SIZING & WIRING AN OFF-GRID SOLAR ARRAY

The key to sustainable energy production in a Tiny Off-Grid House is the efficiency of the photovoltaic (PV) array energy production and reduced voltage loss. The energy density of the battery bank is not the main focus, if the PV array is not even capable of producing enough energy in the shortest amount of sun peak hours to recharge the battery bank.

HOW DO PHOTOVOLTAIC CELLS WORK:

The sun is a huge nuclear fusion reactor that provides free abundant solar energy which can be used as thermal energy or converted in to electrical energy.  Protons—not sun heat— from sunlight strike Photovoltaic (PV) cells where they excite electrons forcing them to travel from the positive N-side towards the negative P-side.  This movement between the electric fields generate electricity with each cell producing approximately 0.5 Volts.

Under standard test conditions: (1000 watts per square meter radiance—one peak sun—at 25°C / 77°F, with an air mass (AM) of 1.5° to 30°C above ambient temperature, the average 400 Watt module maintains an efficiency of 22.3% during the first 25 years of its service life.  This is a significant achievement for photovoltaic cells because in nature plants and algae are only able to harness half of the sunlight in the visible range (400 - 700nm wave length) of the total light spectrum used to power photosynthesis so water (H2O) and carbon dioxide (CO₂) can be metabolized to produce byproducts in the forms of Sugar (Fuel) (C6H12O6) and Oxygen (O₂) it is this solar energy stored as chemical energy when wood is burned for heat, that fuels waters hydrologic cycle that provides kinetic energy for Dams and is the genesis of the mineralized biomass of plants, algae and to some extent animals that humans use as fossil fuels today.  Defenders of fossil fuels (Coal, oil & natural gas) argue that fossil fuels inherently come from clean natural solar energy; that was once metabolized by now deceased carbon based lifeforms. This green washing does not justify the environmental pollution, climate change, extreme weather events and harm to our health that comes from the use of fossil fuels. The combustion of fossil fuels emit harmful emissions that pollute the air we breathe with carbon dioxide, irritants and particulate matter.  There is a better alternative; we can obtain free abundant clean renewable energy directly from the 4.6 billion year old inexhaustible sun which is expected to last another 5 billion years. 

Clean solar PV energy is 100% emissions free, has no mechanical moving parts, requires no fossil fueled transportation, no vibrations, no noise, no combustion, no pollution, no fumes, no thermal discharge waste, no odors, does not heat water to turn steam turbines . . .

The Tiny Off-Grid House solar electrical system is engineered to operate during the months with the lowest sun-peak hours with minimal decrease in efficiency; which will probably be during the winter or rainy seasons; depending on the region. This should obviously guarantee the best performance during the summer months when there are longer and more abundant sun peak hours.

HOW ARE SOLAR CELLS MADE:

Multiple solar PV cells are combined in to a solar module (not to be confused with solar “panels” which are used in solar “thermal” hot water systems; unlike PV modules).  Multiple solar modules are combined to form a large solar array.

Monocrystalline cells are made of crystalline silicon (c-Si).  Silican is the second most abundant natural resource material on the Earth.  Silica rocks, made from purified sand and carbon, are melted at > 2500°F in an electric arc furnace.  The product is 99.0% pure silicon and carbon dioxide which is further refined using the Jan Czochralski method which rotationaly extracts a seed crystal in to a solid cylindrical shape producing a final product that is 9.99% pure silicon ingot.  The ingot is sliced in to very thin wafers 100 - 500 um thick with the corners and sides of the silicon wafers are cut forming space saving angled corners on the module. First generation monocrystalline modules had lots of wasted space due to round silicon wafers.  Other types of solar cells like polycrystalline cells have 90 degree square corners because of the way the silica are layered together offering greater design flexibility during manufacturing.  However, the monocrystalline cells have higher efficiency; requiring smaller cells with equivalent energy production compared to polycrystalline cells.

MONOFACIAL VS BIFACIIAL PV MODULES

A monofacial PV cell harvest sunlight on one light absorbing side of the PV module. Bifacial modules have PV cells on the front & back of the module; harvesting reflected sunlight from reflective surfaces.

