Industries worldwide are heavily dependent on vast amounts of fossil fuels for heating, generating electricity, and powering various processes. This reliance not only places a significant financial strain on these sectors due to the volatility of fuel prices but is also a primary source of greenhouse gas emissions, contributing to the rapid warming of the Earth and altering climate patterns. Minimizing the dependence on fossil fuels would reduce operational expenses and emissions – a critical step toward a more sustainable and environmentally friendly industrial landscape.
Waste Heat Recovery (WHR) systems present a solution for industries looking to achieve these sustainability goals. WHR systems harness thermal energy that would otherwise be lost to the environment and convert it into usable heat or electricity. Capturing and repurposing this waste heat curbs the overall demand for dirty fossil fuels by supplementing the massive energy needs of industry, business, and housing.
WHR systems operate on the principle of heat exchange, efficiently transferring heat from a high-temperature source to a lower-temperature medium. (it’s possible to combine the two as in “direct-contact” which I’ll get back to) This reclaimed energy can then be utilized in five primary applications across dozens of industries and sectors. From powering industrial processes to heating apartment buildings, the applications for WHR systems are vast and varied.
WHR systems are primarily divided into two types: Direct and Indirect systems. Each has unique mechanisms and applications across various industries.
Direct WHR Systems
These systems facilitate the straightforward transfer of heat from the exhaust stream directly to the process or utility fluid without the need for an intermediate fluid or substance. Thanks to minimal thermal resistance, the direct contact between the hot exhaust gas and the cooler fluid offers optimal heat transfer efficiency.
This method is well-suited for applications where large quantities of hot water are required and is extremely efficient in regard to the heat transfer coefficient. The limitation is the recovered water temperature which cannot exceed the point of condensation (approx. 150°F).
For instance, direct WHR systems are often employed in industries with high-temperature exhaust streams, such as metal manufacturing. Furnaces and smelting operations routinely generate exhaust gases that exceed 600°C. The immediate recapture and reuse of this heat can significantly reduce the energy required for reheating processes, which in turn reduces fuel costs and the operational carbon footprint.
Unlike their direct counterparts, indirect systems use a secondary fluid or an intermediate heat transfer medium to convey heat from the exhaust stream to the process fluid. This separation is important for applications where:
● direct mixing of the exhaust gas with the process medium is undesirable due to contamination risks
● when the temperature differential between the two streams does not directly favor efficient heat exchange.
Indirect systems are much more versatile and are used in a range of industries, including power generation, chemical processing, and food production. For example, in power plants, waste heat from exhaust gases can be captured through a heat exchanger and used to preheat feedwater, thus improving the efficiency of steam generation.
In air-to-water WHR systems, which are a subset of indirect systems, the secondary fluid facilitating the heat transfer is water. This configuration is particularly beneficial for applications requiring hot water, steam, or ventilation. It provides a sustainable and cost-effective solution for heat needs across various sectors, including HVAC systems for commercial buildings.
A WHR system's main purpose is to capture and repurpose waste heat, which is achieved through various specialized devices and a carefully configured system design.
The most important component of the WHR process is the heat recovery device. These facilitate the transfer of thermal energy from waste heat sources to usable process streams.
● Plate Heat Exchangers – A series of metal plates with interstitial spaces for fluid flow, plate heat exchangers offer a high surface area for heat transfer, making them efficient for processes where space is at a premium. Thanks to their modular architecture, they can be effortlessly expanded or adapted to meet changing needs. They are commonly used in chemical processing, food and beverage production, and heating systems, where precise temperature management within a compact footprint is important.
● Economizers – These are used primarily in boiler systems to preheat feedwater by recovering waste heat from flue gases. As the hot flue gases pass over these tubes, the thermal energy they carry is transferred to the water inside. This process does not involve direct contact between the flue gases and the water, maintaining the purity of the feedwater. Economizers are widely adopted in industries with steam boilers, such as power generation, chemical manufacturing, and food processing.
Exhaust gas ducts or chimneys are conduits that channel the hot exhaust gases from their source (such as furnaces, engines, or boilers) to the heat recovery device. These ducts must be designed to handle high temperatures and corrosive gases, often requiring insulation and materials resistant to thermal stress and chemical degradation. Properly designed exhaust ducting ensures minimal heat loss during transit.
