Tobacco is a special agricultural product, and China’s tobacco planting area and output rank first in the world[1-2]. Baking is an important part of the tobacco production process, involving complex heat and mass transfer, which is also a high energy consumption process[3]. China’s drying field emits 260 million tons of CO2 and 7.8 million tons of SO2 each year from coal burning, resulting in serious environmental pollution[4]. Given the rich coal resources in China, coal-burning tobacco-curing systems are widely used with low efficiency of heat utilization[5]. The annual increase in coal prices also leads to an increase in tobacco-curing costs.
The importance of tobacco-curing systems not only lies in its great influence on product quality but also on energy consumption and the environment[6]. Heat pump technology has been widely used in many fields due to its advantages of energy saving, high efficiency, reliable operation, and environment friendliness[7]. In recent years, the application of air source heat pumps for tobacco curing has developed rapidly[8-10]. Lü et al.[9] reported that heat pump tobacco-curing systems could save 0.85 yuan compared with coal-burning tobacco-curing systems when producing per kg dry leaf. However, the decrease in ambient temperature reduces the coefficient of performance (COP) of air source heat pumps[11]. In addition, heat pump tobacco-curing systems often need auxiliary electric heating to meet the high heat load of tobacco-curing, thus increasing the energy consumption of the system[12].
The utilization of solar energy provides a solution for energy conservation and CO2 emission reduction in industrial and commercial thermal energy production[13]. At present, solar energy has a widespread application in the drying of agricultural and marine products, such as coffee, peas, tea, medicinal herbs, and fish[14-15]. However, open sun drying requires a long drying cycle, and the product is easily contaminated[16-17]. In addition, solar energy is highly dependent on climatic conditions and has large randomness and fluctuation[18-19].
Solar-assisted heat pump tobacco-curing systems can relieve the burden of independent heat supply by the heat pump and reduce energy consumption[20]. In addition, the flexibility and stability of the system can be enhanced. Peng et al.[21] found that solar-assisted heat pump tobacco-curing systems could save energy consumption by 24.3% compared with heat pump tobacco-curing systems. Xu et al.[22] also found that the cost of energy consumption of solar-assisted heat pump barns is the lowest compared with other tobacco-curing barns. Nevertheless, the solar baking mode of these systems cannot operate at night or in rainy weather. Moreover, the COP of the heat pump is still highly affected by environmental conditions.
Here, a novel tobacco-curing system with a solar-assisted heat pump was proposed and investigated. The system can switch to different operation modes according to environmental conditions. It is also equipped with a heat storage water tank for continuous operation in rainy weather or at night to achieve the maximum utilization of solar energy. The thermal characteristics and the economic and environmental benefits of the system were analyzed, thereby providing an important reference for engineering applications.
1 Methodology
1.1 Tobacco-curing process
To ensure the quality of tobacco leaves, a three-stage tobacco-curing process with low temperature and slow yellowing as the core is generally adopted (i.e., yellowing stage, color fixing stage, and stem drying stage)[23]. This three-stage tobacco-curing process can be further subdivided into seven stages, namely, forced draught (Y1), the middle stage of yellowing (Y2), later stage of yellowing (Y3), early stage of color fixing (F1), later stage of color fixing (F2), early stage of stem drying (S1), and later stage of stem drying (S2), as shown in Tab.1. The total baking time is equal to the sum of the heating-up time and holding time.
The meteorological parameters of different tobacco-curing months in Kunming, Yunnan were selected as the basis for the system design calculation[24]. Yunyan 97 was chosen for calculation, and the mass of fresh leaf before baking is 3 000 kg. The initial water content of the top, middle, and bottom leaves are 82%, 84%, and 86%, respectively, and the final water content is 6.5%.
