The Readville Yard 5 in Dedham will soon be home to a field of solar panels, after the Massachusetts Department of Transportation’s board of directors recently approved a 20-year contract with Gehrlicher Solar America Corp. of New Jersey. Until this spring, when the Massachusetts Bay Transportation Authority decontaminated the site, it was used as storage for railroad-related materials. Now, according to the MBTA, the company will design, install, and maintain a solar “photovoltaic” system at the Dedham location and the Wonderland Garage in Revere. The MBTA hopes the project saves money and increases the agency’s use of renewable energy. The two sites are estimated to cut electricity costs by about $48,900 a year, and now that the contract is signed, Gehrlicher has one year to complete construction.