Cold energy – how Stirling can help energy efficiency

Who knows what a Stirling engine is? Most of us won’t. Well, I didn’t until about a year ago and when I was explained the principle at its core, I had an EUREKA moment.

Let’s go to the fundamentals. Stirling engines work by exploiting temperature differences to move pistons that in turn move a crank that turns a shaft that … you know what an engine is, don’t you?

A Stirling engine looks and feels much like a normal piston engine at least when it comes to power transmission. OK, it may look a little weird at times, I will give you that.

The key difference is that there is no fuel burned to make the pistons move. It’s just the fact that heat tends to expand things and cold makes them shrink.

This happens to ambient air so the difference between two temperatures makes things move.

The bigger the temperature difference, the better the Stirling engine works. It’s a bit like free power – let’s be frank. It’s not entirely free but still must earn its CAPEX. That is the money it has cost to build the thing in the first place. But as soon as you have a stable source of temperature difference – that’s all its takes.

With all this in mind, we would place Stirling engines where there is a lot of exhaust heat to use. But what about exhaust cold?

LNG is a supercooled liquid. It’s cryogenic. I was told that the Stirling engine works as soon as there is a temperature difference of at least 25 degrees Celsius. And as said, the bigger the difference, the better efficiency.

As it works with cold as well as with heat, just imagine what efficiencies could be possible if one used the exhaust heat from a turbine or some other device and the exhaust cold from LNG vaporization on the other side.

We have the mother of all temperature differences and hence a perfect Stirling engine at work. LNG is about minus 160 degrees Celsius cold. Let’s assume for the sake of argument that the waste cold from LNG is just about minus 100 degrees and that we have exhaust heat of 100 degrees from something else (that’s still pretty balmy compared to some exhaust shafts I know) we have a whopping 200 degrees of difference. This must make for a pretty efficient Stirling engine.

A Sterling engine also works best when it runs on a stable regime so it cannot be used as a primary means of propulsion for a vehicle. But if those engines could be made small enough, they could just go everywhere and become a supporting mechanism in a vehicle that runs on LNG as a primary fuel.

Imagine that the waste cold from the LNG is combined with the waste heat from the engine and we have gotten an engine that produces almost free power that could then go into the hybrid drive e-motor. I would run the whole vehicle all electric and produce power with a combination of a gas engine and Stirling engine for additional oomph. It’s only a question of miniaturization.

But imagine even better, a satellite LNG-fueled power plant on an island somewhere in the Caribbean or the Eastern Mediterranean. This LNG is then used to produce stable power for the island community. I already elaborated on small LNG networks and there is another post in the works.

A Stirling engine could again use the waste cold of the LNG directly in situ without any complicated mechanisms and even some exhaust heat from either generators or turbines. Or if that is already used, solar heat can be used also.

The same mechanism can be used on vessels going fully electric and using LNG for power production backed up by a Stirling engine. I am not an engineer or an inventor but a simple methane aficionado but I use my imagination to find new things, at least conceptually.

Will the true innovators among us please get up now, steal the idea, and build a prototype?  Or tell me why that would not work.