Design, construction, and evaluation of a cylindrical-trough solar concentrator with 1.3 m aperture, 2.15 m length, and 0.54 m focal length, with heat-pipe or vacuum tube receiver and one axis tracking system, are presented. Design performance was tested under ASHRAE standard 93-1986 (RA 91). The concentrator system is lightweight and inexpensive since it was made of polymeric membranes and was pneumatically inflated to acquire its cylindrical shape achieving good optical quality. Further implementation of a flat and a cylindrical extension of the concentrating mirror as secondary mirrors was incorporated into the concentrator design in order to compensate for seasonal variations of collected radiation. Total initial investment of $163.30 or $58.5/m2 and efficiencies ranging from 33 to 25% for 25 up to 65°C show an excellent cost-performance ratio. Construction, costs, and efficiencies obtained by us and developed by other groups are compared to emphasize the high cost/benefit ratio and efficiencies of this approach.
Collection of radiant energy from the Sun is used for electric power generation or conversion to thermal energy for industrial processes. Solar thermal collectors may be used in low-temperature applications as in flat-plate collectors (less than 80°C) or of medium temperature with an optical concentration stage, in which the light first impinges on a reflecting surface and then it is redirected towards a receiving element with selective absorbing properties. Here, energy is transferred to a fluid—usually water—thus rising its temperature, and subsequently, it is stored into a thermally insulated tank.
There are two main types of collectors without concentration, used for household processes (washing, drying, etc.) [
Due to their geometry, parabolic-trough collectors focus the incident radiation on a focal line, using evacuated tubes as receivers. This type of collector requires tracking the Sun in one or two axes; this increases its complexity and, consequently, the costs of initial investment, as well as operating and maintenance costs, in contrast to the flat-plate collector systems, which have no moving parts [
The Institute of Renewable Energy (IER) and the Electrical Research Institute (IIE), both in the state of Morelos in Mexico, have developed projects with parabolic-trough concentrators. Their manufacture and materials costs reached $1706 per square meter [
In India the reported cost for building a parabolic-solar collector was (around) $562 per square meter [
Most solar thermal applications for industrial processes have been installed on a relatively small scale and are mostly experimental in nature. Only 85 solar thermal plants for processing heat were reported worldwide in 2008, with an installed capacity of 25 M
Some industrial processes can be driven by medium-temperature parabolic-trough concentrators (PTCs). For instance, there are different designs of PTCs for production of hot water and low-enthalpy steam. These concentrators are modular, with solar collection areas in the range from 2.5 to 5.0 m2 [
In this paper the design of a cylindrical, lightweight, and low-cost concentrator is presented with the objective of assessing its economic viability compared with other systems.
The proposed system for water heating consists of a collector, a storage tank, pipelines, and a solar tracking system. The collector designed in this study was installed on the roof of the Institute of Physics of the Autonomous University of San Luis Potosí at coordinates 22° 36′ 12′′ north and 100° 25′ 47′′ east. The system was north–south oriented, with an east–west Sun tracking. The storage tank was placed next to the cylindrical collector, in a position which does not project shadow on it. CPVC pipes, with a diameter of 1.9 cm, were used and isolated with Armaflex®.
Frequently, a design is accomplished by first decomposing a system into smaller subsystems and then using a progressive series of four operational stages—Feasibility, Preliminary, Detailed Design, and Revision—through which the design is developed to completion, using an iterative scheme [
A cylindrical pneumatic concentrator was designed at a very low cost and weight. It was inflated with air, in order to obtain the cylindrical shape. The use of Mylar® allowed us to reduce costs. The cost of Mylar is $1/m2, which contributes to limit the construction costs of the whole system to a total of $163.30. The length of the concentrator is limited by the extension of the receiver. We used commercially available finned heat-pipe tube with a length of 1.6 m and diameter of 0.068 m. The width of the concentrator was limited by Mylar’s commercial dimensions (1.2 m). It was designed to achieve a focal length of 54 cm, once inflated. This focal length was set in order to have the majority of the deflected rays—considering spherical aberrations of the cylinder—impinging on the receptor for such aperture.
