Thermal Labyrinths in Sustainable Design White Paper by Green Leaf Engineers

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THERMAL LABYRINTHS IN

SUSTAINABLE DESIGN

A Green Leaf Engineers White Paper by Rob Dickie & Mark Kinsella

mailto:[email protected]

THERMAL LABIRYNTHS IN SUSTANABLE DESIGN

Page 2

ABSTRACT

In a world where the refocus on energy consumption is in vogue and modern buildings are

designed to respond to demonstration sustainable technology, or to show off ‘green credits’,

some designers may have forgotten about returning to traditional methods of conditioning

spaces. The ideas may be old but coupled with modern engineering technology and the use of

predicative thermal software simulation packages, the results can be quite startling.

This technical paper will showcase the use of three thermal storage systems working with nature

to provide low cost, sustainable comfort control. There is nothing new in the building physics

behind these ideas - we are just using engineering to modernize them;

Project Elephant – Nairobi Kenya: A traditional subterranean thermal

labyrinth within a courtyard.

British Council – Dubai, United Arab Emirates: The old premises are to be

demolished. We have re-used the rubble to create a pre-cooling heat

exchanger to reduce air conditioning costs.

Ski Resort – East Jindabyne, Australia: Taking a traditional chimney and

modernizing it into a solar collector which preheats the fresh air.


Page 3

CONTENTS

INTRODUCTION 4

PHYSICAL MATHS 7

COMPUTATIONAL FLUID DYNAMICS 9

PROJECT EXAMPLES AND RESULTS 10

1.1 Project Elephant – Nairobi, Kenya 10

1.2 British Council – Dubai, United Arab Emirates 13

1.3 Ski Resort – Jindabyne, NSW, Australia 15

CONCLUSIONS 17

GLOSSARY 18

REFERENCES 19

AUTHOR BIOGRAPHIES 19


Page 4

INTRODUCTION

A material’s ability to be able to retain heat or coolth is described by the phrase ‘thermal mass’. A

materials reaction to temperature is largely measured by its’ specific heat capacity (J/kg.K).

However, to measure the thermal mass or simple heat capacity of a material, or composition of

materials, the physical quantity of that substance must be considered. For example; concrete

(standard density) has a similar specific heat capacity to toughened glass, yet clearly the

concrete has a higher ability to retain heating energy over time.

Some liquids have disproportionately high specific heat capacities, especially when they change

state, liquid to solid or liquid to vapour. Salt or eutectic mixtures are now used across a wide

range of applications from industrial process cooling to everyday use home heat packs.

The technique of storing heat or coolth within a thermally massive structure has been used to

modify the local climate and to provide better comfort levels in the built environment for over two

million years. Societies devoid of the ability to artificially heat or cool their local environment

through the combustion of fuels relied on passive techniques to gain higher levels of comfort.

Perhaps early man’s desire for enhanced comfort drove the process of evolution from the forest

to the cave.

Aztec Adobe Structures, Iranian Ice Ponds and Persian Wind Catchers are some early examples

of man’s ability to harness the local climate and use attributes of it to improve his comfort. These

innovations are perceived to work well because most of the work in attenuating local climate was

already being done by thermally massive construction techniques. The new ‘technology’ just

made the living conditions that little bit better.

The question is, “Why has it taken us so long to understand and harness the benefits of thermal

mass and use it in a range of applications". The Trombe Wall Technique was documented as

early as 1956, and whereas thirteen years later man had developed technology to land on the

moon – progress with ‘thermal mass’ was not as dynamic. Perhaps mankind’s fascination with

industrialisation and consumption of resources in the pretext of development was the

predominant agenda.

It is only recently through our understanding of the effects on our climate, through the

consumption of fossil fuels and the effects of ‘sick building syndrome’ on building users, that we

have begun to relook at some of the passive techniques of improving occupant comfort.

One of these techniques is using the inherent thermal mass of a building to provide a more

constant internal temperature. A well thought out orientation and degree of thermal mass will

allow a higher degree of control over peak temperatures and thereby offer significant reductions

in required plant capacity and inherent capital and energy costs. In the diagram overleaf, the

reduction in plant capacity can be seen in the attenuation of peak temperature whilst the

reduction in energy consumption can be estimated by the area under each curve from the base

condition (dotted line).