In order for Bifacial modules to work effectively on the Tiny Off-Grid House the array will have to be elevated or angled away from the roof in order to allow reflected light to strike the back of the modules. Monofacial modules, are probably the best option for this research, since the array will be installed rooftop where it can maximize the roof tilt angle during sun peak hours.

The average Solar PV modules measures 41.0’’ W (3’ 4”) (1016mm) X 74.0’’ L (6’ 2”) (1835mm).  A minimum of eight 400 Watt modules are needed to adequately recharge the Tiny Off-Grid House Lithium Ion Phosphate battery bank.  To accommodate the 8 PV modules (27’ 4” W) on the roof of a minimum length of 28 feet of roof length is required.

How To Determine PV Array Demensions 41.0” W x 8 Modules = 328”s W Array 328” / 12” = 27.33 Ft (The length of the Tiny Off-Grid House will be 28 FT)

AMBIENT AIR TEMPERATURE IMPACT ON PV PRODUCTION:

Solar PV efficiency increases exponentially as the ambient air temperature lowers. The normal operating temperatures for solar PV modules are -4° to 122°F (-20° to 50°C).  The inverse happens in hotter weather which causes solar PV efficiency to decrease.  The elevated ambient heat excites electrons creating less room in the solar cell to absorb energy from the sun.  Since energy from the PV cell is produced from the difference between the two states of excited/at rest electrons (Voltage), the difference between at rest and excited electrons will be smaller; resulting in less energy produced since the electrons are already excited by hot ambient temperatures.  This may seem like the excitement of electrons caused by heat should generate energy in a PV cell but it does not; only the excitement of electrons caused from sunlight produces energy.  The Solar PV manufacturers provide the temperature coefficient (Tk) to determine the maximum PV voltage (TkVmp) range generated during high and low temperatures; under standard test conditions.

When selecting a solar module, besides the energy density of the module it is also important to choose one with a low Temperature Coefficient. The temperature coefficient is the difference between the resting and excited energy phases of the electrons. The greater the “potential difference” between the two phases the more PV energy is produced.

How To Determine Temperature Coefficient: Ambient air temperature is 32.2°C / 90°F 32.2°C / 90°F - 25°C / 77°F = 7.2°C / 13°F 7.2°C / 13°F x (-0.377%) = -2.71% / -4.90% less PV energy production; respectively

The color of the PV module does impact performance.  Modules with black frames and backgrounds will generate more heat reducing energy production, compared to a PV module with an Aluminum frame and white background.  The white background is a force multiplier by reflecting sunlight on to the PV cells. For aesthetic reasons, most modules are all black to create a uniformed appearance when on the roof. Skirting around a solar PV array creates a clean finish but prevents air flow under the modules that creates passive air cooling of the PV panels through convection.

SUN HOURS VS SUN PEAK HOURS:

Not all daylight hours are the same. Photovoltaic (PV) cells generate DC electricity from the sun photons “most efficiently” during daylight peak hours.  Peak sun hours is when the intensity of the Sun is 1,000 watts of photovoltaic power per square meter.  Also, it must be understood that PV’s modules do not generate electricity from the radiant heat of the Sun, but from Sun Protons.

The total hours the sun is in the sky, from sunrise to sunset, is the “full” sun hours which can be up to 12 hours during the summer months.  During—solar—noon time (true geographic south, not magnetic south) the sun is at its highest altitude in the sky with a solar “azimuth angle” of 180° east or west of true south.  This duration is considered “peak” sunlight hours which can be 7 out of the 12 hours of total sunlight in some regions or during some winter seasons. Solar noon is not determined by the clock—hand pointing at 12:PM—due to regional differences in time and daylight savings time.

How To Determine The KWh Of One 400 Watt PV Module Exposed To 8 Peak Hours, During 12 Hours Of Summer Sunlight 400 watts per module X 8 Peak summer hours daily = 3200 Watts  ÷ 1000* = 3.20 KWh per module daily (1000 watts x 1 hour = 1000 watts hour = 1 KWh) 3.20 KWh x Derate Factor electrical loss of 0.77 = 2.464 KWh per module daily (3.20 Kwh x 0.77 DF* =  2.464 KW) The total solar array should produce: 2.464 KWh X 8 modules = 19.712; R↑ 20.0 KWh’s daily.

