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RO high-pressure pump power and specific energy

The feed pump is the largest energy user in a reverse osmosis plant. How much of it comes back depends on the concentrate pressure and the energy recovery device.

Feed flowm³/h

Feed pressurebarBrackish 10 to 30 bar; seawater 55 to 85 bar.

Recovery%

Pump and motor efficiency

Concentrate pressure as a fraction of feedThe concentrate leaves at 80 to 90% of feed pressure.

Energy recovery device efficiency0 for none; pressure exchangers recover over 90%.

Net specific energy

2.89kWh/m³ permeate

The thermodynamic minimum for seawater is about 0.70 kWh/m³; good seawater plants reach 2 to 3 kWh/m³, brackish plants under 1.

Gross specific energy (no recovery)

4.63kWh/m³ permeate

Pump shaft power

208kW

Hydraulic power

167kW

Power recovered from the concentrate

78kW

Net power

130kW

How it works

Hydraulic power of the feed pump, shaft power through the pump and motor efficiency, and the power an energy recovery device returns from the concentrate (1 bar·m³/s = 100 kW):

P_{h y d} = 100 Q_{F} P_{F} P_{s h a f t} = \frac{P_{h y d}}{η_{p u m p}} P_{r e c} = 100 η_{E R D} Q_{C} P_{C}

(1)

S E C = \frac{P_{s h a f t} - P_{r e c}}{Q_{P}}

(2)

where

$P_{h y d}$: hydraulic power, kW
$Q_{F}, Q_{C}, Q_{P}$: feed, concentrate and permeate flow, m³/s
$P_{F}, P_{C}$: feed and concentrate pressure, bar
$P_{s h a f t}$: pump shaft power, kW
$η_{p u m p}$: pump and motor efficiency
$P_{r e c}$: power recovered from the concentrate, kW
$η_{E R D}$: energy recovery device efficiency
$S E C$: specific energy, kWh per m³ of permeate

with flows in m³/s and pressures in bar. The concentrate leaves the last element at 80 to 90% of feed pressure and more than 90% of that energy is recoverable with a pressure exchanger; the thermodynamic minimum for seawater is about 0.70 kWh/m³. Pretreatment, transfer and permeate pumps, and the second pass if any, are not included.

RO high-pressure pump power and specific energy

How it works

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