Short overview of early developments of the Hardy Cross type methods for computation of flow distribution in pipe networks

Hardy Cross originally proposed a method for analysis of flow in networks of conduits or conductors in 1936. His method was the first really useful engineering method in the field of pipe network calculation. Only electrical analogs of hydraulic networks were used before the Hardy Cross method. A problem with flow resistance versus electrical resistance makes these electrical analog methods obsolete. The method by Hardy Cross is taught extensively at faculties, and it remains an important tool for the analysis of looped pipe systems. Engineers today mostly use a modified Hardy Cross method that considers the whole looped network of pipes simultaneously (use of these methods without computers is practically impossible). A method from a Russian practice published during the 1930s, which is similar to the Hardy Cross method, is described, too. Some notes from the work of Hardy Cross are also presented. Finally, an improved version of the Hardy Cross method, which significantly reduces the number of iterations, is presented and discussed. We also tested multi-point iterative methods, which can be used as a substitution for the Newton-Raphson approach used by Hardy Cross, but in this case this approach did not reduce the number of iterations. Although many new models have been developed since the time of Hardy Cross, the main purpose of this paper is to illustrate the very beginning of modeling of gas and water pipe networks and ventilation systems. As a novelty, a new multi-point iterative solver is introduced and compared with the standard Newton-Raphson iterative method.Web of Science910art. no. 201

Experimental evaluation of correlations used to calculate friction factor for turbulent flow in cylindrical pipes

Los sistemas hidráulicos encargados de la distribución de agua a los usuarios tienen un rol importante en su calidad de vida. Por ello, la determinación de manera precisa del factor de fricción en tuberías, es de gran importancia en el diseño de redes de distribución de agua debido a que influye directamente en el cálculo de caídas de presión (pérdidas de carga) en los sistemas hidráulicos. El objetivo del presente trabajo fue evaluar en forma experimental diferentes correlaciones que describan, mediante una forma explícita, el factor de fricción en una tubería bajo flujo turbulento y la correlación implícita de Colebrook-White. Esto se realizó mediante la comparación de valores numéricos predichos por las correlaciones respecto al valor experimental, encontrándose que la correlación de Colebrook-White predice el factor de fricción con menor porcentaje de error de las 27 correlaciones estudiadas. Para la solución de la ecuación de Colebrook-White se utilizó el método de Newton-Raphson, dado que ésta presenta una estructura recurrente que puede ser llevada a algoritmos y resuelta rápidamente mediante programas computacionales. En este trabajo se desarrolló una herramienta en lenguaje de programación C++ para resolver la ecuación de Colebrook-White.The hydraulic systems that distribute water to users have an important role in their quality of life. For this reason, the precise determination of the friction factor in pipes is of great importance in the design of water distribution networks because it directly influences the calculation of pressure drop (head losses) in hydraulic systems. The objective of the present work was to evaluate in an experimental manner different correlations that describe, in an explicit form, the friction factor in a pipe under turbulent flow and the implicit Colebrook-White correlation. This was done by comparing the numerical values predicted by the correlations in regard to the experimental value, finding that the Colebrook-White correlation predicts the friction factor with the lowest error percentage among the 27 correlations studied. For the solution of the Colebrook-White equation the Newton-Raphson method was used, given that it presents a recurrent structure that can be taken to algorithms and quickly solved by computer programs. In this work, a tool was developed in the C ++ programming language to solve the Colebrook-White equation.Peer Reviewe

Sensitivity Factors for Integrated Energy Systems: A Joined Quasi-Steady-State Approach

Integrated energy systems can increase the use of volatile renewable energy generation while reducing operation cost in the electric power system. The benefits result from shifting energy between energy infrastructures and using the network storage capability of district heating and gas systems. But the more strongly the different energy systems are linked the more complex their operation becomes. To ensure a secure and reliable system operation while using the full potential of integrated energy systems the interactions and the network storage effects of the district heating and gas system must be analyzed. Existing power flow calculation methods of integrated energy systems, however, neglect the network storage effects which result from the dynamic behavior of the district heating and gas system. The dynamic behavior is only investigated if the different energy systems are solved separately. As existing methods do not directly represent the interactions and effects of the dynamic behavior in an integrated energy system, the effect of any unit's power change on the power flows in the integrated energy system can only be determined by a complete power flow calculation, leading to a high computational cost. To reduce the computational cost this thesis derives sensitivity factors estimating the effect of a power change on the system state of an integrated energy system. To derive the sensitivity factors a joined quasi-steady-state power flow calculation method for integrated energy systems is developed extending existing steady-state approaches. For this, the system state of the electric power system, district heating system, and gas system is determined simultaneously, directly representing their interactions. To include the dynamic behavior a gradient method is proposed, which allows temperature and calorific value changes to be tracked in a coupled power flow calculation. The gradient method can accurately depict the dynamic behavior in the joined quasi-steady-state power flow calculation method even with simulation time increments of up to 60 min. Hence, compared to existing methods larger simulation time increments can be chosen to reach the same accuracy, leading to a reduced computation time. The sensitivity factors are on average ten times faster in estimating a new system state after a unit's power change compared to a power flow calculation. Besides the high computational efficiency, they can provide good estimates considering the complexity of the interactions and the dynamic behavior in an integrated energy system. As the joined quasi-steady-state power flow calculation method is based on the steady-state analysis existing use cases can be easily extended to consider the full potential of integrated energy systems. Therefore, the thesis provides system operators with a method to accurately analyze the full potential of Integrated energy systems

