Effect of regeneration heat and energy storage on thermal drying performance in a hardwood solar kiln

Renewable Energy 155 (2020) 783e799

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Effect of regeneration heat and energy storage on thermal drying performance in a hardwood solar kiln Ahmed Khouya ^di University, B.P. 1818, Tangier, Morocco Department of Electrical and Industrial Engineering, National School for Applied Sciences, Abdelmalek Essaa

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 November 2019 Received in revised form 24 March 2020 Accepted 29 March 2020 Available online 3 April 2020

The use of energy efficiency tools and solar energy in wood drying can help reduce the often heavy energy consumption of industrial dryers. The rational management of energy in terms of drying has attracted a lot of attention and the main objective of this work is to adapt the techniques of renewable energy and energy saving in solar kilns. The present work is therefore a contribution to the improvement of energy efficiency and the modeling of a solar wood kiln with thermal storage and heat regeneration. The drying system consists of five main units, a drying chamber, a multi-pass solar air collector, a cylindrical parabolic solar collector, a thermal storage tank and a Closed Feed Air Heater. The investigation carried out in this work is based on the establishment of mass and energy conservation equations in different components of the drying system. The governing equations of heat and mass transfer are solved using the implicit finite difference method. The discrepancies between the experimental and numerical results do not exceed 5%. The results show that the drying time is shorter in June and longer in December. The drying time decreases as the collector area increases and the boards thickness decreases. By incorporating a Closed Feed Air Heater with an effectiveness of 0.75, in the solar dryer, the collector and drying efficiency values are increased from 0.48 to 0.56 and 0.43 to 0.88, in June, respectively. The integration of the thermal storage unit in the solar kiln has the effect of reducing the drying time up to 40 and 60%, in June and December, respectively. Moreover, the combined use of thermal storage and regeneration heat is efficient in reducing the energy consumption ratio (kWh.m
3) up to 50% and 54%, in June and December, respectively. The proposed solution can significantly improve thermal drying performance and thus overcoming the problem of longer drying time, especially in winter. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Solar kiln Drying time Closed feed air heater Thermal storage Performance

1. Introduction The wood drying industry in general is an energy-intensive sector because removing water from a given product requires a lot of heat. Conventional dryers with very high heating power are useful, but their energy consumption and greenhouse gas emissions remain too high [1e3]. In this context, the use of the solar air heater for drying agricultural products has become a common technique in recent years [4,5]. To succeed in such an operation, it is essential to associate with the drying chamber a suitable thermal solar collector that is to say whose service temperatures make it possible to obtain in an attractive range of yield, a reduced drying time and a good quality dried product. It was reported that the thermal efficiency of the double pass solar collector is found to be higher than the single-channel by 34e45% [6e10]. Thermal and mass transfers are two important components during a drying operation. There is research that has developed a

E-mail address: [email protected]. https://doi.org/10.1016/j.renene.2020.03.178 0960-1481/© 2020 Elsevier Ltd. All rights reserved.

drying model to predict the drying time of a stack wood in a conventional energy dryer based on the overall mass transfer coefficient [11e13] and others have shown the effect of the wood dimensions (thickness, length and width), the inclination angle of the solar collector and the thermophysical characteristics on the drying rate in a solar kiln [14,15]. The results demonstrated that the drying time increases as the boards thickness increases and the air velocity decreases. Other studies have investigated the effect of mass flow rate in the thermal solar collector on the wood drying process [16,17]. They showed that the collector outlet air temperature increases as the mass flow rate decreases and that the drying rate is faster in February than in December. Several prediction models of the drying phenomenon have been developed to determine the characteristic drying curves under different operating conditions [18]. The results showed that there is a linear relationship between evaporation capacity and maximum mass flow. This relationship indicates that most of the drying occurs when the extragranular boundaries at transfer are predominant. Some research has shown the effect of ventilation and drying parameters (relative humidity, drying air temperature, drying air velocity) on

784

A. Khouya / Renewable Energy 155 (2020) 783e799

Nomenclature

Symbols A Aco Cp e E h hm I l j Lc m M MR m_ a Nu P Po Pr R Re RH S t dt T U V W x X

Exchange surface rate (m2.m
3) Collector area (m2) Specific heat (kJ.kg
1.K
1) Thickness (m) Enthalpy (kJ.kg
1) Heat coefficient transfer (W.m
2.K
1) Mass transfer coefficient (m.s
1) Solar intensity (W.m
2) Collector width (m) Board position in the stack Collector Length (m) Mass of water (kg) Mass of wood (kg) Reduced moisture content (
) Mass flow rate (kg.s
1) Nusselt number (
) Pressure (Pa) Vacuum rate (
) Prandtl number (
) Universal gases constante (8.314 J mol
1 K
1) Reynolds number (
) Relative humidity (
) Surface (m2) Time (s) Time element (s) Temperature (K) Velocity (m.s
1) Volume (m3) Absolute humidity (kg water/kg dry air) Abscissa (m) Moisture content (kg water/kg dry matter)

Greek letters a Absorptivity coefficient (
) DH Specific latent heat (kJ.kg
1) Dt Time step (s) ε Emissivity coefficient (
) h Efficiency (
) q Temperature of wood (K) l Thermal conductivity (W.m
1.K
1) n Viscosity (m2.s
1) r Density (kg.m
3)

the characteristic curves and the kinetics of wood solar drying [19,20]. It has been found that the activation of night ventilation increased the energy consumption of the dryer and reduced the drying time by almost half. The wood drying is an unavoidable step to bring to the market products with higher added value (Timber, wood frame, firewood, wood briquettes …). Achieving drying under certain economical conditions requires calculating the drying cost with great precision while minimizing energy consumption. Conventional kilns use hot air at controlled drying air temperature, relative humidity and air velocity to evaporate water from the wood stack [21,22]. The boards in the stack are arranged in layers that are separated by narrow strips to facilitate the circulation of hot air between the planks. During the drying process, energy can be supplied by a boiler or an electric heating system. A performed study on the conventional drying of wood beech at temperature ranging from 37 to 67  C

t a ab amb atm b c co dr ea eq f f1 f2 f3 fi g1 g2 H ha ht i ib in k max min o psf r ra reg sat solar sky u w wv 0

Transmission factor(
) Drying air Absorber Ambient Atmospheric Board of wood Convection Collector Drying Exhaust air Equilibrium Fluid (air) Air stream in channel 1 Air stream in channel 2 Air stream in channel 3 Final Glass cover 1 Glass cover 2 Hydraulic Hot air Heat transfer Inlet Insulating background Initial Time increment Maximum Minimum Outlet Fiber saturation point Radiation Return air Regeneration Saturation Solar energy Sky Useful Water Water vapor Oven-dry mass

Abbreviations HTF Heat Transfer Fluid CFAH Closed Feed Air Heater ECR Energy Consumption Ratio PCM Phase Change Material

under oscillating operating conditions showed that the drying time and energy consumption for reducing the moisture content from 0.79 at 0.074 kg water/kg dry matter of a 0.8 m3 wood stack varies from 11 to 16 days and 988 to 1380 kWh.m
3, respectively [23]. An experimental study that evaluated convective wood drying at temperatures ranging from 80 to 120  C indicates a drying time of less than one day for a 100 m3 Radiata pine wood stack [24]. On the other hand, it is estimated that the energy consumption ratio related to the production of hot air oscillates between 887 and 1728 kWh.m
3 and that fan consumption is in turn varies from 68 to 438 kWh.m
3 depending on the board dimensions and the operating conditions of the experiment. Drying wood has always been a delicate and energetic technique. Several solar kiln models have been designed and modelled in recent years [25e28]. These systems constantly exchange with their environment the solar energy captured by the solar collector. The

A. Khouya / Renewable Energy 155 (2020) 783e799

thermal performance of a solar kiln during drying process of Red la climate, a relatively important pine wood is evaluated under Mug region for receiving solar energy in Turkey [29]. This study has shown that it is possible to achieve energy savings of 30% with reference to conventional drying method. Nevertheless, the disadvantage of solar drying is that the solar flux varies from one season to another and according to the months and days. Solar drying is usually conducted in the summer period because it is the season where solar radiation is the most important. Solar energy is only available during the day and its application requires efficient storage of thermal energy, so that excess heat collected during the hours of sunshine can be stored for later use during the night. It is possible to store solar energy in two different sensible and latent forms, using the ability of materials to release or store heat. Different types of thermal storage units have been designed and studied [30e32]. They all differ in the nature of the material used as storage medium and the bed geometry. It was found that the integration of a sensible heat storage system in a solar kiln can be reduced the drying time by about 30% compared to the drying process without storage [33]. The storage of thermal energy by latent heat is more useful than the storage of sensible energy because of the large storage capacity per unit volume/mass at almost constant temperatures. It was shown that the size of the latent heat energy storage system becomes larger as the melting temperature of the selected phase change material (PCM) is high [34]. Extensive literature reviews have been made to learn about the progress and status of research in the field of solar kilns. To the best of the author’s knowledge, there has been little work on methods to improve the energy efficiency of wood dryers. Thus, the objective of this work is to improve existing technologies of solar kilns by incorporating energy efficiency solutions into the drying system. This work is constructed as follows: firstly, an account of the methods and means used to carry out this investigation will be presented. Then, mathematical models to predict the behavior of different units of the solar dryer will be presented. The simulations will be carried out under Tangier climate, in Morocco. The simulations results will make it possible to predict the distributions of temperature and moisture content in the wood stack as well as the estimation of the drying thermodynamic parameters such as the drying time, the thermal efficiency and the energy consumption ratio (kWh.m
3) of dried wood. The influence of some parameters such as the collector area, the wind speed and the surface exchange rate on the drying rate will be presented and analyzed. The effect of incorporating a heat exchanger and a latent heat storage unit on thermal performance of the dryer will be studied and discussed. A thermal performances comparative study of the present dryer model with those of the literature review will also be discussed. This work will conclude with a general conclusion gathering all the results and the statement of possible perspectives that could constitute the follow-up to study. 2. Materials and methods This work focuses on the study of thermal drying performance in a hardwood solar kiln. The model of the dryer studied is shown in Fig. 1. The system works in forced convection, moreover the recycling of the air between the different units of the dryer can be considered. This system consists of: solar air collectors, thermal latent storage unit, hydraulic circuit, heat exchanger, fans and drying chamber. The main objective of this work is to contribute to the improvement of energy efficiency and the modeling of a hardwood solar kiln with heat regeneration and energy storage. It would therefore be very useful to estimate some drying parameters, such as drying time, thermal efficiency and energy consumption.

