Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 3

Project Number: P12472

Solar Stirling generator

Phil Glasser / Lead Engineer, Electrical Engineer William Tierney/ Mechanical Engineer

Bryan Abbott/ Mechanical Engineer Mike Scionti / Mechanical Engineer

Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 3

Abstract

The purpose of this project was to design, build, and implement an autonomous solar Stirling engine with generator to power USB electronics. A single cylinder beta type Stirling engine was selected with a single power piston, a single displacer piston, crankshaft, and flywheel to connect to a permanent magnet DC generator. An Arduino Uno microcontroller monitors the temperature differential between the hot and cold side of the Stirling engine via thermocouples to determine when to kick start the engine to get it running. Generator voltage is converted to 5 Volts USB at a maximum of 10 Watts through a buck-boost switch mode converter.

Introduction

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 have superior performance compared to silicon based massed produced photovoltaic cells in most cases[3]. In addition, although mechanically more complex than photovoltaic’s, 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.

In this first phase of the project, research was done on the different configurations of Stirling engines in existence and a feasibility analysis was completed to select the most practical configuration to meet the engineering specifications of the project. Next, design calculations were completed in order to create an engine and generator that met all of the specifications set forward by RIT.

The engine was then modeled using CAD, materials were purchased, and the engine components were machined and assembled.

Nomenclature

CAD - computer aided design; use of computer technology to aid in the design and production of a product. Typical CAD packages are 3D solid surface modelers.

USB – universal serial bus, commonly used data and power transfer in consumer electronics such as cell phones.

DC – direct current, unidirectional current flow.

SPI - Serial Peripheral Interface Bus, commonly used full duplex communication protocol.

Customer Needs

The goal of the project was to power small electronics through a USB interface, (such as a cell phone, mp3 player, etc.) from a Stirling engine generator powered via the sun. The engine in the system was required to be a heat engine utilizing the Stirling thermodynamic cycle, must receive its heat from the sun, and be able to run autonomously and maintenance free for one year outdoors in a climate similar to Rochester, NY. The system was required to be easy to move and inexpensive. Most importantly the system should not cause any damage to people or surroundings when operated.

Engineerng Specifications

Many of the engineering specifications are driven by USB power specifications and power output needs specified by the customer. The Stirling generator must output at least 10 watts of power at a nominal voltage of 5 volts when operating. The system must not exceed 20 pounds. The system must also be able to operate for an entire year without maintenance, withstanding all weather conditions. The budget allotted to purchase materials for this project is $500.

Theory of Stirling Engines

The Stirling engine operates according to the Stirling thermodynamic cycle, which operates on the principle of a temperature differential to generate power (which is also why they are commonly referred to as heat engines). All single cylinder beta type Stirling engines will have a “hot” side and a “cold” side of the system. The hot side can receive its heat from any source be it fire, the sun, etc. and the cold side is designed to dissipate heat through an active or passive cooling system such as a heat sink, thereby creating the temperature differential. As the working fluid in the cylinder on the hot side of a Stirling engine heats up, it results in an increase of pressure and expansion, forcing the power piston towards the cold side. After the initial expansion of gas on the hot side of the cylinder, the displacer piston will push the gas to the cold side of the engine. On the cold side the pressure will decrease and the working fluid will contract, forcing the power piston back towards the hot side of the cylinder. This process continues repeatedly providing the means for the cyclic action of an engine.

There are a number of different configurations commonly found for Stirling engines, each with its own advantages and disadvantages. The specific design that best fit the needs of this project was the “beta” configuration. This design incorporates a single power piston and a displacer piston, both housed in the same cylinder. The displacer piston is responsible for shuttling the working fluid back and forth. The power piston is sealed to the outside environment of the engine and is responsible for transmitting the power from the expanding and contracting working fluid via linear motion. The linear motion of the power piston is transformed to rotational motion by means of a crankshaft or yoke device. A flywheel is utilized in the system in order to store angular momentum and keep the engine running. The selected design for a beta style Stirling engine can be seen in Figure 1.

Figure 1:CAD representation of the beta style Stirling engine

Mechanical Design Calculations

The implemented design has primarily been a function of “rules of thumb” generated by engineers for different parameters that produces a working finished product. In designing the engine, a conservative approach was taken on all of the efficiency estimations in order to ensure a proper sizing of engine components.

Assuming a 50% efficiency between the power output of the engine and the electrical output at the USB interface, the engine was designed to output 20 Watts of mechanical power. Assuming a conservative 10% efficiency between the solar energy input and the engine output, meant 200 Watts of power was needed to be gathered and concentrated from the sun. Assuming that the sun exerts 1300 W/m2 [4] the mirror size was calculated to have a collection area of 1.658 ft2.

Surface Area= 200W1300Wm2=.154m2=1.658ft2

(1)

Next the amount of volume the power piston had to displace was calculated by utilizing the Beale number, a parameter used to characterize Stirling engines[1]:

Bn=W0PVf

(2)

Where W0 is the power output of the engine (Watts), P is the average mean pressure (MPa), V is the volume of expansion space (cm3), and f is the frequency at which the engine is operating (Hz). A typical Beale number for a high performance Stirling engine is 0.15 [1]. Assuming a typical operating frequency of similar sized Stirling engines, 17 Hz (1020 rpm) and atmospheric pressure, the necessary displaced volume was calculated as 6.39 in3.

The required power piston diameter was calculated using a variant of the Beale Equation [1].

