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HYBRID SYSTEM SIMULATION TO SUPPLY HEATED AIR TO A SOLAR FOOD DRYER

ABSTRACT

The intermittence of solar radiation, due to continuous rainy or cloudy days, is a limitation of simple and small solar dryers. These conditions often make them impossible to use. By including storage systems (thermal accumulation) and/or auxiliary energy sources, drying processes or dehydration can be conducted continuously, even during periods of low insolation. Therefore, the present work simulates and evaluates the thermal and energetic behavior of a hybrid system for heating the air that is directed to the dehydration chamber of a solar food dryer. The software selected for the simulation was TRNSYS. The simulated hybrid system consists of a flat plate solar collector and an arrangement of electrical resistors that guarantee the entry of air, at a constant temperature, into the dehydration chamber. The target temperature selected is 70 oC, and the absence of food products in the chamber is assumed. An arrangement with four electric resistors totaling 1900 W, with three different powers of 1000 W, 500 W, and 200 W proved adequate to guarantee the entrance of air at a constant temperature when considering the climatic conditions of a city in the South of Brazil.

KEYWORDS
electrical resistor arrangement; flat plate solar collector; auxiliary heating system; TRNSYS software

INTRODUCTION

Solar energy is commonly used for the sustainable development of the agricultural sector, standing out as a viable option, particularly in remote rural areas (Mekhilef et al., 2013Mekhilef S, Faramarzi SZ, Saidur R, Salam Z (2013) The application of solar technologies for sustainable development of agricultural sector. Renewable and Sustainable Energy Reviews 18:83-594. DOI: https://doi.org/10.1016/j.rser.2012.10.049
https://doi.org/10.1016/j.rser.2012.10.0...
). A direct application is drying, which reduces spoilage of food after harvest. This technique is widely used in developing countries (Mohanraj & Chandrasekar, 2008Mohanraj M, Chandrasekar P (2008) Drying of copra in a forced convection solar drier. Biosystems Engineering 99(4):604-607. DOI: https://doi.org/10.1016/j.biosystemseng.2007.12.004
https://doi.org/10.1016/j.biosystemseng....
; Queiroz et al., 2011Queiroz AJ de M, Dantas HJ, Figueirêdo RMF de, Melo K dos S (2011) Solar drying of jack fruit almonds. Engenharia Agrícola 31(6):1150-1161. DOI: http://dx.doi.org/10.1590/S0100-69162011000600012
http://dx.doi.org/10.1590/S0100-69162011...
; Shamekhi-Amiri et al., 2018Shamekhi-Amiri S, Gorji TB, Gorji-Bandpy M, Jahanshahi M (2018) Drying behaviour of lemon balm leaves in an indirect double-pass packed bed forced convection solar dryer system. Case Studies in Thermal Engineering 12:677-686. DOI: https://doi.org/10.1016/j.csite.2018.08.007
https://doi.org/10.1016/j.csite.2018.08....
) because of its low cost, low payback time (El Hage et al., 2018El Hage H, Herez A, Ramadan M, Bazzi H, Khaled M (2018) An investigation on solar drying: A review with economic and environmental assessment. Energy 157:815-829. DOI: https://doi.org/10.1016/j.energy.2018.05.197
https://doi.org/10.1016/j.energy.2018.05...
), and for providing increased income to small producers in a sustainable manner.

However, the intermittent behavior of solar radiation and the climatic conditions of the site do not allow the continuous functioning of solar dryers. These characteristics make continuous dehydration of the food impossible, resulting in incomplete dehydration, an unsuitable product, or a product with unsatisfactory characteristics for consumption. The complete drying process, in most cases, is longer than the daily insolation period (El Hage et al., 2018El Hage H, Herez A, Ramadan M, Bazzi H, Khaled M (2018) An investigation on solar drying: A review with economic and environmental assessment. Energy 157:815-829. DOI: https://doi.org/10.1016/j.energy.2018.05.197
https://doi.org/10.1016/j.energy.2018.05...
). Thus, for the simpler solar dryers, the challenge is to ensure that the drying process is continuous, irrespective of periods of low insolation, such as cloudy or rainy days, and no insolation at night.

