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Improvement of the solar drying process of sludge using thermal storage
Journal of Environmental Management 255 (2020) 109883
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Research article
Improvement of the solar drying process of sludge using thermal storage Rodrigo Poblete *, Osvaldo Painemal Universidad Cat� olica del Norte, Facultad de Ciencias del Mar, Escuela de Prevenci� on de Riesgos y Medioambiente, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal storage Sludge Solar drier Landfill leachate
Improvement of the drying process of sludge issuing from the coagulation/flocculation of landfill leachate was investigated using thermal storage systems, which consisted of a rock bed with air preheated by a solar air heater passing through the system. Sludge runs were dried inside the solar dryer with and without thermal storage, as well as outside the dryer. The values for the solar energy accumulated per mass of sludge required to obtain a stable sludge mass were 107.5 kJ/kg (with thermal storage), 240 kJ/kg (without thermal storage) and 580.5 kJ/kg, outside the dryer. The values for the energetic efficiency of the drying process were 38.13% and 16.45% for the processes with and without thermal storage, respectively. The net heat input to the storage system during the charging stage was 420 W and 120 W for the discharging stage, resulting in a global energy efficiency in the storage system of 0.28. The thermal efficiency of the solar drying with and without thermal storage system was 37.8% and 22.2%, respectively.
1. Introduction Landfill leachate is considered an important environmental concern due to its potentially toxic characteristics, which could produce harmful effects in the ecosystem (Li et al., 2016). In this context, complex treatments are required to depurate this kind of water, among them the coagulation/flocculation process. This treatment produces sludge, which can be applied as a soil stabilizer (Khandegar and Saroha, 2013). However, this residue must be examined to ensure its safe application, since some of the substances in the sludge may pose eco-toxicological hazards (Archer et al., 2017), making it necessary to consider them when planning their handling (Ying et al., 2012). Moreover, the use of ferric sludge as an iron source for applying a Fenton process in wastewater treatment (Bolobajev et al., 2014) was studied and the results were similar to those of the classic Fenton process. The huge amount of sludge produced in wastewater treatment plants needs to be removed and handled (Kacprzak et al., 2017) and if this sludge is not adequately disposed of, it will result in a source of envi ronmental pollution (Wang et al., 2019a, 2019b, 2019c). Sludge is a black and ill-smelling wet slurry which may contain pathogens and is therefore considered a biological hazardous waste. It can also contain high concentrations of heavy metals, which are toxic, non-degradable and tend to accumulate along the food chain (Zhang
et al., 2016). Raw sludge is usually discarded, incinerated or disposed of in landfills. However, the enormous amount of sludge produced makes � these options unacceptable from an environmental point of view (Cusido and Cremades, 2012). During the stabilisation process of landfill leachate sludge, humic substances and volatile matters are also pro duced (Yang et al., 2017). Dewatering, using a centrifuge, plate press, belt press or drying bed, is a common method to handle sludge (Spinosa et al., 2011). Dewatering involves the removal of water, which makes the sludge more solid in consistency. Drying does not change the chemical composition of the sludge but it enhances its calorific value. The application of the drying process expedites its subsequent incineration (Piippo et al., 2018). The drying or dehydration of sludge helps to reduce its volume and transport � costs (Swierczek et al., 2018). The sludge-drying kinetics depend on heat and mass transfers at the interface between the product and the air (Ali et al., 2016), solar radi ation, ambient temperature, relative humidity, wind velocity, initial moisture content of the materials to be dried and mass of the product per unit of exposed area (Kamble et al., 2013). The drying process is highly energy-demanding and is therefore usually expensive (Di Fraia et al., 2016). However, solar radiation is a free source of energy available for the drying process (Ali et al., 2016). Several companies dry their wet wastes in open solar systems, without a sunroof or any kind of control device. Under these conditions, the time
* Corresponding author. E-mail address: [email protected] (R. Poblete). https://doi.org/10.1016/j.jenvman.2019.109883 Received 15 June 2019; Received in revised form 18 October 2019; Accepted 16 November 2019 Available online 22 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
required to dry the sludge is very long, especially in winter or rainy seasons. This could be solved by using solar drying technology (Lakshmi et al., 2018), and the use of solar energy for sludge drying has proven to be an adequate solution to reduce processing costs (Di Fraia et al., 2018). For instance, an integrated solar system was studied for drying chamo mile using thermal storage and the result was a moisture reduction of 75% after a 33 h process (Amer et al., 2018). Solar energy is only accessible during the daytime, thus efficient thermal storage makes it possible to use the excess heat collected during the hours of sunshine during cloud cover days and at night (Al-Abidi et al., 2012). Due to the intermittent behaviour of solar energy, an integration of energy storage with solar collectors is useful to make the solar energy source more reliable (Kamble et al., 2013). In a previous study carried out by our research group, we evaluated drying landfill leachate-derived sludge using a solar still, observing that the sludge was successfully dried when peripheral systems (solar collector, solar air preheater and extractor) were used (Poblete and Painemal, 2018). Heat storage is useful to reduce the gap between energy supply and demand, and helps to improve the energy efficiency of solar systems. Thermal energy can be stored as latent (phase change) heat, sensible heat, thermochemical heat or a combination thereof (Khouya and Draoui, 2019). Sensible heat storage makes it possible to enhance the temperature of the heated air and the phase change material providing backup at night or during hours when solar energy is not available (Bhardwaj et al., 2017). The integration of thermal storage into a solar dryer is a positive solution for continuous solar drying (Lakshmi et al., 2018). An adequate configuration of the solar air heated allows maximum use to be made of the thermal power from the sun, thus meeting the requirements to dry the mass (Orbegoso et al., 2017). Natarajan et al. (2017) evaluated the use of aluminium filings, rock and sand beds as thermal storage materials, comparing them to drying processes without the use of such materials. The results showed that the use of thermal storage materials led to effective reductions in the drying time as well as in the moisture removal rate. No research work focused on the drying of landfill leachate sludge using solar systems supported by thermal storage was found. The aim of this research is to study the improvement of the solar drying process of landfill leachate sludge, using thermal storage to accumulate the solar energy required for use in the process.
a)
2. Materials and methods Runs were carried out in order to compare the evolution of the mass of sludge obtained from a coagulation/flocculation process as a pretreatment of landfill leachate prior to a photo-Fenton process. The runs for drying sludge were carried out: a) in the solar dryer without thermal storage; b) in the solar dryer with thermal storage; and c) outside the solar dryer. Fig. 1a shows the schematic diagram of the process, where the air is preheated before entering the solar dryer. After preheating the air, it is pumped to a thermal storage system (rock bed) made up of a tank containing approximately 60 kg of irregular spheroidal rocks. The air was driven across the rock bed and transported to the dryer using an air pump (55 W) and an air extractor (18 W) placed in the back wall of the dryer. The tank was made of 3-cm thick expanded polystyrene (width 40 cm, length 55 cm and height 40 cm), with a thermal conductivity of 0.038 W/(m*C) and an overall heat transfer coefficient of 30 W/m2 � C, working as thermal insulation. Fig. 1b shows details of the thermal storage tank and Fig. 1c depicts a photograph of the systems. The air mass flow rate (ma) was determined considering the air ve locity and the air temperature (Enibe, 2003)[19][19] (see Eq. (1)).
