A waste heat recovery unit (WHRU) is a device that captures heat from parts of a process that produce high temperatures and moves it to another part of the process to improve efficiency. This unit is used in cogeneration, which is a method of using energy from a process for two purposes. Heat can be taken from sources like hot gases from a diesel engine, steam from cooling towers, or water that has been used to cool things, such as in steel manufacturing.

Heat recovery units

Waste heat from exhaust gases in industrial processes or from cooling systems can be used to warm incoming gas before it enters a process. This is a common method for recovering waste heat. Many steel manufacturing plants use this technique to improve production efficiency while using less fuel. Several types of commercial systems transfer heat from hotter areas to cooler ones:

• Recuperators: These are heat exchangers where exhaust gases pass through metal tubes that carry incoming gas, warming it before it enters the process. A heat wheel is an example that works similarly to a solar air conditioning unit.
• Regenerators: These systems reuse heat from the same stream after it has been processed. The heat is collected and used again in the same process.
• Heat pipe exchanger: Heat pipes are highly efficient at transferring heat, moving it hundreds of times faster than copper. They are often used in renewable energy systems, such as in solar collectors. Heat pipes are used in heating systems, where waste heat from a process is moved to other areas through their unique transfer method.
• Thermal wheel or rotary heat exchanger: This system has a round, honeycomb-like structure made of heat-absorbing material that slowly rotates within the air streams of a ventilation system.
• Economizer: In industrial boilers, waste heat from exhaust gases is passed through a recuperator that warms the incoming fluid, reducing the amount of thermal energy needed.
• Heat pumps: These use a low-boiling-point liquid to recover energy from waste fluids.
• Run-around coil: This system includes two or more coils with fins connected by a pump-driven water circuit.
• Particulate filters (DPF): These filters capture emissions by keeping areas near converters and exhaust pipes at higher temperatures, reducing harmful emissions.

A waste heat recovery boiler (WHRB) differs from a heat recovery steam generator (HRSG) because the heated medium in a WHRB does not change its state (such as from liquid to vapor).

Heat to power units

A report by Energetics Incorporated for the U.S. Department of Energy in November 2004, titled Technology Roadmap, and other reports by the European Commission state that most energy from traditional and renewable sources is lost to the atmosphere. These losses occur both at the location where energy is produced (due to inefficient equipment and waste heat) and during the transportation of electricity (due to losses in cables and transformers). Together, these losses account for about 66% of the electricity's value being lost. Waste heat can be found in the final products of certain processes or as a leftover material in industries, such as slag in steelmaking plants. Devices that capture waste heat and convert it into electricity are called WHRUs or heat-to-power units:

• An organic Rankine cycle (ORC) unit uses a special liquid as its working fluid. This liquid has a lower boiling point than water, allowing it to turn into a gas at lower temperatures. The gas then turns a turbine blade, which powers a generator.
• Thermoelectric units (based on the Seebeck, Peltier, and Thomson effects) are also called WHRUs. These units use the temperature difference between two plates to create direct current (DC) electricity.
• Shape-memory alloys can be used to capture low-temperature waste heat and convert it into mechanical movement or electricity.

Applications

• Waste heat from low temperatures (0–120 °C, usually below 100 °C) has not been widely used to create electricity, even though companies using ORC technology have tried. This is because the maximum efficiency of converting heat to electricity is limited (up to 18% for heat at 90 °C and cooling at 20 °C), and after accounting for energy losses, the actual electricity produced is typically only 5–7%.
• Waste heat from medium (100–650 °C) and high (over 650 °C) temperatures can be used to generate electricity or mechanical power through various methods.
• A waste heat recovery system can also meet refrigeration needs, such as cooling a trailer. This setup requires only a waste heat recovery boiler and an absorption cooler. It operates under low pressure and temperature conditions, making it simple to implement.

Advantages

The recovery process will improve the efficiency of the process and lower the costs of fuel and energy needed for the process.

• Reduced pollution: Thermal pollution and air pollution will significantly decrease because fewer hot flue gases are released from the plant, as much of the energy is reused.
• Reduced equipment sizes: When fuel use decreases, the safety and control equipment for handling fuel also becomes smaller. Additionally, gas filtering equipment no longer needs to be as large.
• Reduced auxiliary energy use: Smaller equipment means less energy is needed for systems like pumps, filters, and fans.

Disadvantages

• The initial cost to install a waste heat recovery system might be more than the value of the heat that is saved. It is important to assign a value to the heat that is saved.
• Waste heat is often not very hot. This makes it hard to use the amount of heat found in waste heat effectively.
• Heat exchangers are usually bigger to capture large amounts of heat, which raises the initial cost.
• More equipment means more maintenance costs.
• These systems add more size and weight to power units. This is especially important for vehicles that move around.

Examples

• The Cyclone Waste Heat Engine is made to create electricity by using heat that is usually wasted. It does this through a process called a steam cycle.
• International Wastewater Heat Exchange Systems is a company that works on systems to recover heat from wastewater. These systems are used in places like apartment buildings, public buildings, factories, and areas that share energy among many buildings. The systems use heat from wastewater to make hot water for homes, heat buildings, and cool them.
• In 2014, the Motorsport series Formula One added waste heat recovery units to their cars. These units are called MGU-H.