Numerical analysis of heat pumps models: comparative study between equation-fit and refrigerant cycle based models

An equation-fit (EF) and a refrigerant cycle (RC) based heat pump models have been implemented, validated, analyzed and compared to each other under steady state conditions for a brine to water heat pump. Models validations have been provided through comparisons against experimental data obtained at ISFH. The advantages and disadvantages of the both models have been identified. This work provides significant inputs regarding the selection of a specific model depending on the needs. Analysis of mass flow rates and calculations far from typical catalogue data (non-standard conditions) are provided. The main conclusions can be summarized as: i) the EF model is recommended when the boundary conditions for the estimation and prediction modes are the same and when non-standard conditions are considered; ii) the RC model is the chosen alternative when the mass flow rates are modified from the estimation to the prediction mode.Peer ReviewedPostprint (published version

Analysis of the sensible and total ventilation energy recovery potential in different climate conditions. Application to the Spanish case

Energy recovery elements play a major role in the efficiency and sustainability of building ventilation systems. The use of a sensible or total energy recovery ventilator is a key decision for ventilation systems designers. However, there is a lack of technical tools and developments to support this decision. The authors present a procedure to develop a simple decision tool for designers based on hourly values of the outdoor weather conditions and that can be applied to any kind of building. Results of the procedure are presented in simple-to-use isoline maps and tables. In order to assess credibility of the model used in the procedure, data published in the literature have been used as a reference, showing good accordance. As an example, the procedure has been applied to the Spanish area considering 48 different locations. Results have been presented and discussed. Their analysis shows as the market-accepted recommendation of using energy recovery ventilators in locations with high relative humidity during the summer should be reconsidered.Peer ReviewedPostprint (author's final draft

Numerical Simulation of Turbulent Flows. Multiblock Techniques. Verification and Experimental Validation

Work here presented is the result of basic research in key aspects of the currently available engineering tools and methodologies for the design, optimisation and development of thermal systems and equipment: turbulence modelling, high performance computing and quality tests and procedures so as to assess credibility to the numerical solutions (verification and validation). The thesis comprises six main chapters written in a paper format. Two of them have already been published in international journals, one in the proceedings of a Spanish conference and two in proceedings of international conferences on Computational Fluid Dynamics and heat transfer. The last chapter has recently been submitted for publication to an international journal. Therefore, all the chapters are written so as to be self-contained, complete and concise. As a consequence, some contents of the chapters such those describing the governing equations, or the verification procedure used to assess the credibility of the numerical solutions, are repeated in several of them. Furthermore, as only minor changes have been introduced in the chapters respect to the original papers, each of them reflects the know-how of the CTTC (Heat and Mass Transfer Technological Centre were the research has been carried out) when they were published.Papers presented in chapters 1 and 2 deal with turbulence modelling. A general overview is given on the formulation and numerical techniques of the different levels of turbulence modelling: Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes Simulation (RANS). Main attention is focussed on the eddy viscosity two-equation RANS models. Their formulation is presented in more detail, and numerical solutions of the most extended. Benchmark problems on turbulence modelling are given compared to the available experimental data.Chapters 3 and 4 focus on the use of the multiblock method (domain decomposition method), as a numerical technique that combined with the parallel computing may allow reducing the demanding computational time and memory (high performance computing). The multiblock approach used is based on the conservation of all the physical quantities (fully conservative method) and on an explicit information exchange between the different blocks of the domain. The goal of the work presented in these two chapters is to verify that such a multiblock approach does not introduce additional uncertainty in the numerical solutions.Chapter 5 presents a tool that has been developed at the CTTC for the verification of finite volume computations. In fact, this tool is also partially used and described in the results presented in the previous chapters. Here, it is described and discussed in detail and it is applied to a set of different CFD and heat transfer problems in two and three dimensions, with free and forced convection, with reactive and non-reactive flows and with laminar and turbulent flows.The last chapter shows a complete study for the development of a credible heat transfer relation for the heat evacuated from a ventilation channel. Such study comprises all the different steps that have to be accomplished so as to develop credible and applicable results in mechanical engineering. It comprises a description of the mathematical model to represent the physical phenomena in the channel, the numerical model to solve the set of coupled differential equations of the mathematical model, the construction and testing of an ad-hoc experimental set-up, and a verification and validation (V&V) test that guarantees that the numerical solution is an accurate enough approximation of the mathematical model (verification), and that it properly predicts the reality (validation)

