Entropic Energy: The Rechargeable Battery That Runs On Water

Published on Friday, 06 May 2011 13:15

	By Robin Whitlock Follow @RobinWhitlock66

It has long been known that the difference in salinity between freshwater and seawater is a potential source of enormous entropic energy, providing it can be exploited efficiently. The energy source obtainable from such salinity difference has usually been described in terms of its ‘osmotic power’ or more commonly ‘salinity gradient power’. Methods for utilising this energy source have in the past consisted mainly of reverse electrodialysis and pressure retarded osmosis. In the former process, salt water and fresh water are pumped through a stack of alternating cathode and anode exchange membranes. The chemical difference between each substance generates a charge as they pass over each membrane. This is because there is a lower concentration of charged particles, known as ions, in the freshwater than there is in the saltwater, indeed the difference is of the magnitude of some 60 to 100 times greater in the saltwater than the fresh water. It is this difference that generates the electrical current [1].

This activity falls within the realm of ‘electro-kinetics’ that is to say the area of physics that concentrates on the energy of ions and charged particles in solutions [2]. Pressure retarded osmosis on the other hand works through the process of osmosis whereby in a chamber containing separate quantities of fresh water and sea water, the salt particles in the sea water pull the fresh water through a membrane. This in turn creates greater pressure in the sea water container which can then be used to generate electricity through a turbine [3].

Now it seems there is a third method of exploiting the salinity gradient, according to scientists from Stanford University who are working to develop a new battery which uses seawater as an electrolyte. Unlike the previously mentioned methods, this new process does not require the presence of a membrane since it does not use osmosis in order to generate the current. Rather it uses the entropic power that is momentarily present at the time when fresh water from rivers and streams mixes with salt water from the sea.

Entropic energy is that energy contained within an isolated system and is therefore not available to do work. The term can also be explained as a measure of randomness and disorder within a system or the tendency for all matter and energy to assume a state of inertness [4]. The Stanford University team, led by Dr Yi Cui who is an Associate Professor of Materials Science and Engineering, have discovered a way in which to exploit this change in entropic energy which is essentially chemical energy created as a result of the chemical difference between two liquids and which has been estimated to hold an energetic value of 2.2 kJ (kilojoules) of energy per litre of fresh water [5].

The potential of entropic power was first identified by R. E. Pattle in a letter to the science journal Nature in 1954, in which Pattle wrote “When a volume V of a pure solvent mixes irreversibly with a much larger volume of a solution the osmotic pressure of which is P, the free energy lost is equal to PV. The osmotic pressure of sea-water is about 20 atmospheres, so that when a river mixes with the sea, free energy equal to that obtainable from a waterfall 680 ft. high is lost. There thus exists an untapped source of power which has (so far as I know) been unmentioned in the literature” [6].

In 2009, Doriano Brogioli attempted to exploit the energy of salinity difference by using an electrical double-layer (EDL) capacitor, a method that was first proposed by Gouy, Chapman and Stern. Like Dr Cui’s battery, this method relies on ion difference between salt water and fresh water and also uses the same method, that is to say two electrodes immersed in salt water. Unlike Cui’s research however, Brogioli’s study used porous carbon electrodes whereas Cui has used manganese dioxide and silver. The process, however, is the same, the electrodes are connected to a power supply such that one becomes positively charged and the other negatively charged. Salt water contains positively charged sodium ions and negatively charged chlorine ions, the chlorine ions are thus attracted to the positive electrode and the sodium ions to the negative electrode. An electrostatic force keeps the two sets of ions near their electrodes and in this manner a charge can be stored. Fresh water is then pumped into the device and this causes the two sets of ions to move away from their electrodes and against the electrostatic force, thus creating an electrostatic charge which appears as an increase in voltage between the two electrodes [7].

Cui and his team comment in their paper in the journal Nano Letters that Brogioli’s capacitor suffers from acute industrial problems stemming largely from the presence of impurities in the water and also of dissolved oxygen. They point out that Sales attempted to modify the capacitor with a membrane which improved the energy conversion efficiency and that Gouy et al have also explored this research idea proposing the use of single ion selective nanopores which are used in biomedical devices based on the salinity difference in bodily fluids. Cui et al’s research advances this research by proposing instead that the electrostatic charge from the salinity difference be stored within one of the two electrodes which should be made of a material with a bulk crystal structure. The two electrodes are therefore made of different materials. As with Brogioli’s capacitor, one electrode is an anionic (negatively charged) electrode which interacts with the chlorine ions and the other is a cationic (positively charged) electrode which interacts with the sodium ions. The two electrodes are submerged in fresh water initially, which has a low ionic concentration, and this is subsequently replaced by sea water which has a high ionic concentration. In this way the potential difference between the two electrodes is increased. The salt water is then drained and replaced by fresh water and the cycle begins again. Cui points out that this constant interchange between fresh and salt water could be achieved by some kind of flow system which would greatly assist large scale energy extraction [8].

Cui’s electrodes are made from nanorods of manganese dioxide and silver. Manganese dioxide is better than other materials as it has a surface area that is 100 times larger than other materials making it more available to interaction with the electrically-charged particles in the water. It is also environmentally benign which has the added advantage of being suitable for power plants situated in environmentally sensitive river estuaries.

Dr Cui estimates that the battery has the potential to supply some 2 terawatts of electricity annually, which is about 13 percent of the global energy supply. There are many areas which could be used for mixed entropy plants, but those having the greatest potential include the Amazon basin as well as some of the major rivers of Africa, Canada and the USA. During the course of the research Cui collected Seawater from the California coastline and freshwater from Donna Lake in the Sierra Nevada and achieved a 74 percent efficiency in converting the potential energy in the battery to electric current. He thinks, however, that the battery could be 85 percent efficient and also believes that a power plant using freshwater at a rate of 50 cubic meters per second could generate around 100 megawatts of power which is enough for 100,000 homes [9].

This technology is not without certain drawbacks since there are certain problems to be resolved before any such device can be incorporated into the mainstream energy market. Besides the problems of environmentally sensitivity of estuaries which the choice of manganese dioxide for the electrode is, in part, designed to alleviate, there are various challenges to resolve. One of these is the fact that silver, the material used for the second electrode, is particularly expensive. However a far more serious problem is that the amount of fresh water available in the world for energy use is finite, particularly because it is also needed for drinking water. This is of growing concern in a global society in which water stress is becoming a major concern, particularly exacerbated by climate change.  Indeed the World Policy Institute recently released a report investigating the particular aspect of this issue concerned with the competition for water resources between demands for energy use and as a source of drinking water, something the Institute has called the ‘Water-Energy Nexus’.

It seems the fresh water issue could be solved, in part, by the ability of the mixed entropy process to desalinate seawater thereby providing an extra source of fresh drinking water [10]. Dr Cui also believes that ‘dirty’ sources of water could also be used as the fresh water used in the battery does not have to be clean. Such sources could include surface runoff, waste ‘gray’ water from domestic and industrial sources and also water from sewage. Responding to the environmentally sensitive nature of estuarine areas, another reason for the use of manganese dioxide for one of the two electrodes, alongside that of increased efficiency, is that it is environmentally benign [11].