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Experimental and Theoretical Investigation to
Generate Steam by Parabolic Trough Solar
Collector with Using Different Heat Transfer Fluids
Mohammed Hasan Abbood
1and Mohammed Mohsen Mohammed
21 PhD, Assistant professor, Mechanical Engineering Dep., College of Engineering, University of Kerbala. 2 M.Sc. student, Mechanical Engineering Dep., College of Engineering, University of Kerbala.
Abstract-- This paper describes the mathematical model and experimental tests for parabolic trough solar collector (PTSC). The model is based on detailed energy balances, and it has been applied to evaluate collector thermal performances with different heat transfer fluids. The influence of fluids temperature has been studied from the point of view of heat gain and thermal efficiency. The working fluids which have been selected for this study are hydraulic oil, ethylene glycol based water, and water. An experimental investigation for testing the performance of parabolic trough solar collector (PTSC) is carried out to generate steam at moderate
temperature. The tests have been carried out in KERBALA climatic conditions (32.34º N, 44.03º E) during selective days. The results show that the best performance was in case of using hydraulic oil as heat transfer fluid. The maximum enhancement in thermal efficiency was 31.7% between hydraulic oil and water. While the enhancement in the thermal efficiency reaches about 20.4% between ethylene glycol and water.
Index Term-- Solar collector, Concentrated solar collector, Parabolic trough collector, Receiver tube, Thermal efficiency.
NOMENCLATURE
Aa Collector Aperture area K(Ɵ) Incident Angle Modifier
Ag Area of Glass Cover L Collector length
Ar Area of Receiver Nu Nusselt Number
Cp Specific Heat Pr Prandtl Number
Dgc Glass Cover Diameter Qu Useful Heat Gain by the collector
Dr Receiver Diameter Re Reynolds Number
FR Heat Removal Factor S Absorbed Solar Radiation Per Unit Area
hrad,gc-a
Radiation Heat Transfer Coefficient between
Glass and Ambient Air Ta Ambient Temperature
hrad,rec-gc
Radiation Heat Transfer Coefficient between
Receiver and Glass Cover Tg Temperature of the Glass Cover
hconv,rec-gc
Convection Heat Transfer Coefficient between
Receiver and Glass Cover Tf,i Inlet Fluid Temperature to the Collector
hw Wind Heat Transfer Coefficient Tf,o
Outlet Fluid Temperature from the Collector
Ib Beam Radiation Tw
Water temperature inside the heat exchanger
K Thermal Conductivity UL Heat Loss Coefficient
1. INTRODUCTION
In recent years, competition and development among countries to produce energy cleanly away from traditional ways of producing energy with very large residues on the environment. Many countries now rely more on renewable energy and are working to transform them from one form to another and develop them. One of the most important renewable energy sources that can be depended on the solar energy. Most the application of steam generation for industrial use or electricity production by solar energy, used the method of concentrator solar radiation. Nowadays, parabolic trough solar technology is the most used solar application. The following researches will show the summary for previous theoretical and practical work regarding parabolic trough system.
parabolic trough collector used for solar cooling and heating by using energy balance correlations between the absorber tube, glass tube, and surroundings. The results of the comparison between the mathematical model and the experimental data indicate some differences, including high measured glass temperature and low measured efficiencies. These differences are attributed to heat loss at the supports and the connectors and the low assumption of the absorptivity. Ouagued et al. [4] built a numerical model of a parabolic trough collector under Algerian climate. In this model, the receiver is divided into several segments, and heat transfer balance equations which rely on the collector type, optical properties, heat transfer fluid (HTF), and ambient conditions are applied for each segment. This work led to the prediction of temperatures, heat loss, and heat gain of the parabolic trough. Raj et al. [5] performed numerical and experimental analysis on the performance of the absorber tube of a parabolic trough collector system with and without insertion. Water was used as the heat transfer fluid and three different mass flow rates had been used which were 33Kg/hr, 63 Kg/hr and 85 Kg/hr. It was seen that presence of inserts gave a higher rise in the outlet temperature when compared with the one without insertion which was due to the fact the area of heat transfer was increased with the insertion. It was also observed that the thermal stresses on the tube with insertion were less than that without insertion. A study then done at 2014 by Filho et al. [6] to describes the methodology and the results of an experimental and numerical investigation of the thermal losses of a small scale parabolic trough collector, The numerical simulation when compared to the experimental results for the losses shows the degradation of the vacuum in the annular region of the absorber tube also the collector was tested with solar radiation and the efficiency obtained varied from 0.3 to 0.55.
The present work aims to develop a mathematical model and experimental tests for parabolic trough solar
collector to generate steam by using different types of heat transfer fluid (HTF) and with flow rate of 3 liters per minute (LPM).
