Techno-economic assessment of a greenhouse solar coffee dryer: a case of Coffee arabica L. in Ethiopia

Yam is an important food crop in Ghana. It is a delicacy that is eaten after cooking or frying the fresh sample when cut into slices. The powders of the dried yam can be incorporated into soups, baby food and bread as composite flours. However, the high moisture content of yam when harvested (about 70%wb) makes it susceptible to spoilage leading to reduced shelf life and onward rejection by the consumer. This adds to the waste basket. Drying is a worldwide postharvest method for extending the shelf life of agricultural food products such as yam. An energy source that is cheap and clean has always been from the sun. The open sun method has been utilized severally and for many years, but this is subject to various contaminants and thus reducing its quality. A confined solar drying method is a better option but the energy source is limited to the day’s solar energy. Solar adsorption drying is an opportunity to dry day and night in an efficient and product conserving manner with a reduced drying cycle. Prior to this thesis work, several aspects were insufficiently known to really judge the potential. The thesis tackles some of the bottlenecks and problems: lack of knowledge about the drying properties of yam; how to design appropriate solar collectors for drying and regeneration and to share experiences with the actual implementation of the system to see its possibilities an opportunities. In this thesis the sorption properties of yam which gives an indication of the equilibrium moisture content in relation to water activity at constant temperature, but has been given less attention in literature, is studied. The analytical Crank solution to the Fick’s second law has been utilized in the prediction of drying curves of many food products. However the Crank solution puts limitations on the extent to which the equation could be utilized. The limitation is that the particle of food sample to be dried must be infinite, shrinkage should not exist, while moisture diffusion coefficient must be constant. In practice, these limitations are not realistic since most biological food products go through shrinkage together with variable moisture diffusion. Meanwhile the source of energy for drying has largely come from the sun and has led the use of the so-called open sun drying (OSD) method especially in tropical countries, for so many years. This method exposes the food to weather conditions and contaminants reducing the quality of the dried product. Recent research has shown that a confined drying system is most appropriate in terms of reduced drying time and quality of the final product. The shortcoming of the solar drying (SD) method is that, drying is limited to a time interval of 6-9 hours of the day after which the sun’s energy can no more be explored. Solar adsorption drying (SAD), as an alternative drying to solar drying, is an integrated drying method that helps in drying day and night in a continuous manner. Research in solar adsorption drying systems is limited in literature, therefore its performance is yet to be explored. Solar collectors are an integral part of solar and solar adsorption dryers. However, most researchers construct collectors to see what happens or use heuristic rules, whereas others do not link to dryers. The output air temperature has an effect on the quality of the dried food product, therefore much attention must be given to the output temperature of the air that enters the dryer. On the other hand after adsorption drying, the adsorbent needs to be regenerated. The easiest way to regenerate the adsorbent (silica gel), which requires temperatures over 70°C is by conventional ovens, which is costly. In this thesis, the condition and requirements for solar adsorption drying of yam (Dioscorea rotundata) is investigated. The approach is first to investigate the sorption properties of yam, then the drying characteristics of yam by modifying the Crank solution of the Fick’s diffusion equation, subsequently the model based design and construction of solar collectors for drying and regeneration purposes, and then finally, the proof of principle of solar - adsorption for both drying and regeneration. The first part of this thesis concerns investigating the equilibrium moisture isotherms for yam at 25°C and 50°C, both for sorption and desorption, experimentally using the gravimetric method. The strength of this work is that a Dynamic Vapour Sorption (DVS) analyzer, which is very sensitive to micro changes in weight loss during dehydration or rehydration, was used. A graphical presentation of equilibrium moisture content, as a function of water activity from the outcome of nonlinear regression analyses of desorption and adsorption isotherms respectively, is shown. The findings contradict most of the findings in literature. The parameters and their uncertainty range for each of the models (GAB, Henderson, Halsey, Oswin, Smith, BET and Peleg) are estimated. The strength of the statistical analysis is that an objective criteria is used to select the model that best describes the experimental desorption and adsorption isotherms over the relevant range of moisture contents. This has been presented in a table form where statiscally, the standard error (SE), the percent average relative deviation (PRD), Akaike Information Criterion (AIC), coefficient of performance (R2), and most importantly, the 2-σ bound confidence interval of the parameters, in particular those that bear the same unit as the equilibrium moisture content (Xe), have been taken into consideration. The GAB model was selected to describe both the desorption and adsorption isotherms, because it allows a physical interpretation. Next, Crank’s analytical approximation to Fick’s diffusion equation is used to investigate the effect of moisture dependent sample thickness and diffusivity on the drying behavior of yam (Dioscoreaceae rotundata) cubicles. Separate experiments of drying and shrinkage at constant temperatures of 30, 40 and 50oC were conducted in a cabinet dryer. The comparative study shows an interdependence between diffusivity and shrinkage due to water loss during drying. The analytical expression for the diffusion in a slab results in non-Fickian behavior for smaller cubicles and consequently, results in a higher effective diffusion coefficient. The drying rate trajectory shows two stages as a function of moisture ratio. The drying behavior of yam is better described by observing both drying and drying rate curves concurrently. This gives good agreement between observed and model data by a combination of fractal moisture dependent shrinkage and moisture dependent diffusion. The advantage of this modified Crank’s approximation is that the moisture trajectory of finite food cubicles can be predicted appropriately. Coupled partial differential equations are developed to investigate which collector lengths are appropriate for drying and for adsorbent regeneration. The output air temperature of the model was validated with an experiment. The spatially distributed model is a powerful tool to aid in multi-purpose collector design leading to solar dryer and regeneration system construction. The advantage of this model is that, the spatial mean temperature of the absorber plate, a requirement for the determination of the overall heat loss coefficient, which hitherto is absent in the approach of Duffie and Beckmann, is no more a problem. The result shows a spatially distribution of temperature in air, absorber plate, and glass cover as functions of collector length and time (day) at constant air velocity and presented in a graphical from. While the temperature of absorber plate, glass cover and air are functions of time (day), the air temperature additionally varies directly with collector length. The study shows the operational overall heat loss coefficient, radiative and convective heat gain and efficiency as functions of the operational air velocity. There was a good agreement between the observed and the model air output temperature. The proof of principle of solar adsorption drying (SAD) and its benefits, compared to solar drying (SD) and open sun drying (OSD) and the effects of these drying methods on the quality of different yam cultivars and regeneration of silica gel is considered. The drying methods SAD, SD and OSD are compared in terms of drying cycles. The SAD had, the shortest drying cycle while maintaining the whitish colour of yam, followed by SD and OSD. However, due to the night drying, the SAD dried product is longer exposed to the drying medium (air), leading to slightly lower vitamin C content of dried sample compared to SD dried samples. OSD had the worst effect on vitamin C. The study shows that the different cultivars of yam dried at different rates. Drying has no effect on composition. Regeneration was possible with solar energy but more work must be done to obtain the information needed for improved designs and operation strategies for the desorption. Finally the perspective, impact and the potential application of the study has also been discussed.

