Heat Recovery Time Research Articles

Operating characteristics of an adsorption chiller with heat and mass recovery scheme for low-grade heat source

Waste heat recovery from data center is regarded as one of the most promising application of adsorption refrigeration in the future. To better understanding of the system, the operating characteristics, including temperature, pressure and heat exchange capacity, of the adsorption chiller for such low-grade heat source should be revealed. Unfortunately, they have not been detailedly reported yet. In this paper, lab experiments on a silica gel-water adsorption chiller prototype with heat and mass recovery scheme are implemented. The results show that this chiller can achieve 3.17 kW of average cooling power and 0.482 of coefficient of performance under the conditions of 60 °C hot water inlet temperature, 30 °C cooling water inlet temperature and 22 °C chilled water inlet temperature. The temperature and pressure characteristics reveal that adsorbent bed desorption phase results in the worsening of the deviation from ideal cycle when compared to its adsorption phase. Hence, the heat and mass transfer performance of bed in desorption phase is prioritized to be enhanced. The analysis on heat exchange capacity indicates that mass recovery operation is more beneficial to the adsorption reaction. The optimal heat recovery time is found to be 24 s while the heat recovery efficiency is 0.711.

Latent heat recovery of composite PCM for hybrid BTMSs based on micro-encapsulated phase change slurry

Phase change material (PCM) cooling is widely applied in battery thermal management systems (BTMS) due to high latent heat. To address the long latent heat recovery time of PCM, especially in continuous discharge and charge cycles, a novel hybrid BTMS coupling dual-PCM is designed. The dual-PCM contains the composite PCM as the main heat dissipation and the micro-encapsulated phase change slurry (MEPCS) for latent heat recovery. The experiments are performed to ensure the effectiveness of the hybrid BTMS. The temperature performance of the battery module, solidification rate of composite PCM and pressure drop of the MEPCS are studied as the critical performance indicators. To establish a trade-off between these performance indicators, the interaction effects between inlet velocities and mass concentrations of the MEPCS are investigated. Meanwhile, different ambient temperatures combined with mass concentrations are considered. Results indicate that MEPCS as the coolant has a significant effect on the temperature and solidification performances at low inlet velocity. The higher mass concentration of MEPCS does not mean better heat recovery capacity at the same pressure drop. Considering the fluctuation of the solidification performance, the mass concentration of MEPCS above 10 % is recommended to meet the challenges of different ambient temperatures. The energy efficiency is optimized by adjusting the mass concentration and inlet velocity. The maximum reduction in pressure drop is 39.8 % with high temperature and solidification performances. The solutions can reduce energy consumption and improve the economic benefits for BTMS.

Modeling and Simulation of a Two-Stage Air Cooled Adsorption Chiller with Heat Recovery Part I: Physical and Mathematical Performance Model

In the proposed work, the MATLAB program was used to model and simulate the performance of the investigated two-stage adsorption chiller with and without heat recovery using an activated carbon/methanol pair. The simulated model results were then validated by the experimental results conducted by Millennium Industries. The model was based on 10th order differential equations; six of them were used to predict bed, evaporator and condenser temperatures while the other four equations were used to calculate the adsorption isotherm and adsorption kinetics. The detailed validation is stated in the next paragraphs; for example, it clearly notes that the simulation model results for the two-stage air cooled chiller are well compared with the experimental data in terms of cooling capacity (6.7 kW for the model compared with 6.14 kW from the experimental results at the same conditions). The Coefficient of Performance (COP) predicted by this simulation was 0.4, which is very close to that given by the Carnot cycle working at the same operating conditions. The model optimized the switching time, adsorption/desorption time and heat recovery time to maximize both cooling capacity and COP. The model optimized the adsorption/desorption cycle time (300 to 400 s), switching cycle time (50 s) and heat recovery cycle time (30 s). The temporal history of bed, evaporator and condenser temperatures is provided by this model for both heat recovery and without heat recovery chiller operation modes. The importance of this study is that it will be used as a basis for future series production.

