Sky Days Research Articles

Operation optimization strategy for wind-concentrated solar power hybrid power generation system

This paper presents a new hybrid system to reduce wind curtailment and improve scheduling flexibility. This hybrid system includes a wind farm, a concentrated solar power plant with thermal energy storage, and an electric heater. The major role of the electric heater is to convert the redundant wind power into thermal energy, and the thermal energy is stored in the thermal energy storage of the concentrated solar power plant. The optimal scheduling of this hybrid system is formulated as a mixed-integer linear programming problem to maximize the profit subjected to technical constraints. The effects of the electric heater on the system are studied under different weather conditions. The test results show that the electric heater is helpful for reduction of the both deviation from generation plan and wind curtailment. The maximum relative deviation falls from 5.15% to 0% during the clear sky day, and 47.49% to 31.74% during the partial cloudy day. The wind curtailment rate decreases by 52.59% and 100% for clear and partial cloudy days, respectively. An annual simulation for the system shows that the overall daily cumulative deviations of the new system are significantly decreased, and the wind curtailment can be reduced by greater than 90% for 151 days, validating the effectiveness of the proposed system.

Experiment Investigation on a Solar Asisted Heat Pump Dryer for Chili

The aim of this research is to evaluate the performance of solar assisted heat pump dryer for chilli. The experiments were carried out on three types different of days such as clear day, intermittent cloud sky day and overcast sky day. The experimental results show that the useful heat gain of solar collectors for drying chili of the clear day, intermittent cloud sky day and overcast sky day are 14.5 kWh; 8.91 kWh and 0 kWh, respectively, and the corresponding energy consumptions of solar- assisted heat pump dryer are 19.2 kWh, 21.4 kWh and 26.1 kWh, respectively. The coefficients of performance (COP) in the three cases are 3.45, 3.46, and 3.51, respectively.

Parametric Study on the Thermal Performance and Optimal Design Elements of Solar Air Heater Enhanced with Jet Impingement on a Corrugated Absorber Plate

Previous works revealed that cross-corrugated absorber plate design and jet impingement on a flat absorber plate resulted in a significant increase in the performance of a solar air heater (SAH). Involving these two designs into one continuous design to improve the SAH performance remains absent in the literature. This study aimed to evaluate the achieved enhancement on performance parameters of a SAH with jet impingement on a corrugated absorber plate. An energy balance model was developed to compare the performance parameters of the proposed SAH with the other two SAHs. At a clear sky day and a mass flow rate of 0.04 kg/s, the hourly results revealed that the max fluid outlet temperatures for the proposed SAH, jet-to-flat plate SAH, and cross-corrugated plate SAH are 321, 317, and 313 K, respectively; the max absorber plate temperatures are 323.5, 326.5, and 328 K, respectively; the maximum temperature differences between the absorber plate and fluid outlet are ~3, 9, and 15 K, respectively; the max efficiencies are 65.7, 64.8, and 60%, respectively. Statistical t-test results confirmed significant differences between the mean efficiency of the proposed SAH and SAH with jet-to-flat plate. Hence, the proposed design is considered superior in improving the performance parameters of SAH compared to other designs.

Energetic end exergetic performance of a parabolic trough collector receiver: An experimental study

In this article, an experimental investigation is evaluated with the aim of assessing the thermal performance of a receiver tube of a parabolic trough collector. The parabolic trough collector is designed, constructed and installed in the Laboratory of Thermal Processes, Research and Technology Center of Energy (CRTEn), Borj Cedria. The thermal performance was carried out using both first and second law of thermodynamics. A detailed energy and exergy analysis was performed to evaluate the thermal performance. Throughout the study, the useful energy gain and the receiver tube exergy transferred to the heat transfer fluid, the energy and exergy efficiency as well as the exergy factor were measured. The measurements were conducted during the months of September and October 2015 at Borj Cedria-Tunisia. The result has shown that the thermal performance is strongly influenced by climatic conditions. The average energy and exergy efficiency are found to be higher under clear sky days than the overcast cloudy sky days. The daily energy efficiency varied between 19.7% and 52.6%. While the daily exergy efficiency varied from 8.51% to 16.34%.

