Solar Systems

[Nix]

For those of you who may be curious about how I designed my solar set up. It won't be to everyone taste or needs but, as I like to have a play at such things, it'll hopefully give some insight to a solar back up system.







I've designed my system to be flexible ie so I can, with a flick of a switch, change the charging path of selected solar panels to whichever of my four separate systems I want (one 12v system, two 24v systems and one 48v grid tie system). I'm going to only give a basic outline cos I don't want to bore or put off anyone with tons of detailed info. I encourage any questions/suggestions/constructive criticisms. With electrics, there is no such thing as a silly question, electricity can kill so better to be safe than sorry!

I suppose in the interests of legalities I've got to say that any information, advice or recommendations are to be checked by a qualified electrician if there is any doubt what so ever.

Some basics first that you need to know.

- voltages above 50v are deemed dangerous and can kill so 230v ac is to be treated with respect!
- voltages below 25v ac or 50v dc are relatively safe so touching your 12v battery terminals shouldn't shock you
- for battery banks, you need ultra deep cycle batteries, cheapie car batteries are false economy
- you have to have a controller for charging your battery bank from your solar panels
- solar panels have to match up ie DON'T mix n match different panels of different ratings
- solar panels come in different voltage ranges, decide on 12v or 24v setup before you start buying
- site your solar panels in a southerly direction with no shade obstruction from trees/buildings etc if possible
- know what you want powering so you can choose the right inverter ie modified or pure sine wave
- know your power requirements so you can design your system to cope with your power demands
- DON'T underestimate the currents on low voltage systems, cable size is of utmost importance!
- ALWAYS protect ALL circuits with fuses so that any hic ups are protected without unnecessary damage!
- grid tie inverters must be G83 certified as a minimum requirement
- grid tie system will NOT back up your house in the event of grid failure

For a typical solar powered back up system you will need:

- solar panels
- controller
- battery bank
- inverter
- pv cables/MC4 connectors/protection fuses

I would highly recommend that you plan for a 24v set up. This will halve the current to and from the battery bank compared to a 12v set up. To give you an idea what I'm talking about, for a 1kw fire to be powered by normal mains 230v, you can do it on 1mm cable which only has to take around 4 amps. Matter of fact you can power two 1kw fires with this scenario. However, if a 12v system has to power a single 1kw fire, you'll be talking around 80 amps and have to consider 25mm cable! For those who don't know, this is equivalent to the size of cable going to your mains meter/fuse board ie it's huge and expensive. BTW, most solar panel controllers usually handle both 12v and 24v systems automatically so no worries there.

Remember your school days?

V = IR or (volts) = (current/amps) x (resistance)
W = VI or (wattage/power) = (volts) x (current/amps)

Originally, I planned a small 12v system having an entirely independent 12v circuit in the house to power 12v led lights, car radio, phone chargers, tv, etc. All very well but in reality, a 24v set up will give you so much more flexibility through an inverter though, inherently, it's not as efficient. You really need to have a good idea what you want out of a back up system. This will ultimately dictate what stuff you have to get.

So, the simplest and cheapest set up is a single low powered 12v solar panel going straight to a small standard 12v controller going to a single 12v car battery to power either small 12v items or a small modified sine wave inverter for a very low powered mains supply.

So, how much for the cheapest basic set up?

- 10W solar panel with crock clips here £18.99 delivered
- 10A controller 12/24v (this is not PWM or MPPT, more on that later...) here £8.99 delivered
- 12v car battery, new you're starting at about £35 these days as the lead prices have rocketed but have you got an oldie but goodie lying around?
- 12v inverter 150W modified sine wave here £13.95 delivered. You'll have to chop off the car adaptor and fit some crock clips to the leads to attach to the battery. This one is at least colour coded so you don't get the polarity wrong.

So, if you have an old but usable battery knocking about already, this lot will be delivered to your door for £41.93. Double this if you have to buy a new battery.

I do NOT recommend the above, it's just to give you an idea where it all begins. This lot, power wise, won't last you two seconds.

NB/ you can get mega cheap non-controller solar panels that clamp straight onto a battery but they're for sustained trickle charging only and not for keeping a working battery healthy.

The other end of the scale, you can spend well in excess of £10,000 of your hard earned on a professionally installed system without lifting a finger, except to sign the cheque of course.

If you're like me, you're somewhere in between, it's just a matter of where.

The system I'm finding really versatile is my main 24v set up using:

- up to 5 x 24v 250W (= 1.25kW max) solar panels (switchable so I can have anywhere between 2 - 5 panels connected at any one time)
- 60A PWM (MPPT type also good) controller
- 6 x 12v 125Ah ultra deep cycle batteries (ie 3 x 24v)
- 2kW pure sine wave inverter

From memory, this lot cost me over £2,200 but that does include cabling, connectors, specialised tools, roof rails and brackets, meters, etc, etc. Expensive, I know, but it can back up the whole house if the grid goes down (not the likes of kettles, electric showers etc) and heat our hot water. It should last many many years if treated properly. Solar panels have a life expectancy in excess of 20 years, batteries (if the "depth of discharge" [DOD] is kept shallow) I reckon should be good for at least 10 years, the inverter and controller (if not abused and decent quality) again, 10+ years.

