Energy-economic-environmental analysis of solar drying system: a review

Received Jan 30, 2019 Revised Jul 8, 2019 Accepted Jan 30, 2020 Solar drying is an emerging technology to preserve wide range of products from agriculture to animal-based products. The application of solar dryers, however must be evaluated to determine its benefit and effectiveness. In the evaluation of solar dryer performance, three criteria which are most important to look at are thermal performance, economic cost and environmental implications. Therefore, this paper attempts to review the thermoeconomic analysis and environmental evaluation on various solar drying system. Performance equations in energy–economic–environment analyses for solar drying syistems evaluation are presented. The CO2 emission, carbon mitigation, and earned carbon credit of various solar drying syistem are also presented.


INTRODUCTION
In the recent years of energy research, renewable sources are gaining much attention as the world is shifting from fossil fuel to alternative energy. One of the reasons that drives this shift is due to the increasing demand for energy in the future, that initiate the exploration for a more sustainable energy sources to last to the end of human lifetime. The alarming scene of environmental degradation and pollution is also another main reason that pushes for cleaner, and more responsible energy generation. Solar energy is the most accessible, readily available, and highly potential as renewable source of energy generation. The amount of solar radiation intensity that reaches the outer atmosphere is 1,360 W/m 2 , and after accounting for natural losses, the global radiation that reaches the ground is still high at the range of 800-1000 W/m 2 , on a clear sky sunny day in summer [1]. Due to its energetic potentials, solar energy is converted into useful applications in the form of thermal and electrical energy. Solar energy is widely used in solar thermal technology such as in solar collector systems [2][3][4], in photovoltaic/thermal systems [5][6][7][8][9][10][11][12][13][14][15][16] and in solar drying systems [17][18][19][20][21].
The role of solar thermal is theorized to be able to lower the burden on scarce renewable resources and also to supply renewable energy in conditions where no alternatives are available [22]. While the application for solar thermal systems is widely known in domestic sector, it also provides huge potential for industries to benefit from. Kylili et al. conducted a life-cycle assessment (LCA) on industrial solar thermal system (ISTS) in the Europe, and found significant energy and carbon savings from its application, which ranges from 35 -75 GJ and 2 -5 tonnes of CO2 per kWth depending on the geographical location, respectively [23].

Energy Analysis
Solar dryers take into application of energy conversion from solar to useful thermal energy for drying process. For this purpose, numerous methods and processes were developed and their effectiveness can be evaluated on many merits, such as energy efficiency, time to dry and product quality. In solar drying, thermal performance is a reliable indicator to study the system merits and can be quantified using energy analysis. Energetic performance is based on the first law of thermodynamics, which takes in to account the quantity of energy and the energy change in respect to the change in surroundings [30]. However, the drawbacks of energy analysis is that it only considers energies at inlet and outlet of the system, and sometimes is redeemed as insufficient for system optimization as it neglects the irreversibility and thermodynamic losses [31][32][33].
In general, energetic analysis on solar dryers can be done on two main components; the drying systems and the drying materials. Drying systems of solar dryers includes the solar absorber unit, drying chamber, and movement of heated drying air throughout the system. In short, energy analysis of solar dryer components is commonly done by applying heat transfer and energy balance based on the principle of energy conservation of the first law of thermodynamics. Determination of thermal performance of solar dryers are important to achieve maximum moisture removal while using minimum amount of energy [28].
In literature, there are several indicators that are commonly used to evaluate the thermal capacity of solar dryer components, especially for solar collector unit. The amount of useful heat that can be harness from solar collector can be calculated using heat removal factor, and the incident solar radiation, . value is depended on the material of construction used for collector, as well as the surface area, as suggested by (1) [34].

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(1) The energy used for moisture evaporation can be calculated as [35] (2) Thermal efficiency of solar collector is the ratio of heat gain by air passing through the collector to the energy gained due to solar irradiation, given by [36][37][38].

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(3) Another indicator commonly used in energetic analysis is the thermal efficiency of solar dryers, . Essentially, is the ratio of energy required to evaporate product's moisture to the energy consumed for the drying process. In short, thermal efficiency of the drying system is the ratio of the energy used for moisture evaporation to the energy input to the drying system.
In passive convection dryers, dryer efficiency calculation is based on the air movement due to natural buoyancy, whereas active dryers takes into account the energy input through electrical fans or blowers, given by respectively [39], [40]. Depending on the type of solar drying system, the energy consumed for drying process would need to account for all source of energy generated in the system. In hybrid system, usually photovoltaic-thermal (PVT) hybrid dryers, electrical efficiency of solar collector is quantified as the system takes electricity into energy generation.
The relationship between energy input to solar dryer and amount of water evaporated can also be used to define the performance of the dryer and to compare performance of the dryers. is Specific moisture extraction rate (SMER) in kg kWh -1 relates how much moisture can be removed per unit of energy, whereas specific energy consumption (SEC) is the reciprocal of SMER with units of kWh kg -1 [41] (7) (8) Pickup efficiency, or moisture removing efficiency of drying air is the efficiency measure on moisture extraction using hot air, and it can be calculated using (9) In hybrid systems where energy source comes from other than solar energy, solar fraction is determined to quantify the ratio of energy extraction of heat from solar collector to the overall energy available for the drying process [35]. Solar fraction can be expressed by (10) From the drying material components, effectiveness of drying can be associated with moisture reduction within the samples. The mass of water removed (W) from a wet product can be calculated by [28] (11) Moisture ratio, which is a dimensionless form of moisture content explains the ratio of remaining moisture to be removed at time t over initial total moisture present. In the study of drying, MR is an important tool to understand the kinetics and drying profile as they vary from one material to another. In fact, MR is found to be mostly adequate to describe the drying behavior of some fruits and vegetables as it translates to drying constant, k (s -1 ). This is an important parameter widely used in thin-layer modelling, to obtain drying curve as a function of time [42].

Economic Analysis
While energy analysis is a common approach used to minimize thermodynamic efficiencies within dryer system, thermoeconomic is a different take to estimate the cost-optimal structure and the optimal values of thermodynamic efficiencies in each component [43]. Thermoeconomic is viewed as a promising diagnostic tool, even for complex system [32]. Through economic analysis, solar dryer application has been proved to have undeniable improvement on carbon footprint reduction associated with the energy-intensive drying process. In a review article by Mathew et al., solar dryers are highly effective device with low investment to produce good quality of dried products. The unit cost of useful energy for solar dryers were found to vary from 0.0034 to 0.015 USD per MJ of energy for different types of drying products [44].
El-Hage et al. conducted an economic study to evaluate monetary savings due to application of industrial solar dryers under Lebanese climate. The energy cost saving is determined on monthly basis, where it is dependent on the percentage of time where solar dryer is used, Pr, the dryer energy consumption for operation , and the cost of electricity for one unit of kWh, . Depending on the Pr value which ranges from 0.1 to 1, the energy cost reduction records savings between $130 to $4160 per month for drying of 120kg of various vegetable samples.
From the determined SM and capital cost of the solar dryer, simple payback period (PP) for the dryer system was determined as follows (14) A more detailed economic analysis was performed by ELkhadraoui et al. who evaluated the economics of chapel-shaped greenhouse for red pepper and grape drying in Tunisia [45]. The payback period for the dryer system was determined to be short at 1.6 years. The calculation used takes into account the capital cost of the dryer , inflation rate i, interest rate on long term investment d, and the saving during first year of the dryer . This method of calculation is also used by [40], [46]. (15) Another approach for economic analysis is the incorporation of cost-benefit analysis to compare cost and benefits of solar drying to other means by taking into consideration the size, materials for construction, efficiency, operation, sophistication and sustainability of the driers which vary from countries to country. Past study on economic analysis on solar drying systems as show in Table 1.