ρ ¼ 2:357
3; 7894*10 3 *TðKÞ
The air pump and the air extractor produced a forced convection of the air that enhanced the efficiency of the drying process. The equipment used, such as solar dryer, solar collector and solar air preheater, is described in Poblete and Painemal (2018). In brief, the solar dryer [18][18]has frontal and lateral walls, a 4-cm thick poly carbonate sunroof, 0.2 W/m � C thermal conductivity, 80% optical transmittance and an overall heat transfer coefficient of 3.8 W/m2 � C. On the wooden back wall, which is 3 m wide, 2 cm thick and 1.35 m high, an extractor was placed to remove the steam produced during the drying process. The air entering the solar dryer was preheated using a solar air heater located next to the solar still, with a mass flow of 0.025 kg/s. Air velocity and temperature were measured using a thermo-anemometer (model D8060048, Veto y Cia. Ltda.). The solar air preheater consisted of a wooden base (1.6 m2) with lateral wooden walls (10 cm high) and two glass covers (4 mm thick). Since the experiment was carried out during different days and under different solar irradiation conditions, the amount of incident solar irradiation received by the solar systems was measured and the cumu lative solar energy per unit of mass (Qrad) (kJ/kg) was calculated using
Air flow direction Rocks
Probe thermometer
Tank
Solar dryer
Thermal storage
c)
b)
(1)
Thermal insulation
Solar air heater
Rock bed
Fig. 1. a) Schematic diagram of drying system and thermal storage, b) photograph of thermal storage systems and c) photograph of the systems. 2
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
an adaptation of Malato et al. (2009)[20][20] (Eq. (2)): Qrad;n ¼ Qrad;n
Ad 1 þ Δtn *radg;n * ms
MR ¼
(2)
mw ⋅L � P þ radg;n ⋅Ad ⋅t
Q_ch ¼ ma *Cpa *ðTi;es Z
d d
*100
(6)
To;es Þ
(7) (8)
Q_ch dt
0
Similarly, the instantaneous (Q_dc ) and the net heat (Qdc ) in the storage system during the discharging time were calculated by Eqs. (9) and (10) (Rabha and Muthukumar, 2017): Q_dc ¼ ma *Cpa *ðTo;es Z Qdc ¼
t
Ti;es Þ
Q_dc dt
0
(9) (10)
The global energy efficiency of the storage system (nen,es) is calcu lated as the ratio between the net heat in the storage during the dis charging time and the net heat input to the storage system during the charging time, using Eq. (11) (Dincer and Rosen, 2011): nen;es ¼
Qdc Qch
(11)
The specific heat of the rocks is 0.81 kJ/kg*k, their density is 2245 kg/m3 and their thermal conductivity is 0.13 W/m*K. Thermal efficiency (Et) of the solar drying process was determined using Eq. (12): Et ¼
qu qu ma *Cpa *ΔT ¼ ¼ radg;n *Ad I radg;n *Ad
(12)
where qu is the thermal heat gain of the air (J), ΔT is the difference between the temperature of the air entering and exiting the solar dryer, ma is the mass flow rate of air (kg/s) and Cpa is the specific heat of the air (J/(kg� C)). 3. Results and discussion Fig. 2 shows the evolution of the mass of sludge subject to different drying conditions. It is possible to observe that the sludge dried inside the solar dryer achieves a stable mass faster than the sludge dried under natural conditions (outside the dryer), which is the usual industrial practice for drying sludge. Also, the stabilisation of the mass of sludge using thermal storage was obtained faster than without thermal storage. The solar energy accumulated per mass of sludge (Qrad) was 107.5 kJ/kg with thermal storage, 240 kJ/kg without thermal storage and 580.5 outside the dryer. This study agrees with Lakshmi et al. (2018), who observed re ductions in the drying time when using a solar dryer as compared to the drying process carried out in the open sun. The results obtained using thermal storage are better than those obtained in a previous work car ried out by our research group (Poblete and Painemal, 2018), where a stable mass (20 g of sludge) was obtained using an accumulated energy of 241.6 kJ/kg. Furthermore, it is possible to observe that the mass underwent an evolution along the run. In the case of the run carried out outside the dryer, the mass experienced a reduction during the day, when there is solar irradiation. However, although the drying process stopped at night (as expected), an increment in the mass of sludge was measured in the first hour of the next morning, possibly due to light rains occurring
(3)
where Mi and Mf are the initial and final sludge moistures (kg), respectively. The moisture (M) was calculated according to Prachaya warakorn et al. (2008) using: mt
t
Qch ¼
where L is the latent heat of water vaporisation (J/kg); P is the sum of the power (W) consumed by each equipment in the system over a set time t (h); and mw is the mass of water evaporated from the sludge (kg), calculated using Eq. (4): � mi * Mi Mf mw ¼ (4) 100 Mf
M¼
Me Me
where Me is the equilibrium moisture content. Also, the instantaneous (Q_ch ) and net heat input (Qch ) to the storage system during the charging process were determined according to the method described by Rabha and Muthukumar (2017), using Eqs. (7) and (8), respectively:
where Qrad,n and Qrad,n-1 are the global solar energy accumulated per mass of sludge (kJ/kg) at times n and n-1, respectively; Δtn is the experimental time of the sample (s); radg,n is the mean incident radiation on the solar still (W/m2); Ad is the illuminated area of the solar dryer (m2); and ms is the mass of the sludge (kg). The solar irradiation (radg,n, W/m2) was measured using a pyran ometer (CPM 10, Kipp & Zonen; 285-2800 nm wavelength and 7–14 μV/ W/m sensitivity). The pyranometer was tilted to 30� , i.e. the same angle as the latitude of the testing site (Coquimbo, Chile). For each run, the sludge to be dried (300 g of mass) was placed on an aluminium tray (7 cm high; 1000 cm2 surface area). The mass was chosen to make handling easier. For the runs inside the solar dryer (runs “a” and “b”), trays were placed over the fins in the heated base of the solar still. For the run performed outside the dryer, trays were placed over the soil next to the solar dryer. Each run was carried out in triplicate and lasted until a high degree of dryness was obtained and the mass of sludge was stable. Tray mass and temperature of the process were recorded. Density and volatile compounds in the sludge were determined before and after the drying process carried out using thermal storage, where the volatile solids in the sludge were measured heating the sludge at 550 � C for 4 h as per the Standard Methods for Examination of Water and Wastewater (McCrady, 2008). The temperature of the sludge, polycarbonate, sunroof, concrete base and soil above the tray was measured at the beginning of the runs and at every hour, using a HI 98501-1 thermometer (Hanna; 0.1 � C precision). The temperature of the air inside the solar still, the air coming from the solar air preheater as well as the air before (Ti,es) and after (To,es) entering the rock bed were recorded using a Campbell Scientific, Inc. 109-L34 thermocouple, (0.25 � C precision). Temperature and humidity of the air inside the solar still were recorded using a Campbell Scientific, Inc. CS215-L probe. The energetic efficiency of the drying process (ED), defined as the ratio between the energy in the evaporated water and the energy required to remove the moisture from the sludge, was calculated considering the electric and solar energy consumptions of the equipment (i.e. recirculation pump, air extractor and air pump) and using Eq. (3) (Singh et al., 2006). ED ¼
Mt Mi
(5)
where mt is the weight of wet sludge at instant t, and d is the mass of dry sludge. The moisture content of sludge was determined by the drying and weighing method in accordance with ISO-11465: 1999. The moisture ratio (MR) was determined using Eq. (6) (Shalaby et al., 2014):
3
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
300 With storage
Outside
Night
Night
Mass of sludge (g)
250
Without storage
200
150
100
50
0
0.0
0.4
0.9
1.5
1.6
1.8
1.9
2.0
2.0
2.4
2.6
2.8
Log Qrad Fig. 2. Evolution of mass of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
during the night. This nocturnal moisture reabsorption is the ratio be tween the overnight rise in the moisture of the mass and the moisture content during the day when the drying was carried out (El-Sebaii et al., 2002), resulting in a value of 2.35%. Nocturnal moisture reabsorption was not observed in the drying process carried out inside the solar dryer. With regard to the process carried out inside the dryer, during the first stage of the runs (from 0 to 34.6 kJ/kg of accumulated solar en ergy), the loss of mass was faster for the process without thermal storage than with storage since all the thermal energy coming from the solar air heater was transferred to the drying process (as shown in Fig. 1). However, at nightfall, when the air pump began to drive in cold air, the loss of sludge mass stopped. At night, with thermal storage, the dried
sludge lost mass because the air pump drove in air from the rock bed, which was saving thermal energy from the air heated by the solar air heater during solar radiation hours. A similar observation was made by Lakshmi et al. (2018), who reported a reduction in the moisture of black turmeric during night-time. The ED, calculated using Eq. (3), was 38.13% and 16.45% for the process carried out with and without thermal storage, respectively. The results obtained with thermal storage are better than those obtained by Chaouch et al. (2018), who reported an ED of 18.34% and 15.72% in different seasons of the year. Moreover, the results obtained are similar to those obtained by Abubakar et al. (2018), who observed that the drying efficiency with the storage material increased approximately
1.0
Moisture ratio (MR)
Night
0.8
Without storage
0.7
Outside
Night
With storage
0.9
0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.0
0.4
0.9
1.5
1.6
1.8
1.9
2.0
2.0
2.4
2.6
2.8
Log Qrad Fig. 3. Evolution of moisture ratio of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3). 4
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
13% in comparison with the efficiency without thermal storage. The moisture ratio, calculated using Eq. (6), was variable during the drying process (See Fig. 3). As in the case of the sludge mass, the evo lution of this parameter displayed different trends depending on the drying method used, resulting in a faster drop inside the solar dryer. Similar results were obtained by Rabha and Muthukumar (2017), who observed that the use of this kind of device makes it possible to reduce the moisture content faster. Moreover, the moisture ratio dropped more rapidly when thermal storage was used, due to the effective utilisation of the retained energy (Bhardwaj et al., 2019). These results of mass evolution are related to the temperature of the air. Fig. 4 shows the behaviour of the air temperature before and after the rock bed and the solar radiation. Fluctuations in the solar irradiation recorded along the process are caused by the cloud cover. When the power of the solar irradiation increases, the charging process of the storage thermal system starts up, evidenced by an increase in the temperature of the air both before and after the storage (rock bed), although in the latter case the increase is lower. Conversely, when the solar irradiation decreases, the heat saved in the thermal storage is discharged and the temperature of the air coming from the storage is higher than the temperature of the air before the storage (see Fig. 3). These results are similar to those observed by Lakshmi et al. (2018). The intersection point of the air temperature curves indicates the moment when the charging stopped and the discharging began (Rabha and Muthukumar, 2017). During the charging stage, the highest differ ence in the temperature of the air was 7 � C and, during the discharge stage, 8 � C. The net heat input to the storage system was calculated using Eq. (8) and it directly depends on the difference in the air temperature between the rock bed inlet and outlet. The value was 420 W. For the discharge stage (calculated by Eq. (10)) the value was 120 W. The global energy efficiency of the storage system was calculated using Eq. (11) and was 0.28. Fig. 5 shows the evolution of the temperature of the sludge subject to different drying processes. The temperature of the sludge dried outside the solar dryer was lower than the others, with a peak temperature of 29.5 � C. This occurs because inside this system, the heat is maintained for a longer time due to a greenhouse effect. The temperature of this sludge behaves similarly with solar radiation. Moreover, the solar irradiation affected the temperature of the sludge inside the dryer, i.e., when the solar irradiation dropped the flow
Solar irradiation
Before storage
After storage
Solar irradiation (W/m2)
900
45 40
800
35
700
30
600
25
500
20
400
15
300
10
200
5
100 0 6:20 12:35 18:50 1:05 7:20 13:35 19:50 2:05 8:20 14:35 20:50 3:05 9:20 15:35 21:50 4:05
Time of day (h)
Fig. 4. Evolution of solar irradiation and air temperature before and after storage system. 5
0
Air temperature (°C)
1000
of thermal energy decreased. Similar observations were reported by Dina et al. (2015). It is possible to observe that the temperature of the sludge dried using thermal storage was higher than that of the sludge dried without this process. This resulted in the trend obtained in the drying process for the runs shown in Fig. 2 and the reduction in the drying time (Kant et al., 2016; Qiu et al., 2016). Fig. 6 shows the evolution of the temperature of the heat base inside the solar dryer and the soil outside it during the runs. It is observed that these temperatures display the same trend as the temperatures of the sludge placed on this heat base or soil and that there is a heat transfer between the base and the material to be dried (Wang et al., 2017). Similar results were obtained by our research group in a previous study, where a direct relationship between the temperature of the sludge and the temperature of the heat base was observed (Poblete and Painemal, 2018). During the drying process, the temperature of the air inside the solar dryer was higher than the air outside (see. Fig. 7). This behaviour is explained by the greenhouse effect caused by the translucent envelope of the solar dryer. This envelope makes it possible to maintain the heat from the solar air heater and the heated base, as well as the heat pro duced when the thermal storage systems are working. The higher tem perature of the air inside the solar dryer vis-� a-vis outside makes it possible to improve the drying process and also to reduce the energy consumption required (Bahammou et al., 2019). The thermal efficiency of solar drying with and without thermal storage was calculated using Eq. (12). The results were 37.8% and 22.2%, respectively. These results agree with the findings reported by Natarajan et al. (2017), who obtained similar values for thermal effi ciency in the drying of food using thermal storage. They also agree with Baniasadi et al. (2017), who observed that the thermal efficiency of the solar dryer is enhanced using an energy storage system. Also, the results obtained in this research using thermal storage were higher than those reported by (Wang et al., 2019a, 2019b, 2019c), who obtained a 24.3% of thermal efficiency when drying sludge using a sandwich-like chamber bed. A research study by (Wang et al., 2019a) used rice husk and sawdust as conditioners to improve the efficiency and energy consumption of thermal sludge drying. Although this solution resulted in a reduction in the time and energy required in the process, it also increased the volume of dried sludge.
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
50
With storage
Outside
Night
40
T° of sludge (°C)
Without storage
Night
45
35 30 25 20 15 10 5 0
0.0
0.4
0.7
0.9
1.0
1.5
1.6
1.7
1.9
1.9
2.0
2.0
2.0
2.3
2.4
2.6
2.7
2.8
Log Qrad Fig. 5. Evolution of temperature of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
45 With storage
T° of base and soil (°C)
40
Outside
35 30 25 20 15 10 5 0
0.0
0.4
0.7
0.9
1.0
1.5
1.6
1.7
1.9
1.9
2.0
2.0
2.0
2.3
2.4
2.6
2.7
2.8
Log Qrad Fig. 6. Evolution of heat base temperature from the solar dryer with and without storage and soil outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
The density and volatile compounds in the sludge before the drying process carried out using thermal storage were 0.81 kg/L and 7.92 g/kg, respectively, and 1.2 kg/L and 7.89 g/kg after the drying process. Therefore, a slight loss in the volatile compounds was observed due to the drying process, which may have been due to the partial evaporation of a certain amount of ammonia or volatile fatty acids during the drying process (Pan et al., 2020; Deng et al., 2009). A more in-depth analysis of these losses will be approached in a future research project.
applied in the drying process. The use of thermal storage makes it possible to reduce the energy consumption required for drying sludge. Sludge dried inside the solar dryer reaches a stable mass faster than sludge dried under natural conditions, outside the dryer. The stabilisation of the mass of sludge was obtained faster with thermal storage than without it. The evolution in the mass of sludge was variable along the run. When the run was carried out outside the dryer, the mass experienced a reduction during the day, only to increase in the first hour of the next morning. During the night, a reduction in the mass of the sludge dried using thermal storage was obtained. The results of mass evolution are related to the temperature of the air.
4. Conclusions The use of solar thermal storage processes in the drying of sludge from landfill leachate treatment was evaluated. Although the drying process is highly energy-demanding, solar ra diation is a free available source of energy that could be efficiently 6
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883
60
T° of air (°C)
50 40 30 20 10
6:55
5:00
3:05
0:40
22:45
20:35
18:40
16:45
13:50
11:55
6:45
10:00
4:50
2:55
23:50
20:15
18:20
T inside
15:15
13:20
9:30
11:25
6:25
4:30
2:35
0:40
22:35
20:40
18:45
15:40
13:45
9:55
11:50
8:00
T outside 0
Time of day (h) Fig. 7. Evolution of air temperature inside and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
When the solar radiation decreased, a discharge of the heat storage occurred. This was evidenced by a higher temperature in the air coming from the storage in comparison with the temperature of the air before the storage. The thermal efficiency of the solar dryer is enhanced with the storage of heat.
Chaouch, W.B., Khellaf, A., Mediani, A., Slimani, M.E.A., Loumani, A., Hamid, A., 2018. Experimental investigation of an active direct and indirect solar dryer with sensible heat storage for camel meat drying in Saharan environment. Sol. Energy. https://doi. org/10.1016/j.solener.2018.09.037. Cusid� o, J.A., Cremades, L.V., 2012. Atomized sludges via spray-drying at low temperatures: an alternative to conventional wastewater treatment plants. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2012.03.053. Deng, W.Y., Yan, J.H., Li, X.D., Wang, F., Zhu, X.W., Lu, S.Y., Cen, K.F., 2009. Emission characteristics of volatile compounds during sludges drying process. J. Hazard Mater. https://doi.org/10.1016/j.jhazmat.2008.05.022. Di Fraia, S., Massarotti, N., Vanoli, L., 2018. A novel energy assessment of urban wastewater treatment plants. Energy Convers. Manag. https://doi.org/10.1016/j. enconman.2018.02.058. Di Fraia, S., Massarotti, N., Vanoli, L., Costa, M., 2016. Thermo-economic analysis of a novel cogeneration system for sewage sludge treatment. Energy. https://doi.org/ 10.1016/j.energy.2016.07.144. Dina, S.F., Ambarita, H., Napitupulu, F.H., Kawai, H., 2015. Study on effectiveness of continuous solar dryer integrated with desiccant thermal storage for drying cocoa beans. Case Stud. Therm. Eng. https://doi.org/10.1016/j.csite.2014.11.003. Dincer, I., Rosen, M., 2011. Thermal Energy Storage Systems and Applications. John Wiley & Sons, Inc. https://doi.org/10.1016/B978-0-08-087872-0.00307-3. El-Sebaii, A.A., Aboul-Enein, S., Ramadan, M.R.I., El-Gohary, H.G., 2002. Experimental investigation of an indirect type natural convection solar dryer. Energy Convers. Manag. https://doi.org/10.1016/S0196-8904(01)00152-2. Enibe, S.O., 2003. Thermal analysis of a natural circulation solar air heater with phase change material energy storage. Renew. Energy 28, 2269–2299. Kacprzak, M., Neczaj, E., Fijałkowski, K., Grobelak, A., Grosser, A., Worwag, M., Rorat, A., Brattebo, H., Almås, Å., Singh, B.R., 2017. Sewage sludge disposal strategies for sustainable development. Environ. Res. https://doi.org/10.1016/j. envres.2017.03.010. Kamble, A.K., Pardeshi, I.L., Singh, P.L., Ade, G.S., 2013. Drying of chilli using solar cabinet dryer coupled with gravel bed heat storage system. J. Food Res. Technol. 1, 87–94. Kant, K., Shukla, A., Sharma, A., Kumar, A., Jain, A., 2016. Thermal energy storage based solar drying systems: a review. Innov. Food Sci. Emerg. Technol. https://doi.org/ 10.1016/j.ifset.2016.01.007. Khandegar, V., Saroha, A.K., 2013. Electrocoagulation for the treatment of textile industry effluent - a review. J. Environ. Manag. https://doi.org/10.1016/j. jenvman.2013.06.043. Khouya, A., Draoui, A., 2019. Computational drying model for solar kiln with latent heat energy storage: case studies of thermal application. Renew. Energy. https://doi.org/ 10.1016/j.renene.2018.06.090. Lakshmi, D.V.N., Muthukumar, P., Layek, A., Nayak, P.K., 2018. Drying kinetics and quality analysis of black turmeric (Curcuma caesia) drying in a mixed mode forced convection solar dryer integrated with thermal energy storage. Renew. Energy. https://doi.org/10.1016/j.renene.2017.12.053. Li, J., Zhao, L., Qin, L., Tian, X., Wang, A., Zhou, Y., Meng, L., Chen, Y., 2016. Removal of refractory organics in nanofiltration concentrates of municipal solid waste leachate treatment plants by combined Fenton oxidative-coagulation with photo - Fenton processes. Chemosphere. https://doi.org/10.1016/j.chemosphere.2015.12.069. ~ ez, P., Maldonado, M.I., Blanco, J., Gernjak, W., 2009. Malato, S., Fern� andez-Ib� an Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal. Today 147, 1–59. https://doi.org/10.1016/j.cattod.2009.06.018. McCrady, M.H., 2008. STANDARD METHODS FOR THE EXAMINATION OF WATER AND WASTE-WATER, twelfth ed. Am. J. Public Heal. Nations Heal. https://doi.org/ 10.2105/ajph.56.4.684-a.
