Hoyt C. Hottel

A simple model for estimating the transmittance of direct solar radiation through clear atmospheres

Abstract The clear-day all-wavelength transmittance τ of solar radiation directly through the 1962 standard atmosphere to a surface at altitude A is found to fit a simple mixed-gray-gas model (1 black, 1 gray, 1 clear) with a maximum error of 0.4 per cent. The relation is τ= a o +a 1 e k/cos z where z is the zenith angle of the sun. The constant a0, a1 and k are functions only of altitude and of haze model (visibility range). Allowance is made for the effect of four climate types (tropical, midlatitude summer and winter, and subarctic summer) by modifying the 3 constants by ratios which depend on climate type only (not on altitude or haze model) and which lie between 0.92 and 1.04. This simple model should be capable of being combined with sunshine or cloud records, plus generalizations covering diffuse radiation, to yield predictions of insolation for use in design optimizations.

Rapid devolatilization and hydrogasification of bituminous coal

Abstract Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.

Rapid devolatilization of pulverized coal

The rapid devolatilization of a lignite and a bituminous coal was studied by electrically heating in helium approximately monolayer samples of small particles supported on wire mesh heating elements. The samples were rapidly brought to a desired temperature, held there for a desired time, and then rapidly cooled. Devolatilization rates, measured by weighing samples before and after experiments of known duration, were determined as a function of residence time (0.05–20 sec), temperature (400–1100°C), heating rate (102–104°C/sec), pressure (0.001–100 atm), and particle size (50–1000 μm). Devolatilization kinetics were determined by non-isothermal techniques since substantial reaction occurred during heating even under the most rapid heating rates. Weight loss from both coals was essentially complete within a fraction to a few seconds depending upon temperature, and increased with increasing final temperature up to 900 to 950°C. Weight loss (corrected to its value at a fixed temperature) was found to be independent of pressure, heating rate and particle size for the lignite, i.e., it depended only on temperature and time; but for the bituminous coal it increased with decreasing pressure, decreasing particle size and, to a small extent, increasing heating rate. The general reaction scheme appears to involve thermal decomposition forming volatiles and initiating a sequence of secondary polymerization and char-forming reactions. The kinetics and yields of the primary decomposition are successfully described by a set of independent first-order parallel reactions represented by a Gaussian distribution of activation energies around a mean of 56 kcal/mole for the lignite, with a standard deviation of 11, and 51 kcal/mole for the bituminous coal at 69 atm and 70 μm particle diameter, with a standard deviation of 7. For the bituminous coal it was necessary in addition to allow for pressure- and particle-size-dependent secondary reactions representing competition between char-forming reactions and diffusional escape of volatiles. Attempts to correlate the data in terms of a single first-order reaction lead to an overall activation energy (∼10 kcal/mole) that is considerably lower than the mean activation energy of the multiple-reaction system, and to a different set of kinetic parameters for each set of experimental conditions. Conditions such as lower pressure, smaller particle size, and better particle dispersion which help to diminish the effect of secondary reactions appear to be more important than rapid heating in the production of volatile yields in excess of the volatile content obtained by proximate analysis.

Development of multicolor pyrometers to monitor the transient response of burning carbonaceous particles

A three‐color ratio pyrometer has been developed to obtain surface temperatures and high‐temperature combustion rates of burning carbonaceous particles. The features and performance of this instrument are contrasted to those of a two‐color ratio pyrometer, constructed earlier for similar studies. The three‐color pyrometer employs a visible (0.65 μm) and two near‐infrared (0.80 and 0.975 μm) wavelengths. The instrument uses a single optical fiber to capture radiation emitted from a particle burning in a high‐temperature laminar flow furnace. Monitoring of the combustion events takes place coaxially with the particle flow, from observation windows located at the top of the furnace injectors. Thus, the temperature‐time history of burning particles can be recorded. The radiation flux is split into three beams using dichroic edge filters. Narrow (or medium) bandwidth interference filters guide monochromatic radiation to solid‐state silicon photodetectors. The associated amplification is linear and/or logarithm...

Smoke-scattered light measurement of turbulent concentration fluctuations

Abstract An optical method has been developed for measurement of concentration fluctuations at a point within systems undergoing turbulent mixing. Intensity, two-point correlation and spectral density of mean-square concentration fluctuation have been measured in a jet of smoky air discharged into ambient air. The data agree well with comparable measurements of temperature fluctuations reported by investigators who have used the hot-wire anemometer. Theoretical consideration is given to some unique problems associated with resolution of the new turbulence measuring device.

Apparatus for determining high pressure coal‐hydrogen reaction kinetics under rapid heating conditions

A simple and relatively inexpensive method is described for measuring the rate and extent of the reaction of coal with hydrogen under conditions of commercial interest. A thin layer of pulverized coal is supported on an electrically heated metal screen strip capable of initial heating rates of 600–12 000°C/sec followed by extended time at final temperatures of 400–200°C. The device is enclosed in a vessel designed for hydrogen pressures up to 205 atm. Typical results are illustrated and shown to compare favorably with data from previous investigations.

Reaction kinetics of carbon dioxide with electrode carbon particles

Kinetic data for the reaction of carbon dioxide with electrode carbon particles19 were used in a study of the reaction mechanism. A reactor with a single layer of carbon particles (50–60 mesh) was used throughout. Mass-specific reaction rates under fixed reaction conditions were found to vary with reaction time and were correlated with fractional carbon gasified. All measured rates were converted to values on fresh surface at zero reaction time. The reaction kinetics are well represented by the Langmuir-Hinshelwood type equation, the constants of which were converted to those of a reaction scheme in which oxygen exchange between gas phase and carbon surface is followed by carbon gasification. The equilibrium constants for the oxygen exchange reaction were calculated from the rate constants. Their values were found to agree with those from Grabke28, but were much smaller than those from Ergun9,10.

Important factors in determining the efficiency of TPV systems

Part I divides the TPV system into its five major components—combustor, emitter, radiant transfer system, photocell, and power conditioner. Using the best data available to the authors from experimental studies, plus theoretical analysis where useful, the probable upper bound of efficiency for each part of a practical TPV device is estimated. A probable overall TPV efficiency of 13% is obtained for a combustion flame temperature of 2000 °C and a fibrous ytterbium oxide emitter temperature of 2000 K. If a flame temperature of 2300 °C and emitter temperature of 2000 °C are possible, this efficiency may approach 22%. Experimental plus analytical studies of the combustor, emitter and radiant transfer system are recommended that should determine what the maximum TPV system efficiency is likely to be. These studies would also give the design data needed for building a practical TPV device. Part II is an analytical model of the radiant transfer system for a fibrous or solid emitter plus thermal and photocell shi...

The role of scatter in determining the radiative properties of surfaces

Abstract The efficiency of devices using solar energy as a source of heat and the temperature control of units subject to solar irradiation are strongly dependent on the emissivities and absorptivities of the exposed surfaces. In this paper a fundamental study of the effect of scatter on the radiative properties of surfaces is reported. Experimental values of the directional reflectance of well-defined particulate layers are presented. Reflectances calculated using various theoretical models are compared with the experimental values.

Fifty years of solar energy research supported by the Cabot Fund

This lecture describes the developments in the uses of solar energy during the past fifty years. In particular, the experiences in the design and performance testing of solar heating systems in houses at the Massachusetts Institute of Technology are discussed. The author concludes that the economic value of the sun, as we now know how to use it, is overrated; that while solar water heating is economical in some parts of the US active space heating systems are less so. Further research in photochemistry, photoelectricity and radiative scatter is needed.