Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 s2

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Project Number: P12471

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Solar Stirling engine generator

Tara Dougherty
Mechanical Engineering / Thomas Gamer
Mechanical Engineering
Daniel Thering
Electrical Engineering / David Volzer
Mechanical Engineering

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Abstract

Team P12471 was tasked with designing a working solar powered Stirling engine for Rochester Institute of Technology in May of 2012. A Stirling engine is a heat engine that is operating by rapidly heating and cooling a gas within a piston/cylinder device. The gas is fully contained in the engine, meaning that there is no intake or exhaust, which then classifies a Stirling engine as an external combustion engine, since the heat is applied to the engine externally. The main stipulation of this project was to use solar energy, rather than combustible fuels to produce the heat to run the system. The long term objective of the project was to research into alternative devices that would allow for electrical generation in remote regions with limited access to electricity, as well as applications in space where photovoltaic cells become obsolete and inefficient due to effects of radiation.

Our Stirling engine was of a Beta (inline hot and cold cylinders) style, with a Mykroy disk acting as a thermal break between the hot and cold cylinders. A reflective mylar/fiberglass dish was fabricated to focus the solar energy, and the system featured a DC motor generator and battery bank. Testing of the dish proved successful, as the internal air temperature of the hot cylinder reached a maximum of 163°C. Testing of the engine proved inconclusive, as the system was unable to overcome its own internal friction and/or generate sufficient pressure to start the cycle. Testing of the circuit proved that the power electronics were able to convert three phase power from the generator into 5V DC power at a USB port. Overall the project was a mixed success, but yielded a significant basis for future research utilizing the current platform.

Introduction

Harvesting energy from renewable sources (solar) offers a way to provide power in remote locations using local sources. Photovoltaic systems, which convert sunlight directly into electricity, are commonly used because of the direct conversion of energy to power. However in extraterrestrial applications, radiation can destroy the photovoltaic cells, while a mechanical device such as a Stirling engine would be unaffected. In addition, photovoltaic cells are fragile, while a Stirling engine can withstand considerable abuse and adverse conditions without affecting its performance. In addition, unlike an internal combustion engine, a Stirling engine releases no harmful emissions into the atmosphere.

Scope

The purpose of this project was to design and construct a portable Stirling cycle engine generator that operates using solar energy, or with any kind of heat source (fire, heat lamp, etc.). The Stirling engine generator must produce 10 Watts of power at 5 Volts. The generator must be weatherproof, and be able to last 1 year from the date it is set outside, and the generator must weigh less than 20 pounds, excluding the weight of the solar collector.

History

The Stirling Engine was invented and patented by Robert Stirling in 1816. The engine he began working on followed earlier attempts at making an air engine, but was the first put to practical use in 1818, when the engine Stirling built was used for pumping water in a quarry. The building of Stirling engines has become largely popular among hobbyists but faces challenges of efficient power production compared to photovoltaic cells. More recently, the Stirling engine has since been researched by NASA for the space program.

Background

Today there are 3 main Stirling engine styles: Alpha, Beta and Gamma. An Alpha Stirling Engine contains two power pistons that are in separate cylinders, one of which is hot and one that is cold. Air travels from the hot side to the cold side via piping. The Beta Stirling Engine has one power piston that is arranged coaxially with the displacer piston. In this engine type the displacer piston does not extract any power from the gas that is expanding; its function is to help shuttle the working gas from the hot cylinder to the cold cylinder or heat sink. A key benefit of the Beta configuration is that it is only necessary to seal the cold end, thus simplifying the sealing method and material. The third style of Stirling engine is the Gamma Stirling engine. This engine type is similar to a Beta engine with the exception that the power piston is mounted in a separate cylinder which is located directly alongside the displacer piston cylinder, but both are connected to the same flywheel.

Ideal Stirling Engine Cycle:

The Stirling Cycle consists of four processes which combine to form a closed cycle; two isothermal (constant temperature) and two isochoric (constant volume) processes.