Acknowledgements The authors wish to thank the Laboratorio Central de Cultivos Mar inos (Central Laboratory for Marine Culture) of the Facultad de Ciencias �lica del Norte for providing equipment del Mar of the Universidad Cato support. References Abubakar, S., Umaru, S., Kaisan, M.U., Umar, U.A., Ashok, B., Nanthagopal, K., 2018. Development and performance comparison of mixed-mode solar crop dryers with and without thermal storage. Renew. Energy. https://doi.org/10.1016/j. renene.2018.05.049. Al-Abidi, A.A., Bin Mat, S., Sopian, K., Sulaiman, M.Y., Lim, C.H., Th, A., 2012. Review of thermal energy storage for air conditioning systems. Renew. Sustain. Energy Rev. https://doi.org/10.1016/j.rser.2012.05.030. Ali, I., Abdelkader, L., El Houssayne, B., Mohamed, K., El Khadir, L., 2016. Solar convective drying in thin layers and modeling of municipal waste at three temperatures. Appl. Therm. Eng. 108, 41–47. https://doi.org/10.1016/j. applthermaleng.2016.07.098. Amer, B.M.A., Gottschalk, K., Hossain, M.A., 2018. Integrated hybrid solar drying system and its drying kinetics of chamomile. Renew. Energy. https://doi.org/10.1016/j. renene.2018.01.055. Archer, E., Petrie, B., Kasprzyk-Hordern, B., Wolfaardt, G.M., 2017. The fate of pharmaceuticals and personal care products (PPCPs), endocrine disrupting contaminants (EDCs), metabolites and illicit drugs in a WWTW and environmental waters. Chemosphere. https://doi.org/10.1016/j.chemosphere.2017.01.101. Bahammou, Y., Lamsyehe, H., Kouhila, M., Lamharrar, A., Idlimam, A., Abdenouri, N., 2019. Valorization of co-products of sardine waste by physical treatment under natural and forced convection solar drying. Renew. Energy. https://doi.org/ 10.1016/j.renene.2019.04.012. Baniasadi, E., Ranjbar, S., Boostanipour, O., 2017. Experimental investigation of the performance of a mixed-mode solar dryer with thermal energy storage. Renew. Energy. https://doi.org/10.1016/j.renene.2017.05.043. Bhardwaj, A.K., Chauhan, R., Kumar, R., Sethi, M., Rana, A., 2017. Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb). Case Stud. Therm. Eng. https://doi. org/10.1016/j.csite.2017.07.009. Bhardwaj, A.K., Kumar, R., Chauhan, R., 2019. Experimental investigation of the performance of a novel solar dryer for drying medicinal plants in Western Himalayan region. Sol. Energy. https://doi.org/10.1016/j.solener.2018.11.007. Bolobajev, J., Kattel, E., Viisimaa, M., Goi, A., Trapido, M., Tenno, T., Dulova, N., 2014. Reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment. Chem. Eng. J. 255, 8–13. https://doi.org/10.1016/j.cej.2014.06.018.
7
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Journal of Environmental Management 255 (2020) 109883 Spinosa, L., Ayol, A., Baudez, J.-C., Canziani, R., Jenicek, P., Leonard, A., Rulkens, W., Xu, G., Van Dijk, L., 2011. Sustainable and Innovative Solutions for Sewage Sludge Management. Water. https://doi.org/10.3390/w3020702. � Swierczek, L., Cie�slik, B.M., Konieczka, P., 2018. The potential of raw sewage sludge in construction industry – a review. J. Clean. Prod. https://doi.org/10.1016/j. jclepro.2018.07.188. Wang, P., Mohammed, D., Zhou, P., Lou, Z., Qian, P., Zhou, Q., 2019. Roof solar drying processes for sewage sludge within sandwich-like chamber bed. Renew. Energy. https://doi.org/10.1016/j.renene.2018.09.081. Wang, S., Cui, Y., Li, A., Wang, D., Zhang, W., Chen, Z., 2019. Seasonal dynamics of bacterial communities associated with antibiotic removal and sludge stabilization in three different sludge treatment wetlands. J. Environ. Manag. https://doi.org/ 10.1016/j.jenvman.2019.03.092. Wang, T., Xue, Y., Hao, R., Hou, H., Liu, J., Li, J., 2019. Mechanism investigations into the effect of rice husk and wood sawdust conditioning on sewage sludge thermal drying. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2019.03.074. Wang, Q., Zhu, J., Lu, X., 2017. Numerical simulation of heat transfer process in solar enhanced natural draft dry cooling tower with radiation model. Appl. Therm. Eng. https://doi.org/10.1016/j.applthermaleng.2016.11.155. Yang, K., Zhu, Y., Shan, R., Shao, Y., Tian, C., 2017. Heavy metals in sludge during anaerobic sanitary landfill: speciation transformation and phytotoxicity. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2016.12.019. Ying, D., Xu, X., Li, K., Wang, Y., Jia, J., 2012. Design of a novel sequencing batch internal micro-electrolysis reactor for treating mature landfill leachate. Chem. Eng. Res. Des. https://doi.org/10.1016/j.cherd.2012.06.007. Zhang, Q., Zhang, L., Sang, W., Li, M., Cheng, W., 2016. Chemical speciation of heavy metals in excess sludge treatment by thermal hydrolysis and anaerobic digestion process. Desalin. Water Treat. https://doi.org/10.1080/19443994.2015.1055518.
Natarajan, K., Thokchom, S.S., Verma, T.N., Nashine, P., 2017. Convective solar drying of Vitis vinifera & Momordica charantia using thermal storage materials. Renew. Energy 113, 1193–1200. Orbegoso, E.M., Saavedra, R., Marcelo, D., La Madrid, R., 2017. Numerical characterisation of one-step and three-step solar air heating collectors used for cocoa bean solar drying. J. Environ. Manag. https://doi.org/10.1016/j. jenvman.2017.07.015. Pan, C., Fu, X., Lu, W., Ye, R., Guo, H., Wang, H., Chusov, A., 2020. E Ff Ects of Conductive Carbon Materials on Dry Anaerobic Digestion of Sewage Sludge: Process and Mechanism, vol. 384. https://doi.org/10.1016/j.jhazmat.2019.121339. Piippo, S., Lauronen, M., Postila, H., 2018. Greenhouse gas emissions from different sewage sludge treatment methods in north. J. Clean. Prod. https://doi.org/10.1016/ j.jclepro.2017.12.232. Poblete, R., Painemal, O., 2018. Solar drying of landfill-leachate sludge: differential results through the use of peripheral technologies. Environ. Prog. Sustain. Energy 1–9. https://doi.org/10.1002/ep.12951. Prachayawarakorn, S., Tia, W., Plyto, N., Soponronnarit, S., 2008. Drying kinetics and quality attributes of low-fat banana slices dried at high temperature. J. Food Eng. https://doi.org/10.1016/j.jfoodeng.2007.08.011. Qiu, Y., Li, M., Hassanien, R.H.E., Wang, Y., Luo, X., Yu, Q., 2016. Performance and operation mode analysis of a heat recovery and thermal storage solar-assisted heat pump drying system. Sol. Energy. https://doi.org/10.1016/j.solener.2016.08.016. Rabha, D.K., Muthukumar, P., 2017. Performance studies on a forced convection solar dryer integrated with a paraffin wax–based latent heat storage system. Sol. Energy 149, 214–226. Shalaby, S.M., Bek, M.A., El-Sebaii, A.A., 2014. Solar dryers with PCM as energy storage medium: a review. Renew. Sustain. Energy Rev. https://doi.org/10.1016/j. rser.2014.01.073. Singh, P.P., Singh, S., Dhaliwal, S.S., 2006. Multi-shelf domestic solar dryer. Energy Convers. Manag. https://doi.org/10.1016/j.enconman.2005.10.002.
