Spectral Pyranometer Research Articles

Improved Methodology to Measure Normal Incident Solar Radiation with a Multi-pyranometer Array

An improved methodology to estimate the normal incident solar radiation based on an anisotropic clear sky model using a Multi-Pyranometer Array (MPA) is analyzed in this paper. The MPA is a static platform, which is used to measure solar energy irradiation at different tilt and azimuth angles, and that can be used to estimate the normal incident component without the tracking devices that require more detailed installation and maintenance. The array's sensors are of the photovoltaic type, which require both the spectral and incidence angle corrections. These corrections were made by empirical functions; the one for the incidence angle is generic and the spectral effect is satisfied by calibrating each of the sensors with a Precision Spectral Pyranometer (PSP) with a four parameter change- point linear function. For this study, the calibrated data was verified by calculating their corresponding theoretical value based on the beam component from solar tracking Normal Incidence Pyrheliometer (NIP) and the diffuse component from a Black and White Pyranometer (B&W), which were located on the same test bench.The improved procedure is based on grouping the measurements of two sensors and analytically look for a satisfactory solution; one for the horizontal and south sensors and one for the east and west sensors. The solution for the east and west sensors is expanded to two new solutions by mirroring their readings according to the solar noon; each real measurement and its mirrored values generate a solution. The solutions are grouped to have a more steady and robust expression. A final switching scheme provides the value that was compared with the reference NIP measurement. The analysis of the results showed that the root square mean error (RMSE) of the sample decreased by a 30% in comparison with previous results reported in the literature. The data sample used in this study correspond to clear sky conditions on the fall and winter seasons, so further comparison on other seasons and with regular solar conditions is required.

Determination of Aerosol Characteristics and Direct Radiative Forcing at Pune

Simultaneous measurements of aerosol optical depth and incoming global solar flux were carried out with a MICROTOPS-II Sunphotometer and Eppley Precision Spectral Pyranometer over Nowrosjee Wadia College (NWC, Pune) as the nodal centre, and at Pune University (PU) and IUCAA Girwali Observatory (IGO), in a field campaign mode during December 2010-May 2011. Data was analyzed to determine the surface short-wave (SW) aerosol direct radiative forcing (ADRF, ΔF per unit 0.1 AOD) and to study the characteristics of the aerosols. The results indicate that ADRF shows significant day-to-day variability and co-varies with AOD. The cause of the day-to-day variation in ADRF is an anticorrelation between ADRF/AOD, and differences in the daily maximum minus minimum RH and temperature. At NWC, ADRF ranges between -37.7 W/m^2 (highest) and -5.9 W/m^2 (lowest). For 500 nm, ADRF takes values in the range -17.3 ± 7.1 W/m^2 to -54.2 ± 5.5 W/m^2 at PU, whereas the corresponding values at IGO are -15.1 ± 2.1 W/m^2 and -36.6 ± 6.4 W/m^2. Monthly ADRF is at the minimum level in winter and maximum during the pre-monsoon period. The magnitude of AOD shows significant diurnal variability. In winter, the mean AOD diurnal percentage departure at 500 nm is positive in the morning and negative during the afternoon, and this is reversed in the pre-monsoon period. The diurnal cycle of AOD is related to the prevalent meteorological conditions, surface-based nocturnal temperature inversion in the atmospheric boundary layer (ABL), and influx of aerosols from different source regions.

Performance evaluation of a hot-box reflector solar cooker using a microcontroller-based measurement system

The performance of a low-cost compound box-reflector solar cooker designed and constructed by the department of Mechanical Engineering, at the University of Zimbabwe, was investigated and evaluated using a microcontroller-based measurement system over a period of 3 months. Solar radiation and temperature measurements are sent directly to the computer for monitoring and subsequent analysis using a spreadsheet program. The system is connected to the computer through the RS232 port. Temperature was measured by LM335 temperature sensors, whereas solar radiation was measured by a Kipp & Zonen CM3 thermopile-based pyranometer that was initially calibrated against the Eppley Precision Spectral Pyranometer. Peak temperatures of about 90°C for the food can be attained in about 5 h on a clear day in Bindura, Zimbabwe (18°S, 31°E). A standardized cooking power of 11 W and an overall efficiency of 15% were found for this cooker. Copyright © 2008 John Wiley & Sons, Ltd.