*If the solar array performance is compromised from Derate Factors (NREL Derate Factor of 0.77) such as DC to AC power conversion, PV modules obscured with dust, grime or shading will lower the PV system energy production.

TILT ANGLE OF PV ARRAY:

The angle of the Tiny Off-Grid House roof is designed, brilliantly simple, to maximize the efficiency of the Solar Photovoltaic (PV) array stationary angle of inclination throughout the seasons.  The PV array will be installed on a 30° to 35° sloped roof that faces true south—in northern regions—and is able to harness enough sunlight even when the sun is at its lowest in winter. The 30° to 35°degrees sloped roof is at an angle that makes the array face almost 90° degrees to the angle of the sunlight capturing the maximum sunrays; perpendicular to the array even when the sun path is at its lowest arch during the winter months.  Also, this angle and pitch of the shed roof —to starboard— can facilitate snow and water removal naturally with gravity; along with the added benefit of avoiding low hanging tree limbs from hitting the Tiny Off-Grid House roof when driven on the right-side of a road. The solar angle is determined by the latitude +/-15° since this angle will harness the maximum sunlight during winter.  Land based pole mounted solar arrays have the ability to perform seasonal tilting; even hourly rotations to track the suns path.  During the winter season, the ideal tilt angle increases +15° latitude; conversely, during the summer season the ideal tilt angle decreases -15° latitude.  Seasonal tilting can increase PV module performance by as much as 30%.

In the northern hemisphere the positioning of the PV array should ideally face south at your location latitude plus 15°; since the northern exposure will be shaded.  The orientation will differ from northern (America) and southern (South Africa) hemispheres. Regardless of which hemisphere you are located your solar modules should face the equator.  Tall trees or other obstacles that can cause shading should be avoided because shading compromises the productivity of the PV array; especially if they are wired in series.  Positioning the PV array angle towards direct peak sunlight in winter will more efficiently create PV electricity during the months of least solar production.  However even indirect daylight will generate some power for the PV array.  On cloudy days enough sunlight still diffuses through the clouds to keep PV arrays productive; albeit decreased.

During the winter, snow surrounding a PV array can be a force multiplier increasing solar PV production due to the albedo effect caused by light color snow or ice reflecting sunlight (aka snow blindness) on to the solar PV array. Also cooler temperatures makes the solar array work more efficiently.

PV ARRAY WIRING:

There are three wiring methods of a PV array; each method, or combination of methods, depends on the desired PV performance of the array.

Series wiring connections are positive to negative. Wiring PV modules in series adds the voltages, however, the amperage remains the same. Higher voltage reduces the potential for voltage drop and reduces the wire gauge. Also, a series wiring connection increases the voltage during early morning and late evening low irradiance (Not to be confused with insolation/irradiation; the irradiance is the angle sunlight hits the module). If one of the modules is shaded, the entire series wired array amperage, not the voltage, will be reduced to the lowest module amperage output. This is why PV modules with different voltages and amperage should not be wired in to an array.

Parallel wiring connections are positive to positive and negative to negative. When wiring PV modules in parallel the voltage remains the same while the amperage are added. Parallel wiring mitigates the impact a shaded module would have on an array amperage. Due to an increase in amperage, thicker wires are required.

“Series / parallel” connections are wired first in series then the strings are all wired in parallel. Wiring the array in series / parallel adds each series strings voltage and adds each parallel strings amperage.  However, a series / parallel array can only be achieved with an even number of modules (2, 4, 6, 8 . . .)

The use of zip ties are necessary for wire management under the array. The zip ties must be listed as UV and inclement weather resistant.

Module Level Optimization Tracking:

Individual module optimization & monitoring can be accomplished through the use of Microinverters or DC Optimizers.

Solar modules convert sunlight in to DC power.  Microinverters, located behind each module, converts the DC power in to AC power (Household electricity).  This rooftop energy “conversations” comes at a price; voltage drop.   

DC Optimizers, located behind each module, provides individual module monitoring. The DC power, from the strings, is converted in to AC power at the String Inverter.  The string level DC power has its advantages when it comes to DC battery charging.

