Energy Conversion and Management 72 (2013) 171–178

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Energy Conversion and Management

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Mushroom drying with solar assisted heat pump system

0196-8904/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2012.09.035

⇑ Corresponding author. Tel.: +90 0312 202 87 07.E-mail address: mustafaaktas@gazi.edu.tr (M. Aktas�).

Seyfi S�evik a, Mustafa Aktas� b,⇑, Hikmet Dogan a, Saim Koçak c

a Mechanical Education Department, Technical Education Faculty, Gazi University, Teknikokullar, 06503 Ankara, Turkeyb Energy Systems Engineering, Technology Faculty, Gazi University, Teknikokullar, 06503 Ankara, Turkeyc Mechanical Engineering, Faculty of Engineering and Architecture, Selçuk University, Konya, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Available online 6 April 2013

Keywords:Solar energyMushroom dryingHeat pumpSolar assisted heat pump dryer

In this study, a simple and cost effective solar assisted heat pump system (SAHP) with flat plate collectorsand a water source heat pump has been proposed. Mushroom drying was examined experimentally in thedrying system. Solar energy (SE) system and heat pump (HP) system can be used separately or together. Acomputer program has been developed for the system. Drying air temperature, relative humidity, weightof product values, etc. were monitored and controlled with different scenarios by using PLC. This systemis cheap, good quality and sustainable and it is modeled for good quality product and increased efficiency.Thus, products could be dried with less energy input and more controlled conditions. Mushrooms weredried at 45 �C and 55 �C drying air temperature and 310 kg/h mass flow rate. Mushrooms were dried frominitial moisture content 13.24 g water/g dry matter (dry basis) to final moisture content 0.07 g water/g dry matter (dry basis). Mushrooms were dried by using HP system, SE system and SAHP system respec-tively at 250–220 min, at 270–165 min and at 230–190 min. The coefficients of performance of system(COP) are calculated in a range from 2.1 to 3.1 with respect to the results of experiments. The energy uti-lization ratios (EURs) were found to vary between 0.42 and 0.66. Specific moisture extraction rate (SMER)values were found to vary between 0.26 and 0.92 kg/kW h.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Solar assisted heat pump systems can be classified to conven-tional SAHP systems and direct-expansion SAHP (DX-SAHP) sys-tems. Solar assisted heat pump systems have been studied andused since the last decade in order to increase the quality of prod-ucts where low temperature and well-controlled drying conditionsare needed [1]. The combination of a heat pump and a solar energysystem would appear to reduce many of the disadvantages thateach has when operating separately or alone. During winter condi-tions, the energy that could be collected by the solar collectors, butwhich would be too low in temperature to be useful for directheating, may be used as a heat source for the heat pump. Sincethe solar collection–storage system can supply energy at tempera-tures higher than the ambient air temperature, the capacity andheat pump COP would increase over that if the heat pump worksalone, the peak auxiliary load requirement would be reduced andthe combined heating system (solar and heat pump) will be oper-ated more economically [2].

Both heating and drying can be done with solar assisted heatpump. A detailed review of studies conducted on SAHP systems

is given in Table 1. As it can be seen from the table, SAHP systemshave been evaluated under three groups, water heating, spaceheating and product drying, respectively. Drying is extracting liq-uids in a matter. At the technical drying, external interference isapplied to the drying process and the moisture in the matter is ex-tracted in various methods. Thus, drying is described as reductionof product moisture to the required dryness values at a definiteprocess [16,17]. In the world, 97–98% of dried vegetables thatmade traded dry with hot air under controlled conditions. The fac-tors that affect drying rate are air temperature, air velocity, producttype, moisture content of the product, thickness of product, meth-od of drying, temperature moisture diffusivity and drying kilnstructure. Although mushrooms are fresh consumed vegetable, itincreasingly is consumed as the dried product in recent years[18]. Dried mushrooms, packed in airtight containers can have ashelf life of above 1 year [19].

In this study, PLC-controlled solar assisted heat pump dryerwere designed. Solar drying, heat pump drying and solar assistedheat pump drying in system that were designed and manufacturedwere experimentally analyzed for drying mushrooms. The neces-sary heat for drying system was provided by heat pump condenserand solar energy heat exchanger. Spent energy was checked moredetailed with PLC control. So, energy input was reduced. Also thismanner, more quality product can be obtained, as a result of thecontrolled drying process.

