Empirical radiometric correction of a silicon photodiode rotating shadowband pyranometer

We have improved the silicon photodiode-based rotating shadowband pyranometer[1] to suit it for radiometric observations by deriving empirical corrections to mimic thermopile sensors. We present the results of an intercomparison between our rotating shadowband pyranometer and a conventional complement of first-class thermopile instruments. The daily integrated values for total horizontal, diffuse horizontal and direct normal irradiance measured with our rotating shadowband radiometer agree on average to better than 1%, 3%, and 2.5%, respectively, with the thermopile results, after our empirically derived corrections are applied. In absolute units, 10-minute integrated values seldom differ by more than 20 watts/m² from the thermopile values.     

Determination of the spatial extent of the focal point of a parabolic dish reflector using a red laser diode

The spatial extent (SE) of the focal point (FP) of a parabolic dish reflector has been determined using a laser diode as a source of light. The laser diode measurements were compared with those taken using a Gardon radiometer where the sun is the source of light. This was done in order to determine if the laser diode technique can be used in place of the radiometer technique. A height of 0.267 m as measured from the vertex of the dish reflector has been determined as the optimum height to place the aperture of the receiver by both methods. An optimal radius of the receiver aperture was determined to be 0.16 m by use of the laser diode and 0.13 m by using the radiometer. The laser diode results show good agreement with the radiometer results. This means that the laser diode technique can be used to determine the SE of the FP. It can also be used to determine the height at which to place the receiver's aperture. Thus, the laser diode can be used in place of the radiometer in cases where the radiometer is not available. This technique has the advantage in that it does not require outdoor experimental setups.     

The transient transpiration radiometer: analysis and parameter selection

A large variety of available radiometers require steady-state conditions to obtain output results. This means that in order to quantify the heat transfer, the temperature of the instrument must first be stabilised, consequently lengthening its response time. The purpose of this paper is to present a formulation of the transient response of a transpiration radiometer. The measurement technique of the radiometer under study is based on a porous disc aligned perpendicularly to a flowing gas stream, while one of its faces is exposed to the heat flux to be measured. A discussion of the results is presented using a mathematical model, according to an examination of the simplifying assumptions made in the data reduction process and the examination of the response. Also presented are sensitivity analysis, temperature, heat flux intensity and manufacturing constraints of the relevant design.     

Investigations of solar radiation detectors using a laboratory test facility for solar radiation meteorological instruments

A laboratory test facility for solar radiation detectors has been built and is in operation at the Aerological Station of the Swiss Meteorological Institute (SAP/SMI). This installation is conceived as a universal test bed for solar radiation exposed meteorological instruments, and consists of a commercially available solar simulator, a laser alignment system, a translation mechanism with instrument mounts, and an adjustable projection mirror. The solar simulator produces a well characterized radiation beam which can be filtered to match the terrestrial or outer space solar spectrum with an irradiance of up to one solar constant (1367 Wm⁻²). The instrument mounts and a HeNe laser beam provide a precise and easy alignment of the reference and the test instruments in the radiation beam, allowing for incident angles in the range of 15°–75°. The measurement is based on a comparison of the response of an active cavity absolute radiometer PMO6 with the signal of the test instrument.Detailed investigations of the Haenni Solar 111B type heliometer have revealed important irregularities in the sunshine threshold irradiance angular distribution. Measurements performed with and without the protection glass cover prove the exceedingly high threshold values at large declination angles to be a consequence of enhanced reflections due to the incident angle and inhomogeneities in the glass cover.Very satisfactory results have also been obtained on characterization measurements of pyranometers showing the mean values of the responsivity to be within 0.8% of the calibration values measured at the world radiation center (WRC) at Davos.     

A mathematical model for fixed dome type biogas plant

This paper describes the development of a mathematical model for determining the dimensions of a fixed dome type Deenbandhu model biogas plant, the most popular model constructed in India. The developed model could estimate the different dimensions within an absolute variation of 6%. This will help the designers to determine the dimensions of various capacities of a Deenbandhu model biogas plant for different hydraulic retention times to suit the local climatic conditions.     

