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H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289 Table 1 Summary table of WHR technologies. Technologies Regenerative Burners Recuperative Burners Economisers Waste Heat Boilers Recuperators Regenerators Rotary Regenerators Run around coil (RAC) Heat Recovery Steam Generator (HRSG) Plate Heat Exchanger Heat Pipe Systems Thermoelectric Generation Piezoelectric Power Generation Thermionic Generator Thermo Photo Voltaic (TPV) Generator Heat Pump Direct Contact Condensation Recovery Inirect Contact Condensation Recovery Transport Membrane Condenser Temperature Range Used High High Low – Medium Medium – High Low – High Medium – High Low – Medium Medium – High High Medium – High Medium – High Medium – High Low High Low – High Low – Medium Medium – High Medium – High Medium – High Benefits Saving fuel by preheating the combustion air and improving the efficiency of combustion. Both the exhaust gas and waste heat from the body of the burner nozzle are capture and more heat from the nozzle is generated. The system maximises the thermal efficiency of a system by recovering low-medium temperature heat from the waste flue gas for heating/preheating liquids entering a system. The system is suitable to recover heat from medium – high temperature exhaust gases and is used to generate steam as an output. The technology is used for applications with low – high temperatures and is used to decrease energy demand by preheating the inlet air into a system. The technology is suitable to recover waste heat from high temperature applications such as furnaces and coke ovens and for applications with dirty exhausts. Rotary regenerators are used for low – medium temperature applications and could potentially offer a very high overall heat transfer efficiency. This unit is used when the sources of heat are too far from each other to use a direct recuperator and when cross contamination between the two flow sources needs to be prevented The system can be used to recover the waste heat from the exhaust of a power generation or manufacturing plant to significantly improve overall efficiencies by generating steam that can be used for process heating in the factory or power generation. Plate heat exchangers have high temperature and pressure operating limits and are used to transfer heat from one fluid to another when cross contamination needs to be avoided. Heat pipes have very high effective thermal conductivities, which results in a minimal temperature drop for transferring heat over long distances and long life that requires no maintenance, as they incorporate passive operation. They have lower operation costs when compared to the other types of heat exchangers. The system produces electricity directly from waste heat and eliminate the need for converting heat to mechanical energy to produce electrical energy. The system can be used for low-temperature waste heat recovery and works by converting ambient vibration such as oscillatory gas expansion directly into electricity. The device is used for high temperature waste heat recovery and works by producing electric current through temperature difference between two media without the use of any moving objects These devices are used to directly convert radiant energy into electricity and offer a better efficiency when compared to other direct electrical conversion devices. Heat pumps transfers heat from a heat source to a heat sink using a small amount of energy and can be used to offer economical and efficient alternative of recovering heat from various sources to improve overall energy efficiency. Heat pumps in particular are good for low-temperature WHR, as they give the capability to upgrade waste heat to a higher temperature and quality. the system uses a direct mixture heat exchanger without a separating wall and can be used to transfer heat from immiscible liquid – liquid and solid – liquid or solid-gas. The system provides the advantage of eliminating cross contamination of the flue gas and water and can be designed to work as a filter of a process. The system works by extracting and delivering the hot water back into the system feed water directly from the exhaust gas through a capillary condensation channel. This way, the water is extracted through a membrane channel rather than directly from the flue gas and so the recovered water is not contaminated and does not require filtering. Limitations The system requires additional components such as a pair of heat exchange media and several control valves to function, which can be complex. The burner and the nozzle needs to be inserted into the furnace body, which may require installation and modification of the furnace. The system may need to be made out of advanced materials to withstand the acidic condensate deposition, which can be expensive. An additional unit such as an auxiliary burner or an after burner might be needed in the system if the waste heat is not sufficient to produce the required amount of steam. To maximise heat transfer effectiveness of the system, designs that are more complicated may need to be developed. The system can be very large in size and have very high capital costs. The system is not suitable for high temperature applications due to the structural stresses and the possibility of deformations that can be caused by high temperature This system is found to have a very low effectiveness when compared to a direct recuperator and needs a pump to operate, which requires additional energy input and maintenance The system requires several components to function and may require an additional burner to improve the quality of the recovered waste heat. On the other hand, the system is very bulky and require on site construction. Parameters such as frequent variation in temperature and load must be studied and based on that suitable heat exchanger design must be chosen to avoid failure of the structure of the heat exchanger for the application To achieve an optimum performance from the heat exchanger, appropriate design, material, working fluid and wick type based on the application and temperature range of the waste heat must be studied and chosen. The system has a very low efficiency of 2–5%, however, recent advances in nanotechnology have allowed electrical generation efficiencies of 15% or greater to be achieved The system are found to have a low efficiency, high internal impedance, need for long term durability and very high cost. The functionality of this technology is shown to be limited to high temperature applications and be inefficient, however, several attempts have been made to improve their efficiency and enable their use for low temperature applications The device is found to have a limited operating temperature range and their efficiency decreases as the temperature increases. Having said that, high efficiency PV cells that can withstand high temperature ranges are also available, however, they are expensive and increase system costs. In order to use this system, the method of capturing the waste heat based on its source and grade must firstly be analysed and in that respect, appropriate heat exchanger and system installation needs to be set up. Due to absence of a separating wall in this heat exchanger, particles from the flue gas can be mixed with the water, which may require filtering before exiting the heat exchanger. The system consists of a heat exchanger which in order to minimise corrosion from acidic condensate should be made from advanced materials and can be expensive. The system employs a capillary condensation channel, which in order to minimise corrosion from acidic condensate, may need to be made from advanced materials and can be expensive. 281PDF Image | Waste Heat Recovery Technologies and Applications
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