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The second main category of Stirling engine is a version in which the piston floats freely within a hermetically sealed cylinder thus without an external linkage to drive a generator. This elegant development of the Stirling principle is even more silent than the previous examples and is ideal for domestic use. Sunpower Inc., a US company, plans to introduce a system to the UK in 2007 which is based on a Stirling engine which incorporates an alternator within the cylinder which produces 1 kW of electricity at 50 cycles. This makes it compatible with the grid. The top of the cylinder is heated in the conventional way. The base is cooled by return water from a radiator system which is sufficiently cool to create the pressure difference needed to power the piston. It will be marketed in the UK by Microgen and promises to be an attractive alternative to a conventional domestic boiler (see Fig. 9.10). The UK is a particularly favourable market for this technology for several reasons:

• A high proportion of households are owner-occupied.

• Of the 22 million or so gas consumers, over 14 million have central heating and rising. This is essential to absorb the heat dumped into the cooling water.

Figure 9.7 The components of the 565 kW STM Stirling CHP unit (courtesy of STM Power)

Free Piston Stirling Cooler
Figure 9.8 The STM unit in situ (courtesy of STM Power)



Compression Space


Expansion Space

Beta Engine


| Regenerator

Cooler Heater

Figure 9.9 Principle of the single cylinder beta Stirling engine

• All consumers are linked to a reliable grid electricity supply, necessary to import power when necessary.

• The ratio of heat demand to electricity demand in a typical house is —5:1, which is compatible with the heat-to-electricity ratio output of the Stirling engine.

• As an addition to a conventional central heating boiler the target pay-back time is 3-4 years. This may reduce as the price of gas and electricity continues to rise.

• Microgen claim that, if every suitable home in the UK used this technology, it would reduce its CO2 emissions by 25%.

Sunpower Inc. is also currently developing a four cylinder free piston engine linked to a gas turbine output stage. The pistons are connected to gas compressors. The compressors are then used to drive a gas turbine. The prototype machine uses double acting compressors so that the turbine receives eight pulses of energy for each cycle of the engine. This promises to be a low-maintenance, high-reliability technology, mainly due to the absence of heavily loaded moving parts.

Another application of Stirling technology is being researched in The Netherlands. This links the FPSE to a groundwater source heat pump to become the free piston Stirling heat pump (FPSHP). It is a hermetically sealed system in which the mechanical output of the FPSE using helium is directly connected to a Rankine cycle heat pump using CO2 as its working gas. The alternator within the FPSE powers components such as the water circulation pump, indoor fan, burner blower and electronics. The system will supply heat for

Stirling Alternator

& alternator

Figure 9.10 Sunpower/Microgen free piston domestic Stirling CHP unit

& alternator

Figure 9.10 Sunpower/Microgen free piston domestic Stirling CHP unit domestic hot water, and space heating. It will be capable of operating in either heating or cooling mode.2

To summarize the benefits of the Stirling engine:

• it can use any combustible fuel from agricultural and forestry waste to biogas and natural gas

• it is up to 80% efficient when used for CHP

• it is robust, being of simple construction and requiring low maintenance

• it is quiet running, which makes it suitable for domestic application

• it is durable, offering up to 60,000 hours of life

• it is competitively priced compared with conventional boilers, taking into account the electricity generation.

Later in the decade it is possible that the Stirling engine will play a significant part in the shift to combined heat and power in the domestic sector. This is because it is one of the few CHP units presently available which can be scaled down to suit an individual house. The Microgen FPSE 'personal CHP' can be wall-mounted to produce up to 15 kW of heat as well as 1 kW of electricity.

According to MicroGen the estimated average cost saving in electricity was £200 per year in 2003. Since then electricity prices have risen considerably. Hopefully the UK government will sanction the widespread adoption of net metering on a one-for-one basis as a way of promoting microgeneration. Only then will householders get a fair deal for their home produced electricity. The situation should be helped when the whole of the EU has liberalized the electricity market by 2010 at the latest.

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Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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