A Climatology of Midlatitude Continental Clouds from the ARM SGP Central Facility: Part I: Low-Level Cloud Macrophysical, Microphysical, and Radiative Properties

Abstract A record of single-layer and overcast low cloud (stratus) properties has been generated using approximately 4000 h of data collected from January 1997 to December 2002 at the Atmospheric Radiation Measurement (ARM) Southern Great Plains Central Facility (SCF). The cloud properties include liquid-phase and liquid-dominant mixed-phase low cloud macrophysical, microphysical, and radiative properties including cloud-base and -top heights and temperatures, and cloud physical thickness derived from a ground-based radar and lidar pair, and rawinsonde sounding; cloud liquid water path (LWP) and content (LWC), and cloud-droplet effective radius (re) and number concentration (N) derived from the macrophysical properties and radiometer data; and cloud optical depth (τ), effective solar transmission (γ), and cloud/top-of-atmosphere albedos (Rcldy/RTOA) derived from Eppley precision spectral pyranometer measurements. The cloud properties were analyzed in terms of their seasonal, monthly, and hourly variations. In general, more stratus clouds occur during winter and spring than in summer. Cloud-layer altitudes and physical thicknesses were higher and greater in summer than in winter with averaged physical thicknesses of 0.85 and 0.73 km for day and night, respectively. The seasonal variations of LWP, LWC, N, τ, Rcldy, and RTOA basically follow the same pattern with maxima and minima during winter and summer, respectively. There is no significant variation in mean re, however, despite a summertime peak in aerosol loading. Although a considerable degree of variability exists, the 6-yr average values of LWP, LWC, re, N, τ, γ, Rcldy, and RTOA are 151 gm−2 (138), 0.245 gm−3 (0.268), 8.7 μm (8.5), 213 cm−3 (238), 26.8 (24.8), 0.331, 0.672, and 0.563 for daytime (nighttime). A new conceptual model of midlatitude continental low clouds at the ARM SGP site has been developed from this study. The low stratus cloud amount monotonically increases from midnight to early morning (0930 LT), and remains large until around local noon, then declines until 1930 LT when it levels off for the remainder of the night. In the morning, the stratus cloud layer is low, warm, and thick with less LWC, while in the afternoon it is high, cold, and thin with more LWC. Future parts of this series will consider other cloud types and cloud radiative forcing at the ARM SCF.

Determination of the thermal offset of the Eppley precision spectral pyranometer

Eppley's precision spectral pyranometer (PSP) is used in networks around the world to measure downwelling diffuse and global solar irradiance at the surface of the Earth. In recent years several studies have shown significant discrepancy between irradiances measured by pyranometers and those computed by atmospheric radiative transfer models. Pyranometer measurements have been questioned because observed diffuse irradiances sometimes are below theoretical minimum values for a pure molecular atmosphere, and at night the instruments often produce nonzero signals ranging between +5 and -10 W m(-2). We install thermistor sondes in the body of a PSP as well as on its inner dome to monitor the temperature gradients within the instrument, and we operate a pyrgeometer (PIR) instrument side by side with the PSP. We derive a relationship between the PSP output and thermal radiative exchange by the dome and the detector and a relationship between the PSP output and the PIR thermopile output (net-IR). We determine the true PSP offset by quickly capping the instrument at set time intervals. For a ventilated and shaded PSP, the thermal offset can reach -15 W m(-2) under clear skies, whereas it remains close to zero for low overcast clouds. We estimate the PSP thermal offset by two methods: (1) using the PSP temperatures and (2) using the PIR net-IR signal. The offset computed from the PSP temperatures yields a reliable estimate of the true offset (+/-1 W m(-2)). The offset computed from net-IR is consistent with the true offset at night and under overcast skies but predicts only part of the true range under clear skies.

Quantification of Solar Heating of the Dome of a Pyrgeometer for a Tropical Location: Ilorin, Nigeria

Abstract A two-year database (September 1992–August 1994) on global solar radiation and downward infrared radiation measured at Ilorin, Nigeria (8°32′N, 4°34′E), using a precision spectral pyranometer and pyrgeometer, respectively, was analyzed so as to quantify the dome heating effect for this station in particular and the Tropics in general. Eight clear days with a clearness index greater than 0.60 were chosen, and the analysis was done on both hourly and daily bases. Only those hours in which insolation flux was greater than 100 W m−2 were chosen. The results show that on clear days the flux values could be 12.4% too high on an hourly basis, 10.3% too high on a monthly mean diurnal basis, and 3.7% too high on a daily basis. On a cloudy day the flux was only 0.58% (maximum) too high. These findings compare very favorably with the findings of other investigators. The shading of the dome, as sometimes used by investigators to take care of the dome heating effect, is found to be highly inadequate, especial...

