Aregenerative thermal oxidizer (RTO) is an example of a waste heat recovery unit that utilizes a regenerative process.
Awaste heat recovery unit (WHRU) is anenergy recoveryheat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, usually increased efficiency. The WHRU is a tool involved incogeneration. Waste heat may be extracted from sources such as hotflue gases from a diesel generator, steam fromcooling towers, or evenwaste water from cooling processes such as insteel cooling.
Waste heat found in theexhaust gas of various processes or even from the exhaust stream of aconditioning unit can be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat. Manysteel 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:[1]
Recuperators: This name is given to different types ofheat exchanger that the exhaust gases are passed through, consisting of metal tubes that carry the inlet gas and thus preheating the gas before entering the process. Theheat wheel is an example which operates on the same principle as asolar air conditioning unit.
Regenerators: This is an industrial unit that reuses the same stream after processing. In this type of heat recovery, the heat is regenerated and reused in the process.
Heat pipe exchanger: Heat pipes are one of the best thermal conductors. They have the ability to transfer heat hundred times more than copper. Heat pipes are mainly known in renewable energy technology as being used inevacuated tube collectors. The heat pipe is mainly used in space, process or air heating, in waste heat from a process is being transferred to the surrounding due to itstransfer mechanism.
Thermal wheel or rotary heat exchanger: consists of a circular honeycomb matrix of heat absorbing material, which is slowly rotated within the supply and exhaust air streams of an air handling system.
Economizer: In case of process boilers, waste heat in the exhaust gas is passed along a recuperator that carries the inlet fluid for the boiler and thus decreases thermal energy intake of the inlet fluid.
Heat pumps: Using an organic fluid that boils at a low temperature means that energy could be regenerated from waste fluids.
Run around coil: comprises two or more multi-row finned tube coils connected to each other by a pumped pipework circuit.
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.
A waste heat recovery boiler (WHRB) is different from aheat recovery steam generator (HRSG) in the sense that the heated medium does not change phase.
According to a report done byEnergetics Incorporated for theDOE in November 2004 titledTechnology Roadmap[2] and several others done by theEuropean 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.[3] Waste heat of different degrees could be found in final products of a certain process or as a by-product in industry such as theslag insteelmaking plants. Units or devices that could recover the waste heat and transform it into electricity are called WHRUs orheat to power units:
Anorganic Rankine cycle (ORC) unit uses an organic fluid as theworking fluid. The fluid has a lower boiling point than water to allow it to boil at low temperature, to form asuperheated gas that can drive the blade of a turbine and thus a generator.
Traditionally, waste heat of low temperature range (0-120 °C, or typically under 100 °C) has not been used for electricity generation despite efforts by ORC companies,[citation needed] mainly because the Carnot efficiency is rather low (max. 18% for 90 °C heating and 20 °C cooling, minus losses, typically ending up with 5-7% net electricity).
Waste heat of medium (100-650 °C) and high (>650 °C) temperature could be used for the generation of electricity or mechanical work via different capturing processes.
Waste heat recovery system can also be used to fulfill refrigeration requirements of a trailer (for example). The configuration is easy as only a waste heat recovery boiler andabsorption cooler is required. Furthermore, only low pressures and temperatures needed to be handled.
Reduced pollution:Thermal pollution andair pollution will dramatically decrease since lessflue gases of high temperature are emitted from the plant since most of the energy is recycled.
Reduced equipment sizes: As fuel consumption reduces, the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes.
Reduced auxiliary energy consumption: Reduced equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans,...etc.
Capital cost to implement a waste heat recovery system may outweigh the benefit gained in heat recovered. It is necessary to put a cost to the heat being offset.[6]
Often waste heat is of low quality (temperature). It can be difficult to efficiently utilize the quantity of low quality heat contained in a waste heat medium.
Heat exchangers tend to be larger to recover significant quantities which increases capital cost.
Maintenance of equipment: Additional equipment requires additional maintenance cost.
Units add size and mass to overall power unit. Especially a consideration on the mobile power units of vehicles.
TheCyclone Waste Heat Engine is designed to generate electricity from recovered waste heat energy using a steam cycle.[7]
International Wastewater Heat Exchange Systems is another company addressing waste heat recovery systems. Focused on multi-unit residential, publicly shared buildings, industrial applications and district energy systems, their systems use the energy inwaste water for domestic hot water production, building space heating and cooling.[8]
Motorsport seriesFormula One introduced waste heat recovery units in 2014 under the nameMGU-H.
