Heliostat Field Research Articles

A Model of Integration between a CSP System and a PV Solar Field Sharing a Solid Particles Two-Tanks Thermal Storage

The integration of a CSP tower system with a PV solar field, sharing a thermal energy storage, is modeled and discussed. The tower system uses a new-design solid particle fluidized bed receiver integrated with a thermal storage, where hot particles are directly collected to store daily energy for overnight production of electricity. The PV solar field is aimed to supply the daily energy demand; when there is a surplus of PV energy production, the electric energy is converted to heat and accumulated in the thermal storage too. The integration of the two energy systems is modeled, building efficiency functions for all the sub-components of the integrated plant (heliostat field, receiver, storage, power block, PV field). Yearly simulations are performed for two different locations, Spain and Australia, obtaining that a system with a peak power of 10 MWe CSP + 15 MWe PV can supply—with a limited curtailment—a fraction of more than 60% (respectively, 62% and 68%) of a realistic electric load with a peak demand around 10 MW, to be compared with the 45/47% of the same load obtained adopting a PV-only system with the same overall peak power. In the integrated system, PV directly supplies 40/41% of the load, the remaining 23/28% being produced by the power block (mainly fed by the CSP).

Analysis of tracking characteristics of a heliostat field using a graphical ray tracing procedure

The tracking requirements of all heliostats in a field are not the same, and they vary according to day, time, and latitude. In this work, a graphical ray tracing (GRT) based model is developed and demonstrated with two-axis Azimuth-Elevation (AE) and Spinning-Elevation (SE) tracking techniques. The procedure is simple and obviates the complex rotation-matrices formulation reported in the previous literature. Using the GRT, a new Center-Oriented Spinning-Elevation (COSE) tracking technique is proposed and demonstrated in which, instead of at the target, heliostats are pointed towards the on-field center-point of the tower. Tracking based on this co-ordinate system results in a substantial reduction in rotations of tracking motors and parasitic-energy of heliostats. Motion analysis of eight heliostats placed at different on-field locations and distances from the tower are conducted, and their tracking characteristics are explained in detail with respect to time, latitude, and tracking modes. The effect of on-field position and distance is presented with fast-tracking requirements and reflected mirror images on the receiver plane. The COSE method obtains a better shape of the reflected-image with less aberration on the receiver plane than SE and AE methods. The analysis performed in this work addresses the trade-off between tracking speed and cost while designing a tracking system for heliostat field.

Integrated optical–thermal modelling and techno-economic evaluation of a ‘bladed’ solar-thermal receiver

The receiver in a concentrating solar thermal system is a costly component, and its contribution to system-level techno-economics must be carefully evaluated. This study examined the strategy of adding ‘blades’ to conventional tubular receivers, in the form of additional tube-banks perpendicular to the primary aperture surface, to improve light trapping and reduce size. An integrated optical–thermal model was used, incorporating geometrical configuration, flow path design and aiming strategy, and optical and thermal performance assessed followed by an economic analysis of its dynamic annual performance. A 65MW˙th polar heliostat field and molten nitrate salt working fluid were assumed, and design iterations conducted to maximise overall optical–thermal receiver efficiency. The final design achieved a 30% reduction in optical and thermal losses, equivalent to 2.5% increased efficiency. Even though the final design can sustain higher peak flux, its size could not be reduced without incurring excessive spillage loss. Meanwhile, the added blades did not increase the intercepted radiation. As a result, in the techno-economic evaluation, the approximately doubled tube mass increased the receiver cost by 16%–25%, and only 1–3USD/MWh˙e savings were achieved in the levelised cost of electricity (LCOE). The LCOE of bladed receivers was evaluated for the first time in this study, confirming that bladed receivers in their current form are not promising. However, the study provides insight into the benefits of the related ‘STAR’ concept, where vertical blades are added to cylindrical receivers, which increase the receiver aperture while decreasing the tube cost per aperture area.