Aims: The study aimed to design and develop a greenhouse solar corn dryer that provides an efficient, sustainable, and cost-effective drying solution. The consistent use of traditional and conventional open sun drying (OSD) for agricultural products and yield necessitates the development of an innovation that can maintain quality, expedite the drying process, and potentially replace these techniques by utilizing renewable energy. Study Design: The study employed a Research and Development Design and an experimental quantitative approach to analyze variables and data. Place and Duration of Study: Situated in Brgy. Anlugan, Cabanglasan, Bukidnon and lasted from January 28, 2025, to March 26, 2025. Methodology: First, the sketch and construction of the greenhouse solar dryer (GSD) were done in the locale of the study. Then, to commence the corn drying process, 2 sacks were bought for each method and repeated to conduct 2 trials, totaling up to 8 sacks of corn kernels with high moisture content. Necessary research instruments were used to measure the following parameters: moisture content, temperature, relative humidity, cost, and time. Lastly, samples of corn were randomly taken and filled into bags for testing of aflatoxin levels. Results: There was a significant difference between the performance of the GSD compared to the OSD in terms of moisture content, as P = 0.001. for both trials. Furthermore, there was no significant difference in the relative humidity measured from both methods, yet the GSD managed to store more heat compared to the OSD. Furthermore, aflatoxin levels measured through the ELISA test showed that the former contained samples that were food-grade compared to the traditional method. Lastly, through calculated costs and efficient hours of drying, the GSD was cheaper and took less time compared to the latter. Conclusion: The developed design has the potential to replace the traditional practice of open sun drying, yet it requires more enhancements and further trials to evaluate and potentially replace this method for the benefit of local farmers.

