What is the main advantage of waste heat recovery in industry?

Heat that is produced by a machine that uses energy, as a byproduct of doing work

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Air conditioning units extract heat from a dwelling interior with coolant, and transfer it to the dwelling exterior as waste. They emit additional heat in their use of electricity to power the devices that pass heat to and from the coolant.

Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility (or in thermodynamics lexicon a lower exergy or higher entropy) than the original energy source. 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.

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).

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). One example is waste heat from air conditioning machinery stored in a buffer tank to aid in night time heating. Another is seasonal thermal energy storage (STES) at a foundry in Sweden. 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.[1] 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.[2][3] Another STES application is storing winter cold underground, for summer air conditioning.[4]

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 waste heat can contribute to the urban heat island effect.[5] 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. The burning of transport fuels is a major contribution to waste heat.

Conversion of energy

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Machines converting energy contained in fuels to mechanical work or electric energy produce heat as a by-product.

Sources

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In the majority of applications, energy is required in multiple forms. These energy forms typically include some combination of heating, ventilation, and air conditioning, mechanical energy and electric power. Often, these additional forms of energy are produced by a heat engine running on a source of high-temperature 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. This is commonly referred to as waste heat or "secondary heat", or "low-grade heat". 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. The largest proportions of total waste heat are from power stations and vehicle engines.[citation needed] The largest single sources are power stations and industrial plants such as oil refineries and steelmaking plants.[citation needed]

Air conditioning

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Conventional air conditioning systems are a source of waste heat by releasing waste heat into the outdoor ambient air whilst cooling indoor spaces. This expelling of waste heat from air conditioning can worsen the urban heat island effect.[5] Waste heat from air conditioning can be reduced through the use of passive cooling building design and zero-energy methods like evaporative cooling and passive daytime radiative cooling, the latter of which sends waste heat directly to outer space through the infrared window.[6][7]

Power generation

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The electrical efficiency of thermal power plants is defined as the ratio between the input and output energy. It is typically only 33% when disregarding usefulness of the heat output for building heat.[8] The images show cooling towers, which allow power stations to maintain the low side of the temperature difference essential for conversion of heat differences to other forms of energy. Discarded or "waste" heat that is lost to the environment may instead be used to advantage.

A coal-fired power station. These transform chemical energy into 36&#;48% electricity and the remaining 52&#;64% to waste heat.

Industrial processes

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Industrial processes, such as oil refining, steel making or glass making are major sources of waste heat.[9]

Electronics

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Although small in terms of power, the disposal of waste heat from microchips and other electronic components, represents a significant engineering challenge. This necessitates the use of fans, heatsinks, etc. to dispose of the heat.

For example, data centers use electronic components that consume electricity for computing, storage and networking. The French CNRS explains a data center is like a resistor and most of the energy it consumes is transformed into heat and requires cooling systems.[10]

Biological

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Humans, like all animals, produce heat as a result of metabolism. In warm conditions, this heat exceeds a level required for homeostasis in warm-blooded animals, and is disposed of by various thermoregulation methods such as sweating and panting.[11]

Disposal

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Low temperature heat contains very little capacity to do work (Exergy), so the heat is qualified as waste heat and rejected to the environment. Economically most convenient is the rejection of such heat to water from a sea, lake or river. If sufficient cooling water is not available, the plant can be equipped with a cooling tower or air cooler to reject the waste heat into the atmosphere. In some cases it is possible to use waste heat, for instance in district heating systems.

Uses

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Conversion to electricity

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There are many different approaches to transfer thermal energy to electricity, and the technologies to do so have existed for several decades.

An established approach is by using a thermoelectric device,[12] where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as the Seebeck effect.

A related approach is the use of thermogalvanic cells, where a temperature difference gives rise to an electric current in an electrochemical cell.[13]

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The organic Rankine cycle, offered by companies such as Ormat, is a very known approach, whereby an organic substance is used as working fluid instead of water. The benefit is that this process can reject heat at lower temperatures for the production of electricity than the regular water steam cycle.[14] An example of use of the steam Rankine cycle is the Cyclone Waste Heat Engine.