A 25‐month database of stratus cloud properties generated from ground‐based measurements at the Atmospheric Radiation Measurement Southern Great Plains Site

A 25‐month database of the macrophysical, microphysical, and radiative properties of isolated and overcast low‐level stratus clouds has been generated using a newly developed parameterization and surface measurements from the Atmospheric Radiation Measurement central facility in Oklahoma. The database (5‐min resolution) includes two parts: measurements and retrievals. The former consist of cloud base and top heights, layer‐mean temperature, cloud liquid water path, and solar transmission ratio measured by a ground‐based lidar/ceilometer and radar pair, radiosondes, a microwave radiometer, and a standard Eppley precision spectral pyranometer, respectively. The retrievals include the cloud‐droplet effective radius and number concentration and broadband shortwave optical depth and cloud and top‐of‐atmosphere albedos. Stratus without any overlying mid or high‐level clouds occurred most frequently during winter and least often during summer. Mean cloud‐layer altitudes and geometric thicknesses were higher and greater, respectively, in summer than in winter. Both quantities are positively correlated with the cloud‐layer mean temperature. Mean cloud‐droplet effective radii range from 8.1 μm in winter to 9.7 μm during summer, while cloud‐droplet number concentrations during winter are nearly twice those in summer. Since cloud liquid water paths are almost the same in both seasons, cloud optical depth is higher during the winter, leading to greater cloud albedos and lower cloud transmittances.

New empirical relationships for determining global PAR from measurements of global solar radiation, infrared radiation or sunshine duration

Data on global solar radiation (H), global photosynthetically-active radiation (PAR), downward infrared sky radiation (IR) and sunshine duration (n) for a 2-year duration are analysed on monthly (monthly mean of daily values) basis at Ilorin (8°32′N, 4°34′E), Nigeria. The data were taken using a precision spectral pyranometer, quantum sensor, pyrgeometer and Campbell–Stokes recorder, respectively. Two thermistor circuits were built into the pyrgeometer for monitoring the dome and body temperatures so as to take care of the dome heating effect. From monthly values of IR, PAR and H, a correlation model relating the three parameters was established and is of the form: (1) where fp is the ratio of PAR to H. Further analysis showed that the use of maximum likelihood quadratic fit to relate the three parameters does not improve the coefficient of determination. Also from the monthly mean values of PAR and n, equations of the Angstrom–Prescott one-parameter model of linear and quadratic forms are developed for the estimation of global PAR at this location and locations with similar climatic conditions. The advantage of these models over the traditional method of estimating PAR, by assuming it to be of constant ratio to insolation, is that PAR can be estimated even at locations where insolation values are not available since they make use of only solar-related astronomical parameters (extraterrestrial radiation and maximum sunshine duration) and duration of sunshine. The results show that in practice, the simple linear regression model is sufficient and hence the use of a quadratic model is only slightly justifiable. On developing the models based on the two seasons (rainy and dry) encountered here, the rainy season model showed a high correlation coefficient, R, of 0.9793 against the poor R of 0.5075 for the dry season. The poor correlation during the dry season period is probably due to large differences in the characteristics of the sky during this period. Copyright © 2000 Royal Meteorological Society

A microcontroller-based data acquisition system for solar radiation and environmental monitoring

The hardware design and operation of a battery-powered microcontroller-based data acquisition system (DAS) for unattended remote measurements are presented. The system was designed around the ST62E20 8-bit microcontroller and applied for solar radiation monitoring. The measurement system uses the SolData silicon-cell pyranometer as the solar radiation sensor. The data from the sensor is collected by means of on-chip A/D converter and stored in a serial EEPROM until uploaded to a portable computer. Keeping the DAS in a low-power mode, which is only interrupted when measurements are to be taken or when a computer is connected to retrieve the stored data, minimizes power consumption. An on-chip timer provides an interrupt to awaken the system from its low-power wait mode at 10-min intervals to sample and store the data. At the end of each data collection period, the acquired data will be transmitted to the computer through the RS232 serial port for subsequent analysis. Only unprocessed data is stored in EEPROM. Quality control and data analysis is done off-line in the laboratory to minimize system cost, complexity and system downtime. Field tests and comparisons of this measurement system against the standard Eppley precision spectral pyranometer (PSP) have shown a slightly nonlinear correlation and that the accuracy of this measurement system as applied to solar radiation monitoring is fairly good, typically /spl plusmn/13 W/m/sup 2/.

