To decarbonize electricity, solar panels must be spread over large areas of the globe. The United States could need solar panels covering 61,000 kilometres by 2050 -- a larger area than the Netherlands. Japan and South Korea, which are land-scarce countries, might need to dedicate 5% of their land for solar farms.
It is not an easy decision to decide where to place these panels. There is intense competition for land, which is also required for food production and biodiversity conservation. One solution is floating solar panels (or 'floatovoltaics) that can be installed on reservoirs.
The concept of floating solar panels is promising. There has been an increase in investments and installations. There are many questions about the technology's impact on the environment, as well as its economic, social and technical dimensions.
These knowledge gaps must be filled quickly to avoid overstating the benefits or having it stalled by unforeseeable roadblocks.
Space-intensive solar power requires at least 20 times the area of conventional fossil-fuel plants in order to produce one gigawatt (GW). There are many locations that could be used for large installations. Each environment has its pros and cons.
The deserts are blessed with plenty of sunshine and have little competition for land use. There are always trade-offs. Modeling shows that large areas of dark solar panels could alter the local temperature and airflow patterns in the Sahara. This could lead to droughts in the Amazon and sea-ice loss at the Arctic. The Mojave Desert, in the US southwest, has seen a reduction in the number of cacti which are culturally significant to Native Americans. It can also be difficult to transport energy from desert areas to the places it is required.
Although agricultural fields offer another possibility, researchers are still trying to figure out how the combination of solar panels and crops in 'agrivoltaic systems will impact food production6. Although rooftops, car parks, and highways are all viable options, they are not large enough.
Solar arrays placed on reservoirs can have many benefits. The arrays consist of conventional solar panels that are mounted on floats and anchored by mooring lines. They are 5% more efficient than land-based panels because they are closer to water. The arrays can shield the surface from the sunlight and reduce evaporation. They also retain water for hydropower and irrigation8. The grid infrastructure that carries electricity to consumers is already in place for hydropower reservoirs, which reduces transmission costs. Combining solar and pumped-storage hydropower could solve the twin challenges of producing energy when it is darkest and storing it in reservoirs for when it is bright.
Some hydropower operations may also have a lower carbon intensity (emissions per unit of energy produced). Floatovoltaics could help to reduce them. Many hydropower plants are just as low-carbon than other renewables. Some projects can release as much methane, a powerful greenhouse gas, from submerged plant matter. Solar panels placed on less than 2% of the reservoir's surface can double the production of electricity, which could reduce the carbon intensity, which is an important indicator in climate policy (see Supplementary Information).
Flotovoltaics are a small part of the electricity market. The global installed capacity for floating solar panels stood at 3 GW11 as of 2020. This compares to more than 700 GW of land-based solar systems. The potential for expansion is great, considering the large number of reservoirs around the world -- roughly equal to France's total area. Covering 10% of the world's hydropower reservoirs with floating solar panels would install nearly 4,000 GW of solar capacity9 -- equivalent to the electricity-generation capacity of all fossil-fuel plants in operation worldwide.
Floatovoltaics cost more than land-based, but this is not a major problem. Despite the new market's immaturity, the break-even price of floating solar projects is only 4-88% lower than ground-mounted solar power13. There are many projects underway and the market is expanding fast14. One project, which is expected to be completed in 2024 in Batam (Indonesia), will produce 2.2GW using solar panels that cover 16 km2 of water. This will nearly double global floatovoltaic power production.
Rapid scaling up of any new technology can lead to unanticipated consequences. For example, wind turbines have been known to cause harm to birds and bats. Their installation offshore can also create noise pollution that could affect marine life and interfere with whale migrations.
In both theory and practice, the trade-offs between expanding floatovoltaics as well as environmental, social, and economic goals are largely unknown. Reservoirs, artificial ecosystems, have been criticized for having a variety of negative socio-environmental effects. They can be used to provide habitat for wildlife and a vital role in recreation and fisheries. Many reservoir management functions serve multiple purposes, including hydropower and flood control. Climate change will increase the pressure on reservoirs for multiple purposes.
These trade-offs can increase public opposition to floatingovoltaics, prolong the environmental-impact approval procedure, and deter private investment, thereby limiting the shift towards decarbonization.
We compared the solar-power capacity of large reservoirs to the projected national demand of extra solar energy by 2050 in order to understand the potential for floatovoltaics (see "Floatovoltaic Potential" and Supplementary Information).
Brazil's National Electric Energy Agency passed a resolution in November 2021 regulating joint operation of hydropower plants and floatovoltaic units. Each source is measured separately. This decision should increase the likelihood of Brazilian hydropower producers installing floatovoltaics.
Brazil's hydropower plants receive a remuneration based on their share of national hydropower production and not the amount of energy they produce. This system is known as the Energy Relocation Mechanism and was created to protect against hydrological risks. It also means that any plant installing floating-voltaics in the country would share its profits with all other hydropower producers. These energy markets are now split by the new resolution.
Brazil's Congress passed Federal Law 14300 in 2022. This law allows small-scale floatovoltaics in the decentralized market. To maximize sustainability and to gain investor confidence, specific environmental regulations will still be required.
However, it will be more difficult for island nations, much of Europe, and the Middle East, to use floatovoltaics in order to meet their needs due to smaller reservoirs in small or arid countries and weaker sunlight at higher latitudes. Some industrial countries, such as China, will have a high demand for solar power by the middle of the century. They will need solar on land, and not just floatovoltaics to cover all their reservoirs.
In areas where floatovoltaics are feasible, the question is how much reservoirs could be covered before the downsides outweigh any benefits. Additional coverage can increase logistical problems, social disruptions and adverse environmental effects. Even with 2% coverage, a 300-kilogram reservoir of this size would still leave a huge footprint. It would cover about one-tenth the area of India's 2,245 megawatt Bhadla Solar Park, the largest land-based solar farm in the world.
Global warming is causing water bodies to warm, which can lead to harmful algal blooms15. These effects can be countered by floatovoltaics in lakes and reservoirs. However, they are only effective when the water surface is more than half covered16. There is more to learn about the effects on drinking water quality, drinking water quality, aquatic biota and terrestrial wildlife, as well as downstream ecosystems.
A large amount of a reservoir can be shady, which could cause cascading effects. In nutrient-polluted areas where harmful algal blooms proliferate, reduced light can make it more difficult for photosynthetic organisms like aquatic plants and phytoplankton. The reduced oxygen production could cause harm to fish and other animals. Extreme oxygen depletion could favor methane-producing bacteria which could offset the decarbonization benefits. These effects are likely to be minimal if solar-panel coverage falls below 50%. It is not yet known how severe any one type of impact will be or how impacts will vary depending on latitude, water quality, and other factors.
To evaluate the ecosystems' response to floatovoltaic, large-scale field studies will be necessary. While there have been several test sites, including the Tengeh Reservoir in Singapore, research tends to be more focused on engineering feasibility rather than ecology.
Flotillovoltaics could also compromise the socio-economic use of reservoirs. A project that interferes with fisheries could cause a disruption in the livelihoods of people already affected by reservoir construction. Lake Kariba, bordering Zimbabwe and Zambia, is the home of the most productive reservoir fishery in the world. The artisanal fishing gear that was used to harvest sardines would not be able to be deployed near large floatovoltaic arrays. It is difficult to predict how shading will affect the food webs that support fisheries.
Floating solar panels could cause damage to the environment and reduce recreational use of reservoirs. This will lead to a drop in property prices. Floatovoltaic developers may face resistance from landowners nearby. These concerns should be analyzed by social scientists who will then devise the conditions that would allow floatovoltaic to be accepted by the public.
Developers could face technical challenges that could lead to higher costs. The potential for biofouling panels with bird faeces or microbial biofilms could be more difficult over water than on land and reduce photovoltaic output. It is possible that panels will need to be cleaned frequently. Accessing them safely and easily is essential.
Another factor is inclement weather. High latitudes can lead to unpredictable forces and large operational and maintenance problems. This is what happened with the floatovoltaic plant in Heilongjiang, northeast China. High winds can cause waves and damage in areas affected by tropical cyclones. For example, in 2019, a typhoon struck a Yamakura Dam floatovoltaic project in Japan. The panels were blown up by the wind and lit a fire.
Variable water levels can be a problem. After years of droughts in the Southwest United States, lakes Powell and Mead are now more than 40m below their full levels. Climate change could exacerbate these fluctuations. The need to ensure stable water levels for floating photovoltaics would likely be overridden by the drinking-water supply and irrigation requirements. It is important to develop engineering solutions and include them in project costs.
These plans must be made public to allow industry and society to move quickly towards mutually acceptable practices, and a realistic assessment on floatovoltaic capacity. Investor confidence would be increased by transparency. Researchers need to calculate costs based on the site conditions and then document the results after large-scale projects are live.
Empirical field-based studies are required in addition to modeling. It would be unrealistic to assume that power companies could do all this work. Access to independent researchers and monitoring for the long-term should be required in licensing agreements. As regulators and the industry develop guidance on best practices, it will be crucial that lessons are shared.
The floatovoltaic sector is expected to grow rapidly. To ensure the sustainable and equitable use of the world's resources, science and policy must work together quickly.