Cylindrical Cavity Receiver Research Articles

Convective heat loss from a modified solar cavity receiver with vertical plate fins: An experimental assessment

AbstractExperimental investigations have been conducted to study the convective heat transfer from a cylindrical solar cavity receiver with vertical fins. The experiments were performed under varying surface heat flux levels and at seven inclination angles, ranging from −90° (upward facing) to +90° (downward facing) at 30° intervals. The impact of fins on the heat transfer process was studied by conducting experiments in two scenarios, namely, finned and unfinned cavities. The findings of the study showed that with an increase in cavity inclination, the magnitude of convective heat loss decreased in both finned and unfinned cavities, while the cavity surface temperature increased. At +90° inclination, the convective heat loss and Nusselt number were observed to have the lowest value, while the surface temperature had the highest value. For a downward‐facing cavity, the fins reduced convective heat loss, leading to an increase in cavity surface temperature. The finned cavity performed better for a vertically downward‐facing inclination (+90°) as it had a contribution of only 11% for convection heat loss compared with the unfinned cavity, which had a contribution of 21% for the same. Furthermore, an empirical model was developed based on the experimental results for the Nusselt number, which correlates experimental data with an error margin of ±15%. This model can be used to predict the Nusselt number for different inclination angles and surface heat flux levels. The presence of vertical fins in the cavity was found to be effective in reducing convective heat loss, especially for downward‐facing cavities. Understanding the influence of fin and cavity inclination on convective heat transfer can lead to enhanced efficiency and performance of solar receivers, thereby increasing the overall energy output of the system.

Solar cavity receiver for melting zinc metal

Concentrating solar power technologies can be applied to reduce the cost and carbon footprint of zinc melting processes. This study aims to improve the knowledge related to small-scale solar melting using a dish concentrator. This technology can be applied to zinc production as well as a range of small-scale applications, such as casting, recycling, galvanisation, and thermal storage. An experimental and analytical analysis of a rotating cylindrical cavity receiver for the indirect melting of zinc metal using concentrated solar power is presented. A multi-facet parabolic dish with an incident area of 2.85 m2 was considered together with a rotating cylindrical cavity receiver. The receiver had an aperture diameter of 0.2 m and the capacity for housing 17 kg of zinc. Five experimental test runs were executed, during which up to 73.5 % of the zinc inventory could be tapped from the receiver in its molten state, and average thermal efficiencies of up to 42 % were achieved. A predictive analytical model considering wind speed, wind direction, and direct normal irradiance was developed and validated against experimental data. A heat transfer efficiency factor was experimentally determined to account for voids in the zinc feedstock. The model was used to predict that approximately 41 kg of molten zinc could be tapped from the experimental setup throughout a typical day with a peak direct normal irradiance of about 900 W/m2 and an average wind speed below 2 m/s. A case study highlighted that energy savings of 0.6 kWh are achievable per kilogram of zinc processed by concentrated solar power rather than the conventional induction furnace.

Experimental investigation of generating superheated steam using a parabolic dish with a cylindrical cavity receiver: A case study

Abstract This study uses solar energy to convert steam from a saturated to a superheated state using a solar steam superheater system. This collector system comprised a parabolic dish with a cylindrical cavity. The parabolic dish is of 2 m diameter and 0.83 m focal distance, covered with a reflective surface made of 1,283 rectangular pieces of mirrors, each of 3.5 cm × 4 cm that were fixed in place with glue type (FnTai). The receiver is a stainless-steel cylindrical cavity having a diameter of 17 cm and a length of 25 cm. A helical copper coil with a diameter of 10 mm and a length of 6 m was inserted inside the cylindrical cavity. The experimental work utilized saturated steam produced separately from an auxiliary electric boiler system, which was made along with other system components. A 0.011 kg/s of saturated steam at a temperature of 112°C enters the copper coil and is heated by the solar radiation reflected by the parabolic dish onto the receiver. As a result, the temperature of the steam is increased to 169.5°C at the receiver outlet. It was found that the collector efficiency is 55.6%. In addition, the convection and radiation heat losses are 12.14 and 10.98%, respectively. Also, the heat losses of (spillage, reflection, and conduction) were estimated to be 21.18%. The mass flow rate and pressure of the saturated steam from the boiler and entering the receiver affected the superheated steam production process. The process of superheating the steam, coupled with the subsequent improvement in thermal performance, indicates an increased efficiency of the collector. This is achieved by boosting the generation of useful heat and mitigating heat losses.

Performance evaluation of various nano heat transfer fluids in charging/discharging processes of an indirect solar air heating system

In this study, an indirect charging/discharging solar-driven air heating system is assessed. In the charging process, a nano heat transfer fluid (NHTF) is thermally charged in a solar cylindrical cavity receiver (CCR) and then releases its thermal energy in a discharging process into the air in cross-contact with finned-tube heat exchangers (FTHEXs). In the experiments, a heating box with two consecutive FTHEXs is designed, and Syltherm 800 with Cu nanoparticles operates as the NHTF. The performance of the entire system in both single and dual FTHEXs modes is evaluated using various NHTFs, such as Therminol® 62, Therminol® 66, and Therminol® VP-1, and various nanoparticles, such as Ag, Cu, and Al2O3. It is found that the dual HEXs mode has a better performance than a single HEX mode. Therminol® 66 has a better performance in the charging process in the CCR, and Therminol® 62 exhibits a better thermal performance in both the charging and discharging processes. The average daily thermal efficiency (ADTE) of the CCR and the entire system for considered operational ranges can be improved up to 65.79 and 39.25%, respectively, for a single HEX mode and up to 69.93 and 42.83%, respectively, for a dual HEXs mode.

Parametric analysis of a high-temperature solar-driven propane steam reformer for hydrogen production in a porous bed catalytic reactor

In this study, a solar-driven porous bed catalytic reactor (PBCR) is investigated with the aim of propane reforming and hydrogen production using a clean energy source. The process consists of two major parts: a PBCR equipped with internal heating tubes and a parabolic dish collector (PDC) with its cylindrical cavity receiver (CCR) to supply the thermal energy required for the reforming process. A proper heat transfer fluid (HTF) is thermally charged in the CCR and discharged in the PBCR. In addition to the reforming reaction, the reversible water–gas shift (WGS) reaction takes place in the PBCR. In the reverse WGS, hydrogen produced in the previous stage is consumed and undesired carbon monoxide is generated. This unfavorable condition should be controlled to restrict the production of undesirable byproducts. In the present study, two CCR and PBCR parts are modeled/simulated using the CFD approach, and then validated using available experimental data. After model validation, the impacts of critical parameters such as inlet gas temperature, pressure and composition, bed porosity, PDC aperture diameter, and HTF circulation rate within the system on the performance of the entire system are assessed. Propane conversion, hydrogen yield, and carbon monoxide selectivity (CMS) are chosen as the system's performance indicators. At optimal conditions, for an inlet pressure of 100 Pa and temperature of 300 K, inlet propane mass fraction of 0.05, bed porosity of 0.3, PDC aperture diameter of 3 m, and HTF circulation rate of 0.05 kg/s, the time average propane conversion, hydrogen yield, and CMS during a typical day and a part of nighttime are obtained at about 62.93 %, 10.326, and 1.0122 %, respectively.

Performance Investigation of Cylindrical Cavity Receiver Using Roughened Surfaces

Solar cavity receiver is an integral part of the solar parabolic dish collectors used for absorbing concentrated solar irradiation. The performance of the receivers can be enhanced utilizing different types of heat transfer fluids (HTF) and modified geometries. The thermal performance of a redesigned cavity receiver with solid semi-circular tube attached to its surface was investigated using CFD three-dimensional simulation. The influence of operating and geometric parameters (i.e., inlet mass flow rate and inlet temperature) were investigated. The results obtained shows that, redesigned solar cavity receiver has thermal efficiency of 8.49-10.27% higher than a normal cavity receiver with an inlet flow rate of 0.01-0.04 kg/s under the same environmental and operating conditions. The temperature of outlet of HTF increases with increases in inlet temperature for Therminol66 oil. The comparison of thermal performance using different HTF (i.e., Therminol66, Ethylene Glycol, Water) is also conducted and it is found that Therminol66 oil gives 9% and 17% higher outlet temperature than ethylene glycol and water respectively at intake flow rate of 0.02 kg/s.