785

2.1. The drying chamber The drying chamber of the studied system is composed of a wooden slab with a thickness of 0.03 m, the roof and walls are made of wooden panels in series with a polyurethane foam with a thickness of 0.02 m. The total volume of the drying chamber is 27 m3 with an air volume of 17 m3(a vacuum rate of 0.63). The dimensions of each board are 1 m  0.2 m  0.025 m, and the distance between layers (battening) is 0.025 m. The total surface area Sb, the mass Mb, and the volume of the wood boards Vb are 920 m2, 6500 kg and 10 m3, respectively. The airflow is heated by solar energy in the thermal solar collector and by means of a blower, it circulates in the passage between layers. Thus, the heat and mass transfer are exchanged between the hot air flow coming from the solar collector and the wood stack. The selected wood specie for this work is Beech (Hardwood). It is commonly used in design, construction, furnishing, heating system and industry. 2.2. The solar collector Different variants of solar air collectors likely to meet the drying process have been reported in the literature review [35e43]. The multi-pass type has attracted attention because of the triple warming experienced by the air in the collector [44,45]. The configuration of the solar thermal collector studied in this work is illustrated in Fig. 2. This configuration consists of: - Two glass plates each having a thickness eg ¼ 4 mm, an emissivity εg ¼ 0.88 and an absorptivity ag ¼ 0.05 [46]. It is a selective system transparent to visible solar radiation and opaque to longwave infrared. The glasses transitivity tg is 0.9 [35]. - An absorber painted black with a thickness of 5 mm and an emissivity εab ¼ 0.95, from the point of radiation view, it is opaque diffuse at emission and reflection [45]. - A thermally insulated bottom of thickness eib ¼ 50 mm and an emissivity εib ¼ 0.9 [35,38,45]. - A Heat Transfer Fluid (HTF) weakly absorbing solar energy. It is a gas (air þ water vapor) assimilate to a non-diffusing gray medium [47]. 3. Mathematical modelling of the solar kiln 3.1. Mathematical modelling of the collector In the scientific approach adopted in this work, attention was paid to the evolution of the air temperature at the collector outlet according to its geometric characteristics for a given mass flow rate. In Fig. 2, the solar air collector with the associated heat transfer coefficients are presented. The incident solar energy is transmitted from the first glass to the other then absorbed by the absorber plate. The fluid passes firstly into the upper sheath which is between the two glasses, then makes the return and passes through the intermediate sheath located between the absorber and the second glass. Finally, the fluid continues its warming through the third sheath between the absorber and the insulator. The heat balance equations in different components of the solar collector are written based on the following simplifying assumptions [45e47]: - The heat transfer is unidirectional; - The heat capacities of the glass cover, the absorber and the insulating material are negligible; - The thermal gradient in the material thickness of the solar collector is negligible; - Thermal stratification in the HFT does not exist vertically in the ducts of the device;

786

A. Khouya / Renewable Energy 155 (2020) 783e799

Fig. 1. Schematic diagram of the Solar Kiln.

also absorbs part of the incident solar energy and transmits the heat by convection to the HTF at the level of the upper and intermediate sheath. The energy balance is written as [45,47]:

 
 Aco I tg1 ag2 þ hr;ab;g 2 Aco Tab
Tg2 þ hc;g2;f 2 Aco Tf 2
Tg2  

hr;g1;g2 Aco Tg2
Tg1
hc;g2;f 1 Aco Tg2
Tf 1 ¼0

In the heat fluid of the intermediate sheath: In the intermediate sheath of the solar collector, the heat transfer fluid exchanges heat by convection with the absorber and the insulating body. The heat balance is written [45]:

Fig. 2. Simplified scheme of the solar air collector.

- The tightness of the solar collector is perfect and no air leakage can appear. Based on these assumptions, the thermal balance of each component of the collector is expressed in the following equations: Thermal balance of the glass 1: In the glass cover 1, part of the incident solar energy is absorbed by the latter, a large part is transmitted to the glazing 2 and the rest is lost by convection and by radiation towards the environment. The energy balance is written as [45,46]:

 
 Aco I ag1 þ hr;g1;g2 Aco Tg2
Tg1 þ hc;g1;f 1 Aco Tf 1
Tg1  

hr;g1;a Aco Tg1
Tsky
hc;g1;a Aco Tg1
Tamb ¼0

(1)

In the heat transfer fluid of the upper glass 2: In the upper sheath of the solar collector, the HTF exchanges heat by convection on the one hand with the glazing 1 and on the other hand with the glazing 2. The energy balance is written as [45]:

    dTf 1 ¼ lhc;g1;f 1 Tg1
Tf 1 þ lhc;g2;f 1 Tg2
Tf 1 m_ a Cp;a dx

(3)

m_ a Cp;a

Thermal balance of the glass 2: In the glass cover 2, there is a heat transfer by radiation between the latter and the glazing 1 and then with the absorber. The glass 2

(4)

Thermal balance of the absorber: The absorbent plate absorbs all the thermal energy transmitted by the glass 2, it exchanges the heat by convection with the heating fluid in the intermediate and lower sheath. The absorber also exchanges heat by radiation on the one hand with the glass 2 and on the other hand with the insulating structure. The thermal balance of the absorber taking into account the heat transfer modes previously described above can be written as [45]:

    Aab I tg1 tg2 þ hc;ab;f 2 Aab Tf 2
Tab þ hc;ab;f 3 Aab Tf 3
Tab

hr;ab;g2 Aab Tab
Tg2
hr;ab;ib Aab ðTab
Tib Þ ¼0

(5)

In the heat transfer fluid of the third sheath: In the lower sheath of the solar collector, the HTF exchanges heat by convection on the one hand with the absorber and on the other hand with the insulating structure. The energy balance is written [45]:

m_ a Cp;a (2)

    dTf 2 ¼ lhc;g2;f 2 Tg2
Tf 2 þ lhc;ab;f 2 Tab
Tf 2 dx

    dTf 3 ¼ lhc;ib;f 3 Tib
Tf 3 þ lhc;ab;f 3 Tab
Tf 3 dx

(6)

In the insulating background: For the insulating structure, the heat transfer is radiative with the absorber, but convective with the HFT and the outside air. The thermal balance is established by Ref. [47]:

A. Khouya / Renewable Energy 155 (2020) 783e799

temperature difference inlet-outlet at the solar collector [34]:

hc;ib;a Tamb þ hc;ib;f 3 Tf 3 þ hr;ab;ib Tab Tib ¼ hc;ib;a þ hr;ab;ib þ hc;ib;f 3

(7)

In equations (1)e(7), convective heat transfers inside de thermal solar collector can be determined using equation (8) knowing the Nusselt number Nu (equation (9)), the thermal conductivity of air la and the sheath hydraulic diameter DH of the collector as [47]:

h¼

Nula DH

Nu ¼

(8) 

 0:666Re0:5 Pr1:3

(9) The Reynolds number Re in equation (9) is computed using equation (10) as function of the HFT velocity Uf, its viscosity nf and the sheath hydraulic diameter DH of the solar collector [47]:

Re ¼

Uf DH

(10)

yf

The convective heat transfer coefficient outside the solar thermal collector can be evaluated using equation (11) knowing the wind speed Uamb in the installation locality [45]:

hc;g1;a ¼ hc;ib;a ¼ 2:8 þ 3Uamb

for 0  Uamb  7 m:s
1

(11)

The radiative heat transfer coefficient between the surface y and the surface z is determined by equation (12) as a function of the surface temperature T, its emissivity ε and the constant of Stephan Boltzmann s as [17]:



hr;y;z ¼




s Ty2 þ Tz2 Ty þ Tz 1 εy

þ ε1z
1

 ððy; z Þ

¼ ðg1; skyÞ or ðg1 ; g2 Þ or ðab; ibÞÞ

(12)

The radiative heat transfer coefficient between the glass and the sky is calculated using the equivalent sky temperature (equation (13)) determined as function of the ambient temperature Tamb as [17]:

Tsky ¼ 0:0552ðTamb Þ1:5

tðdr

Qu ¼

  m_ a Cp;a Tf 3 ðx ¼ Lc Þ
Tamb dt

(17)

0

The solar energy captured by the solar collector during the drying process is computed using equation (18) by means of the collection surface Aco and the incident solar irradiation I on the latter [39]: tðdr



for laminar flow Re < 3  105   for turbulent flow Re > 3  105

0:035Re0:8 Pr1:3

787

(13)

The associated initial and boundary conditions are: At t ¼ 0: Tf1 ¼ Tf2 ¼ Tf3 ¼ Tg1 ¼ Tg2 ¼ Tab ¼ Tib ¼ Tamb

(14)

At x ¼ 0: Tf1 ¼ Tamb and Tf2 ¼ Tf3

(15)

At x ¼ Lc: Tf1 ¼ Tf2

(16)

Equations (1)e(7) with the associated initial and boundary conditions are solved using the Runge Kutta 4th order method [47]. The Cþþ programming language was used to compute the outlet air temperature at the solar collector (Tf3 at x ¼ Lc) for drying purposes. This temperature is a function of the mass flow rate (kg.s
1), geometric characteristics (length, width, depth …) and the climatic conditions of the place where it is installed. The physical and geometric characteristics used in this collector model are presented in Table A.1 (Appendix A). The useful energy (Qu) transferred to the drying chamber is computed using equation (17) as a function of the mass flow rate of the fluid m_ a , its specific heat Cpa and the

