EXPERIMENTAL STUDIES ON A NOVEL CHEMICAL HEAT PUMP DRYER USING A GAS–SOLID REACTION

The authors have been studying Chemical Heat Pumps (CHP) from the viewpoints of energy saving and environmental impact. The CHP can store thermal energy in the form of chemical energy by an endothermic reaction, and release it at various temperature levels during heat demand periods by exo/endothermic reactions. The authors have proposed in an earlier study a novel chemical heat pump (CHP) system for environmentally-friendly effective utilization of thermal energy in drying as a chemical heat pump dryer (CHPD). In this exploratory study, we test the effectiveness of operating the proposed CHPDs experimentally. Basic experiments on the CHPDs such as hot dry air production for convective drying are performed on lab-scale CHPD apparatuses using gas–solid reactions in calcium oxide/calcium hydroxide reactant beds. The proposed CHPDs are found to produce hot air by CHP operation for drying. The temperature levels of the produced hot air and the reaction rates/conversions are as good as in the case of hot water supply system using basically same CHP operation. The cold heat for air dehumidification is also found to be generated/recovered by the same CHPD system. The generated heat amounts can be increased by changing the operating conditions although the heat recovery must be enhanced for practical application of CHPDs.     

PROPOSAL FOR A NOVEL CHEMICAL HEAT PUMP DRYER

A new chemical heat pump (CHP) system for ecofriendly effective utilization of thermal energy in drying is proposed from the viewpoints of energy saving and environmental impact. CHPs can store thermal energy in the form of chemical energy by an endothermic reaction and release it at various temperature levels for heat demands by exo/endothermic reactions. CHPs have potential for heat recovery and dehumidification in the drying process by heat storage and high/law temperature heat release. In this study, we estimate the potential of the CHP application to drying systems for industrial use. Some combined systems of CHPs and dryers are proposed as chemical heat pump dryers (CHPD). The potential for commercialization of CHPDs is discussed.     

PROPOSAL FOR A NOVEL CHEMICAL HEAT PUMP DRYER

ABSTRACT

A new chemical heat pump (CHP) system for ecofriendly effective utilization of thermal energy in drying is proposed from the viewpoints of energy saving and environmental impact. CHPs can store thermal energy in the form of chemical energy by an endothermic reaction and release it at various temperature levels for heat demands by exo/endothermic reactions. CHPs have potential for heat recovery and dehumidification in the drying process by heat storage and high/law temperature heat release. In this study, we estimate the potential of the CHP application to drying systems for industrial use. Some combined systems of CHPs and dryers are proposed as chemical heat pump dryers (CHPD). The potential for commercialization of CHPDs is discussed.     

Enhancement of Energy Efficiency of a Chemical Heat Pump-Assisted Convective Dryer

In earlier studies we have shown by simulation and experimental studies that the proposed chemical heat pump (CHP) unit can be used to recover waste heat from dryers and reuse it by storing and releasing heat with upgrading the temperature or by dehumidification. However, the final thermal energy production efficiency of the CHP for drying was found to be low. In this paper we present experimental results to demonstrate the potential for improved heat-recovery/storage and the heat-release/production of hot dry air for batch drying applications using the heat enhancement mode of the CHP. A new laboratory scale experimental CHP dryer system was built utilizing the calcium oxide/calcium hydroxide hydration/dehydration reversible reaction. The aim of this study is to improve the efficiencies of the heat recovery from heat source in the heat-storage step and the hot dry air production in the heat-release step of the CHP for heating up the air to around 100°C. The results of this experimental study utilizing a new reactor design showed that the shallow reactor/heat exchanger could accomplish 94% chemical heat storage and produce 100°C air at better than 75% efficiency for the reaction heat by controlling the preheating condition. The reaction conversion reached 90% in these experiments. The proposed CHP-assisted convective dryer system is found to be energy-efficient over a wide temperature range of industrial interest.     

