Month: December 2008

Article watch: Solar Energy vol 82 issue 11, 2008

From Solar Energy vol 82 issue 11, 2008:

Long-term performance calculations based on steady-state efficiency test results: Analysis of optical effects affecting beam, diffuse and reflected radiation, by Pedro Horta, Maria João Carvalho, Manuel Collares Pereira, Wildor Carbajal

There are a growing number of commercially available solar thermal collector types: flat plates, evacuated tubes with and without back reflectors and different tubular spacing or low concentration collectors, using different types of concentrating optics.

These different concepts and designs all compete to be more efficient or simply cheaper, easier to operate, etc. at ever higher temperatures, and to extend the use of solar thermal energy in other applications beyond the most common water heating purposes.

In view of the proper dimensioning of solar thermal systems and proper comparison of different collector technologies, for a given application, there is a growing need for existing and future simulation tools to be as accurate as possible in the treatment of these different collector types.

Collector heat losses are usually considered to be well determined, under variable operating conditions, through the use of the heat loss coefficients provided by efficiency curve parameters. Yet, the traditional approach to the optical efficiency fails to describe accurately the optical effects affecting the amount of radiation which actually reaches the absorber.

This paper develops a systematic approach to the proper handling of incident solar radiation, folding that with the information available from tests for determination of collector efficiency curve (steady-state) and the way the optics of different collector types uses incident solar radiation and transforms it into useful heat.

Conversion function between the Linke turbidity and the atmospheric water vapor and aerosol content, by Pierre Ineichen

This technical note presents a conversion function between the widely used Linke turbidity coefficient TL, the atmospheric water vapor and urban aerosol content. It takes into account the altitude of the application site.

The function is based on radiative transfer calculations and validated with the help of an independent clear sky model. Its precision is around 0.12 units of T

Posted on by David Lindelöf

Remotely editing files as root with Emacs

I often need to edit files on remote machines or on embedded devices, that is, machines without a monitor and on which a proper editor might not necessarily be installed.

In the past that has always left me with the rather painful choice between vi and nano. Now I have never invested enough time in learning vi beyond the most basic editing commands. And nano is okayish for small edits but hopeless for larger ones.

So I was delighted to learn that you can edit files remotely through ssh with Emacs. If you want to remotely edit aFile on host aHost, open the following file:

/aHost:/path/to/aFile

The built-in Tramp package will take care of the rest. You can even use Dired remotely with this mechanism, an extremely powerful feature.

But what was missing for me was a painless way of editing remote files as root. The Tramp version that’s included in Emacs 22.2.1 was 2.0.57, with which I was unable to remotely edit files as root. The latest version of Tramp, 2.1.14, is in my humble opinion far easier to work with.

To install it, just follow the instructions. I created a directory ~/emacs into which I unzipped the Tramp distribution. I compiled it in place and did not bother installing it system-wide, being the only user of my system.

Then I added the following to my .emacs file:

;; Load most recent version of Tramp for proxy support
(add-to-list 'load-path "~/emacs/tramp/lisp/")
(require 'tramp)
(add-to-list 'Info-default-directory-list "~/emacs/tramp/info/")

With this in place, suppose you want to edit as root the files on aHost. The best is to add the following to your .emacs:

;; Setting for working with remote host
(add-to-list 'tramp-default-proxies-alist
'("aHost.*" "root" "/ssh:yourusername@%h:"))

Now editing remote files on aHost is easy, just open the following:

/sudo:aHost:/path/to/aFile

And that’s about it.

Posted on by David Lindelöf