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:
Recuperators: This name is given to different types of heat 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. The heat wheel is an example which operates on the same principle as a solar 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 in evacuated 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 its transfer 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.
Heat to power units
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. Such units, for example, uses an Organic Rankine cycle with 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.
A WHRB is different from a Heat Recovery Steam Generator (HRSG) in the sense that the heated medium does not change phase.
Waste Heat of medium (120-650°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 and absorption cooler is required. Furthermore only low pressures and temperatures needed to be handled.
Advantages and disadvantages of waste heat recovery
These systems have many benefits which could be direct or indirect.
Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the costs of fuel and energy consumption needed for that process.
Reduction in Pollution: Thermal and air pollution will dramatically decrease since less flue gases of high temperature are emitted from the plant since most of the energy is recycled.
Reduction in the equipment sizes: As Fuel consumption reduces so the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes.
Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans,...etc.
Capital cost: The 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.
Quality of heat: 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.
During the past years, companies have developed many products for the recovery of the waste heat. A new concept is being introduced by Cyclone Power Technologies that uses an external combustion engine design for the waste heat recovery application.
International Wastewater Systems is another company adressing waste heat recovery systems. Focused on multi-unit residential, publicly shared buildings, industrial applications and district energy systems, their systems use the energy in waste water for domestic hot water production, building space heating and cooling.