This is our system model
All clear? Not so much?
We can think of this model as having 5 key functions:
Energy Generation: Using a hybrid photovoltaic/thermal solar array, we can extract both electrical energy and thermal energy from the sun’s rays.
Energy Storage and Arbitrage: We’ll be storing the thermal energy in a large tank filled with some hot material (likely water) and the electricity in a bunch of batteries. The batteries will also be able to sell/buy electricity to/from the grid in order to increase revenues and not waste any excess energy we produce.
Water Pumping: Being a desalination plant, the system would use either horizontal wells that run along the ocean floor and collect seawater or vertical wells that dig below the beach and collect brackish water (less salty, naturally filtered). Additionally, many pumps are required to push water through the filters along the way, especially the Reverse Osmosis (RO) filter.
Filtration: The untreated water squeezed through the RO filter leaving filtered freshwater as the product and a concentrated brine as the waste. This brine is then put through the Forward Osmosis (FO) filter, which gets a little bit more freshwater out of the brine and leaves only a super-concentrated brine as waste.
Salt Production: This super-concentrated brine is poured into wide, shallow tanks that allow the remaining water to evaporate away, leaving untreated salt. This salt can be sold to a company that treats it, thus giving us more revenue.
Wait a sec, what’s this Forward Osmosis business?
Let’s look at the basics:
Osmotic Pressure: Say you put pure water in a tank (or in the picture, a u-tube) and then split the tank perfectly in half with a semipermeable membrane that only allows water to pass. Next, you add a solute (sugar, in the picture) to one side and impose a concentration gradient on the interface between the pure and sugar water. Solutions have a natural tendency to “even themselves out” in terms of solute concentrations but the sugar can’t go anywhere, so instead so water will flow into the side with sugar in a process called osmosis. If you wanted to stop this from happening and instead just keep the two sides at the same height, you’d have to apply a pressure to the sugary side, right? That very same pressure is the Osmotic Pressure.
RO: Reverse Osmosis is what would happen if you applied a pressure much greater than the osmotic pressure to the sugar water; water would flow from the sugary side to the pure water side since it is literally being squeezed out of the sugar water. In short, RO pushes water out of a highly-concentrated solution.
FO: Forward Osmosis uses the tendency of water to flow from a location with less solute to a location with more solute to pull water out of the sugar water. By making a so-called “draw solution” have a really high concentration of some water-soluble polymer, we can pull pure water away from sugar water without needing to apply much pressure. Normally this would just leave us with an even more contaminated volume of water, but the polymer used in the draw solution has the nifty property that it precipitates out of the solution at moderately high temperatures (~80℃), thus giving us just fresh water at the top of the tank and some soon-to-be draw solution at the bottom. In short, FO pulls water into a highly-concentrated solution.