Performance of a solar greenhouse-biogas hybrid dryer for dehydration of water hyacinth (Eichhornia crassipes)

Abstract The development of a solar greenhouse dryer combined with an anaerobic digester for biogas production can reduce carbon emissions and create zero waste. This paper presents the performance of a solar greenhouse biogas dryer (SGBD) that burns biogas to supplement the drying of water hyacinth. The SGBD has an overall efficiency of 87% for 100 kg of fresh water hyacinth. The auxiliary heating system enabled the SGBD to achieve drying temperatures of 24.5 to 45.0 degrees during cloudy and rainy days and produce high quality dried products. There was a fivefold increase in drying capacity and a reduction in drying time of six hours (38%) using the SGBD. End users of the dried product judged it to have improved consistency in color and shape. The SGBD cost less than US$3800 to build with a payback period of five months. This design demonstrates a low-cost and simple technology for small businesses.


Introduction
Water hyacinth (Eichhornia crassipes) is considered to be one of the world's most harmful invasive aquatic weed species due to its negative impact on aquatic ecosystems.There are many ways to manage the proliferation of the weed, including chemical herbicides, biological control agents, and mechanical or manual removal from waterways. [1]Water hyacinth has the potential to be transformed into useful products such as packaging material, or provide the raw material for biogas production, which is an environmentally friendly energy source.
Recently, water hyacinth stems have been dried using solar drying technology to produce animal bedding for the National Laboratory Animal Center at Mahidol University in Thailand.In the animal bedding production process, only the stems were used because the dried stems have good properties for bedding, including high fiber content, soft structure, and absorbency, and provide animals with a dust-free environment. [2]Finding a use for the roots and leaves of the plant can increase its value and minimize waste.Converting the roots and leaves to biogas to provide supplemental energy to a solar dryer is a potential application for leftover plant material.Solar drying is a simple technology and suitable for small scale production.The products can be dried in a closed system that can protect products from contaminants. [3]The solar dryer also provides better quality dried products compared to traditional sun drying.Nevertheless, the solar dryer operates effectively only during clear weather.There are problems with solar dryers being unreliable on rainy or cloudy days, as well as at night when the dried product can reabsorbed moisture, [4,5] resulting in longer drying time or spoilage.Consequently, a hybrid dryer combined with external energy is an option to enhance drying efficiency and produce high quality dried products.
Several previous studies have reported the performance of hybrid dryers using a variety of supplemental energy methods to produce dried agricultural and food products.For example, Leon and Kumar [6] investigated the performance of a hybrid dryer assisted biomass drying system with thermal storage for drying of chili.The continuous drying was operated by the hybrid system which can maintain a constant drying air temperature and air flow rate.The system provides a reduction in drying time and high quality dried chili.Janjai [7] studied a solar greenhouse dryer equipped with an LPG burning system for drying of osmotically dehydrated tomatoes.This hybrid dryer solved problem caused by rainy and cloudy days and provided continuous drying of product.Mu'azu et al. [8] studied a forced air-convection combined with organic waste energy for drying of vegetables.The hybrid dryer achieved shorter drying times and higher quality dried products that were free from dust and contaminants compared to traditional sun drying.Nabnean et al. [9] designed a hybrid dryer using water type solar collector, solar hot water storage tank, and heat exchanger for drying of osmotically dehydrated cherry tomatoes.This hybrid dryer showed a reduction in drying time and high-quality dried products compared to traditional sun drying.The efficiency of the solar collector was 21-69%.The dried products were completely protected from rain and insects.Yassen and Al-Kayiem [10] developed a solar-biomass hybrid dryer for continuous drying of chili.The drying system with Co-Gen provided shorter drying times and higher efficiency than without Co-Gen.Tham et al. [5] studied a solar greenhouse dryer assisted with a heat pump for drying of herbs.This hybrid system reduced the relative humidity of the drying chamber by 10-15%, maintained the relative humidity of 65% during nights or cloudy days, and reduced the drying time by 10%, and guaranteed the consistency of drying efficiency and the quantity of dried herbs.Pankaew et al. [11] investigated the performance of a solar greenhouse dryer combined with a rice husk burner for drying of bananas.The rice husk burner achieved high efficiency and the hybrid dryer demonstrated considerable improvement in drying time and dried product quality when compared to traditional sun drying, and eliminated problems of drying during cloudy and rainy days.Szadzi nska et al. [12] studied a hybrid dryer combined with ultrasound and microwave for drying of red beetroot.The hybrid-intermittent drying showed a reduction in drying time and energy consumption, improving the drying rate, and providing better color, higher betanin retention, and attractive appearance making it an attractive option for the food industry.Ndirangu et al. [13] evaluated the performance of a solar-biomass greenhouse dryer for drying of agricultural products.It was found that the improvement of ventilation and the addition of an auxiliary heat source to the hybrid dryer could increase the drying efficiency.Pankaew et al. [14] studied a solar greenhouse dryer combined with phase change material (PCM) for drying of chili.The hybrid dryer with PCM thermal storage obtained shorter drying times and higher efficiency than without the PCM.This hybrid dryer can be used for drying during adverse weather conditions.Murali et al. [15] developed a solar-LPG hybrid dryer for continuous drying of shrimp.The hybrid dryer had high efficiency and the problem of fluctuating weather conditions was solved.Ndukwu et al. [16] developed a hybrid solar-biomass dryer for drying of plantain slices.The hybrid dryer showed considerable reduction in drying time compared to traditional sun drying and was reported to release lower CO 2 emissions than a conventional solar dryer.Lamrani et al. [17] studied a forced-air convection hybrid dryer for wood drying.The results showed that using a hybrid dryer reduced drying time and CO 2 emissions and provided a continuous drying operation.Abedini et al. [18] designed a cabinet hybrid dryer with an auxiliary infrared heat source for drying of shrimps to achieve good results in terms of dried product quality, drying efficiency, and drying time.Atalay et al. [19] studied a hybrid dryer assisted with solar and wind energy.The hybrid dryer achieved shorter drying time and higher energy efficiency compared to conventional solar dryers.Recently, Andharia et al. [20] designed a mixed-mode solar thermal dryer for drying of marine products.This hybrid dryer provided good quality dried products and demonstrated an economical and environmentally friendly alternative for operation during the absence of sunshine.
Consequently, this research aims to improve the efficiency of product drying using a solar greenhouse dryer equipped with a biogas heating system (SGBD).Water hyacinth roots and leaves were used to produce biogas as an auxiliary heat source for use during offpeak hours of sunlight.Applying this drying process to animal bedding production can result in a zero waste and an environmentally friendly system that converts waste to value and reduces carbon emissions.Moreover, this study can be a guideline for small businesses development to produce dried water hyacinth, as well as other agricultural and food products.

