Sunday, 31 March 2013

A good roof overhead ~ part 7 ~ solar panels are in!

ELECTRICAL PRINCIPLES CLARIFIED BY AN ELECTRICAL ENGINEER 
~ 10TH APRIL 2013 ~

As at the date of publication the solar panels are in place on the roof but have yet to be hooked up electrically.  They look great!  Most of them, 40 in total, are installed in the top roof.  The others have been fitted in a lower section of roof. 

These panels have been manufactured as integral tiling components - quite a step forward from fitting them on brackets as separate gear. 

It must be acknowledged however, that at this latitude the angle of the roof is not ideal for maximizing the suns energy as it is a long way south and the angle at which the sun strikes the panels is relatively oblique.  There is an advantage in being able to choose the angle at which the panels are set. 

Both solutions have advantages and disadvantages. 

In the photograph below John is fitting the final two lengths of ridge flashing:


The tiles and matching solar panels were manufactured by Fangxing Roofing.  You can find out more about them here:
Details of their photovoltaic solar units can be found here:
I propped one of the solar tiles against the balcony railing to photograph it. 


The following data is from their site as linked to above:
Overall size: 410mm x 940
Effective size: 350mm x 900
Thickness: 13mm
Weight: 6.0 kilos

Each Duer Solar Slate with standard power of 30 watts contains 12 pieces of monocrystalline silicon solar cells. 


I am interested in solar power for the independence it can give from services delivered by large networks, as well as the reduction of dependence on what I think of as dirty energy sources.  Although most electricity generated in New Zealand comes from hydroelectric power stations it still utilizes natural resources, rivers and lakes which may have sensitive ecologies, and would otherwise be quite different environments; and the power still has to be conveyed from one place to another requiring vast lengths of wiring, lamp posts and associated materials.  If sunshine can be put to work, for goodness sake, lets use it!

Solar generated electricity is clearly an excellent innovation but how does it work?  
I must say I found most descriptions rather over my head, and like anything else a few building blocks of basic information went a long way.  I share here what I've gleaned:

The best website I've found is this one:
To grasp of how solar panels work a simple ABC of electrical principles is not only helpful but essential, so if this is of interest you might like to start of here:
  • Electricity basics: on this page you will find explanations of what power is, definitions of volts, amps and watts, the difference between DC (Direct Current) and AC (Alternating current), and, importantly, how solar electric cells generate electricity - excellent!
My attempt at interpreting that content turned out to be somewhat flawed, and I was pleased to be set straight by my friend Simon Dalley, who is an electrical engineer.  My introduction here describing John's solar panels sets the scene:
The components within the solar panels where the energy is generated are referred to as photovoltaic or PV cells.  The manufacturer of John's panels refers to them with the more specific technical term: monocrystalline silicon solar cells.  If you look at the photograph above of one of John's panels you can see the 12 solar cells it contains.  These cells are energised by daylight, preferably sunlight, which sets off an electrical reaction within the units.  Okay so far.  Our electrical engineer takes up the explanation here:
This reaction converts light energy to electrical energy - electrons, instead of being stationary in the circuit to which the cell is connected, are driven round it and can perform useful work. Each PV cell produces about half a "volt" of electrical voltage (tension or pressure), and about three (guesstimate) amps of electric "current" (flow) when the sun is shining on it. The amount of "power" (rate at which it can do useful work, measured in watts) from each cell is its volts times its amps, in this case 0.5 x 3 = 1.5 watts.
Cells can be connected in "series", like a string of Christmas lights, ("+" of one cell to "-" of the next), in which case the voltages (tensions) add together. You can think of it like a whole team lined up on the rope in a tug-of-war; each individual player's tension adds to the total. In the case of the Solar Slate, its twelve cells are connected in series like this and give a total voltage of 12 x 0.5V = 6 volts. The current from this string, since there is only one path going via all the cells, is still 3 amps. The overall power is the overall voltage times the overall current: 6 x 3 = 18 watts; you will note that this is also the sum of the generated powers of each of the twelve cells: 12 cells x 1.5 watts = 18 watts total.
Cells, or series-connected strings of cells as in each tile, can also be connected in "parallel", all the "+" terminals connected together and all the "-" terminals connected together. Imagine 12 taps into a trough running at the same time. Each cell (or string) is like a water tap creating its own flow. In this case, the individual currents (flows) add together, but the voltage (pressure) is still that of each individual tap. Again, the total power is the sum of the individual powers, and also, the overall current times the overall voltage.

