This a quick overview of what kind of renewable energy is available. The following document has some useful information for comparing different forms of renewable and conventional energy: Australian Power Generation Technology report
There is an excellent book on solar energy, "Let It Shine" by John Perlin which goes through the history of solar energy right up to modern times.
Photovoltaic cells are arranged on flat panels coated with of treated silicon. When sunlight hits the cells, electricity starts flowing.
This technology has been getting cheaper and cheaper and it is now cost-effective. But the sun does not always shine, so this technology has to be supplemented by other renewable energy and/or energy storage.
Solar thermal power plants work by heating up a body of molten salts or other fluids. This heat is then used to drive turbines which generate electricity. These systems store energy in the form of heat.
Solar thermal electricity is more expensive than photovoltaic electricity. It has fallen out of favour because photovoltaic energy has dropped so much in price. Currently renewable energy is used in conjunction with fossil fuel. In an environment where fossil fuel burning has ceased, this technology may perhaps make a resurgence.
Solar energy can also be used directly for heating. It can be used to heat buildings provided the buildings are oriented to take advantage of the sun. And solar energy can be used to heat water.
This is another cost-effective and increasingly used technology which uses wind to drive a turbine to generate electricity. Wind also fluctuates although it is rare for there to be absolutely no wind. Also, demand peaks are often caused by unusually hot or cold weather. Such weather usually coincides with above-average wind which brings in hot or cold air from different latitudes. In other words, wind energy is unlikely to let us down when we need it most: during heat waves and cold spells.
Wind turbines often kill birds and bats which fly into the path of the blades. Research into minimising this problem is ongoing. One approach already in use is to keep wind turbines away from the areas where birds of endangered species are most likely to fly.
Capturing the energy from water as it moves from a greater height to a lower height and using this energy to create electricity is a well-established and cost-effective technology. However, its application is limited by the availability of suitable sites.
Far below ground, temperatures are a lot higher than at the surface. This heat can be used to generate electricity. Iceland, which has favourable geological conditions, produces 25% of its electricity that way. Conditions in Australia are not as favourable, so this may not be the right technology for us. See Clean Energy Council and Australian Renewable Energy Agency