Global surge in exploiting renewable sources of energy can be attributed to the prevailing campaign and increased awareness of the merits offered by these forms of energy. In addition, the consequences of excessive use of fossil fuels and other non-renewable energy sources are being felt. So far, various forms of renewable energy sources have been explored, namely: wind power, hydropower, biofuels, solar power, geothermal power, and ocean power. Huge quantities of renewable energy can be extracted from the oceans, in the forms of tidal energy, current energy, wave energy, ocean thermal energy, marine biomass energy, and salinity gradient energy (SGE). SGE is least explored despite its global potential being in the same extent as wave energy or thermal gradients, and it is a hundred times higher than that of tidal energy. Existing literature on SGE mainly assess the theoretical SGE potential considering average salinity gradients that are assumed constant over time. However, the thermohaline structure of natural water bodies is highly variable on time and space as a function of tides, changes in freshwater inflows, rain, solar irradiation, and wind regimes, among other meteorological and hydrological forcers.
Large salinity gradients may develop in low inflow coastal lagoons where evaporation exceeds precipitation. If freshwater discharges are present and the morphology of the basin constrains the mixing, three different water zones are likely to be found: freshwater, seawater, and hypersaline. The three zones can allow different configurations of SGE exploitation. Equipped with this knowledge, researchers from the National Autonomous University of Mexico: Dr. Xavier Chiappa-Carrara and Professor Cecilia Enriquez, in collaboration with Dr. Oscar Reyes-Mendoza at The College of the Southern Border and Dr. Oscar Alvarez-Silva at the Northern University in Colombia assessed the feasibility of implementing SGE in hypersaline coastal lagoons with a case study on the coastal lagoon: La Carbonera in Yucatan, Mexico. Their work is currently published in the research journal Sustainable Energy Technologies and Assessments.
In their approach, the researchers first analyzed the variability of salinity and temperature in the three characteristic zones of the coastal lagoon and the correlation of the variables with atmospheric forcing. This was done using 1-year records of in situ measurements. Lastly, the theoretical potential for SGE and the intra-annual variability of that potential were assessed considering the three possible harvesting configurations of mixing among fresh, sea, and hypersaline water.
The authors reported that a high seasonal variability of the energy potential for the three analyzed exploitation configurations was identified, as a function of the seasonal variability of the atmospheric forcers of the thermohaline structure of the lagoon, mainly the wind speed, air temperature, and atmospheric pressure. Additionally, higher frequency variations and extreme events overcome the seasonal variability. The more dramatic variations recorded in the lagoon were a consequence of the nearby transit of Tropical Storm Hanna (Figure 1). It was formed in the Gulf of Mexico on October 21st, 2014, and lasted in the region until October 29th (Figure 1A) with two transits across the Yucatan Peninsula. The effect of this storm on the salinity concentrations in the lagoon (Figure 1B) was notorious at both hypersaline and seawater locations. The salinity strongly decreased as the storm developed, reaching the lowest values of the whole year of measurements. The sensor installed at the mouth of the lagoon (seawater location), was actually lost during this period of increased water runoff towards the ocean. The hypersaline location started with the recovery of its usual salinity values between the 26th and 28th of October, when the storm’s eye was far from the Yucatan Peninsula. However, the storm returned and the salinity concentrations were reduced again on the 29th and remained lower than average until the 5th of November. The disturbance of this storm on the SGE potential is observed in Figure 1C. The energy potential of the three possible configurations of diluted and concentrated waters was strongly reduced and variable during the storm period, following the trend of salinity disturbances. For a real energy production plant, it would imply a total shut down during an event like this, affecting the annual yield and the reliability of the plant. This is the first time that the effects of extreme weather conditions on the SGE potential are reported from field data.
The high variability of the potentials is essential to highlight the importance of implementing long-term measurement schemes of the thermohaline structure of natural systems for the assessment of realistic estimations of the SGE resources and the reliability of the energy exploitation at those systems.
In summary, the study presented a comprehensive analysis of the climatology and possible applications for energy generation in a tropical hypersaline coastal lagoon. Results showed that the thermohaline structure vary significantly in each of the three locations at different time scales (diurnal to seasonal) depending on tides, winds, air temperature, atmospheric pressure, and solar radiation. This allowed to suggest that the highest annual energy yield would be obtained from alternating among the mixing configurations throughout the year, depending on the specific seasonal thermohaline structure of the lagoon.
Oscar Reyes-Mendoza, Oscar Alvarez-Silva, Xavier Chiappa-Carrara, Cecilia Enriquez. Variability of the thermohaline structure of a coastal hypersaline lagoon and the implications for salinity gradient energy harvesting. Sustainable Energy Technologies and Assessments, volume 38 (2020) page 100645.