Figure 1: Ideal Stirling Cycle

The above figure shows a Pressure-Volume (P-v) diagram, and a Temperature-Entropy (T-s) diagram for an ideal Stirling engine cycle. Depending upon the direction of integration the work and heat will either be added or subtracted from the system. In a Stirling cycle heat transfer must occur. The net Work produced is represented by the area 1-2-3-4.

Process 1-2: Isothermal Compression: heat rejection to the low temperature heat sink, work is done on the working fluid.

Process 2-3: Isochoric Heat Addition: Heat addition (energy exchange from regenerator), NO work is done.

Process 3-4: Isothermal Expansion: Heat addition from the high temperature heat sink, work is done by the working fluid (energy exchange to the flywheel)

Process 4-1: Isochoric Heat Rejection: Heat rejection (energy exchange to regenerator), NO work is done

Actual Mechanical Cycle:

Process 1-2: The power piston compresses the gas and the displacer shuttles the gas to the heat absorption chamber. Work is done on the system.

Process 2-3: Heat is added to the system via the solar collector and heat absorber.

Process 3-4: The gas expands causing it to shuttle past the displacer and drive the power piston outward. Work is released by the system. This is the power stroke of the cycle.

Process 4-1: Heat is rejected from the system by the heat exchanger.

System Design:

Design Assumptions:

1. Power generation is not required at night

2. Solar collector will always be facing the sun (not responsible for tracking sunlight)

3. Weight of solar collector is not included in the 20 pound weight limit

4. Modeled after Ideal Stirling cycle

Design Process:

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Table 1: System Design Combinations

Table 2: Pugh Analysis

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Table 1 shows the selected combinations of system components that were considered when original planning began. Table 2 shows the Pugh Analysis which compared the various combinations against a benchmark combination. This Pugh analysis was used to decide on and validate the chosen system configuration.

After completing the Pugh analysis, it was decided that a Beta configuration would provide the greatest potential for power production and reduce weight by having only one engine cylinder. This configuration also introduces mechanical complexity due to the coaxial pistons within the cylinder. A four bar bowtie mechanism was originally chosen to transfer power from the engine to the generator. However, to reduce mechanical complexity, an offset crankshaft using simple turnbuckle linkages was designed as an alternative. As a result of the Pugh analysis a DC generator was chosen with lithium-ion batteries for the charging circuit.

Mechanical Design:

Figure 2: CAD Model – Front Figure 3: CAD Model - Back

Figure 4: CAD Model - Powertrain Figure 5: CAD Model - Linkages

The mechanical design of the Solar Stirling Engine includes several unique features that effectively make use of solar energy to produce electricity. In figure 2, a 20-inch diameter solar dish is shown, made of fiberglass and coated with a reflective mylar film. A copper cylinder is placed at the focal point of the dish where the solar energy is concentrated to generate heat within the cylinder. As seen in figure 4, a heat sink is located at the other end of the engine with a cermaic disk used as a thermal break. A displacer piston used to shuttle the working gas from the hot cylinder to the heat sink as well as a sealed power piston are assembled inside the engine chamber. Figure 3 and figure 5 detail the turnbuckle linkages that connect the pistons to the offset crankshaft. Spring reinforced PTFE o-rings, PTFE/oil impregnated bronze bushings and thrust washers are used to reduce mechanical friction in the system. The crankshaft, made of stainless steel, is assembled using set-screws and supported by steel ball bearings. A flywheel is attached to the crankshaft providing the necessary momentum and stability the crankshaft needs for continuous rotation.

