| Household biogas is being transformed into central gas supply in whole village,Large/medium biogas digesters having broad development prospects.Therefore,the research and development of biogas digester with solar thermal system is of great significance.Large/medium biogas digesters have a lot of problems such as low fermentation temperature in winter,unreasonable control mode and so on.In order to solve the existing problems,this paper studied biogas digester with solar thermal system.Outdoor environment parameters(air temperature,solar radiation,wind speed and wind direction)and fermentation temperature and water temperature in the tank and heat flow in two test site,Kaihua County with combined heating system and Zhuji with solar heating system in Zhejiang Province,were consecutively observed and measured from December 2015 to March 2016.Based on these observed data,the thermal performance of the two different solar heating system were analyzed.Then find out the weak links and make suggestions for improvement.From December 8,2015 to March 31,2016 in Zhuji,which used solar heating system the digester heat dissipating capacity makes up the largest part of the total heating load.The heat transfer between the top of digester and soil,the side of digester and soil,the bottom of digester and soil accounted for 29.39%?29.74%,18.09%?18.39%and 38.41%?39.03%of the total heating load,respectively.From December 8,2015 to March 31,2016 in Zhuji,system was not operating perfectly and didn't reach the cyclic temperature difference because of solar radiation is weak.In the present situation,daily material mass was under the theory of inventory.The upper limit of fermentation temperature was 22 ?.The rate of gas production will be not promoted with rising temperature,even the fermentation temperature exceeds the limit.From December 9,2015 to March 31,2016 in Kaihua County,which combined solar energy with air-source heat pump,the digester heat dissipating capacity made up the largest part of the total heating load.The heat transfer between the top of digester and soil,the side of digester and soil,the bottom of digester and soil accounted for 36.33%?41.98%,19.70%?27.55%and 14.94%?15.80%of the total heating load,respectively.In the present situation,the upper limit of fermentation temperature were 23 ?.The rate of gas production will be not promoted with rising temperature,even the fermentation temperature exceeds the limit.A mathematical model of solar heating system was built based on the conservation law of energy.The accuracy and effectiveness of the model were validated by testing data.Then evaluated the accurace of model by two indicators,the average error and relative error between the measureed values and simulation values.The results showed that the maximum average errors of the fermentation temperature in Kaihua County and Zhuji were 0.159 ? and 0.285 ? respectively,corresponding to the maxium relative errors of 0.73%and 1.47%respectively.So the model meeted requirements.Based on the model in Zhuji,taked simulations by changing the cyclic temperature difference between solar thermal collector and water tank and digester.The results showed that the smaller the cyclic temperature difference,the more heat digester get and the higer the fermentation temperature between December and March in a typical year in Hangzhou,where the climates is similar compare with Zhuji.By the way,the left of the cyclic temperature difference refered to temperature difference between solar thermal collector and water tank,the right of the cyclic temperature difference refered to temperature difference between digester and water tank.When the cyclic temperature difference were(15,15),(15?,11?),(15?,5?),(10?,10?),(10?,5?)and(5?,5?),the average fermentation temperature were 2.49?,3.48?,3.57?,4.64? and 5.41 ? more than(15,15),respectively.At current feed rates,fermentation temperature exceeded 22? between December and March if the cyclic temperature difference were(5?,5?).That is,daily gas production reached a stable level of 68m3.Therefore,the best cyclic temperature difference was(5?,5?),if feed rates stayed the same.Based on the model in Kaihua County,taked simulations at the fermentation temperature of 23 ? in a typical year in Quzhou,where the climates is similar compare withQuzhou.Following main conlusions were obtained:(1)The heat pumpunit model accounted for more than 60%of total power consumption in whole year.(2)The power consumption of low temperature season accounted for 80.29%of total power consulption in whole year.(3)When the area was between 50m2 and 100 m2,combined heating system was superior to air-source heat pump heating system in economical effects.When per square meter of solar thermal collector was under ?700,the net present value was increased with increasing solar collector area.When per square meter of solar thermal collector was ?800,the net present value reached the maximum at the solar collector area of 80m2.After 80m2,the net present value decreased with increasing solar collector area.Finally,a 500 m3 standard reinforced concrete biogas digestor,equipped with solar heating system with air-source heat pump,was designed.Based on the digestor and the model stated before,A corresponding mathematical model of solar heating system was built in a typical year in Hangzhou.There were two situation to simulation:(1)Do not change the heating system and change fermentation temperature only.(2)Change the area of solar collectors,keep the fermentation temperature and heat pump constant.1 · The solar collector area was 184m2 and the unit power of Heat Pump Units was 7.5kW.When the fermentation temperature was increased from 20? to 30? and per cubic meter of biogas was ?1.2,net profit of whole year and low temperature season increased firstly and then decreased.And the Net profit reached the maximum at the fermentation temperature of 29 °C.To get the maximum profit,fermentation temperature should be set at 29 °?.If converted biogas into natural gas,per cubic meter of biogas was ?2.27,net profit of whole year and low temperature season was increased with increasing fermentation temperature.2.The solar collector area was changed from 30m2 to 250m2and the unit power of Heat Pump Units was 7.5kW.When per square meter of solar thermal collector was?500,the net present value was increased with increasing solar collector area and was positive.When per square meter of solar thermal collector was ?600,?700 and ?800,the the corresponding optimal area was 220m2,210m2 and 170m2,respectively.The net present value reached the maximum at the corresponding optimal area. |
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