Thanks to the rise of intermittent renewable energy sources, we’ve seen increased demand for new energy storage technologies, like batteries, pumped storage hydropower, and flywheels. But what if I told you that this little toy, a 200-year-old invention, combined with thermal energy storage might be a promising solution? Let’s explore the Stirling Engine and the future of renewable energy storage.

Not to keep beating a dead horse here, but finding viable energy storage solutions is the only way for intermittent renewable energy generation like solar and wind to solidify themselves as a part of the energy mix. Typically we’re talking about chemical batteries or mechanical storage systems, but we may be able to add the thermal-powered Stirling engine to that list. It’s been reapplied in an innovative way to become another potential option for energy storage. And the whole reason I went down the path of making this video is because of this little toy, but I’ll get to that in a minute.

Engines have been powering the world since the Industrial Revolution, first with dirty, coal-powered steam engines, and more recently, combustion and jet engines. Unlike those, the Stirling engine doesn‘t use steam or fuel. It can run from absolutely any source of heat that‘s applied externally, heating, cooling, and recycling the same air to provide useful power that can drive a crankshaft. 1 2

This engine was developed by the Reverend Robert Stirling, when he was 26 years old and had just been ordained to his first parish. His invention was developed to overcome some problems of steam engines, like operating at high-pressure with a risk of explosions, having low efficiency, and demanding a lot of water to operate as well. An additional benefit, since it doesn’t depend on contained explosions like an internal combustion engine, is that the Stirling engine runs silently. 1 3 4

There are three main types of Stirling engines: alpha, beta, and gamma, which are distinguished by how they move the air between the hot and cold areas 5. Which brings me back to my little friend here, which is a displacer-type Stirling engine. There are five key elements of a Stirling engine:

1) Heat

The power source: heat. It’s where the engine gets all the energy to be used in the process, like a solar mirror concentrating heat from the sun, a coal fire, or even a cup of tea. Seriously, a cup of tea … yes, it’s only going to provide a tiny amount of energy that would be quickly used up as the tea cools down, but it works.

Just placing this on top of a hot cup of tea gets things moving. That heat transferring into the base of the engine leads me to the second key element of a Stirling engine…

2) Gas / Working Fluid

The gas, also called working fluid, is used to move heat energy from the source (the cup of tea) to the heat sink, which I’ll get to in just a second.

The working fluid is sealed in a chamber inside the engine, and in this case it’s just air, but it could be hydrogen, helium, or any substance that remains in a gas state when heated and cooled during the process. 1 As the heated air inside expands, it pushes upwards against a displacer piston, which is the third key element.

3) Pistons

Although there are different types of Stirling engine designs, they usually have two pistons to work … this toy has one. The alpha configuration has a compression and an expansion piston that are placed inside two separated cylinders.

Once the heated gas reaches the top of the chamber, the gas has reached the colder side and the heat sink … which is the fourth key element.

4) The Heat Sink

This is where the heated gas is cooled before going back to the heat source. The heat sink is usually a piece of metal that releases heat into the air, sometimes the body of the machine itself. In the case of medium to high power engines, a radiator is required to transfer the heat from the engine to the ambient air. 1. In this case it’s just the top metal plate. The cooled gas then returns to the hot side to repeat the process all over again, driving the piston inside the machine.

5) Heat Exchanger / Regenerator

And the final key element is the heat exchanger or regenerator. There isn’t one on this little guy, but a heat exchanger is usually placed between the heat sink and the heat source, inside the sealed chamber. It holds heat released from the hot gas moving inside the chamber. When the gas moves back, it recovers heat from it again. The heat exchanger is important because it holds the heat that would be lost to the environment, and if lost, would decrease the efficiency of the machine.1

Alpha Stirling Engine
Alpha Stirling Engine

In the case of the beta and gamma Stirling engines, they have a work piston and a displacer piston. The first piston fits tightly into the cylinder and converts the gas expansion energy into useful work, driving the engine it’s powering. In the beta Stirling engine, both pistons are in the same cylinder, while the gamma configuration has them separated into a hot and a cold cylinder. 1 4 6 7

There’s one important thing to understand about a Stirling engine … it just needs a temperature difference between the heat source and the heat sink (where it ends up) to work. In the case of this toy, what do you think will happen if I put it on top of ice water?