Design and construction of the solar greenhousebiogas hybrid dryer
The structure of the SGBD is shown in Figure 1.The SGBD was located at the Community Enterprise Nara Phirom Group Network, Nakhon Pathom, Thailand, in which the latitude and longitude coordinates of 3.884368 and 100.289212, respectively.The overall system consists of three parts: a solar greenhouse dryer, plug flow digester, and biogas burner.The dimensions of the SGBD and positions of its drying samples are shown in Figure 2.
The SGBD was developed for producing dried water hyacinth for use as animal bedding at the National Laboratory Animal Center (NLAC), Mahidol University.The SGBD has a volume of 40.19 m 3 , enclosed with a parabolic roof structure made of polycarbonate sheets.The SGBD was located in an eastwest orientation to enhance drying effectiveness and optimize the collection of solar radiation. [21]The parabolic structure was chosen because it has a shape that permits solar radiation to pass into the SGBD throughout the day and can effectively handle wind loads.Moreover, the Department of Alternative Energy Development and Efficiency, Thailand recommends the parabolic structure as the main design of solar greenhouse dryer structures. [3,22]Additionally, parabolic-shaped solar dryers provide shorter drying time, higher temperature, and higher efficiency than an even-span shape. [23]The lightweight and transparent polycarbonate sheets were selected due to their ability to create a good greenhouse effect with high transmittance of shortwave solar radiation and low transmittance of infrared energy, as well as good thermal resistance with low density and low thermal conductivity which reduces heat loss to the environment. [24,25]The SGBD floor was made of 2-inch thick foam insulation sandwiched between black metal sheets providing strength, durability, and good heat  absorption.Concrete posts were used to make an elevated platform due to this area being prone to flooding.
The ventilation system involved three 6-inch fans with power of 43.2 W, positioned on the top of the SGBD opposite to the air inlet.Relative humidity sensors (MH13001, measurement error ±3% RH) were used to control the fans automatically, which were powered by a 100-W solar cell panel with a 0.96 kWh battery for solar energy storage (Figure 3).The fans were operated when the relative humidity inside the SGBD was equal or higher than 60% and also higher than the relative humidity outside the SGBD.
Two sets of metal shelves were placed 40 cm apart inside the SGBD.Each set of shelves consisted of four levels.Each level had a drying tray with dimensions of 1.25 m x 2.80 m x 1.40 m, allowing a maximum loading capacity of 100 kg of fresh water hyacinth (Figure 4).Drying was monitored by weighing product samples at 2-h intervals using a digital balance (GHL DJ1002C).
Figure 5 shows the design and dimensions of the biogas production system.The biogas system was designed to digest a mixture of cow dung and water hyacinth roots and leaves.The horizontal plug flow digester has a volume of 22.5 m 3 .Biogas was generated using a mixture of 300 kg of roots and leaves mixed with 300 kg of cow dung.The biogas heating system was used as an auxiliary heat source for the solar greenhouse dryer.The heating system consisted of a biogas burner, 10-inch fan, and air-to-air heat exchanger (Figure 6).The biogas from the plug flow digester flowed through the gas pipeline to the biogas burner.A manual shut off valve was installed to control gas flow rate.Ambient air from outside the SGBD was supplied to the air-to-air heat exchanger and hot air from the heat exchanger was blown into the SGBD.Exhaust gas flowed from the outlet of the heat exchanger to ambient air through the chimney.

