Elsevier

Energy and Buildings

Volume 42, Issue 8, August 2010, Pages 1205-1214
Energy and Buildings

Control tuning of a simplified VAV system: Methodology and impact on energy consumption and IAQ

https://doi.org/10.1016/j.enbuild.2010.02.011Get rights and content

Abstract

Nowadays, with the improvement of living standards, air-conditioning systems have been widely used causing more health problems linked to air-conditioning systems and indoor air quality. When an air handling unit is designed for a conference room, its variable use demands an outdoor air rate control in particular to optimize energy consumption of the new air heating/cooling. Based on an experimental site, the present work shows how to combine air quality and the improvement of energy consumption thanks to a way to define a control strategy. The control problem considered here is the management of air quality associated to a control of the outdoor air rates. A review of classical tuning methods is led in order to select suitable ones. Thus, a linearization of the phenomena must be done to border the system expression in tuning methods. Several classical methods are studied and the family model is chosen: monovariable and multivariable applications are brought to fruition. Simulations and applications permit to enhance energy consumption through the choice of a strict way to tune such control equipments.

Introduction

With the improvement of living standards, air-conditioning systems have been widely used causing more health problems linked to air-conditioning systems and indoor air quality. Problems are summarized in the existing research. Many kinds of AC (air-conditioning) systems used to improve the indoor thermal comfort and IAQ are listed. Ref. [1] reports on a review of a recent research on air-conditioning systems and indoor air quality control to preserve human health. This research is focused on independent control of temperature and humidity system (ICTHS), cooling ceiling and displacement ventilation systems (CC/DV) and dedicated outdoor air system (DOAS). A further study is suggested on air-conditioning systems and indoor air quality control for healthy indoor air environment, besides indoor air purification.

The present study concerns the energy improvement of an AHU (air handling unit) allowing the introduction of a variable outdoor airflow. Its issue consists in adjusting the outdoor airflow rate to fit with hygienic requirements in terms of indoor air quality, minimizing energy consumption due to the heating of the supplied air thanks to the study of a conference room equipped with CO2 sensors with a variable occupation and only in winter. The first step of this approach is to identify the room occupation thanks to CO2 measurements and to find the available controllers from industry. The second one consists in modelling the transfer phenomena by studying the variations of CO2 rate in the room. A review and a selection of a suitable tuning method are done, then, applied. Finally a simulation and a comparison with experimental data are led.

CO2 rate control is well known as a key point to comply with indoor air quality [2], [8]. The impact of a fuzzy reasoning controller on indoor air temperature has been previously studied: [3] investigates the performance of this kind of machine for the control of indoor air quality in naturally ventilated buildings. Simulations have been performed using a new airflow and pollutant transport model, which has been developed and validated for this purpose. Results showed that satisfactory IAQ levels can be maintained, while good stability of the control parameter (i.e. window opening area) is achieved. The performances were not as good as expected, but not negligible compared with the normal conditions of the building use.

Most of the recent studies on variable air volume (VAV) air-conditioning systems are dedicated to energetic studies. Ref. [4] presents a simulation of a VAV air-conditioning system in an existing office building for the cooling mode, and listed about 14 references dealing with innovative work on VAV. Among these, Ref. [5] studies two modern office buildings in Shanghai with typical variable air volume (VAV) systems. Four separate spaces on a standard office floor in each building facing different directions (north, south, east, and west) were selected for thorough site measurements of outdoor airflow rates and indoor air quality (mainly CO2 rate) during typical days of the four seasons. Computer simulations and calculations of outdoor airflow rates and CO2 rate in the four-test spaces on an hourly basis for the entire year were also carried out. In addition to the site measurements, electrical consumption for the two buildings was monthly recorded or estimated. Simulations and calculations of the buildings energy consumption and energy cost were performed using two different outdoor air control strategies of a typical VAV system as well as a fan coil unit (FCU) system.

