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An open-source Modelica library for design and control: Modular and scalable heat pump models using natural refrigerants
Abstract
Achieving climate goals in the building sector, heat pumps using natural refrigerants are particularly prom-ising compared with fossil-fired technologies to decrease emissions. For the efficient design of heat pumps, simulation methodologies are increasingly applied, and the modelling language Modelica has proven suita-ble for developing thermo-hydraulic simulation models. However, Modelica libraries for heat pump models are often very specific or not open-source that reusability is limited. Instead, a uniform modelling architec-ture is necessary that allows using tailor-made modelling approaches depending on the application. There-fore, we propose an open-source Modelica library (https://github.com/RWTH-EBC/AixLib/issues/739) con-taining models for natural refrigerants and modular scalable heat pump components. The models are devel-oped according to a uniform three-layer architecture. Refrigerant models base on the Helmholtz Equation of State validated using RefProp 9.1, whereas component models base on e.g. semi-empirical approaches vali-dated using data from the literature. Additionally, modularity and scalability are proven in simulation exam-ples for all components.
... In comparison to compressor models from the literature, this model handles with real gas behaviour and, therefore, it is able to predict volumetric and isentropic efficiencies depending on variable operating points. The used fluid property models were specially developed to be fast and accurate (Sangi et al., 2014;Vering et al., 2020). The refrigerants R410A, R134a, R744 and R32 are evaluated under dynamic conditions in this work. ...
This paper presents an approach to investigate the refrigerant’s influence on a reciprocating compressor. High-light of this model is the one-dimensional semi-physical approach, which is general fluid independent. Compared to compressor models from literature, this model handles with real gas behaviour. The applied fluid property models have been specifically designed to be fast and accurate. This is given by the implementation of a hybrid approach based on the simplified Helmholtz Equation of State (EoS) and polynomial functions. Considering rotational speeds between 30 s-1 and 70 s-1 and pressure ratios from 5 up to 13, results show a maximum isentropic efficiency of 67 %, 63 %, 66 %, 53 % for the refrigerants R410A, R134a, R744, R32, respectively.
Vapor compression cycles have gained high importance due to increasing concerns regarding the decarbonization of heating and cooling applications. Typically, vapor compression cycles operate under dynamic conditions, making it difficult to predict their performance accurately. Dynamic simulation models present a decent solution for increasing the reliability of the performance assessment. However, the implementation of dynamic simulation models is a complex task, requiring a uniform description and model development. Therefore, this article introduces an open‐source Vapor Compression Library called VVCLib based on a critical literature review of existing libraries for modeling and simulation of vapor compression cycles. The VCLib combines simulation models from the refrigerant through the component to the system level. The simulation models in the VCLib strictly follow a modular and scalable modeling approach. Class diagrams vividly describe the structure of models, making the VCLib easy to use for new users, especially students. The VCLib is open‐source and accessible to support educational and research purposes: https://github.com/RWTH-EBC/X-HD/
Aiming for a sustainable building stock, air-source heat pump systems are a key technology. In residential application, heat pump systems typically consist of a heat pump, an auxiliary heater, two thermal storages (for space heating and domestic hot water), and a system controller. Compared to conventional technologies, the efficiency of a heat pump system is highly dependent on it’s design, (component sizes, flowsheet, and refrigerant choice) and its operating conditions, particularly part load and temperature levels. While design strategies exist on different domains in the literature, there is no integrated strategy to optimally design heat pumps systems. In this work, we use process intensification to consider the heat pump system design and operation simultaneously. Formulating the intensification process leads to a two-stage MINLP optimization problem with multiple objec-tives: Costs and emissions. In the first stage, the design is optimized in an annual dynamic building performance simulation using a surrogate-based optimization procedure. Then, using the same dynamic simulation models, the system controller is optimized in the second stage using a Genetic Algorithm. Our results underline the necessity for integrated optimization process. The integrated process can decrease annualized costs up to 36% and emissions up to 51% compared to standard design procedures.
Purposeful design and efficient operation decrease costs and emissions of heat pump systems. Nowadays, reliable design processes base on covering demands at extreme boundary conditions without considering refrigerants simultaneously. Subsequently, the controller is set up to operate systems resiliently. However, design and operation are mutually dependent and refrigerants influence overall performance. Hence, a methodology is necessary to include operational fluid dependencies already in the design process. Within this work, we present a procedure to design heat pump systems applying a multi-objective Genetic Algorithm (MO-GA). First, MO-GA minimises costs and emissions designing the system’s superstructure based on annual demands. Subsequently, detailed controller parameters are optimised considering natural refrigerants. Compared to German standards, MO-GA reduces costs up to 12% and emissions up to 16% along a pareto front. The optimised PID values improve supply temperature control such that the integrated absolute error is reduced up to 60% compared to conventional lambda tuning.
AixLib is a Modelica model library with focus on modeling the dynamic behavior of buildings, HVAC equipment and distribution networks to enable integrated analyses of energy systems on the scales from single building to city district. The library is available at www.github.com/RWTH-EBC/AixLib. In this paper, we present the library and its integration within the IEA Annex 60. Possible applications are demonstrated using two use cases for the controls of a single building and for the heat demand of a city district. The paper concludes in a discussion of typical applications for AixLib and which developments we plan for the future.
