The aerosol effect on direct normal irradiance in Europe under clear skies

The effect of spatial and temporal variability of aerosol optical depth (AOD) on direct normal irradiance (DNI) under clear skies is studied, with the synergetic use of satellite and ground-based data as well as calculations from a radiative transfer model. The area of interest is Europe; data from May to September during 13 years (2000–2012) are analyzed. The aerosol effect on DNI is high in areas influenced by desert dust intrusions and intense anthropogenic activities, such as the Mediterranean basin and the Po Valley in Italy. In May, the attenuation of DNI from aerosols, over these areas, can reach values up to 35% and 45% respectively, which corresponds to 4 and 6 kWh m⁻² per day. In most areas, even for periods with lower values of AOD, the attenuation of DNI is found to be around 20%, which corresponds to about 2–3 kWh m⁻² less received DNI per day, compared to the corresponding value on an aerosol clean day. However, the DNI has increased during the recent years, due to the decreasing tendency of AOD over most areas of Europe. The increase is around 6–12%, which corresponds to an amount of 0.5–1.25 more kWh m⁻² received per day, compared to a clean day. The percentage differences of daily DNI from the corresponding monthly climatological value reveals that day-to-day differences (due to AOD changes) from the monthly mean, by ±20%, can occur. The significance of the aerosol changes in Europe reveals the necessity for near real-time measurements or forecasts of AOD when reliable estimations of DNI are required.     

Estimation of hourly direct normal from measured global solar irradiance in Spain

The availability of a good data set, registered in six Spanish locations, including several radiometric variables, has been used to test different approaches for estimating hourly direct normal irradiance by decomposition models. Models proposed by different authors have been tested. Following this preliminary study, to improve the k_b–k_t correlations, another geometric variable has been used as a predictor of hourly beam transmittance, k_b, by means of piecewise correlations. The new beam transmittance correlations, which include additional geometric information, reduce the root mean square deviation. In addition, they show a better performance in terms of the determination coefficient of the regression analysis of measured vs calculated values, providing an improved capture of the real world effects than models that are function of the clearness index only. A new model that uses only two ranges of clearness index is proposed. The proposed model shows seasonal dependence and thus we have developed a seasonal version of it. However, the performance of the seasonal version has proved to be similar to the corresponding annual model.     

Evaluation of decomposition models of various complexity to estimate the direct solar irradiance over Belgium

Solar energy production is directly correlated to the amount of radiation received at a given location. Appropriate information on solar resources is therefore very important for designing and sizing solar energy systems. Concentrated solar power projects and photovoltaic tracking systems rely predominantly on direct normal irradiance (DNI). However, the availability of DNI measurements from surface observation stations has proven to be spatially too sparse to quantify solar resources at most potential sites. Satellite data can be used to calculate estimates of direct solar radiation where ground measurements do not exist. Performance of decomposition models of various complexity have been evaluated against one year of in situ observations recorded on the roof of the radiometric tower of the Royal Meteorological Institute of Belgium in Uccle, Brussels. Models were first evaluated on a hourly and sub-hourly basis using measurements of global horizontal irradiance (GHI) as input. Second, the best performing ground-based decomposition models were used to extract the direct component of the global radiation retrieved from Meteosat Second Generation (MSG) images. Results were then compared to direct beam estimations provided by satellite-based diffuse fraction models and evaluated against direct solar radiation data measured at Uccle. Our analysis indicates that valuable DNI estimation can be derived from MSG images over Belgium regardless of the satellite retrieved GHI accuracy. Moreover, the DNI retrieval from MSG data can be implemented on an operational basis.     

Semi-empirical models for the estimation of clear sky solar global and direct normal irradiances in the tropics

This paper presents semi-empirical models for estimating global and direct normal solar irradiances under clear sky conditions in the tropics. The models are based on a one-year period of clear sky global and direct normal irradiances data collected at three solar radiation monitoring stations in Thailand: Chiang Mai (18.78°N, 98.98°E) located in the North of the country, Nakhon Pathom (13.82°N, 100.04°E) in the Centre and Songkhla (7.20°N, 100.60°E) in the South. The models describe global and direct normal irradiances as functions of the Angstrom turbidity coefficient, the Angstrom wavelength exponent, precipitable water and total column ozone. The data of Angstrom turbidity coefficient, wavelength exponent and precipitable water were obtained from AERONET sunphotometers, and column ozone was retrieved from the OMI/AURA satellite. Model validation was accomplished using data from these three stations for the data periods which were not included in the model formulation. The models were also validated against an independent data set collected at Ubon Ratchathani (15.25°N, 104.87°E) in the Northeast. The global and direct normal irradiances calculated from the models and those obtained from measurements are in good agreement, with the root mean square difference (RMSD) of 7.5% for both global and direct normal irradiances. The performance of the models was also compared with that of other models. The performance of the models compared favorably with that of empirical models. Additionally, the accuracy of irradiances predicted from the proposed model are comparable with that obtained from some rigorous physical models.     

