Tuesday, 25 September 2012

Home-made Solar Panels – Part 1 – Construction ...

Earlier this year, we decided to have a go at making our own solar panels.   It's very much a small and experimental array to check out the construction techniques, the costs and the economics of solar generation at our particular location. 

We have a shallow low-level south-facing roof over the kitchen and garage, with an open aspect to the west, so this seemed an ideal location.

We bought a 1kW kit of 6"x3" polycrystalline cells which came complete with rolls of tabbing and busbar wires and also with the flux-pens needed for soldering of the tabs.

I also spotted a job-lot of ex static-caravan windows on eBay and bought 12 for £10 each.  These were single-glazed windows removed from old caravans during refurbishment.  It was a couple of hundred miles round-trip to collect them, but well worth it for ready-glazed aluminium frames.  The sizes I bought were all around 42"x32".

Five cleaned frames trial-fitted into our roof mounting structure
When you're working out how many cells can fit into your frame, bear in mind that the cells are not exactly 6"x3" – in fact, they're usually 150 x 80 mm and 80 mm is almost 4 mm larger than 3", so we know from experience that this can screw up your layouts if you're drawing up the panels when waiting for the cells to be delivered !  Our frames each allowed a maximum of 60 cells, laid in 5 rows of 12.


The first job was to clean the frames, a difficult job because of all the mastic that had been used to seal them in the caravans.  However, a bit of white spirit and a lot of elbow grease brought them back to almost as-new condition.

The next job was to 'tab' the cells, i.e. affix the tabbing wires to the front surfaces with connection tails.  This can be a bit tricky to master at first, but there are many YouTube videos showing various ways of how to do it, and after a short while you'll develop your own particular technique and get the hang of it.  My wife's an expert now !   Use an 80W soldering iron and carry out the soldering with the cells laid out on a piece of glass – we had an large bath-mounted shower panel we removed during the house refurbishment, which was ideal.  

You're bound to break quite a few cells during this process – they're very fragile – or make a few with poor connections, but don't discard them.  They can be used to make additional panels which are still very much serviceable, if not pretty – we've already made one other 36 cell (18V open-circuit) panel comprising many of these 'seconds' for charging a couple of 12V lead-acid leisure batteries (our 13 outdoor security lights are all 12V MR16 4W LED spotlights, and we have a switchbox to use power from the batteries when they're charged-up and from a mains transformer when they're run down – in the summer, we can run these lights two or three nights per week from the batteries).

The tabbed cells are then 'strung' together in rows to form a series string which can be laid out on your backing board.  Leave a gap of 3-5 mm between the cells.  The tails from the front of the tabbed cells are soldered to the connections on the back of the next cell, and so on.  Make sure that the tab tails do not short-circuit on the tracks near the edges of adjacent cells. I haven't included photos of the cell tabbing or stringing because, as I said, they're already well covered elsewhere.


Backing board, stiffened and varnished (yacht varnish)
Setting out the strings of cells
Strings all laid out and busbar connections made

The ends of each string then need to be connected to each other using the bus wire to make a continuous circuit.  Note that the positive side of the cell is the back side – correct polarity is very important when it comes to wiring the panels.

The individual panels are tested in the sunlight using a multimeter.  Aim the panel at the sun, switch to a DC voltage scale and connect the meter leads to measure the open-circuit voltage, i.e. with zero current.  In bright sunlight, this should comfortable exceed the nominal voltage of the cells in series.

ready for testing...
Open-circuit voltage test...

Then, assuming your meter has the facility, switch to a DC amperage scale and reconnect the leads to measure the short-circuit current.  Be very careful, this can produce big sparks under bright sunlight conditions – it may be best to first turn the panel around away from the sun, connect the meter leads and then turn it back again.  This reading may be less than the rated current of the cell, but you should get around 3.0A or so in direct sunlight.

If these tests are OK, it's indicative that the panel is working satisfactorily.  If not, you can identify any bad cells by measuring the voltage and current across each individual cell or groups of cells.  They can then be replaced.

Note that the panels cannot deliver any power at either the short-circuit current or the open-circuit voltage.  The peak power point lies somewhere on a convex curve between the two. 

For example, our panels test at around 36V on open circuit, but when under load the voltage drops to around 24-26V.  Therefore, our nominally 540 Wp array actually delivers considerably less than this, particularly at our latitude and roof angle.

Each of our panels contains 60 cells rated at 1.8 Watt-peak (Wp), and under optimum conditions each cell can nominally deliver 3.6A at 0.5V.  Therefore, connecting 60 cells in series gives a panel voltage of 30V and a nominal power rating of 108 Wp.

Adding panels together is like adding cells - connected in series, the voltages are additive but the current remains the same as a single unit; connected in parallel, the currents are additive but the voltage remains the same.

With five panels in our array, all connected in parallel, the open-circuit voltage remains over 30V but the current rating of the system becomes 18A, giving an array of 540 Wp.

The wiring arrangement needs to be carefully considered when connecting several panels in parallel.  With only two panels, if there's a fault on one then the maximum current that can be back-fed into the faulty panel from the good one is just 3.6 A.  However, with five connected, then a faulty panel could be back-fed with over 14A from the other four, which would destroy the panel.  Therefore, each of the panels is individually protected by a 4A fuse.  With this arrangement, diodes for each panel are not strictly necessary, although we still have them in the circuit at present – I'm considering taking them out since diodes induce a voltage drop and hence very slightly reduce the voltage available from the panels.

