Parabolic Trough Collector System Research Articles

Hybrid solar hydrothermal carbonization by integrating photovoltaic and parabolic trough technologies: Energy and exergy analyses, innovative designs, and mathematical Modelling

Hydrothermal carbonization (HTC) is an excellent process for transforming biomass into solid fuels. Evaluating HTC's environmental benefits requires addressing substantial energy needs for reactant heating. In this regard, two innovative solar systems for powering HTC were investigated: a parabolic trough collector (PTC) that heats a heat transfer fluid (HTF) circulating through a heat exchanger surrounding a batch reactor and photovoltaic panels (PV) that powers a heating collar. Experimental results reveal that the PV system efficiently attains subcritical conditions, whereas the PTC system faces challenges. Exergetic analysis identifies heat transfer as a primary irreversibility factor, resulting in a recorded maximum entropy of nearly 100 W/K. This caused the exergy efficiency to decrease to a minimum of 0.07, dropping the accumulated exergy below 82 W. To address this issue, a validated mathematical model was employed to investigate three optimized configurations: an immersed heat exchanger, a jacketed system, and a hybrid system combining both. The results demonstrated achieving desired HTC conditions at mixing temperatures of 185 °C, 195 °C, and 230 °C, respectively. In contrast, the PV system rapidly attains 220 °C and 40 bar in 70 min. This study proposes innovative solar HTC designs for sustainable hydrochar production using PV and PTC technologies.

Comparative Analysis of Solar Tower and Parabolic Trough Systems for Solar Heat in a Steel Industry

Concentrated Solar Thermal (CST) systems emerge as a promising alternative to replace fossil fuels used in industrial thermal processes due to their high energy density and dispatchability. In this study, a comparative analysis of two CST systems, Solar Tower (ST) and Parabolic Trough Collector (PTC), has been made to replace natural gas used in the steam generation process in Türkiye's largest steel production plant, located in the Mediterranean Region of Türkiye. Both the ST and PTC systems were placed in a field area of approximately 0.4 km2. The results have shown that, on a monthly basis, the PTC system could exceed the plant's thermal energy for two-thirds of the year, while the ST system could meet the plant's energy requirements for one-third of the year. This could reduce CO2 by about 12.7 and 19.1 kilo-tones for ST and PTC at LCOH of about 68.9 and 36.9 EUR/MWh, respectively.

Optimal energy collection with rotational movement constraints in concentrated solar power plants

In concentrated solar power (CSP) plants based on parabolic trough collectors (PTC), the sun is tracked at discrete time intervals, with each interval representing a movement of the collector system. The act of moving heavy mechanical structures can lead to the development of cracks, bending, and/or displacement of components from their optimal optical positions. This, in turn, diminishes the overall energy capture performance of the entire system. In this context, we introduce two combinatorial optimization problems of great interest to PTC plants. The minimum tracking motion (MTM) problem is aimed at detecting the minimum number of movements needed while maintaining production within a given range. The maximal energy collection (MEC) problem seeks to achieve optimal energy production within a predetermined number of movements. Both problems are solved assuming scenarios where the energy collection function contains any number of local maximums/minimums due to optical errors of the elements in the PTC system. The MTM and MEC problems are solved in O(n) time and O(n2mω∗) time respectively, where n is the number of steps in the energy collection function, m the maximum number of movements of the solar structure, and ω∗ the maximal amplitude angle that the structure can cover. The advantages of the solutions are shown in realistic experiments. While these problems can be solved in polynomial time, we establish the NP-hardness of a slightly modified version of the MEC problem. The proposed algorithms are generic and can be adapted to schedule solar tracking in other CSP systems.

Development and Economical Analysis of Innovative Parabolic Trough Collector Integrated Solar Still

