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Solar Panels & Regulators

 

Why Solar Power ?

To be independent for more than a few days, you will need to be able to charge your house and maybe vehicle battery.

This can be done in several ways, you can run your engine for several hours or so, you can run a small generator for the same time or you can have a solar charging system.

The best option is the latter (providing you don't need more than 5-20 Amps or so) - a solar system.    Why ?    Because once it is installed, it is fully automatic, reliable and maintenance free (apart from keeping the panel surface reasonably clean) with a typical trouble free life of 20 years plus.
There will however be some instances when a small generator would be useful.  See Generator page.

Once an appropriate size panel(s) and regulator is installed, you will no longer have to worry about having a few lights on each night and you will also be able to watch a few hours TV each night  because the next day with a reasonable amount of sun shine, the batteries will recharge ready for the next night.
All done automatically without you doing anything.
A single 75W panel will generate approximately 4-4.5 Amps maximum or typically 10 to 20 Amp Hours per day during the summer months or 2-2.5 Amps maximum or typically 5 to 10 Amp Hours per day during the winter months (depending on location).

Also and often overlooked, your battery will be maintained in optimum condition all year round even when your caravan etc is not being used, ensuring maximum life.   A solar panel and decent controller will see to that.
All batteries self discharge when left idle and nothing ruins one quicker than one that is allowed to be in a discharged or semi discharged state. 

Basically it works like this . . .

Solar panels work by converting sunlight into electrical energy.

The basic element of a solar panel consists of a bonded pair of silicon wafers on a conductive backing, one called the 'P' layer and the other the 'N' layer.   The photons of light interact with this PN substrate and a potential difference of approximately 0.6 Volt is generated.

A typical solar panel comprises of 32 to 36 such elements electrically connected in series thereby producing a panel with an open circuit voltage of 18 to 22 Volts. 

The silicon material used in the panel comes in three basic forms, monocrystalline silicon, polycrystalline silicone and amorphous silicon. 


Monocrystalline

Monocrystalline silicone is grown as one large crystal and subsequently cut into thin slices to form the individual cells.   Panels made this way are a little more efficient, around 14-16%, but are also more expensive to produce.   These panels usually comprise 34 to 36 elements producing 20 to 22 Volts open circuit.


Polycrystalline

Polycrystalline silicon is cast in blocks and the final cut slices consists of many smaller crystals.   Manufacturing costs are lower, therefore these panels are little cheaper to purchase.   While the efficiency is a little lower, around 12-14%, the low angle light output can be higher, but they generally do not perform quite as well as monocrystalline types at higher panel temperatures.


Amorphous

Amorphous silicon panels are produced by a completely different and cheaper process by depositing a vaporised silicon directly on to a backing material.   This results in a cheaper panel but the efficiency is half that of mono or polycrystalline types, around 6%.   This means you need twice the panel surface area to achieve the same output.   They do have one advantage however, the amorphous silicon can be applied to a flexible backing such as plastic or thin stainless steel to result in a flexible panel with the ability to be laid on a curved surface.   However, these flexible panels usually have a poor Watt to Dollar ratio.
 

Solar panels are available in a variety of sizes.   The larger the panel size or area, the greater the electrical output.   Common sizes range from 5 Watt to 185 Watt.   The maximum output will only be achieved when the panel is pointing directly at the sun and the panel temperature is 25 deg C or less.   Since our panel will be mounted horizontally, we will never quite see the full output as shown in the table below.

As you can see from the table, the output will vary depending on your latitude as well as the time of day.   For example, if you are in Cairns (latitude of 17 Deg S) your panel output at noon will be 90-95% of the panel's maximum, if you are in Hobart (latitude of 43 Deg S) your panel's output at noon will be around 75% of maximum.   These figures are approximate only and depend on the type of panel.
Also remember that during the winter months the sun is much lower in the sky (greater angle to the panel) and the days are much shorter resulting in approximately half as many Amp Hours collected per day. 

Planning your solar panel size

For our application you should select at least a 75 Watt panel or the largest panel that will fit on your available roof area.   You will soon regret skimping on panel size on a cost basis.

As mentioned earlier, a single 75W panel will generate approximately 4-4.5 Amps maximum or typically 10 to 20 Amp Hours per day.   I believe this is the minimum needed for a practical system.    You could get away with less if you only wanted to operate a light or two and occasionally your water pump.

There is no reason why you can not install two or more panels, roof area and budget permitting.

If you are using a compressor type refrigerator, particularly anything larger than a tiny portable one, you will need to increase this to at least three, preferably four 75-85W panels to be long term self sufficient and independent of mains or generator power.    Anything less and you will find in times of high ambient temperature or cloudy weather that your system is not coping.
Remember, you have also got lights, TV, water pump, maybe inverter etc making demands on the system.

Manufacturers specifications

Solar panel manufacturers provide the following typical data.
These figures are for a 75 W panel,

And measured under the following conditions,

I have included these figures to illustrate the incorrect information supplied by some on how to calculate the panel size you may require for a specific application.
You will notice that the panel above is rated at 75 Watts.