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Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman
Research article
Improvement of the solar drying process of sludge using thermal storage Rodrigo Poblete *, Osvaldo Painemal Universidad Cat� olica del Norte, Facultad de Ciencias del Mar, Escuela de Prevenci� on de Riesgos y Medioambiente, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal storage Sludge Solar drier Landfill leachate
Improvement of the drying process of sludge issuing from the coagulation/flocculation of landfill leachate was investigated using thermal storage systems, which consisted of a rock bed with air preheated by a solar air heater passing through the system. Sludge runs were dried inside the solar dryer with and without thermal storage, as well as outside the dryer. The values for the solar energy accumulated per mass of sludge required to obtain a stable sludge mass were 107.5 kJ/kg (with thermal storage), 240 kJ/kg (without thermal storage) and 580.5 kJ/kg, outside the dryer. The values for the energetic efficiency of the drying process were 38.13% and 16.45% for the processes with and without thermal storage, respectively. The net heat input to the storage system during the charging stage was 420 W and 120 W for the discharging stage, resulting in a global energy efficiency in the storage system of 0.28. The thermal efficiency of the solar drying with and without thermal storage system was 37.8% and 22.2%, respectively.
1. Introduction Landfill leachate is considered an important environmental concern due to its potentially toxic characteristics, which could produce harmful effects in the ecosystem (Li et al., 2016). In this context, complex treatments are required to depurate this kind of water, among them the coagulation/flocculation process. This treatment produces sludge, which can be applied as a soil stabilizer (Khandegar and Saroha, 2013). However, this residue must be examined to ensure its safe application, since some of the substances in the sludge may pose eco-toxicological hazards (Archer et al., 2017), making it necessary to consider them when planning their handling (Ying et al., 2012). Moreover, the use of ferric sludge as an iron source for applying a Fenton process in wastewater treatment (Bolobajev et al., 2014) was studied and the results were similar to those of the classic Fenton process. The huge amount of sludge produced in wastewater treatment plants needs to be removed and handled (Kacprzak et al., 2017) and if this sludge is not adequately disposed of, it will result in a source of envi ronmental pollution (Wang et al., 2019a, 2019b, 2019c). Sludge is a black and ill-smelling wet slurry which may contain pathogens and is therefore considered a biological hazardous waste. It can also contain high concentrations of heavy metals, which are toxic, non-degradable and tend to accumulate along the food chain (Zhang
et al., 2016). Raw sludge is usually discarded, incinerated or disposed of in landfills. However, the enormous amount of sludge produced makes � these options unacceptable from an environmental point of view (Cusido and Cremades, 2012). During the stabilisation process of landfill leachate sludge, humic substances and volatile matters are also pro duced (Yang et al., 2017). Dewatering, using a centrifuge, plate press, belt press or drying bed, is a common method to handle sludge (Spinosa et al., 2011). Dewatering involves the removal of water, which makes the sludge more solid in consistency. Drying does not change the chemical composition of the sludge but it enhances its calorific value. The application of the drying process expedites its subsequent incineration (Piippo et al., 2018). The drying or dehydration of sludge helps to reduce its volume and transport � costs (Swierczek et al., 2018). The sludge-drying kinetics depend on heat and mass transfers at the interface between the product and the air (Ali et al., 2016), solar radi ation, ambient temperature, relative humidity, wind velocity, initial moisture content of the materials to be dried and mass of the product per unit of exposed area (Kamble et al., 2013). The drying process is highly energy-demanding and is therefore usually expensive (Di Fraia et al., 2016). However, solar radiation is a free source of energy available for the drying process (Ali et al., 2016). Several companies dry their wet wastes in open solar systems, without a sunroof or any kind of control device. Under these conditions, the time
* Corresponding author. E-mail address: [email protected] (R. Poblete). https://doi.org/10.1016/j.jenvman.2019.109883 Received 15 June 2019; Received in revised form 18 October 2019; Accepted 16 November 2019 Available online 22 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Environmental Management 255 (2020) 109883
required to dry the sludge is very long, especially in winter or rainy seasons. This could be solved by using solar drying technology (Lakshmi et al., 2018), and the use of solar energy for sludge drying has proven to be an adequate solution to reduce processing costs (Di Fraia et al., 2018). For instance, an integrated solar system was studied for drying chamo mile using thermal storage and the result was a moisture reduction of 75% after a 33 h process (Amer et al., 2018). Solar energy is only accessible during the daytime, thus efficient thermal storage makes it possible to use the excess heat collected during the hours of sunshine during cloud cover days and at night (Al-Abidi et al., 2012). Due to the intermittent behaviour of solar energy, an integration of energy storage with solar collectors is useful to make the solar energy source more reliable (Kamble et al., 2013). In a previous study carried out by our research group, we evaluated drying landfill leachate-derived sludge using a solar still, observing that the sludge was successfully dried when peripheral systems (solar collector, solar air preheater and extractor) were used (Poblete and Painemal, 2018). Heat storage is useful to reduce the gap between energy supply and demand, and helps to improve the energy efficiency of solar systems. Thermal energy can be stored as latent (phase change) heat, sensible heat, thermochemical heat or a combination thereof (Khouya and Draoui, 2019). Sensible heat storage makes it possible to enhance the temperature of the heated air and the phase change material providing backup at night or during hours when solar energy is not available (Bhardwaj et al., 2017). The integration of thermal storage into a solar dryer is a positive solution for continuous solar drying (Lakshmi et al., 2018). An adequate configuration of the solar air heated allows maximum use to be made of the thermal power from the sun, thus meeting the requirements to dry the mass (Orbegoso et al., 2017). Natarajan et al. (2017) evaluated the use of aluminium filings, rock and sand beds as thermal storage materials, comparing them to drying processes without the use of such materials. The results showed that the use of thermal storage materials led to effective reductions in the drying time as well as in the moisture removal rate. No research work focused on the drying of landfill leachate sludge using solar systems supported by thermal storage was found. The aim of this research is to study the improvement of the solar drying process of landfill leachate sludge, using thermal storage to accumulate the solar energy required for use in the process.
a)
2. Materials and methods Runs were carried out in order to compare the evolution of the mass of sludge obtained from a coagulation/flocculation process as a pretreatment of landfill leachate prior to a photo-Fenton process. The runs for drying sludge were carried out: a) in the solar dryer without thermal storage; b) in the solar dryer with thermal storage; and c) outside the solar dryer. Fig. 1a shows the schematic diagram of the process, where the air is preheated before entering the solar dryer. After preheating the air, it is pumped to a thermal storage system (rock bed) made up of a tank containing approximately 60 kg of irregular spheroidal rocks. The air was driven across the rock bed and transported to the dryer using an air pump (55 W) and an air extractor (18 W) placed in the back wall of the dryer. The tank was made of 3-cm thick expanded polystyrene (width 40 cm, length 55 cm and height 40 cm), with a thermal conductivity of 0.038 W/(m*C) and an overall heat transfer coefficient of 30 W/m2 � C, working as thermal insulation. Fig. 1b shows details of the thermal storage tank and Fig. 1c depicts a photograph of the systems. The air mass flow rate (ma) was determined considering the air ve locity and the air temperature (Enibe, 2003)[19][19] (see Eq. (1)).