Experimental procedure
To investigate the performance of the SGBD, water hyacinth was dried using the SGBD and traditional sun drying at the same time under the same weather conditions to compare the efficiency of both drying methods.An empirical study was carried out over three days with different weather conditions in  February, 2022.The water hyacinth was collected from Nara Pirom canal.After harvesting, their roots and leaves were chopped off, then the stems were cleaned and cut into 1-centimeter lengths.Water hyacinth stems were selected based on a previous NLAC report that animal bedding composed of dried stems had better shape, texture, and consistency than other water hyacinth parts. [2]Approximately 300 kg of roots and leaves were obtained for every of 100 kg of stems.
In this study, fresh water hyacinth was dried under two conditions.The first condition used the traditional sun drying method, in which 20 kg of water hyacinth was divided into two groups.The second condition used the SGBD, in which 100 kg of water hyacinth was divided into four main groups.Each SGBD group consisted of four levels of drying trays.Samples of fresh water hyacinth (starting weight of one kilogram) were designated in each group.The samples were weighed at 2-h intervals to monitor weight loss.A sample was considered completely dried when its weight decreased to less than 100 grams, following the criteria of the NLAC, Mahidol University.During the drying experiments, the temperature and relative humidity, both inside and outside the SGBD, along with the level of solar radiation and air velocity were collected.Additionally, the variation of product temperature and weight were recorded from 9:00 am until 3:00 pm.
Product moisture content is the amount of water or liquid that is contained in the product.Normally, it is reported on a wet basis (wb). [22,26,27]The wet basis is the ratio of weight of water per weight of wet sample, which was calculated using Equation 1.
where MC wb is wet basis (%), W w is wet weight (kg), and W d is dry weight (kg).Performance of the solar greenhouse-biogas hybrid dryer

Performance of the biogas heating system
The performance of the heating system is the ratio of heat gained by air in the heating system to heat received as shown in Equation 2. [11] e where m a is the mass flow rate of ambient air to the heat exchanger (kgÁs À1 ), m f is the mass flow rate of biogas to the heating system (kgÁs À1 ), h f is the heating value of biogas (JÁkg À1 ), C pa is the specific heat of air (JÁkg À1 Á C À1 ), T out is the outlet temperature of hot air from the heat exchanger ( C) and T in is the ambient air temperature ( C).

Drying efficiency
The drying efficiency of the SGBD is given as the ratio of output energy of the dryer to input energy to the dryer which is shown as a percentage.Solar radiation input on the dryer was calculated using Equation 3. [11] E where E solar is the solar energy input on the dryer (J), A dryer is dryer area (m 2 ), and S r (t) is solar radiation at time t (WÁm À2 ).
The SGBD output in terms of energy required for vaporization was calculated using Equation 4. [11] E dryer ¼ m r L g (4)   where E dryer is the SGBD output (J), m r is moisture removed (kg), and L g is the latent heat of vaporization of moisture (JÁkg À1 ).Therefore, drying efficiency of the SGBD was calculated using Equation 5. [11] where e eff is the SGBD efficiency (%), E module is the energy output from the solar panel (J), and E biogas is the energy output from the biogas heating system (J).

Cost evaluation of the solar greenhouse-biogas hybrid dryer
The economic evaluation of the SGBD was calculated using the equations adopted from Pankaew et al. [11] as follows.

Drying costs (C drying )
The capital cost (C capital ) of the SGBD was computed from Equation 6. [11] C where C dryer is the cost of the solar greenhouse dryer, C heating is the cost of the heating system, C digester is the cost of the digester, C vent is the cost of the ventilation system, and C labor,c is the labor cost for construction the solar greenhouse dryer.
The annual cost (C annual ) of the SGBD was estimated using Equations 7 and 8. [11] C and where C main,i is the maintenance cost, C op,i is the operating cost at year i, i in is interest rate, i f is inflation rate, and x is economic parameter.The maintenance cost was assumed to be 1% of the capital cost per year.In this study, the operating cost (C op ) is only the labor cost for handling water hyacinth (C labor,h ) as shown in Equation 9. [11] C op ¼ C labor, h Therefore, the drying cost (C drying ) was calculated using Equation 10. [11] C drying ¼ C annual M d (10)   where M d is the amount of dried water hyacinth obtained from this SGBD per year.The drying cost was calculated to be 1.25 USD/kg.

Payback period
The payback period (PB) was estimated using Equation 11. [11] PB ¼ where M f is the amount of fresh water hyacinth used per year, P f is the price of fresh water hyacinth, and P d is price of dried water hyacinth.There is no cost for fresh water hyacinth.
Based on the evaluation data in Table 1 and Equation 11, the payback period of the SGBD was estimated to be five months.

Performance of the solar greenhouse-biogas hybrid dryer
An empirical study was conducted to investigate the performance of the SGBD in February, 2022.During the drying experiments, the solar radiation during drying of water hyacinth was measured (Figure 7).The solar radiation intensity before starting the heating system (7:00 am) ranged from 48-102 W/m 2 .Due to intermittent clouds and rain, the peak solar radiation intensity during the days was 704 W/m 2 on the first day, 823 W/m 2 on the second day, and only 241 W/m 2 on the third day.The traditionally sun dried batch of water hyacinth had to be moved under cover during rainy periods while the SGBD was able to work continuously.This is one factor that accounts for the improved efficiency of the SGBD.
Figure 8 shows the variations of temperature and relative humidity throughout the experiment.At the start of the day (7:00 am), the ambient temperature and temperature inside the SGBD were comparable, ranging from 24.0-25.5C outside and 24.5-28.0C inside the SGBD.The average temperature inside the SGBD supported by the heating system was approximately 7 C higher than the ambient air temperature by 9:00 am.
On the first and second days, both the ambient and the SGBD temperatures increased from 9:00 am to 1:00 pm, then decreased the rest of the day.The maximum temperatures inside and outside the SGBD on the first day were 45.0 C and 34.0 C, respectively.The maximum temperatures inside and outside the SGBD on the second day were 44.5 C and 32.0 C, respectively.Rain on the third day limited the temperature outside the SGBD to a maximum of 30.5 C.][20] The increased temperature inside the SGBD is another factor contributing to the improved efficiency.
While the relative humidity before starting the heating system (7:00 am) was high, ranging from 81.8 to 96.0% inside the SGBD and 95.9 to 96.0% outside, by 9:00 am, the average relative humidity inside the SGBD was approximately 18% lower than the ambient air.
During the first day, the highest relative humidity inside and outside the SGBD was 68.8% and 96.1%, respectively.On the second day, the highest relative humidity inside and outside the SGBD was 77.9% and 96.1%, respectively.On the third day, the highest relative humidity inside the SGBD and ambient relative humidity was 86.7% and 96.2%, respectively.The relative humidity was inversely proportional to the temperature because the warm air increased water holding capacity. [7,25][20] Consequently, the environment inside the SGBD provided higher drying potential than the ambient air due to the auxiliary heat source raising the drying temperature and the properties of the polycarbonate sheets covering the SGBD structure.Generally, thermal radiation was absorbed by the drying material and the black elements inside the SGBD.Additionally, the polycarbonate sheets prevented the reflection of long wavelength infrared radiation, thus reducing heat loss to the surroundings and creating a good greenhouse effect. [5,24,25]The high temperature inside the SGBD resulted in lower relative humidity inside the SGBD than outside (Figure 8).Because of this, the ventilator fans were not activated by the relative humidity control sensors throughout the experiment.
The comparison of average inside and outside product moisture contents (%wet basis) is shown in Figure 9.The moisture measurements revealed that the initial moisture content of water hyacinth samples in each experiment was approximately 89%.The samples placed on tray 4 (top tray) finished drying (sample weight less than 100 grams) at 1:00 pm on the second day with a 19% final moisture content.While the samples placed on tray 3 and traditional sun drying method reached a suitable moisture content of 17% and 12%, respectively, in the same drying time at 11:00 am.Samples placed on tray 2 and tray 1 finished drying at 1:00 pm with a final moisture content of 12% and 8%, respectively, on the third day.
In comparison with the drying time of a conventional solar greenhouse dryer reported in a previous study, [2] the SGBD is able to achieve the same level of dryness in the same amount of drying time using less solar radiation.Consequently, the SGBD offers the potential to maintain better temperatures during cloudy or rainy days.This will enable product to be dried more days of the year.
There were significant differences (Sig.¼ 0.005 at P value ¼ 0.05) between the products dried inside and outside the SGBD, as well as differences in the drying time of products at different levels of the drying trays in the SGBD.The results showed that the samples placed on tray 4 had more efficient drying than the lower trays due to the top tray receiving energy from both direct solar radiation and heat in the dryer. [2,24]Dried product from the SGBD had more consistent and cleaner appearance than product from the traditional sun drying method due to the heating system that improved the drying efficiency by providing consistently higher temperatures, even during variations of weather conditions.
In this study, the biogas heating system was used to warm up the SGBD during the morning (7:00 am À 9:00 am).The biogas consumption was 0.57 m 3 /h with a calorific value of 9 -16 MJ.The production rate of gas was 2.1 Â 10 À6 m 3 /s with 0.21 m 3 of gas generated per kg Volatile Solids.The average gas production rate was 0.18 m 3 /day.The biogas produced in this study was relatively lower than the study of the Department of Alternative Energy Development and Efficiency [28] because the water hyacinth waste and cow dung had not been pretreated and the ingredients were not completely mixed, resulting in longer retention time and lower biogas production rate.While the amount of biogas produced for this system was suitable, future studies to optimize the biogas system can be conducted.Table 2 shows the potential biogas production from various agricultural wastes compared to the water hyacinth waste in this study. [29]he overall effectiveness of the biogas heating system was 73%, while the SGBD had an overall efficiency of 87% for 100 kg of fresh water hyacinth.In comparison, the conventional solar greenhouse dryer reported in a previous study had an overall efficiency of 63% for 100 kg of fresh water hyacinth. [2]anjai et al. [24] found that the drying efficiency is related to the crop weight in the drying process.In their study, the highest load capacity of 150 kg provided the best drying efficiency of 20%.Consequently, it was recommended to operate the solar dryer at the highest possible efficiency.In the study of Boonyasri et al., [30] the solar dryer had the highest efficiency of 56% for drying 40 kg of pork.Pankaew et al. [11] revealed an overall drying efficiency of the hybrid dryer was 13% and the maximum effectiveness using a rice husk burning system was 88%.
A comparison of the SGBD efficiency with traditional sun drying and a conventional solar greenhouse dryer is shown in Table 3.The SGBD is faster than traditional sun drying and improves production because the SGBD has a higher load capacity (100 kg) than the traditional sun drying (20 kg) in the same area size (20 m 2 ).Although the drying time between the SGBD and the conventional dryer is not Table 2. Potential of biogas production from various agricultural wastes. [29]pes of wastes Biogas yield (m significantly different, the SGBD achieved the same level of dryness using less solar radiation showing the benefit of adding the biogas heating system.
Figure 10 shows the characteristics of the water hyacinth stems dried by the SGBD and traditional sun drying methods.Although the drying time of water hyacinth dried in the SGBD was only slightly different from the traditional sun drying, the physical quality of the dried products by the SGBD and traditional sun drying methods showed different characteristics.The SGBD method improved the appearance of dried water hyacinth in terms of consistent color and shape, providing a pale green color, as well as smooth and soft texture, while the traditional sun dried product was dark green with dark spots, had greater shrinkage, and contained contaminants.The accumulation of heat inside the SGBD and the energy enhancement from the biogas heating system allows moisture to evaporate continuously.In contrast, traditional sun drying was disrupted by variable weather conditions, resulting in discontinuous moisture evaporation.In addition to controlling the fluctuation in weather conditions, the SGBD prevents microbial growth, reduces drying time, increases production capacity, enhances product quality, and provides significant protection against dust and insect contamination.The SGBD built for this study is currently being used by the Community Enterprise Nara Phirom Group Network with good success.