This is derived from the peak power voltage (17 V) and peak power current (4.45 A).
            Power = Volts x Amps, that is 17 V x 4.45 A = 75 W
     or    Amps = Power / Volts, that is 75 W / 17 V = 4.45 A

They will tell you that this 75 W panel is good for 6.25 A, their justification for this is by saying this is a 12 V system therefore 75 W / 12 V = 6.25 A.    (You will even read this on web sites selling solar panels and advising how to calculate panel size.) This is absolutely WRONG  This exaggerates the figure by more than 40%.
The peak power current is 4.45 A and this is the most you will see on a correctly functioning system, you can see from the table that even the short circuit current is only 4.75 A.
Even 4.45 A assumes that the panel is facing the sun directly, that the panel temperature is 25 deg C or less and that the panel surface is clean.  Remember this is panel, not ambient temperature, panel temperature is usually much higher and panel output falls with increase temperature.

Their calculations also make the assumption that you will get this 6.25 A for 8-10 hours per day every day.
In my experience a more realistic average approximation is obtained by the simple formula, 
peak power current x sun shine hours x 0.5 (Ipp x 0.5 x H), in this example, 18 AH, this assumes a horizontally mounted panel.
For a north facing panel (for the southern hemisphere) inclined to an optimum angle for your latitude would be more like (Ipp x H x 0.75) or 27AH and a tracking panel (Ipp x H) or 36AH, depending of course on the season and latitude.
Therefore average Amp Hours collected per day by my calculation for this fixed horizontal panel with eight hours of sun shine is around 18 AH and by theirs is a whopping 50+ AH. 
This is why I said earlier you need at least three 75-85 W panels, not one, to be long term self sufficient when running a compressor type refrigerator.    The difference between their figures and mine is a cold beer. 
Also, during winter you can halve that Amp Hour collected figure and although your compressor fridge will be cycling on less often, this will be offset by the longer nights which will mean more use of lights and TV. 

There are regulators available that can extract a little more usable power from the panel by making use of the full peak power voltage but they are expensive and intended for much larger solar arrays. 

Often the on/off cycling time hence average power consumption of  refrigerators is understated. 
I would want to know its average consumption at 35 deg C, not 25 deg C and people do have to open the fridge door from time to time and you can't always put pre-cooled food and drink in, all of which increases the "on" time hence power requirements.

Unless you are prepared to invest in a minimum of 225 to 300 watts of solar panels, I believe you will be better off with an 3-way absorption type gas/12V/240V fridge.

If you want to avoid later disappointments, look at a realistic scenario, not necessarily a worst case scenario but certainly not a best case.

Solar Regulators

Do you need one ?
That depends on two things, the size and type of solar panel and the size of battery you are charging.

In my experience these small self regulating panels (so called because their 32 cell format generates only just over 15 Volts - manufacturers figures) has very limited usefulness in your RV application.

Modern charge regulators in conjunction with larger 36 cell solar panels will result in faster charging and longer battery life with very little maintenance, and far more Amp Hours collected every day.

When choosing for a solar charge regulator, these are some of the features I would be looking for in an "intelligent" regulator.
-	Three step charging cycle, Boost (max current), Absorption (constant voltage), Float (maintenance voltage) phases with periodic Equalisation phase.
	For an explanation of these phases, see Battery Charger page.
-	Ability to charge different battery types, e.g. flooded and gel.
-	Ability to disconnect the battery load when battery voltage falls below a safe preset minimum.
-	Some indication of regulator operation, either with LED or LCD indicator.
-	Ability to handle 20 Amp charge and 20 Amp load disconnect.

My personal recommendation ?

For the non-technical user you can't go past the Steca Delta (marketed by BP as BP Solar GCR20A)

BP Solar have two regulators of interest here, BP GCR 20A & BP GCR 30A. Rated at 20A and 30A respectively, both feature low voltage load disconnect, self learning charging algorithm that automatically adjusts to battery age, capacity and degree of sulfation.
Two LED's monitor regulator operation and fault conditions.

Wet or gel battery types, auto detect 12V/24V operation, monthly equalisation charge, over voltage & over temperature protection are all standard features.
Available with optional LCD display. 

These are an excellent unit made in Germany.



And for the technically minded, the  Plasmatronics PL series.

The PL20 (20A), PL40 (40A), PL60 (60A), features adjustable charging, periodic equalisation, load disconnect, backup generator start, set for wet or sealed VLRA batteries, data logging, LCD display, 12-48V operation. 

These are very sophisticated PWM units with a number of built in programmes and also fully user configurable from the front panel or via a computer. 
Daily data for the last 30 days is stored (batt volts max, batt volts min, Amp Hours in & out, time batt was fully charged, state of charge) and can be viewed on the LCD and may also be downloaded to a computer via the inbuilt serial port.
External shunt and analogue/digital interface are available to measure higher load current, useful for monitoring battery condition where a DC/AC converters are used.     External battery temperature sensor is available and the use of is highly recommended.

This is the one for the technically minded who needs / wants to know everything that's happening and be able to control it.
In my opinion the PL series are the best and most versatile charge regulators available, bar none.
And they are made in Australia too.


Also see Battery , Battery Charger, Batteries & 3-Way Fridge Problems and Generator pages.