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.     

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.     

Performance simulation of an absorption heat transformer operating with partially miscible mixtures

This paper proposes to study the thermodynamics performances of a new absorption heat-transformer cycle, where the separation step is obtained by the cooling and settling of a partially miscible mixture at low temperature. This new cycle has been called an absorption-demixing heat transformer (ADHT) cycle. A numerical simulation code has been written, and has allowed us to evaluate the temperature lift and thermal yield of 2 working pairs. Both high qualitative and quantitative performances have been obtained, so demonstrating the feasibility and industrial interest for such a cycle. Moreover a comparison of the simulation results with performances really obtained on an experimental ADHT has confirmed the pertinence of the simulation code.     

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.     

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.     

Analytical model of solar pond with heat exchanger

This paper presents an analytical model of a three zone solar pond with heat exchange pipes laid in its bottom convective zone. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted are derived as a function of geometrical and operational parameters of the system. The transfer of heat from the pond bottom convective zone to the heat exchange fluid is expressed in terms of a heat removal factor, F_R. Analytical results, characteristic of the optimum performance of the pond, are presented and the criteria for the size and heat transfer characteristics of the heat exchanger are investigated. The annual average efficiency of heat extraction exhibits the asymptotic increase with the increase of length per unit pond area of heat exchange pipe.     

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.     

Effect of heat exchanger on the performance of a shallow solar pond water heater

In this communication, a mathematical model has been developed to predict the performance of a shallow solar pond water heater with a heat exchanger. Explicit heat balance equations are written for the plate temperature and water tank temperature, as well as for the heat extracting fluid temperature, by properly taking into account the absorption of solar radiation in the body of pond water. It is seen that efficiencies may be achieved as high as 60% at water flow rates of 0.1 – 0.2 kg/s m². Thereafter, the efficiency becomes almost constant at higher flow rates.     

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.     

Improvement of an existing solar powered absorption cooling system by means of dynamic simulation and experimental diagnosis

This paper focuses on the validation of a dynamic simulation model used to describe the performance of an existing solar cooling installation located in Zaragoza (Spain). The dynamic model has been developed under the simulation environment TRNSYS. The aim of this simulation model is to dispose of a tool in order to use it to evaluate different energy improvement actions in a real solar cooling installation. This solar cooling installation has been monitored and analyzed since 2007. The COP of this experimental solar cooling system presents a great influence from its heat rejection sink, a dry cooling tower. Once the model was validated with the experimental data obtained from the real installation, it was used to predict the chiller performance with a new geothermal sink, which started to operate in 2009. The present work describes the design and validation model process, as well as the comparison between the model results and the monitoring ones with the geothermal heat rejection system.     

Heat transfer performance of an external receiver pipe under unilateral concentrated solar radiation

The heat transfer and absorption characteristics of an external receiver pipe under unilateral concentrated solar radiation are theoretically investigated. Since the heat loss ratio of the infrared radiation has maximum at moderate energy flux, the heat absorption efficiency will first increase and then decrease with the incident energy flux. The local absorption efficiency will increase with the flow velocity, while the wall temperature drops quickly. Because of the unilateral concentrated solar radiation and different incident angle, the heat transfer is uneven along the circumference. Near the perpendicularly incident region, the wall temperature and absorption efficiency slowly approaches to the maximum, while the absorption efficiency sharply drops near the parallelly incident region. The calculation results show that the heat transfer parameters calculated from the average incident energy flux have a good agreement with the average values of the circumference under different boundary conditions. For the whole pipe with coating of Pyromark, the absorption efficiency of the main region is above 85%, and only the absorption efficiency near the parallelly incident region is below 80%. In general, the absorption efficiency of the whole pipe increases with flow velocity rising and pipe length decreasing, and it approaches to the maximum at optimal concentrated solar flux.     

Thermodynamic analysis of the performance of a solar absorption heat transformer at maximum coefficient of performance

It is proven that a solar absorption heat transformer affected by the irreversibility of finite-rate heat transfer may be modelled as an equivalent combined system consisting of a solar collector and an endoreversible absorption heat transformer, the latter being further treated as a combined cycle having an endoreversible heat pump driven by an endoreversible heat engine. The maximum coefficient of performance of the system is determined, based on the linear heat loss model for solar collectors and the general optimum relation for endoreversible absorption heat transformers. The optimality problems concerning the primary performance parameters of the system are discussed. The results obtained here may serve as a good guide for the evaluation of existing real solar absorption heat transformers or provide some theoretical bases for the optimal design of future solar absorption heat transformers. © 1997 by John Wiley & Sons, Ltd.     

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.     

Influence of generator effectiveness on performance of vapour absorption heat transformers

A thermodynamic analysis performed on a single-stage vapour absorption heat transformer brings out the influence of mass transfer effectiveness of the generator on its performance parameters. The working fluid pairs considered for the heat transformer are R21-DMF and R21-DMETEG. The variations in the performance parameters, namely, heating coefficient of performance, exergetic efficiency, concentration difference and circulation ratio, with the changes in generator effectiveness are studied. As expected, an increase in generator effectiveness results in the improvement of the coefficient of performance, exergetic efficiency and concentration difference across the absorber. The extent of improvement is more pronounced for R21-DMETEG than for R21-DMF. Correlations are presented for quick estimation of performance parameters under different operating conditions.     

