Decoding Stirling Engine Designs

Stirling Engines

The original Stirling engine was developed in 1816 by Robert Stirling as an alternative to the early steam engine which tended to explode. However, its practical use was confined to low power domestic applications. The Stirling engine is noted for its high efficiency (up to 40%), quiet operation, and the ease with which it can use almost any heat source, in this case the sun's heat. This compatibility with renewable energy has become significant because of concerns for the US addiction to oil, the carbon footprint, and climate change consequences. The Stirling engine is a waterless CSP technology.

Over the years many different versions of the Stirling engine have been devised. Initially, the simplest version, the Alpha Sterling engine, will be explained. Afterwards a more modern version, the Siemens configuration with four cylinders will be explained. There are also several versions in between.

The Alpha Stirling Engine

An Alpha Stirling engine contains two pistons in separate cylinders, one hot and one cold. The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is situated inside the low temperature heat exchanger. The working gas (usually hydrogen) inside both cylinders comes in contact with the hot cylinder walls which are heated from an external source. The gas is raised to a temperature of approximately 650ºC. The picture on the right is a simplified Stirling engine in operation.

Add a Regenerator

In a real Sterling engine, a lot of energy is lost in the gas transfer between the two cylinders. Hence a major innovation, called the "regenerator", also invented by Robert Stirling, is used to conserve energy and improve efficiency. The regenerator is an internal heat exchanger and temporary heat storage unit placed between the hot and cold cylinders such that the working fluid passes through it first in one direction and then back in the other. Its function is to retain some of the heat within the system that would otherwise be exhausted to the atmosphere. It does this at intermediate temperatures between the minimum and maximum cycle temperatures. The primary effect of regeneration is to greatly increase the thermal efficiency by 'recycling' internal heat which would otherwise pass out of the engine. A typical regenerator design is a stack of fine metal wire meshes with low flow resistance and with the wire axes perpendicular to the gas flow to reduce conduction in that direction.

The Siemens Stirling Engine

The Alpha engine can be compacted into a small multiple cylinder configuration, enabling very high-power output. A schematic diagram of the "Siemens" configuration is shown below. The Siemens configuration does not use two separate pistons, but rather uses the front and back side of one piston called a double-acting piston. With a double-acting piston, the expansion space of the front side of one piston is connected to the compression space of the back side of the adjacent piston through the in-line heater, regenerator, and cooler. The Siemens configuration involves four cylinders each with a double-acting piston 90 degrees out of phase with the next cylinder. The Siemens configuration greatly improves the engine efficiency over the original alpha arrangement. These engines have the power piston connected to the crankshaft by a second connecting rod, which is itself in another small cylinder to eliminate the lateral forces from the crankshaft from putting pressure on the engine cylinder walls. This arrangement is called a "crosshead". A seal is used between the cross-head and piston to separate the high-pressure area from the low-pressure zone. This allows the crosshead surfaces to stay lubricated in the low-pressure area while preventing fouling in the high-pressure engine system.