In the last decades, several studies have been conducted (Ekechukwu & Norton, 1999Ekechukwu OV, Norton B (1999) Review of solar-energy drying systems II: an overview of solar drying technology. Energy Conversion & Management 40:615-655. DOI: https://doi.org/10.1016/S0196-8904(98)00093-4
https://doi.org/10.1016/S0196-8904(98)00...
; Murthy, 2009Murthy MVR (2009) A review of new technologies, models and experimental investigations of solar driers. Renewable and Sustainable Energy Reviews 13:835-844. DOI: https://doi.org/10.1016/j.rser.2008.02.010
https://doi.org/10.1016/j.rser.2008.02.0...
; El-Sebaii & Shalaby, 2012El-Sebaii AA, Shalaby SM (2012) Solar drying of agricultural products: A review. Renewable and Sustainable Energy Reviews 16(1):37-43. DOI: https://doi.org/10.1016/j.rser.2011.07.134
https://doi.org/10.1016/j.rser.2011.07.1...
; El Hage et al., 2018El Hage H, Herez A, Ramadan M, Bazzi H, Khaled M (2018) An investigation on solar drying: A review with economic and environmental assessment. Energy 157:815-829. DOI: https://doi.org/10.1016/j.energy.2018.05.197
https://doi.org/10.1016/j.energy.2018.05...
; Lamidi et al., 2019Lamidi RO, Jiang L, Pathare PB, Wang YD, Roskilly AP (2019) Recent advances in sustainable drying of agricultural produce: A review. Applied Energy 233-234:67-385. DOI: https://doi.org/10.1016/j.apenergy.2018.10.044R
https://doi.org/10.1016/j.apenergy.2018....
; Anannob et al. 2020Anannob AA, Masuda MH, Dabnichkia P, Ahmedb A (2020) Design and numerical analysis of a hybrid geothermal PCM flat plate solar collector dryer for developing countries. Solar Energy 196:270-286. DOI: https://doi.org/10.1016/j.solener.2019.11.069
https://doi.org/10.1016/j.solener.2019.1...
). Most of them aim to increase the efficiency of solar dryers and even allow their nocturnal use (Queiroz et al., 2011Queiroz AJ de M, Dantas HJ, Figueirêdo RMF de, Melo K dos S (2011) Solar drying of jack fruit almonds. Engenharia Agrícola 31(6):1150-1161. DOI: http://dx.doi.org/10.1590/S0100-69162011000600012
http://dx.doi.org/10.1590/S0100-69162011...
; El Khadraoui et al., 2017El Khadraoui A, Bouadila S, Kooli S, Farhat A, Guizani A (2017) Thermal behavior of indirect solar dryer: Nocturnal usage of solar air collector with PCM. Journal of Cleaner Production 148:37-48. DOI: http://dx.doi.org/10.1016/j.jclepro.2017.01.149
http://dx.doi.org/10.1016/j.jclepro.2017...
), focusing on modeling (Prakash et al., 2016Prakash O, Laguri V, Pandey A, Kumar A, Kumar A (2016) Review on various modelling techniques for the solar dryers. Renewable and Sustainable Energy Reviews 62:396-417. DOI: http://dx.doi.org/10.1016/j.rser.2016.04.0281
http://dx.doi.org/10.1016/j.rser.2016.04...
; Dhanushkodi et al, 2017Dhanushkodi S, Wilson VH, Sudhakar K (2017) Mathematical modeling of drying behavior of cashew in a solar biomass hybrid dryer. Resource-Efficient Technologies 3(4):359-364. DOI: https://doi.org/10.1016/j.reffit.2016.12.0
https://doi.org/10.1016/j.reffit.2016.12...
; Anannob et al. 2020Anannob AA, Masuda MH, Dabnichkia P, Ahmedb A (2020) Design and numerical analysis of a hybrid geothermal PCM flat plate solar collector dryer for developing countries. Solar Energy 196:270-286. DOI: https://doi.org/10.1016/j.solener.2019.11.069
