Prediction of CO2 leakage into biosphere is very important for risk assessment in geological carbon storage projects. Underground CO2 can be transported into biosphere through short term leakage due to fractures of wellbores or cap rocks, which has been extensively investigated, and long term leakage due to diffusion, which has few relevant studies. This paper presents a diffusive model for CO2 gradual leakage into biosphere during a long period after CO2 injection. First, the paper describes a general diffusive model with long term secondary trapping effects for CO2 fluxes from underground into biosphere. Secondly, a simplified one-dimensional model is presented and solved for the CO2 concentrations in groundwater. The results show that the groundwater CO2 concentration will reach the maximum value at about 50 th year after CO2 injection and then slowly decrease due to secondary trapping effects.Moreover, the partition coefficient is the dominant parameter for predicting the groundwater CO2 concentration while the convective mass transfer coefficient plays an insignificant role.
In general,thermal processes can be classified into two categories: heat-work conversion processes and heat transfer processes. Correspondingly,the optimization of thermal processes has to have two different criteria:the well known entropy generation minimization method and the recently proposed entransy dissipation maximization method. This study analyzes the thermal issues in a heat exchanger group,and optimizes the unit arrangements under different constraints based on a suitable optimization crite-rion. The result indicates that the principle of minimum entropy generation rate is valid for optimizing heat exchangers in a ther-modynamic cycle with given boundary temperatures. In contrast,the entransy dissipation maximization is more suitable in heat exchanger optimizations involving only heat transfer processes. Furthermore,the entropy generation rate induced by dumping used streams into ambient surroundings has to be taken into account,except for that originating from the hot and cold-ends of heat exchangers,when using the entropy generation minimization to optimize heat exchangers undergoing a thermodynamic cycle.
Inspired by the property diagrams in thermodynamics,which distinctly reflect the performance and characteristics of thermodynamic cycles,we establish a state equation for heat motion and introduce a two-dimension property diagram,T-q diagram,in heat transfer to analyze and optimize the performance of heat exchangers,where heat flow is a state parameter for heat motion.According to the property diagram,it is convenient to obtain the influences of heat exchanger area,heat capacity rate and flow arrangement on the heat transfer performance during the analysis of heat exchangers and their networks.For instance,when analyzing the heat exchanger network in a district heating system,it is obvious to find that:if both the heat demand and the indoor air temperature in each branch of the network are the same,the total area of heat exchangers,the flow rate of water and the return water temperature in each branch are all the same;if the indoor air temperatures in different branches are different,the temperatures of the waters after flowing through different branches are different,which means that the mixing process of return waters with the same temperature is not an essential requirement to realize the best performance of district heating systems.
The flat-plate solar collector is an important component in solar-thermal systems,and its heat transfer optimization is of great significance in terms of the efficiency of energy utilization.However,most existing flat-plate collectors adopt metallic absorber plates with uniform thickness,which often works against energy conservation.In this paper,to achieve the optimal heat transfer performance,we optimized the thickness distribution of the absorber with the constraint of fixed total material volume employing entransy theory.We first established the correspondence between the collector efficiency and the loss of entransy,and then proposed the constrained extreme-value problem and deduced the optimization criterion,namely a uniform temperature gradient,employing a variational method.Finally,on the basis of the optimization criterion,we carried out numerical simulations,with the results showing remarkable optimization effects.When irradiation,the ambient temperature and the wind speed are 800 W/m2,300 K and 3 m/s,respectively,the collector efficiency is enhanced by 8.8% through optimization,which is equivalent to a copper saving of 30%.We also applied the thickness distribution optimized for wind speed of 3 m/s in heat transfer analysis with different wind speed conditions,and the collector efficiency was remarkably better than that for an absorber with uniform thickness.
