Electricity from Seawater?

July 31, 2009

The Physics Central Web site of the American Physical Society has an article reporting on some recent research by Italian physicist Doriano Brogioli, demonstrating the possibility of generating electricity from the energy released when salt water is mixed with fresh water:

A device that gleans usable energy from the mixing of salty and fresh waters has been developed by University of Milan-Bicocca physicist Doriano Brogioli. If scaled up, the technology could potentially power coastal homes, though some scientists caution that such an idea might not be realistic.

The process, in a certain sense, reverses that of a de-salinization plant, which consumes energy to turn salt water into fresh water.  When fresh water is mixed with salt water, the thermodynamic entropy of the system is increased. leading to a release of energy.  What Brogioli has done is to develop a new kind of salinization cell, in which he uses two chunks of activated carbon (which has a very large surface area relative to its mass) as the “plates” of an electrolytic capacitor.  The cell is initially filled with salt water, and the plates are charged by a power supply.  This cause ions  (e.g., Na+ and Cl) in the salt water to migrate to the oppositely-charged plate.  The cell is then flushed with fresh water.  Diffusion of the ions into the fresh water, working against the electrostatic attraction, produces a positive voltage across the capacitor.  (This is analogous to charging the capacitor, then removing the dielectric.  The capacitance drops but the voltage increases.)   The increased voltage is drained off, and then the cycle is repeasted.  (You might think of it as “Salt, Rinse, Repeat”.)

So far, the idea has only been tested in the lab, but there are hopes that it can be scaled up to practical applications;

A typical cell would require about three dollars worth of activated carbon, and, given a steady flow of water, the cell could produce enough electricity to meet the needs of a small house.

“I don’t see any reason why it should not work,” said Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute at Drexel University in Philadelphia. “Capacitor desalination has been demonstrated and commercialized, and this can be called reverse capacitance desalination. It appears to be a logical approach. Of course the challenge is the practical implementation.

If the process can be scaled up to industrial size, it might be possible to install it in places where fresh and salt water naturally mix, such as river estuaries.  It’s been esyimated that the energy released at a river estuary is the equivalent of a 225-meter waterfall.

Brogioli has demonstrated that it is possible to obtain energy in this way.  I hope a way to scale it up practically is found; at this point, we can use all the alternative approaches to generating energy that we can find.

Numbers with Meaning

July 30, 2009

If you remember taking science classes in school, or teaching them, you may remember that some emphasis was placed on carrying the units of measurement along with the numbers when solving physical problems.  To take a very simple example, if a cyclist rides 9.2 miles  in 30 minutes, then his speed is given by:

Speed = 9.2 miles / 0.5 hour = 18.4 miles per hour

Doing this is just an additional check that the calculation has been done correctly, and that the answer is in the expected units; that is, if we are calculating a speed, we expect an answer in units of distance / time.  If we end up with an answer in cubic feet, something is wrong.  (I’ve done a bit of science teaching, and one of my little brain-teasers for physics students is: a number is given in units of stone-barleycorns per milli-fortnight.  What is the number a measure of?)†

Another reason to preserve the numbers and units together, of course, is to ensure that someone else reading them will interpret them correctly.  There have been some fairly serious problems caused by not getting the units right:

Out of context, a number can be a dangerous thing. In 1991, for example, NASA’s $125 million Mars Climate Orbiter was destroyed because one team of engineers used imperial units of measurement while others relied on metric ones.

The quote is from an article in Technology Review, which describes a new semantic database technology for dealing with numeric data, developed by a Cambridge, MA start-up, True Engineering Technology .  The idea is fairly simple: each number is stored along with its units of measurement, context, tolerance, and semantic tagging information.

To store a number in the system–creating what the company calls a “truenumber”–a user simply types a short phrase into a form on the website … [for example] a user might type, “The distance from New York to London in kilometers is 5,581.”.

The software is being made available in two ways: as a product that companies can install on their own servers and networks, and as a free (of charge) public Web service at the company’s site, something that the company envisions as a “Wikipedia for numbers”.  The market for the commercial version is expected to be engineering companies, primarily, but potentially any organization that works extensively with numbers is a possible customer.

