Photovoltaic system

• 1. Introduction

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  Photovoltaic systems produce electricity from sunlight without a single moving part, without combustion, and without a sound. Right now, they sit on rooftops and stretch across desert fields, quietly converting photons into the electricity that powers homes, satellites, electric aircraft, and the occasional wristwatch. Germany alone has installed 1.4 million of them. The United States has another 440,000. And the global installed capacity, which stood at roughly 200 gigawatts as of 2015, was about 40 times what existed in 2006.

  How does a system made primarily of glass and silicon accomplish all of this? What happens inside a solar cell when light strikes it? Why does a single shadow falling across a small corner of a panel cascade into a dramatic drop in output? And what is standing between a PV system and every home on earth? Those are the questions this documentary sets out to answer.

• 2. From Photon to Plug

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  At the heart of every photovoltaic system is the solar cell, a device that converts photons directly into electricity. A single cell typically produces only about 0.5 volts, far too little to power anything useful on its own. Manufacturers wire many cells together in series inside a weatherproof laminate, forming a module. That module consists of tempered glass on the front, a flexible encapsulant, a fire-resistant rear backsheet, and an aluminium frame.

  Typical panel ratings range from under 100 watts to over 400 watts, and a standard 150-watt module is roughly a square meter in size. Multiple modules wired in series form a string, and multiple strings wired in parallel form an array. The array produces direct current, but the electricity grid, and nearly every appliance in a home, runs on alternating current. That gap is bridged by an inverter.

  As of 2019, the best conversion efficiency for state-of-the-art inverters reached more than 98 percent. String inverters dominate the residential and medium commercial market with about 52 percent market share, while central inverters cover the large commercial and utility-scale segment at around 44 percent. Micro-inverters, which attach to individual panels, hold less than 2 percent of the market but offer panel-level monitoring as a built-in feature.

• 3. The Shadow Problem

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  Photovoltaic cell output is extremely sensitive to shading, a behavior engineers sometimes call the Christmas light effect. When even a small portion of a cell or module falls into shadow while the rest remains in sunlight, the consequences ripple far beyond that shaded spot.

  Here is why. When cells are wired in series, the current through the entire string is limited to whatever the weakest cell can carry. A shaded cell, instead of contributing its normal 0.5 volts, may actually drop 8 volts, absorbing the power produced by 16 other cells in the string. The shaded cell converts that absorbed energy into heat rather than electricity, creating what engineers call a hot spot.

  Bypass diodes, placed between cells or groups of cells, serve as a pressure valve for this problem. When a cell is shaded, current reroutes through the bypass diode, limiting the damage to only the output that the shaded section would have provided. The bypass diode's primary job is preventing hot spots from causing further damage and, in extreme cases, fires.

  Sunlight blocked by dust, dirt, snow, or biological growth, collectively called soiling, causes a related but more gradual problem. In 2018, the global annual energy loss from soiling was estimated at least 3-4 percent, with an estimated revenue loss of between 5 and 7 billion euros worldwide. In one study from Ontario, cleaning flat-mounted solar panels after 15 months increased their output by almost 100 percent. Panels tilted at just 5 degrees, however, were adequately cleaned by rainwater alone.

• 4. Tracking the Sun

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  A fixed solar panel is a compromise. It points in one direction, typically south in the northern hemisphere, and captures whatever sunlight falls on it at varying angles throughout the day. A tracking system changes that equation by tilting the panel to follow the sun's path.

  Single-axis trackers increase total power production by about 20-25 percent compared to fixed-mounted systems. Dual-axis trackers, which adjust for both the daily arc and the seasonal change in the sun's elevation, can push that gain to 30 percent or more. In some configurations, sensors and optimizers can increase viable output by up to 45 percent.

