Solar energy is energy that comes from the sun. It can be used naturally in various ways and makes use of the photovoltaic effect to generate electricity from sunlight. Solar energy is becoming increasingly popular these days with more and more households relying on solar power too as it generates electricity and heat which is entirely sustainable and free.
There are three types of solar energy:
- Photovoltaic solar energy: used to produce electricity;
- Solar thermal energy: used to heat water;
- Passive solar energy: which directly takes advantage of sunlight.
We take a closer look at the basics of solar powered energy explained below:
The Water Analogy
Picture a tank stand 10 metres tall. Imagine a tank on the tank stand, 5000 litre capacity and another 2 metres tall. When the tank is virtually empty – down to its last few millimetres of water – the pressure in the downpipe at the base will be 100kPa. When the tank is full to the brim the pressure at the bottom of the downpipe will be 120kPa.
So any pressure above 100kPa will start the flow of water up the pipe into the tank, and a pressure of 120kPa will be sufficient to finish the job. Any pressure greater than that required to lift the water is superfluous and useless if it does not increase the water flow, which in this case is limited by the capacity of the pump. Holding your thumb over the end of a hose pipe will increase the pressure in the pipe and make the outflow jet further, but will not increase the flow rate.
You want a solar pump to fill the tank. The salesman eagerly points out a good seller. “This is a very powerful pump” he says. You know that power is the product of pressure multiplied by volume of flow. “I don’t care about pressure” you say “anything above 120kPa is superfluous. “How much volume does it pump at 120kPa? I only want to know about litres per hour” He looks at the specs. “It doesn’t say, but at zero head it will pump 500 litres per hour, at 120kPa it should be nearly that” he says. “Let me do some arithmetic“ you say, since clearly demand is going to determine how the supply side is going to look.
The tank serves one or two outlet taps with variable but predictable water usage. The daughters want to wash their hair everyday and there’s the other washing and drinking to consider. You reckon you need 4000 litres per day. Nothing less will do. That means the pump must work for 8 hours every day to keep the tank from going dry. Not a problem on good, sunny days, but on cloudy days, that are already short, such as in winter, there could be a water shortage. What to do?
One option would be to put another tank on the stand containing 5000 litres of backup water to tide you over the bad days, the first tank made to overflow into the second one, and the second one stopping the pump when full. Or you could add another solar pump to double your pumping capacity. Or you could buy a backup petrol-powered pump to kick in on the days the sun doesn’t shine. The second tank is the cheapest option, but could also run out on the second cloudy day. The second solar pump will also be affected by the lack of sunshine though both pumps will still produce on cloudy days albeit at reduced performance. The petrol-driven pump is the safest but least attractive option.
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Back to the Real
Moving this over to the solar electricity context, some things change but most remain the same. The water tank becomes a deep-cycle battery the output pressure of which will vary from 11,4 volts when depleted, to 13,8 volts when fully charged. The capacity of the battery, instead of 5000 litres will probably be about 105 Ampere/hours.
(An Ampere is not a quantity of electricity, but a rate of flow. By definition one Ampere constitutes one electric charge unit, (a coulomb = 6,022 x 1023 electrons), moving past a point in one second. So an Ampere/hour is 3600 charge units, or coulombs, that have moved through the circuit. Coulombs are quantities of electricity, the same way a litres are quantities of water)
Many solar panels produce an output voltage that is way in excess of what is required to charge a battery, and using that figure in conjunction with output current to calculate power, gives an impressive but meaningless number. What is important is the current flow, just like the rate water flow into the tank was what we want to know about.
When estimating campsite solar power requirements, often the only really essential load on the system is the fridge, which is kept at low temperature by a compressor that draws around 36 watts from the battery. LED lighting is also a factor but since it’s used for only an hour or 2 at night, constitutes 10%, or less, of the load imposed by the fridge, and can be done away with when battery power is short.
Dividing fridge’s power consumption of 36W by the voltage (nominally 12V) gives us a current consumption of 3 Amps, while the compressor is running. The duty cycle (how much does the fridge run and how much does it rest?) is an unknown here, but can be guessed at 30% for a fridge not often opened, to a worst-case of 50% on hot days when the pals come over for a beer or 3.
At 3A and 50% duty the fridge will demand 3A x 12 hours, or 36 Amp/hours in any 24-hour period. A typical solar panel will deliver a short-circuit current of 5,5 Amperes, and an open-circuit voltage of 21 Volts. The actual current delivered at 15 Volts, such as when it is charging your 100Ah battery through an (essential must have) regulator must be guessed or measured. Measurement tells us it’s about 4,5A.
