Wifi

This data on a wi-fi box had me looking up the frequencies. FM radio is 88 to 108MHz so these are much higher frequencies. Infrared can be as low as 300GHz. These are radio waves but much shorter wavelengths than the broadcasters. The frequency ranges of different types of radio wave are here. dBm is explained here.

Vega in the constellation Lyra

 

Vega is well known for being a bright star so I was wondering why. It turns out that it is very close at only 25 light years which explains a lot about the brightness. But it is twice as heavy as the sun so that means it burns though its fuel faster and is hotter, a class A star. These hotter stars have much shorter lives; I was amazed to discover that it was only "born" during our Cambrian period. Using data from here, it is have roughly twice the radius of the Sun, so it has 4x the surface area. The temperature is roughly 1.7x. By Stefan's Law, that would mean the luminosity is 4 x 1.7 = 33x more luminous. The data gives 40x but there are quite large ranges in the radius and temperature data.

Sunspots and luminosity

Here are a couple of sunspots from last week. I've been looking into how they form. They are colder patches formed because concentrations of magnetic flux get in the way of convection currents near the outer surface of the sun. The centre of a sunspot, the umbra, is quoted as being 3000 - 4500 Kelvin whilst the surface of the sun is usually 5780 Kelvin. Normally, anything at the temperature of a sunspot would be glowing very brightly and would be quite a red colour. The luminosity is proportional to the fourth power of the absolute temperature by Stefan's Law. This means the usual surface is say 5780^4/4000^4 = 4.4x brighter. The contrast makes it look dark rather than red.

Wind turbine carbon footprint

There was always a lot of talk round here about a rumour that the amount of carbon put into the foundations of a turbine released more carbon than it saved over its lifetime. I'm back with the book "How Bad are Bananas?" by Mike Berners-Lee. On page 153, he has a table which suggests that a 660kW turbine like this should have paid back the carbon released in its manufacture, transportation and installation in 145 days. That's less than half a year. This turbine has been in for more than 20 years. The rumour is not true. The source of the information is this paper. 

Arcturus

 

Last month the constellation Bootes was prominent in the western sky after sunset. I've coloured it in the photograph. The circled star at the bottom is Arcturus, which is the brightest star in our northern skies. It is a red giant so has expanded giving a much larger surface area and hence a high luminosity since Stefan's Law says that luminosity is directly proportional to luminosity. It is also very near, being only 37 light years away. It is a K class star. The classes are OBAFGKM (which I remember as Oh Be A Fine Guy, Kiss Me). K stars are much cooler.

BESS - Battery Energy Storage Systems

 I had thought that we stored excess electrical energy in this country by using pumped storage but it seems that we are now heading towards using fields of lithium ion battery banks. I got the hint from the BBC and from there found the interactive renewables map from this website.

Are wind turbines more powerful in the rain?

The other day, I had a claim repeated to me that says that wind turbines produce more power in rain. It's an appealing idea that I must imagine is based on rain being denser than air. There is a change in momentum when air hits the wind turbine. For the same change of speed, water would produce a bigger change in momentum and thus more force. If this is true, I can find no reference to it online. This paper only deals with Vertical Axis turbines whereas the common ones pictured are horizontal axis. It suggests that VAWTs perform worse in the rain.