Introduction

Tobacco curing (TC) is an energy-intensive drying process for agricultural products. Traditionally, coals are commonly used for flue-cured tobacco heating and curing in China, which is highly consumptive and causes pollution to the surrounding environment1. In order to solve this problem, in recent years, the Chinese government has been committed to replacing coals for clean energy in tobacco heating2,3, such as biomass4,5,6, solar7 and air source heat pump (ASHP) including the open or semi-open heat pump (HP) used for air exchange8,9, among which ASHP for TC has been widely applied and reported10,11.

With the evaporator of HP, the cooling median condenses the water vapor in the air and discharges it12, so as to reuse the heat in the confined space, which is widely used in the constant-temperature drying of agricultural and industrial products13,14,15. Dehumidification in high humidity environment with variable temperatures for TC, Maw et al. used the HP dehumidifier for the first time in 1999 in the bulk curing barn (BCB); however, it was only used for yellowing tobacco leaves16 and tobacco warehouse17 due to technical limitations. Then, Sun et al.18 developed a fully integrated air source HP specifically for tobacco heating, referred to as the “open” HP used for exchange (Fig. 1, (a)). To maximize the utilization of heat generated by the internal moisture in the air discharged from the BCB, Lv et al.19 channeled this air to the HP’s evaporator, significantly enhancing the thermal energy conversion efficiency. This system is referred to as a “semi-open” HP used for air exchange (Fig. 1, (b)). Soon afterwards, Ren designed a closed HP dehumidification system, dehumidifying intermittently the dry tobacco leaves20. Zhang et al.21 successfully adapted the traditional air source heat pump (ASHP) to harness geothermal energy for heating tobacco by positioning the evaporator underground, but the system demonstrated low thermal efficiency22. With the iterative upgrading of cold medium, Frate et al. noted that waste heat recovery systems, like Organic Rankine Cycles, operate with low efficiency when the maximum temperature falls between 85 °C and 100 °C during TC process23. As refrigeration technology advanced, Zhu et al.24 re-examined refrigerant selection and heat load calculations for heating flue-cured tobacco. Moreover, optimizing wind speed parameters can greatly enhance the curing quality of tobacco leaves, as highlighted in reference25,26,27.

However, the heat pump for tobacco heating shows a decreasing trend in the coefficient of performance (COP) of the heating system with increasing altitude, especially in high-altitude areas where this technology cannot be promoted. To handle this problem, and on the basis of previous studies, an integrated HP heating/dehumidification tobacco flue-curing equipment is designed to reduce energy consumption and heat steadily for TC in different coastal areas.

Fig. 1
Fig. 1
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The structure of open or semi-open HP used for air exchange of the current BCB for TC: (a) “open” HP used for air exchange, (b) “semi-open” HP for air exchange. Blue solid line: Low temperature and low humidity circuit, Red solid line: High temperature and high humidity circuit.

Principle of design and configuration

Parameter configuration of TC

Different from the drying of other crops, TC is a complex process of heat and mass transfer, accompanied by complex physical and chemical changes. The whole process requires strict quantitative control on the temperature, humidity and time of the curing environment. Figure 2 shows the temperature control, the air discharge, the water loss rate of tobacco leaves and the heating load in in normal curing. According to the color change of appearance and the state of tobacco leaves, the process can be divided into yellowing, leaf drying and stem drying stages based on bioactivity. At the yellowing stage below 42 °C, leaf cells undergo a series of metabolism with the participation of oxygen, and change from green to yellow, which requires the air to exchange with the outside world to make up for the oxygen consumed in the loading chamber of the barn. The leaf drying and stem drying stage is basically the color-retaining drying of general agricultural products. In the middle leaf drying stage, the water loss rate of tobacco leaves reaches a maximum of 55 kg/h, and the maximum heat load is 1.64 × 105 kJ/h, which requires a large amount of heat evaporation to discharge about 70–80% of the water in leaf blades and main veins. It is also the period when tobacco leaves need the most heat for curing. From Fig. 2(e), a maximum energy power of 39.2 kW·h of heating equipment for TC is required.

Fig. 2
Fig. 2
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The TC process: (a) Curing process curve, (b) Discharge rate of moist air, (c) Water loss rate of tobacco leaf, (d) Heat load curve, (e) Heat load curve.

Dynamic balance of heat

The tobacco leaves absorb heat from the air and evaporate into the air. At the same time, they also produce heat from chemical reactions, which is difficult to calculate. If the tobacco leaves and air in the barn are taken as a whole and the maintenance structure of the barn as the boundary, the dynamic balance of heat between the internal and the external environment becomes simple and clear. The maintenance structure works as a carrier of the lost heat in the curing process, and the heat load can be calculated. Meanwhile, in the closed curing environment, there is no exchange of air, heat and mass with the outside world, so it is discharged from the barn in the form of condensed water.

