Performance analysis of a waste heat driven activated carbon based composite adsorbent – Water adsorption chiller using simulation model

This study aims at improving the performance of a waste heat driven adsorption chiller by applying a novel composite adsorbent which is synthesized from activated carbon impregnated by soaking in sodium silicate solution and then in calcium chloride solution. Modeling is performed to analyze the influence of the hot water inlet temperature, cooling water inlet temperature, chilled water inlet temperatures, and adsorption/desorption cycle time on the specific cooling power (SCP) and coefficient of performance (COP) of the chiller system with the composite adsorbent. The simulation calculation indicates a COP value of 0.65 with a driving source temperature of 85 °C in combination with coolant inlet and chilled water inlet temperature of 30 °C and 14 °C, respectively. The most optimum adsorption–desorption cycle time is approximately 360 s based on the performance from COP and SCP. The delivered chilled water temperature is about 9 °C under these operating conditions, achieving a SCP of 380 W/kg.     

Study on solar/waste heat driven multi-bed adsorption chiller with mass recovery

The study deals with a solar or waste heat driven three-bed adsorption cooling cycle employing mass recovery scheme. A cycle simulation computer program is developed to investigate the performance of the chiller. The innovative chiller is driven by exploiting solar/waste heat of temperatures between 60 and 90 °C with a cooling source at 30 °C for air-conditioning purpose. The performance of the three-bed adsorption chiller with mass recovery scheme was compared with that of the three-bed chiller without mass recovery. It is found that cooling effect as well as solar/waste heat recovery efficiency, η of the chiller with mass recovery scheme is superior to those of three-bed chiller without mass recovery for heat source temperatures between 60 and 90 °C. However, COP of the proposed chiller is higher than that of the three-bed chiller without mass recovery, when heat source temperature is below 65 °C.     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.     

Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller

In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively.     

Study on a re-heat two-stage adsorption chiller – The influence of thermal capacitance ratio,overall thermal conductance ratio and adsorbent mass on system performance

Silica gel/water based adsorption cycles have a distinct advantage in their ability to be driven by heat of near-ambient temperature so that waste heat below 100 °C can be recovered. One interesting feature of refrigeration cycles driven by waste heat is that they do not use primary energy as driving source. From this context, some researchers investigated the performance of multi-stage adsorption refrigeration cycles those can be operated by heat source of temperature 60 °C or lower which are usually purged to the environment, with a heat sink of temperature at 30 °C. However, the performances of multi-stage systems are low. To improve system performance, an analytic investigation on a re-heat two-stage chiller is performed to clarify the effect of thermal capacitance ratio of the adsorbent and inert material of sorption element, overall thermal conductance ratio of sorption element and evaporator along with silica gel mass on the chiller performance. Results show that cycle performance is strongly influenced by the sorption elements overall thermal conductance values due to their severe sensible heating and cooling requirements resulting from batched cycle operation. The effect of thermal capacitance ratio (C_s/C_m) becomes significant with relatively higher mass of silica gel. It is also found that the chiller performance increases significantly in the range of silica gel mass from 4 to 20 kg.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Novel ammonia sorbents “porous matrix modified by active salt” for adsorptive heat transformation: 3. Testing of “BaCl2/vermiculite” composite in a lab-scale adsorption chiller

A composite adsorbent composed of BaCl₂ impregnated into expanded vermiculite has been synthesized and tested in a laboratory scale adsorption chiller. Previous work has established the promising theoretical performance of this adsorbent with ammonia as a refrigerant, in terms of equilibrium uptake, suitable equilibrium temperatures for use in air conditioning applications and good reaction dynamics. Analysis of the adsorption phase revealed a simple exponential approach to equilibrium uptake which was not previously observed in larger scale experiments.It was demonstrated that this material can provide effective operation of the chiller using a low potential heat source (80–90 °C) giving COP as high as 0.54 ± 0.01 and SCP ranging from 300 to 680 W/kg. The specific cooling power depends strongly on the driving temperature difference and the cycle duration.     

Waste heat driven silica gel/water adsorption cooling in trigeneration

A low-grade waste heat driven solid/vapour adsorption chiller has been successfully designed and tested. A simple model was developed to aid the design and predict the performances. The system comprised two identical sorption reactors operating out of phase in order to ensure continuous cold production. One sorption reactor consisted of six commercially available automotive plate/fin heat exchangers in which silica gel grains were accommodated between the fins. The system was tested as to the power delivered at 12 °C and the power density. The average cooling power was 3.6 kW. This is only 72% of the design value and can be largely attributed to the lower heat transfer fluid flow rate through the sorbent reactor. The thermal efficiency, COP, was 0.62 and the power density was 17 kW/m³ for the system as a whole. Higher power densities are possible. At present, the adsorption chiller is integrated in a prototype trigeneration system, which is tested at CRF’s Eco-building in Turin.     

