Energy is available in many forms such as mechanical, chemical, thermal, electric, etc… These energies can be shifted from one form to another at the cost of losing some of their potential, or strength. The exception to this is thermal energy, which represents that loss itself and all forms of energy will transform to this when used.
When accomplishing a task that requires energy, think of the available sources and which forms they take, and if it is necessary to transform them. A river provides mechanical energy, and while you might be able to turn that into electrical, if your task is to lift something then that energy is best left as is. There exist machines which will pump water uphill, at the cost of directly reducing the velocity of water (hydraulic ram pump).
For designing new and efficient systems, an understanding of physics and mathematics is essential. Electric generators rely on electric and magnetic fields, ram pumps rely on fluid dynamics, and solar panels rely on quantum effects. These can be used to predict the behavior of an energy system before designing it, and to produce equations which allow for fine-tuning of a system.