Tab.1 Operating conditions of tobacco baking
Baking stageTdb/℃Twb/℃Heating-up time/hHolding time/hThe bottom leafThe middle leafThe top leafY136366888Y238376122028Y3423812284460F1483816161616F2543910888S160405555S268427777The total baking time/h146170194
Notes: Tdb is the dry-bulb temperature; Twb is the wet-bulb temperature.
1.2 System description
The novel tobacco-curing system is composed of a solar heating system, a heat pump system, a heat storage system, a defrosting system, and a control system, as shown in Fig.1. The system uses solar energy and a heat pump as dual heat sources. In addition, a heat storage water tank serves as an auxiliary heat source for solar heat collection to realize tobacco-curing under different climate conditions and to ensure the quality of the tobacco. To improve the efficiency of heat utilization, an energy recovery ventilator is installed at the exit of the barn to recover heat from the exhaust and reduce heat loss. The hot and humid air after the heat exchange is discharged to the evaporator, which also helps improve the COP of the heat pump.
1—Barn; 2—Heating chamber; 3—Baking chamber; 4, 5—Air distributor; 6, 7—Temperature and humidity sensor; 8, 9—Frequency conversion fan; 10—Grill; 11—Energy recovery ventilator; 12—Partition; 13—First heat exchanger; 14—Second heat exchanger; 15—Condenser; 16—Expansion valve; 17—Evaporator; 18—Inverter compressor; 19—First circulating pump; 20—Second circulating pump; 21—First electromagnetic valve; 22—Second electromagnetic valve; 23, 24—Temperature sensor; 25—Heat storage water tank; 26—First three-way valve; 27—Second three-way valve; 28—Solar collector; 29—Controller
Fig.1 Schematic of the novel tobacco-curing system with a solar-assisted heat pump
The heat storage system has two functions. On the one hand, it can adjust solar heat input at different times to enhance the flexibility of system operation. On the other hand, the heat storage water tank can defrost the evaporator through the second heat exchanger under the condition of low ambient temperature. This setup broadens the operating conditions of the air source heat pump and maintains a high COP. The heat pump system uses R134a as the refrigerant due to its high critical temperature and low boiling point.
A schematic of the novel tobacco-curing system with a solar-assisted heat pump is shown in Fig.2. Sunlight and (or) electricity are the energy input of the system. The control system can switch between different operating modes according to the difference in solar radiation and water temperature.
Fig.2 Schematic of the novel tobacco-curing system with a solar-assisted heat pump
The system can realize three operating modes, namely, solar baking mode (SD), solar-assisted heat pump baking mode (SHPD), and heat pump baking mode (HPD), as shown in Tab.2. For nighttime and rainy days, solar radiation is small, and the novel tobacco-curing system operates at HPD. When the surface temperature of the evaporator is less than 5 ℃, the defrosting system is turned on. At this point, the first and second electromagnetic valves are opened. Driven by the first circulating pump, the hot water in the heat storage water tank flows into the second heat exchanger to realize the hot water-air heat exchange, thereby heating the cold air around the evaporator. The heated air then exchanges heat with the surface of the evaporator to defrost. The defrosting method is simple and does not affect the normal operation of the heat pump, thus ensuring the good quality of the tobacco.
Tab.2 Operation mode of the novel tobacco-curing system
Operation modeControl logicControl methodsSDOnly bakingIs ≥ImsOpen channel a, c of the first and second three-way valve, start the sec-ond circulating pumpBaking+heat storageIs ≥Ims and Tc-Tb ≥15 ℃;When Tt-Tb≥15 ℃, end the heat storageOpen channel a, b, c of the first and second three-way valve, start the second circulating pump. When the heat storage is over, close channel b of the first and second three-way valveSHPDImc≤Is Notes: Is is the average daily solar irradiation, kJ/m2; Ims is the minimum radiation required for solar baking independently, kJ/m2; Imc is the minimum radiation required for the combined operation of the solar and heat pumps, kJ/m2; Tc is the water temperature of the solar collector, ℃; Tt is the water temperature of the heat storage water tank, ℃; Tb is the baking temperature, ℃.
