Waste heat recovery

A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that recovers heat from hot streams with potential high energy content, such as hot flue gases from a diesel generator or steam from cooling towers or even waste water from different cooling processes such as in steel cooling.

Waste heat found in the exhaust gas of various processes or even from the exhaust stream of a conditioning unit can be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat. Many steel making plants use this process as an economic method to increase the production of the plant with lower fuel demand.
There are many different commercial recovery units for the transferring of energy from hot medium space to lower one:

Particulate Filters (DPF) to capture emission by maintaining higher temperatures adjacent to the converter and tail pipes to reduce the amount of emissions from the exhaust

According to a report done by Energetics Incorporated for the DOE in November 2004 titled Technology Roadmap and several others done by the European commission, the majority of energy production from conventional and renewable resources are lost to the atmosphere due to onsite (equipment inefficiency and losses due to waste heat) and offsite (cable and transformers losses) losses, that sums to be around 66% loss in electricity value. Waste heat of different degrees could be found in final products of a certain process or as a by-product in industry such as the slag in steelmaking plants. Units or devices that could recover the waste heat and transform it into electricity are called WHRUs or heat to power units. For example, an Organic Rankine cycle unit uses an organic fluid as the working fluid. The fluid has a lower boiling point than water to allow it to boil at low temperature, to form a superheated gas that could drive the blade of a turbine and thus a generator. Thermoelectric (Seebeck, Peltier, Thomson effects) units may also be called WHRU, since they use the heat differential between two plates to produce DC Power.

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