تجزیه و تحلیل عملکرد سیستم CPC موثر رمان و هزینه
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|28063||2013||10 صفحه PDF||سفارش دهید||7400 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Energy Conversion and Management, Volume 66, February 2013, Pages 56–65
Compound Parabolic Collectors (CPCs) allow large acceptance angle which concentrating the incident radiation and need only occasional tilt adjustments instead of continuous solar tracking. They have, therefore, been found useful in many low concentration applications, where ease of operation, and low cost are important criteria. In addition, researchers have recently shown that in conjunction with other concentrators, CPC can do value addition to Fresnel or heliostat technologies. Also, their usefulness in solar reactors for water disinfection or hydrogen production has been reported. One major limitation of CPC is its height which increases rapidly with an increase in the aperture, rendering the supporting structure bulky and costly. Truncation reduces height, however also reduces the concentration. An improvement in the CPC design has been suggested in this paper, which brings down its height, without much compromise on the concentration ratio. A prototype of this modified CPC design was constructed and tested for thermal efficiencies and achievable temperatures. Results show that the modified CPC design can harness the solar energy to provide low cost Industrial Process Heat.
In commercial solar thermal installations, the Parabolic Trough Collectors (PTCs) are used frequently. However, some other designs are also considered, because of their specific advantages. For instance, a Compound Parabolic Collector (CPC) is capable of concentrating sunlight onto a flat receiver over a full day, without tracking. Unlike other solar concentrators, therefore, CPC does not require elaborate tracking arrangement; and thus provides the advantage of ease of operation. Consequently, during the past 30 years attempts have been made to understand the characteristic features of CPC, and to develop its several applications. 1.1. Compound Parabolic Collectors (CPCs) Concept of CPC was introduced by Winston  in 1974, which is a non-imaging type of concentrator. As seen from Fig. 1, CPC consists of sections of two parabolas ‘A’ and ‘B’, with their focal points at ‘FA’ and ‘FB’ respectively, such that the former lies on curve ‘B’; and the later lies on the curve ‘A’. The dotted portions of the two curves are truncated; and only the parts shown by solid lines constitute the CPC. Angle between the two lines drawn parallel to the axes of parabolas ‘A’ and ‘B’ through ‘FB’ and ‘FA’ respectively is its angle of acceptance ‘2θ’. Rays entering CPC through 2θ’ reach the gap between ‘FA’ and ‘FB’, after a single or multiple reflections, producing a non-imaging type of concentration. The geometrical concentration of the CPC is (d1/d2) where d1 is aperture and d2 is receiver opening (Which is shown as (10) and (11) in Fig. 1). It is also clear from the figure that sections of both the parabolas, forming a CPC are above their respective focal planes. Full-size image (19 K) Fig. 1. The geometry of compound parabolic concentrator (CPC). (1) Parabola A, (2) focus of parabola A, (FA) (3) axis of parabola A, (4) truncated part of parabola A, (5) parabola B, (6) Focus of parabola B (FB), (7) axis of parabola (B), (8) truncated part of parabola B, (9) axis of CPC, (10) aperture of CPC (d1), (11) receiver opening (d2), (12) acceptance angle (2θ). Figure options Muschaweck et al.  showed that except near the equator, asymmetric CPC collectors collect more solar energy than the symmetric ones by advantageous land-use; and with marginally higher construction cost. Mills et al.  have pointed out recommended that the East–West alignment the absorbers of CPC collectors offer for higher annual output as compared to the North–South alignment. In operation, CPC is deployed with its linear receiver aligned East–West, and aperture typically tilted toward south (for locations in the northern hemisphere). The tilt is such that the incident solar rays enter the collector within its acceptance angle; and it is adjusted periodically when the incident rays just spill out beyond that angle. Also, CPC is often truncated at the top in practice to restrict its height; but at the cost of drop in its concentration ratio. 