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Experimental investigation on a solar assisted heat pump in-store drying system
Applied Thermal Engineering 31 (2011) 1718e1724
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation on a solar assisted heat pump in-store drying system Y. Li a, *, H.F. Li a, Y.J. Dai a, S.F. Gao b, Lei Wei b, Z.L. Li b, I.G. Odinez c, R.Z. Wang a a
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China China Grain Reserves Corporation, Beijing, China c Department of Civil and Materials Engineering, University of Illinois at Chicago, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2010 Accepted 13 February 2011 Available online 24 February 2011
Solar powered drying is an environment-friendly technique utilized to dry grain. A novel in-store drying system composed of flat-plate air collectors, a heat pump, fans, air ducts, and grain stirrer is proposed to make full use of solar energy and to reduce the consumption of electricity. A demonstration system was built and tested in Kunming, China. The experimental data was compared with the results derived from a mathematical simulation. The experimental and simulated data demonstrate that the solar assisted heat pump drying system improved the performance of the in-store drying process. An average temperature rise of 8.9 C for the granary inlet air was achieved. The drying rate was increased and the uniformity of grain moisture content was improved. In addition, the grain was maintained in good quality and the level of power consumption was reduced. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: In-store drying Grain Solar air collector Heat pump Equilibrium model
1. Introduction Reducing the moisture content of grains to a safe level is critical for their storage. Grains are typically surrounded by fungi and mold. Environments in which high moisture content is the domain provide favorable living conditions for these microorganisms causing the grains to spoil. A research study on wheat shows that the time for germination to decrease by 10% is 2.5 days and 7 days for wheat which has moisture content of 19% and 17%, (moisture content is abbreviated to m.c. in the following article) respectively [1]. Since the quality of grains is affected by the moisture content, drying has become an important process in the storage of the grains. In-store drying consists in the placement of high moisture content grains in storehouses or granaries ventilated by ambient or heated air. The later two serve as the drying medium to maintain the grains under adequate in-store moisture conditions. A great feature of this methodology is that grain drying and storage occurs in the same location. Compared with the use of high temperature dryer, instore drying is capable of handling large amounts of grain at one time while reducing labor requirement. Due to the low drying temperature and low specific flow rates of in-store drying, the quality and freshness of the grains are effectively maintained [2]. In China, in-store drying by ambient air is the principal grain drying method adopted by various depots in the country. However, there exist several shortcomings regarding this technique that must * Corresponding author. Tel.: þ86 21 34206056; fax: þ86 21 34206309. E-mail address: [email protected] (Y. Li). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.02.014
be addressed and surpassed. Among these include the effectiveness of the method in prolonged drying periods, high power consumption, inhomogeneity in grain-moisture-levels at different locations of the granary, and the dependence of the technique on weather conditions. To make the process less dependent on the weather conditions fossil fuels and electricity can be utilized to ventilate heated air into the grain-storehouse. In order to save energy as well as to make the process more environmental friendly, solar energy can also be utilized as the heat source for in-store drying [3,4]. One shortcoming regarding the application of solar energy is its intermittence or discontinuity which calls for the implementation of an auxiliary heat source. This auxiliary heat source can be in the form of a heat pump. In a previous study, a heat pump was utilized for instore drying. The system was characterized by its energy savings and stable performance [5]. But the system was only composed of heat pump. The combination of solar energy and heat pump for in-store drying has not been found in literature. In this paper, a method called solar assisted heat pump in-store drying is proposed. The specific components of this drying technique consist of a set of solar air collectors, a heat pump and an automatic grain stirrer. The solar air collectors and the heat pump form a joint heat source system that dries the grain. This is a unique combination not only because it makes full use of solar energy but also because it solves the problem of intermittent and non-steady availability of solar radiation. The automatic grain stirrer also improves the uniformity of the grain’s moisture content. A prototype of the in-store drying system has been built and tested in Kunming. The city has sufficient solar radiation and
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724
the location is E102 and N25 in the southwest of China. The study describes the design parameters, conducts tests and examines the experimental data to evaluate the effectiveness of the in-store drying process.
2. Description of the in-store drying system 2.1. Operational system Figure 1 shows a schematic of the solar assisted heat pump instore drying system. The system comprises of four main units: (i) the solar assisted heat pump unit; (ii) the supply and recycle air unit; (iii) the air distribution unit; and (iv) the automatic grain stirrer unit. As the process starts, ambient air is heated or dehumidified by the solar assisted heat pump. The treated air is then sent to the granary through the supply air unit. At the bottom of the granary, air is distributed uniformly to begin the drying process. The air flows through the voids generated by the grains from the bottom to the top. The exhaust air flowing out of the granary is then directed to the evaporator in unit I to recycle the waste-heat. During the drying process, the automatic grain stirrer operates constantly. The in-store drying process contains a movable diaphragm between the condenser and evaporator. The diaphragm has the capability to connect or separate the parts to conduct a heating or dehumidification process, respectively. Because higher temperature is a threat to the stored grain, double heating of solar energy and heat pump for drying air is not suitable. Therefore, parallel connecting method is selected to control the temperature of granary inlet air and reduce the flow resistance. The solar air collectors and the heat pump are arranged in parallel. There are four working modes for the system, which include the solar energy heating mode, the heat pump heating mode, the solar assisted heat pump heating mode, and the heat pump dehumidification mode. Based on the weather conditions, a working mode is selected. If solar radiation is sufficient during the daytime, the system will operate in solar energy heating mode. In case of insufficient solar radiation in daytime, the system will operate in solar assisted heat pump heating mode. In occasions in which solar radiation is unavailable during the daytime, the system will operate in heat pump heating mode. If the ambient humidity is high, the system will operate in heat pump dehumidification mode.
