R.J. Romero

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 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.

Dynamic study of the thermal behaviour of solar thermochemical refrigerator: barium chloride-ammonia for ice production

The results of the theoretical thermodynamic analysis and the dynamic behaviour of the solar heating system of a thermochemical refrigerator, which operates on a heterogeneous solid–gas reaction between barium chloride and ammonia, are presented in this work. The thermodynamic analysis of the barium chloride–ammonia system shows that after energy and mass balance, the global efficiency coefficient (COP) varies very little. The theoretical relative low temperatures of dissociation in this system which are between 50°C and 60°C need simple heating systems such as flat plate collectors are needed, with an advantage over traditional liquid/vapour absorption systems. A simulation of the annual dynamic behaviour of the solar heating system for the operation of a solid-gas reactor is also presented. For an ice production specific cooling load, calculations are made of the different solar fractions of different areas of solar caption as well as the monthly variations of the efficiencies of the refrigeration systems.

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.

Thermodynamic design data for absorption heat transformers. Part seven: operating on an aqueous ternary hydroxide

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 and concentrations, including flow ratios, Carnot coefficients of performance and enthalpy-based coefficients of performance for two effectiveness heat exchanger values have been presented for absorption heat transformers operating on an aqueous ternary hydroxide working fluid consisting of sodium, potassium and caesium hydroxides in the proportions 40:36:24 (NaOH:KOH:CsOH). The interactions of operating temperatures have been illustrated graphically.

Thermodynamics of Fuel Cells

In this chapter the basic thermodynamic and electrochemical principles behind fuel cell operation and technology are described. The basic electrochemistry principles determining the operation of the fuel cell, the kinetics of redox reactions during the fuel cell operation, the mass and energy transport in a fuel cell, etc., are described briefly to give an understanding of practical fuel cell systems. The ideal and practical operation of fuel cells and their efficiency are also described. This will provide the framework to understand the electrochemical and thermodynamic basics of the operation of fuel cells and how fuel cell performance can be influenced by the operating conditions. The influence of thermodynamic variables like pressure, temperature, and gas concentration, etc., on fuel cell performance has to be analyzed and understood to predict how fuel cells interact with the systems where it is applied. Understanding the impact of these variables allows system analysis studies of a specific fuel cell application.