The self-starting solar-powered Stirling engine

Transcription

1 The self-starting solar-powered Stirling engine This project began at the request of an artist who had proposed a Stirling-engine-powered sculpture to a client. The engine only had to run, not really produce any power. Having won the contract, the artist approached me about helping out by building a prototype to demonstrate the feasibility to his client. Two requirements were implicit from the start: First, this heat engine is supposed to run all year when the sun is visible from the sculpture's location. Second, it needs to be able to start itself. Running all year For simplicity, the design does not use solar tracking. A fixed-position engine without solar tracking severely limits options for solar concentration, so this engine has to run on low temperature ratios. After some analysis and experiments, I settled on the basic engine design which runs reliably with an absolute temperature ratio (Thot /Tcold) of This temperature ratio for typical operating temperatures requires about a 35 degree F difference between the hot and cold ends of the engine. Self-Start Solar Stirling The necessary temperature difference is easily obtained in the summer with a flat, horizontal heat collector. The collector is aluminum painted black with an acrylic cover above an air space to reduce heat loss through convection. Although the engine easily ran for around six hours a day in the summer, my measurements indicated that the engine probably would not even start on the shortest day of the year. To obtain four hours of operation on the shortest day of the year, the engine needed to operate with a solar incidence angle of 66 degrees; that is only 24 degrees above the horizon.

2 Tilting the engine to reduce the solar incidence angle was not an option because the side loading of the power piston and displacer would cause too much friction. Instead I used simple solar concentrator to effectively concentrate the sunlight when the sun was low in the southern sky (for the northern hemisphere). This mirror approximately doubles the solar power reaching the collector when the sun is low. The angled acrylic cover also reduces reflective losses that increase with incidence angles above 45 degrees. Heat Collector Mirror Making it self-starting The second problem of starting the engine required an electronic solution. Not only did we need a starter, we needed the controller to know when the engine was capable of running. That meant monitoring the heat collector temperature and cold plate (heat sink) to know when the engine had reached operating temperature. On a normal sunny day the engine will only start once and run until the solar heat is to low for operation. On a day with broken clouds the engine could start maybe half a dozen times or more. I've also found that when the engine is set out on display, viewers enjoy shading the engine until it stops and then watching it start again. A typical day might have 20 engine starts. So the control system also has to monitor the crankshaft to know if the engine was running or not. As part of the development process it seemed a lot easier for the controller to measure and display all the data of interest because the system already had all the sensors that were required. Eventually this will probably evolve into a wireless Ethernet connection that will let us monitor the engine performance remotely to see how the engine is performing and if the engine needs maintenance. The engine start system also needs to be simple and reliable enough to run several thousand times per year and last for years, hopefully without maintenance. Initially I settled with the idea of a starter motor and an actuator to engage the motor when needed. I eventually realized that the reliable starting of a Stirling engine allowed me to eliminate the actuator with a special design that just uses the starter motor. When the engine reaches its operating temperature

3 ratio, a single turn of the crankshaft will always start it. Engine Starter Disengaged

4 Engine Starter Engaged Viewed from the starter disk end, the starter rotates counterclockwise to engage the starter, and then turns clockwise to start the engine and disengage the starter.

5 Controller Schematic As you can see on the schematic, the electrical design is made relatively simple by use of a microcontroller board. In this design I used an Arduino Uno and a motor controller from Adafruit. For some reason the boards that plug into the Arduino are called shields so this is actually referred to as a motor shield. The motor shield is a kit which is easily soldered together. The kit turned out to be an advantage for prototype work because you can customize the board for your application during assembly more easily than modifying an assembled board. For this application I added male headers where needed to simplify connecting signals lines to the Arduino from the sensors and display. Although schematically I show the signals connecting to the Arduino, they actually connect to the motor shield.