Estimation of instantaneous air temperature using remote sensing data

ABSTRACTIn arid areas, the variation of air temperature can be considerable, so instantaneous air temperature (Tai) estimation is needed in different environmental researches. In this research, two different remote sensing data are used for estimating Tai for clear sky days in 2009 in Fars Province, Iran, including atmospheric temperature profile and land surface temperature (LST) data from Moderate Resolution Imaging Spectroradiometer. The Tai from a number of surface weather sites is used to judge the best Tai estimation. Stations’ elevation, latitude, and land cover type are considered to show their effect on Tai estimation. The estimated Tai evaluation focuses on daily and seasonal timescales in the daytime and night time separately. Both LST and vertical temperature profile data produced relatively high coefficient of determination values and small root mean square error value for Tai estimation, especially during the night time. Land cover and elevation vary the error values in Tai estimation more, when LST data is used. In comparison atmospheric temperature profile indicates a smaller error in Tai estimation in spring and summer and in urban land cover type, while using LST data presents a better result in fall and winter especially at night time.

Impact of the photovoltaic system variability on transformer tap changer operations in distribution networks

This study aims to quantify the transformer tap changer operations on distribution networks with large penetration of photovoltaic systems. The influence of solar generation variability on transformer tap changer is investigated by using five different solar variability day types collected in Malaysia with 1 min resolution. The case studies have been performed on a Malaysian representative distribution network. Results show that high solar variability day could increase the transformer's tap changes by 600% as compared to the clear sky day. The transformer's time delay setting plays a vital role in limiting the tap operations.

Thermal Efficiency of Natural Convection Solar Dryer

The purpose of this research is to study the thermal efficiency of natural convection. The working principle of natural convection solar dryer is, once the air in the solar dryer is heated by solar energy, the air relative humidity will drop and floating up through the drying. This air will take moisture out of the product and flow out to the ambient air. It was found from the experiment that, in the duration of 8.00 am – 4.00 pm on a clear sky day, an all-day average ambient air and inside the chamber temperature were 38.34°C and 63.19°C respectively. At the solar radiation intensity of 759.53W/m2, mass flow rate of air was 0.023 kg/s and the thermal efficiency of the solar dryer was 2.59%.

Evaluation of the QUIC-URB wind solver and QESRadiant radiation-transfer model using a dense array of urban meteorological observations

This study assesses the performance of QUIC-URB, a fast-response urban flow model, and QESRadiant, a ray tracing radiation transfer model. Both models are components of the QUIC EnvSim 3D urban micro-scale model, which aims to simulate meteorological variables at high spatiotemporal resolution (~1min, ~1m) in urban settings. The evaluation was performed over a 5.85ha sector of a university campus, in which complex 3D building geometry, vegetation, and various surface materials were modelled. First, wind speeds computed with QUIC-URB were compared to 30-min measurements over 10days at 19 locations. Although results showed a significant underestimation for locations in the wake of buildings, considering model assumptions, its inexpensive computational cost, and measurement uncertainty, the agreement between computed and measured wind speeds is good (r2=0.53, mean absolute error=0.68ms−1). Second, incoming radiation computed with QESRadiant was compared to 2-min measurements over seven clear sky days at 17 locations. Overall, the agreement between computed and measured incoming solar radiation was excellent (r2=0.95). For both models, simulations were run successfully on a standard laptop machine with highly reasonable computational cost, on the order of minutes.