First performance analysis of a silicon-cell microcontroller-based solar radiation monitoring system

This paper demonstrates the operation of a low-cost silicon pyranometer in combination with a self-designed low-power microcontroller-based data acquisition system for monitoring global horizontal solar radiation. The performance and behaviour of this low-cost silicon-cell microcontroller-based data acquisition system (SIMBADAS) are studied on the basis of experimental instantaneous global horizontal solar radiation data obtained between January and October 1997. This measurement system operates unattended, collecting and storing data at 10 min intervals. Data from the SolData silicon-cell pyranometer ( SolData, 1989) are collected via an on-chip A/D converter and stored in non-volatile memory. Only unprocessed data are stored in memory. Quality control and data analysis are done off-line to minimize system cost and complexity. At the end of each data collection period, the acquired data are transmitted to a computer through the RS232 serial port for subsequent analysis. Digital correction of the signal by means of an auto-correction procedure is then done as the data are retrieved by the computer. This paper also presents initial calibration results where the SIMBADAS was compared against the first class reference, the Eppley (PSP) precision spectral pyranometer. Data covering an almost complete range of atmospheric conditions in Harare has been used to develop calibration coefficients for this silicon-based system. Field tests and comparisons with the Eppley pyranometer have shown that the accuracy of the SIMBADAS measurement system is fairly good, typically ±13 W/m 2 except during partly cloudy days in which readings could be vastly different. This error is acceptable for a lot of applications and comparable to other similar methods developed so far.

Effects of clouds and haze on UV‐B radiation

An experiment was conducted over a 6‐month period in Research Triangle Park, North Carolina, to investigate the effects of clouds and haze on ultraviolet (UV) radiation. Data were collected using a Yankee Environmental Systems UVB‐1 pyranometer, an Eppley Laboratory Precision Spectral Pyranometer, and a SCI‐TEC Brewer spectrophotometer. Hourly reports of total cloud cover and surface observations of air temperature, dew point temperature, barometric pressure, and visibility from the National Weather Service located at the nearby Raleigh‐Durham International Airport were also used in this study. An empirical relationship has been formulated for UV‐B attenuation as a function of total solar transmissivity and cloud cover. Cumulus‐type clouds were found to attenuate up to 99% of the incoming UV‐B radiation during overcast conditions. However, these same clouds were found to produce localized increases of UV‐B radiation of up to 27% over timescales less than 1 hour under partly cloudy skies when the direct solar beam was unobstructed. Summer haze was found to attenuate UV‐B radiation in the range of 5% to 23% when compared to a clear day in the autumn. In general, total radiation was attenuated more than UV‐B radiation under cloudy conditions.

Group Calibration of Silicon Cell Pyranometers for Use in an Automated Network

Techniques for simultaneously calibrating a number of pyranometers for use with automated weather stations are examined. The pressure involves taking data from an Eppley precision spectral pyranometer and from up to 11 LI-COR pyranometers using a specially built sensor plate. Results of a group calibration of six pyranometers using 10-minute data over a 3-day period are given. Three best-fit lines were determined for full-scale (5–960 W m−2), midscale (300–650 W m−2) and midrank (400–800 W m−2) datasets. The full-scale calibration technique proved most useful, resulting in a standard error of estimate of less than 20 W m−2 in all cases.

Measurements with a Spectral Pyranometer at Tucson after the Eruption of El chichó Volcano

Abstract Spectral values of the downward hemispheric diffuse and total flux densities at eight wavelengths were measured in Tucson, Arizona following the 1982 eruption of the El Chichon volcano. Characteristics of the volcanic dust (including optical depth, the Junge shaping constant of the size distribution, and the range of the imaginary index) were derived from these data.

Corrections to be Applied to Measurements Made with Eppley (and other) Spectral Radiometers When Used with Schott Colored Glass Filters

This paper demonstrates, in summary form, some corrections to be made to values of spectral flux as measured with Eppley spectral pyranometers. The results of an investigation into the occurrence of what are believed to be systematic errors in scaled hemispherical thermopile systems, employed in pyranometer design, are presented. This investigation was prompted by an analysis of a large mass of solar spectral measurement material, which indicated an apparent change in spectral pyranometer sensitivity under conditions of continuous operation. The study also included pyranometers of natural- and forced-ventilated design. The results indicate that the corrections so far established ought to be of general application, within practical limits, to the present Eppley spectral pyranometer and, perhaps with some modifications, to other constructions.