RESEARCH ARTICLE

Haifeng LI, Yanjun DAI, Jianguo DAI, Xibo WANG, Lei WEI

A solar assisted heat pump drying system for grain in-storedrying

© Higher Education Press and Springer-Verlag 2009

Abstract For grain in-store drying, a solar assisteddrying process has been developed, which consists of aset including a solar-assisted heat pump, a ventilationsystem, a grain stirrer, etc. In this way, low powerconsumption, short cycle time and water content uni-formity can be achieved in comparison with the conven-tional method. A solar-assisted heat pump drying systemhas been designed and manufactured for a practicalgranary, and the energy consumption performance of theunit is analyzed. The analysis result shows that the solarfraction of the unit is higher than 20%, the coefficient ofperformance about system (COPS) is 5.19, and the specificmoisture extraction rate (SMER) can reach 3.05 kg/kWh.

Keywords solar energy, heat pump, airflow, in-storedrying

1 Introduction

Drying is necessary for high water content grains,especially when the grain has to be stored for a longtime. Grain in-store drying, a drying method in which thegrain with high water content is put into the granarydirectly and ambient or heated air is used as the dryingmedium to ventilate the granary, has been adopted by manygrain depots because of its efficiency and simple operation.Of the many drying methods developed, natural ventilation

drying, heated ventilation drying and stir ventilation dryingare the most common ones. Natural ventilation in-storedrying is simple, inexpensive and effective, and has beenused in grain drying fields such as the natural ventilated in-store drying depots in the USA and Canada since the1950s. However, it is often limited by the ambientconditions and its drying process is usually longer.Hence, heated ventilation in-store drying has beendeveloped and popularized since the 1960s in the USAand some other developed countries. Moreover, toeliminate the grain water content gradient caused by in-store drying, grain stirrers, which can bring the grain fromthe bottom to the top surface of the depot, has beendeveloped and applied. Grain butler, a stirrer made inGermany, is well developed and is efficient in keeping thewater content uniform within the grain depot [1,2].As a great agriculture country, China produces massive

rice, wheat and corn. Every year huge amounts of grainneed drying, and the most popular way is the use of a graindryer or a natural ventilation in-store drying system.However, the former not only consumes a lot of fossilenergy but also brings the workers high labour intensitybecause of heavy transportation. The latter often requires along drying time period which consumes a lot of electricalenergy and is subjected to weather conditions. Many kindsof economical grain drying units using renewable energyhave attracted a lot of attention worldwide in the pastdecades. Among them, solar energy drying systems are themost investigated, and solar assisted heat pump technologyis particularly attractive, as the heat pump is a reliable andenergy saving facility. If solar heating is combined with theheat pump, the intermittence and low energy density ofsolar energy can be remedied. Besides, the heat pump canrecycle the sensible and latent heat of the return air, whichwill ensure the efficiency and safety of the grain dryingprocess [3,4].China, a country with a vast territory, has great

differences in climate conditions in different regions. Tosave energy, it is critical that extensive studies beconducted in exploring efficient ways of in-store grain

Received March 11, 2009; accepted May 29, 2009

Haifeng LI, Yanjun DAI (✉), Jianguo DAIInstitute of Refrigeration and Cryogenics, Shanghai Jiao TongUniversity, Shanghai 200240, ChinaE-mail: yjdai@sjtu.edu.cn

Xibo WANGChina Grain Reserves Corporation, Beijing 100040, China

Lei WEIHenan Weilai Machine Engineering, Co., LTD, Zhengzhou 450001,China

Front. Energy Power Eng. China 2010, 4(3): 386–391DOI 10.1007/s11708-010-0003-3

drying under different operation conditions. This paper is areport of the study and analysis of the performance of asolar assisted heat pump system for grain in-store drying.

2 In-store drying process by solar assistedheat pump unit

2.1 Solar assisted heat pump unit for in-store drying

The solar assisted heat pump system is composed of solarair collectors, an air source heat pump, a grain stirrer, and asupply and return fan, etc. Figure 1 shows the schematicdiagram of the in-store drying system.

