The hidden value of water flows: the chemical exergy of rivers

The hydroelectricity potential of rivers is a very well-know parameter used to characterize the availability of a river as a function of its flow and its altitude. However, the chemical potential of water flowing through the rivers is commonly ignored. In its source, water presents high quality and, therefore, it owns an important availability that can be expressed through its chemical exergy value. On the opposite, when it flows into the sea and reaches the thermodynamic equilibrium, it can not be further used and it is converted into a null exergy value. Within these two limit values, the exergy state of the river at its different stages can be assessed. On the other hand, water availability for specific uses depends on its quality. In this way, the almost always hidden value of water, its chemical potential, is highlighted and can be compared to the potential component, since they are expressed in the same units (energy units). In this paper, it is shown that potential and chemical exergy values of rivers rise up with values with the same order of magnitude. That is, the chemical value of a river is, from a thermodynamic perspective, as much as its potential value. The main difference lies in the current available technologies to take advantage of those physical disequilibrium: while hydro-power turbines are a completely proved technology, there are not yet commercial devices to take advantage of the hydro-chemical potential. Results of those estimations for a small Spanish river, the Muga river, are presented in this paper in order to prove the accuracy of the methodology. It is shown that the potential exergy of that river ranges from 2.37 to 7.15 MW, while its chemical exergy is comprised between 2.30 and 8.78 MW for the present state of the river. In addition, several exergy indexes are defined as basic parameters to provide information about the advantage taken from the river, that is, about the water uses within the watershed.