Solar greenhouse dryer system for wood chips improvement as biofuel

Accepted Manuscript Solar greenhouse dryer system for wood chips improvement as biofuel Perea-Moreno Alberto-Jesús, Juaidi Adel, Manzano-Agugliaro Francisco PII:

S0959-6526(16)30924-6

DOI:

10.1016/j.jclepro.2016.07.036

Reference:

JCLP 7608

To appear in:

Journal of Cleaner Production

Received Date: 28 April 2016 Revised Date:

4 July 2016

Accepted Date: 6 July 2016

Please cite this article as: Alberto-Jesús P-M, Adel J, Francisco M-A, Solar greenhouse dryer system for wood chips improvement as biofuel, Journal of Cleaner Production (2016), doi: 10.1016/ j.jclepro.2016.07.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Solar Greenhouse Dryer

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Solar greenhouse dryer system for wood chips improvement as biofuel

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Perea-Moreno, Alberto-Jesús 1; Juaidi, Adel2, Manzano-Agugliaro, Francisco 2,3, *

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Department of Applied Physics, University of Cordoba, CEIA3, Campus de Rabanales, 14071 Córdoba, Spain;

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Dpt. Engineering, University of Almeria, CEIA3, 04120 Almeria, Spain.

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CIAIMBITAL (Research Center on Mediteranean Intensive Agrosystems and Agrifood Biotechnology), University of Almeria, 04120Almeria, Spain. * Author to whom correspondence should be addressed: [email protected]

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Abstract

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The development of cleaner and renewable energy sources are needed in order to reduce fossil fuel

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dependence and mitigate global warming by reducing greenhouse gas emissions. Solar dryers are

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considered as clean technologies and optimal use of these can minimize the use of non-renewable

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energy, and can play an important role in the world's future. Drying for forest products is one of the

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most attractive and cost effective application of solar energy because it may be possible to improve

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forest residues in profitable biomass as biofuel. In this study, a solar greenhouse dryer was used in

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drying experiments of wood chips of Pinus pinaster in the south of Spain. The environmental

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conditions were under autumn season with an average daily solar radiation of 13.74 MJ/(m2·d) during

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the 15 days of the experiment. The experiment analyzed the drying process under indoor conditions of

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several piles of wood chips which was compared to the same shaped piles under open sun drying. The

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results were mathematically modelled to show the advantages of the Solar Greenhouse Dryer for both,

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to reach the 10 % of relative humidity and fewer days were dedicated for this. Thus, these analyses

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reveal that the Solar Greenhouse Dryer is a useful and vital technology for the improvement of wood

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chips as biofuel, it reduces moisture, and it can be applied in many countries because of the fact that it only

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uses solar energy; it is a renewable energy resource which is both free and clean.

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Keywords: Solar Dryer, Drying Technique, Biomass, Wood Chip, Open sun drying

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ACCEPTED MANUSCRIPT 1. Introduction

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Cleaner technologies using renewable energies is essential nowadays for reducing the dependence of

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fossil fuels (Baños et al., 2011) and its negative impact to the environment (Cama et al.

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2013). Bioenergy has been regarded as a promising substitute for fossil fuels (Mizsey and Racz, 2010),

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mainly due to its renewable and carbon neutral nature (Nguyen et al., 2013). Biomass is one of the

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main types of renewable energy sources (Manzano-Agugliaro et al., 2013). In addition to producing

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energy, it contributes to local economic growth and overall sustainable development (Long et al.,

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2013). On the other hand, the biomass residues would appear to be an interesting source of renewable

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energy (Callejón-Ferre et al., 2011). In Spain biomass, wind and hydropower are the main sources of

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renewable energy (Montoya et al. 2014). So, in the Southern of Spain, where exist more than 30000 ha

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of greenhouse crops (Marquez et al., 2011), annual crops as tomato (Clement et al., 2012) or cucumber

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(Clement et al., 2013) offer a huge quantity of agricultural residues that can be profited as biomass for

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gasification (Agugliaro, 2007). Moreover, olive orchards is the most popular tree crop in Spain,