2. PARABOLIC THROUGH SOLAR COLLECTOR
Parabolic trough solar collectors (PTSC) are considered as one of the most mature, successful, and proven solar technologies for electricity generation and steam requirements. The PTSC are typically operated at temperatures range 50-400 ºC. The PTSC system consists of support structure, parabolic trough, and receiver tube. The parabolic trough shaped mirror concentrate sun rays onto receiver tube which is placed in the focal line of the parabola trough as shown in figure (1). The solar parabolic trough is made up of several reflectors. The receiver tube is typically composed of glass cover and an absorber tube with a vacuum between these two elements, to reduce heat losses by convection. The receiver of PTSC usually paint with selective coating in order to maximize solar radiation absorption. The absorbed solar radiation is conveyed to the heat transfer fluid (HTF) that flows inside the absorber tube. The HTF is provided heat to the thermal system directly by using the fluid or indirectly by transferring heat using a heat exchanger.
3. Heat transfer fluid (HTF)
Parabolic trough solar collectors PTSC used heat transfer fluid HTF that flows through the receiver tube, which is collecting and transporting solar thermal energy to storage tanks or heat exchanger. The HTF leaves the parabolic solar collector is utilized to produce steam that used either in industrial processes or to generate electricity. Some criteria must be taken in considered when choosing a HTF such as thermal capacity, the coefficient of expansion, viscosity, boiling point and freezing point. The most commonly HTFs used in the applications of PTSC are air, water, glycol and hydrocarbons oil.
Fig.1. The PTSC system
4. MODEL ANALYSIS
A theoretical model of PTSC has been presented in detail. The model is mainly consisting of three parts. The first part is estimating the solar radiation that reaches the collector. The second part is optical analysis. The purpose of
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experimental data. So the equations of this model can be describe here as:
Optical efficiency
The optical efficiency is defined as the ratio of energy reaching the absorber tube to that received by the reflector aperture [7] :
(1)
The actual amount of absorbed radiation on the receiver is calculated by [8]:
( ) ( ) (2)
Thermal model of PTSC
The thermal model was based on a one-dimensional steady-state thermal-resistance heat transfer model. This model can predict the behavior of any parabolic collector design under any ambient condition and with a variety of work parameters [9]. The heat transfer model considers radiation and convection heat transfer modes from the solar radiation to heat transfer fluid (HTF) as shown in figure (2).
Fig. 2. Thermal resistance model Heat loss coefficient (UL) can be estimate based on receiver area:
[ (
) ]
(3)
For the space between the glass cover and receiver is evacuated, and convection losses are negligible, UL is calculating from the
following equation:
[ (
) ]
(4)
The convection heat transfer coefficient between the glass cover and ambient due to wind (hw) [7]:
(5)
For the convection loss coefficient, the Nusselt number (Nu) and the Reynold’s Number (Re) can be used as follow [7]:
( ) (6)
( ) (7)
(8)
The radiation heat transfer coefficient between glass cover and sky temperature is:
Where sky temperature is calculating from the following equation [10]
(10)
The radiation heat transfer coefficient between glass cover and receiver is:
( ) ( )
( )
(11)
Heat Transfer to Working Fluid
The heat transfer from the receiver tube to the fluid (HTF) must be characterized by turbulent or laminar flow conditions accordingly, the evaluates the Reynolds number of the fluid [11].
(12)
Nusselt number of the fluid :
(13)
( ) ( ) (14)
The heat transfer coefficient to the fluid, is evaluated:
(15)
Overall Heat Transfer Coefficient and heat removal Factor
The overall heat transfer coefficient, Uo, needs to be estimated, this should include the tube walls because the heat flux in a concentrating collector is high. Based on the outside tube diameter, this is given by [11]
[
( )
]
(16)
The collector efficiency factor, F', is given as:
( )
(17)
The collector heat-removal factor FR, it is one of important design parameter where it is a measure of the thermal
resistance encountered by the absorbed solar radiation that reaching the collector. Its value depends on flow rate of working fluid inside the receiver, FR is found by this equation [12]:
[ ( )] (18)
Thermal Efficiency of parabolic trough collector
The thermal efficiency of a PTSC can be defined as the ratio of heat gained by the collector, Qu, to the total incident radiation that is incident on the aperture of the collector
(19)
Where the useful heat gained, Qu, is a function of the outlet and inlet temperature of the heat transfer fluid in the receiver tube as shown below:
( ) (20)
The useful heat collected by the receiver can also be expressed in terms of optical efficiency, heat loss coefficient, heat removal factor, and inlet receiver fluid temperature:
[ ( )] (21)
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( )
(22)
5. Numerical solution for a mathematical model The mathematical model is applied to study the performance of a parabolic trough solar collector (PTSC). This model consists of three parts. The first one is a solar radiation model, the amount of beam radiation incident upon collector is calculating using relationships and equations between the Earth and the sun. The second one is the optical model. In this part, has the ability to estimate the optical efficiency. The last one, is the thermal model, which estimate the amount of energy collected by the working
fluid, heat loss and thermal efficiency. This model is implemented by MATLAB software. There are two types of inputs: input variables and input parameters. The input variables are changeable through the daytime such as date, time, ambient temperature and wind speed. These input variables must be entered into the code in order to run the simulation. The input parameters have fixed values and do not change through day time such as geometrical properties, latitude and longitude of the PTSC location, outer and inner diameter of the glass envelope and the receiver, aperture area of the trough, and optical properties.
