Reflective reservoir covers may be cost effective in hot, dry places
Jeff Haley
September 16, 2023

Using a newly available estimate of damages from global warming and a measurement of Earth’s energy imbalance, the economic value of placing solar reflective covers over water reservoirs to cool the planet, reduce evaporation and reduce maintenance costs can be determined.  In places where the economic loss from evaporation is high and sun is intense, the value of the benefits likely exceeds the cost.  

In 2020,Stefania Bonafoni and Aliihsan Sekertekin demonstrated that albedo (solar reflectivity) of surfaces with a minimum dimension of about 30 meters can be determined using data from satellite images.  In 2022, Smoliak et al. showed that the potential amount of sunlight that could theoretically be reflected by the surface back to space from any location by a reflector that sends reflections in all upward directions can be estimated using model data.  Thus, we can calculate the increased number of megawatts reflected to space by each reservoir cover compared to what would otherwise be there averaged over one year -- the number of Reflected Megawatt Years (ReMYs).  

In 2023, James E Hansen et al. reported that Earth’s mean energy imbalance was 587 million megawatts.  Ideally, we would reflect to space enough solar energy to bring this imbalance to zero and an additional 10% or so beyond achieving balance so the Earth begins to cool, a total of about 650 million megawatts.  van der Wijst et al.(2023) estimated that projected additional warming would likely cost the world economy about 0.5% of its gross domestic product in 2025.  If, by 2025 we could spend less than 0.5% of annual world GDP and thereby send to space 650 million megawatts, that would be cost-beneficial, considering only the economic benefits without also considering other damages and risks such as biodiversity losses, health, and tipping points.

The World GDP in 2025 is projected to be about 116 trillion US dollars.  Thus, reflectivity interventions that reflect 650 million megawatts to space would provide an average benefit in 2025 of US$780 per Reflected Megawatt Year (ReMY).

Estimated economic value of reservoir covers.

Water deeper than a few meters is highly absorptive of solar energy, typically 5% to 7% reflectivity at the latitudes of interest.  The most reflective mass produced man-made materials reflect 92% of solar energy while transmitting 4.9% and will have a higher reflectivity if made thicker.  Reservoir covers can achieve 85% to 89% reflectivity for an 80% gain.

Many water reservoirs and canals can benefit from covers to reduce evaporation, minimize unwanted algae blooms, reduce salinity, and reduce maintenance.  By itself, this is cost-effective for only a few reservoirs or canals.  Subsidizing placement and maintenance of covers that also reflect solar energy to space that would otherwise be absorbed will make it cost-effective to put such covers on many more reservoirs and canals.  Covering reservoirs with floating solar photovoltaic panels may be a better use for many reservoirs near urban areas or hydropower dams, but even if all these opportunities are pursued, there will still be large areas where a reflective cover is the best option.

The Egyptian government has been studying the feasibility of covering 2000 square mile Lake Nasser to reduce evaporation.  Covering one square kilometer of the lake, which has a reflectivity potential of 193 watts per square meter, for a reflectivity gain from about 6% to about 86%, would reflect about 154 megawatts.  The cover will have a limited useful life.  As a rough guess, we assume the average benefit after the first year will be 11 times the first year.  At $780 per ReMY, multiplied by 12 and by 154, the value to the planet of one square kilometer of cover would be about $1.5 million.  This is likely a large portion of the initial cost plus lifetime maintenance cost of the cover, making it  likely cost-effective if we also consider the additional local benefits of reduced evaporation and reduced algal growth.  A proposed structure for a cover made of glass suitable for Lake Nasser has been published.

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