The following numbers are based on what I have seen in the above video or statements the speaker has made and in a few cases, guesses based on what I can see. All my dimensional estimates are strictly eyeball and may easily be off by 20% or even more.

Sunpulse Stirling engine generating electric power

power piston diameter: est. 48 inches
power piston stroke: est. 2.5 inches
displacer diameter: est. 48 inches
displacer stroke: variable, est. 2.5 to 6 inches
flywheel diameter: est. 72 inches
flywheel rim: est. 0.5 inches thick by 4 inches wide
Operating RPM: est. 60 to 90
Heating and cooling pumps: est. 2 inch diameter by 3 inch stroke, double-acting
2×9.4 cubic inches per cycle = 312 grams (for water)
Operating temperatures: Hot oil or water at 5 bar pressure, 150 to 200 deg C. Water cooling, est 25 degC or higher

Estimate operating gas temperature Th=180 deg C, Tc=40 degC
Power output of generator: 1.5 kW
Engine pressure variation: +/- 0.1 bar (approximate in video of gauge)
Hot oil with possibly gravel stored in elevated barrel est. 55 gal barrel

Stirling engine driving water pump

flywheel diameter: est. 48 inches
flywheel rim: est. 2.0 inch diameter steel
displacer stroke: est. 5 inches
claimed pumping output:
400,000 liters/day, zero head
(110,000 gal/day or 9200 gph for 12 hr day)
80,000 liters/day, 10m head
(22,000 gal/day or 1800 gph for 12 hour day)
15,000 liters/day, 50m head
(4100 gal/day or 340 gph for 12 hour day)

total solar collection diameter est. 11 feet including mirrors

There are several interesting features of this engine that I’ve tried to capture in these stills taken from the video. First, the robust power piston with what I would describe as a conical truss. If you want to get significant power out of a low-temperature Stirling engine, you need a big power piston. I estimate the power piston at 48 inches in diameter. combine that with a pressure fluctuation of about 0.1 bar (1.5 psi) shown on the gauge in the video and you have a peak piston force of around 2700 lbs.

An atmospheric Stirling engine such as this will have an approximately sinusoidal pressure versus time with two power peaks per revolution. One half the cycle will be above atmospheric pressure and one half will be below atmospheric pressure . The average force =.64 x peak force or about 1730 lbs. I estimate the power piston stroke at 2.5 inches so for a complete cycle the travel is 5 inches for a total of 8650 in-lbs of work or 721 ft-lbs per cycle. At 60 RPM this would be about 980 watts. This gets you in the ballpark of 1500 watts. At rated power this engine might be turning 90 rpm, the pressure might be even higher, or my power piston diameter and stroke estimates could be way off.

To me the most interesting feature of this engine was the variable-stroke displacer. The following photo labels some of the components. A motor-driven jack screw (which you can see operating in the video) adjusts a connecting rod anchor point on the lower displacer lever. When the connecting rod is close to the pivot the piston travel is long and when it is farther away the travel is shorter.

The reason you want to do this is to be able to quickly regulate the power output of the engine to match the load. The time constant for heating or cooling the engine might be minutes, but you need to adapt to electrical load changes much faster. This mechanism lets you operate the engine at a fixed temperature and be able to vary the displacer stroke to control the RPM. Closed-loop RPM control using this method is much better and less wasteful than say adding or dropping a dummy load to flat-load the engine.

The above photo also shows what I refer to as a rolling fabric seal around the circumference of the power piston that provides an airtight seal to the cylinder. The seal is probably a coated fabric that is flexible but does not stretch under the 1.5 psi operating pressure.

This last photo shows the engine-driven piston pumps that pump both the hot oil or water through the hot end of the engine and the cooling pump. Both of these pumps appear to be double-acting so they pump liquid twice per complete cycle.

This is quite an impressive project. Also see some information at Sunvention.

I want to thank all the people who stopped by to see the solar power Stirling engine in operation. I appreciate the questions, comments and interest in the engine. I regret that I was unable to offer a better response to those that asked about the efficiency. I simply cannot make a meaningful measurement of efficiency on this engine because it has several thermal shorts that are part of the engine. I’ll post more details on efficiency and the low-temperature engine in a future post.

One of my goals in attending the Maker Faire was to get a feel for potential interest in the engine and see if there might be applications that could make use of the engine. My impression is that there is a lot of interest—I just need to deliver the power. I was showing a development engine with an output of about 1/10 watt. I think interest would be better for at least 100 watts, approximately the power a person puts out exercising, or preferably 1kwatt.

One application that sounded particularly promising is the possibility of using the engine to make use of the excess solar-heated water available in buildings that use solar hot water for space heating. Space heating requires considerable power, the cause of you high energy bills in the winter. In the summer these systems are essentially unused. I’ve heard from users that they’ve seen steam coming out of these systems as the temperatures soar in the summer. A hot water engine need not be highly efficient as long as it can generate power to pay for itself plus some extra.

I’m always interested in hearing about potential application where a low-temperature low-power engine might be useful. There are potential applications such a powering fountains or operating kinetic sculpture that might be able to use power in the 10 watt range. One benefit of the type of Stirling engines that I am designing is that they produce torque at low RPM so that they tend to be more useful for mechanical operations. High RPM power often requires a gear train to reduce RPM and increase torque.

As I mentioned at the Maker Faire, I’ll be putting out some free plans on this website for making a simple, small, Stirling engine. I’m still getting a few bugs out (mostly reducing friction) so that it can operate from sunlight. My goal is to have it done by May 18.

After that I’ll put out a simple simulator for Excel that will help you estimate the power output of a Stirling engine design. Essentially it will give you a maximum possible work per revolution and information on pressures during the cycle. It won’t tell you things like what RPM the engine will turn—that depends on the design of your engine and how efficiently you transfer heat and how low the friction is in your engine.

You can see a video of the Maker Faire 2008 Configuration