Set-point Strategy Research Articles

Setpoint control for air heating in a church to minimize moisture related mechanical stress in wooden interior parts

The paper presents the setpoint control for air heating in a church to minimize moisture related mechanical stress in wooden interior parts, with the focus on the preservation of a monumental organ. The setpoint operation of the heating, ventilation, and air conditioning (HVAC) system is evaluated by simulation using MatLab, COMSOL and Simulink models. The main model components are presented and combined in a single integrated Simulink model: (1) a HAMBase Simulink building model for simulating the indoor temperature and relative humidity, (2) a COMSOL partial differential equation model for simulating detailed dynamic moisture transport and related mechanical stresses in the monumental wood (organ) and (3) a Simulink controller model. The main advantage of the integrated model is that it directly simulates the impact of HVAC control setpoint strategies on the indoor climate and the related mechanical stresses in wooden objects, like a monumental organ. As control strategy the limited indoor air temperature changing rate is discussed. Recommendations from international literature suggest that a limitation of the relative humidity changing rate of 2 to 5RH%/h will preserve the interior of churches. This study shows that a limitation of indoor air relative humidity changing rate of 2RH%/h can reduce mechanical stresses by a factor of 2.5, compared to maximum capacity heating.

Dynamic modeling and fuzzy augmented PI control of a high-rise building hot water heating system

Energy consumption of hot water heating (HWH) systems in high-rise buildings is a topic of considerable economic interest. In this study a dynamic model of a HWH system was developed and an intelligent fuzzy logic operating strategy was designed to improve energy efficiency of the system. Dynamic responses of temperature and water mass flow rate in the system were studied. Two types of boiler temperature set-point strategies, one based on average outdoor air temperature method and the other using a fuzzy augmented system, were explored and their performance was compared. From the simulation results it is shown that the strategy based on the fuzzy inference system saved boiler fuel consumption by about 10%.

Optimal setpoint operation to reduce peak drying of a church organ

The paper presents the characteristics of the Walloon Church in Delft (Netherlands) and a description of constraints for the indoor climate, giving criteria for the indoor air temperature and relative humidity with the focus on the preservation of the monumental church organ. The set point operation of the Heating Venting and Air Conditioning (HVAC) system is evaluated by simulation. The next main model components are presented and combined in a single integrated model: 1) a whole building response model for simulating the indoor temperature and relative humidity, 2) a Partial Differential Equation (PDE) based model for simulating detailed dynamic moisture transport in the monumental wood (church organ) and 3) a SimuLink controller model. The building model is validated with measurements. The main advantage of the integrated model is that it directly simulates the impact of HVAC control set point strategies on the indoor climate and the church organ. Two types of control strategies are discussed. The first type is a limited indoor air temperature-changing rate. The second type is a limited indoor air relative humidity changing rate. Recommendations from international literature suggest that 1) a changing rate of 2 K/h will preserve the interior of churches and 2) a limited drying rate is important for the conservation of monumental wood. This preliminary study shows that a limitation of indoor air temperature changing rate of 2 K/h can reduce the peak drying rates by a factor 20 and a limitation of the relative humidity changing rate of 2%/h can reduce the peak drying rates by a factor 50. The second strategy has the disadvantage that the heating time is not constant.

Dynamic simulation of energy management control functions for HVAC systems in buildings

Five energy management control (EMC) functions such as outside air (OA) economizer cycle, programmed start and stop lead time, load reset and occupied time adaptive control strategy are developed and evaluated using a variable air volume heating, ventilating and air conditioning, VAV-HVAC, system level dynamic model as a simulation platform. A real time system embedded with the above EMC functions is presented. The simulation results manifest that energy savings of 17% can be achieved when the system is operated with the EMC functions and optimal set points compared with the system without such functions. These results do point out that the optimal set point strategy is very useful in achieving energy efficient operation of HVAC systems.

