S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671456

per year, while the CO2 reduction was estimated at 82 million

tones per year [4].

According to the Environmental Protection Agency [5] an

average household spends over 40% of its annual energy budget

on heating and cooling costs. On the other hand, office

buildings account for about one-third of all the energy used in

the US, a quarter of which is lost through the inefficiency of

standard windows to retain heat in the winter or deflect heat in

the summer. While regular glass can only allow a constant

amount of light, the switchable window can be tuned or

dimmed, controlling the amount of light passing through. An

estimate of the expected savings in heating, lighting and air-

conditioning costs is $ 11–20 billion per year. According to the

EPA, even a $ 7 billion saving would equate to a reduction in

carbon emissions at power generating plants equal to taking

336,000 cars off the road, while the energy savings would be

enough to light every home in New York City.

1.1. Switchable glazing: state-of-the-art

Liquid crystal (LC) glazing operate on the principle of

electrically controlled light scattering. They consist of liquid

crystal droplets surrounded by a polymer mixture sandwiched

between two pieces of conducting glass. When no electricity is

applied the liquid crystal droplets are randomly oriented,

creating an opaque state. When electricity is applied the liquid

crystals align parallel to the electric field and light passes

through, creating a transparent state. The switch between the

two states is nearly instantaneous. LC windows are hazy

because they scatter rather than absorb light, so there is a fog

factor even when the device is in the transparent state. An LC

glass is either clear or opaque with no in-between states, and

merely scatters light rather than blocking it, which limits it to

certain interior privacy applications. Furthermore it requires

constant energy to maintain the clear state, and thus no energy

saving benefits are provided.

Suspended particle (SPD) smart windows are constructed by

using two panes of glass separated by a conductive film with

suspended, light absorbing, microscopic particles. Microscopic

light-absorbing particles are dispersed within a thin film. When

no electrical voltage is applied to the film, these particles absorb

light, making the glass dark. When voltage is applied, the

particles align and allow light to pass through. By simply

adjusting the electrical voltage manually or automatically, the

amount of light passing through the SPD-glass product can be

controlled. One important disadvantage is that electricity is

required to keep the window transparent.

An electrochromic (EC) window is an active solar control

device whose transmittance in the visible and near IR part of the

spectrum can be reversibly modulated by the application of a low

voltage (typically 3–5 V dc). EC windows function as the result

of transport of charged ions (typically Li+) from an ion storage

layer, through an ion conducting layer into an electrochromic

layer. Electrons are also injected from the external circuit into

this layer for charge equilibration, changing its electronic density

and its optical properties, causing it to absorb visible light, the

result of which is the window darkening. To reverse the process,

the voltage is reversed, driving the ions (and electrons) in the

opposite direction, out of the electrochromic layer, through the

ion conducting layer, and back into the ion storage layer. As the

ions migrate out of the electrochromic layer, it brightens (or

‘‘bleaches’’), and the window becomes transparent again.

An EC window has several advantages compared to

conventional shading and solar control devices. It does not

impede visibility through the window as with blinds and

curtains, while it provides glare control and thermal comfort

management. It has no moving parts and as a result, minimum

maintenance costs. It requires a low voltage power supply (it

can even be powered by photovoltaics) and it can be integrated

into the central power management of the building. It has

practically infinite coloration stages. It can block both direct

and diffuse solar radiation, unlike passive shading devices.

Unlike tinted glass it can become transparent during the early

morning and afternoon hours to improve natural lighting

conditions. Furthermore, it has a low energy consumption

(typically 8 W m�2), which is nearly zero when the glazing is

kept at constant conditions. This is due to the very high open

circuit memory of the device [6]. An EC window can out-

perform the best currently available window systems (in most

applications) and has lower annual energy demand than an

opaque insulating wall [7]. The primary energy benefits are:

reduced cooling, heating and ventilation loads and the ability to

displace artificial electric lighting use by managing daylight

admittance. Additionally the architectural and aesthetic appeal

of a dynamic coating that an EC technology offers is difficult to

quantify but it will be a major contributing factor for its

selection for many building applications. Many design

decisions are made not only on the basis of ‘‘payback’’ but

also on the basis of style and appearance.

Several companies worldwide have announced the develop-

ment of commercial EC glazing for architectural applications

during the last years [8–10]. The performance of commercial

devices is limited by several factors: they have rather limited

transmittance modulation (50–15% for EControl by Pilkington,

72–17% for Asahi and 55–8% for SAGEGlass1 [8–12]. Awider

range (e.g. 75–10%) would allow better control of solar gains.

They suffer from defects that form during extended operation:

these can be dark spots or pinholes (up to 3 mm in diameter and as

many as three spots per m2 [6]) that are no longer

electrochromically active, dust and metallic particles (due to

the preparation technique problems) which cause electrical

leakage and hence low production yields [8]. Large area devices

also suffer from uneven coloration and degrade after prolonged

operation (typically 5% degradation in maximum transmittance)

[6,10]. Furthermore, some of them have a limited lifetime: 6000

coloration/bleaching cycles per 5 years for Pilkington EControl

[6], the devices developed by Asahi are designed for a lifetime of

10 years [8], while SAGEGlass1 samples have expected lifetime

of 20–30 years [9]. As these devices have been produced during

the last few years, there are not yet available comprehensive long-

term operational performance data. Worldwide, there is not any

generally accepted standard test procedure for long life testing of

the EC devices or evacuated glazing, due to the inherent

complexity and the many parameters involved [10].

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1457

The EC devices fabricated thus far are integrated into

insulating windows that consist of a double glazing called IGUs

(Insulated Glass Units), coated with low emittance (low-e

hereafter) coatings and an argon-filled gap. Such structures

achieve mid-plane thermal transmittance (U-value) of about

1.1 W m�2 K�1. This value can be further reduced with

evacuated double glazing, in conjunction with the EC device

resulting in a 3-glass pane EC evacuated window.

