Dual Power Immersion Heater

The automatic load controller for my immersion heater has been working pretty well.  It turns on and off with the varying power of the Sun.  But it left something to be desired when the battery was starting the absorb cycle.

The battery consumed quite a lot of power, but not all of it.  The immersion heater needed 650W to run, so couldn't.  The result was a period of under utilised solar power in the late mornings, with a trace that looks like this:

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By modifying the step down transformer supply for the heater, I created a dual power heater.  I step down the AC Voltage from the solar inverter with a 4kVA "tool transformer".  It outputs 110V AC from the 230V AC input.  This runs the 3kW heater element at just 650W.

Under software control from the PC load manager, the first relay turns the heater on and off, while a new second relay selects the power level of the heater, depending on the solar power available.

I added a pair of 6A diodes in parallel (for power handling, as the peak current when the heater is on is about 8.4A).  This converts the 110V AC into half-wave rectified DC.  The diodes are rated at 600V so they are pretty bullet proof.  This has the effect of reducing the power consumption of the heater from 650W to just 350W; measured with an AC plug-in power meter at the 230V AC input to the transformer.

The thermostat switch on the heater ordinarily wouldn't like DC power, as it would cause arcing when the contacts open, and this would soon destroy the thermostat.  But as this is half-wave DC, it still has the periods of zero Voltage in each 50Hz cycle, so the thermostat contacts can open and close as normal without arcing.

I then had to modify the control software to take advantage of the new dual power heater.

I decided to ditch the purely light level & system power level threshold system, for one that attempts to estimate the array power available for driving the heater loads (at two power levels).

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It has a seed value that is the expected array power (the "system size").  It then applies a self tuning modifier to that base value (plus/minus 200W) and then multiplies that by the measured light strength from the new sensor (as a percentage).  This gives me the estimated "array power".  The "system power" is the real measured output power from the Morningstar charge controller log data.  This includes all loads: battery charge load, other loads (e.g. the fridge), as well as the heater load (if it happens to be on).

By comparing the real power output during times when the battery is likely to be fully loading the array with the estimated "array power", the self tuning parameter adjusts the estimate up or down, so that the estimate gets better.  It has some limits set in the routine, so that it does not tune the estimate at very low light levels that would never be enough to drive the heater load.  It also has some fuzziness in the tuning so that if it is within 1% of the real power, it stops hunting.  If the tuning parameter gets bigger than 200W variance, it starts to modify the base assumption about the "system size", saving the change in a config file for next time the program runs.

Most of the estimate tuning happens in the MPPT bulk charge phase, as that is when the battery will absorb all the power the array can muster, and so the "system power" should equal the estimated "array power".  The idea is that the tuning parameter will compensate for the distributed orientation of the panels (some are East-West, some are South, some are at steep angles, some at shallow angles).  The system will also "learn" how dirty the array is (if there has been no rain for a while, and dust has collected on the panels).

The final step is that when the battery enters the absorption phase, the program looks at the estimated array power and subtracts the current "system power", which includes all non-heater loads plus charge demand, and calculates the "available power" for running the heater load.  If the "available power" is greater than the low (350W) heater setting, but less than the high (650W) heater setting, it turns the heater on, and selects "low power" mode (the diode bypass relay is energised and the normally closed contacts change to be open).  If the "available power" is higher than the high power setting, the diode bypass relay is de-energised, the contacts revert to the normally closed position, and the heater receives the full 110 V AC power.

The resulting power utilisation is more even, with the heater able to use low levels of available power and maintain the tank temperature. 350W is enough to very slowly heat the water, or at least compensate for losses through the insulation.  It all helps.  When it's sunny enough, and the other loads are low enough, the heater can run at full power (650W).

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The above trace also shows the water heater interacting with the cyclic load of the fridge freezer.  While the recorded system power varies considerably, note that the battery is given priority in attaining full charge and holding a steady float Voltage for as long as possible.

During the absorption and float stages of battery charge, the heater decision process also includes some "array power" estimate tuning.  If the heater repeatedly has to reduce power to low power, the tuning parameter slowly drifts downwards.  If the heater has to be shut off due to low power, the parameter decreases more quickly.  As a last resort, if the battery Voltage actually drops below the float threshold set in the load controller, then a much more severe adjustment of the parameter occurs.  This behaviour means that on clear sunny days, the heater is given priority and has a tendency to stay on.  On days with very changeable weather, the heater progressively errs on the side of caution, becoming less and less likely to turn on and more likely to turn off or remain in low power mode.  This favours maintaining the battery charge level.

Use of a half-wave rectifier at high power (350W is a significant load) is normally frowned upon, as it presents a very non-linear AC power load (only half the cycle is used).  The plug-in AC meter did show a very bad power factor (PF = 0.5).  This would result in power being wasted in the wiring and generator as reactive power (current out of phase with the Voltage).  But the 3kW inverter is stable into any power factor load (inductive or capacitive), and in the end, the source of the power is a DC battery or solar panel.  With the very large capacitors in the inverter input (for surge delivery), the DC source is not aware of the non-linear AC load, and merely sees a useful reduction in load.  The "bad" AC load does consume more of the available VA capacity of the inverter than a good power factor load would, but provided the total VA load is less than the permissible load, no harm is done.

Some More Piccys

A couple more piccys of my upgraded PV arrays. 

As well as the movable ones, pointing largely West to extend the generating day almost to sunset, I also rotated the Sharp array on the garage roof to the S-SW a bit.  In the early morning, it didn't generate much, as there is some shading from trees to the West.  In the late afternoon, the power would drop off too, as the sun soon goes round to an oblique angle.  So, rotating it S-SW a bit flattens out the mid-day power curve of the whole system, and extends the peak power generating time longer into the afternoon.

