<Tr> <Td> <Ul> <Li> Anaerobic digestion </Li> <Li> Biofuel </Li> <Li> Geothermal </Li> <Li> Hydroelectricity </Li> <Li> Solar </Li> <Li> Tidal </Li> <Li> Wave </Li> <Li> Wind </Li> </Ul> </Td> </Tr> <Ul> <Li> Anaerobic digestion </Li> <Li> Biofuel </Li> <Li> Geothermal </Li> <Li> Hydroelectricity </Li> <Li> Solar </Li> <Li> Tidal </Li> <Li> Wave </Li> <Li> Wind </Li> </Ul> <Tr> <Th> Sustainable transport </Th> </Tr> <Tr> <Td> <Ul> <Li> Carbon - neutral fuel </Li> <Li> Electric vehicle </Li> <Li> Fossil fuel phase - out </Li> <Li> Green vehicle </Li> <Li> Plug - in hybrid </Li> </Ul> </Td> </Tr>

Explain the principle of generating electricity from hydro energy