Fig. 3. Flow chart of MATLAB
For the solar model, inputs are time, and day date. All the equation and relationship of sun-earth are considered to produce angle of incidence and beam radiation. For optical model, which the inputs are emissivity, absorptivity, transmisivity, and reflectivity of the tube and parabolic trough. Also, Incident angle modifier and End loss factor are solved, which used to estimate the optical efficiency of PTSC. For thermal model in which consist of heat transfer equations by convection and radiation.
To find all temperatures and solve these equations, an iterative process was suggested by Duffie [13]. Procedure that used to solve this equation is by initially guessing a value (closer to ambient temperature) for the outer glass temperature, then solve the heat transfer equation through heat collection element and evaluate the inner glass temperature. The outer glass temperature (initial guess) needs to be checked by performing an energy balance on the glass cover to evaluated new value of outer glass temperature. By comparing the two values of outer glass temperature. If the comparison is not equal, a new guessing value will apply as shown in flow chat, figure (3). after all
temperatures required are evaluated, heat losses, heat gain and thermal efficiency are calculated.
6. Experimental rig and test procedure
A PTSC is a concentrating collector, which the reflector surface has a parabolic form as shown in figure (4). Solar radiation falling on trough then concentrated on the focal line, where a receiver tube was placed. The parabolic collector consists of three troughs, which are arranged in series so that the heat transfer fluid (HTF) will gain heat gradually as it flows through the tubes one by one. By circulating the HTF, thermal energy in the HTF will transfer to water by using a coil tank heat exchanger.
Fig. 4. The PTSC system
Table I Specifications of the PTSC
ITEM Value/Type
Rim angle (θr) 77.72
Focal distance (f) 233.1 mm
Glass cover external diameter (Dg) 58 mm
Receiver external diameter (Dr) 47 mm
Collector aperture width (W) 1040 mm
Concentration ratio (C) 46.8
The effective aperture width (W) 750 mm
Collector length (L) 1800 mm
Parabolic curvature 1220 mm
Total Collector aperture area 3.73 m2
Reflective material Aluminum composite panels
Tracking system One axis auto tracking system
Electrical motor Electrical motor with moving arm
The experimental setup used for testing the PTSC system shown schematically in Fig. 5. It consists of the following (1) support structure, (2) three parabolic troughs (3) evacuated glass tube (4) coil-tank heat exchanger, and (5) auto tracking system.
First, it is very important to clean the reflector from any dirt or dust. Then, pump and tracking motor wires are connected to electricity to running the test. Heat exchanger tank is filled with 30 liters of water (half volume is filled with water only). In the current experiment work, the working fluid is circulated in a closed cycle. The collecting tank is filled with working fluid from the main supply. After this tank had been filled, the outlet from the tank connecting to the pump, then the working fluid is pumped to the inlet of troughs and the trough outlet to the coil inlet then exit from
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Fig. 5. Schematic Diagram of Experimental Setup.
7. Results and discussion
This study presents the experimental and theoretical investigation to generate steam by using a parabolic trough solar collector PTSC system. Thermal performance for PTSC is determined theoretically in MATLAB (R2014a) software by evaluating the parameters that effected on the system such as solar radiation, wind speed, day number, and temperatures of HTF. Also, the thermal performance of PTSC was calculated from measured data experimentally. The measured data was recorded with different heat transfer fluid hydraulic oil, ethylene glycol, and water. The tests were conducted at the college of engineering/ University of Kerbala (44.03° Longitude and 32.34° Latitude), through the clear sky days for the months of July and August from 9:00 A.M until 2:30 P.M,
8. Outlet temperature
In this cases, the data measured experimentally are recorded for three days under clear sky similar weather (25th of July, 5th and 9th of August, 2018). all the necessary data have been measured to study the performance of the PTSC with hydraulic oil, ethylene glycol, and water as HTF. Figure (7) shows the outlet temperature of working fluid from the collector for different HTFs, which is increase through the test day with time passing for all type of the HTF until it reaches to the end of the experiments. This can be referring to the actuality that the incident solar radiation which falls on the collector directly due to auto tracking system of the collector continuously and directly toward the solar radiation. The maximum difference between hydraulic oil and water was 21%, while the difference between ethylene glycol and water was 7%. Figure (8) shows the temperature difference between the outlet temperature and inlet temperature through the solar collector for each working fluid. This is referring to hydraulic oil is better than ethylene glycol and water, which is heated up faster and its temperature rises more. this is due to the low heat capacity of hydraulic oil as compared with the two other fluids.