Prediction of direct and global solar irradiance using broadband models: Validation of REST model

In the present paper, three parametric models Yang, CPCR2 and REST (without considering transmittance due to nitrogen dioxide) have been analyzed for four Indian stations, namely New Delhi, Mumbai, Pune and Jaipur over the period of 1995–2002, under cloudless conditions. These stations have different climatic conditions. The beam radiation at normal incidence as well as global solar radiation at horizontal surface was computed for these locations during all seasons except monsoon (June to September). The computed values of beam and global irradiance was compared with reference values in case of beam and measured values in case of global solar radiation on the basis of percentage root mean square error (RMSE) and mean bias error (MBE). The maximum RMSE is 6.5% in REST model, as compare to 15% in Yang and 11% in CPCR2 model for predicting direct normal irradiance. The predicted global radiation at horizontal is showing maximum RMSE 7% in REST model, 13.4% in Yang and 25.9% in CPCR2 model. This shows that REST model has good agreement with measured data for these Indian stations as compare to other two models.     

Assessing the potential of concentrating solar photovoltaic generation in Brazil with satellite-derived direct normal irradiation

With the declining costs of flat plate and concentrating photovoltaic (PV) systems, solar PV generation in many sunny regions in Brazil will eventually become cost competitive with conventional and centralized power generation. Detailed knowledge of the local solar radiation resource becomes critical in assisting on the choice of the technology most suited for large-scale solar electricity generation. When assessing the energy generation potential of non-concentrating, fixed flat plate versus concentrating PV, sites with high levels of direct normal irradiation (DNI) can result in cost-competitive electricity generation with the use of high concentrating photovoltaic systems (HCPV). In large countries, where the transmission and distribution infrastructure costs and associated losses typical of centralized generation must be taken into account, the distributed nature of solar radiation should be perceived as a valuable asset. In this work we assess the potential of HCPV energy generation using satellite-derived DNI data for Brazil, a large and sunny country with a continental surface of 8.5 million km². The methodology used in the study involved the analysis of global horizontal, latitude-tilt, and direct normal solar irradiation data resulting from the Solar and Wind Energy Resource Assessment (SWERA) Project, and an estimate of the resulting electricity production potential, based on a review of HCPV generators operating at other sites. The satellite-derived solar irradiation data, with 10 km × 10 km spatial resolution, were analysed over the whole country, in order to identify the regions where HCPV might present a considerable advantage over fixed plate PV on an annual energy generation basis. Our results show that there is a considerable fraction of the national territory where the direct normal solar irradiation resource is up to 20% higher than the latitude-tilt irradiation availability. Furthermore, these sites are located in the most industrially-developed region of the country, and indicate that with the declining costs of this technology, distributed multi-megawatt HCPV can be a good choice of technology for solar energy generation at these sites in the near future.     

Uniqueness verification of direct solar spectral index for estimating outdoor performance of concentrator photovoltaic systems

To analyze the impact of a direct spectral distribution of the solar spectrum on the outdoor performance of concentrator photovoltaic (CPV) systems, an index for the direct spectral distribution is needed. Average photon energy (APE), the average energy of a photon in the direct solar spectrum, is one of these indexes. In this contribution, the uniqueness of APE to the direct solar spectral distribution is statistically analyzed to assure that an APE value uniquely yields the shape of a direct solar radiation spectrum. The results have exhibited the uniqueness of the direct normal solar spectrum with each APE value, in which the standard deviations are quite small. Short-circuit current density of the InGaP/InGaAs/Ge triple-junction solar cell in the CPV system is additionally calculated using the direct spectral irradiance with different APE values. It is revealed that APE is a useful index to describe the direct spectral distribution to evaluate the outdoor performance of the CPV systems.     

