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After Philips cancelled their Stirling engine development programme in the 1950’s, little advancement in Stirling engines took place for the next two decades. It is likely at this point that any development of a Stirling engine for commercial gain was considered unfeasible, after seeing the financial failure of the Philips programme. Despite this, a radical new design was conceived in 1970 by D. West, a research scientist at Harwell Laboratories. His engine, pictured in Figure 13, used a liquid piston arrangement and was incredibly simple, using only bent tubes and ball valves and is able to pump water. Essentially, it functions in
the same way as a regular Stirling engine, with a hot and a cold space, and a volume of water which displaces heated or cooled air.

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Figure 13: Liquid-piston, or fluidyne, Stirling engine built by Colin West in 1970 [9]

Another pioneering Stirling engine researcher was William Beale, inventor of the free­piston Stirling engine. This is an engine with few moving parts and no crankshaft, where the piston and displacer motion relies on oscillations controlled by gas pressure and springs or dampers. This is discussed in more detail in Section 2.2.4.

Modern AdvancesIn 1978 an engine was conceived for an entirely new application — powering a submarine. A Stirling engine is actually very suitable for this as they are silent and vibration free (making the submarines harder to detect), they don’t require air for combustion (depending on heat source) and they are surrounded by a very effective heatsink, the sea.

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Figure 14: (a) United Stirling submarine engine and (b) later improved version (right) with wobble-plate drive [9]

Swedish company United Stirling was responsible for making the engine in Figure 14 (a), a 4 cylinder double acting engine. This engine was based on a Philips design that United Stirling purchased the licence agreement for. Several years later, the engine was improved by replacing the complicated drive mechanism with the ingeniously simple wobble-plate drive mechanism as shown in Figure 14 (b). The wobble-plate drive was originally conceived in the 19th century by Sir William Siemens however the concept was never realised until 1949 when Philips experimented with it in the early days of their development programme [9]. This version, developed by R. J. Meijer (formerly of Philips) featured a variable angle wobble plate which overcame many of the failings of the earlier versions.

In 1985 McDonnell Douglas designed a large solar parabolic mirror setup capable of tracking the sun across the sky and focusing its energy on a centrally mounted Stirling engine that could reach up to 1430°C [9]. They had an exclusive deal with united Stirling to use their Stirling engines, which could produce 25kW of electrical power with a very respectable overall thermal to electric efficiency of 31% [9].

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Figure 15: McDonnell Douglas Solar Stirling engine system [15]

These large parabolic mirrors have found their way into several installations worldwide, most notably the recent (2005) joint venture between electricity supplier Southern California Edison and Stirling engine manufacturer Stirling Energy Systems. This agreement will see the installation of some 20,000 solar Stirling dishes into a 1,800 ha area of the Mojave Desert, for a total power generating capacity of 500 MW — the largest of its type in the world at the time of the agreement (until the 900 MW project in Imperial County, Southern California was announced). More recently the project was expanded up to 34,000 dishes totalling 850 MW. The advantages of solar Stirling systems is high efficiency (exceeding that of parabolic troughs and non-concentrated photovoltaics), relatively low cost per kW compared with other solar technology and high life expectancy (the Stirling engine used is the 25kW unit same as that pictured in Figure 14 (a), and has been tested for 26,000 hours of continuous operation) [16].

An interesting and very different type of Stirling engine is the thermoacoustic Stirling engine — an engine that converts heat into intense acoustic power with no moving parts. The sound waves produced can be converted into electricity by way of a linear alternator or electro-acoustic power transducer, or used directly in acoustic refrigerators or pulse-tube refrigerators to provide heat-driven refrigeration [17]. The invention of the engine, pictured in Figure 16, is credited to researchers at the Los Alamos National Laboratory (LANL).

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Figure 16: Thermoacoustic Stirling engine developed by LANL

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