1.3 Thermal calculation method
The operating parameters of the system were calculated in accordance with the energy balance and mass balance. Furthermore, to ensure the pressure balance of the system, the mass flux of exhaust air is equal to the mass flux of fresh air, and the exhaust air temperature is assumed to be equal to the baking temperature. The mass flux of exhaust air is expressed as
(1)
where c is the dehydration rate, kg/h; d1 and d2 are the humidity ratio of preheated fresh air and exhaust air, respectively, kg/kg(a).
The required sensible heat during the curing process is given by[25]
Qe=mtct(Tf-Ti)
(2)
where mt is the total mass of the tobacco leaf, kg; ct is the specific heat of the tobacco leaf, kJ/(kg·K); Ti and Tf are the initial and the final temperature of the tobacco leaf, respectively, ℃.
The required latent heat of vaporization during the curing process is given by[26]
Qa=mwγ
(3)
where mw is the lost water mass, kg; γ is the average required heat to vaporize per kg of water, 2 500 kJ/kg for the yellowing stage, 2 600 kJ/kg for the color fixing stage, and 2 800 kJ/kg for the stem drying stage[10].
Heat loss during the curing process is given by[27]
Ql=(Qe+Qa)s
(4)
where s is the heat loss rate, %. To simplify the calculation, s is set as 10%[4].
The total required heat during the curing process is expressed as
Qt=Qe+Qa+Ql
(5)
The solar heat collection of the solar collector (The heat storage of the water tank is also part of the heat collection of the solar collector. To simplify the calculation, the heat storage is no longer considered separately.) is given by[4]
Qs=IsAcηwηc
(6)
where Ac is the solar collector area, taking 40 m2; ηc is the collector efficiency, taking 50%; ηw is the heating efficiency of hot water, taking 65%.
The heat provided by the heat pump is given by[28]
(7)
where 
is the total mass flux of air, kg/h; ca is the specific heat of air, taking 1.01 kJ/(kg·K); Tci and Tco are the inlet and outlet air temperatures of the condenser, respectively, ℃.
The minimum radiation required for independent solar baking is expressed as
(8)
where QHE is the heat recovered by the energy recovery ventilator, kJ.
The minimum radiation required for the combined operation of the solar and heat pumps (i.e., the minimum irradiation required for the circulating water to achieve a certain temperature rise) is expressed as
(9)
where 
is the mass flux of circulating water, kg/h; tch is the total running time of the collector and the heat storage water tank, h; cw is the specific heat of water, taking 4.18 kJ/(kg·K); ΔT is the temperature rise of circulating water, ℃.
1.4 Economic analysis of the system
The initial investment of the tobacco-curing system is expressed as
M=Me+Mi+Mb+Mp
(10)
where Me, Mi, Mb, and Mp are original equipment cost, equipment installation cost, building material cost, and power distribution and control cost, respectively, yuan.
The COP of the heat pump is expressed as
(11)
where Tk is the condensation temperature, K; T0 is the evaporation temperature, K; ηe, ηcom, ηex, and ηsys are the motor efficiency, compressor efficiency, heat exchanger efficiency, and system efficiency, respectively, %.
The power consumption of the novel tobacco-curing system is expressed as
W=
(2PF+PP+PHP)dt
(12)
where tc is the running time of one baking cycle, s; PF, PP, and PHP are the power of the fan, water pump, and heat pump, respectively, kW.
The total curing cost of the tobacco-curing system is expressed as
R=Rc+Rp+Rl
(13)
where Rc, Rp, and Rl are the cost of coal consumption, operating equipment, and labor, respectively, yuan.
The payback period of the tobacco-curing system compared with the coal-burning tobacco-curing system is given by[29]
(14)
1.5 Environmental benefit analysis
The CO2, SO2, and dust emission reduction of the tobacco-curing system compared with the coal-burning tobacco-curing system are defined by[29]
Ser=iSj
(15)
where Sj is the standard coal saving of the system, kg. For CO2, SO2, and dust, i is 2.493, 0.22, and 0.01, respectively.
