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A solar thermal cooling system using novel nontracking external compound parabolic concentrators (XCPC) has been built at the University of California, Merced and operated for two cooling seasons. Its performance in providing power for space cooling has been analyzed. This solar cooling system is comprised of 53.3 m(2) of XCPC trough collectors whi...
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... clean. They collect and convert solar energy by heating a mineral oil (Duratherm 600) which is circulated by a pump. Heat is transferred to an aqueous solution containing 40% ethyl- ene glycol by a heat exchanger (90% efficiency at operating con- ditions) which is then circulated into a 23 kW LiBr-water double effect absorption chiller (see Fig. 1 and Table 2). The intermediate ethylene glycol loop was installed because the chiller manufac- turer would not guarantee their warranty if a paraffin-based fluid (mineral oil) was pumped through their chiller. However, they have since retracted their issue with paraffin-based substances and in the future an intermediate loop will not ...
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... solar collector will work well on an entirely overcast day but the XCPC, and other nontracking collectors such as flat plates, are still able to collect power. This is especially true on days where clouds are interspersed with sun- shine because clouds act as giant mirrors, amplifying the amount of diffuse light just before crossing the sun (see Fig. 10), and a so- lar collector which accepts diffuse light can take advantage of this ...
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... to overpower our cooling load. As a result we were able to demonstrate the full 8 h solar collection window (2 1 =2 h warm-up, 5 1 =2 h run-time). However, once the collectors were cleaned, they warmed up 30 min faster, maintained much higher temperatures, and quickly overpowered our cooling load causing a low chiller COP and chiller cycling (Fig. 13). As mentioned before, this forced Table 4 Results and definitions-Typical system performance Range (instantaneous) Daily ...
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... and fluid that need to be heated from ambient to operating tempera- tures. In 2011, the system included a large 50 gal (0.19 m 3 ) thermal storage tank with the purpose of evening out fluctuations in solar radiation. This greatly increased the total mass of fluid in the system and increased the system warm-up time by approxi- mately 1 h (see Figs. 15 and 16). Because the collectors have an 8 h collection window, any increase in warm-up time cuts into power production time and should be minimized. Therefore, it is important to design a system with a small heat capacity in the high temperature loops by minimizing pipe diameters, lengths, and fluid volume. During the 2011 tests, the chiller ...
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... Transactions of the ASME solar energy by a couple of hours and in some locations, space cooling may be needed before or after the direct solar powered cooling window. Typical meteorological year (TMY) data was used to determine hourly cooling needs throughout the year for Merced, CA. A hybrid chiller can provide cool- ing from natural gas (Fig. 17) in the morning during the warm-up time of the collectors (see Figs. 15 and 16) and cold storage can lengthen the cooling window into the late afternoon and evening. The use of cold storage is recom- mended over hot storage due to a smaller temperature differ- ence with the ambient and therefore less heat loss (or gain). An oversized ...
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... some locations, space cooling may be needed before or after the direct solar powered cooling window. Typical meteorological year (TMY) data was used to determine hourly cooling needs throughout the year for Merced, CA. A hybrid chiller can provide cool- ing from natural gas (Fig. 17) in the morning during the warm-up time of the collectors (see Figs. 15 and 16) and cold storage can lengthen the cooling window into the late afternoon and evening. The use of cold storage is recom- mended over hot storage due to a smaller temperature differ- ence with the ambient and therefore less heat loss (or gain). An oversized collector array can provide direct solar powered cooling while also cooling a ...
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... The solar COP of the entire system was 0.363 with very little variation. This is the first system of its kind to combine XCPC technology with double effect absorption chilling technology. The purpose of this demonstration was to prove the XCPC's competency at providing thermal power for space cooling and in this sense it was a success. The XCPC (Fig. 18) operates at efficiencies comparable to those reached by track- ing type solar collectors and the power production window coin- cides nicely with the desired window for space cooling. At the same time, the XCPC is able to collect diffuse light, allowing for cooling during cloudy or hazy days, which is especially important in regions ...
Citations
... It is recommended to use a constant average value for the number of light reflections to simplify engineering calculations when evaluating the aperture of the CPC collector. Widyolar et al. (2014) conducted a study on the viability of a CPC collector by utilizing it to power a LiBr-H 2 O-based absorption chiller. This marked the first instance of a double effect solar refrigeration system (SRS) integrating with a CPC collector, achieving temperatures as high as 200 °C. ...
The aim of this research was to develop a model for a solar refrigeration system (SRS) that utilizes an External Compound Parabolic Collector and a thermal energy storage system (TESS) for solar water heating in Chennai, India. The system parameters were optimized using TRNSYS software by varying factors such as collector area, mass flow rate of heat transfer fluid, and storage system volume and height. The resulting optimized system was found to meet 80% of hot water requirements for the application on an annual basis, with an annual collector energy efficiency of 58% and an annual TESS exergy efficiency of 64% for a discharge period of 6 h per day. In addition, the thermal performance of 3.5 kW SRS was studied by connecting it to an optimized solar water heating system (SWHS). The system was found to generate an average cooling energy of 12.26 MJ/h annually, with a coefficient of performance of 0.59. By demonstrating the ability to efficiently generate both hot water and cooling energy, the results of this study indicate the potential for utilizing a SWHS in combination with STST and SRS. The optimization of system parameters and the use of exergy analysis provide valuable insights into the thermal behavior and performance of the system, which can inform future designs and improve the overall efficiency of similar systems.
... Since the 1970s there have been hundreds of papers on the design, performance and application of very many concentrators based upon the CPC principle. To quote only a few examples, there are papers reporting the performance and efficiency of CPC troughs with various absorber shapes, [8][9][10][11] applications to hot water heating, [12][13][14][15][16][17] to refrigeration, [18][19][20] to chemical processes 21 and for water de-salination. 22 In a recent paper 23 it is suggested to mount an assembly of standard double-wall CPC on the side of buildings for water heating applications. ...
We report on the development of non‐tracking solar concentrator troughs designed subject to the condition that they operate efficiently for diffuse irradiation from all angles during daylight hours. Troughs of the usual compound parabolic form were considered initially. Remarkably however, the most efficient shape for the above requirement turns out to be a trough of simple semi‐circular cross‐section incorporating a tube of height equal to the circle radius and with a planar absorber plate placed vertically. This trough has a limited concentration ratio of exactly 2.0 but an intercept factor for incident radiation of 100% for all angles of incidence. This desirable feature is explained from the properties of caustics arising from reflection of incident light at the semi‐circular trough (SCT) surface. This very simplest of trough design has the clear advantages of small size and low‐cost of manufacture. Experimental tests conducted under varying solar irradiation conditions, demonstrate that the simple step of placing an evacuated tube in a SCT can lead to a power output gain of a factor of at least 1.7 on average. Compound parabolic concentrators (CPC) for solar thermal application have been used since around 1970 but, without tracking, they do not capture sunlight over the whole day and are expensive to manufacture. We propose the absolutely simplest design of a semi‐circular concentrating trough which, of suitable dimensions, can capture irradiation incident at all angles. The design utilizes light reflected into caustic surfaces and is already employed successfully in pilot studies for industrial heating and refrigeration.
... In 2011, the UC Merced team demonstrated solar-driven double-effect absorption cooling using the original NS array 22,23 . In 2015 a 45.6 m 2 East-West oriented XCPC collector array was added to the existing North-South array at UC Merced's Castle Test Facility. ...
... This production of hyper saline concentrate (brine) and more importantly, what to do with it, remains a major challenge for desalination systems 27 . In order to facilitate widespread adoption of these systems, improved brine management strategies will be required to reduce disposal costs, limit negative environmental impacts, and safeguard water supplies for current and future generations 22 . ...
... System-level performance, however, is reduced due to additional heat losses in the solar field and distribution piping as well as soiling (dusting) of the solar field. In Fig. 6 for example, the peak thermal (~28 kW) divided by the peak solar (~82 kW) yields a 34% solar to thermal efficiency for the array which is similar to that recorded in [41] for a single array. A calculation of the heat loss in the distribution piping yields~7 kW when operating with an average fluid temperature of 140°C. ...
