The selection of volume ratio of two-stage rotary compressor and its effects on air-to-water heat pump with flash tank cycle Younghwan Ko a , Sangkyoung Park a , Simon Jin a , Byungsoon Kim a , Ji Hwan Jeong b,⇑ a Air Conditioning and Energy Lab., LG Electronics, Seoul 153-802, Republic of Korea b School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea highlights " Two-stage rotary compressor was developed for vapor injection cycle. " The effect of volume ratio of a two-stage rotary compressor on AWHP was investigated. " Experimental investigation on vapor injection air-to-water heat pump. " Higher COP and larger heating capacity than conventional AWHP at cold climate. article info Article history: Received 17 April 2012 Received in revised form 14 August 2012 Accepted 7 November 2012 Keywords: Two-stage rotary compressor Volume ratio Vapor injection Air-to-water heat pump Flash tank abstract A conventional heat pump exhibits performance degradation even though larger heating capacity is needed as the outdoor temperature declines. As a way to prevent the performance degradation, a heat pump with an inverter-driven two-stage rotary compressor and vapor injection (VI) cycle was investi- gated for an air-to-water heat pump (AWHP) system employing a flash tank. The volume ratio of two cyl- inder of a two-stage rotary compressor has significant effect on the performance of the AWHP so that it was experimentally investigated. Based on this result, a two-stage rotary compressor designed with an optimized volume ratio was manufactured and incorporated into the AWHP system. It was found that the VI AWHP system improved the heating capacity by 48% and the COP by 36% compared to those values for the conventional AWHP at water temperature of 60 °C and ambient temperature of 15 °C. This VI AWHP system can be used for cold climate applications. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Highly efficient heat pumps have a great chance of replacing residential gas- and oil-fired boiler systems due to global environ- mental concerns and CO 2 emission regulations. The global market for heat pumps keeps increasing because their efficiency is higher than that of conventional heating systems. However, the coeffi- cient of performance (COP) of a heat pump decreases as the tem- perature of the heat absorption process declines. Furthermore, the heating capacity of air-source heat pumps decreases in cold re- gions so that the application of air-source heat pumps is restricted to regions where winters are relatively warm. Even though an in- verter-driven heat pump can provide a larger heating capacity by increasing the refrigerant flow rate, the performance deterioration of heat pumps in cold climates cannot be avoided. Various modified heat pump cycles have been investigated in order to overcome the poor performance of heat pumps in cold climate. In recent years refrigerant injection techniques, especially vapor injection (VI) techniques, have attracted intensive interest owing to their outstanding performance potential in cold climates. Wang et al. [1] claimed that the injection process could be treated as a ‘‘continuous parameter-varying adiabatic throttling and isostatic mixture time varying process’’. Previous research on VI techniques can be classified into ther- modynamic modeling and experimental studies. For the thermody- namic analysis, Domanski [2] performed numerical simulations of an economizer cycle and reported that the COP improvement was prominent for fluids with higher heating capacity. Using the ther- modynamic model with an economizer and rotary compressors, Vaisman [3] found that R507A and R404A have better system per- formance than do R134a and R410A. Ma and Chai [4] developed a thermodynamic model of a VI system based on R22, and concluded that to maximize the capacity of the system the ratio of the injec- tion pressure to the geometric mean of the suction and the dis- charge pressure should be around 1.2. Siddharth et al. [5] 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.11.021 ⇑ Corresponding author. Address: School of Mechanical Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea. Tel.: +82 51 510 3050; fax: +82 51 512 5236. E-mail address: jihwan@pusan.ac.kr (J.H. Jeong). Applied Energy 104 (2013) 187–196 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy