Optimization of the Energy Output of Osmotic Power Plants

On the way to a completely renewable energy supply, additional alternatives to hydroelectric, wind, and solar power have to be investigated. Osmotic power is such an alternative with a theoretical global annual potential of up to 14400？TWh (70% of the global electricity consumption of 2008) per year. It utilizes the phenomenon that upon the mixing of fresh water and oceanic salt water (e.g., at a river mouth), around 2.88？MJ of energy per 1？m3 of fresh water is released. Here, we describe a new approach to derive operational parameter settings for osmotic power plants using a pressure exchanger for optimal performance, either with respect to maximum generated power or maximum extracted energy. Up to now, only power optimization is discussed in the literature, but when considering the fresh water supply as a limiting factor, the energy optimization appears as the challenging task. 1. Introduction Due to a future lack of fossil fuels and the risk of global climate change caused by CO2 emissions, it appears prudent to aim for an energy supply which is based on renewable energy sources. It appears to be realistic that solar and wind power plants (together with hydroelectric power) will be able to supply the future demand [1]. However, in both cases, the generation depends strongly on natural fluctuations. So additional, renewable base-load capable energy sources are desired. A possible base-load energy source is offered by the use of the mixing entropy when fresh water and salt water (e.g., sea water) are mixed, predominantly at river mouths. In fact, mixing entropy (or rather its reduction) is also derived from solar energy, which drives evaporation from the ocean leading to separation of fresh water and salt water. Its theoretical global annual potential is estimated to be up to 14400？TWh per year [2]. Including economic and ecological boundaries, Statkraft forecasts that in fact only 1600？TWh is usable [3], Zeuner [4] confirms this estimation, which is still of the global demand in electricity in 2008 [5]. There are several concepts to use mixing entropy for generating electric power; a list is given in [4]. One of them is the concept of “osmotic power,” which has been introduced by Sidney Loeb in the seventies; for example, see [6]. Because several components can be adopted from “reverse osmosis” desalination plants, osmotic power plants are the most developed concept to use mixing entropy for generating electric power. In 2009, the Norwegian energy provider Statkraft started operating the first osmotic test power plant [3]. In this paper, the optimal