Creating power for your home, off grid.
Emphasis on nuts-n-bolts, hands-on projects.
I had a look around Maplins for a wireless relay. I'd used one before for an electric garage door at my previous apartment but they were really expensive (about 30 Pounds). It was a high security one with up to four learned remotes and code hopping - ideal for a garage door but overkill for a remote light switch.
Wandering around the supermarket today I noticed a mini wireless door bell kit. I've got one for our front door already but this one was smaller and had a LED that lit up when you pressed the button as well as making a noise. It had 64 key codes to cope with neighbours using the same transmitter and was only 10 Pounds. It's battery operated (a 12V sausage in the remote and 3x AA in the receiver). These things have come on in recent years. The one on my door takes 2x C batteries and seems to last about a year. Considering that the receiver is a radio, that's pretty impressively low power consumption.
Back at home, I took the new door bell receiver apart. I was lucky in that the LED comes on solidly while the digital chip is playing the sound. This made the thing ideal for switching the inverter remote on and off as it has a momentary press switch that toggles the inverter. It's a logic switch and shorting it with a test meter showed 12V at 3mA when working. I didn't want to use a relay in the receiver as it takes too much power and the thing only had 3x AA batteries. Curiously, only two are needed for the receiver, the third is in series but the 4.5V only goes to the music chip via the loudspeaker. I could take out the third battery and the LED still came on when you pressed the button (just no sound).
I figured that an opto-isolator would do the trick for providing another switch in parallel to the press button but didn't have one handy. What I did have was a LED in the receiver and a miniature photodiode salvaged from the position sender of a printing calculator print drum. So I cobbled the two together by cutting the curved top off of the LED to make it flat and superglued the photodiode on to it. As they were both water clear, it didn't work very well as stray daylight kept turning the photodiode on. So I encased the whole thing in some black shrink tubing and it worked a treat.
I stuck the bell button on the wall upstairs and it works just fine. There's a couple of seconds delay before the solar mains comes on as the inverter does some checks and winds itself up before enabling the output (so it can start big loads cleanly). In fact, it's made the upstairs lights better at night. Now when we go to bed we can leave the bathroom light on and turn off the solar mains with the remote. When one of us has to "go" in the night, we can turn on the bathroom light from the other end of the hall.
I'm happy as I can save the solar batteries at night. The wife is happy because she can turn on the bathroom light remotely and the cat is happy because she no longer gets stepped on in the dark while sleeping in the middle of the hall like some kind of rug.
I've been off the air for a while... what with a month out in Japan and then the Christmas season.
There was another sale on those amorphous panel kits again and so I bought another 4x 15W set (ordered on-line while I was still in Japan and it was delivered the day after we came home.)
My wife didn't like the panels being on the lawn so I re-arranged the home-made wooden frames to lean them all up under the kitchen window and took the leap of installing the new array on it's supplied ABS frame up on the garage roof with some extra ballast to keep them from falling over in the wind (they've survived a couple of storms without trouble). Up there it catches more winter rays as it has a clear view of the sky apart from some trees about 100m away. With the Sun only making about 15 degrees above the horizon it casts a shadow very far away but only early in the day so by 10am the Sun has crept westwards into open space for the rest of the day (useful generating fizzles out at about 2.30pm and it's dark by 4pm).
Even on sunny days I've had to give up on using solar power to light the house every night - the most I could get was every other day or every third day without resorting to using grid power to top off the batteries. Good job it's so easy to flip between grid and solar on the sockets I installed in the utility cupboard.
It also now paid dividends to have bought the MPPT charge controller as I had to run a long feed wire from the garage to the charge controller (some 15m) but as I'm running the strings at 24 Volts rather than 12 Volts, it keeps the wire losses down a bit.
The total array is now somewhat oversized for the controller in that it is rated for 200W solar input into a 12V battery but I've actually got 264W installed. The controller won't be damaged by the extra power but the micro-controller is programmed to limit the charge current to 15A so if I do get the stated 264W then some will be wasted in throttling. But in the whole of December, even on crystal clear sunny days, the most I saw from the array was about 80W. In the summer I only ever saw anything like the rated output for an hour or so each day so I think it's no bad thing to have the array oversized a bit as 90% of the time you don't get anything like the STC (standard test conditions) quoted outputs. For a start, you only probably only get STC levels of sunlight at the Equator up a high mountain. The other is that amorphous panels degrade with time faster than crystalline panels so you only get their rated output for the first couple of years anyway... it's the price you pay for them being so much cheaper than crystalline ones.
