Feedforward control of capacity and superheat for a variable speed refrigeration system Li Hua a , Seok-Kwon Jeong b, * , Sam-Sang You c a Department of Refrigeration and Air-Conditioning Engineering, Graduate School, Pukyong National University, Pusan, Republic of Korea b Division of Mechanical Engineering, Pukyong National University, Pusan, Republic of Korea c School of Mechanical and Information Engineering, Korea Maritime University, Pusan, Republic of Korea article info Article history: Received 20 August 2007 Accepted 19 May 2008 Available online 7 July 2008 Keywords: Feedforwad compensator Decoupling model Superheat control Capacity control COP Variable speed refrigeration system abstract This paper deals with an independent control method for the refrigeration system based on general PI control law. In the variable speed refrigeration system (VSRS), the capacity and superheat are respectively controlled by inverters and electronic expansion valves for saving energy and improving performance as well. At first, the decoupling model has been proposed to eliminate the interfering loops, and then the transfer functions are obtained from numbers of experimental tests. Once the new PI controller is designed, the system performance is evaluated by numerical simulations as well as extensive experimen- tal works. Finally, it is found that the independent control scheme can guarantee not only precise control performance but also high COP for the VSRS. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Recently, the applications of inverter refrigeration system for commercial and residential purpose have been increased due to the demand on saving energy and better degree of comfort. It is of- ten necessary to design a practical control system in order to man- age the VSRS for saving energy and guaranteeing high efficiency. A basic refrigeration cycle mainly consists of a compressor, heat exchangers, and expansion valves. In fact, since all such compo- nents in the cycle are connected with various pipes and valves, they exhibit inherent nonlinear characteristics in operational ranges. Hence, it is almost impossible to exactly identify dynamic characteristics and develop dynamic model for the complete refrig- eration systems [1–4]. On the other hand, the conventional control schemes for VSRS are mainly focused on two control variables; the degree of super- heat and the refrigeration capacity. In general the degree of super- heat is mainly controlled by a thermostatic expansion device which could regulate evaporating pressure and refrigerant mass flow rate as well. In order to improve coefficient of performance (COP) of the system, the degree of superheat should be maintained at a constant level by adjusting opening angles of the electronic expansion valve (EEV). Furthermore, the control capability is highly desirable so as to effectively respond to partial loading con- ditions for possible energy saving. Refrigeration machines are usu- ally operating under partial loading conditions with employing on/ off -control for the compressor. However, such a conventional technique to cope with partial loading could deteriorate compres- sor durability to a considerable extent. Therefore, the on/off control scheme is gradually being replaced by a VSRS with an inverter dri- ven compressor. It is noted that in the VSRS, the capacity and superheat can not be controlled independently because of interfer- ing loops inside when the compressor speed and opening angle of EEV are changing simultaneously. A fuzzy control algorithm was implemented by Apreaa et al. [5] to avoid the usage of the dynamic system model. The control algo- rithm could continuously regulate the compressor speed by an in- verter. Li et al. [6,7] proposed control system based on the fuzzy control inference. It offers simultaneous control of the capacity and superheat without troublesome dynamic model. Such method can provide better control performance for the refrigeration sys- tem in spite of its inherent strong nonlinear characteristics. Choi et al. [8] suggested a superheat control method with feed- forward loop for variable speed heat pump system, in which the compressor frequency was determined by an empirical equation using regression analysis. In case of the control system based on the empirical model, it inevitably exhibits fairly large steady-state errors. Since the capacity controller considered the steady-state case only, the superheat controls have huge overshoot and under- shoot. To overcome these problems, Li and Jeong [9] proposed a decoupling model to eliminate the interfering loops between the 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.05.022 * Corresponding author. Tel.: +82 51 629 6181; fax: +82 51 629 6174. E-mail address: skjeong@pknu.ac.kr (S.-K. Jeong). Applied Thermal Engineering 29 (2009) 1067–1074 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng