Plant & Works Engineering

Whitelegg Machines Help with a Stirling Project

Published:  30 November, 1999

It was in 1816 that the Revd. Robert Stirling propounded the idea of a heat engine which he termed an ‘Economiser’ now universally known as a ‘Stirling Engine’.  

The operational concept is that it acted as a heat exchanger, retaining heat from the hot portion of the engine as air passed to the cold side, and released heat to the cooled air as it returned to the hot side and in the process converting energy to mechanical work, with near perfect efficiency. 

Stirling engines were widely developed in the 19Century until the advent of the electric motor superseded it. 

Modern versions of the engine are in existence, having been used for specialist civil and military applications. But the need for the development of alternative power sources and the availability of modern technology and materials have resulted in renewed interest and exciting developments in this area. 

Just one of these is from Microgen Energy Limited, a subsidiary of BG Group. Microgen are specialists in Stirling engine technology and the modern application of micro combined heat and power (microCHP) incorporating their patented, ‘Free-Piston’ Stirling engine.

The Free-Piston Stirling engine has only two moving parts. The operating force is a temperature difference or pressure wave between the top and bottom parts of the engine. In a microCHP appliance, natural gas is used to heat the top of the engine whilst water returned to the appliance via the heating pump is circulated through the lower part of the engine to provide cooling.  

These differing temperatures drive a free floating, sealed piston up and down. This action occurs 50 times a second to generate electricity (50 Hz). 

Both Baxi in the UK and Remeha, a Dutch boiler manufacturer are working with Microgen to incorporate the technology into a new generation of their domestic boilers. 

The Free-Piston Stirling engine  will produce 5kW of thermal heat for a central heating and hot water system, also 1kW of electricity at 230V 50Hz for direct use in the home. In addition, the appliance also houses a second heat exchanger and burner, similar in design to traditional condensing boilers, to provide more heat for the home during periods of high demand. Excess power is exported back to the grid. 

During the development stage of the project Roger Silcock, senior engineer at Microgen, was looking for an effective motor test instrument to test the wound coils of the alternator prior to assembly in the engine. As the unit is completely sealed in use, failure of the alternator/generator was not an option. 

Following research Roger contacted Whitelegg Machines, motor test equipment specialists who demonstrated the Baker D3R digital winding tester

This instrument is designed to maximize testing and predictive maintenance in a lightweight, sturdy format. The D3R’s high precision testing capability allows engine data collection in the workshop or the field. The results can then be printed immediately or stored for later use. As many of Baker's instruments, this machine is IEEE compliant. 

The D3R Digital Winding Tester can perform Resistance, HiPot and Surge tests as well as digitise and store data for future use.  

The Resistance Test verifies the existence of dead shorts within the turn-to-turn coils and shows any imbalances between phases due to turn count differences, along with locating poor wire connections or contacts. The machine with its 4 terminals is ideal for this. 

DC HiPot testing detects faults in groundwall/earth insulation, and also provides a complete polarization index test.  

Surge testing of up to 3Kv detects faults in both inter-turn winding and phase-to-phase insulation systems. Using advanced analog-to-digital conversion hardware, the "D" series captures the surge test waveform, displays it, and allows a print-out to made, if required. 

Scaled up production will be taking place in Japan of the boiler units and the latest Baker automatic testers have been specified for the plant.  


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