6 Electronics in Controller Box The photo shows the controller in its enclosure. A small prototyping board is used to hold the two resistors, trim pot, and the related signal connections. The motor leads are connected to screw terminals on the motor shield. All signal lines to the Arduino use female connectors attached to the headers. The Arduino is powered by either an external 9V battery pack or other external dc source. An on-board 5V regulator is used for external power. When connected to USB the Arduino is powered by the USB. The motor controller supports four dc motors or two stepper motors through four H-bridge drivers capable of driving 600mA continuous current with input voltages from 4.5 to 36V. The power supply for driving the motor can be external. For this design, I use the 9Vdc input to the Arduino that is routed through a protection diode and available as Vin on the Arduino bus. On my design, I used the stepper #2 connection which is also for motors #3 and #4. The motor controller requires most of the standard digital lines so I used some of the analog inputs which are also programmable to function as digital I/O. The TMP36 temperature sensors provide analog outputs. The hall-effect sensor provides a digital low when it detects a magnetic field (south pole). All three sensors were connected to flexible multiconductor cables that are fixed in position with epoxy. The three cables, plus another cable containing the four stepper motor connections, were connected to a D-sub 15-pin connector to mate with the other end on the controller box. The stepper motor does not need to provide much torque to spin this engine, so you could probably select from among many that operate in the range of 9-12 volts and fit the physical space. Unfortunately this Stirling engine can't generate enough power to run the electronics, in fact it probably has an excess power output on the order of 10 mw (on a good day). To run all the above electronics

7 requires a battery or external power source like a photovoltaic panel. If you figure out how to generate useful power with a delta T of 35 degrees F, a large fortune awaits you. I should warn you that the theoretical thermal efficiency limit at this temperature ratio is 6 percent and the practical limit is even less. Sketch Programs on the Arduino are called sketches, again I have no idea why. Whatever they are called, my experiences programming the Arduino have been very positive. There are libraries for the LCD display and motor shield that are easy to use and work reliably. Adafruit provides complete tutorials for assembling and using the motor shield and also for driving the LCD. The online language reference for programming the Arduino include examples that make programming easy. This was my first project using the Arduino and I have since used Arduinos on four other projects. They are so simple to use it is actually fun. I particularly enjoy the IDE and fast software edit cycles. You can edit a program, compile, upload and be testing it in five seconds. I realize there are faster and more powerful microcontroller boards available, but if this meets your needs, it's difficult to imagine anything easier to use on simple development projects. Here is an outline of the functions performed by the program for the engine controller: 1. Measure heat collector and heat sink (cold plate) temperatures and display them. 2. Measure crankshaft speed and display in Hz and rpm. 3. Compute absolute temperature ratio and Th - Tc and display. 4. Initiate engine start if the following conditions are met: a. Temp ratio (Th/Tc) is sufficient (1.065) b. The engine is not turning (hz = 0); c. Increases temp ratio if the start fails. 5. Display the total engine run time since power on. 6. Display the total number of engine starts. 7. Display the total number of failed starts. A failed start is one where the engine runs for 2 minutes or less. 8. Displays the current starting temperature ratio. 9. Displays a log of the engine temperature ratio, Hz, percent of total run time. The temperature ratio ranges are displayed for example as: T106, 1.50, 08 where: T106 = to for Th/Tc 1.50 = average engine speed in cycles/sec (hertz) 08 = 8% of total engine running time was spent at 1.060<= Th/Tc < You'll notice the program needs to display a lot of information on a 2 line x 16 character display. This is done by using a fixed display on the first line that continuously displays Th (heat collector temperature), Tc (heat sink temperature), and engine speed (hertz). These are the most important data and I want them continuously available. The second line cycles through the other information at approximately a 2 second cycle rate per display. The LCD display was selected before I realized how much data I would eventually want to display.

8 Any character LCD using the HD44780 controller should be compatible. A 4 line x 20 character display would have been a better choice. [controller program listing] Building the engine Although I haven't drawn up complete plans for this prototype engine, I have taken numerous detailed photos of all parts of the engine and accompany them with detailed information including how to make it better. I also provide the key dimensions to size the critical components correctly. You can find them on my website The link to the engine information starts at: Here are a few key basics to keep in mind if you want to build a low-temperature Stirling engine like the one I describe: 1. Minimize all sources of friction. The key friction areas are the power piston, the crankshaft, and the displacer shaft. I use ball bearings on the crankshaft and close tolerance fits on the power piston and displacer shaft. 2. Minimize gas (air) leaks. While minimizing friction, you also need compression for the engine to run. There should only be two sources of gas leaks in an engine like this: (1) Around the power piston, (2) Around the displacer shaft. Both of these leak sources must be kept small. 3. These engines are quiet and fun to watch, but you really can't generate much power. If you want more power you need a much higher temperature ratio. Photovoltaic panels aren't as much fun to watch, but they are much better for generating power from sunlight.