Detailed modelling of flat plate solar thermal collectors with honeycomb-like transparent insulation

A detailed numerical model for flat-plate solar thermal collectors based on one-dimensional finite volume techniques was recently presented, see Cadafalch (2009). The model considers a solar thermal device as a pile of components represented by one or several layers characterized by thermal inertia, internal energy generation and heat transfer to neighboring layers. A multi-layer model is then used to evaluate the full flat-plate solar thermal device. The model permits to investigate any configuration and material by combining appropriate layers. Standard components as opaque insulation, absorbers, air-gaps and glasses were addressed in Cadafalch (2009).Here, a numerical model to evaluate honeycomb-like transparent insulation material in the covers as a component of the multi-layer model is discussed in detail. The honeycomb is evaluated coupling radiation, convection and conduction phenomena. The discret ordinate method is used to evaluate media participation in thermal radiation.A comparison of numerical and experimental results is presented and discussed in order to show evidence of the model credibility.Peer Reviewe

A transient model for radiant heating and cooling terminal heat exchangers applied to radiant floors and ceiling panels

Renewable energy technologies for heating and cooling can be often optimized using low temperatures in the terminal heat exchangers for heating and the opposite for cooling. For example, thermal and electrical driven heat pumps have higher coefficient of performance (COP) for low impulse heating temperature and relatively high cooling temperature. If solar energy is directly used for heating purposes also a low temperature is necessary. Using this range of temperatures, a large exchange area is needed in order to obtain the desired conditions. In this sense, radiant floors and ceilings can be used with the objective to reduce the impulse water temperature in winter and increase it in summer to obtain high energy savings. However, a careful design and optimal control strategy are important to reach expected energy savings. Therefore, a model capable to capture transient effects, system control strategies, and it’s coupling with building energy simulation, is of importance. A transient numerical model for radiant floors and ceilings is presented and validated. The model has been implemented in RDmes online web platform and can solve both steady state and transient situations for sizing and predicting respectively. The radiant floor model has been developed to be used in the framework of the IEA-Task44 and the radiant ceiling has been employed in the FREDSOL project. The model developed is based on the composite fin model for radiant floor described by Kilkis et. al. (1994) coupled with a multi layer model and a step-by-step algorithm. The multi layer model solves the transient one-dimensional conduction behavior of the different layers. The two models are coupled considering the heat flow from the pipe as a sink source term in a typical transient conduction problem. The similar concept was used in the collector model presented by Cadafalch (2009). The step-by-step algorithm solves the fluid flow in one dimension. In the paper, an explanation of the mathematical formulation and numerical algorithm is provided in detail. A validation has been realized by means of experimental data comparisons from other references. Computational results have been analyzed and compared with other models presented in the test cases summarized in.Peer ReviewedPostprint (published version

Dynamic modelling of flat plate solar collectors: analysis and validation under thermosyphon conditions

Two collector models were analyzed under thermosyphon solar thermal system conditions: an extension of the physical model described by Duffie and Beckman (1991) and a modified correlation model based on the test efficiency curve obtained from European Standards. Special attention was paid to the body forces term of the momentum equation, a key aspect for thermosyphon system calculations. The models were verified and validated using a virtual test that numerically reproduces efficiency curves according to EN12975 (2006). A virtual test generated to represent thermosyphon unsteady system conditions was used to analyze model response under transient conditions. The Extended Duffie–Beckman model was shown to perform well when submitted to strong unsteady boundary conditions such as inlet fluid temperature, irradiance and mass flow rate. The model based on the efficiency curve was shown to work well for time steps larger than the collector residence time. However, for lower time steps, the model was found to be inaccurate due to the hypothesis of a single control volume for the fluid analysis. For the same reason, besides the assumption of a first order temperature profile in the fluid flow, the model was not capable to predict a physical behavior when submitted to strong variations of the fluid inlet temperature.Peer Reviewe

