Solar Control Glazing for Trucks

(1)

UPTEC ES07 006

Examensarbete 20 p

Mars 2007

Solar Control Glazing for Trucks

(2)

(3)

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Solar Control Glazing for Trucks

Johan Tavast

This thesis concerns the use of solar control and electrochromic glazing in trucks. The purpose has been to study the decrease in solar energy transport into the cab and how to utilize the technology. The solar spectrum consists of both visible light and near-IR radiation, and solar control glazing transmits the majority of the visible light and reflects or absorbs most of the near-IR radiation. Electrochromic glazing has variable transmittance, which enables the driver to regulate the energy flow through the window. A decrease in energy transfer can reduce the use of air conditioning and give a lower peak temperature in the cab. This could generate a better fuel economy and a possibility to reduce the cost of the polymer material in the instrument panel. Five different types of glass were tested in several experiments to determine the reduction in heat transfer. The experiments evaluate the performance of the glass in an environment with simulated solar radiation and forced convection, caused by the speed of the truck. The tests showed that during a sunny day with an irradiance of 1000 W/m2 there is a possibility to lower the cab temperature by 5 °C and the inflow of energy by nearly 90 W/m2.

A survey was carried out to get a subjective assessment of the use of solar control and electrochromic glazing, and other potential technical applications. The general consensus was that the decrease of solar radiation would create a better working environment for the drivers.

Sponsor: Volvo

ISSN: 1650-8300, UPTEC ES07 006 Examinator: Ulla Tengblad

(4)

Sammanfattning

Denna rapport behandlar användningen av solskyddsfönster och elektrokroma fönster i lastbilar. Syftet med arbetet är att undersöka den minskning av solenergiflöde som kan åstadkommas, och hur teknologin bäst kan användas. Solspektrumet består av UV-strålning, synligt ljus och nära-IR-strålning. Solskyddsfönster transmitterar det synliga ljuset, medan UV-strålningen och den nära-IR-strålningen reflekteras eller absorberas. Det finns två huvudtyper av solskyddsfönster; det ena absorberar solstrålningen medan det andra reflekterar solstrålningen. Detta kan antingen göras genom att glassmältan blandas ut med olika metalloxider eller genom att man lägger en beläggning på rutan. De elektrokroma rutorna har reglerbar transmittans, vilket innebär att föraren själv kan variera ljusinsläppet i lastbilshytten genom att göra rutan ljusare eller mörkare. De elektrokroma rutorna består av två glasrutor där den elektrokroma beläggningen läggs in emellan. För att ändra transmittansen läggs en elektrisk spänning över rutan. Den elektriska spänningen driver joner från nickeloxiden i beläggningen till wolframoxiden. När jonerna drivs från nickeloxiden bildas en ny molekyl som har en mörkare färg än ursprungsmolekylen. Jonerna drivs till wolframoxiden som i sin tur bildar en ny molekyl som också är mörkare än sin ursprungsmolekyl. Processen är reversibel då polariteten över rutan ändras.

En minskning av den instrålade energin kan leda till en reducerad användning av luftkonditionering vilket skulle kunna förbättra bränsleekonomin. Möjligheten till en reducerad bränsleförbrukning är en drivande kraft bakom projektet. Enbart i USA används 27 miljarder liter bränsle till luftkonditionering. Användningen av solskyddsfönster skulle också kunna reducera maxtemperaturen i hytten då lastbilen har varit parkerad, detta medför att specifikationen till plasten i instrumentbrädan skulle kunna förändras till en något billigare version. Den högsta uppmätta maxtemperaturen i en lastbilshytt är 110 °C. Denna höga temperatur medför att en värmeresistent plast måste användas. Vidare medför reduktionen av strålningen och temperaturen i kupén att arbetsmiljön i lastbilen blir bättre.

För att undersöka minskningen av den instrålade energin studerades fem olika rutor i flera experiment. De olika typer av rutor som användes var tre solskyddsrutor, en elektrochrom ruta och en normal klar ruta. De tre solskyddsrutorna absorberar delar av solstrålningen för att minska värmeinflödet. Den vanliga rutan användes som referens. För att skala ner experimenten användes en taklucka som grund för experimentens uppbyggnad. Det skulle bli för dyrt att sätta in solskyddsfönster i en lastbil i detta stadium av projektet.

(5)

Resultaten från experimenten visar att det går att reducera energiflödet genom glaset markant. Testen visar att under en solig dag med en ljusintensitet på 1000 W/m2 kan inflödet av energi minskas med upp 90 W/m2 då fordonet står stilla, om man byter en normal klar ruta till ett solskyddsfönster. Minskningen av energiflödet är ännu större då fordonet körs. Experimenten visar också att maxtemperaturen skulle kunna minskas med upp till 5 °C då bilen står parkerad. Resultaten visar också att om elektrokroma rutor används skulle den instrålade effekten minskas med 130 W/m2 och maxtemperaturen minskas med 8 °C då bilen står parkerad.