Nomenclature

c specific heat (kJ/kg K)Fk collector surface area (m2)h specific enthalpy (kJ/kg)ITOT total solar radiation incident upon plate of the collector

(W/m2)Mi initial wet weight (g)Md final dry weight (g)Mt moisture content at time ‘‘t’’, (g water/g dry matter)Mt+dt moisture content at ‘‘t + dt’’, (g water/g dry matter)Me equilibrium moisture content (g water/g dry matter)M0 initial moisture content (g water/g dry matter)_m mass flow rate (kg/s)_mWater the mass of water taken from mushrooms (kg/h)_QC condenser capacity (kW)_QHC heat exchanger capacity (kW)

_WC power input to compressor (kW)_WP power input to pump (kW)_WF power input to fan (kW)_V volumetric flow rate of air (m3/s)g efficiency (%)q density of air (kg/m3)DT temperature difference (�C)

SubscriptsHC heat exchangeri inletia inlet airo outletoa outlet airt time (min)

172 S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178

2. Materials and method

SAHP drying system allows to be used with two different heatproduction systems. In this type of system, heated water by solarcollectors uses as the heat source of heat pump. Mushroom dryingprocess was realized by two systems that are heat pump and solarenergy system from water to air. SAHP drying system designed andmanufactured consists of compressor, condenser, evaporator, heatexchangers, dryer filter, capillary tube, axial fans, PLC, controlequipment, solenoid valves, inverter, sensors for temperature andrelative humidity. SAHP drying system that designed and manufac-tured is shown in Figs. 1–3. The systematic diagram of PLC systemand air flow is shown in Fig. 4.

This system allows two units to be used together or separately.In the case of insufficient heat obtained from solar energy, heatpump enters to the circuit, if the heat is sufficient heat pump exitsfrom the circuit. Heat energy that obtained from solar energy orheat pump condenser is given to the drying room by means of afan which is with frequency inverter and according to the temper-ature can be done speed control. Dryer is composed of five mainparts and three cycles. These include: drying chamber, heat pumpsystem, solar energy system, sensor connections, air ducts and con-nection pipes. Cycles are refrigerating fluid, air and water.

Energy produced in the system is provided from three solar col-lectors that are connected in series and heat pump unit. In case ofin the absence of heat demand in place of use, hot working fluidthat is from solar collector is routed to the heat storage tank. Thesame air circulates in this system that designed and manufactured,but if it is necessary, fresh air from outside can be taken withvariable air volume (VAV) box. The air is circulated by a fan. Fresh

Table 1Some studies conducted on SAHP systems.

Authors Application type Conclusions

Sakai et al. [3] Water and space heating –Chen et al. [4] Lumber drying Solar air colBest et al. [5] Rice drying In good quaChaturvedi et al. [6] Water heating (30–70 Hz):Hawlader et al. [7] Food grains drying COPSys = 6.0;Chyng et al. [8] Water heating COPdaily-total

Kuang et al. [9] Water heating COPmonthly-av

Huang et al. [10] Water heating COPHP-mode =Guoying et al. [11] Water heating COP = 3.98–Li et al. [12] Water heating COPseasonal-av

Trilliant-Berdal et al. [13] Water and space heating COPheating = 3Hepbas�lı [14] Water and space heating eGSHP = 72.33Li et al. [15] Grain drying Solar fractio

air was taken from outside to the system during only solar energyexperiments. However, fresh air was not taken from outside to thesystem during HP and SAHP experiments. Control function is car-ried out by a series of electro-mechanical control elements. Thesecontrol elements consist of equipment that is used compatiblewith each other. These control elements consist of equipments thatare used compatible with each other. Such as PLC control, speedfan, compressor, solenoid valves, frequency converters, hygrostat,control cards that runs synchronized with measuring instrumentwhich is used of routed the system. Temperature and relativehumidity control of air momentarily can be followed by computerscreen in our system. With PLC system, temperature of drying airin system keeps the value of the set from process control equip-ment. Table 2 shows characteristics of various measuring instru-ments used in experiments.

There are three heating modes for the dual source system(Fig. 4). In the first case; system is operated in the direct solar heat-ing mode. PLC system provides at the pre-determined control val-ues. SV1, SV2 and SV5 are off, SV4 is on and the circulation pumpoperates. If excess heat is produced in the collectors; producedheat is directed to the heat storage tank by opening SV5 and usingnumber three water heat exchangers (HC). The heat pump is off,and any excess solar energy is being stored in the tank.

