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This article focuses on maximizing the thermal energy collected by parabolic-trough solar collector fields to increase the production of the plant. To this end, we propose a market-based clustering model predictive control strategy in which controllers of collector loops may offer and demand heat transfer fluid in a market. When a transaction is made between loop controllers, a coalition is formed, and the corresponding agents act as a single entity. The proposed hierarchical algorithm fosters the formation of coalitions dynamically to improve the overall control objective, increasing the thermal energy delivered by the field. Finally, the proposed controller is assessed via simulation with other control methods in two solar parabolic-trough fields. The results show that the energy efficiency with the clustering strategy outperforms by % that of traditional controllers, and the method is implementable in real-time to control large-scale solar collector fields, where significant gains in thermal collected energy can be obtained, due to its scalability.

In this paper, a novel parabolic trough solar receiver-reactor (PTSRR) system of a locally-installed Kenics static mixer (KSM) is proposed for efficient solar thermal hydrogen production. A three-dimensional comprehensive model was established for PTSRRs of the methanol-steam reforming reaction (MSRR) for hydrogen production, by combining the Finite Volume Method and the Monte Carlo ray-tracing method with a MSRR comprehensive kinetic model. The validated model was preliminarily applied to study the effects and mechanisms of the concentrated solar flux nonuniformity and the locally-installed KSM on PTSRR photo-thermal-chemical comprehensive characteristics and performance, taking the methanol flow rate and the catalyst sintering temperature limitation into account. With a preliminary optimization on the concentrated solar flux nonuniformity, the optical efficiency and the solar flux nonuniformity are improved by 6.58% and 30.42% respectively. It is further revealed that these PTSRRs of better concentrated solar flux density nonuniformity also have better thermal-chemical comprehensive characteristics and performance. Novel PTSRRs of the locally-installed KSM have better comprehensive characteristics and performance than corresponding original PTSRRs or even optimized PTSRRs, with a maximum increase in the methanol conversion rate of 6.92%. It thus will operate more safely and more efficiently, by the cost of a little more pump power to overcome corresponding larger flow resistance caused by the locally-installed KSM. From the mechanism, this kind of novel PTSRR of a locally-installed KSM provides a useful option of high potential for improving uniformities of a series of key field variables in the whole photo-thermal-chemical conversion process, and thus improves the comprehensive characteristics and performance of PTSRRs.

Concentrated solar power (CSP) has attracted increasing attention as a renewable energy source with zero carbon emission. For efficient conversion of the externally concentrated heat, a suitable electric generator is crucial. In contrast to the currently widely used steam turbine, the free-piston Stirling electric generator (FPSG) features a flexible power range, high reliability, and zero water consumption. To date, most Stirling electric generators have been integrated with a parabolic dish concentrator in a dish-Stirling system. However, this configuration faces great difficulties in incorporating thermal energy storage or an auxiliary heating facility. To address this, a trough-Stirling concentrated solar power system, in which the parabolic dish concentrator is replaced with a parabolic trough collector (PTC), might be a good candidate. In this study, a free-piston Stirling electric generator integrated with a parabolic trough collector is constructed and tested for solar thermal power generation for the first time, and the performance of the generator and the overall system are experimentally investigated, with Therminol VP-1 being used as the circulating heat transfer fluid. The preliminary on-sun test results show that a maximum output electric power of 2008 W with a thermal-to-electric efficiency of 15% on the free-piston Stirling electric generator can be obtained at a heater head temperature of 300 °C, accompanied by a maximum overall efficiency of 3.0%. Loss analysis indicates that the dominant losses are the end loss and cosine loss of the parabolic trough collector, and that heat loss occurs in the heat collection elements and pipelines, thus providing indications as to where further improvements can be made.

A hybrid concentrating photovoltaic/concentrating solar power spectrum splitting collector has been designed, developed, and experimentally tested. The two-stage optical system pairs a parabolic trough with a compound parabolic concentrating secondary, generating 50X geometric concentration on the thermal absorber. Double-junction indium gallium phosphide/gallium arsenide solar cells integrated into the secondary reflector generate electricity from photons with energy greater than ∼1.4 eV and reflect the remaining lower energy infrared photons to the thermal absorber for optimal spectral utilization. Simulations predict an optical efficiency of 64%, thermal efficiency of 52% at 600 °C, and efficiency of the back-reflecting concentrating photovoltaic subsystem of 6%. The collector has been developed into a working prototype and tested on-sun up to 600 °C using suspended solid particulate (alumina based) heat transfer fluid. The prototype demonstrates an optical efficiency of 63% and a net solar-to-electric efficiency of the concentrating photovoltaic subsystem of 4%. Thermal efficiency is much lower in the prototype collector and the potential causes have been examined. Overall, this work demonstrates a two-stage collector design, the ability of back-reflecting solar cells to operate as spectrum splitting devices, and the feasibility of an internally circulated solid particulate heat transfer fluid which enables high temperature operation beyond what is currently achievable with existing heat transfer fluids.

