Theoretical study on flow and radiation in tubular solar photocatalytic reactor

In this paper, based on the mixture flow model, an optimized six-flux model is first established and applied to the tubular solar photocatalytic reactor. Parameters influencing photocatalyst distribution and radiation distribution at the reactor outlet, viz. catalyst concentration and circulation speed, are also analyzed. It is found that, at the outlet of the reactor, the optimized six-flux model has better performances (the energy increase by 1900% and 284%, respectively) with a higher catalyst concentration (triple) and a lower speed (one third).     

Photocatalytic hydrogen production under direct solar light in a CPC based solar reactor: Reactor design and preliminary results

In despite of so many types of solar reactors designed for solar detoxification purposes, few attempts have been made for photocatalytic hydrogen production, which in our option, is one of the most promising approaches for solar to chemical energy conversion. Addressing both the similarity and dissimilarity for these two processes and by fully considering the special requirements for the latter reaction, a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar reactor has been designed for the first time. The design and optimization of this CPC based solar reactor has been discussed in detail. Preliminary results demonstrated that efficient photocatalytic hydrogen production under direct solar light can be accomplished by coupling tubular reactors with CPC concentrators. It is anticipated that this first demonstration of concentrator-based solar photocatalytic hydrogen production would draw attention for further studies in this promising direction.     

Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration

Photocatalytic water splitting with solar light is one of the most promising technologies for solar hydrogen production. From a systematic point of view, whether it is photocatalyst and reaction system development or the reactor-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions). Optimization of these two issues are therefore given special attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. Our systematic and continuous study in this field lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future.     

Experimental study on direct solar photocatalytic water splitting for hydrogen production using surface uniform concentrators

In this paper, a direct solar photocatalytic water splitting system with surface uniform concentrators (SUCs) is designed. Parameters influencing hydrogen production rates and energy conversion, viz. sacrificial agent concentration, catalyst concentration and circulation speed, are analyzed under typical days. It is found that the system with SUCs has better performances with higher sacrificial agent, higher catalyst concentration and lower speed: double and triple concentration of the sacrificial agent will improve the energy conversion efficiency by 4.52% and 19.35%, respectively; double and triple the photocatalyst concentration will improve the energy conversion efficiency by 81.32% and 200.00%, respectively; energy conversion efficiency under valve-half-closed and the valve-closed conditions are improve by 21.82% and 118.18% comparing with the valve-open condition.     

Experimental study of direct solar photocatalytic water splitting for hydrogen production under natural circulation conditions

Photocatalytic water splitting for hydrogen production provides a promising route for the future hydrogen economy, being operational in the visible light domain with a potential use of solar radiation. An outdoor pilot demonstration of CPC-based photoreactors has been designed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering to assess its effectiveness in solar photocatalytic hydrogen production. Nine sets of CPC-based photoreactors, each of which is 3.6 m² in area and 23 L in volume, are connected, controlled and operated in parallel. The high efficiency photocatalyst (Cd_1-xZn_xS), low concentration sacrifice agents (Na₂S and Na₂SO₃) and deionized water are the raw materials of the pilot system. Two operation models, viz. the natural circulation model and the gas disturbance model, are proposed considering the expense and the efficiency. From our observations, the slurry temperature inside the tubes rises by 20–30 °C from the ambient. The slurry velocity can reach 1.2 m/s in the gas disturbance model, but is as low as 3.5 cm/s in the natural circulation model. The average hydrogen productivity is 184.30 mL/min and accumulated to be 10.321 L/h in the natural circulation model, with the average solar radiation, photocatalyst concentration and sacrifice agents' concentration being 803.8 W/m², 2.77 g/L and 0.1 mol/L, respectively.     

Simplified architectural method for the solar control optimization of awnings and external walls in houses in hot and dry climates

In extremely hot and dry climates, like northwestern Mexico, solar gain reduction in houses using solar passive techniques is important for improving comfort inside the construction and to save costs in electrical cooling during the whole year, because the winter season is also hot in those regions. A new one-dimension method is proposed to analyze the interaction between two common shading devices: awnings and external walls to reduce insulation on the facade and inside the house due to fenestration. The method is demonstrated by optimizing a typical dwelling with an azimuth of 90° (east), which, achieves 45% reduction in direct solar insulation during the summer solstice on the profile of the facade. Results showed that this method is simple and reliable in increasing the shadow on the facade and to block completely the solar beam radiation on the windowpane with optimal relations between these shading devices.     