  Trackers have their limits, though. In diffuse light, such as under cloud or fog, tracking offers little or no benefit because the sunlight is scattered rather than directional. For very large installations, the ongoing maintenance cost of tracking systems becomes a substantial drawback. Pricing trends continuously shift the calculus between adding more stationary panels and using fewer tracking ones. According to data from WoodMackenzie and GTM Research, global solar tracker shipments hit a record 14.5 gigawatts in 2017, representing growth of 32 percent year-over-year.

• 5. Batteries and the Grid

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  Most PV systems built today send their surplus power into the public electricity grid rather than storing it locally. About 99 percent of European solar systems and 90 percent of US systems are grid-connected. When excess power flows outward through a customer's meter, policies like net metering and feed-in tariffs allow households to receive credit or payment.

  Battery storage is still the exception rather than the rule, but the economics are shifting. Lead-acid batteries have long dominated small residential PV systems because of their reliability, low self-discharge rate, and comparatively modest cost. As of 2014, lithium-ion alternatives were about 3.5 times as expensive. In 2015, Tesla Motors launched the Powerwall, a rechargeable lithium-ion battery aimed at the residential market.

  Grids that absorb large amounts of solar power face their own structural challenge. The electricity network was originally designed for one-way flow, from generator to customer. When rooftop systems generate more than a household consumes, electricity flows the other way and can push voltage outside the acceptable range of plus or minus 5 percent. In Queensland, Australia, more than 30 percent of households had rooftop PV by the end of 2017, making this a live operational issue rather than a theoretical one.

  For isolated communities, hybrid systems offer a different solution. On Pellworm island in Germany and Kythnos island in Greece, PV is combined with wind generation. The Kythnos installation reduced diesel consumption by 11.2 percent.

• 6. Cost and Scale

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  For large-scale installations, prices below $1.00 per watt were common by 2012. In Germany, system costs fell to USD 1,389 per kilowatt-peak by the end of 2014. A 50 percent price decrease was recorded in Europe between 2006 and 2011 alone. The cost of electricity generated by PV systems falls by 32.6 percent for every doubling of cumulative installed capacity, a pattern known as the learning curve.

  Modules and panels now account for less than half of a PV system's overall cost. The remainder belongs to what the industry calls the balance of system, which covers inverters, mounting hardware, cabling, and soft costs such as permitting, inspection, labor, and customer acquisition.

  About 70 percent of all solar cells and modules worldwide are produced in China and Taiwan. European and US manufacturers together account for only about 5 percent of global production. Crystalline silicon remains the dominant material, used in 90 percent of modules worldwide, while thin-film technologies have been losing market share.

  The energy conversion efficiency of a conventional solar module climbed from 15 to 20 percent between 2004 and the date the source was written. A group at the Fraunhofer Institute for Solar Energy Systems achieved a laboratory cell efficiency of 44.7 percent by concentrating light to the equivalent of 297 suns. The International Energy Agency projected that solar power could become the world's largest source of electricity by 2050, with photovoltaics contributing 16 percent of global demand.

• 7. Where PV Goes Next

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  Grid parity, the point at which generating solar electricity costs no more than buying it from the grid, had been reached in about 30 countries at the time the source was written. Spain offers a cautionary counterpoint: despite generating around 40 percent of its electricity from photovoltaic and other renewables, and despite cities like Huelva and Seville recording nearly 3,000 hours of sunshine per year, the country imposed a solar tax in 2013 on owners who remained off-grid, with fines reaching up to 30 million euros. That legislation was ultimately repealed in 2018.

  The physical potential of solar power is staggering when stated plainly. The Sahara desert covers more than 9 million square kilometers. Covering just 1 percent of that area, about 90,600 square kilometers, with PV panels could theoretically generate as much electricity as all of the world's power plants combined.

  A PV system recoups the energy used to manufacture it in roughly 2 years under typical conditions. In exceptionally sunny locations, or when thin-film technology is used, that payback period drops to one year or less. Given that module lifespans typically run to 25 years or more, a well-sited system produces clean electricity for two decades beyond the point where it has repaid its own energy debt.