Qsolar ¼

IAco dt

(18)

0

It is useful in any study relating to the solar collector to assess its performance and compare it with that of other configurations existing in the literature. In this work, this performance is estimated by calculating the collector efficiency (equation (19)) during the drying process which is defined as the useful energy transferred to the HTF divided by the solar energy collected on the device [39]:

hco ¼

Qu Qsolar

(19)

3.2. Mathematical modelling of the drying unit Convective drying as well as other drying techniques are used to preserve and store wood products for longer periods by removing some of their moisture content [34,47]. It is a complicated process that involves an inevitable coupling between mass and heat during drying [19]. Mathematical models are necessary because they allow the simulation of heat and mass transfer in wood and predict the influence of weather conditions on this noble material. Depending on the wood destination (frame, firewood, construction and finish, etc.), it must have a reasonable humidity which allows it to be used correctly, as well as good dimensional stability over time [48,49]. To the best knowledge of the author, studies relating to the solar drying kinetics of wood are rare in the literature [14,16,17,25,34]. Thus, this work contributes to the modeling of heat and mass transfers in a forced-convection solar dryer. The modeling is inspired by the methods of the continuous media mechanics applied to hygroscopic products. These methods consist especially in expressing the mass and energy conservation balances in a reference volume. The drying mathematical model used in this work is based on the following simplifying assumptions: - The shrinkage induced by drying is negligible [47]; - The air velocity is uniform along the boards [14]; - Heat transfer between the kiln walls and the environment is negligible [34]; - The mass flow of hot air is diffused equitably between layers [33]; - The position of each board is represented by its distance from the wood stack inlet in the mass flow direction [28]; By adopting all the above assumptions, the drying model equations are expressed in sections 3.2.1 to 3.2.5. 3.2.1. Mass balance for air drying The mass conservation for air drying reflects the water content lost by the wood and gained by the air. It is expressed using equation (20) as a function of the specific humidity W, the moisture content of wood X and other drying characteristics such as the mass

788

A. Khouya / Renewable Energy 155 (2020) 783e799 Table 1 Energy and thermal drying parameters of the present drying system.

Energy needed (kWh) Solar energy (kWh) Energy supplied (kWh) Fans consumption (kWh) Drying time (days) Drying efficiency (
) Collector efficiency (
) Energy consumption ratio (kWh.m
3)

flow rate m_ a and the product dry mass M0 [33]:

m_ a ðW
Wa Þ ¼
M0

dX dt

(20)

3.2.2. Energy balance for air drying The energy balance of the drying air expresses the energy lost by the air and gained by the product. It is formulated by equation (21) as a function of the drying air temperature Ta, that of the product Tb and other characteristics such as the heat transfer coefficient hth and the exchange surface Sb of wood [14]:

 ma Cp;a

 dTa dTa þ Ua ¼ hth Sb ðTb
Ta Þ dt dx

dTb dXðtk Þ ¼ hth Sb ðTa
Tb Þ
M0 DH dt dt

M
M0 M0

(22)

In this work, the instantaneous moisture content of the wood is evaluated by calculating its wet mass M during the drying process (equation (24)). The schematic diagram for performing the calculation is presented in Appendix B. The mass of wood Mj at actual time tk (see Fig. B1) for the jth board can be expressed as [33]:

(24)

where [33].

dMj ðtk Þ ¼
M0

dXj ðtk Þ ðtk
tk
1 Þ dt

drying rate curve. The drying rate equations of different species of wood have been determined experimentally in a previous work [18]. The drying kinetics curves of wood can be obtained from:

dX dX ¼ f ðMRÞ dt dt 1

(26)

where f ðMRÞ ¼ X1 MR þ X2 MR2 þ X3 MR3 . X1, X2, and X3 are constants determined by tests in a previous work [18]. For beech wood:

f ðMRÞ ¼ 1:65MR
1:31MR2 þ 0:66MR3

MR ¼

(23)

Mj ðtk Þ ¼ Mj ðtk
1 Þ
dMj ðtk Þ

1043 8613 4454 724 90.58 0.23 0.52 517.8

(27)

MR is the reduced water content of wood defined as [18]:

3.2.4. Mass conservation for wood The mass conservation equation for a given product reflects the amount of water it releases by evaporation during drying. The moisture content base dry (kg water/kg dry matter) is expressed using equation (23) as a function of the dry and wet product mass as [34,47]:

X¼

December þ January þ February

(21)

3.2.3. Energy conservation for wood In the same way as for the energy balance of the drying air, the heat transfer rate for wood during the drying process is described by the following equation [33]:

mb Cp;b

June 1019 4502 2195 162 20.25 0.43 0.48 235.8

(25)

M0 is the oven-dry mass of the jth board segment, invariant with time and tk-1 ¼ tk e Dt. The change rate in moisture content of a wood segment inside the dryer can be expressed by an equation called the characteristic

X
Xeq Xin
Xeq

(28)

is the drying rate of the first period approached by the
dX dt 1 expression [14,19,34,47]:


 dX
¼ Ahm Xsat
Xeq dt 1

(29)

Mass transfer coefficient is required in any heat and moisture transport calculations. Mass transfer coefficient are often determined using empirical correlations based on measurements of different geometry and flows [50]. In this work, the mass transfer coefficient hm is computed using equation (30) as a function of the heat transfer coefficients hth and other drying parameters such as the sorption isotherm Xeq and the fiber saturation point of wood Xpsf as [51]:

hm ¼

hth Xpsf vRH Cp;a ra r0 vX

(30)

In equation (30), the term vRH vX is referred to the sorption isotherm of wood expressed as [16,17,20]:




 Xeq ¼ b1 Xm HR 1
b2 HR 1
b2 HR þ b1 HR

(31)

where b1, b2 and Xm are expressed as: b1 ¼ 27:73 expð
2135:87 =ðRTb ÞÞ b2 ¼ 1:93 expð
2308:79 =ðRTb ÞÞ Xm ¼
7:33  10
4 Tb þ 0:286 The fiber saturation point Xpsf which occurs for a moisture content between 0.25 and 0.3 kg water/kg dry matter depends on the temperature and is given by the expression [17]:

Xpsf ¼ ðb1 Xm Þ

.

ðð1
b2 Þð1
b2 þ b1 ÞÞ

(32)

In the study of a solar dryer system, it is necessary to know the convective heat transfer coefficient between the product and the

A. Khouya / Renewable Energy 155 (2020) 783e799

fluid flowing inside the dryer. It has been shown that the drying rate increases as the convective heat transfer coefficient increases [50]. The convective heat transfer coefficient can be determined using equation (33) as a function of the Reynolds (Re) and Prandtl (Pr) correlation as [14]:

hht ¼

  8 la  0:5 0:33 0:0664Re for laminar flow Re  2100 Pr >
 > : la  0:023Re0:8 Pr0:3 DH

for turbulent flow ðRe > 2100Þ (33)

where la is the air thermal conductivity computed as a function of the drying air temperature Ta as [47]:

la ¼ 4  10
5 Ta þ 0:0246

(34)

In equation (33), the parameter DH is taken equal to twice the stick thickness [14]. During the drying process, the equilibrium moisture content of a given product is strongly affected by the drying air temperature and relative humidity. It has been shown that the drying rate increases as the relative humidity inside the product decreases [18]. In this work, the relative humidity RH inside and outside the dryer is computed using equation (35) in which W represents the absolute humidity of air and Psat the saturated pressure at given temperature Ta estimated by equation (36) as [14,17,52]:

RH ¼

WPatm ðW þ 0:622ÞPsat ðTa Þ

Psat ðTÞ ¼ 10ð7:625 Ta =ðTa


29:212 ÞÞ
3

(35) for 273 K  Ta  413 K (36)

producing equipment [53]. To remedy this gap, it is necessary to look for ways to raise the fluid temperature before entering the solar collector. The air released outside the dryer is often much hotter than the outdoor air, its capture and reuse could reduce energy consumption and significantly reduce drying costs. Therefore, to improve the performance of the installation, it is economically advantageous to install, between the air exterior and the exhaust air, a Closed Feed Air Heater (CFAH) and cross currents to recover some of the heat during all seasons of the year. The schematic of a CFAH is shown in Fig. 3. This type of heat exchanger could also be used in heating systems, in which the heat is transferred from the exhaust air to the fresh air coming out from the exterior without any mixing taking place. The effectiveness of a regenerator is defined as [52]:

hreg ¼

Eha
Eamb Era
Eamb

It is essential in any study relating to a drying operation to evaluate the performance of the energy conversion systems used. The idea is to assess the overall thermal efficiency of the operation by calculating the ratio between the needs and energy expenditure during the drying process. The overall thermal efficiency of drying is computed using equation (37), it is defined as the energy required to evaporate the water from the initial water content until the boards reach the equilibrium moisture content divided by the thermal energy supplied during the process [34]:

hdr ¼

  mw DH þ mw Cp;w Tw;fi
Tw;in Qu

(37)

Eha ¼ hreg ðEra
Eamb Þ þ Eamb

(39)

The determination of Eha makes it possible to deduce the temperature of the air at the outlet of the heat exchanger using equation (40) as [52]:

Tha ¼


ðEha
DHWha Þ  Cp;a þ Cp;wv Wha

(40)

If the exchange process occurs without condensation at the CFAH, ie if the temperature of the air to be released into the atmosphere is higher than the dew point, the air temperature at the outlet of the heat exchanger can be computed using equation (41) as [52]:

(41)

In general, the efficiency of the CFAH is provided by the manufacturer and in this work, the thermal performance of the drying is evaluated for two effectiveness 0.5 and 0.75. 3.5. Numerical procedure for solving the drying model The governing equations of heat and mass transfer of the drying model are discretized by the implicit finite difference scheme already developed by several authors in many scientific research works [14,28,34,47]. The discretized equations are developed in appendix B. A computer program in C þþ has been developed to perform the calculations step by step from the first board located at the entrance to the stack of wood to the last at the top of the column (see Fig. B1 in Appendix B). This code is connected to that

3.4. Regeneration heat process Conventional energy drying of a given product is costing companies too much [53]. For example, if we take an energy-intensive sector such as processes in the wood drying industry, it is generally considered that drying operations represent about 15% of the total energy consumed by the industrial sector in developed countries [34]. This is an important part, hence the need to find solutions to optimize the process of drying wood in an economic but also ecological approach. In fact, it has often happened that 70% of the energy consumed by a wood kiln could be lost because of the low efficiency of the energy conversion in solar or conventional heat-

(38)

Knowing the effectiveness of the CFAH, the enthalpy of moist air at the inlet of the solar collector is determined as:

Tha ¼ hreg ðTra
Tamb Þ þ Tamb 3.3. Thermal efficiency of the drying system

789

Fig. 3. Synoptic scheme of a Closed Feed Air Heater(CFAH).

