It will not happen overnight, but California is on the verge of a hydro-revolution; the beginning of a radical transformation that will dramatically increase the amount of available water. It has already happened in other parts of the world. From where we stand today, this transformation sounds quite unlikely. There are those in California who believe we should return to a desert landscape, retire what has become the most productive agricultural region in the world and live with what little water nature provides. Fortunately, we have overcome even greater obstacles and water abundance in California is not only possible, it is inevitable. The foundation for this water revolution is already in place.
To put into perspective where California is today regarding its water resources, look at a country that has struggled mightily with the same challenge: Israel. Israel is extremely water scarce; it is ranked number twentyone on the World Resources Institutes listof most waterstressed nations after Kuwait, Oman and Libya. In the fifties, Israels largest freshwater resource, the Coastal Aquifer, had already been overpumped and contained high levels of salt. In 1973, recognizing that conservation alone would not solve the water crisis, Israel began a decades long initiative to implement seawater desalination. By separating freshwater from the ocean, a technologically driven country of only eight million people has since become a net exporter of water. The magnitude of this transformation cannot be overstated; from water scarce to water abundant. From water as a limiting resource to water as a tradable commodity. Today, Israel generates enough freshwater to sell surplus to neighboring countries.
Figure 1: Global water scarcity
In retrospect, the severity of Israels water crisis is precisely what led to a longterm strategic water plan involving desalination. Contrast that to California where one of the worlds largest aqueduct systems, built in the 1970s to redistribute water from the Sierra Nevada Mountains and the Colorado River, has thus far allowed us to sidestep the tipping point that forced Israel to act. From Redding to Los Angeles, water is pumped from snowmelt and storage reservoirs to hundreds of thousands of acres of irrigated farmland and millions of people in the southern part of the state. But the Colorado River Basin, which provides water for much of the western United States, no longer has enough natural recharge to maintain our level of economic growth. A recent NASA study documented the massive supply and demand imbalance, calculating a loss of water since 2004 at over fiftythree million acrefeet roughly equal to the annual water needs of every man, woman and child in the United States.
Figure 2: California drought index
To see the water crisis unfolding first hand, take a drive into Californias Central Valley where hundreds of thousands of acres of valuable farmland are being taken out of production (15% of all food in the U.S. comes from California). Simultaneously, water agencies across the state are relying on dwindling storage reservoirs to provide uninterrupted municipal water service. Our water providers are banking on future wet years to make up for an everincreasing deficit and our remaining water storage is estimated to last only twelve months at our current pace. Simultaneously, climate experts have warned us that severe, extended dry periods like the current one are exacerbated by increased global temperatures and as a result are not merely the seasonal droughts were used to. This new normal has rendered the water delivery system that once provided the state with a reliable source of imported water largely unreliable and severely inadequate. This years water outlook is even worse with senior (riparian) water rights holders being stripped of water supplies that have been held since before 1914. California is now at a critical inflection point, much like Israel was in the 1970s.
Desalination is contentious, especially as a broadbased water solution for California. The most common objections are the high cost and environmental impact. But contrary to popular belief, the cost of not considering desalination is much larger and so are the potential environmental consequences. When we evaluate the cost of desalinated water, we are not talking simply about the cost of a gallon of H2O, we are weighing the economic value of a gallon of water against our productivity (or inversely the lost productivity without it). When we analyze environmental impact, we are not examining solely the local impact of a desalination plant; we are broadly minimizing the irreversible deterioration to our habitat from overdrafting the water supply.
Figure 3: Land subsidence in the Central Valley
To put this issue in perspective, the unregulated groundwater that is currently being overdrawn due to the loss of water deliveries in the San Joaquin Valley has resulted in unprecedented land subsidence (measured in tens of feet) and described by experts as one of the largest geographic alterations on earth. The water deficit in California is being filled by pumping groundwater, but just like overspending the principle in your savings account, unmanaged drawdown of groundwater is extremely difficult to restore and can have lasting consequences. For these reasons, it is critical we keep Israels experience in focus and give desalination a second look.
As a country, California is the worlds ninth largest economy about five times the population of Israel and nearly eight times the gross domestic product. At this scale, desalination needs to be thoughtfully implemented. In most cases, desalination refers to the removal of salt from seawater, but in practice desalination is broader. The practical definition includes recovering potable water from many nonpotable water sources, most often by removing salt. This includes seawater, saline groundwater, industrial process water, wastewater, agricultural drainage and a host of other impaired water sources. In this context, desalination is actually water reuse, which is a form of water efficiency. We know from the energy industry that efficiency improvements are an attractive investment because they capitalize on installed infrastructure. If we expand our view on desalination, it can be more than powerhungry, largescale, seawater plants it can compliment our existing water infrastructure. The key to unlocking this potential is staring us in the face: the sun.