A two-stage high-concentration collector has been developed which pairs a 45° half rim angle parabolic trough with a secondary compound parabolic concentrator to achieve 50× concentration on the thermal absorber. An experimental prototype fabricated and installed at the University of California, Merced was tested on-sun up to 650 °C using a suspended particulate heat transfer fluid (HTF). The prototype collector demonstrated 63% optical efficiency and 40% thermal efficiency at 650 °C. This prototype demonstrates the feasibility of a two-stage linear trough collector at generating high flux and high absorber temperatures, and the ability of the particulate HTF to operate under these conditions.

The trough solar thermal power generation system is one of the most mature solar thermal power generation systems. A novel major arc-shaped linear cavity receiver with a lunate channel based on the black cavity effect principle for parabolic trough solar collectors is proposed in this work. The effects of the inclination angle, collecting temperature, surface emissivity and aperture width on the heat loss are thoroughly analyzed with a two-dimensional numerical model coupling natural convection with surface radiation. In addition, the thermal performance of the proposed linear cavity receiver is compared with that of the Solel’s UVAC series evacuated collector tube. The results show that: (1) The natural convection heat loss is significantly affected by the inclination angle, while the radiation heat loss is mainly affected by the surface emissivity and the collecting temperature. (2) The aperture width of the receiver has a great impact on the thermal performance. The larger the aperture width is, the greater the heat losses. However, the aperture width can also affect the optical performance and the manufacturing cost. The reasonable aperture width for this kind of linear cavity receiver is about 50–70 mm with consideration of all these factors. (3) The proposed linear cavity receiver demonstrates comparative or even better thermal performance as traditional evacuated collector tubes, especially in high temperature range. In general, the proposed linear cavity receiver has the comparative shape and size as the traditional evacuated collector tubes. More importantly, it has the advantages of raising the collecting temperature and reducing the production and maintenance costs. Therefore, it can be used to replace the evacuated collector tube which has poor performance due to long-term operation.

The solar parabolic trough collector is amongst the most mature solar technologies and has become more cost effective in recent years. In this paper, a novel design for the receiver unit is presented which aims to decrease the radiant energy losses while raising the temperature of the working fluid, when compared to conventional systems. The design incorporates the application of different optically active layers in conjunction with a cavity absorber. The cavity geometry and a hot mirror coating at the aperture enable heightened retention of thermal radiation of the receiver. The additional, novel aspects of the background theory of the design is presented and implemented in a simulation. The simulation results for receiver temperature profiles, heat transfer fluid temperature, and efficiencies are shown. It is seen that our proposal has advantages in terms of thermal behavior over conventional designs in that it can exceed the heat transfer fluid temperature and the efficiency of existing alternatives.

This paper presents a 70-kWe indoor solar simulator for concentrating and non-concentrating solar applications. The solar simulator consists of three major components: a lamp array, an optical integrator, and a collimating lens. The lamp array consists of seven radiation modules, each comprising a 10-kWe short-arc xenon lamp coupled to a custom-designed reflector with the shape of a truncated ellipsoid. The optical integrator and collimating lens are intended for homogenizing the energy flux distribution of light from the lamp arry and reducing the divergnce angle of light. The solar simulator can produce concentrated high-flux, medium-flux and non-concentrated quasi-collimated light with continuously adjustable power output. Within a 60 mm-diameter circular target on the focal plane, the measured peak and mean radiative fluxes are 9200 kW/m2 and 5100 kW/m2, respectively, and the corresponding radiative power is 14.45 kW. For a 4 m × 3 m rectangular area illuminated by quasi-collimated light, the measured radiative flux, half divergence angle, and uniformity of the light are 0.94 kW/m2, 1.3°, and 92%, respectively. At 27% of the rated power input of the solar simulator, a parabolic trough collector illuminated by the quasi-collimated light can reach 350 °C; at 93% of the rated power input, the focused radiation can melt tantalum (melting point 3017 °C). The solar simulator provides a universal research platform for a wide range of solar energy technologies, such as solar thermal, solar thermochemical, solar photovoltaics and photocatalysis (with a minor upgrade of the spectrum).

