Parabolic Trough Concentrator Research Articles

Organic Rankine Cycle coupling with a Parabolic Trough Solar Power Plant for cogeneration and industrial processes

Over the last 25 years solar power plants based on parabolic trough concentrators have been developed for the commercial power industry. On the other hand, in recent years, a way to harness the solar energy is to cogenerate through Concentrated Solar Power (CSP) technology coupled to an Organic Rankine Cycle (ORC) with potential applications to industrial processes. In this work we present a study of a small CSP plant coupled to an ORC with a novel configuration since useful energy is directly used to feed the power block and to charge the thermal storage. In order to analyze this novel configuration we consider a case study with cogeneration applied to textile industrial process at medium temperature. It turns out that this configuration reduces the size of the thermal storage disposal. The performance of the solar power plant was simulated with TRNSYS to emulate real operating conditions. We show the design, study and simulation results, including the production and efficiency curves for our load profile. Our results show that our system is a promising option for applications to medium temperature processes where electrical and heat generation is required.

Inspection and Structural Health Monitoring techniques for Concentrated Solar Power plants

Parabolic trough concentrators are the most widely deployed type of solar thermal power plant. The majority of parabolic trough plants operate up to 400 °C. However, recent technological advances involving molten salts instead of oil as working fluid the maximum operating temperature can exceed 550 °C. CSP plants face several technical problems related to the structural integrity and inspection of critical components such as the solar receivers and insulated piping of the coolant system. The inspection of the absorber tube is very difficult as it is covered by a cermet coating and placed inside a glass envelope under vacuum. Volumetric solar receivers are used in solar tower designs enabling increased operational temperature and plant efficiency. However, volumetric solar receiver designs inherently pose a challenging inspection problem for maintenance engineers due to their very complex geometry and characteristics of the materials employed in their manufacturing. In addition, the rest of the coolant system is insulated to minimise heat losses and therefore it cannot be inspected unless the insulation has been removed beforehand. This paper discusses the non-destructive evaluation techniques that can be employed to inspect solar receivers and insulated pipes as well as relevant research and development work in this field.

Evaluation and Assessment of Gravity Load on Mirror Shape of Parabolic Trough Solar Collectors

In order to achieve high optical collector and therefore high solar field efficiencies parabolic trough concentrators in concentrating solar power plants need to maintain their parabolic shape during operation. Additional to shape deviation already induced by the manufacturing process, deformation due to gravity load is an inevitable factor influencing shape accuracy in all types of parabolic trough collectors.This paper characterizes and quantifies the effect of gravity load on mirror shape and resulting slope and focus deviation values. One inner and one outer ideally parabolic shaped mirror of RP3 geometry mounted onto different support structures are evaluated in finite element analyses for all collector angles relevant for operation. The different support structures include two idealized support structures (ideal and elastic case) and one structure including relevant parts of EuroTrough type collectors (cantilever case).Constructional design and stiffness of the support structure significantly determine characteristic and magnitude of deformation. If compared to non-deformed shape, resulting rms values are as high as SDx = 1.7 mrad and FDx = 6.3 mm (inner mirror, elastic case) and SDx = 1.1 mrad and FDx = 5.6 mm (outer mirror, cantilever case). Depending on the type of support structure, minimum and maximum values occur at different collector angles. If compared to 0° (zenith) collector angle, resulting rms slope and focus deviation values are on average smaller than if compared to non-deformed mirror shape. This implies optimizing mirror shape for 0° (zenith) collector angle. However, it has to be considered that support structures for shape accuracy assessment in laboratory as well as support structures used in collector might differ significantly in design and stiffness, thus making it difficult to find one optimum shape for all types of mirror and collector.

A novel application of concentrated solar thermal energy in foundries.

Scrap preheating in foundries is a technology that saves melting energy, leading to economic and environmental benefits. The proposed method in this paper utilizes solar thermal energy for preheating scrap, effected through a parabolic trough concentrator that focuses sunlight onto a receiver which carries the metallic scrap. Scraps of various thicknesses were placed on the receiver to study the heat absorption by them. Experimental results revealed the pattern with which heat is gained by the scrap, the efficiency of the process and how it is affected as the scrap gains heat. The inferences from them gave practical guidelines on handling scraps for best possible energy savings. Based on the experiments conducted, preheat of up to 160°C and a maximum efficiency of 70% and a minimum efficiency of 40% could be achieved across the time elapsed and heat gained by the scrap. Calculations show that this technology has the potential to save around 8% of the energy consumption in foundries. Cumulative benefits are very encouraging: 180.45 million kWh of energy savings and 203,905t of carbon emissions cut per year across the globe. This research reveals immense scope for this technology to be adopted by foundries throughout the world.

