Research On The Thermal Performance And Cooling Load Calculation Method For Radiant Cooling Systems

Radiant cooling systems has been increasingly used due to their potential of energy saving,better thermal comfort,and healthy indoor air quality.However,due to the lack of knowledge and engineering experience on radiant cooling systems,designers and users are still not quite familiar with this technology.Among the key issues that handles the application of radiant cooling systems,the thermal performance(for instance cooling capacity at steady-state condition,the dynamic thermal performance)are of great importance.Currently,there is a lack of practical cooling capacity calculation method or tools for radiant cooling systems;and the cooling capacity is limited by the condensation problem;moreover,there is not much information on the dynamic thermal performance for radiant systems.In addition,cooling load calculation is another key issue because there is no appropriate cooling load calculation method for radiant cooling systems.This thesis did the following research according to the problems mentioned above.Considering that there is a lack of practical cooling capacity calculation method or tool for radiant cooling system in China,this study adopted an equivalent thermal resistance model to calculate the cooling capacity for radiant systems.The model is verified with good accuracy by CFD simulation and experimental data.Besides,current standards and guidebooks on radiant systems mainly focus on the cooling capacity under standard conditions,not enough attention were paid to cooling capacity in non-standard conditions.This study used experimental method to evaluate the cooling capacity of radiant cooling systems influenced by solar radiation.The experimental results showed that solar radiation could increase the cooling capacity up to 3 times,so we proposed a simplified method to evaluate the cooling capacity with the influence of solar radiation.For the issue of insufficient cooling capacity caused by condensation problem,we tried to find the best strategy to increase the cooling capacity while still keep the condensation under control.We found that the more uniform of the radiant surface temperature,the less possibility condensation will show up,and the higher cooling capacity.This indicated that we should try to make the surface temperature as uniform as possible when designing and optimizing the structure of radiant systems.Base on idea,we proposed three improved solutions for the structure of one radiant system type,and evaluated their cooling capacities using a verified CFD model.The CFD simulation results showed that the cooling capacities of the three improved radiant systems can be increased by about 40%,and the surface temperatures are very uniform,which help reducing the possibility of condensation.As to the lack of information on the dynamic thermal performance of radiant systems in current standards and guidebooks,this research used thermal response time through comparison and analysis.We used both experimental and calculation methods to evalute the response time for radiant systems.The experimental results showed that the response time(?95)of radiant systems with pipes embedded in the concrete layer can be about 18 h.We also simplified the testing process for response time of radiant system through a comparison study.For the calculation methods,we set up a dynamic coupling model through combining equivalent thermal resistance model with state space method.The model is verified in good accuracy by CFD simulation and experimental data.We used the coupling model to calculate the response time for most radiant system types through 56874 calculation cases.Through statistical analysis,we arrived at the response time range for most radiant system types.Moreover,there is no appropriate cooling load calculation method for radiant cooling systems.This study analyzed the cooling load conversion process and key issues of radiant cooling systems.We used an improved heat balance model to calculate the cooling load for radiant systems.Compared to other studies,this model can calculate the cooling load of the fresh system,radiant system at both surface and hydronic sides.This model can be used for cooling load calculation and sizing for radiant systems.We also studied the key impact factors on cooling load calculations.The impact factors like:control strategies,design temperatures,heat transfer coefficient on the building envelope surface,and the split of radiation or convection heat of internal heat gains are taken into consideration.We found that all the four factors could impact the cooling load calculations,whether in the form of total cooling load or fresh air load.Therefore,we calculated the following new parameters for radiant systems:heat transfer coefficients on the envelope surface,and the split of radiation or convection heat of internal heat gain.However,due to the complexity of heat balance model,it is not practical for engineering application.Moreover,it's still unknown whether the design and cooling load calculation method in Chinese standard is suitable for radiant cooling systems.This study compared the cooling load for radiant cooling systems and air systems,we found that the cooling load of radiant system can be around 13.4%higher than air systems.The cooling load calculation method in Chinese standard might cause inaccurate results.We used a correction coefficient to simplify the cooling load calculations.The correction coefficient is defined as the ratio of cooling load increase compared to all air systems.Through 4320 calculations cases,we arrived the simplified calculation formula for the total cooling load.The correction coefficient for air systems and hydronic side cooling load are also proposed and analyzed.The results could be used for designing and sizing radiant cooling systems.