2-1/2 HP, one-cylinder Gamma double-acting power piston,
Two displacer heat transfer cylinders (180 degrees out of phase with each other)
water jackets on tops, wood/charcoal furnace on bottoms

http://boydhouse.com/forums/viewtopic.php?t=372

Small one cylinder Gamma, single acting, propane on hot end.
Only thing I might change is I'd make the flywheel spokes a fan, and I would duct the airflow through a "U" shaped tube to add more cooling to the fins on the cool end.

http://www.starspin.com/stirlings/jimdp1-3.html

Please notice the 2-1/2 HP engine (approx 1800 watts) is VERY large. The Stirling engine was developed as a safer alternative to steam engines at a time when steam boilers were frequently exploding. Shaft power is directly related to "PSI X piston face area".

With steam there is a minimum temperature to get the liquid/gas conversion, and that temp is high, and requires a constant high heat input to maintain the temp and pressure as you draw some off to run the engine. Plus the more pressure you use, the smaller the engine needs to be, leading to early overpressure explosions.

The benefit of a Stirling is that it can run at any temperature, as long as there is an adequate difference in temp (D/T or "Delta-Tee"). They can actually run with a heat source between cold and very cold.

The problem is that, unless there is a huge difference in temp, there will be a small D/T, and a small pressure equals a very large engine to produce "X" amount of HP.

Your silage pile may seem very warm, but it is a poor heat transfer medium, meaning as you draw off the heat in some way to warm the hot end of a Stirling, the warm spot in the silage will cool.

A recent pilot plant used high-protein corn to brew Ethanol, and after the carbohydrates had been fermented, the mash still had protein and was fed to the cows next door. The cow manure was then used to produce bio-gas (methane) and was burned to off-set some of the mash evaporization heat at the ethanol plant.

It would take a significant investment initially, whether its worth it to produce bio-gas in your situation, I don't know. If yes, you could save up the methane over time, and then run a generator when you needed to top off the battery.

Stirlings are fun, but with low pressures, you need a big engine in order to get a small output. My interest in them is its very hard to make a solar steam engine (the numbers say it must be huge), but a Stirling that has a big dish collector (there are free 8-footers near me) and a fan with a water mister might give me enough D/T to get a trickle of 12 volts. Weak, but the fuel is free (when the suns shining!)