En attitydundersökning genomfördes för att få en subjektiv bedömning av hur lastbilschaufförer upplever sin arbetsmiljö och hur de ser på användningen av solskyddsfönster och elektrokroma fönster i lastbilar. Målet med undersökningen var att studera behovet av ett sänkt värmeinflöde genom rutorna på lastbilen. Även andra användningsområden för den elektrokroma tekniken i lastbilar studerades.

En enkätundersökning ansågs vara det bästa sättet att nå ut till förarna. Enkäten utformades så att frågorna delades upp i fyra områden: Generell information om föraren och lastbilen, klimatet i lastbilshytten och användningen av klimatanläggningen, solskyddsteknik inriktad mot elektrochrom teknik, samt information om bränsleförbrukning. Först skickades en testenkät ut till en liten grupp förare för att kvalitetssäkra enkäten. Testet visade att frågorna i grund var bra, emellertid behövdes någon som kunde introducera förarna i den elektrokroma tekniken. Totalt utfrågades 39 förare. Förarna tillfrågades genom att ringa stora åkerier i Uppsalaregionen för att stämma möte vid ett tillfälle då så många förare som möjligt fanns tillgängliga. Först hölls en teknisk introduktion, sedan fick förarna fylla i enkäten.

Resultatet från enkätundersökningen visar att de flesta förarna var nöjda med klimatanläggningen i sin lastbil; det skulle emellertid vara positivt om mindre energi strålade in i hytten under soliga dagar. Den elektrokroma tekniken möttes med en viss misstänksamhet eftersom ingen av förarna hade sett tekniken använd innan. Om den elektrokroma tekniken blir mer allmänt känd och om den visar bra tillförlitlighet och livslängd är förarna intresserade. Till exempel skulle en elektrochrom taklucka kunna förbättra kupémiljön genom att släppa in mer dags ljus under en molnig dag respektive mindre dagsljus under en solig dag. Förarna var väldigt positiva till att kunna hålla lastbilshytten kallare när fordonet var parkerat. Konsensus av undersökningen var att förarna tycker att solskyddsfönster till ett rimligt pris skulle vara välkommet.

(6)

1 Introduction

This master degree project, Solar Control Glazing for Trucks, (20 points/30 ECTS) was conducted at Uppsala University, Volvo in Gothenburg and Chromogenics in Uppsala between September 2006 and March 2007. It concludes the MSc programme in Energy Systems (180 points/270 ECTS) at Uppsala University. The project investigates how different sorts of glazing affects the cab environment and fuel consumption of a truck.

The outline of the thesis is divided into four parts: Firstly, a background and the purpose of the project are given. Secondly, the theory behind heat transfer and light transmission is illuminated. Furthermore, four different experiments were conducted to explain the difference in heat flow using different types of glazing. Finally, a survey was carried out to investigate the truck drivers’ opinions on how to improve their working condition using solar control and electrochromic windows.

1.1 Background

The rapid rise of crude oil prices in 21st century has led the vehicle industry to an increased search for fuel reducing technology. There are numerous technical improvements to be made to decrease fuel consumption in trucks. The growing number of commodities in vehicles increase the energy consumption. The major part of the rise in fuel consumption is credited to the increased air condition (AC) and climate control use [1]. The use of AC for cooling vehicle cabs annually uses approximately 27 billion litres of petrol in the U.S., 8.7 billion litres in Europe, and 3 billion litres in Japan (presuming that all the vehicles have air condition) [1]. An air condition system can increase the fuel consumption by 28 % in a conventional car; in hybrid vehicles up to 100 % [1].

Firstly, by installing solar control windows in a truck the peak temperature can be lowered. The size of the AC system can be reduced because it is designed for peak temperature and not for average thermal load. If the car is parked in the sun a 3-12 °C reduction can be accomplished of peak temperature with energy efficient glazing [1-3]. Further, by lowering the peak temperature in the cab it will be possible to change the expensive polymer material used in the instrument panel to a cheaper material. The measured peak temperature found in a cab located in the Arizona desert can reach 110 °C [4], which stresses the use of a thermal resistant polymer material that will not be deformed under harsh conditions. Finally, the personal comfort of the driver and passengers will increase, due to better thermal comfort, especially for the driver or passenger being directly exposed to sun radiation.

(9)

1.1.2 Vehicle Glazing Overview

During the 1990s there was an increase of the size of the glazed area in cars, also IR reflecting coatings became available for cars. In the 2000s the focus turned to comfort and the development of glazing continued with for example electrochromic glazing [5].

The leading automotive glass producers are Saint Gobain, Pilkington, Asahi, PPG and Guardian. The five top producers currently hold 80 % of the world market [5]. During the last decade the focus has turned from performance to safety and comfort. The development in glazing for vehicles springs from major improvements in the float glass industry. Today one kilo of float glass costs around 0.30 Euro [5]. The standard glazing in cars today is still clear glass. The windscreen is laminated glass, while the sidelights are made from safety glass. The normal thickness of car glass ranges from 3-5 mm [6]. There are numerous demands on glazing producers depending on the intended use of the vehicle: Everything from bullet-proof glass to securing the visual comfort of the driver and passengers. An overview of vehicle glazing demands is presented in table 1.