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producing agricultural residues as olive tree pruning (Spinelli and Picchi, 2010) that can be used as

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fuel for heating systems in boiler (López et al. 2010), or the residues of olive oil industry as olive stone

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(Gómez et al., 2016) or olive cake (Oktay, 2006). However, one of the main drawbacks is the water

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content (Casanova-Peláez et al., 2015), that can be studied, analyzed, and finally leads to inexpensive

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solutions, Moreover, it uses cleaner technologies for drying (Labed et al., 2016). Despite its huge

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potential for increasing sustainability, the use of solar energy in industry is still uncommon (Fernández-

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García et al., 2015).

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Due to rising prices of conventional fuel and the increasing demand for greener fuels, wood pellets

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established fuel in the EU and USA (Reed et al., 2012). Pelletization is the process of making compact

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biomass that facilitates cleanliness, handling, and the increase of energetic value per unit of volume

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(Stelte et al., 2012). The main advantages of wood pellets reside in two facts. First, pellet produces

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betetr fuel that Wood residue can be burned directly as heating fuel (Magelli et al., 2009). Secondly,

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they emit less air pollutants than conventional fossil fuel (Cambero and Sowlati, 2014) (Garcia-

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Maraver et al., 2014). It is almost applicable to all types of utility boilers (Goh et al., 2013), and the

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compact size is also an advantage in terms of storage and transport compared with other wood biomass

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(Shahrukh, et al., 2016).Wood pellets are low -cost biomass and can also be considered as renewable fuel

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(Gori et al., 2013), which is an attractive source of energy for large-scale biomass combustion plants

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(Hossain et al., 2016). Wood pellets combustion is a new technology due to fuel scarcity and

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environmental concerns over burning fossil fuels (Zhang et al., 2009), and they have good potential for

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providing thermal energy in the industry or electricity generation. The large scale implementation of

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wood pellets as a biofuel has the potential to replace fossil fuels for heat and power production

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(García-Maroto et al., 2014). Thus, the pellet of wood is an example of a clean and renewable source

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of energy, and is considered to be one of the substitutes of the fossil fuels (Uasuf and Becker, 2011).

67 Almost the whole production and consumption of pellet production worldwide are from Europe and

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North America (Miranda et al., 2015), e.g. from 2006 to 2012, the pellet production grew from 7 to 19

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million tons (Duca et al., 2014). In European countries such as Austria, France, Finland, Germany,

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Italy, Sweden, and Switzerland, wood pellet heating technology is mature enough to compete

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successfully with traditional fossil fuel heating devices (Trømborg et al., 2013). Among the Chemical

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and physical parameters of pellets, the Moisture content (M) expressed in percentage is one of the most

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important (Sjöström and Blomqvist, 2014), for that reason, there is a normative reference, EN 14774-2.

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Drying is the process of removal of moisture from the product to a specified value due to application

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of heat, so the final product was improved as biofuel compared to initial one (Gómez-de la Cruz et al.,

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2014). Dehydration is one of the most ancient technologies used for the conservation of food and

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agricultural products (Labed et al., 2016). The industrial dryers operate based on the principle of

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simultaneous heat and mass transfer where the water of the product is removed (Gómez-de la Cruz et

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al., 2015). The most common dryers of biomass applied in bioenergy plants are convection dryers but

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the use of steam drying technology is increasing (Fagernäs et al., 2010). The advantages and

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drawbacks of the different drying technologies are evaluated based on some parameters such as drying

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way, temperature and exposure time (Ståhl et al., 2004).Regarding the pelleting facilities, one of the

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main conclusions obtained is that the drying is a very costly process due the energy consumption and

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the cost of machinery that means around the 30% of the total cost of the facilities (Uasuf and Becker,

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2011).

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The raw material can contain 60 % of wet basis (wb) moisture, so the drying process becomes

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necessary. In the literature pre-treatments were carried out on raw materials in order to obtain the pellets.