1.2. Applications of CPC The concentration ratio of CPC is known to have relatively low values, as compared to parabolic dish or trough concentrators, and generate lower temperatures. Researchers have worked on many thermal applications of CPC, where ease of operation is important, and moderate temperatures are sufficient. Use of CPC has also been reported for enhancing the power generation of photovoltaic power plants . Because of their ability to concentrate radiation incident through large acceptance angles, CPC have been found to be effective as secondary concentrators in combination with the other concentrating devices . As a result, they have a fruitful role to play in solar tower technology, which generates very high temperatures (800 °C and above). A case study  has shown that, after carefully weighing all options, a preference for using CPC has been proposed for concentrating solar power in solar thermal power production plants, over other competing technologies. Table 1 gives a list of latest published reports on applications of CPC. It illustrates that the CPC is expected to play a crucial role in the emerging technologies for concentrating solar power. Table 1. Summary of previous work published in the literature. No. Reference Details Comment 1  CPC is proposed to be used to enhance the photovoltaic output. Positions of the CPC are adjusted thrice every day – eastward, southward and westward in the morning noon and afternoon respectively. The CPC axis is oriented in the polar direction. A mathematical model computes annular solar gain captured by such a system. In a typical case for latitude 40 °, the annual solar gain would jump from around 13000 MJ/m2 to 40000 MJ/m2, because of 3-position per day adjustments of CPC An added advantage of aligning CPC axis along the polar direction is that the natural convection currents pass below the photovoltaic panels and cool them 2  In a central tower solar power plant, a beam down reflector near the top of the tower greatly simplifies the heat retrieval. It is positioned close to the top focal point of the tower to minimize its size. This creates a broader image at the lower focus. The paper discusses the use of a CPC at the base, to reduce image size there. It also analyzes the effect of truncation of CPC on its cost and performance Central tower technology is the latest solar technology, showing much promise. As discussed here, CPC is likely to help improve its performance 3  Aston University, UK and ‘Solar Energy Centre’, India carried out an exercise to decide the best technology for solar thermal power production in India. The method adopted was the Analytical Hierarchy Process (AHP), which is a well-established tool to arrive at decisions when multiple options and multiple criteria are under consideration. The criteria included efficiency, compatibility with working fluid, reliability, availability, affordability, resource usage and scalability. The choices were parabolic trough, heliostat field reflectors (solar tower), linear Fresnel reflector, parabolic dish reflector and compound parabolic reflector. Ten experts from ‘Solar Energy Centre’, India prepared hierarchy of options. The ‘Compound Parabolic Collector with Fresnel lens’ was the most favored option for projects in Gujarat, India Against the five options of established technologies experts the preferred the use of CPC. They have recommended the combination of CPC with Fresnel. The authors, however, caution that unlike established technologies, problems in an untested technology are not fully known; and its advantages are likely to get highlighted A 2-D Fresnel lens is fitted at the aperture of a truncated CPC. Higher temperatures can be reached, and accuracy demanded in tracking is reduced because of CPC 4  Combination of CPC with evacuated collectors has huge potential for process heat applications. The CPC was truncated, with aperture width of 122 mm and height of 93 mm. The concentration ratio was just 1.8. The entire collector assembly was encased in a partially evacuated casing. This resulted in reduced convective heat losses and higher temperature of the receiver Main contribution to reaching higher temperature was by technique. Also, maintaining partial vacuum in the collector assembly. Negotiation of the extreme weather conditions in the solar field is challenging problems 5  A truncated CPC was used for heating air. Computer simulation was compared with experimental model. The CPC had 0.6 m height, 0.6 m width, and 0.72 m2 aperture. Flat plate absorber had 0.24 m2 area, and concentration ratio was 3. Maximum temperature of out flowing air was 72 °C, when air mass flow rate was 1.3 g. and length of receiver tube was 2.0 m., the corresponding thermal efficiency was 10% The CPC had acceptance angle of 15 ° and the receiver reached temperature of 118 °c. The concentration ratio reduced from 3.8 to 3, because of truncation 6  A CPC based photo catalytic hydrogen production solar reactor has been designed and tested. The concentration ratio of the CPC was 4; and increased the hydrogen output by an order of magnitude from 