Environmental Analysis
In practice, percentage of reductions on fuel consumption depends on the type and solar dryer system. The range of savings recorded can vary from 20-40 percent in hybrid systems, to total fuel elimination in natural ventilation greenhouse solar dryer [50]. Past study on environmental analysis on solar drying systems as shown in Table 2. CO2 mitigation is a tool to measure climate change potential with the opportunity to reduce greenhouse effect emission by capping total annual emissions and letting the market assign a monetary value to any shortfall through trading [51]. In carbon credit model, monetary incentives allow transactions among businesses and individuals to get involve in carbon footprint reduction and at the same time funds reduction schemes globally. Carbon credit is the component of energy analysis. A carbon credit is a generic term for any tradable certificate or permit representing the right to emit one tone of carbon or carbon dioxide equivalent. Carbon trading is also an application of an emission trading approach [22].  [23] 2018 Carbon savings Life-cycle assesment on environmental performance of industrial solar thermal system (ISTS). Large-scale ISTS applications were found to achieve energy and carbon savings ranging from 35 -75 GJ and 2 -5 tonnes of CO2 per kWth, depending on the geographical location. [44] 2018 Various The economic analysis of different driers has been discussed in this article. Embodied energy (EE) is the total energy required to produce any items, things, or services [40]. It is a variable commonly used in environmental analysis, to determine how much energy is associated with producing a unit of system by taking into account the energy used in extraction, processing, manufacturing, and transporting of the materials [52]. The calculations on EE serve as an indicator of the overall environmental impacts of materials and systems, as the energy consumed correlates to CO2 production which contributes to GHG emission. In analysis, EE calculation requires the quantification of the materials used in the construction and maintenance of the dryer over its entire life time. The mass values of the different materials were then multiplied by the embodied energy coefficients of the corresponding materials (EEC), usually expressed in MJ kg -1 to give the total EE for the overall equipment [53].
Energy Payback Time (EPBT) is the time required to pay back the EE, can be calculated as (16) Carbon credit is a tool that represents any tradable certificate or permit that grants the right for businesses or industries to emit one tone of carbon or carbon dioxide equivalent, which is essential in the application of emission trading approach [54]. They provide a way to reduce greenhouse effect emissions on an industrial scale by capping total annual emissions and letting the market assign a monetary [51]. Carbon credit model is commonly used to calculate the carbon mitigation involved with the usage of solar dryers, as well as the earned carbon credit associated. The overall CO2 mitigation over dryer lifetime is calculated as the difference of total CO2 mitigation and total CO2 emission where is the annual thermal output energy of the dryer, is the dryer lifetime, and is the CO2 mitigation per kWh of the dryer. The equation for X is given as follows 0.98 (18) where the first term accounts for power consumption loss, (10%), and second term for energy loss due to transmission and distribution, (45%). Therefore, at given and values, the amount of CO2 mitigation of the system, X is determined to be 2.01 kg.
From the quantified net lifetime CO2 mitigation of the dryer system, earned carbon credit can be calculated by multiplying the value with the cost of carbon credit, D which ranges from USD 5-20 per ton of CO2.

(19)
A simpler environmental analysis was performed by Elhage et al. who studied the amount of CO2 reduction in relation to percentage of solar dryer usage, mass of drying sample and type of food being dried under Lebanese climate. By quantifying the amount of energy consumption per month of the dryer , the amount of CO2 produced , and amount of reduction in CO2 emission , 2 by the system is quantified as , where is the amount of CO2 produced from 1 kWh electricity which differs from one place to another.

CONCLUSIONS
Solar drying is a highly potential application of solar thermal technology. The use of solar dryers for drying of agricultural produce as well as poultry and marine products results in higher product quality through better control of drying process. One approach to evaluate the thermal performance of solar dryers is done through energy analysis which is discussed in detail in this review. Solar dryers also contribute to environmental conservation, as it reduces the energy demand in the food post-harvesting sector. To evaluate