Using PTC And Greenhouse For Heating Biogas Digesters

The digestion temperatures in Northern China do not reach the optimum point for continuous production of gas without heating. In cold weather, it is difficult to generate large amounts of gas with stable performance without heating and the heating must be cheap, reliable, environmental friendly, efficient and have high performance. All these properties are available when using renewable energy as the studies show. The direct and diffuse solar radiations in Xi'anyang area indicative that we can use the solar heating to increase the temperature of digesters, so this study focuses on using solar energy for heating biogas digesters. The relation between ambient temperature and the temperature inside the digesters is one of the most important factors. As shown in chapter two gas production has been significantly affected by temperature inside the digester, when the temperature decreases sharply, gas production also decrees sharply.In chapter four of this study we used EnergyPlus building energy simulation software and real weather data for biogas digester heating simulation with 8,760 hours of simulation. Chapter four was carried out in two parts .The first part used two biogas digesters above ground; one with solar heating and the second without heating. The second part of this chapter is a simulation of two biogas digesters underground; one with solar heating using greenhouse and inlet heating, and the second without heating. It was shown that using solar energy (greenhouse and inlet heating) is better than the other three options. However this method cannot achieve the optimum temperature during all months of the year, so this study proposed a solution by using parabolic trough solar concentrators (PTC) with inlet heating.Using one-tube concentrators in the focal point with fixed PTC gives lower efficiency compared with using two-vacuum tubes, this is because of the variation of the solar altitude angle is very large so using two focal points gives more heating as shown in chapter three.In chapter five one digester under the ground was used as study object, which is located at Runzhen middle school, Shaanxi province, and its volume is 100 m3. The system's raw material was human manure, which was fed to the system every day. The average temperature in January was -1.6oC, while the lowest temperature was -15oC and the average of solar radiation in Runzhen area was 4.5 kwh/m2. The objective of this study was to investigate the feasibility of using solar energy for increasing biogas production. The system used greenhouse and PTC solar heaters to reach optimum temperature. The system was used for heating the biogas digester and classrooms. The fermentation in the biogas digester using PTC worked in normal production during year and in case of a drop in temperature the control unit will switch to the auxiliary electrical heating system in order to heat the digester using hot water to make the system run steadily. In the winter the solar heating system increased biogas production 12 m3 per day . The gas yield was used in the school kitchen being transported by a pipe designed for cooking. The total profit from the PTC system in Runzhen School was 16,104 Yuan , heating classrooms for 60 months and reducing CO2 emission 788.950 ~ 1075.830 tons.Using greenhouse with inlet heating and PTC is the most suitable method for heating economically; therefore we recommended using solar energy for increasing the temperature of biogas digesters. In addition, the rest area of greenhouse can also be used for planting.The new system model depends on parabolic trough solar concentrators (PTC) with tracking system and double face of greenhouse. This new design will achieve the optimum temperature and accordingly high gas production per day without additional daily costs.