How to increase my chance for being admitted to a PhD if my MSc is in a different topic?
The change of topic should not be a serious problem as long as you have a solid CS background.  What is more likely the problem is your choice of new topic, and whether the schools where you are applying have faculty working in green computing.

For PhD programs, your best bet is to send a targeted and well-informed letter to professors working in the field that interests you.  I cannot stress the well-informed part enough - this is more than just looking at the faculty member's web page.  Read some of his/her papers, think up of suggested research directions or potential improvements of the research.  Demonstrate an ability to understand the professor's research and provide novel and meaningful ideas.