790

A. Khouya / Renewable Energy 155 (2020) 783e799

developed for the model of the triple pass solar collector in order to communicate to it at any time the input data which are the temperature, the relative and specific humidity of the air leaving the solar collector. The numerical simulations are conducted under Tangier climate (latitude 35 460 0200 Norte, longitude: 5 470 5900 West), in Morocco. Table C.1 in Appendix C presents certain weather conditions for different months considered. The thermophysical properties of the studied hardwood specie (Beech) are also used for establishing the simulations results [18,50]. The flowchart followed to establish the calculation in different stages of the solar kiln is explained in previous work [34]. The data and information requested by the constructed code such as weather files and sorption isotherm of wood were brought into play stat variables. The electrical energy consumption by the blowers to ensure the required air velocity in the wood stack is estimated to be 8 kWh per 24 h (day and night ventilation). It should be noted that the results of this work are largely oriented towards a better rationalization of energy in terms of hardwood solar drying (Beech). However, this investigation could be compared and applied to wood drying in general.

drying process for two types of solar kiln (conventional and solar kiln) are presented. It should be noted that the experimental results indicated here for the solar dryer are those obtained during the  tests, in a previous work carried out on a solar kiln under Yaounde climate, in Cameroon [20]. For conventional drying, the tests were carried out in a sawmill located in the Chilean region of Bio-Bio [11]. The tested volume of wood in the stack is 0.4 and 2.156 m3 for conventional and solar kilns, respectively. The temperature, air velocity and relative humidity of drying air were remaining constant during the conventional drying tests and equal to 343 K, 1.5 m s
1 and 72%, respectively. It was found that the moisture content decreases as the drying time increases and that after 92 h and 480 h of drying, the wood stack reached the equilibrium moisture content of use for conventional and solar kilns respectively. The average discrepancy between predicted and measured values are 3% and 5% for conventional and solar kilns, respectively. Thus, the perfect agreement indicates that the present drying model is suitable for calculating the drying parameters in the solar kiln under different operating conditions. 5. Results and discussion

4. Validation of solar thermal collector and drying model Fig. 4 shows the comparison of the experimental and numerical evolution of the air outlet temperature in the collector as well as the hourly variation of the solar irradiance on a typical day under the climatic conditions of Coimbatore (latitude 11.0183  N; longitude 76. 9725  E) during the year 2016 [54]. It is obvious that the outlet air temperature and solar radiation increased in the morning with time and decreased from afternoon to evening. It should be noted that experimental data such as outdoor air temperature, solar radiation intensity, and characteristics of the solar collector investigated experimentally in Coimbatore were used to test the evolution of the air outlet temperature in the present model. The results show that the discrepancies between the experimental and numerical results do not exceed 2%, which corresponds to a very satisfactory agreement, which reveals the validity of the numerical results obtained under the conditions considered. In Fig. 5, the experimental and numerical hourly evolution of moisture content of wood during

Fig. 4. Variation of temperature and solar intensity against time.

5.1. Drying without regeneration heat and thermal storage unit The present drying model will first be applied to the solar dryer (drying unit þ solar collector) without storage and regeneration heat. The results of this configuration is presented in section 5.1 and 5.2. The other results relating to the use of efficient means to improve the operation of the solar kiln will be presented later in the other paragraphs according to the complexity of the integrated devices (Section 5.3, 5.4 and 5.5). In the basic approach of this work, a stack of wood with an initial moisture content of 0.4 kg water/kg dry matter was considered (Mi ¼ 6500 kg). The drying time is completed when the final moisture content of wood reaches the equilibrium moisture content assumed to be 0.12 kg water/kg dry matter (Mf ¼ 5200 kg). The amount of water that should be removed during drying process is estimated at 1300 kg. It should be noted that the calculations are made by referring to the weather conditions of the 15th day of each month. The value of mass flow rate, collector area and wind speed used in the calculations is

Fig. 5. Variation in moisture content against drying time.

A. Khouya / Renewable Energy 155 (2020) 783e799

0.25 kg s
1, 24 m2 and 1 m s
1, respectively. The thermal performance of the studied solar dryer is evaluated in terms of drying time, useful energy and thermal efficiency according to the weather conditions of a given month. Figs. 6 and 7 show the evolution in drying air temperature and wood moisture content as a function of the time during the drying process. Results show that air temperature increases in the morning and decreases in the afternoon. The maximum drying temperatures recorded during drying are approximately 313, 314, 321 and 341.5 K, on the 15th day of December, January, February and June, respectively. It is clear that the moisture content of wood decreases as the drying time increases. The results show that the drying rate is faster at the beginning of the drying process and becomes more and more slower as the drying progresses. The reduction in moisture content during June is 0.00115 kg water/kg dry matter per hour for the first 120 h and 0.00036 kg water/kg dry matter per hour on the rest of the drying time. This irregular behavior of the moisture content is probably due to the transport of water between the wood and the air controlled by the external resistance which persists as long as the water content of the wood is above the fibers saturation point (0.25 kg water/kg dry matter) [18]. In this field, the evaporation of water is fast because the driving force to release free water within the boards is important. From Fig. 7, by starting the drying at the beginning of December, it is clear that the boards did not reach the equilibrium moisture content of 0.12 kg water/kg dry matter before the end of this month or end of January, but towards the end of February, a drying time of around three months. This long drying time is explained by the low air temperatures collected at the outlet of the triple pass solar collector, temperatures deemed insufficient to increase the mass transfer potential from the inside to the outside of the product in order to dry it quickly [14,17,34]. Table 1 indicates some drying parameters such as energy required, energy consumption ratio, drying time and thermal efficiency of the solar kiln. The energy supplied to the dryer in order to evaporate 1300 kg of water was 2195 and 4454 kWh, in summer (June), and winter (December þ January þ February) respectively. The drying time is considerably shorter in June (hottest month) and longer in December (coldest month). It was also shown that the consumption of fans is greater in winter than in summer. The

Fig. 6. Variation in drying air temperature as a function of time on the 15th day of December, January, February and June.

791

Fig. 7. Variation in moisture content over time during the hardwood drying process.

drying efficiency is high in June and low in December. On the other hand, the overall efficiency of the solar collector is slightly lower for the drying process carried out in June compared to that carried out during the winter period. These results are in agreement with those obtained experimentally under various operating conditions on vacuum tube solar collectors designed and manufactured by Copperhill Alternate Energy Inc [55]. The results show that the energy consumption ratio (ECR) of dried wood increases as the energy supplied to the dryer increases. The ECR of the drying system was significantly lower in June (235.8 kWh.m
3), because the duration of the day and the solar intensity reached higher values in this month. This important result can be clearly interpreted by the high temperatures harvested from the solar collector during June which resulted in a high drying efficiency. It should be noted that the ECR is an indicator that determines the rate of return (earned interest) and the net present value of an energy-efficient investment. This is a key indicator that could be useful when selling solar-wood drying because many customers want to know how fast the initial large investment will be profitable. Hence, the project of solar drying of wood during summer (June, July and august) could be acceptable because the values of the ERC are very low compared to those of other traditional methods of drying [23,33,57]. For winter, the drying time is longer and therefore the finance losses could be high. Therefore, it is necessary to compare the prices of wood drying using conventional energy before being propelled on the competition market. Some research work reports that the ECR of dried wood can vary between 600 and 1200 kWh.m
3 for conventional drying [22e24,28]. However, in general, the costs of exploiting solar energy are low because it is a free energy that simply depends on the efficiency of thermal conversion devices. To overcome this problem of drying during periods of less sunlight, it should seek to integrate new solutions to reduce the drying time and production costs of the dried wood. These solutions will be discussed in order of merit in the rest of this work. A drying process is an energy-consuming process that is controlled by several parameters such as the characteristics of the drying air (temperature, specific humidity, relative humidity, …), those of the product to be dried (hardwood, softwood, …) and geometric characteristics (length, thickness, …). All these parameters can influence the drying time, the energy consumption and

792

A. Khouya / Renewable Energy 155 (2020) 783e799

the quality of the finished product. In the following discussions, attention was drawn to the importance of the characteristics of the exhaust air leaving the drying chamber. Fig. 8 shows the evolution of the dryer inside and outside specific humidity in June under Tangier climate. In Fig. 9, the evolution of the temperature and the relative humidity of the air inside and outside the dryer is presented. The results show that the dryer inside specific humidity follows the evolution of the air temperature inside the dryer (Fig. 9). The minimum and maximum values of the absolute humidity inside the dryer are approximatively 0.012 and 0.026 kg water/kg dry air, respectively. On the other hand, the outside specific humidity increases at the beginning of drying until reaching a maximum value of 0.045 kg water/kg dry matter then decreases gradually so that it could reach its initial value of the outdoor air at the end drying, this is only possible when the wood is dried for example at 0 kg water/kg dry matter instead of 0.12 kg water/kg dry matter. The absolute humidity parameter is of great importance in particular when recovering a portion of energy at the outlet of the drying chamber (see section 5.3). In fact, when the hot humid air passes through the CFAH, heat can be recovered in a sensible and latent form, which could improve system performance. As shown in Fig. 9, the temperature outside the solar kiln fluctuates between 315 and 325 K, when the thermal regime is established in the wood stack. This leads to a waste of energy and a poor performance since the air normally brought to this great temperature is released directly into the atmosphere. In addition, the relative humidity at the outlet of the solar kiln decreases as the drying progresses. It has been shown that the equilibrium moisture content of wood decreases as the relative humidity decreases [22,23]. Thus, it is considered useful to recover the energy of the air leaving the dryer through the incorporation of an energy saving system, in order to improve its efficiency.