Cost evaluation
The costs and economic parameters of this SGBD for the small community in this study are shown in Table 1.The cost evaluation is based on the prices of the materials and dried products at the time of the study.The SGBD capacity is 100 kg of fresh water hyacinth and the capital cost for installation and construction was 3,750 USD.The Community Enterprise Nara Phirom Group Network can produce 1,200 kg of dried water hyacinth per month for the NLAC which can generate income for the community of around 2,571 USD/month.Based on the costs to build the drying system and the income generated by the sale of the dried products, this SGBD has an estimated payback period of five months.

SGBD advantages
The SGBD developed in this study has several advantages in common with other hybrid solar drying methods.The greenhouse enclosure ensures that dried products are protected from weather disturbances such as rain, dust, or wind, and reduces labor Table 3. Summary for the SGBD efficiency compared with traditional sun drying and conventional solar dryer for water hyacinth.
Parameters SGBD Traditional sun drying Conventional solar dryer [2] Solar radiation range; average (W/m  compared to traditional sun drying because batches can be left unattended, even during rainy days.It also prevents contamination of the dried products by pests.Like some other hybrid systems, the supplemental heat generated by the SGBD reduces the total drying time and increases the batch size allowing the output of products to be increased.However, the SGBD has unique advantages related to its location and purpose.The greenhouse was designed to upgrade a small community enterprise.Because the costs and revenue of the enterprise were well established, the SGBD had the best combination of features.It was built using inexpensive materials readily available in the community allowing the return on investment to be rapid.The construction of the greenhouse and biogas burning system did not require any complex parts and operation of the manufacturing process did not require advanced training.The greenhouse and drying trays were designed to dry a specific material (water hyacinth) and the dried product (animal bedding) had input from a customer to evaluate product quality.
In addition, the SGBD does not generate heat using external energy sources with fossil fuel backup such as LPG or electricity, that other hybrid dryers use.The SGBD design incorporates zero-waste and low-carbon concepts.Most of the waste from the production was the water hyacinth roots and leaves.These residuals had been thrown back into the river which affected water quality or fermented as fertilizer, resulting in the release of methane.The generation of biogas from these residuals was selected for the SGBD since it is considered more cost effective and environmentally friendly than other methods.Compared to other solar combined drying methods, the SGBD reduces electricity consumption by 252 À 2,168 kWh or 907 À 7,805 MJ per year. [7,9,11]Another advantage of the SGBD in this study is its ability to serve as a model that can be easily replicated in other communities or modified to accommodate other dried products.All of these factors were accounted for in the design of the system.

