Earth Notes: Eco Puzzler: Tay Salty Leccy

Updated 2024-02-20.
For those on their way to mastering engineering... How to generate 10MW electricity from the Tay estuary saline gradient? #ecoEngineering #renewables
First in a series of eco puzzles set for real MEng students by "The Prof". This one is about the nascent technology of energy from salinity gradients which could be potentially a useful addition to a portfolio of renewable energy sources.

Text lightly edited from real project work description for MEng students.

Many thanks to "The Prof" for so generously making this material available!

Saline Electricity Generation

By: ""


You will be aware of the moves to increase the amount of energy generated from renewable sources. You will also be aware that this is not straightforward. There is plenty of renewable energy about but its density in the environment is low and so collecting it means a widespread and, therefore expensive, infrastructure. Also, it is difficult to match availability to demand in a simple way; solar energy comes when the sun shines, wind and waves (forms of solar) when the wind blows, tidal according to the moon, and so on. Hence, in thinking about a system for renewable energy for a community, it is important to link as many diverse sources as possible.

This puzzler is about the nascent technology of energy from salinity gradients which could be potentially a useful addition to a portfolio of renewable energy sources. Certainly, it does not necessarily depend on short cycles of weather and rotations of the earth and moon.

The first operating plant was built in Norway (see [archive] Osmotic power). This is tiny but the estimates of potential power production are large, e.g. 3000 MW where the Rhine joints the North Sea.

The project

The project is to carry out the concept/feasibility design of a salinity gradient power station in the Tay Estuary. It will have a target maximum output of 10 MW which should be available at all times. However, to be able to fit in a portfolio of power from wind, tides and, say, nuclear, the power station should be very responsive to changes in the demand/supply balance. As a benchmark, your salinity gradient power station should be able to respond to changes in load as quickly as an open cycle gas-fired power station. Amongst other things, you will need to consider:

  1. which technology to exploit — there are about three, with pressure-retarded osmosis probably being a good starting point
  2. the actual design of the plant
  3. siting, including freshwater intakes, saltwater intakes and brackish water discharges
  4. any necessary pre-treatment of freshwater and saltwater
  5. impoundment of freshwater for buffering output power
  6. environmental impacts
  7. how to eliminate or at least, mitigate, the risks from innovation, the weather, the site conditions and so on.


The economic viability is to be expressed in terms of Energy Return on Energy Invested (EROI).

To calculate this you will need to estimate energy used in construction and decommissioning and the net energy generated over, say, a 30 year operating life.

Skills needed

Such plants will be large installations which will need to intercept, store and treat large fractions of the normal flow in these major UK rivers. Hence, on the face of it, the principal disciplines appear to be chemical engineering, civil engineering and mechanical engineering. However, these plants are operational prototypes based nascent technologies and so limiting skills to the conventional and obvious may rule out interesting solutions.

(Level: Masters; group work, learning activity.)