Nusselt Number Prediction for Oil and Water in Solar Tubular Cavity Receivers

In this study, a numerical model was developed for prediction of Nusselt number in solar cavity receivers. Thermal oil and water were used as the working fluid. A dish concentrator with different shapes of the cavity receiver, including hemispherical, cylindrical, and cubical, was investigated. The different shapes of cavity receiver were studied under the same operating conditions for prediction of the internal heat transfer coefficient correlation for each cavity receiver. The system is investigated under the variation of solar radiation, flow rate, and inlet temperature of solar working fluids. The developed thermal model is validated based on the experimental data for the cylindrical cavity receiver using thermal oil. The results reveal that the hemispherical cavity receiver had the highest cavity heat gain, heat transfer coefficient, and Nusselt number values compared to two other cavity receivers. It could be concluded that the cavity heat gain, and heat transfer coefficient, and Nusselt number amounts had improved with increasing solar radiation, increasing flow rate, and decreasing inlet temperature of the working fluid. Some equations were suggested for prediction of Nusselt number with the variation of solar radiation, flow rate of the working fluid, and inlet temperature of working. It was concluded that application of thermal oil had resulted in higher Nusselt numbers than the use of water as the solar working fluid. Consequently, the application of oil is suggested for high-temperature systems.

A Comparative Study of Multi-form Steam Generators Using Concentrated Solar Power

Solar energy reaching Earth can be used as promising renewable energy to overcome the challenges of steam generation processes. Solar concentrators suffer from fine adjustment of solar radiation concentration and high investment cost. Therefore, multi-configuration receivers named the cylindrical cavity receiver, helical receiver, and the absorber-evaporator-tank have been manufactured from the coiled copper tube and brass plates, respectively. Then, they are tested and compared to improve the performance of steam generators. The performance of the absorber-evaporator-tank receiver has been compared with the cylindrical cavity receiver and helical receiver for a period from 10 am to 12 pm. The present investigation shows that the boiling point of water increasing when the pressure of generated steam increases particularly inside the coiled tube, which affects the dryness fraction of the generated steam. The present results show that the efficiency of the absorber-evaporator-tank is greater than the cylindrical cavity and helical receivers for the tested period. It is found that the thermal efficiency for the absorber-evaporator-tank doubles at noon, which indicates an effective solar receiver for generating steam.

Comparative study of modified conical cavity receiver with other receivers for solar paraboloidal dish collector system.

Solar parabolic dish concentrators are one of the most efficient solar power conversion technologies. The cavity receivers are the most common type, used for reducing the heat losses from the receiver. In this paper, a novel cavity receiver is proposed, and the objective is to compare the novel modified conical cavity receiver with the existing cavity receivers such as cylindrical, conical and modified cavity receivers. The cavity receivers are designed for the parabolic dish of 4-m diameter which is installed at the National Institute of Technology Puducherry, India. Ray-tracing analysis was carried out to determine the size of the receiver. The analysis was carried out for various orientations of the receivers from 0 to 90° with a step size of 15° and also for the cavity temperatures: 300°C, 400°C, 500°C, 600°C and 700°C. Based on the results obtained, the modified conical cavity receiver is found to be the best design in terms of minimum heat losses compared to other receivers. The next best choices are found to be modified cavity, conical cavity and cylindrical cavity receiver. The whole analysis is conducted with a developed model in COMSOL Multiphysics.

Effect of surface material properties and operating conditions on the heat flux and temperature distributions in the cavity receiver of a solar-dish-coupled biomass gasification reactor

In this paper, a new 5.8 kW solar biomass gasification reactor is modeled by means of Monte-Carlo ray-tracing method and computational fluid dynamics. The reactor is coupled to the cylindrical cavity receiver of a parabolic-dish solar concentrator, where the incoming solar heat flux is concentrated. The absorbed heat flux distribution (flux density map) at each receiver surface is determined with the ray-tracing software SolTrace®. The sensitivity of the flux distribution to three uncertain variables is evaluated: the receiver solar absorptance, the receiver reflection type and the tracking error. It was found that the tracking error causes a pronounced increase in the flux peak at the cylindrical surface, while reflection scattering greatly reduces the flux peak at the bottom surface. Solar absorptance redirects the flux from every surface to the cylindrical surface, and so, it increases its flux peak. Reflection scattering reduces reflection losses. Two extreme flux concentration cases were found: 0.95 absorptance and 0.5° tracking error produce a 231 kW/m2 peak at the cylindrical surface; 0.25 absorptance, no tracking error and specular reflection produce a 250 kW/m2 peak at the bottom surface. The heat transfer within the reactor is modeled with ANSYS® CFX® with the flux distributions of the extreme cases as boundary conditions. The receiver temperature contours were obtained. Peak temperature locations coincided with the peak flux locations. For a 1200 K gas outlet temperature, peak temperatures of up to 1470 K were found, which exceed the material limit (1273 K). This issue is avoided by selecting suitable surface properties: solar absorptance <0.42 and a Gaussian specularity error >300 mrad.

