Essentially, I'm wondering (knowing virtually nothing at present), what is at the front of modern research in the realm of energy. Whether this be renewable energy, or simply more efficient ways of using energy (e.g. fuel cells?). I assume nuclear fusion will be mentioned too, so what obstacles are there still to overcome there? Is there one, or several, particular modes of energy production that seem to be the 'front-runner' for the future? Thanks in advance.