Experimental setup of the integrated parabolic trough collector (PTC) with solar still was developed. PTC was designed considering the solar geometry and the physical laws of parabolic shape and the concentrators. Test were conducted at the location with latitude 19.9975ON and longitude 73.7898OE. Theoretical analysis was done using ray tracing and engineering equation solver (EES) software while designing the system. PTC system was developed with dimensions of 1.5 m length, 1 m width and a concentration ratio (CR) of 21.22. Theoretical thermal efficiency was predicted as 48.1% whereas experimental average thermal efficiency is observed as 42.76%. The observed temperature difference between the vapor and the glass cover is about 17 °C and between ambient air and vapor is about 24.4 °C. Maximum water temperature in the conventional solar still was 64.6 °C where as for the PTC coupled solar still was 74.4 °C. PTC coupled solar still is having averagely 37% higher production rate. This has definitely added an advantage because of the higher energy absorption rate compared with the conventional solar still. PTC coupled solar still system has nearly 35% more heat absorption. Total embodied energy of the system is around 896.875 kWh. Total capital cost of the system is Rs. 41300/-. Total annual output of pure water is around 3 L/Day. Estimated energy payback period is around 2.29 years and the total carbon credit earned is Rs. 2165.38 per year.

Hybrid Solar Thermal Energy System for District Heating Application

Solar district heating (SDH) systems can be good alternatives to conventional systems when they are optimized with hybrid configurations and thermal energy storage (TES). In this scope, a hybrid renewable thermal energy system (RTES) model has been built combining flat plate collector (FPC) solar system with parabolic trough collector (PTC) system via a heat exchanger and coupled with TES. To undertake the hybridization of the system, System Advisor Model (SAM) software was modified, which allowed control over configurations and more accurate modelling of heat transfer between the collectors. The model is first compared to an existing hybrid solar district heating systems (DHS) system in Taars, Denmark. The results showed a good correlation with an overestimation of only 6.4% compared to most recent heat output. Then the same system configuration was modeled in different geographic locations to investigate the impact of changes in direct normal irradiance (DNI) to the heat sink thermal output of the hybrid system. The results showed that the annual net thermal power output in California, USA can be three times more than the annual net thermal power output in Taars, Denmark. Finally, multiple hybrid configurations with varying solar field sizes were simulated based on the heat demand of two different university campuses DHS. The results showed that, retrofit applications of this hybrid DHS system coupled with TES could reduce the natural gas consumption of the existing systems between 25% and 41%. The use of hybrid RTES highlighted in this paper can be extended to many more opportunities.

DNI Forecast Tool for the Smart Operation of a Parabolic Trough Collector System With Concrete Thermal Energy Storage

This work presents a basic forecast tool for predicting direct normal irradiance (DNI) in hourly resolution, which the Solar-Institut Jülich (SIJ) is developing within a research project. The DNI forecast data shall be used for a parabolic trough collector (PTC) system with a concrete thermal energy storage (C-TES) located at the company KEAN Soft Drinks Ltd in Limassol, Cyprus. On a daily basis, 24-hour DNI prediction data in hourly resolution shall be automatically produced using free or very low-cost weather forecast data as input. The purpose of the DNI forecast tool is to automatically transfer the DNI forecast data on a daily basis to a main control unit (MCU). The MCU automatically makes a smart decision on the operation mode of the PTC system such as steam production mode and/or C-TES charging mode. The DNI forecast tool was evaluated using historical data of measured DNI from an on-site weather station, which was compared to the DNI forecast data. The DNI forecast tool was tested using data from 56 days between January and March 2022, which included days with a strong variation in DNI due to cloud passages. For the evaluation of the DNI forecast reliability, three categories were created and the forecast data was sorted accordingly. The result was that the DNI forecast tool has a reliability of 71.4 % based on the tested days. The result fulfils SIJ’s aim to achieve a reliability of around 70 %, but SIJ aims to still improve the DNI forecast quality.

Cupric oxide nanofluid influenced parabolic trough solar collector: thermal performance evaluation

ABSTRACT The availability of solar radiation has potential for energy applications, and recently, the parabolic trough collector (PTC) plays a crucial role in heat exchanger applications. The conventional system lacked thermal performance, reducing heat transfer coefficient and thermal efficiency. With this, the main goal of the research is to attempt to enrich the overall thermal capacity of the PTC system by introducing triangular copper fins and CuO nanoparticles (10–20 nm). During the experimental evaluation, the different concentrations (0.1, 0.2, and 0.3%) of CuO blended with water (nanofluid) are permitted to circulate the receiver boundary configured with PTC setup 3.5 m2 at the flow rate of 0.12 kg/s. Effects of CuO nanofluid concentrations on heat gain, heat transfer coefficient, and thermal and exergy efficiency of the system are measured and spot the optimum heat gain of 689.9Wm enriched heat transfer coefficient of 172.3 W/m2K, maximum thermal and exergy efficiency of 70.9% and 37.2% respectively.

