The Use of a Parabolic Solar Concentrator in Nasiriya city, Iraq

In this paper, it presents a detailed analysis of the use of the parabolic solar concentrator in heating and boiling water, as the parabolic solar concentrator was manufactured with a diameter of (96 cm), the focal length of the dish is (72 cm), and the focus depth of the dish is (8 cm). It was made using a parabolic dish with a diameter of (96 cm) and using glossy aluminum foil as a reflective sun ray after being cut into strips 10 cm wide and glued to the inner surface of the dish. Metal tin cans with two capacities (1L and 2L) were used as the absorbent receiver. Experiments were conducted to boil water from the roof of the house in Nassiriya city. The study calculated the optical efficiency of the equivalent solar concentrator, the amount of heat output as a result of the fall of concentrated solar radiation on the receiver, the amount of useful heat gained, the thermal losses from the receiver, the collector efficiency. Many tests were conducted in Nassiriya city weather conditions. The results of the experiments showed that the efficiency of the solar center mainly depends on the diameter of the concentrator dish, the quality of the reflector used, the time of heating the water, and the closing to midday. While the high ambient air velocity leads to a decrease in the receiver temperature by increasing the heat loses to ambient air by convection, thus reducing the efficiency of the solar concentrator; as well as the accuracy of directing the dish towards the sun and determining the focus accurately also affects the efficiency of the solar concentrator.


Introduction
For a long time, researchers presented research studies and many practical experiments on the topic of using solar concentrators in heating, boiling water, and producing steam, and how to improve the efficiency of solar concentrators and reduce heat losses. There was a lot of research conducted on solar concentrators, as the idea of using solar energy to boil water and cook food is not new, as the first scientist to test solar cooking was a German physicist named Tschirnhausen (1651-1708) [1]. Use a large lens to focus the sun's rays and boil water in a clay bowl filled with water for boiling. Subsequently, a lot of research was done on the exploitation of solar energy in cooking and water heating, and improving the efficiency of these concentrators and the rest of their important parameters. [2] Designed, made, and evaluated a solar parabolic dish with a diameter of (2.2 m) and an aperture area of (3.8 ). To improve thermal efficiency, reduce heat loss, and realize steam production from concentrated solar energy, this solar collector is equipped with manual tracking. The sun's heat was concentrated on a black absorbent that was placed in the focal point of a dish concentrator. The researchers found that for the beam solar intensity (800 ⁄ ) at noon, the direct heat that can be obtained was 200 W, and they found that the temperature of the oil outlet reaches 50 at the beginning of the experiment and then increases after 30 minutes until it reaches the maximum value (80 ). The average concentration ratio and average energy efficiency are respectively about (150) and between 40% to 77%. This device can be used in various applications such as pasteurization and detoxification. [3] designed and manufactured a solar concentrator dish with a diameter of (1.6 m), depth of the dish (18 cm), and Focal distance (84 cm) to heat water and produce steam, and its inner surface is covered with a reflective layer with a reflection rate of (76%) they used a stainless-steel receiver with a helical absorber inside.
They also used a tracking system to ensure the vertical sunlight fell on the reflector, and the dish was also equipped with a temperature measurement system and solar energy. [5] experimentally studied the thermal performance of a solar concentrator dish with a conical spiral absorber. A dish with a diameter of (1.4 m) and a focal length of (0.3223 m) and covered with a reflective material with a reflection of (0.86), with truncated coneshaped helical coiled receiver made up of copper and coated with nickel-chrome at a focal point, the lower and upper diameter of the receiver were (0.135 m) and (0.095 m), respectively. The researchers concluded that the light energy captured by the receiver increased by 41% during the day, the average rise in heat losses was 76% compared to the increase in solar radiation and wind speed by 41% and 42%, respectively, the collector instantaneous efficiency decreased by 12.3% during the day and an instantaneous efficiency of 63.9% was achieved.
[6] studied the utilization of solar energy to produce steam using a (1.7 m) diameter concentrator dish. The diameter of the dish was (1.7 m). A brass spiral absorbent with a diameter of (20 cm), a length of (3 m) and a diameter of (12.5 mm) was used. The solar radiation falls on the plate and is reflected in the absorbent vessel which contains the coil that carries the water. The results of this experimental work gave a good indication of the production of steam with a temperature of about 115.7 °C during a short time from the concentration of sunlight (in Iraq). Also, it is possible to produce very hot steam when increasing the length of the copper receiver coil.
[7] studied the thermal performance of a parabolic concentrator with a diameter of (200 cm) and a focal length of (66.5 cm) was studied as an asymmetric parabolic concentrator covered with a reflective aluminum foil of (0.9) reflectivity, they use absorbent volumetric SiC honeycomb size (105 collector to concentrate the rays on two types of copper absorption tubes with a length of 2.4 m, one of which has an outer diameter of (3 cm) and the inner (2.8 cm) painted black, the other dyed black and covered with an outer diameter glass tube (3.6 cm) and an inner vacuum tube (3.4 cm). The device was equipped with automatic tracking of the sun. The study aimed to obtain the necessary heat to heat the water and to study the thermal performance of the solar collector. The results showed that the system efficiency and the beneficial heat energy obtained with the evacuated glass tube were higher than that obtained from the copper tube and were directly proportional to the water mass flow rate and the amount of solar radiation incident on the surface.