The DC Power produced by the solar array goes directly to charge the Tiny Off-Grid House DC battery bank (via MPPT Solar Charge Controller); with no DC to AC power conversions that could lead to voltage drops.

The PV array can not be wired directly to a battery bank; since the voltage fluctuates depending on availability of sunlight throughout the day. Batteries require a specific charging voltage. A solar charge controller is needed to regulate the voltage from the solar array to the battery bank.

There are two types of charge controllers Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT). The Tiny Off-Grid House will use the Victron Energy MPPT since MPPT works better with lithium batteries and at lower temperatures.

WHEN TO USE INLINE FUSES

The NEC 690.9 states inline fusing of the positive conductors of the solar array is necessary when the Short Circuit Current (Isc) of the array exceeds the Maximum Series Fuse Rating, then fuses must be used to match the Maximum Series Fuse Rating. The values can be found on the manufacturer label affixed to the back of each module.

However, if the Short Circuit Current is less than the Maximum Series Fuse Rating, then no fuses are required.

Unlike parallel wiring, fuses and MC4 connectors are not necessary in series wiring of the PV array.

Regardless, a fused or unfused solar array requires a solar disconnect.

The proactive use of fuses in the solar module / array when not required has no negative effects but no benefits either; with no return on investment, since a short-circuit event cannot exceed the maximum fused series rating of the module. Also, a well grounded solar array, with a ground electrical conductor, is the most effective way to deal with electrical surges (Lightening).

GROUNDING THE ARRAY

Each of the eight Aluminum framed conductive PV modules will be grounded with a continuous #4 AWG copper grounding electrical conductor (GEC). The GEC is bonded to each PV module using a tin plated Aluminum grounding lug (UL467) which has little resistance and offers resistance from corrosion. The grounding lugs are attached to each module frame using the manufacturer predrilled holes designed for this purpose. Each PV module frame is electrically isolated with an anodized coating. Grounding lugs are either designed with serrated washers or serrated set screws to penetrate this protective coating of the frame and enhance bonding. Mid clamps are installed between two modules securing the sides with serrated teeth that penetrates the anodized coating, bonding the two modules.

AERO DYNAMICS OF PV ARRAY:

Bernoulli’s Principle can create an area of low pressure as a result of faster air flow above the solar PV array and an area of slower high pressure underneath that has the potential to lift the array from fierce winds or during transport on the Tiny Off Grid House roof.  Proper securement of the solar PV array on to the metal roof is important. Since the solar modules will be secured directly on to the metal roof panels and not floating on rails, the modules will be attached to the standing seams securely with sufficient amounts of non-penetrating AceClamp A2® metal roof clamps that secures each solar module on to the vertical double-lock seams of the Tiny Off-Grid House metal roof. The use of AceClamps will enable the solar array to withstand the weight of ice and snow and the aerodynamic lift forces created from turbulent winds. Also, skirting on the bow side of the array can deflect airflow above the array preventing the array from behaving like a sail.

For more information on solar array metal roof clamps please read “Off-Grid Solar Array Mounting Clamp & Metal Roofing”

AGRIVOLTAICS:

Although the Tiny Off-Grid House will incorporate a rooftop solar array it is worth mentioning the emerging practices in solar ground mounted technology. Ground mounted solar PV arrays has been criticized for clearing and stealing arable land that could be used for agriculture.  This criticism is astounding considering that forest are mainly burned and cleared for cattle ranching; which is the leading cause of deforestation.  However, ground mounted solar PV arrays can coexist along with agriculture and livestock. This dual purpose use of land provides a symbiotic relationship for energy production and agriculture; giving family owned and cooperative farmers another sustainable revenue stream. Through transpiration, the plants below keep the array cool boosting power production. Already on agrivoltaic farms livestock are grazing on pasture growing under elevated ground mounted solar arrays. Plants benefit from the shade of the array and are shielded from heavy rains, hail and sun stress. Plants do have a limit on the amount of sunshine they can metabolize known as light saturation point. Once a plant reaches this maximum rate of photosynthesis the plant starts evaporating water to cool itself. Agrivoltaics reduce the amount of water evaporation from plants reducing water loss and agricultural water usage.

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