Stirling engines in space

During the course of several past discussions on energy efficiency, the issue of Stirling engines has arisen. These machines convert temperature gradients into usable kinetic energy which can be used to drive machinery or generate electricity. According to an article in this month’s Scientific American, they have found a new use. NASA is phasing out the radioisotope thermoelectric generators (RTGs) that have been used to power some space missions in favour of the older and non-radioactive technology.

RTGs work by using plutonium 238 decay to heat a thermocouple, which then produces usable current. The Stirling based system still uses plutonium decay for energy, but uses the heat more efficiently. The plutonium-Stirling combination is about 25% efficient at converting heat to electricity, compared to 6-7% for a conventional RTG. A prototype constructed by Lockheed Martin uses two Stirling engines to drive a generator and produce 100 watts of power. The unit that does so is about 1m long and 30cm wide, weighing 20kg – half as much as an RTG.

Extrapolating from space technology to more mundane uses is generally hazardous – for instance, satellites have solar panels with 35% efficiency, but they cost millions of dollars. That said, the technology does demonstrate that Stirling engines have a role to play in increasing efficiency in some circumstances.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

10 thoughts on “Stirling engines in space”

  1. Stirling Engine Kit

    The intriguing design of this engine, invented and patented in 1816 by Dr. Robert Stirling, is still relevant today. Initially sought as a safer alternative to steam engines (whose boilers could explode), this engine never saw wide spread use because it was generally very heavy and required high temperatures. The principle of the engine is simple: an external heat source and heat sink create a temperature difference in a cylinder and the air inside expands and contracts, moving a piston and a crank shaft in a rotary motion. The energy source can be any source of heat or cold, including solar or even an open flame. The Stirling engine can be more efficient and quieter in comparison to the more familiar internal combustion engine.

    This Stirling Engine Kit is a truly unique gift for the person who thought they had everything – an engine that seemingly runs on air! But this engine can run on the heat coming from your hand. Please note that it is a kit and some assembly is required. Not suitable for children under 3 due to small parts.

  2. NASA Running Low On Fuel For Space Exploration

    “With the end of the Cold War came warmer relations with old adversaries, increased trade and a world less worried about nuclear war. It also brought with it an unexpected downside: lack of nuclear fuel to power deep space probes. Without this fuel, probes beyond Jupiter won’t work because there isn’t enough sunlight to use solar panels which probes closer to the sun use. The fuel NASA relies on to power deep space probes is plutonium-238. This isotope is the result of nuclear weaponry and since the United States has not made a nuclear device in 20 years, the supply has run out. For now, NASA is using Soviet supplies, but they too are almost exhausted. It is estimated it will cost at least $150 million to resume making the 11 pounds per year that is needed for space probes.”

  3. Tiny ‘nuclear batteries’ unveiled

    Researchers have demonstrated a penny-sized “nuclear battery” that produces energy from the decay of radioisotopes.

    As radioactive substances decay, they release charged particles that when properly harvested can create an electrical current.

    Nuclear batteries have been in use for military and aerospace applications, but are typically far larger.

    The University of Missouri team says that the batteries hold a million times as much charge as standard batteries.

    They have developed it in an attempt to scale down power sources for the tiny devices that fall under the category of micro- and nano-electromechanical systems (Mems and Nems).

    The means to power such devices has been a subject of study as vigorous as the development of the devices themselves.

  4. NASA’s Plutonium Supply Dwindling; ESA To Help

    “NASA’s stockpile of the plutonium isotope Pu-238 is at a critical level, causing concern that there won’t be enough fuel for future deep space missions. Pellets of Pu-238 are used inside radioisotope thermoelectric generators (or RTGs) to generate electricity for space probes traveling beyond the orbit of Mars — solar energy is too weak for solar arrays at these distances. Blocked by a contract dispute with Russia to supply Pu-238 and the US Department of Energy that has not been granted funds to produce more of the isotope, NASA lacks enough of the radioisotope to fuel the future joint NASA-ESA mission to Europa. However, the head of the European Space Agency has announced that they have plans to commence a new nuclear energy program to alleviate the situation.”

  5. Until last year, NASA was also working hard on space-going Stirling engines, which could generate as much power as an MMRTG from just one-quarter the amount of plutonium. Stirling converters work something like high-tech steam engines: the heat generated by plutonium decay drives the expansion of helium gas, which in turn moves a set of pistons to provide power. Missions enabled by Stirling technology might have included a boat to sail on the lakes of Saturn’s moon Titan, or a ‘comet hopper’ that can manoeuvre to different places on a comet’s surface. But NASA cancelled the programme in November 2013, citing cost constraints.

    The decision sparked criticism from planetary scientists such as Jessica Sunshine at the University of Maryland in College Park, who is frustrated by what she sees as a lack of long-term planning for how to deal with NASA’s limited plutonium supply. For example, NASA’s latest call for mission proposals — for relatively low-cost Discovery-class spacecraft — does not even allow the use of radioisotopes for anything other than minimal heating of instruments. “How are we getting from DOE’s restarting the programme to NASA’s flying something?” she asks in reference to the plutonium supply. “Where is that path and how long is that going to take?”

    Despite the agency’s decision to cancel the Stirling programme, a small research effort has continued. John Hamley, manager of the radioisotope power systems programme at NASA’s Glenn Research Center in Cleveland, Ohio, and his team have continued studies on 12 Stirling convertors in various configurations, which have been running for as long as 10 years. The aim is to prove that the pistons can work reliably for the long periods of time needed during an extended space mission.

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