The adoption of highly efficient vapour -compression refrigeration, air conditioning and heat pump (RACHP) systems is compulsory to achieve a low -carbon society. Expansion work recovery using a two-phase ejector is widely recognized as one of the most promising measures to improve the energy efficien cy of RACHP units. This holds true for all operation conditions provided that an effective capacity control technique is implemented. In this work a thorough critical review on the current status of the presently available capacity control techniques for two-phase ejectors was carried out. In addition, their pros and cons as well as the comparison of their performance were reported. It was concluded that two -phase ejectors can be properly capacity controlled in large- and medium -scale vapour -compression uni ts. However, a suitable capacity control mechanism for small -scale vapour -compression solutions still requires a major breakthrough and is being intensively discussed among experts in the field. Suggested Reviewers: Federico Mazzelli Ph.D. Prof., Depart ment of Industrial Engineering, University of Florence federico.mazzelli@unifi.it Well-known expert in two -phase ejectors Nilesh Purohit Ph.D. BITS Pilani purohitnilesh89@gmail.com Well-known expert in two -phase ejectors Neal Lawrence Ph.D. RESEARCH ENGI NEER, Creative Thermal Solutions neal.lawrence@creativethermalsolutions.com Well-known expert in two -phase ejectors Adriano Milazzo Ph.D. Prof., Department of Industrial Engineering, University of Florence adriano.milazzo@unifi.it Well-known expert in two -phase ejectors Yosr Allouche Ph.D. Researcher, Department of Energy and Process Engineering, NTNU allouche.yosr@gmail.com Well-known expert in two -phase ejectors A review on current status of capacity control techniques for two -phase ejectors Paride Gullo(a), Martin Ryhl Kærn(a), Michal Haida(b), Jacek Smolka(b) and Stefan Elbel(c) (a) Technical University of Denmark (DTU), Department of Mechanical Engineering, Nils Koppels Allé, Building 403, 2800 Kgs. Lyngby (Denmark) (b) Silesian University of Technology, Institute of Thermal Technology, Konarskiego 22, 44 - 100 Gliwice (Poland) (c) University of Illinois at Urbana -Champaign, Department of Mechanical Science and Engineering, 1206 West Green Street, 61801 Urbana, IL (USA) Corresponding author’s details:  Paride Gullo  Telephone number: +45 45 25 41 21  Fax number: +45 45 25 19 61  Email address es: parigul@mek.dtu.dk , paridegullo@gmail.com Abstract: The adoption of highly efficient vapour -compression refrigeration, air conditioning and heat pump (RACHP) systems is compulsory to achieve a low -carbon society. Expansion work recovery using a two -phase ejector is widely recognized as one of the most promising measures to improve the energy efficiency of RACHP units. This holds true for all operation conditions provided that an effect ive capacity control technique is implemented. In this work a thorough critical review on the current status of the presently available capacity control techniques for two -phase ejectors was carried out . In addition, their pros and cons as well as the comp arison of their performance were reported . It was concluded that two -phase ejectors can be properly capacity controlled in large - and medium -scale vapour -compression units. However, a suitable capacity control mechanism for small -scale vapour -compression solutions still requires a major breakthrough and is being intensively discussed among experts in the field. Keywords: Capacity control ; Expansion work recovery; Multi -ejector; Needle ejector; Vapour -compression system; Vortex ejector *Title Page Highlights  Two-phase ejectors are expected to lead to a significantly more sustainable future  Benefits from two -phase ejectors strongly depend on their capacity control strategy  Capacity control mechanisms for two -phase ejector are comprehensively reviewed  Two-phase ejectors can be capacity controlled in large - and medium -scale units  Capacity control method for small -scale units still requires a major breakthrough 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 1. Introduction Vapour -compression refrigeration, air conditioning and heat pump (RACHP) systems play a key role in modern life, being extensively employed for satisfying many fundamental human needs, such as food preservation, medicine storage, water heating and cooling/ heating of living spaces. However, their massive use leads the