ORC based micro-cogeneration systems for residential application - A state of the art review and current challenges

The environmental awareness and sustainability concerns have reached an unprecedented level of importance that is leading to, among other things, constant improvements in the traditional methods of energy conversion. One of those improvements involves the use of micro-CHP (combined heat and power production) systems for residential applications since the potential in terms of primary energy savings and GHG (greenhouse gases) reductions is considered to be enormous. Among the different technologies available for micro-CHP systems, ORC based ones seem the most suitable and promising option due to its simplicity and its ability to retrofit current heating systems used in residential dwellings. This work reviews the use of ORC for micro-CHP applications taking into account the intrinsic requirements of the residential sector. One of those requirements is the ability to face highly variable thermal demand loads for which a short response time is necessary to ensure. The present research analyzes how manufacturers and the researcher centers are dealing with this key requirement. One of the findings of this work is that the referred requirement is essentially dependent on the ORC-evaporator design and how the primary energy reaches the power cycle, while the remaining ORC main components (expander, pump and condenser) play a minor role on that ability. Additionally, this research also offers an analysis of the micro-CHP potential and of the ORC market evolution, presenting a historical perspective of the technology, current main manufacturers, main application areas and a comparison between the use of an organic fluid and water/steam as working fluid for Rankine cycles.