Effect of thermal trap on the performance of an underground shallow solar pond water heater

A simple analysis of an underground shallow solar pond water heater has been presented. The effect of a thermal trap at the top of the system has also been incorporated in the analysis. Using the model, the effect of various system parameters, viz. thermal trap thickness, heat capacity of water mass, flow rate and duration of flow rate have been studied in detail. Numerical calculations have been made for a typical winter day at New Delhi (India). It is concluded that the system with thermal trap gives better performance in comparison with a system with a movable insulation system.     

Enhancing the thermal efficiency of solar ponds by extracting heat from the gradient layer

An alternative method of heat extraction from salinity-gradient solar ponds is investigated with the aim of increasing the overall energy efficiency of collecting solar radiation, storing heat and delivering this heat to an application. In this alternative method, heat is extracted from the non-convecting gradient layer of a solar pond as well as, or instead of, from the lower convective zone (LCZ). A theoretical analysis of combined gradient-layer and LCZ heat extraction is conducted to obtain expressions for the variation of temperature with depth in the pond, and the temperature gradient with depth. The dependence of the overall energy efficiency of the pond on thickness of the gradient-layer, temperature of delivered heat, and various combinations of gradient layer and LCZ heat extraction rates, including the limiting cases of gradient-layer heat extraction only, and LCZ heat extraction only, is then explored. This theoretical analysis suggests that heat extraction from the gradient layer has the potential to increase the overall energy efficiency of a solar pond delivering heat at a relatively high temperature by up to 50%, compared with the conventional method of heat extraction solely from the LCZ. The potential gain in efficiency using gradient-layer heat extraction is attributed to the lowering of heat losses by conduction to the upper convective (surface) zone that can be achieved with this method. Experimental investigations are proposed to test the predictions of the theoretical analysis in practice, and assess the impact of a number of idealized assumptions made on the findings reported here.     

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%.     

Experimental performance evaluation of a novel heat exchanger for a solar hot water storage system

The performance of a novel heat exchanger unit (‘Solasyphon’) developed for a solar hot water storage system was experimentally investigated. The ‘Solasyphon’ is a simple ‘bolt-on’ heat exchange unit that can be integrated externally to a traditional single-coil hot water cylinder (HWC) avoiding the costly replacement of an existing HWC with a twin-coil HWC. The installation cost of a ‘Solasyphon’ is lower compared to a traditional HWC thus offers greater cost effectiveness. A data acquisition system was designed to compare the thermal performance of an integrated ‘Solasyphon’ HWC with a traditional twin-coil HWC under controlled simulated conditions. The analysis was based on experimental data collected under various operating conditions including different primary supply temperatures (solar simulated); primary supply patterns and draw off patterns. The results indicated that the ‘Solasyphon’ delivered solar heated water directly to the top of the HWC producing a stratified supply at a useable temperature. Under variable solar conditions the ‘Solasyphon’ would transfer the heat gained by a solar collector to a HWC more efficiently and quickly than a traditional HWC. The ‘Solasyphon’ system can reduce installation costs by 10–40% and has a lower embodied energy content due to less material replacement.     

Comparative performance evaluation of fertiliser solar pond under simulated conditions

An experimental test rig for solar pond simulation was developed to study the chosen fertiliser salt, Muriate of Potash (MOP) for use in a solar pond under simulated conditions with provisions to vary the heating input and maintain a particular lower convective zone temperature. The performance, in terms of temperature and density profiles, was studied for MOP and was compared with that of sodium chloride and saltless solar ponds for different heating regimes and lower convective zone temperatures. The formation of three zones viz., upper convective zone, nonconvective zone, and lower convective zone was distinct at all heating combinations for both MOP and sodium chloride salts under simulated conditions. The temperature and density gradients were not affected significantly by intermittent no-heating spells of the solar ponds. Maintaining lower convective zone temperature of 70 °C and above led to the initiation of minor internal convective zone under simulated conditions. The temperature decay of lower convective zone (LCZ) was at lesser rate for different LCZ temperatures associated with both the heating regimes, for a MOP pond over a 24 h period of cessation of heating as compared to sodium chloride and saltless ponds.     

Numerical heat transfer studies of PCMs used in a box-type solar cooker

Theoretical investigations on the phase change materials (PCMs) used as the heat storage media for box-type solar cookers have been conducted in this study. The selected PCMs are magnesium nitrate hexahydrate, stearic acid, acetamide, acetanilide and erythritol. For a two-dimensional simulation model based on the enthalpy approach, calculations have been made for the melt fraction with conduction only. Different materials such as glass, stainless steel, tin, aluminum mixed, aluminum and copper are used as the heat exchanger container materials in the numerical calculations. The large value of thermal conductivity of heat exchanger container material did not make a significant contribution on the melt fraction except for at very low thermal conductivities. Based on the theoretical results, stearic acid and acetamide are found to be good compatibility with latent heat storage system. It is also found that the initial temperature of PCM does not have very important effects on the melting time, while the boundary wall temperature plays an important role during the melting and has a strong effect on the melt fraction. The results also show that the effect of thickness of container material on the melt fraction is insignificant. The results obtained in this paper show that in a box-type solar cooker, acetamide and stearic acid should be used as a latent heat storage materials.