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2.2. Components of the system To evaluate the performance of the heat pump in-store drying system, a prototype was built in Kunming, China. Corn was utilized as the grain. The different components of the system are described as follows. Four flat-plate solar air collectors were manufactured. The dimensions of each collector are 4 m 6 m enclosing a total surface area of 96 m2. An air source heat pump was manufactured with a compressor containing a power rating of 22 kW. The heat pump is arranged in parallel with the collectors. The grain stirrer, also known as a grain butler, utilized in the system was manufactured by a Germany company. The stirrer consists of a motor with a power rating of 2.2 kW and a 4 m length auger. The grain stirrer operates automatically and helps improve the uniformity of the grains’ moisture content. Fig. 2 shows the granary (a), solar collectors (b), heat pump (c), and grain stirrer (d) used in the demonstration system. The air ducts connect the equipment to the ventilation openings located at the bottom of the granary. The air distribution unit and grain stirrer are located inside the granary. This system was utilized to conduct the experiments.
3. Materials and methods 3.1. The granary The granary consists of a large warehouse with dimensions 37.22 m 22.86 m 7.8 m. The grain stored in the granary is corn. Table 1 shows some relevant parameters regarding the granary and grain.
3.2. Parameters and specifications to operate the system The experiment aims to evaluate the performance of this novel in-store drying system by measuring the percentage reduction in moisture content of the corn, the power consumption level and the quality of the grain. The working mode was selected according to the weather conditions. The measured experimental data include the temperature of the granary inlet air, the corn’s moisture content, quality index of corn and the power consumption of the drying
Fig. 1. Schematic diagram of the solar assisted heat pump in-store drying system. 1. solar air collector 2. air source heat pump 3. evaporator 4. condenser 5. movable diaphragm 6. supply fan 7. ventilation supply tube 8. air recycle tube 9. grain stirrer 10. granary.
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Fig. 2. The granary (a), solar collectors (b), heat pump (c), and grain stirrer (d) used in the demonstration system.
process. Other important operating parameters are described in the in the paragraphs below. (1) Ventilation system During the experiment, the ventilation rate of the system was maintained at a steady rate of 34,800 m3/h. The solar collector channel and the heat pump channel remained unblocked regardless of the system’s operating mode. The ventilation rates of the solar collector channel and the heat pump channel were kept at 23,270 m3/h and 11,570 m3/h, respectively. (2) Conditions to turn on the heat pump The heat pump is turned on based on the weather conditions and time of the day. If the solar radiation is sufficient in the daytime and the granary inlet air can achieve a temperature rise of 6 w 8 C compared to ambient air, the heat pump will not be operated and the system operates under the solar collector heating mode. If the solar radiation is not sufficient during the daytime the heat pump will be turned on forcing the system to operate under the solar assisted heat pump heating mode or heat pump heating mode. The Table 1 Parameters of experimental granary. Items
Parameter
Grain Grain temperature Grain moisture content Granary size Height of grain bulk Granary capacity
corn 20 C 12.9% (wb) 37.22 m 22.86 m 7.8 m 5.3 m 3700 t
evaluation of solar radiation is based on the outlet air temperature. A temperature rise of 6 C compared to ambient air is the criterion. In the present study the heat pump dehumidification mode was not utilized due to the low humidity registered in Kunming during the experimental trials. Most of time during the experiment the relative humidity of outdoor air is lower than 40%. Therefore, that is not necessary for dehumidification. (3) The grain stirrer The grain stirrer is used during the in-store drying period. In general, it operates when the moisture gradient registers more than 1% moisture difference in several one-meter-apart locations along the depth of the granary. It is important to mention that because the corn stored in the granary was collected from different locations in China, inhomogeneous moisture content distribution was a norm in the experiment. The initial moisture content difference reached 4.7%. Therefore, the grain stirrer worked continuously during test. (4) Moisture content measurement The grain bulk was divided into five layers and 24 points were selected in each layer. Moisture content measurements were taken in the different layers and locations. In total, 120 test points were used as the measuring moisture sites. 105 C standard moisture measurements were taken at designated time intervals to register the moisture level of the corn. (5) Corn’s quality measurement technique The quality of the grains was measured at the beginning and end of the experiment. The samples were collected at the same time as
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724
the first and final moisture measurements were taken. The quality parameters measured in corn were fatty acid level and unsound kernel.
3.3. Model simulation Due to the limited experimental resources, the initial average moisture content of corn in the granary was lower than the safetystandard storage level. To evaluate the performance of the present in-store drying process, the drying experiment was conducted but did not last long in order to keep the corn in good quality. At the conclusion of the experimental trial, the reduction in moisture content of the corn was less. Therefore, the research team decided to use a mathematical simulation to supplement the experimental portion of the study. This helped to formulate a more comprehensive analysis of the proposed drying method. The deep bed grain drying model was applied to simulate the variation in moisture content inside the granary. The grain drying model can be classified as equilibrium, nonequilibrium and logarithmic type [6]. The logarithmic model is the simplest but its precision is the lowest. The non-equilibrium model is mainly used for high drying temperatures whereas the equilibrium model is suitable for low drying temperatures [7]. The research team decided to use the equilibrium model and proposed some modifications to improve the simulation precision [8,9]; the modified model can be referred to as the quasi-equilibrium model. In this study, the drying process can be categorized as low drying temperature. The mathematical model suggested by Hossain et al. [9] along with the modifications (e.g. empirical thin-layer drying equation) is presented as follows:
k ¼ expðxt y Þ i0:5 h h ð1:8T þ 32Þ 3:353 x ¼ 6:0142 þ 1:453 104 ðrhÞ2 i0:5 104 þ 3:0 108 ðrhÞ2 y ¼ 0:1227 1:461 103 ðrhÞ þ 4:14 105 ðrhÞT 1:044 104 T It should be noted that the mathematical model applied here does not consider the effect of the stirrer. The simulated result is only used to analyze the variation of the average moisture content of the entire granary rather than the actual moisture content distribution. Thus, the later point supports the assumption of not considering the effect of the stirrer. 4. Performance analysis The performance of the demonstration system is evaluated by three indexes: the drying rate; grain quality indices and the power consumption level. 4.1. Drying rate The main objective of the drying process is to reduce the moisture content of the grain. Therefore, the drying rate is one of the most important parameters in the evaluation of system’s performance. The drying rate is defined as moisture content reduction per unit drying time. Its expression is given by:
dM M M2 ¼ 1 Dt dt
(1) Thin-layer drying equation for corn:
M Me ¼ expðkt n Þ M0 Me
(1)
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(5)
where M1 and M2 are the initial and final moisture content of the corn. 4.2. Grain quality indices
(2) Mass balance equation: The equation indicates the variation of humidity drying air based on the theory that moisture lost by grain is equal to moisture gained by air.
