A parabolic solar collector for harnessing solar energy in Bucaramanga, Colombia

In this work, a solar energy collection system based on a parabolic solar collector adjusted to the conditions and availability of energy was designed to examine this type of collection device and evaluate the energy potential when installed in an educational institution. To do this, data from the historical series of solar radiation compiled by the POWER project (Prediction of Worldwide Energy Resources) were analyzed and compared with data from the Institute of Hydrology, Meteorology and Environmental Studies in Colombia (IDEAM)


Introduction
There has been considerable development regarding renewable and alternative energies as technological advances occur worldwide.In the report "Renewable Energy World Status Report" industry trends are presented with respect to biomass, geothermal energy, hydropower, ocean energy, concentrating solar thermal energy, solar heating, and wind power [1]- [3].China, the United States, Turkey, Morocco, Denmark, Austria, and Iceland stand out as the points of greatest energy development [4]- [6].In the case of Colombia, research focused on alternative electrical energy, biomass, and solar through solar panels can be mentioned, however, much importance has not been given to the development of other renewable energies [7]- [9].
In this work, a study was carried out on the use of solar thermal energy using solar collectors and the possibility of using it as energy support for the facilities of an educational institution when installed in free spaces, such as the rooftop, and thus contribute to the diversification energy and self-sufficiency [10]- [12].This is how a solar chart was made with the help of Sun Earth Tools to reflect the position of the sun with respect to the delimited area in which the solar collector will work.Likewise, solar radiation data recorded by NASA and IDEAM were used, which were characterized to determine the expected solar radiation.A parabolic solar collector was designed according to the characteristics of the institution and that would allow greater use of solar energy, considering the variation of solar radiation at different times of the day, different days of the week, and different months of the year [13]- [16].

Solar radiation
Solar radiation is the set of electromagnetic radiation emitted by the sun.It is generally expressed in terms of radiant exposure or irradiance (i.e., instantaneous power of solar radiation received per unit area) and it is expressed in kW/m 2 in the international system of units [17]- [19].Likewise, irradiation is the power that falls per unit area in a specific time and the corresponding unit in the international system is kWh/m 2 .Irradiation makes it possible to calculate energy generation according to a power value that considers variations in solar radiation in the place where the device is installed.The place designated for the installation of the solar energy harnessing system is the rooftop of building B at Unidades Tecnológicas de Santander (UTS) in Bucaramanga.The geographic coordinates of this site are latitude: 7.105 and latitude: -73.1236.

Solar radiation in the UTS
Bucaramanga is a city with a temperate-dry climate in which there are two rainy seasons and two dry seasons.The sun shines about 4 hours a day in the rainy seasons, which occur in the months of March to May and from September to November.On the other hand, the sun shines between 5 and 6 hours in the dry season months (December, January, February, June, July, and August).
The POWER project provides data sets from NASA research that contain solar-related parameters for assessing and designing renewable energy systems.Table 1 shows the average solar radiation (in kWh/m 2 ) in the UTS for each month from 2010 to 2017.These data were derived using radiative transfer models from satellite observations obtained from the NASA Langley Research Center (LaRC) POWER Project funded through the NASA Earth Science/Applied Science Program [20]- [22].The variation of monthly solar radiation in kWh/m 2 from 2010 to 2017 is shown in Figure 1.  1 and Figure 1, the months with the highest average solar radiation were August and July with 5.59 kWh/m 2 and 5.47 kWh/m 2 respectively.On the other hand, the month with the lowest average solar radiation was November with 4.6147 kWh/m 2 .This last data is important to establish the feasibility of the collector design [23]- [26].Likewise, Table 2, shows the characterization of the indirect incidence per square meter over the horizontal surface in the selected place.The Institute of Hydrology, Meteorology and Environmental Studies of Colombia (IDEAM) has a measurement station located approximately 2 km from the UTS, specifically in the Neomundo Convention Center in Bucaramanga (Figure 2).The average solar radiation (in kWh/m 2 ) taken by the IDEAM at the Neomundo station for each month from 2015 to 2017 is shown in Table 3.   POWER project IDEAM

Design of the solar collector
To obtain greater intensity per unit area, solar radiation concentrators can be used that use parabolic surfaces that concentrate the radiation at the focus of the parabola.In its design, the indirect incidence of the sun in November was used because it is the lowest average incidence in the year.A saturated water inlet temperature of 25°C and outlet temperature of 80°C, ambient temperature of 22.6°C; a heat concentration tube of 26.64 mm internal diameter through which water flows at a speed  = 0.5 m/s was also established.Table 4 shows the values of some properties of saturated water at the average temperature (52.5°).
The length of pipe to which a turbulent fluid develops hydrodynamically and thermodynamically is given by:  ℎ ≈   ≈ 10   = 0.2664

Results
The design and modeling of the collector were carried out with the help of SolidWorks and Wolfram software.
Figure 5 shows the base sketch of the collector's parabola made with Wolfram's "Parabolic Trough Solar Concentrator" tool, while Figure 6 show its dimensions.
Figure 5. Parabola of the collector Table 5 shows the results that the simulation of the parabolic solar collector yielded.With an entry angle of 2° of direct radiation, the largest area in contact with the tube is achieved, with which the radiation rectification factor or interception factor that reaches the tube is 1 [30], [31].To the extent that this angle varies, the area of incidence in the tube will be affected and the intercept factor will decrease.The rooftop of building B of UTS has an approximate area of 735 m 2 and, due to its height, solar radiation is not hindered by the shadows of nearby buildings.Considering the dimensions of the rooftop, 24 of these collectors could be installed on it, distributed as shown in Figure 8. Taking that into account, according to the parameters used, four collectors are needed to heat the water to a temperature of 80°C [32]- [34].

Conclusion
The design of a parabolic solar collector has been developed that produces a total power of 4240.64 W, where the average temperature is 52.5 °C, which is high taking into account that not all of the incident energy is used.Therefore, it is recommended to evaluate the preheating situation at the entrance of the initial collector, with the objective of reducing the average temperature and determining the increase or decrease in the number of solar collectors planned for the roof of the building, which in the case of this study was 18.

Figure 2 .
Figure 2. Neomundo measurement station locationFigure2shows the monthly average of the data from the POWER project and the IDEAM.The average solar radiation at both sites (UTS & Neomundo) is between 4.4 and 5.4 kWh/m 2 , with November being the month with the lowest average.To take advantage of solar energy, there are solar charts which are a graphic representation that seek to determine the position of the sun in the sky over time for a specific latitude[27]-[29].We used the "Sun Position" tool from SunEarthTools.com to obtain the solar chart shown in Figure4.

Figure 7 .
Figure 7. 3D model of the solar collector designed

Figure 8 .
Figure 8. Distribution of solar collectors on the rooftop

Table 1 .
Monthly solar radiation in the UTS (kWh/m 2 ) Figure 1.Monthly solar radiation variation According to Table

Table 2 .
Characterization of indirect incidence

Table 3 .
Solar radiation data of the Neomundo station