But having said that, they should last longer than 6 months!!! From the first 60W kit I bought, one panel was DOA (remember the saga to get the panel replaced?) and now another one in that set has just over the holidays spontaneously died! I noticed on a very sunny day just before New Year that the array output was a bit low, so I started testing the strings. One of the 15W panels was completely dead! Not a sausage from it in full Sun. It wasn't physically damaged at all but has gone completely open circuit. And being in a 24V pair, it also meant that I lost the output of the good panel in series with it...
So I put a e-mail into the shop to get it replaced [shudders at the prospect] and swapped out the dead panel for the not-quite-dead cracked panel that I still had left over from the last episode. I hope this isn't going to be a recurring theme with these panels, especially now that some of them are up on the garage roof.
On the bright side (literally), the days are already getting noticeably longer after the mid-winter solstice.
Meanwhile, to get a better handle on how much I can hammer the battery pack on these winter days, I've upgraded the metering on the system. Before, I had a shunt on the battery charge lead from the controller so I could see the charge rate and measure any loads attached to the Morningstar's own load terminals. But obviously with only a 12V phone charger and a couple of LED lamps on the load terminals I wasn't getting any info on what the inverter was up to. The remote for it has a 10 step % gauge for load so 1 bar on the LED ladder means 100W of output load.
I found a couple of cheap DMMs that are very low power and can run for weeks continuously turned on from 3 AAA rechargeable batteries. Even better, I could get them in different colours so the one that measures solar Amps is yellow and the one that measures the inverter load is green.
Both read down to 0.1 mV. The solar one has the shunt that reads 1mV per Amp and the inverter one reads 0.1mV per Amp. So in the photo you have to ignore the decimal point on the green meter which is showing 2A load while the yellow one is showing 0.5A charge.
I deliberately wired the green one to read negative to denote loads whereas the yellow one reads positive for charge. This way they are consistent when the yellow one is showing a negative value at night because there's no solar power and the LED lights are loading the battery.
I devised a novel way of making and tuning the inverter shunt. I soldered one of the sense wires to the end of the inverter positive lead (after the fuse) and then just soldered the other sense wire to a brass drawing pin. The 35mmsq cable is so fat that the pin can go right inside without coming out the other side. Having calculated the right length of inverter wire, I pushed the pin through the insulation and compared the meter measurement with that of an ammeter temporarily replacing the inverter in-line fuse. I could then tune the shunt by pulling the pin out and moving it along the wire without actually cutting the insulation away. When I found the right spot, I just taped over the pin to hold it there and insulate it. Easy.
It's been really windy lately but so far the exposed array on the garage roof is still there. I put two garden parasol ballasts on the thing to weigh it down. I also screwed a new hook in the wall of the house so I could re-route the overhead wire to the junction box more directly, saving 2m of wire. Not sure how much difference it will make but every little bit helps.
Dennis had posted a link to some cheap panel supplier in the US and looking at the offerings there, I could see the next generation of my system. The Morningstar is limited to 15A output but can work at 12 or 24V on the battery side and up to 75V on the PV side. So, it can handle 400Wp of PV if you use a 24V battery. If I get more solar panels I'd be able to use the 24V or 36V nominal panels that occupy the 100-200Wp range and reconfigure the battery pack for 24V operation... Of course, this would mean replacing my 12V inverter with a 24V one, but then I'd be able to use all the same battery cabling and fuses and yet double the inverter output from 1kW to 2kW. 1kW is about as far as you can go on a 12V system before the cable gauges get really silly (and expensive).
Who knows..? Maybe this summer, when the promised mass market commoditisation of solar panels kicks in and prices drop.
I'd have to figure out something for grid battery charging though... My 30A variable power supply only goes up to 15.5V. Oh yeah, I had a near miss the other day... The cheap 30A in-line fuse holder for the charger wasn't up to running at 25-30A continuously. The mini blade fuse and the holder melted together Luckily, I always keep and eye on the charger when it's doing it's thing as it's not an automatic one.
I'm soaking this stuff up like a sponge!
