• Energy Research

Abstract Single Stage Heat Transformer (SSHT) is a device to recovery waste heat by a thermodynamic cycle. In this paper an experimental SSHT prototype was analyzed. This prototype operates with Water/Carrol mixture. Four test runs were carried out in order to evaluate the performance. The heat powers were measured from 0.99 to 1.35 kW for the generator, 0.97–1.33 kW for the condenser, 0.99–1.35 kW for the evaporator and 0.69–0.81 kW for the absorber. Experimental Gross Temperature Lift (GTL) was values from 18.5 to 22.2 °C and the dimensionless Coefficient of Performance (COP) was calculated for those operating conditions from 0.30 to 0.35.

In absorption systems using the aqueous lithium bromide mixture, the Coefficient of Performance is affected by the desorber. The main function of this component is to separate the refrigerant fluid from the working mixture. In conventional boiling desorbers, constant heat flux and vacuum pressure conditions are necessary to carry out the desorption process, and usually, the absorbers are heavy and bulky; thus, they are not suitable in compact systems. In this study, a membrane desorber was evaluated, operating at atmospheric pressure conditions with a water/lithium bromide solution with a concentration of 49.6% w/w. The effects of the solution temperature, solution mass flow, and condensation temperature on the desorption rate were analyzed. The maximum desorption rate value was 6.1 kg/m2h with the following operation conditions: the solution temperature at 95.2 °C, the solution mass flow at 4.00 × 10−2 kg/s, and the cooling water temperature at 30.1 °C. On the other hand, the minimum value was 1.1 kg/m2h with the solution temperature at 80.2 °C, the solution mass flow at 2.50 × 10−2 kg/s, and the cooling water temperature at 45.1 °C. The thermal energy efficiency, defined as the ratio between the thermal energy used to evaporate the refrigerant fluid with respect to the total thermal energy entering the membrane desorber, varied from 0.08 to 0.30. According to the results, a high solution mass flow, a high solution temperature, and a low condensation temperature lead to an increase in the desorption rate; however, a low solution mass flow enhanced the thermal energy efficiency. The proposed membrane desorber could replace a conventional boiling desorber, especially in absorption cooling systems that operate at high condensation temperatures as in warm weather regions.

Abstract A conventional desorber in the Absorption Heat Transformer (AHT) cycle requires a constant heat flux and vacuum pressure conditions to boil the working mixture and separate the working fluid. In this paper, an Air Gap Membrane Distillation (AGMD) unit was adapted as desorber/condenser with water/Carrol mixture in order to demonstrate the feasibility of the desorption process at atmospheric pressure conditions. Two membranes with 0.22 and 0.45 μm pore sizes were used and three temperature levels were tested. The maximum increase in the concentration (Δ X ) was 1.54% w/w (from 60.63 to 62.17% w/w) with a membrane with pore size up to 0.45 μm and a solution temperature of 82.7 °C. The maximum thermal process effectiveness was 17.7% (on average) with a membrane with pore size up to 0.22 μm and a solution temperature of 84.4 °C. Due to the corrosion process, a fouling particle by iron oxide was found on the membrane; this fouling layer could promote the “wetting” process in the membrane after a long operating time.

A thermodynamic analysis of a half-effect absorption cooling system powered by a low-enthalpy geothermal source was carried out. This paper presents modeling of the half-effect absorption cooling system operating with an ammonia/lithium nitrate mixture and based on the first and second laws of thermodynamics, using as energy inputs real data from two geothermal wells located at Las Tres Vírgenes volcanic complex, Baja California Sur, México. Plots of coefficients of performance and exergy efficiency against condenser, evaporator, and generator temperatures are presented for the half-effect cooling system. The results showed that the system was able to operate at generation temperatures between 56 and 70 °C, which were supplied by the geothermal wells in order to produce cooling at temperatures as low as −16 °C, achieving coefficients of performance between 0.10 and 0.36, while the exergy efficiency varied from 0.15 to 0.40 depending on the system operating temperatures.

For absorption cooling cycles using water as a refrigerant, H2O/LiCl mixtures are suitable for replacing conventional H2O/LiBr mixtures. In addition, membrane devices can be used to develop compact and lighter absorption systems, and they can operate with H2O/LiCl mixtures. The present paper describes an experimental evaluation of a membrane desorber/condenser operating at atmospheric pressure. Two operation modes were analyzed: continuous cycle operation and intermittent operation. For the first operation mode, the maximum desorption rate was 3.49 kg/h·m2, with a solution temperature of 90.3 °C and a condensation temperature of 25.1 °C. The lowest desorption rate value was 0.26 kg/h·m2, with a solution temperature of 75.4 °C and a condensation temperature of 40.1 °C. In the second mode, after three operating hours, the refrigerant fluid produced, per 1 m2 of membrane area, 7.7, 5.6, 4.3, and 2.2 kg, at solution temperatures of 90.3, 85.3, 80.4, and 75.4 °C, respectively. A one-dimension heat and mass transfer model is presented. The calculated values of desorption rate and outlet temperatures were compared with the experimental data; a square correlation coefficient of 0.9929 was reached for the desorption rate; meanwhile, for the outlet solution temperatures and the outlet cooling-water temperatures, a square correlation coefficient up to 0.9991 was achieved. The membrane desorber has the advantages of operating at atmospheric-pressure conditions, high condensation temperature, the ability to use different saline solution working mixtures, and different operation methods. These advantages can lead to new absorption systems.

Abstract Conventional desorbers in absorption systems are in general bulky and heavy; hence they are not usually used in compact systems. In addition, depending on the working mixture, they operate in a vacuum or at high pressures, which is in both cases a great disadvantage. In this study, a desorber/condenser was tested and modelled, using the Air Gap Membrane Distillation (AGMD) process, operating at atmospheric pressure conditions, with water/lithium bromide mixture for absorption system applications. The desorber/condenser used a membrane with a 0.45 μm pore size. Thirty-six experimental test runs were carried out at different operating conditions. A one-dimensional heat and mass transfer model was developed in order to describe the vapour water desorption from an aqueous LiBr mixture at atmospheric pressure conditions. The refrigerant desorption rate for the experimental conditions varied from 0.30 to 9.69 kg/m 2 h. The theoretical refrigerant desorption rate calculated by the heat and mass transfer model agreed with the experimental data within ±10%. The results showed that the highest mass transfer resistance was at the boundary layer (1/K bl ) which increases with the increment of X LiBr and decreases with the increment of T LiBr .

The generator is the starter device of single stage heat transformer (SSHT) and its characteristics have main effects on the overall efficiency of this kind of absorption machines. This article reports a study of the generation of steam and changes in the concentration of the working solution (Water/Carrol mixture) into a plate heat exchanger as a function of its horizontal and vertical position by gravity effect. It is considered the analysis of six experimental tests; two were evaluated in a plate heat exchanger in a horizontal position and four in a vertical position (90 degree inclination). The generation of steam and increased concentration of the working solution are more sensitive to the vertical position of exchanger than in horizontal position. The results of numerical-experimental analysis indicates that a heat exchanger in horizontal position, the steam generation and the change in the concentration of the working solution occurring in the middle of the plate (or at greater distance depending to the thermodynamic conditions) and instantly in vertical position (at the input of the plate).

Abstract Parabolic Trough Collectors (PTC) provides thermal solar energy at medium temperature, and in order to increase the thermal level, the solar system can be coupled to upgrading devices, such as Absorption Heat Transformers (AHT). In this paper, a feasibility analysis of the PTC system operating as thermal source of an AHT is described. The PTC and AHT units were tested and, based on the experimental data of each system, a heat transfer analysis was carried out in order to propose a single system. Two case studies were analysed: In the first, the evaporator temperature was close to the generator temperature (84.6 and 85.2 °C respectively) and a simultaneous flow from the heat source was used; in the second case, the evaporator temperature was lower than the generator temperature (79.6 and 86.7 °C respectively) and a serial flow from the heat source was proposed. Results show that, for the absorber temperature of 101 °C, the calculated generator and evaporator heat loads were 1.50 and 1.34 kW respectively in Case 1, and 0.86 kW for both components in Case 2. For Case 1, the PTC system required 6.0 m 2 in order to provide two mass flows of 6.00 × 10 −02 and 5.35 × 10 −02 kg/s for generator and evaporator at 89 °C. For Case 2, one mass flow of 6.60 × 10 −02 kg/s at 89 °C for generator and evaporator must be satisfied by a 3.7 m 2 PTC system.