I will be interested to see if this becomes successful.  I have a vivid memory of one personal experience of numbers being used by someone who didn’t understand them.  I worked for a time, quite a few years ago, for a consulting company that, among other things, published regular reports on the state of the economy and financial markets.  These included figures on the rate of return to various investments over recent periods.  One of the returns reported was for investments in 30-day US Treasury bills.   Now this should have been easy to get, since the prices of T-bills are reported every day in the Wall Street Journal and many other papers.  Yet our firm’s numbers were consistently different from the ones calculated by everyone else.

I asked if anyone had looked into this, and was told that they had, and that the formula used for the calculations had been checked, and verified to be correct. So I went to see the guy that did the calculation, and asked him to humour me by walking through a sample with me.  The formula, as it happened, was exactly right; it’s just that  it didn’t apply for the data that were being used.  Treasury bills are discount instruments.  Unlike, say, a corporate bond that provides a periodic interest payment (typically semi-annually), the T-bills interest is implicit.  If the bill will be redeemed for $100 at maturity, it will initially be sold at a discount, at a price less than $100.  For example, if the bill sells originally for $99, and is redeemed for $100 at maturity, the interest rate over that period is [(100/99) -1] * 100 = 1.01 %.   And therein was the rub: the formula being used assumed that the rate entered was the interest rate, but the number reported in the paper is the discount (which would be 1% in this case).  So the numbers were consistently too low.

Fortunately, the mistake was easily fixed, and the historical data corrected, once the problem was found.  But I can’t help but think that there are many more examples out there, perhaps not as spectacularly disastrous as the Mars Orbiter, but waiting to bite someone nonetheless.

Oh, I almost forgot.  A stone is a British unit of weight, equal to 14 pounds (I weigh about 10½ stone).  A barleycorn is an old English unit of distance, equal to about one-third of an inch.  And a fortnight is a unit of time: fourteen days.   Now weight is a measure of force ( F = ma, from Newton), and we know from elementary mechanics that the definition of work is:

Work = Force · distance

So we have a measure of work per unit time; or, in other words, a measure of power.  (Compare the definition of one horsepower as 33,000 foot-pounds per minute.)

Adobe Security Advisories

July 30, 2009

Adobe has released a couple of security advisories that are worth noting, since they pertain to software that is installed on a large number of PCs.

The first bulletin [APSA09-03] applies to Adobe’s Reader (formerly Acrobat Reader), Acrobat, and Flash Player software, on all platforms.  The Flash Player is very widely used as a browser plug-in, to display multimedia content from YouTube and other sites.  The Reader is similarly used, as a plug-in or as a stand-alone application, to view files in Portable Document Format (.PDF).  Acrobat can be used to create PDF files.  Adobe rates this as a Critical vulnerability, and expectes to release a patch by tomorrow. The bulletin contains information about a possible work-around, by renaming, deleting, or removing access to the ‘authplay.dll‘ library.

The second bulletin  [APSA09-04] is specific to users of the Flash Player as a plug-in for Internet Explorer on Windows; other browsers are not affected.   It is related to the problem with the Active Template Library, for which Microsoft released out-of-schedule patches on Tuesday.  If you have installed that Microsoft patch, you are probably somewhat protected; but Adobe rates this as a Critical vulnerability.   They expect to have a fix available by tomorrow.

Update Thursday, 30 July, 21:00

The fixes for the Flash Player problem in the first bulletin [APSA09-03] are now available for download.  See Adobe Security Bulletin APSB09-10 for instructions and download links.

Update, Friday, 31 July, 16:00

The fixes for Acrobat and Reader have now been released.  The Security Bulletin, APSB09-10, has been updated to reflect this.

