Comparative system analysis of direct steam generation and synthetic oil parabolic trough power plants with integrated thermal storage

Parabolic trough power plants are currently the most commercially applied systems for CSP power generation. To improve their cost-effectiveness, one focus of industry and research is the development of processes with other heat transfer fluids than the currently used synthetic oil. One option is the utilization of water/steam in the solar field, the so-called direct steam generation (DSG).Several previous studies promoted the economic potential of DSG technology (Eck et al., 2008b, Price et al., 2002, Zarza, 2002). Analyses’ results showed that live steam parameters of up to 500 °C and 120 bars are most promising and could lead to a reduction of the levelized electricity cost (LEC) of about 11% (Feldhoff et al., 2010). However, all of these studies only considered plants without thermal energy storage (TES).Therefore, a system analysis including integrated TES was performed by Flagsol GmbH and DLR together with Solar Millennium AG, Schott CSP GmbH and Senior Berghöfer GmbH, all Germany. Two types of plants are analyzed and compared in detail: a power plant with synthetic oil and a DSG power plant. The design of the synthetic oil plant is very similar to the Spanish Andasol plants (Solar Millennium, 2009) and includes a molten salt two-tank storage system. The DSG plant has main steam parameters of 500 °C and 112 bars and uses phase change material (PCM) for the latent and molten salt for the sensible part of the TES system. To enable comparability, both plants share the same gross electric turbine capacity of 100 MWel, the same TES capacity of 9 h of full load equivalent and the same solar multiple of the collector field of about two.This paper describes and compares both plants’ design, performance and investment. Based on these results, the LEC are calculated and the DSG plant’s potential is evaluated. One key finding is that with currently proposed DSG storage costs, the LEC of a DSG plant could be higher than those of a synthetic oil plant. When considering a plant without TES on the other hand, the DSG system could reduce the LEC. This underlines the large influence of TES and the still needed effort in the development of a commercial storage system for DSG.     

Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application

In order to harvest solar energy, thermal energy storage (TES) system with Phase Change Material (PCM) has been receiving greater attention because of its large energy storage capacity and isothermal behavior during charging and discharging processes. In the present experimental study, shell and tube TES system using paraffin wax was used in a water heating system to analyze its performance for solar water heating application. Energy and exergy including their cost analyses for the TES system were performed. Accordingly, total life cycle cost was calculated for different flow rates of the Heat Transfer Fluid (HTF). With 0.033 kg/min and 0.167 kg/min flow rates of water as HTF, energy efficiencies experienced were 63.88% and 77.41%, respectively, but in exergy analysis, efficiencies were observed to be about 9.58% and 6.02%, respectively. Besides, the total life cycle cost was predicted to be $ 654.61 for 0.033 kg/min flow rate, which could be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min. Therefore it can be summarized that total life cycle cost decreases with the increase of flow rate.     

Experimental investigation of hydrogen production integrated methanol steam reforming with middle-temperature solar thermal energy

Developing a hydrogen production method that utilizes solar thermal energy in an effective manner is a great challenge. In this paper we propose a new approach to solar hydrogen production with the integration of methanol steam reforming and middle-temperature solar thermal energy. An experiment on hydrogen production is conducted using a 5-kW solar reactor at 150−300 °C under atmosphere pressure. The 5-kW solar receiver/reactor is fabricated and positioned along the focal line of one-tracking parabolic trough concentrator. As a result, the chemical conversion of methanol can reach levels higher than 90%, and the volumetric concentration of hydrogen in the gas products can account for 66−74% above the solar flux of 580 W/m². The obtained maximum hydrogen yield per mole of methanol is 2.65−2.90 mol, approaching the theoretical maximum value, and the experimentally obtained thermochemical efficiency of solar thermal energy converted into chemical energy is in the range of 30−50%, which is competitive with other high-temperature solar thermochemical processes. A kinetic model of solar-driven methanol steam reforming related to solar flux is also derived based on the experimental data. The promising results demonstrate that this solar-driven hydrogen production method can be feasible in practical applications.     

Packed-bed thermal storage for concentrated solar power – Pilot-scale demonstration and industrial-scale design

A thermal energy storage system, consisting of a packed bed of rocks as storing material and air as high-temperature heat transfer fluid, is analyzed for concentrated solar power (CSP) applications. A 6.5 MWh_th pilot-scale thermal storage unit immersed in the ground and of truncated conical shape is fabricated and experimentally demonstrated to generate thermoclines. A dynamic numerical heat transfer model is formulated for separate fluid and solid phases and variable thermo-physical properties in the range of 20–650 °C, and validated with experimental results. The validated model is further applied to design and simulate an array of two industrial-scale thermal storage units, each of 7.2 GWh_th capacity, for a 26 MW_el round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.     

Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92 min for the smooth tube, 350.75 min for dimpled tube which is reduced about 19.9% and 320.25 min for cone-finned tube which is reduced about 26.9% and 302.75 min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2 Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.     

Investigating the potential for energy,fuel, materials and chemicals production from corn residues (cobs and stalks) by non-catalytic and catalytic pyrolysis in two reactor configurations

The results of thermogravimetric analysis (TGA), non-catalytic and catalytic pyrolysis of corn cobs and corn stalks are reported in this paper. Pyrolysis took place in two different reactor configurations for both feedstocks: (1) fast pyrolysis in a captive sample reactor; and (2) non-catalytic slow pyrolysis and catalytic pyrolysis in a fixed-bed reactor. Experiments were carried out in atmospheric pressure at three temperatures: low temperature (360–380 °C), medium temperature (500–600 °C) and at high temperature (600–700 °C). The results of the experimental study were compared with data reported in the literature. Investigating the potential of corn residues for energy, fuel, materials and chemicals production according to their thermochemical treatment products yields and quality, it can be stated that: (a) corn stalks could be suitable raw material for energy production via gasification at high temperature, due to their medium low heating value (LHV) of pyrolysis gas (13–15 MJ/m³); (b) corn cob could be a good solid biofuel, due to the high LHV (24–26 MJ/kg) of the produced char; (c) additionally, corn cobs could be a good material for activated carbon production after being activated or gasified with steam, due to its high fixed carbon content(～74 wt%); (d) liquid was the major pyrolysis product from catalytic pyrolysis (about 40–44 wt% on biomass) for both feedstocks; further analysis of the organic phase of the liquid products were hydrocarbons and phenols, which make them interesting for chemicals production.     

High-temperature carbonate/MgO composite materials as thermal storage media for double-walled solar reformer tubes

The composite materials of molten alkali-carbonate/MgO-ceramics are examined as thermal storage media in a tubular reformer using a double-walled reactor tube of a laboratory scale. The concept of a double-walled reformer tube is proposed as a solar tubular reformer and involves packing a molten salt/ceramic composite material in the annular region between the internal catalyst tube and the exterior solar absorber wall. The composite materials of Na₂CO₃, K₂CO₃, and Li₂CO₃ with magnesia are tested as thermal storage media. The reforming performances of the composite materials are tested in the cooling mode of the double-walled reactor tube. The experimental result obtained under feed gas mixture of CH₄/CO₂ = 1:3 at 1 atm shows that the use of 80 wt%Na₂CO₃/20 wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5–19 min in comparison to the case without a composite material. In addition, the Li₂CO₃/MgO and Na₂CO₃/MgO composite materials relatively revealed good performances: they prolonged the cooling time by over 10 min in the gas hourly space velocity (GHSV) range of 5000–12,500 h^?1. The application of the reactor tubes to solar tubular reformers is expected to realize stable operation of the solar reforming process under fluctuating insolation during cloud passage.     

Thermal characteristics of manganese (II) nitrate hexahydrate as a phase change material for cooling systems

The imbalance of electrical demand in summer due to cooling system demand is a big problem in many countries. One promising solution is shifting peak demand from early afternoon to night by utilizing natural cold energy resources such as cool outside air during night or running a refrigerator driven by midnight power. In these cases, using the thermal energy storage (TES) of phase change material (PCM) which has a melting point from 15 to 25 °C is one of the most effective ideas. However, few suitable PCMs for this temperature range are at present commercially available. This study aims to evaluate the potential of Mn(NO₃)₂ · 6H₂O (manganese (II) nitrate hexahydrate) as a new PCM for the TES of cooling systems. First, experiments on the modulation of the melting point of Mn(NO₃)₂ · 6H₂O and reduction of supercooling were made by dissolving small amounts of salts in the material. Consequently, MnCl₂ · 4H₂O was found to have good performance with regard to both modulation of the melting temperature and the heat of fusion. Next, a thermal response test was carried out by using a small cylindrical vessel. Results showed that the required temperature levels for charging and discharging the heat of this mixture were clarified. In addition, the price and safety of this material as a PCM are discussed.     

Power,placement and LEC evaluation to install CSP plants in northern Chile

Chile is expecting a 5.4% growth in energy consumption per year until 2030, requiring new and better solutions for the upward trend of its electricity demand. This state leads to select and study one of the potential alternatives for electricity generation: concentrated solar power (CSP) plants. Such renewable technology found in Chile a very favorable condition. Recent researches indicate Atacama Desert as one of the best regions for solar energy worldwide, having an average radiation higher than in places where CSP plants are currently implemented, e.g. Spain and USA. The aim of this study is to present an analysis of levelized energy cost (LEC) for different power capacities of CSP plants placed in distinct locations in northern Chile. The results showed that CSP plants can be implemented in Atacama Desert with LECs around 19 ¢US$/kWh when a gas-fired backup and thermal energy storage (TES) systems are fitted. This value increases to approximately 28 ¢US$/kWh if there is no backup system.     

