A review on thermal analysis of hybrid greenhouse solar dryer HGSD

Abstract

Keywords: Hybrid; greenhouse; heat transfer; mass transfer; solar dryer

Introduction

­For the existence of human life in earth, the food is one of the basic requirements. The rising population puts pressure on the food production. This rising demand of food can be met either by growing more food or storing the produced food for meeting the future need. The reduction in after harvest loss can also help in meeting the need of food products. The development of solar dryers helps in reducing the after harvest loss of various agro and non-agro products. Most of the agro based products gets spoiled due to presence of high moisture content in them.

The drying is considered as the best way to make the product suitable for long time storage [1–4]. In solar dryers the produce is dried to safe moisture level. The minimum quantity of moisture at which the product is safe for long time storage is considered as safe moisture level [5]. The drying of product gives long life to the product and prevents it from microbial attack [6,7]. Apart from solar dryers, artificial dryers were also used for drying at faster rate [8] but these are operated using non-renewable energy sources and also affects the quality of dried product [9].

*Corresponding author. *E-mail address: pushpendra852@gmail.com This paper was recommended for publication in revised form by Regional Editor Jaap Hoffman Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

The solar drying is an efficient technique of utilizing solar radiation for space heating and drying products [10–12] as solar energy is inexhaustible and eco-friendly energy source [13,14]. From very ancient period, the open sun drying is practiced for drying various agro and nonagro products [15,16] but it has lot of limitations like dried product gets affected by birds, pest, rain, microbes etc. [17–19]. Hence the solar dryers were developed to counter the limitations of open sun drying [20] and these are also sustainable and eco-friendly [21,22]. The solar dryers are basically of three types namely direct, indirect and mixedmode dryers [23]. Greenhouse dryers are generally the direct type dryers, which are considered as the best mean to harness solar energy for space heating and drying [24]. It works on the principle of greenhouse effect that implies that the cover of greenhouse dryer allows the short wavelength solar radiation through it while the long wavelength radiation coming from inside remains trapped inside the dryer. This rises the inside temperature of dryer which is required for drying purpose [25,26]. The greenhouse dryers have various applications like drying, aquaculture, soil solarisation, crop cultivation, space heating, etc. [27,28]. The greenhouse is not

limited to drying of agro products but also used for drying non-agro products like cotton, sludge, paper, rubber etc. [29–32]. The greenhouse operates usually in either active or passive modes. Figure 1 shows the detailed classification of greenhouse dryers. In an active greenhouses, the air is forced or induced by some external means like fans blowers, etc. [34]. Thus requiring extra energy for the operation of external devices [35], while the passive greenhouses do not require any such devices. The density difference arising due to temperature change establishes the air circulation in passive dryers [36,37]. In terms of cost, passive dryers are preferred while for faster drying active dryers are preferred [38,39]. Greenhouse dryers operating in active or passive mode give better results in terms of quality, color, drying time, etc. as compared to drying under the open sun [21,40]. Various researches had taken place to improve the efficiency of greenhouse dryers like insulating north wall [29,30,41–43], using mirrors for reflecting infrared radiations [44], using thermal storage material in floor [10,29,32,45,46], etc. The hybrid dryers are also a step towards the use of solar energy in a more effective manner and improving the productivity of greenhouses. Simply a word hybrid means a combination

Figure 1. Classification of the greenhouse on the various basis [24].

of two or more than two. So hybrid greenhouses are those which utilize two sources of energy or utilize single source (solar energy) in different ways. As per our literature review, the methods by which the greenhouses are converted into a hybrid greenhouse are shown in Figure 2. It depicts that the greenhouses are converted into hybrid ones by three methods given below: i) PV/T integrated greenhouse solar dryers [47–52] ii) Greenhouse attached to solar collectors [43,53–57] iii) Greenhouse attached with other air heating devices like biomass or any other fossil fuel burners [58–65] The main objective of this paper is to present the advances that took place in the area of hybrid greenhouse solar dryers. The paper also encapsulates the steps that are adopted by different authors to carry out the heat transfer analysis of hybrid dryer. This helps the researchers in understanding the concept of heat transfer in the dryer so they implement some new ideas to make more efficient dryers. The methodology adopted to make this review paper is shown in Figure 3.

