A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part II: Further development of the controller to improve control sensitivity

The development of the novel direct digital control (DDC)-based capacity controller for a direct expansion (DX) air conditioning (A/C) unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and relative humidity (RH) in a conditioned space served by the DX A/C unit has been reported in Part I of the two-part series. The results of preliminary controllability tests for the novel capacity controller presented in Part I, however, suggested that the controller developed was operational, with acceptable control accuracy but rooms for improvement with respect to control sensitivity. This paper, the second part of the two-part series, reports on the further development of the controller to improve its control sensitivity and the associated controllability test results. Both control accuracy and reasonable control sensitivity were achieved by incorporating a traditional Proportional–integral (PI) controller into the DDC-based capacity controller.     

New absorption chiller and control strategy for the solar assisted cooling system at the German federal environment agency

Typically the cooling capacity of absorption chillers is controlled by adjusting the driving hot water temperature according to the load. Meanwhile the cooling water temperature is controlled to a constant set value. In order to increase the solar cooling fraction and/or to decrease the operating costs of solar assisted cooling systems (SAC-systems) a new control strategy has been developed which controls hot and cooling water temperature simultaneously. Hereby the specific cost of cold – generated from solar or conventional heat – can be reduced. The basic concept of the strategy is explained and results are shown for the SAC-system at the Federal Environment Agency in Dessau, Germany. Here a recently developed absorption chiller is now used instead of a former adsorption chiller. With the new absorption chiller and the control strategy the seasonal energy efficiency ratio SEER is above 0.75, electric efficiency is 35% higher and water consumption is reduced by 70%.     

Evolutionary design of dynamic neural networks for evaporator control

This paper presents a novel neural network (NN) to control an ammonia refrigerant evaporator. Inspired by the latest findings on the biological neuron, a dynamic synaptic unit (DSU) is proposed to enhance the information processing capacity of artificial neurons. Treating the dynamic synaptic activity after the nonlinear somatic activity helps to capture the dynamics demarcated by the Gaussian activation pertaining to the input space. This practice leads to a remarkable reduction in curse of dimensionality. The proposed NN architecture has been compared with two other conventional architectures; one with dynamic neural units (DNUs) and the other with nonlinear static functions as perceptrons. The objective is to control evaporator heat flow rate and secondary fluid outlet temperature while keeping the degree of refrigerant superheat in the range 4–7 K at the evaporator outlet by manipulating refrigerant and evaporator secondary fluid flow rates. The drawbacks of conventional approaches to this problem are discussed, and how the novel method can overcome them are presented. An evolutionary approach is adopted to optimize the parameters of the NN controllers. Then evaporator process model is accomplished as a combination of governing equations and a sub NN resulting in a simple and sufficiently accurate model. The effectiveness of the proposed dynamic NN controller for the evaporator system model is validated using experimental data from the ammonia refrigeration plant.     

Identification and control of multi-evaporator air-conditioning systems

Modern multi-evaporator air-conditioners (MEACs) incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and conventional expansion valves. In this paper, feedback controller design for the MEAC system is first addressed through experimental identification. The identification produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish the stability for the cascade design are given. Due to the deficiency in control inputs, the proposed control structure exhibits steady-state errors in the superheat responses which in turn can produce unacceptable steady-state superheats. To resolve this issue, the reference superheat settings are determined via an optimization procedure so that the resultant steady-state superheats become acceptable. The control experiments indicate that the proposed controller can successfully regulate the indoor temperatures and maintain the steady-state superheat temperatures at acceptable levels.     

Modeling, identification and control of air-conditioning systems

In this paper, feedback controller design for the air-conditioning system is addressed through systematic modeling and identification. Particularly, the physical model of the system reveals the key parameter that dictates energy efficiency, and the identification procedure produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish performance and stability of the cascade design are given. Experiments show that the controller can simultaneously achieve satisfactory transient response in the indoor temperature, and improve energy efficiency at steady state.     

Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems

This paper describes a detailed procedure into the investigation of optimised control strategies for CO₂ cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO₂ system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.     

Thermal performance indicators for refrigerated road vehicles

An experimental evaluation of insulation effectiveness, pull-down times and effectiveness of a mechanical refrigeration unit was performed on five refrigerated panel vans. Additionally, a mapping of temperature variability in the cargo space was carried out during a simulated journey encompassing several door openings. The temperature variability measured in the vans was neither correlated to the effectiveness of the insulation nor the time required to achieve complete cooling of the cargo space. These criteria are common thermal performance indicators used to evaluate refrigerated vehicles in international standards. The efficiency of insulation was correlated to the pull-down times, presenting the possibility of calculating the former as a function of the latter. The temperature variability was correlated with the time required for the unit to recover temperature control after a door opening and the difference between the maximum and minimum temperatures reached during a door opening cycle.     

Innovative design of a magnetocaloric system

In the present paper we consider the problem of optimizing the cooling of a magnetocaloric refrigerator. In this work we first theoretically and then experimentally study the performance of a single material regenerator under different operating conditions. The basic principles of the design and implementation of our magnetic refrigerator prototype are presented as well as a new magnetic assembly of NdFeB permanent magnets.

The design of the equipment uses a movement of relative displacement optimized for the phases of activation and inactivation of the magnetic field. Each part of the equipment is implemented in order to be controlled separately and to allow a large variety of the tests: gear pumps with individual control, sequence of programmable magnetocaloric cycle, unit control by programmable controller and application programming interface by color LCD touch screen, real-time processing data acquisition using a National Instruments System implemented on Independent PC, expelled heat using different standard heat exchangers.     

Applications and control of air conditioning systems using rapid cycling to modulate capacity

Rapid cycling the compressor of an air conditioning or refrigeration system can be used to modulate capacity, thus offering an alternative to a variable speed compressor. This paper explores design tradeoffs to optimize rapid cycling performance based on experimental results using two different evaporators and changing other components of an air conditioning system. Rapid cycling has inherent compressor lift penalties associated with larger mass flow rates, which need to be minimized. Preventing dryout (superheating) in the evaporator during the off cycle, a major penalty as cycles are lengthened, is also important. Evaporator dryout is minimized by increasing the refrigerant side area and reducing off cycle drainage. Combining a flash gas bypass with a suction line heat exchanger was found to maximize performance during the off cycle while allowing increased cycle lengths without incurring major penalties.     

Non-linear predictive control of a vapour compression cycle

A potential energy saving can be obtained by a global optimal setting of each actuator in a vapour compression cycle. This paper describes a predictive optimal control algorithm applying this strategy. At each step, an optimal command profile is computed, upon predictions of variables system evolution, using a simple non-linear model of a vapour compression cycle. Choice of this profile is based on a multiple criterion including cycle efficiency and technological constraints. The approach is experimentally validated on a pilot scale refrigeration plant with variable speed compressor. Trials have been performed and showed a meaningful energy saving.