To put back into the battery what we know the fridge is going to take out, our 4,5A solar panel will need to work at full capacity for 8 hours per day (8h x 4,5A = 36Ah). This could be a big ask on cloudy days (as with the solar pump), and puts us at the very edge of viability. So what are the ways this system can be made less failure-prone?
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As with the second water tank, a second battery is the cheapest option. (Bear in mind that a 100Ah battery is actually a 55Ah battery since you should never discharge a deep cycle battery beyond 45% of nominal capacity. Battery protectors will prevent excessive discharges.). The second battery will provide a full 24 hours of backup supply plus a generous margin. In the same way that the second water tank is filled by the overflow from the first, the second battery will be charged by the battery manager once the primary has had its feed.
Or you could add a second solar panel. It will provide some, or all, of the shortfall on cloudy days, but will be a total waste of money on the sunny ones, if the fridge remains the only load. If other non-essential loads like lighting are added to the system these could be supported by the second panel when the fridge is looked after by the first.
A petrol generator is, once again, the most versatile but least satisfying alternative.
The efficiency of a solar panel depends on a variety of easily overlooked variables. Any shadow, no matter how minor, will profoundly affect the panel’s performance. Because many of the elements are connected in series, blocking one will disable the entire chain, and the shadow of an insignificant-looking branch can stop the entire array from delivering any current at all.
The angle of incidence of solar radiation is also very important. The greater the slant of the sunlight the less the output, so keeping panel facing the sun and out of the shadow of potentially interfering vegetation is almost a fulltime job. The sun will move 45° every 3 hours, and 45° away from the sun will potential power conversion by 30%.
Ideally the panel should be mounted on a shaft that parallels the earth’s axis and that turns 15° per hour, but, since it’s not practical to take such a contraption on a camping trip, you’re forced to have someone move it by hand periodically.
The cross-sectional area of the conductors between the panel(s) and the regulator/batteries is another issue I’ve seen debated. The wire-gauge norm in domestic reticulation of 9A per 1mm² will cause excessive power loss on anything other than very short cables. Multi-strand twin-flex with a cross section of 2,5mm² is easily sourced at any hardware shop and has the capacity to serve two 4,5A panels in parallel without excessive loss.
Although a change in EMF (electromotive force) travels down a conductor at almost the speed of light, the actual charge-carriers, the electrons, move quite slowly (a few metres per hour), and when they are forced to speed up due to a conductor being too thin, the resulting particle collisions mean energy being converted to heat in the wire rather than charge in the battery.
Regulators
All batteries regardless of their tyre are delicate devices that don’t forgive or recover from abuse. They need regulators to protect them from being over charged, and they must never be excessively discharged.
Invoking the water analogy once again, solar charge regulators are similar to the valves found in toilet cisterns. The older type of cistern valve has a float attached to an arm that operates a plunger. The plunger is brought ever closer to the water jet as the level in the cistern rises and slowly stops the influx of water to the tank. This very simple mechanism has one major drawback – it is very slow, particularly in the final phase when the water inflow slows to a trickle, and that trickle is expected to fill the tank sufficiently to stop the inflow entirely.
A more modern, but more complex, mechanism allows the inflowing water to fill the tank at 100% of flow rate until a critical level is reached and the device trips a valve that uses the incoming pressure to instantly squeeze off the inflow.
Electric charge regulators similarly come in two major varieties: there’s the series regulator that gradually decrease the charge current as the battery voltage reaches 13,8V. At no stage is the battery voltage allowed to exceed 13,8V, and any time the voltage drops below this level charging resumes.
The intelligent charger on the other hand starts with a de-sulphating phase where a high speed (200KHz) series of very short power pulses is sent to the battery to soften the hard sulphate layer that tends to build on the plates thereby blocking their charge/discharge availability and reducing the effective battery capacity.
Next comes the bulk charging phase, when up to 16V is applied for a duration time determined by the charger’s internal algorithms. When the charger logic decides that the battery is fully charged it then lowers the charge voltage to a float level that is intended to counter the self-discharge tendency of the battery.
Intelligent chargers are more expensive but reward their owners by getting the batteries charged as quickly and efficiently as possible, whereas series regulators will waste a lot of solar energy by being over-cautious with the charge voltage.
If you have any questions relating to solar power, regulators or electricity, please leave them in the comments below or engage with us on our community Forum.
