OTEC on Enceladus and Europa

G'day,

Would a OTEC system work in the internal ocean of an ice world like Europa
or Enceladus? As I understand it OTEC systems on Earth work because of the
temperture differential between the warm surface water and cold deep water.
There would be a far greater temperture difference between the surface
temperture and the warm liquid water. What would be a good working fluid,
methane?

ta

Ralph