Energy Conversion and Management 50 (2009) 2180–2186

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Energy Conversion and Management

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A review of heat pump drying: Part 1 – Systems, models and studies

Neslihan Colak a, Arif Hepbasli b,*

a Department of Food Engineering, Faculty of Engineering, Pamukkale University, 20070 Denizli, Turkeyb Department of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey

a r t i c l e i n f o

Article history:Received 21 September 2008Accepted 27 April 2009Available online 3 June 2009

Keywords:Heat pump dryerEnergyExergySustainable development

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.04.031

* Corresponding author. Tel.: +90 232 388 400x512E-mail address: arif.hepbasli@ege.edu.tr (A. Hepba

a b s t r a c t

The first heat pump dryer (HPD) patent applications were started in 1973, while recently, there has beena great interest in utilizing HPDs for drying fruits, vegetables and biological materials.

This study deals with reviewing heat pump drying studies and consists of two parts. In the first part ofthis study, historical development of HPDs was briefly given first. Description of these systems was thenpresented. Finally, studies conducted on HPD were reviewed in terms of process efficiency modeling andprogress of quality.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

During the last decades, environmental issues have evolvedfrom pollution and depletion of natural resources, such as climatechange. Fossil fuel consumption contributes to these damages.From the viewpoint of environmental problems, developing sus-tainable and renewable energy sources and improving efficiencyof systems used thermal energy has become important.

The new equipment must be designed in compliance with therecent environmental and energy policies. Also, due to globaliza-tion and market expansion, a newly developed product qualitymust conform to a wide consumer’s preference [1].

Drying is one of the most energy intensive unit operations thateasily accounts for up to 15% of all industrial energy utilizations[2]. In many industrial drying processes, a large fraction of energyis wasted [3].

From 1980s, apart from the rise in energy prices, legislation onpollution, working conditions and safety requirements have be-come more stringent. To meet these requirements and optimizeenergy consumption, new technologies in drying method anddryer design have been in demand [4].

Mujumdar [5] reported that drying consumes up to 70% of thetotal energy in manufacturing wood products, 50% of the total en-ergy consumption in the manufacturing of finished textile fabricsand over 60% of the total energy needed for on farm corn produc-tion. Therefore, energy management is an essential part of dryingprocess and efficient energy conservation contributes significantlyto the overall operating cost [6].

ll rights reserved.

4; fax: +90 232 388 8562.sli).

Recently, there has been a great interest in utilizing HPDs fordrying fruits, vegetables and biological materials [7]. Heat pumps(HPs) are devices for raising the temperature of low grade heat en-ergy to a more useful level using a relatively small amount of highgrade energy [8]. Using HPs in convective hot air dryers has beenrecognized as an ideal area for HP applications [9]. The energy effi-ciencies of conventional dryers are generally very low, a value of35% being representative of the upper performance range [10].Strommen et al. [11] found that HPDs consume 60–80% less energythan conventional dryers operating at the same temperature. Thismakes such dryers a feasible option for users who are not satisfiedwith the comparatively high energy consumption of directlyheated dryers [9].

The main advantages of using HP technology are the energy-saving potential and the ability to control drying temperatureand air humidity. This creates the possibility of a wide range ofdrying conditions [12].

Many technologies have been developed to achieve best prod-ucts at the lowest cost, such as hot air drying, vacuum drying,and freeze drying [13]. But such drying processes are highly energyintensive. Optimal operation of dryer is one of the most cost-effec-tive methods for energy saving. Product quality is another impor-tant factor to be considered simultaneously with energyconservation [14]. HPDs offer several advantages over conven-tional hot air dryers for the drying of food products, including high-er energy efficiency, better product quality, the ability to operateindependently of outside ambient weather conditions. Also, thistechnology is environmentally friendly due to low energy require-ment and no release of gases and fumes into the atmosphere [15].Based on the preliminary studies conducted, the color and aromaqualities of dried agricultural products using HPs were better thanthose products using conventional hot air dryers [16–19].

Nomenclature

COP coefficient of performance (dimensionless)MER moisture extraction (evaporation) rate (kg moisture/

kWh)SMER specific moisture extraction (evaporation) rate (kg

moisture/kWh)SEC specific energy consumption (kWh/kg moisture)E energy rate (kW)Ex exergy rate (kW)

AbbreviationsHP heat pumpHPD heat pump dryerGEHP gas engine driven heat pumpLAC lactic acid bacteriaCHP chemical heat pumpCHPD chemical heat pump dryer

N. Colak, A. Hepbasli / Energy Conversion and Management 50 (2009) 2180–2186 2181

HP systems improve energy efficiency and cause less fossil fuelconsumption, so these systems appropriate to sustainable develop-ment concept.

This study is presented in two parts: the historical developmentof HPDs, system descriptions and reviewing of HPD studies waspresented in Part 1. Classification of HPD systems were given intabulated forms and performance analyses of these systems weredescribed in various ways in Part 2. Also, studies on the perfor-mance evaluation of HPDs and dried product quality were pre-sented, while advantages and disadvantages of the HPDs werediscussed.

2. Historical development of heat pump dryers

The first HPD patent applications were started in 1973 and thefirst HP drying studies in the literature were done by Hodgett [20]and Geeraert [21]. Hodgett [20] reported that; energy consumptionof HPD was less than conventional steam heated dryer, whileGeeraert [21] studied on HP timber drying.

Tai et al. [22] presented advantages of HPD systems and Oliver[23] studied on dehumidifying HPs used extensively for timberdrying. Zylla et al. [24] concluded that SMERs increased as the rel-ative humidity of the dryer outlet air increased.

Cunney and Williams [25] reported that a well designed enginedriven HP could achieve a reduction of about 30–50% in drying en-ergy cost. Newbert [26] showed that the energy consumptioncould be reduced by 40% for drying of malt with a coupled gas en-gine HP (GEHP) dryer.

In 1988, about 7% of the industrial HPs were used for drying.These HPs represented an installed capacity of 60 MW [16]. In1992, Meyer and Greyvenstein [27] analyzed the life cycle cost ofHPD application for grain drying.

Several patents have been granted for products and processes inthe field, and several types of HPDs have been manufactured inter-nationally [12,28].

In 2006, the optimum operating temperatures and the optimumsizes of system components were calculated at which minimumlife cycle cost occurred for the HPD system by Soylemez [29].

Dryer

Evaporator

Condenser

Air cycle Refrigerant cycle

Fig. 1. A schematic illustration of a heat pump drying system.

3. Description of HPD systems

A HP drying system consists mainly of two subsystems; a HPsystem and a drying chamber. HPs can transfer heat from naturalheat sources in the surroundings, such as the air, ground or water,or from industrial or domestic waste, chemical reaction or dryerexhaust air. Drying chamber can be formed as tray, fluid bed, ro-tary or band conveyor. The main components of the general HPunit are an evaporator, a condenser, a compressor and an expan-sion valve.

As shown in Fig. 1, in a HP drying system, the working fluid(refrigerant) at low pressure is vaporized in the evaporator by heat

drawn from the dryer exhaust air. The compressor raises the en-thalpy of the refrigerant of the HP and discharges it as superheatedvapor at high pressure. Heat is removed from the working fluid andreturned to the process air at the condenser. In the drying system,the hot air at the exit of the condenser is allowed to pass throughthe drying chamber where it gains latent heat from the product tobe dried. The working fluid from the condenser is then throttled tothe low pressure line (using an expansion valve) and enters theevaporator to complete the cycle. The humid air at dryer exit thenpasses through the evaporator where condensation of moisture oc-curs as the air goes below dew point temperature.

4. Reviewing the studies on heat pump drying systems

Drying is one of the most energy intensive industrial processes.In developed countries, approximately 10% of fuel is used for thispropose [20].

Since the greatest source of heat losses in conventional dryers isdue to the venting of the moist air [10,30], attention has been fo-cused on waste heat recovery [31].

Considerable studies about HPD systems have been done formany decades. Variation of number of studies on HPD with timewas given in Fig. 2. These studies will be examined under threemain topics.

