thermal mass activation by hollow core slab coupled with night ventilation to reduce summer cooling...

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Building and Environment 42 (2007) 3285–3297

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Thermal mass activation by hollow core slab coupled with nightventilation to reduce summer cooling loads

Stefano Paolo Corgnati�, Andrea Kindinis

Department of Energy (DENER), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

Received 4 July 2006; accepted 28 August 2006

Abstract

This study deal with the analysis of the effectiveness of free cooling ventilation strategies coupled with thermal mass activation to

reduce peak cooling loads.

A numerical simulation of the temperatures distribution of an office placed in Milan, Italy, during the month of July, is conducted on a

Simulinks dynamical model.

No air-conditioned system is present but two different free cooling systems are analysed and compared. Both systems act a primary

ventilation during the day and a night ventilation during the non-occupancy period but the first is a traditional mixing ventilation system,

the other is a thermal mass activation system, i.e. the outdoor ventilation air, before entering the room, flows through the ducts of the

hollow core concrete ceiling slab.

The performances of the two systems are investigated by means of time profile analyses of indoor operative temperatures and by means

of frequency temperature distributions during the occupancy period.

The cooling performances are measured by two different discomfort indexes: one represents the discomfort time percentage during

occupation period, the other the discomfort weighted on the distance of calculated operative temperature from the acceptable

temperature interval.

This paper, in last analysis, tries to highlight the possibilities on cooling loads reduction and on thermal comfort increase in

Mediterranean climate, connected to new strategies for thermal mass activation and night ventilation.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Thermal mass activation; Night ventilation; Cooling loads; Thermal comfort

1. Introduction

In the last years the demand for thermal comfort isconsiderably increased in all European countries especiallyduring the summer period. In consequence of a greaterdemand, highest summery energetic consumptions follow.

In order to rationalize energy consumptions and, at thesame time, to follow the new possibilities offered by thetechnology, it appears useful the development of freecooling strategies for the temperature control or just toreduce the thermal summery loads in rooms, especially inoffice buildings, in which it is possible to use, during the

e front matter r 2006 Elsevier Ltd. All rights reserved.

ildenv.2006.08.018

ing author. Tel.: +39011 5644507; fax: +39 011 5644499.

ess: [email protected] (S.P. Corgnati).

night, high air flow rates without disturbing for drat andnoise.Moreover the office buildings present, usually, high

energetic density levels for the relatively recent capillarydiffusion of computer and other electrical equipments andespecially for the absence, until few years ago, of anintegrated approach among architectural, structural andenergetic aspects in building design.The association of building thermal mass and free

cooling systems considerably improves the temperaturecontrol parameters and can reduce the energy demand formicroclimatic control. Unfortunately there are just fewbuilding elements able to exalt the proper attitude of heatstorage, the thermal mass being usually hidden inside of thestructures.

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Nomenclature

ACH [h�1] air change rates per hours [m] widthS [m2] total areaSnet [m

2] net arear [kg/m3] densitycw [J/(kgK)] specific heat

l [W/(mK)] conducibilityU [W/(m2K)] thermal transmittanceta [1C] indoor air temperaturet0,m [1C] average operative temperaturete [1C] outdoor air temperaturets [1C] ceiling temperatureDst standard deviation

S.P. Corgnati, A. Kindinis / Building and Environment 42 (2007) 3285–32973286

One of these elements is represented by the hollow coreconcrete slab, usually adopted as a panel floor forbuildings. The cylindrical ducts put inside, in order tolighten the floor slabs, are the ideal form to exchangethermal energy between the air flowing through it and theconcrete.

In North European countries the passive coolingthrough the hollow core slab systems demonstrated theirefficiency to reduce cooling loads and to obtain thermalcomfort without air conditioning systems. In the Mediter-ranean countries this technology is less used mainly for tworeasons. Firstly, while in the north of Europe hollow coreslabs are the most used kind of floor, in the Mediterraneancountries just a little percentage of the building floor aremade of hollow core slabs. The second reason is that thenorthern climates are characterized by lower outdoortemperature in summer. But it is not obvious that thiskind of cooling system works in the southern climates asmuch as in the northern climates.

The aim of this paper is to investigate the activation ofbuilding thermal mass by means of outdoor air ventilation,exalting the effect of night ventilation. Object of the studyis the indoor temperatures distribution in a typical openspace office room in Milan (Italy), all along the hottestmonth of July. No air conditioned system is considered,but two different mixing ventilation systems are compared,the first traditional (i.e. the outdoor ventilation air issupplied directly in the ambient), the other characterised bythe concrete slab mass activation developed through thehollow core concrete ceiling slab (i.e. the ventilationoutdoor air flows through the ducts of hollow core concreteslabs before entering the room). Both systems provide dailyand night ventilation and attempt to maintain, as it ispossible, thermal comfort conditions during the occupationperiod. The performance of the analysed ventilationsystems also referred to the office room just ventilatedwith primary ventilation air during the occupation period,in order to give a clear idea of the thermal stress expositionof the office during July.

