Despite many differences in agronomic practices and in cultivated crops, all traditional agricultures shared the same energetic foundation. They were powered by the photosynthetic conversion of solar radiation, producing food for people, feed for animals, recycled wastes for the replenishment of soil fertility, and fuels for smelting the metals needed to make simple farm tools.
The books from Vaclav Smil are a trove of knowledge on energy evolution. This book discusses the evolution of human energy advances over time, from agriculture to weapons.
The book reads more as an academic article, with a plethora of references and sources. One sixth of the book is just references. Very dense in knowledge and explanations, it overwhelms the reader with the sheer depth of analysis.
Smil tries to use largely a single energy unit, joules, to measure everything, from the various techniques to harness animals to work to the different ways to pass water through the watermills. The purpose is to quantify the evolution of human energy efficiency over time.
The book is encyclopedic in its depth and range, truly a history. The book dryness of writing and data is broken by very informative and engaging boxes, explaining various facts and developments.
The only downside is the grammar errors found here and there sometimes.
I was impressed by the precision and correct analysis of energy sources and transformations, missed by many pundits.
Also impressive is the general neutral tone regarding various sources that the author manages to impose.
Overall, an incredible book, THE book on energy history.
Solar energy is a plentiful energy source, radiance energy coming from sun is at the order of 85,000 to 120,000 TW, while current world energy consumption is around 18 TW (IEA, Key Energy Statistics, 2014; Coimbra, Photovoltaic and Photothermal Energy Production: Future of Energy, 2014).
For electricity, solar energy technologies are broadly divided into photovoltaic cells (direct PVs) (direct conversion to electricity: photons to electrons); solar thermal power (using heat, also known as concentrated solar power) and other technologies (solar Stirling engines etc.).
Conventional PV cells have a very low efficiency, only 20%, but multi-junction cells, which absorb photons from different parts of the solar spectrum have efficiencies around 40% (however, they are 100 times more expensive than conventional PV cells).
Solar thermal power has an intermediary step, where solar energy is transformed first into thermal energy than into electrical energy. Mirrors can heat either a central liquid transforming it into steam or a working fluid, usually a high-temperature oil, in small tubes.
The biggest problem with solar energy is its variation, season to season, day to day, morning to evening. Daily variations can be somewhat balanced because daily electricity consumption roughly coincides with higher solar radiance, however storage is a must for long term development of the technology (Coimbra, Photovoltaic and Photothermal Energy Production: Future of Energy, 2014).