Thermal performance evaluation of an indigenously designed small-scale solar parabolic trough collector for manufacturing Indian milk products

ABSTRACT A small-scale solar parabolic trough collector system coupled with a jacketed vessel was designed and evaluated for its performance and quality of the products manufactured in it. The prototype having a 7.026 m2 aperture and 88.38° rim angle was installed in Mehsana, Gujarat, India. The study was conducted in May–June 2022 at different flow rates of sigma therm-k. The useful heat rate ranged from 567.82 W to 861.25 W. The daily average thermal efficiency was obtained in the range of 9.81–14.61% and maximum thermal efficiency achieved during the milk trials was 31.51%. Similarly, maximum absorber temperature achieved during stagnation was 193.4°C and during absorber fluid flow condition was 146.5°C. The maximum convective heat transfer coefficient of 49.5 W/m2K was calculated at Reynold number 1822 and Nusselt number 24.12. It was found that the quality parameters of manufactured milk products were at par with domestic food safety standards. Direct normal irradiance was found to be the most influencing factor followed by the initial milk load. The present system is a potential substitute for various small-scale Indian traditional heat-desiccated milk products, where higher thermal efficiency is not the primary requirement and emphasis is more on the importance of ease of operation.

Design and evaluation of solar parabolic trough collector system integrated with conventional oil boiler

Design and evaluation of solar parabolic trough collector system integrated with conventional oil boiler

Enhancing the efficiency of solar parabolic trough collector systems via cascaded multiple concentration ratios

Solar parabolic trough collector (PTC) technology is clean and economical. However, PTC technology suffers from relatively low optical and thermal efficiencies at high operating temperatures and concentration ratios (CRs). In this work, a novel strategy for cascadingly using solar collectors with multiple concentration ratios in one collector loop is proposed to reduce optical losses and improve the overall performance of the PTC system. Two systems were studied for implementation of the proposed strategy: an ideal system employing multiple optimal CRs in distinct collector loop sections and a multisection system employing multiple practical CRs. An integrated optical model and two-dimensional heat transfer model based on finite volume and radiation heat transfer methods were developed in this study. These models proved to be highly accurate in estimating the thermal properties of solar collectors. At operating temperatures between 293 and 393 °C, the optical efficiency of the multisection system was 6.7% higher than that of a conventional system with a single CR. Under the same conditions, the thermal efficiency was improved by 4.5%. This work is anticipated to provide a reference for solar collector design and a new avenue for enhancing the overall efficiencies of PTC systems.

PERFORMANCE ASSESSMENT OF PARABOLIC TROUGH COLLECTORS UNDER CLIMATIC CONDITIONS OF IZMIR, TURKEY: A CASE STUDY

The geographic location of Turkey offers significant potential for renewable energies, especially solar energy. One promising technology for solar energy generation is the parabolic trough collector. To shed light on the advantages of solar energy and the design of solar power plants, this study aims to estimate the heat gain and thermal efficiency of a parabolic trough collector (PTC) under the climatic conditions of Izmir location, which is one of the cities in Turkey that is most exposed to solar energy. In this regard, a comprehensive mathematical model for the PTC systems is developed. The conventional iterative solving approaches are highly sensitive to the initial condition of the process (i.e., if they are initiated from an improper location of the search space they can be easily trapped into local minima). Thus, in this work, to provide a robust framework, the proposed mathematical structure is converted to a constrained optimization model that can be solved using a general optimization technique. Hereof, a gradient-free based method, so-called fuzzy reinforced differential evolution with virtual mutant method is employed. Firstly, to validate the performance of this method, it is tested on existing data given by Sandia National Laboratory. Subsequently, it is employed to resolve the model for the Izmir climatic conditions, which serves as the case study of this work. The performance and heat gain value of parabolic trough collectors under considered climatic conditions are reported and discussed in illustrative tables and diagrams.