In order to evaluate the capability of the ventilationsystems coupled with the building thermal mass to internalthermal loads variation, the temperatures distributions arecalculated for three different levels of endogenous internalgains: 30, 45 and 60W/m2.

The study was carried out through dynamical simula-tions by a developed dedicated software Simulinks, a tool

of the commercial software Matlabs. From the testreference year meteorological data of July, air and surfacetemperatures (floor, ceiling, internal walls, and windows)were obtained minute-by-minute.In order to give synthetic results and an evaluation of the

ventilation system efficiency, two different comfort indexeswere evaluated: one represents the discomfort timepercentage during occupation period, the other thediscomfort weighted on the distance of the calculatedoperative temperature from the acceptable temperatureinterval.The simulation results provide information about the

possibilities of introducing new strategies for thermal massactivation by outdoor ventilation through the hollow coreconcrete slabs in Mediterranean countries.

2. A literature review

The high daily variation of solar radiation, outdoortemperature and internal loads during the summer, imply,year after year, greater energy consumption. In particularduring peak periods, in order to maintain thermal comfort,it is necessary to remove all excess heat from indoorimmediately after entered or produced; this requiresoversized cooling systems, which are able to handle greatcooling loads for short periods. The building thermal masscan be usefully utilised in order to reduce peak coolingloads, indoor temperature shifting, and so, energy demandin the summer season. The materials constituting thebuilding fabric have high heat capacity, so the building canitself work as thermal storage and help to remove thethermal loads. In the last 25 years, for a renewed interest insummer thermal efficiency in building, a lot of studies triedto quantify the role of building fabric on the indoorcomfort temperatures and on the summer cooling loads. Atthe same time, among the different instruments to activatethe building thermal mass, one of more profitable systemsappeared to be night cooling, made through natural, forcedor hybrid ventilation or by means of a pre-conditioningsystem.Givoni [1] compared different passive and low energy

effectiveness of night cooling systems and discussed theapplicability for different climatic conditions. The ideallocations appeared to be placed in arid regions with nighttemperatures below 20 1C and day time temperaturesbetween 30 and 36 1C. Even if other cooling systems have

ARTICLE IN PRESSS.P. Corgnati, A. Kindinis / Building and Environment 42 (2007) 3285–3297 3287

to be associated for the indoor temperature control duringthe day, night ventilation significantly reduces the length ofperiods and the duration of the working time of additionalcooling systems.

Ruud et al. [2] evaluated the effect of building thermalstorage on peak cooling load in an office building inFlorida, pre-cooling the building at night and during theweekend by means of forced ventilation. The resultsshowed an 18% reduction in cooling energy suppliedduring daytime but no reduction of peak cooling loads.This represents one of the first interesting and completestudies about the association of thermal mass and nightcooling.

Andresen and Brandemuehl [3] simulated the thermalperformance of a typical office space located in St. Louis inorder to study, among other things, the possibility ofreducing electrical consumes by means different strategiesof night pre-cooling. While for the set up, the HVAC is setoff during the not occupied period, for two different pre-cooling strategies, during the not occupied period, thesupply air is set, respectively, to 13 and 10 1C.

Blondeau et al. [4] investigated night ventilation inFrance both by experimental and numerical simulations.Forced ventilation with 8 ACH was tested on a building ofthree level offices and a classroom. The difference betweenthe outside and the inside of the building is considered asthe driving potential of the night time energy removal. Apotential energy efficiency index was defined as the ratiobetween the hourly energy removal from the building, Qn,and the corresponding electric consumption of the fan, Qf.Even if it appears a general index, this coefficient is directlyconnected with the used fans, the extension of airflownetwork and its dimensions. So a comparison of consumesbetween traditional cooling and night ventilation system ispossible just referring to a precise study because of theinfinite possibilities on the design of air flowing network.

Geros et al. [5] tested on three different full scalebuildings in Athens, under different structure, design,ventilation techniques and climatic characteristic: the first amulti-zone air conditioned office building in the suburbanarea of the city, monitored with different air changes (up to30 ACH) during the night, the second an air conditionedoffice building in the city centre, with light structure andhigh internal gains, natural ventilated during the night andthe third a free-floating office building, with heavystructure and medium internal gains, in a low density builtarea in the city centre, with night natural cross ventilation.The study concerns just the indoor temperatures, not theconsumptions. The three main parameters affecting theefficiency are individuated to be: the difference betweenindoor and outdoor during the night periods, the air flowrate applied during the night and the thermal capacity ofthe building.

Kolokotroni et al. [6] studied the possibility of introdu-cing night ventilation for office buildings in moderateclimates, such as UK, presenting plots of summer weatherdata on the bioclimatic chart for three different locations.