Experimental study of a low temperature heat driven re-heat two-stage adsorption chiller

The performance of an advanced adsorption chiller, namely, ‘reheat two-stage’ has been investigated experimentally in the present study. The performances in terms of specific cooling power (SCP) and COP are compared with those of conventional single and two-stage chiller. Results show that the reheat two-stage chiller provides more SCP values than those provided by conventional single-stage chiller while it provides better COP values for relatively low heat source temperature. The reheat two-stage chiller also provides almost same cooling capacity comparing with two-stage chiller for the low temperature heat source, while it provides higher COP value than that provided by two-stage chiller. Experimental results also show that the overall performance of the reheat two-stage chiller is always higher than that of conventional single and two-stage adsorption cycle even the temperature of the heat source is fluctuated between 55 and 80 °C.     

Micro tri-generation system for indoor air conditioning in the Mediterranean climate

Mediterranean countries show two specific features regarding air-conditioning of buildings: a high—and growing—cooling load and high relative humidity, at least in coastal zones. In this contribution we report on the development of an innovative micro scale tri-generation system (power + heating + cooling), equipped with a rotor based desiccant system adapted to the Mediterranean conditions which receives heat for the desiccant regeneration from a combined heat and power (CHP) cycle.The paper presents the design of the advanced desiccant air handling unit which uses a high efficient combination of a vapor compression chiller working at a high evaporator temperature and a desiccant wheel (silica gel). The electricity of the chiller is supplied by the CHP system and the heat to regenerate the desiccant is the waste heat of the CHP. System simulations have been used to optimize the hydraulic design and the operation strategy in order to minimize operation costs and maximize energy savings. Some new component models, e.g. for the advanced desiccant cycle were developed for this purpose. The final design of the entire system consisting of the CHP system, the vapor compression chiller, the advanced desiccant air handling unit and the load system is described. The load system is composed of an air duct network with induction units and a chilled water network with fan-coils in the office rooms.Regarding energy performance results indicate an electricity saving >30% in comparison to state-of-the-art solutions based on conventional technology.     

Adsorption cooling driven by solar collector: A case study for Tokyo solar data

An analytical investigation has been performed to study the possibility of application of solar cooling for the climatic condition of Tokyo, Japan. Silica gel–water adsorption cooling system has been taken into consideration for the present study and lumped parameter model is used to investigate the performance of the system. Based on the solar radiation data it is found that at least 15 collector (each of 2.415 m²) is required to achieve the required heat source temperature (around 85 °C) to run the cooling unit. It is also observed that the solar powered adsorption cooling unit provides cooling capacity around 10 kW at noon with base run conditions, while the system provides solar COP around 0.3, however, the solar collector size can be reduced by optimizing the cycle time.     

α–h Diagram and principle of exergy coupling of GAX cycle

Ammonia absorption chiller systems of a single-stage cycle and a Generator Absorber heat exchanger cycle (GAX) were simulated and studied. At heat source temperatures of T_H = 120 °C, T_M = 25 °C and T_L = 5 °C, the coefficient of performances of the two cycles are 0.589 and 0.776, the GAX cycle is higher 31.8% than the single-stage cycle. And the exergy efficiencies of the two cycles are 15.4% and 27.4%, the GAX cycle is higher up to 77.9%. This paper proposes a new method that adopts the energy quality factor α as a evaluation criterion and also uses the α–h diagram as a thermodynamic analysis tool graphically, and a concept that divides absorption cycle to a heat pump subcycle and a heat engine subcycle. By means of the α–h diagram, the thermodynamic frameworks of the two cycles were illustrated. The comparison analysis indicates that the improvement of cycle performance depends on its thermodynamic perfectibility. In fact, the exergy demand of heat pump subcycle in the GAX cycle is as the same as that of the single-stage cycle, however, the energy cascading use and the exergy coupling framework of the heat engine subcycle in GAX cycle is retrofitted, so that the exergy consumption is reduced and the increased benefit is obtained from the overall cycle.     

Experimental results on operating parameters influence for an adsorption refrigerator

A continuous heat recovery adsorption refrigerator using activated carbon-methanol has been developed. In this system, the heat source to drive the adsorption system can be controlled at a temperature from 60 °C to 110 °C, and the evaporating temperature can also be controlled at any requested value from 0 °C to 15 °C. To realize the operation performance of the system, many sensors of temperature, pressure and flow rate are installed in the adsorbers, the condenser and the evaporator. A lot of experiments have been completed in different operation conditions. Thus, by means of the experimental data, influences of the operating parameters, such as heat source temperature, evaporating temperature, cooling water temperature, cycle time and flow rate of throttling valve and so on, on p-t-x diagram of the cycle, specific cooling power (SCP) and coefficient of performance (COP) have been asserted. And causes of the influence are also analyzed. A series of conclusions are obtained.     

The influence of heat exchanger parameters on the optimum cycle time of adsorption chillers

The research investigated the influences of heat exchanger parameters, such as heat capacity and NTU, on the optimum performance of a single-stage adsorption chiller. Silica gel–water pair was chosen as the adsorbent–adsorbate combination so that low temperature heat source under than 100 °C could be utilized as the driving force.The mathematical model of the adsorption chiller using dimensionless parameters was developed and a global optimization method called the particle swarm optimization was applied in the simulation to obtain the optimum cycle time. The results showed that the smaller heat capacity heat exchanger improved both the maximum specific cooling capacity (SCC) and the COP. While, the larger NTU of the adsorbent bed resulted in the decrease of the COP due to the short cycle time although the maximum SCC was enhanced.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

The effect of cycle boundary conditions and adsorbent grain size on the water sorption dynamics in adsorption chillers

The aim of this work was an experimental study of the temporal evolution of isobaric adsorption uptake (release) for simplest configuration of an adsorbent-heat exchanger unit, namely, a monolayer of loose adsorbent grains located on a metal plate. The study was performed by a large temperature jump method at four various boundary conditions of an adsorptive heat transformation cycle typical for air-conditioning application driven by low temperature heat: T_e = 5 and 10 °C, T_c = 30 and 35 °C and T_HS = 80 °C. The size of the Fuji silica grains was varied from 0.2 to 1.8 mm to investigate its effect on water sorption dynamics. For each boundary set and grain size the experimental kinetic curve could be described by an exponential function up to 80–90% of the equilibrium conversion. Desorption runs are found to be faster than appropriate adsorption runs by a factor of 2.2–3.5, hence, for optimal durations of the isobaric ad- and desorption phases of the chilling cycle should be selected accordingly. The size R of the adsorbent grains was found to be a powerful tool to manage the dynamics of isobaric water ad-/desorption. For large grains the characteristic time was strongly dependent on the grain size and proportional to R². Much less important appeared to be an impact of the boundary conditions which variation just weakly affected the dimensionless kinetic curves for the four tested cycles. The maximal specific cooling/heating power was proportional to the maximal temperature difference ΔT and the contact area S between the layer and the metal plate, and can exceed 10 kW/kg. The heat transfer coefficient α estimated from this power was as large as 100–250 W/(m² K) that much exceeds the value commonly used to describe the cycle dynamics.     

A new methodology of studying the dynamics of water sorption/desorption under real operating conditions of adsorption heat pumps: Experiment

In this paper we proposed and tested a new methodology of studying the kinetics of water vapour sorption/desorption under operating conditions typical for isobaric stages of sorption heat pumps. The measurements have been carried out on pellets of composite sorbent SWS-1L (CaCl₂ in silica KSK) placed on a metal plate. Temperature of the plate was changed as it takes place in real sorption heat pumps, while the vapour pressure over the sorbent was maintained almost constant (saturation pressures corresponding to the evaporator temperature of 5 °C and 10 °C and the condenser temperature of 30 °C and 35 °C). Near-exponential behaviour of water uptake on time was found for most of the experimental runs. Characteristic time τ of isobaric adsorption (desorption) was measured for one layer of loose grains having a size between 1.4 mm and 1.6 mm for different heating/cooling scenarios and boundary conditions of an adsorption heat pump. Maximum specific power estimated from the τ-values can exceed 1.0 kW/kg of dry adsorbent, that gives proof to the idea of compact adsorption units for energy transformation with loose SWS grains.     

Adsorption cooling cycles for alternative adsorbent/adsorbate pairs working at partial vacuum and pressurized conditions

This article presents the performance analysis of both ideal single-stage and single-effect double-lift adsorption cooling cycles working at partially evacuated and pressurized conditions. Six specimens of adsorbents and refrigerant pairs, i.e., ACF (A-15)/ethanol, ACF (A-20)/ethanol, silica gel/water, Chemviron/R134a, Fluka/R134a and MaxsorbII/R134a have been investigated. The relationships between equilibrium pressures, adsorbent temperatures and equilibrium adsorption concentrations (Dühring diagram) are presented. Parametric analyses have been carried out with various regeneration (desorption) and evaporation temperatures. Theoretical analysis for adsorption cycles working in single-stage mode shows that ACF (A-20)/ethanol can achieve a specific cooling effect (SCE) of 344 kJ/kg_ads, which is followed by the silica gel/water pair with 217 kJ/kg_ads at a regeneration temperature of 85 °C. On the other hand, when the regeneration temperature is below 70 °C, single-effect double-lift cycle has a significant advantage over single-stage cycle, at which the SCE is higher due to the reduction in adsorption bed pressure in single-effect double-lift cycle.     

Study on activated carbon/HFO-1234ze(E) based adsorption cooling cycle

In this study, the dynamic behavior of a 4-bed adsorption chiller was analyzed employing highly porous activated carbon of type Maxsorb III as the adsorbent and R1234ze(E), which global warming potential (GWP) is as low as 9, as the refrigerant. The simulated results in terms of heat transfer fluid temperatures, cycle average cooling capacity and coefficient of performance (COP) were obtained numerically. With 80 kg of Maxsorb III, the system is able to produce 2 kW of cooling power at driving heat source temperature of 85 °C which can be obtained from waste heat or solar energy. In particular, it can be powered by the waste heat from the internal combustion engine and therefore is suitable for automobile air-conditioning applications.