https://doi.org/10.1016/j.solener.2019.1...
), in the simulation (Hernandez et al., 2016Hernandez RO, Guerra GLM, Tinôco IFF, Martins JH, Souza C de F, Osorio S JA (2016) Simulation of the internal environment of a post-harvest installation and a solar dryer of coffee. Revista Brasileira de Engenharia Agrícola e Ambiental 20(2):163-168.; Sonthikun et al., 2016Sonthikun S, Chairat P, Fardsin K, Kirirat P, Kumar A, Tekasakul P (2016) Computational fluid dynamic analysis of innovative design of solar- biomass hybrid dryer: An experimental validation. Renewable Energy 92:185-191. DOI: http://dx.doi.org/10.1016/j.renene.2016.01.095
http://dx.doi.org/10.1016/j.renene.2016....
; Basso, 2017Basso DM (2017) Simulação do sistema de aquecimento de ar de um secador solar híbrido de produtos agroalimentícios usando o TRNSYS. MSc Thesis, Porto Alegre, Universidade Federal do Rio Grande do Sul. Available: http://www.bibliotecadigital.ufrgs.br/da.php?nrb=001025068&loc=2017&l=b84e29e817f833c6
http://www.bibliotecadigital.ufrgs.br/da...
), and the experiment (Pangavhane et al., 2002Pangavhane DR, Sawhney RL, Sarsavadia PN (2002) Design, development and performance testing of a new natural convection solar dryer. Energy 27(6):579-590. DOI: https://doi.org/10.1016/S0360-5442(02)00005-1
https://doi.org/10.1016/S0360-5442(02)00...
; Mohanraj & Chandrase kar, 2008Mohanraj M, Chandrasekar P (2008) Drying of copra in a forced convection solar drier. Biosystems Engineering 99(4):604-607. DOI: https://doi.org/10.1016/j.biosystemseng.2007.12.004
https://doi.org/10.1016/j.biosystemseng....
; Amer et al., 2010Amer BMA, Hossain MA, Gottschalk K (2010) Design and performance evaluation of a new hybrid solar dryer for banana. Energy Conversion and Management 51(4):813-820. DOI: https://doi.org/10.1016/j.enconman.2009.11.016
https://doi.org/10.1016/j.enconman.2009....
; Okoroigwe et al., 2013Okoroigwe EC, Eke MN, Ugwu HU (2013) Design and evaluation of combined solar and biomass dryer for small and medium enterprises for developing countries. International Journal of Physical Sciences 8(25):1341-1349. DOI: https://doi.org/10.5897/IJPS2013.3937
https://doi.org/10.5897/IJPS2013.3937...
; Sonthikun et al., 2016Sonthikun S, Chairat P, Fardsin K, Kirirat P, Kumar A, Tekasakul P (2016) Computational fluid dynamic analysis of innovative design of solar- biomass hybrid dryer: An experimental validation. Renewable Energy 92:185-191. DOI: http://dx.doi.org/10.1016/j.renene.2016.01.095
http://dx.doi.org/10.1016/j.renene.2016....
; Bhardwaj et al., 2017Bhardwaj AK, 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 Studies in Thermal Engineering 10:302-314. DOI: http://dx.doi.org/10.1016/j.csite.2017.07.009
http://dx.doi.org/10.1016/j.csite.2017.0...
; Shamekhi-Amiri et al., 2018Shamekhi-Amiri S, Gorji TB, Gorji-Bandpy M, Jahanshahi M (2018) Drying behaviour of lemon balm leaves in an indirect double-pass packed bed forced convection solar dryer system. Case Studies in Thermal Engineering 12:677-686. DOI: https://doi.org/10.1016/j.csite.2018.08.007
https://doi.org/10.1016/j.csite.2018.08....
) of different types of solar driers and sustainable drying techniques.

