School project or something like that, I assume?

It's often a good idea to start with the Wikipedia article in question (http://en.wikipedia.org/wiki/Renewable_energy), find out some relevant issues, and use those keywords for further searches. (I'm not recommending you to quote Wikipedia or believe whatever it says, just to use it as a starting point.)

A quick Google found me these as well: (http://www.suite101.com/content/key-issues-that-renewable-energy-should- address-a114696) and (http://www.cato.org/pubs/pas/pa-280.html) - the second one is a long read, but looks very useful. Try further Googling.

Can't help much with the specifics since I'm a noob on environmental matters :P

I wouldn't necessarily say there are problems with renewable energy, merely problems with plans to implement renewable energy sources. Primary concerns are over the fact that the world's economy is essentially run by oil. Therefore, in order to compete in a global marketplace, clean energy must become more cost-effective compared to coal and other fossil fuels, otherwise the nations dependent on clean energy would immediately face a cut in economic output and a lowered standing in the global competitive market compared to the nations who stayed with coal and fossil fuels.

Another as you said, was space, i.e. where would we place all the energy facilities. Solar energy is currently so inefficient, it would take huge swaths of land to produce significant sources of energy; land that could be used for other tasks, like agriculture and cities. Wind energy may harm natural wildlife (birds in turbines), and to avoid that problem, we could deploy "flying" turbines into the stratosphere to generate energy from the incredibly rapid wind up there. However, then we sacrifice popular transportation routes, and therefore costs of transportation would increase. Therefore, wind energy would have to produce enough energy cheaply to offset that issue.

Nuclear energy, the obvious issue is what to do with waste. Even as we are improving our nuclear facilities to be able to recycle the nuclear waste, there is still an incredible amount of superheated, irradiated water used for cooling, among other wastes of the plant, that cannot be eliminated. There will always be some amount of waste that will last for thousands of years. Additionally, nuclear is perhaps the most dangerous (e.g. Chernobyl), although safety standards are improving (except for in the US which has horribly outdated nuclear facilities).

Geothermal can also be dangerous, because although it can run at all times, in all weather, it may tap into important stress points in the Earth's crust, and rupture fault lines, or perhaps even trigger a volcanic eruption (since most geothermal plants are placed over such "hot spots").

Biofuel is currently in its infancy. For any new renewable technology, it is important to assess how much of an increase in costs in the other areas of life will occur because of the new energy. Take producing fuel from corn: it caused the price of corn in supermarkets to skyrocket. As such, biologists and geneticists turned to engineer a sort of oil-producing algae and bacteria, and while they have successfully bred such organisms, it is currently not economically feasible or cost effective to provide a reasonable pacifier to our fossil fuel needs.

Hydrogen power (splitting of water into H2 and O) seems like the perfect technology (it's only exhaust is water vapor), but safe, and powerful hydrogen fuel cells are still far in the future, and consumer's likely won't see the first hydrogen-powered car for years.

Another difficulty lies in our current infrastructure. It is entirely geared toward a fossil-fuel-based economy: gasoline fuel stations, rubber tires, cement, make up, plastic bags, etc. Without the investment of states and the federal government to 1) update our current infrastructure to the 21st century and 2) take the green economy into consideration when updating the infrastructure, we will not see any kind of revolution towards a cleaner environment.

(Basically, don't try to say renewable energy is bad, but point out the current problems with each kind of potential energy source. Use the fact that all this science is still in its infancy to your advantage).