An Iberian climatology of solar radiation obtained from WRF regional climate simulations for 1950–2010 period

The mesoscale Weather Research and Forecasting (WRF) Model is used over the Iberian Peninsula to generate 60years (1950–2010) of climate data, at 5km resolution, in order to evaluate and characterize the incident shortwave downward radiation at the surface (SW↓), in present climate.The simulated values of SW↓ in the period 2000–2009 were compared with data measured in Spanish and Portuguese meteorological stations before and a statistical BIAS correction was applied using data from Clouds and the Earth's Radiant Energy System (CERES), on board four different satellites. The spatial and temporal comparison between WRF results and observations show a good agreement for the analyzed period, although the model overestimates observations. This overestimation has a mean normalized bias of about 7% after BIAS correction (or 17% for original WRF output). Additionally, the present simulation was confronted against another previously validated WRF simulation performed with different resolution and set of parametrizations, showing comparable results. WRF adequately reproduces the observational features of SW↓ with correlation coefficients above 0.8 in annual and seasonal basis.60years of simulated SW↓ over the Iberian Peninsula were produced, which showed annual mean values that range from 130W/m2, in the northern regions, to a maximum of around 230W/m2 in the southeast of the Iberian Peninsula (IP). SW↓ over IP shows a positive gradient from north to south and from west to east, with local effects influenced by topography and distance to the coast.The analysis of the simulated cloud fraction indicates that clear sky days are found in >30% of the period at the southern area of IP, particularly in the Algarve (Portugal) and Andalusia (Spain), and this value increases significantly in the summer season for values above 80%.

Performance of basin type stepped solar still enhanced with superior design concepts

It is difficult maintaining a minimum water depth in a conventional basin type solar still, as the required area is quite large. To overcome this difficulty, the R&D community proposed a stepped solar still in an attempt to increase the production per unit area by decreasing the thermal inertia of the water mass, where the area of the basin is minimized via the utilization of small trays. However, stepped solar still is still potential for further enhancement. It is leaned from the literature, that adding internal and external reflectors, absorber materials (fins) and external condensers are considered superior in enhancing the absorption, evaporation and condensation processes of the basin type solar still. Combining these design concepts concurrently with stepped solar still to enhance its performance is still a missing link in the literature. In this study, a previous work of stepped solar still is selected to apply these modifications on it and propose it as a new design. Energy balance model is developed to compare the performance of the stepped solar still before and after modification. The energy balance results are obtained by solving the energy balance equations for various elements: absorber plate, saline water and glass cover of the solar still. Hourly solar radiation and hourly ambient temperature of clear sky day conditions are used as input data in the energy balance model. The hourly performance of the stepped solar still is compared before and after modification under the following evaluation parameters: temperature difference between saline water and glass cover, evaporation/convective/radiative heat transfer coefficients, solar still productivity and solar still efficiency. The results showed that the hourly values of evaluation parameters after modification are always higher of that before modification. This increment is tested statistically to confirm its significance. So, the differences in the mean values of each evaluation parameter before and after modification are tested by statistical paired t-test. The test results confirmed that there is a significant difference in the mean values of each evaluation parameter before and after modification. Moreover, the daily productivity of the stepped solar still after modification increased from 6.9 to 8.9kg/m2; this represents 29% enhancement compared to before modification. Finally, based on the results of the evaluation parameters and the statistical test, the thermal performance of the proposed stepped solar still is considerably improved through the new modification.

Calibration of METRIC Model to Estimate Energy Balance over a Drip-Irrigated Apple Orchard

