W. Rivera

Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature

Abstract Mathematical models of single-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/Carrol™ mixtures were developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. Plots of coefficients of performance and gross temperature lifts are shown against the temperatures of the heat supplied by the solar pond. The results showed that the single-stage and the double absorption heat transformer are the most promising configuration to be coupled to solar ponds. With single-stage heat transformers it is possible to increase solar ponds temperature until 50°C with coefficients of performance of about 0.48 and with double absorption heat transformers until 100°C with coefficients of performance of 0.33.

Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COPh for the basis cycle and COPSHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio FR has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.

Experimental evaluation of a single-stage heat transformer used to increase solar pond's temperature

A heat source at temperatures not higher than 80°C was used to simulate the heat input to an absorption heat transformer from a solar pond. An experimental absorption heat transformer operated with the water/Carrol mixture was used to demonstrate the feasibility of these systems to increase the temperature of the heat obtained from the solar ponds. Carrol™ is a mixture of LiBr and ethylene glycol [(CH2OH)2] in the ratio 1:4.5 by weight. Flow ratios, gross temperature, useful heat, and coefficients of performance are plotted for the heat transformer versus temperature and solution concentration. Gross temperature as high as 50°C were obtained. The maximum temperature of the useful heat produced by the heat transformer was 132°C. The COP for the unit was in the range 0.14–0.36.

Experimental evaluation of a single-stage heat transformer operating with the water/Carrol mixture

This paper describes experimental results obtained with a single-stage heat transformer (SSHT). Many combinations of fluid pairs have been proposed although only the water/lithium bromide mixture has been widely used. The experimental work was done using the water/Carrol™ mixture, where Carrol™ is a mixture of LiBr and ethylene glycol [(CH2OH)2] in the ratio 1:4.5 by weight. Flow ratios, gross temperature lifts, useful heat, and coefficients of performance are plotted for the heat transformer vs temperatures and solution concentrations. Because the water/Carrol™ mixture has higher solubility than water/lithium bromide and high experimental values are obtained for the gross temperature lift, it is a preferred mixture.

Theoretical comparison of performance of an absorption heat pump system for cooling and heating operating with an aqueous ternary hydroxide and water/lithium bromide

Abstract This paper compares the theoretical performance of the modelling of a heat pump system for cooling and heating operating with water/lithium bromide and an alternative aqueous ternary hydroxide mixture. The aqueous ternary hydroxide working fluid consists of sodium, potassium and caesium hydroxides in the proportions 40:36:24 (NaOH:KOH:CsOH). Plots of Carnot coefficients of performance and enthalpy based coefficients of performance are shown against the most important temperatures of the system. The results showed that similar coefficients of performance are obtained for both mixtures; however, it was found that the system operating with the alternative mixture may operate with a higher range of condenser and absorber temperatures and the heat delivered by these components can be easily removed by air.

Thermodynamic design data for absorption heat transformers. Part six: Operating on water-carrol

Abstract The Gibbs phase rule and thermodynamic properties of the working pair limit the choice of operating temperatures. For any combination of temperatures, the concentrations in the absorber and the generator and hence the flow ratios are fixed. For any particular working pair, the coefficient of performance is related to the flow ratio. Tables of possible combinations of operating temperatures, including flow ratios, Carnot coefficients of performance nd enthalpy based coefficients of performance have been presented for absorption heat transformers operating on water-carrol (lithium bromide-ethylene glycol). The interaction of operating temperatures has been illustrated graphically.

Evaluation of a heat transformer powered by a solar pond

A single-stage heat transformer operating with the water/lithium bromide mixture was operated to demonstrate the feasibility of the use of these systems to increase the temperature of the heat obtained from solar ponds. Electrical heaters at temperatures not higher than 80°C were used to simulate the heat input to an absorption heat transformer from a solar pond. Gross temperature lifts, useful heat and coefficients of performance are plotted for the heat transformer against temperatures and solution concentrations. Gross temperature lifts as high as 44°C were obtained. The maximum temperature of the useful heat produced by the heat transformer operating with the water/lithium bromide mixture was 124°C. The maximum coefficient of performance for the unit was 0.16.

Theoretical comparison of single stage and advanced absorption heat transformers operating with water/lithium bromide and water/Carrol mixtures

A thermodynamic analysis was carried out to compare the theoretical performance of single stage, two stage and double-absorption heat transformers operating with the water/lithium bromide and the water/Carrol mixtures, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH2OH)2] in the ratio 1:4·5 by weight. A mathematical model to predict the theoretical performance of single stage and the advanced heat transformers is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system for both mixtures. The water/Carrol mixture showed in general to have a better performance than the water/lithium bromide mixture.

Modelling of single-stage and advanced absorption heat transformers operating with the water/carrol mixture

Abstract A thermodynamic analysis was carried out to study the performance of single-stage, two-stage and double-absorption heat transformers operating with the water/Carrol mixture, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH 2 OH) 2 ] in the ratio 1:4.5 by weight. A mathematical model to predict the theoretical performance of single-stage and advanced heat transformers operating with the water/Carrol mixture is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system. A two-stage heat transformer consists of two single-stage heat transformers which can be coupled in three different ways. A double-absorption heat transformer is a single-stage heat transformer to which a dual-purpose absorber/evaporator unit has been added.

Comparative study of a cascade cycle for simultaneous refrigeration and heating operating with ammonia, R134a, butane, propane, and CO2 as working fluids

In this paper, a cascade system for simultaneous refrigeration and heating is simulated with different working fluids. Ammonia, R134a, butane and propane are evaluated in the low-temperature (LT) cycle and carbon dioxide (CO2) is used in the high cycle. The effects of the thermodynamic parameters on the cascade system are evaluated with the aim of finding the best working fluid performance and optimum design parameters. Coefficients of performance (COP) and exergetic efficiencies were estimated for each one of the cycles and for the entire system. The behaviour of these parameters is presented as a function of the internal heat exchanger effectiveness and main operating system temperatures. The results showed that the cascade system using butane in the LT cycle increased the COP up to 7.3% in comparison with those obtained with NH3–CO2. On the other hand, the cascade systems operating with the mixtures R134a–CO2 and propane-CO2 presented similar results reaching COPs up to 5% higher than those obtained with the NH3–CO2 system.