The optical performance of a concentrating solar power (CSP) collector is critical to the overall efficiency of the system. This study presents a detailed optical comparison between three representative CSP collector designs including linear Fresnel, parabolic trough, and central-receiver technologies. Optical models are implemented in SolTrace, which is ray-tracing software developed at the National Renewable Energy Laboratory. The ray-tracing algorithm is used to calculate a collector’s design-point performance as well as its incidence-angle modifiers to evaluate the collector performance at any sun position during a typical meteorological year. The efficiency over a one-year period is then analyzed based on ray-tracing results. Using China Lake (California) as an example, the annual optical efficiency is 60% for the selected parabolic trough collector, 52% for the selected central-receiver technology, and 40% for the selected linear Fresnel collector. The parabolic trough has the highest optical performance among all. The selected central-receiver technology provides the most consistent seasonal production profile over the course of the year due to its two-axis-tracking ability but would suffer most from the increasing solar collector optical error. It is also shown that a dramatic cost reduction is required for the selected linear Fresnel technology to be competitive in the future energy market. Sensitivity of three CSP technologies to the deployment locations and the overall optical-error magnitude is also examined through annual performance analysis. The results will provide insights into a better understanding on inherent technical aspects of different CSP technologies.

This paper is devoted to the analysis of different secondary reflector designs for linear Fresnel collectors. A number of authors have proposed alternative shapes of secondary reflectors in order to enhance the efficiency and the flux intensity at the absorber tube. In this work the primary mirrors layout has been maintained constant for all designs. Thus, only secondary concentrators that do not require a change in the primary mirrors field have been studied. In order to carry out the study a validated Monte Carlo ray trace code has been used, where three optical properties are considered: annual optical efficiency, mean flux intensity and circumferential flux intensity homogeneity. The maximum efficiency reached by each technology, for given optical flux specifications, is obtained. Finally, the optical performance of linear Fresnel collectors with secondary reflector is compared with that of state-of-the-art parabolic trough collectors. It is concluded that the efficiency of Fresnel collectors is around 23% than that of parabolic troughs, although with mean flux intensities around 46% higher and circumferential flux intensities notably more homogeneous (top to bottom flux intensity ratios five times higher).

To alleviate the effect of solar radiation fluctuation on solar thermochemical reaction, phase change material (PCM) is applied in solar-chemical reactor to buffer the temperature vibration induced by solar radiation fluctuation. In order to comprehensively study the effect of PCM on chemical reactor, a two-dimensional model was employed to compare the steady performance of 3 different kinds of solar parabolic trough receiver reactors (SPTRR): without PCM (SPTRR0), with PCM near the outer wall (SPTRR1) and with PCM in the center (SPTRR2). Then, the transient performance of three SPTRRs under continuously pulsed solar radiation and real solar radiation fluctuation are, respectively, analyzed. The results showed that comparable chemical performance can be achieved for all three different SPTRRs although the amount of catalysts in SPTRR1 and SPTRR2 are 13.2% less than that in SPTRR0. When the time interval of cloud presence is 3 min, compared with SPTRR0, the averaged methanol conversions in SPTRR1 and SPTRR2 are, respectively, improved by 10.5 and 9.8% and production selectivity (higher H2/CO) is better. And when under a real solar radiation fluctuation, compared to SPTRR0, the averaged methanol conversions in SPTRR1 and SPTRR2 are relatively improved by about 10.7% and 7.2%. SPTRR1 can more effectively attenuate the effect of solar radiation fluctuation and shows better chemical performance than SPTRR2.

A novel proposal to drive forward linear Fresnel technology within the solar market competitiveness is presented in this paper. It consists of two innovative storage strategies conceived for the management of a flexible three-tank thermal storage system of molten salts, which seeks the enhancement of part-load plant efficiency in alternative East-West oriented Fresnel solar plants. A consistent methodology has been developed in order to assess quantitatively this new proposal by means of annual performance simulations of a global model. This allows the comparison of the aforementioned approach of Fresnel-based plant with reference parabolic trough collectors (PTC) and linear Fresnel collectors (LFC), which are equipped with state-of-the-art two-tank thermal storage of molten salts and North-South solar field orientation. Results show an increase of more than 10% in annual plant efficiency compared to conventional North-South Fresnel plants based in high latitude locations, i.e. Almeria (Spain). This boost in LFC performance would imply narrowing the gap with PTCs, where expensive high optical quality receivers are used. Findings in this paper state that the proposed innovative strategies are proven to be more effective when low quality receivers are used and when the solar fields are located far from tropics.

