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Archive for the ‘Estimation’ Category

The Solar roof

Sunday, December 18th, 2022

David Brin, in a comment on his blog describes Elon Musk as a ‘successful innovator’ rather than an investor or government subsidy truffle pig. Brin seems to be under the impression that Solar City “put up 2 million solar roofs”.

As best I can find, Tesla has only installed a few thousand ‘Solar Roofs’. Electrek reported in 2022 that Tesla was doing 23 installs / week, and was pausing installations. Tesla started mass market deployments of the product in 2020.

Tesla bought SolarCity in 2016. SolarCity does mainly ordinary solar panel installations, and Tesla uses combined figures to make it seem like the ‘Solar Roof’ product is more successful. The Tesla ‘Solar Roof’ costs several times more per watt that ordinary solar panels, and doesn’t make economic sense.

Cooling a cooler

Friday, August 6th, 2010

Can a solar panel be used to cool a beer cooler? The lid of a picnic cooler is big enough to hold a typical 20W solar panel ($60).

How much energy does it take to keep a cooler cold?
Heat conduction Q/ Time = (Thermal conductivity) x (Area) x (Thot – Tcold)/Thickness

Equation from here.

I couldn’t find specifications for cooler insulation. I guess picnicers don’t ask. Figures for styrofoam are available, figure 2 in. of styrofoam. Styrofoam has a thermal conductivity of 0.033 W/m°C (or here).

Let’s assume air temp of 90F, cooler temp of 40F, that gives a delta T of 28 °C.

Heat flows in through the walls of the cooler. A cooler has an internal surface area of about a foot on each of six sides, 6 sq ft, about 0.67 m2.

Plug the figures in, the cooler heats up at rate of 12 Watts an hour. So 12W of cooling should keep it cold.

So the 20W solar panel powering a Peltier cooler with heat sinks inside and outside the cooler should be able to keep a picnic cooler cold during the day. The solar panel will provide less power when the sun isn’t overhead. The Peltier cooler is inefficient, figure 50% efficiency, effectively 10W of cooling when it gets 20W of solar power. Peltier coolers also have a peak temperature difference of about 20°C, so poor heat exchangers will cut its effectiveness.

Extra heating by the having the cooler sitting in the sun, or not enough sunlight on the solar panel, or opening the cooler will decrease the system effectiveness. Perhaps it would only cool the contents to 50°F.

This assumes that the cooler starts cold. This system does not have the power to cool down a room temperature cooler full of liquid. Figuring two and a half gallons of liquid (10 l) in the cooler, it would take nearly a day to cool it from 90°F down to 40°F running off a battery or wall current.

The limits of Peltier cooling is a delta T of about 60°C, achieved by sandwiching several layers of coolers together. A small 4 in. square chamber surrounded by 3-4 inches of Styrofoam only needs one Watt of cooling. Sandwiched Peltier coolers are inefficient. At a guess, the same 20W solar panel would work. At night, the chamber would heat up at a rate of about 1°C an hour, reaching say -15°C at dawn. So small scientific samples could be frozen in a portable, off-grid cooler of this type.

Water on the moon!

Monday, November 16th, 2009

Last October, NASA’s LCROSS mission slammed a spent rocket booster then the LCROSS spacecraft itself into the moon. No debris plume was seen from Earth, but observations from LCROSS of the booster hitting indicate the presence of water on the moon. How much water? Most news accounts don’t say, but the Science magazine article does.

100 kilograms of water was detected from an impact that created a crater estimated to be 20 m wide and 3 meters deep. So 100 kg water in about 500 m3 of regolith = 0.1 g/kg. (Googled a reference giving 2.3 to 2.6 g/cm3 as lunar regolith density).

The article gives a higher estimate for water, 0.1% to 10%, higher than my crude 0.01% estimate. Which is great–enough water to extract easily and live off. Best news for space exploration in thirty years!

LCROSS impact plume
(Credit: NASA)

The lost decade, or thanks for nothing!

Friday, September 25th, 2009

Census data on median income change between 2000 and 2008 by way of USA Today:

Median income 2000-8

US GDP in constant dollars per capita grew 9.7% over this period, so the country grew 10% richer and if it was distributed evenly everyone’s income would have grown roughly 10%. But not–instead most of the $$ went to a small slice of the population, and that doesn’t show up in this table.

Doug Green talk on apoptosis

Tuesday, January 13th, 2009

“106 cells die every second in your body.”

At a rough estimated weight of 10-9 g/cell this works out to 10-3 g per second, 100g per day of cells die.