Improving Spam Detection

July 29, 2009

The Technology Review, published by MIT, has an article today reporting on some research done at Georgia Tech on  improving methods of identifying spam.  Spam is an enormous problem; as I’ve mentioned before, it’s estimated that more than 90% of E-mail messages sent on the Internet are spam.  ISPs and other providers of E-mai services spend a huge amount to detect and remove spam, lest it completely overwhelm legitimate E-mail.

The researchers looked at some contextual characteristics of E-mail that are not normally examined by spam filters, which usually focus on the message content:

The system, known as SNARE (Spatio-temporal Network-level Automatic Reputation Engine), scores each incoming e-mail based on a variety of new criteria that can be gleaned from a single packet of data.

For example, the standard mail transfer protocol [SMTP’] used to transfer mail across the Internet uses port 25.  A normal mail server will have several other ports open for communication, in addition to port 25.  (For example, it might have port 22 open for Secure Shell connections.)   Machines dedicated to sending spam typically only open port 25.  The researchers also found that, by using the approximate mapping of IP addresses to geographic locations, they could identify regions that were particularly likely to harbor spammers.  In addition, spam tends to travel a longet distance, geographically, than legitimate E-mail.

The idea of using additional characteristics to identify spam is in principle a good one, and looking at these characteristics is relatively cheap in terms of resource consumption.  Still, almost since the transmission of the first E-mail, there has been an arms race going on between the spammers and those trying to deter or defeat them.  The results of this research are potentially quite useful, but spam is probably going to be with us for a long time.

US Releases Arctic Ice Images

July 28, 2009

Reuters has a report on the release by the US government of a series of images, taken by intelligence satellites, of Arctic sea ice coverage at six sites around the Arctic Ocean, and some additional sites in the United States.

Some 700 images show swatches of sea ice from six sites around the Arctic Ocean, with an additional 500 images of 22 sites in the United States. The images can be seen online at gfl.usgs.gov/.

The images were released following a recommendation to do so by the National Academy of Science, in order to contribute to the study of climate change.  This is noteworthy for at least two reasons:

  • These images have much higher resolution than the best previously available unclassified data.  The new images have a resolution of about 1 meter, compared to 15-30 meters in the older images.
  • The images were released on the same day as the NAS report, an uncommonly quick response for a government agency.

The higher resolution allows smaller features to be identified that, while not significant individually, may collectively be quite important in understanding and modeling the processes that lead to ice formation and melting.

For example, during the summer months, pools of melted water form on top of Arctic ice floes, and these puddles can stretch across 30 meters. The water in the puddles is dark and absorbs heat, as opposed to the white ice all around them, which reflects heat.

This is actually an instance of a larger problem that affects not only the analysis of climate change, but more prosaic things like weather forecasting.  The physical processes that contribute to the weather, for example, are reasonably well-understood, and can be described, typically, by systems of differential equations.  One of the standard approximation techniques for solviing these equations is the replacement of the differential equations with difference equations on a grid (3-dimensional, in this case).  This method works well when data can be observed and measured across the grid; but in the case of weather, there are many points where the data is missing.

Regardless of one’s view of global warming, the availability of more specific evidence has got to be a good thing.

GPS Spellcheck ?

July 28, 2009

I have mentioned here occasionally  the danger that people can become entirely too dependent on their electronic gadgets and gizmos.  So I was amused to read a story on the Reuters “Oddly Enough” page about a Swedish couple vacationing in Italy, who wanted to visit the Isle of Capri, and the famous Blue Grotto.  Unfortunately, they mistyped the name of their destination into their GPS navigation system, and ended up in the Northern Italian town of Carpi, instead:

“It’s hard to understand how they managed it. I mean, Capri is an island,” said Giovanni Medici, a spokesman for Carpi regional government, told Reuters Tuesday

It seems that other parts of their pre-trip research were a bit lacking, too.  The mistake was discovered when they asked at the local tourist office how to drive to the Blue Grotto.  It’s been a while since I was there, but I’m pretty sure it is still a cave opening to the Gulf of Naples, and is accessible only by boat.

Fortunately, the couple appeared to take the news in good grace, and turned around to drive south.

%d bloggers like this: