A MODEL-PREDICTIVE CONTROLLER FOR AIR HANDLING UNITS

Transcription

1 A MODEL-PREDICTIVE CONTROLLER FOR AIR HANDLING UNITS Y. Sauffer 1, L. Von Allmen 1, E. Onillon 1, S. Arbere 1, E. Olivero 1, D. Lindelöf 2 1: CSEM SA, Jaque-Droz 1, 2002 Neuchael, Swizerland 2: Neuroba AG, Rue de Veyro 9, 1217 Meyrin, Swizerland ABSTRACT Heaing and cooling for hermal comfor are he main consumers of energy in buildings, and here is a growing need o improve he energy efficiency (and hereby reduce CO 2 emissions) of hese building services. The regular increase in energy ariffs only exacerbaes he problem. Building owners are seldom willing o inves in a deep rerofi ha may lower heir energy consumpion, bu are insead willing o replace heir oudaed HVAC sysems. Indeed, offhe-shelf conrollers are ofen based on (only) he oudoor emperaure, and occasionally ake ino accoun he indoor emperaure. In paricular, pracically no commercial sysems ake ino accoun weaher forecass. Consequenly, hese conrol sysems lead o poor comfor and subopimal energy efficiency. In his paper, a novel model-predicive conrol (MPC) algorihm for fan coil unis (FCU) is presened, which aims a reducing he operaional coss while guaraneeing hermal comfor. I is planned o be deployed on a es sie in Greece wihin he second half of The simulaion resuls are presened and compared o a sandard PI conroller. For he MPC based conroller, he rade-off beween he user comfor and he energy consumpion of he will be presened and commened. Simulaions have demonsraed energy savings of up o 57% compared wih he reference conroller. Resuls from field ess are expeced by he end of Keywords: hermal regulaion, MPC INTRODUCTION Thermal comfor regulaion, linked o Heaing, Venilaing and Air Condiioning (HVAC), is one of he main energeic expendiure in buildings. In order o reduce ha consumpion, wihou degrading user comfor, wo disinc, ye complemenary pahs can be aken. Firs, rerofi can be carried ou. Exra insulaion can be added o he walls and roof and he windows can be upgraded. Second, he means of conrolling he emperaure wihin he building can be changed. In his work, he second opion was chosen. I is shown in [1] ha available conrol sysems in buildings rely mosly on convenional echniques such as cooling curves, classical Proporional Inegral Derivaive (PID) conrollers and fuzzy conrollers. These are he mos widely used conrollers in he indusry [2]. While PID conrollers are an improvemen compared o hermosas, hey sill have several issues, mainly due o he difficuly o choose he gain values [3]. To address his problem, self-uning adapive PID conrollers based on recursive leas-squares [4] and fuzzy conrol [5] have been developed. Oher sraegies include adapive conrollers, which have he abiliy o adap according o climae condiions and building properies. Adapive sysems can include parameers esimaion mehods using Recursive Leas-Squares (RLS) algorihms [6], geneic algorihms (GA) [7], nonlinear disurbance rejecion conrollers wih hermal load esimaion and fuzzy conrollers [8]. In order o achieve simulaneous and ofen conradicory energeic and comfor objecives, model predicive conrol (MPC) sraegies have been developed. CISBAT Sepember 9-11, Lausanne, Swizerland 437

2 Ruano e al. [9] have used a muli-objecive geneic algorihm (MOGA) for designing an offline radial basis funcion (RBF) neural nework (NN) model. When compared o a simple onoff conrol sraegy he auhors claimed a 27% reducion in he use of he air condiioner for a beer hermal conrol. Ferreira e al [10] also used a predicive model implemened by RBF NN idenified by a muli-objecive geneic algorihm o minimize energy consumpion while achieving a desired hermal comfor level. In he presen aricle, a Model-Predicive Conrol (MPC) algorihm applied for heaing and cooling is presened. While he developed algorihm is mean o be used o conrol a Fan Coil Uni (FCU), i can easily be adaped for oher devices, i.e. more generally an air handling uni (AHU), however for clariy reasons and given he es sie specificiies, he erm FCU will be used hroughou he aricle. The developed MPC conroller is benchmarked agains a PI conroller. The documen is organized in four main secions. Firs he simulaion environmen, he model-predicive conrol (MPC) algorihm and he es case are presened. Second, he simulaion resuls are presened. Third, he resuls are analysed and discussed. Finally, he works is summarized and an oulook is provided. METHOD Simulaion environmen In order o develop and validae our MPC algorihm, a simulaion environmen had o be developed. Naurally, he various elemens available on he es sie had o be modelled wih enough accuracy, so as o allow he poring of he work o he es sie. The chosen simulaion plaform was MATLAB and Simulink. The main blocks are: Heaer chiller 1 : simulaes he heaing/cooling of he waer which is delivered o he fan coil unis; Room 1 : simulaes he hermal behaviour of he room (including he FCU); Conroller: a PI and our MPC conroller; Simulaion inpus: weaher, emperaure se poins and energy ariffs. MPC conroller The MPC conroller aims a guaraneeing user comfor while minimizing energy expendiure. Accordingly, he objecive funcion is a weighed sum of he wo following erms: Temperaure error (comfor): is role is o penalize deviaions beween he indoor emperaure and is sepoin Power consumpion: is role is o penalize he cos of he energy consumpion. The formulaion of he objecive funcion is shown below (equaion (1)): 1 The work was performed in he framework of he European projec AMBASSADOR (Sevenh Framework Programme Gran Agreemen No ) and is mean o be deployed on a es sie in Lavrion (Greece) in he second half of The heaer/chiller as well as simulaed room model were developed by members of he AMBASSADOR consorium. 438 CISBAT Sepember 9-11, Lausanne, Swizerland