Concerning the climatic conditions, Shaviv et al. [7]studied the influence of temperature difference betweennight and day on the maximum indoor temperature: forfour different locations, placed in Israel, they found alinear correlation between maximum temperature andnight and day temperature difference.Few parameters affects the effectiveness of thermal mass

in buildings: the material properties, the building orienta-tion, location and distribution of thermal mass, the thermalinsulation, ventilation and use of auxiliary cooling systems,climatic conditions and occupancy patterns [8]. Shaviv andCapeluto [9] studied the influence in a hot, humid climate,of four design parameters: building proportions, orienta-tion, shading and area of walls and windows. For thatconditions they demonstrated that with a good buildinginsulation the most sensitive parameters are the windowsdimensions, the summer window shading and the buildingorientation, while proportion, wall shading, have verysmall influence.Cheng et al. [10] measured that the higher the level of

solar radiation and the lighter the building, the moresensitive is the building performance to the envelopecolour. Passing from black to white, inside unventilatedand no window room, the maximum indoor temperaturedecreases of more than 10 1C.Givoni [11] monitored buildings with different mass

levels in a summer of south California under different ratesof ventilation and shading conditions. He noted that theeffect of night ventilation was very effective in lowering theindoor temperatures just for high but not for low massbuildings. An experimental formula was elaborated inorder to predict the maximum indoor air temperature in aparticular day but it is not extendable to structuresdifferent from those studied.The influence of insulation and building thermal mass on

cooling loads was studied by Bojic and Yik [12] for high-rise residential buildings in Honk Kong. The influence ofinsulating the envelope reduced the early space coolingload up to 38% but depending on the positions ofinsulation layers in the wall could either increase or reducethe peak cooling loads, while reducing the peak capacity ofenvelope and partitions leads to a large increase in the peakcooling loads (up to 60%).Kalogirou et al. [13] presented a simulation with the

TRNSYS program of a building placed in Cyprus: thermalmass coupled with night ventilation (3ACH) provides acooling load reduction of 7.5%.For the great number of parameters affecting the

effectiveness of thermal mass, all the theoretical analysis,the experimental reports and the different theoreticalsimulations produced certain results dispersion. It ishowever clear that night ventilation is one of the mosteconomic ways to control summer electricity demand bothin moderate and in hot climates (according to an efficientair ducts network design). The mass structure plays anessential role in the thermal response of the building: a highmass has smaller indoor air temperature variation than

ARTICLE IN PRESSS.P. Corgnati, A. Kindinis / Building and Environment 42 (2007) 3285–32973288

a low mass building; the difference between the maximumand the minimum outdoor temperatures during day andnight is linearly correlated to the maximum indoor airtemperature; the ventilation strategies assume a funda-mental role in the control of electricity demand and anoptimisation is possible according to variable nightventilation rates.

Considering the role of night ventilation, it influences thedaily indoor conditions in different ways: reducing the pickindoor air temperatures, reducing the average of airtemperature especially during the first morning hours,reducing the walls temperatures and creating a time lagbetween the external and internal maximum temperatures [14].

According to the fact that night ventilation is particu-larly suited to office buildings that are usually unoccupiedduring the night time, in the last years a lot of applications,as green buildings, demonstrated the efficiency of nightventilation systems associated to high thermal mass inmoderate climates. For the high variability of climaticconditions and the great numbers of parameters influen-cing the thermal efficiency of a building, a great part of thestudies of last 25 years on building thermal mass concernedthe development of simplified design models for estimatingthe cooling loads, in order to give guidelines to a rationaldesigns of the buildings. Some specific interesting designmodels for office building are reported in [15–18]. Anywayas masterly explained by Givoni [19], architectural meansfor minimising the heat gains of buildings generally are lessexpensive than the application of cooling systems so, evenif passive systems have to be adapted under a specificclimate, it is always possible with an appropriate design.

After a rational design of the fabric and of ventilation orconditioning system, in order to achieve season savings onelectrical consumption, the ventilation strategies have to bewell managed: an interesting field of research of last yearsregards the development of real time transient cooling andheating requirements predicting methods based on thelecture of on-site data [20,21] and different techniques ofoptimisation for minimising energy consumption [22,23].With the great improvement of the summer energy demandthe roles of thermal mass and night cooling are funda-mental in energy control. The study proposed tries toapproach the development of fit passive techniques underItalian climate, not sufficiently studied.

By means of air circulating through the cores of aconcrete hollow core slab, the ceiling provides for bothconvective and radiative cooling; it absorbs part of thermalenergy from the entering outdoor air and, at the same time,acts as a heat sink for the indoor environment. The difficultin modelling the ventilated slab consists in the complexityof the sum of contemporary thermal exchanges inside andoutside the slab, in the description of the air flowingthrough the ducts and in the absence of uniformity oftemperatures distribution on the slab.

One of the first study about the ventilated hollowcoreslab is due to Augenbroe and Vedder [24] who showed, bymeans of both experimental results and a finite elements

model, that the heat conduction in the concrete mass in thedirection of the air flow is negligible in comparison to theheat transfer by air flow and that the heat conductionbetween the cores is negligible in comparison with the heatflows between slab surface and environment. For thisreason, in few studies the surface between ducts isconsidered an adiabatic boundary. By means of theseassumptions it was possible to Augenbroe and Vedder tostudy a simpler one-dimension method in good accordancewith experimental results.Zmeureanu and Fazio [25] simulated one sunny day in