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(Miranda et al., 2009) in the southwest of Spain for Pyrenean oak made: chipped, ground and dried to

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10% ; (Filbakk et al., 2011) for Pyrenean sylvestris in Norway made: piled up two months( the logs),

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chipped, air dried until moisture was between 11% and 14%; (Zamorano et al., 2011) for olive

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branches obtained by pruning in the south of Spain: chipped, and the moisture was reduced below

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15% by a rotating dryer; and also in southwest of Spain for olive pomace where they dried until

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moisture range 10%–15% (Miranda et al., 2012).

96 The earliest method of drying agro-commodities was the sun drying; however it has several

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disadvantages as the damage produced by wind and rain (Patil and Gawande, 2016). But is

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unquestionably cheapest method among all available drying processes because it doesn’t require any

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expensive setup and energy source (Tiwari et al., 2016). Related to industrial processes, open sun

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drying requires large land and drying time (Ayensu, 1997). For renewable energy sources, solar energy

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is the most appropriate for drying systems (Belessiotis and Delyannis, 2011). Solar drying is a

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technological process and works on the principle of greenhouse effect (Pirasteh et al., 2014). There are

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two types of solar drying system: low temperature and high temperature dryers (Koyuncu, 2006).

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These systems which need a simple technology allow to be adapted to the rural regions for drying

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applications (Thirugnanasambandam et al., 2010), also in the most developing countries where

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supplies of non-renewable sources of energy are either unavailable, unreliable or, for many farmers,

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too expensive (Hernández-Escobedo et al., 2015) this technology can be used. However, in the

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literature exist mixed mode greenhouse solar dryers that are combined with photovoltaic–thermal

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(PVT), for grape drying (Barnwal and Tiwari, 2008; Barnwal and Tiwari, 2011), red pepper and grape

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(ELkhadraoui et al., 2015), bitter gourd (Srinivasan and Balusamy, 2015) or cassava chips (Aliyu and

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Jibril, 2009), but these studies are focussed also in the quality of the dried product such as the problem

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of decolouration, and they use a very small greenhouses with glass as coverage material, these

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systems can not be used for wood chips. The main goal of this study is to assess the solar greenhouse

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dryer system for wood chips improvement as biofuel.

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2. Material and methods

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The methodology adopted in this study consists of conceptualization of the solar greenhouse dryer

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configuration, design of the conceptualized dryer, construction of the designed dryer and evaluation of

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the solar greenhouse dryer compared to open sun drying; for this wood chips stacked in different

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shapes of piles were used.

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2.1 The solar greenhouse dryer

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The solar greenhouse dryer built in this study was the feature on conventional greenhouse type tunnel,

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see Figure 1. The dimensions and views are described in Figure 2, where the length (East-West) and 4

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structure was made by 9 arches of 3.70 m with radius covered with thermal plastic film with a

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thickness of 200 × 10-3 mm (800 G). The soil was covered by black plastic film, which was useful for

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the isolation of wood chips from the soil and it also helps to capture heat from the sun. The solar

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greenhouse dryer has two inspection doors, Figure 2-A. The middle the roof has a vent with

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dimensions of 14.85 m × 0.50 m, see Figures 2-C and 2-D.

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Figure 1: Diagram of drying process inside solar greenhouse dryer.

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Figure 2: Solar greenhouse dryer (unit in meters). A) Front view. B) Plan. C) Real front view. D) Perspective and roof ventilation.

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2.2. Solar radiation

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The solar greenhouse dryer was built at coordinates 36º50’10” N of latitude and 4º13’33” W of

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longitude, close to the town of Malaga in the south of Spain. Drying wood chips (Pinus Pinaster)

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in the outdoor condition and inside solar greenhouse dryer began on 30 October 2015 and was

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finished on 15th of the next month. Here, the values of radiation that occurred in the study area

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during the 15 days when this study was conducted are shown below in Table 2. The data were

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provided by a meteorological station close to the studied area.

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2.3. Wood chips and piles

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The wood chips were made on 30 October 2015, with wood chip (Pinus Pinaster) 2.68 cm half-length

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and an average width of 0.28 cm, having an average initial moisture content of 50%.

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The piles of wood chips were the following: two piles outdoors and three piles inside the solar

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greenhouse dryer. Every pile has a volume of one cubic meter. Table 1 shows the features of each

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pile. The Figure 3 shows pictures of each one. Figure 4-A shows the moisture measurement with

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Extech hygrometer M0210. The moisture content also was measured by weighing the piles of wood

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chips, using the hand scale Beurer LS06 (accuracy of 10 g), see Figure 4-B. The Findings of both

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measures gave the same results.

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ACCEPTED MANUSCRIPT Table 1: Shape of each pile Pile Code

Position

1-O-E

Open sun Extended

2-O-C

Open sun Conically shaped 2.00 (diameter)

3-I-E

Indoor

Extended

4-I-C

Indoor

Conically shaped 2 (diameter)

5-I-SC

Indoor

Shaped cordoned 6.00 × 0.70

Form

Plan Dimension (m x m) Height (m) 2.30 × 2.30

0.200 0.956

2.30 × 2.30

0.956

0.238

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Figure 3: Picture of each pile

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3. Results

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The experiments were carried out to test the performance of the designed and constructed solar

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greenhouse dryer compared to open sun drying. In the first step, environmental conditions as relative

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humidity and temperature were compared. In the second step the drying time was studied and in the

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third step the shape of the wood chips was analyzed and they were mathematically modelled.

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Figure 4: Moisture measurement of wood chips. A) Relative Humitidy (Extech MO220 Wood Moisture Meter). B) Weight (Beurer LS06).

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3.1. Indoor vs. Open Sun Drying

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In order to compare the indoor and open sun drying, during a week the temperature and humidity were

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monitoring at different hours of the day. The temperature and relative humidity of the air was recorded

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by Extech MO220 Wood Moisture Meter trade mark equipment, with a range interval of 6 to 99.9%,

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and accuracy of 0.1% for RH and 0.1°F/°C for temperature. The results have shown that, for outdoor

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the maximum temperature was 30.70°C. However, the temperature of the indoor solar greenhouse

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reached 55.90 °C, both measurements were taken at 13:00 p.m., this means difference of 25.20 °C

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higher temperature indoor, see Figure 5 where the difference is highlighted. The mathematical

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modelling for the solar greenhouse dryer and for open sun drying is shown in equations (1) and (2)

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respectively.

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TSGD = - 1.1944 × h2 + 33.817 × h - 184.65;

R2 = 0.872

(1)

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TO = - 0.3806 × h2 + 11.398 × h - 57.815;

R2 = 0.823

(2 )

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ACCEPTED MANUSCRIPT Where T is the temperature in °C and H is hour of the day; SGD is Solar Greenhouse dryer and O is

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Outdoor. For equations 1 and 2, if the maximum of the functions are calculated, it can be observed that

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indoor, at the solar greenhouse dryer, the maximum of 54.7ºC was reached at 14:12 h, while for open

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sun drying the maximum of 27.5ºC was reached at 15 h.

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Figure 5: Difference of the temperatures during the day: Open Sun vs. solar greenhouse dryer.

Figure 6 shows the relative humidity for the same period, and also for open sun drying and in the solar

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greenhouse dryer conditions. The recorded top values of relative humidity were at 9 a.m, being 53.0 %

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outdoor conditions and 38.0 % inside the solar greenhouse dryer. The mathematical modelling for the

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solar greenhouse dryer and for outdoor conditions is shown in equations (3) and (4) respectively.

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RHSGD = 0.7778 × h2 - 23.733 × h + 191.4;

R2 = 0.9765

(3)

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RHO= 0.5556 × h2 - 18.8 × h + 181.8;

R2 = 0.9905

(4)

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Where RH is the relative humidity and H is the hour of the day; SGD is Solar Greenhouse dryer and O

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is Open sun drying. For equations 3 and 4, if the minimums of relative humidity are calculated, it can

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be observed that indoor, at the solar greenhouse dryer, the minimum of 10.36 % was reached at 15:18

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h, while for open sun drying the minimum of 22.76 % was reached at 16:54 h. These data can be used 9

ACCEPTED MANUSCRIPT for knowing the optimal hour for the extraction of matter (wood chips) inside the greenhouse dryer or

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keeping the matter from open sun drying. As the levels of temperature and relative humidity are the

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two main environmental factors that influence the drying time, it is demonstrated that the

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environmental conditions generated inside the solar greenhouse dryer are more favourable for a wood

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chip drying in less time and also suitable for reaching a final moisture content less than what can be

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achieved in drying outdoors for the same place and time of the year. Anyway it must be proved

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experimentally. Also, in the following sections these results are shown.