Design Optimisation of Compound Parabolic Concentrator (CPC) for Improved Performance

A compound parabolic concentrator (CPC) is a well known non-imaging concentrator that will concentrate the solar radiation onto receiver (PV cell). This is also will lead to the decreases of concentration ratio but it will be negligible. In this paper, the flux distribution of untruncated and truncated 2-D hollow compound parabolic trough concentrator (hCPTC) design is presented. The untruncated design has initial height, H=193.4mm with concentration ratio, C_(2-D)=4. This paper presents the optical simulation of compound parabolic trough concentrator using raytracing software TracePro. Results showed that, after the truncation, the height of CPC reduced 45% from initial height with the geometrical concentration ratio only decrease 10%. Thus, the cost of reflector and material dielectric usage can be saved especially at manufacturing site

Experimental Study of a Parabolic Trough Medium Temperature Solar Thermal System

A parabolic trough concentrator (PTC) with a rim angle of 84˚, a length of 4.2 m, and anaperture area of 6.72 m2 was designed and manufactured in a way required relatively simple technology and skill. An experimental system with the PTC as the core part was assembled on the top of the Teaching Building No.7 of Ludong University (in Yantai, China). A PLC solar tracking system is employed to realize an orientation of a north-to-south direction. According to the international and national standard, thermal performance of the PTC was measured under various weather and working conditions. The experimental results reveals that the effective radiation, which considers the optical influences including the shading effect, cosine effect, incident angle modifier and end loss, deserves great attention. Peakefficiencyof72% was obtained from the experiment.

Study on the Performance of the Solar Cooling System Using Cavity Receiver Based on the Neural Network

A solar absorption cooling system with cavity receiverfor demonstration was introduced. The system consisted of a double-effectlithium bromide chiller and a parabolic trough concentrator with cavitysolar receiver.The high temperature heat from the cavity receiver enabled the chiller to operate in double-effect state,and the refrigerant water produced by the refrigerator flowed through the end fan coil toadjust the room temperature.Monitoring data from the demonstration system showed that instantaneous efficiency of the solar cavity receiverwas about 44%in direct solar irradiance of about 500W/m2, the receiver outlet temperature was150oC and thetemperature difference between the receiver outlet and the receiverinlet was 10oC.The lithium bromide refrigerator operated in the state of double effect in good solar irradiation, the refrigeration efficiency was about 1.2, and chilled watertemperature in the storage tank was about 15oC, and the room temperature was maintained at about 25oC. The experimental results showed that the design system could meet the demand of refrigeration and it was verified that the cavity receiver used in double-effect lithium bromide absorption refrigeration scheme was suitable.The relationships between the heat gain, inlet temperature, outlet temperature and flow rate of the working medium in the solar cavity receiver were analyzed, and the main influence factors on the heat gain of the solar cavity receiver were determined. On this basis, acavity receiver heat gain prediction model was establishedbased on a RBF neural network.The simulation results showed that the model predicted values and the measured values had good consistency,so the RBF neural network model could accurately predict the temperature rise of working medium of the solar cavity receiver.

Heat Transfer Enhancement in Absorber Tube of Parabolic Trough Concentrators Using Wire-Coils Inserts

Heat transfer enhancement technology provides many advantages in heat exchanger applications. In the present paper, the heat transfer characteristics and the pressure drops of the absorber tube of Parabolic Trough Concentrators with wire-coils inserts are investigated. Water is used as a heat transfer fluid and simulations are carried out in COMSOL Multiphysics® 4.4, for various flow rates of water as well as for different pitch values of wire-coils insert. It is observed that introduction of wire coils increases the turbulence inside the tube, thereby increasing the Reynolds number and hence the Nusselt number. Value of Nusselt number is observed to be increased by 104% to 330% which indicates the increase in the value of convective heat transfer coefficient and hence the heat transfer rates.