Montreal of an office building with HC&NV system,varying the ventilation rate between the values 4, 12, 24ACH and comparing the ventilation system with one ofconventional design. The model is implicit at the finitedifferences, with time step of one hour. The authorsunderline the impossibility of finding values and simula-tions able to quantify the reduction of cooling loadindependently from the climate and the location. For theirconditions, the authors verify that the ventilation rate, tocool the fabric sufficiently in order to reduce cooling loadsthe next day, has to be increased at night from 4 ACH to12 ACH.Ren and Wright [26] introduced, considering a transient

analysis of an office room with HC&NV system, for theventilated slab a simplified bi-dimensional lumped para-meter model. The authors note that the previous analysesignored the fluid-dynamics properties of the heat transferby airflow cause in [25] and [24] the convection heatexchange coefficient was assumed to be constant.At the same time Winwood et al. [27] concluded that the

major part of the heat transfer takes place at the corners ofair path, according to computational fluid dynamicmodelling of the air flowing through the slab. Ren andWright, assumed the convection heat transfer coefficientaround the corners of air cores approximately 50 timeshigher than that for a plain duct. Comparing the modelwith measured performance data for two sets of tests, themaximum error in air temperature leaving the ventilatedslab was 1, 1 1C while the maximum error for the averagetemperature of the slab was 0, 3 1C. Anyway the correctionof the convection heat transfer coefficient hc on the cornersappears excessive.Russell and Surendran [28] investigated, by means of a

finite difference discretization of a slab in 4691 nodes, thecooling potential of a typical hollow core slab in functionof the number and relative disposition of the active sinks.They set that three active cores, located next to the roomboundary, increase the cooling potential, over a traditionalslab subject to night ventilation, by 335%.Barton et al. [29] used a two dimensional finite difference

model similar to that developed by Ren and Wright but,according to the following studies of Winwood et al. [30]gave less importance to the exchange on the corners: whilethe slab temperature distribution varies sensibly on thecorners the influence on the air temperature is dramaticallyreduced. The theoretical results are in accordance with


experimental data taken from literature. Two differentutilisations of the cores are simulated: 3 and 5 active cores.And as it seems to be logic, five cores work better thanthree as the longer is the air passage, the greater thedumpening effect on the air diurnal temperature range.

Corgnati et al. [31] proposed a simplified finite differencemethod in which the slab is assimilated to a heat exchanger,and in order not to over estimate the contribution ofcorners, the convection heat transfer coefficient is setconstant all along the ducts. In order to point the idealcontribution of the thermal storage of the slab, the internaltemperature of the slab, at each time, is the same of thepart exposed to the room. Starting from this model, thisstudy explores the possibilities of building thermal massactivation by means of night ventilation under the Italianclimate.

3. Numerical model

3.1. Model

A numerical model is used to perform the dynamicsimulations.

The main issue, i.e. the analysis of the phenomena ofheat transfer in transient conditions expressed in its typicaldifferential formulation, has been solved with the explicitmethod. The room has been analysed with a model basedon the finite differences. Each element involved in thethermal balance is described with one or more nodes. Thetemporal profile of the temperatures has been describedwith a constant step.

At each time step the outdoor temperature, the solargain and the internal heat gain with its convective and theradiative fractions are fixed.

At the generic time ti+1 the unknown quantity of thesystem is represented by the indoor air temperature, whoseinstantaneous value is given by the expression:

ta i þ 1ð Þ ¼ ta ið Þ þ Dta ið Þ��floorþ Dta ið Þ

��ceiling

þ Dta ið Þ��external walls

þ Dta ið Þ��inner walls

þ Dta ið Þ��windows

þ Dta ið Þ��solar heating

þ Dta ið Þ��inner heating

þ Dta ið Þ��ventilation

.

Hence, for a generic node i, at the time tk the temperatureis a function of only the thermal flows of the previous timetk�1.

For a stable solution the following convergence criterionmust be verified:

DtoCiP

j1=Ri;j,

where the summation is extended to the contiguous nodes j

to i.The ventilation system through the ducts of the floor

slabs is modelled as a heat exchanger with the finitedifferences.

The thermal exchange between the air flowing in thefloor slab and the concrete through the part of the ductcorresponding to a node, occurs with an instantaneouslogic, i.e. the inlet temperature of the air in a piece of slab isthe same of that exiting from the previous piece. So, thereal time utilised by the air to run along the part of the ductmust be much smaller than the time step used in thesimulation.

Dtr ¼Dx

v� Dt.

The analysis was carried out by means of a special toolSimulinks of the commercial software Matlabs based onblock algebra [31].

3.2. Assumptions, initial and boundary conditions

The most important assumption regards the heatexchanger model of the concrete slab. Each ceiling slabwas divided into 48 nodes. Each slab node is supposed toexchange energy with the ventilation air and with theindoor air. The heat conduction between the cores nodes issupposed to be negligible in comparison with the heat flowsbetween slab surface and environment [24].The convection coefficient for the thermal exchanges

between ventilation air and concrete is calculated as afunction of the ventilation rate, in order not to overestimate the contribution of corners, using the expression,for the Nusselt number:

Nu ¼ 0:023Re0:8

for fully developed flow.The indoor air is considered uniform inside the ambient,

i.e. the air is supposed to be perfectly mixed.The fixed internal heat gains are divided in a quote

convective and radiative assumed to be of 60% and 40%,respectively.The analysed office room is contiguous to rooms at the

same temperatures at each time steps, i.e. none heat flowsare considered among contiguous rooms.The initial conditions were determined, in order to start

with equilibrium temperatures simulating the indoorconditions with the first day outdoor temperatures andirradiations repeated for 3 days. The surfaces temperaturesfounded for each ventilation system at the midnight of thethird day were then used as initial conditions.