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Figure 1: The effects of thermal mass on room temperature.

The room temperature is responding to ‘charge’ and ‘discharge’ phases of the thermal mass. As

cooling energy is released into the space during the day, the thermal mass is said to be in a

discharge phase. When it is in a charge phase, it is absorbing the coolth from the time of the day

when the outside air is at its coolest.

A further refinement on this energy model is to alter the sinusoidal swing of the internal room

condition by increasing the amount of cooling provided to the room during charge phase. This

can be done naturally or mechanically but is reliant on an increased amount of air flow passing

over the thermal mass. The increased air flow effectively increases the face velocity of the air

adjacent to the solid mass and thereby increases the amount of convective heat exchange. Doing

this at optimum times of the day and year allows a greater amount of cooling energy charge and

a better thermal mass effect.

Delivering a project which allows the occupants to be connected to the buildings thermal mass is

difficult and reliant on a number of additional design parameters; acoustics, aesthetics, fire

protection, security, occupied hours etc. The successful integration of thermal mass effects into a

building is purely reliant on the buildings ability to act as a heat exchanger.

As an average across most climate regions it is considered that 80% of the amount of heat

exchange occurs within the first 30mm of the thermal mass (typically concrete or brick). In

addition, if the night cooling, charge, stage is to be effective, the air adjacent to the planar

surface should be moving at approximately 3m/s to generate the convective heat transfer

coefficient large enough to absorb heat from walls and soffits. These two factors make buildings

poorly shaped heat exchangers. Thereby understanding the holistic energy picture of the amount

of cooling energy produced against the amount of mechanical energy required, the decision to

use the building as a heat exchanger becomes counter intuitive.


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Effective methods of improving this status quo have been used successfully over the last few

decades. For example, the Eastgate Office Building in Harare, Zimbabwe shows how air is pushed

through a labyrinth of precast concrete shapes embedded at each level underfloor. This evolution

of thought process begins to decouple the building as a heat exchanger and utilises an element

of ‘extruded or removed’ thermal mass to provide the cooling benefit. This system now has a

number of advantages over its predecessors; the building is no longer solely reliant on exposed

thermal mass (acoustics, aesthetics etc.) and the performance of the thermal mass is

predictable and efficient.

Improvements to thermal and energy modelling software have allowed designers the ability to

desktop model a range of particle beds to be able to design the most effective labyrinth

structures for a range of building locations, types and configurations.


Page 7

PHYSICAL MATHS

The following mathematical model is born out of the first principles of conduction and convection

heat transfer equations; it has been taken from Yang et al [2]. It provides an explanation of final

temperature values within a simple space that utilises thermal mass. The whole envelope of this

system is insulated and the temperature distribution within the space is even. Internal and other

heat gains are neglected;

Two heat balance equations for the system are shown below:

(1)

(2)

Sub equation (2) into (1):

(4)

(5)

From equation 1 we know that:

(6)

Using this replace Ti with Tm:

(7)

(8)

(9)


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For a simple space, the flow of the air past the mass affects the change in temperature. It is

known that by maximising turbulence within the space heat transfer will also increase. Once this

simple system has been established, modifications can be made to the design to suit the

environment it is to be applied to. In a more complex system with a number of air paths the heat

balance equations evolve as follows:

(10)

(11)

h2A2... and h3A3... represent different surfaces of thermal mass. By subbing equation 11 into

equation 10 we have:

(12)

Following the order of equation 7 to 9 a general solution can be again found:

When equation 11 is subbed into equation 10, heat transfer coefficients remain in the general

solution. The heat transfer coefficient is determined by the Reynolds’s number. Therefore a highly

turbulent flow will result in a greater heat transfer. Equations governing the heat transfer

coefficient will be explained in detail in section 4 (British Council).

This model has been tested against real life data accrued (over a period of one year) from two

projects; Eastgate and the Harare International School, both in Harare, Zimbabwe.


Page 9

COMPUTATIONAL FLUID DYNAMICS

Computational Fluid Dynamics (CFD) is a finite element technique that enables a user to study

the dynamics of a fluid’s flow. Mass, momentum and energy conservations of the flow are solved

explicitly at each intersection on a specified grid domain. At each grid point a detailed set of

solutions of flow variables are given. CFD is a sophisticated computer based design technique

that adds detail and efficiency to a design process.

