Originally Posted by Caravan Nobber
I am writing this article in the hope that the informationprovided may prove interesting and hopefully informative. It is my intention to keep the subject as simple as possible, but there are certain technical areas that I will need to mention. First let me say that I am not, nor do I claim to be an expert within this field, but I am a degree qualified electronics engineer and if nothing else this should explain my terrible grammar and spelling!
In December 2011 my wife and I joined the world of caravanning (a little late in life but better late than never) we entered the market by buying a 2006 Bailey Pageant Moselle which we love and have enjoyed on a number of lovely club sites. All of these sites had included an electric hook-up; however, during the June bank holiday we visited a five night music festival where there was no electricity. Prior to this event I began to think about alternative energy sources, mainly because I needed the motor mover to move the van off the road and into its storage area, where reversing was impossible due to the limited turning circle and getting stuck in the middle of a single (busy) road was not an option.
My initial thoughts were, “well the leisure battery is charged by the tow vehicle when driving between destinations, but if the journey is relatively short then the charge will be limited.” With this in mind I turned my attentions to alternative independent sources and almost jumped straight into purchasing a small suitcase generator. Now before I enter into this further please allow me to state that I am in no way trying to deter anyone from using a generator, nor am I saying that a generator is not a good method for producing energy. In actual fact (unlike a solar panel) a generator is capable of producing 240 VAC which will run almost every electrical appliance within the caravan including charging the leisure battery, but I had to consider where I could stow it including fuel and oil, then the fact that even though they are relatively quiet running they do produce noise and exhaust fumes and even when cold there must be a smell from the engine. With this in mind, plus the fact that my car boot is reserved for my two chocolate Labradors I decided to research an alternative.
Solar power, I believe, is the only other option currently available, but realistically can a solar panel (PV) provide sufficient energy to recharge the leisure battery bearing in mind that energy will only be generated during the daytime and that the efficiency will depend considerably on a number of factors such as weather conditions, temperature, time of year and physical position or angle relative to the sun? To my surprise I learned that solar panels work best when cool and that they generate a reasonable amount of energy even during cloudy or wet conditions so this was an encouraging start.
So how much energy can I expect to consume during a typical 24 hour day and how much needs to be replenished? Fortunately most of my appliances utilise gas to operate such as the cooker, water heating, fridge and environmental heating. This only leaves the lighting, water pump extractor fan, heat circulation fan and TV to operate off the 12 volts battery (deliberately not mentioning the motor mover here).
My caravan is fitted with a total of 16 lights and originally each light consumed 10 watts of power, a total of 16 x 10 = 160 watts (or 13 amps), however I have dramatically reduced this load by fitting new MR11 12V 140Lum surface mount LED high power day white bulbs (from Future Green Light via ebay), where each emits the equivalent of 25 watts of light but amazingly only consumes 2 watts of power. This has reduced my lighting consumption from 160 watts down to only 32 watts (or 2.7 amps). There is also a warm white option available that would reduce the piercing brightness of the day white lamps, thus set a mellow atmosphere within the living area. It is quite unlikely that all the lights will be on at any one time but for my load estimate I will be using the worst scenario, maximum all on.
The water pump is only used occasionally and is estimated to draw approximately 50 watts (approximately 4 amps). By far the highest power consumption will be the motor mover and the TV, but the motor mover only consumes power for a relatively short period of time (estimated 10 amps in 5 to 10 minutes) and once the caravan is connected to the tow vehicle, most of the energy consumed by the mover would be replenished during the journey even during a relatively short journey. My TV consumes 47 watts (4 amps) so neglecting the motor mover and circulation fans I am going to estimate that the TV will be used for 5 hours and that all the lights will be on for 6 hours, plus the water pump used for 30 minutes. In this case the current drain on the battery would be approximately:
TV 5hrs usage x 4A = 20A
Lights 6hrs usage x 2.7A = 16.2A
Water Pump 30 minutes usage 4Ahr / 2 = 2.0A
Total = 38.2 amps.
Therefore, approximately 36 – 38 Amps will be consumed from the leisure battery each evening/night, now an 110Ah leisure battery in theory should be capable of supplying 110 amps for one hour before its completely flat, therefore by drawing up to 38 amps per night a fully charged battery should be capable holding up for just under three days without charge. This means that the solar panel will have to replenish about 38 amps per day. Hence if the solar system is capable of supplying an average of 5 to 7 amps then it should take about 5.5 hours to recharge.
Choosing the right solar panel:
(What solar panel best suits my requirements, where best to position it and what energy can I expect from the panel?)
From my research I have identified that there are a number of different types of solar panel available and each do the same thing; convert solar radiation into electricity, but each has different characteristics and prices.
The cheapest (and least efficient) type of solar PV is known as "thin film" or amorphous silicon, although this type of PV is cheap you will need a large surface area to obtain a decent amount of energy from it but it is quite tolerant to high temperatures.
Polycrystalline solar PV are made up of a number of smaller silicon crystals. This is rated as being very effective and is generally cheaper. These are usually dark blue in colour and are widely available.