1.2. Evacuated glazing

Evacuated glazing consists of two sheets of glass hermetically

sealed around their periphery with a vacuum gap between the

glass sheets. An array of tiny support pillars, typically 0.3–

0.5 mm in diameter, are used to separate the glass panes and

prevent the glass panes from touching due to atmospheric

pressure. Low emittance (low-e) coatings are used on one or both

of the internal glass surfaces to reduce radiative heat transfer

from the inside to the outside of the glazing as seen in Fig. 1.

Evacuated glazing have been fabricated with two sheets of

glass rigidly fused together around their periphery using a solder

glass with a coefficient of thermal expansion that matches that of

the glass sheets to be sealed [13]. The edge seal was made by

heating the entire structure to around 450 8C for 1 h and cooling

slowly to room temperature. Subsequent evacuation of the

window was achieved through a pump-out tube with a typical

internal diameter of 1 mm, attached to one of the glass panes [14],

after which the end was melted and sealed. Attempts to produce

evacuated glazing using high temperature laser based manu-

facturing techniques, were unsuccessful due to out-gassing

resulting from the sealing processes [15]. The use of high

temperature edge sealing techniques for evacuated glazing

restricts the choice of low emittance film, incurs high embodied

energy and precludes the use of tempered glass which would

suffer from loss of temper at 450 8C. A low temperature (less than

200 8C) hermetic seal would negate these issues.

Previous research [16,17] has led to the development of a

patented technique for the fabrication of evacuated glazing at

low temperatures. The technique uses indium or indium alloy as

the edge sealing material applied to the surfaces of the glass

sheets to be bonded together. Indium is a well suited sealing

material as it can be used to join materials with greatly

mismatched coefficients of thermal expansion subject to

thermal cycling and has the ability to ‘‘wet’’ or ‘‘cold weld’’

to metals, glass and ceramics. These characteristics are

fundamental in determining the effectiveness of a seal,

Fig. 1. Schematic representation of an evacuated glazing (not to scale).

allowing it to mould itself to the shape of the materials being

sealed together, thereby eliminating air-paths. Indium’s low

vapour pressure at ambient conditions makes it ideal for

vacuum seals, when the durability conditions to be met are

demanding and current polymer based products are inadequate.

1.3. Electrochromic evacuated windows

The authors of this work have been researching electro-

chromics, evacuated glazing and polymers for more than a

decade. During the last 3 years they have collaborated to

develop electrochromic evacuated glazing (see Fig. 2). In this

paper, we present the results of this collaboration. An account

of the materials and techniques that proved to be the most

promising is given. The performance characteristics of the

prototypes that have been developed are presented and

compared to that of competing technologies.

2. Development and fabrication of the electrochromic

device

In an effort to improve the electrochromic properties of the

glazing, various materials have been developed and their

properties have been assessed as shown in Table 1. The thin

films to be described in the following paragraphs have been

fabricated either by e-beam gun evaporation or thermal

evaporation. Their structure, morphology and physical proper-

ties have been assessed by advanced techniques (SEM, XPS,

FTIR, CV and GITT). Various substrates were used, such as

plain glass, ITO coated glass, Pilkington K-GlassTM, alumi-

nium foil and c-Si wafers, depending on the material and the

diagnostic technique used. The vacuum chambers and the

deposition parameters were calibrated for each material.

2.1. Transparent electrical conductors

Transparent electrically conductive coatings are crucial for

the operation of EC devices, as they convey electrical signals

from the external circuit leading to coloration/bleaching. An

optimum transparent conductor for an EC device should exhibit

high electronic conductivity – low sheet resistance and

minimum obstruction to visible light.

Two groups of materials are of particular interest for use as

transparent conductors: (i) doped metal oxides and (ii)

combination films which incorporate metal layers [18,19]. In

most of the fabricated EC devices a SnO2:F coated glass called K-

GlassTM commercially available from Pilkington was used. This

product was found to have the best performance to price ratio as

compared to other similar products (such as ITO, In2O3:Sn).

2.2. Active electrochromic film

The properties of the active electrochromic film play the

most important role in the overall performance of an EC device.

An ideal electrochromic film should have large coloration

efficiency, sufficient ionic and electronic conductivity so as its

coloration/bleaching time is acceptable, cycling reversibility,

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671458

Fig. 2. Electrochromic evacuated glazing.

stability and durability. All these have to be taken into account

to decide which material is the most suitable for viable EC

devices. Molybdenum oxide films exhibit inferior cycling

stability compared to WO3, as shown in Table 1, and thus,

tungsten oxide has been chosen for the development of the EC

component of the glazing [19–21].

2.3. Ion storage and protective layers

The purpose of an ion storage layer is to temporarily store

the ions that color the active electrochromic film. The ion

storage film accepts and stores ions in the bleached state of the

EC device improving its performance. The ion storage film can

Table 1

properties of optimised materials for electrochromic devices

Material Preparation method Thickness

(nm)

Coloration

type

Char

(mC

EC layer

WO3 e-gun, sputtering, sol–gel 350–500 Cathodic 20–4

MoO3 Thermal evap.,

sputtering, sol–gel

300–400 Cathodic �20

TC

SnO2:F Spray pyrolysis >1000 N/A N/A

In2O3:Sn Spray pyrolysis >1000 N/A N/A

ZnS/Ag/ZnS e-gun, sputtering, 40/10/40 N/A N/A

Ion storage–protective layer

V2O5 e-gun, sputtering, sol–gel 300–500 Anodic 30 (m

CeO2–TiO2 Sputtering sol–gel 150–450 Passive 20–5

NiO Sputtering (low yield) 400 Anodic 1

CeO2 e-gun, sputtering, sol- gel 150–500 Passive 10

MgF2 e-gun, sputtering 150–200 Passive 5

exhibit complementary coloration to the EC layer, also

contributing to the increase of the coloration efficiency of

the device. It may also be passive, with no significant change of

its optical properties. Of the materials tested Vanadium

Pentoxide (V2O5) and Magnesium Fluoride (MgF2) were

found to have the best performance for ion storage and

protection of the transparent conductor, respectively [19,20].