Modeling and Simulation of a Two-Stage Air-Cooled Adsorption Chiller with Heat Recovery Part II: Parametric Study

This study is the second part of the theoretical study of “Modeling and Simulation of a Two-Stage Air-Cooled Adsorption Chiller with Heat Recovery”, which is based on developing a theoretical model for a two-stage adsorption chiller with an activated carbon/methanol pair. The following models were conducted numerically using MATLAB. The model was based on 10th order differential equations; six of them were used to predict bed, evaporator and condenser temperatures, while the other four equations were used to calculate adsorption isotherm and adsorption kinetics. In this second part, bed heat exchangers and evaporator and condenser heat exchangers are studied by varying the parametric design of a chiller. This includes but is not limited to activated carbon mass inside a single bed, overall heat transfer coefficient for the bed and evaporator and the mass flow rates of all components comprising the chiller. The optimum values increased the COP from 0.35 to 0.4, while the cooling capacity was slightly changed. The COP is 95% of a Carnot cycle working at hot water temperatures as low as 60 °C, and 90% at hot water temperatures as high as 90 °C. It was found that the simulation model results for the two-stage air-cooled chiller agreed well with the experimental data in terms of cooling capacity (6.7 kW for the model against 6.14 kW for the experimental result at 30 °C cooling water temperature). The model optimized the adsorption/desorption time, switching time and heat recovery time to maximize both cooling capacity and COP. Moreover, the model is used to study the effect of activated carbon mass, size of beds and mass flow rates of cooling, heating, chiller and condenser on both cooling capacity and COP.

The short-term demand response potential and thermal characteristics of a ventilated floor heating system in a nearly zero energy building

Exploiting the thermal storage of building thermal mass is promising to increase the load shifting capacity. In this study, the energy consumption and thermal performance of a novel ventilated electric heating floor in a nearly zero-energy building in an extremely cold region in China were investigated. The numerical model of the heating system was first established and validated, then the energy consumption over one month were simulated through a rule-based control strategy in a traditional power system and a system with demand response, meanwhile, the impact of key structural and operational parameters was also analyzed. Results show that the energy consumption flexibility can be significantly increased through ventilation in both energy systems. Specifically, when ventilation is on throughout the period in the first energy system, the sum of energy consumption is reduced by 62.3% compared to that under non-ventilation mode. In the second energy system, the consumed energy can be reduced through ventilation in downward demand response period by 26.1%, and the energy storage is increased in upward demand response period by 4.6%. Besides, thermal stability can be increased through ventilation, and heat recovery time is reduced.

Performance study of adsorption refrigeration system with composite adsorbent

As an effective way to improve the performance of heat and mass transfer, Composite adsorbent used in adsorption refrigeration has attracted increasing research attention. The aim of the research is to reveal composite theory of silica gel composite adsorbent and to understand adsorption refrigeration system mechanism running by above composite adsorbent. Coarse porous silica gel with different particle diameters was selected as the matrix of composite adsorbent and calcium chloride was selected as additive. The results showed that composite adsorbent's thermal conductivity and adsorption capacity increased significantly compared with pure silica gel. The thermal conductivity could be increased by 58.2%, and the adsorption capacity could be increased by 4–10 times. Finally, the adsorption refrigeration system using silica gel-calcium chloride composite adsorbent was studied. A two-bed adsorption refrigeration cycle running by above composite adsorbent was adopted. The experimental results showed that the optimal values of system cooling capacity and COP were not at the same time. The system cooling capacity reached its maximum at 1500 s and the system COP reached its maximum at 1200 s. The best heat recovery time was 20 s. The SCP of the system was up to 288.1 W kg−1, which could reduce the volume of the adsorption refrigeration system effectively.