Table 1: The customer’s different needs for glazing in their vehicle are presented in the left column and the glass producer’s response is presented in the right column.

Customer needs Glass products as a response to needs

Laminated sidelights Acoustical Comfort

Double glazing with sealed air space

Tinted absorbing glass – green glass is the standard

Thermal comfort

IR reflective coating

Solar Control Tinted band on the top of the windscreen Defrosting

Defrosting /Deicing Heating coatings on heating wires UV protection (anti-fading) PVB

Easy cleaning Coatings facilitating cleaning (for back mirror) Controlled Transmission Few products with electrochromic glazing

Visual Comfort

Low reflection Low reflection coatings

Space Large roof (moon roof)

Privacy Low transmission (<25 %) with tinted glass or coated glass which hides interior

Aesthetics

Enamel bands around many glass parts Laminated sidelights

Security

Bullet-proof glazing

People safety

Safety glazing (tempered or laminated)

Table modified from [5].

1.1.3 Solar Control Glazing

(10)

normal glazing. Developments in solar control windows have led to coatings where a large part of visible light is transmitted while NIR radiation is reflected or absorbed. These coatings are made up of multiple layers, where the active component in the coating usually is silver [7]. These layers are soft and must be protected by using a laminated glazing.

1.1.4 Electrochromic Glazing

An electrochromic window has the ability to change its optical properties when an electrical charge is applied across the pane. The glass goes from a high light transmitting state to a partly reflecting and absorbing state over the solar spectrum, 300-2500 nm. The optical properties of the window can be changed by altering the voltage applied to the pane. The state is reversible, i.e. if the polarity of the charge is reversed the pane reverts to its original state. Figure 1 shows an overview of an electrochromic device. It consists of 5 different layers sandwiched between two panes of glass or a polymer material. Firstly both of the outer protective glass or polymer layers are coated with a transparent conducting layer, serving as an electrical contact. Secondly, an electrochromic layer is added on both sides. Finally, an ion-conducting electrolyte is laminated between the electrochromic layers.

Figure 1: A cross section of an electrochromic device showing the ion flow from the ion storage nickel oxide to ion receiving tungsten oxide. The figure is from [8].

(11)

1.2 Purpose

The main goal of the project is to investigate how much solar control and electrochromic glazing can lower the energy transferred into the cab, both when the truck is driven and parked. The advantage of using an electrochromic window would be that the transmittance could be lowered while the truck is parked, subsequently decreasing the heat transfer into the cab. Consequently by using solar control and electrochromic glazing less air condition will be utilized. Further, the fuel consumption of the truck is decreased and a smaller air condition unit can be installed. Reducing the radiation leads to a lower cab temperature, creating a more comfortable working environment for the drivers. With a lower peak temperature it might also be possible to change the expensive polymer material currently used in the instrument panel to a cheaper one. Finally, a subjective investigation concerning the drivers needs and designs, and how to implement the new technology in the most effective way was performed.

1.3 Method

This degree project is divided into two parts; the fist part concerns how to experimentally investigate the heat transfer reduction into the cab using different solar control windows, the second part subjectively evaluates the attitude of the drivers towards solar control and electrochromic glazing, and how to employ the technology in the best possible way.

1.3.1 Experiments

The energy flow through the cab was investigated through four experiments. In the experiments five different types of glass panes was used as presented in table 2. Table 2: The different types of glass used in the experiments

Clearfloat -A normal clear pane.

Optifloat Green -A green tinted absorptive pane, with fairly high light transmittance. Arctic Blue -A blue tinted absorptive pane, with low light transmittance.

Sun Protection -A dark absorptive pane, with very low light transmittance. Electrochromic -A pane with variable light transmittance.

The Clearfloat pane was used as a reference pane. The Optifloat Green pane is a tinted absorptive pane and has visible light transmitting of 75 %, a quality needed to fill the legal requirements of a windscreen and it is the most important pane to compare to the reference pane. Arctic Blue pane is also a tinted absorptive pane and has better solar control qualities than Optifloat Green. The Arctic Blue pane does not legally transmit enough visible light; however, it is still interesting compared to the Optifloat Green pane, which could be replaced with a more advanced sun protection window. The Optifloat Green and the Arctic Blue panes are both tinted panes and are not coated with a solar control coating. The Sun Protection pane has a very low visible light transmittance and is more interesting for use as a sunroof glazing. The electrochromic pane has a variable transmittance ranging from about 75-20 %, which makes the electrochromic panes interesting for sunroofs and to lower the light transmittance when the vehicle is parked to keep the cab cooler. The advantage with an electrochromic window would be that it could lower its transmittance below the legal limit, when the truck is parked, which would reduce the heat transmittance significantly.

(12)

3.1. The second experiment investigates the heat leakage through a truck cab. Experiment 2 is described in section 3.2. A sunroof was used in experiment 3 and 4 to scale down the

experiment and reduce costs. To simulate the compartment a small insulated box was fitted to the sunroof. In experiment 3 the heat leakage of the panes and of the insulated box was determined, as described in section 3.2. In experiment 4 a sun simulation tank in Gothenburg was utilized to evaluate the performance of the test panes in a simulated solar environment.

1.3.2 Subjective Evaluation of Glazing Improvements

To be able to provide the drivers with a better commodity and to get their opinion of how to improve their working environment a survey was carried out. No previous known

investigation of the use of solar control and electrochromic glazing in trucks had been done. It would be a very important tool to get to know the driver’s opinions. The only survey with some similarities is “Combined Sun Protection & Blackout System” [9], which focuses on how to improve the current sun protection and blackout system. The participants of the survey were truck drivers.

The survey was performed as a questionnaire with pre-stated questions. The composition of the questions was discussed and decided with our supervisor at Volvo. Firstly, a test

questionnaire was sent out to truck drivers. The test showed that a person needed to be present when the respondents filled in the questionnaire to give a brief technical introduction, answer questions and ask follow up questions.

The questionnaire was carried out through contacting different hauling businesses in the Uppsala region. A meeting was set up when most drivers where available to answer the questionnaire. In total 39 drivers were questioned. The survey is described in section 6.

2 Theory

The theory illustrates how the solar radiation is transferred through the glazing. Firstly, solar radiation and the solar spectrum are described. Secondly, heat transfer by means of

conduction, convection and radiation is depicted. Finally, window physics is considered.

2.1 Solar Radiation

(13)

Figure 2: Spectral distribution of solar flux at the surface of the earth and in space

When the solar radiation reaches the atmosphere the photons of certain wavelengths interact with different gaseous molecules, hence the gaps in the solar spectra at the earth’s surface are created.

2.1.1 Direct and Diffuse Radiation

Direct radiation is sunlight that reaches the earth’s surface directly from the sun. Light scattered by the atmosphere reaches the target surface at different angles, as shown in figure 3. Direct and diffuse solar radiation is together called global solar radiation. A simple way to describe direct and diffuse radiation is to imagine a building, the direct radiation causes the sharp shadow of the building. The diffuse radiation enables us to see details in the ground covered by the shadow [10].

(14)

The amount of sun light reflected by the ground differs widely depending on the surface covering. Bare rocks and gravel will reflect much more than a grassed area. Snow will reflect an even larger amount of the sun’s radiation [10].The ratio between direct, diffuse and albedo radiation is strongly influenced by atmospheric conditions. Water vapour and pollution increases the amount of diffuse radiation. A desert climate has much more direct radiation than for example a subtropical climate.

2.2 Heat transfer

The second law of thermodynamics describes the universal law of increasing entropy. A common enunciation of the second law is stated by Rudolf Clausius “Heat cannot of itself pass from cooler to a hotter body”. Heat can be transferred in three different ways: conduction, convection and radiation. The driving force of heat flow through a window is the temperature difference of the bodies on either side [11].

2.2.1 Conduction

Conduction is heat transfer through molecular interaction in a fluid or solid body. In a solid opaque material conduction is the only means of heat transfer. Fourier’s law from 1822 is foundation for heat transfer problems:

x T q_cond ∂ ∂ − = λ (1)

The heat transferred through a section qcond, is dependent on the proportionality factor λ and temperature gradient -∂T/∂x, change in temperature per unit length. The proportionality factor λ (W/m2

,K) is the heat conductivity of the material and is specific for each material.

2.2.2 Convection

Convection is heat transport in a gas or a fluid via blending. In “free” convection the gas or fluid blends through density differences, which occur due to temperature differences. An example of free convection is downdraft from windows. In forced convection the gas or fluid is forced to move by a pump or a fan. An example of forced convection is when air is forced through the AC-system in a vehicle. The temperature difference between a solid wall and a gas leads to heat transfer by convection. The major part of the temperature change occurs in a thin layer close to the wall. The thickness of this layer depends on the nature of the fluid, its speed, the shape of the wall, etc. The convective heat flow is expressed as [11]

(

s a

c

c T T

q =α −

)

(2)

(15)

2.2.3 Radiation

Energy can be transferred from one body to another with electromagnetic waves. Heat radiation as all electromagnetic radiation moves with the speed of light and can be transferred through vacuum. All bodies warmer than 0 K emits radiation. When radiation hits an object in gaseous, fluid or solid form the radiation will either be absorbed (α), reflected (ρ) or transmitted (τ). Consequently [12] 1 = + +ρ τ α (3)

Electromagnetic radiation consists of photons, which travel through space as waves. The photon energy is proportional its frequency, according to [12]

λ hc hv

E = = (4)

where the photon energy E (J) is given by Planck’s constant h (6.626⋅10-24

Js), the speed of light in vacuum c (2.998⋅108_{m/s) and the wavelength λ (m). The equation shows that photons} with shorter wavelengths has more energy, i.e. blue light has more energy than red light. A body that absorbs all radiation that falls upon it is called a black-body and it reemits the maximum amount of radiation at the body’s given temperature [13]. The type and amount of energy that a black-body emits is directly related to its temperature. Black-bodies with a temperature below 700 K produce little radiation in the visible spectra and thus appear black. Above 700 K the black-body starts to glow red and then changes colour as the temperature increases to orange, yellow, white and finally blue. Planck’s law of black-body radiation predicts the power per surface area and wavelength interval λ± dλ/2 [12]

1 1 2 5 2 − ⋅ = ⋅kT hc e hc I λ λ (5)

where h is Planck’s constant (6.626⋅10-24_{Js), k is Boltzman’s constant (1.381⋅10}-23

J/K) and c is the speed of light (2.998⋅108

m/s). The power emittance per unit area is found by integrating equation 5. This relation is described by Stefan-Boltzman’s law [12]

4 T

I =σ (6)

where σ is the Stefan-Boltzman’s constant (5.67⋅10-8

W/m2,K).

2.3 Window Physics

The solar radiation that falls on a window is reflected, transmitted or absorbed by the glass according to equation 3. Reflection is defined as

(16)

where the reflectance is the fraction of the reflected incident radiation per wavelength, IR(λ), that hits the pane and the incident reflected , I(λ). The transmittance is defined as

( )

_{( )}

( )

λ λ λ τ I I_T = (8)

where the transmittance is the fraction of the transmitted incident radiation per wavelength, IT(λ) that hits the pane and the incident radiation, I(λ). The absorptance is defined as

( )

_{( )}

( )

λ λ λ α I IA = (9)

where the absorptance is the fraction of the absorbed incident radiation per wavelength, IA(λ) that hits the pane and the incident radiation, I(λ). For each wavelength

( )

λ I_R

( )

λ I_T

( )

λ I_A

( )

λ

I = + + (10)

the solar incident radiation equals the reflected, transmitted and absorbed part. This is illustrated in figure 4.

Figure 4: Sun radiation being transmitted, reflected and absorbed when it hits glass.

The radiation absorbed in the glass is reemitted to the surroundings. When the window is in thermal equilibrium it absorbs as much as it emits, according to Kirchhoff’s law of radiation

A(λ) = ε(λ) (11)

(17)

2.3.1 Total Heat Transmittance, U-value

The heat leakage through a window is described by the U-value (W/m2K). A model of a simple window is presented in figure 5. It shows how heat is transferred through a window pane by convection, conduction and radiation.

Figure 5: For simplicity the window pane is heated from one side by a hot plate with temperature Tp. The heat flow through the pane is q1. The conductance through the pane is

determined by the heat transfer coefficient h1. The heat flow from the pane surface with

temperature Ts, to the ambient temperature Ta is q2. The convection and radiation from the

surface is determined by the heat transfer coefficient h2 and h3.

According to the first law of thermodynamics in this steady state system

2 1 q

q

q= + (12)

where q is the total energy output of the hot plate. The heat conductance through the pane depends on the temperature differences between Tp and Ts.

(

Tp Ts

)

h

q₁ = ₁ − (13)

The heat transfer coefficients of convection and radiation from the surface to the surrounding air are defined by:

(

h h

)(

Ts Ta

q2 = 2 + 3 −

)

(14)

The total thermal transmittance per unit area U is defined as

(

Tp Ta

)

U

(18)

where P is the power and A is the area. Using equation (12) to (15) gives 3 2 1 1 1 1 h h h U = + + (16)

Furthermore, looking at a single glazed window the U-value is defined as [14]

i e h h U 1 1 1 ₌ ₊ (17)

Where he and hi are the heat transfer coefficients towards the exterior and interior. The heat transfer coefficient he is dependent on the emissivity, the wind speed near the glazing and other

climatic factors. The internal heat transfer coefficient is given by [14]

he = hc+ hr (18)

Where hc is the convection conductance and hr is the radiation conductance. The radiation conductance for normal glass is [14]

837 , 0 4 , 4 ⋅ ε = r h (19)

For uncoated glass the surface emissivity is ε = 0,837. There are several expressions used for the heat transfer coefficient toward the interior, hi. The expression used for buildings as a standard is presented in equation 19. The mobility of a vehicle complicates the heat transfer caused by convection. The convection conductance is expressed as [15]

v

hc =4,7+1,9 (20)

where v is the wind speed (m/s). For simplicity the external and internal conductance values are standardised for building given as [16]

[

glass

]

W m K h e _e e 2 / 23 837 , 0 ; 837 , 0 4 19+ ⋅ = = = = ε ε (21) and

[

glass

]

W m K h i _e i 2 / 8 837 , 0 ; 837 , 0 4 , 4 6 , 3 + ⋅ = = = = ε ε (22)

(19)

2.3.2 Total solar energy transmittance, g-value

Light transmittance through an ordinary window is very high; up to 90 % of the visible light is transmitted. About 6-7 % is reflected and the rest is absorbed. Hardly any UV radiation is transmitted through a normal window [9]. The total solar energy transmitted through a window, also called g-value is defined in [16] given as

i e q T

g= + (23)

where the direct solar transmittance Te and the heat transport factor qi is the heat transfer factor towards the inside of the pane, shown in figure 6.