2.4. Electrolyte materials for electrochromic windows

applications

The approach for the development of electrolytes for EC

devices was to identify, prepare and study new and efficient

ge capacity

/cm2)

Luminous transmittance

(%) (bleached/colored)

Stability durability

0 80/10 Stable, more than 5000

voltammetric cycles

85/20 Unstable above 5000 cycles

�90 Hard coating, stable up to 350 8C�90 Hard coating, stable

�85 Soft coating, stable up to 250 8C,

optical interference problems

aximum) 70/60 Sol–gel films are unstable due

to phase transitions

0 80 10% reduction of charge capacity

after 300 cycles

70/50 Stable, 1200 cycles

90 Stable

95 Stable, optically neutral

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1459

polymer based materials. The aim was to optimise parameters

such as the molecular structure of the polymer, salt type and

concentration, and to compare with existing state-of-the-art

materials. A specific aim was to explore how to influence the

performance of the EC devices through a judicious choice of

the electrolyte. Two classes of transparent electrolytes were

focused upon. The first was a conventional, state-of-the-art gel

electrolyte system based on poly(methyl methacrylate)

(PMMA), lithium perchlorate salt (LiClO4) and propylene

carbonate (PC). In the second class of electrolytes we combined

Lewis acidic boron and Lewis basic ethylene glycols in the

molecular structure of the polymer. The latter electrolytes were

expected to favour the lithium transport number and the transfer

of lithium at the interface with the WO3 layer. In the present

work, the electrolyte materials were not only used as ion

conducting materials, but also as the only source of lithium ions

for the electrochromic material during the coloration process.

LiClO4 was used throughout the project because it showed

beneficial properties in terms of solubility, conductivity and

stability, and is relatively inexpensive.

2.4.1. State-of-the-art electrolytes

Electrolytes consisting of LiClO4, PC and PMMA typically

show a high level of ionic conductivity. Liquid electrolytes

were first prepared from PC and LiClO4 to contain 9–11 wt.%

salt. Gel electrolytes were then produced by adding 7 wt.%

PMMA to the liquid electrolytes. The gel electrolytes were

optically clear viscous liquids and reached the highest

conductivities of the present electrolytes, in the range of

3 � 10�3 S/cm at 30 8C.

2.4.2. Preparation and selection of boron-containing

electrolytes

A large number of polymers and liquid oligomers were

synthesised according to the general scheme shown in Fig. 3.

The polymers were prepared by using boron trioxide (B2O3) to

link different poly(ethylene glycol)s (PEGs) and triethylene

glycol monomethyl ether or diethylene glycol monomethyl

ether via the formation of boronate esters [22–24]. PEGs with

different molecular weights were used as molecular main chain

segments and the latter two glycols were used to introduce

branches or side chain segments. The influence of the polymer

structure on the properties of the electrolytes was assessed by

evaluating the ionic conductivity, thermal stability and optical

clarity [22]. The electrolytes exhibited practically the same

thermal stability range as the neat polymer, i.e. at least up to

150 8C. All the electrolytes were in the rubbery state at room

temperature, with the glass transition temperatures below

�50 8C. The conductivity was in the range between 2 and

8 � 10�5 S/cm at 30 8C [22–24]. A number of electrolytes

Fig. 3. Synthesis of boron-containing polymers via con

containing different polymers and salts were evaluated in the

optimisation process in which the electrolyte possessing the

most favourable balance of properties for use in electrochromic

applications was selected for further evaluation [22]. The

optimised electrolyte was designated B300T-LiClO4-34 and

was based on a polymer prepared from boron trioxide,

triethylene glycol monomethyl ether and PEG with a molecular

weight of 300 g/mol. It further contained 1 mol salt per 34 mol

ethylene oxide units of the polymer, corresponding to a salt

content of approx. 7 wt.%.

Liquid electrolytes were prepared using boron-containing

oligomers and different lithium salts. The interest of these

oligomers was linked to their use as plasticizers (or solvents) to

produce gel electrolytes in which both the ionic and boron

diffusion are faster than in the polymer electrolytes described

above. The synthetic pathway used for these oligomers was

similar to the one used for the boron-containing polymers. The

ionic conductivity of these electrolytes were found to be in the

range 5 � 10�5 to 1.6 � 10�4 S/cm at 30 8C. The low viscosity

of liquid electrolytes increases the risk of leakage from the EC

device. To increase the viscosity of the electrolyte, polymers

may be added to the liquid electrolytes to form gel electrolytes.

In the present work, gel electrolytes were produced by adding

PMMA and LiClO4 to the oligomers. The resulting gel

electrolytes were very sticky and viscous materials with ionic

conductivities at approximately 4.0 � 10�4 S/cm at 30 8C.

Detailed descriptions of the properties of the different boron-

containing electrolytes have been reported previously [22–25].

The preparation method is relatively uncomplicated and makes

use of inexpensive base chemicals. Furthermore the method

offered control over the viscosity and the mechanical properties

of the electrolytes. In general, the electrolytes were optically

transparent, sticky and thermally stable below 150 8C. The

evaluation of EC devices containing optimised boron-contain-

ing electrolytes and different lithium salts showed that devices

containing electrolytes based on LiClO4 had higher levels of

coloration and higher coloration efficiencies. The electrolytes

based on plasticizers and solvents showed interesting properties

of coloration, especially those containing boron. Finally it

should be mentioned that the presence of liquids in these

electrolytes made the stability of the sealing of the EC device

critical and problems of moisture ingress appeared and had to

be dealt with.

2.5. Proposed optimum electrochromic device

The materials development work presented above is

summarised in the following optimum configuration of the

EC device: K-GlassTM/350 nm WO3/0.25 mm B300T-LiClO4-

34 electrolyte/150 nm MgF2/K-GlassTM.

densation reactions of B2O3 and different glycols.

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671460

The materials selected for the above configuration exhibited

the best properties of all those tested in terms of cycling

stability, coloration speed, mechanical and optical properties,

as can be seen in Table 1. Their properties have been described

in previous paragraphs. The thickness of each layer has been

optimised experimentally.

