Performance improvement of absorption heat transformer

In this study, a mathematical model of absorption heat transformer (AHT) operating with the aqua/ammonia was 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 and used a special ejector located at the absorber inlet. By the use of the ejector, the obtained absorber pressure becomes higher than the evaporator pressure and thus the system works with triple-pressure-level. The ejector has two functions: (i) aids pressure recovery from the evaporator and (ii) upgrades the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. The other advantage of the system with ejector is increased absorber temperature. Therefore, pressure recovery and pre-absorption in the ejector improves the efficiency of the AHT. Under the same circumstances, when compared to an AHT with and without an ejector, the system's COP and exergetic coefficient of performance (ECOP) were improved by 14% and 30%, respectively and the circulation ratio (f) was reduced by 57% at the maximum efficiency condition. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The maximum upgrading of the solar pond's temperature by the AHT was obtained at 57.5 °C and gross temperature lift at 97.5 °C with coefficients of performance of about 0.5. The maximum temperature of the useful heat produced by the AHT was 150 °C. In addition, exergy losses for each component in the system were calculated at different working temperatures and the results of both systems with and without an ejector were compared. Exergy analysis emphasised that both the losses and irreversibilities have an impact on the system performance and exergy analysis can be used to identify the less efficient components of the system. Exergy analyses also showed that the exergy loss of the absorber of AHT with ejector was higher than those of other components.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was 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. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

Exergy and energy analyses of absorption heat pumps

This paper presents a detailed thermodynamic analysis of absorption heat pumps and heat transformers. First and second law thermodynamic methods are both applied to calculate the respective energy efficiency (COP) and the energy efficiency (n) for the systems as well as the individual components. Using LiBr/H₂O as an example, two three-dimensional T-S-X diagrams for an absorption heat pump and a heat transformer are constructed by calculating the thermodynamic properties and the chemical potentials. From the calculated COP and n values, it is concluded that AHP for heating has better thermodynamic performance than both AHP for cooling and heat transformer.     

Wood chip drying with an absorption heat pump

This paper presents the thermal and economic analysis of a mobile wood chip drying process. The dryer was subjected to different operating conditions, which were studied in order to determine the optimal characteristics of the dryer in terms of energy consumption. In addition, the impact of the exterior climatic conditions on the dryer's performance was also evaluated. The performance of the dryer coupled with an absorption heat pump was modeled in steady-state conditions under different operating parameters. Finally the system's energy performance was compared to the performance of two other systems (a wood burning furnace and a waste-heat recovery system). The results demonstrate that single-stage absorption heat pumps are only interesting when the set point temperature of the drying air is below 60 °C. Otherwise, a two-stage absorption heat pump must be used. In terms of energy and financially, this type of drying is very costly. Of the three processes that were studied, heat recovery systems proved to be the most energy efficient and economic solution.     

Experimental performance of ternary solutions in an absorption heat transformer

In this work, results from experiments with ternary solutions in an absorption heat transformer are presented. The experiments were performed under controlled conditions using water/lithium chloride/zinc chloride and water/calcium chloride/zinc chloride solutions as working pairs. The results showed that the gross temperature lift is increased with regard to the results obtained using binary solutions because the concentration of the solutions was enhanced. The water/lithium chloride/zinc chloride solution showed a generally better performance than the water/calcium chloride/zinc chloride mixture. The highest gross temperature lift for the former solution was 37·5°C for an absorber temperature of 96°C. This result compared favourably to that previously obtained for water/lithium bromide in the University of Salford. © 1998 John Wiley & Sons, Ltd.     