DH ¼
rg Ga
Two quality indices of corn are measured in this study. Fatty acid and unsound kernel can indicate the quality change of the grains. The increase in value of the two indices (i.e. from initial to final measurements) translates into a decrease in the quality of corn. 4.3. Power consumption level
DM Dz Dt
(2) The specific power consumption, Ew, to reduce the grain moisture content by 1%, is selected to describe the power consumption level. Ew is defined as:
(3) Energy balance equation:
P
The equation indicates the variation in air or grain temperature in the finite difference form
ðCv Cw ÞT þ rvap DM i Dt Cg þ Cw M þ ðCw Cv ÞDM þ Gr a ðCa þ Cv HÞ D z
DT ¼ h
g
(3) The thin-layer drying equation used in this study is an empirical equation for corn at low drying temperature; this was developed by Sabbah [3]:
M Me ¼ exp kt 0:664 M0 Me where
(4)
Ew ¼
W G$ðM1 M2 Þ
(6)
where W is the power consumption of the system’s individual components (kWh), G is the total weight of corn in the granary (t), and M1 and M2 are the initial and final moisture content, respectively. 5. Results and discussion 5.1. Temperature and humidity of granary inlet air The in-store drying experiment was conducted from April 30th to May 5th, 2009. The effective ventilation time was 42 h. The temperature and humidity of the granary inlet air varied with the
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Fig. 5. Measured variation of average moisture content of corn. Fig. 3. Temperature comparison of granary inlet air and ambient air.
ambient conditions and working mode of the drying system. Figs. 3 and 4 are the temperature and relative humidity comparison between granary inlet air and ambient air during the experimental period, respectively. It was found that after air flows through the solar air collector and heat pump, the temperature of the granary inlet air rises while the relative humidity decreases. During the experimental period, the average temperature difference between ambient air and granary inlet air reached about 8.9 C. The relative humidity of the granary inlet air varied between 13.6% w 37.7% which is lower than ambient air. The use of the solar assisted heat pump unit to heat the ambient air improves the drying capability of the air significantly. 5.2. Variation of corn moisture content Variation of average moisture content is shown in Fig. 5. The value decreased from 12.9% to 12.5% during the 42 h of ventilation drying time. The drying rate is higher than another in-store drying experiment conducted in China [10]. Besides, Fig. 5 shows the simulation results of moisture content variation based on the model presented above. The simulation is according to the actual temperature and humidity data of the granary inlet air. It can be seen that the actual drying time is close to the simulation result obtained from the mathematical model. The tendencies closely
Fig. 4. Relative humidity comparison of granary inlet air and ambient air.
match. Thus, the model proposed above is adequate to simulate the actual grain drying process. As explained in Section 3.3, due to the limitations of experimental resources, the initial moisture content of the experimental corn is low and the experimental time is short. To evaluate the new in-store process under more typical conditions, a mathematical simulation was applied. The actual size of the granary is 37.22 m 22.86 m 7.8 m and 5.3 m height of bulk grain is applied in the simulation. The ventilation rate is 34,800 m3/h. The initial moisture content of corn used in the simulation is 16.6% (wb). Since the drying experiment was conducted in May in Kunming, the typical weather data of Kunming for the month of May was used in simulation. Two simulation results were obtained. One is the natural air ventilation drying with the ambient data directly. The second one is the heating-ventilation-drying, in which the granary inlet air temperature is 8.9 C above the ambient air due to the heating from the solar assisted heat pump unit according to the experiment. Simulation results are shown in Fig. 6. It can be found that, under typical weather conditions in Kunming during the month of May, the natural air in-store drying is not sufficient. The drying rate is rather slow and it took nearly a month to reduce the moisture content of about 1%. In contrast, the performance of the solar
Fig. 6. Comparison of simulation results under two different conditions (based on the weather conditions in Kunming during the month of May).