A memory game with molecules

Monday, November 17th, 2008

I had an idea for a game. It’s a memory game, the idea is to flash a molecule on the screen for a few seconds in the left hand window, then in the window to right the player builds the molecule. That’s basically the game. The player learns to recognize interesting chemicals, learns to break down larger molecules into functional groups as a way of remembering them, and perhaps learns what they are.

As the molecule fades it would be replaced with a picture that goes with the molecule–oranges for citric acid, as a memory aid or a clue for the chemically astute player.

The game could be made easier by having the molecule fade out slowly, or flashing on periodically, or visible through a port.

I don’t really want to write a molecule editor myself, that would take a lot of time and also it turns out to have been done by chemist/programmers many times. Yeah! Some very good molecular editors are out there. I was particularly impressed with Molinspiration WebME editor. Two problems though, it’s 2D and not open source.

Looking further, I found BKchem and molsKetch both of which look good and are GPL licensed but are 2D. Jamberoo is Java based but the molecule editing worked too slowly for a game.

Avogadro is 3D, is GPL licensed so the source code is available, and works on Linux/OSX/Win. It looks good and works well, so I think it would make a good starting point for a game.

Vitamin C in Avogadro:
Avogadro screen shot

Stats: extinction

Tuesday, October 4th, 2005

From PZ Myers, unsourced:

“0.1% of all the species that have existed are currently extant, and the average lifetime of a species is roughly 10 million years”

Bio stats page

How effcient is your brain?

Wednesday, September 28th, 2005

A high-end CPU these days uses nearly 100W of power. But it doesn’t have nearly the computing power of a human brain. AI is in good part, perhaps mainly, a software problem, but raw computing power seems lacking too. So how many of today’s CPUs would it take to build a computer with human intelligence? Say at least 10,000 Opterons.

This 10K CPU computer system would use 1MW of power. So how does that compare to a human brain? A person runs on 2000 Kcal / day.

At 86400 s / day this is 23 cal / s x 4.187 calories / J = 97 J / s. J / s are equal to Watts so this is 97 W.

Say 40% of the body’s energy is used by the brain. Then a person’s brain uses 40 W, as much a weak light bulb. Which is order-of-magnitude correct–your head is a little cooler than a weak bulb but the bulb is smaller.

So a human intelligent computer would use 1MW of power while a person’s brain uses 40W. The human brain is 25,000X more effcient than a computer. This says a few things about today’s computers. They are terribly inefficient, at least for the types of computations an AI needs to do. And today’s AI software design doesn’t capture the organization of biological computers. We have 1000 CPU systems today, and could build 10K CPU systems. But no system today is as clever as a mouse. Today’s AI may have crested the house fly brain goal post. But the lack is clearest in the hardware architecture.

Nanobacteria, an estimate

Thursday, April 21st, 2005

I heard an assertion nanobacteria exist along with some speculation about them and so on. It was news to me and got me wondering if I had missed a discovery announcement, so I ran some numbers:

Consider a large nanobacteria, 30 nm by 100 nm. It has an approximate volume of 7e-23 m3.

How much DNA could this volume contain? Pop up a DNA molecule in Chime and get an estimate of dimensions: 20 bp run for 6.4 nm, so 0.32 nm / bp. The double helix has a radius of 1.3 nm.

Length of DNA with a volume equal to the cell = 7e-23 m3 / [ (1.3 nm)2 x π ] = 1.3e-5 m
Bps = length / bp length = 1.3e-5 m / 0.32 nm = 42 kb of DNA.

A low average bacterial gene length is 300 bp, so a nanobacteria could hold a maximum of 42 kb / 300 bp = 139 genes. The minimum number of genes for an independent living organism is 300 – 500, so a nanobacteria can’t have enough genes. It can’t exist. But let’s run the other numbers, on proteins.

A typical protein is 3 nm, lets call it a cube with a volume of (3 nm)3 = 2.7e-26m3.
A nanobacteria can hold 7e-23m3 / 2.7e-26m3 = 2600 proteins total. Low but not a hard limit on existence.

Now let’s put this together and describe a nanobacteria. 30% of the volume is water and small molecules. Of the remainder, let’s say 2/3 is DNA and 1/3 is protein. The nanobacteria contains enough DNA for 65 genes, and 600 proteins. Which means nanobacteria can’t exist, they are too small. There are quite a few unlikely bits in biology (pretty much every rule in biology has exceptions), so I’ll qualify it and say it is quite unlikely that nanobacteria exist. And to my estimates, add the evidence from projects to mass sequence DNA extracted from the environmental samples–if nanobacteria exist, their DNA would turn up in these projects, and no new phylum of sequences has emerged.

An E. coli by contrast is 1 µ x 3 µ and can hold 600 Mb of DNA and 20 million proteins. It’s genome is actually 4.6 Mb, and the extra space is taken up by a cell wall and protein.