Later, we tested the same design using anodized aluminum foils (20901L) fabricated by Alanod, as the reflecting surface, instead of Mylar, in order to compare both systems finding equivalent performances. As part of the design of the cylindrical collector, the aluminum foil is not cut or rolled: aluminum foils are installed on the structure without modification; the cylindrical profile is given by the shape of the structure. The total cost of the whole system using Alanod was $234.1.
Receivers use selective AlN coatings as absorber because, with a nonselective absorber, radiation losses would dominate at high temperatures, in such a way that eliminating convection alone would not be very effective [
Heat-pipe has a metal absorber mounted within a single envelope vacuum tube. The absorber is thermally connected to a finned heat-pipe, containing acetone that boils and evaporates, transporting by convection the heat absorbed towards a bulb inserted into a manifold, in which the heat is transferred by conduction to the working fluid. Since a glass-to-metal seal is used to couple the heat-pipe and the vacuum tube, the cost of these receivers is higher in comparison with simple evacuated tubes.
The heat-pipe is tilted to ensure that once acetone releases its heat into the water, thus condensing, it will flow back to the heated collector to repeat the cycle in which heat always flows in one direction (upwards) driven by convection. This thermal-diode effect is very useful in designing solar thermal collectors, because it automatically shuts the collector off and prevents heat loss by radiation when there is insufficient solar radiation (radiation absorption usually takes place in large areas but they will remain at low temperatures due to their lower vertical position). Also, heat-pipes have lower heat capacity than ordinary liquid-filled absorber tubes, thus minimizing warm-up and cooldown losses [
The supporting structure for the mirror surface shown in Figure
Design of the structure of the cylindrical concentrator.
This support structure is covered on the top with a transparent plastic film (polycarbonate) and the reflecting curved surface is of aluminum-metalized plastic film (Mylar). Reflectance of this Mylar, as reported by the manufacturer, is 95%. It has a thickness of 0.05 mm, a density of 139 kg/m3 or a surface density of 0.02085 kg/m2, and a price of $1 per m2.
The Mylar reflective surface is placed oriented to the Sun’s incoming direction and is protected with the transparent polycarbonate film. This polycarbonate has a transmittance of 88%, a thickness of 0.13 mm, a surface density of 1.8 kg/m2, and a price of $3.0/m2. The polycarbonate coating is used to protect the Mylar from dust and water (weathering), which causes oxidation of the metalized surface and reflectance losses by detachment of the metallic film, thus increasing the durability of the reflective surface. In addition to the protection provided by this polycarbonate film, its main function is to provide a closed surface, capable of containing air at a pressure slightly greater than atmospheric (around one standard atmosphere). This makes it possible to provide a pneumatic structure with the desired curvature to the entire reflecting surface, by means of Pascal’s principle. Air was injected by a domestic air compressor into the chamber through a valve fixed in the concentrator structure, so the air causes the Mylar to tense and adapt, perfectly fitting the cylindrical shape, as shown in Figure
Cylindrical mirror wrapped with Mylar.
Following the geometric procedure reported in [
Another important parameter in the cylindrical collector is the collector concentration ratio,
The geometric concentration ratio,
Collector parameters.
Collector dimensions | 1.3 m × 2.15 m |
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0.54 focal distance |
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1.3 |
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0.096 |
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2.79 aperture area of the cylindrical collector (m2) |
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13.54 |
Two identical rings (Figure
A base of tubular profile was designed with roller bearings to support the structure and allow it to rotate. All this structure obeys the principle of lightness, easiness to move, service, and transport of the entire system (Figure
A first flat secondary mirror, which helps to capture further radiation from the Sun, which does not interfere on the receiver due to the lower altitude of the Sun during winter (summer), was designed, thus increasing the net opening area at this season, as shown in Figure
The vertical secondary mirror, of Mylar, is placed on the north-side (for the north hemisphere) ring of the concentrator. This mirror helps to capture some of the radiation when the Sun has an inclination due south. The rays first reflect in the mirror to the concentrator and back to the collector (Figure
Northern flat secondary mirror.