Monocrystalline solar is the most efficient but as you would expect it is more expensive than thin film or polycrystalline solar.
Just to mention, there is also a premium type available called a hybrid or HIT (Heterojunction Incorporating Thin Film) solar. It incorporates a monocrystalline overlaid with a thin film layer which apparently makes the best power conversion efficiency material, but as you would expect this is the most expensive option.
So with all this variety which should I choose?
With the solar panel being such an important part of the system I decided to look towards the Monocrystalline, it being the most efficient but the three major things that dictated my final choice were: physical size, power output and cost. The PV output power is normally quoted in watts but what really charges the battery is current, so to obtain a better understanding it is worth converting the watts to current. Fortunately this can easily be achieved by using a simple equation called Ohms law.
Where Power (Watts) = Current (I) x Volts (V)
With the power output quoted in Watts, we can easily transpose the value from watts to obtain the value of current:
Current (I) = Power (Watts) / Voltage (V).
Remember as previously mentioned the calculated values are based upon perfect 100% efficiency (perfect conditions) Also, to charge a 12 volts battery the voltage will be higher typically 14 to 14.5 volts for a full charge. Therefore, a 100 Watt solar panel would theoretically provide 100/12 = 8.3 Amps.
A good quality mains battery charger would normally be capable of charging batteries something near 5 to 8 Amps so for this reason, including losses in efficiency I decided not to use a solar panel of less than 100 Watts which in “theory” could produce up to 8 Amps
My final choice made, I purchased an SSP-120W 12v PV monocrystalline solar panel from Sunshine Energy, it is housed within an aluminium frame that makes it perfect to mount into my intended position. The panel is supplied with 5m of cable with MC4 type connectors fitted to the ends, which removes the need to calculate the cable size. Also, these cables can be extended but would incur a small (loss) in efficiency due to volts drop over the cable length. The physical dimensions for this panel are 1480mm x 680mm x 35mm and an important factor is that it only weighs 10.1 kg, it’s hi-impact resistant and tolerant to high winds.
Where to fit the PV:
The choice of position for me was a fairly simple one. Fortunately my roof space near the front of the van is large enough to mount the panel and it is slightly angled forward, although not the perfect angle facing the sun this would still be a reasonably good position to capture energy throughout the day without the need to have to rotate it towards the sun. Also, at this elevation it should be better positioned to miss most shaded areas that could otherwise be encountered at ground level.
Naturally not everyone will want to fit a PV to the roof of their caravan or there may not be sufficient room for one, so a ground mounted panel would undoubtedly be just as good but may require more thought when setting up and may have to be rotated periodically towards the sun during the day.
Mounting the panel
As previously stated I have mounted the panel on the caravan roof, but to avoid holes within the roof I have used six special mounting brackets purchased from Lensun, four corner mounts plus two middle to prevent flex or sag at the middle of the panel. With all six mounting brackets secured to the panel’s aluminium frame using self-tapping screws the whole assembly was then carefully positioned in the middle of the roof and glued using Sikaflex 252 adhesive.
There is however (not for the faint hearted) a need to drill one hole large enough for the two cables from the solar panel to enter the van. Here I have used a cable entry housing which is also glued to the roof using Sikaflex 252 that also provides a sound moisture seal. To protect the power cables from sharp edges when entering through the roof I have used a short length of plastic tube over the cables then finally to ensure a perfect moisture seal I applied a liberal amount of caraflex IDL 99 (available from most caravan accessory shops).
From the roof cable entry the two power cables are routed neatly within a small electrical conduit following the original van wiring down to the power distribution box and leisure battery where it meets the solar charge controller.
Solar charge controllers (The brains of the system):
Up to now I have discussed the need to select the right type of solar panel but now I need to consider how the power generated will be applied to the leisure battery.
Lead acid batteries can easily be permanently damaged if not recharged correctly and one example of this would be over charging or boiling it, generally most solar panels output far more voltage than what the battery requires to charge, so for this reason solar panels are not normally connected directly to the battery, instead the energy produced by the panel is regulated by a solar charge controller, which ensures that the battery is safely changed and maintained in its best condition.
Most modern charge controllers regulate the battery charge by what is known as a 3 stage charge cycle.
Stage 1 is known as the Bulk phase, where the voltage rises to the Bulk level (usually 14.4 to 14.6 volts). This is when the battery draws maximum current as the bulk level voltage is reached. Following this phase the absorption stage begins.
During the absorption stage voltage is maintained at between 14.4 to 14.6 volts for a defined time (normally about an hour), during which the current flow gradually reduces as the battery reaches a fully charged state.
After the absorption time phase the voltage is then reduced to the float level (approximately 13.4 to 13.7 volts) and the battery normally draws no more than about 0.2A, until energy is once again consumed and the next cycle begins.
Types of controller:
There really are, in my opinion, only two modern types of charge controller available that are really worth evaluating. The first is a pulse width modulated (PWM) type and the second type is a maximum power point tracking (MPPT) system, commonly known as a tracker. The right choice of controller I believe is paramount to achieve the greatest efficiency.