2 Results and Discussion
2.1 Tobacco-curing characteristic analysis
Fig.3 shows the temperature of preheated fresh air and exhaust air in the tobacco-curing barn in different baking stages. The fresh air and the exhaust air exchange heat through the energy recovery ventilator, thereby increasing the temperature of preheated fresh air. With the increase in exhaust air temperature, the temperature of preheated fresh air in different months all showed an upward trend.
Fig.3 Temperature of preheated fresh air in different months and the temperature of exhaust air
In addition, the temperature of preheated fresh air in each stage was the highest in July because the mean ambient temperature in July was the highest in Kunming, Yunnan; the heat recovery of the exhaust air and the temperature of preheated fresh air were also correspondingly higher. This result indicates that ambient temperature is the main factor that affects the temperature of preheated fresh air under the same heat recovery rate and exhaust air temperature.
Fig.4 shows the thermal characteristics in different baking stages, taking the curing process of the top leaf as an example. Data of each stage in the figure show averages. The dehydration rate, the mass flux of exhaust air, and the heat load have similar changes with the baking time. Similar results have been found by Lü et al.[10] and Yao[6]. These three parameters all reach their maximum values in F2 but decrease significantly in S1. The water evaporation rate of the tobacco leaf has a decisive effect on the changes of these three parameters. The water content of tobacco leaf in the early baking stage is high, and the water vapor pressure on the surface of the tobacco leaf is greater than that in the baking chamber; thus, the free water on the surface of the tobacco leaf evaporates quickly. With the increase in baking time, the water vapor pressure on the surface of the tobacco leaf decreases, the vaporized surface layer gradually moves inward, and the water transfer resistance increases, resulting in a significant reduction in dehydration rate in the stem drying stage[30]. In addition, the average mass flux of exhaust air in each baking stage in July is more than that in other months. The capacity of the system to dehumidify may
decline due to the hot and humid weather, thereby increasing the mass flux of the exhaust air of the system[31]. As shown in Fig.4(c), the heat loss rate under different environmental conditions is assumed to be 10%; thus, the heat load of the system in different months has minimal difference and mainly depends on the latent heat of vaporization required by tobacco-curing. Moreover, the maximum average heat load is approximately 30 kW.
The bottom leaf is used as an example to analyze the operating mode of the novel system and the heat supply of each heating equipment in April, as shown in Tab.3. Given the great average daily solar radiation in April, the utilization rate of the solar energy of the system is significantly high. In addition, given a large amount of total required heat, solar collectors hardly achieve separate operations. Therefore, the entire tobacco-curing process relies more on the heat pump, and solar energy can relieve the burden of the independent heat supply by the heat pump and reduce energy consumption.
Tab.3 Operation mode of the novel tobacco-curing system in April (Is=18.830 MJ/m2)
Baking stageIms/(MJ·m-2)Imc/(MJ·m-2)Operation modeQHE/MJQs/MJQHP/MJY136.8371.424SHPD43.185244.790 234.091Y244.2973.428SHPD103.991244.790 331.075Y356.91310.617SHPD322.042244.790 1 234.958F162.87216.270SHPD493.519244.790 1 389.884F2104.96217.180SHPD521.124244.790 1 119.714S123.9524.217SHPD127.920244.790 66.590S29.5731.808SD54.828124.4450
Fig.5 shows the COP of the heat pump at different baking stages. Under certain environmental conditions, the required heat pump condensing temperature continues to increase, whereas the COP of the system gradually decreases when the baking temperature increases. In addition, in the same baking stage, the COP of the system in July was higher than that of other months, indicating that the increase in ambient temperature helps increase the COP of the system. This result is obtained because the increase in ambient temperature helps improve the evaporation temperature of the evaporator. According to Carnot’s theorem and Eq.(11), the increase in evaporation temperature helps improve the COP of the heat pump.
