Solar Design

250buggys

Member
I have an area on my porch roof that is South facing and thought that would be an ideal spot to put a few panels up and that is where it all started........

Next step was to construct a frame but I didn't want the weight of the panels to be directly on the roof causing extra loading, leaks etc. What I did was to use Unistrut for the frame and cantilever brackets mounted on the wall so the frame was tied in with 8 fixings.

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The 35mm2 connection shown here is an earth strap, I figured if it got a bolt I didn't want it feeding back to the charge controller or inverter.

I used Hilti Fix resin to bond in the mounting studs.

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With decent feed in tariffs a thing of the past and the current option of grid tied systems that let you use your generated solar before any excess being exported to the grid I couldn't see working when you are out all day when the sun is up and back when it is not generating.

I decided to put together an off grid system that fed into a battery bank when not using the solar.
The primary focus was to heat water as this is one of the biggest loads and would contribute to a good saving and would be easy-ish to calculate the pay back period.

The first mistake I made was to use the 3kW immersion heater already in the tank and sized my inverter accordingly. I found with such a big load it was just cycling the battery bank.

I opted for a 24V system. Reason being a 24V inverter uses half the current a 12V inverter does. With my 3kW water heater on I was seeing a 125 Amp current draw from the battery. A 12 Volt inverter would have pulled 250 Amps. A 48 Volt system would have been the best design solution but 48 Volt pure sign wave inverters even though they are manufactured never come up on ebay 2nd hand.

My battery bank is 280Ah at C10 rating
280Ah / 125A = 2.24 so this looked like the bank was well undersized for the load.

By swapping out my 3kW for a 1 it would reduce my load by a third

280Ah / 41A = 6.71 which gets me closer to my 280 Ah C10 rating

By not achieving the C10 rating it reduces the amount of capacity so to mitigate I only cycle down to 80% 20% DOD (depth of discharge).

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I have now swapped out for a solar friendly 1kW immersion element but could have gone for a 1.5kW to speed heating up.


The panels I used were 270 Watt, I have 3 on the roof and another around the back West facing, the 4th panel helps to mop up the afternoon and evening sun maximising the daily solar harvest. So just over a 1kW array.

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Discounting the oven as that is effectively hard wired in and would require a change over switch in the distribution the next biggest load is the washing machine. Looking at current draw it probably has a 2kW or there abouts heating element in. It is no trouble to run this off the inverter and again helps to contribute to pay back.

Solar panels seem to be getting cheaper but what is actually happening is that the cost is the same but you get more Watts for your £. The design life of a panel is 20 - 25 years. I saved on installation costs and the bit that is expensive is the solar charge controller and inverter. The cabling is inexpensive, to reduce volt drop in my cable runs I used 6mm CSA 2 core for the DC. The solar fitters always use 4mm singles for some reason.

My solar charge controller I first used was a PWM, these have now mainly been superseded by MPPT variants. I was hesitant to make the change because of cost reasons but my solar harvest increased by 25% as the MPPT actively scans the power curve and hunts for power.

When I ran the PWM charge controller I fed each 24 Volt panel into a junction box in parallel to sum the current. Now I have moved over to MPPT I have reconfigured the array to be wired in series to sum the Voltage. The MPPT looks at the combined system Voltage and converts it to 24 Volts at the most efficient way to charge the bank and feed the inverter.

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To monitor the system there is a Victron BMV, the loads are wired through a shunt and gives you an accurate read out of your load current draw, power consumed, power harvested and solar in. Also system Voltage, battery bank capacity plus a whole load of other useful features.


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The charge controller is really clever it gives you so much information to help monitor your system, trend data and improve your design.
I use the Victron connect app to monitor system data via the built in bluetooth and am working on a telemetry unit to host this remotely on an Odroid C2 so I can monitor my system from anywhere.

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This pic shows
Solar coming in at 1.2kW
Array Voltage 111V
Array Current 11 A
Battery Voltage 24V
Battery charging at 48.4 A

I was seeing a yield of around 6.5 kWh on a blue sky day, blue sky and no cloud from when the sun rises to when the sun sets. I call these Super Solar days and there are not many of these in a year! There is always a little cloud and over the course of 8 hours this detracts quite a bit from the daily harvest. The sun seems to come out in the morning and disappear or come out in the afternoon. It can be a grey day and then the sky clears completely at night which happens quite often.

The 6.5 kWh was seen in April so in the summer months I thought I may see 8 or 10 with the extra day light hours. What happened was I never saw more than 6.5. I found out that the panels are more efficient when they are cold in the spring, this leads to higher system voltages and the angle of the low sun seems to help too. In the summer the sun was higher and the panels were much hotter and seem to struggle. This was all good data to help with system improvements and upgrades.

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The charge controller gives good feedback and you can see when we have had a week of really good weather and when not so good.

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To be continued.....
 

