The energy resource is pressurised brine
A seawater RO plant produces permeate and concentrate. Concentrate leaves the membranes at a pressure that remains high; dissipating it entirely through a valve wastes recoverable energy. An isobaric exchanger transfers some of that energy directly to incoming feed. Other arrangements return mechanical work to the pumping system.
Energy Recovery and Danfoss documentation shows solutions for different train sizes. Selection depends on the flow through the device and its pressure envelope, rather than daily permeate output alone. Check part-load duty and seasonal changes as well as the nominal operating point before choosing a unit.
Separate hydraulic power from electricity consumption
For flow Q in m³/h and pressure difference Δp in bar, approximate hydraulic power is P = Q × Δp / 36, in kW. Estimate the corresponding electricity demand by dividing by combined pump, motor and drive efficiency. This unit conversion is an original screening balance, not a replacement for equipment curves.
Specific energy consumption, SEC, is electrical power divided by permeate flow, expressed in kWh/m³. Always state which equipment is inside the measurement boundary. A high-pressure calculation excludes intake, pretreatment, auxiliary pumping, washing, post-treatment and general services unless they are explicitly added. Comparing numbers with different boundaries can reverse a purchasing decision.
Worked example: the energy carried by concentrate
Assume 25 m³/h feed, 10 m³/h permeate and 15 m³/h brine: 40 % recovery. For simplicity, use a 60 bar pressure increase, recoverable brine at 58 bar and 85 % combined pumping efficiency. Without energy recovery, the calculated pumping duty is 25 × 60 / (36 × 0.85) = 49.0 kW, equivalent to 4.90 kWh/m³ of permeate.
If an idealised balance uses 95 % of the brine hydraulic energy, recovered power is 15 × 58 × 0.95 / 36 = 23.0 hydraulic kW. Subtracting this from feed hydraulic work and applying the same 85 % efficiency to the remainder gives approximately 22.0 electrical kW, or 2.20 kWh/m³.
This second estimate assumes transferred energy replaces pumping work and excludes additional integration loads. It is neither a commercial-device prediction nor whole-plant consumption. Mixing, leakage, pressure losses and booster-pump duty must be resolved with the selected arrangement and equipment curves. The numbers illustrate why the brine stream matters.
| Example quantity | Value | Meaning |
|---|---|---|
| Feed / permeate / brine | 25 / 10 / 15 m³/h | Flow balance excluding auxiliary losses |
| Pumping without pressure recovery | 4.90 kWh/m³ | Calculated high-pressure function only |
| Idealised hydraulic scenario | 2.20 kWh/m³ | Remaining work under the stated assumptions |
| Guarantee-ready plant consumption | Requires design | All agreed equipment and test conditions |
Small plants need a suitable minimum operating point
An oversized device may operate outside its efficient window. Check minimum flow, inlet pressure, backpressure, filtration, startup, flushing and control. The iSave data sheet, for example, specifies hydraulic and protective requirements that belong to the installation alongside the device efficiency. Include these requirements when comparing footprint and cost.
Hold seawater quality, temperature, product specification, recovery and availability constant when comparing alternatives. If one proposal simultaneously changes membranes, pumps and recovery, separate the contribution of each change. A manufacturer’s maximum advertised saving cannot be copied directly into a project calculation without validating the operating conditions.
Turn measured savings into a purchase decision
Annual electricity saving equals the difference in SEC multiplied by annual usable-water volume, using the same measurement boundary. Apply the project electricity tariff and subtract additional maintenance. Compare this result with installed investment: device, pumps, drives, piping, instrumentation, automation and commissioning. State which estimates are supplier quotations and which remain assumptions.
A plant operating for few hours can justify a different solution from a continuously operated plant with the same nominal output. Request a consumption guarantee with test conditions and included equipment. For retrofits, measure flow, pressure and power beforehand so that improvements caused by unrelated changes are not attributed to the recovery device. The resulting baseline also helps verify the improvement after commissioning.
Common questions
Does 95 % efficiency mean 95 % less electricity?
No. It describes a transfer function under specified conditions. Some feed still needs pressurisation, and the plant has losses and other electrical loads.
Is the device selected by permeate flow?
Permeate flow helps establish the balance, but the device must be selected for the actual flows and pressures at its connections throughout the operating range.