So first off my opinion
This is another one that initially sounds like somebody has found a loophole in physics.
The surface of the ocean is warm.
Deep ocean water is cold.
Let’s build a power station between them.
Electricity!
Obviously there is considerably more engineering involved than that.
But Ocean Thermal Energy Conversion — OTEC — really does use the temperature difference between warm surface seawater and cold deep seawater to generate electricity.
Which immediately raises the question:
How much difference in temperature can there possibly be?
Enough, apparently.
If you’re in the right part of the world.
So where does the energy actually come from?
Ultimately, the Sun.
Solar radiation warms the surface of the ocean.
But that heat doesn’t warm the entire ocean equally.
Deep ocean water can remain considerably colder.
So we end up with a temperature difference.
And temperature differences can be useful.
A conventional thermal power station also relies on temperature differences.
We create a hot side by burning fuel or using nuclear fission.
We have a colder side.
Then we operate a heat engine between them.
OTEC is doing something similar.
Except nature has already created the hot and cold reservoirs.
How does it generate electricity?
One approach is called a closed-cycle system.
Warm surface seawater is used to heat a working fluid with a low boiling point.
The working fluid boils.
The resulting vapour drives a turbine.
The turbine drives a generator.
Then cold seawater pumped up from deep in the ocean cools the working fluid back into a liquid.
And around it goes again.
So:
Warm ocean → Working fluid boils → Turbine → Generator → Cold ocean → Working fluid condenses
That’s actually rather clever.
But the temperature difference isn’t huge
And that’s the problem.
A conventional thermal power station can operate between some pretty serious temperatures.
OTEC is trying to generate useful power from a relatively modest temperature difference.
That limits efficiency.
So you potentially need to move enormous quantities of seawater through the system to generate a useful amount of electricity.
Now we’ve got pumps.
Big pumps.
And those pumps need electricity.
So some of the electricity we’ve just generated has to be used simply to keep the system operating.
That’s where the engineering challenge starts becoming obvious.
Geography matters enormously
You can’t stick one of these off the coast of Skegness and expect miracles.
Unfortunately.
OTEC works best where warm surface water sits above much colder deep water.
That generally means tropical ocean regions.
You also need access to sufficiently deep water.
So once again we’ve found an energy technology where geography decides whether the idea is brilliant or useless.
Solar panels can work almost anywhere to some degree.
OTEC very definitely cannot.
But there could be useful side effects
This is where it gets more interesting.
If you’re already pumping large amounts of cold deep seawater towards the surface, that water might potentially have other uses.
Cooling buildings.
Supporting certain types of aquaculture.
Potentially helping with freshwater production depending on the type of OTEC system.
So perhaps judging the plant purely on electrical efficiency misses part of the picture.
We’ve seen this before with Combined Heat and Power.
If a system produces several useful outputs, looking at only one of them can give you a misleading view of its value.
Then I start thinking about the data
This system would produce some lovely datasets.
Surface water temperature.
Deep water temperature.
Water flow.
Working fluid temperature.
Working fluid pressure.
Pump energy consumption.
Turbine output.
Generator output.
Heat exchanger performance.
Weather.
Ocean conditions.
Then I’d want to know the really important number:
Net electricity output.
Not how much electricity the generator produced.
How much electricity was actually left after operating the pumps and everything else needed to keep the process running?
That’s a very different number.
Gross output can tell a lovely lie
Imagine the generator produces 10 MW.
Fantastic.
Then you discover the plant needs 4 MW to run its pumps and supporting equipment.
You’ve actually got 6 MW available.
That’s obviously a deliberately simple example, but it demonstrates something I think is important in energy data.
The biggest number isn’t necessarily the useful number.
You need to understand the system boundary.
What are we counting?
What are we excluding?
What did it cost us energetically to produce the number on the dashboard?
That’s as much a data question as an engineering one.
The ocean itself becomes an input dataset
This is another bit I like.
The fuel isn’t arriving by pipeline.
There isn’t a coal train.
There isn’t a gas connection.
The environmental conditions surrounding the asset determine what it can produce.
So oceanographic data becomes operational energy data.
Temperature profiles.
Currents.
Depth.
Seasonal variation.
Potentially even climate patterns over longer periods.
That’s a fascinating crossover.
A dataset that an oceanographer sees as information about the sea might be seen by an energy company as information about available generation.
Same data.
Completely different question.
So where is my head at?
I don’t think OTEC is going to become a major part of Britain’s electricity system.
The North Sea isn’t exactly famous for its tropical surface temperatures.
But globally I think it’s fascinating.
Particularly for islands and tropical regions where the geography actually makes sense.
What interests me most is that it’s another example of energy hiding in a gradient.
There isn’t necessarily anything dramatic happening.
One bit of water is warmer than another bit of water.
That’s it.
But if the difference is large enough and you can engineer a system around it, you can generate electricity.
Sometimes energy engineering seems to be largely about noticing that two things are different and figuring out how to put a turbine between them.
That’s where my head is at the moment.
Gareth Winterman