A field experiment was carried out to calibrate and evaluate the METRIC (Mapping EvapoTranspiration at high Resolution Internalized with Calibration) model for estimating the spatial and temporal variability of instantaneous net radiation (Rni), soil heat flux (Gi), sensible heat flux (Hi), and latent heat flux (LEi) over a drip-irrigated apple (Malus domestica cv. Pink Lady) orchard located in the Pelarco valley, Maule Region, Chile (35°25′20′′LS; 71°23′57′′LW; 189 m.a.s.l.). The study was conducted in a plot of 5.5 hectares using 20 satellite images (Landsat 7 ETM+) acquired on clear sky days during three growing seasons (2012/2013, 2013/2014 and 2014/2015). Specific sub-models to estimate Gi, leaf area index (LAI) and aerodynamic roughness length for momentum transfer (Zom) were calibrated for the apple orchard as an improvement to the standard METRIC model. The performance of the METRIC model was evaluated at the time of satellite overpass using measurements of Hi and LEi obtained from an eddy correlation system. In addition, estimated values of Rni, Gi and LAI were compared with ground-truth measurements from a four-way net radiometer, soil heat flux plates and plant canopy analyzer, respectively. Validation indicated that LAI, Zom and Gi were estimated using the calibrated functions with errors of +2%, +6% and +3% while those were computed using the standard functions with error of +59%, +83%, and +12%, respectively. In addition, METRIC using the calibrated functions estimated Hi and LEi with error of +5% and +16%, while using the original functions estimated Hi and LEi with error of +29% and +26%, respectively.

Blue Skies Over Beijing: Olympics, Environments, and the People’s Republic of China

During the 2008 Olympic Games, after years of environmental regulations, two months of short-term measures, and opportune weather, Beijing measured a record number of “blue sky days,” at the same time reassuring international athletes and journalists the air was safe for competition and Beijing residents. We use this case to understand how environmental objectives are achieved in sport. Using Bruno Latour’s object-oriented political ecology, we describe the events leading to, during, and after the Games. We argue environmental objectives are possible when environments are made public; this means environmental objects—such as skies and particulate matter—must be assembled and then articulated or, in other words, brought forward and made capable of speech.

Plume Segmentation from UV Camera Images for SO2 Emission Rate Quantification on Cloud Days

We performed measurements of SO2 emissions with a high UV sensitive dual-camera optical system. Generally, in order to retrieve the two-dimensional SO2 emission rates of a source, e.g., the slant column density of a plume emitted by a stack, one needs to acquire four images with UV cameras: two images including the emitting stack at wavelengths with high and negligible absorption features (λon/off), and two additional images of the background intensity behind the plume, at the same wavelengths as before. However, the true background intensity behind a plume is impossible to obtain from a remote measurement site at rest, and thus, one needs to find a way to approximate the background intensity. Some authors have presented methods to estimate the background behind the plume from two emission images. However, those works are restricted to dealing with clear sky, or almost homogeneously illuminated days. The purpose of this work is to present a new approach using background images constructed from emission images by an automatic plume segmentation and interpolation procedure, in order to estimate the light intensity behind the plume. We compare the performance of the proposed approach with the four images method, which uses, as background, sky images acquired at a different viewing direction. The first step of the proposed approach involves the segmentation of the SO2 plume from the background. In clear sky days, we found similar results from both methods. However, when the illumination of the sky is non homogeneous, e.g., due to lateral sun illumination or clouds, there are appreciable differences between the results obtained by both methods. We present results obtained in a series of measurements of SO2 emissions performed on a cloudy day from a stack of an oil refinery in Montevideo City, Uruguay. The results obtained with the UV cameras were compared with scanning DOAS measurements, yielding a good agreement.

A new ground-based FTIR spectrometer reference site at Shadnagar (India) and preliminary columnar retrievals of CH4 and N2O

ABSTRACTA Fourier Transform Infrared (FTIR) spectrometer has been operational since March 2014, at National Remote Sensing Centre (NRSC), Shadnagar, which is a suburban site of central India. Direct solar radiation spectra recorded by the FTIR during clear sky days at a high spectral resolution of 0.01 cm‒1 were analysed using the line-by-line radiative transfer algorithm (LBLRTA) to retrieve the initial results of column-averaged concentrations of methane (XCH4) and nitrous oxide (XN2O) in the atmosphere. Error residuals between the measured and estimated atmospheric transmissions are in the range of 0.40–0.80% for CH4 and 0.20–0.50% for N2O, respectively. The maximum (minimum) XCH4 and XN2O were, respectively, 1.88 ± 0.04 ppm (1.67 ± 0.02 ppm) and 316.15 ± 5.85 ppb (278.92 ± 4.02 ppb). Columnar concentrations of CH4 and N2O retrieved from the Infrared Atmospheric Sounding Interferometer (IASI) were compared against retrieved XCH4 and XN2O using the ground FTIR. The observed differences between the two data sets lie between −0.06 ppm and +0.10 ppm for CH4 and between +32 ppb and ‒10 ppb for N2O.

