Session: 06-02 Power to Heat Solutions

Paper Number: 79404

79404 - On Integrated Fluid Screening and Turbomachinery Design for Optimized Industrial Heat Pumps 

Heat pumps represent a key technology for recovering excess heat from a variety of sources for use in commercial, industrial and residential sectors while striving to meet future net-zero goals. Since industry is one of the largest heat-consuming sectors and industrial waste heat is a comparatively hot heat source (30 to 70 °C), the development of high-temperature heat pumps for this application is an important area of research. These systems operate with heat sink temperatures generally above 100 °C and heating capacities ranging from 20 kW to 20 MW. Despite recent research efforts, there are still technological challenges that need to be overcome to further increase the cycle efficiency (expressed as COP) and TRL of high-temperature heat pump technology.

A fundamental issue is the selection of a suitable high-temperature refrigerant that must meet a number of criteria, including no ozone depletion potential, low global warming potential, non-toxicity, suitable thermodynamic properties, large-scale availability, and cost-effectiveness. These constraints limit severely the choice of possible working fluids.

The efficient design of components offers further optimization potential. In particular, the compressor represents the core component of a heat pump and its performance largely determines the overall efficiency of the cycle. Consequently, improving compressor performance is generally regarded as one of the main drivers for improving heat pump efficiency. In the field of industrial heat pumps, the use of centrifugal compressors is particularly attractive because they are scalable, compact, can handle high flow rates, and can achieve higher efficiencies than reciprocating compressors. Furthermore, they offer the possibility of oil-free operation through gas foil or magnetic bearings.

The compressor efficiency considered for the thermodynamic cycle design of a heat pump is generally estimated under the assumption of ideal operation. However, such an approach, where no information on actual compressor performance is evaluated or fed back to the cycle design, can lead to inaccurate predictions of state points. Uncertainties grow if the same compressor efficiency is assumed for cycles operating with different fluids and machine sizes. In addition to this, process parameters defined by the cycle design can lead to unfeasible or uneconomical compressor designs. Moreover, even with comparable thermodynamic design parameters and efficiencies, the design of compressors for different working fluids is bound to lead to different results, alone due to different thermophysical properties (e.g. speed of sound affecting the compressor choke point).

Therefore, it is important to integrate fluid selection as well as turbocompressor design and performance prediction into the design process of heat pumps. In this work, we present an approach to the contextual optimization of working fluid and compressor design for efficient heat pumps. First, suitable fluids are selected for a high-temperature heat pump cycle by means of an optimization strategy. A conventional single-stage layout is considered for this investigation based on a parametric zero-dimensional thermodynamic cycle model. The fluid screening accounts for binary mixtures and aims at providing high cycle efficiency and volumetric heating capacity. By applying a meanline approach, preliminary designs of centrifugal compressors and performance assessments are then conducted for the cycles operating with the most promising fluids and for the refrigerant R1336mzz-Z. The latter represents a state-of-the-art reference fluid for high-temperature applications. The meanline approach is validated using test cases from the literature. Resulting cycle and compressor specifications are compared and energetic and turbomachinery-specific aspects, including sizing considerations, are discussed. The most favourable compressor designs are reviewed by 3D CFD performance assessments based on preliminary blading geometries derived from main dimensions of the meanline approach.

By adjusting the thermodynamic properties of the working fluid through the use of mixtures and applying an appropriate turbocompressor design, it is shown that an increase in the COP value by up to about 40% is possible compared to cycles operating with R1336mzz-Z.

Presenting Author: Renan Emre Karaefe Ruhr-Universität Bochum, Faculty of Mechanical Engineering, Chair of Thermal Turbomachines and Aeroengines

Presenting Author Biography: Professional Experience (Academic)<br/>Since 02/2017 Chair of Thermal Turbomachines and Aeroengines, Ruhr-Universit&#228;t Bochum, Bochum, Germany, Research assistant and PhD student (supervisor: Prof. Dr. Francesca di Mare)<br/><br/>Research topics:<br/>- Aerothermodynamic design and analysis of turbomachines for liquid air energy storage (LAES) application<br/>- Supercritical CO2 centrifugal compressor design and analysis<br/>- Heat pump centrifugal compressor design and analysis<br/><br/><br/>07/2016 - 01/2017 Chair of Fluid Process Engineering, Paderborn University, Paderborn, Germany, Research assistant<br/><br/>Research topics:<br/>- Modelling and simulation of energy systems comprising phase change materials<br/><br/><br/>Education<br/>10/2012 - 10/2014 Mechanical Engineering, Ruhr-Universit&#228;t Bochum, Bochum, Germany, Major field of study: Energy and Process Engineering, Degree: Master of Science<br/><br/>10/2008 - 08/2012 Mechanical Engineering, Ruhr-Universit&#228;t Bochum, Bochum, Germany, Major field of study: Energy and Process Engineering, Degree: Bachelor of Science

Authors:

Renan Emre Karaefe Ruhr-Universität Bochum, Faculty of Mechanical Engineering, Chair of Thermal Turbomachines and Aeroengines
Pascal Post Ruhr-Universität Bochum, Faculty of Mechanical Engineering, Chair of Thermal Turbomachines and Aeroengines
Francesca Di Mare Ruhr-Universität Bochum, Faculty of Mechanical Engineering, Chair of Thermal Turbomachines and Aeroengines
Valerius Venzik RWTH Aachen University, E.ON Energy Research Center, Institute for Energy Efficient Buildings and Indoor Climate
Paul Wotzka Institution: RWTH Aachen University, E.ON Energy Research Center, Institute for Energy Efficient Buildings and Indoor Climate
Dirk Müller RWTH Aachen University, E.ON Energy Research Center, Institute for Energy Efficient Buildings and Indoor Climate
Riley Bradley Barta Technische Universität Dresden, Bitzer-Chair of Refrigeration, Cryogenics and Compressor Technology