A novel beam-down,gravity-fed,solar thermochemical receiver/reactor for direct solid particle decomposition: Design,modeling, and experimentation

A novel solar-thermochemical reactor for the reduction of ZnO powder using concentrated sunlight has been designed, constructed and tested. The purpose of the reactor is to accomplish the first step in a two-step water-splitting process to generate hydrogen renewably from sunlight using the ZnO redox cycle. Abbreviated as GRAFSTRR (Gravity-Fed Solar-Thermochemical Receiver/Reactor), the reactor is closed to the atmosphere, and features an inverted conical-shaped reaction surface along which reactant powder descends continuously as a moving bed, undergoing a thermochemical reaction at high temperature upon exposure to highly concentrated sunlight within the reaction cavity. Heat transfer and Zn production within the cavity have been modeled, as well as the influence of effective reactant particle size on reactive surface area. Initial experiments using a high-flux solar simulator successfully demonstrated the mechanical stability of the reactor and primary systems, namely particle entrainment in the vortex flow, moving bed adhesion to the reaction surface, and the solid particle delivery and exit mechanism. This paper presents the GRAFSTRR concept, select design choices, and a summary of pertinent findings from experimental and numerical investigations.     

Catalytic thermochemical reactor/receiver for solar reforming of natural gas: Design and performance

The advantages of thermochemical conversion of concentrated solar energy using catalytic processes are discussed. The design of a solar volumetric thermochemical reactor/receiver (TCRR) with catalytic absorber, method for synthesis of catalytically activated ceramics, and preparation of catalytic absorber have been described. The prototype TCRR was tested in the high flux solar furnace at the DAC, Cologne by using the dioxide reforming of methane. The tests were performed to check the main concept of the TCRR design and catalytic absorber, to study the influence of solar flux distribution, the reagent flows and their ratio on the productivity or conversion, determine the reagent's conversion depending on the focal point disposition with respect to the absorber, and to study the efficiency of the thermochemical conversion. The chemical and total efficiencies of the CO₂–methane conversion were calculated using the experimentally measured concentrations of the reaction products. The highest overall efficiency achieved in these experiments was 30% with the Ni–Cr catalytic absorber.     

Solar photocatalytic degradation of gaseous formaldehyde by sol–gel TiO2 thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol–gel TiO₂ thin film. In a pseudo-first-order Langmuir–Hinshelwood (L–H) model, the maximum reaction rate constant obtained is 0.148 min⁻¹ under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol–gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells.     

Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices

A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m×2.0 m×0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m². To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse.     

Field solar photocatalytic purification of pesticides-containing rinse waters from tractor cisterns used for grapevine treatment

The objective was to assess in a vineyard the effect of purifying by solar photocatalysis the title rinse waters (presently rejected or, extremely rarely, cleaned in specific installations) in terms of efficacy and on-site ease-of-use for the wine grower. The on-site, self-functioning, solar purifying unit included a corrugated-steel inclined plate of area S=1 m² onto which a TiO₂-coated thin material had been stuck, a 100-l tank, and an aquarium-type pump powered by a photovoltaic panel. For a vineyard of area A=0.15 km², the rinse water (about 80 l) corresponding to each of four typical vine treatments was analysed (major pesticides for each treatment, TOC, Microtox test and, in one case, BOD₅) by independent laboratories, before and after purification for 8 days. These analyses showed that the S/A ratio tested was insufficient. From the relatively low final organic content reached in one case, it is calculated that a three-time higher S/A ratio might suffice, but new trials are necessary to determine whether it is valid for the other typical cases. Inferred contribution of inorganic ions to the post-photocatalytic treatment toxicity points out to the need for an additional detoxification. However, even with a too small S, the photocatalytic treatment markedly improved the quality of the rinse waters. These field experiments have also demonstrated that the purifying prototype is robust, and easy to install and use on site by the wine grower.     

Scalable optical fiber reactor for photocatalytic H2 production: Addressing scattering issues

Scattering is one of the main challenges in scaling up photocatalytic water splitting using the most prevalent powder catalysts. This can be overcome by decoupling the reaction medium from light transmission, as in the case of optical fibers. Here we explore utilizing optical fibers coated with 5 wt% CuO supported on TiO₂ for photocatlytic H₂ production from water-methanol mixtures. CuO/TiO₂ is a well studied photo catalyst in which photoreduced Cu species are known to act as sensitizers for inducing visible light activity. Lower activity of sequentially coated systems indicates that appropriate interfaces of active Cu and TiO₂ with water are desirable. The scalability of such optical fiber-based systems along with potential in non-potable turbid water media are demonstrated. Maximum activity of 22 μmoles of H₂ in 8 h was obtained with 50 mg of catalyst coated on optical fibers, which increases linearly with increase in fiber numbers, whereas, drastic reduction in activity is observed in powder catalyst upon increasing the catalyst quantity. A one-to-one comparison of 700 mg of catalyst in powder form and coated on optical fibers indicates more than one order enhancement in activity in the optical fiber based system. In addition, ∼70% retention in activity in highly turbid non-potable water was observed as compared to powdered system which shows complete reduction in the activity by 99.99%.     

Swine sewage as sacrificial biomass for photocatalytic hydrogen gas production: Explorative study

This is the first-time reporting on the photocatalytic production of hydrogen gas (H₂) via water splitting in the presence of swine sewage (SS) as sacrificial agent. H₂ was evolved upon both UV-A and solar light irradiation in presence of platinised titania as the catalyst. The influences of irradiation time, catalyst amount, pH and SS concentration on the reaction yield were investigated. Under the best conditions, up to 50 μmol of H₂ were produced from aqueous SS (1.7% v/v, COD 82 mg L⁻¹), with good within-laboratory reproducibility (RSD <3%, n = 3). The batch-to-batch reproducibility was verified, too. Despite the complexity of the matrix, the amount of evolved H₂ was about a half of that obtained by use of model sacrificial agents as glucose, sucrose and glycerol, known to be excellent electrondonors. The catalyst, recovered after irradiation, was regenerated by mere washing with acidic water and conserved part of the activity for reusing for at least 3 runs. The proposed procedure is at low environmental impact, does not release carbon dioxide and proceeds also under solar light with reaction yields up to 94% compared to those observed under UV-A radiation. This explorative study demonstrates that a waste such as SS is a rewarding and inexpensive feedstock to produce a high-value chemical as hydrogen gas. The results suggest that further work should be addresses in this direction.     

Design optimization of thermoelectric devices for solar power generation

We present an improved theoretical model of a thermoelectric device which has been developed for geometrical optimization of the thermoelectric element legs and prediction of the performance of an optimum device in power generation mode. In contrast to the currently available methods, this model takes into account the effect of all the parameters contributing to the heat transfer process associated with the thermoelectric device.The model is used for a comparative evaluation of four thermoelectric modules. One of these is commercially available and the others are assumed to have an optimum geometry but with different design parameters (thermal and electrical contact layer properties).Results from the model are compared with experimental data of the commercial thermoelectric module in power generation mode with temperature gradient consistent with those achievable from a solar concentrator system. These show that it is important to have devices optimized specifically for generation, and to improve the contact layer of the thermoelements accordingly.     

Thermal modeling of passive and active solar stills for different depths of water by using the concept of solar fraction

This communication presents the thermal analysis of passive and active solar distillation system by using the concept of solar fraction inside the solar still with the help of AUTOCAD 2000 for given solar azimuth and altitude angle and latitude, longitude of the place. Experiments have been conducted for 24 h (9 am to 8 am) for New Delhi climatic conditions (latitude 28°35′N, longitude 77°12′E) during the months of November and December for different water depths in the basin (0.05, 0.1 and 0.15 m) for passive as well as active solar distillation system. Analytical expressions for water and glass cover temperatures and yield have been derived in terms of design and climatic parameters. It is observed that

(i) the solar fraction plays a very important role at lower values of solar altitude angle;

(ii) the internal convective heat transfer coefficient decreases with the increase of water depth in the basin due to decrease in water temperature;

(iii) there is a fair agreement between the experimental observation and theoretical prediction during daytime as compared to that during the night.

Thermal modeling of integrated roof air heater for passive solar space heating of a non-air-conditioned building

This communication presents the periodic heat transfer analysis for solar space heating of an unconditioned building with an integrated roof air heater. The system consists of an air duct within the roof such that the air is continuously or intermittently forced to circulate the cooler room air through the inlet of the air duct. Time dependence of the air flow is represented by a step function of time for daily operation and, hence, has been expressed as a Fourier series in time. The analysis takes into account air ventilation, ground heat conduction and furnishings. The effects of depth of the air duct from the outer surface of the roof and the magnitude and duration of air flow rate on indoor air temperature have been studied for a typical cold winter day in Delhi. It is seen that a time dependent air flow through the duct is desirable from the point of view of increasing the indoor air temperature in the case of a bare roof. However, in the case of a blackened and glazed roof, continuous air flow is needed for increasing the room air temperature. The results are desirable from the point of view of efficient space heating of solar passive buildings.     