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724 Table 2 Corn quality variation during the experiment. Item
Before the experiment
After the experiment
Fatty acid (mg KOH/100 g) Unsound kernel (%)
35.1 6.2
40.2 6.8
assisted heat pump in-store drying is excellent. With the help of the solar assisted heat pump unit, the drying air can achieve a temperature rise of 8.9 C. It takes about 240 h for the moisture content of corn to decrease from 16.6% to 14.5% (wb). Hence solar assisted heat pump system improves the in-store drying rate greatly. 5.3. Uniformity of corn moisture content Experimental results show that the uniformity of corn moisture content has been improved. In terms of test points, the maximum moisture content difference is 4.7% among 120 test samples before the experiment, while the value decreases to 2.8% after the experiment. The improvement in moisture uniformity can be attributed to the utilization of the grain stirrer which mixed the corn from different layers. Higher moisture content corn from the top layer is moved to a lower layer and this can be dried to a greater extent. Therefore, the moisture content difference decreases. 5.4. Variation of corn quality Variation of corn quality during the experiment is shown in Table 2. Due to the low temperature and low velocity of drying air, the variation of corn quality is not significant. Fatty acid rises from 35.1% to 40.2%, which is still in the range of quality maintenance. The percentage of unsound kernels has a smaller increase, which is possibly caused by the grain stirrer. Overall, it can be concluded that the solar assisted heat pump in-store drying did little damage to the quality of drying grain. 5.5. Power consumption level Power consumption level is another important index to evaluate the in-store drying process. Lower power consumption is expected. The utilization of solar energy and the high efficiency heat pump makes this possible. During the experiment, the electrical equipment includes a compressor, supply fans, evaporator fan, axial fans and grain stirrer. The total power consumption is 1860 kWh. The index Ew defined as power consumption per grain ton to reduce the moisture content by 1% is selected to describe the power consumption level. According to the calculations, Ew for the experiment is 1.24 kWh (1%mc∙t), which is much lower than the requirement level of 2.0 kWh (1%mc∙t) for corn in-store drying drafted by Chinese agricultural ministry. Besides, the value is also lower than other in-store grain drying experiments that are performed in many grain depots in China [10e12]. 6. Conclusion A novel in-store drying process is proposed, which is composed of a solar air collector, a heat pump, fans, air ducts and a grain stirrer. The combination of this equipment leads to the creation of a unique in-store drying system. The utilization of solar energy results in significant reduction in conventional energy consumption for in-store drying. The use of heat pump not only supplies heat to the drying air, but also solves the problem of the intermittent availability of solar energy. Automatic grain stirrers can improve the uniformity of moisture content. A demonstration
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system was built and tested in Kunming, China. Based on the experimental and simulated results, the following conclusions were obtained. The solar assisted heat pump in-store drying process can reduce the moisture content of grain effectively and quickly. Because of the utilization of solar energy and heat pump, the temperature of the granary inlet air is higher than that of the ambient air, while relative humidity is lower than ambient air. In our experiments, the average temperature rise is 8.9 C. Both the experimental and simulated results show that the drying time decreased. The solar assisted heat pump in-store drying process can also improve the uniformity of moisture content. Different layers are mixed because of the utilization of a grain stirrer. The maximum moisture content difference was 4.7% among 120 test samples before the experiment, and decreased to 2.8% after the experiment. The solar assisted heat pump in-store drying process does little damage to the quality of drying grain. Variations of fatty acids and unsound kernels during the experiment were small and still in the range of quality maintenance. The solar assisted heat pump in-store drying process has a low power consumption level. Power consumption per grain ton to reduce the moisture content by 1% is 1.24 kWh (1%mc∙t), which is much lower than the official standard in China. Acknowledgements This work is supported by the National Key Technologies R&D Program under the contract No. 2006BAD08B06-2. Nomenclature
Ca Cg Cv Cw Ew G Ga H k n M M0 Me M1 M2 rvap rh T t W wb x y z
rg
specific heat of dry air, kJ/kg K specific heat of dry grain, kJ/kg K specific heat of water vapor, kJ/kg K specific heat of water, kJ/kg K specific power consumption, kWh (1%mc∙t) weight of drying grain, ton mass flow rate of air per unit bed cross section, kg/m2 s absolute humidity of drying air, kg/kg drying air parameter of thin-layer drying equation parameter of thin-layer drying equation moisture content of grain, dry basis initial moisture content, dry basis equilibrium moisture content, dry basis moisture content before drying, wet basis moisture content after drying, wet basis latent heat of vaporization of water, kJ/kg relative humidity of drying air temperature of drying air, C drying time, s power consumption, kWh wet base, quality ration of moisture and entire grain parameter of thin-layer drying equation parameter of thin-layer drying equation height of the bulk grain, m bulk density of grain, kg/m3, dry basis
References [1] C. Karunakaran, W.E. Muir, D.S. Jayas, N.D.G. White, D. Abramson, Safe storage time of high moisture wheat, Journal of Stored Products Research 37 (2001) 303e312. [2] T. Yutthana, P. Somkiat, Experiments on in-store paddy drying under tropical climate: simulation and product quality, in: Proceedings of the 13th International Drying Symposium, Beijing, China, 2002.
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[3] O.V. Ekechukwu, Review of solar-energy drying systems II: an overview of solar drying technology, Energy Conversion and Management 40 (1999a) 615e655. [4] O.V. Ekechukwu, Review of solar-energy drying systems III: low temperature air-heating solar collectors for crop drying applications, Energy Conversion and Management 40 (1999b) 657e667. [5] P.C. Fu, K. Li, S.W. Liao, Analysis of application and results of heat pump technology for in-store drying of grain, Grain Storage 36 (2007) 16e18. [6] V.K. Srivastava, J. John, Deep bed grain drying modeling, Energy Conversion and Management 43 (2002) 1689e1708. [7] S. Cenkowski, D.S. Jayas, S. Pabis, Deep bed grain drying e A review of particular theories, Drying Technology 11 (1993) 1553e1581.