Please keep it coming.
I hope Sharkey doesn't mind me hijacking his solar forum for what's patently just my blog
I've just been reading a web page about some new Ah meters that look interesting. Sharkey has got an E-Meter and I know a supplier that can get them but was put off by the self consumption of the LED display on it. This Victron one uses a LCD display and claims a self consumption of 4mA for 12V batteries. It can work on any battery from 9-90V DC so you can use it even if you decide to reconfigure your batteries.
The specs for the E-Meter are here:
The E-Meter is quite an old design now (the user guide dates from 1998) and it's self consumption varies from 28mA to 150mA. However, the E-Meter is more suited to very high voltage packs (like in his car) and can read nett kWh as well as Ah (more useful for cars).
- Seasoned Nomadicista
- Posts: 340
- Joined: Mon Jun 13, 2005 12:19 am
- Location: Winlock, WA
The solar array they used was a new type of system that uses vacuum tubes to collect heat.
The theory sounds fine but the price was more than a little steep.
I am thinking at the cost of our local PUD rates the pay back on their system would be about 40 years.
I am all for not having to pay for power off of the grid.
But so far, all of the alternatives require such a substantial investment that the pay back will never happen with the really low PUD power rates we enjoy.
Currently we are paying less than a nickel per KwH.
Having said that, I am really enjoying the discussions in regards to alternative power.
As the alternatives become more affordable and as the power grid becomes more and more unaffordable the pay back curve is going to get to the point where it would be foolish not to make the investment.
Actually, the E-Meter is a pain on high voltage battery packs, as it has a maximum voltage input of 30 volts. Using it on the EV requires a prescaler to divide the pack voltage by 10 for voltage readings and a DC-to-DC converter to isolate the 12 volt auxiliary battery in the car from the traction pack to supply 12 volts to power the meter.the E-Meter is more suited to very high voltage packs and can read nett kWh as well as Ah (more useful for cars).
In my estimation, being able to read ampere-hours is essential on any battery pack that powers a load and doesn't have either a grid-powered charger that can fill it immediately and completely, or else an oversized PV / hydro / wind system that can do that job. Consuming power from any battery pack without the ability to tell depth of discharge and how much partial charge has been put back into the battery is a good way to deep-six your expensive batteries.
Discharging too deeply damages batteries, as does operating them for extended periods at a partial discharge. If your charge source fills the batteries each time they are cycled, then you don't have to worry as much about the latter. If you have a fixed load that draws a known number of ampere-hours from the batteries before complete recharging, then you don't have any problems related to the former.
Voltage readings alone are not adequate to determine battery state of charge. If you wait until your low voltage cut out shuts down the system, then you have probably damaged your batteries. Better to be able to read the ampere-hours drawn from the cells and reduce/remove load when the practical battery capacity has been approached, usually 80% discharged, maximum.
Having to scroll through four different display modes in the E-Meter is semi-annoying, but in all my systems, I have dedicated analog voltage and ammeters, so I usually only use the Ah display unless I need a precise measurement of a value in digital format. I've never used the kWh display in the E-Meter, which it has to be programmed to display.
There are many other choices for digital system displays, I'm sure that there are better choices than the E-Meter.
I've been tinkering again...
I stumbled upon this web site for a very different type of state of charge meter and apart from getting my credit card out again it prompted some re-wiring.
The site has loads of useful info on batteries and charging and monitoring and wiring and split charge systems, mostly for boat users but equally applicable to houses with off-grid mixed power. I just don't have a problem with galvanic hull corrosion
Anyway, the upshot was that I re-wired the battery bank to balance it better. I'd used 6mmsq interlinks and that meant that to deliver the inverter surge rating, I'd connected it to one battery directly in the set (the no.3 battery) and the load was sort-of shared by the other batteries but it wasn't ideal at all. I'd also put the chargers (solar and grid) on different points too... A bit of a shambles really.
As it says here http://www.smartgauge.co.uk/batt_con.html you should never end-feed a parallel bank or tap it in the middle or connect it together with wet string
So here's the new battery pack with 35mmsq interlinks formed of a length that goes into the end battery and then goes along to the other end as a bus. The other taps are made with short lengths woven and soldered into the bus. You wouldn't believe how hard it is to solder 35mmsq cables. My 100W soldering iron didn't even dent it. I had to use a big clamp iron that I bought for soldering copper pipes.
