Nowadays, we are facing the chalenge of global temperature increase on the planet, due to gases released by industrial processes and social ways of living. Most people don´t even realized that most of electric energy is produced by the combustion of fossil fuels (ex: in portugal 60% of the electric energy produced is based on the combustion of fossil fuels) This process has some inefficients inerents to the transportation of the energy and the effecience of the production process, the overall energy balance is of about(50%), that means that half of the energy released by the combustion is lost on the process. Some industries produce their own energy (cogeneration) increasing the efficient to up to 60%, by using thermal energy released on the combustion, this is not an alternative energy but it´s a rational use of energy preventing the losses described above.
Solar energy, is a free energy and many processes are being studied (adsorption refrigeration), is a process being developed to substitute conventional air conditioners (see : Solar driven refrigerator).
Hidrogen is told to be an alternative energy source, i personaly doubt it (see : hidrogen - is this the answer?). Proven technologies are the use of wind energy, and the substitution of fossil fuels, by natural fuels like bio-ethanol.