2.6. Deposition of materials for the electrochromic

components

Tungsten oxide thin films were deposited on K-GlassTM

samples, which were used as the transparent conductor

substrates for the prototypes. The samples were cut to a

dimension of 40 cm � 40 cm and thoroughly cleaned before

the layer deposition. Pre-cleaning was done using a CE

certified infrastructure, while distilled water and chemicals

were used for the final cleaning. The thin films used for the

fabrication of the EC prototypes were deposited in a specially

designed vacuum chamber capable of preparing samples with

dimensions up to 40 cm � 40 cm. It consists of a stainless

steel chamber measuring 60 cm diameter � 90 cm height,

fitted with a multi-pocket electron gun, motorised rotating

substrate feedthrough and an in situ thickness monitor. All the

necessary vacuum components for the operation of the system

(pumps, feedthroughs, valves, pressure gauges, fittings,

control units, etc.) have also been installed. The chamber

design ensured a �10% variation in thickness across the

40 cm � 40 cm samples, which proved to be adequate to

prevent noticeable color deviations. In this chamber, more

than one layer could be deposited without interrupting the

procedure, thus allowing multilayer coatings suitable for

transparent conductors to be prepared. The deposition

parameters and the thickness of the films were monitored

in situ in order to achieve the desired properties. Some of the

above mentioned parameters (e.g. film thickness, substrate

cleaning, substrate temperature) were found to seriously

affect the quality of the produced samples. Optimisation of

these parameters was undertaken based on the prototype test

results.

3. Development and fabrication of the evacuated

glazing

3.1. Sample preparation and manufacture

Evacuated glazing were manufactured from two panes of

4 mm thick float glass, with a low-e coating on one or both of

the internal glass surfaces and/or a low-e coating on one of

the external glass surfaces. The use of a low-e coating on one

external glass surface allowed for the subsequent fabrication

of a 3-glass pane electrochromic evacuated glazing. The

glass panes were cut to a size of 40 cm � 40 cm or

50 cm � 50 cm and cleaned and dried. Each glass pane

was heated on a hot plate and the edge seal of widths from 6

to 12 mm applied. The glass panes were cooled to room

temperature and the array of support pillars positioned. This

consisted of stainless steel pillars, 0.4 mm in diameter and

0.15 mm high, arranged onto one glass sheet in a regular

25 mm square grid pattern. The pillar array ensured the glass

panes did not touch due to atmospheric pressure when the

space between the sheets was evacuated. The two glass

sheets were located together with the applied edge seal layers

on the internal glass surfaces.

The prepared glazing was located in a purposely designed

vacuum chamber [16] that facilitated, in a single stage process,

evacuation and sealing. The chamber was evacuated to a

pressure in the range 4 � 10�6 to 4 � 10�7 m bar and the

sample heated uniformly to the temperature required to melt

the edge seal material. The heating lamps were switched off

and the evacuated glazing cooled to room temperature. The

chamber was purged to atmospheric pressure and the glazing

removed.

3.2. Testing and modification of the fabrication process

A number of evacuated glazing were fabricated from two,

50 cm � 50 cm panes of 4 mm thick Pilkington K-GlassTM. An

initial visual inspection of the glazing under a diffused

fluorescent light revealed significant stress patterns around each

of the supporting pillars clearly indicating the presence of a

vacuum. The glazing, when tested for thermal performance in a

guarded hot box calorimeter, as described in Section 5.4, had

poorer than predicted thermal performance, even though well

defined stress patterns were still evident around the support

pillars. Samples produced from two sheets of Pilkington K-

GlassTM, 50 cm � 50 cm, had exposed glass U-values in the

region 2.5–3.5 W m�2 K�1.

The reduced thermal performance was due to out-gassing

from the internal glass surfaces leading to the gas pressure

increasing to a level at which gaseous conduction in the vacuum

cavity became significant. To resolve this, a number of changes

were made to the glass cleaning regime and the materials used

and further glazing were fabricated from two 50 � 50 cm panes

of Pilkington K-GlassTM. When tested they exhibited much

improved thermal performance with U-values down to

1.17 W m�2 K�1 for the exposed glass area. These were in

close agreement with the predicted simulation U-values of

1.1 W m�2 K�1. The cleanliness of the glass surface was

critical for the production of a durable, high performance

vacuum glazing.

During thermal testing the U-value of the fabricated glazing

increased, over a period of several days, to levels similar to

those achieved for the initial ‘‘poor’’ evacuated samples, i.e.

2.5–3.5 W m�2 K�1. From visual examination it was evident

that significant degradation to the edge seal had occurred. This

was attributed to moisture ingress resulting from condensation

that occurred on the cold glass surface during thermal testing.

Subsequent glazing were fabricated with a secondary seal,

comprising a two part epoxy adhesive, applied around the edge

of the glazing to protect the primary vacuum sealing material.

After several days of thermal testing, the U-value of glazing

with a secondary seal remained constant with no degradation

apparent. The secondary seal also provided the sample with

increased structural rigidity.

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1461

Table 2

Evacuated glazing types manufactured

Glass type (trade name) Sample size (cm) Emissivity of low-e glass Orientation of low-e coating

2 sheets Pilkington K-GlassTM 50 � 50 0.15/0.15 Both K coatings on inside

2 sheets Pilkington K-GlassTM 50 � 50 0.15/0.15 1 K coating inside 1 K coating outside

2 sheets Pilkington K-GlassTM 40 � 40 0.15/0.15 Both K coatings on inside

2 sheets Pilkington K-GlassTM 40 � 40 0.15/0.15 1 K coating inside 1 K coating outside

1 K-GlassTM, 1 Guardian 1.4DT 40 � 40 0.15/0.12 K coating inside 1.4DT inside

1 K-GlassTM, 1 Guardian 1.4DT 40 � 40 0.15/0.12 K coating outside 1.4DT inside

1 K-GlassTM, 1 Pilkington Optitherm 40 � 40 0.15/0.06 K coating outside Optitherm inside

3.3. Support pillar design

Pillars used in the manufacture of the evacuated glazing

were produced from stainless steel by a process of chemical

etching in which a photo negative of an array of pillars was

transferred to a sheet of stainless steel the thickness of the pillar

(0.15 mm) and covered with photo resist. The material

surrounding the pillars was subsequently etched away to half

its thickness on one face, turned over and etched through from

the opposing face to produce the array of pillars. This ensured

that both faces of the pillar were similar in size. Pillars were

manufactured with dimensions ranging from 0.3 to 0.5 mm in

diameter by 0.15 mm high.

