- Electrical and Computer Engineering - University of Canterbury - New Zealand
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Stirling Engine Info.

Stirling Engines in General

Basics

Stirling engines work by expanding and contracting gas in a piston, just like other kinds of engines such as petrol, diesel and steam engines. The big difference is that the gas Stirling engines use is completely self-contained or ‘closed’ so does not need to come from some sort of fuel source (like igniting petrol, or steam from a boiler). All you need to get a Stirling engine to go is a temperature difference of some sort that expands and contracts the enclosed gas.

The easiest way to produce a high temperature difference is to burn things like waste material, oil, gas, hydrogen, etc. Burning a wide variety of stuff may be very handy, but tends not particularly environmentally friendly. Since only a temperature difference is required, you could use solar power or other naturally occurring heat sources to run a Stirling engine.

Stirling engines can be very energy efficient (more than 50% in some cases which is better than most high efficiency gas or steam turbines and way better than internal combustion engines, which struggle to reach 25%), so even the ones designed to burn fuel produce less pollution than other types of engine.

A modern high temperature Stirling engine.

The main problem with Stirling engines is that it is difficult to accelerate them quickly, so they aren’t really much good for using in cars or trucks (although there are examples where people have tried it).

History

First invented by Robert Stirling in 1816, Stirling engines have been around for a while now. There has been quite a lot of work carried out on variations of the original engine and today some quite sophisticated Stirling engines are commercially available. However, owing to the historically plentiful supply of petrochemical products, internal combustion engines still dominate markets where Stirling engines are much better suited both economically and environmentally.

A 10 kW commercially available Stirling engine.

Uses

Apart from cars and trucks, there are a huge number of possible uses for Stirling engines where quick acceleration is not an issue. For transportation they can be used in trains, aeroplanes, and ships (some American nuclear submarines use Stirling engines because they are so efficient and more importantly, quiet). Other well-suited uses are in pumps, electrical generators, and industrial process.

Although Stirling engines are better suited to high power uses (kilowatts and higher), they can be designed for smaller, more domestic applications. For example, a New Zealand company (Whisper Tech) has produced a Stirling engine based system called WhisperGen, that runs on natural gas and provides household electrical power and heating.

WhisperGen by Whisper Tech, a natural gas powered Stirling engine designed to provide domestic electrical power and heating.

Low Temperature Differential Powered Stirling Engine

Motivation/Concept

Most Stirling engines are designed to be as energy efficient as possible, especially those that burn stuff for the heat source. Energy efficiency is better at higher temperatures, so most Stirling engines try to maximise the temperature of the hot part of the engine.

Commercially available solar powered Stirling engines are usually very expensive owing to the large mirrors used to collect and focus sunlight onto a ‘hotspot’ (like using a magnifying glass to focus sunlight into a point hot enough to burn paper). This focussing of light to make a hotspot is important if high efficiency is desired.

A commercial solar powered Stirling engine. Note use of expensive curved mirror.

It is possible to down-play the importance of energy efficiency, and concentrate more on designing a low temperature differential powered Stirling engine that is relatively cheap andhas a low environmental impact.

When talking about solar power, the amount of power you want out of any device determines how much solar energy you need to collect. The more sunlight you need to collect, the bigger the area your collector needs to be. So, a high efficiency Stirling engine will need a smaller collector than a low efficiency one for the same output power. But in New Zealand (especially in rural settings), land space is often not a hugely critical factor (as it would be in Tokyo for example) so a large, low cost and low maintenance solar collector can be used. The really exciting aspect of a low temperature difference solar Stirling engine is that it can still work on very cold, overcast days (a very common condition in New Zealand winters). In this situation, the ground temperature just a metre or so below the surface is warm enough to produce the temperature difference needed to make the engine work. Similarly, it is possible to set up the engine to run overnight if the air temperature falls well below ground temperature.

Another important feature low temperature differential engines is that it can be easily adapted to use heat created from non-solar sources such as sub 70ºC geothermal heat, waste industrial heat, decomposing/composting vegetation or even the burning of waste vegetation/materials (as a last resort)

Uses

Since solar conditions vary significantly and may even not allow the engine to function, solar-powered engines should be used to feed energy into some sort of storage facility. For example, an electric generator can be attached to the engine which charges up a bank of batteries over time. The energy stored in the batteries can then be easily converted to normal mains AC electrical power and used as an individual or isolated mains supply. In fact, it is possible to do away with batteries altogether and feed the electrical power directly back into the national power grid which can be credited to your electricity power bill! In another example, the engine could be connected to a pump to fill a storage tank or reservoir from a low-lying water source.

When using a more constant heat source (eg., geothermal), it is possible to use the power for direct connection to an output application. A good example would be for an isolated domestic electric power generator, where the Stirling engine drives an electric generator that is conditioned to directly produce a mains voltage power supply. For larger power, the generated electricity could be used in National co-generation.

In Conclusion

By sacrificing energy efficiency for low temperature operation in the Stirling engine, alternative and cheap means of creating a temperature difference can be utilised to produce useful output power. These engines can be cheap to build and can be used to supplement power requirements in farms, domestic residences, and other applications where land space is readily available.

For a list of graduate research projects involving Stirling engines and power electronics, follow this link.