Equipment structure

Figure 3 shows the appearance and internal structure of the integrated equipment, which is mainly composed of the heating, dehumidification and auxiliary heating systems. With the compressor 2 (11), a heating evaporator (1), a heating condenser (12) and pipelines, the heating system is responsible for rising the air temperature, and discharging the heat of walls, doors, windows and the ground in the barn; the dehumidification system is made up of the compressor 1 (10), dehumidification evaporator (4), dehumidification condenser (5) and pipes. In the airtight environment, it controls the discharging of water in tobacco leaves in the form of condensate. The auxiliary heating system consists of electric heating wires (8), providing auxiliary power heat in case of extreme weather conditions such as low temperature or rain. Heating evaporator (1) is shared by the heating and dehumidification systems, while in case of frost, the curing console turns off the heating evaporator (1), turns on the dehumidification system condenser (5) and fan the heat into the desiccant dehumidification system condenser (4), thus completing defrosting. If the HP system fails to meet the heating demand for TC during TC process, the electric auxiliary heating wire (8) is activated to provide supplemental heat.

Fig. 3
Fig. 3
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Schematic diagram of the heating/heat dehumidification structure of ASHP in a BCB (1. Heating evaporator — 98 m2, 2. Internal hot-air outlet valve — Length and width with 100 and 30 cm, 3. Condensed water, 4 Dehumidification evaporator — 70 m2, 5. Dehumidification condenser — 85 m2, 6. Defrosting fan—φ450 centrifugal fan with 1.5 kw·h, 7. External cold-air inlet valve — Length and width with 80 and 30 cm, 8. Electric auxiliary heating wire — diameter φ9.52 mm, 9. Electronic on/off valve — DPF(TS1)3.2 C-01, 10. Compressor (1) – 7 kW·h, 11. Compressor (2) – 7 kW·h, 12. Heating condenser — 130 m2, 13. Circulation fan, 14. Hot air outlet, 15. Partition wall between, 16. Tobacco beam, 17. Barn door, 18. Air inlet, 19. Outdoor awning).

Curing control workflow

The TC controller is operated by STM32F103RBT6, and technicians can set or modify the values of dry-bulb temperature (DBT) and wet-bulb temperature (WBT) online. The workflow of the control system is shown in Fig. 4. When the curing starts, the control device determines the curing stage of tobacco leaves by identifying the set DBT and td value. The heating system controls the Ax quantization of the synchronous signal according to the difference of t between the actual DBT ta in the barn and the set DBT td. The 5 Fx speed gear values correspond with t≤−1.5 °C, −1.5 °C < t≤−1.0 °C, −1.0 °C < t≤−0.4 °C, −0.4 °C < t≤−0.1 °C and t>−0.1 °C respectively. The greater the difference in the target DBT, the greater the heating power output will be when the frequency conversion technology is used to automatically switch gears, and vice versa. When the set value of td is greater than 42 °C, the system will close the doors of air intake and humidity discharge and start the dehumidification system until the end of the curing. Similarly, according to the difference of h between ha and hd, the Fx quantization dehumidification operation with 5 synchronization signals is set. In order to reduce the switching frequency of the HP, FNmin and ANmin as the minimum operation power of dehumidification and heating, is operated continuously during the curing. The sensor detectors incorporate high-quality DS18B20 sensing components imported from the United States, Provides precise temperature measurements with an accuracy of ± 0.5 °C within a range of −10 °C to + 85 °C. Features an elegant and compact injection-molded design, available in one-to-two or one-to-three configurations, Equipped with a sheath design that facilitates easy tying with cotton yarn and can be conveniently attached to water storage bottles.

Fig. 4
Fig. 4
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Schematic diagram of flue-cured tobacco heating control.

Configuration of major devices

By consulting relevant literature in the calculation of heating and dehumidification of this equipment28,29, the power of the compressor for heating and dehumidification is 7 kW•h. The cold medium is R134a, and the area of heating evaporator and condenser are made of copper tube in the diameter φ9.52 mm, flat plate wave plate and hydrophilic aluminum, 98 m2 and 130 m2, respectively. Electrical auxiliary heating is made with wires at a heating power of 18 kW•h. Prior to HP heating for TC, independent testing of the power grid focuses on ensuring that the voltage deviation stays within ± 7%, the harmonic distortion rate is capped at 5%, with odd harmonics not exceeding 4% and even harmonics limited to 2%.

