The Stirling engine and its role in microCHP

The Stirling Engine, or external combustion engine as it is sometimes known, was invented in 1816 by the Reverend Robert Stirling, a Scottish minister. It predates the ‘internal combustion engine’ with which everyone is familiar by a considerable time but has never been fully developed because of high material costs and the high cost per unit power. 

 

These machines are very suitable for use in small scale combined heat and power units, known as microCHP, rated at less than 5kW electrical output.

 

They do have advantages over conventional internal combustion engines in that they can run on any available heat source which could be anything from natural gas, biomass fuel and even solar power (see Fig 1) if the suns heat is concentrated on the heat side of the engine.  

 

They also have no valves and they are sealed and no injection into the operating chambers is required.  Also, Stirling engines have a relatively low operating pressure, so they are safer than higher pressure engines and can run without an air supply. Importantly, the only exhaust gases are from the external source. As this source could be from a renewable source, then the Stirling engine can be truly described as being capable of fully ‘renewable power’ generation. 

 

These engines are also remarkably quiet which makes them suitable for situations such as for use in homes. They theoretically can have an efficiency of up to 40% but generally they are about 30% efficient at present.

 

There are some disadvantages in that Stirling engines take some time to start and are not as flexible in varying power output as conventional engines.

 

 

A Stirling engine combined with a gas or other fuel fired boiler can produce heat and power output for use in a home and replace a more conventional heat only boiler, a combined heat and power unit (CHP). 

 

These are currently being manufactured and installed by a number of manufacturers, but are still expensive compared to a heat only boiler. The rated electrical output of these units is at present about 1kW which is sufficient to power a house, but the electrical power is only available when the unit is producing heat for heating or hot water. 

 

Currently, such units are not available in an ‘off grid’ mode, where they can be operated independently of an external mains supply. They have to be connected to an electrical source and are eligible for a Feed in Tariff (FIT) of 10p per unit of electricity when installed by an MCS qualified installer.

 

 

The operation of a Stirling engine is best described with a series of illustrations. This unit - as described - is a single cylinder version. Other versions of the engine use two cylinders with pistons connected by external flywheel or crankshaft. 

 

The heat source is at the bottom of the cylinder with the cool part of the cylinder at the top. This cool area could be water cooled in some types of engine which helps achieve a good temperature difference between the two ends of the cylinder. 

 

The illustrations do not show the ‘regenerator’ which is a heat store placed between the hot and cool areas which helps retain some heat and improves the efficiency of the unit. This may be as simple as metal gauze which allows a free flow of air but retains some of the heat. 

 

The engine has two pistons – a power piston and a displacer piston. The power piston produces power applied to the external crankshaft and flywheel. This is necessarily a pressure tight fit into the cylinder. The other piston is a ‘displacer’ which moves freely within the cylinder and allows air or the gas used within the engine to flow around it and pass from the hot side of the cylinder to the cool side.

 

If we take one complete cycle - first the power piston (at the top of the cylinder) is at the bottom of its cycle and has compressed the gas and the displacer piston is at the top of its cycle and so most of the gas is at the hot end of the cylinder. The two pistons are connected to the same flywheel - see Fig 2.

 

As the gas is heated it expands and increases in pressure and pushes the power piston upwards and turns the common flywheel - see Fig 3.

 

The displacer piston is moved further down into the cylinder and pushes the hot gases past it into the cooled portion of the cylinder - see Fig 4.

 

The cooled gas is now compressed by the power piston being pushed down by the flywheel. The displacer piston is now at the bottom of its cycle and the gas at the heated end starts to expand starting another cycle - see Fig 5.

 

The operation is very simple and small units can achieve very high speeds. In the very small units which work on low temperature differences, such as the model units advertised as working off the heat of a cup of tea, then the flywheel has to be stared manually!

 

At this point, perhaps take a look at a (mostly) wooden working model of a Stirling engine built by Voltimum managing editor James Hunt. This fun toy is certainly not sustainable (it runs off a candle or mentholated spirit lamp) and it produces almost no power, but it does show a real Stirling engine working, albeit with the cylinders (the only metal parts) aranged differently from the figures in this article - see the YouTube video below:

 

 

Stirling engines, when incorporated into a boiler system, can give overall energy conversion efficiencies of around 90+%. They are at present expensive, which is limiting their market growth, but as production increases then prices will come down. 

 

The qualification for feed in tariffs makes them more economic, but these tariffs are only available at present for the first 30,000 units installed.

 

 

Stirling engines are not only available in small power outputs, but are now being manufactured commercially at 35 kWe and above for use in more conventional CHP schemes. 

 

The use of a boiler and large power output Stirling engines can make a building completely ‘zero carbon’ if the boiler uses renewable resources such as biomass and the electrical output of the Stirling engine can provide the power requirements of the building.