This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike electricity or fuel energy. This is commonly referred to as waste heat or 'secondary heat', or 'low-grade heat'. A heat engine can never have perfect efficiency, according to the second law of thermodynamics, therefore a heat engine will always produce a surplus of low-temperature heat. Often, these additional forms of energy are produced by a heat engine, running on a source of high-temperature heat. These energy forms typically include some combination of: heating, ventilation, and air conditioning, mechanical energy and electric power. In the majority of energy applications, energy is required in multiple forms. Machines converting energy contained in fuels to mechanical work or electric energy produce heat as a by-product. The burning of transport fuels is a major contribution to waste heat. The biggest point sources of waste heat originate from machines (such as electrical generators or industrial processes, such as steel or glass production) and heat loss through building envelopes. On a biological scale, all organisms reject waste heat as part of their metabolic processes, and will die if the ambient temperature is too high to allow this.ДЄnthropogenic waste heat is thought by some to contribute to the urban heat island effect. Another STES application is storing winter cold underground, for summer air conditioning. An example of using STES to use natural waste heat is the Drake Landing Solar Community in Alberta, Canada, which, by using a cluster of boreholes in bedrock for interseasonal heat storage, obtains 97 percent of its year-round heat from solar thermal collectors on the garage roofs. The heat is stored in the bedrock surrounding a cluster of heat exchanger equipped boreholes, and is used for space heating in an adjacent factory as needed, even months later. Another is seasonal thermal energy storage (STES) at a foundry in Sweden. One example is waste heat from air conditioning machinery stored in a buffer tank to aid in night time heating. Thermal energy storage, which includes technologies both for short- and long-term retention of heat or cold, can create or improve the utility of waste heat (or cold). Instead of being 'wasted' by release into the ambient environment, sometimes waste heat (or cold) can be used by another process (such as using hot engine coolant to heat a vehicle), or a portion of heat that would otherwise be wasted can be reused in the same process if make-up heat is added to the system (as with heat recovery ventilation in a building). Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation. Waste heat has lower utility (or in thermodynamics lexicon a lower exergy or higher entropy) than the original energy source. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. They emit additional heat in their use of electricity to power the devices that pass heat to and from the coolant Air conditioning units extract heat from a dwelling interior with coolant, and transfer it to the dwelling exterior as waste.

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