Similar to exhaust gas ducting, process fluid ducts transport the medium (water, air, or another fluid) that receives the recovered heat from the heat recovery device to its end-use application. This could involve circulating water to a boiler feedwater system, directing heated air for space heating, or any number of industrial processes requiring thermal energy. The design of process fluid ducting must consider the fluid's properties, required flow rates, and temperature to optimize heat transfer and minimize energy loss.
In indirect WHR systems, a heat transfer medium (such as thermal oil, steam, or water) carries heat from the exhaust stream to the process stream through a closed circuit. This circuit includes:
● pumps for circulating the medium
● expansion tanks to accommodate volume changes due to temperature fluctuations
● sometimes auxiliary coolers or heaters.
A sophisticated control and monitoring system regulates the operation of heat exchangers, pumps, and valves based on real-time data on temperatures, flow rates, and pressures. It ensures that the WHR system operates within its design parameters, maximizes heat recovery, and responds dynamically to changes in the heat source or demand.
A bypass arrangement is often included to divert exhaust gas away from the heat recovery device under certain conditions, such as during maintenance or when the heat is not needed. This feature allows the primary process (like power generation or manufacturing) to continue uninterrupted, providing key operational flexibility.
Waste Heat Recovery (WHR) systems, by virtue of their design, provide significant energy savings and sustainability benefits across a broad spectrum of industries. Looking into the five key implementations of WHR systems, we can see the sector-specific uses for each.
One primary application of WHR systems is preheating combustion air. In this situation, heat exchangers facilitate the transfer of thermal energy from waste heat to the air intended for combustion.
By the time this air is introduced into the combustion chamber, it has been significantly warmed, reducing the amount of fuel required to raise its temperature to the necessary level for efficient combustion.
Specific industries that can benefit from such implementations include:
Power Generation – Exhaust gas economizer systems can markedly improve combustion efficiency within power plants by preheating the combustion air. This means that for the same amount of fuel, the plant can produce approximately 3% more energy and emit 2% fewer GHGs.
Building Sector – For commercial and residential buildings, preheating combustion air for natural gas-fired furnaces and water heaters can lead to about 3% savings in fuel and a corresponding 3% reduction in GHG emissions (DOE, 2015).
Metal and Steel Industry – Preheating combustion air can dramatically reduce the energy needs for high-temperature furnaces for smelting and casting.
Cement Industry – Cement production involves the use of kilns that operate at extremely high temperatures. Waste heat is utilized to preheat air in kilns, lowers the energy requirements for cement production, and reduces the reliance on fossil fuels.
Another key application of WHR systems is generating steam from recovered waste heat. Once captured, the waste thermal energy is transferred to water in a heat exchanger. The hot water reduces the workload of the boiler used to generate steam. The steam is then used to drive turbines to produce additional electricity or is used directly in heating applications for a range of industries:
Power Generation – Steam generated from recovered waste heat can then drive a turbine, producing additional electricity without the need for extra fuel. This process can increase a plant's output by up to 30% and its overall thermal efficiency, as it maximizes the utility of the heat generated from fuel combustion.
Laundry and Dry-Cleaning Services – Can utilize steam generated from WHR systems for energy-intensive washing and ironing processes.
Manufacturing/Processing – Many manufacturing processes require heat at various stages, such as drying, curing, or chemical processing. This is especially relevant in industries with high thermal demands, such as:
● Chemical and Petrochemical Industry – The generation of steam is integral for processes like distillation, evaporation, and facilitating chemical reactions, where precise temperature control is needed to ensure product quality and consistency.
● Paper and Pulp Industry – Steam generated from WHR systems can be used for pulping, drying, and bleaching.
● Food & Beverage Industry – Steam heat can be used for large-scale operations involving cooking, baking, and sterilization processes.
● Textile Industry – Steam can be used for dyeing, drying, and conditioning textiles.
3. Producing Electricity or Mechanical Power
Converting waste heat directly into electricity or mechanical power through thermoelectric generation is an innovative use of WHR systems. This involves using thermoelectric generators (TEGs), which are devices that convert temperature differences directly into electrical voltage through the Seebeck effect. When one side of the thermoelectric material is heated (by waste heat), while the other side is kept cool, an electrical current is generated across the material.