In the second case; in the drying system, with condensationheat pump system and dehumidification unit are active. Dehumid-ification status while the heat pump circuit; SV1 and SV2 off, SV3and SV6 are open and number two of the pump is on. When theenergy storage tank temperature is below the control value, thecold water in the storage tank is pumped to the heat exchanger 1(dehumidifier). The pumped cold water heats up and it returns to

lector, 1.5 HPlity rice, COP = 5.30COPh = 2.5–4.0gevap-coll = 0.080, gair-coll = 0.77

= 1.7–2.5 (year around); Twater = 57.2 �Cg = 4–6; gcoll = 40 60%2.58; COPhybrid-mode = 3.32

4.32; Twater = 55 �Cg = 5.25; gcoll = 1.08; eSys = 21%; Twater = 50.5 �C.75, 60% for the first 11 months (for hot water solar fraction)%, eSDHWS = 14.53%, eSys = 44.06%, Exergetic COP; COPGSHP = 0.245, COPSys = 0.201

n 20%, COPs = 5.19, SMER = 3.05 kg/kW h

Fig. 1. Computer control screen.

Fig. 2. A schematic view of solar assisted heat pump drying system.

S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178 173

Fig. 3. Various view of drying system.

Flow Schema (Air and water)

Air flow Water flow

Control Unit

Process control

equipment (inverter, PLC

etc.)

Fresh air

Return air inlet

Condenser (Refrg.)

Evaporator (Refrg.)

Aux. condenser (Refrg.)

Solar collectors

Fan

HC 1 (Water)

& Dehumidifier

(Water)

HC 2 (Water)

HC 3 (Water)

Outlet air

Fig. 4. The systematic diagram of PLC system and air–water flow.

174 S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178

storage tank, so that heated water uses as a heat source for the heatpump.

In the third case, in the drying system, with condensation heatpump unit and solar energy unit are active. In the event of insuffi-cient heat from solar energy, heat pump is activated, if there is suf-ficient heat from solar energy is switched off. Depending on themagnitude of the product drying load, the excess solar energy iscollected in the energy storage tank. When the storage tempera-ture is either at its minimum temperature or when needed, auxil-

iary heat (to heat storage tank from heat exchanger 3) is suppliedand the heat storage tank is used as a heat source for theevaporator.

System was organized two separate closed system that is sepa-rate heat pump and solar collector each other. In drying system,the total required amount of heat is provided by solar heat exchan-ger and the heat pump condenser. Water heated by solar collectorsystem is used as the heat source. This is used both the drying pro-cess and as the heat source of heat pump.

Table 2Characteristics of measuring instruments.

Instruments Properties Range Accuracy Uncertainty

Temperature and humidity sensors 0–10 VDC, 15–35 VA/DC, �50–125 �C, 5–95% RH ±0.1 �C ±0.07 �C±0.1 RH ±0.08 RH

Temperature sensors 10 mV/�C �55 to + 150 �C, ±0.1 �C ±0.06 �CPressure transmitter 4–20 mA 0–10 bar ±0.2% ±0.12%Velocity and temperature

measurementNTC sensor �20, +70 �C, 0–20 m/s 0.01 m/s, 0.1 �C ±0.009 m/s

Solar meter 0.3–1.1 lm spectrum 0–1200 Watts/m2 3% ±1.7%Load cell 2 mV/V 0–50 kg ±0.02 kg ±0.012 kgDigital balance 220 V 0–6100 g ±0.01 g ±1.2 gControl panel 0.1–600 Hz, 230 V-1phase, 460 V-3phase, 15-temperature and

6-humidity sensors, 8-analog inlet, 8-digital inlet, 4-analog outlet(0–10 V), 6-relay, 2-SSR, etc. Contactor, reader card, sensor board,PLC, switch and, etc.

0.1–600 Hz 0.01 Hz0–60 s

–

Table 3Experimental results.

Measurement HP 45 �C HP 55 �C SE 45 �C SE 55 �C SAHP 45 �C SAHP 55 �C

SMER (kg/kW h) 0.26 0.46 0.77 0.92 0.47 0.74COP 2.1 2.3 – – 2.9 3.1EUR (%) 0.42 0.47 0.6 0.66 0.51 0.53Time, min 250 220 270 165 230 190Collector efficiency (%) – – 48% 58% 46% 51%Initial and final MC (g water/g dry matter) 13.24–0.07 13.24–0.07 13.24–0.07 13.24–0.07 13.24–0.07 13.24–0.07

MC

0

2

4

6

8

10

12

14

0 30 60 90 120 150 180 210 240 270 300Drying time, min

Moi

stur

e co

nten

t(g

wat

er/g

dry

mat

ter) HP 45 °C

HP 55 °C

SE 45 °C

SE 55 °C

SAHP 45 °C

SAHP 55 °C

Fig. 5. Variation in moisture content as a function of drying time.

