On the effect of boundaries in two-phase porous flow

In this paper we study a model of an interface between two fluids in a porous medium. For this model we prove several local and global well-posedness results and study some of its qualitative properties. We also provide numerics

Experimental study on transcritical Rankine cycle (TRC) using CO2/R134a mixtures with various composition ratios for waste heat recovery from diesel engines

A carbon dioxide (CO2) based mixture was investigated as a promising solution to improve system performance and expand the condensation temperature range of a CO2 transcritical Rankine cycle (C-TRC). An experimental study of TRC using CO2/R134a mixtures was performed to recover waste heat of engine coolant and exhaust gas from a heavy-duty diesel engine. The main purpose of this study was to investigate experimentally the effect of the composition ratio of CO2/R134a mixtures on system performance. Four CO2/R134a mixtures with mass composition ratios of 0.85/0.15, 0.7/0.3, 0.6/0.4 and 0.4/0.6 were selected. The high temperature working fluid was expanded through an expansion valve and then no power was produced. Thus, current research focused on the analysis of measured operating parameters and heat exchanger performance. Heat transfer coefficients of various heat exchangers using supercritical CO2/R134a mixtures were provided and discussed. These data may provide useful reference for cycle optimization and heat exchanger design in application of CO2 mixtures. Finally, the potential of power output was estimated numerically. Assuming an expander efficiency of 0.7, the maximum estimations of net power output using CO2/R134a (0.85/0.15), CO2/R134a (0.7/0.3), CO2/R134a (0.6/0.4) and CO2/R134a (0.4/0.6) are 5.07 kW, 5.45 kW, 5.30 kW, and 4.41 kW, respectively. Along with the increase of R134a composition, the estimation of net power output, thermal efficiency and exergy efficiency increased at first and then decreased. CO2/R134a (0.7/0.3) achieved the maximum net power output at a high expansion inlet pressure, while CO2/R134a (0.6/0.4) behaves better at low pressure

Preliminary experimental comparison and feasibility analysis of CO2/R134a mixture in Organic Rankine Cycle for waste heat recovery from diesel engines

This paper presents results of a preliminary experimental study of the Organic Rankine Cycle (ORC) using CO2/R134a mixture based on an expansion valve. The goal of the research was to examine the feasibility and effectiveness of using CO2 mixtures to improve system performance and expand the range of condensation temperature for ORC system. The mixture of CO2/R134a (0.6/0.4) on a mass basis was selected for comparison with pure CO2 in both the preheating ORC (P-ORC) and the preheating regenerative ORC (PR-ORC). Then, the feasibility and application potential of CO2/R134a (0.6/0.4) mixture for waste heat recovery from engines was tested under ambient cooling conditions. Preliminary experimental results using an expansion valve indicate that CO2/R134a (0.6/0.4) mixture exhibits better system performance than pure CO2. For PR-ORC using CO2/R134a (0.6/0.4) mixture, assuming a turbine isentropic efficiency of 0.7, the net power output estimation, thermal efficiency and exergy efficiency reached up to 5.30 kW, 10.14% and 24.34%, respectively. For the fitting value at an expansion inlet pressure of 10 MPa, the net power output estimation, thermal efficiency and exergy efficiency using CO2/R134a (0.6/0.4) mixture achieved increases of 23.3%, 16.4% and 23.7%, respectively, versus results using pure CO2 as the working fluid. Finally, experiments showed that the ORC system using CO2/R134a (0.6/0.4) mixture is capable of operating stably under ambient cooling conditions (25.2–31.5 °C), demonstrating that CO2/R134a mixture can expand the range of condensation temperature and alleviate the low-temperature condensation issue encountered with CO2. Under the ambient cooling source, it is expected that ORC using CO2/R134a (0.6/0.4) mixture will improve the thermal efficiency of a diesel engine by 1.9%

Influence of geology and hydrogeology on heat rejection from residential basements in urban areas

Urbanization and limited land availability have resulted in the increased utilization of underground structures including residential basements in largely populated cities such as London, with an average addition of 200 basements per year in some boroughs. Residential basements kept at a comfortable temperature level throughout the year significantly contribute to heat fluxes in the subsurface as well as an increase in ground temperature. Understanding the ground thermal status is crucial in managing the significant geothermal energy potential in urban areas as well as the sustainable development of the urban underground, and in maintaining the energy efficiency of underground structures. In this proof-of-concept study, a 3D finite element approach accounting for coupled heat transfer and groundwater flow in the ground was used to investigate the influence of ground conditions on the heat rejection rate from basements. A detailed analysis was made of ground, above ground and underground built environment characteristics. This study demonstrates that the amount of heat from basements rejected to the ground constitutes a significant percentage of the total heat loss from buildings, particularly in the presence of groundwater flow. The extent of thermal disturbance in the ground varies depending on the ground characteristics. The volume of thermally disturbance ground inversely correlates with the groundwater flow rate in ground mainly consisting of highly permeable material. However, a direct correlation exists when the thickness of permeable soil layer decreases. A larger horizontal to vertical ratio of ground thermal disturbance is observed when the thickness of permeable soil layer increases

An Engineering Analysis Method for Deep Geothermal Energy

At present there are already many deep geothermal projects allowing us to have a better understanding of deep geothermal energy. However, there are still many issues to be solved for more reliable use of deep geothermal energy. This thesis proposes an engineering analysis method to assess the performance of a typical deep geothermal system, which is a doublet system with one injection well and one extraction well. A convective heat transfer boundary between the aquifer and overburden layers is applied for the axisymmetric problem based on the Lauwerier model for the first time. A new analytical solution is deduced using a series of Laplace transforms. The interaction between the injection well and extraction well is first neglected. Compared with other relevant analytical solutions, this new analytical solution comprehensively includes both heat conduction and heat advection in the aquifer and the heat flux between the aquifer and overburden layer. As long as the relevant parameters are obtained, the new analytical solution can intuitively illustrate the temporal and spatial temperature distribution within the aquifer. It can be used to determine the location of the extraction well and to evaluate the extracted geothermal power of the hot water aquifer. The convective heat transfer boundary at the interface does not only reflect the actual heat transfer process at the interface, but also models the heat transfer process in the overburden layer. It is shown that the dimensionless equivalent heat transfer coefficient is expressed as a function of the dimensionless injection rate and the dimensionless thermal conductivity of the overburden layer so the new analytical solution effectively incorporates the properties of the overburden layer. A series of FE simulations are conducted, and the analytical model is curve fitted to the FE results to evaluate the values of the dimensionless equivalent heat transfer coefficient. Based on the results of the curve fitting exercise, two empirical equations are proposed for typical cases. Applying the analytical solution coupled with these empirical equations and along with proper error estimates, it is possible to conduct a simple and rapid evaluation of the geothermal potential of a particular site. The revised analytical solution in this thesis is novel as there is no other analytical or semi analytical solution for the doublet system considering the heat conduction and heat advection in the aquifer and the heat flux between the aquifer and overburden layer. The revised analytical solution extended the analytical solution for a single injection well to a doublet scheme by considering the interaction effect between the injection well and the extraction well. The expression of the critical distance between two wells is obtained so that the best location of the extraction well in a doublet system can be determined. The spatial and temporal temperature distribution in the aquifer for a doublet scheme can be given by the revised analytical solution when the well distance is greater than the critical distance. It is found that it is valid to use a single well model to simplify a doublet scheme when the extraction well is far away from the injection well. The temperature of the extracted water against different time, injection rates and well distances can be obtained via the revised analytical solution. The revised analytical solution is compared with the experimental data and the numerical solutions and it is found that they match with each other well. The effect of a natural fault/fracture that exists in the aquifer on the performance of a doublet system, namely the temperature distribution and extracted temperature, is evaluated. By comparing the line model with the domain model, it is found that the line model is valid and computationally efficient. It is found that the acceleration effect of the fracture on thermal movement is the greatest when the fracture is located at the midpoint of the two wells. When the fracture is shifted towards the injection (extraction) well, the acceleration effect decreases and then becomes the deceleration effect. The deceleration effect of the fracture is the greatest when the fracture is located at the injection (extraction) well. The expressions of the critical angle for any position of the fracture in the doublet system are obtained. Equipped with these expressions, it is possible to decide whether the doublet system is still efficient during its life span once the cold water injection rate and the geometry and properties of the fracture are given.B

Thermal performance of a solar assisted horizontal ground heat exchanger

This paper presents an experimental study of a solar assisted horizontal ground heat exchanger system (HGHEs) operating as a daily heat storage unit. Initially, several soils were assessed as sensible heat storage mediums, with sand and gravel selected as the most appropriate. Then, a HGHEs was designed and connected to a 15m2 test room with a heating load of 1kW at Nottingham Trent University. Heating cables, simulating solar input, were used to heat the soil in the HGHEs to 70℃, then a heat transfer fluid (HTF), was circulated through a closed loop heat exchanger to extract the stored heat. The parameters of soil backfill and HTF mass flow rate were investigated in the HGHEs. Several output flowrates ranging between 0.1 to 0.6L/min were tested, producing discharge times varying between a few hours to a few days. The HTF mass flowrate was found to be the most significant parameter, affecting the HGHEs thermal capacity and heat exchange rates. The sand filled HGHE produced approximately 50% more hot water (T>35℃) during a longer duration achieving an efficiency of 78% compared to the gravel filled HGHE with a lower system efficiency of 58%. Insulating the HGHE system was found to reduce heat losses and avoid temperature fluctuations in the HGHEs. Overall, the results show the hot water quantity, temperature range and duration produced from the system were in line with low temperature district heating guidelines and can be applied to some household heating applications incorporating low flows and low temperatures