So what happens when I put this onto ice water? It will actually slow down and start turning in the opposite direction. Why? I’ve just reversed which side is the hotter side. The ambient room temperature air makes the top metal plate the warmer side. It all comes down to the temperature difference between the plates. In the end a Stirling engine can be powered by any number of sources, like a combustion fuel, waste heat, or solar heat. 1 6

The thermal efficiency of Stirling engines reaches values up to 40%, while the efficiency of similar Otto and Diesel engines are 25% and 35%, respectively. In 1986, for example, the MOD II automotive engine, which utilized Stirling technology with pressurized hydrogen as the working gas, reached a thermal efficiency of 38.5%, much higher than a spark-ignition internal combustion engine of the same power. 8 9

But where are Stirling engines used? Well, not many places. You’re not going to see them in something like a car because it takes too much time to ramp them up and down, but the technology is useful in targeted cases. Like a cogeneration unit, which combines a Stirling engine with something like a natural gas generator. The Stirling engine can repurpose the natural gas generator’s waste heat as the heat source to produce mechanical energy. This is something that you can find in the industrial and agricultural industries. On top of that, it’s also utilized in submarines, nuclear plants, and even solar power. In one application, the machine is placed at the focus of parabolic mirrors to convert solar energy into electricity, as the example of the 1.5MW Maricopa Solar power plant installed in Arizona, which reaches 31% efficiency. 10 11

But it’s the use of Stirling engines and their incredibly efficient conversion of thermal energy into mechanical energy that may provide another great storage option. A Swedish company, Azelio, is already a leading supplier of Stirling engine-based renewable energy solutions, which now focus on distributed and dispatchable solar electricity, using the Stirling engine for Thermal Energy Storage (TES). 12

In TES systems, thermal energy is stored by heating or cooling a material, so that the stored energy can be used later, either for heating and cooling applications or for power generation. Depending on the technology, the energy can be stored and used for hours, days, or even months, which helps to address seasonal variability in energy supply and demand. Concentrated Solar Plants are the most widespread application of TES, where the storage enables them to dispatch electricity 24/7. The main storage technology used is Molten Salt Thermal Storage which accounts for 75% of TES as of 2017. 13 14 15

But, Azelio has been developing a different approach. Its technology combines Stirling‘s technology with TES, being charged from solar PV systems or wind generators. The technology is capable of providing 13 hours of clean and reliable electricity for continuous operation. In addition, Azelio‘s technology requires no replacements and zero down-time during servicing, with an incredible lifespan of 30 years. 14 16

But how does it work?

First, energy coming from concentrated solar power, wind or solar PV is utilized to heat up a phase change material, in this case, aluminum, to 600°C. Reaching this temperature causes the material to change its phase state, maximizing the energy density to store that energy for a very long time. This stored thermal energy is used to power up a Stirling engine, using a heat transfer working fluid. The output of the engine is then connected to an electric generator to produce electricity with zero carbon emissions. The storage has a capacity for 13 hours of electricity delivering at nominal power, and longer when adjusting output to shifting demand. The system will also deliver heat at 65°C, which is useful for industrial heating. 17

Each unit is composed of a storage unit and a Stirling engine with a peak power discharge rate of 13 kW and a heat discharge of 26 kW, and the conversion rate from heat to electricity is around 30%. The modular feature of Azelio‘s technology provides building installations from 0.1 MW up to 100 MW. 14 18 19

The company installed a storage facility in the 580 MW Noor Ouarzazate solar complex in Morocco in March of last year, and just this past December began to install its technology in one of the world‘s largest solar parks in Dubai, where it will be part of a mini-grid composed of panels and batteries. 20 21

There are also early deals in the works with Jet Energy in Francophone Africa and SVEA Solar in Sweden. So far all of these arrangements account for 426 MW of power with 5.4 GWh of storage capacity. All this provides a solid base to start series production, which is scheduled for later this year. 22

But how does it stack up to other energy storage solutions? The data is still somewhat limited, but according to a Life Cycle Assessment made by the Swedish research institute RISE, Azelio‘s technology is (23 g CO2/kWh) 29% lower in CO2-equivalent emissions than a Li-ion battery system. Even when considering that the batteries were only replaced once over a 25-year lifecycle (32 g CO2/kWh), and 96% lower than a high-efficiency diesel generator (523 g CO2/kWh). 19 While it looks to be a promising solution, an effective cost comparison to other storage technologies will depend on the start-up‘s projects and its technology development over the next few years.

My fascination with this little guy sent me down an interesting path I wasn’t expecting. I had no idea this kind of technology was being used as a possible clean energy storage solution until I started digging around. Is the Stirling Engine the answer to the future of renewable energy? The jury is still out, but it looks promising. I love seeing old technologies getting repurposed in new ways like this.

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