When the panels have been tested OK, then the cells and wiring can be 'encapsulated' on the board using a specialist proprietary silicone compound.  At around a third of the total cost of the finished panel, the Q-Sil, Polastosil, Silguard encapsulants etc are all relatively expensive, but don't be tempted to try any other means of encapsulation unless you have equipment to heat-shrink EVA sheets – you'll find that in the end these flexible and UV-stabilised silicone rubber compounds are the only feasible way of protecting your cells, which is necessary because otherwise moisture present in the air will eventually corrode the cell tracks and interconnections and severely reduce the life of the panels.

two-part encapsulant compound
curing after encapsulant pour
showing +ve connection wire and frame sealing strip
final testing before inserting in frame...

After encapsulation on the backing board, the board can be installed in the glazed frames.

finished panel - board is fixed by screws through holes in frame

We bought a 22-60V 1,000W grid-tie inverter for our system.  600W and smaller versions are also available and are much cheaper, but their reliability is questionable when operating near their maximum rating and the internal cooling fans tend to run continuously, draining more of your precious power.  

The grid-tie inverter simply plugs into the household grid via any standard socket outlet, converts the DC voltage from the panels to 220-240V AC and synchronises with the household frequency to allow the power to be delivered into the home.   Note that these units are significantly different from inverters which you can buy to run AC appliances from the car cigarette lighter socket – a grid-tie inverter produces true sine-waves and automatically detects and synchronises with your home supply.  They also automatically and continually seek the optimum power point of your solar array.   An added safety feature is that if your house electricity supply is interrupted or turned off at the main, the inverters isolate and stop pushing 240V AC into the wiring. 

Most of these low-cost grid-tie units come out of China, and we were very wary of mounting them inside the house.  A few hundred watts doesn't seem much, but if the unit should become faulty there's a potential fire risk.  Therefore, we built a power cabinet and fixed it on an outside wall.

showing power cabinet & solar inverter - the other smaller
inverter is for the wind turbine, more on that in a future post...

Our system is relatively low-powered when compared to professional systems which can make enough energy to be able to feed back into the national grid and earn you money.  However, they must be installed by certified companies in order for you to benefit from these 'feed-in' tariffs.  Therefore, there's little point in our system ever producing more energy than can be immediately consumed, otherwise we'd simply be feeding the grid for free !  

five panel array in-situ and operational

However, as in most households, we have a 'base-load' of appliances which are continually switched on, such as the fridges and freezers, our security cameras and their associated recording computer, my laptop etc, which can absorb most of the power from the array.  This base load is generally around a few hundred watts.


Part 2 of this series on solar panels will be about our operational data and the costs & economics of our particular installation...

4 comments:

  1. Intriguing - I've occasionally toyed with the idea of running a GTI to carry some of the base load and I'm surprised you can use such a small amount of roof area to good effect. I've used 12V MR11 LED bulbs and simply bridged them across the 12V line from the battery via a 3A fuse- this system works even through the shortest day.

    One small request from a reader - using tags or categories would make off-grid articles easier to find ;) It's great stuff but challenging to unearth!

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    1. Hi. Many thanks for the tip on tags etc. It simply never occurred to me before ! I've now given it a go, but it might still need a little tweaking.

      On your other comment about the solar array economics, you're right, it's not a good investment at all but it was fun to construct, and it's a useful experiment to establish what is possible from solar energy at our location.

      I've a few further posts (hopefully now easier to find !) on the performance and economics.

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    2. Thank you - tags absolutely sorted now :) So I can now go and learn more - fantastic!

      With the economics we keep wrestling with that at the smallholding, which is an island site. The borehole pump is 500w, though with an evil power factor. And every time solar comes up I have the job of shooting it down - you can buy an awful lot of petrol for the generator for the capex, and the batteries etc. I was half minded to look at wind, but you've saved me the bother!

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    3. Hi. Proprietary solar panels are a lot cheaper now - even less than it cost me to build my own - but I don't think they're yet an economic proposition, at least not as a direct substitute for mains electricity, but clearly you don't have that alternative at the borehole pump location.

      How often does the pump run, and for how long ? If it's only occasionally, then a couple of 120W panels may be all that's required to keep the batteries topped-up between pump starts. And the pump might be rated at 500W but does it actually draw that much ?

      I'm guessing here, but a couple of used panels for £120, a charge controller (<£20), and a couple of used car batteries from the local scrapyard - say less than a couple of hundred quid to give it a try, although as you say that's a lot of petrol ! But the petrol generator had a cost too, at least to buy it initially.

      Not knowing the water system requirements, it's difficult to comment further. The hydraulic power required is the product of the flow and the pressure, the pressure being the static lift head plus the friction losses in the discharge piping, all of which can be calculated if you've a 'characteristic curve' included in the pump documentation.

      And if you're on a smallholding, you might be able to put up a much larger wind turbine than I can erect here. One of the rules is basically that if it falls over it must only be able to fall on your land, not over into someone else's. But it's probably only likely to be a remotely economic proposition if you build it yourself or buy a used turbine very cheaply.

      Depends on whether you only think about it in economic terms, or you'd like the fun and challenge of building and maintaining it as a hobby project ...

      I haven't yet totally give up on wind power, although I don't know why ! - maybe just a perverse dislike of being beaten...

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