# Salinity gradient: Correlation

Prices it moves with, and why

Source page: https://www.materialinsights.org/energy/e_salinity/correlation/

Where fresh river water meets the sea, the salinity gradient holds free energy, but capturing it needs large membranes that are costly and foul easily. Membrane cost and life, not the resource, set its value.
 No public price Salinity gradient has no public monthly price series: A free natural flow; no market price, only capture costs.

#### What its price moves with

 Energy [OTEC OTEC and salinity power](https://www.materialinsights.org/energy/e_otec/)Main converter

Salinity gradient power, by pressure-retarded osmosis or reverse electrodialysis, is grouped here with OTEC. Only small pilots, such as in the Netherlands, have run, because power output per square metre of membrane is still low.
 No public price for OTEC and salinity power, so the link is explained in words only.

 Commodity [H₂O Water](https://www.materialinsights.org/commodities/water/)Resource

The gradient depends on fresh river water reaching the sea, and that water is wanted for farms and cities. Lower river flows in dry seasons cut the gradient available. River estuaries are therefore the natural sites for these plants.
 Not enough overlapping price history to measure.

 Commodity [NaCl Salt](https://www.materialinsights.org/commodities/salt/)Resource

Saltier brines, such as from desalination plants or salt works, give a steeper gradient and more power per membrane. Pairing salinity power with brine outflows is one route being tested.
 Not enough overlapping price history to measure.

 Chemical [PTFE Fluoropolymers (PTFE, PVDF)](https://www.materialinsights.org/chemicals/fluoropolymers/)Capture cost

Reverse electrodialysis uses ion-exchange membranes, some based on fluorinated polymers like those in fuel cells. Membrane cost is the main barrier, so cheaper polymer membranes would decide its value. Membrane fouling also raises running costs.
 No public price for Fluoropolymers (PTFE, PVDF), so the link is explained in words only.

How to read these numbers

**Correlation of monthly changes** (the number in the coloured pill) compares how much each price moved, month by month, since 2006 or the first month both series exist. It runs from −1 to +1: near +1 the two usually rise and fall in the same months, near 0 there is no link, and a negative number means they tend to move in opposite directions. Above 0.7 is strong, 0.4 to 0.7 moderate, 0.2 to 0.4 weak.

**12-month changes** compare year-on-year moves, which shows whether two prices share the same cycles even when the month-to-month noise differs.

**Price function** is a straight-line fit of the two prices on logarithmic scales: price ≈ k × (partner price)^β. β is how much this price moves, in per cent, for a 1% move in the partner over the whole period, and R² is the share of the variation the fit explains. Two prices that both drift up with inflation can fit well without any real link, so read it together with the monthly correlation.

**Lead or lag** checks whether the monthly correlation is clearly higher when one price is shifted by 1 to 6 months.

Correlation is not cause. The reasons on each card explain the economic link; the numbers show how strongly it has shown up in prices. Proxy series are US producer price indexes or related prices and can blur the real link.

Data: World Bank Commodity Price Data (Pink Sheet), October 2026; US Bureau of Labor Statistics producer price indexes, IMF primary commodity prices and US EIA prices via FRED, Federal Reserve Bank of St. Louis, retrieved November 2026. Calculations by this site.

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_salinity/index.md)
- [Overview](https://www.materialinsights.org/energy/e_salinity/overview/index.md)
- [How energy resides](https://www.materialinsights.org/energy/e_salinity/resides/index.md)
- [Abundance](https://www.materialinsights.org/energy/e_salinity/abundance/index.md)
- [Fluctuation in time](https://www.materialinsights.org/energy/e_salinity/time/index.md)
- [Across the Earth](https://www.materialinsights.org/energy/e_salinity/geography/index.md)
- [Impact on nature](https://www.materialinsights.org/energy/e_salinity/nature/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_salinity/links/index.md)