3.4. Fabricated evacuated glazing samples

To improve the thermal performance of the electrochromic

evacuated glazing a number of low-e coated glass types were

used. The range of glazing fabricated is summarised in

Table 2. Glazing were manufactured using two panes of

Pilkington K-GlassTM or one pane of Pilkington K-GlassTM

and either one pane of Pilkington OptithermTM or one pane of

Guardian 1.4DT. OptithermTM is classified as a soft low-e

glass with an emissivity of approximately 6% and Guardian

1.4DT is classified as a hard low-e glass with an emissivity of

approximately 12%.

In Table 2, the orientation of the coating refers to whether the

low-e coatings on the two glass panes were both on the internal

vacuum cavity side or whether one coating was on the vacuum

cavity side and one coating was on the external glass surface.

The size of initial evacuated glazing fabricated was

50 cm � 50 cm, this was however reduced to 40 cm � 40 cm

to accommodate the maximum size of fabricated electro-

chromic glazing.

4. Development of the electrochromic evacuated

prototype

4.1. Fabrication of the electrochromic evacuated

prototypes

4.1.1. 4-glass prototypes

At the early stages of the prototype development 4-glass

devices have been fabricated in the following manner. The two

halves which constitute the electrochromic part of the window

(K-GlassTM/WO3 and K-GlassTM) were bonded together by use

of pure silicone sealant, which did not react with the electrolyte.

The cavity formed between the glass panes was filled with the

electrolyte which was inserted through a small entry point

towards one corner of the sample. The edges were sealed

peripherally and tested for leakage before it was fully

functional. Subsequently the evacuated glazing was laminated

to the EC glazing to form a 4-glass electrochromic evacuated

glazing. This method of manufacture facilitated the develop-

ment and fabrication of the evacuated and electrochromic parts

of the prototype in parallel, however with 4-glass panes the

weight and cost of the prototype would be increased and visual

transmittance was reduced.

4.1.2. 3-glass prototypes

For the fabrication of the 3-glass electrochromic evacuated

prototype, the 40 cm � 40 cm K-GlassTM/WO3 part of the EC

window was bonded directly to the completed evacuated

glazing component. For the 3-glass prototype the evacuated

glazing was fabricated with two low-e coated glass panes, one

of which had the low-e coating on its outer surface. This

provided the transparent and electrically conductive layer

required for the electrochromic part of the glazing.

All the problems/defects observed during the fabrication of

the prototypes were addressed successfully (see detailed

Fig. 4).

4.2. Problems/defects of the electrochromic evacuated

prototypes

During fabrication of the electrochromic evacuated proto-

types several problems were encountered which are described

in Fig. 4. Modifications were made to the fabrication process

and the significantly improved prototypes have been prepared.

The most critical problems addressed are:

1. I

nhomogeneous WO3 films: The new vacuum chamber has

been designed specifically for the preparation of

40 cm � 40 cm samples, taking into account all the

parameters involved in the deposition process (such as the

source-substrate distance, sample rotation, etc.). Indeed,

examination of the thin films prepared in this chamber has

shown a �10% variation in thickness across the

40 cm � 40 cm samples, which is adequate to prevent the

appearance of noticeable color deviations.

2. W

O3 film detachment due to insufficient cleaning of theglass substrates: The 40 cm � 40 cm samples have been

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671462

Fig. 4. Observed defects of the electrochromic evacuated prototypes.

cleaned using a CE certified industrial process. This method

has proved sufficient to eliminate the defects observed in the

initial samples.

3. I

nhomogeneous distribution of the applied electric field:A special conductive adhesive tape has been bonded on the

edges of the K-GlassTM coated surfaces. It has been used for

the application of the coloration voltage, considerably

enhancing the electrical field uniformity. Further develop-

ment of appropriate conductive line patterns is needed in

order to eliminate these inhomogeneities.

4. E

Photo 1. 3-Glass prototype in the as-prepared state.

lectrochromic device peripheral leakage: A crucial

parameter affecting the lifetime of the samples was found to

be the peripheral sealing. Several methods were investigated

but finally the combination of special glass spacers, two

component epoxies and silicone sealant, was employed. This

sealing method was found to have the best results as it

offered satisfactory prevention against moisture ingress and

eliminated the leakage problems.

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1463

Photo 2. 3-glass Prototype in the colored state.

Fig. 5. Applied voltage pulse and measured current during potentiostatic

coloration.

In Photos 1 and 2, the prototype is shown in the as-prepared

and the fully colored state.

5. Testing of the prototypes

5.1. Electrochromic properties

The thickness of the WO3 layer and the liquid electrolyte in

the 40 cm � 40 cm prototype was 350 nm and 0.8 mm,

respectively. The sample was colored galvanostatically, with

the application of a constant current pulse, and the color

intensity was regulated by the current application time. The

inserted charge during each coloration cycle was 4.6 mC/cm2.

In Photos 1 and 2, the prototype is shown in the as-prepared

and the fully colored state. The luminous transmittance of the

prototype decreased from 0.63 to 0.02 in the colored state,

giving a Contrast Ratio value of 32:1. The device could be fully

bleached by reversing the polarity of the applied current pulses.

The coloration efficiency (CE) of the prototype at 550 and

650 nm was: CE550 = 50.2 cm2/C and CE650 = 92.3 cm2/C,

respectively.

