Solar thermochemical cycles for hydrogen production (STCH)

In the last decade, there have been major developments in the generation of green fuel sources to replace fossil fuels with the aim of reducing greenhouse gas emissions. Hydrogen is one of these sources that can be used either in the power generating fuel cells or as a direct combustion fuel. To produce large hydrogen on a large scale successfully, the whole process plant ought to utilise clean energy and avoid fossil derived sources as much as possible. This study focuses on a large scale solar driven hydrogen production in form of a closed sulphur-ammonia thermochemical water-splitting cycle. The cycle utilises both the thermal and quantum components of free and widely available solar irradiation to drive the oxygen and hydrogen generation respectively. The only fluid input to the cycle is water, and only output is hydrogen and oxygen gases. However, in the most common type of sulphur-ammonia cycle, the hydrogen producing step is undertaken by a high energy demanding electrolyser at high operating temperature. In this research, the electrolyser step is replaced by a solar driven photocatalytic reactor step, as a mean to make the cycle more economical and greener. To begin with, an energy efficient and economically viable photoreactor based on LED lightning, which could mimic the quantum part of solar irradiation, was designed and benchmarked. Light intensity and flux analysis of the LED lightning indicated that cool LEDs would be the most suitable option for a reactor using visible light driven cadmium sulphide (CdS) photocatalyst particles. To further increase the photostability of the CdS catalyst and improve its photonic yield, a suitable cocatalyst was searched for. Among the cocatalysts under investigation, cobalt phosphides (CoxP) was found to be most promising for an increased hydrogen evolution at a reduced cost, if compared with more common noble metals such as Pt and Pd. Different synthesis method and synthesis parameters showed different CoxP composition could be tuned and optimised, resulting in different hydrogen evolution yields. The CoP synthesised through an organometallic method loaded on CdS by sonication, showed exceptionally high hydrogen yield compared to the other tested CoxP/CdS, and even more than the most referred Pt/CdS, when an aqueous lactic acid solution was used in the reactor. All investigations involved optimising different working parameters such as; cocatalyst loading on CdS, reactor particle loading, solution concentration variation and irradiation flux. For a solar driven sulphur ammonia thermochemical cycle, the working fluid would be ammonium sulphite rather than lactic acid and therefore, the same various CdS photocatalytic composites and various working parameters were tested again but using ammonium sulphite solution instead. Once again, the organometallic synthesised CoP/CdS composite performed excellent with a comparable hydrogen yield than the more understood and elaborated Pt/CdS. For the photocatalytic reactions of both lactic acid (aq.) and ammonium sulphite (aq.) a mathematical predictive model of the hydrogen production was developed. The model was based on a pseudo-steady state approach of the Langmuir-Hinshelwood adsorption isotherm, which further incorporated the total radiative flux effect into the model. The Langmuir-Hinshelwood part reflected the mechanism of the surface adsorption/desorption and the surface reaction rates whereas the irradiation part of the model was based on an approximate solution of radiation field theory (RTE) and the derivation of geometrical positions of the particles in a cross-section of the reactor which was further expanded to incorporate the total visible radiative flux density received by the particles in the whole photoreactor volume. One of the main advantages of the model was the successful incorporation of both the measured photocatalyst’s optical scattering and absorption coefficients in the relevant LED output range of 410-500 nm, increasing the accuracy of the model. The obtained model parameters were then successfully validated for a range of photocatalytic hydrogen generation experiments conducted at various reaction conditions. These results show that the developed model can be used to predict photocatalytic hydrogen production satisfactory regardless of the radiation type, reactor size or catalyst particle, with minor computational effort or use of any commercial software. The obtained model information is suggested to work as a supporting aid for any photocatalytic reactor scale-up, which can easily be altered to either reduction product (hydrogen) or oxidation product if the most dominating initial reactions are known. A feasibility study of a complete thermochemical cycle was also done, where implementing several groups of parallel hydrogen photoreactors was suggested as the best option for large scale hydrogen production with the main purpose of replacing the energy demanding electrolyser in the thermochemical cycle. The parallel photoreactor configuration can be easily implemented and be operationally efficient in any thermochemical cycle configuration. Finally, it was shown that an increased hydrogen efficiency (due to an increased photonic energy conversion) and an improved hydrogen economy of the cycle (due to use of cheaper catalyst & reactor materials and a reduced power consumption) can be achieved

Comprehensive adsorption and irradiation modelling of LED driven photoreactor for H2 production

A newly developed hydrogen generation model describing both adsorption and irradiation mechanisms for an externally irradiated 10-LED photoreactor is presented and validated against experimental data. The surface reaction mechanism of the model is based on a pseudo-steady state Langmuir-Hinshelwood kinetic which incorporate the total radiative flux effect. The irradiation mechanism of the model is based on an approximate solution of the Radiative Transport Equation (RTE) together with the derivation of the geometrical positions of the particles in the cross-section of the reactor. This enables calculation of the total visible radiative flux density received by the particles in that area. Integration of the calculated total radiative flux density over the longitudinal reaction depth accounts for the total received photon flux by all particles inside the whole photoreactor. One of the main features of the irradiation mechanism is incorporation of the photocatalyst’s optical scattering and absorption coefficients, which are obtained by the spectrophotometric measurements in the LED output range of 410-500 nm. A least-square best fitting procedure is used to determine the model parameters, where they are successfully validated for a range of photocatalytic H2 experiments conducted at different catalyst loadings, photolyte concentrations, and incident radiation fluxes. The results indicate that the developed model can predict photocatalytic hydrogen production satisfactory with minor computational effort or use of any commercial software. The obtained information provides a coherent framework for the scaling-up and design of the LED-photoreactors

Is high waist circumference and body weight associated with high blood pressure in Iranian primary school children?

Introduction and objectives: The prevalence of overweight, abdominal obesity and hypertension among children has increased worldwide including Iran over several decades. We carried out a study to provide current estimates of the prevalence and trends of hypertension, overweight and obesity along with the relationship between weight status and hypertension in Iranian school-children. Method: This study was carried out among 1184 fifth-grade students, whose ages ranged from 11 to 14 years. Body weight, body mass index (BMI), height and waist circumference (WC), systolic (SBP) and diastolic blood pressure (DBP) were measured. Results: 22.04 and 5.32 of students were overweight and obese, respectively. The prevalence of overweight and obesity was significantly higher among girls than boys (all p = 0.02), whereas the prevalence of hypertension was significantly higher in boys than girls (p = 0.001). Although 27 of boys and 24.32 of girls had abdominal obesity, no significant associations were reported between abdominal obesity and sex (p = 0.12). The prevalence of hypertension in children with normal weight, overweight and obesity, was 3, 9.7 and 17.8 , respectively (p < 0.01). We have obtained that the mean values of weight and WC were significantly higher in boys than girls. Based on linear regression, every 1 cm increase in abdominal circumference leads to an estimated DBP and SBP increase of 0.173 and 0.164 mmHg, respectively (p < 0.05). Conclusions: This study showed a high prevalence of hypertension and obesity in a school-based population in Tehran, Iran, in which the prevalence of hypertension was significantly and positively correlated with weight and WC. © 2016, Springer International Publishing Switzerland