Powering Spacecraft with TEGs
Chris - Powering space craft is a challenge. Traditional chemical batteries go flat and for deeper space explorations where the Sun don't shine, solar power is out of the question. Instead, the European Space Agency is working on units that rely on radioactivity to provide the heat needed to run a Thermo-Electric Generator. Dr. Richard Ambrosi is from the Space Research Centre at the University of Leicester where he does the work that helps to power our missions to Mars. Hello, Richard.
Richard - Hi, Chris.
Chris - I've made passing mention of a couple of them but what are the main problems associated with powering probes in space?
Richard - Well, the primary problem is when you don't have access to the Sun. So, if you want to explore the more distant cold, dark, inhospitable environments, then you need an alternative power source and nuclear power is one of those alternative power sources. It turns out that even if you want to operate very close to the Sun, you have a problem where solar panels generate a lot of heat so they become less efficient and therefore, nuclear power can make a big difference. Other examples include surviving the lunar night or exploring dark, cold lunar craters and certainly, if you want to operate continuously day and night on Mars, travel large distances and function for a very long period of time, you need to develop alternative power sources.
Chris - When space scientists who were working on things like the Apollo missions and those early probes were looking at those very problems, what sorts of things did they come up with as a solution? Did they jump literally on this 200-year-old bit of physics and say, Thermo-Electric Generators is the way to go?
Richard - Well, they did essentially that. The use of radio isotope Thermo-Electric Generators in space goes back to the '60s. The US has been very successful in launching a number of these systems in exploring the moon and exploring deep space. And Russia has been very successful in using Thermo-Electric Generators to convert heat generated from reactors in space into electricity. Russia has launched more than 30 reactors into space.
Chris - So in essence, you have a radioactive source, a piece of plutonium or strontium, or something that radioactively decays vigorously and produces heat and that gives you the hot side of your Thermo-Electric semi-conductor like we were discussing. I would guess that space being cold as it is, 3 degrees above absolute zero, that's quite a good cold side.
Richard - It is but it's actually quite challenging, developing a system that provides you with the constant heat source, constant delta T, and allows you to radiate any of the unconverted heat into your environment without impinging on the overall mass of your radiator structure or your thermal management system that has to dissipate the excess heat. So it's actually quite challenging, developing an efficient, a mass efficient system where you get a lot of power for the mass involved.
Chris - How radioactive are these sources and how big are they? So if I have a 1-tonne satellite or something or probe which I'm going to send off into deep space, what mass will be the generator, the Thermo-Electric Generator and how much radioactivity or radioactive material is in there?
Richard - Well, if we look at the US systems, we're talking about 100 to 250-watt units. The mass efficiency ranges from about 3 watts per kilogram to 7 watts per kilogram depending on the flavour.
Chris - But isn't that quite a lot of radioactive material?
Richard - Well it's not the radioactive material that's the bulk of the mass. It's the whole system, so you have to take into account that the radioactive material is encapsulated in a containment system. It's surrounded by an aeroshell to allow it to survive worse case re-entry into the atmosphere scenarios. You then have a radiator structure in all of the thermal management system that goes with it. If we're talking about a system for a European design that will have a mass efficiency of about 2 watts electric per kilo and we're talking about 100 watts electric then the whole system should weigh about 50 kilograms to 100 kilograms at most.
Chris - So when you're designing the systems that you are working on to send things like probes to Mars to explore the surface of other planets and so on, what do you have to do in terms of engineering in the safety? We've had one Russian mission that failed to leave Earth and has come back into the atmosphere fairly recently, Phobos-Grunt so, what do you have to do to make sure that the radioactivity in there isn't going to pose a threat?
Richard - Well you have to make sure that the radioactive material is completely encased in a system that will prevent the dispersal of the material, irrespective of the re-entry situation; So whether it lands in the ocean or lands on the ground. You have to also design in safety features that allow it to withstand launch pad explosions. These are all requirements that feed into a launch safety framework and the US and Russia have launch safety frameworks for launching radioactive material into space and Europe will have the challenge of developing its own launch safety framework.
Chris - You mentioned that these units are a couple of hundred watts. That's not very much, when you think that my computer, just the computer is 300W. So, for running really quite high end systems, that's not very much. Do they basically use the generator to charge up a battery or a big capacitor so you have something to give you surges of current to run energy intense bits of equipment for short times? Is that how they work?
Richard - Well, you can use multiple units. So for example, in a mission requiring 600 watts if your unit generates 100 watts, you would use 6 units. You wouldn't necessarily need to use batteries. You could use them in combination with batteries. But space instruments and space systems are designed to use, to minimise on the amount of power that they need to operate.
Chris - So what are the big challenges that you are now trying to overcome because we've been working on these things for 40 or 50 years, haven't we and successfully too? So what are the big challenges that still remain to be overcome?
Richard - Well for Europe, the challenge is to develop its own capability in building both radioisotope Thermo-Electric Generators and radioisotope heater units. Europe will be using an alternative isotope to what's being used in the past so the challenge will also be to be able to produce the isotope in significant quantities.
Chris - And why do we need our own capacity? Why can't we just go to NASA and say, "We'll borrow one of yours."?
Richard - Well there is a general shortage of plutonium production and Europe has access to americium-241 which is in the separated plutonium in sellafield in the UK. So it would be more cost effective for Europe to use a material that is available.
Chris - Super! Richard, thank you very much. That's Richard Ambrosi. He's from the University of Leicester.