Passive energy, a term usually used in reference to passive solar energy, is natural energy which is directly harnessed to achieve a desired goal. By contrast, active energy is energy which is used to generate electricity. Passive energy systems require little energy or effort to maintain, and they are designed to replace traditional energy sources, reducing overall energy use rather than just switching the source of the energy.
A number of systems fall within the umbrella of passive energy. In fact, there's a high probability that the structure you are in right now takes advantage of this energy in some form or another. For example, it may have south facing windows if it is in the northern hemisphere, so that it can take advantage of sunlight for warmth. Many people use this type of energy unconsciously, as seen when people orient their furniture in a way which allows them to take advantage of natural light and warmth instead of using artificial systems for heating and lighting.
Passive energy can be used for heating and cooling in lieu of active systems. Passive heating systems can include passive solar tanks to heat water for bathing and cooking, along with windows orientated towards the sun, solariums to gather and trap heat which can be dispersed through a structure, and arrays of materials like tile which absorb heat and radiate it later, keeping temperatures stable in a building without the need for a climate control system.
Passive energy can also be used to drive air currents to improve air circulation and for cooling purposes. Other examples of passive cooling systems include curtains and shades which block sunlight during the heat of the day, or landscaping to cool a structure. Planting trees and shrubs will keep a structure cool in summer, and can help a structure stay warm in winter, by creating an insulating buffer.
People have been using passive energy in construction for hundreds of years, with many of the steps taken to harness this energy being commonsense. Structures specifically designed to be environmentally friendly often integrate passive systems as part of their design, and passive systems can also be added to the design of a conventional structure. One great advantage of this type of energy is that it usually does not require money or energy for maintenance and function, which means that once the system is installed, it will work for years. This is in contrast with active systems, which often require periodic replacement or repair, and may demand regular maintenance.