5.2. Cycling reversibility

The evaluation of lifetime expectancy for the EC devices

was carried out via accelerated cycling reversibility experi-

ments. During these tests the EC device was continuously

cycled between its fully colored and fully bleached state in

order to achieve the desired number of cycles. Various

degradation factors can be set and monitored, such as: decrease

of the Contrast Ratio, evolution of non-responsive areas,

delamination of layers, etc. Typical EC devices were subjected

to cycling reversibility experiments in order to evaluate the

effect of continuous coloration/bleaching on their performance.

Cycling reversibility of EC prototypes was evaluated by

potentiostatic methods (see Fig. 5). Deep cycling scenarios

(cycling from fully colored to fully bleached state) have been

selected, aiming to test the prototype’s capabilities and to bring

forward possible defects due to severe or extended use. Various

coloration/bleaching scenarios have been evaluated in order to

choose the most appropriate aiming to achieve rapid coloration-

bleaching and to preserve the life of the devices. The results

indicate that the tested devices can endure more than 5000

cycles or more than 10 years of operation in real conditions for

office buildings.

The K-GlassTM/WO3/electrolyte/K-GlassTM devices pre-

sented problems during bleaching due to insertion of Li ions in

the plain K-GlassTM. Such devices were found to degrade after

1000–2000 deep cycles. To overcome this problem an

additional MgF2 protective layer has been used. K-GlassTM/

WO3/electrolyte/MgF2/K-GlassTM devices have been fabri-

cated and tested. They were found to withstand more than 5000

cycles, which corresponds to a period of 8–12 years of

operation for a commercial office building.

During continuous cycling it was observed that the amount

of charge flowing through the device was reduced, due to

trapping of Li ions in the WO3 film. The trapped Li ions appear

to return to the electrolyte during relaxation intervals. In real

working conditions, EC devices will not be subjected to

continuous cycling, but to rather short coloration periods

followed by large relaxation intervals. This is expected to

extend the lifetime of the devices.

5.3. UV testing of prototypes

Resistance to degradation as a result of exposure to UV is

considered crucial for a useable service life of real EC devices.

Practically in an EC device the only component sensitive to UV

radiation is the polymer electrolyte. In order to evaluate the

performance of the prototypes containing the boron polymer

electrolytes, a suitable solar simulator system has been installed

providing an acceleration factor of 1:120.

The solar simulator system incorporates a UV enhanced

Xenon lamp mounted in a lamp housing. The 7 cm � 7 cm EC

sample was placed in front of the condenser to achieve

maximum irradiation. The active (UV irradiated) sample area

was 4–5 cm2. The sample’s operation and coloration/bleaching

capability was confirmed every 24 h of irradiation. After

irradiation for an equivalent period of 12 years the only

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671464

degradation effect observed in some samples was the evolution

of small bubbles, which were attributed mainly to the

evaporation of solvent or gas dissolved in the electrolytes

(nitrogen or argon from the protective atmospheres in the glove

box or during the preparation/synthesis processes). These

bubbles did not affect the performance of the devices.

Furthermore, an improvement in coloration performance was

observed which can be attributed to an alteration in the polymer

electrolyte properties.

5.4. Thermal testing

The guarded hot box calorimeter used for thermal testing

consisted of two chambers separated by a centre mask wall

which contained the evacuated glazing under test. The

calorimeter was fabricated from Styrofoam IB-X (an extruded

polystyrene insulating foam with a thermal conductivity of

0.028 W m�2 K�1) with chamber walls 0.15 m thick and a

mask wall 0.3 m thick. One chamber was chilled to

approximately 5 8C while the control chamber was maintained

at 25–30 8C. The control chamber contained a guard box

manufactured from 0.018 m thick plywood. The guard box

contained a dc heater, air circulation fans and a matt black

copper baffle plate positioned between the heater and the

glazing under test. The control chamber contained an ac heater

and air circulation fans. The chilled chamber contained air

circulation fans and an evaporator coil with a matt black copper

baffle plate positioned between the evaporator coil and the

glazing sample under test. Table 3 shows measured U-values

for a range of fabricated evacuated glazing and electrochromic

evacuated glazing. The effect of the sample size on the

measured U-value can be clearly seen where the exposed glass

U-value increases from 1.17 to 1.35 W m�2 K�1 as the sample

size decreases from 50 cm � 50 cm to 40 cm � 40 cm. The

poorer thermal performance can be attributed to the edge effect

where the increased thermal transmittance through the metal

edge seal increases the thermal transmittance of the glazing

close to the edge of the sample, raising the overall U-value for

the glazing. The influence of the edge effect on the total window

U-value will thus be greater for a smaller sample.

The reduced thermal performance of the electrochromic

evacuated glazing in comparison to the evacuated glazing can

be attributed to the additional glass thickness and the size of the

sample. The EC glazing consisted of an evacuated glazing

manufactured from two panes of 4 mm thick glass with an

Table 3

Thermal performance of evacuated and electrochromic evacuated glazing

Glass type Sample size

(cm)

Mea

valu

2 K glass both K inside 50 � 50 2.38

2 K glass both K inside 40 � 40 3.03

2 K glass 1 K in, 1 K out 40 � 40 4.14

1 K glass, outside 1 Guardian 1.4DT, inside 40 � 40 2.16

1 K glass, outside 1 Optitherm, inside 40 � 40 1.76

Electrochromic evacuated glazing, K-GlassTM & Guardian 40 � 40 2.7

additional 4 mm thick glass sheet laminated to one side by

means of the electrochromic material. This effectively

produced an evacuated glazing with one pane of 4 mm thick

glass and one pane of glass 8–9 mm thick. Simulations have

shown [26] that when the sample size is small (less than

1 m � 1 m) increased glass thickness increases the thermal

conduction through the edge seal and lowers the overall thermal

performance of the glazing system. With an increase in glazing

size the proportional effect of the increased thermal conduction

through the edge seal is reduced by the highly insulating

properties of the evacuated glazing away from the edge seal

region.

