Water, Salt and Energy - A Perspective on Desalination

by Steve Smallcombe

Water and salt (sodium chloride) like each other. A lot! The oceans are saltwater and so is our blood and tears. Water and salt get along together just fine. In fact, if you mix salt with freshwater, energy is released in what is called the Gibbs Free Energy of mixing which we will abbreviate, ∆Gmix. One manifestation of this energy of mixing is osmosis - a process where a solvent like water will flow through a membrane from one side to the other in order to equalize the concentration of, e.g. salt on both side of the membrane. In the case of freshwater and saltwater or seawater, the osmotic pressure is considerable with over 400 pounds of pressure per square inch (PSI) at the membrane. That's four times the pressure used for a nail gun. To relieve this pressure water will naturally flow from the freshwater side to the seawater side, in a process called Forward Osmosis, until the height of the seawater column is 875 ft higher than the freshwater side! That is a lot of pressure.

 

 

In the 70s, scientists realized there was a huge potential to obtain clean energy from mixing water streams with different salt concentrations, and that freshwater flowing into the ocean released considerable energy that could be captured without any use of fuel or emission of carbon dioxide. Today electrical generation plants at the mouth of rivers in Europe and Scandinavia take advantage of this natural salinity gradient by resisting or retarding the osmotic flow, and using that pressure to create electricity with what is called Pressure Retarded Osmosis. It's estimated that using 10% of our river water this way could generate electricity for 500 million homes.

If we mix water with salt, or even mix freshwater with saltwater, energy is released. It stands to reason that if you want to reverse that process and extract either the salt or the water from seawater, that it will take energy. In science, the study of temperature, heat and energy is called thermodynamics. Basically thermodynamics says that when it comes to energy, you have to be like an accountant and run a balance sheet. You are not allowed to create or destroy energy, but rather must figure out where energy goes to or comes from any time you want to do something. Thermodynamics is often summarized by saying that there is no such thing as a perpetual motion machine.

That brings us to desalination where indeed we want to separate water from saltwater, and the laws of thermodynamics say that it will take just as much energy to separate water from salt as was released when the two were mixed, ∆Gmix. One of the most practical methods for desalination is called Reversed Osmosis where instead of extracting energy from the osmotic pressure, we can apply energy and well more than 400 PSI pressure, and force the system to go backwards.

The more freshwater we take from the seawater, the more concentrated the brine becomes and the more energy that is required for the next drop of water to pass through the membrane. That is why most desalination plants only remove about 40% or the water and the rest is returned to the sea as concentrated brine.

In reverse osmosis, or any method used for desalination, you have to pay back the energy of mixing, ∆Gmix. The science is clear, the minimum theoretical energy associated with desalination is independent of method, is fundamental and determined by the laws of thermodynamics.

Of course some methods are less efficient than others. Methods that involve a phase change such as when heat is used to turn water into steam take even more energy, but the energy used above the minimum theoretical energy is potentially and often practically recoverable. So if you read a press release about a new desalination method that recovers the energy - it's true, but only to the extent that the energy recovered is the energy spent that is in excess of the minimum theoretical energy, ∆Gmix. If you are going to end up with freshwater separated from saltwater a certain amount of non-recoverable energy, ∆Gmix, needs to be spent. Period.

Some new high tech membrane such Graphene and or carbon nano-tubes can potentially lower the energy costs involved in pumping water, but those saving are fairly minimal compared to ∆Gmix. The image above is from a lecture by Menachem Elimelech, Department of Chemical and Environmental Engineering, Yale University, illustrates the fact that the pressure at current membranes is only 10-20% above the theoretical minimum energy or thermodynamic limit and that high permeability membranes will therefore have a negligible effect on energy use. Seawater desalination will always be very energy intensive as dictated by the laws of thermodynamics. Period.

If California and the planet had all the energy we needed and more. and all of that energy was clean and did not contribute to climate change, then desalination would be a no brainer. But that is not the case. Right now a vast majority of the energy we use is coming from fossil fuels which when burned are the leading cause of climate change. California is is on a path to reduce energy use and convert the energy we do need from fossil fuels to renewables. We are making progress, but have a long ways to go. There is a phrase Negawatts, not Megawatts, which means is that it is far easier and cheaper to not use energy than it is to provide more energy. Reducing our energy consumption by using more energy efficient products and processes is equally if not more important than bringing new energy sources online.

Scientist say that the minimum energy likely to ever be achieved for a seawater desalination plant is roughly 10 times the energy currently used for freshwater sources. Today's plants are within a factor of 2 of that limit so some progress on energy savings is possible, but there is a limit to how much reduction is possible, and unfortunately that limit involves using a lot more energy than is used for current freshwater sources. Water is already California's largest use of energy and advocating for a technology that greatly increases energy use is a bad idea.

If we want to understand the role of desalination in our water portfolio going forward, it is a matter of understanding the true energy cost of desalination and using that in determining where and when it is appropriate. If you start a discussion on desalination by talking about the vast oceans as a virtually unlimited source of freshwater, if we just were smart enough to take it, then one very much over emphasizes the importance of seawater desalination to our future water portfolio. We don't have the renewable energy to make that happen, and to use fossil fuels for that purpose would be a disaster. Of course there are plenty of climate change deniers who want to do exactly that, and it is critical we don't ofter them any political cover or encouragement. No energy is free or ever will be, as all energy has a cost.

Seawater desalination is an important part of our water portfolio, but it must be considered a method of last resort, i.e. all other methods of water conservation and water recycling/reuse must be implemented first. This is the position of the Sierra Club, the NRDC and other environmental organizations.

Fortunately the State and coastal cities seem to understand this and the other negative environmental consequences of desalination, and are currently actively considering or implementing potable water reuse as a preferred alternate. Potable water reuse takes advantage of a major source of water that is local, i.e. water from the local wastewater treatment plant, and thus cuts down on the energy involved in transporting water from a distance source.

Treatment of wastewater for reuse is often called "desalination", as is desalination of brackish water (water with some, but a relatively low salt level, such as found in estuaries), but since there is significantly less salt involved with these water sources, it uses a lot less energy, and therefore is to be greatly preferred to seawater desalination.

Unfortunately all of the above is often lumped together in the press as "desalination" leading people to believe the the desalination plants at some costal cities in California such as Cambria and Sand City are using seawater desalination. They are not. Both use brackish water and in the case of Cambria, brackish water from a beach well near an estuary is mixed with wastewater from the nearby domestic wastewater treatment plant. Even then it is recognized that use of this water source will double local water bills and is only to be used in an emergency.

Seawater desalination may seem like an obvious solution to our water problems, but the fact that it is not in widespread use today in California is a clue that there are some drawbacks, serious environmental problems in this case. It is an inconvenient truth.