The critical thing is that all the charge sources and loads are connected to the diagonal opposite (+) and (-) points (bottom left and top right on the picture). While this means that load current has to now pass through three interlinks from each battery, it also means that each battery has the same length of interconnect to the load point. The result is an almost balanced pack. Ideally, you could connect them in a star pattern to balance them fully but then you need longer leads and a pair of massive terminals off the pack to join them at.
Just as I was finishing the re-wire, the courier arrived with the SmartGauge - timing!
It's another flush wall mount deal so I'll have to make a back box for it but here it is perched with all the other meters. For such an expensive bit of kit, it's made of pretty flimsy plastic but I suppose it would look ok when actually mounted in a box or recessed in a panel.
Without understanding how you can measure the SoC of a battery without knowing the current going in and out, I'll have to just take Chris Gibson's word for it - that and the British Army who apparently have bought this kit for their vehicles... But then the army has a track record for buying expensive stuff that doesn't work so I'm not sure of the marketing value of that reference customer
So far it looks as though "it does what it says on the tin". Very simple to set up and nothing to really play about with (unlike Ah counters where you have to program them with the bank size, charge efficiency and Peukert constants and so on). This one just asks you for what type of battery you have (flooded, gel, AGM, hybrid low maintenance, Calcium maintenance free, or carbon fibre) and err... that's it. It then starts to work and shows either Volts or % State of Charge.
My pack was somewhere near fully charged today when I was wiring it up again and the SmartGauge defaults to 75% when first set up. It was really sunny today so the solar system pumped the pack up to nearly 100% (it went into absorption mode for a couple of hours but didn't quite make it to float mode) and the gauge read 97%. I used the lights and watched a movie and it went down to 74% over 6 hours. It's supposed to get better at estimating over time as it starts out assuming a new battery of the type you set in it's modelling but then learns about how the battery has aged over a few cycles and feeds that info back into the model to tune it. So you don't have to even tell it how big the bank is because it always tells you how fully charged the bank is relative to it's actual capacity. This is important because discharging to 50% of capacity means 50% of current capacity (not the 440Ah it was on the day you first used it). After a year of heavy use it could be 90% of the rated capacity (396Ah) so you should no longer assume that you can take half of 440Ah any more. Now you should be only pulling 198Ah to keep within 50% DoD. You should also be pulling less Amps too - as the lower actual capacity is also measured at the 20 hour rate so that drops the rate of discharge allowable too.
It seems to be doing something intelligent (not just being a Volt meter) as during the run tonight the pack voltage (as shown on my old big LED display) sagged to 12.2V and the SmartGauge was going down as well. But when I turned off some things, the pack gradually recovered over 20 minutes to 12.5V but the SmartGauge just slowed its decent of the SoC %.
First off, a disclaimer: The information in this post is provided based on my research into the wiring regulations applicable in the UK. Bear that in mind when reading it and research your own local wiring regulations or leave such matters to a professional and qualified electrician. I make no claim to the accuracy or suitability of the information contained in this post and accept no liability for any consequences as a result of it's use.
Grounding & Earth Bonding
Grounding the PV panels, DC systems and inverter seems to fall into two camps. Some new panels (thin film and rear contact types) need to be positive grounded to avoid a polarising effect on the panel that reduces their output. This is because if you negative Earth them, there may exist a positive leakage charge on the panel (on wet days, for example) with respect to Earth and this causes the polarisation. If you Earth the positive side then the whole panel is at Earth potential or below so the correct polarisation of the panel is maintained at all times.
This is a bit of a pain for anyone using a traditional system where the negative side is grounded. You need a special solar charge controller (as most are designed for negative grounding) and possibly even a special mains inverter - although the advice was that you must use an inverter with galvanic isolation (which I presumed to mean a transformer between the DC and AC sides). If the negative DC input of the inverter is tied to the Earth and you then try to use a positive Earth PV system, you'll effectively short out the inverter DC side... Not good. So the inverter DC inputs have to be isolated from the inverter chassis and house Earth in such a system.
As far as grounding PV panels for safety, it's not so much of an issue for a system that only runs 12V or 24V nominal solar panels (although a 24V system like mine can show working voltages during final stage battery charging of up to 42 Volts). It is very important for grid-tie systems that can run at up to 600 Volts DC.