D=8W0BnπPf1/3≈2.3 in

(3)

From this point the design decisions were based upon the rules of thumb mentioned in Stirling Engines [1]. The stroke of the power piston was set to half of the power piston diameter (1.15in), and the length of the displacer piston was made to be twice the diameter of the power piston (4.6 in). The length of the power piston was arbitrarily set at 1 in and the chamber length was set at 8.5 in.

An important aspect of the design was the flywheel. The flywheel was designed to keep the engine rotating by storing angular momentum and only allowing a moderate amount of variation in the speed of the engine by use of the following formula [5]:

J=Uωavg2Cs

(3)

Where J is the angular moment of inertia, U is the average work per cycle, ωavg is the average rotational speed, and CS is a constant that determines the amount of variation of speed. Smaller values of CS correspond to higher inertias and thus less variation, so a value of 0.03 as chosen with “some variation acceptable” [5].

After general design calculations were made, some materials had to be selected. One material of extreme importance is the chamber. By use of the Beale equation, stress equations, and heat transfer equations, Walker formulated the following ratio for power to heat transfer rate [1]:

PQc=cpσfLl2K

(2)

Where Qc is the rate of heat transfer, K is thermal conductivity, L is the length of the cylinder, l is the length of the stroke, and σ is the maximum permissible stress. This relationship shows that the ration of power to heat transfer rate relies heavily on strength of material, speed of the engine and the length of the cylinder and stroke. This means that an ideal material for a Stirling engine would be one with high strength and low thermal conductivity, making thin walled stainless steel a great candidate[1].

To minimize the size of the flywheel and obtain the proper moment of inertia, it was made out of steel. The crankshaft was designed such that the power piston would have a stroke length of 1.15 in and made out of steel. In addition, the power piston and all shafts were made out of steel. In order to minimize weight, the base plate, all brackets and the displacer piston were made from aluminum. PTFE piston rings were chosen for to seal the outer power piston with the cylinder and the displacer rod to the inner power piston. Since Stirling engines tend to suffer heavily from friction, PTFE was chosen because of its low coefficient of friction with steel. Ball bearings were chosen for the main bearings of the crankshaft and bronze sleeve bearings were chosen for each side of the connecting rods.

The final calculations done to complete the mechanical design work were determining interferences in parts that were to be press/shrunk fit. A standard interference of .001 in was selected for the press fit. Minimal attention was given to calculating loads on bearings or the stresses on the parts due to the very low values of torques and forces present in the system.

Electrical Design Considerations

In order to meet the system specification of USB output, which according to the USB specification is DC 5V ±0.25V, mechanical power from the Stirling engine needed to be converted to electrical power. A 67W permanent magnet DC motor was chosen to be used as a generator to convert rotational energy to electricity, and as a means to “soft start” the engine autonomously to meet the autonomous operation specification.

This “soft start” is necessary to overcome the torque of the motor until the Stirling engine starts rotating enough ramps up the output torque. The chosen method to perform this “soft start” is to simply drive the motor with the power conditioning disabled from a battery for a brief period of time.

In order to power the embedded microprocessor that controls the “soft start” a 6V lead acid battery was chosen. To determine the power rating in amp-hours, several assumptions were made. A power draw of 5V at 25mA, for 24 hours a day, over 7 continuous days of operation, with a depth of discharge of 60%, and a temperature de-rating of 20% given an average temperature in Rochester NY of 53º, and the system voltage of 6V, equation (4) was created to determine the battery sizing at 7Ah [3].

Vop*Iop*hours op *1D.O.D.*Temp DeratingVBattery=Rated Amphours

5*0.025*24*7 *1.6*1.26=7 Ah

(4)

A buck-boost converter was selected as a means to convert the variable voltage output from the generator to the required 5V output for USB, and another for the 7V output to charge the 6V battery. The LM5118 was chosen as the buck-boost controller, due to its flexibility, wide input voltage range, and design tools available. Now owned by Texas Instruments, the National Semiconductor WebBench® design tool was used to select initial component values for the buck-boost circuits, and then customized to provide 0805 sized SMD components, small quantity availability, and ease of soldering based on the pad layout. The final designed schematic can be seen in Figure 4.

The PCB, Arduino, battery, and enclosure were modeled or imported from STEP files into SolidWorks© to confirm the sizing of the box shown in Figure 2. The PCB was designed and modeled in Altium Designer© shown in Figure 3, then converted to GERBER files, and sent to Advanced Circuits (http://www.4pcb.com/) for fabrication.

Figure 2: Enclosure containing Arduino, buck-boost board, and battery

Figure 3: Buck-boost board rendering

Figure 4: Buck-boost board schematic diagram (larger diagram in appendix)

Two issues with the original schematic were noted: misplacement of current sense resistor voltage sense traces and missing connection of the output voltage to the Vout sense pin of the LM5118 buck-boost controller. Once these issues were corrected, the battery charge and USB output buck-boost circuits acted as expected. The schematic has been updated to reflect the proper connection, however a new PCB has not been laid out reflecting these trace changes. One peculiarity that was noted is that if the Arduino is powered off and the power conditioning board on, the motor PMOS FET is driven into an active mode and the motor spins. This is not a use case, as the Arduino is always on or in a low power state when the system is active. In order to activate the system, a 5A fuse should be installed into the fuse holder and the Arduino switched to on. This power the buck-boost board and Arduino making the system active. To disable the electronics for storage, remove the main 5A fuse from the fuse holder.