Study on the heat transfer and sorption characteristics of a consolidated composite sorbent for solar-powered thermochemical cooling systems

In this paper, a new consolidated composite sorbent made from barium chloride and expanded graphite is presented for solar-powered thermochemical sorption cooling systems. A larger sorption capacity and volume cooling density can be obtained with chemisorption systems when compared with those based on physicosorption. The heat transfer and sorption characteristics of the composite sorbent were investigated. Experimental results showed that the chemical composite sorbent can effectively utilize solar energy or low-grade waste heat sources with temperature ranging from 75 to 90 °C, and it could incorporate 0.61 kg of ammonia per kg of the reactive salt. The temperature evolution in the reactor was strongly influenced by the physicochemical reaction, whereas the transient heat transfer properties in the reactive composite material were different during the decomposition and the synthesis phases owing to the variation of the ammonia content and solid configuration inside the metallic salt complex. The rate of conversion in the reactor was very sensitive to the working temperatures and pressures, and the COP (coefficient of performance) obtained with the consolidated composite sorbent varied between 0.50 and 0.53 when the evaporation temperature ranged from 0 to 15 °C at a generation temperature of 80 °C.     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Modelling and performances of a deep-freezing process using low-grade solar heat

A solar deep-freezing process has been designed. It aims at cooling down a cold box to about −20 °C, using simple flat plate solar collectors operating at 70 °C. This original process involves two cascaded thermochemical systems based on the BaCl₂/ammonia reaction. Its working mode is discontinuous as it alternates between a regeneration mode during daytime and a cold production mode during nighttime. A global dynamic model involving the various system components allows the simulation of the process; it predicts the evolution of the components temperatures and the rates of chemical reactions of the system. It also allows the dimensioning of the system components to maintain a 500 l cold box at −20 °C during the 6 sunniest months of the year under typical Mediterranean weather conditions and provide over 80% of the total yearly cooling needs of this box. This requires a solar collector area of 5.8 m² and 39 kg of reactive salt. The predicted coefficient of performance (COP) is about 0.1 over the year, and the net solar COP, taking into account the collector efficiencies, is 0.05.     

New deep-freezing process using renewable low-grade heat: From the conceptual design to experimental results

An exergy-based analysis applied to ideal thermochemical dipoles allowed to design an original process that could use low-grade energy, produced from a thermal solar collector at around 70 °C, to provide low-temperature cold, below −23 °C, in order to store deep-frozen food. The ideal coefficient of performance (COP) of this system is 0.5 and the exergetic yield is 1. Taking into account the process enthalpies and the sensible heat of the reactants, the COP_thermo is 0.17. The process functioning is described in this paper. It alternates between a regeneration mode during daytime and cold production mode during night-time. An experimental prototype was designed and built. It proved the feasibility of the concept and showed an experimental COP of about 0.06, which is similar to the up-to-date solar cooling systems, but at higher cold temperatures. The mean annual exergetic yield of the process is about 0.06.     

Experimental investigation of a natural zeolite–water adsorption cooling unit

In this study, a thermally driven adsorption cooling unit using natural zeolite–water as the adsorbent–refrigerant pair has been built and its performance investigated experimentally at various evaporator temperatures. The primary components of the cooling unit are a shell and tube adsorbent bed, an evaporator, a condenser, heating and cooling baths, measurement instruments and supplementary system components. The adsorbent bed is considered to enhance the bed’s heat and mass transfer characteristics; the bed consists of an inner vacuum tube filled with zeolite (zeolite tube) inserted into a larger tubular shell. Under the experimental conditions of 45 °C adsorption, 150 °C desorption, 30 °C condenser and 22.5 °C, 15 °C and 10 °C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric cooling power density (SCP_v) and mass specific cooling power density per kg adsorbent (SCP) of the cooling unit are 5.2 kW/m³ and 7 W/kg, respectively.     

Novel composite sorbent for resorption systems and for chemisorption air conditioners driven by low generation temperature

The utilization of a composite sorbent (NaBr and expanded graphite) in chemisorption air conditioning systems driven by low-grade heat source, and in resorption systems with simultaneous heating and cooling effects was experimentally investigated using bench-scale prototypes. The mass of ammonia desorbed and adsorbed was measured, and used to calculate the specific cooling capacity. The sorbent produced 219 kJ kg⁻¹ of cooling at 5 °C and 510 kJ kg⁻¹ at 15 °C, when the heat source temperature was 65 °C and the heat sink temperature was 30 °C. The air conditioning system mean specific cooling power (SCP), and mean coefficient of performance (COP) were calculated based on the desorbed and adsorbed masses, and on the variation of temperature in the reactors. For the same heat source and heat sink temperatures mentioned above, the air conditioning system had a SCP of 129 ± 7 W kg⁻¹ and a COP of 0.46 ± 0.01, when cooling occurred at 15 °C. Regarding the utilization of the composite sorbent in resorption machines, the prototype was tested for production of cooling/heating at −5/50 °C, and at 10/70 °C. In the former condition, the COP was only 0.02, but in the latter condition, there was a tenfold increase in the COP, and the combined coefficient of performance and amplification reached 1.11, which indicates the energy saving potential of resorption systems using the studied sorbent.     