[8] S. Soponronnarit, Energy model of grain drying system, ASEAN Journal on Science and Technology for Development 5 (1988) 43e68. [9] M.A. Hossain, B.K. Bala, M.A. Scatter, Simulation of natural air drying of maize in cribs, Simulation Modeling Practice and Theory 11 (2003) 571e583. [10] H. Liu, L.L. Zhang, L.M. Ren, The in-store drying experiment of high moisture content by mechanical ventilation, Journal of Henan University of Technology 28 (2007) 22e25. [11] G.F. Yang, Application of mechanical ventilation to reduce moisture content of paddy in large warehouse, Grain Storage 34 (2004) 13e15. [12] S.X. Yi, H.X. Sheng, F.F. Wang, Test of ventilation and lowering moisture on late indica rice of high moisture content in large warehouse, Grain Storage 35 (2005) 32e34.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation on a solar assisted heat pump in-store drying system Y. Li a, *, H.F. Li a, Y.J. Dai a, S.F. Gao b, Lei Wei b, Z.L. Li b, I.G. Odinez c, R.Z. Wang a a
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China China Grain Reserves Corporation, Beijing, China c Department of Civil and Materials Engineering, University of Illinois at Chicago, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2010 Accepted 13 February 2011 Available online 24 February 2011
Solar powered drying is an environment-friendly technique utilized to dry grain. A novel in-store drying system composed of flat-plate air collectors, a heat pump, fans, air ducts, and grain stirrer is proposed to make full use of solar energy and to reduce the consumption of electricity. A demonstration system was built and tested in Kunming, China. The experimental data was compared with the results derived from a mathematical simulation. The experimental and simulated data demonstrate that the solar assisted heat pump drying system improved the performance of the in-store drying process. An average temperature rise of 8.9 C for the granary inlet air was achieved. The drying rate was increased and the uniformity of grain moisture content was improved. In addition, the grain was maintained in good quality and the level of power consumption was reduced. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: In-store drying Grain Solar air collector Heat pump Equilibrium model
1. Introduction Reducing the moisture content of grains to a safe level is critical for their storage. Grains are typically surrounded by fungi and mold. Environments in which high moisture content is the domain provide favorable living conditions for these microorganisms causing the grains to spoil. A research study on wheat shows that the time for germination to decrease by 10% is 2.5 days and 7 days for wheat which has moisture content of 19% and 17%, (moisture content is abbreviated to m.c. in the following article) respectively [1]. Since the quality of grains is affected by the moisture content, drying has become an important process in the storage of the grains. In-store drying consists in the placement of high moisture content grains in storehouses or granaries ventilated by ambient or heated air. The later two serve as the drying medium to maintain the grains under adequate in-store moisture conditions. A great feature of this methodology is that grain drying and storage occurs in the same location. Compared with the use of high temperature dryer, instore drying is capable of handling large amounts of grain at one time while reducing labor requirement. Due to the low drying temperature and low specific flow rates of in-store drying, the quality and freshness of the grains are effectively maintained [2]. In China, in-store drying by ambient air is the principal grain drying method adopted by various depots in the country. However, there exist several shortcomings regarding this technique that must * Corresponding author. Tel.: þ86 21 34206056; fax: þ86 21 34206309. E-mail address: [email protected] (Y. Li). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.02.014
be addressed and surpassed. Among these include the effectiveness of the method in prolonged drying periods, high power consumption, inhomogeneity in grain-moisture-levels at different locations of the granary, and the dependence of the technique on weather conditions. To make the process less dependent on the weather conditions fossil fuels and electricity can be utilized to ventilate heated air into the grain-storehouse. In order to save energy as well as to make the process more environmental friendly, solar energy can also be utilized as the heat source for in-store drying [3,4]. One shortcoming regarding the application of solar energy is its intermittence or discontinuity which calls for the implementation of an auxiliary heat source. This auxiliary heat source can be in the form of a heat pump. In a previous study, a heat pump was utilized for instore drying. The system was characterized by its energy savings and stable performance [5]. But the system was only composed of heat pump. The combination of solar energy and heat pump for in-store drying has not been found in literature. In this paper, a method called solar assisted heat pump in-store drying is proposed. The specific components of this drying technique consist of a set of solar air collectors, a heat pump and an automatic grain stirrer. The solar air collectors and the heat pump form a joint heat source system that dries the grain. This is a unique combination not only because it makes full use of solar energy but also because it solves the problem of intermittent and non-steady availability of solar radiation. The automatic grain stirrer also improves the uniformity of the grain’s moisture content. A prototype of the in-store drying system has been built and tested in Kunming. The city has sufficient solar radiation and
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724
the location is E102 and N25 in the southwest of China. The study describes the design parameters, conducts tests and examines the experimental data to evaluate the effectiveness of the in-store drying process.
2. Description of the in-store drying system 2.1. Operational system Figure 1 shows a schematic of the solar assisted heat pump instore drying system. The system comprises of four main units: (i) the solar assisted heat pump unit; (ii) the supply and recycle air unit; (iii) the air distribution unit; and (iv) the automatic grain stirrer unit. As the process starts, ambient air is heated or dehumidified by the solar assisted heat pump. The treated air is then sent to the granary through the supply air unit. At the bottom of the granary, air is distributed uniformly to begin the drying process. The air flows through the voids generated by the grains from the bottom to the top. The exhaust air flowing out of the granary is then directed to the evaporator in unit I to recycle the waste-heat. During the drying process, the automatic grain stirrer operates constantly. The in-store drying process contains a movable diaphragm between the condenser and evaporator. The diaphragm has the capability to connect or separate the parts to conduct a heating or dehumidification process, respectively. Because higher temperature is a threat to the stored grain, double heating of solar energy and heat pump for drying air is not suitable. Therefore, parallel connecting method is selected to control the temperature of granary inlet air and reduce the flow resistance. The solar air collectors and the heat pump are arranged in parallel. There are four working modes for the system, which include the solar energy heating mode, the heat pump heating mode, the solar assisted heat pump heating mode, and the heat pump dehumidification mode. Based on the weather conditions, a working mode is selected. If solar radiation is sufficient during the daytime, the system will operate in solar energy heating mode. In case of insufficient solar radiation in daytime, the system will operate in solar assisted heat pump heating mode. In occasions in which solar radiation is unavailable during the daytime, the system will operate in heat pump heating mode. If the ambient humidity is high, the system will operate in heat pump dehumidification mode.
