The output of a natural, 'sustainable' energy source is usually in the form of a steady flow which is quite unsuited to the sudden surges of power needed by a tramway. To cope with the sudden demand, the apparatus would need to be uneconomically large in comparison with the average energy requirements.
As more sustainable energy sources are connected to the national electricity grid, tramways will find themselves using more 'green' energy with the surges being met from other plant. In contrast, internal combustion engines cannot, at present, use any sustainable energy sources.
Storage methods, such as batteries, which could supply the peak demands and recharge slowly during the interval between, have low efficiency and in themselves damage the environment during manufacture and disposal.