Waste heat for drying, water and food: Science and thermodynamics

Why desalting seawater with heat costs so much energy

This tile covers end uses that can soak up low-grade heat that would otherwise be thrown away: drying, thermal desalination, warming greenhouses and fish tanks, regenerating carbon-capture solvents, and driving absorption chillers. The appeal is that they can use heat too cool for boilers or furnaces; the catch is that they must sit next to the heat source.

Grade of energy

Multi-effect distillation (MED) desalination runs at about 70 C and multi-stage flash (MSF) at 90 to 110 C, so both can use exhaust or cooling heat that boilers and furnaces cannot. A review of high-temperature heat pumps identifies drying, pasteurising, sterilising, evaporation and distillation as key uses for heat at 90 to 160 C. These uses matter because so much waste heat is low grade: ORNL estimates about 570 TBtu a year of US industrial waste heat below 149 C. Amine carbon-capture solvent is stripped of CO₂ in a reboiler at about 130 C, which suits low-pressure steam or upgraded waste heat.¹³⁴⁸

Exergy versus energy efficiency

Thermal desalination uses a lot of heat: the IRENA-ETSAP brief gives about 80.6 kWh of heat per cubic metre of fresh water and applies the same figure to MSF and MED, plus 2.5 to 3.5 kWh (MSF) or 1.5 to 2.5 kWh (MED) of electricity. Seawater reverse osmosis (RO) needs only 3.5 to 5.0 kWh of electricity. On a quality basis the gap narrows: 80.6 kWh of heat at 70 C holds about 10.6 kWh of work potential (Carnot factor about 0.13, calculated), but that is still more than RO uses. Thermal routes therefore only make sense when the heat is genuinely free and would otherwise be wasted. A model of an MED plant with thermal vapour compression, based on Libya's Zawya plant, produces about 6 kg of water per kg of steam (gain output ratio). Carbon capture is just as heat-hungry: standard 30% MEA solvent needs about 3.5 GJ of heat per tonne of CO₂ (3.46 GJ optimum in a Copenhagen pilot; a cement design study states 3.53 GJ, although its own heat and CO₂ flows imply about 4.4 GJ, calculated).¹¹⁰⁸⁹

Media compatibility and corrosiveness

Desalination handles hot brine, which scales and corrodes; Red Sea and Gulf water carry about 41,000 and 45,000 ppm of dissolved solids and have high biofouling potential. Flue gas used directly for drying can contaminate food or products, so indirect exchangers or a water loop are often added, and condensing flue gas is acidic. Amine solvents break down if overheated, so reboiler steam temperature must be kept in check.¹⁴

Variants compared^132118912

Sources

1. Water Desalination Using Renewable Energy, Technology Brief I12 IEA-ETSAP and IRENA, 2012
2. Not Just Hot Air: Low-Cost Decarbonization through Heat Recovery (Zahlan and Talan, Cadmus), ACEEE Summer Study on Energy Efficiency in Industry ACEEE, 2021
3. High Temperature Heat Pumps: Market Overview, State of the Art, Research Status, Refrigerants, and Application Potentials (Arpagaus, Bless, Uhlmann, Schiffmann, Bertsch), 17th International Refrigeration and Air Conditioning Conference, Purdue (conference paper) Purdue e-Pubs, 2018
4. Technology Assessment on Low-Temperature Waste Heat Recovery in Industry, ORNL/TM-2021/2150 (Thekdi, Nimbalkar et al.) Oak Ridge National Laboratory, 2021
5. Comparative techno-economic analysis of the integration of MEA-based scrubbing and silica PEI adsorbent-based CO2 capture processes into cement plants (Jaffar et al.), Journal of Cleaner Production 414, 137666 Elsevier (open access, Ulster University repository), 2023
6. Results from pilot-scale CO2 capture testing using 30 wt% MEA at a Waste-to-Energy facility (Vinjarapu et al.), Applied Energy 355 Elsevier (via RePEc IDEAS), 2024
7. Energetic, Exergetic, and Economic Analysis of MED-TVC Water Desalination Plant with and without Preheating (Eshoul et al.), Water 10(3), 305 MDPI, 2018
8. UK: 29 hectares of new tomato greenhouses to use waste heat (trade press) HortiDaily, 2019
9. Brevik CCS Facility Profile Global CCS Institute, 2026