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2.2. Components of the system To evaluate the performance of the heat pump in-store drying system, a prototype was built in Kunming, China. Corn was utilized as the grain. The different components of the system are described as follows. Four flat-plate solar air collectors were manufactured. The dimensions of each collector are 4 m 6 m enclosing a total surface area of 96 m2. An air source heat pump was manufactured with a compressor containing a power rating of 22 kW. The heat pump is arranged in parallel with the collectors. The grain stirrer, also known as a grain butler, utilized in the system was manufactured by a Germany company. The stirrer consists of a motor with a power rating of 2.2 kW and a 4 m length auger. The grain stirrer operates automatically and helps improve the uniformity of the grains’ moisture content. Fig. 2 shows the granary (a), solar collectors (b), heat pump (c), and grain stirrer (d) used in the demonstration system. The air ducts connect the equipment to the ventilation openings located at the bottom of the granary. The air distribution unit and grain stirrer are located inside the granary. This system was utilized to conduct the experiments.
3. Materials and methods 3.1. The granary The granary consists of a large warehouse with dimensions 37.22 m 22.86 m 7.8 m. The grain stored in the granary is corn. Table 1 shows some relevant parameters regarding the granary and grain.
3.2. Parameters and specifications to operate the system The experiment aims to evaluate the performance of this novel in-store drying system by measuring the percentage reduction in moisture content of the corn, the power consumption level and the quality of the grain. The working mode was selected according to the weather conditions. The measured experimental data include the temperature of the granary inlet air, the corn’s moisture content, quality index of corn and the power consumption of the drying
Fig. 1. Schematic diagram of the solar assisted heat pump in-store drying system. 1. solar air collector 2. air source heat pump 3. evaporator 4. condenser 5. movable diaphragm 6. supply fan 7. ventilation supply tube 8. air recycle tube 9. grain stirrer 10. granary.
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Fig. 2. The granary (a), solar collectors (b), heat pump (c), and grain stirrer (d) used in the demonstration system.
process. Other important operating parameters are described in the in the paragraphs below. (1) Ventilation system During the experiment, the ventilation rate of the system was maintained at a steady rate of 34,800 m3/h. The solar collector channel and the heat pump channel remained unblocked regardless of the system’s operating mode. The ventilation rates of the solar collector channel and the heat pump channel were kept at 23,270 m3/h and 11,570 m3/h, respectively. (2) Conditions to turn on the heat pump The heat pump is turned on based on the weather conditions and time of the day. If the solar radiation is sufficient in the daytime and the granary inlet air can achieve a temperature rise of 6 w 8 C compared to ambient air, the heat pump will not be operated and the system operates under the solar collector heating mode. If the solar radiation is not sufficient during the daytime the heat pump will be turned on forcing the system to operate under the solar assisted heat pump heating mode or heat pump heating mode. The Table 1 Parameters of experimental granary. Items
Parameter
Grain Grain temperature Grain moisture content Granary size Height of grain bulk Granary capacity
corn 20 C 12.9% (wb) 37.22 m 22.86 m 7.8 m 5.3 m 3700 t
evaluation of solar radiation is based on the outlet air temperature. A temperature rise of 6 C compared to ambient air is the criterion. In the present study the heat pump dehumidification mode was not utilized due to the low humidity registered in Kunming during the experimental trials. Most of time during the experiment the relative humidity of outdoor air is lower than 40%. Therefore, that is not necessary for dehumidification. (3) The grain stirrer The grain stirrer is used during the in-store drying period. In general, it operates when the moisture gradient registers more than 1% moisture difference in several one-meter-apart locations along the depth of the granary. It is important to mention that because the corn stored in the granary was collected from different locations in China, inhomogeneous moisture content distribution was a norm in the experiment. The initial moisture content difference reached 4.7%. Therefore, the grain stirrer worked continuously during test. (4) Moisture content measurement The grain bulk was divided into five layers and 24 points were selected in each layer. Moisture content measurements were taken in the different layers and locations. In total, 120 test points were used as the measuring moisture sites. 105 C standard moisture measurements were taken at designated time intervals to register the moisture level of the corn. (5) Corn’s quality measurement technique The quality of the grains was measured at the beginning and end of the experiment. The samples were collected at the same time as
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724
the first and final moisture measurements were taken. The quality parameters measured in corn were fatty acid level and unsound kernel.
3.3. Model simulation Due to the limited experimental resources, the initial average moisture content of corn in the granary was lower than the safetystandard storage level. To evaluate the performance of the present in-store drying process, the drying experiment was conducted but did not last long in order to keep the corn in good quality. At the conclusion of the experimental trial, the reduction in moisture content of the corn was less. Therefore, the research team decided to use a mathematical simulation to supplement the experimental portion of the study. This helped to formulate a more comprehensive analysis of the proposed drying method. The deep bed grain drying model was applied to simulate the variation in moisture content inside the granary. The grain drying model can be classified as equilibrium, nonequilibrium and logarithmic type [6]. The logarithmic model is the simplest but its precision is the lowest. The non-equilibrium model is mainly used for high drying temperatures whereas the equilibrium model is suitable for low drying temperatures [7]. The research team decided to use the equilibrium model and proposed some modifications to improve the simulation precision [8,9]; the modified model can be referred to as the quasi-equilibrium model. In this study, the drying process can be categorized as low drying temperature. The mathematical model suggested by Hossain et al. [9] along with the modifications (e.g. empirical thin-layer drying equation) is presented as follows:
k ¼ expðxt y Þ i0:5 h h ð1:8T þ 32Þ 3:353 x ¼ 6:0142 þ 1:453 104 ðrhÞ2 i0:5 104 þ 3:0 108 ðrhÞ2 y ¼ 0:1227 1:461 103 ðrhÞ þ 4:14 105 ðrhÞT 1:044 104 T It should be noted that the mathematical model applied here does not consider the effect of the stirrer. The simulated result is only used to analyze the variation of the average moisture content of the entire granary rather than the actual moisture content distribution. Thus, the later point supports the assumption of not considering the effect of the stirrer. 4. Performance analysis The performance of the demonstration system is evaluated by three indexes: the drying rate; grain quality indices and the power consumption level. 4.1. Drying rate The main objective of the drying process is to reduce the moisture content of the grain. Therefore, the drying rate is one of the most important parameters in the evaluation of system’s performance. The drying rate is defined as moisture content reduction per unit drying time. Its expression is given by:
dM M M2 ¼ 1 Dt dt
(1) Thin-layer drying equation for corn:
M Me ¼ expðkt n Þ M0 Me
(1)
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(5)
where M1 and M2 are the initial and final moisture content of the corn. 4.2. Grain quality indices
(2) Mass balance equation: The equation indicates the variation of humidity drying air based on the theory that moisture lost by grain is equal to moisture gained by air.