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Figure 6: Relative humidity (%) during the day: Outdoors vs Solar greenhouse dryer.

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3.2. Drying Time for Wood chips drying: indoor vs. Open Sun Drying

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The initial moisture of wood chips was 50 %, this is the value for the beginning day of the drying

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process. For each pile of wood chips three samples were extracted, three in top layer, three in the

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middle layer and three in the bottom layer. These samples were measured daily by a hygrometer

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(Extech MO220, see Figure 4) in order to record the daily contents of humidity. This evaluation

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continued until the time needed to reach a moisture content of relative final humidity in humid base

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around 10 %, this is 15 days after the beginning. A lot of space between wood chips enhances air

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movements inside the pile (Pecenka et al. 2014) with subsequent reduction of moisture content (van 10

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Blijderveen et al., 2010) and microbial activity. Thus, to avoid the rotting of the wood chips, daily, the

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piles were mixed by hand.

231 It is necessary to mention that the door management was for controlling the relative humidity inside

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the solar greenhouse dryer. In the three first days, the humidity evaporated from the wood chips which

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reached up to levels of 40 % inside the greenhouse, hence the doors were open during the first three

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days all the time in order to achieve at least 30 % of relative humidity in the wood chips. After that, the

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doors remained closed the rest of the time, except for two hours a day and a half hour at night.

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Table 2 shows the measured values of each pile. It is observed that solar greenhouse dryer piles reach

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less humidity in less time, being the best method for the pile 5-I-SC (Shaped cordoned), thus inside the

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solar greenhouse dryer, for the best pile (5-I-SC) it can reach 10 % of relative humidity in 8 days,

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instead of the best pile outdoor reach 20 % of RH this day, of course this pile has the disadvantage that

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occupies 8 times more surface than the others.

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Table 2: Relative Humidity (RH) for all piles and Radiation Data, in year 2015 Radiation (MJ/(m2·d))

Relative Humidity (%)

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Day

1-O-E

2-O-C

3-I-E

4-I-C

5-I-SC

14.90

50.00

50.00

50.00

50.00

50.00

2

15.60

38.65

37.67

35.52

31.65

29.00

15.50

23.68

26.90

29.22

29.60

28.75

15.30

24.23

29.25

26.03

28.67

24.15

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13.80

24.77

26.50

24.98

27.65

19.98

6

12.80

21.13

29.85

21.72

24.38

12.05

7

15.10

19.57

26.32

20.95

21.60

11.77

8

1450

21.30

19.53

20.73

19.67

10.04

9

13.10

20.95

20.05

16.25

19.67

10

13.30

15.32

18.78

14.98

17.54

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14.20

17.22

15.00

14.85

16.45

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12.70

12.88

19.37

13.77

14.30

13

13.70

16.03

22.80

10.00

10.00

14

13.60

18.87

20.92

15

8.00

15.47

13.85

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The drying process involves simultaneous: (i) heat transfer from the surrounding to the surface of the

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product being dried combined with heat transmission within the material; and (ii) mass transfer from

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inside the product to its surface, followed by external transport of moisture to the surroundings (Di

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Scala and Crapiste, 2008). Solar dryers and drying modes can be classified on Active Dryers with

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forced circulation and Passive Dryers with natural circulation (Leon et al. 2002), and the combination

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of both called mixed-mode. However, standard test procedures for evaluating the performance of solar

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dryers are not available (Bena and Fuller, 2002). The main solar dryer evaluation parameters for

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thermal performance are the drying time/rate, the drying temperature and relative humidity (Fudholi et

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al., 2010; Belessiotis, and Delyannis, 2011).

