(151b) One Year Operation of a Salinity Gradient Solar Pond in Northern Cyprus- Experimental Investigations and CFD Simulation

Long Term Operation of a Salinity Gradient Solar Pond in Northern
Cyprus- Experimental Investigations and CFD Simulation

¹Madina
Obaidullah, ²Sultan Kadyrov, ³Soudabeh Gorjinezhad, ⁴Onur
Taylan, ⁵Mehdi Amouei Torkmahalleh, ⁶Goodarz Ahmadi

1-      Sustainable
Environment & Energy Systems, Middle East Technical University – Northern
Cyprus Campus, Guzelyurt, Kalkanli, Mersin 10, Turkey

2-      Chemical
Engineering Program, Middle East Technical University Northern Cyprus Campus,
Guzelyurt, Mersin 10, Turkey

3-      Civil
Engineering Department, School of Engineering, Nazarbayev University, 53
Kabanbay batyr ave., Astana, 010000, Kazakhstan

4-      Mechanical
Engineering Program, Middle East Technical University Northern Cyprus Campus,
Guzelyurt, Mersin 10, Turkey

5-      Chemical
Engineering Department, School of Engineering, Nazarbayev University, 53
Kabanbay batyr ave., Astana, 010000, Kazakhstan

6-      Mechanical
Engineering Department, Clarkson University, Potsdam, NY, 13699- 5725

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. Studies indicate that the temperature in solar ponds may in general
reach up to 70-80°C implying that the thermal energy from solar ponds can be
useful to applications with low grade energy demand [1]. One of the most
important advantages of solar pond over other renewable energy sources, such as
solar collectors, is its lower cost of investment [1]. Solar pond is
environmentally friendly in particular when it is used for electricity
generation. The heat obtained from solar pond can be converted into electric
power even at low temperatures [2]. In this regard, organic Rankine cycle
engines are generally run using the temperature difference in a solar pond. For
applications where the organic fluid fails to operate due to low temperature
difference, thermoelectric generators can be a good candidate to replace
organic Rankine cycle engines for power generation [3].

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). 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 responsible for
energy storage, and is known as the lower convective zone (LCZ) [5]. The
working principles of solar ponds are quite simple. In the absence of salinity
gradient, the solar radiations reaching a pond are mostly absorbed by the lower
water levels and will cause the water to heat up. The heated water is then
risen up to the pond surface due to buoyancy effect and loses its thermal
energy to the atmosphere. Thus, the main reason behind imposing salinity
gradient to solar ponds is to create a density gradient restricting such
buoyancy driven natural convection and as a result to trap the thermal energy at
the bottom of the pond [4].

Northern Cyprus is enriched in
solar energy in particular during summer. The performance of a solar pond in
this region has not been evaluated yet. To take advantage of solar energy in
Northern Cyprus, a salinity gradient solar pond has been constructed and
operated at Middle East Technical University Northern Cyprus Campus (METU NCC)
located at Guzelyurt, Northern Cyprus (Figure 1). This
study is primarily concerned with describing the development, validation and
use of CFD for computer modeling of flow and thermal analysis of the
experimental salinity gradient solar pond at METU NCC. The pond has started
operating since Oct. 2014 and experimental results are being recorded
accordingly. The CFD commercial software considered to be utilized is ANSYS
FLUENT and the challenge is to visualize the transient effect of temperature distribution,
salinity concentration, velocity field and the degree of influence of wind
speed on the overall stability and performance of the solar pond over time. 

Figure
1.
Constructed Solar Pond at Middle East Technical University (Northern Cyprus)

  2. Materials and
Experiment

A cylindrical salinity gradient
solar pond of 61 cm diameter, 55 cm height and 1.2 cm thickness installed and
operated since October 8^th, 2014 at Middle East Technical University
Northern Cyprus Campus (METUNCC) located at Guzelyurt, Northern Cyprus. Three
inlet ports were installed to carefully add the solutions to the pond to create
the three salinity layers. Also, these inlets were used 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 divided in three
zones. The bottom zone (LCZ) consisted of 75 liters of saturated salt solution
(concentration C). The middle zone (NCZ) consisted of three equally divided
sub-layers, each 15 liters in volume, with 0.75C, 0.5C and 0.25C
concentrations. The top zone (UCZ) consisted of 15 liters of fresh water. The
rest of the pond was left empty. The pond was equipped with three sampling
valves to withdraw samples from each layer to monitor the salt concentrations
using conductivity measurements. Six thermometers (2 in each zone) 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 other time of the day was performed
as needed. The bottom and sides of the pond were insulated with a 2.5cm thick
thermal insulator. The inner surfaces of the pond were painted black. Excess
(undissolved) salt was added to the bottom of the pond, to ensure saturation of
the bottom layer. The ambient temperature and solar radiation were monitored
using a sun tracker (Kipp and Zonen) placed next to the solar pond. Wind
velocity and relative humidity were also monitored throughout the experiment.

  3. Computational Scheme
and Modeling

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. A two-dimensional
computational domain is considered for investigating the transient behavior of
the pond representing a cross-section of the solar pond. Accordingly, the
rectangular domain of consideration is that of the existing pond dimensions
with 61 (cm) in width and 50 (cm) in height. 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
ambient air, is taken to be at ambient temperature with an average wind speed according
to local meteorological data. The salinity concentration of the LCZ is kept at saturated
level, while the top surface is set to zero concentration, initially.

The modeling of the pond is divided
into two phases. Firstly, a simple model is going to be developed in which it
is assumed that the concentration level remains the same due to the present
regular injection of brine solution to the system. Accordingly, the thermofluid
properties will be assumed to be primarily a function of temperature and will
be evaluated for each layer at its initial concentration. In the second phase,
the model will be enhanced by defining the concentration as a varying parameter
to the system. In this regard, the variation of properties with concentration
can be taken into account and the significance of salt diffusion and its
influence on the stability of the pond can be investigated.

4. Results and Discussion

Figure 2 shows the
average monthly solar pond temperature as well as the insolation variations
from October 2014 to April 2015. As can be seen, the pond has been functioning
as the temperature differences among the three layers have been established.  The
pond temperature variations are well correlated with the ambient temperature and
insolation variations. The temperature at all layers decreased from October
2014 to end of December 2015, but then beginning January 2015, it increased.

Figure 1. Average monthly
temperature and insolation variations of the solar pond

  References

[1.]       Sukhatme,
S., & Nayak, J. (2008). Solar energy: Principles of thermal collection and storage (3rd ed.). New Delhi: Tata McGraw-Hill.

[2.]       Akbarzadeh,
A., Bernad, F., Casas, S., Gibert, O., Cortina, J. L., & Valderrama, C.
(2013). Salinity gradient solar pond: Validation and simulation model. Solar
Energy, 98,
366-374.

[3.]       Singh,
B., Gomes, J., Tan, L., Date, A., & Akbarzadeh, A. (2012). Small Scale
Power Generation using Low Grade Heat from Solar Pond. Procedia
Engineering, 50-56

[4.]       Mehdizadeh,
M., & Ahmadi, G. (2014). Two-dimensional Computer Simulation of Salinity
Gradient Solar Pond Operation. Proceedings of ASME 2014 Fluids
Engineering Summer Meeting, FEDSM2014.

[5.]       Jaefarzadeh,
M. (2005). Thermal behavior of a large salinity-gradient solar pond in the city
of mashhad. Salinity Gradient Solar Pond. Retrieved October 14,
2014, from http://profdoc.um.ac.ir/articles/a/203017.pdf