Solar Power Design Basics – Solar Panels
The four major components we shall look at before we design a basic system are the following:
- Charge Controller or regulator
- Solar Panel
Let’s start with the solar panel.
A few fun facts about solar panels, before we chat out the calculations:
- Solar panels must face the right direction, so as to remain in the sun’s path as much as possible. In the Southern hemisphere like here in
Zimbabwe, they must be tilted facing North and in the Northern Hemisphere e.g Europe, they must face South.
- No matter what any salesman tells you, there is no such thing as “shade tolerant” solar panel. Any form of shade on a panel will reduce its output drastically. A very cloudy day will wipe out up to 80% of your panels out.
- All areas on earth are different regards the amount of time the sun is available to effectively shine “directly” onto the panels. For example at the North pole this time swings from 1 hour availability to 23 hours depending on the time of the year, while in Zimbabwe that time can vary from 5 hours in winter to 7 hours in summer – confused? A solar panel generates the most power when it is placed perpendicular to the sun – meaning the sun’s rays must hit it as square and as direct as possible. This only happens acceptably for around 5.5 hours on average in Zimbabwe.
Now let’s do a quick calculation to conclude our basic understand of solar panels. Quite a few people believe that a solar panel of any size will supply the power they require. Not so, a solar panel is not magic! If you use a 60 watt light for one hour at night you will need full sun on a 60 watt solar panel for one hour plus to generate what you have used & store it in a battery. There is thus a direct relationship between what you use during daytime, and what you must store to use at night.
Your solar panel will typically be 12 volt rated and more for the larger panels. For our purposes & calculations we will use 12 volts. This 12 volt rated panel will typically have an output of around 17-22 volts! You need that excess voltage for power to flow from your panel to your battery (remember volts = pressure).
In perfect laboratory conditions a 100 watt solar panel will produce 100 watts of power at 12 volts from full sun. Typically the same panel will produce around 7 amps under the same conditions.
Assuming the same laboratory conditions and the available sun time of 5.5 hours, the total produced power would be:
Panel amps x time exposed to the sun = total amps produced and delivered to the battery. Thus:
7 amps x 5.5 hours = 38.5amps
And further applying Ohms Law and using our system voltage of 12 volts, real power (watts) available would be:
38.5amps x 12 volts = 462 watts
In conclusion dividing our 60 watt load into our available power above, our bulb will burn for:
462 watts / 60 watts = 7 hours and 42 minutes.
Now remember this is under perfect conditions, with a perfect solar panel, perfect regulator and perfect battery, this will however help us to understand the basics. Next week we will attach the solar panel to the regulator and discover a little more about the relationship between solar panels and the regulator.
Solar Power Design Basics – Solar Charge Controllers
The Solar Charge controller or regulator is placed between the battery and the solar panel, and in DC systems also between the load and battery. This critical component can be defined as a device that prevents the solar panels from over-charging the battery when it is full of charge, and also serves to protect the battery from being dangerously over-discharged by the loads attached.
The charge controller basically takes the erratic raw power produced by the solar panels and attempts to make it smooth before passing it onto the battery for storage.
Solar charge controllers come in various sizes with various capabilities. The most important and common to 90% of charge controllers is the “3 stage charging capability”.
Let’s try and simplify .
“3 stage charging”
The most efficient way to charge a solar battery is to first flood it with as much power as you can (we will discuss this in detail when we talk battery’s) this stage is called bulk charging. During this stage the battery is charged at the highest current (amps) and volts allowable by the battery’s structure and size. Once the battery is near full, the second stage starts at a lower voltage and less current (amps) this stage is called the absorption stage. During this stage the battery is pushed slowly and further to its full state. Once at full charge the battery is now maintained at that full position in what is commonly known as the trickle charge stage. During this final stage the battery is held at the full voltage with an occasional small burst of current and voltage.
Regulators differ on how well they all perform these stages of charge. The cheapest regulators may not deliver on these stages and subsequently shorten the lifespan of your batteries.
Now controller/regulator size. The design of your system including battery bank size, size of solar panels to be used, and even the power to be used, will determine the size of your regulator. Let’s look at an example below.