5.2. Effect of certain parameters on drying performance 5.2.1. Effect of collector area and velocity on drying rate The numerical results of this influence are illustrated in Fig. 10. Results showed that the drying time decreases as the collector area increases. This is because when the area of the thermal collector is large, the temperature of the air harvested at its outlet increases, and consequently, the water evaporation on the product surface also

Fig. 8. Specific humidity variation as a function of time in June.

Fig. 9. Variation of temperature and relative humidity against time in June.

increases. To select a suitable solar collector area, several drying parameters such as, the amount of wood in the stack and its initial moisture content, the mass flow rate and the drying energy consumption must be taken into account during its design. According to the results based on the drying time and energy consumption for the reference case studied, the recommended collector area may be about 24 m2. It has been demonstrated that a 36 m2 collector area leads to an increase of the drying air temperature above 100  C and could affect the properties of dried wood products [56]. Fig.10 shows also that the wind speed has a detrimental effect on the behavior of the drying system. The results show that as the air velocity increases as the drying time increases. This is because as wind speed increases, thermal losses to the outside of the collector increase. It should be noted that the wind speed near the solar collector remains dependent on the

Fig. 10. Variation of moisture content against drying time. Influence of collector area and wind speed.

A. Khouya / Renewable Energy 155 (2020) 783e799

weather conditions of the installation location of the drying system. To achieve the best performance of the drying system, it is recommended to install the solar collector in a location with lower wind speeds. 5.2.2. Effect of size and boards thickness on drying rate The surface exchange rate (m2.m
3) of a wet wood is defined as the ratio between the exposed wood area to air and the total volume of wood in the stack. Privileging a high surface exchange rate means for a fixed stack volume to favorite the fast heating of the product and therefore its water evaporation. This section proposes, based on a purely geometrical analysis, to compare the wood drying kinetics with respect to the thickness and the size. Table 2 shows the drying thermal parameters for different sizes and thickness of wood boards during the drying process in June. The results show that a decrease in the size of boards of the same thickness leads to limiting the useful energy for drying and to reducing the energy consumption ratio (kWh.m
3) of dried wood. It is also obvious that the higher the exchange surface of the wood exposed to the drying air flow, the shorter the drying time. As A increases, the water evaporation at the surface of the product is rapid which reduces drying time and improves the thermal efficiency of the drying system. The results show that for a given volume of wood, it is possible to modify the boards size arranged in the stack to decrease or increase the drying time of the same amount of wood without varying the porosity. In practice, the wood stack covered with a number Nb of given boards, the hot flow at each board is influenced by the other Nb-1. Thus, a decrease in the size of the wood boards in the stack results in an increase of the number of wood boards in the solar kiln and therefore an increase of the surface exchange rate between air and wood, hence reduced drying time. On the other hand, an increase in the thickness of the boards causes the opposite effect. It has been shown that the drying time increases as the thickness increases. These results are in good agreement with those obtained on food products during convective drying at different sizes and forms [58].

793

between 307 and 314 K. It is clear that this temperature is higher than the maximum ambient air temperature (see Table C1 in Appendix C). The maximum drying temperatures recorded at the outlet of the solar collector are 341.5 K and 447.5 K for the baseline and Case A respectively. In Fig. 11, it is evident that the temperatures harvested using the heat regeneration process are higher than those without the use of the CFAH exchanger. These results indicate that the use of a CFAH with an efficiency of 0.5 has the effect of reducing the drying time up to 29% compared to the drying method without regeneration process. In Table 3, the drying parameters for Case A and B are indicated. The results show that the higher the efficiency of the exchanger, the higher the temperature at the outlet of the solar collector and more energy was also diffused into the wood stack. Based on these results, the incorporation of a recuperative heat exchanger in solar kiln is more efficient than a simple drying process. It has shown that all thermodynamic parameters are significantly improved for Case B than Case A. For Case B, the drying process has been achieved after 8.62 and 34 days of drying, a reduction of 57.4 and 62.4%, in June and December respectively. The results also showed that the energy consumption ratio is considerably reduced for Case B than Case A. Energy saving is defined as the difference between the consumed energy (Fans consumption þ Useful Energy) with and without the use of the CFAH. The maximum energy saving was 1202 and 2763 kWh, in June and December, respectively. By incorporating a Closed Feed Air Heater with an effectiveness of 0.75, in the solar dryer, the collector and drying efficiency values are increased from 0.48 to 0.56 and

5.3. Effect of regeneration heat on thermal drying performance In the previous sections, it has been shown that the efficiency of the solar dryer without using energy saving processes does not exceed 0.43, in June (hottest month). Heat storage, the subject of which will be discussed in the next paragraph, may reduce drying time but not energy. The reduction of the energy bill involves the use of methods making it possible to improve the energy efficiency of heaters. This section of paper is intended to analyze the effect of recovering some of the heat from the humid air leaving the dryer using a Closed Feed Air Heating system on drying parameters for two effectiveness: hreg ¼ 0.5 (Case A) and hreg ¼ 0.75 (Case B). Fig. 11 shows the hourly evolution in temperature and moisture during the wood drying process using a CFAH with an efficiency of 0.5, in June. The outlet air temperature at the CFAH fluctuates

Fig. 11. Variation of moisture content and temperature against drying time. Influence of the heat regeneration process.

Table 2 Energy and thermal drying parameters of the present drying system. Influence of the size and the boards thickness.

Wood volume (m3) Boards Dimensions (m) Number of boards Nb Exchange surface rate (m2.m
3) Useful energy (kWh) Fans consumption (kWh) Drying time (days) Energy consumption ratio (kWh.m3)

Size 1

Size 2

Size 3

Size 4

Size 5

10 1  0.2  0.025 1998 92 2195 162 20.25 235.8

10 0.5  0.2  0.025 3996 94 2146 157 19.58 230

10 2  0.2  0.025 999 91 2207 163 20.33 237

10 1  0.2  0.05 999 52 3185 234 29.3 342

10 1  0.2  0.1 500 32 6010 440 55 645

794

A. Khouya / Renewable Energy 155 (2020) 783e799

Table 3 Energy and thermal drying parameters of the present drying system. Influence of the heat exchanger efficiency. Drying parameters

Maximum drying temperature (K) Fans consumption (kWh) Useful energy (kWh) Energy saving (kWh) Drying time (days) Drying efficiency (
) Reduced drying time (days) Collector efficiency (
) Energy consumption ratio (kWh.m
3)

June

December þ January

Case A Case B

Case A

Case B

347 115 1651 591 14.37 0.58 5.88 0.52 176.6

321 434 2980 1764 54.2 0.3 36.38 0.6 341.4

328 272 2143 2763 34 0.43 56.58 0.72 241.5

359 69 1086 1202 8.62 0.88 11.63 0.56 115.5

0.43 to 0.88, in June, respectively. The collector and drying efficiency values also increased from 0.52 to 0.72 and 0.23 to 0.43, in December, respectively. These results are extremely important. It can be concluded that drying could be achieved with a lower energy consumption and a relatively short time using the process of heat regeneration between the solar collector and the drying chamber, all months considered. 5.4. Effect of thermal storage on drying rate In this paragraph, the thermal performance of the solar kiln coupled to a latent heat energy storage system is presented and discussed. Two phase change materials RT58 and RT64 were tested using the calculation algorithm inspired from a previous research work [34]. The thermal properties of these PCM are presented in Table C.2 (Appendix C). It should be noted that the thermal properties of the two materials tested are the same except for the melting temperature and the latent heat of solidification. RT64 and RT59 pass from solid phase to liquid phase at temperatures of 337 and 331 K, respectively. Moreover, the latent heat of fusion of RT64 is 230 kJ kg
1, whereas for RT58, it is equal to 170 kJ kg
1. The choice of these two materials is motivated by the fact that the outlet fluid temperature at the parabolic solar collector could be reached 353 K during the four seasons of the year as the collector size and the mass flow rate are taken into account in the calculation model [14,34]. Fig. 12 shows the effect of the integration of the thermal storage unit in the solar kiln on the drying rate during the wood drying process, in June. The simulations results are performed using RT64 as PCM. The maximum drying air temperature recorded at inlet of the solar kiln was about 343 and 341.5 K with and without thermal storage, respectively. The minimum drying air temperature recorded inside the solar kiln was about 322 and 296 K with and without thermal storage, respectively. The results show that drying process using RT64 as PCM allows a reduction of 40%, in terms of drying time, in June. The design parameters of the thermal storage unit to meet the drying demand in June and December are also shown in Table 4. From these results, it is clear that the month of June has the shortest drying time and that December is the longest. These results can be explained by the climatic conditions that are different from one month to another. The storage period is too long in December and small in June indicating that the size of the thermal storage system is bigger and more added costs are required to ensure the drying during winter. In fact, in June, the number of tube required for the sizing of the thermal storage unit is about 36 with a distance of 0.21 m between two adjacent tubes. The storage tank has a diameter of 1.75 m and a volume of 7.27 m3. The mass and the volume of PCM are 1928 kg and 2.19 m3 respectively. It is also noticed that the thermal storage size is slightly larger for RT58 than RT64. This difference is due to the thermal properties such as latent heat and melting temperature which are slightly higher for RT64 than RT58.