4. Numerical study

4.1. Room description

The examined office room has a floor surface of(12� 4.8)m2 and a height of 3m. It is located in a mediumfloor of a multi-storey building.The room, shown in Fig. 1, presents three external walls,

each one with a window of 4.8m2, exposed respectively toeast, south and west.


The external walls are made of lightweight concreteblocks, modelled as an homogeneous material with thegeometrical and thermal characteristics listed in Table 1. Inthe dynamical analysis with the finite differences, threenodes correspond to each wall.

The partitions are supposed in lightweight aggregateconcrete based on expanded clay. They divide rooms at thesame temperatures. Their geometrical and thermal char-acteristics are shown in Table 2. In the dynamical analysiswith the finite differences, three nodes correspond to thepartition.

With reference to the hollowcore slab, the width of eachslab is 1.2m so that four slabs are used to compose thefloor. The hollowcore concrete slab is divided from theupper floor by means of an adiabatic surface.

The lower side of the slabs constitutes the ceiling of theroom so that the slab exchanges thermal energy both

Fig. 1. The simulated office room.

Table 1

External walls: thermal and geometrical data

Geometrical characteristics

Orientation s [m] S [m2] Sn

East 0.40 14.4 9.

South 0.40 36 31

West 0.40 14.4 31

Table 2

Partitions: thermal and geometrical data

s [m] S [m2] r [kg/m3]

Partitions 0.14 50 1100

inside, with the air flowing through its ducts, and outsidefor natural convection with the air of the room and forradiation with the other room surfaces. Each slab is lightedby means of four cylindrical ducts with circular sectionwith a diameter of 0.25m as shown in Fig. 2. The air paththrough the four ducts of each slab is 29m long (Fig. 3).Geometrical and thermal characteristics of the hollowcoreslab are shown in Table 3. In the dynamical analysis withthe finite differences, one node corresponds to the tiledfloor, 48 to the ceiling.The three windows located at the three external walls are

oriented, respectively, East, South and West, with a surfaceof 4.8m2 each. Each window is composed by two clearglass sheets and an air space, respectively, of 0.006 and0.012m of with: the U value is 3W/(m2K). By means of asolar shading the solar radiation entering the room throughthe windows can be reduced up to 40%.The office room occupancy period ranges between 9 a.m.

and 6 p.m. During this period, three different kinds of usesare examined through three different internal gain levels:

Thermal-physical characteristics

et [m2] r [kg/m3] 1

6 cw [J/(kgK)] 8

.2 l [W/(mK)] 0

.2 U [W/(mK)] 0

cw [J/(kgK)] l [W/(mK)] U

840 0.32 1.

Fig. 2. The hollowcore concrete slab section.

—Light level
30W/m2
—Medium level
45W/m2
—Heavy level
60W/m2
The outdoor climatic conditions are taken from thehourly meteorological collection of the test reference yearrelating to the city of Milan and to the month of July.

600

80

.7

.67

[W/(m2K)]

45

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Fig. 3. The simulated hollowcore slab.

Table 3

Hollowcore slab: thermal and geometrical data for a surface of

(1.2� 12)m2

s [m] Snet [m2] r [kg/m3] cw [J/(kgK)] l [W/(mK)]

Tiled floor 0.06 14.4 2300 1071 —

Adiabatic surface 0.005 14.4 — — —

Concrete slab 0.05 14.4 2400 900 0.67

Hollowcore slab 0.40 0.284 2400 900 0.67

Table 4

Ventilation logics during occupancy period

tapte or taotcomf ACH ¼ 2 h�1

ta4te and taXtcomf ACH ¼ 5 h�1

ta4te and taXtcomf,inf and ACH ¼ 5 h�1 ACH ¼ 5 h�1

Table 5

Ventilation logics during not occupancy period

IF AND AND THEN (h�1) UNTIL

teots ts420 1C ta421 1C ACH ¼ 5 teots & tsX18 1C




S.P. Corgnati, A. Kindinis / Building and Environment 42 (2007) 3285–3297 3291

4.2. Ventilation strategies

In order to assess the thermal performance of thermalmass hollowcore slab system coupled with night ventilation(TM+NV), three different configurations of ventilationstrategies were simulated and analysed. The studies wereperformed considering that no Air Handling Units (AHU)system works to modify the ventilation air psychometricproperties.

In particular, the following ventilation strategies wereexamined:

4.2.1. Daily ventilation, DV

During the occupancy period, the room is ventilatedwith 2 h�1 air change rates not flowing through thehollowcore slab but fit in the room by a traditional mixingventilation system. During the non-occupancy period, theroom is not ventilated.