Breaking the 2kWp Barrier

Some nice weather this week meant a spot of gardening and some more woodwork...

The other 48Wp amorphous array has been upgraded to 160Wp, using a couple of the new Klearskies panels.  That leaves four more lolling about in the living room, waiting for a place to go. :D

The nearer one is the new one, and I move it around in the day, depending on how low the battery was from the previous night, or if the weather is forecast to be sunny only in the morning, or only the afternoon.  This brings the total installed capacity up to 2080Wp.

I can't actually use all of it at the same time as the Morningstar TriStar MPPT controller is only rated for 60A at 24V, or some 1600W (they seem to rate it for the float Voltage of 27V on a 24V system).  I've got 1740Wp installed (would generate just over 64A), but it points in different directions, so will never make the total power at the same time.  Orienting the panels in different directions just extends the time that the whole system runs without using the battery, and gives the battery the best chance of reaching full charge.

It's been incredibly sunny the last few days, and I've been able to move the 160W mobile array to track the sun from the SE in the morning, to the setting Westerly light, further increasing the "battery free" run time.

New Mobile 170Wp Array

Starting to deploy the new panels I bought... :D

First up is replacing the old 12Wp panels on my mobile array.









Which now looks like this...

The panels are mounted high on the frame so that they aren't shaded by plants growing up at the base or by long shadows from other plant pots on the path.

This new array points W-SW, so that it starts to come into prime generation when parts of the main S facing array are beginning to be shaded by the neighbours conservatory and the garden fence. For now, the other 48Wp Topray array is still on the lawn but now pointing SE to catch the early morning light. The 30Wp plastic pair of Topray panels has been disconnected and I'll probably sell them at a car boot.

To accommodate the extra power generation, I doubled the feed wires (up to about 2.6mmsq).  I've also ordered some proper MC4 waterproof connectors.  I'm beginning to collect the materials that will be needed to install some of the panels on the roof, where it will be too dangerous to be messing about with chockblock connectors.  Much easier just to plug the modules together.  I'll also have to buy some proper solar cable that can stand the weather for a long time.

Generating Boost!

After missing out at the police auction in Bristol that was selling loads of recovered stolen PV dirt cheap, and missing out on some other auctions on line, I got lucky this week...

Eight 80-85W mono panels (650Wp) for £940... £117.50 each!
Two are 85W models (a BP and a very old looking GB Sol) and the rest are pretty new Chinese "Klearskies 2000" 80W models.  One had a load of garden fence paint splashed on it but an hour with some water and the Japanese "magic sponge" soon got rid of that.  Quite a feat, considering the glass is textured.

Now all I have to do is figure out where to mount them. :D

Strange as it may seem, I'm thinking of selling one of them, as that way I'll have an even number of 80W panels overall.  I'd only gone on to eBay to look for one panel to match up to the odd panel I have on the garage (I bought 5 panels last time).  Although I could use all eight, it would leave me with an odd panel again and it's too much of a temptation to buy "just one more" to make it an even number.

If I sell one, then I won't have that itch to scratch.

The guy was selling them to pay for his MCS / Part P training and then he'll do his house with a proper grid tied array as his exam install to get certified.  So now I'm sponsoring PV installers to get through college and do things properly by bodging my own system...  There's an irony.

He donated the suspicious 600W Chinese GTI that he'd used with this lot at the back of his garden.

I had a quick look at the innards and was glad that I did.  A capacitor on the output display power supply had swelled up and burst.

If magic smoke hadn't already issued forth, it soon would have.  As its only the Watt meter on the output and not part of the inverter itself, I bodged on a similar capacitor (but alas too small to make the meter work, but safe enough to make it not work reliably, if that makes sense).

An initial play with it in the garden yesterday showed that the inverter works, sort of.  I hooked up two 80W panels in series (the inverter works from 28-52V DC) and it made some noise and a plug in AC meter said something like 40W output (it wasn't sunny).  But then it sort of stopped altogether and restarted if I turned the DC off and on again.  Not sure if this thing works properly at all (or ever did...).  But as he gave it to me as a freebie, it can't hurt to play with it a bit.  On the other hand, he did mention that he used to have a 1kW one, but it exploded.

It has some appeal though... plugging in maybe just 80W of grid tied power to offset the base load that I can't get off grid.  I could use the old pair of 40W Kyocera panels on the garage and just put the GTI in the garage, plugged into the spur there.  I've some leeches that can't go on solar power (like the clock in the electric cooker, the central heating boiler, and so on). With a little bit of grid tied power I could null most of that energy import out without losing power to the grid (and I can't get paid for or worse, the import meter may count up even when power is flowing out of the house).

Of course the next thing will be arguing with the FIT bods about having expanded my DIY system. They may throw their toys out of the pram and kick me off the FIT scheme altogether (fine by me) or they might agree to pro-rate the kWh payments to exclude the additional capacity. I think they are already calculating it wrong for my existing system; I got my second payment a few weeks ago (only the second statement though as I didn't get approved until December) and it doesn't look quite right.

I'll also now have too much PV for the 60A Tristar controller so I'm going to use some of these panels on a E-W system. I'll get rid of the mickey mouse 12W & 15W amorphous panels and use the new ones to boost morning and late afternoon production, as off-grid needs a long flat power curve rather than lining all 2400Wp up at the noon sun and over loading the controllers and battery. I can't even use 2400W as the immersion heater is geared down to just 650W. But it will mean that I can use that 650W for longer than before and run other things like the fridge longer without hitting the battery at all.

It might even be time to bite the bullet and get a roof rail kit and talk to the BC bods about allowing me to install this kit on the roof, where it belongs.