5.5. Durability testing of the evacuated glazing

Durability testing of the fabricated evacuated glazing and

electrochromic evacuated glazing prototypes was undertaken in

a purpose built environmental test-rig manufactured from

Styrofoam IB-X. The environmental chamber consisted of two

chambers separated by a central mask wall. When under test the

chambers were clamped together against the central mask wall

with compressible foam attached to the front face of each

chamber wall creating an airtight seal between the mask wall

and the chambers.

One chamber was a control chamber with an inner guard

box and the other was an environmental chamber. The guard

box was manufactured from styrofoam and contained a 90 W

dc heater and a series of dc axial fans which ensured mixing

of the heated air. A matt black copper baffle plate was

positioned in the guard box between the heater and the

glazing sample under test. The control chamber contained an

air-conditioning unit which maintained the temperature at

20 8C when undertaking the transient thermal cycling tests. It

also contained 3, 60 W ac heaters used to maintain a

temperature balance with the guard box during thermal

transmittance testing and a series of ac air circulation fans to

ensure good air mixing.

The environmental chamber contained an evaporator linked

to a chiller which lowered the air temperature to �35 8C. The

chamber contained a 2.4 kW heater and air circulation fans

which heated the chamber to 50 8C. A matt black copper baffle

plate was positioned between the heater/evaporator and the

glazing under test. Type T thermocouple sensors were used to

measure the air temperatures, baffle plate temperatures and

glass surface temperatures.

sured C

e (W m�2 K�1)

Measured U-value

exposed glass (W m�2 K�1)

Measured U-value

centre pane (W m�2 K�1)

1.17 1.06

1.35 1.22

1.42 1.28

1.07 0.96

0.96 0.86

1.31 1.18

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1465

Fig. 6. Typical temperatures measured during a thermal test cycle for a

40 cm � 40 cm evacuated glazing constructed from one pane of Pilkington

K-GlassTM and one pane of Pilkington Optitherm.

5.6. Thermal cycling

A 40 cm � 40 cm evacuated glazing fabricated from one

pane of Pilkington K-GlassTM and one pane of Pilkington

Optitherm was located in a uPVC frame and positioned in the

centre mask wall. The temperature in the environmental

chamber was lowered to �35 8C over a period of

approximately 4 h where it remained stable for a further

30 minutes. The temperature was then raised to 50 8C over a

period of approximately 3 h where it remained constant for a

further 30 minutes. This represented one complete 8 h cycle

test. A graphical representation of the temperature profiles in

the chambers and of the glazing surfaces is shown in Fig. 6.

The air temperature in the control chamber was maintained at

approximately 22 8C during the thermal cycling tests. The

temperatures chosen were considered to represent the

extremes to which the glazing would be exposed to under

real environmental conditions. These cycles were repeated to

test the durability of the developed edge seal. After 15 cycles

the U-value of the evacuated glazing was re-calculated, a

decrease in thermal performance of 0.13 W m�2 K�1 was

observed.

5.7. Solar simulator testing

Evacuated glazing samples were tested under radiation from

a solar simulator lamp. The solar simulator used in this work

had a 1200 W OSRAM HMI lamp with a zoom optical system

and uniform focusing, a cooling fan and iris diaphragm. These

lamps are ac-operated discharge lamps in which the luminous

arc burns in a dense vapour atmosphere comprising mercury

and rare earth halides.

An evacuated glazing 40 cm � 40 cm, fabricated from one

pane of Pilkington K-GlassTM and one pane of Pilkington

Optitherm was incorporated in a uPVC frame and secured in the

centre mask wall of the durability test-rig. The wall was sealed

to the environmental chamber and the air temperature in the

chamber was lowered to �25 8C. The lamp was positioned

about 2 m in front of the mask wall and a solar radiation

intensity of 600 W m�2 was used to illuminate a quarter section

of the glazing sample under test, replicating the effect of part

shading of the glazing which could occur in real operational

conditions. After solar radiation testing the sample was re-

tested for thermal transmittance with no degradation in U-value

observed.

6. Comparison with equivalent products

The fabricated prototypes compare favourably with equiva-

lent state-of-the-art devices and products as presented in the

following paragraphs.

6.1. Electrochromic devices

In recent years many laboratories and industrial companies

worldwide have fabricated and tested EC devices suitable for

glazing applications. Indeed, during the period 1998–2004, 40

such devices have been presented in the literature [27], having

the following properties:

� D

imensions: The EC devices presented in the literature have

active areas varying from 2 to 4800 cm2. However, of the 40

devices reported only five exceed 100 cm2 and of them, only

three exceed 1000 cm2. (the area of the fabricated prototype

is 1600 cm2).

� C

ycling times: The time required for coloration or bleaching

of the devices depends on their dimensions. For large devices

(area > 1000 cm2), cycling times exceed 200 s. (the fabri-

cated prototypes can be colored in about 100 s).

� C

ycling durability: This also varies from a few hundred

cycles (in the case of most sol–gel devices) to 3 � 105 cycles

for sputtered or e-gun deposited films. Degradation of the

devices is gradual and propagates around defects, pin holes,

etc. It is therefore very important for the devices to be defect-

free and substrates to be thoroughly cleaned prior to film

deposition. The device degradation has been attributed to

environmental exposure (UV radiation, thermal stress) and to

interaction with the atmosphere (photo-induced degradation

in the presence of atmospheric oxygen, moisture ingress,

etc.). The most durable devices have been cycled at low

contrast ratios probably to prolong their life. (the fabricated

prototypes withstand more than 5000 deep cycles, which

corresponds to a period of 8–12 years of operation for a

commercial office building).

� O

ptical properties: Of the 40 devices reviewed, 15 exhibit a

Tlum value that exceeds 70% in the bleached state. In the

colored state, only 8 devices present Tlum lower than 10%.

The contrast ratio of the vast majority of the devices is about

5:1 or less, and only two of them exceed 10:1. (the fabricated

prototypes have a contrast ratio of 32:1 with Tlum ranging

from 63% to 2%).

