Development and evaluation of transfer functions for a variable speed compressor

Abstract

In this study, transfer functions of electronic expansion valve and variable speed scroll compressor in a chiller system are developed based on an experimental analysis. Dynamic behaviours of water temperature at chiller output and refrigerant superheat at evaporator outlet have either the first-or the second-order linear models, respectively. Different from the existing modelling studies in literature novel models are presented for variable speed refrigeration system with scroll compressor and liquid type evaporator. The scroll compressor used in the current study is not produced for variable speed operation however it is operated with some limitations in the range of 30 to 50 Hz through an inverter. Similar restrictions are presented for the electronic expansion valve as 10 per cent to 35 per cent opening range because of operational limitations. To validate transfer functions of the variable speed scroll compressor and electronic expansion valve, mean square error values for all transfer functions are calculated. The mean square error values vary between 0.0126 and 0.3116. In addition, all transfer functions are designed and tested using corresponding proportional-integral-derivative controllers in order to see their applicability. The results indicate that energy savings are a consequence of operating the chiller at smaller frequencies than the basic 50 Hz while partial load occurred.

Model predictive control of chilled water temperature for centrialized HVAC systems

“…In [6], it is assumed that 1 ( ) w A z  and 1 ( ) w B z  are constant, even with different cooling loads. However, since the cooling load may vary by a large amount when the environment of the conditioned space changes, they are considered to be functions of w m  in this work.…”

“…Dynamic characteristics of heat transfer in evaporator can be determined by cooling load on the water side and working condition on the refrigerant side. For control purpose, denote wo T y  as system output and compressor speed cp F as input signal u , the following linear model can be built from [6]:…”

“…But some unmeasured variables such as outdoor air temperature are not involved, which may lead to undesired output of neural networks. [6] employed first and second order transfer functions to approximate the dynamic of system near the operating condition. But this kind of model could only be expected to work well near the identified working condition and may fail to give satisfied performance when working condition is away from the identified situation.…”

Operation of Compressor and Electronic Expansion Valve via Different Controllers

“…EEV is the first controllable equipment in VSRS (Aprea et al, 2006a,b;Lazzarin and Noro, 2008, 2011. For this controller, two inputs and one output variable were defined in Fig.…”

“…Therefore, it has been widely used in Heating Ventilation Air Conditioning and Refrigeration (HVAC&R) systems (Jiangjiang et al, 2006). Recently, energy consuming is a strict issue in designing new refrigeration system Ekren et al, , 2011Nasutin andHassan, 2006, Sahin et al, 2010). EEV and VSC have important effect on efficiency of system energy consumption.…”