Micro-scale Combined Heat and Power

Technology Overview

The market for domestic, micro-scale combined heat and power (CHP) systems  is expanding rapidly worldwide. The current small and micro-scale CHP offering is generally based on established and proven technologies of Stirling or internal combustion (IC) engines. Stirling engine systems are hindered by poor electrical efficiencies at small scale; current commercially available systems have ~12% efficiency at 1kW output power. IC engines can provide much better efficiencies, >20% at 1kW power output, but suffer from noise and vibration, in addition to higher emission levels. Both systems further suffer from high manufacturing costs, resulting in long payback periods even with governmental incentives. 


The technology described herein provides a radically new concept for small and micro-scale combined heat and power generation, based around an external combustion, reciprocating Joule cycle (RJC) system. The RJC engine has been studied by a number of researchers with very promising results. However, the energy converter in previous systems, which is based on conventional, crankshaft engine technology, is unable to provide the required conversion efficiency.

Technology Features & Specifications

In order to realise the full potential of the RJC engine, the proposed system uses a double-acting free-piston engine power converter with high operational flexibility and extremely low mechanical losses. Studies performed so far have suggested that efficiencies higher than those of current IC engine system can be obtained while maintaining the advantages of a mechanically simple system. Due to the external combustion, fuel flexibility and emissions levels will be very low and comparable to existing micro scale CHP system based on Stirling engine systems.

Potential Applications

The micro-scale CHP technology can be used for a number of applications and specifically could be an alternative to conventional boiler or to any stationary CHP systems. Typically such systems require significant capital investment. With government energy incentives, achieving a payback time of three years or less is potentially achievable with this invention.

Customer Benefits

Through the use of an efficient thermodynamic cycle and low engine frictional losses, the free piston CHP concept can achieve a fuel economy superior to that of conventional technologies, improving the viability of micro-scale CHP system technology and giving significantly reduced CO2 emissions.

Benefits include:

  • Fuel flexibility
  • Low emission levels
  • Short term payback

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