| The heat storage effect of the north wall in conventional active heat storage solar greenhouse(AHSSG)performs well,and plays an important role in improving the indoor thermal environment at night.However,it also has some disadvantages,which restricted the application of AHSSG,such as low heat storage capacity,low efficiency of heat transfer ducts,unsteady air motion,and complex construction technologies.In this study,based on the structural characteristics of the north wall in AHSSG and preliminary results of our research team,we studied the heat transfer characteristics of the north wall from three aspects: heat regenerator,heat conductor,and heated airflow mode.Furthermore,assembly construction structure of AHSSG and its performance were investigated.The main results are as follows:(1)A new phase change materials additive for soil curing agent(PCC)was configured.Incorporation of PCC into the soil,the compressive strength was increased significantly,and the compressive soil strength was positively correlated with the content of PCC.The different density of compacted soil as control,the average compressive strength of solidified soil with more than 8% PCC(PCC+S)and the density of 2.0 g·cm-3soil compaction(S2.0)were more than 2 MPa,and can be used in the construction of AHSSG.By scanning with an electron microscope,weak links between soil particles became thin film and acicular cementation,and the crystallization characters of phase-change materials could be clearly observed after PCC was added.The internal factors and energy storage mechanism of the strengthening of PCC+S were explained from the micro scope.It was found that 8% PCC+S and S2.0 have good thermal performance and can be used as heat storage material for AHSSG north wall.The test results of the micro-shrinkage model of walls showed that the storage and release heat of the back wall in 8% PCC+S is 3 499.8 and 3 285.7 k J respectively,which was 13.6% and 15.0%higher than that of S2.0.(2)A test bed for testing the heat transferring performance of different materials was fabricated.The results showed that the heat transfer effect of steel mesh skeleton-geotextile composite pipe(SFG)was the best.The ability of heat transferring in STG was 3.9 timeshigher than that in galvanized iron pipe(GI),2.1 times higher than that in polyvinyl chloride pipe(PVC),which indicated that the heat transfer effect of ventilated air duct was better than that of closed air duct.By using computational fluid dynamics(CFD)analysis,the maximum relative error of the three heat transfer duct calculation models was 4.4%,which indicated that the model could be used to study the heat transferring performance of different materials and the distribution of heat storage soil.Based on simulations,we found that the heat transfer effect of PVC half tube(h-PVC)was better than PVC,the effect of heat transfer of strongly rammed pure soil pipe(SPS)was better than weakly rammed pure soil pipe(WPS).(3)Three types of AHSSG north wall models were constructed including the top in and out air movement mode(W1),the side in and out air movement mode(W2),and the side in and top out air movement mode(W3).The maximum relative error between W1 and the test result was 13.4%,which indicated that the numerical simulation had good agreement with the field measurement.The simulation results showed that the effective thermal storage range of W1,W2,and W3 were 700 ~ 800,500 ~ 600,and 600 ~ 700 mm,respectively.Analyzing the air distribution of cross sections at 8 m from the inlet.W1 was better than W3,and W3 was better than W2 regarding the heat storage performance of AHSSG.(4)Three types of AHSSG indoor environment in winter were tested including the original AHSSG(G1),the backfill-assembled AHSSG(G2),and the module-assembled AHSSG(G3).Results of 31 consecutive days(2017-12-22 to 2017-12-22)measurement showed that the air temperature in G3 was slightly better than that in G2,air temperature in G3 and G2 were all better than that in G1.On a typical sunny day(2017-12-31 9 a.m.~ next day 9 a.m.),the heat storage wall thickness of three AHSSG were 700~800mm,800~900mm,and 700~800mm,respectively.On a typical cloudy day(2018-1-14 9 a.m.~ next day 9 a.m.),the heat storage wall thickness of three Greenhouse were 300~400mm,500~600mm,and500~600mm,respectively.The tomato growth status in G3 was the best,was the worst in G1.The cost of per-square-meter of G1,G2,and G3 were 461.1RMB,389.9RMB,and299.0RMB,respectively,therefore,the construction cost of G3 was significantly reduced.(5)The initiative and passive heat transfer characteristics of the AHSSG was analyzed individually.Results showed that,on a typical sunny day,the heat storage of G1,G2,and G3 active circulation heat storage system were 120.36 MJ,215.27 MJ,and 73.94 MJ,respectively.Energy efficiency ratio(K)were 15.48,49.83,20.77,respectively.Therefore,the heat quantity of G2 active circulation heat storage system was the largest,which had the best energy-saving effect.On a typical sunny day,the passive heat storage of G1,G2,and G3 back wall were 280.58 MJ,412.06 MJ,and 415.56 MJ,respectively.The heat storage of G1,G2,and G3 were 142.01 MJ,281.55 MJ,and 299.88 MJ,respectively,more than heat release.Ona typical cloudy,the heat release of three solar greenhouses day were 63.57 MJ,46.91 MJ,and 40.27 MJ,respectively.The heat release of G1,G2,and G3 were 51.36 MJ,29.05 MJ,and 3.44 MJ,respectively,more than heat storage.Therefore,G3 stored the most heat during the day time,released the least heat during the night time.On a typical sunny day,the heat storage contribution rate of these three AHSSG active circulation heat storage system were30.02%,34.32%,and 15.11%,respectively.Results showed that module-assembled AHSSG had a better heat storage performance and a lower cost than the original AHSSG.Therefore,module-assembled AHSSG was highly recommended due to its practical and reliable properties. |
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