It seems that every day, there’s a new form of energy being developed that’s set to revolutionize the industry. Without a sound understanding of the scientific, engineering, and financial challenges involved, economists, energy company executives, and government policy analysts have an ever challenging time discerning the hype from the feasible.
Often proxies are used to determine the plausibility of breakthroughs, such as the prestige of the journal the information was published in, the prestige of the institution the innovation was found in, or how established the company is that is promoting the latest technology.
From an engineering standpoint, it is often frustrating to put things in context while the hype is being generated. At the same time, it’s often expected of engineers to distill information using metaphors, examples, and illustrations to describe what the latest revolution is all about. The fear that is stricken in the souls of non-STEM individuals when they view an equation, as shown below, is lamentable.
A great example of this phenomenon is the latest breakthrough in the field of Osmotic Power, otherwise known as Blue Power.
Getting energy or producing useful work through the difference in salt concentrations is a natural biological phenomenon: fish use this when breathing, and humans use this throughout the body, including in the blood. Its application to electrical generation has been relatively modest, mostly due to issues with membrane technology (plants and animals have cell walls, which complete this function quite well).
Pressure-retarded osmosis (PRO)
There are two schools of thought when it comes to getting energy through osmosis. The first is to let the water through, creating a larger head pressure at the salty side:
This membrane can then be used to run a turbine:
To get an understanding of how this technology could be applied, two equations need to be considered.
Or in simpler terms:
(Click to enlarge)
How quickly the membrane moves ions is related to how many holes the membrane has, or how dense the holes are. The pressure across the membrane is related to the thickness and the size of the holes: larger holes mean smaller pressure differentials. The osmotic pressure difference is hard to control in nature, being related to the salinity of the ocean water and the fresh or brackish water being considered. The term ?π can be written as:
(Click to enlarge)
Here, 0.9 mol/Litre is used as the difference between salt water and moderately brackish water, rather than the ideal, salt to fresh water. For a good comparison, 325 PSI is 750 feet of head, or about the height of the Hoover dam. Related: 30 Years After The Disaster: Ukraine Plans Huge Solar Farm In Chernobyl
However, the osmotic pressure difference is only one part of Equation 1 above, and does not take into consideration practical limitations. A plant operating on this basis was built in Norway in 2009, creating about 2-4 kW of power: about enough to power two microwaves. The plant operated until 2013, after it was determined that the membrane required was very large was very expensive to manufacture and install. For the same installation costs, a Rolls-Royce natural gas power plant could be built to produce almost 4000 times as much power, 15 MW.
Reverse electrodialysis (RED) osmosis
The second school of thought is to let the ions through from the salt water to the fresh water side, creating brackish water on both sides, but also an electrical flow across the sides:
RED operates very similarly to a battery, and has advantages over PRO, not requiring a turbine, which is a maintenance item, to operate. Recently, a university in Switzerland, the Ecole Polytechnique Fédérale de Lausanne (EPFL), which is a highly respected school, published an article in the journal Nature, a highly respected academic journal, talking about a breakthrough in RED technology: specifically a breakthrough in membrane technology applied to RED.
The new RED-based osmotic nano-generator used a 0.65nm thick MoS2 membrane. The hole in the membrane was 5nm in diameter, and the solutions on either side of the membrane contained potassium chloride. Although the theoretical application potential of this technology is on the scale of 2000 nuclear plants, there are still quite a few significant technical challenges to make this type of power feasible:
Making metre square MoS2 membranes has not been done. It’s worth attempting, but the time to commercial viability here is lengthy.
The holes in the membranes are 5nm in diameter: this is smaller than any known virus and smaller than most coal dust that clogs people’s lungs. To ensure that the membrane stays clean is not a trivial task.
The lead scientist for the breakthrough noted that one potential application is a nano-generator to power nano sensors: many scales of magnitude smaller than grid or even distributed generation scale. With further advances in ultra-thin membrane technology and boron-nanotubes, energy densities can be shrunk for nano-pore applications such as these membranes.
This type of research is vital to finally move us away from nuclear energy after all other fossil fuel use has stopped. The time scales involved for commercial and industrial production are upwards of 25-50 years. Luckily, with nuclear power allowing us to bridge the gap, we have the time to develop these types of technologies. The hype surrounding these types of breakthroughs however, can be detrimental to the larger task at hand of greening the grid.
By Matt Slowikowski for Oilprice.com:
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