Investigation of the Influence of Inner Geometries of the Receiver Tube of a Parabolic Trough Collector System Used for Water Heating

Solar collectors are devices that help to harvest solar thermal radiation and convert into other forms of energy. Because of its low fabrication cost and simple design, the Parabolic Trough Collector (PTC) is one of the finest alternatives for medium-temperature (i.e., in 80°𝐶𝐶-250°𝐶𝐶 range) needs among all types of concentrated solar collectors. The absorber tube or the receiver tube is one of the main critical components of a PTC. Thermal radiation of the sun falling onto a parabolic reflector is reflected to its focal line where the receiver tube is placed. Therefore, the performance of a PTC system significantly depends on Receiver Geometries (RGs). A locally developed PTC system with a new and accurate tracking system was designed and fabricated in the current study and steam generation inside the receiver was considered in the performance analysis. The experimental analysis was performed using seven different RGs. The experiments were carried out by measuring generated steam every 10 minutes for all the RGs, and simultaneously, solar insolation data was taken by a domestic solar Photovoltaic (PV) system with a capacity of 5 kW. The steam generation data was taken only when the power output of the reference solar PV system was ≥ 4kW. Having tested seven different RGs, the researchers were able to improve the steam generation to a maximum of 10.0% compared to the reference geometry using direct experimental data. This was 8.1% in accordance with the statistical approach via multiple linear regression model. A three-dimensional mathematical model for two-phase flow was formulated for a closed domain. Restricting this model into twodimensional geometry, the steam generation process was simulated as a TPF problem using ANSYS Fluent software. The volume fraction profiles clearly show the phase transition, which leads to evaporation and condensation inside the tube.

Developing optical and thermal models with experimental validation of parabolic trough collector for moroccan industrial heat applications

This work presents the development of advanced optical and thermal models for parabolic trough collector systems (PTCs) through an extensive experimental study utilizing a PTMx-24 collector from Soltigua company, installed at Green Energy Park (GEP) research facility. The optical model leverages Tonatiuh software, employing the Monte Carlo Ray Tracing (MCRT) method. Moreover, a MATLAB script was adjusted to account for Incidence Angle Modifier (IAM) fluctuations in the optical analysis, while incorporating receiver thermal parameters for the thermal analysis. This study assesses the potential of PTCs in the territory of Solar Heat for Industrial Processes (SHIP) over various seasons within a semi-arid climate, offering invaluable insights for sustainable energy utilization. The validation of the models was conducted across four distinct days characterized by diverse meteorological conditions. The optical model demonstrates a remarkable correlation of 97.04 % between the MATLAB script and experimental results. Additionally, results from Tonatiuh software exhibited an important accuracy of 95.92 %. The thermal model showcased outstanding precision, revealing a correlation of 98.5 % compared to experimental results, signifying its adaptability to collectors of varying geometries and parameters. These findings highlight the predictive potential of these models, aiding in the decision-making process to determine the system's performance on specific days with specific meteorological conditions.

Thermal performance analysis of eccentric double-selective-coated parabolic trough receivers with flat upper surface

The Parabolic trough collector (PTC) system combined with molten salt energy storage system has gathered increasing attention owing to its advantages of high operating temperature and stable power output. However, the substantial heat loss experienced by parabolic through receivers (PTR) under high temperatures imposes limitations on the thermal efficiency and power generation potential of the PTC system. In this study, a novel eccentric double-selective-coated parabolic trough receiver is proposed. The upper surface of the receiver is flattened to further reduce radiation heat loss. Various distances from collector focus are explored to determine the most suitable opening angle of secondary selective coating, ensuring the highest thermal efficiency. The results show that, although the eccentricity causes partial optical loss, the reduced upper surface area and the resultant larger secondary coating opening angle contribute to a significant reduction in heat loss and an improved thermal efficiency of the novel receiver. The thermal efficiency of the new absorber can be improved by up to 20.59 % (under direct normal irradiance of 600W/m2 and a receiver surface temperature of 600 °C). Furthermore, the higher the emissivity and the lower the absorptivity of the solar selective coating, the better the performance of the novel absorber tube.

Techno-economic analyses of solar thermal process heat integration at South African beverage producers

South Africa's industrial sector relies primarily on coal and gas to meet process heat requirements (700–750 petajoules per annum). Much of this energy is used to fire steam boilers operated at less than 200 °C. This study determines the viability of solar thermal technologies as a replacement for fossil fuels for the supply of steam in large beverage facilities in South Africa. The Gauteng and Western Cape provinces are considered, which account for two-thirds of South Africa's beverage production. The sizing of collector fields and potential solar fractions are determined using typical ready-to-drink beverage packaging hall energy requirements. The cost of heat from parabolic trough collector systems is modelled using quasi-dynamic simulations in Polysun, and heat storage configurations are optimised. This study demonstrates that solar thermal projects would be viable alternatives to heavy fuel oil, diesel, gas, and even coal - should the cost of coal remain above US$250 /tonne (2020 real-term values). Transition to solar thermal energy for process heat in this sector is thus strongly recommended.