Fig. 12. Variation of moisture content and temperature against drying time in June. Effect of thermal storage.

It should be noted that the drying process with storage is not achieved before the end of December, but in January because it is a colder month where solar intensity is lower. The most important result is that the drying time in December was reduced from 90.58 to 36.5 days using RT64 as PCM (a reduction of 60%). It can be concluded from these results that hardwood drying can be achieved with a relatively short time thanks to the integration of a thermal storage unit in the solar kiln. In addition, the judicious choice of the appropriate PCM must be made on the basis of several criteria that may come into play, such as wood species, sunshine, melting temperature, heat of fusion, etc [34].

5.5. Effect of the use of thermal storage and heat regeneration on thermal drying performance The drying industry in general is an energy-intensive sector because the elimination of water from a given product requires a lot of heat. The use of conventional dryers is very useful, however the costs related to the production of dry matter are relatively very high. To achieve this, it is essential to promote the development of new energy efficiency tools while ignoring outdated practices that are contributing to increase energy consumption. In order to improve markedly the thermal performances of the present solar dryer, special attention has been paid in this section to the behavior of the drying system when it is integrated with a thermal storage Table 4 Energy and thermal drying parameters of the studied drying system. Influence of thermal storage. Drying parameters

Daily storage time (hr) Masse of PCM (kg) PCM volume PCM (m3) Tank volume (m3) Tank diameter (m) Number of tube Tube pitch (m) Drying time (days) Reduced drying time (days)

December þ January

June RT64

RT58

RT64

RT58

11 1928 2.19 4.09 1.31 36 0.21 12 8.25

11 2924 3 4.46 1.21 94 0.11 15.88 4.37

14 3877 4.4 9.05 1.96 42 0.28 36.5 54.08

14 5934 6.13 7.33 1.76 101 0.16 43.56 47

A. Khouya / Renewable Energy 155 (2020) 783e799

tank and a heat exchanger installed before the solar collector enters. Table 5 shows the drying parameters values when the thermal storage unit and a Closed Feed Air Heater are integrated to the solar kiln. All parameters shown here for this strategy of drying are calculated for an exchanger efficiency of 0.5 and using RT64 as PCM. The beneficial effect of the combined use of thermal storage and heat regeneration system on drying rate was obvious especially for the case of drying in December (Fig. 13). In fact, for all situations and cases studied in Section 5.1 to 5.4, of this work, the drying has never been completed before the end of December (Tables 1e4). The results show that the thermal regeneration process alone allows a reduction of 40% in terms of drying time compared to the baseline case. The combined use of heat regeneration and thermal storage at the same time can reduce drying time from 90.58 to 15 days, a reduction of 75.58 days in December. The drying method is also efficient to reduce the energy consumption ratio (kWh.m
3) by 50 and 54%, in June and December, respectively. It has been found that the drying time and energy required to reduce the moisture content of 300 boards with a dimensions of 0.4  0.025  0.025 m from an initial moisture content of 1.145 kg water/kg dry matter to a final moisture content of 0.11 kg water/kg. kg of dry matter varies from 4 to 6 days and from 230 to 341 kWh using a conventional dryer, respectively [22]. This proposed solution effectively improved the drying efficiency by 51 and 91% in June and December, respectively. These results could confirm that the present dryer model offers the best thermal performance in terms of efficiency, energy consumption ratio and drying time. 5.6. Comparison of the thermal performance of the present solar dryer with other existing models In order to analyze the thermal performance of the present solar dryer, the results obtained using the present model are compared with those of four wood dryers indicated in the literature review (two solar kilns and two conventional dryers) [11,17,23,57]. The names of the sites hosting the mentioned dryers are also given to get an idea of the climatic and geographical conditions in which the drying was carried out. Table 6 summarizes the thermal performances comparison results between these five dryers. First, the results indicate that wood species dry faster in a conventional dryer than in a solar dryer when the drying air temperature is high. In fact, for the Bio-Bio model, it takes less than a day to dry 100 m3 of Pine radiata at a constant temperature of 393 K and an air speed of 8 m s
1. Using the Tangier dryer model, the required drying time to reach the moisture content of 0.12 kg water/kg dry matter for a 10 m3 hardwood stack (Beech) is only 4 days when the heat recovery and thermal storage are combined in the system. The thermal performance of the Tangier dryer is significantly higher than that of the conventional Belgrade dryer operating at drying air temperatures varies from 310 to 335 K. The results show that the thermal energy consumption to produce a cubic meter of wood in a conventional and solar way varies from 988 to 1380 kWh, 887 to 1728 kWh and 168 to 1237 kWh for the dryer of Chile, Belgrade and Tangier, respectively. The results also show that the Tangier solar dryer has the best thermal performance compared to the solar kilns indicated in Table 5 Energy and thermal parameters of the present drying system. Effect of thermal storage and heat regeneration.

Drying time (days) Reduced drying time (days) Drying efficiency (
) Energy consumption ratio (kWh.m
3) Reduced energy consumption ratio (%)

June

December

4 16.25 0.65 117 50

15 75.58 0.44 237 54

795

Fig. 13. Variation in moisture content against time during the drying process in December, January and February. Effect of thermal storage and heat regeneration.

Table 6. This is the result of the integration of the latent storage and heat recovery unit in the current solar dryer. In fact, the drying time of 4 days required to dry 10 m3 of wood stack from an initial moisture content of 0.4 kg water/kg dry matter at that of equilibrium of 0.12 kg water/kg dry matter was never recorded for the two solar dryers presented for comparison. The results show that the drying time required to reduce the Eucalyptus moisture content from 0.53 to 0.15 kg water/kg dry matter for the Brisbane model was 38 days. The drying time also varies from 20 to 24 days for the Cameroon Model using a wood stack of Iroko with an initial and final moisture content of 0.4 and 0.1 kg water/kg dry matter, respectively. The difference between these three dryers lies in the quality of the heat production devices to which are added more or less favorable weather conditions for drying in the case of the Tangier design. The Cameroonian model solar dryer does not use thermal storage, but the drying temperatures are almost identical to those of the Tangier model. However, the maximum collector efficiency of the Tangier model (0.72) was much higher than that of the Cameroon Model (0.36) and Brisbane kiln (0.53). This improvement in the collector efficiency is explained by the triple circulation of air in the solar collector of the Tangier model. Regarding the energy consumed during drying, there is a big difference in producing a cubic meter of dried wood between the Brisbane model (182.5 kWh.m
3) and the Tangier one (168 kWh.m
3), especially when the latter design combines thermal storage and heat regeneration process. On the other hand, the electrical energy consumption ratio of the fans ranges from 3.2 to 72.4 kWh.m
3 for the Tangier kiln and is estimated to be 10 kWh.m
3 for the Brisbane model. The electrical consumption of the fans also varies from 63 to 438 kWh.m
3, for the Bio-Bio Model, which is a conventional dryer. The overall results show that the present model could offer the best thermal performance compared to the other dryers presented for comparison in terms of thermal efficiency, energy consumption and drying time. 6. Conclusions and future work The investigation conducted in this work concerns the contribution to the improvement of energy efficiency and the modeling of a solar wood dryer with latent storage and regeneration heat

796

A. Khouya / Renewable Energy 155 (2020) 783e799

Table 6 Comparison of the thermal performance of different solar dryers. Drying characteristics Energy source Wood species Wood stack volume (m3) Drying air temperature (K) Air velocity (m.s
1) Xi (kg water/kg dry matter) Xfi (kg water/kg dry matter) Drying time (days) Fans consumption(kWh.m
3) Collector efficiency (%) Energy consumption ratio (kWh.m
3)

PRESENT STUDY (TANGIER)

PREVIOUS WORK [57] (BRISBANE)

PREVIOUS WORK  [17] (YAOUNDE)

PREVIOUS WORK [23] (BELGRADE)

PREVIOUS WORK [11] (BIO BIO)

Solar Beech 10 283 to 360 1 to 5 0.4 0.12 4 to 90.58 3.2 to 72.4

Solar Eucalyptus 10 293 to 323 0.5 0.53 0.15 38 10

Solar Iroko 2.156 293 to 360 1.4 0.4 0.1 20 to 24 e

Conventional Beech 0.8 310 to 335 e 0.79 to 0.99 0.074 to 0.088 11 to 16 e

Conventional Radiata pine 100 358 to 393 5.6e8 1.45 0.1 less a day 63 to 438

0.48 to 0.72 117 to 517

0.19 to 0.53 182.5

0.29 to 0.36 e

e 988 to 1380

e 887 to 1728

process. This study was carried out by establishing thermal and mass balances on the various components of the solar collector, the drying chamber and the CFAH. The governing equations of heat and mass transfer were solved using the Runge Kutta 4th order method for the solar collector and the implicit finite difference method for the drying model. The numerical simulations are conducted under Tangier climate. The discrepancies between the experimental and numerical results did not exceed 5% for the solar collector and drying model, which corresponds to a very satisfactory agreement, which revealed the validity of the numerical results obtained under the conditions considered. The results showed that the drying time is shorter in June and longer in December. The drying and collector efficiency were 0.43, 0.23, 0.48 and 0.52, in June and December, respectively. The energy consumption ratios of dried wood were 235.8 and 517.8 kWh.m
3, in June and December, respectively. The effect of certain drying parameters on drying kinetics was studied and the results showed that the drying time decreased as the solar collector area increased and the wind speed decreased. The dying time also increased as the boards thickness increased. In order to improve the thermal drying performance, a Closed Feed Air Heater has been incorporated into the system to recover all or part of the energy normally released into the atmosphere by the solar kiln. The use of a Closed Feed Air Heater with an effectiveness of 0.75, has the effect of reducing the drying time up to 57.4 and 62.4%, in June and December, respectively. The integration of the thermal storage unit can be reduced the drying time to around 40%, in June. Moreover, the combined use of the thermal storage and the CFAH in the drying system has the effect of reducing the energy consumption ratio by about 50% and 54%, in June and December, respectively. This proposed solution can be significantly improved the thermal drying performance and thus overcoming the problem of longer drying time during the winter. Comparing the results of this dryer model with those of other solar and conventional kilns has shown that the present model could offer the best thermal performance in terms of thermal efficiency, energy consumption and drying time. The thermal drying parameters have been improved thanks to the regeneration process, since the heat of the humid air which is normally released into the environment is now used to preheat the air entering the thermal storage and the solar collector. This, in turn, decreases the amount of heat required for the drying process. Future work: It should be noted that the wood drying is an unavoidable step to bring to the market products with higher added value. Achieving drying under certain economic conditions requires calculating the drying cost with great precision while