Influence of operational parameters and material properties on the contact heat transfer in rotary kilns

The heat transfer by contact between the covered inner wall surface and the solid bed in a rotary kiln was experimentally investigated. An indirect heated batch rotary drum with a diameter of D = 600 mm and a length of L = 450 mm has been used. Inside the drum, a rotating and stationary measuring rod was installed, each assembled with 16 k-type thermocouples to measure the temperatures. Thus, the radial and circumferential temperature gradients within the solid bed were measured, and the contact heat transfer was determined from the heating of the bed. The effect on the contact heat transfer coefficient was estimated for the operational parameters of the drum, rotational speed (1, 3, 6 rpm) and filling degree (10%, 15%, 20%); the solid bed parameters, particle diameter (0.7, 1.3, 2.0 mm) and thermo physical properties (conductivity, heat capacity, density). As test material, quartz sand, glass beads and copper beads were used. The measured heat transfer coefficients were compared with theoretical calculations of four model approaches from the recent literature. A good agreement with just one of these model approaches could be achieved.     

Assessing the use of simple dye-sensitized solar cells for drinking water chlorination by communities with limited resources

Dye-sensitized ZnO and TiO₂ photoelectrochemical cells were constructed using recycled waste materials and readily accessible household chemicals to assess whether it would be feasible for low-income communities to utilise solar energy for drinking water chlorination. Prussian Blue sensitized ZnO cells utilising ferro/ferricyanide and iron/copper redox couples for charge transfer produced open circuit potentials of between 0.19 and 0.53 V, and short circuit currents in the range 0.3–1.5 mA cm^?2. Although the power output from these cells was significantly lower than those using the iodide/triiodide redox couple for charge transfer, the significantly lower cost of construction of cells using alternative electrolytes could make these cells accessible to poor communities for producing small amounts of solar electricity for drinking water chlorination.     

Transient analysis of modified cuboid solar integrated-collector-storage system

A novel solar water heating system, modified cuboid solar integrated-collector-storage (ICS) system with transparent insulation material (TIM) has been designed and developed, which combines collection and storage in a single unit and minimizes the nocturnal heat losses. A comprehensive study has been carried out to evaluate the heat transfer characteristics inside the enclosure of the system to enhance the collection and storage of solar energy. The transient behavior of the modified-cuboid solar integrated-collector-storage system is investigated numerically to evolve optimum configuration. The optimum design for the system is obtained by carrying out a numerical parametric study with different geometry parameters like the depth of the cuboid (d = 2, 5, 8, and 12 cm), and inclination angles (10°, 20°, 30°, and 50°). The inside heat transfer coefficient of the ICS system, stratification factor and water temperature distribution inside the enclosure have been predicted by numerical simulation. Average heat transfer coefficient at the bottom surface of absorber plate is 20% higher for depth of 12 cm as compared to the 2 cm depth of cuboid section, after 2 h of heating. The stratification factor also increases from 0.02 to 0.065 as depth of the system increases from 2 cm to 12 cm. There is a marginal effect of inclination angles of the system on the convection in the enclosure. As the inclination angle increases from 10° to 50°, the average heat transfer coefficient increases from 90 W/m² K to 115 W/m² K. But the stratification factor is comparatively high for lower inclination angles. With the optimum design parameters, a field experimental set-up was built and the numerical model was validated for efficient heat collection and storage in a modified cuboid ICS system. The model is in good agreement with the experimental results.     

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO₂-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.     

Preparation and thermal energy properties of paraffin/halloysite nanotube composite as form-stable phase change material

Phase change materials (PCMs) have attracted extensively interests in solar storage. In the study, we prepared a new kind of composite PCM by impregnating paraffin (P) into halloysite nanotube. The as-prepared composite PCM was characterized by TEM, FT-IR and DSC analysis techniques. The composite can absorb paraffin as high as 65 wt.% and maintain its original shape perfectly without any paraffin leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 65/35 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite was added into the P/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 60.78% and 71.52% compared with the composite without graphite, respectively. Due to its high adsorption capacity, high heat storage capacity, good thermal stability and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application.     

Development of a system model for a hydrogen production process on a solar tower

An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800 °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100–1200 °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting.Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks.This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process.     