Energy and Cost Estimation for Application of Chemical Heat Pump Dryer to Industrial Ceramics Drying

Abstract

In this article we estimate the potential of a new chemical heat pump dryer (CHPD) application to an industrial ceramics drying process from the viewpoints of energy and cost saving. A CaSO₄/H₂O/CaSO₄·1/2H₂O hydration/dehydration CHPD system and a CaO/H₂O/Ca(OH)₂ hydration/dehydration CHPD system were examined. The CHPD systems store heat and simultaneously release the increased amount of heat at different temperature levels by using two chemical heat pumps (CHP) in their heat-enhancement mode. Furthermore, we propose enhanced systems using chemical heat pipes (CHPipe) for their environmental and cost merits. As a result, the consumed energy and the cost of using the CHPD systems in the industrial ceramics drying process are found to decrease to less than half of the conventional drying process using gas-fired boilers. For example, the energy efficiency and the cost of the present drying process are 28.4% and 604 × 10³ (JPY/month) (JPY: Japanese Yen), respectively. The energy efficiency and the cost of the proposed CHPD system are found to be 79.7% and 216 × 10³ (JPY/month), respectively, based on our experiments.     

SIMULATION OF HYDRATION/DEHYDRATION OF CaO/Ca(OH)2 CHEMICAL HEAT PUMP REACTOR FOR COLD/HOT HEAT GENERATION

ABSTRACT

A chemical heat pump (CHP) utilizes reversible reactions involving significant endothermic and exothermic heats of reaction in order to develop a heat pump effect by storing and releasing energy while transforming it from chemical to thermal energy and vice versa. In this paper, we present a mathematical model and its numerical solution for the heat and mass transport phenomena occurring in the reactant particle bed of the CHP for heat storage and cold/hot heat generation based on the CaO/Ca(OH)₂ reversible hydration/dehydration reaction

Transient conservation equations of mass and energy transport including chemical kinetics are solved numerically subject to appropriate boundary and initial conditions to examine the influence of the mass transfer resistance on the overall performance of this CHP configuration. These results are presented and discussed with the aim of enhancing the CHP performance in next generation reactor designs.     

Saffron Drying with a Heat Pump–Assisted Hybrid Photovoltaic–Thermal Solar Dryer

Saffron is the most expensive spice and Iran is the largest producer of this crop in the world. Saffron quality is profoundly affected by the drying method. Recent research has shown that hybrid photovoltaic–thermal solar power systems are more efficient in comparison with individual photovoltaic and thermal systems. In addition, heat pump dryers are highly energy efficient. Furthermore, they are suitable for heat-sensitive crops such as saffron. Therefore, in the present study, the performance of a hybrid photovoltaic–thermal solar dryer equipped with a heat pump system was considered for saffron drying, in order to obtain a high-quality product and reduce fossil fuel consumption. The effect of air mass flow rate at three levels (0.008, 0.012, and 0.016 kg/s), drying air temperature at three levels (40, 50, and 60°C), and two different dryer modes (with and without the heat pump unit) on the operating parameters of the dryer was investigated. The results of the investigation showed that total drying time and energy consumption decreased as air flow rate and drying air temperature increased. Applying a heat pump with the dryer led to a reduction in the drying time and energy consumption and an increase in electrical efficiency of the solar collector. The average total energy consumption was reduced by 33% when the dryer was equipped with a heat pump. Maximum values for electrical and thermal efficiency of the solar collector were found to be 10.8 and 28%, respectively. A maximum dryer efficiency of 72% and maximum specific moisture extraction rate (SMER) of 1.16 were obtained at an air flow rate of 0.016 kg/s and air temperature of 60°C when using the heat pump.     

A Control Strategy for a Chemical Heat Pump Dryer

This article presents results of an experimental investigation on the controllability of hot air production using a pair of chemical heat pumps (CHP). The objective of this study is to determine how a CHP-assisted batch dryer can be operated effectively. The CHP uses the well-known CaO/H₂O/Ca(OH)₂ hydration/dehydration reaction, which is reversible. The hot air temperature can be controlled by adjusting the reactor temperature, and pressure, as well as thermal power supplied to it. It is shown that hot air can be produced in both the heat storage and heat release steps of the CHP.     

Investigation on Drying Performance and Energy Savings of the Batch-Type Heat Pump Dryer

Simulation of the heat pump cycle and the drying process has been carried out to obtain the design parameters of the dryer. The analysis indicates that a specific moisture extraction rate (SMER) greater than 3.4 kg/kWh can be obtained. A box-type heat pump dryer has been developed and investigated for the performance of drying of shredded radish. Heat pump drying took 1.0–1.5 times longer than hot air drying. However, the heat pump dryer showed considerable improvement in energy savings. The SMER of the heat pump dryer was about three times higher than that of the hot air dryer.     