4.1. Process efficiency

The performance of the HPDs can be determined by the mois-ture extraction rate (MER), the specific moisture extraction rate(SMER), the coefficient of performance (COP), energy efficiency,exergy efficiency or economic analysis. These methods are ex-plained with formulas in the second part of this study. The percent-age distribution of methods used for determining the HPDefficiency is given in Fig. 3.

Fig. 2. Variation of number of studies on HPD with time.

Cost6%Ex

6%

SEC8%

E16%

COP 23%

SMER 41%

Fig. 3. Distribution of performance assessment methods used in HPDs.

0

2

4

6

8

10

SME

R, k

g/kW

h

10.0 – Heat pump dehumidification at 40oC and 90% RH [21]

8.8 – Heat pump dehumidification at 40oC and 80% RH [21]

4.5 – Steam compression dryer [80]

0.3 – Conventional drying of malt [81] 0.3 – Conventional drying of oak [82]

Fig. 5. Effectiveness of various drying systems [34]. (See above-mentioned refer-ences for further information.)

2182 N. Colak, A. Hepbasli / Energy Conversion and Management 50 (2009) 2180–2186

Hodgett [20] reported that the HPD with an average SMER of3 kg/kW would use less energy than a steam heated dryer with athermal efficiency of 75% or a direct fired dryer of 58% efficiency.According to Oliver [23], SMER of the HPD systems were 0.57 kg/

Coefficient of Performance

1

2

3

4

5

0 2 4 6 8

Single-stage, motor driven HPD

Single-stage, motor driven HPD with subcooling

Single-stage, engine driven HPD

Single-stage, engine driven HPD with subcooling

Two-stage, engine driven HPD with subcooling

Fig. 4. COP of different heat pump configurations [33].

kW and 1.02 kg/kW when the drying temperatures were 50 �Cand 80 �C, respectively.

The performance of a compressor is very dependent upon thetemperature range between the evaporator and condenser. Manyof the commercial HPDs consist of a single-stage vapor compres-sion cycle [32]. A schematic illustration of COP for different HPconfigurations is given in Fig. 4 [33].

A summary of moisture removal efficiency for selected dryers isshown in Fig. 5. The HPDs are approximately 10 times as effectiveas traditional drying systems [34].

Jolly et al. predicted the performance of a HP assisted continu-ous dryer in [35].

Meyer and Greyvenstein [27] studied on the economic feasibil-ity of the HPD as applied to grain drying. It was found that therewas a minimum operating period that makes the HPD more eco-nomical than other dryers [36].

Strommen and Jonassen [37] and Alves-Filho and Strommen[38] described the development of novel, counter-current HPD flu-idized-bed dryers with high SMERs for the drying of heat sensitiveproducts.

The pilot plant electric HPD [39] developed by the ElectricityCorporation of New Zealand had a peak SMER of 7.94 kg/kWh at50 �C and 80% relative humidity.

Perera and Rahman [15] presented general comparison of HPDwith vacuum and hot air drying. As can be seen in Table 1, HPDsare more energy efficient than conventional hot air dryers.

Performance assessment and cost analysis of a HP fruit dryerwere done by Soponronnarit et al. [40]. In this study, energy con-sumption was 9.93 MJ/kg water evaporation and total operatingcost was 0.38 US$/kg water evaporation of which 0.16 was energyconsumption cost, 0.04 was maintenance cost and 0.18 was fixedcost.

Table 1General comparison of heat pump dryer with vacuum and hot air drying [15].

Hot air drying Vacuum drying HPD drying

SMER (kg H2O/kWh) 0.12–1.28 0.72–1.2 1.0–4.0Drying efficiency (%) 35–40 670 95Capital cost Low High ModerateRunning cost High Very high Low

Fig. 6. Schematic of the control volume for the heat and mass transfer during thedrying process [57].

N. Colak, A. Hepbasli / Energy Conversion and Management 50 (2009) 2180–2186 2183

Achariyaviriya et al. [41] developed a mathematical model forHPDs and simulated the performance of an open loop dryer, aclosed loop dryer and partially closed loop dryer. They reportedthat COP decreased when the fraction of evaporator bypass air in-creased for all systems of HPD.