Figure 6: The absorbed energy reemitted to the interior by qi and to the exterior by qe

The solar radiation absorbed by the pane is divided into two parts

e i e =q +q

α (24)

As presented in figure 6. The direct solar transmittance over the solar spectrum is given by [16]

( ) ( )

( )

∫

= ₂₅₀₀ 300 2500 300 λ λ λ λ λ τ τ d S d S e (25)

(20)

specific range of wavelengths. Thereby higher intensity wavelengths will contribute more to the total transmittance. The direct solar reflectance is given by [16]

( ) ( )

( )

∫

= ₂₅₀₀ 300 2500 300 λ λ λ λ λ ρ ρ d S d S e (26)

where S is the intensity distribution function over the solar spectra. The calculations of the transmittance and reflectance are standardized in ISO 9050.The absorptance is given by combining equation 3, 12 and 13.

e e

e τ ρ

α = 1− − (27)

The secondary heat transport factor is dependent on the energy absorbed by the window and the convection on both side of the window and is given by [16]

(

)

e e i i i A h h h q ⋅ + = (28)

where hi is the heat transport coefficient towards the inside given by equation 18, he is the heat transport coefficient towards the outside. At steady state this absorbed power is re-emitted to the interior and to the exterior of the glass. The inward going fraction qi, is an important part of the total solar energy transmittance. For a single Clearfloat glass Te is approximately 83 % and g is 86 %. This is only a slight difference but for solar control glass there can be a significant difference [9].

3 Experiments

To investigate the difference in heat flow into the cab between the test panes, four experiments were performed. The experiments are devised to test the performance of different types of glazing in a truck. The goal of the experiment is to find out the reduction of heat flow into the cab and the reduction of peak temperature in the cab. To investigate which type of pane is best suited to use in a truck five different test panes were examined. It is important to remember that the reduction in peak temperature was only valid for the experiments when the car is parked, due to the fact that the AC system is on when the truck is driven so the cab never reaches peak temperature. The test panes that were used are the original dark pane for the sunroof from St. Gobain, two different solar control panes from Pilkington, an electrochromic “smart window” from Chromogenics and a clearfloat pane, which was used as a reference. These panes are chosen because of the different optical properties, the solar control glass was tinted with different metal oxides, not coated.

(21)

3.1 Optical properties of the Test Panes, Experiment 1

This experiment intends to examine the optical properties of the five different test panes used in the study. The transmittance and reflectance in the visible spectra and the NIR spectra was measured. To find the total solar energy transmittance g, given in equation 23, the direct solar transmittance and reflectance is calculated using equation 25 and 26. The test panes absorptance is calculated with equation 27. The heat transfer factor qi, is calculated by combining equation 21 and 22 in equation 28.

The in-house constructed spectrophotometer is based on a SpectraPro-275 spectrophotometer from Acton Research Corporation. The SpectraPro-275 is 0.275 meter triple grating spectrophotometer/spectrograph. The spectrophotometer is used to simulate solar radiation and can measure transmittance and reflectance. The range of the spectrophotometer is divided into two parts, the first range from 300 nm to 1100 nm and the second range from 1100 nm to 2500 nm. The measurement steps between 300 nm and 1100 nm is 10 nm, and the measurement steps between 1100 nm and 2500 nm is 50 nm. The software utilized for the experiment was Hewlett Packard’s VEE version 4.01. Figure 7 shows a basic sketch of the spectrophotometer.

Figure 7: An overview of a spectrophotometer.

(22)

3.2 Heat leakage of the Truck Cab, Experiment 2

The U-value of the cab is obtained by heating the cab until thermal equilibrium is reached, i.e. when the temperature inside the cab reaches maximum compared to the temperature outside the cab. The heat source is placed inside the cab and has a fixed power output. By measuring the cab temperature and the outside temperature the U-value can be determined using equation 15.

The experiment was carried out in a large garage at Volvo in Gothenburg, with constant indoor temperature. The heat fan was from “Engströms Mekaniska Verkstad AB”. The power output has two settings, 600 W and 1200 W. A PM300 energy meter from UPM was used to measure the power output of the heat fan. The temperature was measured with a FLUKE 52 K/J thermometer. The cab was a standard L2H2 cab as shown in figure 8a and the placement of the measurement equipment is shown in figure 8b.

Figure 8a: A standard size L2H2 cab. Figure 8b: The heat fan was placed on the floor of the cab and the thermometer was placed at the driver’s chest level.