Fig.5 COP of the heat pump in different baking stages
2.2 Comparison with other tobacco-curing techniques
Fig.6 shows the comparison of power consumption between the novel tobacco-curing system and heat pump tobacco-curing system (no heat recovery and solar heat collection). When baking the same part of the tobacco leaf under the same environmental conditions, the power consumption in the novel tobacco-curing system is significantly lower than that of the heat pump tobacco-curing system. According to calculations, the power saving rate of the novel tobacco-curing system is 25.9%-35.1% compared with the heat pump tobacco-curing system. In addition, the curing process of different parts of a tobacco leaf reached the minimum power consumption in April in the novel tobacco-curing system. The average daily solar radiation was the highest, and the humidity ratio of the air was low in April for Yunnan. Thus, the curing condition was superior, and the power consumption of the system reached the minimum value. In Yunnan, July is the rainy season, air humidity is high, and the average daily solar radiation is lower than that in April; thus, the power consumption of the novel tobacco-curing system in July is higher than that in April[32]. However, in the heat pump tobacco-curing system, ambient temperature is the main factor affecting system power consumption; thus, the system power consumption reached the lowest in July. Moreover, the novel tobacco-curing system and the heat pump tobacco-curing system reached the maximum power consumption in January; thus, tobacco-curing is not suitable in January. In the two kinds of tobacco-curing systems, the order of the power consumption of different parts of the tobacco leaf is as follows: top leaf > middle leaf > bottom leaf. The longest baking time of the top leaf leads to the maximum power consumption of the system, whereas the shortest baking time of the bottom leaf leads to the minimum power consumption.
Fig.6 Comparison of power consumption between the novel tobacco-curing system and heat pump tobacco-curing system
In accordance with the thermal calculation results, equipment selection and cost evaluation of the novel tobacco-curing system were conducted. Tab.4 shows the initial investment of three kinds of tobacco-curing systems. The cost of building materials for the novel tobacco-curing system and heat pump tobacco-curing system consists of the cost of color steel plates and polyurethane insulation. Equipment installation costs are calculated at 13.5% of equipment and building materials costs. Through a comprehensive consideration of various costs, the initial investment of the novel tobacco-curing system increased by 30 989 yuan, and the heat pump tobacco-curing system increased by 18 390 yuan compared with the coal-burning tobacco-curing system. Although the initial investment of the novel tobacco-curing system is relatively high, the tobacco company subsidizes the new energy and heat pump tobacco-curing technology in a large proportion and thus can reduce its investment cost[33].
Tab.4 Comparison of initial investment costs of the three tobacco-curing systems Yuan
SystemMeMiMbMpTotalNovel system60 5609 3248 50713 50091 891Heat pump system49 4607 8268 50713 50079 293Coal-burning system28 4603 84221 8006 80060 902
Tab.5 compares the curing costs, which mainly include energy-consumption cost and labor cost, of the three tobacco-curing systems. Compared with the coal-burning tobacco-curing system, the novel tobacco-curing system and the heat pump tobacco-curing system have a higher degree of automation and avoid the manual coal-feeding process; thus, they can effectively reduce the labor cost by more than 70%. Furthermore, the dry leaf curing cost of the coal-burning tobacco-curing system is 2.67-3.96 yuan/kg, and that of the novel tobacco-curing system is only 0.86-1.06 yuan/kg, which saves more than 60% of the curing cost. Therefore, the cost reduction effect of the novel tobacco-curing system is evident. In addition, the order of the dry leaf curing costs of different parts is as follows: bottom leaf > middle leaf > top leaf, which is exactly the opposite of the order of total curing costs. The top leaf has the lowest initial water content and the highest mass of dry leaf; thus, the dry leaf curing cost is the lowest.