Researches In Hybrid Greenhouse Dryer

The produces are mainly dried to safe moisture levels so that it can be preserved for a long time. The safe

moisture level (SML) of some of the crops is shown in Table 1. As solar drying is the best process of crop conservation and hybrid dryer are further making the drying process more efficient. Different modifications are employed by researchers in past to improve the dryer efficiency and the hybrid dryer is one of the development. To make the dryer hybrid, the different methods employed are shown below along with the effect of those methods on various heat transfer parameters is given in Table 2 and Table 3. PV/T integrated HGSD The dryers are usually made hybrid by attaching the PV/T modules for generating electric energy to operate the fan and other auxiliary devices. PV/T panels are those which not only produce electricity from solar energy but also use the incident solar radiation for heating the air inside the dryer. One of the ways used to attach PV/T in HGSD is illustrated in Figure 4. Barnwal and Tiwari (2008) developed the PV/T integrated HGSD at IIT, New Delhi, India. The dryer was even spanned roof type having 6.5 m2 floor area and enclosed with polyethylene sheet. DC fans operated by PV modules were used to force air inside the dryer. Matured (GR-II) and pre-matured (GR-I) grapes were dried under the open sun and also inside the dryer and the observations of both were compared. The heat

Sweet Potato Mango chip Mango seed Banana Melon Guava Carrot Grapes Apricot

transfer coefficient for the GR-II type was found greater than GR-I type [47]. Barnwal and Tiwari (2008) carried out the test on the greenhouse dryer using the thermal loss efficiency factor. The dryer was operated in no-load under active as well as in passive mode. The thermal loss efficiency was observed higher in forced mode [48]. Tiwari et al. (2016) developed the PV/T integrated mixed-mode hybrid dryer having ground area of 1.066 m2 and was enclosed with 3mm glass. Two DC fans operated by PV panels were provided for forced circulation of air. MATLAB 2013a was used for numerical computation of thermal models developed considering parameters like the temperature of the crop, greenhouse, PV module, etc. [90]. Eltawil M. A. et al. (2018) developed the tunnel type dryer attached with flat-plate collector and PV panels. The dryer had a ground area of 2 m2 and was enclosed with 2mm thick plexiglass. The dryer was tested with and without load and also with and without a thermal curtain. The potato was dried inside the dryer and the drying rate was observed at a different air flow rate. The drying efficiency reaches a maximum value of 34.29% when air flows at the rate of 0.0786 kg/s with a thermal curtain above the potato slices [91].

Nayak et al. (2011) evaluated the dryer performance by drying various samples of mint. In 21 hours, the mint was dried from 80% (wb) to 11% (wb) moisture content. The dryer had an efficiency of about 34.20%. The CO2 mitigation and carbon credit earned by the dryer in its lifetime was also evaluated [51]. Greenhouse attached with solar collectors Solar collectors were also used by different researchers to preheat the air externally in the collectors and then supplying it to the greenhouse drying chamber. The schematic representation of the HGSD attached to the solar collector is shown in Figure 5. Zaineb Azaizia et al. (2017) developed the hybrid greenhouse having a ground area of 14.8 m2 and a center height of 3 m. The plexiglass enclosed dryer was attached with a solar collector having area 2 m2. The mathematical model for the proposed system was established using the TRNSYS program. The effect of air flow rate, collector, and drying area on the humidity and temperature inside the dryer was investigated. The result shows that the optimum collector area was 2 m2 with an optimum airflow rate of 250 kg/h and an optimum drying area of 40 m2 [53].

Table 2. Effect on different heat transfer parameters observed by different researchers in hybrid dryers Authors

Moisture evaporation in natural convection was found better than in forced convection.