The inverter is rated for a certain input voltage, e.g. 48 V. The tiles are connected in whatever combination of series and parallel that will supply this voltage, e.g. several strings of 8 series 6V tiles are then connected in parallel.
A more detailed article giving an overview of solar power technology can be found via the link below.  When I looked at the content on this page I was heartily glad I had read about the theory of solar electricity as outlined in 'Electricity basics' linked to above, and didn't need to plough through this much more detailed content:

Of course there is more to it than that.  Their library page outlines coverage: 
These clips looked useful too:
There were a few points that I continued to find difficult however, one of which was the difference between wiring panels in series or in parallel: why would solar panels linked into groups (described as 'in series') provide more voltage than the same number of panels wired independently of each other (described as 'in parallel')?  The answer is that they all provide the same amount of power, but grouping them together ...  Here I hand over to our electrical engineer once more: 
A simile that could be used is this: imagine each cell as a machine that pumps golf balls (electrons) up to a height of 0.5 metres (volts) and then emits them from a spout. The golf balls run down a channel (circuit), achieving useful work on the way, maybe turning a paddle wheel or something, and back into a bin at the bottom of the machine, from where they get pumped up again. 3 golf balls are emitted per second, representing a flow of 3 amps.
Connecting in series is like having 12 machines stacked up to a height of 6 metres, receiving golf balls each from the one below or, for the bottom one, the return chute. Same current (number of balls per second) but height is multiplied by 12.
Connecting is parallel is like having 12 machines side by side, each adding their balls to a (larger) channel which is now taking a flow of 36 balls per second. But they're still only going up to a height of half a metre (0.5 volts).
When the sun is dimmed by clouds, the number of golf balls per second decreases.
Re-stating this in basic electrical terms, amps are a measure of the electrical current, and voltage is the force with which they move about, so, in relation to the wiring of solar panels...
  • In series, the voltage (force) is multiplied by the number of panels
  • In parallel, the amperage (current) is multiplied by the number of panels and the voltage remains the same.  
Applying the principle of 'in series' wiring to John's solar panels each of which has an output (force) of 6 volts, when these panels are wired up into groups of 8, the output from each group could be as much as 48 volts - when sunshine is producing a maximum effect. 
This video helped me gain this understanding:  (Do overlook the use of a weird synthetic voice at the beginning - it's very brief!)


Okay, so once the wiring of the panels is all hooked up in series, what are they hooked up to?  If general household usage is the goal the wires will plug into a device called an inverter, which converts the electricity generated from one form to another:  This is necessary because solar panels, like batteries, generate electricity in the form of what's called a Direct Current, usually referred to as DC.  Household electricity is provided from the 'mains' power supply in the form of an Alternating Current, usually referred to as AC, so the Direct Current from the solar panels needs to be converted to an Alternating Current.  The inverter converts DC to AC.   

I initially found the difference between AC and DC baffling as most explanations describe AC as going 'backwards and forwards'.  This diagram from Wikipedia article on Alternating Current was helpful.  The vertical axis shows current or voltage, and the horizontal axis time:

Diagram courtesy of Wikipedia, thank you.

Our electrical engineer explains about the backwards and forwards movement of AC:
AC really does move backwards and forwards, and not basically forward. This motion is represented by the sine wave in the diagram, whose negative (backward) half cycle is equal and opposite to its forward one.  Why?  Because that just turns out to be the easiest form of electricity to generate with a rotating machine, which is what all mechanical generators consist of.  You might recall from trig that a point on a rotating wheel describes a sine wave with its x or y displacement: x = r cos (t) and y = r sin (t) where t is time and r is the radius of the wheel. Mains electricity is AC of course, and pulses backward/forward at a rate (frequency) of 50 cycles per second (Hertz).
(I never did get competent at trigonometry!)