A comparative study of SPCTRAL2 and SMARTS2 parameterised models based on spectral irradiance measurements at Valencia, Spain

Results obtained using the parametric models SPCTRAL2 and SMARTS2 for the urban area of Valencia, Spain, have been analysed and compared with experimental measurements at ground level obtained with two Li-cor 1800 spectroradiometers with a 6 nm resolution. The study used two different input parameters in both models for the aerosol characterisation: the aerosol optical thickness at 0.5 μm, τ_aλ(0.5), and the Angstrom turbidity coefficient β. The results obtained show that both algorithms reproduce quite correctly the spectral irradiance experimental values when an urban aerosol model parameterised by the τ_aλ(0.5) value is considered. In all the cases the deviations are lower when SMARTS2 code is used.     

Comparative study of three different catalyst coating methods for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFCs) with membrane–electrode assemblies (MEAs) made separately by three different catalyst coating methods, namely, air-spray, electro-spray and dual-mode spray, is evaluated. Platinum–ruthenium (PtRu) is incorporated as a catalyst for the anode. Several techniques (XRD, FE-SEM, and TEM) are used to examine whether the coating method affects the morphological features of the PtRu catalyst, whereas cyclic voltammetry is used to evaluate the active surface area. The cell polarization curves attained for the three coating methods that use different methanol concentrations are compared to determine the best method. It is found that the PtRu catalyst coated by the dual-mode spray shows the most uniform nanoparticle distribution and the highest active surface area. The DMFC performance is best when the dual-mode spray is employed (165 mW cm⁻² at 2 M methanol).     

Effects of cathode flow fields on direct methanol fuel cell-simulation study

Flow-field design of direct methanol fuel cell (DMFCs) plays an important role affecting the cell performance. Previous studies suggest that the combination of anode parallel flow field and cathode serpentine flow-field present the best and stable performance. Among these, cathode flow-field holds higher influence than that of anode. However, more detailed experiments needed to be done to find out the reasons. In this study, CFDRC half-cell models are adopted to simulate the flow phenomena within serpentine, parallel and grid flow field. We find that gas is well distributed within serpentine flow field while barren region are observed within parallel flow field. These factors contribute to the cell performance greatly. In addition, the durability test of DMFCs using parallel flow field is improved when the flow rate is increased or the current is uphold at inferior, so the barren region maintained at an acceptable level.     

Sulfonated poly(arylene ether sulfone) membranes blended with hydrophobic polymers for direct methanol fuel cell applications

In this paper, proton exchange membranes for direct methanol fuel cells were prepared by blending sulfonated poly(arylene ether sulfone) with poly (vinylidene fluoride-co-hecafluoropropylene)(PVdF-HFP) and polyethersulfone (PES) to decrease methanol permeability while maintaining high proton conductivity. The content of the second polymer, such as PES and PVdF, in the blend membranes was controlled at 10–40 wt% based on SPAES. In order to investigate the effects of the second polymer content in the blended membranes, parameters of the prepared membranes related to fuel cell performance were characterized, including their morphology, mechanical properties, methanol permeability, and proton conductivity. Surface roughness of the blend membrane was increased by the introduction of a hydrophobic polymer. Mechanical properties of the PES/SPAES blend membrane were improved owing to interaction between the sulfonic acid groups in SPAES and PES. However, the tensile strength of the PVdF/SPAES blend membrane was decreased by due to the poor compatibility of SPAES and PVdF. The methanol permeability in the blended membranes decreased with increasing content of PES and PVdF. The SPAES/PES blend membranes exhibited good proton conductivity and lower methanol permeability than the SPAES membrane. The SVdF15 blend membrane showed the highest selectivity due to the absence of methanol crossover and a small decrease of proton conductivity. These blend membranes are suitable for DMFC applications.     

Poly (vinyl alcohol)/3-(trimethylammonium) propyl-functionalized silica hybrid membranes for alkaline direct ethanol fuel cells

A novel hybrid membrane based on poly (vinyl alcohol)/3-(trimethylammonium) propyl-functionalized silica (PVA-TMAPS) is prepared by a simple solution-casting method. The properties of the PVA-TMAPS membranes are characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD). It is found that the thermal stability of the membranes increases with the addition of TMAPS particles. Moreover, the study of the effect of different weight ratios of PVA to TMAPS on the OH⁻ conductivity shows that the membrane with a ratio of PVA:TMAPS = 90:10 exhibited the highest OH⁻ conductivity. Finally, it is shown that the application of the alkaline membrane (PVA:TMAPS = 90:10) to an direct ethanol fuel cell can yield a peak power density of about 50 mW cm⁻² at 60 °C.     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.     

