Thermal Mass and Night Ventilation Utilising Hidden Thermal Mass

Nick Barnard

FaberMaunsell

Marlborough House

Upper Marlborough Road

St Albans AL1 3UT

Summary

This paper presents the development and implementation of a system (CoolDeck) that

utilises thermal mass in conjunction with night ventilation. The system has been developed to

improve the performance of systems where air is ventilated through false ceiling and floor

voids to access hidden mass. The system improves thermal interaction between the

circulating air and the thermal mass of the slab. Phase Change Material has been integrated

into the system to augment the thermal mass of the slab and provide additional thermal

storage. A case study is presented demonstrating the effectiveness of the system in a

refurbishment application.

1. Introduction

Thermal mass can be used in conjunction with night ventilation of a building to provide

passive cooling. Outside air is circulated through the building where it comes into contact

with and cools the building fabric. The cooling that is stored in the building fabric is then

available to offset heat gains the following day and keep temperatures within comfort limits

(Figure 1).

Night ventilation is most effective in moderate climates such as the UK where the diurnal

swing is sufficient that ambient temperatures at night fall below normal daytime internal

comfort temperatures. It is suitable for buildings with periodic daily loads such as offices. The

use of thermal mass in conjunction with night ventilation can be used to minimise / eliminate

the need for mechanical cooling. In the absence of mechanical cooling, comfort conditions

can be maintained in applications with low to moderate cooling loads.

Many buildings have false ceilings and floors that act to thermally isolate slabs from the

conditioned space, i.e. they are hidden. To achieve thermal transfer between the slab and

the space, air can be used as the transfer medium and circulated between the ceiling / floor

void and the space. However, the cooling performance of these types of system is limited due

to poor rates of heat transfer between the circulating air and the slab, typically 2 to 3 W/m2K

(1).

Improving surface heat transfer between the circulating air and the thermal mass has the

potential to significantly improve the performance of systems using floor and ceiling voids.

FaberMaunsell and Corus have worked collaboratively to research and develop one such

concept aimed at achieving this. The concept is to attach sheeting elements to the slab surface

in the void and circulate air by a fan through the narrow paths formed by sealing along the

edges (Figure 2). Air flows in through the gaps the ends and out through a spigot connection

in the middle (see arrows). The sheeting elements are made of 0.9 mm sheet steel. The

elements themselves are not intended to provide a significant amount of thermal storage. The

turbulent air flow created through the paths enhances heat transfer between the slab surface

and the circulating air. The system is currently referred to as CoolDeck.

Figure 1: Thermal mass + night ventilation Figure 2: CoolDeck element

2. Development

A key parameter in the design of the sheeting elements is the air flow velocity and the

height of the air path. Reducing the height of the air path increases the air velocity and flow

turbulence and so in turn the rate of heat transfer. However, pressure drop and heat transfer

are linked by the common underlying mechanism of turbulent air movement. The penalty of

increasing the turbulence of the air to enhance heat transfer is an increase in the differential

pressure and hence of the fan energy required to drive the air movement. The height of the air

path was therefore designed so that a reasonable balance was struck between heat transfer

enhancement and fan energy consumption.

Admittance values (2) are a measure of the conductivity of a material under dynamic heat

flow conditions. For a 24 hour charge/discharge cycle a dense concrete slab will typically

have an admittance value Yc of 10 to 20 W/m2K for heat flow in the material itself (ie

ignoring surface heat transfer). This compares with a surface convective heat transfer

coefficient hs which is normally in the region of 2 to 3 W/m2K. The overall heat transfer

coefficient has between the air and the slab (hs at the surface and Yc for the material itself

considered as conductances in series) can be approximated by:

has = (hs x Yc) / (hs + Yc)

Normally hs

Figure 3: Test rig (numbers denote measurement points)

Figure 4: Test results

UK CIBSE Guide C4 (3) equations C4.5 and 4.8 have been used to calculate the friction

factor, taking into account surface roughness. Equation C4.4 was used to calculate the

pressure drop per unit length. Heat transfer coefficients for the smooth tube case have been

calculated from UK CIBSE Guide C3 (4) equation C3.21:

Nu = 0.023 x Re0.8 x Pr0.33

Where:

Nu is the Nusselt number

Re is the Reynolds number

Pr is the Prandtl number

The Reynolds-Colburn analogy (5) was used to calculate the heat transfer coefficient for a

rough tube:

St.Pr2/3 = f/2

Where:

St is the Stanton number hc/(r.u.cp)

Pr is the Prandtl number

f is the friction coefficient

hc is the surface convective heat transfer coefficient (W/m2K)

r is the air density (kg/m3)

u is the air flow velocity (m/s)

cp is the air specific heat capacity (J/kgK)

As can be seen in Figure 4, the test results lie between the two theoretical extremes. To cope

with uncertainty in the surface roughness a worst case approach has been used for subsequent

design work assuming smooth tube values for surface heat transfer coefficients and rough

tube values for pressure drop.

3. Implementation

A system has subsequently been installed in Stevenage Borough Councils offices (Figure 5)

as part of the Councils Best Value approach to dealing with office comfort conditions. The

offices had been suffering from summer time overheating as a result of increased use of

computers. Rather than resort to a fully air conditioned solution it was decided to try to

implement passive measures where feasible and effective.