Performance analysis on a new multi-effect distillation combined with an open absorption heat transformer driven by waste heat

In this paper, a new water distillation system, which consists of either a single- or multi-effect distiller combined with an open absorption heat transformer (OAHT), has been proposed.The new integrated system can be used for distilling waste water with high amounts of SiO₂ from heavy oil production, and the resultant distilled water can be supplied to steam boilers to produce high quality steam which in turn is injected into oil reservoirs to assist with heavy oil recovery. The thermodynamic cycle performances for these new integrated distillation systems were simulated based on the thermodynamic properties of the aqueous solution of LiBr as well as the mass and energy balance of the system. The results indicate that combined with OAHT, the waste heat at 70 °C can be elevated to 125 °C and thereby produce steam at 120 °C in the absorber, which is able to drive a four-effect distiller to produce distilled water. For a single-effect and four-effect distiller, the coefficients of performance (COP) are approximately 1.02 while the performance ratios are 2.19 and 5.72, respectively. Therefore, the four-effect distillation system combined with an OAHT is more thermally effective and is an ideal option to process the waste water in oilfields.     

Solar absorption heat transformer applications to absorption refrigerating machines

This paper concerns the study of a two-stage vapour absorption system (heat transformer and refrigerating machine) employing the refrigerant absorbent combinations of LiBr-H₂O and NH₃-H₂O successively. The system consists of coupling the previous absorption cycles so that the first-stage absorber produces heating water to circulate in the generator of the second stage. The performances of a solar installation (two stage system plus solar collector) had been tested in Rabat (Morocco). It is found that the system can be operated at lower hot source temperatures and, thus, it can be supplied either from flat plate collectors or from thermal effluents.     

A review of working fluids of absorption cycles

A complete review of working fluids of absorption cycles is presented. Absorption cycles contain absorption heat pumps, absorption chillers and absorption transformers. Working fluids are divided into five series according to different absorbents.     

温度参数对升温型溴化锂吸收式热泵性能系数影响程度分析

通过对升温型溴化锂吸收式热泵进行理论分析,得到在相同工况条件下,发生温度变化对系统性能系数的影响程度最大,但小于吸收温度和冷凝温度影响程度之和.并利用焓浓度图,结合前人数据对理论分析进行了验证.     

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

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 pond's 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.     

A review of absorption refrigeration technologies

This paper provides a literature review on absorption refrigeration technology. A number of research options such as various types of absorption refrigeration systems, research on working fluids, and improvement of absorption processes are discussed.     

Comparative assessment of alternative cycles for waste heat recovery and upgrade

Thermally activated systems based on sorption cycles, as well as mechanical systems based on vapor compression/expansion are assessed in this study for waste heat recovery applications. In particular, ammonia-water sorption cycles for cooling and mechanical work recovery, a heat transformer using lithium bromide-water as the working fluid pair to yield high temperature heat, and organic Rankine cycles using refrigerant R245fa for work recovery as well as versions directly coupled to a vapor compression cycle to yield cooling are analyzed with overall heat transfer conductances for heat exchangers that use similar approach temperature differences for each cycle. Two representative cases are considered, one for smaller-scale and lower temperature applications using waste heat at 60 °C, and the other for larger-scale and higher temperature waste heat at 120 °C. Comparative assessments of these cycles on the basis of efficiencies and system footprints guide the selection of waste heat recovery and upgrade systems for different applications and waste heat availabilities. Furthermore, these considerations are used to investigate four case studies for waste heat recovery for data centers, vehicles, and process plants, illustrating the utility and limitations of such solutions. The increased implementation of such waste heat recovery systems in a variety of applications will lead to decreased primary source inputs and sustainable energy utilization.     

Mass and energy storage analysis of an absorption heat pump with simulated time dependent generator heat input

This communication presents an assessment of the feasibility of energy storage via refrigerant mass storage within an absorption cycle heat pump with simulated time dependent generator heat input. The system consists of storage volumes with the condenser and absorber of the conventional absorption cycle heat pump to store liquid refrigerant, weak and strong solutions during the generation period, which are required for the heat pump operation during the generation off period. A time dependent mass and energy storage analysis based on mass and energy balance equations for various components of the heat pump system has been carried out to evaluate energy storage concentration and storage efficiency for combined and separate storage schemes for the weak and strong solutions. Two possible performance modes, viz constant pumping ratio or the constant flow of the strong solution from the absorber to the generator have been analysed: the latter is preferable over the former from a practical point of view. Numerical computer simulation has been made for a typical winter day in Melbourne (Australia) with the desired heating load specified. It is found that the concept of refrigerant storage within the absorption cycle heat pump is technically feasible for efficient space heating. The energy storage concentration in the condenser store is slighly higher while that in absorber store is slightly lower for the separate storage mode as compared to the combined storage. However, the combined storage has an advantage of less storage volume and hence is more cost effective than separate storage and the disadvantage of limited system operation due to the decrease of solution concentrations.     

