My son have an interesting school project to look into sustainable energy source for an island. The island will be the size of Singapore. One of tasks is the review of sustainable energy. He is required to find out the pros and cons of each energy sources. Fossil fuel is the default energy source for current usage.

Popular Sustainable Energy
Natural energy can come from light, wind or water. Popular choices of alternative energy are solar power, windmill or tidal. Solar farm remains the most popular choice in Singapore due to the constant sun in equator. Windmill is another popular choice but it is suited for large open space with strong winds. This choice is definitely out due to densely build up areas. Tidal energy is the most interesting but we seldom see these implemented. This could also be due to the small coastlines of Singapore.
Sustainable Ecosystem
The sustainable ecosystem is also a big challenge. The landscape have not been designed to hassle these energies. We have been so reliance on fossil fuels that the entire ecosystem consist of it. The switch to sustainable energy will be painful and costly. After all, existing demand and critical mass of fossil fuels are keeping the cost low, leaving consumers little incentive to switch to cleaner energy.
It will be a long journey to move to sustainable energy. Initiatives like solar farm are simple efforts to build these ecosystem. These pace are still slow but will Earth wait for consumers to switch. Only time will tell!