Steam chest for high temperature steam turbine

Cleaner Coal Technology ProgrammeAvailable from British Library Document Supply Centre-DSC:3291.521(246) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

A transient model for radiant heating and cooling terminal heat exchangers applied to radiant floors and ceiling panels

Renewable energy technologies for heating and cooling can be often optimized using low temperatures in the terminal heat exchangers for heating and the opposite for cooling. For example, thermal and electrical driven heat pumps have higher coefficient of performance (COP) for low impulse heating temperature and relatively high cooling temperature. If solar energy is directly used for heating purposes also a low temperature is necessary. Using this range of temperatures, a large exchange area is needed in order to obtain the desired conditions. In this sense, radiant floors and ceilings can be used with the objective to reduce the impulse water temperature in winter and increase it in summer to obtain high energy savings. However, a careful design and optimal control strategy are important to reach expected energy savings. Therefore, a model capable to capture transient effects, system control strategies, and it’s coupling with building energy simulation, is of importance. A transient numerical model for radiant floors and ceilings is presented and validated. The model has been implemented in RDmes online web platform and can solve both steady state and transient situations for sizing and predicting respectively. The radiant floor model has been developed to be used in the framework of the IEA-Task44 and the radiant ceiling has been employed in the FREDSOL project. The model developed is based on the composite fin model for radiant floor described by Kilkis et. al. (1994) coupled with a multi layer model and a step-by-step algorithm. The multi layer model solves the transient one-dimensional conduction behavior of the different layers. The two models are coupled considering the heat flow from the pipe as a sink source term in a typical transient conduction problem. The similar concept was used in the collector model presented by Cadafalch (2009). The step-by-step algorithm solves the fluid flow in one dimension. In the paper, an explanation of the mathematical formulation and numerical algorithm is provided in detail. A validation has been realized by means of experimental data comparisons from other references. Computational results have been analyzed and compared with other models presented in the test cases summarized in.Peer Reviewe

Numerical analysis of heat pumps models: comparative study between equation-fit and refrigerant cycle based models

An equation-fit (EF) and a refrigerant cycle (RC) based heat pump models have been implemented, validated, analyzed and compared to each other under steady state conditions for a brine to water heat pump. Models validations have been provided through comparisons against experimental data obtained at ISFH. The advantages and disadvantages of the both models have been identified. This work provides significant inputs regarding the selection of a specific model depending on the needs. Analysis of mass flow rates and calculations far from typical catalogue data (non-standard conditions) are provided. The main conclusions can be summarized as: i) the EF model is recommended when the boundary conditions for the estimation and prediction modes are the same and when non-standard conditions are considered; ii) the RC model is the chosen alternative when the mass flow rates are modified from the estimation to the prediction mode.Peer Reviewe

Numerical analysis of heat pumps models: comparative study between equation-fit and refrigerant cycle based models

An equation-fit (EF) and a refrigerant cycle (RC) based heat pump models have been implemented, validated, analyzed and compared to each other under steady state conditions for a brine to water heat pump. Models validations have been provided through comparisons against experimental data obtained at ISFH. The advantages and disadvantages of the both models have been identified. This work provides significant inputs regarding the selection of a specific model depending on the needs. Analysis of mass flow rates and calculations far from typical catalogue data (non-standard conditions) are provided. The main conclusions can be summarized as: i) the EF model is recommended when the boundary conditions for the estimation and prediction modes are the same and when non-standard conditions are considered; ii) the RC model is the chosen alternative when the mass flow rates are modified from the estimation to the prediction mode.Peer Reviewe