Tab.5 Comparison of operation costs of three tobacco-curing systems
SystemPositionMass of fresh leaf/kgMass of dry leaf/kgPower consumption/(kW·h)Coal consumption/kgEnergy consumption cost/yuanLabor cost/yuanTotal cost/yuanDry leaf curing cost /(yuan·kg-1)Novel systemTop leafMiddle leafBottom leaf3 0003 0003 000540485-566513474-556449464-549000340-396122462-5180.86-0.96332-389107439-4960.86-0.97325-38491416-4750.93-1.06Heat pump systemTop leafMiddle leafBottom leaf3 0003 0003 000540690-837513676-826449666-823000483-586122605-7081.12-1.31473-578107580-6851.13-1.34466-57691557-6671.24-1.49Coal-burning system3 500-4 500400-500220-2601 100-1 400924-1 1624201 344-1 5822.67-3.96
Notes: Electricity price is 0.7 yuan/(kW·h), and coal price is 0.7 yuan/kg. The coal-burning tobacco-curing system is equipped with three people per 5 kang, and the novel and heat pump tobacco systems are equipped with three people per 20 kang. The labor cost per kang is 100 yuan/(person·d).
July to September is the main season of tobacco leaf picking and baking in Yunnan.Assume that the tobacco leaf is baked eight times in a year (3 000 kg tobacco leaf per kang), and the curing cost is averaged for economic analysis. As shown in Tab.6, the payback period of the heat pump tobacco-curing system is 2.0-2.6 a, whereas that of the novel tobacco-curing system is 3.0-3.7 a, which is slightly higher than that of the heat pump tobacco-curing system; nevertheless, the novel tobacco-curing system is relatively economical and feasible. The high initial investment of the novel tobacco-curing system increases the system’s payback period. Thus, further research is needed to reduce its initial investment.
Tab.6 Payback period of two different tobacco-curing systems
PositionPayback period/aNovel tobacco curing systemHeat pump tobacco curing systemTop leaf3.02.0Middle leaf3.12.2Bottom leaf3.72.6
The energy consumption of the three tobacco-curing systems was converted into standard coal for calculation. The conversion coefficient of power was 0.404 kg/(kW·h), and the conversion coefficient of raw coal was 0.714 3 kg/kg. The emission reduction of different parts of tobacco leaf during the curing process was calculated, and the average value was taken, as shown in Tab.7. The novel tobacco-curing system has a significant reduction in pollution reduction, reducing 15 586 kg CO2 annually. This result indicates that the novel tobacco-curing system has considerable environmental benefits.
Tab.7 Emission reduction of two different tobacco-curing systems
System Emission reduction/kgCO2SO2DustNovel system15 5861 37563Heat pump system13 6741 20755
3 Conclusions
1) The thermal calculation indicated that dehydration rate was a key parameter during the tobacco-curing process. The mass flux of exhaust air and the heat load had similar changes to the dehydration rate, and both reached the maximum in the late color fixing.
2) Economic analysis indicated that the dry leaf curing cost of the novel tobacco-curing system was only 0.86-1.06 yuan/kg. In addition, the annual CO2, SO2, and dust emission reduction of the novel system could reach 15 586, 1 375, and 63 kg, respectively.
3) Compared with the heat pump tobacco-curing system, the novel system has important economic and environmental benefits, but the payback period is slightly longer. Future research should focus on reducing the initial investment of the novel tobacco-curing system to reduce its payback period.
[1]Hu R S, Wang J, Li H, et al. Simultaneous extraction of nicotine and solanesol from waste tobacco materials by the column chromatographic extraction method and their separation and purification[J].Separation and Purification Technology, 2015, 146: 1-7. DOI:10.1016/j.seppur.2015.03.016.
[2]Banoži
M, Babi J, Joki S. Recent advances in extraction of bioactive compounds from tobacco industrial waste—a review[J]. Industrial Crops and Products, 2020, 144: 112009. DOI:10.1016/j.indcrop.2019.112009.