Materials and methods

Test materials

The experiment was conducted in the Curing Platform in Wangluo Town, Xiangxian County, Xuchang City, Henan Province and Sangtuo Town, Pengshui County, Chongqing City in August 2020–2022. The tested variety was Zhongyan-100, the tobacco field with uniform nutrition, and the normal mature middle leaves were harvested and placed in the loading chamber of BCB. According to local customs, the moisture content of fresh tobacco was 83 ± 1%, and the tobacco loading capacity of fresh tobacco was ± 4,500 kg. The length × width × height of the air-flow descending BCB is 8.0 m×2.7 m×3.5 m. The HP is assembled by CSSC Sunrui (Luoyang) Special Equipment Co., LTD.

Temperature data collection

The curing controller is capable of recording and storing the built-in DBT and WBT, whose sensors meet the specifications set by the 2009 National Tobacco Bureau’s “Technical Specifications for Bulk Curing Barns”. Sensors Configuration: Main line: 5 m, Upper shed branch line: 2.5 m, Lower shed branch line: 1.5 m. On starting the machine, the system will automatically record the setting and the actual DBT through the control panel, and the power consumption will automatically record once every 4 h.

Ventilation inspection between blades

When the DBT was 42 °C, multiple wind speed sensors (WD4150C2, Hangzhou Jili, China) were used to measure the wind speed at horizontal and vertical positions inside the BCB loading chamber. There were 15 wind speed sensors in the every horizontal (Fig. 5(a)), and with total 45 sensors in the upper, middle, and lower positions. To undertaking the standard work of wind speed sensor, the specially designed small cages were designed to separate direct contact between sensors and tobacco leaves (Fig. 5(b)). The ANSYS FLUENT 2022 R1 (https://www.ansys.com/it-it/products/fluids/ansys-fluent) module of the finite element analysis software (ANSYS, USA) was used to analyze the flow of wind speed under natural ventilation and dehumidification of the loading chamber in natural ventilation dehumidification and HP dehumidification. The wall boundary condition is equal wall temperature. Under the fresh tobacco full loading, the tobacco loading area is considered as a porous medium. The reference porosity, viscous resistance coefficient, inertial resistance coefficient30, and thermal conductivity coefficient of tobacco leaves31 in ANSYS FLUENT 2022 R1 software were set 0.8, 4 × 107, 690 and 0.046 w/(m·k).

Fig. 5
Fig. 5
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Method of wind speed detection in the BCB loading chamber (cm) ((a) Foundation of the loading chamber, (b) Protective device for sensors).

Indicator judgment method

Refer to the testing method of general technical specifications (Chinese national standard GB/T 17758 − 2010) to test the COP and SMER of ASHP drying unit.

COP of the HP

COP characterizes the performance of the HP, which is defined as the ratio of the heat produced by the HP to the driving energy consumed. The formula is as follows:

$$COP=\frac{HP}{E}$$
(1)

where, HP is the heat capacity of the condenser, kW; E is energy consumption that drives the HP system to work, kW•h.

SMER of the drying system

SMER mainly reflects the comprehensive performance of the drying system of the HP. It is defined as the moisture quality of the material removed per unit to the energy consumed, and can be expressed as follows:

$$SMER=\frac{M}{E}$$
(2)

where, SMER is the dehumidification ratio, kg/(kW•h).

Experimental data uncertainty analysis

The HP heating parameters are recorded under varying weather conditions to validate the rationality of the system design.

Data processing

Origin 2020 was used to draw the change chart of performance index in the BCB. DPS7.0 was used to analyze the significance of data differences.

Results analysis

Correlation between the external temperature and the heating system during TC

The ability of the HP to meet the heating demands for tobacco drying serves as a key indicator of whether the system design is reasonable. Since external environmental temperature is a variable and uncertain factor in the experiment, it significantly influences the outcome and must be considered when evaluating the system’s effectiveness. The maximum heat loss in the process of TC through the walls, roof, doors, windows and the ground appeared in the later stem drying stage of the leaves, when the DBT in the barn exceeded 60 °C. Table 1 shows the DBT changes inside and outside the HP curing barn during the test from August 5 to September 22, 2022, with a large difference in daytime temperatures. Under the HP heating mode, the average temperature in the BCB during the dry stage of the later stem drying stage was 64.47 ± 2.39 °C. Under cloudy and rainy conditions with the temperature lower than 18.5 °C, the electric heating device should be started. Based on whether HP for tobacco heating can meet the heating requirements for TC, the external environmental temperature is an uncertain factor affecting the experiment, the rationality of the system design can be judged.

Table 1 The lowest ambient temperature of BCB during the high-DBT heating of the HP unit.
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HP’s accuracy in DBT and WBT control

As can be seen from Fig. 6, the average and maximum temperature differences between the actual and the set reference DBTs in the loading chamber are 0.31 °C and 1.0 °C (the 98th hour), respectively. The actual WBT is consistent with the set WBT, with the average and maximum WBT at 0.06 °C and 0.5 °C respectively (the 93rd hour), and stable rising of WBT, indicating stability in the heating process of TC, and precision in DBT and WBT control, hence satisfying the required environmental temperature and humidity.