Wind barriers, according to their sizes and shapes, can effectively control, shift and even modify the airflow field in their downstream. These structures can accelerate the wind flow over the mirror field and move the airborne particles away from the mirrors. For example, in concentrated solar power plants it is highly desirable to engineer the system in a way that fewer particles are deposited within the solar field, in particular, onto mirror surfaces. Therefore, design optimization of dust barriers could significantly impact the mirror soiling and favourably reduce the cleaning water consumption of a solar power plant. This study focuses on the optimization of a solid wind barrier around a parabolic trough collector plant for their protection against dust soiling. The presented simulation results show that an optimum solid wind barrier is able to direct large amount of particles (in this study, more than 86%) to pass over the solar field with very small fraction (around 0.8%) being deposited on the mirrors. In addition, it was found that the barrier wall is more effective in deflecting the larger particles from the solar field.

Solar thermal systems are advantageous since it is easier to store heat than electricity on a large scale. As such, concentrated solar power is receiving considerable interest among researchers, developers and governments. Several concentrated solar power technologies have been developed including the solar tower, the parabolic trough technology, solar dish and linear Fresnel systems. Among them, the parabolic trough solar collector is a proven technology used dominantly for both industrial process heat and power generation. This technology has matured over the years, and its advancement has become the topic of numerous research studies which were the counter driving force of the field. Particularly in recent years, a significant amount of theoretical and numerical studies have been conducted to assess and improve the performance of parabolic trough solar collectors. This review methodologically holds colossal knowledge of current and past studies to assess the optical and thermal performances of parabolic trough solar collectors, modeling approaches and the potential improvements proposed on behalf of the parabolic trough solar collector design. The optical modeling approaches are identified to be analytical and ray-tracing. The review of thermal modeling approaches presents the steady and transient heat transfer analyses of single and two-phase (with direct steam generation) flows. Also, the computational fluid dynamics models used to analyze the physics of parabolic trough solar collectors with a better insight are reviewed and presented. Finally, the studies conducted on the performance improvement of parabolic trough solar collectors are separately examined and presented, these include novel designs, passive heat transfer enhancement, and nanoparticle laden flows.

This paper presents a novel computing method of optical efficiency fitting formulas and its applications for parabolic trough solar collectors, by combining the Monte Carlo ray-tracing methodology with the population-based particle swarm optimization algorithm. The expressions of the optical efficiency fitting formulas are deduced by means of analyzing the Monte Carlo ray-tracing data samples with a variable reduction technique, while the fitting formula coefficients are obtained using the particle swarm optimization algorithm. The computing time of the annual optical efficiency reduces to less than 10−7 s, from about a day as previously calculated by a once powerful Monte Carlo ray-tracing model. Thus, it makes the once time-consuming long-time average optical efficiency calculations or corresponding population-based optimizations much more feasible and efficient. A dynamic particle swarm size factor is also proposed to improve the particle swarm optimization performance, which reduces nearly 90% of the particle swarm optimization computing time. Numerical results calculated by the Monte Carlo ray-tracing model and the obtained optical efficiency fitting formulas were compared with the corresponding reference data. Good agreements were obtained, proving that the computing method and model are reliable. The obtained optical efficiency fitting formula of parabolic trough solar collectors is then applied successfully to efficiently study the characteristics of long-time average optical efficiencies under different geometrical configurations, geographical locations, tracking models and corresponding optical or geometrical optimizations. This proposed novel method offers a useful option of high potential for further applications on efficient optical efficiency related analyses and corresponding population-based optimizations for parabolic trough solar collectors.

The use of flow inserts is a common passive technique for enhancing the thermal performance of parabolic trough solar collectors. The objective of this work is to investigate a novel insert with a star shape. The shape of this insert is optimized by examining different combinations of its geometric characteristics. Totally 16 different cases are studied with the fin length to be varied from 15 mm to 30 mm and the fin thickness from 2 mm to 5 mm. The evaluation criteria of the examined cases are the thermal efficiency, the exergy efficiency and the overall efficiency of the solar collector. The exergy efficiency and the overall efficiency are suitable criteria for evaluation the Nusselt number increase with the simultaneous friction factor increase. The collector is examined for inlet temperatures between 400 K and 650 K with a volumetric flow rate equal to 150 L/min. According to the final results, the thermal efficiency, the exergy efficiency and the overall efficiency are increased for all the examined flow inserts. The thermal efficiency enhancement is higher with the inlet temperature increase and it can reach up to 1%. Moreover, it is found that greater dimensions of the insert lead to higher performance enhancements. The pressure drop is found to increases many times with the use of inserts but the pumping work value is extremely low in all the cases. The analysis is conducted with SolidWorks Simulation Studio with a validated model.

