(468f) Offset-Free Model Predictive Control of Vapor Compression Cycle | AIChE

(468f) Offset-Free Model Predictive Control of Vapor Compression Cycle


Mhaskar, P. - Presenter, McMaster University
House, J. - Presenter, Johnson Controls
Salsbury, T. - Presenter, Johnson Controls

The goal of energy efficiency is to make a device produce the same output by using less energy. This is clearly desirable in general, but must always be weighed carefully against any hidden costs required to achieve it. For example, improving the efficiency of one device in isolation might lead to more energy use somewhere else, thereby off-setting the overall benefit. There are also fundamental limits to how efficient energy conversion from one form to another can be. As we approach these limits, the potential for absolute improvements diminish accordingly.

One way to improve energy efficiency that has little or no hidden energy penalties is by modifying the way that a device is controlled. If no change is made to a device beyond improving the control algorithm, this is almost always a net benefit. Studies have shown that HVAC systems in buildings frequently operate above their theoretical minimal energy use level because of poor control design. Current practice fails to take into account interactions within a system and also handles constraints in a way that is separate and often damaging to control performance. Energy use is also not usually featured as an objective in the control performance and is handled instead by setting set-points in an ad-hoc way based on experience.

Model predictive control (MPC) is a control methodology that has reached a certain level of maturity in several industries but has not yet been applied to buildings at any substantial scale.  MPC is designed to handle many of the shortfalls of existing control mentioned earlier including interaction between variables, constraints, and the incorporation of higher level objectives such as energy use. The work reported here involves applying MPC methods to vapor compression systems of the type used to provide cooling in buildings through equipment such as roof-top units. An MPC strategy is developed and tested for manipulating the electronic expansion valve and also the speed of a variable speed compressor. The control objective is to regulate the air temperature being blown over the evaporator subject to constraints on the superheat. An optimization objective is formulated to minimize the energy used by the compressor while meeting control objectives and constraints. A linear (identified) model is used for the air and superheat temperature predictions accounting for variations in the air and load conditions outside the vapor compression system.

An MPC formulation is devised to counter the inevitable plant model mismatch and achieve offset-free tracking.  A detailed simulation model is used to evaluate and compare the performance of the MPC strategy with various combinations of traditional control strategies, including the use of MPC at a supervisory level. Results will be presented to demonstrate the improved temperature regulation and energy savings achievable using the MPC strategy.