MR

0,00

0,20

0,40

0,60

0,80

1,00

0 30 60 90 120 150 180 210 240 270 300Drying time, min

Moi

stur

e ra

tio

HP 45 °C

HP 55 °C

SE 45 °C

SE 55 °C

SAHP 45 °C

SAHP 55 °C

Fig. 6. Variation of MR moisture ratio according to time.

DR

0,000,020,040,060,080,100,120,140,160,180,20

0 30 60 90 120 150 180 210 240 270 300

Drying time, min

Dry

ing

rate

(g

wat

er/g

dry

mat

ter.

min

) HP 45 °C

HP 55 °C

SE 45 °C

SE 55 °C

SAHP 45 °C

SAHP 55 °C

Fig. 7. The variation in the drying rate according to the drying time for differentfunctions of the mushroom.

Drying times of the mushrooms

0102030405060708090

100

0 30 60 90 120 150 180 210 240 270 300Drying time, min

%

HP 45 °C

HP 55 °C

SE 45 °C

SE 55 °C

SAHP 45 °C

SAHP 55 °C

Fig. 8. Drying times of the mushrooms.

S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178 175

Coefficient of performance of heat pump;

COPHP ¼_Q C

_WC

ð1Þ

The coefficient of performance of the overall heating system(COPSys) can be defined as

COPSys ¼R _Q Sys

R _WSys

¼_Q C

_WComp þ _WF1 þ _WF2 þ _WP1 þ _WP2

ð2Þ

Relative humidity of drying air

0

10

20

30

40

50

60

70

0 15 30 45 60 75 90 105

120

135

150

165

180

195

210

225

Drying time, min

%Mixture

Td

Tc

Th

Dryingcabinet

Td: Temperature after dehumidifier Tc: Temperature after condenser Th: Temperature after heat exchanger

Fig. 11. Air relative humidity of measurement points for 45 �C SAHP dryer room.

Collector water temperatures

0

10

20

30

40

50

60

70

0 15 30 45 60 75 90 105

120

135

150

165

180

195

210

225

Drying time, min

oC

1.Ci1.Co2.Co3.CoTank

1.Ci: 1. Collector inlet

1.Co: 1. Collector outlet

2.Co: 2. Collector outlet

3.Co: 3. Collector outlet

T: Tank

Fig. 9. Collector water temperatures for 45 �C SAHP dryer room.

Air temperatures

0

10

20

30

40

50

60

0 15 30 45 60 75 90 105

120

135

150

165

180

195

210

225

Drying time, min

oCReturn

Mixture room

Td

Tc

Th

Drying cabinet

Td: Temperature after dehumidifier

Tc: Temperature after condenser

Th: Temperature after heat exchanger

Fig. 10. Air temperatures at the measurement points for 45 �C SAHP dryer room.

176 S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178

The SMER can be defined as the energy required to remove 1 kgof water and may be related to the power input to the compressor(SMERHP) or to the total power to the dryer including the fan powerand the efficiencies of the electrical devices. SMER described by thefollowing equation [16];

SMERSys ¼_mWater

R _W¼

_mWater

_WComp þ _WF1 þ _WF2 þ _WP1 þ _WP2

ð3Þ

EUR is energy utilization ratio in drying cabinet and describedby the following equation [16];

S. S�evik et al. / Energy Conversion and Management 72 (2013) 171–178 177

EUR ¼_mia � ðhia � hoaÞ

_mia � cp � ðT ia � TaaiÞð4Þ

The moisture ratio (MR) and drying rate (DR) during dryingexperiments were calculated using the following equations:

MR ¼ M �Me

M0 �Með5Þ

DR ¼ Mtþdt �Mt

dtð6Þ

where M, M0, Me, Mt and Mt+dt are the moisture ratio, moisture con-tent, initial moisture content, equilibrium moisture content, mois-ture content at ‘‘t’’ and moisture content at ‘‘t + dt’’ (g moisture/g dry matter), respectively, ‘‘t’’ is drying time (min). However, themoisture ratio (MR) was simplified to M/M0 of the (M �Me)/(M0 �Me) [20]. Initial moisture content of the mushroom was cal-culated from the following equation:

