Keywords are important! The words you use will determine how successful your search results are. Think about what you know already and what you need to find out about. For example, let’s assume you have a topic title e.g. "Energy Efficient Housing". How well do you know this topic? If this is an entirely new topic to you, the first stage will be general background reading (not journal articles). Having a better understanding of the topic, will give you a better idea of the vocabulary/keywords used, the sorts of questions you might want your dissertation/essay to answer and the sub-topic/chapters that you will research.
The reason you would want to ask yourself questions is that although "Energy Efficient Housing" is a good starting point, when it comes to looking for research conducted in this area, you may want to be both broader...and more specific e.g.: To have a broad overview of this topic, you might want to look at:
Thinking about more specific areas of interest, you might want to look at:
Therefore, sometimes you have to use broad concepts/keywords (to get a better understanding of a topic) and narrow concepts/keywords when you want to pinpoint more specific research/developments.
Remember also, think about synonymous and related terms as well as alternate spellings and root words etc (e.g. "load" is the root of "loads" and "loading").
The likelihood is that each sub-topic/chapter/area will have different keywords that you will use to find the information you want i.e. you will never find all the information you want with one search for 'Energy Efficient Housing', rather it is likely that you will do several different searches specific for each chapter/area of interest.