Calculating regulator/controller size
When calculating regulator size, you will need to remember the maximum system voltages as follows:
12 volts DC peaks at 15 volts
24 volts DC peaks at 30 volts
48 volts DC peaks at 60 volts
Simply add up the size of your solar panel array and divide the number by the system voltages peak volts. For example a 12 volt system peaks at around 15 volts DC. If we took a total system solar panel size of 200 watts the regulator/charge controller size will be as follows:
200watts divided by 15 volts = 13.3 amps our recommended safe regulator/controller size becomes 15 amps.
There we go, not so complicated right? When purchasing or selecting a regulator, please take some time to learn more about the types of regulators available and fully understand their weaknesses and strengths before making your final decision.
The most common types of available charge controllers are as follows:
- Switching regulators (the cheapest)
- Diversion charge controllers
- PWM Charge controllers
- MPPT Charge Controllers
Understanding The Equipment – The Solar Battery (part 1)
In order for your standard solar system to be fully useful, you need energy storage. This is where batteries come in! Here we are dealing with mostly lead acid batteries and although you can use Lithium Phosphate batteries (at somewhat of an expense). Modern lead-acid batteries come in flooded (FLA), gelled (GEL) and absorbed glass mat (AGM) types just to name a few. For the purpose of this article let’s assume that they are all the same.
First let’s look at the technical terms that define a Solar battery. These are basically Voltage, Capacity, Cycle Depth and Battery Life. Combined these form the basis for deciding the type of storage required for your power system.
Lead acid batteries are made up of 2 volt blocks or cells. Link 6 of these in series and you get a 12 volt block much like that which we use in starting motor vehicles.
Probably the most important information about a battery, this is measured in amp-hours (Ah) and is the amount of energy stored in the battery. This is measured by discharging a full battery at an industry standard amperage over a standard period of time. This rating is given to us by the manufacturer and serves as a base line for calculating the battery size we need for any use.
Thus in summary battery capacity is the battery’s discharge over time. For example a battery rated 100Ah at C20 would have been rated based on the battery being discharged at 5amps over 20 hours or 100amps / 20 hours = 5 amps per hour.
In the same breath a 200Ah battery at C20 would provide 10amps discharge over 20 hours. Going back to calculations or power and our dear friend Mr Watts here, we can calculate the amount of real power stored in a full battery. Using that formulae Amps x Voltage = Power, a 200Ah battery thus has a theoretical total, 200 x 12 = 2,400watts of power stored. (note:- a full battery would be approximately 2.28 volts per cell, while an “empty” would be around 1.8 volts per cell)
“Real Battery Capacity”
What I like to term “real battery capacity” is what we consider usable power. Consider a bottle of cooking oil. If we were to measure its contents to the absolute millilitre of oil, we would find that what we put into the bottle is not what we can get out. There will always be a bit left behind, that is un-usable. We can then say that the oil which we can get out is the usable volume of oil.
Although for different reasons, this analogy describes the battery’s real capacity in the same way. With a battery though, the reason we cannot take everything out is that amongst other, reasons a battery needs to have a little bit of power left in it to protect it against damage. The recommended maximum discharge level is 50% of the nominal capacity of any given battery size. Beyond this we consider the battery as going into deep discharge.
Looking at the last calculation we did to determine the full real power capacity of a 200Ah battery, if we factor in the recommended maximum discharge level of 50% the available power is now: 200 x 12 = 2,400watts x 50% = 1,200watts
Putting this into context and applying the theory from our article on power. The 1200 watts available would power a 60 watt load for: 1200 watts / 60 watts = 20 hours.
It is important though before going on to note that a solar battery does differ from a car battery. A car battery is designed to provide huge surges of power in short bursts to start your car for example, and then to charge up quickly again. These are called cranking battery’s and are NOT meant for the storage of power. The battery’s we are discussing today are considered deep cycling batteries. These batteries are designed to store power and discharge it at a steady slow rate up to a maximum depth of 80% of their nominal capacity without damage.
