Terry G. Lenz

Thermal performance of solar concentrator/cavity receiver systems

Abstract To utilize solar energy at a high temperature, a parabolic dish/cavity receiver configuration is often used. The energy loss mechanisms of such a system are analyzed. System efficiency is defined as the power absorbed by the working fluid circulating in the cavity divided by the solar power falling on the concentrator aperture. Power profiles produced in cavities of varying geometry with concentrators of varying rim angle are also discussed. It is found that varying concentrator rim angle and cavity geometry can greatly affect the cavity power profile without a large effect on system efficiency. Cavity isothermality often requires a nonlinear power profile , particularly in a thermochemical system. The methodology described can be used to optimize concentrator/cavity design variables.

Performance studies for an experimental solar open-cycle liquid desiccant air dehumidification system

Abstract A nominal 10.5-kW (3-ton) open-cycle liquid desiccant dehumidification system has been designed, installed, and successfully operated at the Solar Energy Applications Laboratory, Colorado State University. Packed bed units were used to dry the air in the dehumidifier and to concentrate the desiccant in the regenerator. Liquid distribution in the regenerator was studied for two systems: a gravity tray distributor, and a spray nozzle system. Higher capacities (40–50% increase) and lower pressure drop (30–40% reduction) for the air flow were observed with the spray system. Cooling capacities of 3.5–14.0 kW (1.0–4.0 refrigeration tons) were achieved for both the regenerator and dehumidifier. Functional relationships correlating the independent variables to the rate of vaporization in the regenerator and rate of condensation in the dehumidifier were obtained by statistical analysis of the experimental data. These studies thus provide data and correlations useful for design guidance and performance analysis of similar open-cycle liquid desiccant cooling systems, particularly for the liquid/vapor contact units.

Computer-based molecular mechanics techniques for accurate prediction of thermodynamic properties of chemically reactive systems

Abstract Computer-based molecular mechanics (or force field) techniques are proposed as logical current methods for accurate prediction of the thermodynamic properties of chemically reactive systems involving nontrivial molecules. Although chemists have increasingly employed molecular mechanics models in their predictive and interpretive work, chemical engineers have largely ignored these developments. We illustrate the potential power of these molecular mechanics models by applying them to the prediction of thermodynamic equilibrium constants, K eq ( T ), for the two Diels-Alder reactions: 1,3-cyclopentadiene + C 2 H 2 ⇌norbornadiene (A) 1,3-cyclopentadiene + C 2 H 4 ⇌norbornene. (B) Our results agree quite well with literature data for reactions (A) and (B). In addition, our computational approach requires substantially less time than laboratory study (with obvious economic benefits), and further critique of important results is often quickly possible by carrying out additional computations for related systems.

MEASUREMENT OF WATER DIFFUSIVITY IN AQUEOUS LITHIUM BROMIDE AND LITHIUM CHLORIDE SOLUTIONS

A relatively simple method was employed for measurement of water diffusivity in aqueous lithium bromide and lithium chloride solutions. The twin bulb apparatus used for these measurements was developed using an analogy between this apparatus and the conventional diaphragm cell apparatus. Tritiated water (TOH) was used as a tracer for these experiments because of its chemical similarity and proximity to the molecular weight of water. High tracer activity used at the beginning of the experiments allowed the use of relatively shorter time duration for each experiment (s;≈ 20 h) and a quasi-steady state equation to calculate the diffusivity from the observed tracer activity data Initially, the water diffusivity in lithium bromide solutions for concentrations varying from 0.5 M to 3 M (22.1 weight percent) was measured to obtain a comparison with published values. The lithium bromide concentration was further varied from 3 M to 11 M (57.4 weight percent) to obtain data in the concentration range usually employ...

Thermodynamic measurements for the Diels-Alder adduct of anthracene and maleic anhydride

In the present experimental work, the energy of combustion of the crystalline Diels-Alder adduct of anthracene and maleic anhydride: C18H12O3, was measured with a model 1241 Parr automatic calorimeter and a Parr model 1710 calorimeter controller. The standard molar enthalpy of combustion of the (anthracene + maleic anhydride) adduct at po = 0.1 MPa was determined to be ΔcHmo(C18H12O3, cr, 298.15 K) = −(8380.0±5.9) kJ·mol−1. The molar enthalpy of fusion of this adduct at its melting temperature (534.07 K), as measured by a 910 DuPont d.s.c. and a 9900 DuPont thermal (analyzer + digital computer), was found to be (36.3±4.2) kJ·mol−1. The other thermodynamic properties of the (anthracene + maleic anhydride) adduct derived from those measured properties are: ΔfHmo(C18H12O3, cr, 298.15 K) = −(418.2±6.4) kJ·mol−1 and ΔfHmo(C18H12O3, 1, 298.15 K) = −(389.7 ± 7.7) kJ·mol−1.

Thermodynamic molecular mechanics force field: Modified QCFF program

The QCFF program originated by Warshel and Karplus4a was modified to compute accurate thermodynamic properties So, C  po , (H  To – H  0o )/T, and ΔH  fo for various acyclic and cyclic alkenes and alkadienes. Modifications consisted of adjusted bond angle, dihedral angle, bond stretch, and bond energy parameters that improved calculated vibrational frequencies, zero point energies, and thermodynamic functions. Supplemental torsional potential energy functions that were added to existing torsional functions led to greatly improved relative conformer energies and ΔH  f0 values. It was shown that inclusion of hindered internal rotation leads to significantly better agreement of calculated thermodynamic functions with observed values for acyclic alkenes at high temperatures. The calculated thermodynamic properties of the alkenes and alkadienes were deemed sufficiently accurate for calculation of standard enthalpies and Gibbs free energies of gas phase chemical reactions at various temperatures.

Experimental Studies with a Solar Open-Cycle Liquid Desiccant Cooling System

ABSTRACT A nominal 3 ton (10.5 kW) open-cycle liquid desiccant cooling system has been designed, installed and operated at the Solar Energy Applications Laboratory, Colorado State University. Performance data for the regenerator, operated in a decoupled mode, were obtained for a variety of inlet conditions. Depending on the operating conditions, capacities of 1.4 – 3.0 refrigeration tons (4.9 –10.3 kW) were obtained in the regenerator. Liquid distribution in the regenerator was studied for two systems: a gravity tray liquid distributor and a spray nozzle manifold. Higher capacities (40–50% increase) in the regenerator were obtained with the spray nozzle system. Another advantage of the spray nozzle arrangement, as compared to the tray liquid distributor, was a reduction in pressure drop (by 30–40%) for the air flow across the regenerator. The independent variables affecting the capacity were correlated by statistical analysis of the data.