External Cylindrical Receiver Research Articles

Exploring efficiency limits for molten-salt and sodium external cylindrical receivers for third-generation concentrating solar power

Third-generation concentrating solar-thermal power (CSP) systems are proposed which aim to halve the cost of electricity from CSP. This work demonstrates that a transition from state-of-the-art cylindrical molten nitrate salt receivers with a peak concentrated solar flux of 1MW/m2 and a heliostat field optical error of 3mrad to future cylindrical sodium receivers with a peak flux of 1.8MW/m2 and a heliostat field optical error of 1mrad could enable the exergy output of CSP receivers to be increased significantly, by 25% at design point, while keeping the general system design very similar. This comparison is conducted on a large dataset established with a detailed field and receiver energy balance model considering field aiming, peak receiver flux and hydrodynamics. It is based on design point simulations, and is independent from cost considerations. At high temperatures, the optimal configurations achieve their gains only with very accurate heliostats and when the peak flux is increased, while at low temperatures, there is nothing to be gained from very high peak flux limits on external tubular receivers. This underscores the critical importance of high heliostat accuracy for high-temperature CSP systems design. For the third-generation, high-temperature (740 °C) sodium receiver configuration considered, it is found that the upper limit to thermal–optical efficiency of optimised receivers at design-point is around 88 %.

Evaluation of external tubular configurations for a high-temperature chloride molten salt solar receiver operating above 700 °C

Next-generation concentrating solar power (CSP) tower technologies target operating temperatures exceeding 700 °C to increase the thermal-to-electric conversion efficiency. Molten chloride salts are one possible alternative to current commercial molten nitrate salts to enable the higher operating temperature. This paper analyzes the predicted optical, thermal-fluids, and structural performance of traditional external tubular solar receiver configurations applied with a chloride salt heat transfer fluid (HTF) and inlet/outlet temperatures of 500 °C/735 °C, and considers sensitivity analysis and optimization relative to receiver sizing, tube sizing, number of panels, flow circuit configurations, and solar flux concentration under constraints on internal velocity, pressure drop, wall thickness, and required creep-fatigue lifetime. The high temperature conditions increase the significance of inelastic deformation mechanisms such as creep relative to that expected in commercial 565 °C nitrate salt designs. High-temperature creep and creep-fatigue damage in the metal alloy tubes are the key factors that limit allowable solar flux concentration and achievable receiver thermal efficiency at the near-800 °C wall temperature conditions. For a traditional external cylindrical receiver configuration, the design parameters and conditions capable of satisfying all constraints produced, at best, a design point receiver efficiency of 78.2%, or 80.5% when excluding receiver intercept efficiency. Variation in the optimal receiver performance relative to uncertainty in the binding maximum velocity, minimum wall thickness, and minimum lifetime constraints is presented.

Coupled optical and thermal simulation of the thermal performance of a 50 MWe external cylindrical solar receiver

The performance of the central receiver is directly related to the efficiency and power generation capacity of the solar tower power plant. In this paper, by combining the analytical approximation method and the finite volume method, the coupled optical and thermal performance of a 50 MWe external cylindrical receiver was numerically evaluated. The thermal performance of the receiver under design parameters was first simulated, and the model was verified by the comparison with the method in Zhou et al. [Renewable Energy 164, 331–345 (2021)]. The effects of key parameter and uniformity of incident flux distribution on the thermal performance were then extensively investigated. Finally, the thermal performance changes in one day were explored. The results showed that direct normal radiation (DNI), salt inlet mass flow, salt inlet temperature, and coating absorption rate strongly affected the receiver's thermal performance. Compared with the non-uniform incident flux distribution, the accurate salt outlet temperature could also be obtained under the uniform incident flux distribution, but the thermal efficiency was underestimated by 0.5%, and the wall temperature on the front side of the tube was generally underestimated, reaching 5.6% at the location of the maximum wall temperature. Evolution trend of the salt outlet temperature and maximum wall temperature on vernal equinox was similar to that of the DNI distribution. From 14:00 to 18:00, they decreased by 18.3% and 12.3%, respectively. The maximum wall temperature from 13:00 to 16:00 exceeded the design value of 700 °C, which was detrimental to the safe operation of the receiver and might cause decomposition of molten salt in the tube.

Influence of longitudinal clips in thermal stresses and deflection in solar tubular receivers

Mechanical boundary conditions in tubular receivers of solar power tower plants have a main role in the thermal stress distribution and tube deflection. Longitudinal supports, particularly, has an strong influence on stress and displacements, since they prevent the tube bending.In this work, the influence of longitudinal supports, on tube deflection and stress has been studied in external-cylindrical receivers, using an analytical methodology, which it is able to take into account the tube geometry in the deflection calculation. Therefore, real tube geometry with elbows can be considered. Results for two aiming strategies, one equatorial and another that flattens the heat flux, have been compared for different clips distances, from 1 to 9 meters.The analytical methodology developed in Matlab provides lower computational cost than the numerical model developed in Abaqus. Results show that clip distribution has a significant impact on thermal stress. For clips distance of 2 meters or lower, the generalised plane strain solution provides the stress distribution along the tube accurately, with a tube deflection lower than 1 millimetrer. When clips distance increases, the longitudinal stress distribution differs from the plane strain case, and the deflection increases to non-desirable values. Deflection is greater at tube ends, and aiming strategies that flatten the heat flux increases the displacement in that regions.

Heat losses and thermal stresses of an external cylindrical water/steam solar tower receiver

A receiver serves as a pivotal component in the solar power system as it is responsible for the light-heat conversion. Extensive research has been carried out on cavity receivers while external receivers have been neglected hitherto. Considering this imperative research gap, this work endeavours to narrow the gap by numerically analyzing the thermal performance of an external cylindrical receiver. A methodology is proposed to determine the efficiency of a cylindrical shaped receiver. A heliostat field is simulated using Monte-Carlo Ray Tracing tool to obtain heat flux distribution on the receiver. The peak heat flux obtained, i.e., 425 kW/m2 lies at the centre of the receiver’s front. By designing a tube layout and using boiling heat transfer correlations, temperature at the receiver’s surface and water are obtained. Numerical analysis and simulations are then carried out to evaluate receiver’s thermal efficiency in six different wind directions and four different wind velocities between 3 m/s and 12 m/s. Natural convection and radiation losses were also considered. Combined heat transfer coefficients obtained through numerical simulations are compared with the previous experimental data. The effect of wind in a single direction is precisely evaluated by dividing the cylinder into panels and evaluating heat losses on each panel individually. The thermal efficiency evaluated oscillates between 71% and 77% based on wind velocity, and the results are validated against the real power plants and experimental data for cylindrical solar receivers. A tube at receiver’s centre having the highest temperature gradient is then selected to evaluate thermal stresses. The equivalent stress obtained is less than the yield strength with safety factor > 2.5.

Correlation between central receiver size and solar field using flat heliostats

In Central Receiver Systems (CRSs), thousands of heliostats track the sunrays and reflect beam radiation on to a receiver surface. The size of the reflected image and the extent of reflection from the heliostats are one of the important criteria that need to be taken into account while designing a receiver, since spillage losses may vary from 2 to 16% of the total losses. The present study aims to determine the size of an external cylindrical receiver, such that the rays reflected from all the heliostats in the field are intercepted. A dimensionless correlation with respect to tower height and receiver size (diameter and height) as a function of heliostat size and its position is discussed in the paper. This correlation could be used as a first-order approximation to estimate the receiver dimensions. When applied to the Ivanpah Solar Electricity Generating Station (ISEGS) plant, the correlation yields satisfactory estimation of receiver dimensions.

Preliminary design of heliostat field and performance analysis of solar tower plants with thermal storage and hybridisation

Solar tower technology has gained considerable momentum over the past decade. In a solar tower plant, a single receiver is used and the power collected by the heliostat field is strongly coupled to the tower height and its location with respect to the field. The literature available focuses largely on the component-level details of the heliostat field, ray-tracing mechanisms, receiver heat transfer analyses, etc. However, there is no systematic method to determine the preliminary optimum tower height and solar field together with an estimation of a solar tower plant’s technical performance. The present paper provides a methodology to estimate the design performance of a tower plant (with storage and hybridisation) using an external cylindrical receiver. The methodology is validated with the Gemasolar plant in Spain and the Crescent Dunes plant in the United States of America. The optimum efficiency is 16% for the Gemasolar plant (20MW and 15h of storage) and 18.3% for the Crescent Dunes plant (110MW and 10h of storage). Further, the methodology is applied to hypothetical plants in Jodhpur as a case study and the results are analysed. The trends seen are similar to that observed for parabolic trough plants.

Aiming strategy model based on allowable flux densities for molten salt central receivers

Abstract This study presents an aiming model to properly point heliostats at cylindrical molten salt receivers in Solar Power Tower. By means of two iterative algorithms (search and fit), the proposed strategy attempts to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints are translated into allowable flux densities ( AFD ) that are handled by the model. The computer code accommodates the flux images produced by each heliostat in a field to accurately fit the AFD limit. In this paper, a Gemasolar-like field–receiver system serves to illustrate the aiming model. Compared to the equatorial aiming, receiver interception is slightly lower using the proposed strategy, but the receiver integrity is ensured; peak flux is significantly reduced up to 23%. It has been found that a favorable flux density profile generally has its peak displaced to the salt entrance at each receiver panel. Since external cylindrical receivers consist of a combination of up-flow and down-flow panels, the optimal flux profile is challenging for contiguous panels with contrary demands. In spite of that, remarkable matching is achieved by the fit algorithm. Because of its fast computation and automatic operation, the resulting tool can be applied to real-time control of existing heliostat fields and the integrated design of the coupled systems field and receiver.

Numerical investigation on uniformity of heat flux for semi-gray surfaces inside a solar cavity receiver

Solar cavity receiver is quite often utilized in the solar tower power system. It is a photo-thermal conversion component, which absorbs solar radiation energy and heats the working fluid. The safety and thermal efficiency of solar cavity receiver can directly affect the efficiency of the entire power system. In this paper, a modified combined method is proposed to simulate the thermal performance of a saturated water/steam solar cavity receiver. Compared with the combined method presented previously, this modified one can describe the whole steady process of the receiver and accurately estimate the steady-state efficiency and the mass flow rate of the generated steam in the boiling tubes. Under the given boundary condition of incident solar flux, the modified method is adopted to find an optimized distribution of absorptivity in the solar wavelength band for the absorber surfaces in order to improve the uniformity of surface heat flux distribution. The simulation results show that the uniformity of surface heat flux is improved effectively and the maximum mean square deviation of the external tube temperature drops from 32.4°C to 23.6°C after optimizing. Based on the relatively uniform heat flux distribution, the effect of thermal emissivity of the absorber surfaces on the thermal performance of the cavity receiver is further investigated. Being different from those of parabolic trough and external cylindrical receivers, both the radiative and the convective heat loss of the present cavity receiver decrease with increasing thermal emissivity, which improves the receiver’s efficiency. These results can provide a useful reference for the design and selection of surface coatings for the cavity receivers.