minimizing energy consumption. Conventional dryers with very high heating power are useful, but their energy consumption and greenhouse gas emissions remain too high. Therefore, future work will investigate the effect of the combined use of the water-water heat pump, the parabolic trough concentrated solar collector and energy storage on thermal parameters and economic performance during the wood drying process. A drying model that taken into account the life cycle energy, thermal losses and financial analysis during the solar dryer life will also be proposed and discussed. Declaration of competing interest The author declares that he has no competing financial interests or known personal relationships which could have seemed to influence the work reported in this document.

CRediT authorship contribution statement Ahmed Khouya: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Writing - original draft, Project administration, Resources, Validation, Visualization, Writing - review & editing.

Appendix A. Triple pass solar collector. Triple pass solar collector parameters used to establish the simulation results:

Table A.1 Specifications of the solar air collector. Parameters Length (m) Larger (m) Depth (m) Absorptivity of the glass cover (
) Transmissivity of the glass cover (
) Absorptivity of the absorber plate (
) Emissivity of the absorber (
) Emissivity of the glass cover (
) Emissivity of insulation face (
)

Values 6 4 0.15 0.05 0.9 0.9 0.95 0.88 0.9

A. Khouya / Renewable Energy 155 (2020) 783e799

Appendix B Discretized heat and mass transfer equations of the drying model: Drying is a relatively complex phenomenon because of the large number of parameters involved is due to the entanglement of heat and mass exchanges. To solve the equations of the drying model, it is necessary to resort to the spatial discretization of the drying chamber. Each board is spotted by a position from the entrance of the stack in the direction of the mass flow (Fig. B.1). The time step is Dt. The water contents and the temperatures at time (k) in a position j are calculated from those in the positions j-1, j and j þ 1 at the previous time k-1. The discretized equations are given below.

797

The specific heat of the wood during the drying process was calculated using equation (B.4) or (B.5) as a function of the moisture content X and other drying parameters such as the specific heat of oven dry wood Cp,0 (equation (B.6)) and the specific heat of water Cp,w (equation (B.7)) [14]:

Cp;b ¼

  Cp;0 þ X:Cp;w if X > Xpsf 1þX

(B.4)

Cp;0 þ Cp;w 1þX     þ X
0:06191 þ 2:3610
4 q
1:33 10
2 X if X  Xpsf

Cp;b ¼

(B.5) Mass balance for air drying The mass conservation on a drying air segment adjacent to the board (Fig. B.1) allows to calculate the absolute humidity Wjþ1 at time tk using equation (B.1) as [34]:


 dXj ðtk Þ m_ a : Wjþ1 ðtk Þ
Wj ðtk Þ ¼
M0 : dt

(B.1)



Cp;0 ¼ 0:1031 þ 0:003867 q

kJ:kg
1 :K
1



(B.6)

Cp;w ¼ 10
3     kJ:kg
1 :K
1  5543:35
8425:29:10
3 q þ 0:01305q2 (B.7) The specific latent heat DH was calculated using equation (B.8) as [47]:



DH ¼ 4186:8 ð597
0:56 qÞ J:kg
1



(B.8)

The specific heat of the drying air was computed by equation (B.9) as [47]:

Cp;a ¼ 10
3  ð1835
0:734 ðTa
273:15Þ Þ



kJ:kg
1 :K
1



(B.9)

The air velocity around the stack is computed using equation (B.10) as a function of the vacuum rate Po, the mass flow ratem_ a and the section Sdr of the dryer [47]:

Fig. B.1. Temperature and mass rate balance on drying air and wood boards.

Ua ¼

m_ a

(B.10)

ra Sdr Po

The air viscosity is computed using equation (B.11) as [47]: Energy balance for air drying A heat balance on the air before and after heat and mass exchange with the jth segment makes it possible to calculate Ta,jþ1 (tk) [33]:

 ma Cp;a

Ua : Ta;j ðtk Þ m_ a : Cp;a Ta;jþ1 ðtk Þ
Ta;j ðtk Þ þ eb  

Ta;j
1 ðtk Þ þ hht Sb Ta;j ðtk Þ
qj ðtk Þ ¼ 0

na ¼ 1; 66:105



 Ta þ 273; 18 0;756 273; 18

(B.11)

Appendix C

(B.2) Maximum and minimum values of some climatic conditions during the months of January, February, June and December for the Tangier city:

Energy balance for wood A heat balance on the jth wood segment as shown in Fig. B1, allows to calculate the board temperature q,j (tk) using equation (B.3) as [33]:

 
mb :Cp;b
qj ðtk Þ
qj ðtk
1 Þ ¼ hht Sb Ta;j ðtk Þ
qj ðtk Þ Dt dXj ðtk Þ
M0 :DH: dt

(B.3)

Table C.1 Weather conditions for different months.

Tmax (K) Tmin (K) RHmax (%) RHmin (%) Imax (W.m
2)

January

February

June

December

290 277 96 35 515

296 283 95 43 560

310 298 75 47 1050

292 280 96 54 500

798

A. Khouya / Renewable Energy 155 (2020) 783e799

Thermal storage parameters used to establish the simulation results: [23] [24]

Table C.2 Properties of the studied phase change materials [34,59]. Properties Melting point (K) Specific latent heat of fusion (kJ.kg
1) Density (liquid phase) (kg.m
3) Density (solid phase) (kg.m
3) Specific heat capacity (solid) (kJ.kg
1.K
1) Specific heat capacity (liquid) (kJ.kg
1.K
1) Thermal conductivity (solid/liquid) (W.m
1.K
1)

RT64

RT58

337 240 780 880 2 2 0.2

327e331 170 760 900 1.8 2.4 0.2

[25] [26]

[27]

[28]

[29]

References [30] [1] K.J. Chua, S.K. Chou, Low-cost drying methods for developing countries, Trends Food Sci. Technol. 14 (2003) 519e528. [2] S. Johnsson, E. Andersson, P. Thollander, M. Karlsson, Energy savings and greenhouse gas mitigation potential in the Swedish wood industry, Energy 187 (2019) 115919, https://doi.org/10.1016/j.energy.2019.115919. [3] J. Theresia, G.A. Widyadana, D. Wahjudi, Optimal kiln dry allocation for dry timber preparation to minimize cost, Jurnal Teknik Industri (24) (2019) 43e48, https://doi.org/10.9744/jti.21.1.43-48. [4] M.A. Karim, M.N.A. Hawlader, Development of solar air collectors for drying applications, Energy Convers. Manag. 45 (2004) 329e344. [5] L. Bennamoun, A. Belhamri, Design and simulation of a solar dryer for agriculture products, J. Food Eng. 59 (2003) 259e266. [6] L.B.Y. Aldabbagh, F. Egelioglu, M. Ilkan, Single and double pass solar air heaters with wire mesh as packing bed, Energy 35 (2010) 3783e3787. [7] B.M. Ramani, A. Gupta, R. Kumar, Performance of a double pass solar air collector, Sol. Energy 84 (2010) 1929e1937, https://doi.org/10.1016/ j.solener.2010.07.007. [8] K. Aoues, N. Moummi, M. Zellouf, A. Benchabane, Thermal performance improvement of solar air flat plate collector: a theoretical analysis and an experimental study in Biskra, Algeria, Int. J. Ambient Energy (32) (2011) 95e102, https://doi.org/10.1080/01430750.2011.584469. [9] C.A. Komolafea, L.O. Oluwaleyeb, O. Awogbemib, C.O. Osuekea, Experimental investigation and thermal analysis of solar air heater having rectangular rib roughness on the absorber plate, Case Stud. Ther. Eng. 14 (2019) 100442, https://doi.org/10.1016/j.csite.2019.100442. [10] W. Gao, W. Lin, T. Liu, C. Xia, Analytical and experimental studies on the thermal performance of cross-corrugated and flat-plate solar air heaters, Appl. Energy (84) (2007) 425e441. [11] R.A. Ananias, E. Mougel, A. Zoulalian, Introducing an overall mass-transfer coefficient for prediction of drying curves at low-temperature drying rates, Wood Sci. Technol. 43 (2009) 43e56, https://doi.org/10.1007/s00226-0080216-3. [12] R.A. Ananias, B. William, A. Mara, S. Carlos, B.K. Roger, Using an overall mass transfer coefficient for prediction of drying of Chilean coigüe, Wood Fiber Sci. 41 (2009) 426e432. [13] R.A. Ananias, P. Perez, C. Salinas, D. Elustondo, Drying schedules for canelo wood, Dry. Technol. 31 (2013) 282e285. [14] B. Lamrani, A. Khouya, A. Draoui, Energy and environmental analysis of an indirect hybrid solar dryer of wood using TRNSYS software, Sol. Energy 183 (2019) 132e145, https://doi.org/10.1016/j.solener.2019.03.014. [15] D.M. Elustondo, L. Oliveira, Model to assess energy consumption in industrial lumber kilns, Maderas Cienc. Tecnol. (11) (2009) 33e46. onard, A. Zoulalian, [16] S.T. Merlin, Z.B. Bonoma, L. Bennamoun, L. Monkam, A. Le Modeling of coupled heat and mass transfer during drying of ebony wood using indirect natural convection solar dryer, Dry. Technol. (37) (2019) 1863e1878, https://doi.org/10.1080/07373937.2018.1544144. , R. Romain, R. Yann, Mathematical modelling and nu[17] S.T. Merlin, Z. Andre merical simulation of a simple solar dryer for tropical wood using a collector, Appl. Therm. Eng. 131 (2018) 356e369. termination des courbes caracte ristiques de se chage [18] A. Khouya, A. Draoui, De ces de bois, Revue des Energies Renouvelables 12 (2009) 87e98. de trois espe chage solaire du bois : Mode lisation simplifie e du se chage [19] N. Bekkioui, Se choir solaire a  parois vitre es, the se de doctorat, d’une pile de bois dans un se  Mohammed, vol. 5, Faculte  des Sciences, Rabat, 2009. in: Universite , R. Romain, R. Yann, B. Beguide , Modeling and simulation [20] S.T. Merlin, Z. Andre of an industrial indirect solar dryer for Iroko wood (Chlorophora excelsa) in a tropical environment, Maderas Cienc. Tecnol. 13 (2017) 29e40. [21] K. Phonetip, B. Ozarska, B. Belleville, G. I Brodie, Comparing two intermittent drying schedules for timber drying quality, Dry. Technol. (37) (2019) 186e197, https://doi.org/10.1080/07373937.2018.1445638. [22] A. Berrocal, R. Moya, B. Bond, M. Rodriguez-Solis, F. Munoz, D. Perez, Schedule