Analytic ray tracer on GPU for central receiver systems

An accurate ray tracer is essential to calculate the efficiency of central receiver systems. Furthermore, its runtime has a direct influence on the applicable optimization method for finding optimal layouts of heliostat fields. Developing a ray tracer that achieves both objectives is the main focus of this study. There exist several different ray tracing techniques, starting from the classical Monte Carlo method to analytical convolution methods. Within this work, we introduce a new analytical ray tracer with high accuracy at low runtime. This is achieved by integrating over the bivariate Gaussian distributions used in our convolution ray tracer. In such a way the raytracer is capable of including the effect of multiple solar rays without the need of simulating each ray. Besides the overall efficiency, the new ray tracer can also compute accurate flux maps with thousand times fewer rays than the Monte Carlo method. To further improve the overall model accuracy, a new equation to convolute the sun, slope and tracking error is derived. We show that it perfectly matches the results of a separate accounting for these errors. The newly integrated convolution ray tracer is developed on the same platform as a bidirectional Monte Carlo ray tracer and a convolution method. Besides a large cross-validation of the results, this allows for a reasonable direct run time comparison. With extensive case studies, the quality of the solution, the run time, and various other aspects are investigated. Furthermore, all ray tracers are also implemented on the GPU improving the run time compared to the CPU version by a factor of 50. We show that the integrated convolution ray tracer running on the GPU achieves an accuracy of 99.95% for an annual simulation in 0.7 s for the PS10 and in 1.8 s for the Gemasolar.

Genetic algorithm optimization of heliostat field layout for the design of a central receiver solar thermal power plant

The heliostat field layout in a central receiver solar thermal power plant has significant optical losses that can ultimately affect the overall output power of the plant. In this paper, an optimized heliostat field layout based on annual efficiency and power of 50 MW for the local coordinates of Quetta, Pakistan, is proposed. The performance of two different heliostat field layouts such as radial staggered and Fermat's spiral distribution are evaluated and different design points in a year are considered for the analysis. The field layouts are then optimized using a rejection sampling based Genetic Algorithm (GA). It considers the output power and mean overall efficiency for vernal equinox, summer solstice, autumnal equinox, and winter solstice as objective functions. The GA optimizes the heliostat field parameters, namely, security distance (DS), tower height (TH), heliostat width to length ratio (WR), and the length of heliostats (LH). The study system was developed in MATLAB for validation. It was observed that for the radial staggered layout, the number of heliostats decreased by 364 and the efficiency was improved by 8.52 % using GA optimization relative to unoptimized results field layout. The annual efficiency for Fermat's spiral configuration was improved by 14.62 % and correspondingly, the number of heliostats decreased by 434.

Precise and fast spillage estimation for a central receiver tower based solar plant

An accurate and computationally cheap method to estimate the optical efficiency of a heliostat field associated with a central receiver tower (CRT) system is essential for an optimized design. In this article, an improved method is developed to estimate the intercept efficiency of a field with flat heliostats. Based on the slant distance, a judicious combination of two different schemes, i.e. Elliptical Gaussian Distribution (EGD) and Pseudo Ray Tracing (PRT), is employed to estimate flux in an image plane. The flux from the image plane is translated to the receiver to estimate spillage. The proposed method is rigorously validated against the state of the art tool, SolTRACE, under various conditions which demonstrates robustness and accuracy of the method. The deviation of intercept efficiency from SolTRACE is small (within 1.5 %). However, such a high degree of accuracy is achieved at a meagre computational cost (below 15 %) compared to SolTRACE even without parallel computing. Thus, the intercept efficiency or spillage estimation of a given heliostat field is rendered ten times faster by the method presented in the current article without a loss of accuracy. An earlier study by the same authors showed the accurate and fast estimation of shading and blocking efficiency. The present study adds to the previous effort and proceeds towards the objective of achieving a fast and precise tool for the design and analysis of heliostat fields. The proposed tool is developed in Python environment which exploits the parallel computing features based on NumPy, Pandas and multiprocessing.

The effect of HelioPod™ mobility on concentrated solar thermal project economics

The production of heat for industrial processes is one of the significant contributors to greenhouse gas emissions, with the mining and minerals processing industry in South Africa as a substantial user of high-temperature process heat. South Africa produces 80 % of the world's manganese. The sintering of manganese in South Africa is fossil fuel intensive and emits nearly a million and a half tons of CO2 per year. Concentrating Solar Thermal (CST) produces heat that can be utilised at a significantly lower cost than diesel firing, which is currently used for sintering. However, with CST being a nascent technology, most mining operations are reluctant to commit to its use. These risks can be minimised for the customer using an Energy Supply Company (ESCo or energy-as-a-service company) to provide heat to the mine.For the ESCo, one of the most significant risks is the potential for the mining customer to terminate the contract before the end of the life of the CST energy supply asset. Using a solar field based on heliostats, such as the HelioPod™, that can be redeployed presents a level of insurance against this risk. In this analysis, the economic advantage of the HelioPod™ solar field-based CST system is compared to that of a conventional permanent pedestal-based heliostat field. The comparison was made assuming early termination of the contract and scenarios where the CST facility was redeployed once or twice during the nominal life of the facility. The discounted cash flow analysis shows that reusing the heliostat field leads to significant economic improvements. The study shows that the conventional permanent heliostat system reduces the project NPV@7 % by up to 28 % compared to the HelioPod™ system with one move in the project's life.

Design and evaluation of a novel heliostat-based combined cooling, heating, and power (CCHP) system: 3E analysis and multi-criteria optimization by response surface methodology (RSM)

Concerning the global challenges arising from fossil fuel-dependent power cycles, including environmental concerns and fuel transportation issues, solar-based technologies have appeared as a remarkable alternative. Hence, the present study introduces a novel approach for combined cooling, heating, and power generation through a fossil fuel-independent Brayton cycle combined with an advanced multi-heat recovery process. Furthermore, an intelligent process is performed to optimize the newly developed system. This system employs a Brayton cycle, modified by a recuperator and intercoolers, integrated with a heliostat field. Furthermore, a thermal energy storage unit provides the input energy continuously. The heat recovery process encompasses a dual-effect absorption chiller, a Kalina cycle, a heating generation subsystem, and a liquefied natural gas cold energy utilization unit. The proposed system is examined from energy, exergy, and economic points of view and is optimized through an intelligent process based on response surface methodology. Therefore, ten distinct decision variables are identified, while the objective functions encompass the exergetic efficiency and the sum unit cost of products. The accuracy of the regression models is examined through the analysis of variance. Therefore, the optimal state demonstrates a sum unit cost of products and an exergetic efficiency of 24.75 $/GJ and 21.72 %, respectively.

Towards an environmentally friendly power and hydrogen co-generation system: Integration of solar-based sorption enhanced gasification with in-situ CO2 capture and liquefaction process

The sorption-enhanced gasification systems, which integrate the gasification process with an in-situ CO2 capture system, have emerged as environmentally friendly solutions. This study proposes an innovative solar-based SEG system aimed at co-generating power and hydrogen while ensuring environmental sustainability. The suggested system comprises municipal solid waste gasification, in-situ calcium looping CO2 capture process, steam and humid air gas turbine secondary power cycles, and a CO2 liquefaction system. Comprehensive analysis including energy, exergy, and exergoeconomic evaluations are conducted to assess the overall system performance. The annual electrical energy efficiency of the system is calculated to be 11.9%, resulting in a net electrical power generation of 19.48 MW. The annual total energy efficiency is determined to be 54.8%. To convert the captured CO2 into a liquid form, a dual-pressure Linde-Hampson cycle with a coefficient of performance of 1.9 is employed. Among the system components, the carbonator reactor exhibits the highest exergy efficiency at 88.7%, while the sorption-enhanced gasifier, calciner, and combustion chamber show relatively higher exergy destruction. The heliostat field is identified as the most expensive component in the SEG system. The levelized cost of electricity (LCOE) for the produced electricity is calculated to be 60.1$/MWh.

Validation of a closed-loop aim point management system at the Jülich solar tower

Due to modeling errors and disturbances caused by clouds, Solar Power Tower (SPT) plants consider high safety margins during operation leading to a significant loss of revenues. This conservatism contradicts the expansion of SPT plants despite their potential. In this work, an aim point management system is deployed at the open-volumetric receiver of the Jülich solar tower. It employs All Sky Imager (ASI)-based nowcasts as well as flux density and temperature measurements. While the temperature measurements provide feedback for an outer receiver control loop, the flux density measurements offer feedback for an inner heliostat field control loop of a cascade control. The receiver controller estimates flux density setpoints for the inner control loop. By comparing these flux density setpoints with measured flux densities, a static optimal control technique compensates for modeling errors and increases the intercept. This control requires a fast aim point optimization, the enhanced ant-colony optimization meta-heuristic. Furthermore, a feed forward controller compensates for disturbances due to clouds by scaling the heliostats’ reflected power used in the system model by DNI nowcasts. At the Jülich solar tower, the static optimal control compensates for changes in the allowable flux density in less than three control steps. The cascade control controls the subreceivers in a narrow temperature range of 15K, which has never been achieved before at this tower.

Optimization Model of Heliostat Field Layout based on Genetic Algorithm

In this paper, starting from the relevant data of the heliostat field, we firstly built an accurate computational model and established a 3D coordinate system with the tower as the origin, and visualized the data through Matplotlib to present the results clearly. Finally, we constructed a reliable mathematical geometric model and explored how to optimize the heliostat field by a genetic algorithm: maximizing the output power per unit area with the annual power already determined.

Performance comparison of three supercritical CO2 solar thermal power systems with compressed fluid and molten salt energy storage

In recent years, the supercritical carbon dioxide (sCO2) Brayton cycle power generation system has gradually attracted the attention of academics as a solar thermal power generation technology. To achieve the stable and effective use of solar energy, three sCO2 solar power generation systems were studied in this paper. These systems included a molten salt thermal storage system, a compressed CO2 energy storage system, and a combined molten salt thermal storage and compressed CO2 energy storage system. The thermodynamic analysis model of the sCO2 power generation system is constructed, and a mathematical model of the solar power tower system is produced. The operating conditions, thermal efficiency, exergy efficiency, and economics of these three systems on typical days are investigated and compared. The results indicate that the power generation system coupled with compressed CO2 storage has higher thermal and exergy efficiencies than the other two systems with the same daily heliostat field. The thermal efficiencies can be as high as 6.56% and 5.91% respectively, and the exergy efficiencies as high as 5.76% and 6.22% respectively. The system coupled with compressed CO2 energy storage is more cost-effective and has a shorter payback period than the system coupled with molten salt thermal storage system and the system with both forms of energy storage.

Techno-economic assessment of a numbering-up approach for a 100 MWe third generation sodium-salt CSP system

This work presents the design and techno-economic analysis of a 100 MWe concentrated solar power (CSP) system using a supercritical CO2 power block with 700 °C input temperature. Aiming to leverage the relatively higher efficiency of small heliostat fields and potential multi-build discounts, a numbering-up approach is examined, developing four alternative configurations (1×100, 2×50, 3×33, and 4×25MW˙e), in which each module has its own dedicated tower, heliostat field, receiver, thermal storage and power block. A comprehensive techno-economic model is combined with detailed annual simulations to yield levelised cost of energy (LCOE) estimations and analyse the potential of system numbering-up for high-temperature next-generation CSP systems based on liquid heat transfer fluids (HTFs). The simulations are verified against the System Advisor Model with differences in the LCOE calculations within ±1.0%. Comparing the four systems shows that a 1×100MW˙e system leads to an LCOE of 54.88USD/MWh˙e, lower than for the numbered-up modules. However, the LCOE difference between configurations with one and two modules is moderate, with the 2×50MW˙e configuration showing an LCOE of 55.99USD/MWh˙e (+2%). Despite their higher annual conversion efficiencies, the 3×33MW˙e and 4×25MW˙e systems are more capital-intensive and escalate LCOE by 6.9 and 12.2%, respectively. With size-dependent power block efficiency, further LCOE escalation with numbered-up systems is observed, however, multi-build savings could potentially reverse this cost escalation and a 13.9–19.6% saving on the two-module system would allow them to break even.