What is more, the building studied in [5] is only used for intermittent occupancy. Continuous operation of ventilation system is not needed to achieve good indoor air quality during occupation periods. This kind of buildings has a great energy saving potential which is not harnessed enough yet. Indeed, energy loss can be avoided by promoting natural means and managing mechanical ones. In Ref. [5], an energy efficient ventilation system is based on control strategies which adjust the ventilation according to time and/or occupancy. Ref. [6] demonstrates that the robustness of an experimental scale model can be used to provide recommendations on the management and the control of ventilation systems in case of intermittent occupancy.

Other innovative strategies have been investigated. Ref. [7] presents the development of a multi-input multi-output (MIMO) control strategy which consists in controlling simultaneously the indoor air temperature and humidity (which affects occupants’ thermal comfort and indoor air quality) by varying the speeds of both compressor and supply fan in an experimental direct expansion AC system. This approach is very interesting and this paper complies with this kind of modelling and control design.

The main difficulty of this study stands in the coupling of outdoor air rate and air quality [22]. This coupling is modelled hereafter, in order to enhance the control thanks to a correctly designed controller. First a linearization of the process has to be realized to come close to control tuning parameter techniques available in the literature (linear systems). The systematic parameters tuning is then detailed and carried out. The methods of tuning for linear continuous systems are built on for this work: first of all by the monovariable approach (monovariable model method and “partial matched model” method) and next by the assimilation to a multivariable system (multivariable model method). The programming is realized by means of a graphic software (Simulink), working under Matlab environment [9].

Section snippets

Building data

The studied building is a conference room in a French University. Ref. [10] underlines the importance of IAQ in school buildings: a poor IAQ, due to an unsuited ventilation rate, can cause a reduction in the students’ performance. This room can hold approximately 300 people. It is equipped with four rectangular return air inlets located on the ceiling, on the side of the amphitheatre and six circular outlet air grilles, four distributed above the steps and two above the rostrum. There are no

Modelling the phenomena, process linearization

The indoor air quality is characterised by the concentration of (metabolic) carbon dioxide in the air. In the framework of our study, the air quality in the room is managed by controllers action on the airflows. The controller input is a weighting measurement of several CO2 sensors. The selected model is therefore the perfect mixed one. In the case of the gaseous pollutants concentration control in a zone (perfect mixed model), the relation between inputs (airflow rate coming from outdoor

Methodology for the possible tuning methods selection

Different classical methods of tuning the controllers were reviewed. Some of them were selected because of their appropriateness to the defined problem and their easiness in programming.

Fig. 7 presents briefly the adopted way for the method selection.

Analysis of several methods

In few cases (constant occupation), the introduced pollutants airflow can be considered as constant for a limited period (conference rooms). In this case the input, a real non-controllable input parameter, u1 = q/V is supposed constant. The system

Simple method model

This calculation is done for the conference room considered in part 1. The numerical values are as follows:

  • V = 1244 m3,

  • Qo = 1500 m3/h (time constant of the phenomena: τ = V/Qo = 0.83 h,

  • q = 2.97 × 10−6 m3 CO2/s. persons [15] (occupation of 40 people),

  • Co = Ce = 390 ppm, Ci0 = 676.5 ppm, set point Cr = 1000 ppm.

In the studied case (see Eq. (4)): G2(p) = F2(p) is a 1st order transfer functionG2(p)=b01+a1p=V/Q0(CeCi0)1+V/Q0p

Identifying term to term:

  • the gain: G = (Ce  Ci0V/Qo = −852,353 ppm s;

  • the time constant of the system: T1 = 

Conclusion

After working on a mathematical study of the transfer function simplified for the pollutant rate in a conference room, several designing techniques of controller were selected according to monovariable approaches, for the simplified problems and to multivariable approaches to take into account complex and coupled problems.

The controllers were then tested in simulation, and experimental data site were used to complete the study. The advances realized during this work can find a natural extension

Acknowledgements

The authors want to thank ADEME (Agence de l’Environnement et de la Maîtrise de l’Energie, the french Energy Agency) for financial support of this study thought the measurements collected in the project” Implantation et test de méthodes de retro and on-going commissioning” (Establishment and test of methods of retro and on-going commissioning).

The authors want also to thank the journals’ reviewers of a previous draft of this paper.

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