As heat is applied to the copper cylinder via solar energy, the Stirling cycle begins and the pistons begin to reciprocate in and out of the chamber. The linear motion of the pistons is converted to rotational motion resulting in rotation of a pulley system using timing belts and a gear ratio of 16:1. The pulleys turn the generator which produces electricity and transfers it via the electrical circuits to the USB output.
Electrical Design:

Design of the electrical circuits started with the power rectification and voltage regulation portions. Maximum generator power output was rated at 50W; the system was designed to handle up to 2A at 24V at the input. The three main portions of the power circuit are the diode Rectification Bridge, 5V, and 12V voltage regulators. The 12V regulator, REG1, is used to power the battery charger and motor controller. 5V regulated DC power is supplied to the USB port by REG2. The 48 pin chip is the motor controller which turns on 3 sets of 5A power MOSFETs to power the generator during a start operation. The power MOSFETs are connected to the battery and are used as switches to apply power to the three phases of the generator. Circuitry other than the MOSFET control signals are used to determine speed, direction, and protection for the 5A MOSFETs. CMP1 is a comparator that is used to measure current going into the 12V voltage regulator, and read the thermistor temperature. An XOR gate is used to control when the voltage regulators, motor controller and charger should be on or off. The basic operation is; if the thermistor is sensing sufficient heat turn on the motor controller to start the engine. When enough back current is sensed going to the 12V voltage regulator the Stirling engine is under its own power. Then turn the voltage regulators on and the motor controller off.

Testing Criteria:

Solar Collection: Test for focus point using sun (preferred) and a piece of wood (detection of charring), or lamp and light meter.

Thermal: Test for heat transfer characteristics by taking temperature readings of hot and

and cold cylinder as heat is applied to hot cylinder.

Mechanical: Test engine free of electrical load, record flywheel speed. Check for durability

of mechanical linkages and fasteners following test.

Electrical Charging: Continuity check using multi-meter

Electrical Storage: Continuity test using multi-meter, and measure time to completely charge/discharge batteries

System: Check self-starting capability (turn system on from room temperature conditions), measure hot/cold end temperatures, measure output power, measure output voltage, and record generator output.

Durability: Soak engine for 10 minutes with a hose and then turn system on, place system in wind tunnel for 10 min at 30 mph and check operation, place system in freezer for 4+ hours and check for adhesive separation, and run continuously for 24 hours then shut down and check for wear and tear.

Testing Results:

Solar Collection: Testing of the solar dish was successful. The curvature of the dish created a focal point, approximately 2 inches wide, located at the end of the copper cylinder.

Thermal: Testing for heat transfer characteristics was successful. Temperature readings were taken at several locations on the hot cylinder, heat sink, and ceramic disk to analyze the effectiveness of the system to maintain a hot temperature on the hot cylinder while remaining cold as possible at the heat sink. The following graph shows the results of this test.

Power Circuit: Using a power drill to turn the generator the power circuit was able to convert variable voltage three phase AC power to 5V DC power. Efficiency tests of the circuit were also run using lab power supplies. The result was a total end to end efficiency of 70% with a 5ohm load.

Mechanical: Mechanical testing of engine proved successful. Due to in-operation of engine

From heat source, a hand operated drill was used to turn crankshaft at 300 rpm. The linkages and mechanisms moved as designed and all fasteners and connections remained sufficiently attached despite vibrations from engine operation.

Electrical Charging: The electrical charging board is a COTs product. It was tested on a 2.2AH four cell lithium polymer battery. The charger was able to charge the battery to its rated voltage level of 13V.

Electrical Storage: Using a 5ohm load the four cell lithium polymer battery was discharged over a short period of time. The battery was able to produce over a 2A output continuously. It is rated at 30A max output.

System: This test could not be completed due to the engine not being able to operate from a heat source as a complete system. However, a hand operated drill was used to turn the generator and produce 5 Watts of electricity with an input equivalent to 300rpm.

Durability: This test could not be completed due to the engine not being able to operate from a heat source as a complete system.

Conclusions and Recommendations: The Stirling Engine Generator was successfully built and tested, but unfortunately it was unable run on its own. Despite the engine not functioning as a complete system, each subsystem, thermal, mechanical, and electrical, functioned successfully as designed. Most design objectives were satisfied and the result was a durable, weatherproof, Stirling engine with the potential to generate electricity.