was not able to sustain its thermodynamic cycle.
The engine was connected to an electrical power generation system by a V-belt drive transmission. The electrical infrastructure consisted of a Windzilla permanent magnet DC generator for power generation, a Xantrex C35 charge controller for regulating voltage and current, a Scorpion 12V absorbed glass mat battery for power storage, and a 1kW 1Ohm resistor as a diversion load. The engine and all electrical components were housed in a custom built steel frame.
By the conclusion of the project, a solar collection system and electric power generation system were designed, fabricated, and tested. The engine modifications to reduce internal friction and dead volume were successful; however, these improvements were not enough to sustain cyclic operation of the engine.