MCdb ¼Mi �Md

Md� 100 ð7Þ

Energy production and efficiency of collector in solar collectorcan be found by the following equation;

_Q ¼ _m � c � DT ¼ _m � c � ðTout � TinÞ ¼ _V � q � c � ðTout � TinÞ ð8Þ

Instantaneous thermal efficiency of solar collector [21];

g ¼_Q

FK � ITOTð9Þ

3. Experimental results and system performance analysis

Mushroom exact dry weight was determined. 2000 g mush-rooms were dried in the system for each experiment. When weightof mushrooms reaches to 150.4 g, it was calculated to end thedrying process. According to dry basis, initial moisture contentamount 13.24 g water/g dry matter, amount of final moistureof dry matter 0.07 g water/g dry matter was obtained. The mois-ture content of mushroom that used in the experiments was found92.98% from average of full dry weight three different samples.The water activity value is very important criterion for prod-ucts storage in long time without degradation. In general, wateractivity (aw) of a multi-product are relatively low. It is recom-mended that water activity of typically dried food should be lesslevel than 0.6. For this reason, it has been decreased below thevalue.

Mushrooms were dried at 45 �C and 55 �C temperatures 310 kg/h mass flow rate. Experiments were carried out with only solar en-ergy, only heat pump and whole system (solar assisted heat pump)at 45 �C and 55 �C temperatures and accordingly mushrooms weredried 250–220 min, 270–165 and 230–190 min respectively. Dry-ing periods is different although there was no difference betweendrying temperatures. The formation of this difference has affectedfactor such as the periods to reach the desired temperature, to beopen or closed of dehumidification process and other conditions.While the utilization rate of energy were a tendency to increaseat the beginning of the drying. Because of a reduction in theamount of moisture in the production, when it is entered to thedrying period that is decreasing speed, this rate was reduced. Thecoefficients of performance of system (COP) are calculated in arange from 2.1 to 3.1 with respect to the results of experiments.The energy utilization ratios (EURs) were found to vary between0.42 and 0.66. Specific moisture extraction rate (SMER) valueswere found to vary between 0.26 and 0.92 kg/kW h. The experi-mental results are shown in Table 3. Solar energy (SE) system isthe best efficient system in term of energy consumption, SMER,EUR according to experimental results. The best SMER value is SE

(55 �C) drying process. Also, SE (55 �C) drying process has theshortest drying time. Average solar water collector efficiency is50–75%.

Variation in moisture content as a function of drying time isshown in Fig. 5. Variation of moisture ratio from different functionsand temperatures versus drying time for solar assisted heat pumpdryer (SAHPD) is shown in Fig. 6. The variation in the drying rateaccording to the drying time for different functions of the mush-room is shown in Fig. 7. The drying times of the mushroom areshown in Fig. 8.

Collector water temperatures, air temperatures and air relativehumidity values were monitored in each experiments. However, inthis study, it was given only the data which is belong to experi-ment that is 45 �C temperature in SHAP dryer (Figs. 9–11).

4. Conclusions

The study carried out on the obtained experimental results is asfollows:

� This system allows to economic dried of products all yearround by minimizing or disabling the human factor in thedecision.� According to experimental results, it has been recommended

that the usage of SAHP system is useful for thermal efficiency.Also, SAHP system COP values better than HP system due toassisting of solar energy.� The drying process can be continued with solar energy in day-

time and a heat pump system can be used other times (atnights).� Mushrooms were dried from 13.24 g water/g dry matter to

0.07 g water/g dry matter with less energy input.� It was tried to determine the drying characteristics of mush-

room and effects of their quality. Considering the drying timeand quality, in good quality dried mushroom was achieved at45 �C drying air temperature and 310 kg/h mass flow rate.� It was found that an obtained mushroom that is at the end of

the experiments was complying with long-term storage. Inaddition, the samples from dried mushroom have been ana-lyzed in terms of image and taste. It was not recorded any dete-rioration in these products.� The drying process is made much more quickly than according

to nature drying.� With this system, other agricultural products (tomato, red pep-

per, hazelnut, strawberry, etc.) and industrial products (lumber,plastic, etc.) can be dried.� In receiving unwanted moisture inside the product, heating

does not be made quickly because of the risk that is degradationof product. In case of rapid and extreme heating, pores in theouter sections will be closed in product. So, the remaining mois-ture in the inner parts it will be difficult to go out thereby dry-ing outer surfaces are destroyed.

Acknowledgement

The authors gratefully acknowledge Gazi University fortheir support in this study under Research Projects #:07/2010-14.

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