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

Why desalting seawater with heat costs so much energy

Source page: https://www.materialinsights.org/energy/e_waste_heat_uses/rc-sci/

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

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
Favourable

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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf)^[3](https://docs.lib.purdue.edu/iracc/1876)^[4](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)^[8](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf)
 MED operating temperature about 70 C^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) MSF operating temperature 90 to 110 C^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) US industrial waste heat below 149 C about 570 TBtu/yr^[4](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf) MEA capture reboiler temperature about 130 C^[8](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf)

#### Exergy versus energy efficiency
Mixed

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).^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf)^[10](https://www.mdpi.com/2073-4441/10/3/305)^[8](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf)^[9](https://ideas.repec.org/a/eee/appene/v355y2024ics030626192301557x.html)
 MSF and MED heat use about 80.6 kWh thermal per m3 (290 kJ/kg)^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) Seawater RO electricity 3.5 to 5.0 kWh per m3^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) Work potential of 80.6 kWh heat at 70 C about 10.6 kWh (calculated)^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) MEA reboiler duty about 3.5 GJ per t CO2 (3.46 to 3.53 reported; up to about 4.4 implied, calculated)^[8](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf)^[9](https://ideas.repec.org/a/eee/appene/v355y2024ics030626192301557x.html)

#### Media compatibility and corrosiveness
Mixed

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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf)^[4](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf)
 Gulf seawater dissolved solids about 45,000 ppm^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) Red Sea dissolved solids about 41,000 ppm^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf)

#### Variants compared^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf)^[3](https://docs.lib.purdue.edu/iracc/1876)^[2](https://www.aceee.org/sites/default/files/pdfs/ssi21/panel-3/Zahlan.pdf)^[11](https://hortidaily.com/article/9149975/uk-29-hectares-of-new-tomato-greenhouses-to-use-waste-heat)^[8](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf)^[9](https://ideas.repec.org/a/eee/appene/v355y2024ics030626192301557x.html)^[12](https://www.globalccsinstitute.com/wp-content/uploads/2026/05/Brevik-CCS-Facility-Profile.pdf)

| Use | Heat needed | Energy or cost figure | Comment |
|---|---|---|---|
| **MED desalination** | About 70 C | 80.6 kWh heat plus 1.5 to 2.5 kWh power per m3 | 8% of global desalination capacity |
| **MSF desalination** | 90 to 110 C | 80.6 kWh heat plus 2.5 to 3.5 kWh power per m3 | 26.8% of capacity, mostly Gulf |
| **Seawater RO (benchmark)** | None | 3.5 to 5.0 kWh power per m3 | 60% of capacity |
| **Drying (grain, timber, sludge)** | Up to about 160 C via heat pumps | Grain dryer project paid back in 7.28 years | Seasonal; product contamination risk |
| **Greenhouses, aquaculture, algae** | Low; warm water | 29 ha UK tomato project on sewage-works and CHP heat, GBP 120 million | Seasonal; needs land beside the source |
| **Carbon-capture solvent regeneration (MEA)** | Reboiler at about 130 C | 3.46 to 3.53 GJ per t CO2 | Brevik cement capture sized to the waste heat available |

#### Sources

1. [Water Desalination Using Renewable Energy, Technology Brief I12](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/IRENA-ETSAP-Tech-Brief-I12-Water-Desalination.pdf) 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](https://www.aceee.org/sites/default/files/pdfs/ssi21/panel-3/Zahlan.pdf) 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)](https://docs.lib.purdue.edu/iracc/1876) Purdue e-Pubs, 2018
4. [Technology Assessment on Low-Temperature Waste Heat Recovery in Industry, ORNL/TM-2021/2150 (Thekdi, Nimbalkar et al.)](https://info.ornl.gov/sites/publications/Files/Pub164247.pdf) 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](https://pure.ulster.ac.uk/ws/files/121039922/1_s2.0_S0959652623018243_main.pdf) 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](https://ideas.repec.org/a/eee/appene/v355y2024ics030626192301557x.html) 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](https://www.mdpi.com/2073-4441/10/3/305) MDPI, 2018
8. [UK: 29 hectares of new tomato greenhouses to use waste heat (trade press)](https://hortidaily.com/article/9149975/uk-29-hectares-of-new-tomato-greenhouses-to-use-waste-heat) HortiDaily, 2019
9. [Brevik CCS Facility Profile](https://www.globalccsinstitute.com/wp-content/uploads/2026/05/Brevik-CCS-Facility-Profile.pdf) Global CCS Institute, 2026

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_waste_heat_uses/index.md)
- [Overview](https://www.materialinsights.org/energy/e_waste_heat_uses/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_waste_heat_uses/correlation/index.md)
- [Engineering and integration](https://www.materialinsights.org/energy/e_waste_heat_uses/rc-eng/index.md)
- [Economics and contracts](https://www.materialinsights.org/energy/e_waste_heat_uses/rc-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_waste_heat_uses/rc-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_waste_heat_uses/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_waste_heat_uses/links/index.md)