Ripen fruits have a higher heat transfer coefficient thus faster moisture removal rate.

The dryer thermal loss efficiency factor was almost equivalent to the solar air flat plate collector which is about 80%.

For faster drying and better dryer efficiency, room and crop surface temperature needs to be higher.

Pretreatment (Blanching) reduces the drying time of the product as compared to the untreated product.

For the better collector and tunnel dryer efficiency, an optimum airflow needs to be maintained as a higher airflow rate decreases the efficiency and vice versa.

The storage life of the product is higher at the lower value of moisture content.

Flat plate solar air collector attached greenhouse solar dryer

The drying rate is strongly influenced by the area of the crop surface. It decreases with an increase in area as more moisture is to be evaporated.

Dehumidification of supply air can increase the moisture evaporation rate from the crop surface.

The drying capacity of air inside the greenhouse increases with a decrease in its relative humidity and an increase in its flow rate. •

Forced convection gives a better result than natural convection.

Dryer efficiency and moisture evaporation rate is affected by collector efficiency, so the collector should be designed and fabricated with great care. Suggested to recirculate the air going out of the dryer to utilize available energy in a better way and improve dryer performance.

Solar collector inside the dryer increases the convective heat transfer coefficient which signifies a higher conversion rate of solar energy into useful heat energy.

Greenhouse solar dryer attached with hot water storage system

During day time, water is heated by circulating it inside the dryer and then storing it in the storage tank. That hot water was used to continue the drying operation during night time. Thus water is used as the thermal energy storage medium.

Due to improper circulation of air, the moisture evaporated from the product makes the room air saturated and thus reduces the drying rate. So proper ventilation is to be provided.

Greenhouse attached with evacuated tube collector and auxiliary heater

The developed simulation model was used to optimize the size of the solar collector and thermal storage system.

Greenhouse attached with crop dryer having packed bed thermal storage

Thermal energy storage reduces the temperature fluctuations during night time and thus drying at night becomes more consistent.

Carried out the energy and exergy analysis of the proposed hybrid setup.

The dryer efficiency can be increased by reducing heat losses through the wall, chimney, and heat absorbed in its metallic components.

For high moisture content crops like tomato, the drying rate needs to be faster, which is possible in the solar dryer rather than in the open sun.

The heat utilization factor is directly related to the heat generation inside the dryer.

Width of air flowing channel and thickness of packed bed affects the crop temperature.

Figure 5. Greenhouse dryer integrated with a flat plate collector.

Eltawil M.A. et al. (2018) developed the tunnel type dryer coupled with flat-plate collector and PV panels at King Faisal University, Saudi Arabia. The peppermint is dried inside the dryer in one, two, and three layers for evaluating dryer performance. The maximum drying time was 360 min inside the dryer while 420 min in the open sun. The use of a thermal curtain gives better quality mint as compared to drying under the open sun. The dryer had an efficiency of 30.71% and an energy payback time of 2.06 years [54]. ELkhadraoui A. et al. (2015) carried out the economic analysis and performance evaluation of the dryer. The dryer operated in forced convection mode and used for drying red peppers and grapes. The dryer payback duration was 1.6 years. The result shows that the drying time for grapes and red pepper was 50 hours and 17 hours respectively inside the dryer while it was 67 hours and 24 hours respectively in the open sun [55]. A semi-cylindrical greenhouse dryer with a flat-plate collector was constructed by Mehta P. et al. in 2018. Fish was dried inside the dryer to test its performance. To predict the collector temperature at its outlet, a mathematical model was established and then solved by the SageMath programming language. The drying time of fish was 18 hours inside the dryer while it takes 38 hours in the open sun [56]. Chauhan and Kumar (2016) constructed the

