(467e) One Year Operation of a Salinity Gradient Solar Pond in Northern Cyprus

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

¹Soudabeh Gorjinezhad,
¹Sultan Kadyrov,  ²Mohammad Askari, ¹Negar Zare Pakzad, ¹Mehdi Amouei Torkmahalleh, ³Goodarz
Ahmadi

1-     
Chemical
Engineering Program, Middle East Technical University Northern Cyprus Campus, Guzelyurt, Mersin 10, Turkey

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

3-     
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  SEQ Figure \* ARABIC 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. Thus, in the first
phase, the concentration will not be considered as a modeling parameter and it
is aimed to primarily study the variation of the temperature using the most
simplified model possible. 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. In addition, the results between
both cases and the experimental results will be compared to justify the
computational complexity added to the model.

4. Results and
Discussion

Figure
2 shows the average monthly solar pond temperature variations from October 2014
to February 2015) as well as the average monthly ambient temperature and
irradiance. 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 variations. The temperature at all
layers decreased from October 2014 to end of December 2015, but then beginning
January 2015, it increased as solar irradiance increased.

Figure 1. Average
monthly temperature 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