Sunny Days for Silicon: The Rise of Solar Energy olar energy — the conversion of sunlight into electricity or heat — is the most abundant energy source available on Earth. The sun delivers more energy to the planet's surface in one hour than the entire human civilisation consumes in a year. For most of the twentieth century, the technology to harness this resource at scale was either unavailable or prohibitively expensive. The opening decades of the twenty-first century have seen a transformation so rapid and so profound that solar power has moved from the margins of the energy economy to its centre, threatening to displace fossil fuels as the primary source of electrical power within a generation. The dominant technology for converting sunlight into electricity is the photovoltaic (PV) cell, first developed in the 1950s at Bell Laboratories in the United States. A PV cell consists of a layer of semiconductor material — typically silicon — through which electrons flow when struck by photons of light, generating a direct electrical current. Individual PV cells are assembled into modules, and modules are combined into arrays to produce the solar panels familiar from rooftops and solar farms. The efficiency of a solar cell — the proportion of incoming light energy that is converted to electricity — has increased dramatically through successive generations of research and development. Commercial silicon panels available today achieve efficiencies of between 20 and 24 percent, compared with around 6 percent for the first practical solar cells of the 1950s. The cost of solar electricity has fallen faster than almost any other energy technology in history. Between 2010 and 2020, the cost of electricity from utility-scale solar installations fell by approximately 89 percent. This decline was driven by improvements in manufacturing efficiency, scaling of production, and learning effects as the cumulative quantity of solar panels installed grew. In many parts of the world, new solar power is now the cheapest source of electricity ever built. The cost trajectory continues downward, and analysts project that solar will become even more dominant as efficiencies improve and supply chains mature. The rapid growth of solar power has not been without challenges. The intermittency of solar generation — the fact that panels produce electricity only when the sun shines — creates difficulties for electricity grids that must balance supply and demand in real time. The geographic concentration of the best solar resources in deserts and low- latitude regions means that the benefits of solar power are not evenly distributed, and that capturing them may require the construction of long-distance transmission infrastructure to deliver power to population centres far from generation sites. The
integration of large quantities of variable renewable generation into existing grid infrastructure requires new approaches to system management, storage, and demand flexibility. Battery storage is increasingly seen as the key to unlocking solar's full potential. Lithium-ion batteries, the same technology used in electric vehicles, can store surplus solar electricity generated during the day for use in the evening and overnight. The cost of battery storage has fallen almost as rapidly as the cost of solar panels, and paired solar-plus-storage systems are becoming competitive with other sources of firm, dispatchable power. Longer-duration storage solutions — capable of storing energy for days or weeks rather than hours — are an active area of research and development, with technologies including flow batteries, compressed air storage, and green hydrogen production being explored. Beyond electricity generation, solar thermal technologies use sunlight to produce heat for industrial processes, space heating, and water heating. Concentrating solar power systems use mirrors or lenses to focus sunlight onto a small area, heating a working fluid that drives a conventional turbine. While solar thermal technologies have been overshadowed commercially by the dramatic cost reductions in photovoltaics, they retain advantages in applications where high-temperature heat rather than electricity is required, and in contexts where thermal storage can provide dispatchability more cheaply than battery storage.