A combination of thermal mass and solar blinds have been used to achieve comfort

conditions. Exposing the soffit by removal of the false ceiling was not favoured due to

aesthetic, coordination, acoustic and winter heating concerns (6). The CoolDeck solution was

therefore adopted to utilize the mass hidden in the ceiling void.

During the summer, cool outside air is introduced into the offices by window fans at night

(Figure 6). Ceiling fans circulate this air under the CoolDeck elements to store the cooling in

the thermal mass. During the following day, the CoolDeck ceiling fans operate to release the

stored cooling.

Figure 5: Stevenage Borough Council offices Figure 6: CoolDeck concept

The ceiling void is approximately 220 mm deep. A modular arrangement has been adopted

with four CoolDeck elements per circulating fan. This is suitable for the zone-by-zone

approach used for this building. The CoolDeck elements are sized at approximately 1800 mm

x 450 mm. The elements are constructed from sheet steel with returned edges along the length

for rigidity. A spiggot connection is located centrally for connection to the fan system.

Spacers/seals are fixed along the underside edges to form and seal the air path from the

spiggot to the ends of the element. All the other components and fittings used in the system

are standard. Installation of the system was undertaken by Stevenage Borough Council.

Estimated costs are in the region of 40/m2, compared to an estimated cost for the air

conditioning option of 180/m2.

Monitoring was undertaken in areas throughout the building in the summer before the

refurbishment works were undertaken and again in the following summer with the remedial

measures in place. Results for one of the west facing areas with the system installed are

shown in Figures 7 and 8 to give a comparison of thermal conditions before and after.

These indicate that a reduction in the region of 5 K in internal temperatures has been achieved

relative to ambient temperatures. This is consistent with the modelling predictions from the

initial thermal analyses. Approximately 12 K of the reduction is attributed to the solar blinds,

the remaining 3-4 K to the CoolDeck system operating in conjunction with night ventilation.

West zone before

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Figure 7: Monitoring before

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Figure 8: Monitoring after

In addition to the reduction in temperatures there is also appreciable air movement created by

the CoolDeck system. It has also been noted that the areas with night ventilation are fresher

in the mornings. This perceived improvement in indoor air quality has been attributed to the

night ventilation.

The design Coefficient of Performance (cooling / fan energy) is in the region of 20. This is

due to system pressure drops being kept to a minimum through the use of window fans to

bring outside air into the space and small modular systems to recirculate air between the space

and the CoolDeck elements (fans specified to circulate approximately 5 l/s/m2 @ 30 Pa).

In subsequent refurbishment work, phase change material has been integrated with the

CoolDeck elements to increase their thermal storage performance (Figure 9). The phase

change material is a salt that changes phase between approximately 20oC and 24oC. The phase

change temperature needed to be high enough such that it could be frozen by summer night

ambient temperatures, and low enough that it could provide a cooling effect to the occupied

space.

The quantity of phase change material has been selected to provide approximately the same

thermal capacity as the slab, doubling the storage capacity of each of the CoolDeck elements.

The phase change material is contained in flat pouches approximately 10 mm thick (5.8 kg

per element). These lie in the CoolDeck elements so that the air passing through the gap

exchanges heat with the slab above and the phase change material below.

The cost of the system using the phase change material is approximately the same as that for

the system without the cost of the phase change material being offset by savings due to a

reduction in the number of elements and fittings. The main benefit to the client of using the

phase change material was the reduction in the number of elements and ductwork that needed

to be coordinated into a limited ceiling void.

Use of the phase change material in this application has proved cost effective due to the high

rates of surface heat transfer being achieved by the CoolDeck elements. These ensure that

energy storage per m2 is a lot higher than say for a phase change material exposed in the

occupied space. The phase change material is used more effectively and less area is required,

making it a cost effective solution. Initial monitoring results indicate that the system is

working well (Figure 10).

Figure 9: Integration of PCM

PCM System Monitoring

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Figure 10: PCM System monitoring results

Acknowledgements

The author would like to thank the following for their contribution to the work; Corus, Senior

Hargreaves, Stevenage Borough Council, Climator.

References

1. Barnard N: Dynamic energy storage in the building fabric, BSRIA Technical Report

TR9/94.

2. CIBSE Guide A: Design Data, section A3: Thermal properties of buildings and

components, 1986.

3. CIBSE Guide C: Reference Data, section C4: Flow of fluids in pipes and ducts, 1986.

4. CIBSE Guide C: Reference Data, section C3: Heat transfer, 1986.

5. Holman J P.: Heat transfer, 6th Edition, McGraw-Hill 1986.

6. Barnard N, Concannon P, Jaunzens D.: Modelling the performance of thermal mass, BRE

Information Paper IP 6/01, Garston, Watford, Construction Research Communications Ltd,

2001.

Further Reading

1. Arnold D: Building mass cooling case study of alternative slab cooling technologies,

CIBSE National Conference, Harrogate, October 1999.

2. Turnpenny J R, Etheridge d W, Reay D A, Novel ventilation system for reducing air

conditioning in buildings Part 2: testing of a prototype, Applied Thermal Engineering, 21,

1203-1217, 2001.

Thermal Mass and Night Ventilation Utilising HNick BarnardFaberMaunsellSummaryAcknowledgementsReferencesFurther Reading