Folder 0 MISCELLANEOUS TOPICS ( PV cells, Microwave radiation, FOOD ......)
PV cells
The world is desperate for energy. Both food and energy are needed to support life and we now know that we can only use energy that doesn't pollute the world so we must rely on "RENEWABLES" . For many years our ancesters were wise and used wind, water and sunshine but then coal, oil and gas burst into our lives. This completely upset nature's balance, we realise we have to go back to renewables. Added to our list, within the sunshine area, we now have Photovoltaic (PV ) cells : these are wonderful. They are an invention of the semiconductor age and convert sunshine into electricity.
A brief history is given in the attached file Beq.pdf but it was only in 1954 that the true beginning occured as the Bell Laboratory developed the first silicon PV cell Bell_Solar.pdf .
Here you can see how solar cells are made:
First we start with molten Silicon and "pull" a SINGLE CRYSTAL from the melt. It is not pure Silicon as there is a precise amount of Boron present to make it p-type. This is machined into a cylindrical form and then sliced into thin wafers as shown:-
The wafers are polished to a mirror finish and then the fabrication process begins --
Changing the p-type material into n-type is interesting, The Phosphorus atoms diffuse only a SMALL distance and their spare electrons fill up the "holes" at the Boron atoms but there are many more Phosphorus atoms then are needed to do this and so there are lots of spare Phosphorus atoms which then have spare electron. Hence the top thin layer becomes n-type as shown:-
For each of these processes the cells are handled with care and this is possible as a 3" diameter wafer conveniently sits between the fingers. Any larger cell would cause problems.
The cells must be connected in series to give a desired voltage (often 15 - 18 V) and secured in a frame with a tranparent cover to be WEATHER RESISTANT and this is termed a MODULE.
The modules are then attached to a roof or other frame and all the interconnecting wires are brought to a site for battery charging or for making a connection to the grid.
The above cells are said to be made from MONO Silicon but there is another process whereby molten silicon is poured into a mould and that produces a square block of POLYCRYSTALINE silicon. This material was considered too fragile to be processed by the "hands-on" methods used at Ferranti but now that most of the work is automated the polycrystalline cells are very cost competative with their mono cousins.
Now, we see from the above diagram that "all the action" takes place in a very thin layer of two or three micrometers around the p-n junction. It was therefore felt that a thin film of silicon formed from Silane gas was all that was needed and it could be deposited onto a glass sheet. However it was found that this silicon was amorphous (not crystalline with all the atoms arranged neatly in rows). With this silicon it was impossible to form a p-n junction so the idea looked to be a failure. Luckily it was found that, if the silicon was deposited from a gas mixture of Silane and Hydrogen, a p-n junction could be formed. Therefore we have yet another cell called an amorphous silicon PV cell.
All these cells have been manufactured in the past in the UK - Ferranti 1960 -1980 ,,, BP 1980 - 1995 ,,, Sharp 1999 - 2013 but each company has had to terminate production for reasons unknown.
It is tragic that the UK has failed in this important industry
MICROWAVES
Quite a portion of my time has been spent on MICROWAVES --- yes, cookers included !!!!!!! as these domestic items have proved to be a Godsend to most families.
The vital piece of theory is given in the article LL where we see that IF THE LINE IS MATCHED then there is no reflected wave and all power is tranferred from one point to the next. A piece of apparatus, used by the author in the 1950's, is theNarda Impedance VSWR unit and an illustration can be downloaded from the link given below:- "Google or use link - Narda "
From voltage measurements and the positions of minima, impedance values could be calculated BUT IT MUST BE STRESSED THAT THE MEASUREMENTS WERE FOR A SINGLE FREQUENCY ONLY!!!! (Note - an AD 8318 logarithmic detector gives one a much better way of identifying "minima" -- with this detector these give high voltages of about 2 V.) The mounted detector is shown below and this can be powered from the USB port as only a small current is required.
