Or can we combine both the concepts as energy efficient fault tolerant cloud computing? Will that be feasible and acceptable? Or else can anyone kindly suggest alternate ideas so that my research activities could get the right direction?
in my opinion fault tolerance in cloud computing will be good for you, you will able to get more information in this area and you can publish research papers in this area easily and area has lot of scope.
I believe this depends on your specialization. Efficient use of Energy could be much better solved by the hardware manufacturers to provide a reduced energy consumption scheme. In cloud environments, it is expected that the servers would be up and running almost 24 x 7 x 365 due to the expected number of simultaneous requests/hits from allover the world. Any mechanism to provide efficient utilization of energy should consider a sleep interval for the servers which is not the case in heavy traffic clouds.
I suggest you may work on efficient task scheduling in clouds to provide near optimum workload that will eventually lead to better utilization of energy.