Cogeneration and trigeneration

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Waste of the by-product heat is reduced if a cogeneration system is used, also known as a Combined Heat and Power (CHP) system. Limitations to the use of by-product heat arise primarily from the engineering cost/efficiency challenges in effectively exploiting small temperature differences to generate other forms of energy. Applications utilizing waste heat include swimming pool heating and paper mills. In some cases, cooling can also be produced by the use of absorption refrigerators for example, in this case it is called trigeneration or CCHP (combined cooling, heat and power).

District heating

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Waste heat can be used in district heating. Depending on the temperature of the waste heat and the district heating system, a heat pump must be used to reach sufficient temperatures. These are an easy and cheap way to use waste heat in cold district heating systems, as these are operated at ambient temperatures and therefore even low-grade waste heat can be used without needing a heat pump at the producer side.[15]

Waste heat can be forced to heat incoming fluids and objects before being highly heated. For instance, outgoing water can give its waste heat to incoming water in a heat exchanger before heating in homes or power plants.

Anthropogenic heat

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Anthropogenic heat is heat generated by humans and human activity. The American Meteorological Society defines it as "Heat released to the atmosphere as a result of human activities, often involving combustion of fuels. Sources include industrial plants, space heating and cooling, human metabolism, and vehicle exhausts. In cities this source typically contributes 15&#;50 W/m2 to the local heat balance, and several hundred W/m2 in the center of large cities in cold climates and industrial areas."[16] In , the overall anthropogenic annual energy release was 168,000 terawatt-hours; given the 5.1× m2 surface area of Earth, this amounts to a global average anthropogenic heat release rate of 0.04 W/m2.[17][18]

Environmental impact

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Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense urban areas.[19] It is one contributor to urban heat islands. Other human-caused effects (such as changes to albedo, or loss of evaporative cooling) that might contribute to urban heat islands are not considered to be anthropogenic heat by this definition.

Anthropogenic heat is a much smaller contributor to global warming than greenhouse gases are.[20] In , anthropogenic waste heat flux globally accounted for only 1% of the energy flux created by anthropogenic greenhouse gases. The heat flux is not evenly distributed, with some regions higher than others, and significantly higher in certain urban areas. For example, global forcing from waste heat in was 0.028 W/m2, but was +0.39 and +0.68 W/m2 for the continental United States and western Europe, respectively.[21]

Although waste heat has been shown to have influence on regional climates,[22] climate forcing from waste heat is not normally calculated in state-of-the-art global climate simulations. Equilibrium climate experiments show statistically significant continental-scale surface warming (0.4&#;0.9 °C) produced by one AHF scenario, but not by current or estimates.[21] Simple global-scale estimates with different growth rates of anthropogenic heat[23] that have been actualized recently[24] show noticeable contributions to global warming, in the following centuries. For example, a 2% p.a. growth rate of waste heat resulted in a 3 degree increase as a lower limit for the year . Meanwhile, this has been confirmed by more refined model calculations.[25]

A scientific paper showed that if anthropogenic heat emissions continue to rise at the current rate, they will become a source of warming as strong as GHG emissions in the 21st century.[26]

See also

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References

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&#;Waste heat recovery&#; is the process of &#;heat integration&#;, that is, reusing heat energy that would otherwise be disposed of or simply released into the atmosphere. By recovering waste heat, plants can reduce energy costs and CO 2 emissions, while simultaneously increasing energy efficiency.

Several common consumer items recover waste heat. For example, consider turbocharged cars, which are provided by multiple car manufacturers.

In regular, non-turbocharged cars, the internal combustion gasoline engine expels hot gas through the car&#;s exhaust after its fuel is burned. That gas contains both heat and kinetic energy&#;a portion of which can be recovered. Turbocharged engines divert the hot gas to a turbine, which is used to spin an air compressor. The compressed air is routed to the engine&#;s combustion chamber with the vaporized gasoline, resulting in a more efficient ignition and greater power made with lower fuel consumption.

The energy benefits of industrial waste heat recovery can be similar, and some examples are examined in this article.