Cycle Depth and Battery Life
If you bought a new battery today and put it on your shelf in the shed and somehow managed to keep it charged every day, the battery would last about 10 to 15 years. This rate is determined by the speed at which the internal parts degrade due to the acid they are stored in.
However you are not going to do that, you are going to use it. In this case the factor that determines its life span becomes cycling, or basically how many times you charge and discharge it along with how deep you discharge it before recharging it.
A shallow cycle would be for example 10% discharge, a deeper cycle between 30 & 50%, while a deep cycle could be 80% of the battery’s capacity.
Discharging a battery up to 80% regularly will reduce the battery’s life dramatically. Typical life spans of batteries are rated in cycles vs depth of discharge as follows:
- 4000 cycles to 10%
- 3300 cycles to 30%
- 2500 cycles to 50%
- 1500 cycles to 80%
From the above information we can determine a battery life expectancy: A good AGM or GEL solar system battery discharged on average 10% per day could be expected to have a battery life of around 4000 days or 10.9 years. While discharging the battery to 80% every day, you could expect a battery life of around 4.1 years.
Understanding The Equipment – The Solar Battery (Part 2)
As we continue the discussion around the solar battery this week, we have been asked to take a closer look at the battery types. After we have done that, we will take what we learnt last week and size a battery for a 12 volt solar power system.
As we suggest last week modern lead-acid batteries come in flooded (FLA), gelled (GEL) and absorbed glass mat (AGM). Let’s take a closer look at what this actually means.
The flooded deep cycling battery is the oldest and most common type of battery available used in the industry. This battery is defined as a maintained, wet-cell or flooded battery. The battery’s internal plates are covered by a liquid called electrolyte that is a mixture of battery acid and battery water. Depending on how hard the battery works, the battery water will need to be replenished every couple of months.
The vapour process that sees the battery losing water, means that the batteries must in most cases, be installed in a very well-ventilated area. Although this battery requires a fair amount of attention, when looked after it has potentially long lifespans of use up to 12 years have been recorded. It does however suffer from chemical failures related to liquid conditions such as sulphating which gradually reduces the battery life span when it is not looked after properly.
The GEL battery falls into the family of “Sealed Maintenance Free” (SMF) group of batteries. The gelled battery’s construction is similar to the flooded battery with the major difference being that the electrolyte is in the form of a gel. The battery does not require any topping up of fluid. The battery is able to hold a charge better than a flooded battery as well as deeper discharges when they happen. Because the battery’s electrolyte is a gel it does not have to be kept or installed upright, and suffers less sulphating issues giving it longer life spans for the same discharge rates, with the added bonus of being virtually maintenance free.
Also maintenance free, the absorbed glass mat (AGM) is based on the same principals as other lead acid batteries. The battery’s plates have a very fine fibreglass mat placed between them. The mat is impregnated with the electrolyte making the battery spill proof, usually lighter than both the FLA and GEL types, the battery is the most efficient of all the three types. Like the GEL battery the AGM battery can be charged at a greater rate, and because the electrolyte is in a mat, they are considered less hazardous than the flooded type of battery. The AGM battery is also cheaper than the GEL battery but more expensive than the flooded type.
I hope we have answered the question about battery types in as simple but understandable fashion as possible. In future we will look at selecting the right type for the right application. Let us look at sizing a battery for a 12 volt system.
Sizing your battery bank
Now assuming that all the batteries above are the same and taking the consumption calculation we looked at when we discussed Energy Management. The total energy required was 330watt hours of power.
We want our battery to last for a long time, so we shall assume a maximum depth of discharge of 20%. Now let’s do some calculations. If 330watt hours are 20% depth of discharge what size is the total capacity (X) of the battery?
X = 330/20% making X 1650watt hours.
The battery size will be watt hours / battery bank voltage (system voltage) = Amp Hours.
Battery total size will be 1650 whrs / 12 volts = 137.5Ah.
Rounding up to the next commercially available size, our selection would be the 150Ah battery to operate at 20% depth of discharge.
The total consumed power on a daily basis will be a maximum 30Ah. This is what the solar panel will need to replace on a daily basis. Getting interesting right?