高温热泵在除湿转轮空调系统中的性能

提出一种新型的除湿转轮与高温热泵联合运行的空调系统,该系统利用热泵的蒸发器对除湿后的热空气进行降温处理,同时将冷凝器释放的热量为除湿转轮提供再生能耗,在系统内部实现冷量和热量的抵消,既降低系统能耗又减少环境污染。为此研制了采用工质R142b的空气源热泵,将机组置于可模拟转轮处理空气和再生空气状态的标准空气焓差室对其性能进行测试。通过改变室外侧环境温度和进入冷凝器的风量研究R142b在空气源热泵机组中的循环性能和排气压力。结果表明：当蒸发器环境温度为（45±0.2）℃时,冷凝器进风温度为（27±0.2）℃时,灌注R142b的空气源热泵可产生79.2℃的热风,满足转轮的再生温度要求,且排气压力在压缩机的正常工作压力范围内。     

鱿鱼热泵-热风联合干燥节能研究

以热泵干燥阶段的温度、分阶段干燥的水分转换点、后期热风干燥阶段的温度为影响因素,以单位能耗除湿值（SMER）和总挥发性盐基氮值（T—VBN）两者的综合指标Y为最终评价指标进行响应面优化分析,得出了联合干燥最佳工艺参数：热泵干燥温度（X1=54．3℃）、水分转换点（X2=32．3％）和热风干燥温度（X3=43．5℃）。在最优工艺条件下比较HPD、HPD＋AD、AD三种干燥方式下的样品品质和耗能,联合干燥得到的鱿鱼干品质高于热风干燥,而且降低了干燥能耗38．67％。     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Drying Kinetics and Antioxidant Phytochemicals Retention of Salak Fruit under Different Drying and Pretreatment Conditions

Kinetics of hot air drying and heat pump drying were studied by performing various drying trials on salak slices. Isothermal drying trials were conducted in hot air drying and heat pump drying at a temperature range of 40–90°C and 26–37°C, respectively. Intermittent drying trials were carried out in heat pump drying with two different modes: periodic heat air flow supply and step-up air temperature. It was observed that the effects of relative humidity and air velocity on drying rate were significant when moisture content in salak slices was high, whereas the effects of temperature prevailed when the moisture content was low. As such, it was proposed that drying conditions should be manipulated according to the moisture transport mechanisms at different stages of drying in order to optimize the intermittent drying and improve the product quality. Generally, loss of ascorbic acid during drying was attributed to thermal degradation and enzymatic oxidation, whereas the loss of phenolic compounds was mainly due to thermal degradation. Experimental results showed that heat pump drying with low-temperature dehumidified air not only enhanced the drying kinetics but produced a stable final product. Heat pump–dried samples retained a high concentration of ascorbic acid and total phenolic compounds when an appropriate drying mode was selected.     

热泵微波联合干燥系统研究

对热泵微波干燥的实用性及其性能进行了模拟分析和试验研究 ,试验及计算结果吻合较好。分析了空气旁通率、压缩机转速及空气质量流量等主要设计及运行参数对干燥性能的影响。与蒸汽加热干燥比较的结果表明 :通过精心设计热泵微波干燥系统 ,可使其能耗与蒸汽加热干燥相当。     

Performance simulation of a wall-type reactor in which exothermic and endothermic reactions proceed simultaneously, comparing with that of a fixed-bed reactor

By combining endothermic and exothermic reactions in one reactor, a mutual utilization of thermal energy involved in reactions is expected to produce a saving energy and a cost-down for running in industrial reaction process. In this case, a wall-type reaction system is thought to be suitable because such reaction system is good at exchangeability of thermal energy by conductive heat transfer. This study supposed a wall-type reaction system consisting of endothermic and exothermic reaction channels stacked up and a fixed-bed reaction system of the same configuration, and compared them by numerical simulation in the case where endothermic and exothermic reactions progress simultaneously.

In the fixed-bed reaction system, heat transfer in the catalyst bed takes place by convection, and this transfer becomes the rate-limiting process. Accordingly, occurrence of hot spot in the exothermic channel and shortage of thermal energy in the endothermic channel were predicted. This trend became distinct by making the feed gas directions flowing in the two channels countercurrent and by stacking the channels in multiple tiers. In the wall-type reaction system, however, the temperature distributions in the exothermic and endothermic channels almost conformed to the set temperatures, and the temperature difference between channels was small. Even if the feed gases flowed in countercurrent and even if the channels were stacked several deep, this trend did not change. In the wall-type reaction system, the exchange of thermal energy would take place efficiently by conductive heat transfer between the endothermic and exothermic channels. Furthermore, it was inferred that the wall-type reaction system would provide a stable operation in mutual utilization of thermal energy.     