RACHP sector to be dramatically energy intensive, contributing to global warming significantly . Refrigeration and air conditioning alone are accountable for about 20 % of the worldwide electrici ty consumption (IIR, 201 9). Peters (2018) claim s that the cooling sector is bound to incre ment its global energy consumption by more than 90% by 2050 in comparison with levels defined in 2018 , if the cooling sector is not improved . This growing thermal need is a driving force for greenhouse gas (GHG) emissions worldwide. According t o Isaac and van Vuuren (2009), GHG releases related to cooling applications could double by 2030 , if no appropriate actions are adopted. Therefore, climat e change mitigation by enhancing energy efficiency of RACHP units becomes increasingly important. In this regard, the European Union committed to cut GHG emissions by at least 40% below 1990 levels by 2030 and, simultaneously, to enhance the energy efficie ncy by at least 32.5% by 2030 (European Commission, 2019) . A two-phase ejector used for expansion work recovery is well-recognized as a promising solution allowing for highly efficient vapour -compression RACHP systems ( Elbel, 2011; Sarkar, 2012; Sumeru et al., 2012 ). The adoption of a suitable capacity control strategy is crucial to two -phase ejectors in order to maximize the coefficients of performance ( COPs ) of vapour -compression RACHP systems over all the operation conditions (Sumeru et al., 2012). This is due to the fact that the performance of two -phase ejectors is significantly governed by their four characteristic dimensions (i.e. suction nozzle, motive nozzle, mixing chamber and diffuser), which need to be permanently suited to the operating conditio ns so as to implement an appropriate capacity control along with an effective expansion work recovery (Liu et al., 2012c ). According to Elbel and Lawrence (2016), the adoption of a two -phase ejector leads to COP enhancements between 10% and 30% for R744 va pour- compression units at the design conditions . However, the authors also highlighted the crucial importance of controlling the ejector, since its performance at var ious outdoor temperature s and cooling capacit ies (i.e. off -design operations) is dramatica lly penalized . Lucas and Koehler (2012) showed experimentally that a n R744 two-phase ejector with fixed -type geometry (i.e. passive ejector, meaning no active control implemented) results in reductions in COP up to 17%, as the gas cooler pressure was 0.5 MPa above the optimal operation condition . The experimental results by Poolman et al. (201 8) highlighted that the use of a passive ejector causes an increase in annualized energy consumption by 6% compared with the same solution without ejector in cont ainer refrigeration units. Hu et al. (2014) experimentally proved that, for a given operating condition, different nozzle diameters can entail differences in energy efficiency ratio (EER) by more than 40%. Slightly different geometries could lead to consid erably different COPs at the same boundary conditions (Hu et al., 2014; Zhu and Elbel, 2016a, 2016b). It is worth remarking that most of the RACHPs, regardless of their scale and purpose, operates most of the time at off -design operation. The need for a no vel capacity control technique for two -phase ejector is explained by Zhu and Elbel (2016a, 2016b) as follows: “ A variable geometry ejector with adjustable needle in the motive nozzle can optimize ejector cycle performance under different conditions, but th is design is complicated and costly, and more frictional losses are incurred because of the additional surface area introduced which results in lower nozzle and ejector efficiencies. This provides motivation to develop a new technology to control the motiv e nozzle restrictiveness ”. Despite the wide number of review works on two -phase ejectors (Elbel, 2011; Sarkar, 2012; Sumeru et al., 2012; Chen et al., 2013; Besagni et al., 2016; Elbel and Lawrence, 2016; Tashtoush et al., 2019; Besagni, 2019; Aidoun et a l., 2019; Ringstad et al., 2019), their crucial role in steering society towards sustainability (Elbel, 2011; Sarkar, 2012; Sumeru et al., 2012) and the key importance of their capacity control mechanism to achieve this target (Sumeru et al., 2012), no rev iew work is currently available targeting the capacity control mechanism of these devices, to the best of the authors’ *Manuscript (final without changes marked) Click here to view linked