Ameen and Bari [42] investigated that clothes could be driedunder a controlled environment by using condenser waste heat.The drying rate for this system was 32.9% higher than that of thecommercial dryer and 205% higher than natural drying.

Typical SMER values for atmospheric freeze drying with HPs arein the range of 4.6–1.5 kg/kWh [43]. Likewise, the SMER of indus-trial vacuum-freeze drying is in the range of 0.4 or below [12,44].

Adapa and Schoenau [45] studied about energy analysis of re-circulating HP assisted continuous bed drying of specialty crops.They found the re-circulating HPD was 22% more energy efficientand resulted in 65% reduced drying time compared to conventionaldryers incorporating electric coil heaters.

Colak and Hepbasli [46] performed exergy analysis of drying ofapple in a HPD. In this study, exergy destructions and exergy effi-ciencies of the dryer were determined at different drying airtemperatures.

Soylemez [29] calculated the optimum operating temperaturesand the optimum sizes of system components at which minimumlife cycle cost occurred for the HPD. In this study, net overall lifecycle cost was approximately 5500 US $.

Fatouh et al. [47] conducted a study about comparison of the HPdrying characteristics of different herbs. It was found that smallsize herbs without stem needed low specific energy consumptionand low drying time.

Energy and exergy analysis of timber dryer assisted HP wasstudied by Ceylan et al. [48]. As the moisture content evaporatedfrom the timbers in HPD decreased, energy utilization in the dryeralso decreased and exergy efficiency changed according to the dry-ing period.

Hancioglu Kuzgunkaya and Hepbasli [49] made an exergeticevaluation of drying of laurel leaves in a vertical ground sourceHPD. In this study, exergy efficiency values for the drying processon the product/fuel basis were found to be 9.11–15.48% at 40and 50 �C, respectively [49,50].

Colak and Hepbasli [51] performed an exergy analysis of dryingof blanched carrot in a ground source HPD at three different dryingair temperature (45, 50 and 55 �C).

Peregrina et al. [52] presents an economic study of a novel ther-mal fry-drying technology which transforms sewage sludge andrecycled cooking oil (RCO) into a solid fuel. In this study, combinesystem which using the open heat pump with fry-drying allows anefficient energy recovery.

Colak et al. [53] studied on exergetic assessment of drying ofmint leaves in a HPD.

4.2. Modeling

Jolly et al. [35] developed a model for the continuous operationof an HPD. Clements et al. [54] used this model to predict the per-formance of a HP assisted continuous dryer.

Many theoretical studies on heat and mass transfer in HPD sys-tems have been conducted [55]. Mathematical modeling provides atool to enable drying rate and efficiency to be predicted under arange of conditions [56]. The heat and mass transfer during thedrying process is shown schematically in Fig. 6 [57].

Thin layer drying kinetics is also commonly used to design, sim-ulate and optimize complex drying processes. Accurate predictioncan determine the best quality of end product as well as reductionin process time [58]. Thin layer drying kinetics is needed to designfull scale HP drying systems. Rahman et al. [59] have studied tomeasure and model the desorption isotherm of peas, and to mea-

sure and model the HP air drying kinetics of peas by two compo-nent model.

Alves-Filho et al. [1] described a simulation component modelfor a multiple fluidized bed HPD, with two independent dryingloops and two refrigerant circuits.

Various intermittent drying schemes were selected by Ho et al.[6] to optimize the set of optimizing parameters to enhance dryingkinetics, product quality parameters and heat recovery during HPdrying.

Adapa et al. [55] developed a simplified mathematical model ofa HP assisted crop dryer.

Simulation for convective drying of heat sensitive materials in aslab form was presented by Islam et al. [57].

Chestnuts were dried in a HPD under several experimental con-ditions by Moreira et al. [60]. This study concluded that, the dryingkinetics is faster when the driving force is higher and the physicalresistances are eliminated. The use of different varieties of chest-nut does not yield to significant differences in the drying kinetics.