The prototype was installed in a north–south configuration, tracking the Sun from east to west. The final design of the cylindrical collector for supratropical latitudes is shown in Figure
Final design of the cylindrical collector showing southern cylindrical and northern flat secondary mirrors for latitudes > 23° 26′ 14′′.
The weight of the entire prototype was 22.3 kg, resulting in a surface density of 8 kg/m, less than a half of the one reported by [
In the design for domestic purposes, the bulb of the heat-pipe is inserted into a thermally insulated reservoir of a 60-liter water tank, covered by 7.5 cm wide polyurethane foam [
To detect the Sun’s position relative to the cylindrical concentrator, a pair of Light-Dependent Resistors (LDRs) are attached to the concentrator and separated with a small plate in such a way that each LDR will receive an equal amount of light, only if the concentrator is pointing directly towards the Sun: two comparators are used to sense the differential resistance produced by these two LDRs, and they activate a tracking motor to tilt the concentrator on its axis, when the differential resistance becomes too large. An H-drive-transistor switching circuit takes the comparators’ output signals and amplifies them to drive a permanent-magnet DC motor one way or the other. This motor move is a Dayton 1LPV8 motor of 12 V DC at 2.1 A, with a power of 1/30 HP. The DC motor is coupled to a reduction gearbox. The final frequency of rotation is 6 rpm. Two limit switches were added for additional security of the motor and the structure, in emergency situations, and to restore the initial position at the end of the solar day.
The optical efficiency depends on the optical properties of the materials involved, the geometry of the collector, and the various imperfections arising from the construction of the collector [
Modeling of the shape and placement of reflecting surfaces was made possible with commercial ray-tracing software programs SolTrace and Tonatiuh. SolTrace [
Both programs use the width and length of the receiver, solar radiation incidence angle, the radius of curvature of the mirror, its aperture, and reflectance as input parameters for the calculation of the cylindrical collector.
Results from ray-tracing with SolTrace and Tonatiuh software showed that not all rays reached the focus. The rays that hit on the receiver had an incidence percentage of 94%. Nevertheless we have not characterized the focal spot by absolute measurements; photographic analysis reveals consistency with these simulations.
Thus, considering twice the reflectance of polycarbonate, twice the reflectance of glass, once the reflectance of Mylar, and once the absorptance of the selective surface of the heat-pipe fins, one gets an optical efficiency of the system of
The instantaneous thermal efficiency can be obtained from an energy balance for the receiver of the collector. The instantaneous efficiency is defined as the rate at which useful energy
In order to characterize the efficiency of the system, a copper jacket heat exchanger was made and the bulb of the heat-pipe was inserted in it. Water was circulated in the jacket at 1.5 liters per minute, and the temperature was measured and registered at the inlet and outlet of the jacket every 10 seconds. Length of the bulb (jacket) is 6 cm while the diameter of the outer (inner) surface is 1.5 cm, giving 28.27 cm2 of thermal contact surface for heat interchange.
The ASHRAE 93-1986 (RA 91) standard provides test methods for determining the thermal efficiency of concentrating collectors [
Diagram of the experimental system.
The system is evaluated considering a recirculation closed-loop. Water from the storage tank is pumped to the water jacket heat exchanger, where it is heated and then flows back into the storage tank.
The experiments have been conducted on various days from 10:00 am to 4:00 pm local time in Central Mexico with a mean solar radiation in the range of 600–900 W/m2 and a mean room temperature in the range of 20–25°C.
For data acquisition of temperature the measuring equipment was a Digi-Sense Model 92000–00 from Cole Parmer. It has an ability to read up to 12 thermocouples, with a resolution of 0.1°C, and can be programmed to measure at different time intervals.
Production costs of this prototype are broken down to show how cheap it is, compared with commercially available designs.
As can be seen in Table
Cost of the solar cylindrical collector.