The solar air collectors are connected one by one inparallel. The solar heating unit and the heat pump unit arealso arranged in parallel to reduce flow resistance. Both thecondenser and the evaporator of the heat pump are thefinned tube heat exchanger. Considering the grain bulk’sresistance, the solar collector fan, the condenser and theevaporator fan are installed and combined with the airsupply system to form an integral drying ventilationsystem. Figure 2 shows the heat pump unit for in-storegrain drying.The system can work in three operation modes, the solar

assisted heating and ventilation mode, the heat pumpheating and ventilation mode, and the heat pumpdehumidification and ventilation mode. The detailedworking process is shown in Fig. 3, in which Fig. 3(a)illustrates the solar assisted heating and ventilation modewhich can be adopted on sunny days. The working processof this mode can be explained by Fig. 1. On a sunny day,when the supply and return fan is turned on, air valves 1and 2 are moved to the position of the solid line. Ambientair is heated by the solar collectors and the heat pumprespectively. The airflows that come from the solar

collectors and the heat pump are mixed by the supplyfan and then sent to the granary to dry the grain. The returnair from the granary is introduced to the evaporator for heatrecovery. During the drying process, the stirrer keepsworking so that a uniform drying effect can be ensured.The heat pump heating and ventilation mode, feasible at

night, is shown in Fig. 3(b). In this mode, the collector fanis turned off and the air valve 3 is closed. Air valves 1 and 2are moved to the position of the solid line. Ambient air isheated by the heat pump, and then sent to the granary todry the grain. Also, the return air from the granary isintroduced to the evaporator for heat recovery.The heat pump dehumidification and ventilation mode,

used on rainy or cloudy days, is shown in Fig. 3(c).In this mode, the collector fan is turned off and airvalve 3 is closed. Air valves 1 and 2 are moved to theposition of the dashed line. Ambient air is introduced firstinto the evaporator where the air is cooled and dehumi-dified, and then sent into the condenser for reheating.Thereafter, it is sent to the granary for grain drying. Itshould be noted that during the drying process, the stirrerkeeps working all the time so that a uniform drying effectcan be ensured.

2.2 Air distribution in granary

To have a good in-store drying effect, both reasonable airdistribution in the granary and appropriate air temperatureand humidity are strictly required for ventilation dryingwith heating, without heating, or with dehumidification.For a safe and efficient in-store drying, it is important thatthe process air be supplied to the granary quickly anduniformly. In this study, a new type of CEDZ all-steelventilation duct manufactured by a domestic company, asshown in Fig. 4, is utilized. This ventilation duct is called aventilation cage, and is designed according to press-ventilated airflow. The cage, with a large opening ratio, alow ventilation resistance, a flexible changing duct lengthand distance (among ducts), and less dead corners forventilation, is composed of the main duct, the branch duct,the elbow, the plug, the tee joint, the fastener, and thedistributor, etc.

Fig. 1 Schematic diagram of a solar assisted heat pump forin-store drying

1,2,3–air valve; 4–solar air collector; 5–condenser; 6–evaporator;7–exhaust fan; 8–expansion valve; 9–compressor; 10–supply fan;

11–stirrer; 12–granary

Fig. 2 Multifunction heat pump unit for in-store grain drying

Haifeng LI et al. Solar assisted heat pump drying system 387

2.3 Grain stirrer

To ensure the uniformity of drying and to solve theproblem of the grain at the bottom of the bulk beingdried exceedingly while that at the upper part not beingdried sufficiently, a stirrer is also necessary because itcan guarantee drying uniformity in the vertical directionwith a lower power consumption apart from thereasonable air distribution unit. Therefore, a grain stirrercalled the grain butler, fabricated by a Germancompany, as shown in Fig. 5, is used. The stirrer increasesthe contact area of the process air and the grain, so that thedrying time can be reduced.

