Study of an innovative micro-CHP system fuelled by LPG

The work deals with an innovative micro-combined heat and power (micro-CHP) unit based on Stirling engine technology. Although simplicity of design, low-noise operation and multi-fuel feeding make Stirling engines a promising micro-CHP technology for dwelling sector, power-to-heat ratio is lower than alternative technologies (i.e. reciprocating engine, fuel cell, microturbine) resulting in poor global efficiency during low heat demand periods. Switching the system off and buying energy from the grid is demonstrated to be the best managing strategy in those periods, leading to fewer operating hours and longer payback time, especially in the Mediterranean areas. The present study investigates the application of a regenerative heat exchanger to recover part of the thermal power discharged by the unit when the heat demand is low to increase the electric efficiency of the micro-CHP device and its power-to-heat ratio. The heat exchanger main design parameters are defined and the unit performances are discussed on the basis of simulation data. Fuelling the system with liquefied petrol gas (LPG), an electrical efficiency increase of 7% is achieved thus offering interesting energy savings in dwelling sector. In addition LPG is characterized by lower emission than other fuels and can be applied in many areas not connected to the natural gas grid, improving the security of supply. A detached house is considered to assess the technoeconomic unit performances with or without the regenerator. The solution studied guarantees: i) higher Primary Energy Saving (PES) ii) six month reduction in the payback time, iii) 800 increase in operating hours, iv) higher end-user self-efficiency, aiding to reduce the electric input from the grid during peak periods and minimize the risk of congestion of the network. Finally a sensitive analysis is developed, demonstrating that a micro-CHP equipped with this innovative solution is less affected by the variation of the LPG purchasing cost