The fabricated prototypes compare favourably with the

state-of-the-art electrochromics (as presented above), with

regard to optical properties, coloration times, size and

durability.

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–14671466

A complete Energy and Life Cycle analysis for a

40 cm � 40 cm prototype EC window has been performed

and is described in Refs. [4,28]. Combining various parameters

(i.e. control strategy, expected lifetime, climatic type, expected

purchase cost) significant conclusions have been drawn for the

development and the potential applications of the device

compared to other commercial fenestration products. The total

production energy was found to be 2261 MJ, 91% of which is

allocated to the aluminum frame, about 7% to fabrication

processes and the remaining 2% to the embodied energy of the

raw materials. Various control strategies for the EC window

operation have been compared with respect to the induced

heating – cooling loads and energy savings. The reduction of

the purchase cost (to 200 s m�2) and the increase of the

lifetime (above 15 years) are the two main targets for achieving

both cost and environmental efficiency.

When the EC window prototype is used instead of a single

glass in cooling dominated areas for a maximum expected

lifetime of 25 years, the reduction in energy consumption can

reach up to 54%, corresponding to 6388 MJ. Furthermore, the

total energy savings provided will be 33 times more than the

energy required for its production while the emission of 615 kg

CO2 equivalent per electrochromic glazing unit can be avoided.

Based on the conducted analysis, 0.9 years of operation of the

prototype window are required to compensate the production

energy of the plain EC device. The total cost savings range from

228 to 569 s m�2 glass for 10 and 25 years of EC window

operation, respectively.

6.2. Insulating glazing

The evacuated glazing technology will compete in the high

performance insulating glazing (IG) market. There is a diverse

range of commercially available IG units with varying thermal,

physical and optical properties depending upon their end use

application.

� T

he thermal performance or U-value of an insulating

glazing is primarily controlled by the emissivity of the glass

used in its construction (low-e glass) and the number of

glass panes used. The size of the glazing also influences U-

value; smaller glazing generally have higher U-values.

Double glazing units fabricated with soft low-e glass such

as Pilkington OptithermTM or SuncoolTM with an emissivity

in the range 0.04–0.06, incorporating a gas fill such as argon

have been reported with centre of pane U-values in the

range 1.1–1.3 W m�2 K�1. The fabricated evacuated glaz-

ing prototypes have measured centre of pane U-values

down to 0.86 W m�2 K�1 for similar glass types and a size

of 40 cm � 40 cm. A similar size double glazed unit

comprising low-e glass with a gas fill cavity had a measured

U-value of 1.8 W m�2 K�1. Therefore the fabricated

evacuated glazing compares favourably with IG units in

terms of thermal performance. Additionally simulations

show evacuated glazing has the potential to achieve centre

of pane U-values down to 0.5 W m�2 K�1 for larger sample

sizes.

� I

nsulating glazing have been reported with U-values in the

range 0.2–0.4 W m�2 K�1, however the improved thermal

performance is achieved through the use of additional glass

panes with units comprising three, four and even five glass

panes. Consequently these glazing have a thickness of up to

50 mm with a commensurate increase in weight of up to

50 kg m�2. This presents difficulties particularly with

framing arrangements and installation of the units. Consisting

of only two 4 mm glass panes the weight of the evacuated

glazing is 20 kg m�2. Additionally for each pane of glass

which is added to an insulating glazing to improve thermal

performance there is an undesirable increase in cost and a

significant reduction in the transmission of visible light

through the glazing.

The physical attributes of the electrochromic evacuated

prototype make it particularly desirable for applications where

the glazing thickness is an issue of concern. Standard double

glazing is on average 24 mm thick comprising two panes of

4 mm thick glass with a 16 mm cavity. In contrast the evacuated

glazing has a total thickness of 8.15 mm comprising two panes

of 4 mm glass separated by a 0.15 mm gap, while the overall

thickness of the electrochromic evacuated glazing is approxi-

mately 13 mm. This narrow thickness increases market

opportunities for both the evacuated glazing and the electro-

chromic evacuated technology.

7. Conclusions

Materials suitable for EC devices such as transition metal

oxides, ion storage layers and transparent conductors have been

identified. Tungsten oxide (WO3) proved to be the most

promising electrochromic material combining high transmit-

tance modulation and stability. A series of WO3 thin films have

been prepared and characterised using state-of-the-art techni-

ques. Ion storage (V2O5 and CeO2) and protective (MgF2)

layers have also been prepared and tested. Furthermore,

suitable polymer electrolytes based on the LiClO4 salt and

boron-containing branched poly(ethylene glycols) have been

produced and optimised as components of EC devices.

EC devices with dimensions up to 40 cm � 40 cm were

fabricated in a purpose built vacuum chamber. They exhibit

favourable optical characteristics, cycling reversibility and

durability. Refinement of an existing vacuum sealing

technique has been undertaken to provide a durable,

contiguously-sealed evacuated glazing. Fabrication of evac-

uated glazing has been undertaken in a vacuum chamber

designed and constructed in order to facilitate a single stage

process of evacuation and sealing. Samples of evacuated

glazing measuring up to 50 cm � 50 cm have been produced

with U-values down to 0.86 W m�2 K�1. An electrochromic

evacuated prototype has been prepared attaching a

40 cm � 40 cm WO3 coated glass on an evacuated component

(3-glass arrangement). The device exhibited excellent optical

performance, with contrast ratio up to 1:32 (visible dynamic

transmittance range Tlum,bleached = 63% and Tlum,colored = 2%),

and coloration efficiency up to 92 cm2/C. Frames suitable for

S. Papaefthimiou et al. / Energy and Buildings 38 (2006) 1455–1467 1467

use with electrochromic evacuated glazing have been

designed, manufactured and tested.

The prototypes were tested for durability and performance

by indoor and outdoor experiments. These tests provided

definitive results for the performance and durability of the

devices and have been used as feedback for any necessary

improvements.

Acknowledgement

This work was supported under the EU FP5 project

ELEVAG, contract no. ENK6-CT-2001-00547.

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