Sensible solar

Solar panels have been around for years, and they are getting both better and cheaper. If you are a regular wild camper, an investment in remote charging will make life a whole lot easier.

The wild camping movement has it right. You do want to be out in the sticks, enjoying the scenery all to yourself. The thing is, you will also have to be fully off the grid, with no Eskom to keep the fridge running and the kettle on the boil. In any event, remote travel in Africa almost guarantees that you cannot count on 220V power wherever you stop.

So, when it comes to self-contained vehicle travel, it’s best to be able to tap into what Africa offers in great quantities: power from the sun. What you need is a bank of solar panels and a couple of other items to keep the fridge compressor running, your LED lights bright, and things like your laptop, camera batteries and mobile phone charged.

A basic system

What do you need in your vehicle to achieve this? A simple system will consist of three essentials:

  1. A bank of solar panels with enough power output to meet your power draw needs.
  2. As solar outputs range from 17-21V, a solar charge controller (or more intelligent Maximum Point Power Tracker) is required to modulate the power going to your battery, which needs no more than 14.4V for safe charging.
  3. A deep-cycle secondary battery of at least 105Ah output with thick plates that can handle many deep discharges over time. There are many options on the market now, apart from the old lead-acid standard, including AGM and lithium batteries. Cost may be a deciding factor.

If you are prepared to outlay more on achieving a more flexible portable power system, there are three items that are sure to add a lot of utility. What are these?

  1. An intelligent DC to DC charger. This will enable your vehicle’s alternator to charge the primary cranking battery and your secondary battery while you are driving, giving it enough charge to last through your evening power needs. A higher output device will charge the battery faster. 25 Watts is the standard, though a 40W charger will add a bigger margin of safety.
  2. As DC to DC chargers struggle to get a secondary battery to full charge, and since most overlanders and campers don’t drive 10 hours a day, you will need an intelligent 220V charger as a backup. It should be plugged in whenever you reach a suitable 220V power source.
  3. If you would like to charge camera batteries and laptops, in addition to using other devices that run off 220V, then a DC to AC inverter (which converts Direct Current power from solar and battery sources to Alternating Current) will be a useful addition to the system.

The science

When particles of light, or photons, hit the photovoltaic cells that make up a solar panel, electrons are knocked free to generate electricity. A solar panel comprises 20-40 linked photovoltaic cells, which are typically made up of two very thin slices of silicon, usually 0.1mm thick, which is the semiconducting material used in most electronic circuits.

Photovoltaic (PV) simply means to convert sunlight to energy and to work; a solar PV cell needs to create an electric field by separating opposite charges. This is where the two layers come in. In the manufacturing process, the top slice of the cell is infused with phosphorus to give it a negative charge; this is known as N-type silicon. The bottom half comprises a slice of silicon infused with boron to give it a positive charge, known as P-type silicon. At the junction of the two layers, an electrical field is formed, such that when sunlight knocks an electron free, the field pushes that electron out of the junction and towards the conductive metal plates that link the PV cells. That’s the magic: free Direct Current (DC).

*Did you know: Photovoltaic means “relating to the production of electric current at the junction of two substances exposed to light”.

Panel types & efficiency

Two main types of silicon wafers are used to make portable solar panels. They use different manufacturing processes, and when formed into panels, have a different appearance.

Monocrystalline types are made using the more expensive Czochralski process, in which silicon wafers are sliced from a crystal ingot pulled from a vat of molten polysilicon formed by converting silica sand under intense pressure and temperature. The panels made from this material are the most efficient, with a sunlight conversion rate of 20 to 24%. These PV cells have the edges cut off the corners, and the panels appear dark black in colour.

Polycrystalline solar panels are made using a slightly less costly casting process, in which fragments of silicon crystal are melted and cast into large rectangular blocks, before being cut into thin wafers using a diamond wire. Panels made using this method appear blue in colour and have square PV cells with no cut-offs at the corners. They are typically less efficient, with a sunlight conversion rate of 15 to 17%.

How to choose?

