Byard D. Wood

Performance predictions of alternative, low cost absorbents for open-cycle absorption solar cooling

To achieve solar fractions greater than 0.90 using the open-cycle absorption refrigeration system, considerable sorbent solution storage is necessary. Sorbent solutions currently under consideration, such as aqueous solutions of lithium chloride and lithium bromide, may be too costly to exploit the open-cycle storage concept. Having identified the absorber as the system component whose performance is affected the most by a change in absorbent, an absorber model was selected from available literature pertaining to simultaneous heat and mass transfer. Low cost absorbent candidates were selected and their physical properties were either located in the literature, measured, or estimated. Absorber operating parameters were selected and the model was then used to estimate absorber performance for each absorbent in terms of cooling capacity per unit of absorber area. After specifying system parameters such as absorber capacity and cooling load, the absorber area, absorbent cost, and sorbent solution pumping power and storage volume were estimated for each candidate. The most promising of the absorbents considered was a mixture of two parts lithium chloride and one part zinc chloride. The estimated capacities per unit absorber area were 50–70% less than those of lithium bromide; however, the lithium bromide cost for a system sized to cool a 190 m2 residential structure was estimated to be eight times that for the lithium-zinc chloride mixture. Both the lithium-zinc chloride mixture and lithium bromide solutions had estimated pumping powers of less than 0.1 kW. The solubility of the lithium-zinc chloride mixture at absorber conditions was improved over that of lithium bromide, reducing the risk of solidification of the solution.

The interfacial turbulence in falling film absorption: effects of additives

Abstract The results of vertical falling film experiments on the absorption of water vapor to aqueous lithium bromide solutions with an additive, 2-ethyl-1-hexanol, are reported. During the absorption, the film becomes highly turbulent. Consequently, the heat and mass transfer is significantly enhanced by turbulent mixing. In addition, the instability mechanisms are detailed. In the vicinity of water absorption, surface-tension gradients due to the lower LiBr concentration, the lower additive concentration, and the higher temperature at the interface, can favor instability of the falling film.

Absorption of water vapour into falling films of aqueous lithium bromide

Abstract In this study, experiments have been performed for water vapour absorption into 50 and 60 mass% aqueous lithium bromide solution films flowing down a vertical surface to investigate the effects of liquid diffusivity values, molecular properties of the concentrated solutions and non-absorbable gases. The experimental results for wavy films over a film Reynolds number range of 15–90 indicate larger dimensionless mass transfer rates than for strictly laminar flow when the diffusivity of water in a concentrated lithium bromide solution is less than that in a dilute solution. The complete set of results shows that the physical property data for lithium bromide solutions including the diffusivities measured by Kashiwagi are sufficient to explain mass transfer behavior.

A numerical modeling of an absorption process on a liquid falling film

A simple and promising numerical model is developed for simultaneous heat and mass transfer in smooth falling-film absorption. The systems of LiClH2O and LiBrH2O are solved because they are the most popularly used materials in absorption heat pumps and chillers. The results are in good agreement with those of complicated formulations in the literature and the experimental data.

A new look at long term collector performance and utilizability

Abstract A simple technique has been developed to calculate monthly collection efficiency or monthly utilizability for solar thermal flat-plate collectors. It is applicable to south facing tilted collectors operating with a fixed fluid inlet temperature although extensions to other more generalized uses of utilizability are discussed. The heart of the technique is an empirically determined performance map that makes possible quick evaluations of changes in collector design, geographic location and collector inlet temperature. The collector input variables are those that are commonly measured in most thermal test procedures; geographic input variables are the mean monthly temperature and K T (the Liu and Jordan clearness factor). The procedure was developed for monthly optimum fixed tilts but a simple correction can be made to incorporate arbitrary monthly fixed tilts. The method, in general, gives good results compared to long term hourly simulation. The technique also allows one to determine under what operating conditions collector performance begins to depend on site-to-site solar radiation/weather variability and what uncertainties can be expected from its use.

Mass Fire Model: An Experimental Study of the Heat Transfer to Liquid Fuel Burning from a Sand-Filled Pan Burner

Abstract The heat transfer and mass transfer phenomena to a burning array of fuel elements are considered for the various phases of the burning episode which are obtained by burning a fixed amount of liquid fuel in a 152 cm sand-wick pan burner. Quantitative experimental measurements include burning rate, wick temperature distribution, and flame radiation heat flux distributions to the fuel surface as a function of time after ignition. The radiation heat flux is measured with four radiometers which view the flame from beneath the fuel bed with a prescribed view-angle. The total heat flux from the fire plume to the fuel surface is determined from the burning rate and wick temperature histories. A comparison of the aforementioned results is given for non-luminous and luminous flames using methanol and acetone respectively. A comparison of the total heat flux data and the radiation heat flux data indicates that radiation contributes between 20 and 40 percent of the thermal load to the fuel surface for the me...