Poly(isatin-piperidinium-terphenyl) anion exchange membranes with improved performance for direct borohydride fuel cells

In this work, an effective design strategy for anion exchange membranes (AEMs) incorporating ether-bond free and piperidinium cationic groups promote chemical stability. A series of poly (isatin-piperidium-terphenyl) based AEMs were synthesized by superacid catalyzed polymerization reaction, followed by quaternization. The effect of functionalization on the performance of poly (isatin-N-dimethyl piperidinium triphenyl) (PIDPT-x) AEMs was investigated. Highly reactive N-propargylisatin was introduced into the backbone to achieve high molecular weight polymers (η_a = 2.06–3.02 dL g⁻¹) leading to robust mechanical properties, as well as modulating 1.78–2.00 mmol g⁻¹ of the ion exchange capacity (IEC) of the AEMs by feeding. Apart from that, the rigid non-ionized isatin-terphenyl segment provides AEMs improved dimensional stability with a swelling ratio of less than 12% at 80 °C. Among them, PIDPT-90 exhibited a higher OH⁻ conductivity of 105.6 mS cm⁻¹ at 80 °C. The alkali-stabilized PIDPT-85 AEM was presented, in which OH⁻ conductivity retention maintained 85.6% in a 2 M NaOH at 80 °C after 1632 h. Afterward, the direct borohydride fuel cells (DBFC) with PIDPT-90 membrane as a separator showed an open-circuit voltage of 1.63 V and a peak power density of 75.5 mWcm⁻² at 20 °C. This work demonstrates the potential of poly (isatin- N-dimethyl piperidinium triphenyl) as AEM for fuel cells.     

Sensor-less control of methanol concentration based on estimation of methanol consumption rates for direct methanol fuel cell systems

Adequate control over the concentration of methanol is critically needed in operating direct methanol fuel cell (DMFC) systems, because performance and energy efficiency of the systems are primarily dependent on the concentration of methanol feed. For this purpose, we have built a sensor-less control logic that can operate based on the estimation of the rates of methanol consumption in a DMFC. The rates of methanol consumption are measured in a cell and the resulting data are fed as an input to the control program to calculate the amount of methanol required to maintain the concentration of methanol at a set value under the given operating conditions of a cell. The sensor-less control has been applied to a DMFC system employed with a large-size single cell and the concentration of methanol is found to be controlled stably to target concentrations even though there are some deviations from the target values.     

Fabrication and characterization of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) proton-conducting composite membranes for direct methanol fuel cells

A high performance poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) (PVA/MMT/PSSA) proton-conducting composite membrane was fabricated by a solution casting method. The characteristic properties of these blend composite membranes were investigated by using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, methanol permeability measurement, and the AC impedance method. The ionic conductivities for the composite membranes are in the order of 10⁻³ S cm⁻¹ at ambient temperature. There are two proton sources used on this novel composite membrane: the modified MMT fillers and PSSA polymer, both materials all contain the -SO₃H group. Therefore, the ionic conductivity was greatly enhanced. The methanol permeabilities of PVA/MMT/PSSA composite membranes is of the order of 10⁻⁷ cm² s⁻¹. It is due to the excellent methanol barrier properties of the PVA polymer. The peak power densities of the air-breathing direct methanol fuel cells (DMFCs) with 1M, 2M, 4M CH₃OH fuels were 14.22, 20.00, and 13.09 mW cm⁻², respectively, at ambient conditions. The direct methanol fuel cell with this composite polymer membrane exhibited good electrochemical performance. The proposed PVA/MMT/PSSA composite membrane is therefore a potential candidate for future applications in DMFC.     