This chapter presents developments and potentials of solar drying technologies for drying fruits, vegetables, spices, medicinal plants, and fish. Experimental performances of different types of forced solar dryers such as solar tunnel dryer, improved version of solar tunnel dryer, roof-integrated solar dryer, and greenhouse-type solar dryers are addressed. Drying time in the solar dryers was significantly less than that required for natural sun drying and the products are quality products in terms of color and texture. Simulated performances of solar tunnel dryer, improved version of solar tunnel dryer, roof-integrated solar dryers, and greenhouse solar dryers are presented. The agreement between the simulated and experimental results is very good. A multilayer neural network approach was used to predict the performance of the solar tunnel dryer and the prediction of the performance of the dryer was found to be excellent after it was adequately trained.

The current study focuses on the performance of Solar Greenhouse Dryer for drying of grapes for raisin making in Solar Dryer and Open sun condition in Western Maharashtra. The Maharashtra state ranks first in the production of grapes, probably Western Maharashtra produces nearly 800 thousand tons of grapes every year. The major wastage of grapes is due to low sugar content, glossy appearance, shrinkage, excess water in berry, scorching and size variation. Therefore, there is a need to preserve the grapes by drying and making raisins for a non-seasonal requirement. The experiment was conducted for drying of grapes in Solar Greenhouse Dryer and Open Sun condition from 1st of April to 4th of April for 48 hours. The initial weight of the grapes to be dried was 500 grams for both Solar Greenhouse Dryer and open sun drying conditions. The experiment was conducted at Bahe, Borgaon, Tal-Walwa, Dist- Sangli, Maharashtra, India located at 17.115o N and 74.33o E. The main aim of using DOE i.e., Response Surface Modelling is to get an optimum region for drying of grapes in Solar Greenhouse Dryer, from the Surface plot; the region of maxima and minima was obtained.

Solar drying is the oldest preservation technique of agricultural products using several types of solar crop dryers based mostly on solar energy, which is abundant, renewable, and sustainable. This chapter presents an experimental analysis to investigate the performance of two solar drying systems that were used to dry red pepper: solar greenhouse dryer and indirect solar dryer. The solar greenhouse dryer consisted of a flat-plate solar collector and a chapel-shaped greenhouse. Also, the indirect solar dyer consisted of a flat-plate collector and a drying chamber. The results show that the constant rate period is absent from the experimental drying curves. The experimental drying curves show only the falling rate period. Thus, the drying time was 17 hours in the solar greenhouse dryer and 52 hours in the indirect solar dryer. The drying data were fitted to eight different mathematical models. The fit quality of the proposed models was evaluated by using the coefficient determination (R2), reduced chi-square (χ2), and root means square error (RMSE). The experimental tests show that the solar greenhouse dryer was found to be satisfactory and competitive to the indirect solar dryer.

Solar drying is an ancient and inexpensive technique used for the preservation of agricultural item. Solar drying involves the removal of moisture content from crop. It is very important that the solar crop drying system should be cost-effective. Different methods of solar drying have been developed like open sun drying and greenhouse solar drying. The recent development in this area is greenhouse photovoltaic thermal mixed-mode drying in which electricity is also produced while crop drying is done. This system is quite useful for people living in remote areas. The solar drying of commercial industrial crops such as cotton, jute, sugarcane, tobacco, and ground nut has been popular and feasible. There is a need to invent cheaper solar drying methods to meet the demands of farmers of developing countries.