The smaller footprint and greater efficiency of monocrystalline panels make them the premium choice for mobile uses. The look of this type of solid solar panel has not changed much over the years. They still comprise a thin film of PV cells, sandwiched between a sheet of protective glass on the front, with a protective film on the back. This assembly is usually mounted in an aluminium or plastic frame to add protection and rigidity.

A newer development is flexible panels, suitable for permanently mounting it to curved surfaces, such as the roof of a camper or mobile home. Some suppliers choose to join a number of these panels together, protected in a plastic or nylon material outer, forming a very light, easily portable fold-out panel set.

When buying a panel, look for quality hardware:

  • The junction box at the back houses the bypass diodes, which prevent the current from flowing in the wrong direction, i.e. back to the panel when it is shady or the panel is dirty. This junction is where the output wire emerges and should be solid, dust- and waterproof (IP67 rated).
  • The cable should be heavy-duty to reduce voltage drop losses and at least 10 metres long, so you can set up the panels in full sun for most of the day, away from the vehicle (which you probably want in the shade).
  • Try to buy a solar panel with more than a simple solar voltage charge controller. Solar panels put out 18V on average in full sun, which is too high for charging a battery. Rather ensure it is paired with a more intelligent MPPT (Maximum Power Point Tracker), which both optimises voltage input from the panel – up to 30% better – and stabilises the output.
  • Be sure the connectors are good and compatible with the system on your vehicle. The industry standard is the weather-resistant MC4 connector (for 4mm diameter wire), though I have seen Brad Harrison plugs successfully used.
  • Check the physical dimensions and weight of the panel – will it fit in your vehicle easily.
  • Does it come with a decent canvas bag with a heavy-duty zip? This will make a difference when packing and transporting the panel.
  • Does it come with a solid leg system that can handle stronger winds and passing children? You want to set up the panel at an angle of 30 degrees, facing north, to optimise its exposure to the sun. If it is easy to move, adjustments will be far less of a chore.

Panel sizing

There are a few simple ground rules when working out how much output, measured in Watts, you will require from your solar panel array. Of course, a larger panel will collect more power from the sun in less time. Remember, the power draw will come from your battery, which should not be depleted too often below 50% depth of discharge, or its life will be substantially shorter.

What is the total panel output per day? Remember that, in reality, there are only five to seven hours of full sun during the day, with less in winter and more in summer. The typical average? Six hours. A strange reality is that at over 40 degrees Celsius, panel output drops by 1-3V, so the optimum is at slightly lower temperatures. Then take into account losses due to shade, dirty panels and hardware losses/inefficiency – take off 25%.

Apply this formula: Watts (solar panel rating) x Hours (sunlight exposure) divide by 0.75 (losses) = Total Watts per day. Then to derive the energy it supplies the battery, in Amps, use this formula: Panel output in Watts divided by the output voltage of the panel (usually 18V) = XX Amps.

Next, you need to establish the power draws of your appliances, usually given in Watts, and estimate how long each will be used for every 24-hour cycle. Add these all up to get the likely daily draw on the battery, which should be replenished as far as possible each day. For example, a fridge drawing 2.5 Amps per hour, running for 12 hours a day, would draw 30 Amps (2.5 x 12). To get the draw in Watts, multiply the Amps by 12V (the battery), which is 30 x 12 = 360 Watts.

The panel output calculation can also be done in Amp-hours, based on the battery output (typically 105Ah, reduced to around 80Ah as batteries never work at full output in reality). Take this example: Add the power draws from a fridge (30Ah), mobile phone (2A x 2 hours = 4Ah), LED light (1A x 4 hours = 4Ah) and laptop drawing from an inverter (13A x 2 hours = 26Ah). The total draw would be 38Ah direct from the battery, plus 26Ah from the inverter = 70Ah.

What size panel is in this scenario? Solar power over six hours (input from panels) divide draw by hours (70 divided by 6) = 11.6 Amps per hour needed. Multiply 11.6 Amps per hour x 18V (average panel output) = 210 Watts.

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