[3]Siddiqui K M. Analysis of a Malakisi barn used for tobacco curing in East and Southern Africa[J].Energy Conversion and Management, 2001, 42(4): 483-490. DOI:10.1016/S0196-8904(00)00066-2.
[4]Qiu Y, Li M, Hassanien R H E, et al. Performance and operation mode analysis of a heat recovery and thermal storage solar-assisted heat pump drying system[J].Solar Energy, 2016, 137: 225-235. DOI:10.1016/j.solener.2016.08.016.
[5]Zhang Y W, Yi Z X, Zhou Q M. Effect of different bulk curing barns on baking energy consumption costs and upper leaves quality of flue-cured tobacco[J]. Journal of Gansu Agricultural University, 2019, 54(5): 112-120. DOI:10.13432/j.cnki.jgsau.2019.05.014.(in Chinese)
[6]Yao Y. Energy saving efficiency of heat pump tobacco leaf bulk curing system with heat recovery unit[D]. Hefei: University of Technology, 2017. (in Chinese)
[7]Badiei A, Golizadeh Akhlaghi Y, Zhao X, et al. A chronological review of advances in solar assisted heat pump technology in 21st century[J].Renewable and Sustainable Energy Reviews, 2020, 132: 110132. DOI:10.1016/j.rser.2020.110132.
[8]Tian X Y, Li X T, Gao X B, et al. The comparative study of benefits between air source heat pump and coal curing barn[J]. Journal of Anhui Agricultural Sciences, 2016, 44(6): 106-108. DOI:10.13989/j.cnki.0517-6611.2016.06.036.(in Chinese)
[9]Lü J, Wei J, Zhang Z T, et al. Experimental study on performance of heat pump system for tobacco leaf flue-curing[J]. Transactions of the Chinese Society of Agricultural Engineering, 2012, 28(S1): 63-67.(in Chinese)
[10]Lü J, Wei J, Zhang Z T, et al. Theoretical analysis and comparison of performance on heat pump system for flue-cured tobacco based on isenthalpic and isothermal process[J]. Transactions of the Chinese Society of Agricultural Engineering, 2012, 28(20): 265-271.(in Chinese)
[11]Gong G C, Tang J C, Lü D, et al. Research on frost formation in air source heat pump at cold-moist conditions in central-south China[J].Applied Energy, 2013, 102: 571-581. DOI:10.1016/j.apenergy.2012.08.001.
[12]Daghigh R, Ruslan M H, Sulaiman M Y, et al. Review of solar assisted heat pump drying systems for agricultural and marine products[J].Renewable and Sustainable Energy Reviews, 2010, 14(9): 2564-2579. DOI:10.1016/j.rser.2010.04.004.
[13]Kabir E, Kumar P, Kumar S, et al. Solar energy: Potential and future prospects[J].Renewable and Sustainable Energy Reviews, 2018, 82: 894-900. DOI:10.1016/j.rser.2017.09.094.
[14]
evik S. Experimental investigation of a new design solar-heat pump dryer under the different climatic conditions and drying behavior of selected products[J].Solar Energy, 2014, 105: 190-205. DOI:10.1016/j.solener.2014.03.037.
[15]Prakash O, Kumar A. Historical review and recent trends in solar drying systems[J].International Journal of Green Energy, 2013, 10(7): 690-738. DOI:10.1080/15435075.2012.727113.
[16]El Hage H, Herez A, Ramadan M, et al. An investigation on solar drying: A review with economic and environmentalassessment[J]. Energy, 2018, 157: 815-829. DOI:10.1016/j.energy.2018.05.197.
[17]Castillo Téllez M, Pilatowsky Figueroa I, Castillo Téllez B, et al. Solar drying of Stevia (Rebaudiana Bertoni) leaves using direct and indirect technologies[J].Solar Energy, 2018, 159: 898-907. DOI:10.1016/j.solener.2017.11.031.