In this research, a novel cavity receiver for the parabolic trough solar collector (PTC) is presented. A center tube and two inclined fins act as the absorber to absorb the solar energy concentrated by the parabolic trough reflector. Its manufacture is simple. Compared to the tube bundle absorber, it is more convenient to connect in series and avoids the possible non-uniform distribution of the flow in practical applications. An experimental setup was constructed to test the thermal performance of this kind of cavity receiver. The experimental collector efficiency was in the range of 34.18–48.57%. The used physical parameters and installation error in the experiment resulted in the relatively low collector efficiency. A three-dimensional heat transfer model was established and validated by the test results. Effects of thermal conductivity of the insulation materials, emittance of the glass cover and absorber, and optical parameters on the thermal performance of the cavity receiver were estimated in detail. The results quantitatively indicated the importance of improving the physical property parameters of the collector, e.g. if the optimal parameters were selected, the collector efficiency could reach 64.25%, which was comparable to the metal-glass evacuated tube receiver. It indicates that the performance improvement potential of this kind of cavity receiver is huge and the results can theoretically guide the improvement in manufacture and installation. The present study is beneficial for promoting the large-scale application of the PTC in a simple and convenient way and the development of solar thermal technologies. The experimental test of the collector with the optimized parameters of this research and detailed cost analysis will be performed in the future work.

This study aimed to investigate the applicability of the solar heating system in different geographical regions with different meteorological conditions, of which parabolic trough solar collectors (PTCs) were operated with the absorption heat pumps (AHP) and oil/water heat exchanger (OWHE) at medium and low operating temperature respectively. The heat transfer model for PTCs was constructed with a lumped parameter method and validated by experimental results. The thermal performance of the system was evaluated by the index of primary energy ratio (PER). The results showed that on overcast days with low direct normal irradiance (DNI), the operation of the PTC + AHP/OWHE system was not cost-effective. On cloudy days with high DNI lasting for a short period (e.g. 2 h), the operation of the PTC + OWHE system was better than that of the PTC + AHP system as the latter needed more preheating energy before the system operation. While on sunny days with high DNI lasting for a longer period (e.g. 8 h), the PTC + AHP system was suggested to be operated as it owned higher PER values. Besides, the solar energy distributions in China could be divided into four categories, i.e. rich (S1), relatively rich (S2), available (S3) and absent (S4). In S1 region the PTC + AHP system was suggested to be operated. In S4 region the PTC + AHP/OWHE system was not suggested to be operated. While in S2 and S3 region, whether the operation of the PTC + AHP/OWHE system was suggested depending on the meteorological conditions. With these findings the developing strategy of the solar heating system in different geographical regions can be devised and the operating strategy on a specific day with different weather conditions can be developed, which is helpful to guide the application of solar energy and improve the energy structure in domestic heating.

A two-stage high-concentration collector is designed which pairs a 45° half rim angle parabolic trough with a compound parabolic concentrator (CPC) secondary to achieve 50X concentration on the thermal absorber. An experimental prototype fabricated & installed at the University of California in Merced is tested on-sun up to 650 °C using a suspended particulate (alumina) heat transfer fluid (HTF). Early deformation of the secondary reflector was fixed by implementing a passive cooling technique. The prototype collector demonstrates 63% optical efficiency and 40% thermal efficiency at 650 °C and pathways towards improving both optical and thermal efficiency have been identified. Obstacles encountered during testing are discussed, which highlight the need for further investigation into the heat transfer mechanisms within the fluidized particulate stream. This prototype demonstrates the feasibility of the two-stage linear trough collector at generating high flux, high temperatures, and the ability of the particulate alumina HTF to operate under these conditions.

With the aim to enhance the reliability and overall heat transfer performance of a parabolic trough receiver, concentric rod and eccentric rod are introduced as turbulators, and the flow and convective heat transfer characteristics of molten salt in a parabolic trough receiver are analyzed. A three-dimensional model was developed and has been validated with experimental results and empirical equations. Highly non-uniform heat flux was provided by a novel parabolic trough collector. The result shows that both concentric rod insert and eccentric rod insert can enhance the heat transfer performance effectively. For a parabolic trough receiver with a concentric rod insert, with the increasing of dimensionless diameter B, the normalized Nusselt number is about 1.10 to 7.42 times over a plain parabolic trough receiver. The performance evaluation criteria can't reasonably evaluate the effect of B growth on the comprehensive heat transfer performance. By introducing integrated performance factor, it can give a reasonable solution, and it shows that the integrated performance factor has a significance decreases with the increase of Reynolds number when B is larger than 0.8. With B increasing, the integrated performance factor of parabolic trough receiver with concentric rod insert decreasing under a certain Reynolds number. For an eccentric rod insert, the performance evaluation criteria and the integrated performance factor decrease with the increasing of Reynolds number under a certain dimensionless eccentricity H. The performance evaluation criteria decreases from about 1.84 to 1.68 times over a plain parabolic trough receiver when H is 0.8. Moreover, the temperature distribution can be uniformed and the maximum temperature on the absorber tube also can be remarkably reduced with the increasing of B and H under a certain Reynolds number, which helps to reduce the thermal deflection and increase the reliability for a parabolic trough receiver.