ρ ¼ 2:357
3; 7894*10 3 *TðKÞ
The air pump and the air extractor produced a forced convection of the air that enhanced the efficiency of the drying process. The equipment used, such as solar dryer, solar collector and solar air preheater, is described in Poblete and Painemal (2018). In brief, the solar dryer [18][18]has frontal and lateral walls, a 4-cm thick poly carbonate sunroof, 0.2 W/m � C thermal conductivity, 80% optical transmittance and an overall heat transfer coefficient of 3.8 W/m2 � C. On the wooden back wall, which is 3 m wide, 2 cm thick and 1.35 m high, an extractor was placed to remove the steam produced during the drying process. The air entering the solar dryer was preheated using a solar air heater located next to the solar still, with a mass flow of 0.025 kg/s. Air velocity and temperature were measured using a thermo-anemometer (model D8060048, Veto y Cia. Ltda.). The solar air preheater consisted of a wooden base (1.6 m2) with lateral wooden walls (10 cm high) and two glass covers (4 mm thick). Since the experiment was carried out during different days and under different solar irradiation conditions, the amount of incident solar irradiation received by the solar systems was measured and the cumu lative solar energy per unit of mass (Qrad) (kJ/kg) was calculated using
Air flow direction Rocks
Probe thermometer
Tank
Solar dryer
Thermal storage
c)
b)
(1)
Thermal insulation
Solar air heater
Rock bed
Fig. 1. a) Schematic diagram of drying system and thermal storage, b) photograph of thermal storage systems and c) photograph of the systems. 2
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Journal of Environmental Management 255 (2020) 109883
an adaptation of Malato et al. (2009)[20][20] (Eq. (2)): Qrad;n ¼ Qrad;n
Ad 1 þ Δtn *radg;n * ms
MR ¼
(2)
mw ⋅L � P þ radg;n ⋅Ad ⋅t
Q_ch ¼ ma *Cpa *ðTi;es Z
d d
*100
(6)
To;es Þ
(7) (8)
Q_ch dt
0
Similarly, the instantaneous (Q_dc ) and the net heat (Qdc ) in the storage system during the discharging time were calculated by Eqs. (9) and (10) (Rabha and Muthukumar, 2017): Q_dc ¼ ma *Cpa *ðTo;es Z Qdc ¼
t
Ti;es Þ
Q_dc dt
0
(9) (10)
The global energy efficiency of the storage system (nen,es) is calcu lated as the ratio between the net heat in the storage during the dis charging time and the net heat input to the storage system during the charging time, using Eq. (11) (Dincer and Rosen, 2011): nen;es ¼
Qdc Qch
(11)
The specific heat of the rocks is 0.81 kJ/kg*k, their density is 2245 kg/m3 and their thermal conductivity is 0.13 W/m*K. Thermal efficiency (Et) of the solar drying process was determined using Eq. (12): Et ¼
qu qu ma *Cpa *ΔT ¼ ¼ radg;n *Ad I radg;n *Ad
(12)
where qu is the thermal heat gain of the air (J), ΔT is the difference between the temperature of the air entering and exiting the solar dryer, ma is the mass flow rate of air (kg/s) and Cpa is the specific heat of the air (J/(kg� C)). 3. Results and discussion Fig. 2 shows the evolution of the mass of sludge subject to different drying conditions. It is possible to observe that the sludge dried inside the solar dryer achieves a stable mass faster than the sludge dried under natural conditions (outside the dryer), which is the usual industrial practice for drying sludge. Also, the stabilisation of the mass of sludge using thermal storage was obtained faster than without thermal storage. The solar energy accumulated per mass of sludge (Qrad) was 107.5 kJ/kg with thermal storage, 240 kJ/kg without thermal storage and 580.5 outside the dryer. This study agrees with Lakshmi et al. (2018), who observed re ductions in the drying time when using a solar dryer as compared to the drying process carried out in the open sun. The results obtained using thermal storage are better than those obtained in a previous work car ried out by our research group (Poblete and Painemal, 2018), where a stable mass (20 g of sludge) was obtained using an accumulated energy of 241.6 kJ/kg. Furthermore, it is possible to observe that the mass underwent an evolution along the run. In the case of the run carried out outside the dryer, the mass experienced a reduction during the day, when there is solar irradiation. However, although the drying process stopped at night (as expected), an increment in the mass of sludge was measured in the first hour of the next morning, possibly due to light rains occurring
(3)
where Mi and Mf are the initial and final sludge moistures (kg), respectively. The moisture (M) was calculated according to Prachaya warakorn et al. (2008) using: mt
t
Qch ¼
where L is the latent heat of water vaporisation (J/kg); P is the sum of the power (W) consumed by each equipment in the system over a set time t (h); and mw is the mass of water evaporated from the sludge (kg), calculated using Eq. (4): � mi * Mi Mf mw ¼ (4) 100 Mf
M¼
Me Me
where Me is the equilibrium moisture content. Also, the instantaneous (Q_ch ) and net heat input (Qch ) to the storage system during the charging process were determined according to the method described by Rabha and Muthukumar (2017), using Eqs. (7) and (8), respectively:
where Qrad,n and Qrad,n-1 are the global solar energy accumulated per mass of sludge (kJ/kg) at times n and n-1, respectively; Δtn is the experimental time of the sample (s); radg,n is the mean incident radiation on the solar still (W/m2); Ad is the illuminated area of the solar dryer (m2); and ms is the mass of the sludge (kg). The solar irradiation (radg,n, W/m2) was measured using a pyran ometer (CPM 10, Kipp & Zonen; 285-2800 nm wavelength and 7–14 μV/ W/m sensitivity). The pyranometer was tilted to 30� , i.e. the same angle as the latitude of the testing site (Coquimbo, Chile). For each run, the sludge to be dried (300 g of mass) was placed on an aluminium tray (7 cm high; 1000 cm2 surface area). The mass was chosen to make handling easier. For the runs inside the solar dryer (runs “a” and “b”), trays were placed over the fins in the heated base of the solar still. For the run performed outside the dryer, trays were placed over the soil next to the solar dryer. Each run was carried out in triplicate and lasted until a high degree of dryness was obtained and the mass of sludge was stable. Tray mass and temperature of the process were recorded. Density and volatile compounds in the sludge were determined before and after the drying process carried out using thermal storage, where the volatile solids in the sludge were measured heating the sludge at 550 � C for 4 h as per the Standard Methods for Examination of Water and Wastewater (McCrady, 2008). The temperature of the sludge, polycarbonate, sunroof, concrete base and soil above the tray was measured at the beginning of the runs and at every hour, using a HI 98501-1 thermometer (Hanna; 0.1 � C precision). The temperature of the air inside the solar still, the air coming from the solar air preheater as well as the air before (Ti,es) and after (To,es) entering the rock bed were recorded using a Campbell Scientific, Inc. 109-L34 thermocouple, (0.25 � C precision). Temperature and humidity of the air inside the solar still were recorded using a Campbell Scientific, Inc. CS215-L probe. The energetic efficiency of the drying process (ED), defined as the ratio between the energy in the evaporated water and the energy required to remove the moisture from the sludge, was calculated considering the electric and solar energy consumptions of the equipment (i.e. recirculation pump, air extractor and air pump) and using Eq. (3) (Singh et al., 2006). ED ¼
Mt Mi
(5)
where mt is the weight of wet sludge at instant t, and d is the mass of dry sludge. The moisture content of sludge was determined by the drying and weighing method in accordance with ISO-11465: 1999. The moisture ratio (MR) was determined using Eq. (6) (Shalaby et al., 2014):
3
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Journal of Environmental Management 255 (2020) 109883
300 With storage
Outside
Night
Night
Mass of sludge (g)
250
Without storage
200
150
100
50
0
0.0
0.4
0.9
1.5
1.6
1.8
1.9
2.0
2.0
2.4
2.6
2.8
Log Qrad Fig. 2. Evolution of mass of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
during the night. This nocturnal moisture reabsorption is the ratio be tween the overnight rise in the moisture of the mass and the moisture content during the day when the drying was carried out (El-Sebaii et al., 2002), resulting in a value of 2.35%. Nocturnal moisture reabsorption was not observed in the drying process carried out inside the solar dryer. With regard to the process carried out inside the dryer, during the first stage of the runs (from 0 to 34.6 kJ/kg of accumulated solar en ergy), the loss of mass was faster for the process without thermal storage than with storage since all the thermal energy coming from the solar air heater was transferred to the drying process (as shown in Fig. 1). However, at nightfall, when the air pump began to drive in cold air, the loss of sludge mass stopped. At night, with thermal storage, the dried
sludge lost mass because the air pump drove in air from the rock bed, which was saving thermal energy from the air heated by the solar air heater during solar radiation hours. A similar observation was made by Lakshmi et al. (2018), who reported a reduction in the moisture of black turmeric during night-time. The ED, calculated using Eq. (3), was 38.13% and 16.45% for the process carried out with and without thermal storage, respectively. The results obtained with thermal storage are better than those obtained by Chaouch et al. (2018), who reported an ED of 18.34% and 15.72% in different seasons of the year. Moreover, the results obtained are similar to those obtained by Abubakar et al. (2018), who observed that the drying efficiency with the storage material increased approximately
1.0
Moisture ratio (MR)
Night
0.8
Without storage
0.7
Outside
Night
With storage
0.9
0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.0
0.4
0.9
1.5
1.6
1.8
1.9
2.0
2.0
2.4
2.6
2.8
Log Qrad Fig. 3. Evolution of moisture ratio of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3). 4