(23)

Figure 9: A schematic over the general dimensions of the cab.

The total area of the truck is 20.3 m2. The area of the glazing is 2.5 m2. The results of experiment 2 are presented in section 4.2

3.3 Heat leakage of the test panes, Experiment 3

To experimentally determine the U-value of the test panes the heat flow through the sunroof needs to be measured. Equation 15 was used to determine the U-value in experiment 3. To create an environment for the experiment a constant power input is fed to an insulated box, shown in figure 10a and 10b. Inside the box a heat source was placed to create a constant power. In this case a normal 100 W light bulb was used as the heat source. Precautions were made to make sure that the energy from the lamp was not radiated through the glazing, the lamp screen was covered and directed to a corner of the box. Further, the efficiency of the light bulb mostly generates IR-radiation, for which glass is opaque.

(24)

Figure 10a: The inside area the box was 1.2 m2. Figure 10b: The insulated box with

attached sunroof

The walls and bottom of the box were constructed of 50 mm thick polystyrene, with a U-value of approximately 0.5 W/m2K. The box elements were glued together with Montage Ultra, from Plastic Padding. The extra 50 mm frame was fixed to the box to give the construction extra support. The sunroof was fixed in place by the walls of the box and duck tape. The panes were also fixed to the sunroof with tape, so they could easily be removed and to prevent air leakage at the edges of the pane. The glass area of the sunroofwas approximately 0.22 m2 as shown in figure 11. The sunroof is normally fitted with a protection glass from Saint: Gobain, with a light transmittance of approximately 10 %.

(25)

The heat leakage from the box was first tested in experiment 3a. The sunroof was replaced with a polystyrene lid to investigate how well insulated the polystyrene part of the box was. A 100 W lamp was mounted inside the box together with a thermometer to provide the heat. To find the heat leakage the lamp was turned on and the temperature inside and outside the box was measured. The temperature maximum was measured together with the temperature outside the box, so that the heat leakage could be found using equation 15. The test was concluded when the temperature rise was lower than 0.1 °C for 30 minutes. The test was also performed using a fan to simulate convection, the fan is shown in figure 12.

Figure 12: A drum fan was used to create the convection and an anemometer was used to measure the wind speed.

Furthermore, in experiment 3b the U-value of the box was tested with the artificial convection. In experiment 3c the sunroof was fitted to the box as in figure 10b. Again the 100 W lamp was used as an internal heat source. The experiments where completed when the temperature rise was less than 0.1 °C per 30 min. The lamp was directed to the bottom of the floor to minimize the light transmittance through the sunroof pane. Experiment 3d was conducted in the same way as experiment 3c except that the sunroof was exposed to the artificial convection. The results are presented in section 4.3.

3.4 Heat Leakage of the Test Panes in a Simulated Environment,

Experiment 4

To find the heat transfer reduction and decrease in maximum temperature the panes were tested in an environment with artificial sun and simulated wind speed at Volvo’s sun simulation tank in Gothenburg. The purpose of experiment 4a was to investigate the heat leakage of the windows with simulated sun. Experiment 4b was conducted to investigate the heat leakage with simulated sun and simulated conductance. The dark state of the electrochromic pane was not tested, due to technical problems.

3.4.1 The Sun Simulation Tank

(26)

behind the windscreen of a cab. The light source has a poor UV spectrum. The lamps used in the sun box are “Osram Power Star HQI-T 250 W/D” [17]. The luminance spectra of the lamps are presented in figure 12a [18] and the lamp is shown in figure 12b.

Figure 12a: The luminance of the “Osram Figure 12b: “Osram Power Star HQI-T 250 Power Star HQI-T 250 W/D” lamp [18]. W/D”

3.4.2 Mechanics and exposure

The 70 lamps are mounted in a sheet metal frame that is suspended in the air by a crane and a pulley, which is supported by a metal structure, which is shown in figure 13a and 13b.

Figure 13a: sun simulation tank, lights on Figure 13b: sun simulation tank lamp frame

The lights can be adjusted both in height and angle. In this way light intensity and exposure can be varied.

(27)

UV-resistant black paint. The rear side is insulated with a 5 mm thick plate of heat resistive polymer. A circulation fan is fitted within the tank to compensate for the temperature differences that might occur.

3.4.4 Test method and procedure

Control and regulation of light intensity is achieved by a digital light meter and a detector with a range between 400-500 nm. The light ramp is adjusted in height to correct the light intensity. The minimum distance between the test object and the ramp is 600 mm. To be able to determine the temperature and the irradiance the box is equipped with thermometers and irradiance measurement devices. Figure 14 shows the locations of the thermometers and the irradiance measurement equipment.

Figure 14: The sun tank and measurement points are: T1 measures the temperature inside the box and is cowered by a sheet of cardboard paper and is thereby not influenced by the direct radiation from the lamps. T2 measures the temperature inside the box and is subject to the direct radiation from the lamps. T3 measures the temperature inside the tank, outside the insulated box and is subject to direct radiation from the lamps. T4 measures the temperature inside the room where the tank is placed. Ts measure the black body temperature at the same level as the test pane outside the box. I1 measures the Irradiance inside the box. I2 measures the irradiance at outside the box at the level of the test pane.

