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australia:camping_solar

solar power for camping

Introduction

  • current solar panels are based upon silicon cell technologies which combine the output of a number of cells
  • these are usually rated at around 80-350W when tested at Standard Test Conditions of 1000W/sqm incident sun, 25degC and AM1.5 air mass spectral conditions.
    • in real usage, incident light is likely to be 800W/sq.m or less and thus one often uses nominal operating testing conditions of 800W/sq.m, 20degC with a 3.5kph breeze to keep it cool
  • the larger the power rating, the larger the surface area needs to be to create that power output for a given panel efficiency
  • sellers (especially those on Ebay) may over-estimate the output of panels, test actual outputs or do the maths:
    • The standard test of efficiency and output is done with sunlight falling on the panel with an intensity of 1000 watts per square metre and 25degC. The efficiency of the panel will be in the specs, and should be around 17%. So the panel should be producing 1000 x 17 / 100 = 170 watts per square metre of its area. So to get the actual output of the panel just use the dimensions of the panel, calculate its area in square metres, and multiply that by 170.
  • traditional panels have glass fronted panels mounted in an aluminium frame making them bulky and heavy
  • more modern “soft fold” panels are much lighter, more compact and more convenient but considerably more expensive than the older aluminium framed ones
  • for max. efficiency place at correct angle to the sun (at midday, this angle should be your latitude, whereas in the morning or afternoon, the angle will need to be more than this), this may increase output of a 200W panel from 7A laying flat to 9A placed at a more optimum angle (11A/18V in perfect conditions), plus this allows better ventilation to keep them cooler and they wont kill the lawn as may occur if you lay them flat
  • older or cheaper panels may not have blocking diodes in each section and the output may become zero if even one section falls under shade of a tree
  • a portable panel is generally more useful when camping instead of a fixed roof panel
    • it allows you to park your vehicle in the shade and place your panels in the sun (you may need an Anderson plug extension cord for this)
    • it allows you to optimise the angle of the panel to the sun throughout the day
    • you can combine the output with a fixed roof panel output for added versatility
    • however, they are at higher risk of theft and damage and obviously you do need to set them up
  • you can combine two panels into one controller which should give the combined current
    • just use an Anderson plug splitter (2 plugs going into one plug)
  • you also need a solar regulator to charge a battery and ability to choose the correct charge mode for certain batteries such as LiFePO4
    • MPPT solar regulators add around 10% output compared to PWM solar regulators
    • be aware that some Chinese manufacturers falsely badge their PWM controllers as MPPT!
    • the solar controller input max current should be at least 20% higher than the panel's short-circuit current
    • the solar controller input max voltage should be at least 4% higher than the panel's open circuit voltage.
    • some in-vehicle DC-DC chargers also have a solar regulator built-in such as the Redarc ones.

types of panels

  • you should purchase Class A (or perhaps class B) rated solar cells, lower classes have too many defects and much less efficiency
  • 3 main types:
    • polycrystalline - cheapest, individual cells have maximum efficiency of around 22%
    • monocrystalline - most efficient, individual cells have maximum efficiency of around 26.5% but most panels are rated around 20-23%
    • amorphous - expensive, thinner, more flexible and durable, slightly better in cloudy conditions but only 10% efficiency so need more area
  • new shingle cell design allows for better efficiency as1):
    • there are no busbars (ribbons) required
    • can be joined together in overlapping shingle manner resulting in no gaps between the solar cells
    • they can be combined in parallel rather than series which allows partial shading to be not as problematic EXCEPT when shade is in vertical axis and this may actually result in less power output than conventional panels
    • produce more power per sq. metre as less inactive area
    • improved reliability as lower busbar failures and more resistant to external forces
    • more aesthetically pleasing as no visible circuitry

solar panel specifications explained

  • the maximum wattage in ideal conditions = voltage at max. power x current at max. power
  • the open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and this occurs at zero current and for most commercial silicon cells at 300degK is around 0.6V.2) Most 12V solar panels have a total open circuit voltage of around 21-23V and generally is highest mid-morning when the panel has not become too hot. This can be measured with a multimeter across the open ends of the wires attached to the panel. The solar controller specifications for input voltage must be greater than this otherwise the controller will shut off.
  • the short-circuit current ISC is the largest current which may be drawn from the solar cell (this can be measured by passing the current through a multimeter configured to measure amps but care must be taken to avoid arcing) and this depends upon:
    • area of the solar cell
    • light intensity hitting the cell
    • wavelength spectrum of incident light (most silicon cells have sensitivity 0.4 to 1.1 micron wavelengths, glass removes most of the wavelengths shorter than 0.4microns)
    • optical properties of the cell - absorption, reflection (surface should be matte with minimal reflection)
    • collection probability of the cell (depends chiefly on the surface passivation and the minority carrier lifetime in the base)
  • over-heating of panel reduces efficiency so keep them well ventilated
    • open-circuit voltage for a silicon solar cell (which is usually around 600mV) falls by about 2.2mV (or around 0.4%) for each 1degC rise in temperature (there is minimal change in short-circuit current though) 3)
    • the panel's Normal Operating Cell Temperature (NOCT) is an indicator of how well it dissipitates heat - the lower the better as it heats the panel up less during operation.