Annual performance evaluation (energy and exergy) of fully covered concentrated photovoltaic thermal (PVT) water collector: An experimental validation

In the present communication, an experimental set up of single unit of fully covered concentrated photovoltaic thermal (CPVT) collector is designed and fabricated to achieve thermal as well as electrical gain. This system is installed at the rooftop of the building of Centre for energy studies, IIT Delhi, India. Here, semitransparent PV module is used to cover the collector. The monthly observations are taken in clear sky day condition for (September 2015 to August 2016). Two cases are considered on the basis of receiver rotation according to sun movement, to study the annual behaviour of present system, case (i): fixed position and case (ii): manual maximum power point tracking technique (M-MPPT). The manual tracking is adopted for 3 times/day in a 3h interval (09.00–12.00–15.00h). Consecutive days are selected for taking observation for both cases (i-ii) in each month throughout the year. The validation has been found to be fair agreement between theoretical and experimental results by calculating correlation coefficient (r) and error (e). It was found that case (ii) is dominating for overall thermal energy as well as exergy gain. The annual net thermal energy and exergy, obtained by case (ii), were 1.25 and 1.19 times higher than for case (i).

The stability of the radiative regime does influence the daily performance of solar air heaters

The dependence of the daily photothermal conversion performance on the stability of the radiative regime has been rarely treated in literature and only for systems based on water collectors. The objective here is to estimate whether the daily performance of solar air collectors is dependent on the radiative regime characteristics other than the level of daily solar irradiation. Results are obtained by comparing the performance of two solar air collectors whose design is almost similar but one has a porous absorber and the other has a U-corrugated absorber. First, the daily performance of the collectors are analyzed experimentally during clear sky days in Bucharest (Romania, South Eastern Europe). The instantaneous performance of the collector based on porous absorber is generally higher than that of the collector based on U-corrugated absorber. Second, dynamic models are developed and validated against measurements obtained in Bucharest. Two new performance indicators specific to time dependent operation are defined. It is shown that these indicators equal each other at steady state but in transitory operation they have different values. Simulations are performed for collectors operation under the climate of Timisoara (Romania). Eight days, covering all four seasons and belonging to different relative sunshine classes and different radiative regime stability levels are selected. At daily level, the collector based on porous absorber is more effective than the collector based on U-corrugated absorber. When the instantaneous performance is considered, the U-corrugated absorber may provide better results in the morning. These findings do not depend on the stability of the radiative regime. The dispersion of the instantaneous performance values is higher for less stable radiative regimes during days with medium cloudiness (daily relative sunshine σday between 0.4 and 0.7) and during days with fully stable radiative regime and overcast sky (σday=0.0). Each collector has a specific behavior when the level of radiative stability and the class of relative sunshine change. During time intervals of transitory operation the traditional coefficient of performance may be used as a substitute of the time averaged value of the instantaneous coefficient of performance. However, the traditional collector efficiency is not a reliable measure of the time averaged value of the instantaneous efficiency.

Estimation of water requirements for a drip-irrigated apple orchard using Landsat 7 satellite images

A study was carried out to evaluate the METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) model to estimate water requirements or actual evapotranspiration (ETa) for a drip-irrigated apple orchard located in the Maule Region, Chile (lat. 35°25'S; long. 71°23'W; 189 m above mean sea level). For estimating ETa using the METRIC model, seven satellite images (Landsat 7 ETM+) acquired during clear sky days were used from the 2012-2013 growing season. The performance of METRIC was evaluated using measurements of ETa from an Eddy Covariance system (EC) at the time of satellite overpass (11:30 h). The statistical analysis indicated that the METRIC model overestimated ETa values by about 7% with a root mean square error (RMSE), mean absolute error (MAE) and index of agreement (d) of 0.42 mm d-1, 0.29 mm d-1 and 0.88, respectively. Main errors of the METRIC model were associated with the selection of the hot pixels which were difficult to obtain for some satellite images.