4.2.2. Daily and night ventilation, DV&NV

During the occupancy period, the room is ventilatedwith 2 or 5 h�1 air change rates per hour (Table 4), notflowing through the hollowcore slab but fit in the room bya traditional mixing ventilation system. During the non-occupancy period, the room is ventilated with 5, 8, 10 or12 h�1 air exchanging rates. The ventilation rate is functionof the indoor and outdoor air temperature.

4.2.3. Daily and night ventilation through the hollowcore

concrete slabs, HC&DV&NV

During the occupancy period, the room is ventilatedwith 2 or 5 h�1 air exchanging rates flowing through thehollowcore concrete slab (Table 4). During the non-occupancy period, the room is ventilated with 5, 8, 10 or

12 h�1 air change rate per hour, as a function of thehollowcore and outdoor air temperature (Table 5).The basic daily ventilation system DV does not need any

control: the only aim of such ventilation is to guarantee thesuitable number of ACH for indoor air quality. Instead,DV&NV and HC&DV&NV systems need a criterion toactivate one of four different ACH levels.Referring to HC&DV&NV system, which is the main

object of the study, the ventilation purposes duringoccupancy and non-occupancy periods are different.During the occupancy period the first purpose consists inmaintaining adequate IAQ level and possibly acceptabletemperature; during the non-occupancy period the systemtries to refresh the hollowcore concrete slab until itstemperature is sufficiently low to help the room cooling inthe following day.In the occupancy period, the ACH assumes the value of

2 or 5 h�1, as a function of the relationship betweenoutdoor air temperature and indoor air temperature, aspresented in Table 4.In the non-occupancy period, the ACH assumes the

value of 5, 8, 10 or 12 h�1, as a function of the relationshipbetween outdoor air temperature and slab averagetemperature, as shown in Table 5.It is important to highlight that in the peak summer

period, which is analysed in the present study, usually theindoor air temperature does not influence the ventilationstrategy because the outdoor temperature during the night


assumes values round the minimum comfort indoortemperature.

In the same way or likewise it should be noted that theslab and the indoor air reference temperatures, fixed in thisspecific case, respectively, at 18 and 21 1C, have to bechanged along the year, as a function of the desired indoortemperature levels.

In particular, in the peak summer period, the heat storedduring the occupation time often risks to be so high thatalso during night period the ventilation air flowing out ofthe ceiling slab is not able to reduce the indoor temperatureless than 21 1C. As a consequence, in the peak summerperiod the main cooling action is delegated to the ceilingslab: hence the night control strategy is driven only by theslab temperature in order to reduce its temperature at itsminimum possible level.

During occupancy hours, the operative temperatureacceptable interval is fixed at (2672) 1C.

5. Results and discussion

5.1. Generalities

The simulation results are presented by means of timeprofile indoor air temperatures, from which the systembehaviour is clear in its fundamental aspects, by means offrequency distribution graphics, from which a syntheticquantitative analysis is introduced, and finally analysed interms of discomfort indexes.

With reference to the discomfort indexes, in order tosynthetically evaluate the NV+TM thermal performancein maintaining acceptable temperature levels inside theoffice, two indexes are defined:

–

Ta

Av

con

Int

30

45

60

Discomfort over-temperature Time Percentage, DTP
– weighted Discomfort temperature Index, DI
The DTP index measures the discomfort time during theoccupancy period: it is the percentage of time where theindoor temperature overcomes the fixed temperature upperlimit, fixed at 28 1C.

The DI index is calculated by the following expression:

DI ¼X

wi taðiÞ � tcomf ;sup

� �,

wi ¼ taðiÞ � tcomf ;sup

� �,

ble 6

erage indoor operative temperatures, t0,m, and the indoor temperature

figurations

ernal loads [W/m2] DV

t0,m [1C] Dst [1C]

34.91 3.74

37.65 4.55

40.37 5.35

DI ¼X

taðiÞ � tcomf ;sup

� �2.

Hence, it is the sum of the weight difference between airtemperature and the upper comfort limit. The weight factorwi is represented by the difference itself [32].These indexes, together with the indoor air temperature

profiles and the frequency distributions analysed for eachperformed simulation, give an appropriate tool to assessthe performance of each ventilation system.The time profiles and frequency distributions are here

graphically presented, for brevity, just for the case studywith 45W/m2 of internal gains. The results obtained for thewhole analysed combinations are synthetically shown inTable 6, in terms of average indoor operative temperaturest0 and the indoor temperature deviation standard Dst

during the occupancy period.