The current study focuses on the life cycle assessment and an economic valuation of a natural convection solar greenhouse dryer in Western Maharashtra, India. The Solar Greenhouse Dryer is an active device that gains solar radiation incident on to the surface of the dryer and along with wind energy, it removes moisture from agricultural yield. The combination of solar energy and wind energy removes moisture from agricultural yield. The Solar Greenhouse Dryer is primarily used in rural settings; hence, it is very important to analyse the environmental and economic aspects associated with the dryer to obtain the maximum benefit from the dryer with less investment possible. The experiment involving a natural convection solar dryer was conducted at Bahe, Borgaon, Tal-Walwa, Dist- Sangli, Maharashtra, India located at 17.115o N and 74.33o E. The environmental parameters taken into considerations during the analysis covered energy, energy payback time and CO2 emissions, mitigation and carbon credits earned by the dryer. The economic analysis of the solar dryer consists of the annual cost of the dryer, the salvage value, the annual saving obtained and the payback period respectively.

Modification approach of Northern Wall to improve the performance of solar greenhouse dryers: A review

Solar greenhouse dryer system for wood chips improvement as biofuel

The study presents the performance of a small‐scale greenhouse solar dryer (GHSD) to demonstrate hygiene and superiority over traditional methods for medicinal Adulsa and Durva leaves. The GHSD dryer lowered the initial moisture of Adulsa leaves from 73.99% (w.b.) to the final 10.63% (w.b.) under natural and forced convection in 3 and 2.5 h, respectively. Similarly, the initial moisture of Durva leaves was reduced from 67.33% (w.b.) to 11.65% (w.b.) under natural and forced convection in 2.75 and 2.5 h, respectively. Furthermore, mathematical modeling with non‐linear regression was implemented to characterize the drying of Adulsa and Durva leaves, which showed that Midilli–Kucuk and Modified Midilli were the most superior fits for reporting the drying of Adulsa leaves in GHSD and open sun drying, respectively, whereas Verma and Midilli–Kucuk models were superior fits for characterizing the drying of Durva leaves in GHSD and open sun, respectively. Furthermore, the quality of the dried Adulsa and Durva leaves was evaluated, where total phenolic content (TPC) and total flavonoid content (TFC) values were marked higher in the leaves dried by the GHSD as compared to open sun drying for both Adulsa and Durva leaves. Moreover, during antioxidant analysis, the IC50 values among the leaves dried in GHSD were recorded to be greater for the natural convection greenhouse solar dryer (NCGHSD) for both Adulsa and Durva (40.67 and 122.24 μg/mL) compared to open sun drying (101.88 and 158.20 μg/mL). A quality assessment was performed on dried Adulsa and Durva leaf samples, and they exhibited appropriateness for pharmaceutical industry drug production.Practical applicationsGreenhouse solar dryer is an eco‐friendly, sustainable, clean, and hygienic way to dry herbs and medicinal plants. The greenhouse solar dryer can shorten the drying time by 50%–60% compared to natural stand in the sun (open sun drying) and improve the quality of the dried herbs, medicinal plants, and reduce the decay rate of the dried products. These leaves can be best suited for Ayurveda medicine industries for herbal powder (Churna). Being an economically advantageous greenhouse solar dryer improves the shelf life of the dried leaves, reduces their density, and lower transportation costs, leading to boost farmers' income in herbal farming and nutritional security.

