Presumably promised in the medium and long terms to be a significant development, most solar energy conversion technologies are not mature enough yet to compete with conventional energy on a large scale. There is, of course, an exception: hydro, and some forms of renewable energy that are beginning to be economically viable.

Technical constraints are mainly threefold:

• the available power density is relatively low, which entails large areas of collectors and high material costs;

• the variability of the source is high: solar energy fluctuates a lot, which often requires complex control systems;

• the need to store. Being an energy-flow, it requires storage for most applications, which poses a problem because we do not presently store energy in good conditions.

Economic constraints are twofold:

• a high capital cost. Even when properly managed, the technologies are relatively expensive in investment, while their operating costs are generally low;

• the need for a back-up. In case of unavailability of the source, another energy is often required, imposing additional costs that can become significant.

Their development requires the control of their complexity and appropriate financing mechanisms, which may be difficult to implement at a decentralized level.

Depending on their origin, we can distinguish five major categories of renewable energy:

• geothermal energy, provided by the Earth;

• direct solar energy (thermal and photovoltaic);

• indirect solar energy (wind, hydro, biomass);

• energy of the sea;

• energy from draft animals

In developing countries, food is generally produced, processed and transported through energy of men and animals. Thus energy of draft animals is and will be for a long time the main source of energy to ensure farm work and transportation in these countries. The conversion efficiency could be greatly improved if farming, food and health equipment were modernized, and if they were incorporated in other rural systems such as the production of grain, meat, milk, fiber, fuel and fertilizer (animal waste).