Fig. 6
Fig. 6
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Real time changes of DBTs and WBTs in the loading chamber of BCB.

Overview of wind speed and flow field in BCB

Figure 7 shows the wind flow of tobacco leaves under two conditions of dehumidification in the closed BCB before and after 42 °C. In independent regional distribution and numerical range of the grid test, the uniformity of the wind speed distribution of tobacco leaves that depended on the air inlet and outlet of the BCB was significantly lower than that relying on HP dehumidification, especially in the middle of the front half of the loading chamber near the wall of the heating chamber, which was in the low wind speed area.

Fig. 7
Fig. 7
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Tip speed under TC ((a) The HP dehumidification (b) The dehumidification based on cold air and wet discharge doors).

Performance analysis of HP

The performance of each key DBT in the HP for the TC is shown in Fig. 8. 38 °C is the maximum water loss DBT in the yellowing stage, while 45 °C in the dry stage. The COP gradually increases and decreases, with the maximum value of 4.35 kW/(kW•h) and the average value of 2.85 kW/(kW•h). Between 42 °C and 54 °C, the COP of the BCB declines sharply, due to the change of the TC control program without the heat recovery of the high temperature wet air discharged from the barn. Above 60 °C, the dry tendon of tobacco leaves needs a large amount of heat supply; Affected by the high temperature heating, the COP of the heating system decreases.

Fig. 8
Fig. 8
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Change of performance index in HP during TC process.

The SMER of the system is closely related to the TC process. At 38 °C and 48 °C, the SMER is higher because the tobacco leaves lose more water. Above 60 °C, the SMER decreases because only the main vein of tobacco leaves loses water and the loss rate decreases. Therefore, the SMER in the whole curing process is 3.06 kg/(kW•h), and reaches 4.51 kg/(kW•h) in the early stage of leaf drying.

Economic analysis

According to the results showed in Table 2, the average dehumidification capacity and the SMER are 23.85 kg/h and 3.06 kg/(kW•h), respectively. Due to the fact that the dehumidification method of this designed device is less affected by external altitude or environmental temperature, the curing cost of 1 kg of dry tobacco is only 0.77 Yuan, which is significantly lower than the reported cost in the literature of open or semi-open HP used for air exchange19,21, with obvious advantage in energy saving. The power consumption of the HP decreases as the average daily temperature increases, but it will conserve more energy in sunny days later in the curing season.

Table 2 Economic index of the HP for TC.
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Discussion

The average COP of the designed HP unit is 2.85 kW/(kW•h), lower than that of the single heating equipment32,33. However, through the analysis of flue-cured TC with single HP heating, the structure of the equipment helps the evaporator to recover the heat from the wet air discharged from the barn, especially in the later stage of TC, when the air temperature of the permeable evaporator increases the COP of the HP. Nevertheless, under the influence of ambient air mobility, some high temperature moisture discharged from the barn diffuses into the atmosphere. By contrast, when the DBT of the integrated equipment reaches 42 °C, the BCB is in a sealed state, and there is no high temperature moisture diffusion caused by the exchange of internal and external air. Therefore, the SMER of the integrated machine is 3.06 kg/(kW•h) in the TC process, higher than that of the single heating equipment32,34, with the curing cost per kg of dry tobacco significantly lower than that in the existing literature18, resulting in low COP and high energy consumption in dehumidification.

In the closed cycle of HP dehumidification, the distribution uniformity of the flow field of the wind speed in the BCB is significantly better than that of the open HP in the early stage, and it is also better than other heating literatures under the same working condition35. Heat transfer in the BCB is carried by the wind speed, and the uniformity of the wind speed basically represents the distribution of the temperature36. Uniform wind speed is conducive to a uniform DBT and WBT for tobacco leaves, as well as the consistency of TC control and the improvement of TC quality37. Even so, further research is needed to deal with the change in the flow field of the wind speed caused by the inlet and non-inlet of air, with the same circulating fan of the same power in the BCB.

Conclusion

In this paper, an equipment of ASHP heating/dehumidification is designed, by following the law of flue cured TC process, and adopting the frequency conversion technique to quantify the heat supply and control of the DBT and WBT. After years of testing and optimization, the 2 compressors of the HP equipment are able to operate together, and adapt to different working conditions in the TC process, where the DBT and WBT in the BCB fluctuate slightly around the set value, hence meeting the requirements of heating and dehumidification in different stages of TC. The COP of the system reaches the highest of 4.35 kW/(kW•h), and the dehumidification of tobacco leaves 3.06 kg per kW•h. The cost of 1 kg of dry tobacco is less than 0.8 Yuan. In a nutshell, the integrated ASHP will bring great economic benefits in broader applications in drying other products.