8.1 Heat gain through the collector
The useful heat gained was calculated from the inlet and outlet temperatures through the collector, specific heat, and flow rate of HTF. Figures (9) and (10) show the
experimental and theoretical heat gained from the collector with time at 3 LPM flow rate respectively. It was noticed from experimental results at figure (9), the maximum heat gained enhancement between the hydraulic oil and water was 29%, while the maximum enhancement between the ethylene glycol and water was 16.5%. The useful heat gain for the hydraulic oil is better than the other two HTF, and the heat gain for ethylene glycol is better than the water and lower than the hydraulic oil. This is because of the difference in the thermal properties of each type of fluid used and their ability to raise its temperature. The comparison between the experimental and theoretical results show that the maximum difference is 10.4%, 16%, and 14.9% for hydraulic oil, ethylene glycol based water, and water respectively. This is due to several factors such as the assumptions that assumed during the implementing of the model, applying different equations, correlations, and Earth-sun relationship in the model, the effect of dust and clouds on the solar radiation that measured experimentally is significant when compared with theoretical solar radiation, and also thermal and optical losses that calculated in this model can be lower than that the actual losses that have been significantly influenced by the results of experimental heat gain.
8.2 Heat gained by the heat exchanger tank
The heat gained by the water in the heat exchanger tank is computed every 15 minutes through the tests with different HTF (hydraulic oil, ethylene glycol, and water). Figures (11), (12), and (13) show the comparison between the three types of HTF. It is found that the temperature of water in the tank of heat exchanger increase with time passes. As a result of the solar radiation increase with time passes. It is increased from 48.2oC to 95.4oC, 47.9oC to 92.1oC, and 47.8oC to 90.6oC for hydraulic oil, ethylene glycol, and water, respectively.
heat exchanger, and their relation with a variation of solar radiation during the test period.
8.3 Thermal efficiency
The collector thermal instantaneous efficiency is determined from useful heat gain, solar radiation, and aperture area, to analyze the performance of the PTSC system. Figure (14) shows the comparison between the experimental thermal efficiency for different HTF hydraulic oil, ethylene glycol based water, and water. The maximum difference in thermal efficiency was 31.7% between hydraulic oil and water. While the difference between
thermal efficiency reaches about 20.4% between ethylene glycol and water. It is clear that hydraulic oil gives better thermal efficiency than other HTF, and thermal efficiency of ethylene glycol is higher than water but lower than hydraulic oil with the same flow rate. This is due to the different heat capacity of HTF. Figure (14) and (15) show the experimental and theoretical thermal efficiency. The comparison between the experimental and theoretical results show that the maximum difference is 9.7%, 15.1%, and 21.1% for hydraulic oil, ethylene glycol based water, and water respectively. This is due to several factors such as dust, clouds, wind speed, the transmissivity of the glass
cover, and absorptivity.
Fig. 7. outlet temperatures for different HTF with time at 3 LPM flow rate
Fig. 8. temperatures difference between inlet and outlet for different HTF with time at 3 LPM flow rate
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Fig. 9. experimental heat gain for different HTF with time at flow rate 3 LPM
Fig. 10. theoretical heat gain for different HTF with time at flow rate 3 LPM
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Fig. 11. experimental heat gained by water and steam temperature with time for hydraulic oil as working fluid
Fig. 12. experimental heat gained by water and steam temperature with time for ethylene glycol as working fluid
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Fig. 13. experimental heat gained by water and steam temperature with time for water as working fluid
Fig. 14. experimental thermal efficiency at different HTF with time
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Fig. 15. theoretical thermal efficiency at different HTF with time
9. Conclusions
A mathematical model has been implemented for studied the thermal performance of the PTSC so as to generate steam. This model takes into consideration the heat transfer equations and correlations and heat transfer mechanism that including radiation and convection. An experimental investigation has been carried out for analyzing the thermal performance of PTSC experimentally so as to generate steam at medium temperature. In this study, the experimental tests carried out with different heat transfer fluids HTFs. The performance of the PTSC system depends on the measured parameters such as ambient temperature, inlet temperature, outlet temperature, and solar intensity. A peak experimental efficiencies close to (33.2%, 28.5%, and 22.7%) were obtained for parabolic trough collectors with hydraulic oil, ethylene glycol based water, and water respectively.
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