Đề thi thử số 18 — Reading 40 câu (có bài đọc)

Solar power has undergone a remarkable transformation over the past two decades, evolving from a niche technology used primarily for satellites and remote applications to one of the most competitive sources of electricity in many parts of the world. This transformation has been driven by dramatic cost reductions, supportive government policies, growing concerns about climate change, and significant technological improvements. In 2023, solar photovoltaic (PV) systems generated more than 5 percent of global electricity, a figure that is projected to grow significantly in the coming years.

The dramatic decline in solar PV costs is perhaps the most striking aspect of the solar revolution. The cost of solar panels has fallen by approximately 99 percent since 1976, and by about 90 percent since 2010 alone. This unprecedented cost reduction—often referred to as "Swanson's Law," which states that the price of solar panels tends to drop 20 percent for every doubling of cumulative shipped volume—has been driven by manufacturing scale, technological improvements, and intense competition. As a result, solar power is now the cheapest source of new electricity generation in most countries, and in some places, it is even cheaper than operating existing coal or gas plants.

Beyond cost reductions, solar technology has improved significantly. Modern solar panels convert about 20 to 22 percent of the sunlight that hits them into electricity, compared to about 15 percent for panels made just a decade ago. New technologies on the horizon, including perovskite cells and tandem cells, promise even higher efficiencies and lower costs. Perovskite cells, made from abundant and inexpensive materials, can be manufactured using simple processes and even made into flexible, transparent films that could turn windows and building facades into power generators. Tandem cells stack different materials to capture more of the solar spectrum, with laboratory efficiencies now exceeding 30 percent.

The rapid growth of solar has important implications for the broader energy system. As solar becomes a larger share of electricity generation, the intermittent nature of sunlight—producing power only during the day and being affected by clouds—creates challenges for grid stability. Solutions are emerging, including battery storage, smart grids, demand response, and the integration of solar with other renewable sources like wind, which often produces more power at night. Solar is also being deployed in new and creative ways: floating solar farms on reservoirs, agrivoltaics combining agriculture with solar generation, and building-integrated photovoltaics that replace conventional building materials. As technology continues to improve and costs continue to fall, solar power is poised to play an even larger role in the global energy mix.

Humans have been harnessing the wind for thousands of years—from sailing ships to windmills that ground grain and pumped water. The modern wind energy industry, however, is a much more recent phenomenon, with the first utility-scale wind turbines appearing in the 1980s. Since then, wind power has grown into one of the most important sources of renewable energy worldwide, with global wind power capacity increasing more than fifteen-fold over the past two decades. Modern wind turbines are engineering marvels, with blades that can be longer than a football field and nacelles that house sophisticated control systems.

Onshore wind power, installed on land, has been the dominant form of wind energy to date. Onshore wind is now cost-competitive with or cheaper than fossil fuel generation in many markets, making it an attractive option for new electricity capacity. Countries like Denmark now generate more than 50 percent of their electricity from wind, while many other countries have set ambitious targets for wind power expansion. However, onshore wind has limitations. Suitable sites with strong, consistent winds are limited, and many of the best locations have already been developed. Local opposition, often related to visual impact, noise, and effects on wildlife, can also slow deployment.

Offshore wind, installed in the ocean, is rapidly emerging as the next frontier of wind energy. Offshore wind resources are typically stronger and more consistent than onshore, allowing for larger turbines and higher capacity factors. The world's largest offshore wind farms, with capacities exceeding 1,000 megawatts, can power hundreds of thousands of homes. Modern offshore turbines, with capacities of 15 megawatts or more, dwarf their onshore counterparts. Floating offshore wind technology is opening up new possibilities, allowing wind farms to be sited in deep waters previously inaccessible to traditional fixed-bottom turbines. This dramatically expands the potential of offshore wind, as most of the world's best wind resources are in deep waters far from shore.

Despite its many advantages, wind power faces challenges. The intermittency of wind—calm periods when little power is generated—requires backup or storage capacity. Manufacturing and installing large wind turbines involves significant use of resources, including steel, concrete, and rare earth metals for permanent magnets in some designs. There are concerns about impacts on birds and bats, though careful site selection and modern turbine designs are reducing these effects. Some communities have raised objections to wind farms based on aesthetics, noise, or property values. Despite these challenges, wind power is likely to remain a critical component of the global transition to clean energy, with continued cost reductions and technological improvements making it increasingly competitive and accessible.