Rahman et al. [61] improved a two dimensional theoreticalmodel for heat and mass transfer through composite heat sensitiveproducts. To validate the predictions of the model, experimentalstudies were conducted in a batch type HP assisted dryer usingrectangular shaped potato and apple slices as model compositedrying objects.

4.3. Progress of quality

Researchers have guide to investigate the applicability of newdrying methods for obtain a balance between high qualities ofdried products and low operating costs. In some studies, the highquality of the dried products was highlighted as the major advan-tage of the HPD [16,17,62].

During a drying process, different amounts of degradation ofcolor, flavor and texture may occur. Studies about comparison ofquality degradation between HPD and conventional drying meth-ods for several materials were also done [15]. In many studies,

2184 N. Colak, A. Hepbasli / Energy Conversion and Management 50 (2009) 2180–2186

the common conclusion was that the HPD offers products of betterquality with less energy consumption [17,62–64].

Depending on the drying conditions, food products may under-go various degrees of browning, shrinkage, loss of nutrients, and soon [65]. Foods, like fruits and vegetables, consist of water, carbohy-drate, protein and fractions of lipids. These compounds are easilymodified in a high temperature drying condition and result in deg-radation in food quality [66].

During drying, important changes in structural properties canbe observed as water is removed from the moist material. Shrink-age occurs because food polymers cannot support their weight and,therefore, collapse under gravitational force in the absence ofmoisture [67].

Major advantages of HPD for drying food products are the po-tential improvements in the quality of resultant products [15].

Van Blarcom and Mason [63] shown that when the macadamianuts were dried by HPD at 50 �C drying air temperature, browningproblem did not occur.

Hawlader et al. [65] found that HP drying of onion slices led toan energy saving of about 30% with better product quality, whencompared with conventional dryers.

Britnell et al. [68] investigated the microbiological aspects ofHPD. Due to the lower drying temperatures (<55 �C) used in aHPD, there is the potential for microbiological spoilage. This re-search to-date indicates the HP drier does not support a largemicrobiological population on the coils or at any other sitethrough-out the drier.

Table 2Studies conducted on progress of quality [1,2,4,6,7,16,39,62,63,65,68–71,73–77,79,83–88]

Investigators Type of heat pumpdryer

Product dried

Type Initial moisturecontent (%)

Rossi et al. [62] Air-source HPD VegetableBirchall [86] Air-source HPD Ginger 82–85

Potatoes 80Britnell [68] Air-source HPDVan Blarcom and Mason

[63]Air-source HPD Macadamia

nuts20

Payne [87] Air-source HPD Mango 85Strommen and Kramer

[16]Modified atmosphereHPD

Fish –

Wood [85] Air-source HPD Ginger –Hawlader et al. [4] Air-source HPD Yam –Alves-Filho et al. [1] Air-source HPD Apple 95Prasertsan and Saen-

Saby[17]Air-source HPD Wood,

bananaChua et al. [2] Air-source HPD BananaHo et al. [6] Air-source HPD Potatoes 4.2Perera [69] Air-source HPD Apple –Cardona et al. [70] Air-source HPD LAB –Dandamrongrak et al.

[83]Air-source HPD Banana –

Namsanguan et al. [71] HPD and SSD Shrimp 30, 40 (wb)Hawlader et al. [73] Modified atmosphere

HPDGuava,papaya

–

Hawlader et al. [7] Modified atmosphereHPD

Ginger –

Hawlader et al. [65] Modified atmosphereHPD

Apple –

Alves-Filho et al. [74] Atmospheric freezeHPD

Red pepper 27.78(db)

Nathakaranakuleet al.[76]

SSD and HPD Chicken meat 43, 67 (db)

Stawczyk et al. [84] Atmospheric freezeHPD

Apple cubes –

Sunthonvit et al. [75] Air-source HPD Nectarines –Jangam et al. [77] Air-source HPD Sapota 68–73 (db)Coogan and Wills [79] Air-source HPD White radish –Alves-Filho et al. [88] Air-source HPD Protein 85

Chua et al. [2] evaluated the feasibility of applying selectedstepwise drying schemes to drying banana pieces to reduce dryingtime and improve product color.