The Effect of Solar Reflective Cover on Soak Air Temperature and Thermal Comfort of Car Parked under the Sun

Parking a vehicle under the sun for a short period of time can rapidly increase the interior air cabin temperature no matter in clear sky days or even in partially cloudy days. These circumstances can be anxieties to car occupants upon entry. The aim of this paper is to evaluate experimentally the effect of solar reflective cover (SRC) on vehicle air temperature and cabin thermal comfort. Experimental measurements of parked cars were conducted in UKM, Bangi city, Malaysia (latitude of 2.9° N and longitude of 101.78° E) under partially cloudy day where average ambient temperature is 33°C. The experimental measurements cover the following cases: case (I): car with/ without SRC (at different measurement time); Case (II): using two identical cars concurrently (SRC versus baseline); Case (III): using two identical cars concurrently (solar reflective film (SRF) versus baseline) and Case (IV): using two identical cars concurrently (SRF versus SRC). Experimental results dedicated to case (I) revealed that the maximum cabin air temperature with SRC (39.6°C) is significantly lower than that of baseline case (57.3°C). This leads to temperature reduction improvement of 31% and the difference between the cabin and the ambient air temperature was minimized by approximately 73%. In addition, the results revealed that the air temperature at breath level of car with SRC dropped to comfort temperature (27°C) after 7 min while baseline car reached comfort temperature after 14 min. Results of the other cases are discussed inside the paper. Overall, it is learned that SRC is found superior as an efficient thermal insulation system limits solar radiation transmission into the cabin through the glass; keeps cabin air temperature close to the ambient temperature; and provide acceptable thermal environment to the occupants as they settle into their parked car.

The influence of the El Niño Southern Oscillation on heat waves in India

The present study investigated the relationship between the El Nino Southern Oscillation (ENSO) and heat wave characteristics, such as duration, annual number of heat wave days and maximum temperature of heat wave events, in India. El Nino is associated with an eastward shift in the Walker circulation and breakdown of circulation patterns, resulting in delay in the onset of the Indian Summer Monsoon (ISM). Because most heat waves in India occur during the pre-monsoon season, the present study showed that heat waves during El Nino years were longer and hotter, and it is argued that this is related to a delay in the onset of the ISM. Further, it was found that the shift in the circulation pattern due to El Nino resulted in (1) the weakening of southwesterlies in the Arabian Sea, and (2) the occurrence of a large number of clear sky days over India. These explain the occurrence of warmer and longer duration heat waves during El Nino years. These results imply that, if El Nino activity increases in the future, heat waves in India are likely to intensify.

A paradox for air pollution controlling in China revealed by "APEC Blue" and "Parade Blue".

A series of strict emission control measures were implemented in Beijing and surrounding regions to ensure good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit and 2015 Grand Military Parade (Parade), which led to blue sky days during these two events commonly referred to as “APEC Blue” and “Parade Blue”. Here we calculated Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and Ozone Monitoring Instrument (OMI) NO2 and HCHO results based on well known DOAS trace gas fitting algorithm and WRF-Chem model (with measured climatology parameter and newest emission inventor) simulated trace gases profiles. We found the NO2 columns abruptly decreased both Parade (43%) and APEC (21%) compared with the periods before these two events. The back-trajectory cluster analysis and the potential source contribution function (PSCF) proved regional transport from southern peripheral cities plays a key role in pollutants observed at Beijing. The diminishing transport contribution from southern air mass during Parade manifests the real effect of emission control measures on NO2 pollution. Based on the ratios of HCHO over NO2 we found there were not only limited the NO2 pollutant but also suppress the O3 contaminant during Parade, while O3 increased during the APEC.