Energy storage is widely regarded as the holy grail of the clean energy transition. While renewable energy sources like solar and wind are now cost-competitive with fossil fuels in many markets, their intermittent nature—producing power only when the sun shines or the wind blows—poses a fundamental challenge to grid stability. Energy storage systems can capture excess energy when production exceeds demand and release it when production falls short, effectively turning intermittent renewables into reliable, dispatchable power sources that can meet electricity needs around the clock.

Battery storage, particularly lithium-ion batteries, has seen the most dramatic cost reductions and deployment in recent years. The cost of lithium-ion battery packs has fallen by approximately 90 percent since 2010, driven by the same factors that have reduced solar and wind costs: manufacturing scale, technological improvements, and intense competition. Large-scale battery installations, sometimes called "battery farms," are increasingly common, with some projects exceeding hundreds of megawatt-hours of storage capacity. These systems can provide grid services like frequency regulation, peak shaving, and backup power, while also enabling higher penetration of renewable energy.

Beyond lithium-ion, a range of other storage technologies is being developed and deployed. Flow batteries, which store energy in liquid electrolytes, offer the advantage of being easily scalable by simply adding more liquid. They are particularly well-suited to long-duration storage of 8 to 12 hours or more. Pumped hydro storage, which uses excess electricity to pump water uphill to a reservoir and then releases it through turbines when needed, is the most established form of grid-scale storage, accounting for over 90 percent of global storage capacity. Compressed air energy storage, thermal storage, and even gravity-based storage (using weights or rail cars) are among the other technologies being explored.

The future of energy storage looks increasingly bright. Costs continue to fall, new chemistries are being developed that promise even better performance, and innovative business models are emerging. Some companies are exploring "virtual power plants," which aggregate thousands of small batteries (like those in electric vehicles or home solar systems) to provide grid services. As renewable energy continues to grow and the need for storage becomes more pressing, energy storage will play an increasingly central role in the clean energy transition, helping to ensure that the sun and wind can power our world 24 hours a day, seven days a week.

Hydrogen has long been touted as the fuel of the future. When burned or used in a fuel cell, it produces only water as a byproduct, making it a potentially clean alternative to fossil fuels in sectors that are difficult to electrify, such as steel production, cement manufacturing, heavy shipping, and aviation. A global "hydrogen economy" could, in theory, replace the fossil fuel economy in these hard-to-abate sectors, dramatically reducing greenhouse gas emissions while providing the energy and materials modern society needs.

However, not all hydrogen is created equal. Today, approximately 95 percent of hydrogen is produced from natural gas in a process called steam methane reforming (SMR), which releases carbon dioxide as a byproduct. This hydrogen is known as "grey hydrogen" because of its associated carbon emissions. Capturing and storing the CO2 produced in SMR can create "blue hydrogen," which has lower but still significant emissions. The truly clean option is "green hydrogen," which is produced by splitting water into hydrogen and oxygen using electricity from renewable sources—a process called electrolysis. Green hydrogen has no carbon emissions associated with its production, making it a truly clean fuel.

The promise of green hydrogen is significant, but so are the challenges. Electrolysis is currently more expensive than SMR, requiring either technological breakthroughs or policy support to become cost-competitive. Green hydrogen is also less energy-efficient than direct electrification—using renewable electricity to make hydrogen, then using the hydrogen in a fuel cell, loses some energy at each step. For applications where direct electrification is possible—such as most road vehicles, home heating, and many industrial processes—batteries and direct electric power are typically more efficient. Hydrogen makes the most sense in sectors where direct electrification is difficult or impossible.

Despite these challenges, momentum behind green hydrogen is growing. The European Union has launched a hydrogen strategy aiming for 10 million tons of renewable hydrogen production by 2030. Countries including Japan, South Korea, Australia, and the United States have announced major hydrogen initiatives. Costs are falling as electrolyzer technology improves and production scales up. Pilot projects are demonstrating the use of green hydrogen in steel production, shipping fuel, and other applications. While green hydrogen is unlikely to be a panacea for all climate challenges, it is likely to play an important role in decarbonizing sectors that are otherwise difficult to address, contributing to a more sustainable energy future.