25 ml/h to 300 ml/h, other parameters remaining constant CPC provided low cost concentration of solar insolation and enhanced the reactor output 7  An advanced solarunit is described to facilitate building integration of photovoltaic/thermal power generation unit. In the proposed design, the CPC works as a secondary reflector, concentrating further solar radiation, arrived via a domed linear Fresnel lens, on a photovoltaic/thermal power generating module CPC is used here as building integrated component in conjunction with Fresnel and photovoltaic 8  A solar disinfection (SODIS) reactor was designed to treat 25 l of water, where the tube containing infected water was placed at the focus of a CPC. The results showed that complete inactivation of bacteria occurred in 6–7 h, with no re-growth of bacteria in 24–48 h Water disinfection is a major problem even today in many remote places in India, and also some other countries in Asia and Africa. CPC can be effectively used for this purpose 9  A SODIS reactor in conjunction with CPC, and an exposure period for infected water of 8 h under static conditions, considerably improved quality of water. Moreover, the reactor is designed to enable treatment of large volumes of water Use of CPC for treating contaminated water continues to attract attention of researchers 10  A low temperature electric generation system, using a CPC and an Organic Rankin Cycle (ORC) is proposed. Organic fluid is preheated in a Flat Plate Collector (FPC) prior to entering CPC. Heat storage units with different phase change materials for FPC and CPC are also incorporated in the system The use of organic Rankin cycle appears to have potential Table options 1.3. Vacuum Tube, heat pipe and CPC Vacuum Tube Collectors (VTCs) are non-concentrating, non-tracking solar collectors, capable of producing moderate temperatures, because convective heat losses from their receivers are reduced to minimum. One major application of (VTC) is heating of water. Chow et al. compared performance and cost of VTC water heaters in the market of Hong Kong with flat plate solar water heaters there. They concluded that efficiency for VTC is comparatively higher, but so is cost. Consequently, payback period is same for both; but VTC have an edge in temperatures attained . VTC are often associated with heat pipes. Work has also been reported on the fluids used in heat pipes. Esen et al. reported the experimental comparison for three refrigerants R-134a, R407C and R410A in case of water heaters employing heat pipes . Balzar et al. used a VTC and a long heat pipe to heat the oven plate of a solar cooker. They could attain the temperature of 252 °C for a pot containing 5 l of oil . Stumpf and others further used a double stage heat pipe thermal coupling in a solar cooker; and compared its performance with a solar cooker using a flat plate collector, and that using a single stage heat pipe. They found that the performance of the first type of solar cooker was significantly better . A further development is a combination of VTC and heat pipes with CPC. Along with ease of operation this combination has advantages of higher temperatures and better performance. The study of comparative performance of a VTC coupled with CPC, and a flat plate collector has been reported by Zambolin and Del Col . Similarly, Nkwetta et al. compared performances of concentrated and non-concentrated VTCs, and concluded that the efficiency of the former is 30% higher . Buttinger and others designed a CPC with a basic concept of integration of an absorber tube and reflectors inside a low pressure enclosure. Thus it was a combination of conventional CPC with VTC, and could be used for process heat application . Nkwetta and others have also reported about a solar air conditioning system powered by a CPC with acceptance angle of 20° and concentration ratio of 1.95, coupled with a VTC and a heat pipe. They estimated its optical efficiency to be 79.13 . Though CPC has advantage of ease of operation, it has certain limitations. The foregoing discussion suggests that its limitations on temperatures and efficiencies can be overcome by coupling it with VTC and heat pipes. 