things to consider when buying

  • what style best suits your needs - portable or fixed to a vehicle roof
  • your budget
    • but beware, a cheap panel may break very easily (most are quite fragile) or under-perform and may represent false economy
  • what power output you need - aim to get more than you need - most campers will want 100-200W
  • does it have blocking diodes so that shade on, or damage to, one section doesn't stop the whole output?
  • size and weight
  • durability
  • manufacturer reputation and warranty
  • beware of scammers online who falsify output etc - do your research on them!
  • how easy is it t set up on an angle to optimise efficiency
  • how will you keep it well ventilated to keep it cool
  • how will you prevent it from being stolen
  • weather-resistance including hail damage but also resistance to being blown over in the wind - ability to peg down is useful
  • how will you angle it to optimise the sun's direction - some have built in legs to assist with this

effect of angle of sun to panels

  • panels are designed to work optimally when the sun is perpendicular to the surface
  • in the middle of the day at the Summer solstice, the sun will be angled away from the perpendicular by the same angle as the location's latitude less 23deg and by the Spring and Autumn equinox's at midday the angle is just the latitude
    • thus for Victoria at around 38degS latitude, the panels need to be angled 15deg at midday in summer and more at other times of the day or in other seasons (38deg in Sept and March at midday)
    • ie. elevation angle of the sun = 90 - latitude + declination of sun
    • ie. angle of sun from zenith = latitude - declination of sun
  • this has 2 main effects of solar panel efficiency:
    • reduction of intensity due to increasing Air Mass
      • Intensity perpendicularly direct onto panels = 1.353 x 0.7 (AM^0.678)
      • Air Mass = 1/cos(angle of sun)
      • Air mass at zenith (0deg) = 1; Air Mass at Summer Solstice midday at 38deg latitude = 1.09;
      • at latitude 38degS the intensity on a panel tilted perpendicular to direct sunlight at midday is:
        • Jan 1.03kW/sq.m
        • April/Sept 0.98kW/sq.m (by 7am it has already reached 0.55kW/sqm and by 9.30am 0.9kW/sq.m)
        • June 0.8 kW/sq.m
    • reduction of intensity due to incident angle on panels
      • intensity of sun onto a horizontal surface = intensity when perpendicular x sin(elevation of sun)
        • thus at 38degS at midday in Apr/Sept elevation = 90-38 = 62deg thus 88% of perpendicular intensity
      • intensity of sun onto a tilted surface = intensity when perpendicular x sin(elevation of sun + angle of tilted surface to horizontal)

examples of commercially available 12V panels in Australia in 2021

aluminium frame panels

    • 683x760x60mm folded in half with panels exposed; rear-attached PWM solar regulator compatible with LiFePO4; 11.4kg; 5m cable with Anderson plugs; durable design but no bypass diodes?; $AU209

lighter folding large panels 500-900mm

  • HardKorr 200W solar panels with crocskin cell armour $579-$699 incl. HardKorr 15A 5-stage PWM Solar Controller for Lithium with Anderson plugs; 9kg; 4 panels 710x530x40mm folded; weatherproof; unlike cheaper panels, shade on one area does not impair all area outputs; 11A per hour on a clear sunny day (ie. 1A/hr fridge needs 3hours charge/day in the sun;
    • A Grade monocrystalline shingle solar cells; each section of the blanket is wired in parallel so works in partial shade; Anderson plug; rear legs for tilted use each with eyelets for pegging; 20A MPPT regulator but not for lithium; 4.4m Anderson ext. lead; folds to 550x670x50mm opens with 5 panels 2.5m wide; 9kg; 1680D polyester weave protection plus water resistant PVC coating; $AU439
    • bypass diodes for partial shade; 855x630x10mm folded in half; 6.7kg; PWM regulator but not Li; 5m MC4 cable; no Anderson connectors; build quality looks good; $AU309;

lighter blanket style folding to 500mm or less

    • flexible amorphous cell technology more able to withstand damage - can walk on it but large area (18 sections 1860×1185 unfolded) very expensive $AU2400 w/o controller!! 4.8kg;
    • 15 sections 1720×930 unfolded; 7.2kg; $AU2066 w/o controller
    • $AU379; much more compact than their crocskin panels, similar output on testing, blocking diodes but less durable and controller is only a PWM which does not do lithium. 3×4 array of 12 panels folds to 350mm x 375mm x 70mm; 7kg;
    • 280W; partial shade charging; max. 15A 18V; 2×4 array of 8 panels folds to 500 x 400 x 30 mm; 7.2kg; $AU699
    • 6kg; $AU219; no blocking diodes; PWM not lithium; Anderson plugs;
    • no bypass diodes; Anderson plug; 4.4m Anderson extension lead; PWM regulator but not for lithium; 400x510x40mm folded opens to 4 panels 1.54m wide; 4.3kg; NOCT 47degC; $AU139
  • various brands such as Mobi on Ebay sellers such as Outbax (but they appear to have a poor service reputation and they seem to over-rate the output of their panels significantly 4) ) who have various wattage “2021 tech” blankets with controllers for lithium at discounted prices - perhaps a nice size for output is a 300W Mobi for $AU219 but you may need to consider output may only be 200W max and these controllers apparently interfere with AM radio reception and also apparently get hot if used as output devices (use your battery to connect to devices to resolve this)

semi-flexible thin sheet panels to mount on van roofs

  • don't expect as efficient outputs if they lie flat instead of angled to the sun, and they should be mounted on hollow core panels to provide ventilation, but the advantage is longer duration in the sun as they can be active all the time not relying on user to set them up.
    • ultrathin, flexes to 248deg. $AU289
    • flexes to 30deg; diodes for partial-shade; $AU159
    • IP67 rating; tough PET construction bends to 30deg; MC4 connectors; sturdy corner eyelets; bypass diodes; 1070x670x20mm; 2.85kg; NOCT 47degC; $AU109; a few of these could be transported in the boot of a station wagon which may make this an attractive option for some users, in addition to those who will want to attach to their roof rack.
australia/camping_solar.txt · Last modified: 2021/03/16 22:04 by gary1