3 N 2 minimize K C P ) Occ K T Tˆ ( P, weaher) P, 2 E C 1 ( (1) Wih: C P ) K max( P,0) K min( P,0) (2) ( hea cool Where: P : power applied during he inerval K C : weighing coefficien for he comfor erm K E : weighing coefficien for he energeic erm (K E = 1 - K c ) C : cos of using he FCU ( ) a inerval Tˆ T : emperaure se-poin Tˆ : emperaure given by he building emperaure predicion model Occ : binary variable used o discard he discomfor compuaion when here is no occupancy in he room (i.e. when Occ is se o zero) K hea : he cos of heaing ( /W) K cool : he cos of cooling ( /W) N: number of ime inervals over he predicion horizon Beside he objecive funcion, wo addiional funcions are required: The building emperaure predicion model: I is based on an ARMAX model and akes as inpus: he FCU power, he oudoor emperaure and he solar irradiance. This model is used o predic he evoluion of he room emperaure over he predicion horizon. The FCU cos model: I predics he power needed o process he air. The model is based on he physics of he heaer/chiller and essenially compues he cos associaed wih reaing he air. Simulaion condiions and simulaion cases The following boundary condiions were used for all he simulaions: Weaher daa: Neuchâel (Swizerland) Temperaure se poins: 2 scenarios (see Figure 1): o scenario 1 (full occupancy: Occ = 1): 20 C during dayime, 22 C during nighime; o scenario 2 (parial occupancy: Occ = 0): 20 C during dayime, no occupancy (i.e. occupancy parameer Occ = 0) during nigh-ime (i.e. free se poin). Dayime: 9 am o 6 pm, nigh-ime: 6 pm o 9 am). CISBAT Sepember 9-11, Lausanne, Swizerland 439

4 Figure 1: Illusraion of he effec of Occ = 1 or 0 in he objecive funcion. In he op line, comfor is o be achieved all he ime (even during he nigh). In he boom line, he emperaure error is no compued during periods wihou occupancy. Firs a series of shor simulaions (~60 days) were performed in differen condiions. The following seings were esed: full (Occ = 1) vs parial occupancy (Occ = 0) scenarios, summer versus winer exernal condiions and various values of he comfor-energeic radeoff, i.e. parameer K c aking values beween 0 and 1. Finally, one year simulaions were underaken o assess he algorihm over long duraions. All he simulaion cases and associaed resuls are summarized in Table 1. In addiion, for a specific simulaion wih high comfor (i.e. K C = 1 and Occ = 1) an illusraion of he measured and desired room emperaure is depiced in Figure 2. RESULTS The simulaion resuls are provided in Table 1. Noe ha he mean emperaure error is defined as he average over he simulaion of he absolue value beween he desired room emperaure and he measured room emperaure. Table 1: Tes cases wih associaed simulaion parameers and simulaion resuls (lef). Focus on he effec of Occ on he energy expendiure (righ) DISCUSSION The effec of he comfor-energy rade-off parameers K c, K E, and he effec of aking ino accoun non-occupancy by discarding he comfor erm in he objecive funcion when here is 440 CISBAT Sepember 9-11, Lausanne, Swizerland

5 parial occupancy (scenario Occ = 0) versus assigning anoher se poin emperaure during he same period (Occ = 1), are presened below. Firs, he effec of K c is highlighed in Figure 2. We can noice an almos linear relaionship beween he mean emperaure error and he consumed energy. I can also be observed ha our MPC algorihm consumes less han he PI conroller (MPC: 2130 kwh, PI: 2200 kwh), for a beer comfor level (mean emperaure error MPC: 0.12 C, PI: 0.18 C). Figure 2: Toal energy consumpion as a funcion of he mean emperaure error, obained by changing he K c parameer (lef). Desired and measured room emperaure for a high comfor (K c = 1) simulaion (righ). Second, he effec of he occupancy parameer Occ is shown in Table 1 and compared wih our reference PI conroller in Figure 2. I can be observed ha leing he sysem free when here is parial occupancy (Occ = 0) reduces he energy consumpion almos by a facor wo for a similar comfor value (during he occupancy periods). I is o be noed ha he MPC anicipaes he heaing and cooling needs before he ransiion from un-occupied o occupied, which mainains an accepable level of comfor. The PI conroller is unable o perform such preempive acions. I can be seen ha as expeced, he K c and Occ parameers affec he comfor and energy expendiure. In addiion, he MPC algorihm achieves a beer comfor for a lower energy expendiure, especially in he parial occupancy scenario (Occ = 0), i.e. when non-occupancy is exploied in he opimizaion. Finally, a similar behaviour was observed when saring he algorihm a various imes of he year or during all-year simulaions. CONCLUSION AND OUTLOOK This aricle presened an MPC algorihm developed for FCU conrol. The opimizaion is based on an objecive funcion, which includes a comfor and an energeic/cos erm. The user has he possibiliy o adjus he rade-off beween hese wo erms wih a single and simple parameer, ranging beween 0 (only energy/cos opimizaion) and 1 (only comfor opimizaion). In addiion, he sysem can ake advanage of unoccupied periods. Simulaion resuls have shown ha, by aking ino accoun he energy/cos in he opimizaion and/or by exploiing he unoccupied periods, he energy consumpion could be drasically reduced while mainaining user comfor. The algorihms will be deployed in he es sie in Lavrion (Greece) during he second half of ACKNOWLEDGMENT The work has been developed under he projec AMBASSADOR, ha receives funding from he European Union Sevenh Framework Programme Gran Agreemen No CISBAT Sepember 9-11, Lausanne, Swizerland 441

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