Effects of slip length and hydraulic diameter on hydraulic entrance length of microchannels with superhydrophobic surfaces

This paper investigated effects of slip length and hydraulic diameter on the hydraulic entrance length of laminar flow in superhydrophobic microchannels. Numerical investigations were performed for square microchannels with Re ranging between 0.1 and 1000. It is found that superhydrophobic microchannels have a longer hydraulic entrance length than that of conventional ones by nearly 26.62% at a low Re. The dimensionless hydraulic entrance length slightly increases with the increasing slip length at approximately Re<10, and does not vary with the hydraulic diameter. A new correlation to predict the entrance length in square microchannels with different slip lengths was developed, which has a satisfying predictive performance with a mean absolute relative deviation of 5.69%. The results not only ascertain the flow characteristics of superhydrophobic microchannels, but also suggest that super hydrophobic microchannels have more significant advantages for heat transfer enhancement at a low Re.     

The thermal radiance of clear skies

Measurements of the longwave radiance of the sky were made during the summer of 1979 at Tucson, Arizona; Gaithersburg, Maryland; and St. Louis, Missouri. The global longwave radiation (wavelengths greater than 3 μm) was monitored with a pyrgeometer and the distribution of this radiation in several spectral bands at five different zenith angles was monitored with a spectral radiometer. This paper presents results for the global sky radiation during clear sky conditions. The spectral radiometer was used to calibrate the pyrgeometer and to detect the presence of clouds. The results can most appropriately be summarized in terms of the correlation between the global sky emissivity _sky and surface dewpoint temperature T_dp(°C). The global sky emissivity is defined as the ratio of sky radiance to σT_a⁴, where T_a is the absolute air temperature near the ground, and σ is the Stefan-Boltzmann constant. Based on 2945 night-time measurements in all three cities we find _sky=0.741 +0.0062T_dp with a standard error of estimate of 0.031. A similar relationship with almost identical coefficients holds during daylight hours.     

Low-cost, non-selective, solar-radiation detectors

The theory and basic design of a low-cost, flat-response thermoelectric detector is presented, which can be adapted for use as a pyranometer or net radiometer. This device can also be used as a submersible radiometer for radiation transmission measurements in water. The analysis allows the calculation of the responsivity and optimal design of a detector. Theoretical results are corroborated by experimental data obtained using a laboratory prototype.     

Global solar radiation estimation

This paper presents the comparison between the observed and estimated values of global solar radiation on horizontal surfaces obtained from the linear Angstrom type of correlation of Rehman and Halawani [1] for 52 cities spread in 11 countries; viz. India, Egypt, Sri Lanka, Spain, Zimbabwe, Yemen, Sudan, Italy, Zambia, Hong Kong, and Malaysia. The comparisons are also made with the local linear models wherever available. The estimated values are compared with the measured values in terms of percent error, mean bias error (MBE), mean absolute bias error (MABE), root mean square error (RMSE), and mean percent error (MPE). This study finds that the model of Rehman and Halawani [1] is capable of giving estimates within an acceptable mean percent error of 5% and less for 33% of cities and between 5–10% for 50% of cities.     

Simple method for spectral evaluation of radiant energy emitted from far infrared heaters

It is necessary to know the spectral emissivity of a heater to ensure efficient heating. This paper proposes a useful method for spectral evaluation of radiation energy emitted from far infrared heaters. The measurement device consists of one radiometer and two band‐pass filters. In order to confirm the experimental results, numerical simulations were also carried out with the spectral emissivity of the heaters and the respective spectral sensitivities of the radiometers. The results obtained by these methods show that the far infrared heater radiates high amounts of energy in the far infrared region even at high temperature. It means far infrared heaters can transmit power effectively to materials which exhibit high emissivity in the far infrared region. This method can classify a type of far remote infrared heaters with the near infrared/far infrared energy ratio. By using this data, the authors also proposed a simple identification method of spectral emissivity of far infrared heaters. This inverse method can obtain a rough value of the spectral emissivity of commercial heaters while maintaining the heating conditions. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20396