Technical performance of two solar drying technologies was evaluated: Solar Greenhouse Drying (SGD) with auxiliary heating system, and Direct Solar Drying (DSD) in order to evaluate its effect on antioxidant activity (AA) and total flavonoids of blackberry (rubus spp) waste destined. The SDG and DSD results were compared with those of the dehydrated samples in an electric stove (ES). The fresh and dried fruits were evaluated; the blackberry seedless pulp was used. The AA and flavonoids showed degradation of 70% and 20% compared to the fresh sample. For both compounds, SGD is the one that offers the greatest conservation.Keywords: solar drying; antioxidant capacity; flavonoids; blackberry pulp (rubus spp); solar energy

The use of solar energy in drying of perishable crops such as tomatoes is a good alternative to the problem of post-harvest processing in tropical eastern African countries. A review of the literature revealed that most of the solar crop drying systems developed during the last five decades have small loading capacity and cannot operate during the night. Therefore, an integrated solar greenhouse dryer system [SGDS] with Clay-CaCl2 desiccant energy storage system was designed and tested. Such SGDS have the advantage over other solar systems of high loading capacity and structural simplicity. In addition, they have relatively good thermal crop drying performance compared to most solar dryers. However, their main limitation, like most solar dryers, is their inability to dry at night. Therefore, to enhance night-time drying capacity, a prototype SGDS integrated with a low-cost Clay-CaCl2 desiccant energy storage system was designed, fabricated, and tested. The drying performance of this prototype was evaluated using loads of fresh tomatoes during October – December 2019 at Nairobi, Kenya. The dryer was able to dry fresh tomatoes from 93.9% (mcwb) to 8.3% (mcwb) within 27hours with solar greenhouse drying efficiency of 23% during daytime and desiccant drying efficiency of 19.9% during nighttime. The drying rate for the two-day light drying was 0.985kg/h and 0.875kg/h respectively and that in night drying using desiccants was 0.34kg/h. Based on these results, it was concluded that prototype solar greenhouse dryer with Clay-CaCl₂ energy storage has great potential for drying perishable produce such as tomatoes in tropical countries.

Recently, the quality of food solar dried has been associated to solar irradiance, irradiation and UV-light. Different materials used as covering materials in direct solar dryers had different optical properties, thus, different quality should be expected in solar food drying. The objective of this study was to compare two direct solar dryers builder with different covering materials: Direct Solar Drying with Poly (methyl methacrylate) (DSD), and Greenhouse Solar Drying with polyethylene (SGD), based on their three aspects: chemical composition, heat-mass transference, economic-environmental effect. Chemical composition was evaluated as the total phenolic compounds (TPC), flavonoids (TF) anthocyanins (TA) and antioxidant activity (AA) of blackberry pulp (Rubus spp). Dehydrated blackberry pulp contains lower TA than raw samples because TA is sensitive to solar irradiation and UV solar irradiation. TPC, TF and AA averages do not show significant differences among all the drying processes. Evaluation of energy and heat transfer in the evaporative and diffusive periods of the drying technologies led to an assessment of the return of investment period for DSD and SGD technologies for different power sources. The results suggest payback periods between 39 and 121 months, considering only the 6 months of harvest duration without using the solar dryers the rest of the year.

A literature research on the drying quality of agricultural products with using solar drying technologies

ABSTRACTAn even span solar greenhouse dryer was built and applied to dry Java tea (Orthosiphon aristatus) and Sabah snake grass (Clinacanthus nutans Lindau). Findings showed that the solar greenhouse dryer performs satisfactorily during clear weather except at nighttime and rainy day due to product rehydration which is heavily influenced by high relative humidity from ambient air. Integrating of heat pump into the solar greenhouse dryer has successfully reduced the room relative humidity by 10–15%. Also, heat pump has mitigated the product rehydration issue by maintaining room relative humidity at maximum of 65% throughout the drying period. The drying rate of Java tea was improved three to fourfold, i.e., from 0.004–0.008 to 0.018–0.025 g H2O/g DM min, whereas 10% of drying time was saved for both Java tea leaf and Sabah snake grass leaf with the assistance of heat pump system. Meanwhile, the supply of dry air from the heat pump system with a magnitude of 0.25–0.50 m/s helps in enhancing the drying rate of the herbs as well as minimizing the nonuniformity of drying temperature and relative humidity inside the solar greenhouse dryer.