Heat pump assisted distillation. VII: A feasibility study on heat transformer assisted distillation systems

The problems in matching a heat driven heat transformer to a distillation process in heat pump assisted distillation are discussed. The performance of such a system is a function of the temperature difference across the distillation column. Plots are given of the calculated coefficients of performance against the temperatures of the top and bottom products. In addition plots are given of the calculated coefficients of performance against the generator temperature when the generator is independently heated. The percentage of energy saved is also plotted against coefficient of performance.     

Numerical simulation and performance assessment of a low capacity solar assisted absorption heat pump coupled with a sub-floor system

A prototype low capacity (10 kW) single stage Li–Br absorption heat pump (AHP), suitable for residential and small building applications has been developed as a collaborative result between various European research institutes and industries. The primary heat source for the AHP is supplied from flat plate solar collectors and the hot/chilled water from the unit is delivered to a floor heating/cooling system. In this paper we present the simulation results and an overview of the performance assessment of the complete system. The calculations were performed for two building types (high and low thermal mass), three climatic conditions, with different types of solar collectors and hot water storage tank sizes and different control systems for the operation of the installation. The simulations were performed using the thermal simulation code TRNSYS. The estimated energy savings against a conventional cooling system using a compression type heat pump was found to be in the range of 20–27%.     

Performance of ejector heat pumps

An ejector-compression heat pump can use low-grade thermal energy in the neighbourhood of 93.3°C (200°F) to provide space cooling and heating. This paper applies the existing ejector theory to estimate the performance of an ejector heat pump system at various operating conditions. The study includes parametric, sensitivity and off-design analyses of the heat pump performance. The performance enhancement options and desired ejector geometry are also examined. Refrigerants 11, 113 and 114 are three of the halocarbons most suitable for the ejector heat pump system. The estimated coefficients of performance for a simple ejector heat pump are 0.3 for the cooling mode and 1.3 for the heating mode at a sample operating condition in which the refrigerant (R-11) boiling temperature is 93.3°C (200°F), condensing temperature 43.3°C (110°F) and evaporating temperature 10°C (50°F). A 24 per cent performance improvement is predicted for a heat pump with two-stage ejectors and regenerative heat exchangers. The off-design performance is relatively insensitive to the evaporator temperature variations.     

Comparison of the modeling of a solar absorption system for simultaneous cooling and heating operating with an aqueous ternary hydroxide and with water/lithium bromide

This paper compares the theoretical performance of the modeling of a solar absorption system for simultaneous cooling and heating operating with water/lithium bromide and alternative aqueous ternary hydroxide mixtures. Aqueous ternary hydroxide working fluid consists of sodium, potassium and cesium 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 evaporator temperature. The results showed that, in general, the system with the hydroxide mixture may operate with higher coefficients of performance than the system with the lithium bromide mixture. Also it was shown that the system with the hydroxide may operate with a higher range of temperatures.     

Influence of absorber effectiveness on performance of vapour absorption heat transformers

The effects of mass transfer effectiveness of absorber on the performance of single stage vapour absorption heat transformers were studied by making a thermodynamic analysis. The refrigerant absorbent pairs considered were R21-DMF and R21-DMETEG. The variations in performance parameters such as coefficient of performance, exergetic efficiency, concentration difference and circulation ratio with mass transfer effectiveness of the absorber at various operating temperatures were computed. Increasing the effectiveness of absorber resulted in increases in coefficient of performance, exergetic efficiency and concentration differential across the absorber. The improvements in coefficient of performance and exergetic efficiency with mass transfer effectivensss of the absorber were more pronounced for the R21-DMF pair than for R21-DMETEG. Correlations are presented for quick estimation of performance under different operating conditions.