Experimental study with operational solar‐sorption cooling

Building integrated solar systems have been considered as a reasonable system for building heating, cooling and hot water supply. Various types of solar collectors, such as plate type, evacuated tube type and solar air collector, have been used as the heat source, whereas adsorption chillers, absorption chillers and desiccant dehumidification systems have been considered to match the above solar heat sources. Now, such sorption chillers are more matured, but their coupling with suitable solar heat source is not well researched. Experimental study has been done in this paper to analyse four kinds of typical solar air‐conditioning system with different sorption chillers and solar collectors. Copyright © 2012 John Wiley & Sons, Ltd.     

Multi-pressure absorption cycles in solar refrigeration:: A technical and economical study

A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 140°C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 140°C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 140°C or 200°C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy.     

Study of the effect of finned tube adsorber on the performance of solar driven adsorption cooling machine using activated carbon–ammonia pair

Solar refrigeration represents an important application of solar energy due to the excellent matching between the high sunshine and the refrigeration needs. Solar adsorption refrigeration devices are among the significant techniques used to meet the needs for cooling requirements. Several solar refrigeration systems have been proposed and are under development such as sorption systems including liquid/vapor, solid/vapor absorption, adsorption, vapor compression and others. The purpose of this paper is to identify the influence of a cylindrical adsorber on the performances of a solar adsorption refrigerating machine. The adsorber heated by solar energy contains an activated carbon–ammonia pair; it is composed by many cylindrical tubes welded using external fins. A model based on the conservation equations of energy and mass in the adsorber has been developed and well described. Using real solar irradiance data as well as many initial conditions, the model computes for each point and in the considered time interval during the day, the temperature, the adsorbed mass, the pressure inside the adsorber and the solar performance coefficient (COP). The results show that the optimal diameter of the adsorber with fins is greater than the one without fins. Moreover the mass cycled in the case of an adsorber equipped with external fins is more significant than the one without fins, and the maximal temperature reached in the adsorber with fins attains 97 °C while in the adsorber without fins reaches 77 °C. Thus, the performances of the solar adsorption refrigerating machine with an adsorber equipped with fins are higher than the machine without fins.     

Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store

A domestic-scale prototype experimental solar cooling system has been developed based on a LiBr/H₂O absorption system and tested during the 2007 summer and autumn months in Cardiff University, UK. The system consisted of a 12 m² vacuum tube solar collector, a 4.5 kW LiBr/H₂O absorption chiller, a 1000 l cold storage tank and a 6 kW fan coil. The system performance, as well as the performances of the individual components in the system, were evaluated based on the physical measurements of the daily solar radiation, ambient temperature, inlet and outlet fluid temperatures, mass flow rates and electrical consumption by component. The average coefficient of thermal performance (COP) of the system was 0.58, based on the thermal cooling power output per unit of available thermal solar energy from the 12 m² Thermomax DF100 vacuum tube collector on a hot sunny day with average peak insolation of 800 W/m² (between 11 and 13.30 h) and ambient temperature of 24 °C. The system produced an electrical COP of 3.6. Experimental results prove the feasibility of the new concept of cold store at this scale, with chilled water temperatures as low as 7.4 °C, demonstrating its potential use in cooling domestic scale buildings.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.     

Experimental study of condensation heat transfer of R-134a flow in corrugated tubes with different inclinations

This experimental study is performed to investigate condensation heat transfer coefficient of R-134a flow inside corrugated tube with different inclinations. Different inclinations of test condenser ranging from − 90^° to + 90^° and various flow mass velocities in the range of 87 to 253 [kg/m²s] are considered in this study. Data analysis showed that change in the tube inclination had a significant effect on condensation heat transfer behavior. At low mass velocities, and low vapor qualities, the highest condensation heat transfer coefficient was obtained for α = + 30^° which was 1.41 times greater than the least one obtained for α = − 90^°. The results also showed that at all mass velocities, the highest average heat transfer coefficients were achieved for α = + 30^°. Based on the experimental results, a new empirical correlation is proposed to predict the condensation heat transfer coefficient of R134a flow in corrugated tubes with different inclinations.     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

An investigation of the solar powered absorption refrigeration system with advanced energy storage technology

This paper presented a new solar powered absorption refrigeration (SPAR) system with advanced energy storage technology. The advanced energy storage technology referred to the Variable Mass Energy Transformation and Storage (VMETS) technology. The VMETS technology helped to balance the inconsistency between the solar radiation and the air conditioning (AC) load. The aqueous lithium bromide (H₂O-LiBr) was used as the working fluid in the system. The energy collected from the solar radiation was first transformed into the chemical potential of the working fluid and stored in the system. Then the chemical potential was transformed into thermal energy by absorption refrigeration when AC was demanded. In the paper, the working principle and the flow of the SPAR system were explained and the dynamic models for numerical simulation were developed. The numerical simulation results can be used to investigate the behavior of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the heat loads of heat exchanger devices and so on. An example was given in the paper. In the example, the system was used in a subtropical city like Shanghai in China and its operating conditions were set as a typical summer day: the outdoor temperature varied between 29.5 °C and 38 °C, the maximum AC load was 15.1 kW and the total AC capacity was 166.1 kW h (598.0 MJ). The simulation results indicated that the coefficient of performance (COP) of the system was 0.7525 or 0.7555 when the condenser was cooled by cooling air or by cooling water respectively and the storage density (SD) was about 368.5 MJ/m³. As a result, the required solar collection area was 66 m² (cooling air) or 62 m² (cooling water) respectively. The study paves the road for system design and operation control in the future.     