For example, solar dryers with an auxiliary power source need less time to reach the desired humidity levels (Mekhilef et al., 2013Mekhilef S, Faramarzi SZ, Saidur R, Salam Z (2013) The application of solar technologies for sustainable development of agricultural sector. Renewable and Sustainable Energy Reviews 18:83-594. DOI: https://doi.org/10.1016/j.rser.2012.10.049
https://doi.org/10.1016/j.rser.2012.10.0...
). Among the auxiliary energy sources, biomass (Okoroigwe et al., 2013Okoroigwe EC, Eke MN, Ugwu HU (2013) Design and evaluation of combined solar and biomass dryer for small and medium enterprises for developing countries. International Journal of Physical Sciences 8(25):1341-1349. DOI: https://doi.org/10.5897/IJPS2013.3937
https://doi.org/10.5897/IJPS2013.3937...
; Sonthikun et al., 2016Sonthikun S, Chairat P, Fardsin K, Kirirat P, Kumar A, Tekasakul P (2016) Computational fluid dynamic analysis of innovative design of solar- biomass hybrid dryer: An experimental validation. Renewable Energy 92:185-191. DOI: http://dx.doi.org/10.1016/j.renene.2016.01.095
http://dx.doi.org/10.1016/j.renene.2016....
; Dhanushkodi et al., 2017Dhanushkodi S, Wilson VH, Sudhakar K (2017) Mathematical modeling of drying behavior of cashew in a solar biomass hybrid dryer. Resource-Efficient Technologies 3(4):359-364. DOI: https://doi.org/10.1016/j.reffit.2016.12.0
https://doi.org/10.1016/j.reffit.2016.12...
) and electricity (Amer et al., 2010Amer BMA, Hossain MA, Gottschalk K (2010) Design and performance evaluation of a new hybrid solar dryer for banana. Energy Conversion and Management 51(4):813-820. DOI: https://doi.org/10.1016/j.enconman.2009.11.016
https://doi.org/10.1016/j.enconman.2009....
) stand out. Indirect drying with natural convection (Pangavhane et al., 2002Pangavhane DR, Sawhney RL, Sarsavadia PN (2002) Design, development and performance testing of a new natural convection solar dryer. Energy 27(6):579-590. DOI: https://doi.org/10.1016/S0360-5442(02)00005-1
https://doi.org/10.1016/S0360-5442(02)00...
) or forced drying (Mohanraj & Chandrasekar, 2008Mohanraj M, Chandrasekar P (2008) Drying of copra in a forced convection solar drier. Biosystems Engineering 99(4):604-607. DOI: https://doi.org/10.1016/j.biosystemseng.2007.12.004
https://doi.org/10.1016/j.biosystemseng....
) provide better quality products than direct drying, in which the product is exposed to the sun. Also, forced convection shortens the drying time. Regarding thermal energy storage systems, water reservoirs (Queiroz et al., 2011Queiroz AJ de M, Dantas HJ, Figueirêdo RMF de, Melo K dos S (2011) Solar drying of jack fruit almonds. Engenharia Agrícola 31(6):1150-1161. DOI: http://dx.doi.org/10.1590/S0100-69162011000600012
http://dx.doi.org/10.1590/S0100-69162011...
), water reservoirs with electrical resistors and heat exchangers (Amer et al, 2010Amer BMA, Hossain MA, Gottschalk K (2010) Design and performance evaluation of a new hybrid solar dryer for banana. Energy Conversion and Management 51(4):813-820. DOI: https://doi.org/10.1016/j.enconman.2009.11.016
https://doi.org/10.1016/j.enconman.2009....
), sand (Mohanraj & Chandrasekar, 2008Mohanraj M, Chandrasekar P (2008) Drying of copra in a forced convection solar drier. Biosystems Engineering 99(4):604-607. DOI: https://doi.org/10.1016/j.biosystemseng.2007.12.004
https://doi.org/10.1016/j.biosystemseng....
), stone bed (Shamekhi-Amiri et al., 2018Shamekhi-Amiri S, Gorji TB, Gorji-Bandpy M, Jahanshahi M (2018) Drying behaviour of lemon balm leaves in an indirect double-pass packed bed forced convection solar dryer system. Case Studies in Thermal Engineering 12:677-686. DOI: https://doi.org/10.1016/j.csite.2018.08.007
https://doi.org/10.1016/j.csite.2018.08....
), granite bed (Nemś et al., 2018Nemś M, Nemś A, Pcyga P (2018) A Granite bed storage for a small solar dryer. Materials 11(10):1969-1984. DOI: https://doi.org/10.3390/ma11101969
https://doi.org/10.3390/ma11101969...
), and materials with phase change - PCM (Kant et al., 2016Kant K, Shukla A, Sharma A, Kumar A, Jain A (2016) Thermal energy storage based solar drying systems: A review. Innovative Food Science & Emerging Technologies 34:86-99. DOI: http://dx.doi.org/10.1016/j.ifset.2016.01.007
http://dx.doi.org/10.1016/j.ifset.2016.0...
), such as paraffin (Bhardwaj et al., 2017Bhardwaj AK, 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 Studies in Thermal Engineering 10:302-314. DOI: http://dx.doi.org/10.1016/j.csite.2017.07.009
http://dx.doi.org/10.1016/j.csite.2017.0...
; El Khadraoui et al., 2017El Khadraoui A, Bouadila S, Kooli S, Farhat A, Guizani A (2017) Thermal behavior of indirect solar dryer: Nocturnal usage of solar air collector with PCM. Journal of Cleaner Production 148:37-48. DOI: http://dx.doi.org/10.1016/j.jclepro.2017.01.149
http://dx.doi.org/10.1016/j.jclepro.2017...
), can be used.