Economizer

Stack economizers, commonly used to heat water, are among the simplest type of waste heat recovery. Plant workers may be familiar with these common devices that recover waste heat. Boiler stack economizers use heat energy from the gas expelled in the heating process into the stack to heat boiler feed water and reduce the amount of energy required to make steam. According to the US Department of Energy, installation of a boiler feed water economizer can raise the thermal efficiency of a boiler and reduce fuel consumption by 5 - 10%.

Waste Heat Boiler (WHB)

Using a principle similar to economizers, waste heat boilers recover heat generated in furnaces or exothermic chemical reactions at industrial plants. These locations may contain significant energy that should not be wasted up a stack. Instead, this energy can be captured to generate low-to-medium pressure steam in a waste heat boiler (WHB). A WHB can also be used to remove the heat from a process fluid that needs to be cooled for either transport or storage, and generate steam from that heat. The steam generated in WHB may be used for heating applications, or to drive turbines that generate electricity, compress vapors, or pump liquids. WHB steam may contain significant wetness, so it is recommended that a high efficiency separator and steam trap combination is installed to ensure that the WHB delivers optimal quality steam to the recipient process.

Heat Recovery Steam Generator (HRSG)

Many highly efficient industrial plants with cogeneration or combined cycle systems use a gas-turbine (essentially a jet engine) to generate electricity then create steam from the waste heat using a heat recovery steam generator (HRSG). This section will explain how this process works and how an HRSG comes into action.

Consider the previous example of a turbocharged car engine, but instead change the motor to a jet engine. The gas-turbine/jet engine is fired using natural gas and its exhaust contains extremely hot vapor that would simply be expelled to atmosphere if some of the heat and kinetic energy were not captured. So, how can that waste heat be used as was done with the turbocharged auto? The expelled hot gas needs to drive another turbine, so the exhaust is passed through a HRSG, which creates superheated steam that drives a downstream steam-turbine. The turbine can either drive a generator (combined cycle system) or just use the steam in process applications (cogeneration or combined heat and power (CHP)). HRSGs can have either a single steam drum (as shown in the animation below) or multiple steam drums and pressures. There are also both unfired varieties with natural circulation (shown below) and varieties with duct firing, which is additional heating. Duct firing increases steam generation and quality, and has the ability to create superheated steam and even greater power at a turbine.

Absorption Chiller

Some energy efficient CHP systems may add refrigeration by incorporating absorption chillers that use steam created from waste heat.

The mechanism of absorption chillers can be broken down into the following stages:

• Diluted absorbent liquid (60% lithium bromide salt, 40% water) is preheated with condensate as it moves from the absorber to the generator.
• In the generator, the absorbent liquid is heated by steam, which causes some of the water contained within it to vaporize, making the liquid more concentrated.
• The concentrated absorbent liquid flows downward back to the absorber to capture more water in its continuous loop. At the same time, the water vapor moves to the condenser.
• In the condenser, cooling water causes the vapor to liquify and flow downward toward the evaporator, which has near vacuum pressure.
• Just prior to entering the evaporator, the condensed water flows through an orifice or expansion valve. This restriction limits flow, and the water again vaporizes to a cold mist at 4.5°C [40°F]. This is where refrigeration occurs. The low temperature vapor pulls heat out of the chilled water loop, lowering its temperature by about 5°C [9°F].
• The mist is heated when it absorbs heat from the chilled water loop, but the strong attraction of the concentrated salt solution pulls the water into the concentrated absorbent into the absorber, creating near vacuum pressure in the evaporator in the process.

• Evaporator: Actual refrigeration occurs here. Water passes through a valve, creating a low temperature mist. Sprayed over the chilled water loop, it absorbs heat and cools the refrigerant.
• Absorber: Concentrated absorbent attracts water mist from the evaporator, creating near vacuum pressure while diluting the absorbent.
• Generator: The 60/40% mixture absorbent is heated, evaporating the water, with the concentrated (regenerated) absorbent returning to the absorber.
• Condenser: The water vapor supplied from the generator is condensed by transference of heat to cooling water.

Steam Thermocompression

Thermocompression is another heat recovery approach that can utilize the energy in an otherwise wasted stream, such as low-pressure steam. This is the process of mixing high-pressure and low-pressure steam to create an intermediate pressure using steam compression equipment such as TLV&#;s SC System, which is shown below.

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