As one moves up the RF frquency spectrum after about 10 MHz it is increasingly necessary to make sure the circuit is MATCHED. This means that a signal flows from a source -- through a component -- and into a detector will always proceed without reflections occuring otherwise the signal may not reach the detector. Lecher lines demonstrate this effect and the following video , MVI 0698.AVI , shows how a Standing Wave detector can be used to see where the microwave energy is going - a strong standing wave pattern indicates that all the energy is being reflected back to the source, a small pattern shows that the energy is being transmitted. ( I am grateful for my grandson's help in making this video). Getting the energy out of a line is quite a task. We see that an open line is a very good reflector. BUT by making a gradual change from a line to the outside world with, say a horn, one can see that most of the energy WILL radiate into space. This process of getting each load to look like the Characteristic Impedance of the line is called "MATCHING". In years gone by one was happy to get a microwave circuit matched at a single frequency; now we need "broadband" matching and something like a FIREFOX instrument from Keysight Technologies will help us in this task. Please see the TEN excellent videos from Jake Daly and David Von Workum on "Firefox Fundamentals" (www.keysight.com) .
A STANDING Wave demonstration in a coaxial line is carried out with a Narda apparatus click . The file can be copied if needed MVI 0698.AVI (this is not streamed so will take several minutes to load)
A further excellent demonstration of standing waves on a microstrip line is illustrated on the following video .
Fairly recently modern technology has made microwave experimentation that much easier. A single board analyser is available at a very modest cost. With a reflectometer attached any reflection can be measured over a whole frequency range of MHz to GHz.
A fuller description is given in the attached file MA .
FOOD
Food is the primary source of our bodily energy - the sun heats planet earth up to a habitable temperature near to 293 degrees K ( Kelvin) or 20 degree C (Celsius) and then the "burning" / digestion of food lifts the body temperature to about 36 degree C; thus food is essential for mankind's survival and, indeed, for that of other members of the animal kingdom.
Although history tells us that humans together with the animal world were hunter/ gathers in the distant past, it was the development of agricultural methods that moved the human species to our modern world . In simple terms it was appeciating that by staying "put" at a specific location on the globe, one could practice land cultivation and animal husbandry from year to year giving a more predicable, stable and plentiful (?) supply of food.
A good account of "Food and agriculture" is given in Scientific American, SA, September 1976 in which the whole issue is devoted to the subject. A later article in SA , on the economics of food production , is given in September 1980, page 74 and this may be viewed here .
A type of farming that was practiced in North America and Europe for centuries is best described in the article " Farming in New England" and this was probably a sustainable way of life. I experienced this kind of farming in the WW II years on a small hill farm in North Yorkshire, UK, and it always felt that one was working with nature - rotation of crops reduced desease and pests naturally without the need of chemicals and soil fertility was maintained by the application of animal dung.
Further ideas are given on the web page of HOMESTEADER.
In LESS THAN A LIFE-TIME (my life!) farming in the UK has changed beyond recognition. Most arable crops require spraying three of four times with pesticides, herbicides and the like ... for efficient growth and machinery is involved at all stages of crop management.
Although many would say that this evolution of farming is a necessary fact of life https://www.tannertrading.co.uk/blog/farming-agriculture/the-benefits-of-modern-farming-practices/ it is difficult to envisage how modern farming practices, which involve so much investment in machinery and are so dependent on chemicals , can continue for many more decades ---- and what happens next will be a journey into the unknown.
As a little light relief one may wish to read a short introduction to farming written by my nephew
Forty Shades of Green David Welford (ISBN 978-0-9956121-3-6 ) published by Fryup Press, Whitby, North Yorkshire
(if you have difficulties obtaining this text please contact thompsonfrank943@gmail.com ).
The story resumes in a later folder regarding sustainability
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Some additional points
oddments in PHYSICS
The first experiment is about the Earth's Electric Field. We all know about the two other fields on the Earth; GRAVITY keeps our feet firmly on the ground and with the Earth's MAGNETIC field we make sure our feet are going in the right direction with the aid of a compass. Most people know little about electricity in the atmosphere until they are caught in a thunder storm and the sky is filled with bright flashes of light that stream across the sky. The following article gives a bit more information and, if you make a link to the Reading University site (a link is given in F_M) this shows how the electric field (normally about 100 V/m) varies with time... see F_M .
A little more detail about the apparatus is given in the video
This video presentation of a Field Mill is a large file --- takes SEVERAL mins to load
Another article of interest is the pathway from electronics to microwaves EM .