In this manuscript, the performance of a commercial thermal evaporator powered using solar energy is described. The solar collector, known as the external compound parabolic concentrator (XCPC), is an emerging technology which combines nonimaging optics and metal-glass vacuum tube technology to provide high operating temperatures from a stationary collector. Paired with a commercial thermal evaporator, it provides a renewable heat alternative to fossil fuels for wastewater volume reductions. To assess performance, the team at UC Merced installed a 10 gallon-per-hour (0.037 m³/hr) thermal evaporator and connected it to a 30 kW solar thermal collector field. The solar array operates between 130 and 150 °C, providing thermal energy to drive evaporation for 8 h each day. The single-stage thermal evaporator has a specific energy consumption of 776 kWh/m³. Performance of the array was then used to develop an annual performance model to extrapolate performance for different locations and thermal loads (e.g. other commercial and emerging water treatment and desalination technologies). Extrapolating performance yields an annual thermal generation of ~500 kWhth useable heat delivered to the process load at 150 °C per square meter of solar field, although the team estimates a commercial installation can generate closer to 625 kWh/m²/year. Thus a commercial installation could process 2.5–3 m³ of product water per square meter each year. At these temperatures the thermal energy can also be used to decarbonize a number of other industrial processes such as evaporation, desalination, drying, and more.
... In 2011, the UC Merced team demonstrated solar-driven double-effect absorption cooling using the original NS array 22,23 . In 2015 a 45.6 m 2 East-West oriented XCPC collector array was added to the existing North-South array at UC Merced's Castle Test Facility. ...
... This production of hyper saline concentrate (brine) and more importantly, what to do with it, remains a major challenge for desalination systems 27 . In order to facilitate widespread adoption of these systems, improved brine management strategies will be required to reduce disposal costs, limit negative environmental impacts, and safeguard water supplies for current and future generations 22 . ...
... The proposed system of above 100 °C with high performance was superior to other non-tracking thermal collector systems. Widyolar et al. (2014) and Winston et al. (2014) experimentally proved that the XCPC solar collector is capable of providing thermal power for space cooling applications. ...
... (g) Several intermediate angle CPCs are combined to form a "fly eye" second stage or terminal concentrator which helps to meet the requirement of a tower with a central receiver without sacrificing concentration (Rabl, 1976b). (h) It is capable to collect diffuse light even during cloudy or hazy days, which is especially important in regions with high diffuse percentage ( Widyolar et al., 2014;Winston et al., 2014). (i) The CPC solar collector may be cheaper than PTC for achieving temperature up to 300 °C, due to non-requirement of tracking system and also collects a part of diffuse radiation ( Karwa et al., 2015). ...
... Sagade et al. (2014) conducted experimentation on the prototype compound parabolic trough for its performance study with different combinations of receivers and its surface coating. Widyolar et al. (2014) and Winston et al. (2014) experimentally studied the non-tracking XCPC solar collector performance in driving the absorption chilling system and this was the first system with the combination of XCPC and double effect absorption chilling technologies. Gu et al. (2014) analyzed experimentally and optically an innovative portable solar collector with a non-tracking CPC solar collector with flat plate absorber. ...
... As a result of these tests, a 20 kW array of EW XCPC collectors was installed on-site at the Castle Test Facility at UCM alongside a previous array of NS collectors [34], (see Fig. 23). ...
The design and development of a commercial-ready medium-temperature solar thermal collector, the external compound parabolic concentrator (XCPC), is presented in which a nonimaging reflector is paired with an evacuated tube absorber for efficient and low-cost heat collection between 100 and 250 °C. The absorber geometry is optimized under the constraint of being assembled with an ultrasonic welding machine, with a final pentagon-shaped absorber selected. The modified absorber shape, gap loss, and truncated reflector result in a geometric efficiency of 93% compared to an ideal CPC. The final prototype has a 4.56 m² aperture and simulations predict an optical efficiency of 71% and thermal efficiency of 50% at 200 °C. Experimental test results (optical, thermal, stagnation) have confirmed an optical efficiency of 62% and a thermal efficiency near 50% at 200 °C with a final stagnation temperature of 333 °C. A detailed economic analysis reveals the technology can be installed for $0.58/watt and deliver a levelized cost of heat at 3.01 cents per kWh over a 20 year lifetime. This is equivalent to the current cost of natural gas in the United States, which underscores the potential of this technology to assist in decarbonizing the thermal energy sector.