Comparison of Linear and Point Focus Collectors in Solar Power Plants

Solar tower based plants are seen as a promising technology to reduce the cost of electricity from solar radiation. This paper assesses the design and overall yearly performances of two different solar tower concepts featuring two commercial plants running in Spain. The first plant investigated is based on Direct Steam Generation and a cavity receiver (PS-10 type). The second plant considers an external cylindrical receiver with molten salts as heat transfer fluid and storage system (Gemasolar type). About the optical assessment performed with DELSOL3, a calibration of heliostat aim points was performed to match available flux maps on the receiver. Moving to results, the PS-10 type has higher optical performances both nominal design and yearly average. This is due both to the field size and orientation which guarantee a higher efficiency and to the receiver concept itself. About power production, the molten salts allow higher temperature and consequently conversion efficiency than PS-10. The solar-to-electricity efficiency is equal to 18.7% vs. 16.4% of DSG cavity plant. The obtained results are strictly related to the set of assumptions made on each plant component: when available real plant data where used. The two solar tower plants results were also compared to corresponding commercial linear focus plants featuring the same power block concept. Gemasolar type shows a higher solar-to-electricity efficiency compared to a parabolic trough plant with storage (18.7% vs. 15.4%) because of the higher maximum temperatures and, consequently, power block efficiency. PS-10 is better than a linear Fresnel DSG (16.4% vs. 10.4%) because of the higher optical performances.

Evaluation of Annual Efficiencies of High Temperature Central Receiver Concentrated Solar Power Plants with Thermal Energy Storage

The current study has examined four cases of a central receiver concentrated solar power plant with thermal energy storage using the DELSOL3 and SOLERGY computer codes. The current state-of-the-art base case was compared with a theoretical high temperature case, which was based on the scaling of some input parameters and the estimation of other parameters based on performance targets from the Department of Energy SunShot Initiative. This comparison was done for both current and high temperature cases in two configurations: a surround field with an external cylindrical receiver and a north field with a single cavity receiver. The optical designs for all four cases were done using the DELSOL3 computer code; the results were then passed to the SOLERGY computer code, which uses historical typical meteorological year (TMY) data to estimate the plant performance over the course of one year of operation. Each of the four cases was sized to produce 100 MWe of gross electric power, have sensible liquid thermal storage capacity to generate electric power at full rated production level for 6hours, and have a solar multiple of 1.8.There is a fairly dramatic difference between the design point and annual average performance. The largest differences are in the solar field and receiver subsystems, and also in energy losses due to the thermal energy storage being full to capacity. Another notable finding in the current study is the relatively small difference in annual average efficiencies between the Base and High Temperature cases. For both the Surround Field and North Field cases, the increase in annual solar to electric efficiency is <2%, despite an increase in thermal to electric conversion efficiency of over 8%. The reasons for this include the increased thermal losses due to higher temperature operation and operational losses due to start-up and shut-down of plant sub-systems. Thermal energy storage can mitigate some of these losses by utilizing larger thermal energy storage to ensure that the electric power production system does not need to stop and re-start as often, but solar energy is inherently transient. Economic and cost considerations were not considered here, but will have a significant impact on solar thermal electric power production strategy and sizing.

Simulation of an integrated steam generator for solar tower

Abstract Optical and thermal simulation of a new solar tower steam generator is presented. The steam generator, placed on top of a solar tower, has two integrated receivers, external for boiling the steam and a cavity for its superheating. These two parts of the solar steam generator are facing different sections in a surrounding heliostat field and therefore can be operated independently. The evaporation part is shaped as an external cylindrical receiver which creates a cavity with an aperture for the superheating part. This novel arrangement allows high optical and thermal efficiencies. This concept of the integrated solar steam receiver is illustrated through a modeling of a large-scale plant producing superheated steam at 550 °C and pressure of 150 bars. The decoupling of the steam superheating part from the evaporation makes the system more economical, easy and safe to operate.

RATED MW FROM A HELIOSTAT FIELD ON CYLINDRICAL EXTERNAL RECEIVER

Some of the reflected beam radiation from a heliostat field bypasses the receiver surface. The spillage factor which is a measure of how much of reflected beam radiation actually intercepted by the receiver surface, is calculated and plotted for easy access. The variation of ihe spillage factor with tower height, external cylindrical receiver size, dimensionless radial distance from the tower is computed and plotted. The value of the rated MW energy absorbed by an external cylindrical receiver, is investigated, and its relations to the tower height, the site location and the field radius are given. The effect of changing the radial spacing on the rated MW and the total number of heliostats in the field is also computed and depicted. The developed set of charts for the spillage factor are believed to be very useful for solar central receiver system design.