200 Year Old Technology Makes it into Space – The Stirling Radioisotope Generator

For those of you who have not heard about the Stirling engine, the technology was first proposed by Scotsman Robert Stirling way back in 1816 as an alternative to nasty steam engines, which had a habit of exploding and killing people with high-pressure steam. In nineteenth century steam engines, water inside the pressure vessel was in two phases – steam vapour and pressurised liquid, so in the case of a rupture there was an instant expansion of hot liquid into steam.

Often called an ‘external combustion engine’, Stirling engines are a sealed system with the cylinders inside working with a gas, such as air or nitrogen, which exists in a single phase.

The physical layout of the Stirling engines varies, but all have a ‘power’ piston and a ‘displacer’ that works in concert with the power piston to maintain the constant volume conditions. Each engine has a hot and cold end, with a heat exchanger at each. Inside the engine is a ‘regenerator’,  which is a physical material that stores part of the heat as it flows inside the engine and is crucial to its operation.

Stirling engines have been demonstrated at temperatures well below 100oC. The Ultra Low Temperature Difference Stirling engine was demonstrated to operate at a hot side temperature of just 0.5oC. Like any heat cycle, it is driven by temperature difference, so a low hot side temperature must be balanced by an even lower cold side where heat can be rejected. In practice low temperature differential Stirling engines require a very large surface area for heat transfer and are consequently more expensive to manufacture than high temperature Stirling engines.

The real advantage of Stirling engines lies in their heat source flexibility. The same Stirling engine can operate with a wide range of fuels and over a wide range of temperatures.

NASA have been working for some time on a small Stirling engine for use as a power supply on spacecraft. Called the Advanced Stirling Radioisotope Generator (ASRG), it is driven by the heat from radioactive decay.

Around 1kg of Plutonium 238 forms part of the module. This generates a thermal output of around 500 Watts. The heat drives a small, single cylinder Stirling engine that produces around 140 Watts of electrical power.

Like all Stirling engines, the ASRG is a closed-loop engine. It’s internal working gas will be helium. In its single cylinder the up-down motion of the power stroke is converted into an AC electrical output by a linear alternator. This is then converted to the DC required by on-board systems.

Why would NASA bother putting something with that many moving parts on a spacecraft? Well for a start, Stirling engines are very reliable, and a large part of the work the NASA is undertaking is focussed on reliability studies for the ASRG. But primarily, the ASRG will be four times more efficient per unit mass than the Radioisotope Thermoelectric Generator (RTG) it replaces. That is an impressive increase in efficiency. The RTG modules have been standard on spacecraft for the last 40 years, and use the temperature differential in thermocouples to produce power.

To reduce vibration, two ASRG  units will be mounted opposite each other and synchronised so their pistons move in opposite directions to eliminate mechanical noise.

An RTG system has a typical efficiency of around 5-7%, disappointingly low considering it is driven by 850oC from the Plutonium power source. The ASRG’s Stirling generator would operate at around 38% efficiency with the same 850oC hot end (with heat rejected the lonely depths of space at 90oC). In practice the ASRG’s hot end temperature, and consequently, net efficiency is expected to be a little lower.

The ASRG was demonstrated for the first time in 2012 – the first demonstration of a new nuclear system for power production since 1965. There are also moves to produce more Plutonium, again for the first time since 1965.

The ASRG could be available as early as 2015, and is designed to have a 14 year mission life.

Larger versions have been proposed to power a potential Moon base, and also a Mars base under the NASA Fission Surface Power project. So far a 40kWe version has been trialled in NASA labs (minus the nuclear fuel source i.e. just the Stirling engine component with conventional heat applied). This 40kWe version is likely to be the size of a trash can, and would provide surface power for decades with little or no maintenance.

Around 40 kWe is about the size of generator you need to power a small hybrid-electric vehicle. Maybe NASA would consider selling Plutonium cars to the public? It would be cool to drive around for a couple of decades and never fill up. When you are not driving you could plug it into your house and power both you and your neighbours.

Hey, it’s nice to dream:)

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