Search strategy and terms for identifying relevant patent profiles

Previous bibliometric studies have explored the many different ways in which patent records can be correctly identified for a given field or topic \cite{Verbeek_2002,Schmoch_1997,Albino_2014,Rizzi_2014,Mao_2015,Dong_2012,Landscape_alternative_energy,helm2014renewable}. Whilst filtering of search results based on technology classification categories is generally preferred where possible to ensure a more rigorous search strategy \cite{Albino_2014}, it is also advisable to keep the steps that supplement or remove patents from searches queries to a minimum to maintain data consistency and repeatability \cite{helm2014renewable}. As such, the search queries used in this analysis are based primarily on filtering by International Patent Classification (IPC) or Cooperative Patent Classification (CPC) labels. Where possible the IPC categories applied have been reused from previous studies in order to replicate existing search queries so as to extract comparative datasets, or have been based on expert defined groupings such as the European Patent Office’s Y02 classification which specifically relates to climate change mitigation technologies. Otherwise keyword search terms and IPC labels are combined that focus on the appearance of closely adjoining instances of the search terms (or of their common synonyms) to be matched. The use of IPC technology category filters in this manner ensures that a higher level of relevance and repeatability is achieved. Based on these preprocessing steps, the final search queries used for the technologies to be considered are presented in Table \ref{table:search_terms}.