Self-crosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells

A series of novel anion exchange membranes based on poly(arylene ether sulfone) were fabricated. And the synthesized 1, 1, 2, 3, 3-pentamethylguanidine was used as a hydrophilic group. Bromination reaction rather than chloromethylation was used for the preparation of target conductive polymers. Fourier transform infrared spectroscopy (FTIR), ¹H NMR and mass spectrometry (MS) were used to characterize the as-synthesized polymers. The ratio of hydrophilic to hydrophobic monomers was varied to study the structure-property of the membranes. The performance of the membrane with both hydrophilic/hydrophobic segments was improved over the membrane with sole hydrophilic segments. The self-crosslinking structure of the as-prepared membranes is partly responsible for their very low methanol permeability with the minimum of 1.02 × 10⁻⁹ cm⁻²⋅S⁻¹ at 30 °C and insolubility in organic solvents considered. The structural dependence of water uptake is in the range of 25–87%. The as-prepared membranes did not suffer from serious membrane swelling. The ionic exchange capacity (IEC) reached a maximum of 1.21 mmol⋅g⁻¹. The ionic conductivity of the membrane in deionized water is 6.00 and 13.00 × 10⁻² S⋅cm⁻¹ at 30 and 80 °C respectively.     

Comparative study between platinum supported on carbon and non-noble metal cathode catalyst in alkaline direct ethanol fuel cell (ADEFC)

In this study, we reported a comparison study between the performance of the commercial non-noble metal cathode electrode (made by Hypermec™ K14 catalyst - provided by Acta S.p.A.) and cathode electrode containing 10 wt% Pt/C, in the alkaline direct ethanol fuel cell (ADEFC) under different conditions.Further electrochemical investigations have been done by RDE and driven mode cell to compare the intrinsic activity and selectivity of 10 wt% Pt/C and Hypermec™ K14 transition metals cathode catalysts. It is worthwhile to point out that Hypermec™ K14 cathode catalyst shows a remarkable selectivity to oxygen reduction and it has a superior intrinsic activity in oxygen reduction reaction (ORR) especially in terms of volumetric current density (A/cm³). Test results of active DEFC made by non-noble cathode catalyst showed superior performance compared to the cell made by 10 wt% Pt/C cathode catalysts in terms of power density and OCV at 60 °C and ambient pressure. This result is related to the higher ORR kinetic of non-noble metal cathode catalyst in alkaline media.     

A study on the overall efficiency of direct methanol fuel cell by methanol crossover current

This paper was presented to determine the methanol crossover and efficiency of a direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol concentration, methanol flow rate, cathode flow rate, and cathode backpressure. The methanol crossover measurements were performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply. The membrane electrode assembly (MEA) with an active area of 5 cm² was composed of a Nafion 117 membrane, a Pt–Ru (4 mg/cm²) anode catalyst, and a Pt (4 mg/cm²) cathode catalyst. It was shown that methanol crossover increased by increasing cell temperature, methanol concentration, methanol flow rate, cathode flow rate and decreasing cathode backpressure. Also, it was revealed that the efficiency of the DMFC was closely related with methanol crossover, and significantly improved as the cell temperature and cathode backpressure increased and methanol concentration decreased.     

Proton-conducting membranes with high selectivity from cross-linked poly(vinyl alcohol) and poly(vinyl pyrrolidone) for direct methanol fuel cell applications

A series of hydrocarbon membranes consisting of poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA) and poly(vinyl pyrrolidone) (PVP) were synthesized and characterized for direct methanol fuel cell (DMFC) applications. Fourier transform infrared (FT-IR) spectra confirm a semi-interpenetrating (SIPN) structure based on a cross-linked PVA/SSA network and penetrating PVP molecular chains. A SIPN membrane with 20% PVP (SIPN-20) exhibits a proton conductivity value comparable to Nafion^® 115 (1.0 × 10⁻² S cm⁻¹ for SIPN-20 and 1.4 × 10⁻² S cm⁻¹ for Nafion^® 115). Specifically, SIPN membranes reveal excellent methanol resistance for both sorption and transport properties. The methanol self-diffusion coefficient through a SIPN-20 membrane conducted by pulsed field-gradient nuclear magnetic resonance (PFG-NMR) technology measures 7.67 × 10⁻⁷ cm² s⁻¹, which is about one order of magnitude lower than that of Nafion^® 115. The methanol permeability of SIPN-20 membrane is 5.57 × 10⁻⁸ cm² s⁻¹, which is about one and a half order of magnitude lower than Nafion^® 115. The methanol transport behaviors of SIPN-20 and Nafion^® 115 membranes correlate well with their sorption characteristics. Methanol uptake in a SIPN-20 membrane is only half that of Nafion^® 115. An extended study shows that a membrane-electrode assembly (MEA) made of SIPN-20 membrane exhibits a power density comparable to Nafion^® 115 with a significantly higher open current voltage. Accordingly, SIPN membranes with a suitable PVP content are considered good methanol barriers, and suitable for DMFC applications.