Determination of design space and optimization of solar water heating systems

In this paper, a methodology is proposed to determine the design space for synthesis, analysis, and optimization of solar water heating systems. The proposed methodology incorporates different design constraints to identify all possible designs or a design space on a collector area vs. storage volume diagram. The design space is represented by tracing constant solar fraction lines on a collector area vs. storage volume diagram. It has been observed that there exists a minimum as well as a maximum storage volume for a given solar fraction and collector area. Similarly existence of a minimum and a maximum collector area is also observed for a fixed solar fraction and storage volume. For multi-objective optimization, a Pareto optimal region is also identified. Based on the identified design space, the solar water heating system is optimized by minimizing annual life cycle cost. Due to uncertainty in solar insolation, system parameters and cost data, global optimization may not be utilized to represent a meaningful design. To overcome this, a region of possible design configurations is also identified in this paper.     

Novel twin reactor for separate evolution of hydrogen and oxygen in photocatalytic water splitting

Photocatalytic water splitting with separate H₂ and O₂ evolution is crucial because it eliminates the explosion potential and hydrogen-purification cost. A novel twin reactor was designed to separate the evolution of hydrogen and oxygen in photocatalytic water splitting under visible light. A modified Nafion membrane was employed to segregate the two photocatalysts in the twin reactor so that hydrogen and oxygen can be evolved separately. Conventional Z-scheme catalysts, Pt/SrTiO₃:Rh and WO₃, were used as hydrogen-photocatalyst and oxygen-photocatalyst, respectively. Fe²⁺ and Fe³⁺ were added in the reaction solution as electron-transfer mediator. The ratio of evolved H₂ and O₂ was in agreement with the stoichiometric ratio (2:1) of hydrogen and oxygen of water. An average hydrogen generation rate of 1.59 μmol/g-h was achieved in the twin-reactor system, which was twice as much as that in the conventional Z-scheme system. The improved H₂ yield was due to the prevention of the water-splitting backward reaction in the twin reactor.     

Multi-criteria optimization of an integrated energy system with thermoelectric generator,parabolic trough solar collector and electrolysis for hydrogen production

In this research paper, a newly energy system consisting of parabolic trough solar collectors (PTSC) field, a thermoelectric generator (TEG), a Rankine cycle and a proton exchange membrane (PEM) is proposed. The integration is performed by establishing a TEG instead of the condenser as power generation and cooling unit thereafter surplus power output of the TEG is transferred to the PEM electrolyzer for hydrogen production. The integrated renewable energy system is comprehensively modeled and influence of the effective parameters is investigated on exergy and economic indicators through the parametric study to better understand the system performance. Engineering equation solver (EES) as a potential engineering tool is used to simulate the system and obtain the desired results. In order to optimize the system, a developed multi-objective genetic algorithm MATLAB code is applied to determine the optimum operating conditions of the system. Obtained results demonstrate that at optimum working condition from exergy viewpoint, exergy efficiency and total cost are 12.76% and 61.69 $/GJ, respectively. Multi-objective optimization results further show that the final optimal point which is well-balanced between exergy efficiency and total cost, has the maximum exergy efficiency of 13.29% and total cost of 63.96 $/GJ, respectively. The corresponding values for exergy efficiency and total cost are 10.01% and 60.21 $/GJ for optimum working condition from economic standpoint. Furthermore, hydrogen production at well-balanced operating condition would be 2.28 kg/h. Eventually, the results indicate that establishing the TEG unit instead of the condenser is a promising method to optimize the performance of the system and reduce total cost.     

Evaluation of conventional and high-performance routine solar radiation measurements for improved solar resource, climatological trends, and radiative modeling

The solar renewable energy community depends on radiometric measurements and instrumentation for data to design and monitor solar energy systems, and develop and validate solar radiation models. This contribution evaluates the impact of instrument uncertainties contributing to data inaccuracies and their effect on short-term and long-term measurement series, and on radiation model validation studies. For the latter part, transposition (horizontal-to-tilt) models are used as an example. Confirming previous studies, it is found that a widely used pyranometer strongly underestimates diffuse and global radiation, particularly in winter, unless appropriate corrective measures are taken. Other types of measurement problems are also discussed, such as those involved in the indirect determination of direct or diffuse irradiance, and in shadowband correction methods. The sensitivity of the predictions from transposition models to inaccuracies in input radiation data is demonstrated. Caution is therefore issued to the whole community regarding drawing detailed conclusions about solar radiation data without due attention to the data quality issues only recently identified.