Evaluation of operational strategy of cooling and thermal energy storage tanks in optimal design of multi generation system

In this study, optimal design of solar combined cooling, heating, power, freshwater (FW) and hydrogen (CCHPWH) generation system using parabolic trough collectors (PTC), proton exchange membrane (PEM) electrolyzer and multi-effect evaporation with thermal vapor compression (MEE-TVC) is investigated. The CCHPWH + PTC system according to thermal energy storage (TES) tank with constant partial load (CPL) for chillers (case 1) and CCHPWH + TES + PTC including cooling energy storage (CES) tank system with variable partial load (VPL) strategy for chillers (case 2) in hot climate are compared to each other, and with CCHPWH system with CPL strategy (case 3) and with VPL strategy (case 4), respectively. The optimal results illustrated 0.64% reduction in the TAC of CCHPWH + TES + PTC + CES system with VPL strategy as compared to CCHPWH + TES + PTC system with CPL strategy. Notably, it was observed 4.55% and 4.67% decrement in the TAC of CCHPWH + TES + PTC + CES system with VPL strategy and CCHPWH + TES + PTC system with CPL strategy as compared to CCHPWH system with VPL and CPL strategy, respectively. As well as, energy and exergy efficiency of the CCHPWH + TES + PTC + CES system with VPL strategy in the optimal mood increased 11.85% and 19.05% in comparison with CCHPWH + TES + PTC with CPL strategy. Eventually, proposed cases were optimized and discussed from energy, exergy, economic, environmental.

Efficacy of natural circulation loop based parabolic trough collector- An analytical approach

ABSTRACT Realizing the importance of the natural circulation loop (NCL), which is a passive heat transport device, an analytical study is undertaken to demonstrate the functioning of an NCL-based parabolic trough collector (PTC). Multiple validated analytical models were incorporated to evaluate the system performance. The parametric study involved variations in loop height, loop length, and solar radiation. The loop flow rate enhances by increasing its height or the receiver length. At maximum radiation, 1.3 kg/s flow rate was observed for loop geometry of 5 m height and 7.8 m length against 0.79 kg/s for 1 m height. The flow rate predicted for a 10 m length and 1 m height loop was 0.87 kg/s. Since instability rises with height and heat loss and receiver temperature increase with receiver length, a trade-off between the two is necessary to function NCL-based PTC properly. Otherside, though the higher heat output with an increase in the receiver length is evident, a noticeable drop in thermal efficiency (≈7%) during the peak hours of solar radiation is a matter of concern. Further, the outlet temperature was not affected by the loop height for the defined flow condition of the process fluid. Like in conventional PTC systems, the process fluid’s exit temperature or flow rate can also be maintained constant during a day operation by varying the flow condition. The proposed model can deliver ≈ 1500 liters of hot water in six hours with an average 68% thermal efficiency without external power, in contrast to the necessity of notable pumping power and associated overhead charges in conventional PTC. Further, the system’s stability was predicted using linear stability analysis, and stability maps were generated for different loop heights and found that as the loop height decreases, the unstable zone diminishes. Though the steady-state data points are in the unstable flow region, stable operation is expected by incorporating pertinent instability restraining technique, that is, loop tilt.

Energy, exergy, economic and environmental (4E) evaluation of a solar-integrated energy system at medium–high temperature using CO2 as the parabolic trough collector (PTC) working medium