5.2. Time profile analysis

The indoor operative temperature profiles for mediuminternal gains (45W/m2) are shown in Fig. 4, where also theoutdoor temperature profile is plotted. The curve on top isthe case of daily ventilation (DV). Obviously, it representsthe worst scenario, with a temperatures range variationbetween 24 and 44 1C. During the occupancy period, thetemperature is ever outside the acceptable limits: day afterday, during the warmest period of the year, the heat storedin the room increases. As the aim of the daily ventilation isthe IAQ, the ventilation during the occupancy periodtraduces in added thermal load for the building; this timeprofile gives an idea about the thermal stress of the room.The fact that night ventilation is absent produces that justthe dispersions through the walls help to diminish theindoor temperature. The DV system represents the land-mark for the other systems. From this point of view thetemperature profile with DV represents the heat effect onthe room temperature produced by the internal loads, bythe solar gains and by the primary ventilation without anytemperature control. Moreover, it is should be noted thatthe indoor operative temperature is much higher then theoutdoor air, for the effect of the building thermal inertia.As a consequence it is remarkable that an inertial system asthe medium-heavy building object of our study can worsenthe indoor temperatures keeping inside the heat enteringthe room or inside produced: so the thermal inertia has to

standard deviation, Dst, during the occupancy time for the analysed

DV&NV HC&DV&NV

t0,m [1C] Dst [1C] t0,m [1C] Dst [1C]

25.73 1.47 24.41 1.72

26.31 1.69 25.21 1.89

27.38 1.85 26.03 2.07

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11

13

15

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19

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33

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45

Day

DV DV & NV HC & DV & NV te

20

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22

23

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26

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Day

DV & NV HC & DV & NV

1 2 3 4 5 6 7 8 9 10

Tem

pera

ture

[°C

]Te

mpe

ratu

re [

°C]

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

(a)

(b)

Fig. 4. Time profile temperatures for 45W/m2.


be managed carefully in order to improve the indoortemperature control.

The two other curves in Fig. 4a represent the indoor airtemperatures with night ventilation. For clearness they areeven plotted in Fig. 4b. As the night air change rates arehigh, the temperatures range sensibly reduces.

The hollowcore slab ventilation enables to improvefurther on the indoor air temperature damping peak. Bythe analysis of HC&NV and HC&DV&NV profiles, theimproved thermal mass effect is clearly highlighted, asshown in Fig. 5 where the difference between the indoor airtemperatures for the HC&DV&NV and DV&NV ispresented. During the night time, this difference diminishes

and at times is positive: the ceiling slabs give up theheat stored during the day to the ventilation air introducedin the room. As a consequence, the indoor air temperatureis colder for a simple night ventilation system representedby the DV&NV with respect to HC&DV&NV.Nevertheless changing from night to day time, andespecially during the occupancy period, the same differenceis negative: the average difference is �0.7 1C.As a consequence, the hollowcore ventilation improvesthe system efficiency in maintaining acceptabletemperatures during occupancy periods, in comparisonwith a traditional ventilation system, being equal the airflow rate.

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-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

1 2 3 4 5 6 7 8 9 10

Day

Tem

pera

ture

[°C

]

Occupation period Δt

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Fig. 5. Time Profile Temperatures Difference between HC & DV & NV and DV & NV for 45W/m2.


As the hollowcore slabs during the night give up thestored heat to the air entering the room, it’s besidespossible to use a higher ACH then with a traditional systemwithout reducing too much the indoor air temperature. Or,at a parity of ACH, reaching later the ta limit, theventilation can work for a longer time, restoring uniformityat the indoor temperatures. So the inertial system, properlyactivated, gives positive effects even in terms of redistribu-tion of heat stored in the building.

In Table 5 the ventilation logics are presented: during thenon-occupancy period, when the ceiling temperature over-comes a limit value, a ventilation rate is activated andcontinues until the ceiling reaches the safely temperature oruntil the occupation period begins.

5.3. Frequency distribution analysis

The frequency distribution analysis focus on the indoorair temperature analysis during the occupancy period,where acceptable temperature levels have to be maintained.

In Fig. 6, the results about DV are presented: less then10% of the occupancy period the indoor operativetemperature is below the upper limit of 28 1C. The rest ofthe time the indoor air temperature turns over 39 1C up to43 1C even if outdoor temperature is lower.

A lighter building structure with the same DV systemwould introduce higher indoor temperatures during occu-pation time but mainly in phase with outdoor temperaturethan the structure studied. This underlines the importanceof managing in the most suitable way the thermal mass inorder to obtain positive effects on the peak cooling loadsand temperatures.

Fig. 7 presents the temperatures distribution forDV&NV system. Balancing the heat loads introduced in

the room during the day with the effect of the nightventilation, the peak temperatures in occupation timemaintains below 30 1C. Moreover, the 80% of the indoorair temperature values are inside the acceptable range.Fig. 8 shows the performances with the DV&HC& NV.

The peak temperatures are lower than 30 1C. It isremarkable that about the 85% of the temperature valuesare below the upper temperature limits of 28 1C: indeed upto 20% of the temperatures during the occupation periodare lower than the minimum value of 24 1C. It is evidentthat the use of this system allows obtaining temperaturevalues sensibly lower than also with DV&NV.In order to compare effectively DV&NV and

DV&HC&NV, in Fig. 9 the distribution of the indooroperative temperatures differences, during the occupationtime, between HC&DV&NV and DV&NV in shown: theceiling thermal mass results to be positively activated,giving indoor temperatures lower than a simplenight ventilation system with the same air flow rate. Theindoor air temperature maintains always lower withDV&HC&NV, mainly ranging from �1.5 to �3.5 1C. Ifinstead the operative, the air temperature was consideredthe difference was greater: this for the fact thatHC&DV&NV provides lower air temperatures but higherceiling temperature, during the day.

