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Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.
In some circumstances the use of an enabling technology, either diurnal thermal energy storage or seasonal thermal energy storage (STES, which allows heat or cold storage between opposing seasons), is necessary to make energy recovery practicable. One example is waste heat from air conditioning machinery stored in a buffer tank to aid in night time heating. Another is an STES application at a foundry in Sweden. Waste heat is recovered and stored in a large mass of native bedrock which is penetrated by a cluster of 140 heat exchanger equipped boreholes (155mm diameter) that are 150m deep. This store is used for heating an adjacent factory as needed, even months later. An example of using STES to recover and utilize natural heat that otherwise would be wasted is the Drake Landing Solar Community in Alberta, Canada. The community uses a cluster of boreholes in bedrock for interseasonal heat storage, and this enables obtaining 97 percent of the year-round space heating from solar thermal collectors on the garage roofs. Another STES application is recovering the cold of winter by circulating water through a dry cooling tower, and using that to chill a deep aquifer or borehole cluster. The chill is later recovered from the storage for summer air conditioning. With a coefficient of performance (COP) of 20 to 40, this method of cooling can be ten times more efficient than conventional air conditioning.
A common utilization of this principle is in systems which have an exhaust stream or waste stream which is transferred from the system to its surroundings. Some of the energy in that flow of material (often gaseous or liquid) may be transferred to the make-up or input material flow. This input mass flow often comes from the system's surroundings, which, being at ambient conditions, are at a lower temperature than the waste stream. This temperature differential allows heat transfer and thus energy transfer, or in this case, recovery. Thermal energy is often recovered from liquid or gaseous waste streams to fresh make-up air and water intakes in buildings, such as for the HVAC systems, or process systems.
Energy consumption is a key part of most human activities. This consumption involves converting one energy system to another, for example: The conversion of mechanical energy to electrical energy, which can then power computers, light, motors etc. The input energy propels the work and is mostly converted to heat or follows the product in the process as output energy. Energy recovery systems harvest the output power and providing this as input power to the same or another process.
An energy recovery system will close this energy cycle to prevent the input power from being released back to nature and rather be used in other forms of desired work.
There is a large potential for energy recovery in compact systems like large industries and utilities. Together with Energy conservation it should be possible to dramatically reduce the world energy consumption. The effect of this will then be:
A 2007 Department of Energy study found the potential for 135,000 megawatts of combined heat and power (which uses energy recovery) in the U.S., and a Lawrence Berkley National Laboratory study identified about 64,000 megawatts that could be obtained from industrial waste energy, not counting CHP. These studies suggest about 200,000 megawatts—or 20% -- of total power capacity that could come from energy recycling in the U.S. Widespread use of energy recycling could therefore reduce global warming emissions by an estimated 20 percent. Indeed, as of 2005, about 42 percent of U.S. greenhouse gas pollution came from the production of electricity and 27 percent from the production of heat.
It is, however, difficult to quantify the environmental impact of a global energy recovery implementation in some sectors. The main impediments are
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