Stirling Cycle

The Stirling Cycle is a method of extracting useful motive force from an available temperature gradient. Essentially, it takes the energy from the temperature differential and uses it to drive a piston-based mechanism, typically called a Stirling Cycle Engine, or just Stirling Engine. The Stirling Cycle Engine is a specific form of Carnot Cycle device.

Applications

The Stirling engine is useful in many applications where efficiency and reliability are of paramount importance. It is used in solar power conversion, where the heat exchange between solar-heated furnaces and ambient temperature provide the differential. At the other end of the scale, tiny versions of it have been used to power spacecraft systems and small isolated generators.

In addition, two of the most immediately relevant uses involve powering vehicles. There have been pilot projects involving automotive prototypes (cars, vans, trucks) which have travelled thousands of miles under Stirling power. The concept is also quite useful in designing and operating conventionally-powered (or in some cases nuclear powered) submarines.

How it Works

There are numerous possible configurations of the Stirling engine. However, for simplicity’s sake, here is one of the more simple (if less efficient) ones – a single-cylinder implementation. The cylinder (illustrated below) has two pistons within, separated by an unmoving thermal buffer material (usually a porous metal). The two resulting chambers are filled with a fluid whose pressure is highly temperature-sensitive – a favorite is liquid Helium at 10 atmospheres, for example.

                     Stirling Cycle Engine

Ambient/Heated end                      Cold/Ambient end
                           v---thermal buffer
----------------------------------------------------------
|        |  |     A     --XXX      B       |  |          |
|--------|  | Warm gas    XXX--   Cold gas |  |----------|
|        |  |           --XXX              |  |          |
----------------------------------------------------------
Piston A --^                                ^-- Piston B

As the heated end absorbs energy, the fluid in chamber A expands. This forces piston A back as well as some fluid through the porous thermal buffer (where it transfers some heat to the buffer material) into chamber B. Piston A is connected to a useful load. Eventually, the fluid in chamber A reaches the limits of its expansion. At this time, the gas that has forced its way through the thermal buffer into chamber B has begun to contract due to the lower temperature. Even as Piston A rebounds and begins transferring gas to chamber B through its momentum, the gas in B is shrinking, which pulls piston B (whose backside is at ambient pressure) to the left. As it continues to the left, it forces some gas back through the thermal buffer, where it picks up some warmth left there from its trip to the right, and begins the expansion cycle again.

This cycle continues. It sounds a bit like perpetual motion, but remember: the useful work is performed not by the motion of the fluid/gas from one cylinder to the other, but from the expansion and contraction of said fluid. This occurs due to heat/cooling sources applied to the cylinder chambers; the energy being input into the system from those sources is what in fact drives the mechanical motion.

This is a simple version. Multiple piston/chamber versions are possible, where the cooled fluid from one piston enters the heated chamber of the next cylinder in line, and so on. Also, rather than using a thermal buffer, some versions use a float or weight that travels back and forth and helps ‘push’ fluid from one side of the cylinder to the other and back again in order to collect maximal energy from the heat differential.

Advantages

The advantages are legion. First of all, all that is required to power this engine is a constantly available heat (or cold!) source relative to the ambient environment. This is much easier to find/maintain than a constant supply of combustible fuel! Second, the device is extremely simple, requiring few moving parts especially compared to an internal combustion engine. Third, it is therefore more reliable (and often more efficient) than said ICEs.

Disadvantages For one, it is difficult to use in moving applications without a method of moving the heat/cold source. For most vehicles, this involves burning fuel anyway. For another, it is difficult to gain much torque from these engines unless they are quite large; the useful force produced per unit of displacement is much lower than a gasoline engine. As a result, this engine is much more useful for constant, slow speed applications than highly variable loads or speeds. It's ideal for submarines, which want to be quiet and can't afford the noise or air intake/exhaust from a reciprocating engine.