3 Demonstration drying granary andperformance index

3.1 Demonstration drying granary and solar assisted heatpump system

Based on the requirements for in-store grain drying, agranary in Kunming, Yunnan Province is chosen fordemonstration. In Yunnan, the grain is usually harvested inOctober. Hence, the weather data on a typical sunny day inOctober is selected for system design. It is assumed that themaximum solar radiation on a day is 894W/m2, and theaverage value can reach 750W/m2. The highest

Fig. 3 Three working modes of the drying process with solar assisted pump(a) Solar assisted heating and ventilation; (b) heat pump heating and ventilation; (c) heat pump dehumidification and ventilation

Fig. 4 Ventilation cage on the ground Fig. 5 Grain stirrer

388 Front. Energy Power Eng. China 2010, 4(3): 386–391

temperature is 20ºC and the lowest is 9.5ºC. The solarheating unit can work 8 hours per day.The designed area of the solar collectors composed of 20

units is about 100 m2. The dimensional size of the heatexchanger is 1100 mm�1500 mm�130 mm, and the areafor heat exchanging is 183 m2. The designed air flow rateof the solar collector, the supply air through the condenserand the exhaust air is 7000 m3/h, 23000 m3/h, and20000 m3/h, respectively. Considering the flow resistanceinside the grain bulk, a supply fan with an input power of19 kW and a designed air flow rate of 30000 m3/h isselected. The input power of the exhaust fan is 2.5 kW andthe designed air flow rate is 20000 m3/h.Detailed parameter of the granary is presented in

Table 1.

3.2 Performance index

The performance indexes of the solar collector includesolar thermal efficiency which indicates the performance ofthe solar collector itself, and the solar fraction which standsfor the contribution of the solar heating unit to the totalenergy consumed by the whole system.The solar thermal efficiency, defined as the ratio of

usable heat collected by the solar collector to the solarradiation received on the collector surface for a certain timeperiod, is expressed as [5,6]

ηc ¼!

t2

t1Qudt

Ac!t2

t1ITdt

: (1)

The solar fraction, defined as the ratio of usable heatobtained from the solar collector to the total required heatof the system for a certain time period, can be expressed as

SF ¼!

t2

t1Qudt

!t2

t1ILdt

: (2)

The performance of the heat pump unit is usuallyevaluated with the coefficient of heating performance(COPH), whose definition is given as

COPH ¼ Qcon

Pcom: (3)

For the studied solar assisted heat pump system for in-store grain drying, the performance of the whole systemcan be evaluated by another index, COPS, expressed as

COPS ¼ Qcon þ Qu

Pcom þ Pc: (4)

As for the power consumption of the in-store drying,dehydration quantity per unit of power can be used as itsevaluation index, whose definition is given as [7]:

SMER ¼ dehydration  quantity

input  power, (5)

where the unit of the dehydration quantity is kg, and theunit of the input power is kWh.

4 Result and discussion

4.1 Performance of solar collector

Here, the evacuated tubular solar collector is considered,because the product is easily available at a relatively lowcost. The solar heating unit includes many solar collectormodules, each of which is composed of an adiabaticheader, 52 evacuated tubes, inner coupler casing and fixedfittings. Figure 6 shows the solar radiation intensity and theinlet-outlet temperature of the solar collector on the testingday. It is seen that on a typical day, the temperature rise ofthe collector is 15ºC–30ºC. The outlet temperature is about35ºC–45ºC which can meet the requirements for graindrying. Therefore, the utilization of an evacuated tubularsolar collector is reasonable for the in-store drying system.The thermal efficiency of the solar heating unit is shown inFig.7. The thermal efficiency of the collector is about

Table 1 Parameter of granary for simulation

items parameter

grain variety round-grained rice

grain temperature/°C 20

grain moisture content/% 16–17

granary size/m�m�m 37.22�22.86�7.8

height of grain bulk/m 6

granary capacity/t 3000

wall thickness/m 0.79

roof structure stress arch board

Fig. 6 Inlet-outlet temperature and solar radiation intensityversus time

Haifeng LI et al. Solar assisted heat pump drying system 389

60%–70%, high enough to reduce the number of thefacilities and save the working area.