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In a previous experiment of the literature where mixed mode greenhouse solar dryer were used, highest

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temperatures inside the greenhouse could be reached in comparison with open sun conditions: 20 ºC

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more than open sun (Aliyu and Jibril, 2009; Srinivasan and Balusamy, 2015: Tiwari et al., 2016).,

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these results are consistent with that obtained in this work, but the relative humidity can not be

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compared due the nature of the material to be dried or the type of greenhouse solar dryer (mixed).

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For wood, previous studies were carried out, such as the solar wood dryer tested at University of

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Arkansas (De Vore et al., 1999), where three to four months were necessary for drying wood to 9%

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moisture; special attention have received also the storage systems. Another study makes simulations of

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a wood solar dryer but based on non empirical data and using glasshouse (Bentayeb et al. 2008).

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The required energy for drying wood in conventional dryers ranges from 600 to 1000 kWh m−3

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depending on wood type and thickness (Awadalla et al. 2004). If the wood is dried for example from

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50% moisture content to 20%, the lower heating value (LHV) increases from 0.81 MWh/m3 to 0.89

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MWh/m3, i.e. 10% (Rinne et al., 2014). This is due to the energy wasted for the evaporation in the

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combustion process decreases accordingly to the decrease of the moisture content (MC) of the fuel,

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and, on the contrary, the cost of separate drying increases (Ruiz et al., 2013).

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ACCEPTED MANUSCRIPT Modelling of the wood drying process is advantageous for system design, development of control

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strategies and system optimisation ((Bentayeb et al. 2008). The relative humidity of piles versus drying

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days is shown in Figure 8. It is apparent that the drying rate decreases continuously with moisture

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content or drying time. There is not any constant rate drying period in these curves. If the data is

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mathematically modelled, as shown in Table 3, where different equation were checked, it is observed

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that Logarithmic trend has got better R² in every pile. As it is represented in Figure 7, with this trend

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line, it is clear that the pile 5-I-SC (Shaped cordoned) has got the best results, but also, it has the

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drawback of the huge area occupied. Regarding the shape of the piles, the extended show better results

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in both cases as expected, compared to conical shape. For the outdoor piles (1-O-C and 2-O-C), their

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trend lines are over the others after the third to fifth days, and these lines don’t reach the 10 % of

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relative humidity. This means, that piles in open sun drying acquire moisture, probably at night, due to

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the proximity of the sea, less than 10 km. This also shows an unexpected aspect of the solar

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greenhouse dryer, it protects against environmental moisture. Then it guarantees reaching the desired 10

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% in less time, and also maintains the reached humidity conditions.

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Table 3. Mathematical modelling of drying process. RH = Relative Humidity; d = drying day. Logarithmic trend

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Exponential trend Equation

R²

1-O-E

RH = 36.192 × e-0.067d

0.7123

RH = -10.99 × ln (d) + 45.552 0.8573

2-O-C

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Piles Code

RH = 39.629 × e-0.064d

0.7222

RH = -11.52 × ln (d) + 44.096 0.8625

3-I-E

RH = 44.832 × e-0.108d

0.9471

RH = -13.83 × ln (d) + 46.99

4-I-C

RH = 44.896 × e-0.101d

0.9273

RH = -12.84 × ln (d) + 46.211 0.9399

5-I-SC

RH = 54.779 × e-0.221d

0.9495

RH = -18.34 × ln (d) + 47.529 0.9532

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Equation

R²

0.975

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3-I-E 4-I-C 5-I-SC 1-O-E 2-O-C Trend 3-I-E Trend 4-I-C Trend 5-I-Sc Trend 1-O-E Trend 2-O-C

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Relative Humidity RH (%)

50

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RH = -11.52×ln(d) + 44.096

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RH = -10.99×ln(d) + 45.552 RH = -12.84×ln(d) + 46.211 RH = -13.83×ln(d) + 46.99

RH = -18.34×ln(d) + 47.529

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Drying day (d)

Figure 7: Logarithmic trends of relative humidity (%) during the drying days (d): Outdoors condition

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vs. Solar greenhouse dryer

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For the modelling of the drying process, inside the solar greenhouse dryer, 3 phases are clearly defined

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to dry the piles that are drawn in Figure 8, these were as follow:

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1) Where the speed of drying was very fast in a short time, because the process balances the

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moisture content of the wood chips with the circulating air. This means to reduce the moisture

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to values which are close to 30% in the first 24 to 48 hours. This was called fast dry.