Physical
Geometry of The Parabolic Solar concentrator.

Optical Analysis Model:
The parabolic collector geometry is fundamental to guarantee the proper functioning of the prototype; an error during the geometric calculation would represent the deviation of the solar the parabolic collector geometry is fundamental to guarantee the proper functioning of the prototype; an error during the geometric calculation would represent the deviation of the solar rays; consequently, the absence of temperature at the focal point, which would give way to obtaining low thermal efficiency.
To calculate the parabola, mathematical analysis was performed to find the values that satisfy the design criteria, like: of a parabolic dish ( ), Depth of concentrator dish (h), Focal length of a dish ( ), Aperture area of dish ( ) Rim angle of a dish ( rim ), and concentration ratio.
The scheme used for the analysis is shown in Figure (  The surface area of this parabola is given by the following equation [2,10]: Where: The aperture area of the dish is [11]: The focal length of the dish is given by the following equation [12]: Where a rim angle of the dish. The effect of the rim angle on the focal point position at the same diameter can be shown in Figure (2). It is shown that the focal length is decreased when the rim angle increases [13]. Figure (1) shows the main parameters of parabolic dish geometry [12]. The first is defined as the ratio between solar heat flux over on absorber ( abs) and solar flux (beam solar intensity) falling on an aperture area of a dish ( b) as shown in the following equation: It is considered a true concentration ratio because it indicates the optical losses [15]. The optical concentration ratio is not related to thermal losses and efficiency because not indicate the absorber area.
The geometrical concentration ratio is defined as a ratio between aperture area (Aa)to the absorber area ( ). It affects the choice of the receiver area which affects thermal losses.
The geometric concentration ratio can be represented in the following equation [16]: The optical efficiency is defined as a ratio between the radiation absorbed by the receiver ( ) to the radiation captured by the aperture area of the concentrator ( ) [16,17]. The following equation describes the optical efficiency: Where: s: Energy captured by the reflector.
The other definition of optical efficiency is a product of many properties of dish and absorber surface such as reflectivity of material, absorptivity and transmissivity of absorber material, shape factor (interception factor), and the effect of incident angle of solar radiation [10,17].
which can write be in the following equation: Where is the factor of un-shading or shape factor [11]: Where: : Aperture area.
= a that shaded by the receiver on the concentrator is dish reflectance, τα is transmittanceabsorptance product [13] is the intercept factor of a receiver, which is defined as the ratio of the energy intercepted by the receiver to the energy reflected by the focusing device [18] ( ) For all the concentrates and receivers used in our research: ≈1 And ( ) is the angle of incidence. As the solar parabolic dish concentrator maintains its optical axis always pointing directly towards the sun to reflect the beam, which means the incidence angle of the solar beam into the dish is zero degrees, and the cosine loss equals zero.
The reflectivity of aluminum foil which is used in this project was 0.72. And The reflectivity of pieces of the mirror was (0.70-0.84) The transmissivity-absorptivity product was 0.94 for black paint [10]. The effect of incident angle can be neglected [10]. The range of the optical efficiency is between (0.85 -0.9) for high reflective mirrors [3,10].

