Sun emits enormous amount of heat and light energy. Sun in essence is a nuclear reactor in which the hydrogen-helium fusion creates the heat and light energy we see. Earth being a tiny speck considering the size and the mass of the sun, receives only a very tiny fraction of the total energy of the sun. If human beings find a way to tap about one half of the energy incident on the sun we would be producing power and electricity that would be un manageable !
Solar energy will continue to exist for at least another million years. That is when we might expect sun to enter into a different phase in the star lifecycle. Only then can we must worry about another alternate source of energy to sun. At this rate of pollution we might be extremely lucky if our earth was able to survive one tenth of that duration. Further as soon as soon dies our earth would no longer exist in which case we might not be around to worry about energy sources!
There are two different ways to make use of the solar power incident on the earth; the active method and the passive method.
In the active method solar cells are arranged in solar panels. These solar cells are photovoltaic in that they convert the incident light energy into electricity. These solar panels could be connected to dc batteries for storage purposes. This should be enough to handle night lighting and winter lighting.
The solar cells are not cheap. Building a big enough panel to produce energy for a city would require hundreds of square feet of area and cells. This alone is the singular reason why solar energy has not replaced the conventional sources of energy. But with the advent of miniature chip technologies like the nanotechnology, we are not far off from building a realistic solar source that could power a substantial part of the earth. Even now a good number of countries have realized the importance of preventing our environment. They have recognized the enormous potential of sun and have started using it commercially.
The passive solar powering involves building apartments and blocks to make use of the sun directly. Without tapping into the energy part of the sun one can make big use of the sun. Some of the utilities include ventilation, water circulation, cooling and heating. All these processes make use of thermodynamic concepts to directly use the solar heat.
Green house gases are a bane to our world. But green house in general is an effective agricultural practice to grow certain types of crops which need high temperatures and humidity all around the year. Green houses use the sun and its heat energy to trap the heat in an enclosed shelter to accelerate the growth of plants.
Thus this ever present source of power could be tapped to meet every conceivable need of mankind with little intelligent effort on the part of human beings!