LI Na LI Cheng-xiang

（College of Electromechanical Engineering,Qingdao University， Qiangdao Shandong， 266000）

0 Introduction

In recent years,the solar energy aided biogas production system has been concerned considerably because of benefits of reducing pollutant emissions and energy saving[1].The studies of a solar energy aided biogas production system mainly include the mechanism and conditions of biomethanation[2],the utilization of biogas[3],the simulation and design of a solar energy aided biogas production system[4],etc.Studying the system of the combination of solar energy and biogas used for floor radiant heating and as fuel has an important practical significance to the northern countryside,where the biomass energy and solar energy are rich but still using some conventional energy (coal,oil,etc.)for heating and burning.From this context,a mathematical model which was used to predict the performance and feasibility of this system was established.

1 The system design

The system combined solar energy with biogas boiler for floor radiant heating and fuel which is designed for a rural house of Qingdao,is presented in Fig.1.

2 Mathematical model

2.1 Collector system

Where Qsolaris the heat from solar collectors；Isis the solar irradiation；Asc(=25m2)is the area of solar collectors；ηsc(=0.35)is the efficiency of solar collectors.

2）Biogas boiler

Where Tab,outis the outlet water temperature of the boiler；m˙ab,bgis the biogas flowrate in the boiler；ηab(=0.8)is the efficiency of the boiler；Ql,bg(=18.516×103kJ/kg)is the low grade calorific heat of the biogas.

3）Water tank

Where mwt,w(=1025kg)is the water mass in water tank；m˙h’,w(=0.151kg/s)is the flow rate of WP1；cp,w(=4.186 kJ/(kg·K))is the specific heat of water；Twt,wis the average temperature of water；Th’,outis the water temperature supplied to boiler；Th’,inthe inlet water temperature from bioreactor or heat consumer into water tank；Qwt,lossis the heat loss from water tank.

Qwt,loss=Ut,eAwt（Twt,w-T0） （4）

Where Ut,e(=0.45W/(m2·K))is the heat extraction coefficient of water tank；Awt(=18.1m2)is the area of water tank；T0(=289K)is the air temperature in work room.

2.2 Heating system

Where Th,outis the outlet water temperature of the heat consumer；Qh(=3.6kW)is the heat load of the heat consumer；ηh(=0.85)the efficiency of the heating system.

2.3 Biogas system

1）Bioreactor

Where mbdis the mass of the digester；cp,d(=4.2 kJ/(kg·K))is the specific heat of the digester；Tb,inand Tb,outare the inlet and outlet water temperatures of bioreactors,respectively；Tb,dis the temperature of the digester；hA(=8.792kW/K)is the heat transfer performance coefficient of the heat exchanger；m˙mis the flow rate of the incoming manure；Tm(=289k)is the temperature of the incoming manure；Qb,lossis the heat loss from bioreactors.

Where Ubf(=0.138W/(m2·K)),Ubr(=0.143W/(m2·K))and Ubw(=0.132W/(m2·K))are the average heat transfer coefficients of bioreactors’ floor,roof and walls,respectively；Abf(=12.56m2),Abr(=14.50m2)and Abw(=50.24m2)are the area of bioreactors’ floor,roof and walls,respectively；Tais the ambient air temperature.

2）Kinetic methane production rate models

Where γvis the kinetic methane production rate；B0(=0.48 l CH4/g VS)is the ultimate CH4yield；S0(=70 g/l VS)is the influent Volatile Solids concentration；HRT (=25d)is the hydraulic retention time；μmis the maximum specific growth rate；K is the kinetic parameter.

3）Mass conservation of methane

Where m˙bgis the total flowrate of biogas produced in bioreactors；ρCH4(=0.717 kg/m3)and ρbd(=996 kg/m3)are the densities of methane and digester；xCH4(=0.4)is the mass fraction of the methane.

2.4 Fuel supply system

In Qingdao rural areas,the biogas fuel consumption per day for cooking is about 0.5m3(0.1m3for breakfast at 7 o’clock,0.15m3for lunch at 12 o’clock and 0.25m3for dinner at 18 o’clock)for a family.

1）Compressing process

Where Wcis the shaft power input；k is the adiabatic exponent of biogas；R is 0.305 kJ/(kg·K)；Pbg,c,inand Pbg,c,outare the inlet and outlet pressures；Tbg,c,inis the inlet temperature of the compressor；ηcis 0.59；m˙′bgis the mass flowrate of biogas；xH2Ois 0.0035；xH2Sis 0.0065；Ph,bgand Th,bgare the pressure and temperature of the holder；v is the specific volume；a and b are the Van der Waals constant of biogas.

2.5 Permissible temperature drop

3 Result and discussion

The total solar irradiance on the horizontal surface during this time period is shown in Fig.2.The sunny daytime is generally from 7：30 to 17：30,and the total solar irradiance on the horizontal surface presents cyclical changes every day.In addition,the highest point of the solar irradiation is at 12：00 to 13：00.

Fig.3 shows manure temperature and biogas production rate for 10 January days in the different ambient air temperature.In the heating season,the mean values and the deviation of the manure in the bioreactor are 35℃ and ±1℃ ,respectively.When the manure temperature in the bioreactor is lower than 307.15K,opening the valve M4 and closing the valve M5,the heat of maintaining the usual fermentation is provided by the hot water out of the heat consumer.The manure temperature in the bioreactor is higher than 309.15K,opening the valve M5 and closing the valve M4.The biogas production rate changes with the manure temperature and the average value is 1.481452 kg/h.With the ambient air temperature rising,the cycle of the manure temperature and biogas production rate increases because of the thermal loss decreasing.

Fig.4 and Fig.5 show the biogas consumption rate in the boiler decreasing with the increase of the water temperature in tank because the water tank and the boiler supply heat to the heating and fermentation together.Under different ambient air temperature,the water temperature and biogas consumption rate appear different cyclical changes and the values of them have a huge increase and decrease on account of the periodic changes of the manure temperature.The water temperature ranges from 305.67-321.24K and the biogas consumption rate ranges from 0.27-2.38 NM3/h.In addition,the average values of the biogas consumption rate are 29.49NM3/d,31.37NM3/d and 32.38NM3/d corresponding to-14℃,0℃ and 14℃,respectively.

4 Conclusion

The operating characteristics of the system were simulated,and effects of the ambient air temperature on the performance of the system were studied.Analyzing the results of the simulation,the conclusions could be drawn as follows：

41 The manure temperature in the bioreactor ranges from 307.17-309.15K and the biogas production rate changes with the manure temperature.The average value is 1.481452 kg/h.In addition,with the ambient air temperature rising,the cycle of the manure temperature and biogas production rate increasing.

4.2 The use of water tank could increase the utilization efficiency of the energy,overcome the circadian rhythm of the solar energy and ensure the inlet temperature of the water out of the accessory boiler ranging from 305.67-321.24K.

4.3 The solar energy doesn’t maintain the heat of the floor radiant heating and fermentation,starting-up the boiler.What’s more,the biogas consumption in boiler decreases with the ambient air temperature rising.

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