Water is energy and the problem of insufficient access to freshwater is fundamentally about energy. Often referred to as the energywater nexus, this is the link between the energy used to produce water and vice versa. While desalination does indeed require more energy when compared to alternative options, the type of energy we use makes all the difference. Sources of energy are not created equal and have different costs and consequences depending on the type. The energywater nexus is therefore not just about improving energy efficiency or optimizing the amount of energy we use to procure water, but also where this energy comes from. The choice of how we get from point A to point B, from energy input to water output, is absolutely critical.
The energy to desalinate seawater is high, but it also requires energy to treat water and energy to move water. The energywater nexus states that you cant generate one without the other, so no water source can escape its corresponding energy footprint. Solar has emerged as an abundant energy source that is transforming our daily lives in unthinkable ways. Through net metering, rooftop solar is creating thousands of tiny power producers, each generating periods of excess power being put back onto the grid. In conjunction with energy storage, residential and commercial solar will continue to free up thousands of megawatthours of electricity that can be utilized elsewhere. Users of electricity are becoming generators. Sources of demand are becoming sources of supply. These same fundamental principles apply to water and solar energy is the enabling technology. With access to an abundant source of affordable energy we can actually eliminate water scarcity; Israel has already proven this.
Figure 4: WaterFX solar desalination demonstration
With this thesis in mind, my colleagues and I set out to conduct an experiment on the west side of the San Joaquin Valley in partnership with the Panoche Water and Drainage District and ATSI Engineering and with financial support from the California Department of Water Resources (through Proposition 50). The questions we sought to answer: could solar energy be used to make desalination more affordable, more sustainable and highly scalable? Could we tap agricultural water to turn the states largest water users into producers? If so, the result could mirror what has happened in energy; rather than relying on largescale, centralized generation, smaller distributed projects across the state could free up hundreds of thousands of acrefeet of water through reuse, reducing overall demand on the water grid. Irrigated farmland is the single largest use of water in California (approximately 4050% when you exclude environmental allocations), so this is an ideal place to start. The conclusion that we came to, if implemented properly, is that solar desalination can be highly profitable.
Figure 5: Simple solar still
A still (short for distillation), which is a device for separating water, has been in existence for hundreds of years. It is incredibly simple; just apply heat to boil water, capture and cool the steam and the remaining impurities are left behind. The steam is condensed into pure water. Most stills burn fuel to generate the necessary heat, but solar stills require only sunlight. Just like a glass of cold water on a hot day, heat from the sun causes water to evaporate and condense on the surface of the still where it is removed. Solar stills do not require fuel, but are typically small and only produce up to 20 gallons of fresh water per square foot of solar collection area per year. Since the tradeoff when transitioning to solar energy is fuel for land (surface area to collect the necessary solar energy), solar stills must be much more efficient to be practical for water production. A Concentrated Solar Still (CSS), therefore, is a way to increase the production using concentrating solar energy to generate higher temperatures (up to 350 degrees C) allowing the heat to be efficiently recycled. With this approach, the performance of a simple solar still is increased by a factor of thirty, providing 600 gallons of freshwater per square foot per year.
Figure 6: Salt accumulation in the soil
Treating irrigation water can be difficult as it contains elevated levels of certain compounds (e.g. calcium sulfate, calcium carbonate and silica) that are challenging due to scaling. Continuous, reliable treatment without scaling can be achieved, however, by leveraging thermal desalination (called multieffect distillation or MED), a technology that has fallen out of favor due to the high fuel consumption. However, when integrated with solar heat as the energy source, the result is a robust water treatment process that can be used to treat a wide array of water sources. A solar desalination demonstration plant with this design has been built and operated continuously, showing that it is possible to reclaim over 93% of the subsurface drainage water prevalent in the Central Valley, while minimizing or eliminating brine discharge (the salt can be removed as a solid coproduct, referred to as zero discharge).
