The Solar Experiment

OK, so this article is the one that is responsible for this blog in the first place. It's about what it takes to operate your fridge off-grid, i.e powered entirely off solar. I said that I wanted to try this with ours this weekend (16-Feb-2019). Well, that's not the way my weekend turned out. Why? Hmm, well for starters, because there wasn't any sun. Not in the last 5 days actually. Which is of course not a great start to a solar project. But also, because the parts that I have ordered have not yet arrived :-(

What have I got and what are the parts that I'm waiting for? Well, I have a 265W Solar Panel that I bought on auction for R1,000. I have a mix and match of batteries in varying states of order. And lastly, I have an older 500W 12V Inverter. I am waiting for 2 parts:

So yes, I need to cop out delay for a bit. Until the parts arrive and a new weekend dawns. Until then, watch this space...

Mon, Feb 18, 2019. OK, so the DC to DC converter got delivered today. It was shipped via Pargo, so I'll pick it on my way from work tomorrow. Which I then did on the Tuesday. And got it wired in on the Saturday to an old battery. Everything seemed to be going well, the battery was charging and then, when I got home the next Monday (25-Feb-2019) - poof, the battery read 0V and there was suddenly a dead short between the terminals.

The battery came from an old stack that got overcharged a while ago, so I did know that it was not quite OK, but really, dead short on getting charged? It has a sight-glass providing feedback on the state of the battery and that is now bright yellow. On the label it says Low Electrolyte - do not charge or use. OK, so this battery is gone. And the DC-DC Converter? Well, the input capacitors have popped. As I touch one of them, I feel that the outside plastic wrapper is cracked, meaning that it got pretty hot. Oh no - I will have to try and repair :-(

Tue, Feb 26, 2019. So while still a little miffed about the blown input capacitors on the 20A DC to DC converter, this MPPT Controller arrived. And so I wired it in on Thursday. On another old battery that I had standing around (no bright yellow sight-glass, at least not yet). The controller somehow just feels better. It has proper screw terminals and it actually weighs enough that it doesn't move all over the place as you connect the wires. It also has built-in Bluetooth, which can be used with an App on your phone to view the status of the controller and the battery. It's big deal however, is that it is an MPPT (maximum power point tracking) controller. This lets the controller vary the output current to extract the maximum power from the panel. This is not only more efficient overall, but specifically so in lower-light conditions like early morning, late afternoon or during panel shading. The controller has been running for a few days now, mostly just charging the old batteries. It has not really had the opportunity to prove its worth yet, and it can't until I give it something decent to charge. I'll update this blog once I've got something more impressive to show...

In the meantime... Well, I've been thinking about the capacity that I'll need to take that fridge off-grid. Inside the fridge, there's a sticker that details the power consumption as 320W. I just have this gut feeling that it is less. How do I know? Well, that's a story on its own, but yes, I just know. What I don't know, is how much less. So we'll have to measure it. I've been building a monitoring device for some time, but I haven't finished the software. So in the meantime I'll just use a Sonoff POW R2. While this is not my device of first choice, it will give me something better to work with than gut feel...

Click to view fullsize And so it does. The pic on the right doesn't show it well, but after roughly 28 hours, we are at 2.07 kWHr consumed. Which means the fridge averages at around 2,070 WHrs/ 28 hrs = 74W, so about a quarter of what the label says. Obviously we need to average this over a bit of a longer period to get better results. After all, on a weekend, you have teenagers opening he fridge all the time, which must affect the consumption in line with their impact on the fridge contents.

But OK, let's do some sums on that 74W. In a day, we would consume 74W x 24Hrs = 1,776WHrs. My solar panel can theoretically deliver 265W, and there are roughly 8 hours of usable sunlight per day. In those 8 hours, it should deliver 265W x 8hrs = 2,120 WHrs. That is (2,120 - 1,776) = 344 WHrs more than we need. Sure, it is unlikely that our panel can deliver a constant 265W or that it can capture a full 8 hours of sun a day (6 is probably a more realistic estimate), but it is in the general ballpark, so let's continue.

What about the 16 hours of the day without sufficient sunlight? Well, that has to come from a battery. And that battery needs to be charged from the excess production of the panel. First, let's calculate the energy that the battery needs to deliver in the 16 hours. Well, that's 74W x 16hrs, so 1,184WHrs. The excess energy of the 265W panel available to charge the battery is (265W - 74W) x 8 Hrs = 1,528WHrs. So yes, the panel is big enough to power the fridge during the day and put enough charge into a battery for overnight supply. How about the size of the battery?

Well, the power that a battery can deliver is the voltage of the battery (in Volts) times the current through it (in Ampere). For how long? That depends on the size of the battery. The bigger it is, the longer it can deliver a specific power output. As such then, the capacity of batteries is rated in Ampere-Hours. This is the number of hours that a battery can deliver 1A of current at its rated voltage. As an example, a battery rated at 100AHrs, can either deliver 1A for 100Hrs, 2A for 50 Hrs, 3A for 33Hrs all the way to 100A for one hour (in theory at least). For a 12V battery, the current it needs to deliver to provide 74W is 74W/12V = 6.17A. It has to do so for 16Hrs, so the battery capacity needs to be 6.17A x 16 Hrs = 99 AHrs.