A novel 1 MWe mid-temperature solar thermochemical power generation system is proposed to improve solar conversion efficiency in this work. The system consists of a parabolic trough solar collector, solar receiver/reactor, syngas storage tank, internal combustion engine (ICE) and heat exchange devices. The fed methanol is evaporated and flows into the solar receiver/reactor for decomposition and to produce syngas (H2 and CO), with the required reaction heat provided by the parabolic trough collector. Different from typical solar power technologies, in the proposed system, solar thermal energy is converted to syngas as chemical energy, which can be stored or utilized efficiently by an ICE for power generation. The thermodynamic performance evaluation results indicate that the annual energy efficiency and solar-to-electric efficiency reach to 33.78% and 18.29%, respectively. A small-scale pilot system with the capacity of 20 kWe is constructed, and for the first time, an industrial-scale mid-temperature thermochemical power generation is realized, which experimentally validates the effectiveness of the novel solar power technology. The research findings provide an alternative means for improving solar conversion performances.

Within the continuous improvement effort of parabolic trough collector (PTC) performances, a three-dimensional numerical model is carried out to investigate the turbulent flow and potential heat transfer enhancement in a novel parabolic trough receiver (PTR) where the conventional straight and smooth tube (CSST) receiver is replaced by a newly designed S-curved/sinusoidal one. Heat transfer performance and pressure drop penalty comparisons between CSST receiver and S-curved/sinusoidal tube receiver are conducted using synthetic oil as a heat transfer fluid (HTF). The validity of the model has been tested by comparing the numerical results with the available experimental data of the sinusoidal pipe heat exchangers. As a consequence of the shape of the novel tube receiver, the heat density distribution on the outer surface varies in both longitudinal and circumferential directions while it varies only in the circumferential direction on the corresponding CSST receiver. The analysis of the HTF flow through the novel PTR S-curved/sinusoidal tube showed the emergence of vortices at bends. It is established that, without any additional devices, the mean Nusselt number is expected to increase by 45 % – 63 % , while the friction coefficient increases by less than 40.8 % , which leads to a maximum performance evaluation criteria about 135 % . The maximum circumferential temperature difference of the novel PTR S-curved tube decreases below 35 K for almost all the range of the mass flow rates and should result in the reduction of thermal stresses and heat losses.

This paper presents a dynamic three-dimensional volume element model of a parabolic trough solar collector coupled to an existing semi-finite optical model for simulation and optimization. The spatial domain in the volume element model is discretized with lumped control volumes (i.e., volume elements) in cylindrical coordinates according to the predefined collector geometry. The spatial dependency of the model is therefore taken into account without the need to solve partial differential equations. The proposed model combines the laws of thermodynamics and heat transfer as well as empirical correlations to simplify the modeling and expedite the computations, and the resulting system of ordinary differential equations is integrated in time for temperature. The model was validated with the experimental data provided in the literature, and was employed to evaluate the sensitivity of the collector performance described by the first and second law efficiencies to receiver length, annulus gap spacing, concentration ratio, incidence angle, inlet fluid temperature and flow rate. This work also examined the effects of inlet fluid temperature and temperature differential on dynamic collector performance in the transient case study. Results showed that the first law efficiency was most sensitive to the inlet fluid temperature with the maximum variation of 30%, whereas the incidence angle and concentration ratio affected the second law efficiency the most with the maximum variations of 375% and 300%, respectively. The remaining parameters featured trivial effects in all cases. In the transient analysis, higher temperature differential and lower inlet fluid temperature yielded higher total heat gain while the total exergy gain was insensitive to both parameters. The first law efficiency should therefore be of greater importance than the second law efficiency in the control of dynamic collector performance based on these two parameters.

Regarding energy storage in concentrated solar power plants, thermocline technology is considered to be a cost effective but less efficient solution than conventional two-tank. However, thermocline storage charge and discharge are usually stopped when the varying outlet temperature reaches an arbitrarily chosen value. It is shown here that the stop of the thermocline charge depends on the overheating risk in the solar collectors, while the stop of the discharge is defined by the steam generator requirements. As a consequence, the temperature thresholds that must be defined by the experimental constraints are dynamic. Using these dynamic thresholds on an experimental setup comprising a 230 kWh thermocline tank and a 150 kWth parabolic trough solar field led to a charge efficiency of 95.7% and a 93.5% discharge efficiency. Thus, the varying outlet temperature of a thermocline storage system is not an issue when integrated in a concentrated solar power plant.