Therefore, the use of an auxiliary energy source and/or a thermal energy storage system, such as thermo-accumulation, is presented as an alternative to minimize the effects arising from the intermittence and seasonality of solar radiation. However, for simple solar dryers, it is important to highlight that their main characteristics (low cost, easy handling, and inexpensive maintenance) should be maintained, particularly for applications in remote rural areas.

In this context, this study aims to perform the thermal and energy evaluation of a hybrid air heating system through simulation. Using this system, we aim to ensure the entry of air, at a constant temperature, into the dehydration chamber. The system comprises a flat plate solar collector and an electrical resistor and is simulated in the TRNSYS software (Klein et al., 2014Klein SA, Duffie JA, Mitchell JC, Kummer JP, Thornton JW (2014) TRNSYS users manual. Madison, University of Wisconsin, 17 ed.).

MATERIAL AND METHODS

The simulated model, shown in Figure 1C, is similar to a prototype, shown in Figures 1A and 1B, which does not yet have a hybrid heating system and is located in the metropolitan region of Porto Alegre, RS, Brazil. The climate of this region is humid subtropical and presents the four well-defined seasons of the year. However, it shows great variability in the elements of meteorological weather because it is located in a transition zone. The most covert season of the year starts around May 1 and ends in early February.

FIGURE 1
Prototype solar collector (A) and forced ventilation system (B). Model used in the simulation (C): (1) collector inlet and forced ventilation system, (2) flat plate solar collector, (3) collector outlet, (4) electrical resistor arrangement, (5) plenum, and (6) dehydration chamber.

The simulated solar collector is of the flat plate type, with a simple glass cover similar to the prototype collector shown in Figure 1A. The air circulation takes place under the absorber plate, and the flow is forced, as shown in Figure 1B. It also has constant flow of 0.028 kg s-1 (Basso, 2017Basso DM (2017) Simulação do sistema de aquecimento de ar de um secador solar híbrido de produtos agroalimentícios usando o TRNSYS. MSc Thesis, Porto Alegre, Universidade Federal do Rio Grande do Sul. Available: http://www.bibliotecadigital.ufrgs.br/da.php?nrb=001025068&loc=2017&l=b84e29e817f833c6
http://www.bibliotecadigital.ufrgs.br/da...
). Its dimensions are 2 m long by 1 m wide. The geographical north orientation and 42° slope used in the simulation are the same as the prototype.