Performance of a combined cooling heating and power system with mid-and-low temperature solar thermal energy and methanol decomposition integration

In this paper, a new distributed energy system that integrates the mid-and-low temperature solar energy thermochemical process and the methanol decomposition is proposed. Through the solar energy receiver/reactor, the energy collected by a parabolic trough concentrator, at 200–300°C, is used to drive the decomposition reaction of the methanol into the synthesis gas, and thus the solar thermal energy is converted to the chemical energy. The chemical energy of the synthesis gas released in the combustion chamber of a micro gas turbine is used to drive the combined cooling heating and power systems. Energy analysis and exergy analysis of the system are implemented to evaluate the feasibility of the proposed system. Under the considerations of the changes of the solar irradiation intensity, the off-design performances of the micro turbine and the variations of the load, the design and off-design thermodynamic performances of the system and the characteristics of the chemical energy storage are numerically studied. Numerical results indicate that the primary energy ratio of the system is 76.40%, and the net solar-electricity conversion rate reaches 22.56%, which is higher than exiting large-scale solar thermal power plants. Owing to the introduction of a the solar thermochemical energy storage in the proposed system, the power generation efficiency is insensitive to the variations of the solar radiation, and thus an efficient and stable utilization approach of the solar thermal energy is achieved at all work condition.

Continuous flow solar thermal pasteurization of drinking water: Methods, devices, microbiology, and analysis

The ability to simply and robustly pasteurize drinking water would present tremendous worldwide human health benefits. Ingestion of unsafe drinking water is a leading cause of sickness and death in the developing world. A simple method to use concentrated solar power is presented here with two complimentary numerical models. The first model allows a prediction of water temperatures within the concentrator and enables a user to vary operating parameters to assess the impact on the temperatures. The second model relates the temperatures to pathogen inactivation kinetics so that predictions of pathogen populations can be made. It is found that with a modest size system that includes a parabolic trough concentrator, a thermally activated valve, and a fluid-to-fluid heat exchanger, near complete pathogen inactivation can be achieved. Comparisons are made between simulated and experimentally determined temperatures. The comparison demonstrates the veracity of the model.

Explicit expressions for temperature distribution and deflection in absorber tube of solar parabolic trough concentrator

The portion of absorber tube of parabolic trough, facing the reflector, receives concentrated rays and the portion facing the sun receives direct incident rays resulting in circumferential non-uniform flux distribution. It leads to circumferential non-uniformity in the temperature of absorber tube. Thus, the absorber tube experiences differential expansion that results in tension and compression in its different parts leading to bending of the tube. Using the distribution of solar flux on the absorber tube incorporating the effects of Gaussian sun shape and optical errors, explicit expressions for finding the absorber’s temperature distribution and corresponding deflection in the central axis of absorber tube (from the focal line of trough) are derived in the current work. Deflection due to the weight of the absorber tube is also accounted. The absorber tube is considered to be supported at its ends. To allow the absorber tube to elongate freely, supports are chosen such that they can move axially. Two types of conditions are considered: (i) the ends of absorber tube are allowed to rotate in the planes passing through focal line of the trough and (ii) rotation is not allowed. Keeping solar radiation, ambient conditions, receiver’s dimensions, trough’s aperture width, fluid and material’s properties of absorber tube fixed, calculations have been carried out to study the effects of desired rise in fluid temperature, optical errors and rim angle of trough on absorber’s temperature distribution and deflection in the absorber tube. For the chosen system dimensions, fluid’s properties and absorber’s material, as fluid’s temperature rise increases from 0.1°C/m (averaged over the receiver’s length) to 0.5°C/m, the maximum circumferential difference in absorber’s temperature increases from 9°C to 23°C and the maximum deflection increases from −5.9mm to −13.7mm (positive and negative signs indicate deflections away and towards the vertex line of the trough respectively). As optical errors increase from 0mrad to 20mrad, the maximum circumferential difference in absorber’s temperature decreases from 14°C to 11°C and the maximum deflection decreases from −8.8mm to −7.2mm. As rim angle increases from 60° to 140°, the maximum circumferential difference in absorber’s temperature decreases from 16°C to 7°C and the maximum deflection decreases from −9.0mm to −1.7mm.

Comparative study of different solar-based photo catalytic reactors for disinfection of contaminated water

In this work, two solar-based photo catalytic reactors having different geometries and configurations for the disinfection of contaminated water were designed and tested. These solar-based reactors are stair-type open exposure and parabolic trough concentrator (PTC). For the first type, the stairs were coated with TiO2 using sol-gel method, whereas the parabolic type concentrator used a titania-coated rod. For monitoring of disinfection process, the parameters such as solar irradiation exposure time and bacterial deactivation counts were investigated carefully. On contrary to commercial slurry-based photo reactor reported in the literature, thin film titania-coated reactor’s performance was investigated in this work. Moreover, detailed designed aspects of the reactors, process parameters, and reactor operation are presented and discussed. The target aqueous matrix was drinking water developed in laboratory having 106 CFU/mL bacterial strain. Experimental results revealed that during 20 min of solar radiation exposure, 28 and 75% of the initial bacterial strain disinfection was recorded in staircase and PTC photo reactor, respectively. Moreover, results revealed that 1.75 kJ/L of UV energy is required in PTC photo reactor to achieve 75% reduction of initial bacterial population, whereas 4.1 kJ/L of UV is required to produce the same result in staircase photo reactor. The PTC showed more effective inactivation of E coli bacteria than the staircase-type photo reactors. This study is highly significant for designing of large-scale PTC reactors for clean water supplies for rural population and remote locations.