DH ¼
rg Ga
Two quality indices of corn are measured in this study. Fatty acid and unsound kernel can indicate the quality change of the grains. The increase in value of the two indices (i.e. from initial to final measurements) translates into a decrease in the quality of corn. 4.3. Power consumption level
DM Dz Dt
(2) The specific power consumption, Ew, to reduce the grain moisture content by 1%, is selected to describe the power consumption level. Ew is defined as:
(3) Energy balance equation:
P
The equation indicates the variation in air or grain temperature in the finite difference form
ðCv Cw ÞT þ rvap DM i Dt Cg þ Cw M þ ðCw Cv ÞDM þ Gr a ðCa þ Cv HÞ D z
DT ¼ h
g
(3) The thin-layer drying equation used in this study is an empirical equation for corn at low drying temperature; this was developed by Sabbah [3]:
M Me ¼ exp kt 0:664 M0 Me where
(4)
Ew ¼
W G$ðM1 M2 Þ
(6)
where W is the power consumption of the system’s individual components (kWh), G is the total weight of corn in the granary (t), and M1 and M2 are the initial and final moisture content, respectively. 5. Results and discussion 5.1. Temperature and humidity of granary inlet air The in-store drying experiment was conducted from April 30th to May 5th, 2009. The effective ventilation time was 42 h. The temperature and humidity of the granary inlet air varied with the
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Fig. 5. Measured variation of average moisture content of corn. Fig. 3. Temperature comparison of granary inlet air and ambient air.
ambient conditions and working mode of the drying system. Figs. 3 and 4 are the temperature and relative humidity comparison between granary inlet air and ambient air during the experimental period, respectively. It was found that after air flows through the solar air collector and heat pump, the temperature of the granary inlet air rises while the relative humidity decreases. During the experimental period, the average temperature difference between ambient air and granary inlet air reached about 8.9 C. The relative humidity of the granary inlet air varied between 13.6% w 37.7% which is lower than ambient air. The use of the solar assisted heat pump unit to heat the ambient air improves the drying capability of the air significantly. 5.2. Variation of corn moisture content Variation of average moisture content is shown in Fig. 5. The value decreased from 12.9% to 12.5% during the 42 h of ventilation drying time. The drying rate is higher than another in-store drying experiment conducted in China [10]. Besides, Fig. 5 shows the simulation results of moisture content variation based on the model presented above. The simulation is according to the actual temperature and humidity data of the granary inlet air. It can be seen that the actual drying time is close to the simulation result obtained from the mathematical model. The tendencies closely
Fig. 4. Relative humidity comparison of granary inlet air and ambient air.
match. Thus, the model proposed above is adequate to simulate the actual grain drying process. As explained in Section 3.3, due to the limitations of experimental resources, the initial moisture content of the experimental corn is low and the experimental time is short. To evaluate the new in-store process under more typical conditions, a mathematical simulation was applied. The actual size of the granary is 37.22 m 22.86 m 7.8 m and 5.3 m height of bulk grain is applied in the simulation. The ventilation rate is 34,800 m3/h. The initial moisture content of corn used in the simulation is 16.6% (wb). Since the drying experiment was conducted in May in Kunming, the typical weather data of Kunming for the month of May was used in simulation. Two simulation results were obtained. One is the natural air ventilation drying with the ambient data directly. The second one is the heating-ventilation-drying, in which the granary inlet air temperature is 8.9 C above the ambient air due to the heating from the solar assisted heat pump unit according to the experiment. Simulation results are shown in Fig. 6. It can be found that, under typical weather conditions in Kunming during the month of May, the natural air in-store drying is not sufficient. The drying rate is rather slow and it took nearly a month to reduce the moisture content of about 1%. In contrast, the performance of the solar
Fig. 6. Comparison of simulation results under two different conditions (based on the weather conditions in Kunming during the month of May).
