Matthew S. Elliott

Model-based predictive control of a multi-evaporator vapor compression cooling cycle

This paper presents a decentralized control architecture for multiple evaporator vapor compression systems using model-based predictive control. Vapor compression systems are widely used for heating, air-conditioning and refrigeration, and constitute a major part of total US energy use. Advanced control strategies have the potential to significantly increase energy efficiency, while delivering the necessary amount of cooling capacity. This paper proposes a decentralized control approach based on a study of interacting dynamics, wherein the cooling capacity of each evaporator is controlled by a multi-input, multi-output MPC controller and standard PI controllers are used to regulate system pressures by modulating compressor speed and discharge valve opening. This is in contrast with traditional single-input, single-output control approaches, which can result in undesired dynamic behavior. The efficacy of the proposed control architecture is demonstrated on an experimental system.

A model-based predictive supervisory controller for multi-evaporator HVAC systems

Multi-evaporator vapor compression cooling systems are representative of the complex, distributed nature of modern HVAC systems. Earlier research efforts focused on the development of a decentralized control architecture for individual evaporators that exploits the constraint-handling capabilities of model predictive control while regulating the pressure and cooling setpoints. This paper presents a global controller that generates the setpoints for the local controllers; this controller balances the goals of cooling zone temperature tracking with optimal energy consumption. To accommodate the inherent limitations of the system, a Model Predictive Control (MPC) based approach is used. The improved efficiency and the effects of the tuning parameters are demonstrated upon an experimental system.

Superheat Control: A Hybrid Approach

This paper examines the perennial problem of evaporator superheat control. While standard mechanical devices can operate effectively under design conditions, many behave poorly as conditions vary or under transient operation, resulting in degraded system performance, such as thermostatic expansion valve (TEV) hunting. Technological advances enabling electronic control help alleviate these problems by allowing more sophisticated control approaches to regulating superheat—for example, electronic expansion valves (EEVs). However, poorly tuned EEVs can still exhibit undesirable behavior, and frequent valve adjustments raise concerns about device longevity. In this work, we propose a cascaded control approach, which regulates evaporator pressure and superheat and is achieved with a feedback control device that uses a hybrid of mechanical and electronic feedback. Analysis of the fundamental dynamic behavior of evaporator superheat motivates this approach, while experimental evaluation of two separate systems demonstrates the efficacy of the approach as compared to standard control devices, such as TEV and EEV.

Cascaded superheat control with a multiple evaporator refrigeration system

Variable refrigerant flow (VRF) systems are increasingly being used in commercial buildings in preference to chilled water or ducted air systems. The closely coupled dynamics of VRF systems, however, create a need for effective control strategies to ensure safe and effective operation. Of particular interest is the regulation of evaporator superheat. In this paper a cascaded architecture for controlling superheat is applied to a multiple evaporator system, and compared to traditional PID-controlled evaporators. The effects of the architecture on dynamic coupling are explored, and efficacy is demonstrated with experimental results.

A control architecture solution to superheat nonlinearity

Evaporator superheat control is an important aspect of the operation of refrigeration and air conditioning systems; since the majority of cooling in these systems occurs through evaporation of two-phase refrigerant, the energy efficiency is improved by reducing the amount of superheat present. However, allowing refrigerant to leave the evaporator without completely vaporizing risks catastrophic damage to the compressor, so superior control is required at low superheat levels. One of the most significant challenges present in this control problem is the presence of significant nonlinearities in the response from the control input, e.g. expansion valve position, to evaporator superheat. This paper reveals how a particular control architecture inherently compensates for both the static and dynamic nonlinearities that dominate the valve-to-superheat transient response. Furthermore, the control implementation only requires temperature measurements, which are frequently available in ordinary HVAC systems. Modeling and experimental results confirm the reduction of nonlinearities using the proposed approach, and the authors discuss the effect of actuator limitations on the nonlinearity compensation.

Evaporator Superheat Regulation via Emulation of Semi-Active Flow Control

Proper regulation of evaporator superheat is essential to ensuring safe and efficient operation of vapor compression cooling systems. Typical mechanical control devices may behave poorly under transient disturbances or as operating conditions vary, degrading system performance. Electronic expansion valves partially alleviate these problems by allowing more sophisticated control approaches, but frequent valve adjustments raise concerns about device longevity. A cascaded control approach to superheat regulation has been shown to provide significant improvements in superheat control, utilizing a hybrid of mechanical (passive) and electronic (active) feedback devices. This paper examines the emulation of a semi-active flow control device using a MEMs based actuator with high bandwidth, few moving parts, and no risk of fatigue failure. Experimental evaluation reveals this to be a comparable approach to the hybrid valve design. Moreover, further examination reveals that actuator characteristics are the limiting factor in achieving similar levels of performance using standard electronic valves.Copyright

Pareto Optimal Setpoints for HVAC Networks via Iterative Nearest Neighbor Communication

HVAC systems in large buildings frequently feature a network topology wherein the outputs of each dynamic subsystem act as disturbances to other subsystems in a well-defined local neighborhood. The distributed optimization technique presented in this paper leverages this topology without requiring a centralized optimizer or widespread knowledge of the interaction dynamics between subsystems. Each subsystem’s optimizer communicates to its neighbors its calculated optimum setpoint, as well as the costs imposed by the neighbor’s calculated set-points. By judicious construction of the cost functions, all of the cost information is propagated through the network, allowing a Pareto optimal solution to be reached. The novelty of this approach is that communication between all plants is not necessary to achieve a global optimum, and that changes in one controller do not require changes to all controllers in the network. Proofs of Pareto optimality are presented, and convergence under the approach is demonstrated with a numerical and experimental example.Copyright