Experimental Performance of a Solar Thermoelectric Cogenerator Comprising Thermoelectric Modules and Parabolic Trough Concentrator without Evacuated Tube

A prototype practical solar-thermoelectric cogenerator composed of (1) a primary component of a pile of solar-selective absorber (SSA) slab, thermoelectric (TE) modules, and a depressed water flow tube (multichannel cooling heat sink, MCS), and (2) a parabolic trough concentrator with aperture area of 2 m A— 2 m and east–west focal axis was constructed. Its cogeneration performance under the best climatic and solar insolation conditions in Guangzhou, China was tested. For simplicity, the evacuated glass tube to cover the primary component was eliminated from the system. Six Bi2Te3 TE modules were arranged in series, directly bonded to the rear surface of the solar absorber slab. The hot-side temperature of the TE module reached up to 152°C. The experimentally obtained instantaneous results for the solar to electrical conversion efficiency, heat exchange coefficient of the MCS, and overall system efficiency under the best environmental and solar insolation conditions were about 1.14%, 56.1%, and 49.5%, respectively. To justify these values, an equivalent thermal network diagram based on a single-temperature-node heat transfer model representing the respective system components was used to analyze the thermal transfer and losses of the system. Finally, electrical power of 18 W was generated, with 2 L/min of hot water at 37°C being produced and stored in the insulated container.

An air-based corrugated cavity-receiver for solar parabolic trough concentrators

A tubular cavity-receiver that uses air as the heat transfer fluid is evaluated numerically using a validated heat transfer model. The receiver is designed for use on a large-span (9m net concentrator aperture width) solar parabolic trough concentrator. Through the combination of a parabolic primary concentrator with a nonimaging secondary concentrator, the collector reaches a solar concentration ratio of 97.5. Four different receiver configurations are considered, with smooth or V-corrugated absorber tube and single- or double-glazed aperture window. The collector’s performance is characterized by its optical efficiency and heat loss. The optical efficiency is determined with the Monte Carlo ray-tracing method. Radiative heat exchange inside the receiver is calculated with the net radiation method. The 2D steady-state energy equation, which couples conductive, convective, and radiative heat transfer, is solved for the solid domains of the receiver cross-section, using finite-volume techniques. Simulations for Sevilla/Spain at the summer solstice at solar noon (direct normal solar irradiance: 847Wm−2, solar incidence angle: 13.9°) yield collector efficiencies between 60% and 65% at a heat transfer fluid temperature of 125°C and between 37% and 42% at 500°C, depending on the receiver configuration. The optical losses amount to more than 30% of the incident solar radiation and constitute the largest source of energy loss. For a 200m long collector module operated between 300 and 500°C, the isentropic pumping power required to pump the HTF through the receiver is between 11 and 17kW.

Simultaneous Multiple Surface method for Linear Fresnel concentrators with tubular receiver

Linear Fresnel concentrators (LFR) are known for their potential cost-effectiveness, reliability and wide range of possible solar applications. However, this technology is still facing the challenge of increasing its overall performance, especially in the Concentrated Solar Power (CSP) field where Parabolic Trough and Central Tower Receiver concentrators are the predominant solutions. In order to increase the performance of conventional LFR concentrators it is necessary to increase their optical performance as these concentrators are still far from the theoretical limits of concentration. This paper presents a new Fresnel concentrator designed with the Simultaneous Multiple Surface (SMS) method for two reflective (X) surfaces (Fresnel XX SMS) and with a tubular receiver. This design also promotes a good light uniformity on the receiver. A comparison is made between this new Fresnel XX SMS and two present day available concentrators (a Fresnel with CPC (Compound Parabolic Concentrator) and a PT (Parabolic Trough) concentrator), as well as a calculation of the total amount of collected energy (kWh), before thermal losses, for a particular location, Faro (Portugal). Furthermore, a new definition of CAP (Concentration Acceptance Product) is proposed, as the standard definition does not fully take into account the optical nature of LFR concentrators. The paper ends with a discussion of the results obtained, their impact and possible applications in the future.

