Cass Balzer writes: “When the old Medford (Mass.) Public Library (MPL) opened in 1960, sustainability wasn’t even part of the conversation. Thanks to a municipal bond and strong local support, however, in 2017 the library had the opportunity to construct a new building that was not only modern but also environmentally friendly. MPL is one of the many libraries across the country on a mission to contribute to a greener future. The most ambitious of this group are committing to net-zero energy,” by producing as much energy as they consume.