Waste heat is the heat generated by the machine, or other processes that use energy, as a by-product of doing the work. All such processes emit heat as a fundamental result of the law of thermodynamics. Waste heat has a lower utility (or in thermodynamic lower lexicon of exergy or higher entropy) than the original energy source. Wasted heat sources include all kinds of human activities, natural systems, and all organisms, for example, refrigerators warm the air of the room, internal combustion engines produce high temperature exhaust gas, and electronic components become warm when operating.
Instead of being "wasted" by the release to the surrounding environment, sometimes wasted (or cold) heat can be exploited by other processes (such as using a heat engine coolant to heat a vehicle), or some of the heat that would otherwise be reused in the same process if heat make-up is added to the system (such as with heat recovery ventilation in the building).
Thermal energy storage, which includes technology for both short and long-term heat or cold retention, can create or improve utility from waste heat (or cold). One example is the exhaust heat from an air-cooled engine stored in a buffer tank to aid night warming. Another is the seasonal thermal energy storage (STES) at a foundry plant in Sweden. The heat is stored in the bedrock that surrounds a group of heat-proof gauges equipped with drill holes, and is used for heating the room at adjacent plants as needed, even months later. An example of using STES to utilize natural waste waste is the Drake Landing Solar Community in Alberta, Canada, which, using a bunch of drill holes in bedrock for intakeasonal heat storage, earns 97 percent of the year-round heat from a solar thermal collector on a garage roof. Another STES app keeps cold cold underground, for summer air conditioning.
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.
Anthropogenic heat waste is considered by some to contribute to the urban heat island effect. The largest source of waste heat comes from machinery (such as electrical generators or industrial processes, such as steel or glass production) and heat loss through building envelopes. The burning of transportation fuels is a major contribution to waste heat.
Video Waste heat
Energy conversion
Machines that convert the energy contained in fuel to mechanical work or electrical energy generate heat as a by-product.
Maps Waste heat
Source
In most energy applications, energy is required in various forms. This form of energy usually includes several combinations: heating, ventilation, and air conditioning, mechanical energy and electric power. Often, these additional forms of energy are generated by a hot engine, running at a high-temperature heat source. The heat engine can not have perfect efficiency, according to the second law of thermodynamics, therefore the heat engine will always produce a low-temperature heat surplus. This is usually referred to as waste heat or "secondary heat", or "low heat". This heat is useful for most heating applications, but it is sometimes impractical to transport heat energy over long distances, unlike electricity or fuel energy. The largest proportion of total waste heat comes from power plants and vehicle engines. The largest single sources are power plants and industrial plants such as oil refineries and steelmaking plants.
Power plant
The electrical efficiency of a thermal power plant is defined as the ratio between input and output energy. Usually only 30%. The pictures show a cooling tower that allows a power plant to maintain the low side of the temperature difference essential for conversion of heat difference to other energy forms. Flue or "Waste" of heat lost to the environment can be used for profit.
Industrial process
Industrial processes, such as oil refining, steelmaking or glass manufacturing are the main sources of waste heat.
Electronics
Although small in terms of power, waste heat dissipation from microchips and other electronic components, is a significant engineering challenge. This requires the use of fans, heatsinks, etc. To get rid of heat.
Biological
Animals, including humans, create heat as a result of metabolism. In warm conditions, this heat exceeds the level required for homeostasis in warm-blooded animals, and is removed by various thermoregulation methods such as sweating and panting. Fiala et al. modeling human thermoregulation.
Disposal
Low heat temperatures contain very little capacity to perform work (Exergy), so heat qualifies as waste heat and is rejected into the environment. The most economical is the rejection of heat like water from the sea, lake or river. If sufficient cooling water is not available, the plant must be equipped with a cooling tower to reject the discharge heat into the atmosphere. In some cases it is possible to use waste heat, for example in heating the house with cogeneration. However, by slowing the release of waste heat, this system always requires a reduction of efficiency for the major users of thermal energy.
Usage
Cogeneration and triggeration
Waste from heat byproducts is reduced if cogeneration systems are used, also known as Combined Heat and Power (CHP) systems. Limitations of the use of product heat arise mainly from the cost/engineering efficiency challenges in effectively exploiting small temperature differences to produce other forms of energy. Applications utilize waste heat including swimming pool heating and paper mills. In some cases, refrigeration can also be produced by the use of absorption refrigerators for example, in this case called triggers or CCHP (combined cooling, heat and power).
Pre-heating
Waste heat can be forced to heat the incoming fluids and objects before it gets very hot. For example, the exhaust water can provide its waste heat to the water entering in the heat exchanger before heating at home or power plant.
Electrification of waste heat
There are many different approaches to transferring heat energy to electricity, and the technology to do so has been around for decades. The organic Rankine cycle, offered by companies like Ormat, is a well known approach, in which organic matter is used as a medium of work, not water. The benefit is that this process can reject heat at lower temperatures for electricity production than the regular water vapor cycle. An example of the use of the Rankine steam cycle is the Cyclone Waste Heat Engine. Another established approach is to use thermoelectric, as offered by Alphabet Energy, where temperature changes across semiconductor materials create voltage through a phenomenon known as the Seebeck effect. The related approach is the use of thermogalvanic cells, in which temperature differences generate electrical current in electrochemical cells.
Anthropogenic heat
Anthropogenic heat is the heat generated by humans and human activity. The American Meteorological Society defines it as "Heat released into the atmosphere as a result of human activity, often involving the burning of fuel.Sources include industrial plants, space heating and cooling, human metabolism, and vehicle exhausts.In these source cities usually contribute 15 - 50 W/m 2 to local heat balance, and several hundred W/m 2 in the center of major cities in cold climates and industrial estates. "
Estimates of anthropogenic heat generation can be performed with a total of all the energy used for heating and cooling, running equipment, transportation, and industrial processes, plus those directly emitted by human metabolism.
Environmental impact
Anthropogenic heat is a small effect on rural temperature, and becomes more significant in densely populated urban areas. This is one of the urban heat island contributors. Other human-induced effects (such as albedo changes, or loss of evaporative cooling) that may contribute to urban hot islands are not considered anthropogenic heat by this definition.
Anthropogenic heat is a much smaller global warming contributor to greenhouse gases. In 2005, although anthropogenic waste heat flux was significantly high in certain urban areas (and may be high regionally.For example, the exhaust flux is 0.39 and 0.68 W/m 2 for the continental United States and Europe West, respectively) globally it accounts for only 1% of the energy flux created by anthropogenic greenhouse gases. The global coercion of waste heat was 0.028 W/m 2 in 2005. This statistic is predicted to increase as urban areas become larger.
Although waste heat has been shown to have an effect on the regional climate, climate coercion from waste heat is not usually calculated in state-of-the-art global climate simulations. The equilibrium climate experiments show statistically significant continental heating (0.4-0.9 Â ° C) produced by a 2100 AHF scenario, but not based on current estimates or 2040. A simple global scale estimate with different growth rates than the recently actualized anthropogenic heat shows a real contribution to global warming, in subsequent centuries. For example, 2% p.a. the growth rate of waste heat produces a 3 degree increase as the lower limit for the year 2300. Meanwhile, this has been confirmed by the finer model calculations.
See also
- District heating
- Waste heat recovery unit
- Heat recovery steam generator
- Pinch analysis
- The relative cost of electricity generated by various sources
- Hot island city
- Urban metabolism
References
Source of the article : Wikipedia
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