north wall insulated greenhouse at Energy Centre, MANIT, Bhopal, India. The greenhouse was tested with and without a solar collector. The dryer was operated in passive mode and tested in a no-load condition. The dryer performance in terms of heat utilization factor (HUF), coefficient of performance (COP), heat loss factor, etc. were calculated. The result shows that the dryer with a collector has better HUF and COP than the dryer without a collector [43,57]. Other air heating devices attached to greenhouse The other method applied to the greenhouse dryers is the attachment of auxiliary air heating devices like biomass or LPG burner. These devices supply the hot air during the off sunshine period also. This increases the operating time of dryer so its drying time also gets reduced. The schematic representation of the HGSD attached to solar collectors and water as a thermal heat storage medium is shown in Figure 6. The HGSD with auxiliary heaters and thermal storage materials is presented in Figure 7 (a) and Figure 7 (b) respectively. Deeto S. et al. (2017) developed a greenhouse attached with a solar collector and heat storage unit. The floor area of the dryer was 0.3 m2 and mounted on a black PVC sheet. A water storage tank of 180-liter capacity and insulated with polyurethane foam was attached to the dryer for storing

Figure 6. HGSD with solar collector and water as thermal energy storage material.

Figure 7. (a) HGSD with auxiliary air heater arrangement (b) HGSD with a layer of thermal energy storage material. hot water. The coffee beans were dehumidified from 55% to 12% (wb) in 12 hours. The model suitable for coffee drying was also determined [58]. Hamdani et al. (2018) manufactured the tunnel type greenhouse dryer attached with biomass burner at Samudra University, Aceh, Indonesia. The drying area was 2.08 m2 and the dryer was enclosed with a transparent plastic sheet.

The Queenfish was dried for evaluating the dryer performance. Wood was used as fuel for supplying hot air during off-sunshine hours. The result shows that in 15 hours only the fish was dried to 12% moisture level [59]. Kıyan M. et al. (2013) proposed a hybrid greenhouse attached with a hot water storage unit and fossil fuel heater. The mathematical model had been developed for the proposed setup.

Table 3. Effect of modifications observed by some of the researchers in their solar dryers Type of solar dryer

Dryer with a heat exchanger having a reflector and corrugated sheet

·D  rying time was decreased by about 30 hours as compared to open sun drying (OSD).

· Th  e air temperature inside the direct type dryer reached maximum to 55.1°C and 38.4°C for natural and forced mode respectively.

· Th  e room air temperature rose to above 50°C during the summer season of Germany. [93]

· The drying rate was observed higher in the forced mode. Dryer attached with air heater in series

· Th  e drying efficiency increases from 3.56 to 11.24% with an increase in loading capacity from 1.08 to 4.33 kg/m2 respectively.

·W  ith an increase in the thickness of the potato chips from 5 to 18 mm, the drying efficiency decreases from 10.61 to 3.11% respectively. PV attached greenhouse dryer

·D  ryer takes about 2 days less than OSD for drying peeled longan and banana. The room temperature varies between 30-60°C.

· The payback time of the developed commercial setup is only 2.3 years. Dryer with heat exchanger and water type solar collector

· Th  e air temperature inside the hybrid dryer reaches to the maximum value of 65°C.

· The average exergy efficiency of the prosed setup was 30%.

· Th  e use of solar energy for cocoon drying saves the electrical energy by 0.75 kWh/kg.

The drying time for drying 1000 kg of banana, chili, and coffee was reduced by 2 days in dryers as compared to OSD. The maximum room temperature was 60°C.

· Th  e use of solar collectors increases the convective heat transfer coefficient by about 22 W/m2°C and room temperature by 10°C.

· The quality of banana chips was found best at the air flow rate of 0.0338 m3/s.

· Th  e drying time of 100 kg tomatoes is 4 days and the payback time of setup is 1.37 years. [97]

· The air temperature inside the dryer varies between 35–60°C. [98]

· Th  e dryer was operated in forced mode and the air temperature varies from 50-80°C inside the drying chamber.

· Th  e maximum energy and exergy efficiency of the modified dryer was 16.8% and 21.4% respectively. [101]

· At bottom flow condition, the maximum collector temperature reaches to 45°C.