A 99 AHr battery, when using lead-acid, is considered to be of small to medium size. It is typically twice the size of what you find in a small car like a CitiGolf. Common standard sizes in the ballpark are 102AHr and 105AHr. They typically sell for around R2,000. It's also an almost exact match to the batteries that I've been messing with (105AHr Deltec Voyager Deep Cycle).

So this all sounds as if it's an almost perfect match. But is it really?

Well, the calculation that we made assumes 100% power delivery from the panel, 8 hours a day. As stated, that performance is unlikely. What about clouds? And rainy days? Well, we didn't cater for those. Our calculation also assumes 100% energy conversion efficiency, which is not inline with reality. But worst of all, it assumes that we can discharge the battery completely. If you do that with a lead-acid battery, you seriously shorten its lifetime. We will need to double the battery capacity to halve its depth of discharge, and probably double it again to allow for overcast or rainy days. Which sortof means you need 4 x 100AHr batteries. At R2,000 a shot, that becomes expensive.

So where to from here? Well, let me get 2 of my old batteries into a semi-decent state, attach a load that at least approximates that of the fridge, and measure the result over a longer period. Guess where my next weekend is going to be spent?

Sun, Mar 10, 2019. Well, so where my weekend did not go rather. But it wasn't totally wasted. I moved the 265W Solar Panel from being propped up against a wall to a more suitable resting place on the garage roof. This way, the panel escapes our dogs' daily adventure around the panel... I drilled some holes for the wiring and got the charge controller mounted on the wall. And I chose the best of the two 105AHr Deltec Voyage batteries, connected them in series, and to the charge controller. So yes, at least geared to do those load measurements. The school holidays start next week. I am not directly affected, but it means I'll probably work from home to keep an eye on the youngsters. And hopefully the batteries. I prefer being around here if one decides to go yellow on me...

Mon, April 1, 2019. OK, so what a latter part of March! Eskom, Eskom and Eskom. Or rather No Eskom, No Eskom, No Eskom. And that's no April Fools joke. Stage 4 load shedding kicked right back in from around 15-March. Here in Johannesburg, this pretty much meant we would be scheduled to be without power for 4 hours every day. Except, whenever the 4 hours had passed, power would not return :-(. We had several days of no power, or days with a only a few minutes of power. 24 minutes on 18 March, 56 minutes the next day and so on.

This is the actual inverter!But ey, here was a load situation that I could test the solar panel performance on! As I said before, I had an older 12V inverter lying around. So I reconfigured the battery pack for 2 x 12V in parallel, set the charge controller for 12V output and connected the inverter. Next, I ran a long extension lead to my office and connected my PC and network. Voilla! - everything works just fine and it's all running off solar. The charge controller reckons I am drawing around 150W from the panel and it's producing enough that no supplementary current is required from the battery pack. And it's quiet! Not even an inkling of generator noise. At last I can work without worry, and in blissful peace and quiet. Wonderful! And then not. After an hour, power suddenly drops away. Not completely, because a battery charger inside my office is running, but all the 220V stuff stops working. What now? Damn, something has popped inside the inverter. It now only outputs 110V. As if it emigrated to America. Is this a hint?

Hmm, so so much for running off Solar. In fact, so far the tally for the experiment is not looking too good. The DC to DC converter popped after 2 days. Now, the inverter after 1 hour. Both parts were operating at way below their rated power. Are modern products just simply engineered with an automatic fail mode once they are either out of warrantee, or at a location where return shipping is just not feasible? Or does it just mean, simply don't buy cheap products from the far east?

I say that, but later in the week when Eskom power does make a brief re-appearance, and then playing hard to get, a string of other equipment starts failing. I lose our Garage Door Motor controller board (dead). I lose two power packs supplying one of my network switches. And one morning, I find a UPS latched up after an outage in the middle of the night. It is completely unresponsive even when unplugged. I need to remove the battery to turn it off. As I slide the battery out, it is too hot to touch. I estimate at least 65ºC. Needless to say, the UPS battery is cooked.

Click to view fullsize But there are also some positives. Well, first of all, we've had power with much fewer interruptions for about a week now. Probably an election ploy, but ey, count your blessings. In terms of the experiment, well the Victron Solar charge controller is performing beyond expectation. And I've run the energy monitor on the fridge for a month now (see the updated pic on the right). The final tally for the fridge is 48.58 kWHr in 30 days, so 67 Watts on average. This does not change the quick cost calculation above, but it certainly confirms that running the fridge off solar is not complete pie in the sky. It's just, well, at the computed price point, it feels a bit underwhelming. 1 Fridge on Solar for a like R7,000 investment?

The situation here is a little different though. Back in 2008, when load shedding first started, I put our Internet connectivity, some PC's, the TV and some lights onto a home-brew backup power solution. I based it on batteries, inverters and grid-powered battery chargers. Solar in those days was just too expensive and anyway, grid power pretty much did return after a 4 hour outage, so there was never a problem getting the batteries recharged. But now? Well, you cannot bargain on only 4 hours without power anymore. In fact, rather bargain on days. Which leaves you with the choice of a generator or the sun.

On my next post, I'll detail what I am going to do next...