| Solar energy is the most abundant clean and renewable energy in the world.Applying the solar heating technology in buildings can effectively reduce the consumption of the fossil energy and environmental pollution.However,for the high-altitude cold regions such as western Sichuan and Tibet which have strong heating demand and abundant solar resource,current solar heating technology could not effectively solve the heating problem due to technology defects and is urged to be perfect.As there is a mismatch between the solar energy instability and the heating demand stability,the thermal energy storage(TES)device is necessary for the solar heating system,especially in the high-altitude cold regions.The traditional storage tank takes use of the sensible heat storage mechanism which means low heat storage density and large space requirement as well as great heat loss.In addition,too small tank will lead to water vaporization when the solar radiation is very strong while too large tank will lead to low water temperature which could not meet the terminal demand.For the heat storage device,both storage mechanism and storage matching need the new exploration.Compared to the sensible heat storage,the latent heat storage with phase change material(PCM)offers a higher heat storage density,lower temperature swing as well as lower requirements of space and weight.Therefore,latent heat storage has captured significant research attention in recent years.The solar latent heating technology is attributed to the interdisciplinary field of upstream and downstream disciplines.The upstream disciplines mainly focus on the development of new phase change material and the charging/discharging characteristics of the PCM.However,the reasonable matchting of the latent heat thermal energy storage(LHTES)device in the solar heating system is lack of research attention for the downstream disciplines.Meanwhile,the heat storage medium mostly is?water&PCM?in current research,completely using PCM as the heat storage medium is rare.In the actual application,the solar latent heating system is lack of guidance of the technical code.However,several engineering projects have appeared due to the social demand,which shows the importance and urgency of the reasonable matchting of the LHTES device in the system.Around this key point,this thesis conducted the following works:(1)The solar heating system composition and the classification of the TES device were introduced.The coupling relation of the solar heating system was deconstructed.Based on the target parameters of storage capacity and storage temperature,the TRIZ was introduced to analyze the physical contradiction of the storage capacity of TES deivice under different climatic conditions,and the solution of physical separation of the TES device was offered.The physical separation included three methods:the heat storage medium variation,the physical boundary conditions variation and the modular design.The modula design was suitable for the LHTES device,which meant that a LHTES device was composed of several LHTES units,making the engineering design simple and flexible.(2)Based on the modular design,numerical and experimental study was developed to investigate the charging characteristic of a LHTES unit including a helical coil heat exchanger and paraffin/expand graphite composite PCM.A transient 3D numerical model considering the buoyancy effect was developed by FLUENT 15.0 based on the enthalpy-porosity method to simulate the unsteady melting process of the LHTES unit.Temperature measurements using evenly spaced thermocouples were conducted,and the temperature variation at three locations inside the LHTES unit was recorded.Reasonably good agreement was achieved between the numerical prediction and the temperature measurement for the typical case with initial temperature 20°C,inlet heat transfer fluid(HTF)temperature 60°C and inlet HTF flow rate 0.25m~3/h,which confirmed the numerical simulation accuracy.Effects of the HTF inlet flow rate,HTF inlet temperature,buoyancy and arrangement on the charging performance of the LHTES unit were investigated.For a constant HTF inlet temperature(60°C),the charging times of case 1~4 maintained almost the same(10000s).Increasing the HTF inlet flow rate from 0.25 m~3/h to 1.0 m~3/h showed almost no enhancement of the average charging power.The total charging heat of case 1~4 remained at 7200kJ and8000kJ for the numerical result and experimental result,respectively.Therefore,the HTF flow rate should maintain a relatively low value to reduce the energy consumption of the pump.The HTF inlet temperature can greatly affect the charging performance.Higher HTF inlet temperature resulted in a larger charging power and HTF inlet/outlet temperature difference.For a constant HTF inlet flow rate(0.25m~3/h),the charging time dropped from 13000s to 6000s and the mean charging power increased from 0.55kW to1.40kW as the HTF inlet temperature increased from 55°C to 70°C.However,the further enhancement by increasing the HTF inlet temperature was not apparent and the LHTES unit showed best compatibility with heat source when the HTF inlet temperature was 60-65°C.The numerical results showed that the natural convection induced by buoyancy greatly promoted the melting process of PCM.For case A(considering the buoyancy effect)and case B(not considering the buoyancy effect),the PCM average temperature of case A is 23.7%higher than that of case B at 10,800 s.The average liquid fraction of case A is 90.9%higher than that of Case B at 10,800 s.Thus indicates that the natural convection greatly affects the simulation results,especially during the gradual heating-up stage,therefore,the buoyancy effect of the PCM should be considered in actual applications.The numerical results showed that vertical arrangement of the LHTES unit was a little better than the horizontal arrangement for the charging process of PCM.However,difference of the PCM average temperature mainly appeared in the end of the melting process(less than 5%)and the charging times were almost the same.