How To Maximize Solar Output Where The Sun Hardly Shines

Electricity from solar photovoltaics (PVs) is the fastest-growing source of new electric power worldwide. The growth is due to the dramatic cost decrease of PV over the last several years and the surging demand for clean, renewable energy.

However, the transition from fossil fuels to a completely clean, renewable energy economy requires an enormous expansion of PV and other renewable energy sources beyond what has been installed to date. The benefits of such an expansion are to eliminate 4-7 million deaths per year worldwide arising from fossil fuel and biofuel air pollution, to avoid catastrophic global warming, and to avoid social and political instability arising from the gradual depletion of fossil fuels.

The expansion of solar PV in countries in the tropics and subtropics, which are exposed to a lot of direct sunlight, makes sense to most people. However, the expansion in higher latitudes, such as Canada, Northern Europe, Greenland, Iceland, Northern Russia, and Alaska, has met with more skepticism. The reason is that higher latitudes receive less direct overhead sunlight during the year than do lower latitudes. However, by tilting PV panels or designing them to track the sun over time, solar PV panels can receive far more sunlight than if they lie flat on the ground.

The main purpose of this study, published earlier this year in the journal, Solar Energy, was to quantify, worldwide, the enhancement in solar PV output with tilted and tracked panels relative to horizontal panels. Almost all solar PV panels installed on rooftops are and will continue in the future to have a fixed tilt angle, thus they will not rotate. Panels in large solar PV power plants will have either fixed tilt angles or rotate. Another purpose of the study was to calculate the ideal, or optimal, tilt angle of fixed-tilt panels in each country of the world.

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Optimal tilt angles were derived from the National Renewable Energy Laboratory’s PVWatts solar tracking program for each country. For large countries, optimal tilt angles in different parts of the country were calculated.

Optimal tilt angles were then used as input into the global three-dimensional climate computer model, GATOR-GCMOM (Gas, Aerosol, Transport, Radiation, General-Circulation, Mesoscale, and Ocean Model) to estimate the incident sunlight hitting optimally tilted panels, 1-axis vertically tracked panels (fixed in the east-west direction but swiveling south-to-north around a horizontal axis), 1-axis horizontally tracked panels (fixed in the south-north direction at an optimal tilt angle but swiveling east-to-west around a vertical axis), and 2-axis tracked panels (tracking the sun exactly in all directions).

The incident radiation in each case was then compared with that to a flat, horizontal panel on the surface of the Earth. Globally- and annually-averaged in 2050, the ratios were ~1.19, ~1.22, ~1.35, and ~1.39, respectively. In other words, for example, optimally tilted panels received an average of 19% more sunlight than did horizontal panels. Panels that tracked the sun perfectly (2-axis) received 39% more sunlight than did horizontal panels. However, at high latitudes, the ratios were much higher. For example, at 50° North (near Brussels, Prague, Kiev, and Winnipeg), they were all in the range 1.3-1.5. At 60° North (near Helsinki, Oslo, and Anchorage), they were 1.4-1.6. At 70° North (near Nuorgam, Alta, and Hammerfest), they were 1.5-1.8. At 80° North (near Eureka and Nord), they were 2.1-2.4.

The study also found that 1-axis horizontal tracked panels received only 1-3% less sunlight than did 2-axis tracked panels at most all latitudes. As such, using 2-axis tracking, which is more complicated, is not necessary in most places.

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Further, 1-axis horizontal tracking provided much greater output than did 1-axis vertical tracking below 65o N and S, whereas output was similar elsewhere. Tracking of any kind provided little benefit over optimal tilting above 75o N and 60o S. For reference, Iceland, whose southern border is the furthest north of any country in the world, centers at 65 oN.

The benefits of both tilting and tracking generally increase with increasing latitude. In fact, surprisingly, annually averaged, more sunlight reaches tilted or tracked panels from 80-90o S (over the South Pole) than over any other latitude.

Another finding was that tilting and tracking benefit more the cities at a given latitude with less aerosol and cloud cover than they do cities with more aerosol and cloud cover at that latitude.

In sum, for optimal utility PV output, 1-axis horizontal tracking is recommended, except at the highest latitudes, where optimal tilting is sufficient. However, decisions about panel configuration also require knowing tracking equipment and land costs, which were not evaluated in this study. Installers should also calculate optimal tilt angles for their location for more accuracy. Models that ignore optimal tilting for rooftop PV and utility PV tracking may underestimate significantly country or world PV potential.

The major result of this study is that there is an enormous potential for solar PV output, even in high latitudes, where most people think the sun hardly shines. By capturing sunlight directly when the sun is low, even close to the horizon, it is possible to enhance substantially solar PV output, making PV viable most everywhere in the world. This is good news for future life on Earth.

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These findings are described in the article entitled World estimates of PV optimal tilt angles and ratios of sunlight incident upon tilted and tracked PV panels relative to horizontal panels, recently published in the journal Solar Energy. This work was conducted by Mark Z. Jacobson and Vijaysinh Jadhav from Stanford University.

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