The simulated hybrid heating system consists of the solar collector (items 1 to 3 of Figure 1C) and a set of electrical resistors (item 4 of Figure 1C) coupled from its output (item 5 of Figure 1C). The evaluation is carried out using the TRNSYS software, which simulates the behavior of systems on a transient basis for a period of one year on an hourly basis and for a total of 8760 hours. The software has a modular structure and each component, Type, models the operation of a part of the system (Klein et al., 2014Klein SA, Duffie JA, Mitchell JC, Kummer JP, Thornton JW (2014) TRNSYS users manual. Madison, University of Wisconsin, 17 ed.)

The meteorological data are obtained directly from the TRNSYS software database for the city of Porto Alegre (latitude 30° 1′ South and longitude 51° 13′ 43″ West). These data are generated by the Meteonorm software (Klein et al., 2014Klein SA, Duffie JA, Mitchell JC, Kummer JP, Thornton JW (2014) TRNSYS users manual. Madison, University of Wisconsin, 17 ed.)

To model the hybrid system shown in Figure 2, the following Types were used: weather data (Type15), flat plate solar collector with simple glass cover (Type561), energy integrator (Type24), output data export (Type25), electrical resistor arrangement (Type930), and schedule (Type14h).

FIGURE 2
Type Arrangement used in the hybrid heating system simulation.

The temperature used in the drying process of food products varies according to the type of product and, in some cases, for the same product different drying temperatures are used, as studied in Cruz et al. (2012)Cruz PMF da, Braga GC, Grandi AM de (2012) Composição química, cor e qualidade sensorial do tomate seco a diferentes temperaturas. Semina: Ciências Agrárias 33(4):1475-1486. DOI: http://dx.doi.org/10.5433/1679-0359.2012v33n4p1475
http://dx.doi.org/10.5433/1679-0359.2012...
. The value for the target temperature, Tset, of 70 oC was selected based on the following works available in specialized literature: Queiroz et al. (2011)Queiroz AJ de M, Dantas HJ, Figueirêdo RMF de, Melo K dos S (2011) Solar drying of jack fruit almonds. Engenharia Agrícola 31(6):1150-1161. DOI: http://dx.doi.org/10.1590/S0100-69162011000600012
http://dx.doi.org/10.1590/S0100-69162011...
, Corrêa et al. (2015)Corrêa LC, Andrade ET de, Martinazzo AP, D'Andrea EM, Sousa FA de, Figueira VG (2015) Cinética de secagem, contração volumétrica e análise da difusão líquida do figo (Ficus carica L.). Revista Brasileira de Engenharia Agrícola e Ambiental 19(8):797-802. DOI: http://dx.doi.org/10.1590/1807-1929/agriambi.v19n8p797-802
http://dx.doi.org/10.1590/1807-1929/agri...
, Basso (2017)Basso DM (2017) Simulação do sistema de aquecimento de ar de um secador solar híbrido de produtos agroalimentícios usando o TRNSYS. MSc Thesis, Porto Alegre, Universidade Federal do Rio Grande do Sul. Available: http://www.bibliotecadigital.ufrgs.br/da.php?nrb=001025068&loc=2017&l=b84e29e817f833c6
http://www.bibliotecadigital.ufrgs.br/da...
, and Nunes et al. (2017)Nunes JS, Lins ADF, Gomes JP, Silva WP da, Silva FB da (2017) Influência da temperatura de secagem nas propriedades físico-química de resíduos abacaxi. Agropecuária Técnica 38(1):41-46. DOI: dx.doi.org/10.25066/agrotec.v38i1.29991
dx.doi.org/10.25066/agrotec.v38i1.29991...
. In such cases, the temperature of 70 oC was among the temperatures used for different types of food products.

RESULTS AND DISCUSSION

Three days were selected for the thermal and energy evaluation of the hybrid system. These days represent the following situations: day when there was the largest demand of extra energy (Aug/03), day when there was the smallest demand of extra energy (15/Dec), and day when there was the largest constant demand of extra energy (02/Oct), in the period between 8 h and 18 h in real solar time.