Y. Li et al. / Applied Thermal Engineering 31 (2011) 1718e1724 Table 2 Corn quality variation during the experiment. Item
Before the experiment
After the experiment
Fatty acid (mg KOH/100 g) Unsound kernel (%)
35.1 6.2
40.2 6.8
assisted heat pump in-store drying is excellent. With the help of the solar assisted heat pump unit, the drying air can achieve a temperature rise of 8.9 C. It takes about 240 h for the moisture content of corn to decrease from 16.6% to 14.5% (wb). Hence solar assisted heat pump system improves the in-store drying rate greatly. 5.3. Uniformity of corn moisture content Experimental results show that the uniformity of corn moisture content has been improved. In terms of test points, the maximum moisture content difference is 4.7% among 120 test samples before the experiment, while the value decreases to 2.8% after the experiment. The improvement in moisture uniformity can be attributed to the utilization of the grain stirrer which mixed the corn from different layers. Higher moisture content corn from the top layer is moved to a lower layer and this can be dried to a greater extent. Therefore, the moisture content difference decreases. 5.4. Variation of corn quality Variation of corn quality during the experiment is shown in Table 2. Due to the low temperature and low velocity of drying air, the variation of corn quality is not significant. Fatty acid rises from 35.1% to 40.2%, which is still in the range of quality maintenance. The percentage of unsound kernels has a smaller increase, which is possibly caused by the grain stirrer. Overall, it can be concluded that the solar assisted heat pump in-store drying did little damage to the quality of drying grain. 5.5. Power consumption level Power consumption level is another important index to evaluate the in-store drying process. Lower power consumption is expected. The utilization of solar energy and the high efficiency heat pump makes this possible. During the experiment, the electrical equipment includes a compressor, supply fans, evaporator fan, axial fans and grain stirrer. The total power consumption is 1860 kWh. The index Ew defined as power consumption per grain ton to reduce the moisture content by 1% is selected to describe the power consumption level. According to the calculations, Ew for the experiment is 1.24 kWh (1%mc∙t), which is much lower than the requirement level of 2.0 kWh (1%mc∙t) for corn in-store drying drafted by Chinese agricultural ministry. Besides, the value is also lower than other in-store grain drying experiments that are performed in many grain depots in China [10e12]. 6. Conclusion A novel in-store drying process is proposed, which is composed of a solar air collector, a heat pump, fans, air ducts and a grain stirrer. The combination of this equipment leads to the creation of a unique in-store drying system. The utilization of solar energy results in significant reduction in conventional energy consumption for in-store drying. The use of heat pump not only supplies heat to the drying air, but also solves the problem of the intermittent availability of solar energy. Automatic grain stirrers can improve the uniformity of moisture content. A demonstration
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system was built and tested in Kunming, China. Based on the experimental and simulated results, the following conclusions were obtained. The solar assisted heat pump in-store drying process can reduce the moisture content of grain effectively and quickly. Because of the utilization of solar energy and heat pump, the temperature of the granary inlet air is higher than that of the ambient air, while relative humidity is lower than ambient air. In our experiments, the average temperature rise is 8.9 C. Both the experimental and simulated results show that the drying time decreased. The solar assisted heat pump in-store drying process can also improve the uniformity of moisture content. Different layers are mixed because of the utilization of a grain stirrer. The maximum moisture content difference was 4.7% among 120 test samples before the experiment, and decreased to 2.8% after the experiment. The solar assisted heat pump in-store drying process does little damage to the quality of drying grain. Variations of fatty acids and unsound kernels during the experiment were small and still in the range of quality maintenance. The solar assisted heat pump in-store drying process has a low power consumption level. Power consumption per grain ton to reduce the moisture content by 1% is 1.24 kWh (1%mc∙t), which is much lower than the official standard in China. Acknowledgements This work is supported by the National Key Technologies R&D Program under the contract No. 2006BAD08B06-2. Nomenclature
Ca Cg Cv Cw Ew G Ga H k n M M0 Me M1 M2 rvap rh T t W wb x y z
rg
specific heat of dry air, kJ/kg K specific heat of dry grain, kJ/kg K specific heat of water vapor, kJ/kg K specific heat of water, kJ/kg K specific power consumption, kWh (1%mc∙t) weight of drying grain, ton mass flow rate of air per unit bed cross section, kg/m2 s absolute humidity of drying air, kg/kg drying air parameter of thin-layer drying equation parameter of thin-layer drying equation moisture content of grain, dry basis initial moisture content, dry basis equilibrium moisture content, dry basis moisture content before drying, wet basis moisture content after drying, wet basis latent heat of vaporization of water, kJ/kg relative humidity of drying air temperature of drying air, C drying time, s power consumption, kWh wet base, quality ration of moisture and entire grain parameter of thin-layer drying equation parameter of thin-layer drying equation height of the bulk grain, m bulk density of grain, kg/m3, dry basis
References [1] C. Karunakaran, W.E. Muir, D.S. Jayas, N.D.G. White, D. Abramson, Safe storage time of high moisture wheat, Journal of Stored Products Research 37 (2001) 303e312. [2] T. Yutthana, P. Somkiat, Experiments on in-store paddy drying under tropical climate: simulation and product quality, in: Proceedings of the 13th International Drying Symposium, Beijing, China, 2002.
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[3] O.V. Ekechukwu, Review of solar-energy drying systems II: an overview of solar drying technology, Energy Conversion and Management 40 (1999a) 615e655. [4] O.V. Ekechukwu, Review of solar-energy drying systems III: low temperature air-heating solar collectors for crop drying applications, Energy Conversion and Management 40 (1999b) 657e667. [5] P.C. Fu, K. Li, S.W. Liao, Analysis of application and results of heat pump technology for in-store drying of grain, Grain Storage 36 (2007) 16e18. [6] V.K. Srivastava, J. John, Deep bed grain drying modeling, Energy Conversion and Management 43 (2002) 1689e1708. [7] S. Cenkowski, D.S. Jayas, S. Pabis, Deep bed grain drying e A review of particular theories, Drying Technology 11 (1993) 1553e1581.
[8] S. Soponronnarit, Energy model of grain drying system, ASEAN Journal on Science and Technology for Development 5 (1988) 43e68. [9] M.A. Hossain, B.K. Bala, M.A. Scatter, Simulation of natural air drying of maize in cribs, Simulation Modeling Practice and Theory 11 (2003) 571e583. [10] H. Liu, L.L. Zhang, L.M. Ren, The in-store drying experiment of high moisture content by mechanical ventilation, Journal of Henan University of Technology 28 (2007) 22e25. [11] G.F. Yang, Application of mechanical ventilation to reduce moisture content of paddy in large warehouse, Grain Storage 34 (2004) 13e15. [12] S.X. Yi, H.X. Sheng, F.F. Wang, Test of ventilation and lowering moisture on late indica rice of high moisture content in large warehouse, Grain Storage 35 (2005) 32e34.