Study on the 400 mm 4 × 2 combined slab amplifier system

This paper provides the experiment results of a 400 mm 4 × 2 combined slab amplifier system for high-power laser devices based on a new pumping configuration, which involves 37 nm-diameter pulse xenon lamps, a multilamp arrangement, an operating voltage of 31 kV and a uniform pumping arrangement comprising 12 and 14 xenon lamps in the middle and side flashlamp cassettes, respectively. The experimental results show that the small signal gain coefficient was 5.60% cm−1. The pump-induced wavefront distortion accumulated by six slabs of Nd:glasses is 1.37λ peak-valley (PV) and the pointing offset value of pump beam is about 0.32 μrad/slab/pass. After discharging, the system was purged with clean gas at 0.4 m s−1. The cleanliness in the cavity returned to level 100 within 20 s, and the heat recovery time of the system was about 2 h.

Exergy Analysis of Advanced Adsorption Cooling Cycles.

This study conducted an exergy analysis of advanced adsorption cooling cycles. The possible exergy losses were divided into internal losses and external losses, and the exergy losses of each process in three advanced cycles: a mass recovery cycle, heat recovery cycle and combined heat and mass recovery cycle were calculated. A transient two-dimensional numerical model was used to solve the heat and mass transfer kinetics. The exergy destruction of each component and process in a finned tube type, silica gel/water working paired-adsorption chiller was estimated. The results showed that external loss was significantly reduced at the expense of internal loss. The mass recovery cycle reduced the total loss to 60.95 kJ/kg, which is −2.76% lower than the basic cycle. In the heat recovery cycle, exergy efficiency was significantly enhanced to 23.20%. The optimum value was 0.1248 at a heat recovery time of 60 s. The combined heat and mass recovery cycle resulted in an 11.30% enhancement in exergy efficiency, compared to the heat recovery cycle. The enhancement was much clearer when compared to the basic cycle, with 37.12%. The observed dependency on heat recovery time and heating temperature was similar to that observed for individual mass recovery and heat recovery cycles.

Heat Recovery Time of Adsorption Cooling System

In this study, a two-dimensional numerical model of finned-tube type adsorbers was developed and used to examine heat recovery time to improve the performance of an adsorption cooling system. The optimal heat recovery time, which resulted in the highest COP, was determined for a range of heat source temperatures (60–90[Formula: see text]C) and cycle times (600–1200[Formula: see text]s). The introduced heat recovery process enhanced COP, but also reduced SCP. This penalty became more serious when the hot water temperature was low and cycle time was short, which serves as a guideline for when heat recovery should be adopted in a given operating condition.

Experimental study on operating features of heat and mass recovery processes in adsorption refrigeration

Combined use of heat and mass recovery processes in adsorption refrigeration is an efficient way to improve the system performance. Several kinds of heat and mass recovery processes have been proposed. In this study, a serial heat recovery process between two adsorbers and a mass recovery-like process between two evaporators is selected. Their operating features in a practical silica gel-water adsorption refrigeration system are experimentally studied. Operating features and performances of heat and mass recovery process can be presented in terms of inlet and outlet temperatures and transferred heat of two adsorbers and two evaporators. The results show that residual heat transfer fluid in the adsorbers and evaporators significantly influences the heat and mass recovery processes, respectively. In the case of this study, optimal ranges of heat and mass recovery time is 25–45 s and 5–50 s, respectively. Heat recovery process can produce negative effect when heat recovery time is beyond 45 s and it is inefficient when heat recovery time is below 25 s. Cooling effect is remarkably yielded when mass recovery time is within the range of 5–50 s.

Design and experimental study of a silica gel-water adsorption chiller with modular adsorbers

A silica gel-water adsorption chiller driven by low-grade heat is developed. System configuration without any vacuum valves includes two sorption chambers, a 4-valve hot/cooling water coupled circuit and a 4-valve chilled water circuit. Each sorption chamber is composed of one adsorber, one condenser and one evaporator. The design of this chiller, especially the design of modular adsorber, is suitable for low-cost industrial production. Efficient and reliable heat and mass recovery processes are also adopted. This chiller is tested under different conditions and it features the periodic variations of temperatures and cooling power. Through the experimental study, the optimal cooling time, mass recovery time and heat recovery time are 720 s, 40 s and 24 s, respectively. Besides, the obtained cooling power, COP and SCP are 42.8 kW, 0.51 and 125.0 W kg−1, respectively, under typical conditions of 86/30/11 °C hot water inlet/cooling water inlet/chilled water outlet temperatures, respectively.

Study of the new composite adsorbent of salt LiCl/silica gel–methanol used in an innovative adsorption cooling machine driven by low temperature heat source

An adsorption cooling machine with LiCl/Silica gel–methanol was designed and tested, which can work for air conditioning and cold storage. The machine can be driven by low temperature heat source, such as solar energy and industrial waste heat. The composite adsorbent of LiCl/Silica gel has higher adsorption capacity and the methanol has a higher working pressure. So, the cooling performance, system's reliability and the adsorbent's mass transfer performance can be improved. The adsorption machine was experimentally investigated. The test results show that the cycle time and heat recovery process has more influence on COP (Coefficient of Performance) than on cooling capacity. The mass recovery process has significant influence both on cooling capacity and COP. When the cycle time is prolonged from 460 s to 760 s, the cooling capacity and COP increased by 4.3% and 20.6%, respectively. When the hot water inlet temperature, cooling water inlet temperature, cooling medium outlet temperature and heat recovery time are 75 °C, 31 °C, 5 °C and 60 s, respectively, the cooling capacity is improved by 6.3% by heat recovery process while the COP is improved by 27.3%. When the mass recovery time extends from 50 s to 120 s, the cooling capacity and COP increase by 68.4% and 53.3%. When the hot water inlet temperature is about 88 °C, the cooling water inlet temperature is about 25 °C, the adsorption machine produced -4 °C of cooling medium, the cooling capacity and COP were about 1.0 kW and 0.13, respectively.

Experimental investigation on the optimal performance of Zeolite–water adsorption chiller

This paper presents the performance testing of Zeolite adsorption cooling system driven by low grade waste heat source extracted from prime mover’s exhaust, power plant’s exhaust and the solar energy. The adsorbent FAM Z01 is used as an adsorbent in the adsorption chiller facility. Owing to its large equilibrium pore volume, it has the high affinity for the water vapor adsorbate. The key advantages of the Zeolite adsorption cooling system are: (i) it has no moving parts rendering less maintenance, (ii) the energy efficient means of cooling by the adsorption process with a low temperature heat source, (iii) the use of vapor pipes are replaced by self actuating vapor valves rendering smaller footprint area and (iv) it is environmental friendly with low carbon footprint. The experimental investigations were carried out for Zeolite adsorption chiller at different key operating conditions namely (i) heat source temperature, (ii) the cycle time and (iii) the heat recovery time. It is investigated that performance of coefficient (COP) of this system could be as high as 0.48 while the waste heat source temperature is applicable as low as 55°C.

Experimental study on the dynamic characteristics of adsorption heat pumps driven by intermittent heat source at heating mode

The influence of intermittent heat source on adsorption heat pumps is an unavoidable problem when the system is put into practice. This work studies an adsorption heat pump system driven by intermittent waste heat supplied at 110 °C and dynamic characteristics are obtained under 8 working conditions with water temperature at about 41 °C. The cycle time is 40 min including 120 s heat recovery time. This paper also gives the parameter variation regulations of the system driven by intermittent heat source. The experimental results show a decrease of specific heating power (SHP) and coefficient of performance (COP) is up to 26% and 8.7%, respectively.

Experimental Analysis on the Dynamic Characteristics of a Heat-Regenerative Adsorptive Air-Conditioning System

Based upon a prototype heat powered heat-regenerative adsorptive air conditioning system, the effects of step times (heat recovery, mass recovery, and half-cycle) on the coefficient of performance (COP) and specific cooling power (SCP) are studied. The characteristics of real thermodynamic cycles and dynamic SCP are also analyzed. Strong dynamic behavior was found in that the COP and SCP varied with step times. It is found in experiment that a long heat-recovery time usually leads to a lower SCP. But both the COP and SCP can be increased 16% and 6%, respectively, if a very short time—0.5 min of mass recovery is initiated before heat-recovery. The appropriate half-cycle time for the prototype is about 20–25 minutes for which the SCP and COP are excellent.