If nothing else, you should ground the PV panel frames to offer some protection from lightning strikes.
My panels fall into two categories. One type are made with plastic frames so there's nothing to ground (but I might ground the negative leads). The other type is the more usual aluminium framed kind that should be grounded. I'm not so bothered though. The aluminium ones are leaning against the house wall so it's more likely that the house will be struck by lightning than the panels.
The other aspect is the AC side of the inverter. The inverter Earth is connected to the house mains Earth so that whichever supply is being used, the chassis of connected equipment is at the same potential so that a short to Earth will blow a fuse and not leave the chassis floating at 230 Volts. This is especially important in my living room where some things like metal table lamps are on solar AC and some things are on grid AC.
Neutral to Earth Bonding
A less obvious safety decision to be made is that of whether to Neutral bond the Earth on the inverter. In the UK there are two types of house wiring scheme (so called TN-S earth and TN-C-S). These relate to how the Earth and Neutral are connected. In the diagram below you can see the TN-S system that houses prior to about 1970 used and the more modern TN-C-S system used almost exclusively since the 1990s. In case you're wondering, the T means Terre (French for Earth - don't ask me why they used the French word...)
The difference is not that the Neural is bonded to the Earth, but where it is bonded. The TN-S system has it bonded at the local sub-station, whereas the TN-C-S system has it bonded at your house where the meter is (or just outside the door, near where the utility Earth rod is). The main reason for the difference was that it allows the utility company to run just two wires around the streets (Live and Neutral), whereas they used to run Live, Neutral and Earth separately. In the TN-C-S system they instead bond the Neutral to Earth rods here and there along the way so that if someone cuts the Neutral along the way (with a road digger), there are many other Earth points for the current to escape.
Now coming back to your inverter in the home. Most are supplied with the output Earth connected to the chassis of the inverter and you connect the chassis terminal to the house Earth. That's obvious. What they don't explain are the Neutral bonding options. Some inverters can be configured with the Neutral output bonded to the Earth, some come pre-configured that way, some come with it not connected and some will blow up if connected that way! You have to check the user guide or ask the manufacturer.
Mine came with the Neutral not bonded to the Earth but it is allowed to do so.
Reading around the subject it seems that the main reason for bonding the Neutral to the Earth is so that Residual Current Detector (RCD) breakers will work properly.
The RCD works by measuring the current going out on the Live wire and comparing it with the current coming back on the Neutral wire. In normal operation they should be exactly the same. If there is a difference (the residual) then some of the current is leaking to Earth somewhere it shouldn't (like through you!). If the leakage current exceeds the RCD trip current (often 30mA) then the breaker opens. They don't set the trip current to zero because there is always some current leaking out of bits of kit and the generally accepted lethal dose is 30mA for humans. A RCD won't guarantee to save your life but it's better than trusting to the 13A fuse in the plug, which will happily pass enough current to definitely kill you without blowing. As an aside, I once blew a 125mA fuse with my little finger and lived... but don't try that at home kids
With the Neutral not bonded to Earth, the Neutral and Live wires will float up to -115V and +115V (where the polarity denotes the phase of the voltage). So between the Neutral and Live you will see 230V AC but between either and Earth you will only see 115V. If you are not using an RCD breaker in the inverter output then this is thought to be somewhat safer as you'd only get a 115V shock from a Live or Neutral wire to Earth. Obviously, it's not going to help if you grab the Live and Neutral wires, as you'll get the full 230V.
With the Neutral bonded to Earth, the Neutral will sit at 0V and the Live will be pushed up to the full 230V. With a RCD in the circuit, a shock from the Live wire is going to be your only risk but the high voltage means it is going to be more likely to trip quickly in the event of some fault or water getting into a bit of kit (or you sticking your pinky where it don't belong). For US inverters that only run at 115V, the chances of the RCD working are even less because without the Neutral-Earth bond, the Live will only be sitting at 57.5V. Enough to be hazardous but unlikely to be enough to trip a RCD through a human.
That's why the 115V version of my inverter comes with the Neutral bonded to Earth by default and they state in the manual that you should use a RCD breaker on the output.
