Graduate / Statement of Purpose for a PhD in Wind Energy [6]

This is really proffessional sounding! I am just 6 months into school at 53. I am by far no pro, but check your spelling and comma usage. (2nd par.) should read a five year degree & graduated is misspelled; (3rd par.)drop comma after Electrical Machines; (5th par.) Filed should be field & unites should be units.

Comma splice in (6th par.) Should read: During my Master studies, not only have I learned about water, waste, soil and air pollution treatment and management, but I also learned about conventional, sustainable and renewable energy and technologies.

Check last sentence of (7th par.)
(8th par.) comma after 'In the past,'; 'work result has' or 'work results have'; individually on a project,
(9th par.) 1st sentence is a run on sentence; put a comma after process, or make it 2 sentences.
It sounds like it needs to be very professional. The liitle mistakes could make a difference. Like I said, I am by far no pro, but punctuation and writing is what I am studying right now.

Graduate / Statement of Purpose for a PhD in Wind Energy [6]

I'm not sure what you are trying to say here. Maybe try:
With the combination of previous technical and economical experiences, I am confident that I will live up to the expectations with an aftereffect of one hundred percent success.

A cover letter is the first thing they read. If you don't convince them (breifly) that you are qualified for the position here, they more than likely won't read anything else. Let them know that you know something about the company (so they don't think you picked them out of a phone book.

I hope this helps you!!