Breaking the ice - Get to know Lake Vostok

One of my favorite stories of the past couple of weeks has been the near completion (or completion by some accounts) of the Lake Vostok drilling project in Antarctica. Lake Vostok, for the unfamiliar, is a sub glacial lake in central Antarctica located more than 12,000 feet below the surface ice. Due to the extreme pressures underneath the ice, the lake consists of liquid water at about -3°C below freezing (remember your phase or PV diagrams!). Well, the reason this story is so exiting is the possibility of discovering life in an extremely isolated environment. The lake itself looks to have been isolated from the surface for around 15 to 25 million years, so any surviving organism would be extremophile to say the least. Discovery of life here though would have major implications for the prospects of life on other planetary bodies in our solar system. Europa (of Jupiter), for instance, is a classic example of a planet with icy surfaces with potential for sub-glacial oceans. 

Anyhow, back to the drilling project. The Russian have been drilling core sample at the Vostok station in Antarctica since the 1970’s, but only in 1998 did a joint venture penetrate to nearly 12,000 feet. At the time, the researchers called a halt to the drilling process out of fear of contamination of the lake due to drilling agents. As a result, there has been much controversy as to how to proceed to complete the drill into the lake. The Russian have been using Kerosene to prevent the bore hole from refreezing, but many environmentalists worry that the kerosene will flood into the lake and contaminate the water. It seems to me (after a couple of back of the envelope calculations) that the kerosene column pressure should not be greater than the lake pressure. Resulting in the lake forcing water up into the borehole, rather than the other way around. I can see the dilemma though… we don’t want to destroy something as pristine as Lake Vostok. It’ll be quite interesting to see how things proceed.

When prediction fails, and why.

In preparation for my dissertation proposal (hopefully sometime this semester), I’ve been diligently patching all potential logical flaws in my research assumptions. One of the overarching assumptions that I had been operating on was the inadequacy of existing energy storage prognostics techniques (batteries in particular) for non-constant loads. My brief original forays into the literature indicated this to be true, but I never independently confirmed this fact until Friday. Well, my treacherous spelunking in the literature allowed me to differentiate existing prediction algorithms, and I found that a shocking number or algorithms rely on an archaic empirical relationship, Peukert’s law. This law directly relates the run time of the battery to the current drawn. The peculiarity of battery systems, modeled by Peukert’s law, is the total available charge diminishes given higher currents (in other words, the efficiency of the battery drops). However, I ran some tests with real battery draw data and confirmed what a paper by Doerffel in 2006 illustrated… the invalidity of Peukert’s law in a vast majority of realistic cases.

The first tests that I ran used battery data with constant currents. In this case, Peukert’s law should apply. Accounting for the uncertainties in the parameters, we can generate the probability distribution of predictions. As shown in the figure, Peukert’s correlation is well founded. However, if we look at real battery data given a periodic load (on/off only), this correlation crumbles. As I mentioned above, a shocking number of published techniques still rely on this empirical correlation for prediction for realistic systems… completely discounting the inaccuracy. Dynamic battery models, on the other hand, directly account for variability in the load… given Occum’s razor, simpler prediction actually requires dynamic models.