International commitments on greenhouse gases, renewables and air quality warrant consideration of alternative residential heating technologies. The residential sector in Ireland accounts for approximately 25% of primary energy demand with roughly half of primary home heating fuelled by oil and 11% by solid fuels. Displacing oil and solid fuel usage with air source heat pump (ASHP) technology could offer household cost savings, reductions in emissions, and reduced health impacts. An economic analysis estimates that 60% of homes using oil, have the potential to deliver savings in the region of €600 per annum when considering both running and annualised capital costs. Scenario analysis estimates that a grant of €2400 could increase the potential market uptake of oil users by up to 17% points, whilst a higher oil price, similar to 2013, could further increase uptake from heating oil users by 24% points. Under a combined oil-price and grant scenario, CO2 emissions reduce by over 4 million tonnes per annum and residential PM2.5 and NOX emissions from oil and peat reduce close to zero. Corresponding health and environmental benefits are estimated in the region of €100m per annum. Sensitivity analyses are presented assessing the impact of alternate discount rates and technology performance. This research confirms the potential for ASHP technology and identifies and informs policy design considerations with regard to oil price trends, access to capital, targeting of grants, and addressing transactions costs.
[en] This paper presents the results of an on-going project to develop ThermoCycle, an open Modelica library for the simulation of low-capacity thermodynamic cycles and thermal systems. Special attention is paid to robustness and simulation speed since dynamic simulations are often limited by numerical constraints and failures, either during initialization or during integration. Furthermore, the use of complex equations of state (EOS) to compute thermodynamic properties significantly decreases the simulation speed. In this paper, the approach adopted in the library to overcome these challenges is presented and discussed.
Within the use of renewable energies in buildings sev-eral dynamic influences and interactions have to be re-garded. For example, the performance of heat pump systems depends sensitively on weather or geological effects as well as on building and user behaviour. An energetic optimization requires an intelligent coupling and control of all components of the building services installation, the building envelope and the influences mentioned. The Modelica libraries developed at the Institute for Energy Efficient Buildings and Indoor Climate allow a detailed analysis of the overall system including the various influences and interactions.
Purposeful design and efficient operation decrease costs and emissions of heat pump systems. Nowadays, reliable design processes base on covering demands at extreme boundary conditions without considering refrigerants simultaneously. Subsequently, the controller is set up to operate systems resiliently. However, design and operation are mutually dependent and refrigerants influence overall performance. Hence, a methodology is necessary to include operational fluid dependencies already in the design process. Within this work, we present a procedure to design heat pump systems applying a multi-objective Genetic Algorithm (MO-GA). First, MO-GA minimises costs and emissions designing the system’s superstructure based on annual demands. Subsequently, detailed controller parameters are optimised considering natural refrigerants. Compared to German standards, MO-GA reduces costs up to 12% and emissions up to 16% along a pareto front. The optimised PID values improve supply temperature control such that the integrated absolute error is reduced up to 60% compared to conventional lambda tuning.
Heat pumps are a promising technology for heating (and cooling) domestic buildings that provide exceptionally high efficiencies compared with fossil fuel combustion. There are in the region of a billion heat pumps in use world-wide, but despite their maturity they are a relatively new technology to many regions. This article gives an overview of the state-of-the-art technologies and the practical issues faced when installing and operating them. It focuses on the performance obtained in real-world operation, surveying the published efficiency figures for hundreds of air source and ground source heat pumps (ASHP and GSHP), and presenting a method to relate these to results from recent UK and German field trials. It also covers commercial aspects of the technologies, the typical savings in primary energy usage, carbon dioxide emissions abatement that can be realised, and wider implications of their uptake.
The additive model for the convective and nucleate boiling components originally suggested by Bergles and Rohsenow (1964) for subcooled and low-quality regions was employed in the Kandlikar correlation (1990a) for flow boiling in smooth tubes. It is now extended to augmented tubes and compact evaporators. Two separate factors are introduced in the convective boiling and the nucleate boiling terms to account for the augmentation effects due to the respective mechanisms. The fin efficiency effects in the compact evaporator geometry are included through a reduction in the nucleate boiling component over the fins due to a lower fin surface temperature. The agreement between the model predictions and the data reported in the literature is within the uncertainty bounds of the experimental measurements.
Novel CO2 heat pump systems for domestic hot water supply are modelled using an object-oriented thermodynamic model library written in Modelica. Due to the variety of mathematical structures of thermodynamic models and the possibilities of object-oriented modelling languages, powerful libraries are often very complex and hard to understand. The scope of the presented library lies on good readability and usability for both users and model designers. The basic structure and design concepts of the library are outlined and also the most important model concepts are explained. The heat pump system is modelled and simulation results are compared with test stand measurements. Simulation results are used to analyse the design of the system and to get information about possible further improvements.
Technical Thermodynamics: Basics and Instructions for solving tasks
- H Herwig
Herwig H. et al., 2016. Technical Thermodynamics: Basics and Instructions for solving tasks. Springer
Vieweg, Berlin, 2nd edition, 2016th DOI: 10,1007/978-3-658-11888-4.
HelmholtzMedia:Modelica library for the calculation of fluid properties from a Helmholtz energy equation of state
- M Thorade
Thorade, M., 2017. HelmholtzMedia:Modelica library for the calculation of fluid properties from a Helmholtz
energy equation of state (EoS), 17.09.2017. URL: https://github.com/thorade/HelmholtzMedia.
Transiente Modellierung eines Verdichters zum Vergleich von niedrig-GWP Kältemitteln für Kompressionswärmepumpen
- Vering
Vering et al., 2018, Transiente Modellierung eines Verdichters zum Vergleich von niedrig-GWP Kältemitteln
für Kompressionswärmepumpen, BauSIM Konferenz 2018.