Energy and exergetic performance analysis of a hybrid solar multi-stage Brayton cycle with different working fluids

An energy and exergy model for a hybrid multi-stage Brayton cycle solar thermal plant is presented, incorporating an arbitrary number of compression stages with intermediate cooling and expansion with reheating. In hybrid operation, the cycle receives thermal energy from a solar concentration system of a heliostat field and a central tower complemented by reheaters and an external main combustion chamber of natural gas. The proposed model considers the irreversibility of the plant's components, and direct solar radiation is estimated with the Daily Integration Approach model. The model is validated and implemented with the Solugas experimental plant parameters and is applied in Barranquilla, Colombia. Additionally, this work presents a comparative analysis of different plant configurations using air, carbon dioxide and helium as working fluids. Comparing the power, the energetic and exergetic efficiencies, and the destruction of exergy on an average day of the year, the maximum points of these variables are also found as a function of the pressure ratio. Observing that the two-compression-one-expansion CO2 cycle presents maximum fuel conversion rates and the slightest destruction of total exergy.

Proposal of a new design of central solar receiver for pressurised gases and supercritical fluids

This work presents a novel design of microchannel central receiver for pressurised gases and supercritical fluids in solar tower plants. It consists of a radial arrangement of vertical absorber panels that converge on the central axis of the tower. The absorber panels comprise compact structures, whose compactness is increased in one flow pass compared to the previous one, as the fluid is heated. This concept reduces radiation heat losses due to its light-trapping geometry and increases heat transfer to the thermal fluid without over penalising its pressure drop.For the receiver assessment, it has been developed a thermal resistance model characterising the fluid heating along the panel height and the temperature gradient between parallel channel rows of the compact structure across the panel thickness. Once the thermal and optical boundary conditions are defined, an optimisation analysis of the main geometrical parameters of the receiver has been accomplished. The receiver performance is evaluated by means of a global exergy efficiency referred to the solar subsystem, which computes the receiver heat losses, the fluid pressure drop and the optical efficiency of the heliostat field in which the receiver is integrated. For each parametric optimisation, the configuration that maximises this efficiency is identified.

Flux augmentation and temperature enhancement through secondary parabolic concentrator and air recirculation for a central receiver tower system

Flat volumetric receivers for central receiver tower (CRT) systems face two critical shortcomings: light spillage nearby the receiver and low exit air temperature due to suction of air in ambient condition. Here, a thorough design modification is proposed and analyzed to address both the issues by introducing a secondary optical addendum based on the principle of compound parabolic concentrator (CPC). As a case study, existing and modified designs of PS10 plant with flat heliostats are compared. For the same receiver, the modified plant increases the intercept efficiency by 6% whereas the incorporation of CPC augments the concentrated flux by 18%. A glass covered secondary optical element not only reduces operational difficulties due to absorber-soiling, but also provides an opportunity of heated air recirculation. If the heated air is partially recirculated, it can provide a control parameter, i.e. the air return ratio (ARR), by which the exit air temperature can be enhanced. It is demonstrated here that temperature as high as 1775 K is achievable for the modified PS10 plant when significant fraction of heated air is recirculated. Hence, the present study opens up optimization routes for CRT based plants based on limited area heliostat field and volumetric porous absorber through CPC based secondary optical element and fractional recirculation of heated air.

Multi-tower heliostat field optimization by means of adiabatic quantum computer

In this work, the possible use of quantum annealers for the optimization of multi-tower systems is studied. These systems are constituted by multiple solar towers and a set of mirrors (heliostats) reflecting the incident sun radiation toward a receiver on top of a tower of choice. The optimization consists in choosing on which tower each of the heliostats should focus the reflected radiation in order to optimize the global collected energy and, therefore, is inherently discrete. In this work, we formulated it in Quadratic Unconstrained Binary Optimization (QUBO) form and solved it with the use of quantum annealers. Results are shown for systems ranging from 2 to 9 towers and from tenths to thousands of heliostats. It is demonstrated that quantum annealers, used in hybrid mode, are effective in finding good solutions. The results are compared with the ones obtained with classical annealers showing comparable performances.

Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels

This study analyzes the technical performance, costs and life-cycle greenhouse gas (GHG) emissions of the production of various fuels using air-captured water and CO2, and concentrated solar energy as the source of high-temperature process heat. The solar thermochemical fuel production pathway utilizes a ceria-based redox cycle for splitting water and CO2 to syngas – a tailored mixture of H2 and CO – which in turn is further converted to liquid hydrocarbon fuels. The cycle is driven by concentrated solar heat and supplemented by a high-temperature thermal energy storage for round-the-clock continuous operation. The study examines three locations with high direct normal irradiation using a baseline heliostat field reflective area of 1 km2 for the production of six fuels, i.e. jet fuel and diesel via Fischer-Tropsch, methanol, gasoline via methanol, dimethyl ether, and hydrogen. Two scenarios are considered: near-term future by the year 2030 and long-term future beyond 2030.In the near-term future in Sierra Gorda (Chile), the minimum fuel selling price is estimated at around 76 €/GJ (2.5 €/L) for jet fuel and diesel, 65 €/GJ for methanol, gasoline (via methanol) and dimethyl ether (DME), and 42 €/GJ for hydrogen (excluding liquefaction). In the long-term future, with advancements in solar receiver, redox reactor, high-temperature heat recovery and direct air capture technologies, the industrial-scale plant could achieve a solar-to-fuel efficiency up to 13–19 %, depending on the target fuel, resulting in a minimum fuel selling price of 16–38 €/GJ (0.6–1.3 €/L) for jet fuel and diesel, and 14–32 €/GJ for methanol, gasoline, and DME, making these fuels synthesized from sunlight and air cost-competitive vis-à-vis e-fuels. To produce the same fuels in Tabernas (Spain) and Ouarzazate (Morocco) as in Sierra Gorda, the production cost would increase by 22–33 %. Greenhouse gas savings can be over 80 % already in the near-term future.

Techno-economic-environmental study of an innovative solar-boosted system to produce low-emission hydrogen and methanol: ANN-based optimization

In the present study, techno-economic-environmental optimization of a solar-boosted energy system generating power, freshwater, and methanol as a promising clean fuel has been undertaken. A heliostat field with variable mirrors is used in a gas turbine cycle to decrease the necessity of burning much hydrocarbon fuel. By using pressure swing adsorption, 80% of the contained carbon dioxide of the exiting stream of the gas turbine is recovered and stored. In addition, by exploiting the waste heat of the exiting stream of the gas turbine, an organic Rankin cycle, a multi-effect distillation with thermal vapor compression, and a single effect absorption chiller are driven. Furthermore, a proton exchange membrane electrolyzer is utilized to generate hydrogen and portions of stored hydrogen and carbon dioxide go through the methanol synthesis reaction to produce desired methanol. A comprehensive parametric study through the 4E analysis has been carried out to detect the influential design parameters to set the decision variables for the optimization process. In the end, due to the complexity of the system, a deep neural network is developed to lower the computational time. The findings of the deep neural network-based optimization show the optimal solution in which the total cost rate of the system, exergy efficiency, and the emitted carbon dioxide values are 1.26 $/s, 46.25%, and 0.58 kg/s, respectively.

Co-optimisation of the heliostat field and receiver for concentrated solar power plants

In a concentrated solar power (CSP) tower plant, it is essential to understand the performance of the subsystem formed by the heliostat field and the receiver, operated with an optimal aiming strategy that guarantees the safety and lifetime of the receiver while maximising performance. State-of-the-art studies optimise the heliostat field, aiming strategy and the receiver independently. However, the field and the receiver are interdependent and co-optimisation of the field-receiver subsystem is necessary to obtain the optimal configuration. Fast and accurate annual performance assessments of the subsystem are needed to calculate the annual energy output and the Levelised Cost of Energy (LCOE) of the full CSP system. In this study, a co-optimisation method is proposed based on coupled instantaneous optical, thermal and mechanical models integrated into a system-level model for annual simulation of full system and economics. The resulting system-model is then used for design optimisation based on a genetic algorithm. Several techniques are implemented to make this complex and computationally expensive problem tractable. The proposed method is used to optimise the design of two systems composed of a surround field and a liquid sodium-cooled cylindrical external receiver for first the annual performance and then the LCOE. The good behaviour of the method is confirmed by a sensitivity study. The LCOE-based optimisation leads to a less efficient system than the efficiency-based optimisation but a higher capacity factor. The methods presented are applicable to other CSP plant configurations, including state-of-art molten salt power tower plants.