You may have heard that entropy is a measure for disorder, or chaos, but this is not quite true and in any case a useless definition for the layman's understanding of what renewable energy is all about.

Instead think in terms of 'energy quality', which means what effort is required to make an energy source available to the user. The higher the effort, the lower the quality of it, which means the higher the entropy is.

Thus, if the source of consideration has a low temperature, or occurs widely distributed over large areas, much work (effort) must be done to bring it into one point of usage and/or into a usable form - it has a low quality (high entropy). This work must be deducted from what we finally get out from it and thus the lower the quality of the source, the lower the efficiency of gaining useful energy from it becomes.

Moreover, if fossil fuels are used to do the extra work (like harvesting, transporting and preparing bio-mass, for example), the 'green' aspect of it really becomes very disputable. This basic principle, (being a consequence of the second law of thermodynamics) is largely, usually totally ignored in everything you read on the subject of renewable energy sources.

Hence, solar (panels) and wind energy (propellers), as well as bio-fuels, all being widely distributed over large areas, have per definition a low efficiency as usable energy sources, regardless what technologies are used. The result is that they never can become commercially viable in large scale applications (the tax-payer's money is wasted on large-scale projects in the field, being for the purpose of political prestige only).

There are two exceptions though:

1. Hydro-electric power. The forces of nature (solar energy) collect rain water in high situated reservoirs, ready for us to use (high quality, low entropy source). However, it is only available in mountainous areas and thus of no significance on a global scale (mountainous Norway on the contrary gains 80% of its electricity from water dams, Sweden 50%)

2. Heat-pumps. The forces of nature (wind = solar energy) bring ambient heat (temperature of the surrounding atmospheric air) from basically unlimited areas to the cold side of the machine, that pumps it up to a higher temperature on its other, hot side and that can be used for heating homes. The temperature levels are too low for the generation of electricity (with steam turbines), but for direct heating of homes, heat-pumps are infinitely superior to solar panels.

Hydrogen does not occur in free form on Earth and is therefore NOT an energy source. At best it can be used as an energy-converter (like in fuel cells) and always at (far) lower than 100% efficiency - more energy is needed to produce Hydrogen, than what is gained from using it. Those who argue differently, do not know what they are talking about (likewise cars running on water only  - another violation of the first law of themodynamics).