Vol 112, No 4 (2023)

This study presents recommendations for selecting a circuit design for low-capacity hydrogen liquefaction plants with production rate up to 20 kg/h or 0.48 tpd (ton per day). Main design criteria considered are specific energy cost, as well as capital costs and overall characteristics of the system. Theoretical and real hydrogen liquefaction cycles are reviewed. Mathematical models of different circuits are built considering real parameters of the typical equipment. The advantages and disadvantages associated with certain solutions are identified, and the hydrogen-liquefaction energy efficiency trends are analysed. According to the results, the main of the circuits for low-capacity hydrogen liquefaction plants are selected as per the obtained results.

AIMS: Theoretical and real hydrogen liquefaction cycles are reviewed, and circuit design is mathematically modeled considering the typical equipment’s real parameters.

MATERIALS AND METHODS: Hydrogen-liquefaction cycles are modeled using Aspen HYSYS. Further optimization and parameter selection are conducted using the MATLAB module “Global Optimization Toolbox.”

RESULTS: Advantages and disadvantages associated with certain technological solutions are identified, and the hydrogen-liquefaction energy efficiency trends are analyzed.

CONCLUSIONS: This study compares energy consumptions for liquefaction of various gases, showing the feasibility of energy consumption reduction for hydrogen liquefaction. The importance of continuous ortho–para conversion or increase in number of conversion stages via energy consumption reduction is presented. The main features of refrigerant cycles are described, and a precooling cycle using a mixed refrigerant is selected. Mixed-refrigerant precooling cycle and liquid nitrogen precooling are compared in terms of economic efficiency. The main issues of refrigerant selection are described, and the basic principles of modeling and parameter selection for a small-capacity hydrogen-liquefaction cycle are presented. A low-temperature helium cycle is modeled with the precooling circuit based on a mixed-refrigerant cycle. We reveal an optimum range of precooling temperatures for decrease in overall specific power consumption using a mixed refrigerant in a small-capacity hydrogen liquefaction plant of 80K–100K.

2-stage Rotary CO₂ compressors are currently in use in refrigeration and heat pump applications, including occasionally including vapour injection. The need of correctly assessing the compressor’s efficiency by means of a computationally fast model arises for cycle performance prediction and optimization. In response, an innovation on an existing semi-empirical modelling approach has been introduced in this paper to describe 2-stage Rotary compressors. The model has been validated with experimental data of a CO₂ compressor available in open literature, giving maximum errors of around ±3% for mass flow and ±1.82% for power prediction. Calibration of a simplified 1-stage model is also performed to analyse to which extent it is necessary to complexify the model, and to identify key or neglectable modelling elements. It is found that doing this increases the error of the compressor performance prediction. Model analysis has been done to identify the inefficiencies introduced by each physical effect considered.

This article is a translation of the article by Vega J, Cuevas C, Dickes R, Lemort V. Application of a semi-empirical modelling approach to a Two-Stage Rotary CO2 compressor. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021.

DOI: 10.18462/iir.nh3-co2.2021.0027 Published with the permission of the copyright holder.

In this paper, the Dymola modelling tool is used to study the influence of ejector design onto the whole heat pump cycle working with carbon dioxide. The cycle is built using the components provided by the TIL Modelica library. It is found that the ejector models in TIL are quite limited, namely by their inability to properly capture the on-design plateau and rapid decrease in performance in off-design operation. Therefore, an in-house state-of-the-art ejector model, originally developed in Python, is implemented as a Dymola object. This model is then calibrated onto CO₂ experimental data. The operation of a simple CO₂ heat pump system is investigated, with focus on the ejector sizing at fixed geometry. It is found that there exists an ejector size that maximises the COP of the cycle. Furthermore, critical ejector pressure is not reached at this optimum COP point; the ejector is operating well under the on-design regime.

This paper presents the results of a 24-month field monitoring of CO₂ transcritical units designed to satisfy all the thermal needs of food retail stores, namely refrigeration, heating and cooling. The units, adopting state-of the art technologies, such as two phase ejectors and parallel compression, are developed within the H2020 project MultiPACK. The monitored supermarkets are located in Italy, in two different climatic areas, featuring different heating and cooling demands for indoor comfort, as well as diverse building type and display cabinets mix. Long lasting measurements allow for evaluation of energy and efficiency related KPIs, together with operational indicators. These figures are relevant for benchmarking with traditional solutions.

This article is a translation of the article by Artuso P, Minetto S, Rossetti A, Tosato G, Marinetti S. Two years of data monitoring of all-CO₂ retail stores within the MultiPACK project. In: Proceedings of the 9th IIR Conference on the Ammonia and CO₂ Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0025 Published with the permission of the copyright holder.