氯化钙─甲醇化学热泵换向循环过程理论研究

研究将一种可改善气固化学热泵循环性能的周期性强制流向变换的动态操作引入双反应器氯化钙─甲醇化学热泵系统，在文献［３～５］的基本循环过程研究的基础上，建立了描述这类换向循环过程的动态分布参数模型。由此通过数值模拟分析了固相床层内的热波现象。对不同换向操作方式的研究结果表明：周期反向操作确实可以改善常规的同向操作的循环性能，而半周期反向操作则比常规的性能还要差。     

Calculation Steps for the Design of Different Components of Heat Pump Dryers Under Constant Drying Rate Condition

Heat pump–assisted dryers are an alternative method for drying heat-sensitive food products at low temperature and less relative humidity with lower energy consumption. The mathematical models of a heat pump dryer consist of three submodels; namely, drying models, heat pump models, and performance models. Heat and mass balance of both refrigerant and air circuits in all components of the system are used for development of mathematical models. The models are used for design of different components of heat pump dryers operating under constant drying rate condition. A simple stepwise design procedure for batch-type, closed-loop heat pump dryer is also presented.     

Performance of heat pump drying system integrated into a blood dryer

ABSTRACT

In general, most heat losses in industrial dryers arise due to the discharge of humid air. Using heat pump drying systems, heat from the exhaust humid air can be recovered, thus improving the energy efficiency substantially. In this study, the performance of heat pump integration in a blood dryer was examined. Computer simulation models of the original high-temperature (180°C) dryer and the proposed system with heat pump integration and auxiliary heating were developed. Different heat pump systems and working fluids were investigated to determine the best performing heat pump system. In this case, it was found that an R245fa heat pump system with a subcooler is the best solution. When using an absorption heat pump, the results showed that a type I absorption heat pump with H₂O–LiBr as working fluid pair performs the best. In addition, the economic benefit as well as the optimum operating conditions of the dryer with integrated heat pump were also determined.     

HEAT PUMP DRYING OF AGRICULTURAL MATERIALS

Heat pump-assisted drying is an energy-efficient process because the heat is recoverable. However, the economic feasibility of the heat pump dryer (HPD) is debatable because high grade energy (electricity) is used. In this study, sawn rubber wood and bananas were dried in an experimental HPD. The moisture extraction rates (MER) and the specific moisture extraction rates (SMER) of wood drying and banana drying decreased rapidly with the drying time but the compressor power was relatively constant. If the final moisture content of wood is less than 10%, the maximum average MER and SMER were 2.854 kg/h and 0.572 kg/kWh, respectively. Banana drying yielded the highest average MER of 2.710 kg/h when the drying load was highest. The corresponding SMER was 0.540 kg/kWh. Economic analyses among the HPD, an electrically-heated hot air dryer and a direct-fired dryer revealed that the HPD had the lowest operating cost.     

辅助冷凝器冷却水量对闭式热泵干燥系统性能的影响

针对目前工业生产中金属件清洗干燥工艺中常用干燥方式存在能耗高且环境不友好的现状,提出并建立了一种采用直接串联式辅助冷凝器的闭式热泵干燥系统。基于所搭建的性能测试台,实验研究了流经辅助冷凝器的冷却水量这一关键参数对系统运行工况参数、制热/制冷量、系统功耗、性能系数（COP）以及单位时间除湿量（MER）和单位能耗除湿量（SMER）等性能参数的影响。结果表明：冷却水量为30.6kg/h时,冷凝器出风温度达72.8℃;随着冷却水量的增大,系统制热/制冷量、功耗、冷凝器出风温度及MER均呈现下降趋势,COP维持在5.6左右;MER最高可达3.80kg/h,SMER最高可达1.44kg/(kW·h),MER和SMER的变化趋势相反,故生产实际中需要综合考虑冷却水量对两者的影响;另外,在实验研究工况下最大冷却水出水温度达65.2℃,可为工业生产提供可应用的热水,使能源得到充分地回收利用。研究结果可为闭式热泵干燥系统在金属件清洗干燥工艺中的应用及其节能降耗提供新的思路和参考。