想 means you are just thinking about it and have not acted out .for instance i wanna be a president 。(我想当总统) 要 its a stronger will than 想 and you just make up ur mind to do it .like since its 6 oclock i have to go home for dinner now. (已经6点了,我要回家吃晚饭了) 想要 kinda indecisive .like may i talk to you later ,i wanna have a rest now .(我晚些时候和你谈，可以吗？我现在想要休息一下) in one word ,there is not much difference between them .

我要吃饭。(already sure, can't be change.) 我想吃饭。(not sure yet, perhaps can change) 我想要吃饭。(much more sure than 想 )

我要去四川. 表示动作即将发生.(will) 我想要去四川. 只是自己的一个想法,能去还是不能去还不能确定.(want ) 我在想问题.(think)

I am going to travel around the world (我要去环球旅行， already decided， or you are going to do sth。。。要 means want or decided to。。) I want（wish）to go travel around the world ( just a popped up idea( not seriously or unlikely to happen) 我想去环球旅行 I want （ wish）to go travel around the world 我想要去环球旅行 ( more formal and with stronger emotions）,for another instance, I want to be the chairman of PRC.(我想要当主席）。Generally the latter 2 words are interchangeable , first one is more likely to be used in daily conversation, and the latter is more appropriate to be used in writing.

我要一杯可乐。 我想上大学。 我想要一个家。