The models were solved by simulation software MATLAB/ Simulink. A case study had been done on the greenhouse at Aιlιm University, Ankara, Turkey to check the feasibility of the developed models [60]. Dilip Jain (2005) developed a hybrid solar dryer in which the tray-type dryer including thermal storage material at the bottom was attached to the north wall of the greenhouse. During day time, the heated air from the greenhouse is supplied to the drying cabinet through blower while during off sunshine hours the heat stored in the thermal storage material was used for drying. The greenhouse had a floor area of 24 m2. The thermal model was developed to determine the effect of size and mass flow rate of air on the crop temperature [61]. Ahmad Fudholi et al. (2016) developed the hybrid dryer incorporating solar collectors, PV

panels, and diesel burners. The developed dryer is located at Johor, Malaysia. Silver Jewfish was dried inside the dryer to evaluate its energy and exergy. The result shows that fish was dried from 64% to 10% (wb) in 8 hours. Also, the dryer exergy efficiency lies between 17-44% [62]. Aritesty and Wulandani (2014) developed the greenhouse solar dryer attached with a biomass burner. The dryer consist of 144 trays and 3 blowers. Wild ginger was dried to test the dryer performance. The dryer was tested in no-load and also in two loads condition i.e. 21 kg and 60 kg. The result shows that drying time and drying efficiency increases with increase in load [63]. Serm Janjai (2012) constructed the parabolic shaped greenhouse attached with LPG gas burner at Nakhon Pathom, Thailand. The dryer had a ground area of 160 m2 and enclosed it with a

polycarbonate sheet. PV panel operates the nine DC fans provided to maintain the air circulation. The highest temperature recorded inside the dryer was 65 °C. The tomatoes were dried from 54% to 17% (wb) in 4 days [64].

Steps To DO The HEAT And MASS Transfer Analysis Of Dryer

Energy Balance inside the dryer Energy balance is the balancing of incident energy coming from different sources with the energy released or stored within the system. For the complete drying system, the energy and moisture balance was proposed by Zaineb et al (2017) as given by Eq. 1 and Eq. 2. Energy balance Eq. for drying system, Ctca

Where Ta is the air temperature, Ctca is the thermal . capacitance of air,. t is the time, Qsa is the total heat gain . from all surfaces, Qif is the heat gain through infiltration, Qvt . is gain through the ventilation system, Qgdc is the convective . heat gain through instruments and occupants and Qcgz is the convective heat gain due to the coupling of zones. Moisture balance M ef =

Here, Mef is the effective moisture capacitance, ωo, ωg and ωvt is the humidity ratio of outside air, greenhouse air and air passing through vent respectively, m.if and m.vt are the infiltration and ventilation mass flow rate respectively. For the greenhouse chamber, the energy balance is given by Tiwari et al (2016) as follows:

 1 − α g [1 − Fn ] + ρ ′Fn  ∑ I i Ai τ i + hnr (T z = 0 − Tr ) An   dT (4) +hgr T y = 0 − Tr Ad = M a Ca r dt  a Ca (Tr − Ta ) + ∑U i Ai (Tr − Ta ) + m

Energy balance Eq. for separated drying chamber,  a Ca (Tch − Ta ) + hch (Tch − Ta ) Ach hcz (Tcz − Tch ) Ac = m

Where, hcp is the convective heat transfer coefficient from crop to drying chamber, Tcp, Trm, To are the crop surface, room, and ambient temperature respectively, Acp and Ad are crop surface and dryer area respectively, m.a is the mass flow rate of air and Ugr is the overall heat transfer coefficient from greenhouse room air to ambient through canopy cover. For the greenhouse type dryer, if the drying cabinet is kept separate from the greenhouse chamber, the energy balance Eq. for the air inside the drying cabinet is given as [61],

Where Ca is the specific heat capacity of air and Ach is the area of drying chamber. Energy balance for the dryer operating in no-load condition [56],   h fg  a Ca (Tdo − Tdi ) = Aa hpf  Tpl − Tca − Tca − Tgl  m hpf  