Modified atmosphere HP drying was studied by Perera [69] fordrying of apples. In this study, dried apples showed excellent colorand retention of vitamin C, while the overall dried product qualitywas very high.

Cardona et al. [70] studied on HP dehydration of Lactic Acid Bac-teria (LAB). The aim of this study was to determine under whatpreparation and drying conditions LAB can be dehydrated in aHPD without unacceptable deterioration of viability and activity.The shelf life stability of the dried products at different storagetemperatures was also evaluated.

Namsanguan et al. [71] examined a two-stage superheatedsteam drying followed by HP drying. In this study, the effect oftempering between these two stages of drying was investigatedin order to possibly reduce the drying time. The drying character-istics, along with various quality attributes of dried shrimp wereinvestigated.

Two stage drying such as superheated steam drying in the firststage following by HP drying second stage or opposite was found tobe an alternative drying technology as it can combine the advanta-ges of different drying techniques to improve the product quality[71,72].

Hawlader et al. [73] showed that modified atmospheric HP dry-ing at about 45 �C and relative humidity around 10% led to betterphysical properties. The color of HP drying of apple, guava and

.

Air

Final moisturecontent (%)

Velocity(m/s)

Temperature(�C)

Relative humidity(%)

12 1.8 35, 45, 55 –10 1.3–3 55

<501.5 – 50 10

16 1.5 35, 45, 55 –– 0–3 �20 to 50 20–90

– 0.25, 1 35, 45, 55 –– 1 40, 45, 50 30, 40, 5034 – – –

2.5 30–35 19.8–43.21.0 1.6 25, 30, 40 32–44– – – –– 1.71 10–40 21.725 (db) 3.1 50 –

20 (wb) 0.91 50 –– 0.7 45 10

– 0.7 45 10

– 0.7 45 10

0.75 (db) 1.5–2.5 �3, �5, �10,20

–

0.11 (db) – 55

– – �4, �8, �12,�16

–

18–20 (wb) 1.6 25 106–7 (db) 1 40 15<10 – – –– 1, 2.3 �5, 25 –

N. Colak, A. Hepbasli / Energy Conversion and Management 50 (2009) 2180–2186 2185

potato under inert environmental conditions was similar to vac-uum or freeze drying.

The effect of air temperature on drying kinetics and productquality of atmospheric HP drying of red peppers was studied byAlves-Filho et al. [74]. In this study, an evaporation temperaturesuch as 20 �C could be successfully combined with sublimationto intensify the red and yellow colors in the dried pepper to reducedrying time and costs compared to vacuum-freeze drying.

Sunthonvit et al. [75] found that a HPD was the best system forpreservation of volatile compounds in sliced dried nectarines interms of lactones and terpenoids followed by cabinet dryer andtunnel dryer.

The effects of superheated steam temperature and moisturecontent of chicken at the end of the first stage drying on the dryingkinetics and quality of the dried chicken in terms of its color,shrinkage and rehydration ability were evaluated by Nathakara-nakule et al. [76].

Jangam et al. [77] studied about dehydration of sapota fruit. Inthis study, the drying behavior of sapota pulp was studied using aconvective dryer, a low-temperature heat pump dryer, and a freezedryer. Fiala and Guidetti [78] showed that, a closed-circuit HPD isappropriate for drying of medicinal plants. Coogan and Wills [79]found flavor change of white radish dried in HPD is less than hotair dryer.

Studies conducted on progress of quality were given in Table 2.

5. Conclusions

For development of sustainable energy, three important techno-logical changes have been required: energy economies on the de-mand side, efficiency improvements in the energy production,and renewing of fossil fuels by various sources of renewable en-ergy. In this regard, HPD systems improve energy efficiency andcause less fossil fuel consumption.

In this study, HPD systems were reviewed generally. In the sec-ond part of the study, HPD applications are classified in tabulatedforms with types of heat pump, while dryers and products are ta-ken into consideration.

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