Having said all the above, it is still advisable that you bond the inverter Neutral to Earth so that the whole house is on the same scheme and you'll know that the Neutral is always at Earth potential (not that I expect you'd be poking your fingers into any live sockets, but I've seen electricians do it and they might get a surprise if the Neutral is "live").
Meanwhile back at the farm (the solar farm that is), I've been watching the SmartGauge and it seems to work well. It's been gradually getting to realise how the battery pack responds to discharge and charge over a few daily cycles and now seems to have the measure of the beast. At this time of the year, with only about 60% of the summer Sun strength available, the array manages to put around 20% back into the pack on a clear sunny day.
On the days when the pack has been "topping off", absorbing the last 15% of charge, sitting at 14.4V, it's been gassing quite a lot and I'd begun to notice the nauseous effects of being near the pack if I forgot to open the air vent above the patio door. So, I went down to the hardware store and got some 3mm plastic tube - the sort you use for fish tank air lines. I noticed that the batteries have a commoned vent system that leads to a hole in the end of the battery top. Co-incidentally a 3mm poly tube fits snugly in that hole and so a few minutes with a pair of scissors and some push-fit T pieces had all the noxious gasses piped up and out of the house through the hole where the solar cable comes in.
Before I pushed the tube outside, I did a little experiment - a somewhat dangerous one but then the best ones always are . I taped a plastic sandwich bag to the end of the tube to see just how much gas these batteries make. In a few sunny hours, I had a half full bag of hydrogen and oxygen. Quite surprising really as the batteries were only charging at 13.2V, so not at their really gassy phase when they are above 14.2V. I thought water wasn't supposed to electrolyse below 14.2V but I collected a load of something... I took it outside and released it to be on the safe side. I once did an experiment with a mixture of butane and air in a sandwich bag and it almost broke the windows...
Peering into the cells, the water seemed to have gone down somewhat from when they were new 5 months ago and so I dug out an old measuring cylinder I had from my film developing days and measured out 50ml of de-ionised water into each cell to make up for the water lost to gassing over time.
I might go for AGMs or gel batteries next time - no dangerous gasses and no topping up. Flooded cells are like babies... too much feeding and nappy changing for my liking.
Some other safety tinkering on the system involved adding a new inverter output lead with a Neutral to Earth bond in the plug and a spare portable RCD breaker I had. However, it didn't like the power saving mode of the inverter. I had programmed the inverter to shut down if the load was below 20W. When in sleep mode, it "pings" the AC line to see if a load has been connected and wakes up. Trouble is the RCD has a very sensitive circuit and the relay kept pinging as well, no good for the contacts and quite noisy. So I had to disable the power saving mode so that the output is just either on or off. The inverter draws 1.2A when on but idle compared to 0.25A when sleeping. It's a small price to pay for the added safety.
In fact, I had been toying with the notion of turning the power saving mode off anyway, as the 20W limit was a bit too high for some loads. My laptop, phone charger, camera charger and some CFL lamps all used to not start properly. The lamp would flash but not ignite and the chargers would stop charging before their batteries were full because the inverter would go to sleep before they were finished.
In researching about some new SunPower contactless front plate solar cells, I discovered that these panels must be positive grounded or they won't work properly. They have frame to junction leakage currents that polarise the junction and it puts out less power or in severe cases can temporarily stop working. Traditionally, PV DC systems are negative grounded and so charge controllers and other kit are designed that way. I fired off an e-mail to the Morningstar people to ask if their MPPT controller could be connected up like this:
They replied that they'll look into it and get back to me.
The other requirement is that the AC inverter be designed with a transformer for full isolation between the DC and AC circuits, otherwise bad things will happen if you ground the positive DC side. Fortunately, my inverter is such a design, so all I need to confirm is whether the charge controller can work like that too.
Some more general tinkering today. Reduced the feed wire from the junction box outside by 1.15m by drilling a hole through the door frame to take a shorter route from the PV junction box to the controller. Shaves another 0.45W of losses off the system and makes the outside of the door look more tidy. I also cable clipped the gas vent tube to the door frame in a place where water can't run down and get into the tube. I'm sure that gas pressure would stop water running into the tube but this way I won't have to worry about it.