In some WHR systems, instead of or in addition to generating electricity, the captured thermal energy can be used to produce mechanical power through the use of Organic Rankine Cycle (ORC) systems or Stirling engines. These can convert thermal energy into mechanical work that can drive machinery or generate electricity.
Power Generation – Generated electricity can then support auxiliary equipment or be fed back into the grid. Such implementations can increase power output by 5% and reduce emissions by 4%.
Data Centers – Data centers, known for their high energy consumption primarily due to cooling needs, also generate substantial amounts of waste heat. By converting this waste heat into electricity, data centers can achieve greater energy efficiency and potentially use the generated power to offset some of their substantial energy demands.
Agriculture – Waste heat from biomass boilers or other energy systems can be converted into additional power in agricultural operations, particularly those with large greenhouse facilities. This power can support various agricultural processes, including irrigation systems, crop processing equipment, or climate control systems within greenhouses.
Automotive Industry – The manufacturing and testing phases generate a considerable amount of waste heat from processes like paint drying, engine testing, and various metallurgical procedures. WHRs could transform this waste heat into mechanical power or electricity, moving the industry toward more sustainable manufacturing practices.
Semiconductor and Electronics Manufacturing – This industry is characterized by energy-intensive processes, including wafer fabrication, assembly, and extensive testing phases, all of which generate significant amounts of waste heat. Utilization of this heat for power lighting or other auxiliary systems within a facility can reduce its overall environmental impact.
WHR systems can also repurpose heat that would otherwise be lost to the environment from industrial processes and use it to preheat water before it enters a domestic hot water system. By starting with water that is already warmer than it would be otherwise, the energy demand on water heaters is substantially reduced.
Hotels and Leisure Centers – Significant amounts of waste heat are generated from HVAC systems WHR systems can capture this heat and use it to preheat water for showers, swimming pools, baths, and other guest amenities.
Hospitals and Healthcare – These facilities have a critical need for sterilization processes that consume large amounts of hot water to create steam. WHR systems in these settings can capture heat from various sources, including medical equipment cooling systems and boilers, to preheat water needed for sterilization, patient care, and other uses. This provides a cost-effective method to meet the facility's hot water demand and contributes to a reduced carbon footprint.
Residential Buildings – WHR systems can harness waste heat from common HVAC systems or even the collective heat generated within the building. The preheated water reduces the workload on individual or centralized water heaters, leading to decreased energy consumption and lower utility bills for tenants.
Commercial Office Buildings and Shopping Malls – Waste heat from HVAC systems, lighting, or large equipment can be efficiently captured and used to preheat water for restrooms, food court operations, and maintenance purposes.
Repurposing waste heat to maintain comfortable indoor environments in commercial buildings, industrial facilities, and even residential spaces is perhaps the most commonly applied implementation of WHR systems.
Once captured, waste heat is transferred via a heat exchanger to a working fluid (such as water or a refrigerant) in the WHR system. For heating applications, the process might directly use the heated fluid to warm indoor spaces. For cooling applications, the waste heat is used to power absorption chillers or similar systems that utilize a thermal-driven cycle to produce cooling rather than relying on conventional electrically powered refrigeration cycles. The warmed or cooled medium is then circulated throughout the building or facility using a network of pipes for space heating or ducts for space cooling.
HVAC Industry – Recovered heat can be rerouted to provide additional space heating during colder months, substantially reducing the demand for conventional heating systems. During warmer periods, the same waste heat can drive absorption chillers, producing cooling without the need for additional electrical input.
Agriculture and Farming – This sector can use WHR systems to create optimal growing conditions within greenhouses. Waste heat from various agricultural processes or external sources is used to maintain the desired temperature, promoting year-round growth and enhancing crop yield. In dairy operations, WHR can be used to regulate temperatures, offering improved livestock comfort.
Mining Industry – In the mining industry, maintaining a comfortable and safe working environment within operational buildings is key due to the extreme temperatures often encountered.
Warehousing and Distribution Centers – Warehouses and distribution centers face the challenge of maintaining specific temperature ranges to preserve product integrity, especially in facilities that store temperature-sensitive goods. WHR systems can provide heating or cooling as needed to maintain a stable environment.