Low-temperature MED desalination driven by a heat pump
How a coastal plant making 20 m³ of distilled water a day in a low-temperature MED unit can drive it with a water-source heat pump on the final condenser water, and what it saves against a diesel-fired hot-water generator.
▶Video: Waste-heat recovery heat pumps
V10 · 2–2.5 minLifting warm water from condensers, effluent and process cooling to useful heat
Video coming soon01Problem statement
- MED needs a heat source. Low-temperature MED needs hot water at 70–75 °C, and without plant waste heat this comes from a fuel-fired heater.
- Condenser heat is rejected to sea. The final condenser rejects heat at 35–45 °C, close to the amount the first effect takes in.
- Fuel per cubic metre. Fuel makes up most of the running cost of each cubic metre of distillate.
- Losses between fuel and process. Burning diesel in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant is a coastal process plant that needs about 20 m³ a day of high-purity distillate for boiler feed and process use. It runs an 8-effect low-temperature MED unit with a top brine temperature of about 65 °C.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| MED first-effect heat input | Hot water 72 → 67 °C | 20 m³/day distillate × 2,330 kJ/kg ÷ GOR 7 | 1,849 |
| Feed seawater final heating to top brine temperature | 58 → 65 °C | 50,000 L/day feed, ρ 1.025, cp 3.95, 58 → 65 °C | 394 |
| Storage and pipework standing losses | – | about 5% | 112 |
| Total useful heat | 2,355 | ||
Averaged over 20 running hours, the heat pumps need about 118 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 5 × SE-WWUHT-27 (135 kW total) (SE-WWUHT water-source, R245fa) delivering 75 °C, covering the day's 2,355 kWh in about 17.4 running hours.
- A 4,500 L insulated storage tank decouples the heat pumps from the process peaks.
- Heat source: MED final-condenser cooling water at about 40 °C. The heat pump takes over part of the final condenser duty, so less seawater has to be pumped for cooling.
- The diesel-fired hot-water generator stays connected behind a normally closed valve as standby.
- Part of the final-condenser cooling water, at about 40 °C, is routed through the heat pump evaporator.
- The heat pump returns that heat at 75 °C to the first-effect heater and the final feed heater.
- MVC units already use their own mechanical compression. This note covers MED plants driven by hot water.
Selected equipment
| Model | SE-WWUHT-27 × 5 |
|---|---|
| Heating capacity | 27 kW each, 135 kW total |
| COP | 2.93 estimated for a 40 °C source (the data sheet is rated on a 60/55 °C source) |
| Rating | water or steam to 120 °C, R245fa |
| Supply temperature | 75 °C |
| Storage | 4,500 L insulated tank |
Capacity is sized from the heat demand above. COP is an estimate until Sunniva engineering confirms the design.
Control philosophy
TT-101 at the tank top starts the heat pump below 70 °C and stops it at 75 °C. TIC-201 holds top brine temperature at 65 °C by modulating hot water to the first effect. TIC-202 holds feed seawater temperature. The diesel heater stays connected as standby.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per m³ of distilled water | – | – |
| CO₂ per year | – | – |
06Benefits
Heat recycled within the plant
Heat the condenser would reject is lifted and used again in the first effect.
Low scaling regime
Running the first effect at 65 °C keeps the MED in a low-scaling temperature range.
Less cooling seawater
With the heat pump taking part of the condenser duty, less seawater needs pumping for cooling.
Lower energy cost
Each kWh of electricity delivers about 2.9 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Built-in redundancy
Several machines share the load. If one is serviced, the others and the storage carry most of the day, with the existing heater in reserve.
07Engineering pointers
Site data to collect
- Distillate demand and quality
- MED effects, GOR and top brine temperature
- Condenser cooling water flow and temperatures
- Current heat source and fuel use
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
Design notes
- Seawater materials. Use titanium on all seawater and brine-side exchangers and an isolating loop between the condenser water and the heat pump.
- Antiscalant and venting. Keep antiscalant dosing and non-condensable venting unchanged. They matter more than the heat source for MED reliability.
- Source stream. The source must give up roughly two-thirds of the heat delivered. Confirm its flow and temperature over a full production day, and filter it before the evaporator.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
- Performance figure. The COP on this page is an engineering estimate, not a data sheet value. Sunniva engineering will confirm it for the actual source and supply temperatures.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.