The daily profile of the ambient temperature (Tamb) for the three days is shown in Figure 3A. The daily temperature profiles of the absorber plate (Tp) which is part of the collector, and the air temperature at the outlet of the collector (Tair) are shown in Figures 3B (Aug/03), 3C (Dec/15), and 3D (Oct/02), respectively.

FIGURE 3
Ambient Temperature (Tamb), Plate Temperature (Tp), and Air Temperature (Tair) in (A) three days, (B) August 3, (C) December 15, and (D) October 2.

The cloudy day is the one that presents the smallest difference between the maximum and minimum Tamb, 1.6 oC (Oct/02), in contrast with the days of greater irradiation, in clear sky conditions, 10.2 oC (Aug/03) and 12.1 oC (Dec/15), as shown in Figure 3A. Regarding the maximum difference between Tp and Tair, we have 8.6 oC in Aug/03, 8.4 oC in Dec/15, and 0.6 oC in Oct/02.

The difference is 2.8 oC when assessing the maximum difference between Tp and Tamb for the day of low irradiation. That is, the value of Tp is very close to the value of Tamb throughout the day, so that the rate of heat transfer (losses) between the collector and the environment is practically negligible, which is desirable.

However, the small maximum difference between Tp and Tair results in a negligible heat transfer rate for air heating. Therefore, in situations like these, much if not all, of the energy needed to heat the air must be supplied by the auxiliary heating system.

For clear sky days, the heat transfer rate between the plate and the air is higher, even though there is a greater loss of energy from the collector to the environment, as the maximum differences between Tp and Tamb are 40.3 oC in Aug/03 and 39.0 oC in Dec/15. In these situations, the solar collector is able to deliver some of the energy needed to heat the air.

The maximum values of Tair are 41.3 oC on Aug/03, 63.5 oC on Dec/15, and 18.5 oC on Oct/02. In only one of the three days selected, the maximum Tair arrives close to the value of Tset. This result shows that the flat plate solar collector alone is not enough to guarantee the heating of the air up to Tset, even on the day with the highest daily direct irradiation in the collector plane. That is, without the hybrid system (Sun + auxiliary energy source) the dryer is not able to provide the thermal conditions for continuous dehydration.

Concerning solar radiation, the software provides the energy incident in the collector plane every hour per area unit, instead of the flow (irradiance, G, in W m-2), which is instantaneous. Thus, the concept of mean hourly irradiance, Gm, in W m-2, is defined as the ratio between the direct hourly irradiance in the collector plane and the period of one hour. The behavior of Gm is shown in Figure 4A.

FIGURE 4
Mean hourly irradiance in the collector plane (A) and total hourly energy (Qtot) presented in terms of extra energy and useful energy gain for August 3 (B), December 15 (C), and October 2 (D).

In the case of August 3, the maximum value of Gm (922 W m-2) occurs at 12 h, as seen in Figure 4A. The useful energy gain, that is, the net rate at which energy is transferred to the fluid through the solar collector (Klein et al., 2014Klein SA, Duffie JA, Mitchell JC, Kummer JP, Thornton JW (2014) TRNSYS users manual. Madison, University of Wisconsin, 17 ed.), is shown in Figure 4B. As expected, the behavior of the useful energy gain resembles the irradiance behavior, reaching the maximum value of 3183 kJ m-2 at 12 h. That same behavior is observed on the other days.

The total hourly energy (Qtot) is the energy required to raise Tair to Tset temperature within one hour. That is, it is the sum of the useful energy gain and extra energy, with the latter being supplied by the auxiliary system. In the present case, the largest extra hourly energy demand occurs at 8 a.m. (6808 kJ m-2) while the smallest demand occurs at 12 p.m. (3000 kJ m-2), as shown in Figure 4B.

Qtot's behavior throughout the evaluated period is what is expected for a clear day, decreasing throughout the day due to the thermal inertia of the system. However, due to the significant difference between Tamb and Tset, the amount of energy needed for heating the air is greater than the amount needed for day Dec/15, as shown in Figure 4C.