An adaptive aerodynamic approach to mitigate convective losses from solar cavity receivers

We explore the potential for adaptive air barriers to mitigate convective heat losses from cavity receivers, especially at large tilt angles, using both suction and blowing nozzles. A heated scaled-down cylindrical cavity receiver was fitted with nozzles on four sides of the aperture, at 30° to the aperture plane. The tilted cavity receiver was mounted in a large wind tunnel to allow systematic variation of wind speed and direction. The effectiveness of different nozzle arrangements was calculated from the measured convective heat losses for a series of different mitigation strategies. The results reveal that a suction nozzle mounted at the bottom of the aperture is more effective than a blowing nozzle mounted at the top of aperture for tilt angles of θ= 45°, and under different wind speeds. The effectiveness ranged from 0 at a low suction flow rate to ~80% at a high suction flow rate. With the use of three suction nozzles, at the bottom and both sides of aperture, the effectiveness decreased markedly for both tilt angles and all wind speeds. For more challenging conditions of a 45° tilt angle and 45° yaw angle, the most effective approach is the use of suction through nozzles aligned diametrically opposite to the wind, while other nozzle combinations were ineffective in mitigating losses. Finally, the results highlight the need to apply an adaptive aerodynamic strategy that can respond to measured changes in the environmental conditions to achieve the highest thermal efficiency.

A hybrid adaptive neuro-fuzzy inference system integrated with equilibrium optimizer algorithm for predicting the energetic performance of solar dish collector

Solar energy exploitation has a vital role to fulfill sustainability and decrease the usage of non-renewable energy resources. Solar parabolic dish collector (SPDC) is an effective alternative to fossil fuels due to its high efficiency. Nevertheless, performance prediction, optimization, and working fluid selection of SPDCs are highly complex problems and need complicated calculations and/or costly time-consuming experiments. Artificial intelligence-based algorithms have been proven to be beneficial in modeling different solar systems. Therefore, this study proposes an improved method to predict the thermal performance parameters of SPDC with a cylindrical cavity receiver using a modified algorithm of the adaptive neuro-fuzzy inference system (ANFIS) integrated with equilibrium optimizer (EO). In the developed algorithm, EO is employed as a new metaheuristic approach to enhance the prediction accuracy of ANFIS via determining the optimal values of ANFIS parameters. To evaluate the performance of the developed method, ANFIS-EO is compared with ANFIS and the conventional artificial neural network. The three models were applied to compare and predict the temperature difference of working fluid, heat gain, and energy efficiency of cylindrical receiver for SPDC operating with two different solar working fluids, namely, multi-wall carbon nanotubes/thermal oil nanofluid and pure thermal oil. Moreover, five statistical criteria are utilized to evaluate the performance of the investigated algorithms. The statistical performance results showed that the ANFIS-EO technique had the best prediction accuracy among the three models, and can be regarded as a powerful optimization tool for predicting the energetic performance of SPDC. The predicted results obtained by the ANFIS-EO have an excellent determination coefficient of 0.99999 for all predicted responses.

The Power and Efficiency Analyses of the Cylindrical Cavity Receiver on the Solar Stirling Engine

The technique of solar dish and Stirling engine combination is the most challenging and promising one. For the efficient conversion of the externally concentrated heat to the usable power, we studied the influences of the wall temperature, inclination angle, and open area ratio of the receiver on the Stirling engine power and efficiency. The theoretical analysis of the heat exchange element of the solar Stirling engine was performed, and the simulation model of the cavity absorber was built and analyzed. The temperature cloud and heat loss trends of the receiver under different wall temperatures, inclination angles, and opening ratios were illustrated. When the wall temperature of the absorber changes from 700 to 1000 K, the efficiency of the engine has increased by 8.8% from 21.34% to 30.11%. The higher the temperature, the higher the efficiency. As the inclination angle of the absorber increases from 0° to 60°, the efficiency of the engine is increased by 7.7% from 21.1% to 28.8%. With the increases of the aperture ratio, the engine output and efficiency reduced. The engine efficiency at the aperture ratio of 0.5 is 4% larger than that at the aperture ratio of 1.

