Terra Therma is a European Commission funded research project under the Frame Work 6 programme.
In this project a Stirling motor driven heat pump is developed for home heating. In this way the lost heat generated by the Stirling motor is efficiently used as an additional heat input to the heat pump.
The heating system developed exists out of a novel type of domestic heat pump comprising a linear CO2 compressor driven by a natural gas-fired Stirling engine combined with a novel arrangement of slantered ground source heat probes. The main novelty of the system is that the working fluid of the engine and heat pump are identical and that therefore the complete embodiment can be placed inside a single hermetic shell.
The project showed that CO2 is an acceptable fluid for the contradicting requirements of the heat pump and the Stirling engine and that a practical product package can be designed around this concept. Calculations showed that a PERh of 1.8 is possible and therefore, since natural gas costs are generally lower than electrical costs per unit of energy, a substantial cost savings is possible to the end user.