Model Based Nonlinear Control of Refrigeration Systems

Student thesis: Master thesis (including HD thesis)

  • Gustav Høgh
  • Rasmus Nielsen
Refrigeration systems are traditionally controlled using a thermostatic expansion valve. This thesis presents a new control strategy. The new strategy uses the compressor to control the superheat and an electronic expansion valve to control the cooling capacity. The refrigerant system is modelled to allow for the use of model-based feedback and feedforward. This yields better control of the superheat level. An analysis of the system efficiency is made to find the optimal superheat setpoint. Operating the system at the optimal superheat will reduce the energy needed to provide a given cooling capacity. The optimal superheat is found to be approximately 12 degrees celsius. At lower temperatures the efficiency is decreasing due to refrigerant spray. An autonomous method of detecting the optimal superheat is presented. The control problem is comprehensive investigated and a number of controllers are developed. They are all capable of stabilising and controlling the system and utilise both linear and nonlinear control theory. This thesis proves that is possible to control the refrigeration system using the new control strategy. Furthermore it is shown that it is possible to maintain the correct superheat over a wide range of working points.
LanguageEnglish
Publication date2008
Number of pages115
Publishing institutionAalborg Universitet
ID: 14421132