(118a) Solar Energy Harvesting and Storage Using a Salinity Gradient Solar Pond Under the Northern Cyprus Climate Conditions-Experimental Investigations and CFD Simulation

Authors: 
Eroglu, D., Middle East Technical University Northern Cyprus Campus
Habib, A. R. R., Middle East Technical University Northern Cyprus Campus
Gödelek, S., Middle East Technical University Northern Cyprus Campus
Askari, M., Middle East Technical University Northern Cyprus Campus
Pakzad, N. Z., Middle East Technical University Northern Cyprus Campus
Kahraman, O., Middle East Technical University
Kadyrov, S., Chemical Engineering Program, Middle East Technical University Northern Cyprus Campus

—  Today, renewable energy sources gain importance day by day. It is crucial to develop devices and processes to supply energy from non-polluting and renewable energy sources for sustainable development of the world. Solar pond is an example of such devices that basically collects solar energy and stores it as thermal energy for a long period. Temperature in solar ponds can reach up to 100oC indicating that thermal energy from solar ponds can be useful to various applications with low grade energy demand. It was reported that the solar ponds have the annual collection efficiency in the range of 15–25 %. The heat obtained from solar pond can be converted into electric power even at low temperatures. Solar pond plants depending on their size can generate rated power up to 5 MW itself or up to 80 MW as a hybrid solar electric generation system. One of the most important advantages of solar pond compared to the other energy sources is lower investment cost. Another important property is that solar pond is environmentally friendly in particular when it is used for electricity generation by driving a thermo-electric device or an organic Rankine cycle engine. The CO2 emission for such processes is zero when a single solar system is used for electricity generation. Also, it was reported that the CO2 emission can be reduced up to 65% when a hybrid system is operated for power generation. Solar ponds normally consist of three different salinity layers. The first layer, known as the upper convective zone (UCZ), is located at the top of the pond, and contains the least salinity level. The second layer, whose salinity level increases with depth, is called non-convective zone (NCZ) and located at the middle of the pond. This layer is responsible to act as an insulator to prevent heat from escaping to the UCZ, maintaining higher temperature at deeper zones. The last layer made of a saturated salt solution, is located at the bottom of the pond, known as the lower convective zone (LCZ), and is responsible for energy storage. The performance of the solar pond may decrease with the increasing evaporation rate, and decreasing the salinity gradient. Thus, it is critical to prevent these phenomena. Northen Cyprus is enriched in solar energy in particular during summer. The performance of a solar pond in this region has not been evaluated as yet. To take advantage of solar energy in Northern Cyprus, a salinity gradient solar pond with 62 cm diameter and 50 cm height has been constructed and operated since October 8th, 2014 at Middle East Technical University Northern Cyprus Campus (METU NCC) located at Guzelyurt, Northern Cyprus. The pond has been divided in three zones, UCZ (15 liters), NCZ (45 Liters) and LCZ (75 liters). The LCZ has made of a saturated (C ) salt solution. The NCZ is consisted of three (each one 15 liters) equally divided sub-layers with 3C/4, C/2 and C/4 concentrations. The UCZ was made of fresh water. Six thermometers (3 for each layer) were installed to monitor the temperature variations at 9 a.m., 1 p.m., 5 p.m., and 10 p.m every day. Temperature recording at 6 am. was performed as needed. The pond was equipped with three sampling valves to withdraw samples from each layer to monitor the salt concentrations using a conductivity method. Three inlet ports were also installed in the pond to add proper amount of solutions to each zone during the experiments to compensate for evaporation and sampling losses as well as surface washing. The pond has been insulated and the inner surfaces of the pond were painted black. The bottom of the pond has been left with extra salt to ensure the saturation of the bottom layer. The ambient temperature, solar radiation, wind velocity and relative humidity have been also monitored. The solar pond is planned for operating for a long term enabling a realistic comparative study with the existing energy storage system, flat-plate solar collectors, installed at the METU NCC.  The results of the experiments from October 8th to November 10th, 2014 show that the pond is working properly with an increasing temperature trend from top to the bottom of the pond. Approximately 10oC temperature difference was observed between the LCZ and UCZ which is in agreement with previous studies conducted in this season of the year at different geographical regions. However, it was found 50 to 70% reduction in temperature differences between the pond salinity layers from October 27th to November 3rd, 2014. The reason was found to be due to approximately 25% reduction in solar irradiance while having a maintained salt gradient during this time period indicating the key role of solar radiation in establishing the temperature gradient across the pond. Another key finding was the fast salt diffusion rate from LCZ to the UCZ of the pond. It takes approximately two weeks to increase the salt concentration in the UCZ from zero to 100 g/l and 25 days to reach a uniform salinity between the UCZ and NCZ. Thus, washing the surface of the pond with fresh water was found to be absolutely necessary to maintain the salinity and temperature gradient. The dynamic simulation using a CFD commercial software, ANSYS FLUENT, is being performed to investigate the impact of the climate variations (ambient temperature, irradiance, wind speed and relative humidity) on the performance (temperature distribution and velocity field) of the solar pond. The geometry and meshing of the solar pond were constructed by ANSYS Workbench Design Modeler and Meshing environment. FLUENT version 14.5 was used as the CFD solver. In accordance with most salinity gradient solar pond studies, a two-dimensional computational domain representing a cross-section of the solar pond is considered for investigating the transient behavior of the pond. Accordingly, the rectangular domain of consideration is that of the existing pond dimensions. The lower wall of the rectangle is assumed to be the ground and upper boundary to be the top water level open to atmosphere. Vertical walls and the bottom surface of the pond are set to be impermeable and thermally insulated with the corresponding thermal conductivity value of the insulation material. Also the top surface, due to contact with open air, is taken to be at ambient temperature with an average wind speed according to local meteorological data. The main objective of this study is to experimentally investigate the performance of a salinity gradient solar pond for over 6 months operation in Northern Cyprus. Additionally, a CFD model is being developed  and experimentally validated  to understand the key phenomena impacting the performance of the existing experimental salinity gradient solar pond at METU NCC. This model enables us to evaluate the performance of a larger scale solar pond which is aimed to be operated in the future at METUNCC. The first phase of our study was presented in AIChE annual meeting 2014, Atlanta, GA as on oral presentation. As the experiments and modeling progress we would like to present our finding in AIChE spring meeting 2015.

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