250buggys

Member
I have just found the limitations of the new forum, can only post 10 pics :)

This trend shown below I took shows something quite strange, plodding along seeing over 36 Amps coming in and the charge controller drops off every 10 15 minutes or so. I can only assume that this is the charge controller re-establishing itself on the MPPT curve again. It would be handy f there was a setting in the options to allow the user to set this to maybe every half hour or even 20 mins as over 8 -10 hours of daylight dropping off every 10 mins is not very efficient.

It would also be handy to have a manual button to reset MPPT as my 4th panel isn't fixed so I can play about with its pitch to gain more Amps. If it is in shade and I put it in the sun it can take a while for it to get back on the power curve so a manual button would help otherwise I have to isolate the whole array to force a change.

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To date over the last year I have harvested almost 500kWh, sounds better if you say half a Mega Watt hour! :)

Current improvements

Now I have my panels in series I don't need 24 Volt panels and have opted for some 490 Watt variants to replace the 3 on the roof. The 4 270 Watt panels will go round the back and provide a West facing array.

(4 x 270) + (3 x 490) = 2550 so combined will give me 2.5kW

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System now generates more solar than needed taking the load off the battery bank.

I saw these 655 Watt panels and could have been a good option but would have given me possibly too much current for battery charging, better suited for grid tied systems.



Looking at energy consumption I am currently saving £10 a month. The payback is a little complicated to work out as I am now heating water with electricity where as previously I was heating it with gas. The cost per kWh of gas is cheaper than that of electricity so can only be attributed at that rate. With the 2nd array on line I should see a significant saving and think I should be self sufficient with 10kWh per day. The only issue is the standing charge which equates to about £7 a month. I remember when there wasn't a standing charge and you only paid for what you used....

Future improvements
Next plan is to get my hands on a 24V 1kW Wind turbine and have this power a 800 Watt heater for some background heating to take the load off the central heating. This should work even when the sun goes down and help to reduce the gas bill further.
 
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250buggys

Member
I spotted an East facing array the other day. There really is no logic in putting panels anywhere, the UK gets little to no sun so you really need to oversize your array and it needs to be South or South West facing. Any shading at all and don't bother.
 

Phaeton

Moderator
Staff member
Brilliant, I really want to get into this, still finding it a bit confusing, you have the panels producing the charge, the MPPT controlling it into the batteries, then from the batteries into an inverter, does that then just hook into your normal electric ring or is there something between? I know FIT has gone & replaced with SEG which appears more hassle than it's worth as I would imagine any system I put up we would use all the electricity anyway. How far can you go with this before you have to pay somebody to do the connections? I'm old school & don't like anybody doing work on my premises, as I have little or no control over what they do.
 

250buggys

Member
I am of a similar opinion.

Non technical people were trained to fit smart meters and issues were caused as a result. The chap who previously read your meter worked for the electricity board and would inspect your distribution board and tighten the connections as the AC tends to vibrate and loosen the connections over time causing arcing and heat. (The new circuit breakers, RCBOs have arc fault protection and are expensive). Anyway the people who read meters can't even read anymore I had a huge bill and it was because he got his figures mixed up. I now input my own meter readings every month online and refuse to have a smart meter but that is another story.... Don't let anyone in you don't trust.

If you want a permanent solution you will need to follow BS7671 wiring regs. I think it is a grey area around inverter systems especially off grid but there is development work currently as this is the future. It is pretty simple and pretty popular for the excess solar to dump into an immersion. The battery system can be expensive but you will find we get more grey days than not. If I didn't have a battery bank the washing machine would be on pause every time a cloud came over.



There is nothing stopping you following the regs knocking up a system and getting a spark around to sign it off. You just need to convince the other half, the first time I had the boiler PCB on the kitchen table re-flowing the joints as there was an intermittent fault I got it properly in the neck, it wasn't until I explained I hadn't touched the gas side I was off the hook...
 

Danny

Administrator
Our garden gets sun all day long so we have been looking at solar for a while... but until we decide if we are staying or going then it is on hold :D

Looks like a good project
 

250buggys

Member
Our garden gets sun all day long so we have been looking at solar for a while... but until we decide if we are staying or going then it is on hold :D

Looks like a good project
Thanks, if I ever moved it wouldn't take long to take it apart and bring.
 

250buggys

Member
This is how my system typically started.


Bought a 2nd hand inverter off ebay, had a car battery already. Put a couple of panels on the shed roof and had power in the shed. Enough to solder, a lighting circuit and run power tools. If the sun is out it doesn't even use the battery reserve but means you can still work in the evenings. During the day run the mower, hoover the car. The kWh soon add up and it's deducted off your AC bill.

Those Antares now Cotek inverters are excellent, we used them where I used to work and never really saw any failures. We use Victron where I work now but are really expensive and also a lot 2 nd hand.
 
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