Cost-effective solar power generation systems are of vital importance. The efficient use of full-spectrum sunlight has drawn widespread attention in solar power generation. Here, a 2 kWe hybrid prototype coupling monocrystalline silicon photovoltaics and solar syngas fuelling a heat engine is proposed and experimentally tested. Using double-layer parabolic trough mirrors as the concentrator, the full-spectrum sunlight is split into different wavebands. Split visible and near-infrared sunlight are provided for the photovoltaics and converted into electricity, while both ultraviolent and far-infrared sunlight are used to drive methanol decomposition and produce solar syngas as a fuel for internal combustion engines. At a given solar irradiance intensity, the testing results show that the net solar-to-electricity efficiency of the hybrid prototype is 20.2%. Furthermore, comparison experiments are carried out between the hybrid prototype and the individual power system. The results show that the efficiency of the hybrid prototype is 5.2% higher than that of individual concentrated solar photovoltaics. This study provides an innovative approach to synergistically couple photovoltaic and solar thermal fuel for the cascading use of full-spectrum sunlight.

This study evaluates the benefits of integrating a full renewable dual back-up system (biomass and Thermal Energy Storage (TES)) in Concentrated Solar Power (CSP) plants. Two plants of 50 MWe capacity each are modelled and simulated, based on Parabolic Trough and Solar Tower technologies, with the integration of a biomass grate boiler in parallel to the power island. The Analytic Hierarchy Process is used as a Multi-Criteria Decision Method to compare the performance according to technical, economic, and operational criteria of 7 operating strategies. These strategies have been defined for integrating the biomass block for five levels of TES (No-TES, 5, 10, 15 and 20 h). The results show that the participation of biomass back-up favours the operation of the system as a base-load plant, increasing the capacity factor (CF) up to 71%, the net electric efficiency up to 10%, and reducing the cost of generation down to 56%, compared to stand-alone CSP plants. For the considered solar resource (Seville, Spain), reasonable generation costs (0.153 USD/kWh) can be achieved for a balanced trade-off between biomass and TES while allowing a firm energy supply (CF ≥ 80%) and reducing the required flexibility to the boiler. In addition, generation with a high solar share (over 50%) can be achieved with the proposed dual supporting system, favouring access to solar-driven incentives, as well as reducing the sensitivity of the system to the risks associated with biomass supply.

The solar-to-electric conversion potential of hybrid solar CPV/CSP parabolic trough collector (PTC) systems incorporating spectral beam splitting (SBS) is examined. Single and novel two-stage PTCs are paired with four different solar cells (c-Si, CdTe, GaAs, InGaP) and simultaneously simulated for the first time with spectral and temperature optimization to maximize solar-to-electric conversion. All cases show improvement, with the limiting case of an ideal, lossless optical filter enhancing conversion efficiency over typical PTC CSP plants by 25% with c-Si and 50% with GaAs. Typical interference (dichroic) filter systems demonstrate greater conversion efficiencies than systems with novel semi-transparent and back-reflecting solar cell beam splitters. Two-stage systems recover high operating temperatures in the CSP subsystem under partial illumination without reducing the overall conversion efficiency, enhancing the dispatchability of the thermal power. Economic analysis reveals a hybrid solar CPV/CSP beam splitting system with interference filters and c-Si cells can be built for $2.39/W. A new economic model comparing hybrid CPV/CSP to side-by-side PV/CSP is presented, demonstrating the limitations of c-Si paired systems and the potential of III-V cell SBS systems.

A new design and retrofit approach for a concentrated photovoltaic thermal (CPV-T) system based on a parabolic trough collector is presented. The design differs from previous hybrid architectures, employing a significantly simplified topology in order to reduce costs while keeping electrical efficiency high. To achieve this ambitious goal, the absorber tube of a conventional parabolic trough collector used in thermal systems is replaced by a newly developed hybrid absorber equipped with multi-junction solar cells. This offers the advantage that existing solar thermal parabolic trough facilities can be easily retrofitted, making the system scalable and cost-effective. A scaled prototype of the system was designed and tested in Graz, Austria. During on-sun tests an average electrical system efficiency of 26.8 % with a simultaneous thermal efficiency of 48.8 % was measured using a geometric concentration ratio of 150 (DNI based). This leads to an overall average system efficiency of 75.5 %. Moreover, a solar cell peak efficiency of 30 % was achieved, one of the highest measured solar-to-DC efficiencies for a parabolic trough-based solar collector. Special attention was paid to the temperature difference between the solar cells and the heat transfer fluid in order to ensure sufficient cooling of the cells and maximize thermal efficiency. The electrical, thermal and overall efficiencies at different heat transfer fluid temperatures were measured (17 ° C - 90 ° C) and their coefficients were derived. This work serves as an experimental proof-of-concept for the application of solar cells in parabolic trough collectors, thereby opening up new possibilities for cost reduction of CPV-T systems.

