Desalination and Energy Consumption

An even better solution is to use co-located nuclear power. Advantages include high capacity factor for the desalination plant, co-generation with electricty, no transmission losses and the ability to use thermal methods for desalination. (Nuclear heat being very cheap.)

Sorry, Bobbi, but you really show signs of not understanding the basic subject matter.

There’s this thing called osmotic pressure that is fundamental to reverse osmosis.  To move a fluid against an osmotic pressure differential, you need to apply a greater external pressure.  Pressure times volume has the units of energy, so that sets an irreducible minimum amount of energy that you can approach but never reach, let alone undercut (second law of thermodynamics).

If you are generating fresh water via RO, your minimum cost of water is set by your cost of energy plus the cost of amortization plus O&M.  If you operate intermittently to run on surpluses of RE, you’ll need more equipment per unit of water generated and the price goes up.  Ditto if you need more water storage to go over periods of low or no production, though storing water is very cheap.

The best way to do this is to use some source of energy with a high duty cycle and really low cost.  As Jesper mentioned, this works really well with nuclear power.  Multi-stage flash distillation takes more energy than RO, but the energy is heat rather than electric power.  Seawater leaves scale on surfaces if it’s made too warm, so warm-ish mostly-spent steam from the final turbine stages of a nuclear plant matches very well with flash distillation; most of the available physical work energy has already been extracted, so you’re not even losing very much electric power production.  My SWAG is that low-temperature nuclear steam heat costs something on the order of 0.1¢/kWh, maybe less.

Of the 71% of water on earth, only 4% is drinkable

This nonsense sentence actually should read that 71% of Earth’s surface is covered with water and of all the water, 4% is drinkable. That’s 12 million cubic miles.

The author’s first draft, I assume?

The best way for desalination plants to minimize their energy consumption is by using renewable energy to power the facility.

Apparently renewable kWhs last longer than conventional ones?

High-speed electrical pumps on desalination plants consume more energy than is needed.

A design problem?

Regarding the motivation for seawater desalinization, there are many problems with trying to rely exclusively on rainwater for municipal water supplies:
– Rainfall amounts vary from year to year, and lawns need more water during drought years (due to low humidity)!
– For growing nations like the US, rainfall is a zero sum game, adding new consumers means the existing consumers must use less (and cyclic water shortages create tension between stakeholder groups such as home users, farmers, and golf courses).
– Fairly dividing up water from rivers that pass through multiple state (or nations) always involves very contentious negotiations.

Using well water is not much better, as underground aquifers are stocked with rainwater that fell hundreds of years ago, and replenish very slowly. When we over-use these resources, we steal from our grandchildren.
Desalinization avoids all of those problems, by allowing cities to generate as much fresh water as they need.

Desalinization is not as cheap as rainwater, but it is affordable, as evidenced by growing desalinization use in the Middle East. (iirc) Studies done in California have found desalinization plants are cost competitive with building new aqueducts to bring in water from out of state.

Californians seem to want to buy desalinized water during droughts, and rainwater at other times. That’s fine, and it saves energy; but society still has to pay for desal plants, even when they are not used.

The American Membrane Technology Association has a short paper that puts the energy cost of SWRO in perspective. It’s an industry source with a stake in SWRO, but their numbers check out. As stated in the paper, the energy cost of supplying all water to the average American home via SWRO is less than the energy cost of running an older, less efficient refrigerator for a year, and about equal to the energy cost for a newer high-efficiency refrigerator. That’s still a lot of energy — a 10% increase in the average home’s annual energy use — but it’s far from a killer issue if the energy is from clean sources.

The cost of fresh water from SWRO is still many times higher than what the farm industry is accustomed to paying for purchased irrigation water, so we won’t be seeing the current model of Big Ag running on desalinated water any time soon. However, the single biggest change on the horizon for water consumption would render the cost of purchased fresh water almost irrelevant. That’s a transition to high productivity “indoor farming” with near 100% recycling of water.

Another important trend for municipal water supplies is increasing use of recycled wastewater. The energy cost of wastewater treatment to the level required for “blue pipe” reuse is about half of what’s needed for SWRO. And that’s really a worst-case cost. It applies to treatment of common sewer water in which 99% of what goes in is relatively clean, but then mixes with the 1% of “black water” that really requires extensive treatment. Widespread adoption of standard, code-approved grey water systems for lawn and landscape watering could easily cut per-capita municipal water consumption by half.

A third important trend is just getting started in a few locations on an experimental basis. That’s direct injection of storm runoff waters to recharge aquifers. In essence, running irrigation wells in reverse when there’s a temporary surplus of fresh water that would otherwise overflow surface reservoirs and be run into the sea. There’s actually quite a lot of potential there. It’s likely to be more important in the future, since less and less precipitation will be stored naturally as winter snowpack in the mountains.

About reducing the energy consumption of SWRO:

Don’t expect graphene membranes to make much difference, per se. The energy cost of RO desalination is almost entirely a function of osmotic pressure across the membrane, not physical resistance to the transport of water. A theoretically ideal membrane wouldn’t perform significantly better than the membranes we already have if used in the same physical configurations and flow rates.

Energy consumption in current SWRO systems is about 4x higher than the theoretical minimum, as calculated from static osmotic pressure for normal seawater. But most of the difference has nothing to do with non-ideal membrane performance. It’s mostly due to concentration polarization at the membrane wall. The passage of water through the membrane while leaving the salt ions behind means that there is a “pile up” of salt ions at the membrane surface. The resulting salt concentration at the membrane surface can be several times greater than bulk concentration of salt in seawater. That gives rise to a much higher osmotic pressure across the membrane.

The only way to reduce the pressure from concentration polarization is to accelerate the transport of salt ions away from the membrane against the flow of water through the membrane. That can be done by reducing the specific rate of water transport (transport rate per area of membrane), allowing more time for salt ions to diffuse into the reject flow that carries salt away from the membranes. That improves the energy productivity of the SWRO membrane stack, but reduces its output rate and capital productivity. If graphene membranes can be made cheaper than current membranes, that will be a favorable tradeoff. But if they’re more costly per square meter, forget it.

Another way to reduce the pressure from concentration polarization is to operate the SWRO stack at higher temperature. Salt ions have more mobility at higher water temperatures and diffuse faster. With other parameters the same, pressure across the membrane will be reduced and energy efficiency improved.

All of which suggests that an SWRO plant would be an ideal partner for a nuclear power plant. The desalination plant would operate full time, but at a variable rate setting. At lower rate settings, the desalination plant will draw less power and produce less fresh water output per hour, but the output reduction will be proportionately less than the power reduction. The unit’s net energy efficiency will be improved. The combination of nuclear plant and variable rate desalination plant creates a flexible power system that can load-follow and serve as an efficient zero-carbon backup source for variable renewables. Efficiency of fresh water production is further improved by using cooling water from the reactor to run the SWRO stacks at elevated temperatures.

Bobbi, among some other problems already mentioned, you miss the point that RO requires pumping seawater to extreme pressures – 800 to 1,000 psi, which is equivalent to pumping it up 2,000 feet vertically. This has little relation to taking a shower where the energy used is primarily to heat the water.

Powering RO with renewables is technically feasible but until we have excess renewable energy, its use comes with a penalty since fossil energy will ultimately make up the difference. The same result comes with using nuclear and neither approach solves the problem associated with seawater uptake and hyper-saline disposal.

In reality there is no water shortage on this planet, only a spatial disparity in freshwater availability and human freshwater requirements. The natural water cycle provides more than enough for all our needs but because a lot of that precipitation falls in un-populated watersheds, it travels back to the sea, lightly used, to spend another 2,000 years of residence time in the ocean.

This mismatch of water resources and population has become exacerbated under the vagaries of our changed climate. However, there are two obvious solutions: 1) move the people or 2) move the water. The second option is easier and in various ways has been done since Roman times. Today, there is a better, less-costly solution – the transport of contained freshwater over the ocean – not in tankers but in purpose-built flexible containers that can move millions of gallons at a time. By towing these waterbags slowly and exploiting renewable energy sources, this transport can be done with little or no carbon emissions.