Worth noting the obvious that energy prices will only continue to climb as demand becomes more intense and supplies become more depleted. I estimated that my system will pay for itself within about 2 years. After that, it's all free energy. Also, in the event of SHTF, energy-wise, I should be in a better position than most, whether that's a good thing or not I don't know.

NB2/ Batteries. All batteries are lead acid. The difference is in the construction. In general, the Lead Acid battery is made up of plates, lead, and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35% sulfuric acid and 65% water solution. This solution is called electrolyte which causes a chemical reaction that produce electrons.
The Gel Cell and the AGM batteries are specialty batteries that typically cost twice as much as a premium wet cell. However they store very well and do not tend to sulfate or degrade as easily as wet cell. There is little chance of a hydrogen gas explosion or corrosion when using these batteries; these are the safest lead acid batteries you can use. This is why I keep my AGM battery in the garage, very little explosion risk. The other batteries can leak explosive fumes and are kept outside in a vented enclosure.
Lead acid batteries include the starting battery in your car. They also include deep cycle batteries that have thicker plates than starting batteries. Thinner plates (in the electrolyte) can deliver higher current, necessary for starting loads in a motor vehicle. Thicker plates are better suited to long slow discharges which is why they weigh a ton.
The Absorbed Glass Matt (AGM) construction allows the electrolyte to be suspended in close proximity with the plates active material. In theory, this enhances both the discharge and recharge efficiency.
The gel cell is similar to the AGM style because the electrolyte is suspended, but different because technically the AGM battery is still considered to be a wet cell. The electrolyte in a GEL cell has a silica additive that causes it to set up or stiffen. The recharge voltages on this type of cell are lower than the other styles of lead acid battery.

NB2/ DOD or "depth of discharge" for deep cycle batteries. This is the amount of charge you take away from a battery when using it. AGM deep cycle series offer approximately 250 cycles @ 100% DOD, 500 cycles @ 50% DOD and 1250 cycles @ 30% DOD. If a voltmeter's connected, I personally never want to see a battery go under 11.9v (24.0v for 24v systems) when under load. You can go much lower but you shorten the battery's life span. Nice thing about inverters is that they have a cut off at a certain low voltage to save your batteries :-) Here's an example of usage:

say you've got a load connected to your 100Ah battery which draws 5 amps. Working on 50% DOD, 50% of 100Ah = 50Ah, 50Ah/5A = 10 hours usage. You can easily double this time by wiring two 100Ah batteries in parallel and so on. Most manufacturers I've seen quote 500 recharge cycles at 80% DOD. Only do 20% DOD and the recycling can jump up to 2,500 cycles!

PWM and MPPT Controllers. Basic controllers seem to just pass all available charge to the battery and stop when they reach a certain voltage although I suspect some will even over charge. PWM (Pulse Wave Modulation) controllers are the next step up. They monitor and regulate the battery far better by varying the charge pulses/voltages as the battery becomes recharged. This, in effect, prolongs the life of a battery and is far more efficient. MPPT (Maximum Power Point Tracking) is better yet. Here it converts high voltages from solar panels and converts the charge to more amperage. This means that the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced. It's worth noting that you get what you pay for eg the 60A PWM controller I have for one of my systems was expensive but can be programmed to suit whatever battery type I use, cut off voltages I want and monitors all that's going on etc.

Inverters. Two types, modified sine wave and pure sine wave. Modified are electronically manipulated waves that are squarish in nature so basically more on/off in it's voltage delivery whereas pure sine wave is what you have in your house mains ie a smooth curved wave which is gentle on your electrical items and not abrupt. There are some items that don't like modified sine waves and can be damaged by them like power tool chargers. The big difference is in price, basic modified sine wave inverters are only a fraction of the price of a proper pure sine wave inverter which will handle anything.

Solar panels. Here you have to know what you're planning. There are panels that are only for 12v outputs and panels that are only for 24v outputs. You can wire two 12v panels in series to give 24v but forget about knocking a 24v panel down to 12v without a lot of hassle. Very important to match up your panels, not only in voltage but in ampage. One unmatched panel can and will affect the whole system adversely. The electrical law here is thus:

Panels in series, add the voltage of each panel, but limited to the lowest amp panel in the string.
example:

20V/3A + 12V/4A + 22V/6A + 18V/4A => 216W ie 72v x 3A (the lowest ampage panel). You are wasteing the watts from the higher amp panels.

Panels in parallel, will drop to the lowest voltage panel but the amps will add.
example (same panels):

20V/3A + 12V/4A + 22V/6A + 18V/4A => 204W ie 12V x 17A but you are wasting the volts of the higher panels.

throw away the 12v panel and you get:

20V/3A + 22V/6A + 18V/4A => 234W ie 18V x 13A The next highest voltage factors into your calculation.

So you can see, it's not just a matter of buying any panel and connect to any battery with any controller or use any inverter!

Congratulations if you haven't fallen asleep yet! I'll stop here to see if I've put everyone off...