This paper proposes a solar-integrated energy system at medium–high temperature (i.e., working temperature >300 °C) for power generation, desalination, and sodium hydroxide (NaOH) production. The system consists of four subsystems, including parabolic trough collector (PTC), organic Rankine cycle (ORC), multi-effect distillation (MED), and electrodialysis. The PTC system uses CO2 gas as the working medium to obtain medium–high temperature heat sources. The electrodialysis system electrolyzes the brine to obtain NaOH and HCl, thus avoiding environmental pollution. Energy, exergy, economic and environmental analysis were adopted to evaluate the system's performance, and six critical parameters were investigated. The results showed that the system production of desalinated water, NaOH, and HCl was 106.75 kg/h, 10.61 kg/h, and 48.46 kg/h, respectively, when the direct normal irradiance was 500 W/m2. The net output power of the ORC and the power consumption of the electroanalysis system reached 23.07 kW and 5.00 kW, respectively. The power consumption of the PTC compressor was 8.01 kW. The exergy destruction of solar collector accounted for the most significant proportion, reaching 76.98%. The exergy efficiency of the system reached 12.42%. Among the six different ORC working mediums (R134a, R152a, R600, R600a, R717, and Pentane), Pentane performed the best in the integrated system, while R134a performed the worst. The annual income of the system could reach 27,645 $, with a simple payback period and payback period of 8.62 years and 10.12 years, respectively. The net present value could reach 1,103,722 $. The system could reduce CO2 emissions by 5.64 tons per year. The research in this paper contributes to understanding the overall characteristics of solar-integrated energy systems when using gaseous working mediums to achieve medium and high-temperature thermal storage. It provides a reference for the development of more efficient solar-integrated energy systems.

Thermo-economic assessment of hybrid Kalina cycle and organic Rankine cycle system using a parabolic trough collector solar field

This study presents a solar powered hybrid power system consisting the Kalina cycle (KC) and a bottoming organic Rankine cycle (ORC). LS-2 parabolic trough collector (PTC) is modeled in solar field with two different kinds of working fluid: Syltherm 800 and MXene/Syltherm 800 nanofluid. MXene particle (Ti3C2) is used with 0.1 wt% due to their outstanding thermal absorption properties. The use of MXene-based nanofluid has shown remarkable thermophysical properties, resulting in a significant increase in the useful heat generated by 11.10%. The hybrid system utilizing the MXene-based nanofluid demonstrates a substantial improvement in work utilization. Moreover, the maximum energetic and exergetic efficiency achieved with the MXene-based nanofluid are 6.095% and 6.55%, respectively, indicating its superior performance in converting solar energy into usable power with greater efficiency. When considering the overall cost of the MXene-based solar field, it was found to be approximately 7.63% higher compared to the Syltherm 800-based parabolic trough collector system. However, the Levelized cost of electricity (LCOE) for the Syltherm 800-based solar field was calculated as 0.1351 $/kW-hour, while for the MXene/Syltherm 800-based solar field, it was slightly lower at 0.1324 $/kW-hour. Additionally, the Payback period was reported as 5 years and 9 months for the Syltherm 800-based solar field, and 5 years and 7 months for the MXene/Syltherm 800-based solar field.

Optimisation and techno-economic assessment of parabolic trough collector system in Malaysia, China, and the United States

We evaluated the economic feasibility of installing PTC plants in several sites in Malaysia, China, and the United States with high solar irradiation or solar power plants. The average values of LCOE, NPV and CF for PTC power plants installed in the United States under optimum conditions are 11.92 USD cents/kWh, 47.95 million USD and 75.40%. Meanwhile, Labuan and Coara in Malaysia are two sites that are ideal for the installation of PTC plants, while Datong in China is the most favourable location for the installation of PTC power plants among the locations evaluated. Therefore, an equation relating the DNI to the optimum LCOE can be established, which recommends installing the PTC power plant in locations with DNI higher than 5.98 kWh/m2/day to replace the coal-fired electricity plant. Abbreviations: Avg: Average; Ch: China; Com: Commercial; CPV: Concentrated Solar Photovoltaic; CSP: Concentrated Solar Power; EES: Engineering Equations Solver; EIA: Energy Information Administration; HTF: Heat Transfer Fluid; IEA: International Energy Agency; Ind: India; IRENA: International Renewable Energy Agency; LFR: Linear Fresnel Reflector; Lit Rev: Literature Review; M salt: A Quinary Salt mixture doped with Hybrid; Graphene/TiO2 Nanoparticles; Mal: Malaysia; PPA: Power Purchase Agreement; PTC: Parabolic Trough Collector; PV: Photovoltaic; Res: Residential; SAM: System Advisor Model; SDG: Sustainable Development Goal; SM: Solar Multiple; SPD: Solar Parabolic Dish; SPT: Solar Power Tower; STE: Solar Thermal Energy; TES: Thermal Energy Storage; UN: United Nations; US: United States