[18]Xu B, Wang D Y, Li Z H, et al. Drying and dynamic performance of well-adapted solar assisted heat pump drying system[J].Renewable Energy, 2021, 164: 1290-1305. DOI:10.1016/j.renene.2020.10.104.
[19]Pirasteh G, Saidur R, Rahman S M A, et al. A review on development of solar drying applications[J].Renewable and Sustainable Energy Reviews, 2014, 31: 133-148. DOI:10.1016/j.rser.2013.11.052.
[20]Hasan Ismaeel H, Yumruta
R. Investigation of a solar assisted heat pump wheat drying system with underground thermal energy storage tank[J].Solar Energy, 2020, 199: 538-551. DOI:10.1016/j.solener.2020.02.022.
[21]Peng Y, Wang G, Ma Y, et al. Discussions on energy saving ways of heat pump and solar energy bulk curing barn[J]. Journal of Henan Agricultural Sciences, 2011, 40(8): 215-218. DOI:10.15933/j.cnki.1004-3268.2011.08.044.(in Chinese)
[22]Xu Y Q, Yang N, Wang X Q, et al. Effects of different energy types of curing barn on tobacco leaf quality, economic benefit and energy consumption[J]. Acta Agriculturae Jiangxi, 2018, 30(9): 49-53. DOI:10.19386/j.cnki.jxnyxb.2018.09.11.(in Chinese)
[23]Gong C R, Zhou Y, Yang H W. Introduction for three stage curing of flue-cured tobacco[M]. Beijing: Science Press, 2006. (in Chinese)
[24]National Bureau of Statistics. China statistical yearbook[M]. Beijing: China Statistics Press, 2019. (in Chinese)
[25]Ali S D, Ramaswamy H S, Awuah G B. Thermo-physical properties of selected vegetables as influenced by temperature and moisture content[J].Journal of Food Process Engineering, 2002, 25(5): 417-433. DOI:10.1111/j.1745-4530.2002.tb00575.x.
[26]Yahya M, Fudholi A, Sopian K. Energy and exergy analyses of solar-assisted fluidized bed drying integrated with biomass furnace[J].Renewable Energy, 2017, 105: 22-29. DOI:10.1016/j.renene.2016.12.049.
[27]Tan W S. Study and design of solar energy-heat pump combined drying[D]. Taian: Shangdong Agricultural University, 2016. DOI: 10.7666/d.D833722. (in Chinese)
[28]Ming T Y, Li B G. Application research of combined drying tea with solar energy and heat pump system[J]. Acta Energiae Solar Sinica, 2017, 38(10): 2730-2736.
[29]Liu L. Application of solar energy and heat pump technology in dense tobacco curing barn in southern anhui[D]. Hefei: Anhui Jianzhu University, 2019. (in Chinese)
[30]Guo S L. Design of solar energy-heat pump combined drying device and experimental studies on drying tilapia[D]. Zhanjiang: Guangdong Ocean University, 2010. (in Chinese)
[31]Mohanraj M. Performance of a solar-ambient hybrid source heat pump drier for copra drying under hot-humid weather conditions[J].Energy for Sustainable Development, 2014, 23: 165-169. DOI:10.1016/j.esd.2014.09.001.
[32]Zhao X S, Luo H L, Qi Z M. A comparative study on performance of biomass pellet fuel dense barn and coal-fired dense barn[J]. Journal of Kunming University of Science and Technology, 2019, 44(2): 69-74. DOI:10.16112/j.cnki.53-1223/n.2019.02.010. (in Chinese)
[33]Chen H L, Zhang Z Y, Cheng X H, et al. Comparison of application effect between heat pump and coal-fired curing barn[J]. Journal of Henan Agricultural Sciences, 2015, 44(12): 135-139. DOI:10.15933/j.cnki.1004-3268.2015.12.030. (in Chinese)