Figure 7: WaterFX solid salt co-product
Saline drainage water is pumped into the plant containing around 15,000 parts per million (ppm) of total dissolved solids and steam from the solar collector is used to concentrate the salts up to 200,000 ppm by boiling the water. The steam is cooled and condensed and becomes a source of ultrapure water for reuse. The salts, now in a highly concentrated form, can be separated into valuable downstream products such as gypsum (e.g. wallboard). The treatment of drainage water solves two problems; eliminating saline discharge that would otherwise flow to the rivers and increasing the overall water supply with a new source. This positively tilts the value proposition for desalination, as there is added economic incentive to minimize disposal in addition to capturing the value of the water.
To provide context around what can be achieved with solar desalination, consider the numbers. Every acrefoot per year of water production requires roughly 500 square feet of land (total solar collection area). This number will improve to around 250 square feet over a fiveyear period through improvements in technology and packing density. Therefore, an acre of land in California, using an average direct normal irradiance (DNI) of 6 kWh/m2/day, has the capacity to generate 6080 AF/yr of water; this is up to forty times the per acre water consumption of an average California crop (~2 AF/yr). This means that an acre of solar desalination can satisfy the water needs of forty acres of irrigated farmland. This is an extremely attractive land utilization factor because only two percent of the land needs to be dedicated to water production and this multiple will improve. With access to an abundant source of saltwater, an area encompassing two square miles would provide 100 million gallons per day; sufficient fresh water for a city the size of Las Vegas (600,000 people).
When compared to seawater desalination using fossil energy, solar energy can be used to dramatically cut operating costs. Using commercially available equipment, a cost of $450 per acrefoot of water can be achieved, which compares to $900 per acrefoot at the Carlsbaddesalination facility in San Diego. While the cost reduction curve for different solar technologies will vary, the solar industry as a whole is in its infancy and this cost advantage will increase over time. In analyzing the competitive economics of solar desalination, we must consider the effects of salt accumulation in the Central Valley, which has converted large tracts of land into dedicated drainage regions. Considering this, solar desalination is a more economically attractive option as it not only eliminates the fallowing of land due to drainage water, but also serves as a longterm, reliable source of water. There is an estimated one million acrefeet of drainage water available in the Central Valley that can be sustainably treated and looking beyond drainage water, there is a virtually unlimited supply of saline groundwater that can be converted to fresh water. With this approach, it is entirely possible for California to chart a new course towards true water sustainability.
Solar desalination is unique in that it is not simply conventional desalination powered by solar energy. Solar is unmatched as an energy source in terms of its accessibility and that is why it has emerged as such a powerful solution. Solar energy can be implemented by almost anyone, anywhere, and the barrier to adoption is very low. The definitive end game for any technology is simplicity and solar is the ultimate embodiment of simplified power generation, bypassing the complexity of largescale projects and putting the capability directly in the hands of the user. This is precisely what we need to accomplish with water. Solar energy has the ability to make desalination a massmarket solution, at even the smallest scales and this is where it will become an impactful solution for water reuse. In our vision of the future, concentrated solar stills will become as common on California farms as the iconic grain silos of the Midwest.
There is a multitude of ways to design sustainable desalination systems. We have considered one approach that can be deployed cost effectively in the near term, but solar photovoltaic (PV) can also be used in conjunction with reverse osmosis (semipermeable membranes) or electrodialysis (removing ions with charged electrodes) as another platform for solar desalination in certain applications. As part of this experimental project, weve established a collaborative model by adopting an opensource philosophy. Rapid, iterative development and sharing of information by a collective of users is the fastest path to the best solution. Thus, our hope is that other scientists, engineers and entrepreneurs will be inspired to follow, better what we have developed and improve the technology dramatically. In doing so, we aim to propagate an expanded view on how desalination can be implemented in this era of climate change.
Before we rush to reduce desalination to an option of last resort, lets take the opportunity to weigh all the permutations. Desalination is by no means a silver bullet and must be considered in context, alongside conservation, storage and groundwater management. However, to remove it entirely from our longterm thinking would be a mistake. Desalination has unique characteristics: it is reliable, it is certain and it is scalable. Establishing a comprehensive water plan for California without these features would have economic consequences: reliability is what drives predictable revenues for industries that rely on water, certainty is what allows water planners to meet defined targets and scalability ensures a broad impact on our water supply. Desalination is additive and can become an asset by enhancing the water initiatives already in place.
The hydro-revolution is coming, but it will require patience and fortitude. Israel has shown us the way, but we have the tools to stand on Israels shoulders and go one step further. We do not need to live with insufficient water this is a temporary condition that is curable, just like a treatable disease. Using clean energy to produce clean water is like a vaccine, preventing and even reversing the spread of water scarcity. For Israel, it once seemed impossible too.