Control Structure Selection for Reactor, Separator, and Recycle Processes

We consider control structure selection, with emphasis on “what to control”, for a simple plant with a liquid-phase reactor, a distillation column, and recycle of unreacted reactants. Plants of this kind have been studied extensively in the plantwide control literature. Our starting point is a clear definition of the operational objectives, constraints, and degrees of freedom. Active constraints should be controlled to optimize the economic performance. This implies for this case study that the reactor level should be kept at its maximum, which rules out many of the control structures proposed in the literature from being economically attractive. Maximizing the reactor holdup also minimizes the “snowball effect”. The main focus is on the selection of a suitable controlled variable for the remaining unconstrained degree of freedom, where we use the concept of self-optimizing control, which is to search for a constant setpoint strategy with an acceptable economic loss. Both for the case with a given feed rate where the energy costs should be minimized and for the case where the production rate should be maximized, we find that a good controlled variable is the reflux ratio L/F. This applies to single-loop control as well as multivariable model predictive control.

THE DEVELOPMENT AND EXPERIMENTAL TESTING OF A FUZZY CONTROL SYSTEM FOR BATCH DISTILLATION

The present work describes the development and implementation of fuzzy control algorithms in order to control on-line the overhead product composition of a batch distillation column. Firstly, the influence of design parameters was evaluated through computational simulations and then the algorithms were experimentally tested by monitoring a pilot column. Binary mixtures of n-hexane/n-heptane were distilled. Temperature measurements and vapor-liquid equilibrium data are the basis for the inference of overhead and bottom compositions. Two different operational strategies were used for the experimental runs: constant overhead product composition and previously determined set-point trajectory. Using the first strategy, the performance of the fuzzy controllers is compared to the performance of conventional feedback digital controllers. Experimental results show that fuzzy control presents a better performance than the conventional digital feedback control and also that fuzzy controllers were able to deal successfully with variable set-point strategy, albeit using constant design parameter values. Under conventional control, the average reflux rate implemented was higher than the average reflux rate implemented with fuzzy algorithms. Consequently, the process becomes less time- and energy-consuming under fuzzy control. Since fuzzy methodology is a promising new way of looking at process control problems and their solutions, the results of this work could provide control system designers with a better evaluation of the potential worth of fuzzy control.

Genetic algorithm design of neural network and fuzzy logic controllers

This paper discusses the design of neural network and fuzzy logic controllers using genetic algorithms, for real-time control of flows in sewerage networks. The soft controllers operate in a critical control range, with a simple set-point strategy governing “easy” cases. The genetic algorithm designs controllers and set-points by repeated application of a simulator. A comparison between neural network, fuzzy logic and benchmark controller performance is presented. Global and local control strategies are compared. Methods to reduce execution time of the genetic algorithm, including the use of a Tabu algorithm for training data selection, are also discussed. The results indicate that local control is superior to global control, and that the genetic algorithm design of soft controllers is feasible even for complex flow systems of a realistic scale. Neural network and fuzzy logic controllers have comparable performance, although neural networks can be successfully optimised more consistently.

Optimization of a periodic biological process for nitrogen removal from wastewater

The Bio-Denipho® process, a “phased isolation ditch” technology, varies both aeration pattern and flow path in a continuous flow multi-reactor system to force oscillation of organic and nutrient concentrations in process reactors. Using a six-phase cycle, desired biochemical transformations (e.g. nitrification, denitrification), are accomplished at different times in the same reactor. We used an industry-standard biokinetic model (IAWPRC) to develop and test three control strategies of increasing sophistication: (a) fixed phase lengths, (b) use of constant set points to switch between phases thus resulting in variable phase lengths, and (c) use of switching set points which are a function of on-line measurements (criteria functions). These strategies were optimized for nominal diurnal operating conditions, with the objective of minimizing effluent soluble nitrogen, subject to an upper bound on the oxygen transfer coefficient. In addition, performance of the strategies was simulated against 1-, 2-, and 5-day sustained peak loads. Under nominal diurnal conditions, the performance obtained with each strategy was comparable. Under peak loading conditions, both of the switching set point strategies gave substantially better performance than the fixed phase length strategy. Criteria functions were marginally better than constant switch points.