Thermal Analysis:
Useful heat that was exploited by the receiver is equal to the heat absorbed by water in the receiver. It can be calculated by subtracting the heat energy losses of the receiver from the heat The rate of thermal losses is separated into radiation ( rad ) and convection ( con ) losses. Equations (17) and (15) give the formulas for estimating these quantities [3]: abs ⨯h air ( abs am ) The heat convection coefficient between absorber and ambient can be calculated by the following equation [19]: A mathematical model of the radiation heat losses from the absorber surface can describe in the following equation (18).
So that, the collector efficiency of the system can be written in the following equation [17].
: useful energy delivered to the working fluid.
: the energy incident on the concentrator's aperture.

Practical Modeling Analysis:
Below we list the calculations of samples of experiments in which a parabolic solar concentrator model was used, which was manufactured using a satellite dish, the diameter of the dish is 96 cm, and aluminum reflective paper was affixed with a reflectivity rate of 72%, and using a receiver with two different capacities (1 liter, 2 liters) are metal cans that had been painted black.

Experimental Results and Discussion:
The parabolic solar concentrator was tested in Nasiriyah, southern Iraq, at the site (31.058° N 46.2573° E) [20]. Two experiments were conducted before midday, in which the solar concentrator with a diameter of (96 cm) was used once with a receiver with a volumetric capacity of (1 liter), and the second experiment was conducted with a receiver with a volumetric capacity of (2 liters. Also, two other experiments were conducted after midday with the same solar concentrator and receivers under a clear sky as shown in Figure (   The following outputs were calculated: The optical efficiency of the parabolic solar concentrator ( opt ), energy captured by the reflector ( ), the heat lost due to the heat radiation convection ( ), the amount of heat gained ( useful ), as well as the measurement of the efficiency of the solar cooker( c ). This requires recording the important experimental parameters that we need in computing the important outputs: such as solar beam intensity ( ) falling on an aperture area of the dish , which is measured with a solar meter, and water temperature ( ), the receiver surface temperature at the location of the concentrated ray incidence ( ), the ambient temperature ( ), these temperatures are measured by thermocouples, as well as the ambient air velocity around the working device ( ), which is measured by an anemometer. This data is calculated and recorded every 20 minutes, and using this data for calculating the above parameters ( opt , s, , useful, and c ). And then draw curves that show the change of these variables during the period for heating water from its initial temperature until reaching the boiling point.    As for Figure (8

Conclusions:
This research paper presented a practical study on the use of the parabolic solar concentrator in heating and boiling water in Nasiriyah city, southern Iraq, where the model of this concentrator was designed using simple and cheap materials that are available in the local market. The following conclusions were reached: 1. The shape factor depends on the area of the thermal receiver shade reflected on the solar concentrator and it does not depend on the time of the experiment nor the volumetric capacity of the thermal receiver.
2. The optical efficiency can be improved and thus increase the amount of concentrated heat by using a high-reflection reflector and using a thermal receiver with high absorptivity, low emissivity, and reflectivity, and does not depend on the volumetric capacity of the receiver nor on the time of the experiment. 3. To reduce the thermal losses from the thermal receiver, the focus area on the receiver surface should be reduced and the solar concentrator should be used to boil water when the ambient air velocity is low, and the thermal losses depend on the focus area, the receiver surface temperature, size, and quality of the receiver and the experiment time.
4. The increase in the amount of useful heat used by the receiver and the collector efficiency depends on the diameter of the concentration dish, its optical efficiency, the size and quality of the receiver, the amount of concentrated heat reflected from the concentrator, and the thermal losses of the receiver.