الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
Abstract
Energy demand is growing rapidly due to the continuous population growth, high living standards, and fast economic development. Egypt mainly relies on fossil fuels as the primary source of energy. Using fossil fuels which emit high concentrations of greenhouse gases impacted our environment severely. Therefore, searching for renewable, abundant, clean, and feasible energy resource became a necessity. Biogas resulted from anaerobic digestion of organic wastes is considered one of the important renewable energy sources. The AD process is highly affected by various factors such as fermentation temperature, pH value, and C/N ratio. Fermentation temperature is one of the most important factors that affects biogas production. Biogas digesters operate under wide range of temperature (<25°C-70°C). In Egypt, temperature in winter is usually between 10 °C to 20 °C which is not suitable for biogas production. Thus, an external thermal energy source should be used to enhance the production.
In this study, the fermentation inside the biogas digester was thermally enhanced in winter season using solar parabolic trough collector in order to reach the optimum temperature range that suits biogas production.
The experiment was conducted at Solar Energy Laboratory, Department of Agricultural Engineering, Faculty of Agriculture, Ain Shams University, Shubra El-Khaimah, Egypt (latitude 30° 06’ N, longitude 31° 14’ E) through December 2018 and January 2019. The experimental system is consisted of various parts as follows:
Digestion System:
Two 50 L polyethene barrels were turned into experimental digesters. The first digester is the treatment digester. It is thermally insulated from all sides by glass wool. A mechanical mixing system is installed in this digester. It consists of an electrical motor and a stirring shaft which has 3 blades of length 100 mm. A simple circuit using the Arduino card was used to turn on the motor automatically every 4 hours for 10 minutes to equally distribute the thermal energy along the digester and to avoid a heterogeneous slurry that leads to layers formation. A copper serpentine with a diameter 9.5 mm was installed inside the digester near its wall to act as a helical heat exchanger. A gas outlet was installed on the cap of the digester. The second digester is the control digester. It was only provided with the gas outlet.
Parabolic Trough Collector:
The PTC is the solar technique that is used to enhance the fermentation temperature in the treatment digester. It is made of stainless-steel sheet that is 1740 mm length and 700 mm width. An evacuated tube is placed on the focal line of the PTC. A tracking circuit and two DC motors were used to keep the PTC always orientated towards the sun during the day and benefit from the greatest amount of solar radiation. The PTC is supported by a galvanized iron frame that is tilted with a 30° angle.
The Experiment Procedures:
The feedstock (cattle dung) was collected from a farm in El-Qanater. A sample was taken and has been analyzed chemically. After that, 20 kg of cattle dung were added in both digesters. Also, 20 liters of water were added to reach 10% T.S. Then, the two caps were sealed into the digesters as the feedstock was added only once at the beginning of the experiment (batch mode). The working fluid (water) is heated inside a copper U-tube which is placed inside the evacuated tube by solar energy. The water is pumped from the water tank that is placed half meter above the PTC to the heat exchanger inside the treatment digester. Heat exchange process takes place between the heated water and the slurry. There are two thermocouples placed inside both digesters one at the top of the slurry and the other at the middle to measure the temperature on an hourly basis. The middle one in the treatment digester is connected with a temperature controller. When the temperature reaches 36 °C, (the desired optimum temperature), the controller gives a signal to a control circuit to switch off the pump. On the other hand, the fermentation process in the control digester takes place at the ambient temperature. The produced biogas is collected from both digesters in cylindrical tubes via the displacement method.
The fermentation temperature values and biogas yields of both digesters were compared. The obtained results were as follows:
1. Solar energy is a promising renewable source for providing the biogas digesters with the required thermal energy so that to be within the desired temperature range during the cold days of winter.
2. The efficiency of the collector reached 53%
3. Using the solar parabolic trough technique raised the temperature inside the digester by 20.6%.
4. The average maximum temperature reached during the day inside the control digester was 24.5 °C at the noon period (from 12 pm to 2 pm), while the average maximum temperature reached during the day inside the treatment digester was 32.2 °C at 1 pm.
5. The accumulated biogas productivities of the control digester and the treatment digester during the experiment were 9684.7 mL/kg. T.S., and 24649.69 mL/kg. T.S., respectively.
6. Increasing the fermentation temperature resulted in the increase of biogas productivity, where the productivity of the treatment digester is 2.5 times higher than the productivity of the control digester.
Recommendation:
As for future work:
Use additional source for thermal energy such as electrical heater or store solar energy to maintain the optimum temperature continuously without fluctuations in cloudy and partly cloudy days of winter.