5.4. Discomfort analysis

The analysis of the discomfort indexes allows tosynthetically judge the performance of the differentanalysed ventilation systems. The results are shown forinternal gains of 30, 45 and 60W/m2.In Fig. 10 the discomfort temperature time percentage in

occupation time is shown.

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500

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3500

4000

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Fre

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0%

20%

40%

60%

80%

100%

Fre

quen

cy p

erce

ntag

e

Frequency Cumulated Frequency

acceptable interval

Temperature [°C]

Fig. 6. DV, internal gains 45W/m2—frequency distribution.

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 350

500

1000

1500

2000

2500

3000

3500

4000

Fre

quen

cy

0%

20%

40%

60%

80%

100%

Fre

quen

cy p

erce

ntag

e

acceptable interval


Temperature [°C]

Fig. 7. DV & NV Internal gains 45W/m2—frequency distribution.


For each internal gain levels, the DTP curve for DVassumes very high values, ranging from about 92% up toabout 97%. For DV&NV and HC&DV&NV the DTP issignificantly lower than for DV, but it is evident that theDTP highly increase with the increasing of the internalgains, ranging respectively about from 7% to 37%, andfrom 0 to 20%. It is also evident that with HC&DV&NVhighly better performance can be obtained.

For the DV&NV system, only the case with low internalgains presents acceptable indoor temperature, i.e., the DTPmaintains under 10%; such result is obtained withHC&DV&NV system for medium internal gains, wherethe 8.5% of discomfort percentage is evaluated. Growinginternal loads, the cooling effect of the activated concretecontinues but it is not sufficient to guarantee keep

discomfort times below 20%. It is important to remarkthat in this study the integration of the HC with an AirHandling Unit was not examined: obviously such acombination driven with an appropriate operating logiccan lead to a decrease of DTP keeping low the energyrequirements.In Fig. 11 the Discomfort Index, as a function of the

internal gain levels, is shown in semi-logarithmic scale forthe different ventilation systems. The difference betweenHC&DV&NV and DV&NV gives a precise indicationabout the maximum energy saving related to the couplingof the ventilation system with the hollowcore concrete slabwith the same air change rate per hour.The difference is significant: at low internal gains (0K2

with HC&DV&NV and around 2400K2 with DV&NV)

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0102030405060708090

100

DT

P

30 45 60

Internal gains [W/m2]

HC & DV & NV DV & NV DV

Fig. 10. Discomfort time percentage in occupation time.

0

500

1000

1500

2000

2500

3000

3500

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Fre

quen

cy

0%

20%

40%

60%

80%

100%

Fre

quen

cy p

erce

ntag

e

acceptable interval


Temperature [°C]

Fig. 8. HC & DV & NV Internal gains 45W/m2—frequency distribution.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

-2.5 -2 -1.5 -1 -0.5 0

Temperature difference [˚C]

Fre

quen

cy

0%

20%

40%

60%

80%

100%

Per

cent

age

freq

uenc

y


Fig. 9. Temperatures difference between HC&DV&NV and DV&NV

internal gains 45W/m2—frequency distribution.

1

10

100

1000

10000

100000

1000000

10000000

30 45 60

Internal gains [W/m2]

DI

[K2 ]

HC & NV & DV NV & DV DV

Fig. 11. Discomfort Index in occupation time.


and the difference tends to increase with the internal gainsgrowing (at high internal gains the HC&DV&NV grows at1400K2 while the DV&NV rise at 39000K2).

6. Conclusions

A numerical model at the finite differences is used todescribe by a Simulinks model the operative temperaturesdistribution of an open space office room placed in Milan,Italy, during the month of July. Only few study on thepossibilities offered by thermal mass activation systemsthrough outdoor ventilation strategies are referred inliterature to the Mediterranean, and especially Italian,climate. This paper tries to match this question.The performances of two free cooling systems are examined

and compared: one is a traditional mixing ventilation, theother is characterized by the thermal mass activation of theceiling hollow core slabs. Both systems use great outdoor airchange rates during the night to pre-cool the office. In fact,the analysed simulated office was exposed to a great thermalstress as is shown by the office temperatures analysisventilated just with the primary ventilation and not with anight ventilation nor an HVAC help.


Simulations show that the hollow core ventilation systemoffers an average operative temperature lower of more of1 1C at the same ventilation rates for each of threeendogenous internal gains simulated (30, 45, 60W/m2) incomparison with the traditional system. Considering forthe case of 45W/m2 the temperature distribution analysis,the better performance is shown by a temperature profilemoved in the field 23–29 1C in comparison with thetraditional system (24–30 1C). Moreover, more than 90%of occupation period presents temperature below themaximum acceptable limit of 28 1C (this percentagedecrease to about 80% with DV&NV), and about 30%of values are lower than 24 1C, highlighting the capabilityof the system to produce significant under-temperatureeffects.

This study shows that the night ventilation, better ifcoupled with mass activation, can drastically help onreducing summer cooling loads and on improving thermalcomfort, also in Mediterranean climate.

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