Optical analysis and heat transfer modeling of a helically baffled cavity receiver under solar flux non-uniformity and windy conditions

Optical and thermal analyses of a parabolic dish collector (PDC) with spiral baffles embedded in the annular space of a cylindrical cavity receiver was studied in windy conditions under realistic non-uniform solar flux distribution. The asymmetric distribution of solar flux on the cavity walls was obtained by a nonsequential ray tracing and finite element (FEM) coupled simulation technique. The performance of the system was analyzed in windy conditions, for two different wind directions of inward and outward of the receiver aperture. The slope error of the dish surface has a significant effect on the distribution of reflected rays on the receiver walls. For higher slope errors, the dishes that are more concave and for smaller slope errors more flat dishes give higher thermal efficiency and have better performance. For a dish collector with a focal point of 1.5 m, when the slope error of the dish surface is only 10 mrad, about 13% of the total solar flux received by the dish is lost. When the slope error of the dish surface increases, the system thermal efficiency decreases, and this reduction is more severe for larger focal points and more flat dishes. The effect of wind direction on the system performance for inward aperture mode is more than that of outward mode, and by increasing the wind speed from 1 to 10 m/s, the system thermal efficiency is reduced up to 50%.

Experimental investigation on heat-transfer characteristics of a cylindrical cavity receiver with pressurized air in helical pipe

A concentrating solar system is established for the experimental investigation on a cylindrical cavity receiver. With a built-in helical pipe, the receiver is loaded at the focus point of a parabolic dish, driven by a double-axis tracking system. The helical pipe contains pressurized air as the heat transfer fluid (HTF), heated by concentrated solar irradiation. The experimental investigation analyzed operation parameters effects on both heat losses and the thermal efficiencies, which are direct normal irradiance (DNI), the inlet temperature of HTF and the mass flow rate of HTF. This study also proposed an innovative mathematical model for experiment data processing. The results indicated the increase of DNI from 201.7 W/m2 to 504.3 W/m2 led to a decrease of the thermal efficiency, the increase of the inlet temperature from 150 °C to 200 °C emerged a same tendency. The increase of the mass flow rate from 0.05 kg/s to 0.11 kg/s was on the contrary, since it decreased total heat losses. Good agreements were achieved by compared numerical simulation results of previous studies to the experiment results. Subsequently, a method for thermal performance enhancement was summarized through experimental investigation results, which could be a mentionable improvement for cavity receiver research.

CFD Analysis on Heat Loss from Solar Cavity Receiver of Parabolic Dish Receiver System

In a solar cavity receiver the overall efficiency depends on the thermal losses occurring from it. These losses mainly comprises of three types i.e. conduction, convection and radiation. Out of which convection losses constitutes of the majority of these losses. In this paper the CFD software analysis of a cylindrical cavity receiver of length 30 cm and internal diameter 20 cm is carried out to study the temperature variation within the cavity receiver due to heat losses. The analysis is carried out for internal wall temperature of 500K, 1000K, and 1500K. The cavity receiver’s inclination angle varied from 0, 30, 45 , 60 to 90 at ambient temperature 293 K.

A comparative study of different optimised mirrors layouts of Linear Fresnel concentrators on annual energy and exergy efficiencies

This paper is aimed at investigating optimised mirror layout effects of Linear Fresnel Concentrators (LFC) by evaluating the annual energy and exergy efficiencies. Subsequently, a north–south central receiver LFC with cylindrical mirror profile and Trapezoidal Cavity Receiver (TCR) were considered. Two distinct mirrors reference configurations such as the sun and specific references and four different layouts such as the constant width and shift, constant width and variable shift, variable width and constant shift and variable width and variable shift studied separately and for each of them, the widths and shifts of mirrors optimised through an analytic method. A TCR was designed for every layout, and its heat loss was estimated from numerical modelling to derive the annual energy and exergy values. It was concluded that considered mirrors reference configurations and mirrors layouts play an important role in energy and exergy efficiencies. Furthermore, variable width layouts have a higher impact on energy and exergy amounts than variable shift layouts due to their effects on the receiver dimension and its heat loss. Finally, besides the highest efficiency of variable width and shift layout in specific reference, the constant and variable width layouts demonstrate superior performance in sun and specific references, respectively.