CFD has been incorporated into the design process of the following thermal labyrinth systems.

These models are relatively simple allowing a realistic set of solutions to be found. Focus has not

been on eliminating all error from the meshing of the models but rather providing a general, yet

accurate set of solutions. The CFD component from the program IES Virtual Environment was

used to create models of the 3 systems used in the project examples. Boundary conditions were

set in each of the models and a k-ε turbulence model was applied to the system. Due to the

simplicity and symmetry of the models the k-ε was seen to be sufficient to express the convective

heat flux of the turbulent flow.

The output temperatures given from the CFD modelling were compared to mathematical results.

There has been relatively little variation between both results which allows the CFD results

produced to be put forth as an accurate representation of the heat transfer within the system. It

is concluded that the insight gained by the CFD analysis is decisive for arriving to an improved

design in this application.

In all cases, physical models and real life testing were offered to clients as a further degree of

certainty and to solve any of the potential difficulties in the construction process before final

installation occurred. In each instance this option was not exercised, primarily due to the

accuracy between the numbers of the spreadsheet analysis and the computational fluid

dynamics.


Page 10

PROJECT EXAMPLES AND RESULTS

Three projects have been selected to showcase the opportunities for the reduction in cooling and

heating energy using thermal mass effectively. They have been selected as they represent

differing climate, building types, construction skill levels and particle bed set-out

Each of the projects use nothing more than the efficient use of diurnal temperature swings and in

some instances solar radiation, to make large reductions in energy consumption.

1.1. Project Elephant – Nairobi, Kenya

This is a 3000m² mixed use development in Nairobi, Kenya. It is a secure building with very

specific design criteria for security and communication. It is on a green field site with full access

to local utilities.

The climate in Nairobi is conducive to natural ventilation techniques however a restrictive

security brief dictated the overall massing of the building and the degree of ‘openness’.

Technology utilised in the project had to be suitable for the skill of the local contractor and more

importantly the local facility managers.

Figure 2 - Project Elephant, Sunken courtyard (air intake)

A sunken courtyard was introduced to allow natural light to basement levels; this provided the

opportunity to create a passageway for fresh air to the basement air handling plant. A thermal

labyrinth within the steps of the courtyard auditorium was created which used not only the

diurnal swing of the outside air but effectively coupled the steady ground temperature into pre-

cooling outside air.

A series of concrete baffles were designed to provide the structural framing required whilst

partially obstructing air flow and increasing the degree of turbulence and thereby heat transfer.


Page 11

To create a mathematical model which would replicate the heat transfer process in this particular

particle bed the base mathematical model in equation 18 used specific variables determined

from equations 13 - 16 to give a more precise estimation. Theory from the flow of fluids across

banks of tubes is taken from Incropera & DeWitt’s Fundamentals of Heat and Mass Transfer [1].

This theory is relevant to numerous industrial applications. Flow conditions within the bank are

dominated by boundary layer separation effects and by wake interactions, which in turn influence

convective heat transfer. In this case the cement baffles within the duct will be considered as the

tubes in the cross flow. Relevant conversion co-efficient have been applied to this model.

The design uses the following formula:

(13)

(14)

(15)

Where C and m are constants used for non-cylindrical tubes in cross-flow.

(16)

We can rearrange to find h:

(17)

(18)

This can be re-arranged to be:

(19)


Page 12

Taking hourly weather data for Nairobi, T inlet, and using a spreadsheet based analysis we can

predict the temperature of the air entering the building and thereby quickly show the effect of

cooling. During the peak summer months we have effectively cooled our supply air by 4.6C. This

equates to a reduction in peak cooling load of fresh air by 42%. Moreover, during mid seasons,

we are able to passively cool the entire building using several tiers of the sunken courtyard as we

increase the air volume flow rate and reduce mechanical cooling.

Figure 3 – Spreadsheet analysis of thermal labyrinth performance and verification using CFD.

10

15

20

25

30

35

0 10 20 30

Tem

pe

ratu

re (

de

g.C

)

Time (hrs)

January (summer) labyrinth performance

Tinlet

Toutlet


Page 13

1.2. British Council – Dubai, United Arab Emirates

The current facility needed modernizing and thus the British Foreign Office required the existing

development to be demolished with the new facility to be rebuilt slightly larger at 2500m². The

building was envisaged to be predominantly a teaching facility with ancillary administrative

offices, the new British Council in Dubai was to touch on sustainability but within the confines of

a very limiting budget. The scheme involved the reuse of the concrete blocks from the demolition

phase which instead of transporting off site to landfill, we used their embodied thermal mass

characteristics by placing them in pits in the ground and blowing air across them.