The simplest type of charge controller outputs a fixed voltage while charging the battery, irrespective of the level of charge in the battery. It then monitors the battery and opens the output circuit once the charging process reaches a certain level. Older charge controllers accomplished this mechanically by using relays, but the more recent charge controllers make use of pulse width modulation (PWM). This process is rather complex so I will simply state that when the battery starts to reach its fully charged state, the amount of power being transferred to it gradually decreases as opposed to it being simply cut off as with the earlier relay types. The battery is then held or floated at its fully charged state by the PWM. This type of charge controller can extend battery life and reduce stress on the battery and will keep it charged indefinitely.
MPPT Maximum Power Point tracking
My initial thoughts when I first encountered the MPPT charge controller (tracker) were that it must be a mechanical device that moves the solar panel to track the sun during the day, but this is definitely not the case.
The MPPT system utilises all the benefits of the PWM system but has one major advantage over the normal PWM charge controller. The MPPT tracks the energy generated by the solar panel, at the same time monitors condition of the battery. It then matches the output of the solar panel to the battery voltage which ensures that maximum current charge (amps) is achieved throughout all the variations during the charge cycle, which means that you are getting nearly the full capacity from the solar panel.
As an example: From a solar panel rated at 100 watts, you would never obtain the full 100 watts from it unless the battery being charged is at the optimum charge voltage.
Using the previously mentioned Ohms law, where power (watts) is equal to volts x amps or P=V*I.
A regular charge controller without the ability to track would have the charge voltage set at a specific value meaning that for only one short instant in time during the charge cycle the maximum current flow would occur.
Let’s say at a battery voltage of 12.4 volts the maximum current flow occurs, where a 100 watt solar panel rated at 6 amps at 16.5 volts (6 amps times 16.5 volts = 100 watts) would only be charging at 6 amps times 12.4 volts = 75 watts meaning that we have just lost 25% of the 100 watts capacity!
The MPPT controller compensates for the lower battery voltage by delivering closer to 8 amps into the 12.4 volt battery maintaining the full power of the 100 watt solar panel, 100 watts = 12.4 volts times 8 amps = 100 watts (P=V*I).
It’s that simple, but as you can see from the example an MPPT charge controller can make the difference between a reasonably good system and a system running at its maximum efficiency.
My final decision made, I purchased a high efficiency 20A MPPT solar charge controller from Photonic Universe Ltd, again via EBay. I have to state here that I could have bought a much cheaper 10A version that would have been more than capable of handling the job, but I opted for the larger model thinking that I may wish to add another solar panel which can easily be achieved by simply wiring more in parallel, however the final result from the 120W panel was amazing making an additional panel unnecessary.
Monitoring the system
Wanting to fully understand the efficiency of my system without the need to fit an ammeter I decided to invest in a MT-5 monitor also from Photonic Universe Ltd, which is designed to simply plug straight into my charge controller via a supplied 2M cable. The LCD monitor provides feedback regarding the energy generated by the PV, the level of charge at the leisure battery and the amount of current flowing into the battery.
The MPPT tracker also has a load output that is designed to drive 12 volts systems direct such as a 12 volt fridge. You can also connect a sign wave inverter to this output to achieve a 240 volts AC supply because the source is derived direct from the leisure battery.
As an experiment I connected a 500 watts sign wave inverter to supply the 240 Volt fridge but as expected the load was 12.5 amps, which is 5 to 6 amps greater than the power generated by the panel. Needless to say that if used constantly this would eventually drain the battery flat, however I do anticipate that when the fridge reaches its optimum temperature the consumption will fall considerably, but realistically I do not believe this could work as a permanent supply.
To conclude, I have since used this system to its full during the music festival it fully maintained the leisure battery despite the fact that it rained the whole time plus the motor mover did the job when I arrived home. I have also used it at a number of sites while using the TV, Radio or CD player plus lights during which time the system has never failed to fully recharge the battery.
The maximum energy observed to date while charging the battery plus holding up the load is 7.8 amps additionally the battery has been constantly charged while the van has been stowed providing me with full power without the need for mains power when stored.
The total cost of this system (less fitting) was little over £400 about the cost of a budget suitcase generator.
12V 120 Watt Solar Panel PV Monocrystalline from Sunrise Energy (ebay) £184.95
20A MPPT Charge Controller from Photonic Universe Ltd (ebay) £129.99
2 Piece (One Pair) mounting blocks from Solar Supplier (ebay) £17.00
4 Corner mounting brackets from Lensunsolar (ebay) £41.83
Roof cable entry from Solar Supplier (ebay) £7.50
Sikaflex 252 Adhesive from allrightbutt £18.50
Remote meter from Photonic Universe (ebay) £31.99
Self-tapping screws General hardware store (Just a few pence).
Note also that as mentioned there is a much cheaper 10A version of MPPT available from the same supplier for £79.99 that will easily handle the 120 watts panel.
John Ratcliffe (Ross On Wye).