The ocean thermal energy conversion or OTEC has a large potential for providing energy to humanity at least in tropical zones. it is often considered as a technology which will provide a constant energy in contrary to other renewable energy such as wind or waves. Different processes can be developed to extract this energy and generate electricity but all of them use the surface hot water as the hot source. However this temperature is subject to change. This study addresses the effect of the hot water temperature change and mainly its decrease on different processes that could be used to convert the thermal energy in electricity. A slight decrease of this temperature of 0.25°C can reduce the net power produced by more than 10%. Controls on different parameters can be implemented on these processes. They will allow to almost cancel the temperature effects but these controls must be very sensitive to very small temperature changes.