On this day, the ambient temperature is higher and, consequently, the energy required for heating the air will be lower than in other cases. The largest demand for extra energy occurs at 8 a.m. (4291 kJ m-2) and the smallest demand occurs at 1 p.m. (748 kJ m-2), as seen in Figure 4C. The useful energy gain reaches its maximum at 1 pm (3808 kJ m-2).

For day Oct/2, it can be observed that the useful energy gain value is practically zero. As Tamb is practically constant and of low value, Qtot remains nearly constant throughout the day, around 5000 kJ m-2.

As in the thermal analysis, the energy analysis from Figures 4B, 4C, and 4D show that the heating provided by the solar collector alone is not enough to heat the air to the Tset temperature. We again see the need to use an auxiliary system.

As previously mentioned, the auxiliary power source is electrical (set of resistors) with variable power. In this sense, knowing the extra energy required, it is possible to determine the mean power (Pm) of the set of resistances for each hour, as stated in Table 1, and scale the hybrid system. The maximum and minimum values of Pm are between 208 W and 1891 W.

TABLE 1
Ratio between the extra energy and the total hourly energy, and the respective mean power of the electrical resistances (Pm) for each hour of the simulated days.

Since the selected days represent situations where there was the highest demand for extra power, lowest demand for extra power, and highest constant demand for extra power, a set consisting of two 200 W resistors, a 500 W resistor and a 1000 W resistor, totaling 1900 W, is enough to supply the power needed to heat the air to Tset for any day of the year.

Figure 5 presents the behavior throughout the year of the total energy in terms of useful energy gain and extra energy. In all months of the year, the use of the auxiliary heating system is required. The lowest demands for extra energy are in December and January, since the extra energy is equivalent to 55.3% of the total monthly energy. In June, we have the highest demand for extra energy, since the extra energy is equivalent to 88.5% of the total monthly energy.

FIGURE 5
Annual profile of the total monthly energy in terms of useful energy gain and extra energy

Once the hybrid air heating system is installed, the solar dryer will be able to ensure that the drying process is continuous, regardless of periods of low insolation, such as cloudy or rainy days, or no insolation at all. The chosen auxiliary source, electrical resistors, meets the desirable characteristics for simpler solar dryers, i.e. low cost, easy handling, and inexpensive maintenance.

CONCLUSIONS

This study performed the thermal and energy evaluation of a hybrid air heating system through simulation. With this system, the aim was to guarantee the entry of air, at a constant temperature, into the dehydration chamber. This condition is of paramount importance, as it allows the continuous drying of food, avoiding incomplete dehydration and/or products outside the conditions proper for consumption.

For the type of simulated solar dryer, the use of an auxiliary air heating system proved to be mandatory. In no month of the year, the use of the solar collector alone is sufficient to heat the air to 70 °C. In the best of the cases evaluated, the maximum air temperature at the collector outlet was equivalent to 90% of the target temperature. The energy evaluation confirmed the necessity of using this type of system and allowed the scaling of the electrical resistors.

The period in which there is the greatest demand for extra energy is between April and October. The highest demand occurs in June and the lowest demand occurs in December and January. From the energy analysis, a set-up consisting of four resistors, two of 200 W, one of 500 W, and one of 1000 W of power, totaling 1,900 W, was designed. In the simulation, this arrangement was able to guarantee the entry of air at a constant temperature, equal to 70° C, in the dehydration chamber.

In this manner, the dehydration of food can be performed continuously in periods of low insolation, such as cloudy or rainy days, with the use of the resistor arrangement. Nevertheless, the operating time of the dryer and the drying time can be increased, since the auxiliary heating system allows its operation in the first hours of the day and in the period before sunset, in which both have low insolation.

ACKNOWLEDGEMENTS

The authors would like to thank the Post Graduate Program in Mechanical Engineering, PROMEC, of the Federal University of Rio Grande do Sul, UFRGS, and the Coordination for the Improvement of Higher-Level Personnel, CAPES.

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Publication Dates

  • Publication in this collection
    22 Apr 2020
  • Date of issue
    Mar-Apr 2020

History

  • Received
    11 Sept 2018
  • Accepted
    04 Mar 2020
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