Effects of environmental factors on the conversion efficiency of solar thermoelectric co-generators comprising parabola trough collectors and thermoelectric modules without evacuated tubular collector

Solar thermoelectric co-generators (STECGs) are an attractive means of supplying electric power and heat simultaneously and economically. Here we examine the effects of environmental factors on the conversion efficiencies of a new type of STECG comprising parabolic trough concentrators and thermoelectric modules (TEMs). Each TEM array was bonded with a solar selective absorber plate and directly positioned on the focal axis of the parabolic concentrator. Glass tubular collectors were not used to encase the TEMs. Although this makes the overall system simpler, the environmental effects become significant. Simulations show that the performance of such a system strongly depends on ambient conditions such as solar insolation, atmospheric temperature and wind velocity. As each of these factors increases, the thermal losses of the STECG system also increase, resulting in reduced solar conversion efficiency, despite the increased radiation absorption. However, the impact of these factors is relatively complicated. Although the electrical efficiency of the system increases with increasing solar insolation, it decreases with increasing ambient temperature and wind velocity. These results serve as a useful guide to the selection and installation of STECGs, particularly in Guangzhou or similar climate region.

Potential improvements in the optical and thermal efficiencies of parabolic trough concentrators

A detailed 3D heat transfer model is used to analyze the improvement potential of a typical parabolic trough concentrator (PTC) system with step-wise idealizations of system components. Sigmoid functions are used to idealize and optimize the optical behavior of the absorber tube selective coating. Reflectance, absorptance, and transmittance behavior is evaluated for the glass envelope, assessing the potential performance of a system with an ideal selective glass. Optical properties of the primary concentrator mirror, such as reflectivity as well as tracking and surface errors, are successively idealized to reveal the differences between ideal and real concentrators. In addition, the effect of the supporting structure is analyzed by reducing the number of heat collection elements (HCE) per mirror module, lowering the shaded area between HCEs, and removing the structure altogether. Applying these component idealizations individually yielded increases in thermal efficiency between 4.3% and 7.3% compared to a selected benchmark PTC design, while a combination of all idealizations resulted in an increase of 23%. The analysis of an alternative PTC design has shown that the idealization approach is robust in that it predicts similar increases in thermal efficiency. In a second step, several secondary mirror designs are evaluated and optimized, including a partially reflective glass surface, insulation in the vacuum annulus with reflective surfaces, as well as aplanatic mirror and tailored secondary designs. The analysis of systems with secondary optics revealed potential increases in thermal efficiencies between 0.8% and 1.6% compared to the benchmark design.

Studies on performance characteristics of a solar parabolic trough concentrator with a variable area absorber

The design of a suitable absorber that incorporates an efficient heat transfer augmentation technique is one of the vital parameters that affect the performance of a solar parabolic trough concentrator (PTC). The variable area absorber discussed in this paper provides appreciable augmentation in heat transfer without increasing the pressure drop materially and thus helps in enhancing the performance efficiency of PTC significantly. A computer aided parametric study of this phenomena has been dealt in this paper. The performance of PTC of proposed design has been analysed mathematically and a rigorous simulation model has been developed. The mathematical analysis has been substantiated by elaborate experimental data.

Combined Cooling Heating and Power System with Integration of Middle-and-low Temperature Solar Thermal Energy and Methanol Decomposition

In this paper, a novel distributed energy system, which contains the process of mid-and-low temperature solar energy thermochemical hybridization with methanol is proposed. Through the solar energy receiver/reactor, solar thermal energy collected by a parabolic trough concentrator, at 250°C -300°C, drives the decomposition reaction of methanol into solar fuels of syngas, thus converts to chemical energy. The chemical energy of syngas releases in the combustion chamber of a micro gas turbine to drive the combined cooling heating and power systems. Extra produced solar fuel reserves a gas tank. Energy analysis and exergy analysis of the system are implemented, and the design and off-design performance of the system and the character of chemical energy storage under variable solar radiation are discussed. As a result, the primary energy ratio of the system is 76.40%, and the net solar-to-electricity rate reaches 22.56% much higher than the exited large-scale solar thermal power plant. As the solar thermochemical energy storage contained in the system, the generating efficiency becomes insensitive to the solar radiation, and thus the efficient and stable utilization of solar thermal energy is achieved at all work condition.

Experimental and Numerical Analysis of Thermal Losses of a Parabolic Trough Solar Collector

Parabolic trough concentrators are currently the most developed and most commonly used solar technologies for electricity generation. The thermal losses play an important role in the performance of the system. This paper describes the methodology and the results of an experimental and numerical investigation of the thermal losses of a small-scale parabolic trough collector. The collector is 3m wide and 4m long, with evacuated tubes with selective coatings.