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2) The slope of the drying curve is constant, reaching values of 20 to 30 % of RH for 3-I-E and 4I-C respectively. This takes 4 days. 3) The slope of the drying curve is decreasing very slowly in order to achieve the 10 % of RH. This takes 7 days. 14

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Figure 8: Drying phases were determined in the solar greenhouse dryer.

Over half percent saving in drying time was achieved compared with the open sun drying method if

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shaped cordoned piles are used. Previous studies indicated that natural drying is a viable and methods

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to enhance energy efficiency of wood based fuel products are effective but considers spring and

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summer as seasons for drying (Röser et al., 2011), and other studies highlighted the positive effect of

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covering piles to lower moisture content over the winter months (Jirjis, 1995; Numi et al., 2007). In

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our study, with these good results, it demonstrated the effectiveness of solar greenhouse dryer in the

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autumn season. Of course, it can be used also in summer, when no crops are possible inside the

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greenhouses in these latitudes (Manzano-Agugliaro and Cañero-Leon, 2010; Marquez et al., 2011).

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Then, another advantage of this solar greenhouse dryer is that it can be used for either crops or solar

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dryer, unlike other solar dryer that are only used for this purpose (Phusampao et al., 2014). In

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summary, the field experimental results showed that the performance of the solar greenhouse dryer

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was very encouraging when compared to open sun drying method.

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Additionally, other advantages are that the problems of insects, mould, fungi and rodent infestation can

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be however removed since the products were protected in a drying process; also and dirt and dust can

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be eliminated, thereby producing better quality products (Aliyu and Jibril, 2009).

320 5. Conclusion

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In this study, we have first conducted a comparative study between open sun drying understood as

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open sun and indoor the designed solar greenhouse dryer, in order to determine the difference between

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both environmental conditions. The solar greenhouse dryer can achieve 25.20 °C higher temperature

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and 20 % less relative humidity than open sun drying for the experimentally tested in the south of

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Spain. Thus, it was demonstrated that the environmental conditions generated inside the solar

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greenhouse dryer are more favourable for a wood chips drying in less time and also suitable for

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reaching final moisture content less than what can be achieved in drying outdoors for the same place

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and time of year. The models of piles tested show best results for the extended or cordoned against

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conically shaped for the same volume. The wood chips can be dried, 10 % of relative humidity

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achieved in 13 days in normal conditions, with an average solar radiation of 13.74 MJ/m2 (autumn

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season). It could be concluded that, the use of the solar greenhouse dryer allows improving the wood

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chips as biofuel by reducing the relative humidity in a shortest period of time when it is compared to

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open sun drying.

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Figures Caption Figure 1: Diagram of drying process inside solar greenhouse dryer Figure 2: Solar greenhouse dryer (unit in meters). A) Front view. B) Plan. C) Real front view. D) Perspective and roof ventilation. Figure 3: Picture of each pile

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Figure 4: Moisture measurement of wood chips. A) Relative Humitidy (Extech MO220 Wood Moisture Meter). B) Weight (Beurer LS06).

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Figure 7: Logarithmic trends of relative humidity (%) during the drying days: Outdoors condition vs.

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Figure 6: Relative humidity (%) during the day: Outdoors vs Solar greenhouse dryer.

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Figure 5: Difference of the temperatures during the day: Open Sun vs. solar greenhouse dryer.

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Highlights Solar dryers are considered as clean technologies because only use solar energy A solar greenhouse dryer was used for drying wood chips of Pinus pinaster

The results show the advantages of the Solar Greenhouse Dryer

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Several piles of wood chips were compared in outdoor and indoor conditions

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The use of the solar greenhouse dryers allow to improve the wood chips as biofuel