Google custom search allows you to create vertical search engines, focussing on specific topics. You can shortlist sites to be searched, and results will be either limited to these sites, or the shortlisted sites will be prioritised in the search result.
For a custom search engine on sustainable energy, see this link.
So far, 9 websites have been listed (LE, IEA, ECEEE, ACEEE, ASE, …). Comments welcome which sites we should add. Also some feedback on testing the engine is welcome.
If we’re satisfied, the search form and search results can be seamlessly integrated into various websites of our network and partners.
You can see a mockup of how the search engine would work on
http://www.leonardo-energy.org/drupal/node/1970