[31]

[32]

[33] [34]

[35] [36]

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48] [49] [50]

[51]

[52]

modification of drying rate to decrease the drying time of Juvenile Tectona Grandis L, Wood, Wood Fiber Sci. 49 (2017) 373e385.  Gorisek, Energy consumption of beech G. Mili c, B. Kolin, N. Todorovi c, Z. timber drying in oscillation climates, Drv. Ind. 65 (2014) 309e314. R.A. Ananias, J. Ulloa, D.M. Elustondo, C. Salinas, P. Rebolledo, C. Fuentes, Energy consumption in industrial drying of radiata pine, Dry. Technol. (30) (2012) 774e779, https://doi.org/10.1080/07373937.2012.663029. D. Luna, J.P. Nadeau, Y. Jannot, Solar timber kilns: state of the art and foreseeable developments, Renew. Sustain. Energy Rev. 13 (2009) 1446e1455. K. Phonetip, G.I. Brodie, B. Ozarska, B. Belleville, Drying timber in a solar kiln using an intermittent drying schedule of conventional laboratory kiln, Dry. Technol. (37) (2019) 1300e1312, https://doi.org/10.1080/ 07373937.2018.1496337. K. Phonetip, B. Ozarska, G. Harris, B. Belleville, G.I. Brodie, Quality assessment of the drying process for Eucalyptus delegatensis timber using greenhouse solar, Euro. J. Wood Wood Prod. 77 (2019) 57e62, https://doi.org/10.1007/ s00107-018-1364-2. H.S.F. Awadalla, A.F. El-Dib, M.A. Mohamad, M. Reuss, H.M. S, Hussein, Mathematical modelling and experimental verification of wood drying process, Energy Convers. Manag. 45 (2004) 197e207. € Ünsal, H.V. Go € rgün, E. Avci, Energy efficiency in lumber drying H. Korkmaz, O. -sample of drying red pine (Pinus Brutia) using solar energy, J. Polytech. (2019), https://doi.org/10.2339/politeknik.528103. A.A. Al-Abidi, S.B. Mat, K. Sopian, M.Y. Sulaiman, C.H.L. Abdulrahman, Review of the thermal energy storage for air conditioning systems, Renew. Sustain. Energy Rev. 16 (2012) 5802e5819. G. Alva, L. Liu, X. Huang, G. Fang, Thermal energy storage materials and systems for solar energy applications, Renew. Sustain. Energy Rev. 68 (2017) 693e706. M.R.I. Ramadan, A.A. El-Sebaii, S. Aboul-Enein, E. El-Bialy, Thermal performance of a packed bed double-pass solar air heater, Energy (32) (2007) 1524e1535. D. Luna, J.P. Nadeau, Y. Jannot, Model and simulation of a solar kiln with energy storage, Renew. Energy 35 (2010) 2533e2542. A. Khouya, A. Draoui, Computational drying model for solar kiln with latent heat energy storage: case studies of thermal application, Renew. Energy 130 (2019) 796e813, https://doi.org/10.1016/j.renene.2018.06.090. H. Mzad, K. Bey, R. Khelif, Investigative study of the thermal performance of a trial solar air heater, Case Stud. Ther. Eng. 13 (2019) 100373. M.W. Kareem, K. Habib, S.A. Sulaimon, Comparative study of single pass collector and double pass solar collector filled with porous media, Asian J. Sci. Res. 6 (2013) 445e455. S. Chand, P. Chand, Performance evaluation of solar air heater equipped with louvered fins, Int. J. Heat Technol. 36 (2018) 741e751. H. Vettrivel, P. Mathiazhagan, Comparison study of solar flat plate collector with single and double glazing systems, Int. J. Renew. Energy Resour. 7 (2017) 266e274. lez, S.F. Larsen, A. Hern S.M. Gonza andez, G. Lesino, Thermal evaluation and modeling of a double-pass solar collector for air heating, Energy Procedia 57 (2014) 2275e2284. L. Evangelisti, R. De Lieto Vollaro, F. Asdrubali, Latest advances on solar thermal collectors: a comprehensive review, Renew. Sustain. Energy Rev. 114 (2019) 109318. W. Baig, H.M. Ali, An experimental investigation of performance of a double pass solar air heater with foam aluminum thermal storage medium, Case Stud. Ther. Eng. 14 (2019) 100440. A.A. El-Sebaiia, S. Aboul-Eneina, M.R.I. Ramadana, S.M. Shalaby, B.M. Moharram, Thermal performance investigation of double pass-finned plate solar air heater, Appl. Energy 88 (2011) 1727e1739. A. El Khadraoui, S. Bouadila, S. Kooli, A. Guizani, A. Farhat, Solar air heater with phase change material: an energy analysis and a comparative study, Appl. Therm. Eng. 107 (2016) 1057e1064. K. Sopian, H.T. Liu, S. Kakac, T.N. Veziroglu, Performance of a double pass photovoltaic thermal solar collector suitable for solar drying systems, Energy Convers. Manag. 41 (2000) 353e365. D. Jain, Modeling the system performance of multi-tray crop drying using an inclined multi-pass solar air heater with in-built thermal storage, J. Food Eng. 71 (2005) 44e54. S. Aboul-Enein, A.A. El-Sebaii, M.R.I. Ramadan, H.G. El-Gohary, Parametric study of a solar air heater with and without thermal storage for solar drying applications, Renew. Energy 21 (2000) 505e522. tudes expe rimentale et nume rique d’un procA. Khouya, Contribution aux e chage de bois, The se de doctorat, Universite  Abdelmalek Essaa ^di, essus de se  des Sciences et Technique de Tanger, 2008. Faculte M.U.H. Joardder, M. Mourshed, M.H. Masud, State of Bound Water: Measurement and Significance in Food Processing, Springer, 2018. lisation des structures en bois en environnement variable, S. Merakeb, Mode se de doctorat N vol. 43, Universite  de limoges, France, 2004. The A. Khouya, A. Draoui, Experimental and theoretical analysis of heat and moisture transfer during convective drying of wood, Int. J. Adv. Res. Eng. Technol. 5 (2014) 17e29. _ A. Kajalavi R. Baronas, F. Ivanauskasa, I. Juodeikiene, cius, Modelling of moisture movement in wood during outdoor storage, Nonlinear Anal. Model Contr. 6 (2001) 3e14. A.Y. Cengel, A. M Boles, Thermodynamic, an Engineering Approach, McGraw-

A. Khouya / Renewable Energy 155 (2020) 783e799 Hill Education, 2014, 8 editions. (Accessed 7 January 2014). [53] E.V. IZW, Application of Industrial Heat Pump, vol. 35, IEA Heat Pump Programme Annex, 2014. Final Report. [54] K. Seshachalam, V.A. Thottipalayam, V. Selvaraj, Drying of carrot slices in a triple pass solar dryer, Therm. Sci. 11 (2017) 389e398. [55] S.E. Zubriski, K.J. Dick, Measurement of the efficiency of evacuated tube solar collectors under various operating conditions, J. Green Build. 7 (2012) 114e130. kov [56] A. Konopka, J. Baranski, T. Hura a, I. Klement, The influence of high temperature wood drying conditions using air-steam mixture on its properties, Annals of Warsaw University of Life Sciences e SGGW, Forest. Wood Technol.

799

90 (2015) 107e114. [57] H. Mahmudul, Mathematical Modelling and Life-cycle Energy and Financial Analysis of Solar Kilns for Wood Drying, Thesis, Faculty of Engineering and Information Technologies, The University of Sydney, Australia, 2017. [58] K.H. Ouoba, F. Zougmore, H. Desmorieux, Effect of initial size and shape importance on masse transfer during convective drying, Food Nutr. Sci. 9 (2018) 1514e1524, https://doi.org/10.4236/fns.2018.912109. [59] D.N. Nkwetta, P.E. Vouillamoz, F. Haghighat, M. El-Mankibi, A. Moreau, A. Daoud, Impact of phase change materials types and positioning on hot water tank thermal performance: using measured water demand profile, Appl. Therm. Eng. 67 (2014) 460e468.