What is the energy efficiency of biofuels as an alternative to fossil fuels? Ethanol is produced from biological materials such as sugarcane or corn. How is ethanol produced as a biofuel (chemical methods/biotechnology) and how energy efficient is it?
Proper selection and use of biofuels can be more sustainable environmentally than use of fossil fuels.
This is a very complex problem, and not really a single answer of "efficiency". Efficiency is a ratio of energy input to energy output. It is always less that 1.0 for ANY system due to the Second Law of Thermodynamics. OTHER factors may also be considered, such as impacts on other aspects of society or the environment.
Ethanol is an excellent example, as its production requires a great deal of energy input, results in a great deal of emissions, and often impacts other useful societal factors, such as food supply. SO, just looking at the "end product" use of ethanol vs. gasoline, for example, is NOT evaluating either its efficiency as a fuel nor its sustainability as a fuel source.
There are similar issues with the full "lifecycle" of many so-called "hydrogen economy" mechanisms.
With that said, there are many advantages to particular types of biofuel production and uses. Particularly, oils and fuels derived from non-food plants that can be grown in areas unsuitable for food crops. Also, capture and use of biogas generated from wasted that are produced whether we use them or not can add a substantial "sustainability" boost to the original materials lifecycles.
When comparing energy options, ALWAYS look at the WHOLE lifecycle, from equipment and energy needed to produce it, to the actual energy availability, to the waste products (there are always some) and especially its potential impacts on other environmental and social factors.