Among the solar concentrating technologies, parabolic trough collector (PTC) is the most mature and cost-effective technology for medium and high-temperature levels (150–400 °C). This paper investigates the utilization of internally finned absorbers in LS-2 PTC module for various operating conditions. Twelve different longitudinal fins are tested and compared with the smooth case. The analysis is performed with SolidWorks Flow Simulation, using a validated model by literature results. Generally, it is proved that both greater length and thickness lead to higher thermal enhancement and to higher pressure losses. Various methods are presented for evaluating together the thermal efficiency or Nusselt number enhancement versus the increase in pressure drop or in the friction factor. Taking into consideration four different criteria, the absorber with 10 mm fin length and 2 mm fin thickness is found to be the overall optimum case. For this case, the thermal efficiency is enhanced about 0.82%, the Nusselt number increase 65.8%, while the friction factor and the pressure losses are about the double compared to the smooth case.

Flat plate collectors have relatively low efficiency at the typical supply temperatures of district heating networks (70–95 °C). Parabolic trough collectors retain their high efficiency at these temperatures. To maximize the advantages of flat plate collectors and parabolic trough collectors in large solar heating plants for a district heating network, a hybrid solar collector field with 5960 m2 flat plate collectors and 4039 m2 parabolic trough collectors in series was constructed in Taars, Denmark. The design principle is that the flat plate collectors preheat the return water from the district heating network to about 70 °C and then the parabolic trough collectors would heat the preheated water to the required supply temperature of the district heating network. Annual measured and simulated thermal performances of both the parabolic trough collector field and the flat plate collector field are presented in this paper. The thermal performance of both collector fields with weather data of a Design Reference Year was simulated to have a whole understanding of the application of both collectors under Danish climate conditions as well. These results not only can provide a design basis for this type of hybrid solar district heating plants with flat plate collectors and parabolic trough collectors in the Nordic region, but also introduce a novel design concept of solar district heating plants to other high solar radiation areas.

A new method of analyzing the thermal performance of parabolic trough collectors (PTCs) for solar thermal applications is established using similarity principle and dimensional analysis, through which different types of PTCs can be studied via a single scaled physical model. Six dimensionless numbers for the PTC are drived and are used to build a scaled PTC model. A coupled approach combining Monte-Carlo ray-tracing method with finite-element is developed to analyze the performance of PTCs. Experimental data from the literature are employed to calibrate the numerical model. Compared with the results of four typical cases of non-scaled experimental data obtained from the literature, differences in average efficiencies of the scaled-down models are within 0.75%, thereby validating the scaled model and similarity method for analyzing PTCs with vastly different length scales. Effects of direct normal irradiance (DNI) and temperature difference between the receiver fluid and ambient air ΔTab on the efficiency of PTCs are further analyzed using the scaled model. The simulation results indicate that the collector efficiency increases with the augmentation of the DNI, whereas it decreases with the increase in ΔTab. The similarity analysis method provides a new perspective for solar-thermal research by demonstrating the possibility of performing experiments on PTC on a much-reduced length scales.

Concentrating solar power towers, which include a steam-Rankine cycle with molten salt thermal energy storage, is an emerging technology whose maximum effectiveness relies on an optimal operational and dispatch policy. Given parameters such as start-up and shut-down penalties, expected electricity price profiles, solar availability, and system interoperability requirements, this paper seeks a profit-maximizing solution that determines start-up and shut-down times for the power cycle and solar receiver, and the times at which to dispatch stored and instantaneous quantities of energy over a 48-h horizon at hourly fidelity. The mixed-integer linear program (MIP) is subject to constraints including: (i) minimum and maximum rates of start-up and shut-down, (ii) energy balance, including energetic state of the system as a whole and its components, (iii) logical rules governing the operational modes of the power cycle and solar receiver, and (iv) operational consistency between time periods. The novelty in this work lies in the successful integration of a dispatch optimization model into a detailed techno-economic analysis tool, specifically, the National Renewable Energy Laboratory’s System Advisor Model (SAM). The MIP produces an optimized operating strategy, historically determined via a heuristic. Using several market electricity pricing profiles, we present comparative results for a system with and without dispatch optimization, indicating that dispatch optimization can improve plant profitability by 5–20% and thereby alter the economics of concentrating solar power technology. While we examine a molten salt power tower system, this analysis is equally applicable to the more mature concentrating solar parabolic trough system with thermal energy storage.