Tca, Tpl, Tgl is the collector air, plate, and glass temperature respectively, Tdi and Tdo is the air temperature at the inlet and outlet of dryer and hpf is the convective heat transfer coefficient from plate to the fluid. For parabolic shaped hybrid greenhouse with LPG burner for auxiliary heating of air, the energy balance Eq. used for air inside dryer [64], dTa = Aa hc , p − a Tp − Ta + A f hc , f − a T f − Ta dt dM p + D p ApC pv ρ p Tp − Ta + ρaVout C paTout − ρaVinC paTin dt +U c Ac (Tam − Ta ) +  1 − Fp 1 − α f + 1 − α p Fp  I t Ac τ c

VdH = Ain ρa H inVin − Aout ρa H out Vout dt dM p + D p Ap ρd Tp − Ta dt

Here, ρa is air density. To calculate the convective heat transfer coefficient, Barnwal and Tiwari (2008) proposed the Eq. for active as well as passive mode: For Active Mode,  mevp  ln   = lnC + nln ( Re ) + nln ( Pr )  R 

C, n, and R are constants, Re, Pr is Reynolds, and Prandtl number respectively. Reynolds number is the ratio of the

inertia to viscous force within the fluid while the Prandtl number is the ratio of momentum to thermal diffusivity of fluid. For Passive Mode,  mevp  ln   = lnC ′ + n ′ ln (Gr ) + n ′ ln ( Pr )  R 

Mass evaporated from crop per hour (mevp) is usually calculated to know the quantity of moisture evaporating from the crop surface per hour,

 0.016K h  P Tcp − Rh ⋅ P (T0 )     × Nu mevp =    Lcr ⋅ LHV  

Kh is the thermal conductivity of humid air, Lcr is characteristic Length, LHV is the latent heat of vaporization, Rh is Relative humidity of the air, and P(T) is partial vapor pressure at temperature T. Nusselt number (Nu) is the dimensionless number that shows the increase in heat transfer due to convection over conduction while the Grashof number (Gr) is usually the ratio of buoyancy to viscous force. Nusselt number is usually calculated to compute the value of the convective heat transfer coefficient. Moisture removed from the dried product was calculated using [54,91]; MC =

The relation used to determine the moisture content at any time t, moisture removal rate (MR), and the drying rate of the crop is given by Eq. 13 to Eq. 15 [55]. m − md Mt = t md

The ratio of moisture removed from the crop at any time ‘t’ to the initial weight of crop is termed as moisture removal rate and is calculated as, Mt Mo

Here, md is dry material in the crop, min, mfi, mt is the initial mass, final mass, the mass of crop at any time t, M0 is the initial moisture content in the crop.

The efficiency of the drying system Instantaneous thermal loss efficiency factor under passive and active mode is given by Eq. 16 and Eq. 17 respectively [48].

U gr ∑ At − ∑ Apv + U pv ∑ Apv  (Trm − T0 )  ηi ,nt = 1 −  I (t ) A fa

Na is the number of air exchange, I(t) is solar insolation intensity inside the dryer, Afa, At, Apv is the floor, tray, and PV area respectively. Upv is the overall heat transfer coefficient from greenhouse room air to surrounding through the PV module, which is calculated as,

U L ∑ At = U gr ∑ At − ∑ Apv + U pv ∑ Apv Where UL is taken 6 W/m2°C [48]. Electrical efficiency of PV Panel attached to a dryer for supplying the electrical power to fan is given as [48],  0.8Vocv I scc  ηel =   × 100  Apv I pv 

Vocv is the open-circuit voltage, Iscc is the short circuit current and Ipv is the solar radiation intensity normal to PV panel. Daily drying efficiency was calculated by Nayak et al. using the relation given by Eq. 19 as [51],