Dynamic behavior of a stand-alone hybrid power generation system of wind turbine,microturbine, solar array and battery storage

This paper presents dynamic behavior and simulation results in a stand-alone hybrid power generation system of wind turbine, microturbine, solar array and battery storage. The hybrid system consists of a 195 kW wind turbine, an 85 kW solar array; a 230 kW microturbine and a 2.14 kAh lead acid battery pack optimized based on economic analysis using genetic algorithm (GA). At first, a developed Lyapunov model reference adaptive feedback linearization method accompanied by an indirect space vector control is applied for extraction of maximum energy from a variable speed wind power generation system. Then, a fuzzy logic controller is designed for the mentioned purpose and its performance is compared with the proposed adaptive controller. For meeting more load demands, the solar array is integrated with the wind turbine. In addition, the microturbine and the battery storage are combined with the wind and solar power generation system as a backup to satisfy the load demand under all conditions.A supervisory controller is designed in order to manage energy between the maximum energy captured from the wind turbine/solar arrays, and consumed energies of the load, dump load, battery state of charge (SOC), and generated energy by the microturbine. Dynamic modeling and simulation are accomplished using MATLAB Simulink? 7.2.     

Technical and economic assessment of grid-independent hybrid photovoltaic–diesel–battery power systems for commercial loads in desert environments

Solar photovoltaic (PV) hybrid system technology is a hot topic for R&D since it promises lot of challenges and opportunities for developed and developing countries. The Kingdom of Saudi Arabia (KSA) being endowed with fairly high degree of solar radiation is a potential candidate for deployment of PV systems for power generation. Literature indicates that commercial/residential buildings in KSA consume an estimated 10–45% of the total electric energy generated. In the present study, solar radiation data of Dhahran (East-Coast, KSA) have been analyzed to assess the techno-economic viability of utilizing hybrid PV–diesel–battery power systems to meet the load requirements of a typical commercial building (with annual electrical energy demand of 620,000 kW h). The monthly average daily solar global radiation ranges from 3.61 to 7.96 kW h/m². NREL's HOMER software has been used to carry out the techno-economic viability. The simulation results indicate that for a hybrid system comprising of 80 kWp PV system together with 175 kW diesel system and a battery storage of 3 h of autonomy (equivalent to 3 h of average load), the PV penetration is 26%. The cost of generating energy (COE, US$/kW h) from the above hybrid system has been found to be 0.149 $/kW h (assuming diesel fuel price of 0.1 $/L). The study exhibits that for a given hybrid configuration, the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets for a given hybrid system. Emphasis has also been placed on unmet load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), cost of PV–diesel–battery systems, COE of different hybrid systems, etc.     

Techno-economic evaluation of off-grid hybrid photovoltaic–diesel–battery power systems for rural electrification in Saudi Arabia—A way forward for sustainable development

The burning of depleting fossil fuels for power generation has detrimental impact on human life and climate. In view of this, renewable solar energy sources are being increasingly exploited to meet the energy needs. Moreover, solar photovoltaic (PV)–diesel hybrid system technology promises lot of opportunities in remote areas which are far from utility grid and are driven by diesel generators. Integration of PV systems with the diesel plants is being disseminated worldwide to reduce diesel fuel consumption and to minimize atmospheric pollution. The Kingdom of Saudi Arabia (K.S.A.) being endowed with high intensity of solar radiation, is a prospective candidate for deployment of PV systems. Also, K.S.A. has large number of remote scattered villages. The aim of this study is to analyze solar radiation data of Rafha, K.S.A., to assess the techno-economic feasibility of hybrid PV–diesel–battery power systems to meet the load requirements of a typical remote village Rawdhat Bin Habbas (RBH) with annual electrical energy demand of 15,943 MWh. Rafha is located near RBH. The monthly average daily global solar radiation ranges from 3.04 to 7.3 kWh/m². NREL's HOMER software has been used to perform the techno-economic evaluation. The simulation results indicate that for a hybrid system composed of 2.5 MWp capacity PV system together with 4.5 MW diesel system (three 1.5 MW units) and a battery storage of 1 h of autonomy (equivalent to 1 h of average load), the PV penetration is 27%. The cost of generating energy (COE, US$/kWh) from the above hybrid system has been found to be 0.170$/kWh (assuming diesel fuel price of 0.1$/l). The study exhibits that the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets. Concurrently, emphasis has been placed on: un-met load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as: PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), COE of different hybrid systems, etc. The decrease in carbon emissions by using the above hybrid system is about 24% as compared to the diesel-only scenario.     

Symmetric redox supercapacitor with conducting polyaniline electrodes

Polyaniline doped with HCl (Pani-HCl) and LiPF₆ (Pani-LiPF₆) are prepared and used as the active electrode material of symmetric redox supercapacitors. The system using Et₄NBF₄ as an electrolyte solution has lower internal resistance and larger specific discharge capacitance, and thus, it is suitable for use in a polyaniline redox supercapacitor. The capacitance of Pani-HCl decreases during ∼400 cycles and then becomes constant at ∼40 F g⁻¹. On the other hand, the polyaniline electrode doped with lithium salt like LiPF₆ shows a specific discharge capacitance of ∼107 F g⁻¹ initially and ∼84 F g⁻¹ at 9000 cycles.