Recovering thermal energy from industrial waste heat is a crucial step in improving energy efficiency and reducing environmental impact. This comprehensive guide delves into the various technologies and methods available, providing detailed technical information, formulas, and examples to help you effectively harness this valuable energy source.
Heat Exchangers: Maximizing Waste Heat Recovery
Heat exchangers are the workhorse of industrial waste heat recovery, transferring heat from one medium to another without allowing the two to mix. These devices can recover up to 90% of the waste heat, with an efficiency of up to 80%.
Shell and Tube Heat Exchangers
Shell and tube heat exchangers are commonly used in the temperature range of 100°C to 300°C. The heat transfer rate in a shell and tube heat exchanger can be calculated using the following formula:
Q = U × A × LMTD
Where:
– Q is the heat transfer rate (W)
– U is the overall heat transfer coefficient (W/m²·K)
– A is the heat transfer area (m²)
– LMTD is the logarithmic mean temperature difference (K)
For example, a shell and tube heat exchanger with a heat transfer area of 50 m², an overall heat transfer coefficient of 500 W/m²·K, and a logarithmic mean temperature difference of 20 K can recover up to 500 kW of waste heat.
Plate Heat Exchangers
Plate heat exchangers are another popular choice for industrial waste heat recovery, particularly in the same temperature range as shell and tube exchangers. The heat transfer rate in a plate heat exchanger can be calculated using the following formula:
Q = m × cp × (T₂ – T₁)
Where:
– Q is the heat transfer rate (W)
– m is the mass flow rate of the fluid (kg/s)
– cp is the specific heat capacity of the fluid (J/kg·K)
– T₂ is the outlet temperature of the hot fluid (K)
– T₁ is the inlet temperature of the hot fluid (K)
For instance, a plate heat exchanger with a mass flow rate of 10 kg/s, a specific heat capacity of 4.18 kJ/kg·K, and a temperature difference of 30 K can recover up to 1.25 MW of waste heat.
Organic Rankine Cycle (ORC): Harnessing Low-Temperature Waste Heat
The Organic Rankine Cycle (ORC) is a thermodynamic cycle that converts low-temperature waste heat, typically below 300°C, into useful work. The efficiency of an ORC system can be calculated using the following formula:
η = (Wnet / Qin) × 100%
Where:
– η is the thermal efficiency of the ORC system (%)
– Wnet is the net work output (W)
– Qin is the heat input to the system (W)
For example, an ORC system with a net work output of 1 MW and a heat input of 5 MW can achieve a thermal efficiency of 20%.
Kalina Cycle: Improved Efficiency for Low-Grade Waste Heat
The Kalina cycle is a modified version of the Rankine cycle that uses a mixture of two refrigerants instead of a single refrigerant. This allows the cycle to operate at lower temperatures and achieve higher efficiencies than the traditional Rankine cycle.
The efficiency of a Kalina cycle can be calculated using the following formula:
η = (Wnet / Qin) × 100%
Where:
– η is the thermal efficiency of the Kalina cycle (%)
– Wnet is the net work output (W)
– Qin is the heat input to the system (W)
A Kalina cycle system can achieve an efficiency of up to 25%, with a power output ranging from a few kW to several MW.
Thermoelectric Generation: Direct Conversion of Waste Heat to Electricity
Thermoelectric generators (TEGs) convert heat directly into electricity using the Seebeck effect. They are particularly useful for low-temperature waste heat recovery, typically below 200°C.
The power output of a thermoelectric generator can be calculated using the following formula:
P = (S²ΔT²) / (ρL)
Where:
– P is the power output (W)
– S is the Seebeck coefficient (V/K)
– ΔT is the temperature difference across the thermoelectric material (K)
– ρ is the electrical resistivity of the thermoelectric material (Ω·m)
– L is the length of the thermoelectric material (m)
For example, a thermoelectric generator with a Seebeck coefficient of 200 μV/K, a temperature difference of 100 K, an electrical resistivity of 1 × 10^-5 Ω·m, and a length of 1 cm can produce a power output of approximately 4 W.
Heat Pumps: Recovering Waste Heat for Heating and Cooling
Heat pumps are devices that transfer heat from a low-temperature source to a high-temperature sink, using a refrigerant. They can be used to recover waste heat from industrial processes and utilize it for space and water heating or cooling.
The coefficient of performance (COP) of a heat pump is a measure of its efficiency, and can be calculated using the following formula:
COP = Qh / W
Where:
– COP is the coefficient of performance
– Qh is the heat delivered by the heat pump (W)
– W is the work input to the heat pump (W)
A well-designed heat pump can achieve a COP of up to 5, meaning it can produce five times more heat than the electrical energy consumed.
Absorption Coolers: Harnessing Waste Heat for Cooling Applications
Absorption coolers are similar to heat pumps, but they use a heat source instead of electricity to drive the cooling process. They can be used to recover waste heat from industrial processes and utilize it for cooling applications.
The efficiency of an absorption cooler can be calculated using the following formula:
η = (Qc / Qh) × 100%
Where:
– η is the efficiency of the absorption cooler (%)
– Qc is the cooling capacity (W)
– Qh is the heat input to the system (W)
Absorption coolers can achieve an efficiency of up to 80%, with a cooling capacity ranging from a few kW to several MW.
By understanding the technical details, formulas, and examples provided in this comprehensive guide, you can effectively harness the thermal energy from industrial waste heat and improve the overall energy efficiency of your operations.
References:
- Firth, A., Zhang, B., Yang, A., & Aidong, L. (2019). Quantification of global waste heat and its environmental effects. Waste Management, 85, 307-318.
- U.S. Department of Energy. (n.d.). Waste Heat Recovery Basics. Retrieved from https://www.energy.gov/eere/iedo/waste-heat-recovery-basics
- Oxford University. (n.d.). Quantification of Global Waste Heat and Its Environmental Effects. Retrieved from https://ora.ox.ac.uk/objects/uuid:954f2732-a89d-43e4-95e7-9914a2c0eb36/files/m828b6aa5b77d3e04733ad5255ca63999
- Cengel, Y. A., & Boles, M. A. (2015). Thermodynamics: An Engineering Approach (8th ed.). McGraw-Hill Education.
- Dincer, I., & Rosen, M. A. (2013). Exergy: Energy, Environment and Sustainable Development (2nd ed.). Elsevier.
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