Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Experimental analysis of the influence of the expansion valve opening on the performance of the small size CO 2 solar assisted heat pump Sabrina N. Rabelo a,b, ⁎ , Tiago F. Paulino c , Willian M. Duarte a,d , Antônio A.T. Maia a , Luiz Machado a a Graduate Program in Mechanical Engineering, Federal University of Minas Gerais (UFMG), Belo Horizonte, MG, Brazil b University of Itaúna, School of Mechanical Engineering, Itaúna, MG, Brazil c Department of Materials Engineering, Federal Center of Technological Education of Minas Gerais (CEFET-MG), Belo Horizonte, MG, Brazil d University of Belo Horizonte (UniBH), Belo Horizonte, MG, Brazil ARTICLE INFO Keywords: Experimental analysis Expansion valve opening R744 DX-SAHP ABSTRACT In a Direct Expansion Solar Assisted Heat Pump (DX-SAHP), a valuable part of the energy is provided from solar radiation. The solar radiation can fluctuate during the day affecting the performance of the system. The impact of a fixed opening in the expansion device on the system performance is not well known, especially for a small- size solar assisted heat pump. In this context, it is presented an experimental analysis of the influence of the expansion valve opening on the performance of a CO 2 DX-SAHP. Experimental tests were carried out considering different solar radiations conditions. The value of the expansion device opening that leads to the maximum COP was almost the same, regardless of solar radiation. In consequence, it was concluded that, for a small CO 2 DX- SAHP, using a basic cycle configuration a static expansion device as a capillary tube would be suitable, reducing the costs of this equipment. 1. Introduction The use of heat pumps for heating water instead of electric heaters or gas heaters is one way to reduce the emissions of greenhouse gases. The performance of a heat pump can be improved through the in- tegration with a solar thermal collector. Buker and Riffat (2016) pre- sented different studies on SAHP and they found three usual setups used to produce only domestic hot water: (i) Parallel type indirect expansion Solar Assisted Heat Pump, (ii) Series type indirect expansion Solar As- sisted Heat Pump, (iii) Direct expansion Solar Assisted Heat Pump (DX- SAHP). Due to the distinct performance criteria and environmental conditions existing in each study, it is difficult to perform a proper comparison among the systems used in each study (Buker and Riffat, 2016; Mohanraj et al., 2018; Mohanraj et al., 2018). In a DX-SAHP, an important part of the energy required for its op- eration is obtained from solar radiation and the latent heat received by the condensation of the water vapour present in the atmospheric air (Li et al., 2007). These factors combined contribute to a better coefficient of performance (COP). Omojaro and Breitkopf (2013) presented a comprehensive review of the DX-SAHP and reported that 75% of the studies performed concerning these systems, the final application was water heating. Different refrigerants have been used in a DX-SAHP, such as: R12 (Chaturvedi and Abazeri, 1987; Chaturvedi et al., 1998; Ito et al., 1999), R22 (Li et al., 2007; Kara et al., 2008; Kong et al., 2011), R134a (Moreno-Rodriguez et al., 2013; Sun et al., 2014; Kong et al., 2018), R744 (Oliveira et al., 2016; Faria et al., 2016; Paulino et al., 2018), R410A (Kong et al., 2017) and R407C (Mohamed et al., 2017). Nowadays, many of the scientific works on these systems are focusing on the right choice of refrigerant and on the parameters that contribute to improving the COP (Chata et al., 2005; Duarte et al., 2019; Rabelo et al., 2019). The refrigerant fluid to be employed in the DX-SAHP must be se- lected based on not only on its favorable physical characteristics, but also on its environmental impact. This last parameter must be in con- formity with the agreements established in the international protocols of Kyoto (1997) and Montreal (1987) to control the ozone layer de- pletion and the production of greenhouse gases. CO 2 meets these re- quirements. It is a natural refrigerant with zero ODP and negligible GWP (global warming potential). In addition, CO 2 has a good heat transfer coefficient, high volumetric capacity, no toxicity, and non-in- flammability (Pearson, 2005; Calm, 2008). For these reasons, CO 2 has been widely used in heat pumps, as discussed by Ma et al. (2013). At the Gustav Lorentzen Conference on Natural Refrigerants (2018) in 2018 almost 40% of the papers refereed to CO 2 as refrigerant fluid, compared https://doi.org/10.1016/j.solener.2019.08.013 Received 4 May 2019; Received in revised form 3 August 2019; Accepted 7 August 2019 ⁎ Corresponding author at:School of Engineering, UFMG, Av.Antônio Carlos, 6627, Belo Horizonte, MG CEP 31270-901, Brazil. E-mail address: sasanogueirarab@hotmail.com (S.N. Rabelo). Solar Energy 190 (2019) 255–263 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T