The resulting particle bed was an extremely compact and effective heat exchanger. The size of

particle was critical in balancing fan power, cooling capacity and thereby energy savings. Outside

air would be pre-cooled by the block store and supplied to the roof mounted air handling plant

before being supplied to the building.

This block store utilises the theory of a packed bed of solid particles taken from Incropera &

DeWitt’s Fundamentals of Heat and Mass Transfer [1]. Within the packed bed a large amount of

heat or mass transfer surface area can be obtained in a small volume. The irregular flow that

exists in the voids of the bed enhances transport through turbulent mixing.

For a packed bed:

(20)

where

(21)

By using Reynolds’s analogy that:

(22)

We can rearrange to find h:

(23)

(24)

This can then be used in the heat equation where:

(25)

To find the log mean temperature variable we can use the following equation:

(26)

This can be re-arranged to be:

(27)


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This mathematical model is used to find the outlet temperature when the surface and inlet

temperature are known. Figure 4 provides a graphical representation of the outlet values found

within the rock store.

Figure 4– Spreadsheet analysis of block store performance and verification using CFD.

Again, by using weather data to predict outside air temperature, a spreadsheet analyisis can

quickly and accurately inform on the sizing of particles and therefore the density of the block

store. Once this has been designed to a scheme design standard and cost of implemetation

understood, finite element analysis in the form of CFD can then be used to fine tune the results.

In this instance the degree of variance from the mathematical model to the CFD analysis was

less than 5%.

Attenuating the peak of Dubai’s hot summer days through the reuse of perceived deleterious

material from a demolition phase currently saves $6000/year in the reduction of electricity costs.

It has reduced the peak cooling load of the chiller by over 35%.

With an estimated pay back period of around 2-3 years, it is anticipated that the three block

stores will save $545,000 over an expected 20 year building lifetime.


Page 15

1.3. Ski Resort – Jindabyne, NSW, Australia

East Jindabyne is the locality for the redevelopment of an existing Motel into a much larger 5 Star

Resort comprising of a central hotel, lodges, holiday homes and permanent residences.

The intention is to develop an eco-village which is world class but not a demonstration product.

The brief for the development is that it should achieve a low carbon footprint effortlessly.

A two and three story structure was originally modelled as the best surface to volume ratio and

suited the topography of the site. The views and therefore glazing are located on the southern

aspect of the villas and a circulation staircase to the north. This orientation did not lend itself to

traditional passive winter solar gains.

Figure 5 - South Eastern aspect

Traditional Ski Lodges use stone or masonry central fire places - not only to heat the immediate

spaces at night times or in very cold weather, but also to re-radiate heat throughout the dwelling

long after the fire has been extinguished.

A similar philosophy was adopted but the ‘combustible fuel’ was solar and the chimney was at

the rear, north facing stairwell.

A solar chimney works by allowing solar gain to heat thermal mass to a higher temperature than

would be experienced if that surface was exposed to the outside air temperature. Cold, outside

air for the villas could be drawn in at the lower level of a solar chimney, then heated and supplied

to the villa from high level. Designing the stair treads to be open ended and metallic added a

degree of turbulence to the air stream and being made from a conductive material this further

picked up the solar gain very quickly. The balance of the metal treads of the stair core and the

thermal mass of the surrounding structure created a unique solar labyrinth.

By using mathematical models developed for labyrinths and adding a component for radiant

gain, an appropriate model for this system can be developed.


Page 16

The heat equations for the system are shown below, note a radiant gain is added:

(28)

The Radiant gain can be written as:

(29)

To linearise this:

(30)

(31)

Sub equation 29 into equation 28:

(32)

(33)

Figure 6– Spreadsheet analysis of solar labyrinth performance and verification using CFD.


Page 17

CONCLUSIONS

The opportunities involved with the utilisation of Thermal Labyrinth Systems have been

presented. Applications and environments that such systems can function within are extremely

diverse, as shown with the three projects: Project Elephant, Jindabyne and the British Council,

Dubai.

The benefits associated with the three systems were also found and proved mathematically. By

finding the temperature difference of the air as it had passed through each system, a value of the

energy reduction could be found. To find this temperature difference, heat balance equations

and basic principles were used. Within each system, specific fundamental theory was applied,

relevant to the physical set-out of each system. Project Elephant employed theory associated with

the cross flow of fluids on a bank of tubes. Small discrepancies can be found as the set-out of

this system does not exactly match the set-out of a bank of tubes in cross-flow, however these

are negligible. Jindabyne used the heat balance equations as well as incorporating a radiant gain.

The British council used the theory associated with packed beds. Within all systems a

temperature difference was found during the day-time when occupancy loads were at a

maximum allowing a reduction of energy to be a product of incorporating each system to the

building.

CFD was used to validate the temperature outputs occurring in these mathematical models. It

was employed to back-up the methodology in finding each outlet temperature. The results given

from the CFD closely matched those of the mathematical models. A realistic idea of each

system’s performance was provided through a graphical representation of the temperature

contours.

The present work provides a detailed study into how thermal mass can be used to reduce energy

consumption. Although, the systems analysed are very basic in the concept of utilisation of

thermal mass, through progression of this concept and by using further adaptations to design,

this technology can be employed to ultimately reach a universal goal of maximising sustainable

design within this industry.


Page 18

GLOSSARY

- Interior surface area of mass (m2)

- Bed channel cross sectional area in packed beds (m2)

- Area of particles in packed beds (m2)

- Heat capacity of mass (J/kg K)

- Heat capacity of air (J/kg K)

- Diameter (m)

- Interior convective heat transfer coefficient (W/m2K)

- Radiant heat transfer coefficient (W/m2K)

- Colburn j factor of heat transfer

- Mass (kg)

- Number of tubes in bank of tubes

- Number of tubes along length of bank of tubes

- Nusselt numbers

- Transverse number of tubes in bank of tubes

- Prandtl number

- Heat transfer rate (W)

- Ventilation rate (m3/s)

- Radiant heat gain (W)

- Reynolds’s number

- Transverse distance between tubes in cross flow

- Stanton number

- Log mean temperature difference

- Temperature of inlet (K)

- Temperature of mass (K)

- Temperature of outlet (K)

- Time (s)

- Fluid velocity (m/s)

- Emisivity

- Porosity of packed bed

- Viscosity (kg/s.m)

- Density of air (kg/m3)

- Stephen-Boltzmann constant

Subscripts

- diameter

- Maximum fluid velocity

- Surface


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REFERENCES

Incropera F.P, DeWitt D.P. 2002. Fundamentals of Heat and Mass transfer. John Wileys & Sons,

New York.

Yang, L, Li, Y 2008. Cooling load reduction by using thermal mass and night ventilation.

Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong

Kong, China.

AUTHOR BIOGRAPHIES

Rob Dickie

BEng (Hons) CEng EurING MCIBSE RPEQ

"A quality building services strategy works in complete synergy with the built form. The results

are not only cost effective but sustainable."

Rob is a founding director of Green Leaf Engineers. His career began in Zimbabwe, where his

training and experience as an Energy Engineer pushed him to the cutting edge of a developing

industry called Building Services. He has since worked in Europe, the Middle East and now in

Australia.

He works front end with developers, architects and cost consultants to develop and deliver

projects which are technically advanced and unique.

He is highly dedicated and proactive in incorporating sustainable aspects in projects of every type

and scale, believing that a sustainability agenda is often the driver for lean, cost effective design.

Rob is a guest lecturer at London’s South Bank University and is a frequent contributor to

industry journals, including Construction Weekly in Dubai and Australia’s Architectural Review.

Mark Kinsella

IMechE

Mark is in his final year of a mechanical undergraduate degree at the University of Queensland.

Mark joined the Building Services team at Green Leaf Engineers when the company was formed

in April. Mark’s abilities to reduce complex engineering problems into simple manageable steps

shows his technical prowess and thought maturity, which is deemed far ahead of his peers.

Mark has a keen interest in sustainable design within the building sector, and is currently

completing a thesis on the optimisation of the design of an active thermal mass system.

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Thermal Labyrinths in Sustainable Design White Paper by Green Leaf Engineers