4.2 Performance of solar assisted heat pump unit

According to the design requirements, the heat supplied bythe heat pump will account for about 75% of the total heatproduced by the whole system. Hence, the performance ofthe heat pump may have an important impact on the dryingeffect. A multifunction heat pump unit set is manufacturedaccording to the special requirements for in-store graindrying. The performance of the heat pump is shown inFigs. 8 and 9 when the ambient temperature is 20°C andthe temperature rise is about 15°C, respectively. Figure 8shows the heat capacity of the heat pump. On a typical day,the heat capacity is about 102–115 kW, which indicatesthat the performance of the heat pump is relatively steadyand can ensure the safety of the drying process. If thesystem works in the heat pump heating mode, because thetemperature rise of the air is not high, the COP of the heatpump is more than 4.72. If the solar heating unit is alsoinvolved, the heating performance of the whole system canbe improved significantly. It is seen from Fig. 9 that themaximum COPS of the whole system can reach 6.25.During daytime, the average COPS can reach 5.66. If nighttime heating is also considered, the average COPS of thesystem will be up to 5.19.

4.3 Performance of in-store drying

Figure 10 shows the simulation result of in-store drying ofthe granary listed in Table 1 with the solar assisted pumpsystem on fifteen typical days. The simulation is based onthe deep-bed drying model [8,9]. The computing programis written according to the method proposed by A. Iguaz[10]. The actual mass of the stored grain is about 3000 t.The initial temperature and the water content of the grain is20ºC and 17% respectively. The simulation result

demonstrates that the drying process remains in a constantdrying rate stage during the whole drying period. Thevariation of grain temperature is small, which is limitedwithin 1°C. It takes 10 days to reduce the water content ofthe grain by 1.5%, and about 15 days to reduce the watercontent by 2%.

Fig. 7 Thermal efficiency of solar air collector Fig. 8 Heat capacity of the heat pump

Fig. 9 COP of the system

Fig. 10 Simulation of grain drying on typical days

390 Front. Energy Power Eng. China 2010, 4(3): 386–391

SMER is selected as the evaluation index of powerconsumption for in-store grain drying. A higher value ofSMER means lower power consumption. According to thecomputation, the SMER of the in-store grain dryingprocess by the solar assisted pump system is 3.05 kg/kWh,higher than other in-store grain drying experiments done inmany grain depots in China [11–13], indicating that the in-store drying method by the solar assisted heat pump systemcan meet the requirement for low power consumption.SF is selected as the evaluation index of the contribution

of the solar collector to the total system. In terms of energysaving, the utilization of solar energy is important for thesystem. The higher value of SF means that more solarenergy is utilized, which increases the COP of the totalsystem. In this study, the demonstration granary is inKunming where solar energy is abundant. According to thecomputation, the SF of the system in this study is morethan 20% under the conditions of a typical day. On sunnydays the SF can reach 28%.

5 Conclusion

The characteristics and the performance of a solar assistedheat pump system for in-store grain drying have beenstudied. As a brief summary, the novelty and the majorresults of the study are to be emphasized as follows:1) The in-store drying process with the solar assisted

heat pump system is completed with a solar assisted heatpump heating unit, a ventilation cage unit and a grainstirrer, etc.2) The solar assisted heat pump heating can ensure the

efficiency, low-power consumption and the safety of thedrying process; the ventilation cage can ensure that thedrying in the horizontal direction is uniform; the grainstirrer can guarantee that the drying in the vertical directionis uniform.3) On typical sunny days, the average thermal efficiency

of the solar air collector can be more than 0.6 and the solarcollector can work 8 hours. In the heating and ventilationmode operated only by the heat pump unit, the temperaturerise of the process air is about 15°C, and COPH is about4.5. In the heating and ventilation mode operated by thesolar assisted heat pump as a whole, the COPS of thesystem can reach 5.19.4) As for the contribution of solar energy to the total heat

of the system in the designed condition, the solar fraction is

more than 20%. In terms of the drying effect, thedehydration quantity per unit power consumption is3.05 kg/kWh, including the power consumption of thecompressor and the fans.

Acknowledgements This work was supported by the National KeyTechnologies R&D Program (No. 2006BAD08B06-2).

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