Conclusion
This study demonstrated the benefits of a solar greenhouse dryer combined with a biogas heating system that is inexpensive, has a simple design, and is easy to build.This SGBD produced five times more product per batch compared to traditional sun drying.In addition, this SGBD had an overall drying efficiency of 87% for a batch of 100 kg of fresh water hyacinth, a 24% improvement over a conventional solar greenhouse dryer.The overall effectiveness of the biogas heating system was 73% and produced a minimum drying time of 1 1 = 2 days during the study.Furthermore, the biogas in this study was generated by a zero-waste system using the roots and leaves of the water hyacinth as an energy production material that reduces greenhouse gas emissions.The SGBD produced water hyacinth dried to moisture content of 8% À 19% (wet basis) from an initial content of 89%.The combination of conventional greenhouse design supplemented with heat from the combustion of biogas made the average temperature inside the SGBD approximately 10 C higher than outside.The physical quality of dried water hyacinth samples from the SGBD were judged by users to have better appearance in terms of color and shape than the traditional sun dried product.Besides improving product quality, the SGBD solved the problems of cloudy and rainy days and prevented contamination of product by insects and dust.The total cost for parts and assembly of the SGBD was low and the investment had a payback period estimated to be five months.The design, capacity, and material used for biogas production can be easily modified to create the SGBD systems for preparing other types of dried products.These features make the SGBD a viable option for small community enterprises.Consequently, this SGBD provides an efficient approach for rapid and consistent drying of products with economical value.

Figure 1 .
Figure 1.The overall system of the SGBD.

Figure 2 .
Figure 2. The dimensions of the SGBD and positions of drying samples (S).

Figure 3 .
Figure 3.The ventilation control system.

Figure 4 .
Figure 4.The characteristics and dimensions of the drying shelves.

Figure 5 .
Figure 5.The design and dimensions of the biogas production system.

Figure 6 .
Figure 6.The design and dimensions of the biogas heating system.
for constructions of the solar greenhouse dryer (C dryer ) 2,755 USD Cost of ventilation system (Fans, RH Sensors, Timer, Solar charge controller, Battery deep cycle 80 Ah, C vent ) 275 USD Cost of biogas digester 22.5 m 3 (C digester ) 306 USD Cost of biogas heating system (C heating ) 107 USD Labor costs for construction of the solar greenhouse dryer (C labor,c ) 306 USD Labor costs per batch for handing water hyacinth (C labor,h ) 1,178 USD Quantity of fresh water hyacinth per batch (M f ) 100 kg Quantity of dried water hyacinth per batch (M d ) 10 kg Repair and maintenance costs (C main ) 1%/yr.Sale price of dried water hyacinth (P d ) 2 USD/kg Expected life of the dryer 15 years Remark: Average exchange rates in February, 2022 from the Bank of Thailand (1 USD ¼ 32.67 Baht).

Figure 7 .
Figure 7. Variation of solar radiation intensity during the day.Ã The experiment was carried out in February, 2022.

Figure 8 .
Figure 8. Variation of temperatures and relative humidity with time of the day.Ã The experiment was carried out in February, 2022.

Figure 9 .
Figure 9. Removal of water hyacinth moisture content (%wet basis) over time.Ã The experiment was carried out in February, 2022.ÃÃ Statistically significant difference at p ¼ 0.05, with Sig.value ¼ 0.005.

Figure 10 .
Figure 10.Comparison of dried water hyacinth characteristics for traditional sun drying (A) and the SGBD (B).

Table 1 .
Cost evaluation of the SGBD for community.
There is no pretreatment for the water hyacinth waste in this study.