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13% in comparison with the efficiency without thermal storage. The moisture ratio, calculated using Eq. (6), was variable during the drying process (See Fig. 3). As in the case of the sludge mass, the evo lution of this parameter displayed different trends depending on the drying method used, resulting in a faster drop inside the solar dryer. Similar results were obtained by Rabha and Muthukumar (2017), who observed that the use of this kind of device makes it possible to reduce the moisture content faster. Moreover, the moisture ratio dropped more rapidly when thermal storage was used, due to the effective utilisation of the retained energy (Bhardwaj et al., 2019). These results of mass evolution are related to the temperature of the air. Fig. 4 shows the behaviour of the air temperature before and after the rock bed and the solar radiation. Fluctuations in the solar irradiation recorded along the process are caused by the cloud cover. When the power of the solar irradiation increases, the charging process of the storage thermal system starts up, evidenced by an increase in the temperature of the air both before and after the storage (rock bed), although in the latter case the increase is lower. Conversely, when the solar irradiation decreases, the heat saved in the thermal storage is discharged and the temperature of the air coming from the storage is higher than the temperature of the air before the storage (see Fig. 3). These results are similar to those observed by Lakshmi et al. (2018). The intersection point of the air temperature curves indicates the moment when the charging stopped and the discharging began (Rabha and Muthukumar, 2017). During the charging stage, the highest differ ence in the temperature of the air was 7 � C and, during the discharge stage, 8 � C. The net heat input to the storage system was calculated using Eq. (8) and it directly depends on the difference in the air temperature between the rock bed inlet and outlet. The value was 420 W. For the discharge stage (calculated by Eq. (10)) the value was 120 W. The global energy efficiency of the storage system was calculated using Eq. (11) and was 0.28. Fig. 5 shows the evolution of the temperature of the sludge subject to different drying processes. The temperature of the sludge dried outside the solar dryer was lower than the others, with a peak temperature of 29.5 � C. This occurs because inside this system, the heat is maintained for a longer time due to a greenhouse effect. The temperature of this sludge behaves similarly with solar radiation. Moreover, the solar irradiation affected the temperature of the sludge inside the dryer, i.e., when the solar irradiation dropped the flow
Solar irradiation
Before storage
After storage
Solar irradiation (W/m2)
900
45 40
800
35
700
30
600
25
500
20
400
15
300
10
200
5
100 0 6:20 12:35 18:50 1:05 7:20 13:35 19:50 2:05 8:20 14:35 20:50 3:05 9:20 15:35 21:50 4:05
Time of day (h)
Fig. 4. Evolution of solar irradiation and air temperature before and after storage system. 5
0
Air temperature (°C)
1000
of thermal energy decreased. Similar observations were reported by Dina et al. (2015). It is possible to observe that the temperature of the sludge dried using thermal storage was higher than that of the sludge dried without this process. This resulted in the trend obtained in the drying process for the runs shown in Fig. 2 and the reduction in the drying time (Kant et al., 2016; Qiu et al., 2016). Fig. 6 shows the evolution of the temperature of the heat base inside the solar dryer and the soil outside it during the runs. It is observed that these temperatures display the same trend as the temperatures of the sludge placed on this heat base or soil and that there is a heat transfer between the base and the material to be dried (Wang et al., 2017). Similar results were obtained by our research group in a previous study, where a direct relationship between the temperature of the sludge and the temperature of the heat base was observed (Poblete and Painemal, 2018). During the drying process, the temperature of the air inside the solar dryer was higher than the air outside (see. Fig. 7). This behaviour is explained by the greenhouse effect caused by the translucent envelope of the solar dryer. This envelope makes it possible to maintain the heat from the solar air heater and the heated base, as well as the heat pro duced when the thermal storage systems are working. The higher tem perature of the air inside the solar dryer vis-� a-vis outside makes it possible to improve the drying process and also to reduce the energy consumption required (Bahammou et al., 2019). The thermal efficiency of solar drying with and without thermal storage was calculated using Eq. (12). The results were 37.8% and 22.2%, respectively. These results agree with the findings reported by Natarajan et al. (2017), who obtained similar values for thermal effi ciency in the drying of food using thermal storage. They also agree with Baniasadi et al. (2017), who observed that the thermal efficiency of the solar dryer is enhanced using an energy storage system. Also, the results obtained in this research using thermal storage were higher than those reported by (Wang et al., 2019a, 2019b, 2019c), who obtained a 24.3% of thermal efficiency when drying sludge using a sandwich-like chamber bed. A research study by (Wang et al., 2019a) used rice husk and sawdust as conditioners to improve the efficiency and energy consumption of thermal sludge drying. Although this solution resulted in a reduction in the time and energy required in the process, it also increased the volume of dried sludge.
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Journal of Environmental Management 255 (2020) 109883
50
With storage
Outside
Night
40
T° of sludge (°C)
Without storage
Night
45
35 30 25 20 15 10 5 0
0.0
0.4
0.7
0.9
1.0
1.5
1.6
1.7
1.9
1.9
2.0
2.0
2.0
2.3
2.4
2.6
2.7
2.8
Log Qrad Fig. 5. Evolution of temperature of sludge dried with storage, without storage and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
45 With storage
T° of base and soil (°C)
40
Outside
35 30 25 20 15 10 5 0
0.0
0.4
0.7
0.9
1.0
1.5
1.6
1.7
1.9
1.9
2.0
2.0
2.0
2.3
2.4
2.6
2.7
2.8
Log Qrad Fig. 6. Evolution of heat base temperature from the solar dryer with and without storage and soil outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
The density and volatile compounds in the sludge before the drying process carried out using thermal storage were 0.81 kg/L and 7.92 g/kg, respectively, and 1.2 kg/L and 7.89 g/kg after the drying process. Therefore, a slight loss in the volatile compounds was observed due to the drying process, which may have been due to the partial evaporation of a certain amount of ammonia or volatile fatty acids during the drying process (Pan et al., 2020; Deng et al., 2009). A more in-depth analysis of these losses will be approached in a future research project.
applied in the drying process. The use of thermal storage makes it possible to reduce the energy consumption required for drying sludge. Sludge dried inside the solar dryer reaches a stable mass faster than sludge dried under natural conditions, outside the dryer. The stabilisation of the mass of sludge was obtained faster with thermal storage than without it. The evolution in the mass of sludge was variable along the run. When the run was carried out outside the dryer, the mass experienced a reduction during the day, only to increase in the first hour of the next morning. During the night, a reduction in the mass of the sludge dried using thermal storage was obtained. The results of mass evolution are related to the temperature of the air.
4. Conclusions The use of solar thermal storage processes in the drying of sludge from landfill leachate treatment was evaluated. Although the drying process is highly energy-demanding, solar ra diation is a free available source of energy that could be efficiently 6
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Journal of Environmental Management 255 (2020) 109883
60
T° of air (°C)
50 40 30 20 10
6:55
5:00
3:05
0:40
22:45
20:35
18:40
16:45
13:50
11:55
6:45
10:00
4:50
2:55
23:50
20:15
18:20
T inside
15:15
13:20
9:30
11:25
6:25
4:30
2:35
0:40
22:35
20:40
18:45
15:40
13:45
9:55
11:50
8:00
T outside 0
Time of day (h) Fig. 7. Evolution of air temperature inside and outside the dryer. Error bars represent the standard deviation of the results (n ¼ 3).
When the solar radiation decreased, a discharge of the heat storage occurred. This was evidenced by a higher temperature in the air coming from the storage in comparison with the temperature of the air before the storage. The thermal efficiency of the solar dryer is enhanced with the storage of heat.
Chaouch, W.B., Khellaf, A., Mediani, A., Slimani, M.E.A., Loumani, A., Hamid, A., 2018. Experimental investigation of an active direct and indirect solar dryer with sensible heat storage for camel meat drying in Saharan environment. Sol. Energy. https://doi. org/10.1016/j.solener.2018.09.037. Cusid� o, J.A., Cremades, L.V., 2012. Atomized sludges via spray-drying at low temperatures: an alternative to conventional wastewater treatment plants. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2012.03.053. Deng, W.Y., Yan, J.H., Li, X.D., Wang, F., Zhu, X.W., Lu, S.Y., Cen, K.F., 2009. Emission characteristics of volatile compounds during sludges drying process. J. Hazard Mater. https://doi.org/10.1016/j.jhazmat.2008.05.022. Di Fraia, S., Massarotti, N., Vanoli, L., 2018. A novel energy assessment of urban wastewater treatment plants. Energy Convers. Manag. https://doi.org/10.1016/j. enconman.2018.02.058. Di Fraia, S., Massarotti, N., Vanoli, L., Costa, M., 2016. Thermo-economic analysis of a novel cogeneration system for sewage sludge treatment. Energy. https://doi.org/ 10.1016/j.energy.2016.07.144. Dina, S.F., Ambarita, H., Napitupulu, F.H., Kawai, H., 2015. Study on effectiveness of continuous solar dryer integrated with desiccant thermal storage for drying cocoa beans. Case Stud. Therm. Eng. https://doi.org/10.1016/j.csite.2014.11.003. Dincer, I., Rosen, M., 2011. Thermal Energy Storage Systems and Applications. John Wiley & Sons, Inc. https://doi.org/10.1016/B978-0-08-087872-0.00307-3. El-Sebaii, A.A., Aboul-Enein, S., Ramadan, M.R.I., El-Gohary, H.G., 2002. Experimental investigation of an indirect type natural convection solar dryer. Energy Convers. Manag. https://doi.org/10.1016/S0196-8904(01)00152-2. Enibe, S.O., 2003. Thermal analysis of a natural circulation solar air heater with phase change material energy storage. Renew. Energy 28, 2269–2299. Kacprzak, M., Neczaj, E., Fijałkowski, K., Grobelak, A., Grosser, A., Worwag, M., Rorat, A., Brattebo, H., Almås, Å., Singh, B.R., 2017. Sewage sludge disposal strategies for sustainable development. Environ. Res. https://doi.org/10.1016/j. envres.2017.03.010. Kamble, A.K., Pardeshi, I.L., Singh, P.L., Ade, G.S., 2013. Drying of chilli using solar cabinet dryer coupled with gravel bed heat storage system. J. Food Res. Technol. 1, 87–94. Kant, K., Shukla, A., Sharma, A., Kumar, A., Jain, A., 2016. Thermal energy storage based solar drying systems: a review. Innov. Food Sci. Emerg. Technol. https://doi.org/ 10.1016/j.ifset.2016.01.007. Khandegar, V., Saroha, A.K., 2013. Electrocoagulation for the treatment of textile industry effluent - a review. J. Environ. Manag. https://doi.org/10.1016/j. jenvman.2013.06.043. Khouya, A., Draoui, A., 2019. Computational drying model for solar kiln with latent heat energy storage: case studies of thermal application. Renew. Energy. https://doi.org/ 10.1016/j.renene.2018.06.090. Lakshmi, D.V.N., Muthukumar, P., Layek, A., Nayak, P.K., 2018. Drying kinetics and quality analysis of black turmeric (Curcuma caesia) drying in a mixed mode forced convection solar dryer integrated with thermal energy storage. Renew. Energy. https://doi.org/10.1016/j.renene.2017.12.053. Li, J., Zhao, L., Qin, L., Tian, X., Wang, A., Zhou, Y., Meng, L., Chen, Y., 2016. Removal of refractory organics in nanofiltration concentrates of municipal solid waste leachate treatment plants by combined Fenton oxidative-coagulation with photo - Fenton processes. Chemosphere. https://doi.org/10.1016/j.chemosphere.2015.12.069. ~ ez, P., Maldonado, M.I., Blanco, J., Gernjak, W., 2009. Malato, S., Fern� andez-Ib� an Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal. Today 147, 1–59. https://doi.org/10.1016/j.cattod.2009.06.018. McCrady, M.H., 2008. STANDARD METHODS FOR THE EXAMINATION OF WATER AND WASTE-WATER, twelfth ed. Am. J. Public Heal. Nations Heal. https://doi.org/ 10.2105/ajph.56.4.684-a.