The efficiency of the dryer, when attached with fan or pump [62],

Qfn and Qpm are the thermal energy of the fan and pump respectively. Overall efficiency or system efficiency or energy efficiency indicates the performance of the entire drying system comprising of solar collectors, drying chamber, and other energy sources. The overall efficiency of the hybrid dryer (ηdr) attached with auxiliary air heating devices is given as [58,102],

mc is the mass of crop for drying, Act is the area of collector and Eas is the energy supplied from additional energy sources. Exergy efficiency is an important parameter to indicate the performance of solar dryers. It is the ratio of exergy available at the output (Ex­out) to the exergy input to the hybrid dryer (Exin) [103,104]. It is given by,

Exergy input to the hybrid dryer is the summation of exergy by the sun, the exergy of PV module, exergy of solar collector, and exergy of any other auxiliary devices. The exergy at the output is the summation of exergy consumed in evaporating the moisture from crop surface (Exevp) and exergy of work (Exwork). Exergy efficiency presents a more realistic situation as compared to energy efficiency. As energy efficiency tells that how much work is obtained from the available energy while exergy efficiency tells that how much real work is obtained from the maximum possible available work (ideal condition). Exergy analysis calculates the losses more accurately than energy analysis. Aritesty and Wulandani (2014) proposed the different relation to calculate the Dryer efficiency [63],

Other heat transfer parameters Total energy and exergy gain by the PV/T coupled dryer is given by Eq. 24 and Eq. 25 [90].

. Qu,th,ex is the thermal exergy gain. Solar energy going inside the dryer is given [91] as, t

m.t is the mass flow rate at time t, Toc, and Tic are outlet and inlet temperature of solar collector respectively. The daily thermal output of the dryer can be determined as [51], Q th = M c × Latent heat of evaporation

The heat required to evaporate the moisture content in the drying product [56], Q = M c × LHV ×

ηdr and ηcl are dryer and collector efficiency. Amount of useful energy inside the drying cabinet [58],  a Ca (Toc − Tic ) Qu = m

Energy taken by hot air to the ambient can be calculated using the relation [59],  a (hh − h0 ) Qh = m

Qbio, Qso, and Qel are bio, solar and electrical energy respectively.

Energy going outside the dryer and flat plate collector attached to the dryer is given as [91],

hh and h0 are enthalpies of hot and ambient air respectively. Chan et al. (2015) proposed the Eq. governing the crop drying as given by Eq. 33 [65], ∂M  ∂ 2 M 2 ∂M  − Dv  2 + × dt r ∂r   ∂r

Where Dv is the thermal diffusivity, M is the moisture removal rate and r is the radial distance. The convective heat transfer coefficient of air is calculated to determine the heat transfer occurring inside the dryer by using Eq. 34 [43,57].

3  Tgrd − Trm   hca = 0.884 ×   P Tgrd − Rh ⋅ P (Trm ) (Trm + 273)    +    268900 − P T grd  

Table 3. Value of heat transfer coefficient for different solar dryer Crop Dried

Conclusions

Hybrid dryers are the future of drying systems, as it not only eliminates the various drawbacks of conventional direct and indirect dryers but also improves the drying rate and drying efficiency. The application of thermal energy storage material in hybrid dryers increases the operating time in a day. This same thing can be achieved by using auxiliary heaters with dryers. The increased operating time, faster drying rate, and higher room temperature make the hybrid dryer suitable for drying high moisture crops. The convective heat transfer coefficient is the most important heat transfer parameter that affects the drying rate as well

as drying time. In hybrid dryers, the value of h is about 2-5 times higher than other conventional dryers. The airflow rate is another important parameter that needs to be optimized as it affects the evaporation rate from crop surface to room air. Although the capital cost of hybrid dryers is high that can be compensated by less payback time. As the payback time of commercial hybrid dryers is about 2-3 years only. The use of dehumidifiers can be one modification in hybrid dryers for further boosting up their efficiency. The application of nanoparticle in the dryer can be also one major field for research that might make the drying process faster.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

References

1. The article references can be accessed from the .pdf file.

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