Effect of use of MWCNT/oil nanofluid on the performance of solar organic Rankine cycle

In the current study, a solar-driven organic Rankine cycle (ORC) system was thermodynamically, economically and environmentally investigated. A focal point concentrator with two different cavity-shape receivers was investigated as the ORC heat source. More specifically, the cylindrical and the hemispherical cavity receivers were examined and compared as the most usual and promising choices. MWCNT/oil nanofluid and R113 were used as the solar heat transfer fluid and ORC working fluid respectively. The main aim of this research is an investigation of different cavities in the solar dish and the investigation of the impact of the use of nanofluids in the solar system by different points of view. The results of this work showed that the hemispherical cavity receiver with nanofluid is the most efficient choice with 21.4% system efficiency, while the use of pure thermal oil in the hemispherical cavity leads to 18.9%. On the other hand, the use of the cylindrical cavity leads to 17.8% and 15.8% system efficiency with nanofluid and pure thermal oil respectively. The levelized cost of electricity (LCOE) was 0.077 €/kWh and 0.076 €/kWh for the cylindrical and hemispherical cavity receiver respectively. Moreover, it was concluded that the solar ORC system with the hemispherical cavity receiver as the ORC heat source had resulted in more positive environmental influence related to the cylindrical one.

Improvement in the Flux Uniformity of the Solar Dish Concentrator System through a Concave Quartz Window

A nonuniform and high-strength heat flux load would reduce the working efficiency, safety, and in-service life of a cavity receiver. Four types of concave quartz windows, including conical, spherical, sinusoidal, and hyperbolic tangent, were proposed to be used in the cylindrical cavity receiver of a solar dish concentrator system, which can improve the flux uniformity and reduce the peak concentration ratio of the receiver. For each concave quartz window, 36 structural schemes were offered. Based on the Monte Carlo ray-tracing method, the results showed that the nonuniformity coefficient of the receiver was 0.68 and the peak concentration ratio was 1320.21 by using a plane quartz window. At the same time, when the receiver is in the best optical performance, it is the receiver with sinusoidal, conical, spherical, and hyperbolic tangent quartz windows, respectively. The optical efficiency of the receiver with the above four types of quartz windows was basically the same as that of the receiver with the plane quartz window, but their nonuniformity coefficients were reduced to 0.31, 0.35, 0.36, and 0.39, respectively, and the peak concentration ratio was reduced to 806.82, 841.31, 853.23, and 875.89, respectively. Obviously, the concave quartz window was better than the plane quartz window in improving the flux uniformity. Finally, a further study on the sinusoidal quartz window scheme of all of the above optimal parameter schemes showed that when the installation position of the receiver relative to the dish concentrator was changed, the flux uniformity of the receiver could continue to improve. When the surface absorptivity of the receiver was reduced, the optical efficiency would be reduced. For the parabolic dish concentrator with different focal distance, the concave quartz window can also improve the uniformity of the flux distribution of the cylindrical cavity receiver.

Experimental insights into the mechanism of heat losses from a cylindrical solar cavity receiver equipped with an air curtain

We report on the effectiveness of an air curtain to mitigate convective heat losses from a heated cylindrical cavity receiver operated at fixed tilt (15°) and yaw angles (0°). The cavity was heated electrically with a controller to maintain a constant inside temperature of 300 °C, varying wind speed, air curtain velocity and discharge angle. It was found that the greatest convective heat losses occur over the lower internal surfaces of the cavity for all cases, spanning both natural and forced convection regimes, while a discharge angle of 30° relative to the face of the cavity is more effective than a parallel curtain, which was found to increase heat losses. It was also found that, for a discharge angle of 0°, increasing the velocity of the air curtain leads to higher convective heat losses. However, for a curtain discharge angle of 30°, increasing the air curtain velocity can reduce heat losses by up to 60%. The measured distribution of air temperature across the aperture plane and convective heat losses through the surface were used to provide insight into the causes of these observations. These results suggest that, for tilted, tower-mounted cavity receivers, the orientation of an air curtain should be directed with a component towards the wind, rather than parallel to the aperture plane.