Derry, New Hampshire, has turned a retired landfill into an energy source. A 2.25-megawatt solar project at the site is now operating, and Encore Renewable Energy says it could cut taxpayer-funded electricity costs by about $4 million over the next 25 years. Here's what to know Built on a closed, capped municipal landfill, the installation is already generating power, Solar Power World Online reported. Encore Renewable Energy developed the proje…