There's an outfit near me that claims on their web page to be selling 175Wp Sharp NT-175E1 monocrystalline panels for 645 Pounds (or 3.68 per Watt). I'm tempted as it's not that much more than the amorphous ones I've been buying for 3.10 per Watt. I sent them an e-mail to see if they will let me collect the goods and pay for them at the same time... You can never tell with "too good to be true" offers whether they are a rip-off. So in cases like this I like to collect the goods personally as then you'll soon find out if they are scam artists or not.
For some reason they will only sell the Sharps in pairs but it would be awesome to have 614Wp hooked up. Judging from my mid-winter measurements, that's just about how much PV you need to run a 30W load even on a cloudy December day. That is, after all, why I started this project - to power my work laptop for 8 hours a day without using grid power.
On Friday, Alex called me in the morning (a rare surprise, vendors who actually call you back when they say they will). Sharp had confirmed that the NT series were end-of-line and the ND-170E1F polycrystalline model was replacing the NT model. They are a bit of an odd voltage, being 48 cells and with a Vmp of 23.2V... Too much to use on 12V batteries without an MPPT controller and too low to charge 24V batteries. Most panels I've seen are 36, 40 or 72 cells.
This posed a problem as the NT series was 35Vmp (the same as my series-parallel array) and using a much lower voltage panel like the ND series would cause quite a lot of wasted power from the imbalance into the MPPT controller.
I checked out another supplier while mulling the options. They did even cheaper polycrystalline panels but on quizzing the supplier, I became suspicious about the warranty. They were importing cells from the US and assembling the panels locally. When I asked about the warranty he would only be drawn on the frame. The performance warranty was to be provided by the original cell manufacturer. Not good. Basically, the warranty he offered really wasn't worth anything as he'd just point the finger at the cell maker and the cell maker probably wouldn't accept a module that they hadn't built.
Speaking to Alex again, he could offer me the ND-170E1F panels at 590 Pounds each and throw in free delivery as he had some business to attend to in my area, so he wouldn't need to use DHL. So I accepted and he said he'd deliver on Tuesday.
Over the weekend, I decided to get round the different voltage problem and general saturation of the 15A controller in the sunnier months by buying a second Morningstar MPPT controller. This, I would use just for the new panels and I'd run them in 46.4V series rather than 23.2V parallel. If I ever get round to upgrading the battery and inverter side to 24V operation, the MPPT controller is supposed to charge 24V batteries most efficiently with a 51V array and the long cables to get to the garage roof would suffer less loss at the high voltage. I briefly toyed with the notion of buying a much bigger Outback MX-60 but it still looked like over-kill and was much more expensive than another Morningstar. Besides, if I can configure the battery and inverter for 24V operation, the pair of Morningstars will be able to cope with at least 800Wp of panels and having two controllers gave me the ability to optimising the different voltage arrays.
I also started looking at roof mounts and mailed a couple of questions to Alex over the weekend about these plastic mounts that Ubbink/Renusol make http://www.renusol.com/index.php?id=11&L=1. They are made from the same plastic that water butts are made from and you don't "fix" them to the roof but just fill them with gravel or stones to ballast them down.
On Monday, I got a reply from Alex, saying that while they can get the ConSole mounts, they were a special order item and usually for large installations, rather than just one or two. On the bright side, he said that he could deliver the solar panels a day early.
He turned up on time and helped me unpack and check the panels and make up some MC plug leads that you need special tools to assemble.
He'd brought along some 4mm UV resistant solar cable and gave me a good discount on it, leaving me with a 35m off-cut he had spare at the workshop. Although he'd brought black and red cables, he only had a red bit that was longer than 5m in a single piece so I had to just tape up the negative bare end with black tape. The MC connectors on the panel end of the run are polarised so you can't plug them in the wrong way round anyway.
Here's my new Sharp panels, propped up on the garden chairs (a traditional rite of passage for any new solar panel). I'll have to figure out some wood and hardware for mounting them on the garage roof.
You can also see our cute but dangerous ginger kitten/cat on the fence. He fixes you with a ultra-cute gaze and then punches you in the face when you try to hug him for too long.
As a test, I wired up the panels to the existing controller (the new one might come later this week) and even in the dull light near sunset, they managed to put 0.5A into the battery. I dug out some old diodes and put them on both the old and new array feeds so that the 35V amorphous array wouldn't get back-fed by the 46.4V Sharp string. The MPPT controller settled on a peak at 32V and put 0.7A into the battery.