Synthetic models of distribution gas networks in low-carbon energy systems

L'abstract è presente nell'allegato / the abstract is in the attachmen

Development of a microcomputer based design system for air management of buildings

Expert systems are computer programs that seek to mimic human reasoning. Currently, expert systems are being used for the design of heating, ventilation, and air conditioning (HVAC) systems. The present work involves developing of several smaller expert systems known as knowledge bases, and integrating them in one simple package. The aim of the research is to develop a such computer code for HVAC system designers which will considerably reduce man-hours during the whole design process, improve the productivity, increase the design quality, and give the customers more options to choose the best and optimum design. This thesis describes the development of a computer code, which has the ability to give all the design requirements for HVAC systems. This work which can be considered as a step towards HVAC Expert Systems, which outlines step by step calculation procedure to determine essential elements of heating and cooling loads such as U-value, air infiltration, solar heat gain, heat storage, psychrometric charts and the sunlit area of the exterior surfaces. The code (HVACSYS) consists of a main menu program and several auxiliary programs for gathering data, completing calculations, and printing project reports. The developed code is also connected with the AutoCAD package to give the final design of the HVAC systems. In the AutoCAD package, a special menu for HVAC systems design has been added (HVACCAD). This menu is developed for customizing the AutoCAD package in order to make the code interactive. Finally, a case study has been considered in which solutions were obtained using an existing package and also the developed package. Comparison of the solutions illustrates the usefulness of the new package adequately

Strategie di ottimizzazione di reti energetiche complesse in presenza di generazione distribuita

This project has been conceived in order to analyze and try to answer to the new issues created by complex energy networks and distributed generation scenarios. The carried our research project can be divided into two main parts. The first one focuses on thermal energy networks, i.e. on district heating networks: existing case studies have been modeled and analyzed with an in-house developed software and the new concept of smart district heating has been developed (different configurations for the bidirectional heat exchange at final users have been developed and tested). The second part of the PhD project aims to the definition of optimal strategies within complex energy networks, when thermal, electric and cooling energies are required by a certain number of users, eventually in a distributed generation scenario. In particular, a specific calculation code based on genetic algorithms has been developed and different cases studies have been analyzed, also in isolated energy grid application. Finally, a comparison between the genetic algorithm approach and a mixed integer linear programming based software has been carried out during a research period abroad, at EPFL University in Switzerland

Development of computer programs for fast computation of g-functions and automated ground heat exchanger design

Ground-source heat pump systems utilizing vertical borehole ground heat exchangers are energy efficient. The most accurate method for sizing the boreholes is based on simulations which make use of superposition of a unit-step thermal response g-function. Due to long calculation times, a library of g-functions was constructed in the 1990's and is still utilized in ground heat exchanger design tools. Literature has been silent concerning the expansion of the library or further improvements to the closed source g-function calculation program.g-Functions can now be computed by utilizing a recently developed open-source program, pyfunction. Version 1.1 of pygfunction had excessive memory consumption for large borehole fields, motivating development of an alternative program written in C++, named cpgfunction. The program, when coupled with an adaptive discretization scheme, provided the ability to compute a new expanded library of g-functions. A pure C++ fork of cpgfunction, named cpgfunctionEP, has been integrated into the whole building energy simulation program, EnergyPlus, as a third-party g-function calculation tool. Early versions of cpgfunction were faster than pygfunction, though recent developments in pygfunction have broken the computation barrier for g-function calculation and reduced memory consumption. G-functions are computed live time in a newly developed Ground Heat Exchanger Design Toolbox (GHEDT), written in Python. As part of the work described in this thesis, many additional developments in pygfunction have been made.A fast monthly hybrid time step simulation is developed. This simulation is enhanced and automated compared to an existing program capable of ground heat exchanger sizing; Ground Loop Heat Exchanger Professional (GLHEPRO). The GHEDT sizing results are validated against GLHEPRO. GHEDT provides a novel development of automated selection of a borehole field by placing boreholes within user-defined geometric constraints. The advanced methodology searches borehole fields constructed in domains. When simulated at a constant height over the domain, a discontinuous unimodal excess temperature function is formed. An integer bisection routine enables fast selection of a desired field. The most capable design routine automatically places boreholes within a constrained polygonal available land description excluding a polygonal unavailable drilling zone, where a building or utilities may be located

Combined analysis of electricity and heat networks

The use of Combined Heat and Power (CHP) units, heat pumps and electric boilers increases the linkages between electricity and heat networks. In this thesis, a combined analysis was developed to investigate the performance of electricity and heat networks as an integrated whole. This was based on a model of electrical power flow and hydraulic and thermal circuits together with their coupling components (CHP units, heat pumps, electric boilers and circulation pumps). The flows of energy between the electricity and heat networks through the coupling components were taken into account. In the combined analysis, two calculation techniques were developed. These were the decomposed and integrated electrical-hydraulic-thermal calculation techniques in the forms of the power flow and simple optimal dispatch. Using the combined analysis, the variables of the electrical and heat networks were calculated. The results of the decomposed and integrated calculations were very close. The comparison showed that the integrated calculation requires fewer iterations than the decomposed calculation. A case study of Barry Island electricity and district heating networks was conducted. The case study examined how both electrical and heat demands in a self-sufficient system (no interconnection with external systems) were met using CHP units. A solution was demonstrated to deliver the electrical and heat energy from the CHP units to the consumers through electrical and heat networks. The combined analysis can be used for the design and operation of integrated heat and electricity systems for energy supply to buildings. This will increase the flexibility of the electricity and heat supply systems for facilitating the integration of intermittent renewable energy