1.4. Modifications in CPC geometry Table 1 brings out many useful applications of CPC. One drawback of CPC, however, is that its height increases rapidly with aperture. Another limitation is its comparatively low concentration ratio. Height can be reduced by ‘truncation’, but it further brings down the concentration. In order to overcome these limitations, and to improve the performance, researchers have modified geometry and design of CPC reflectors and collectors. Kaiyan et al.  designed a new compound collector in the following way. A single parabola is cut at the plane above its focal plane. The two halves of the parabola are then shifted towards Y axis (axis of symmetry). Consequently, the two halves come closer to each other, and the original single focal point of the original (single) parabola gets split into two. Two plane mirrors are placed at the bottom, at the gap between the two halves. They act as secondary reflectors, resulting into improved concentration of the incident radiation. This new compound collector has a reduced height, higher concentration ratio and a much lower acceptance angle, as compared to the standard CPC. Its ray tracing analysis has been carried out by Zheng et al. . Souliotis et al. modified the reflector and receiver design of CPC, so that it acted as a storage device as well. The reflector was a truncated, asymmetric CPC comprising three parts: a section of one parabola with vertical axis; a section of another parabola with horizontal axis and a concave reflector joining the two sections. The receiver was made up of two concentric cylinders; where the gap between the two was partially evacuated to reduce the heat losses and water was stored in the inner cylinder. The geometrical concentration ratio of the collector was 1.17, and the temperature of water reached up to 80 °C. Temperature of water was measured with respect to time and was observed that heat gets retained by water until the next morning. CPC, normally, is in the shape of a trough and has a two dimensional symmetry. Senthilkumar et al. , however, have reported a 3-D CPC with spherical receiver with a half acceptance angle of 4 °. The geometrical concentration ratio was 1.8 (aperture width 540 mm, and the sherical receiver having of 100 mm radius). The aothors have claimed that the optical efficiency of the 3-D CPC is significantly higher than a 2-D CPC; and it can produce low pressure steam with 38% thermal efficiency. Dai et al.  have compared in 2011, a two-stage solar concentrator formed by a Dish Concentrator (DC) in tandem with a 3D CPC (CPC-DC) with a single DC. Numerical analysis shows that the interception efficiency of DC is 4% higher than CPC-DC; but the concentration ratio of the later is twice that of the former. Apart from the concentration ratio and the ratio of height to aperture, one more parameter that influences the quality of output delivered by a CPC. The objective is even collection of radiation on the receiver pipe. This is because, for example, when a CPC is used for accelerating photocatalytic reactions, the reaction rate strongly depends on the photon absorption rate; and better results follow a uniform distribution of energy on the absorber surface. Colina-Marquez et al. simulated reflections of direct solar radiation in a CPC to study the layout of incident and reflected rays on the collector. The incident radiation on the receptor was plotted displaying the daily direct energy distribution in the absorber. The purpose of the exercise was to understand the radiation distribution on the collector. From the forgoing discussion, it is clear that a CPC has a wide range of applicability even among latest emerging technologies of solar thermal. The design preferably be modified to have a reduced height, without a heavy compromise in the concentration ratio, and with collected radiation distributed as uniformly as possible on the receiver. In view of this wish list, it was thought desirable to undertake a systematic investigation of a CPC design having the abovementioned advantages. Accordingly, the geometry of CPC was modified to incorporate all the above three factors. Such an improved design was tested, particularly for the application of the Industrial Process Heat.
نتیجه گیری انگلیسی
This paper proposes improved design of compound parabolic reflector as compared to those reported in the published literature. The improved design substantially reduced the reflector height without compromising the concentration ratio. We have designed and investigated a prototype, with aperture width 2 m, acceptance angle 3 ° and receiver pipe of 48 mm outer diameter had a height of around 1 m. For the same aperture width, acceptance angle and receiver dimensions, the height for conventional, un-truncated CPC is around 38 m. Thus the modified CPC design needs substantially reduced support structure, and consequently, expected to be economical and easy to handle. (2) We have demonstrated the generation of steam at 120 °C and its beneficial use for the absorption refrigeration. (3) The proposed CPC design is easy to fabricate requires simple mechanism for tilt adjustment and the production cost is around US $ 70/m2 of aperture area, which makes it cost competitive with existing methods using fossil fuels for producing IPH at moderate temperatures.