I'll have to watch things tomorrow. If it gets too sunny, the blocking diodes are only rated at 6A on the Sharp string and a 3A one on the amorphous array... I've seen the old array putting out 5A on a good winter day lately so if it's too sunny we could have a diode BBQ on our hands. With over 600W of power potentially on tap, I'll actually have to start taking DC electrical safety seriously
Well, I got the new controller and have it all fixed up. Had to work on it until 2am as it needed the wiring and a new box with the two disconnect switches, fuses and so on fitting. I started work on it after dark as there's no other way to work on such a large array of solar panels safely. If you had just one or two, you could cover them up with a tarp or something but now that there's over 50 volts floating around the DC side, I have to be a bit more careful than before.
I got a good deal on the new Morningstar controller. The previous supplier I bought from had put up their price by 17% as the Pound has got much weaker against the USD. But I found another supplier selling it at the old price. They honoured the price but I had to wait a couple of days for them to get stock in. The day after I placed the order they must have realised that the price from the distributor had shot up because they then updated their price list on the web shop with the same 17% increase that the other shop had done. If you're running a shop, it pays to keep tabs on how much your stock costs
Note the large separation between the controllers. These units use passive cooling with a big heat sink forming the back of the unit and the wall mounting plate. It means the wall absorbs some of the heat but also that there are warm air convection currents going up from the unit. So I spaced them a good distance apart. Ideally, you'd put them side by side so they each get cool air but this arrangement is ok.
Morningstar mention on their web FAQ that it is ok to parallel their charge controllers like this (but you can't parallel the load terminals). What they didn't mention was that you should only enable equalise charging on one controller. You could enable it on all of them but the units don't talk to each other and that means they will make independent decisions on when to auto-equalise the battery bank. As this is partly determined by the previous days depth of discharge and a 28 day calendar scheduled equalise, that could result in the bank being repeatedly equalise charged during the month, leading to overcharging and excessive water loss or even positive grid corrosion. If you're using gel batteries then it makes no difference as they don't receive equalise charges ever (it damages them).
I'm hoping that the bulk charge cycle will end normally on each controller as they decide to go into absorption mode based on the battery voltage and then they monitor the current tail-off until either the current stops falling or a time-out occurs and then they switch to float mode. In theory, the two controllers should switch at the same time as they are both monitoring the battery voltage and charge current accepted.
I got another of those meters too so I can see how each array performs.
The new panels are about 25% shaded by the garage roof here and you can see that the output is quite low compared to the amorphous array. The amorphous panels don't suffer so badly with shading because the cells are arranged in strips that go the whole length of the panel so it's hard for a cell to get completely shaded. These Sharps though have the individual cells wired up in a snake pattern across the surface so with a strip of shadow along the bottom of the panels it completely shades a whole bunch of cells. The bypass diodes help a bit but you do lose a lot of power.
Already though, I'm making 18A and so it was the right thing to do to buy another controller. Each one has an output limiter of 15A into the battery so I'd have already saturated the original controller. Now I can push up to 30A into the battery.
I also picked up a whole load of aluminium and wood to start work on building the roof mount for the Sharps so that they can go up on the garage roof and out of the shadows. More on that to come.
I've over-cooked it a bit with these new panels. It got very clear and sunny this afternoon and the Sharp panels overloaded the controller. At first it could cope with the extra power, limiting the battery output current to just over 15A but as the sun intensified, suddenly the controller started to cycle the output off and on completely. After about 20 minutes, the peak daylight had passed and it stopped cutting out and resumed the 15A limited output. So there's a limit to how much over current it can handle. I had hoped that it could at least cope with 340W as the controller is rated for 400W, if used with a 24V battery.
So I'm going to have to come up with some kind of diversion load that cuts in to the PV feed on really bright days. I'm thinking about a current sensing relay that would cut in a load on the PV feed once the battery current reaches 15A. Four 21W 12V car brake bulbs in series might work (as they'd have to withstand up to 46V on a sunny day). They would soak up about 80W of excess power.
Of course the best answer would be to reconfigure the battery for 24V but that would mean trading in my 12V inverter for a 24V one and getting a DC-DC converter so I could run my 12V appliances from the 24V battery bank...
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