Unglazed collector/regenerator performance for a solar assisted open cycle absorption cooling system

Abstract This study includes the results of an investigation on a prototype solar assisted absorption cooling system. The liquid absorbents, for example, lithium chloride or lithium bromide used in this type of system absorb water, the refrigerant, in the absorber and require energy input for a subsequent desorption process. An ordinary black shingled roof was used as a collector/regenerator for the evaporation of water to obtain a strong solution of absorbent for use in the absorber. The rate of evaporation from the collector/regenerator determines the overall cooling capacity of the system. Experiments were conducted on a 11 m × 11 m (36 ft × 36 ft) collector/regenerator to measure solution flow rates and concentrations at inlet and outlet, and temperatures at several locations of the collector. Altogether, there were 100 sets of data taken under various environmental and flow conditions. An iterative solution of the equations describing the conservation of mass and energy led to the prediction of local concentrations of the absorbent solution, and local heat and mass transfer coefficients. Correlations for nondimensional heat and mass transfer parameters were developed in terms of local Reynolds number, Grashof number, Prandtl number, Schmidt number, and N , the ratio of buoyancy force due to mass transfer to buoyancy force due to heat transfer. These heat and mass transfer correlations were used for the simulation of performance of the collector/regenerator. A parametric study of the collector/regenerator performance enabled the identification of important variables. A good agreement between these results and those from a warm, humid climate indicates that the simulation model can be used as a tool for the design of systems operating under similar conditions. The experimental results also show a regeneration efficiency varying between 38 and 67%, and the corresponding cooling capacities ranged from 31 to 72 kW (8.8 to 20 tons).

Results and analysis of a round robin test program for liquid-heating flat-plate solar collectors

Abstract A round robin test program was conducted to determine the intercompatibility of thermal performance data on 2 liquid-heating flat-plate solar collectors. Efficiency tests were performed at 21 test facilities, distributed across the United States, using a common test procedure. The results were statistically analyzed and showed a relatively large spread in the measured values of collector efficiency. Data from approximately half the facilities were selected for detailed analysis. A collector analytical model was used to show that less than 1 3 of the chi-square for a second order polynomial could be attributed to different environmental conditions from facility to facility. In general, data from a single facility were consistent and the majority of scatter was attributed to systematic uncertainties from facility to facility. When the data from six participants reportedly adhering to the requirements of ASHRAE Standard 93-77 were analyzed, the scatter was found to be within normal limits expected for the test procedure.

Control problems in solar domestic hot water systems

Abstract The settings on the controller for a solar domestic hot water system can have major impact on the “ratings” obtained from a 1-day test or simulation. The settings become more critical as the effectiveness of any freeze protection heat exchanger decreases. This paper develops equations for optimal controller settings that will maximize the simulated performance. The practice of using solar radiation that is constant over hourly periods in both experimental- and simulation-based rating procedures is shown to cause problems. Recommendations are made for controller settings that yield a “fair” rating comparison.

Effects of a Nonabsorbable Gas on Interfacial Heat and Mass Transfer for the Entrance Region of a Falling Film Absorber

An analytical solution is presented for the effect of air (nonabsorbable gas) on the heat and mass transfer rates during the absorption of water vapor (absorbate) by a falling laminar film of aqueous lithium bromide (absorbent), an important process in a proposed open-cycle solar absorption cooling system. The analysis was restricted to the entrance region where an analytical solution is possible. The model consists of a falling film of aqueous lithium bromide flowing down a vertical wall which is kept at uniform temperature. The liquid film is in contact with a gas consisting of a mixture of water vapor and air. The gas phase is moving under the influence of the drag from the falling liquid film. The governing equations are written with a set of interfacial and boundary conditions and solved analytically for the two phases. Heat and mass transfer results are presented for a range of uniform inlet air concentrations. It was found that the concentration of the nonabsorbable gas increases sharply at the liquid gas interface. The absorption of the absorbate in the entrance region showed a continuous reduction with an increase in the amount of air.