The technology

How the documented pilot worked

Photovoltaic seawater reverse osmosis, explained from the original site and the 2015 paper.

The process

Six steps from seawater intake to stored permeate.

  1. 01

    Seawater intake

    A beach well draws seawater through the sand itself — a natural first filter that protects everything downstream.

  2. 02

    Ultrafiltration pretreatment

    0.03-micron membranes clarify the feed, extending RO membrane life and letting brine streams be blended to lower discharge salinity.

  3. 03

    High-pressure pumping

    An axial-piston pump (APP) — the most efficient positive-displacement pump available — is driven directly by DC solar through a variable frequency drive.

  4. 04

    Reverse osmosis

    Seawater is pushed through semi-permeable membranes at ~680 psi. Fresh water passes through; the cross-flow of brine carries dissolved salts away. No heat, no phase change.

  5. 05

    Energy recovery

    An axial-piston motor (APM) captures the pressure still held in the brine and returns it directly to the pump shaft — one of the first uses of paired APP/APM recovery.

  6. 06

    Clean water, managed brine

    Permeate below 300 TDS is stored for use day or night; blended brine returns to the sea at reduced salinity. Sunlight in, fresh water out.

Measured water quality

From 33,000 TDS seawater to below 300 TDS permeate

The logarithmic scale keeps both measured concentrations legible. The permeate contained less than 0.91% of the feed-water TDS.

Source: Demand Response Desalination (2015), page 9. The paper reports permeate “lower than 300” TDS; 300 is plotted as an upper bound. Original page.

Measured T-1 module flow

What happens to 21.5 L/min of feed seawater

The paper identifies 33% as the most cost-effective measured yield for the pilot.

Source: Demand Response Desalination (2015), pages 7 and 9. Output flows are calculated from the paper's feed flow and recovery values. Original page.

Where the leverage is

Further energy reductions are limited.

The paper argued that reverse osmosis had already benefited from more than sixty years of technical improvement and that every desalination process remains bounded by the thermodynamic minimum required to separate water from salt.

Its conclusion was cautious: as the process approaches that minimum, opportunities for further efficiency improvements decline. More attention should therefore be paid to coordinating water production with the availability of inexpensive energy.

From the original white paper

Further energy reductions are limited

Figure A places the historical fall in reverse-osmosis power consumption beside the thermodynamic minimum at different salinities and recovery rates. It is reproduced from the paper rather than redrawn from estimated data.

Original white-paper Figure A showing the historical fall in reverse-osmosis power consumption and minimum energy at several salt concentrations
Source: Yan Kunczynski and Ben Burger, Demand Response Desalination, Figure A, 2015. See the original page.

Core technologies

Five pieces of engineering that make it work.

01 Ultrafiltration

Membrane pretreatment

Membrane-based pretreatment (0.03-micron ultrafiltration) protects and extends the life of the RO components, and allows brine streams to be blended — significantly reducing the salinity of what returns to the sea.

02 VFD

DC-native variable frequency drives

Unique to this application, the variable frequency drives accept DC current directly from the solar array and batteries — eliminating expensive inverters and creating a naturally flexible, dispatchable load.

03 APP

Axial-piston high-pressure pump

The most efficient positive-displacement high-pressure pump available — light, low-maintenance, and well matched to variable solar input.

04 APM / energy recovery

Pressure-recovery motor

An axial-piston motor recovers the pressure still held in the brine stream after desalination and applies it directly to the torque of the pump shaft — one of the first uses of paired APP/APM energy recovery.

05 RO

Reverse osmosis

A purely mechanical process that pushes fresh water through a semi-permeable membrane while the cross-flow of brine carries dissolved solids away — no heat, no phase change.

Why modularity mattered

Adjust electrical demand while maintaining water supply.

A conventional plant usually operates large pumps within a narrow range. The proposed modular plant changed demand by changing the number of modules in operation.

  • Treated water can be stored, so the timing of production does not have to match the timing of use.
  • In the pilot, independent modules switched on and off at preset battery-voltage levels.
  • The paper proposed applying the same multi-module principle to grid conditions and energy prices.

Calculated from the proposed module

A plant adjustable in approximately 22 kW steps

The paper envisioned plants containing 5-500 independently controlled modules. Every additional module would add or remove about 22 kW of load.

Source: Demand Response Desalination (2015), pages 9 and 11. Loads shown are arithmetic products of the paper's 22 kW module demand, not measured plant results. Original page.