Hi Nhat Px, hope that I can help.  
  
The question you posted didn't mention anything about "human being are facing the shortage of fossil fuels" at all. In that case may I suggested that to avoid putting anything not appear on the question to your introduction paragraph.  
  
Forgive me if I was wrong, but I guess the question is about "whether the use of alternative source of energy a positive or negative development", so you may have to choose either side just like the way when you are dealing with "Agree or Disagree" questions. I heard some may choose so call 'Balancing approach' which gives the answer of supporting both sides. I admire those who able to give thus the answer in the short time frame (i.e. within 40 minutes on an IELTS test). For me I will not take that risk as I know myself very well - I am not the good writer still! Oops, sorry for the off topic, in short I would suggest you to stick with one side and state your position in the introduction paragraph. This help readers to prepare what they expected to see in the body paragraphs.  
  
And for the second paragraph, it sounds to me that the topic sentence of it is to tell readers the use of "conventional fuels" are expensive. And the reason behind is that 1. the harvestation cost is high and 2. the transportation cost is high. I'm not good in grammar but again, as a reader, I think it would be nice if you can make a little amendment to the sentences such as list all the causes first and then tell the consequences. And I believe it will help you to gain some marks on "coherence and cohesion" as well.  
  
Similar approach as 2nd paragraph to the 3rd one. From there you mentioned that:  
  
Its residents definately suffer ...  
So let me try breaking your idea down to:  
- Shanghai residents suffer from toxic air  
- Shanghai residents cannot go outside without face mask or other protective clothes (protective clothes... are you serious!?)  
- Shanghai factories release contaminated air  
- those contaminated/toxic air generated by the factories using traditional fuels  
  
Now, please don't laugh at me, I'm trying very hard to rearrange your sentence... you know if I'm the good writer, I do not need post my childish essaies here...  
  
...factories there still burn a lot of traditional fuels when in production. Thus the process releases a lot of pollutants such as the toxic contaminated air. This brings the detrimental health impact to the citizen and hence they can only go out with facemask and protective cloth to avoid the inhale of those toxic gas.  
  
Ok, let's stop laugh out loud and move on the 4th paragraph. I guess you are trying to tell readers how good it is to use those "alternative source of energy". And one of your topic sentence is:  
  
Using alternative energy allows society to take full advantage of natural power and maintain our environment balance and health.  
May I suggest that try to tell us more WHY it is the case (i.e. why such the approach allows society to take full advantage of ...)? And also any supporting information for your answer (i.e. if your answer of the above topic sentence is: well, it is clean. Then please explain why it is clean, such as: well, it doesn't like fossil fuels that when burn it release toxic gas and carbon dioxide, which bala bala... it is the most evil enemy of the plant, etc)?  
  
And for the conclusion part, I guess you had been influenced by your introduction paragraph so you bring along the information/task that not mentioned on the original question. Say if my guess was right that it is a agree/disagree type of question, then in the conclusion part you may have to restate your point of view (costly and dirty of using traditional source of energy, how good on using that alternative source of energy) as well as your choose side (the agree I believe).  
  
Sorry, lengthy, again, I'm not a good writer, even to make sentence short... thanks for your patience.