Solar Powered Stirling EngineMegan McHugh | B.S. Sustainable Built Environments | Spring 2017 Capstone Showcase

Mentor: Cho Lik Chan, Ph.D. | Department of Aerospace and Mechanical EngineeringMachine Shop Supervisor: Joseph Hartley | Instructor: Joseph Iuliano

Introduction• The main focus of the research was to design and optimize a solar

powered Stirling engine after a review of relevant literature.• In 1816, Robert Stirling invented the Stirling engine, a device with cyclic

compression and expansion of the working fluid at different temperature levels.1 This operation is known as a closed regenerative thermodynamic cycle, and a net conversion of heat to work is accomplished by the volume change regulating the flow.2

• Low-power range solar thermal conversion units consist of three main sub-systems: the solar receiver, the thermodynamic gas circuit, and the drive mechanism. Stirling engines are considered among the most effective of these units and improvements in performance can be made based on changes in the main sub-systems.3

• The solar powered Stirling engine was patented in 1987 by Roelf J. Meijer. Using a large dish facing the sun, the rays of sunlight can be reflected onto a focus point at the center of the dish to collect solar energy as a source of heat. The heat then powers the Stirling engine connected to the solar dish collector and produces electricity, which makes the system a viable alternative energy source.4

Potential Sustainable Impacts• The solar powered Stirling engine has additional applications as a pump, which is important as it is

cost effective and can be used for water pumping in areas of the world where there is low access to clean water.

• Pumping systems employed in underdeveloped, arid regions for irrigation have a maximum water cost target of 6 cents/m3.5

• Sunvention Sunpulse developed a solar thermal water pump at 2.4 cents/m3, which meets the Word Bank target as assessed by TÜV labs.

• Photovoltaic pumping systems currently cost 8.4 cents/m3 and gasoline pumping systems cost 8.58 cents/m3.

Results• Data was collected at 5 second intervals for both temperature in °F (Figure

1) and speed in rpm (Figure 2) with the heat extinguished after 100 seconds.

• Measurements were taken without the parabolic dish attached, but instead with a butane torch to optimize engine performance before changing the heat source.

Conclusion• The modified design of the Stirling engine was successful in running

provided the appropriate amount of heat, however, further changes could lead to a higher efficiency.

• Measurements with the solar dish revealed that the focal point of the parabolic mirror was between 4.5” to 5” resulting in a 135 °F average cylinder temperature, which is not sufficient to power the Stirling engine.

• Adjustments to the solar dish placement with spacers and more testing is required to use it as the heat source. Research will continue during the summer to optimize the system.

Methodology• The constructed gamma configuration Stirling engine was based on the

MIT 2.670 model.6

• Materials for the Stirling engine parts were aluminum, brass, and steel.• Modifications were made to the base design and the dimensions of

certain parts:• Displacer cylinder: 3.38” to 6.00”• Displacer piston: 2.00” to 5.00”• Connecting rod: 2.75” to 5.75”

• Assembly and testing with the parabolic dish required welding goggles and gloves for protection from the sun’s UV rays.

• The temperature of the cylinder and between the cooling fins (Figure 1) was measured in degrees Fahrenheit (°F) using a digital multimeter with a thermocouple.

• The speed of the flywheel (Figure 2) was measured in revolutions per minute (rpm) with a tachometer.

Funded by:

References1 Stirling, R. (1816). Patent No. 4081. Edinburgh, Scotland: Chancery.2 Thombare, D.G., & Verma, S.K. (2006). Technological development in the Stirling cycle engines. Renewable & Sustainable Energy Reviews. 12, 1-38.3 Mancini, T., & Heller, P. (2003). Dish Stirling systems: an overview of development and status. Journal of Heat Transfer, Trans. ASME. 125, 135-51.4 Meijer, R.J. (1987). U.S. Patent 4707990. Washington, DC: U.S. Patent and Trademark Office.5 Ardron, Mitra. Sunvention SunPulse Water – Solar Thermal Pump. 2010. Print.6 Morris, S.J. (1996). Development of the machine shop instruction and the Stirling engine project for 2.670: ME Tools (Master's thesis). MIT, Cambridge, MA.

Modes of Operation

Forms of Piston Coupling

Forms of Cylinder Coupling

• Single/double acting• Single/multi- phase• Resonant/Non-

resonant

• Rigid coupling• Gas coupling• Liquid coupling

• Alpha coupling• Beta coupling• Gamma coupling

Figure 1. Temperature (°F) at the end of the cylinder or between the cooling fins versus time (s) for the prototype Stirling engine.

Figure 2. Speed (rpm) for all trials versus time (s) for the prototype Stirling engine.

Materials (left to right): Neiko tachometer, SainSmart digital multimeter with thermocouple, and welding goggles.