... Merced, CA InstallationThe Merced installation shown inFigure 22is a 20 kW array of the E/W collectors described in this paper installed alongside an earlier generation 20 kW N/S array. These arrays are used to provide thermal power for various projects including solar cooling (Widyolar et al. 2014), solar drum drying(Ferry et al. 2016), and is currently being commissioned for solar wastewater evaporation.Some representative data is presented below in Figures A, and B for September 8 th , 2016. The total solar energy incident on plane of the collector array between 11:45 pm and 5:00 pm was 175.5 kWh, resulting in a thermal generation of 76.9 kWh and 70.6 kWh of cooling from the chiller. ...
The design and development of new medium-temperature solar thermal external compound parabolic concentrator (XCPC) is presented. A nonimaging reflector is paired with an evacuated tube absorber for efficient heat collection between 100-300 °C. Several absorber geometries are simulated, with the final selection of a modified pentagon absorber shape chosen for manufacturability. The modified absorber shape, gap loss, and truncated reflector result in a geometric efficiency of 93% compared to an ideal CPC. Several selective coatings are compared for down-selection, and a Mylar reflective film with ~89% reflectance is chosen for its low cost and durability. The final prototype has a 4.56 m 2 aperture. Simulations predict an optical efficiency of 71% and thermal efficiency of 50% at 200 °C and experimental test results have confirmed an optical efficiency of 62% and a thermal efficiency near 50% at 200 °C. The stagnation temperature was measured at 333 °C. Worldwide installations are presented.
... Briefly, sunlight strikes the reflective surface of curved collectors and focuses onto an evacuated-tube manifold containing a copper pipe through which heat transfer fluid is pumped. The heated fluid can then be circulated through a load; the Atwater system has been previously demonstrated with a double effect chiller (Widyolar et al., 2014). An advantage of the particular design of the XCPC collector over other solar thermal technologies is that it can concentrate sunlight and reach high temperatures (100 • C-200 • C) without the need for solar tracking. ...
Fruit and vegetable pomaces are co-products of the food processing industry; they are underutilized in part because their high water activity (aw) renders them unstable. Drum drying is one method that can dry/stabilize pomaces, but current drum drying methods utilize conventional, high-environmental-impact heating mechanisms. In this work, a small-scale double drum dryer (20 cm length × 15 cm diameter) was interfaced with a 98.3 m² External Compound Parabolic Concentrator (XCPC) [solar thermal collector] array designed to produce up to approximately 40 kW of heating power. The conditions for drying prune and tomato pomaces were optimized on this system via a split-plot design. The design had 4 variables: added water, added maltodextrin carrier, dwell time, and drum surface temperature. Moisture content, aw, and color of the dried pomaces were assessed to determine the effectiveness of the drying. Both pomaces were rendered shelf-stable (aw < 0.6) for all tested conditions. However, prune pomace exhibited a narrower range of aw values than did tomato pomace. Conditions for adequate drying with minimal color change (and thus expected minimal nutrition loss) were established. This work demonstrates the potential for solar thermal energy to provide the heat for drum drying fruit and vegetable pomaces.
... Other papers concentrate on a wide variety of applications, indicating the versatility of CPC solar concentrators. For example, a non-tracking external CPC (XCPC) with a cylindrical absorber has been implemented [7] to drive successfully a chiller for air conditioning, which is one of the applications described in this paper. In Refs. ...
We report the development of a solar thermal collector module based on our proposed design (Ratismith et.al., 2014) of a large acceptance-angle multiple-parabolic trough which surrounds a standard evacuated cylindrical tube containing a planar absorber plate. The concentrator accepts diffuse solar radiation with an intercept factor of near 100% and so is suitable particularly for tropical climates. The module incorporates a novel direct metal-to-water contact resulting in an improved efficiency of heat transfer to the working liquid. Comparison with the performance characteristics, principally power output and temperatures attained, of a commercially-available non-concentrating assembly of evacuated absorber tubes is made. The experimental results, obtained by testing under typical conditions of solar irradiation throughout the day in Bangkok, indicate that the improvement over a non-concentrating collector, suggested theoretically on the basis of ray-tracing studies, is attained in practice. The superior performance of the concentrating collector indicates its suitability both for residential and industrial applications.