(3)The mathematical modelling of the flat plate solar collector and the solar collecting system design were analyzed.Both solar collector-TES device matching mode and solar collector-TES device-heating terminal matchting mode for the traditional heat storage tank were analyzed.It was found that the characterization parameters of both matchting codes could not reflect the coupling charging performance and the storage temperature.It was not reasonable to just consider the storage capacity without the storage temperature.In fact,only the storage heat which satisfied the requirement of the storage temperature was the effective storage heat.Based on the coupling relation analysis of the solar latent heating system,solar collector-LHTES device match mode and corresponding match characterization parameter including solar collecting factor M,climatic factor C and structural factor S were proposed.Effective charging time and matching evaluation approach of the LHTES device were subsequently proposed.Taking the LHTES unit in chapter 2 for example,five design cases with different solar collecting factors(combination of different HTF mass flow rates and different solar collecting areas)were evaluated by the proposed approach and corresponding system thermal performance was analyzed.The evaluation results showed that the water mass flow rate had a slight effect on the thermal performance for a constant solar collecting area(2m~2)under the typical climatic condition of Lhasa.Increasing the HTF mass flow rate from 0.1m~3/h to 0.3m~3/h contributed to the decrease of the HTF peak value.During the effective charging time,the maximum average liquid fractions of case 1~3 all reached to 0.99 which meant that the PCM had completely melted.Meanwhile,the HTF peak temperatures of case 1~3 were 65.7°C,62.2°C and59.8°C,respectively.All three values were below the water evaporating temperature in Lhasa(86°C).Case 1~3 were all reasonabe matched.However,case 1(0.1m~3/h)showed the best performance due to the lowest HTF flow rate.It can be observed that the mass flow rate had insignificant effect on the thermal performance of the solar latent heat storage system through the simulations for the certain range.Therefore,once a safe operation of the system was guaranteed,a lower HTF mass flow rate was recommended to be applied in actual applications due to an energy consumption reduction from the circulate water pumps.For a constant HTF flow rate(0.1m~3/h),the solar collecting area had significant effect on the thermal performance of the solar latent heat storage system through the simulations.During the effective charging time,the PCM did not melt completely for case 4(Ac=1m~2)with a maximum liquid fraction of 0.5,which indicated that the solar colleting area 1m~2 was not sufficient and case 4 was not reasonably matched.The PCM of case 5 completely melted,however,the HTF peak temperature of case 5 was 87.4°C,which was higher than the local water evaporating temperature(86°C).Therefore,the operation of case 5 could not be safe and case 5 was not reasonably matched.In the design process of the solar heating system,the solar collecting area for each LHTES unit should be kept within a feasible range,as a smaller area will lead to the incomplete melting of PCM while a larger one can reduce to the inefficient operation.High economic investment as well as large laying area for solar collectors due to the increase of the solar collecting area should also be taken to consideration.(4)The matching evaluation approach of the LHTES device was used in a solar latent heating engineering project in Chongqing.The flat plate solar collector-LHTES device coupling modelling was validated by experiment.Based on the coupling modelling,the LHTES device matching was evaluated and analyzed.The evaluation results showed that the LHTES device was not reasonably matched;both the storage capacity and the HTF flow rate were too large.When the number of the LHTES units decreased from 6 to 3 and total HTF flow rate decreased from 2.28m~3/h to 0.6m~3/h,the LHTES device could be well matched.It can be seen that the storage capacity could be reduced by 50%and the pump energy consumption could be reduced by nearly 98%.Therefore,the LHTES device design had great impact on the investment and the energy consumption of the solar latent heating system,and therefore it should be carefully demonstrated.Solar latent heating is a potential new technology.Good heat transfer performance and reasonble matching of the LHTES device both are the key points for the application of the solar latent heating technology.The works including the charging performance of a high efficient LHTES unit,the matching approach study of the LHTES device and the matching evaluation of the LHTES device in the actual engineering project have been conducted in this thesis and can offer some theoretical and technological support for the widely application of the solar latent heating technology. |
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