Acknowledgements The authors wish to thank the Laboratorio Central de Cultivos Mar inos (Central Laboratory for Marine Culture) of the Facultad de Ciencias �lica del Norte for providing equipment del Mar of the Universidad Cato support. References Abubakar, S., Umaru, S., Kaisan, M.U., Umar, U.A., Ashok, B., Nanthagopal, K., 2018. Development and performance comparison of mixed-mode solar crop dryers with and without thermal storage. Renew. Energy. https://doi.org/10.1016/j. renene.2018.05.049. Al-Abidi, A.A., Bin Mat, S., Sopian, K., Sulaiman, M.Y., Lim, C.H., Th, A., 2012. Review of thermal energy storage for air conditioning systems. Renew. Sustain. Energy Rev. https://doi.org/10.1016/j.rser.2012.05.030. Ali, I., Abdelkader, L., El Houssayne, B., Mohamed, K., El Khadir, L., 2016. Solar convective drying in thin layers and modeling of municipal waste at three temperatures. Appl. Therm. Eng. 108, 41–47. https://doi.org/10.1016/j. applthermaleng.2016.07.098. Amer, B.M.A., Gottschalk, K., Hossain, M.A., 2018. Integrated hybrid solar drying system and its drying kinetics of chamomile. Renew. Energy. https://doi.org/10.1016/j. renene.2018.01.055. Archer, E., Petrie, B., Kasprzyk-Hordern, B., Wolfaardt, G.M., 2017. The fate of pharmaceuticals and personal care products (PPCPs), endocrine disrupting contaminants (EDCs), metabolites and illicit drugs in a WWTW and environmental waters. Chemosphere. https://doi.org/10.1016/j.chemosphere.2017.01.101. Bahammou, Y., Lamsyehe, H., Kouhila, M., Lamharrar, A., Idlimam, A., Abdenouri, N., 2019. Valorization of co-products of sardine waste by physical treatment under natural and forced convection solar drying. Renew. Energy. https://doi.org/ 10.1016/j.renene.2019.04.012. Baniasadi, E., Ranjbar, S., Boostanipour, O., 2017. Experimental investigation of the performance of a mixed-mode solar dryer with thermal energy storage. Renew. Energy. https://doi.org/10.1016/j.renene.2017.05.043. Bhardwaj, A.K., Chauhan, R., Kumar, R., Sethi, M., Rana, A., 2017. Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb). Case Stud. Therm. Eng. https://doi. org/10.1016/j.csite.2017.07.009. Bhardwaj, A.K., Kumar, R., Chauhan, R., 2019. Experimental investigation of the performance of a novel solar dryer for drying medicinal plants in Western Himalayan region. Sol. Energy. https://doi.org/10.1016/j.solener.2018.11.007. Bolobajev, J., Kattel, E., Viisimaa, M., Goi, A., Trapido, M., Tenno, T., Dulova, N., 2014. Reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment. Chem. Eng. J. 255, 8–13. https://doi.org/10.1016/j.cej.2014.06.018.
7
R. Poblete and O. Painemal
Journal of Environmental Management 255 (2020) 109883 Spinosa, L., Ayol, A., Baudez, J.-C., Canziani, R., Jenicek, P., Leonard, A., Rulkens, W., Xu, G., Van Dijk, L., 2011. Sustainable and Innovative Solutions for Sewage Sludge Management. Water. https://doi.org/10.3390/w3020702. � Swierczek, L., Cie�slik, B.M., Konieczka, P., 2018. The potential of raw sewage sludge in construction industry – a review. J. Clean. Prod. https://doi.org/10.1016/j. jclepro.2018.07.188. Wang, P., Mohammed, D., Zhou, P., Lou, Z., Qian, P., Zhou, Q., 2019. Roof solar drying processes for sewage sludge within sandwich-like chamber bed. Renew. Energy. https://doi.org/10.1016/j.renene.2018.09.081. Wang, S., Cui, Y., Li, A., Wang, D., Zhang, W., Chen, Z., 2019. Seasonal dynamics of bacterial communities associated with antibiotic removal and sludge stabilization in three different sludge treatment wetlands. J. Environ. Manag. https://doi.org/ 10.1016/j.jenvman.2019.03.092. Wang, T., Xue, Y., Hao, R., Hou, H., Liu, J., Li, J., 2019. Mechanism investigations into the effect of rice husk and wood sawdust conditioning on sewage sludge thermal drying. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2019.03.074. Wang, Q., Zhu, J., Lu, X., 2017. Numerical simulation of heat transfer process in solar enhanced natural draft dry cooling tower with radiation model. Appl. Therm. Eng. https://doi.org/10.1016/j.applthermaleng.2016.11.155. Yang, K., Zhu, Y., Shan, R., Shao, Y., Tian, C., 2017. Heavy metals in sludge during anaerobic sanitary landfill: speciation transformation and phytotoxicity. J. Environ. Manag. https://doi.org/10.1016/j.jenvman.2016.12.019. Ying, D., Xu, X., Li, K., Wang, Y., Jia, J., 2012. Design of a novel sequencing batch internal micro-electrolysis reactor for treating mature landfill leachate. Chem. Eng. Res. Des. https://doi.org/10.1016/j.cherd.2012.06.007. Zhang, Q., Zhang, L., Sang, W., Li, M., Cheng, W., 2016. Chemical speciation of heavy metals in excess sludge treatment by thermal hydrolysis and anaerobic digestion process. Desalin. Water Treat. https://doi.org/10.1080/19443994.2015.1055518.
Natarajan, K., Thokchom, S.S., Verma, T.N., Nashine, P., 2017. Convective solar drying of Vitis vinifera & Momordica charantia using thermal storage materials. Renew. Energy 113, 1193–1200. Orbegoso, E.M., Saavedra, R., Marcelo, D., La Madrid, R., 2017. Numerical characterisation of one-step and three-step solar air heating collectors used for cocoa bean solar drying. J. Environ. Manag. https://doi.org/10.1016/j. jenvman.2017.07.015. Pan, C., Fu, X., Lu, W., Ye, R., Guo, H., Wang, H., Chusov, A., 2020. E Ff Ects of Conductive Carbon Materials on Dry Anaerobic Digestion of Sewage Sludge: Process and Mechanism, vol. 384. https://doi.org/10.1016/j.jhazmat.2019.121339. Piippo, S., Lauronen, M., Postila, H., 2018. Greenhouse gas emissions from different sewage sludge treatment methods in north. J. Clean. Prod. https://doi.org/10.1016/ j.jclepro.2017.12.232. Poblete, R., Painemal, O., 2018. Solar drying of landfill-leachate sludge: differential results through the use of peripheral technologies. Environ. Prog. Sustain. Energy 1–9. https://doi.org/10.1002/ep.12951. Prachayawarakorn, S., Tia, W., Plyto, N., Soponronnarit, S., 2008. Drying kinetics and quality attributes of low-fat banana slices dried at high temperature. J. Food Eng. https://doi.org/10.1016/j.jfoodeng.2007.08.011. Qiu, Y., Li, M., Hassanien, R.H.E., Wang, Y., Luo, X., Yu, Q., 2016. Performance and operation mode analysis of a heat recovery and thermal storage solar-assisted heat pump drying system. Sol. Energy. https://doi.org/10.1016/j.solener.2016.08.016. Rabha, D.K., Muthukumar, P., 2017. Performance studies on a forced convection solar dryer integrated with a paraffin wax–based latent heat storage system. Sol. Energy 149, 214–226. Shalaby, S.M., Bek, M.A., El-Sebaii, A.A., 2014. Solar dryers with PCM as energy storage medium: a review. Renew. Sustain. Energy Rev. https://doi.org/10.1016/j. rser.2014.01.073. Singh, P.P., Singh, S., Dhaliwal, S.S., 2006. Multi-shelf domestic solar dryer. Energy Convers. Manag. https://doi.org/10.1016/j.enconman.2005.10.002.
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