Study on a compact silica gel–water adsorption chiller without vacuum valves: Design and experimental study

A compact silica gel–water adsorption chiller without vacuum valves was manufactured and experimentally studied. This chiller contains two adsorption/desorption chambers and one chilled water tank. Each adsorption/desorption chamber consists of one adsorber, one condenser and one evaporator. The chilled water tank is adopted to mitigate the variation of the chilled water outlet temperature. A mass recovery-like process, which is a heat recovery process between the two evaporators, was carried out in this chiller. A novel heat recovery process was also fulfilled after the mass recovery-like process to improve the coefficient of performance (COP). The cooling power and COP were 9.60 kW and 0.49 respectively when the average hot water inlet temperature, cooling water inlet temperature, and chilled water outlet temperature were 82.0, 31.6 and 12.3 °C, respectively.     

Lithium chloride - Expanded graphite composite sorbent for solar powered ice maker

Consolidated composite material made from expanded graphite (EG) powder impregnated with LiCl salt is proposed for use in solar powered adsorption ice makers. Laboratory experiments were done to test the adsorption and desorption performance of the sorbent under different temperature conditions suitable for solar energy utilization. More than 75% of the reaction between LiCl and ammonia was completed after 30 min of synthesis at evaporation temperatures of −10 and −5 °C and adsorption temperature between 25 and 35 °C. Under the same period, it was possible to obtain 80% conversion in the desorption phase, when the generation temperatures ranged between 75 and 80 °C, and the condensation temperature varied from 25 to 35 °C. The highest average specific cooling power during the synthesis phase was 117 W per kg of the block. The calculated theoretical coefficient of performance (COP) under different cycle conditions was nearly constant at 0.47. Moreover, the new composite sorbent showed higher Specific Cooling Capacity (SCC), compared to activated carbon (AC)/methanol pair. Experiments done with blocks with different proportion of EG, showed that the proportion of EG influence the cooling capacity per unit mass of salt and had almost no influence on the cooling capacity per unit mass of the block. Moreover, the reaction enthalpy (ΔH) and entropy (ΔS) were calculated from experimental data obtained experimentally, and confirmed previous reported values.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

3-D numerical and experimental models for flat and embedded heat pipes applied in high-end VGA card cooling system

The present study utilizes the three-dimension numerical and experimental methods to investigate the optimum thermal performance of a flat heat pipe-thermal module application in high-end VGA card cooling system, and compares that with a traditional copper metal based plate embedded three 6 mm diameter heat pipe-thermal module under three dissimilar inclination angles of 0°, 90° and 180°. The optimization for the thermal modules researches into various fin material, thickness and gap. Results show that the flat heat pipe-thermal module has the best thermal performance at high power GPU of 180 W and inclination angle of 180°. Simulation results show in good agreement with experimental results within 5%. Therefore, the thermal performance of a flat heat pipe-thermal module can be accurately simulated and analyzed by employed the manner introduced in this paper and is able to cope with the higher heat flux GPU over 62.5 W/cm² in the future.     

Estimation of internal heat transfer coefficients of a hybrid (PV/T) active solar still

In this paper, an attempt is made to estimate the internal heat transfer coefficients of a deep basin hybrid (PV/T) active solar still. The estimation is based on outdoor experimental observation of hybrid (PV/T) solar still for composite climate of New Delhi (latitude 28°35′N and longitude 77°12′E). The internal heat transfer coefficients are evaluated by using thermal models proposed by various researchers. The comparison of hourly yield predicted using various thermal models to the experimental has also been carried out by evaluating the correlation coefficient and percentage deviation. It is observed that, Kumar and Tiwari model (KTM) better validate the results than the others model. The average annual values of convective heat transfer coefficient for the passive and hybrid (PV/T) active solar still are observed as 0.78 and 2.41 W m⁻² K⁻¹, respectively at 0.05 m water depth.     

Vapor feed direct methanol fuel cells with passive thermal-fluids management system

The present paper describes a novel technology that can be used to manage methanol and water in miniature direct methanol fuel cells (DMFCs) without the need for a complex micro-fluidics subsystem. At the core of this new technology is a unique passive fuel delivery system that allows for fuel delivery at an adjustable rate from a reservoir to the anode. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The innovative thermal management mechanism is the key for effective operation of the fuel cell system. The vapor feed DMFC reached a power density of 16.5 mW cm⁻² at current density of 60 mA cm⁻². A series of fuel cell prototypes in the 0.5 W range have been successfully developed. The prototypes have demonstrated long-term stable operation, easy fuel delivery control and are scalable to larger power systems. A two-cell stack has successfully operated for 6 months with negligible degradation.