I can't believe that yet another year has passed! It seems no time at all since I was scratching my head trying to remember what happened during the previous year. Well, I think that the main event that stands out above all others is the rendezvous between ESA's Rosetta space probe and the comet Churyumov-Gerasimenko, and the crowning achievement of the mission, depositing the lander Philae on the comet's nucleus.
O.K – the lander could have landed in a better position to receive sunlight to charge its batteries, but it still managed to send back masses of data from the various exploration experiments on board. And consider for a moment what an achievement this has been.
The project began with the launching 10 years ago! The probe then spent a decade passing through the main asteroid belt, making fly-by's of two asteroids and then in 2007 a fly-by of Mars, and this was followed by two more asteroid fly-by's in 2008 & 2010. To conserve energy it was then put into hibernation from which it was successfully awakened on January 20th 2014. When the rendezvous with the comet took place in August, the probe was 673 million kilometers from the Sun. The next triumph was putting Rosetta into an orbit around the comet, not at all easy because of the comet's odd double lobed shape and tumbling motion.
Then in November, right on schedule the lander Philae was dispatched but the 'harpoons' designed to anchor the lander, failed to deploy. This caused Philae to 'bounce' away from its intended landing site. When it fell back, the comet had rotated and Philae came down close to a cliff that shaded it from the sunlight preventing it from re-charging its batteries. Nevertheless, it managed to send back masses of useful data before the batteries finally died.
The comet's next perihelion passage will take place in August 2015 when it will be 186 million kilometers from the Sun. During this time, Rosetta might still be in orbit around the comet and as it has a planned lifetime of about 12 years, barring accidents, the amount of data it will provide will be truly incredible. Yes, I think we can say that the Rosetta Mission has been an unqualified success, a mission of which the 2000 people in total who contributed to it over a decade can be justly proud. But, please someone give Matt Taylor some advice about choosing a suitable celebration shirt and tell Monica Grady to "calm down a bit" – there's a lot more exciting stuff still to come.
oOo
Next meeting The next lecture meeting will be on January 15th 2015 when Prof. Peter Simpson of Imperial College will talk on Fracking. This promises to be an interesting topic for discussion. I look forward to seeing you at the meeting.
Peter R Wallis
In the last paragraph of my article on batteries in the April 2014 newsletter, I referred to an important future requirement: batteries to store power for the nation's electric power grid. Many new sources of power are intermittent in nature, wind turbines and solar arrays; we need to store energy when they are working and release it to the grid when they are not. Mass and size will not matter as much as materials cost, low maintenance cost and long life. A recent article by a large team at the US laboratory MIT presents the results of their work on a lithium-antimony-lead liquid battery which potentially meets the performance specifications for such stationary energy storage [1].
They chose a Li/Sb based cell as it had already been shown to give a relatively high voltage of about 0.9 volts (compared with only 0.2v for Mg/Sb ) and both lithium and antimony are cheap and readily available. They chose to use liquid rather than solid electrodes as the latter can be limited in life due to corrosion and film problems. The liquid lithium is the negative electrode, the electrolyte is of molten lithium salts and liquid antimony is the positive electrode; these three liquids self-segregate by density into three distinct layers; no membranes or separators are necessary – they can limit the system life. Now lithium melts at 180°C, but pure antimony melts at 631°C; this would require an operating temperature of some 700°C, dangerously high. However, they were able to reduce the melting temperature of the positive electrode by alloying it with lead; Sb/Pb proportions of 18:82 mol% brought it down to 253°C and the cell operated well at some 450°C. They were able to show that the introduction of lead did not reduce the cell voltage significantly.
To demonstrate the scalability of the system, they made cells with a 62 Ah capacity which operated with a similar performance to those of 1.9 Ah. To optimise system costs they used a 40:60 positive electrode composition with a LiF-LiCl-LiBr eutectic electrolyte, operating at 500°C. They used these test results to demonstrate over 20 cycles and estimate the electrode costs to be 65 US dollars per kWh.
It has always struck me that the way we generate and distribute electricity is somewhat 'eccentric'. Because we have no facility to store electricity, we have to generate it on demand. This requires a lot of power plants and machinery to sit there on standby waiting for a peak in demand. With existing generators we can switch them on when demand is high but you certainly can't do this with windmills or tidal generators. They will generate when the wind blows and the tide is high and that will not often coincide with a time of peak demand. The problem is to make reliable cost effective 'batteries' that can handle the high voltages required by the grid. This is now beginning to look quite feasible. In addition, low consumption appliances and domestic lighting for example, could work using LED's fed from a 24 volt supply via batteries. The batteries would be on constant charge using power generated at off peak times. They could also receive current from solar panels which only produce power during daylight. So we could have batteries for grid-level energy located in power stations coupled with low voltage systems installed in our houses.
1: Wang K. Et al, "Lithium-antimony-lead liquid metal Battery for grid-level energy storage", Nature, 16th October 2014. [back]oOo
Older members will be saddened to hear that Joan Wolff, wife of past President Prof. Heinz Wolff (1968 – 1987) has recently passed away after a long illness. We send our condolences to Heinz and his family at this sad time.
Last updated 28-Jan-2018