Quantity | Description | Price/unit |
Cost |
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2 tranches | Tubular profile |
11.42 | 22. 85 |
2 tranches | Tubular profile |
11.24 | 22.48 |
4 | Hubs of bicycle | 1.27 | 5.10 |
1 | Plastic tank | 2.25 | 2.25 |
1 | Tank 60 liters | 2.25 | 2.25 |
4 m | Polycarbonate | 0.9 | 3.6 |
1 | Construction of Sun-tracking circuit | 7.49 | 7.49 |
1 | Heat-pipe tube | 7.49 | 7.49 |
1 | Heat exchanger | 7.49 | 7.49 |
2 m | Insulating tubing | 0.97 | 1.95 |
2 m | CPVC pipe 3/ |
1.87 | 1.87 |
4 | Unions for pipeline | 1.50 | 6.00 |
1 | Engine C.D. | 22.48 | 22.48 |
6 m | Mylar | 0.75 | 0.75 |
Manufacturing costs | 50.00 | 50.00 | |
Total | 163.30 | ||
Total cost per m2 | 58.50 |
The prototype was tested according to the ASHRAE 96-1986 standard [
The heat-pipe was inserted into the water jacket heat exchanger. The storage tank was filled with 11.0 liters of water that were circulated through the heat exchanger at 1.5 L/min. The temperature was measured at the inflow and at outflow of the heat exchanger.
Thermal efficiency was calculated by considering the temperature rise across the heat exchanger and using the thermal efficiency equations ((
Thermal efficiency data for heat-pipe with concentration and heat-pipe without concentration and Vidriales Escobar [
The efficiency curve for both heat-pipe with concentration and heat-pipe without concentration, shown in Figure
The test conducted using the heat-pipe tube under concentration conditions resulted in an efficiency of 0.30, decreasing just slightly for a wide range of temperatures. The test conducted using the heat-pipe tube without concentration results in an efficiency of 0.9 for low temperatures, but efficiency drops sharply when temperature increases.
We then compared our results with those of Vidriales Escobar (2007), who used a parabolic concentrator, obtaining a curve presented here, with the parameters in Table
Parameters of collector of Vidriales Escobar.
Collector dimensions | 1.049 m × 2.44 m |
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0.25 focal distance |
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1.049 |
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0.0364 |
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2.56 aperture area of the cylindrical collector (m2) |
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11.17 |
The test reported by Vidriales Escobar was conducted for 10 liters of water with a flux of 0.35 L/min and 872.6 W/m2 of irradiance at ambient temperature of 32.85°C, on March 31, 2006. The test starts with an inlet temperature of 45.35°C. We can observe that our cylindrical collector has almost twice the efficiency reported by Vidriales Escobar.
Different concentration systems that include inflatable structures have been developed by some groups intended for deployable antenna or concentrated photovoltaic systems in Earth or in space: Cassapakis and Thomas [
Some considerations about durability and resistance to outdoor conditions are that Mylar has a longevity larger than a year without presenting aluminum detachment if it is correctly oriented with the aluminum layer at the interior of the inflated mirror as we and Deyle [
A novel cylindrical-trough pneumatically inflated concentrator is presented. Good, stable performance in low-temperature applications is achieved as compared against other similar concentrators reported in the literature. Lightweight and good-optical-quality concentrators result from combining thin frame structures with flexible polymeric membranes, giving a competitive 8 kg/m2 surface density in comparison with typical few tenths of kg/m2 reported for similar systems.
Implementation of secondary mirrors in order to further enhance the collection of solar energy and compensate for seasonal variations in Sun-energy capture due to declination changes is reported.
In addition, we have shown with this design that a far lower cost can be achieved for a pneumatically inflated lightweight concentrator. We have compared our costs of production with others reported in the literature, showing that our approach lowers 29 times the initial investment, with the construction costs of the system kept below a total of $163.30.
This collector shows good potential as a source of process heat for developing countries, where it could be an appropriated technology at competitive costs achieving good thermal efficiency.
The authors declare no conflict of interests.
The authors wish to thank Enrique Martínez Hernández for his technical assistance in the construction of the concentrator and Gregor Zieke for reading the paper. This work was supported by Inter-American Development Bank ATN/KK-11514-RG, ATN/13990-RG, Universidad Autónoma de San Luis Potosí C14-FAI-08-67.67, CONACYT Ciencia Básica 221961, and CONACYT for Scholarship 327853.