Power systems group Rolls-Royce has announced the sale of its wholly-owned subsidiary, Tidal Generation to Alstom for an undisclosed sum.The sale of Tidal, which designs, develops and manufactures tidal turbine technologies, is expected to complete within the next few months. At the end of 2011, Tidal has gross assets of £10.6m. Rolls-Royce shares declined 0.59% to 846.50p by 14:05.NR