Talk to us
Low-temperature MED desalination driven by a heat pump
How a coastal plant making 20 m³ of distilled water a day in a low-temperature MED unit can drive it with a water-source heat pump on the final condenser water, and what it saves against a diesel-fired hot-water generator.
▶Video: Waste-heat recovery heat pumps
V10 · 2–2.5 minLifting warm water from condensers, effluent and process cooling to useful heat
Video coming soon01Problem statement
- MED needs a heat source. Low-temperature MED needs hot water at 70–75 °C, and without plant waste heat this comes from a fuel-fired heater.
- Condenser heat is rejected to sea. The final condenser rejects heat at 35–45 °C, close to the amount the first effect takes in.
- Fuel per cubic metre. Fuel makes up most of the running cost of each cubic metre of distillate.
- Losses between fuel and process. Burning diesel in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant is a coastal process plant that needs about 20 m³ a day of high-purity distillate for boiler feed and process use. It runs an 8-effect low-temperature MED unit with a top brine temperature of about 65 °C.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| MED first-effect heat input | Hot water 72 → 67 °C | 20 m³/day distillate × 2,330 kJ/kg ÷ GOR 7 | 1,849 |
| Feed seawater final heating to top brine temperature | 58 → 65 °C | 50,000 L/day feed, ρ 1.025, cp 3.95, 58 → 65 °C | 394 |
| Storage and pipework standing losses | – | about 5% | 112 |
| Total useful heat | 2,355 | ||
Averaged over 20 running hours, the heat pumps need about 118 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 5 × SE-WWUHT-27 (135 kW total) (SE-WWUHT water-source, R245fa) delivering 75 °C, covering the day's 2,355 kWh in about 17.4 running hours.
- A 4,500 L insulated storage tank decouples the heat pumps from the process peaks.
- Heat source: MED final-condenser cooling water at about 40 °C. The heat pump takes over part of the final condenser duty, so less seawater has to be pumped for cooling.
- The diesel-fired hot-water generator stays connected behind a normally closed valve as standby.
- Part of the final-condenser cooling water, at about 40 °C, is routed through the heat pump evaporator.
- The heat pump returns that heat at 75 °C to the first-effect heater and the final feed heater.
- MVC units already use their own mechanical compression. This note covers MED plants driven by hot water.
Selected equipment
| Model | SE-WWUHT-27 × 5 |
|---|---|
| Heating capacity | 27 kW each, 135 kW total |
| COP | 2.93 estimated for a 40 °C source (the data sheet is rated on a 60/55 °C source) |
| Rating | water or steam to 120 °C, R245fa |
| Supply temperature | 75 °C |
| Storage | 4,500 L insulated tank |
Capacity is sized from the heat demand above. COP is an estimate until Sunniva engineering confirms the design.
Control philosophy
TT-101 at the tank top starts the heat pump below 70 °C and stops it at 75 °C. TIC-201 holds top brine temperature at 65 °C by modulating hot water to the first effect. TIC-202 holds feed seawater temperature. The diesel heater stays connected as standby.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per m³ of distilled water | – | – |
| CO₂ per year | – | – |
06Benefits
Heat recycled within the plant
Heat the condenser would reject is lifted and used again in the first effect.
Low scaling regime
Running the first effect at 65 °C keeps the MED in a low-scaling temperature range.
Less cooling seawater
With the heat pump taking part of the condenser duty, less seawater needs pumping for cooling.
Lower energy cost
Each kWh of electricity delivers about 2.9 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Built-in redundancy
Several machines share the load. If one is serviced, the others and the storage carry most of the day, with the existing heater in reserve.
07Engineering pointers
Site data to collect
- Distillate demand and quality
- MED effects, GOR and top brine temperature
- Condenser cooling water flow and temperatures
- Current heat source and fuel use
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
Design notes
- Seawater materials. Use titanium on all seawater and brine-side exchangers and an isolating loop between the condenser water and the heat pump.
- Antiscalant and venting. Keep antiscalant dosing and non-condensable venting unchanged. They matter more than the heat source for MED reliability.
- Source stream. The source must give up roughly two-thirds of the heat delivered. Confirm its flow and temperature over a full production day, and filter it before the evaporator.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
- Performance figure. The COP on this page is an engineering estimate, not a data sheet value. Sunniva engineering will confirm it for the actual source and supply temperatures.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.