Why do we use AC rather than DC?  The best explanation I could find was in Wikipedia's article about the historic struggle that went on between inventors and those whose businesses depended on them:
As things stand in the modern world, both AC and DC have their advantages and disadvantages
Again our electrical engineer clarifies the overview:
The AC has the great advantage of being able to be "stepped up" or "stepped down" in voltage by a transformer. Converting this to DC is extra hassle and DC also doesn't work in a transformer.  But DC at a high voltage is unbeatable for sending large amounts of electricity efficiently over long distances, as in New Zealand's HVDC inter-island link.

Batteries and solar cells, on the other hand, intrinsically generate DC. An "inverter" is required to convert this to AC at a voltage and frequency suitable for normal home appliances.
Very many thanks to Simon, my obliging and patient advisor!

Back to the story of our neighbours' roof: 
The solar panels were installed as planned but an unexpected shortage of the larger tiles needed to fit alongside the solar panels led to a lengthy hiatus while more were ordered from China.  Even though airfreighted there was a further lengthy delay when the pack reached New Zealand Customs; they held onto the tiles for far longer than could be considered necessary or reasonable, but eventually they arrived.  The photograph below shows the top roof completed apart from the one edge and its ridging:


Before the last of the tiles were put in place John had the job of fishing out the cables belonging to the panels, which he did with a long pole with a hook on it.  There were some tense moments as it is a very confined space but all were located and brought to a point where they could be worked on inside the house!


Once the remaining large tiles and the last lengths of ridging were screwed into place the roof looked splendid.  Looking at the tiles on these neighbouring slopes side by side, you can see how much larger the solar panels and their adjacent tiles are from those on the other side of the ridging. 


That photograph was taken at sunrise which is why the background is in sunlight and the roof still in shade.

Before work could begin in the lower sections of roof the scaffolding which had been providing access to the top level had to be taken down.  John had one last job to do up top, which was to clean the roof.  He explained that the drilling of holes in the ridging for the necessary screws generated small fragments of steel and butynol underlay which needed to be carefully removed.  It was a delightful sight however, and I feel sure that this photograph is one of a kind: not many people vacuum their roofs so this one is doubly historic!


The remaining tiles were fitted into the lower roof, which you can see below:


Once the solar panels were installed in the roof the job of hooking them up and putting them to work was set to one side: one can only do so much at a time. 

However, interim testing was carried out.  An electrician paired up and tested all the leads.  Here they are hanging down through the hole made in the ceiling to gain access:


A selection of plugs designed for the hook-up had been tipped out on the nearby bed - not your average bed-time toys.  Note the beautiful old light bulb at the top, which John produced for testing:


Let there be light!  The light bulb sprang into action: 


That light bulb is an electrical curiosity: made by Philips in Holland, date unknown, it carries 82.4 volts / 6.6 amps, an oddball amount.  Other packaging information states 10,000 LUM. (presumably lumens), E40 Street Seriel.  Hmm.  The screw fitting is unusually large.  

I'm aware that there will be those who find my interest in this sort of thing baffling.  In response I must say, with feeling, that mostly we don't look at things properly.  Once you learn to do so life can never be dull - there is so much to notice; also, Dad was an electrician, so I have at least a cursory knowledge of these sorts of things.  It is beautiful.  If you doubt me, find another that has been as pleasingly crafted.

[Note: I've removed from this section the photograph of the kettle and the related paragraph as it proved to be inaccurate and unhelpful!]

Before the solar generated electricity can be put to use further gadgetry will have to be installed.  This has to wait as more work needs to be done to complete the exterior of the house before the winter weather closes in. 

The final chapter about the solar panels will have to come later: the story of how well they will function in relation to household usage and how much solar generated electricity will contribute to saving on the cost of power provided by the local supplier.  

In the meantime, those readers who are interested in learning more about both the theoretical and practical aspects of solar panels may find this video useful.  I learnt a lot from it.

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