The temperature T1-T4 was measured with two FLUKE 52 K/J thermometers and Ts was measured with a black body thermometer, part of the sun simulation tank. I1 and I2 were measured with a precision type pyrometer (Model, SOLAR 118 HAENNI) placed in a horizontal plane adjacent to the collector.

3.4.5 Sun Simulation, Experiment 4a

(28)

of 365 K the irradiance produced by the lamps was about 1000 W/m2 at the height of the test pane. The box and measurement equipment are presented in figure 15.

Figure 15: The insulated box inside the sun simulation tank.

The test was concluded when the temperature rise was less than 0.1 °C per 30 minutes. The temperature was monitored during the test and it reached its peak after 4-5 hours. The results from experiment 4a are presented in section 4.4

3.4.6 Sun Simulation and Convection Simulation, Experiment 4b

The purpose of experiment 4b was to find the temperature difference between the box at measurement point T1 and T2 compared to T3, with a simulated convection corresponding to a vehicle moving at 65 km/h. The specific speed 65 km/h was chosen because it is a normal driving speed for a truck and the limits of the fan output power. The box was placed as in figure 16a. The box was put at an angle against the fan to better simulate a truck driving as presented in figure 16 b.

Figure 16a: The sun simulation tank and industrial fan set at an angle to simulate a speed

(29)

Figure 16b: The placement of the fan and the box for the convection test.

The setting of the experiment was otherwise the same as in experiment 4a. The balance temperature was reached in approximately 3 hours. The results from experiment 4b are presented in section 4.4

4 Results

Sections 4.1 to 4.4 present the results of the respective experiment. In section 4.1 the optical properties of the test panes are illustrated. In section 4.2 the energy transmittance of the test panes attached to the sunroof is presented. In section 4.3 the truck cab is scaled down to a smaller size and the U-value is determined for the sunroof. In section 4.4 the heat transfer in a simulated environment is investigated. Finally, the results from the experiments were scaled up to a full size cab to determine the reduction of heat transport and maximum temperature.

4.1 Optical Properties of the Test Panes

(30)

Figure 17: Transmittance, reflectance and absorptance of the Clearfloat pane.

The transmittance peaks at 550 nm, with τ = 88.5 %. Figure 17 shows that the Clearfloat pane lets through most of the energy in the visible and NIR range. The reflectance is constant at 7 %. The absorptance varies between 4 -18 % and stabilizes at 9 % in the NIR part of the spectrum.

The Optifloat Green glass transmittance, reflectance and absorptance for visible light and NIR are presented in figure 18.

Figure 18: Transmittance, reflectance and absorptance of the Optifloat Green pane

(31)

The Arctic Blue glass transmittance, reflectance and absorptance for visible light and NIR are presented in figure 19.

Figure 19: Transmittance, reflectance and absorptance of Arctic Blue pane

The transmittance of visible light peaks first at 470 nm, with τ = 58.4 % and again at a smaller peak at 550 nm, with τ = 53.2 %. To give the test pane this characteristic the glass is mixed with iron-oxide and other metal oxides, the features of which are hard to distinguish. The absorptance of the Arctic Blue varies between α = 35-80 % and reaches its highest value at 1100 nm. The reflectance decreases steadily and reaches a constant level after 1100 nm. The Sun Protection glass transmittance, reflectance and absorptance for visible light and NIR are presented in figure 20.

(32)

The average transmittance of visible light is τ = 10 %. The transmittance increases up to 30 % in the NIR spectra. The absorptance of the Sun Protection pane varies between 66-95 %. The reflectance is constant at ρ = 4-5 %.

The electrochromic glass transmittance, reflectance and absorptance for the bleached state are presented in figure 21. The measurements were provided by Chromogenics Swden AB

Figure 21: Transmittance, reflectance and absorptance for the bleached state of the electrochromic pane.

The electrochromic glass transmittance, reflectance and absorptance for the dark state are presented in figure 22. The measurements were provided by Chromogenics Sweden AB.

Figure 22: Transmittance, reflectance and absorptance for the dark state of the electrochromic pane.

(33)

needed interpolation to fit the standard in [13]. The standard uses a fixed step, see [14]. The measurements are presented in appendix 1. The final results from the optical measurements are presented in table 3.

Table 3: An overview of the optical properties of the test panes, where τe is the direct solar

transmittance, ρe is the direct solar reflectance, αe is the direct solar absorptance, τvis is the

visible light transmittance, ρvis is the visible light reflectance, αvis is the visible light

absorptance, U is the heat leakage per area unit and unit temperature and g is the total solar energy transmittance. τe (%) ρe (%) αe (%) τvis (%) ρvis (%) αvis (%) U (W/m2K) g (%) Clearfloat 83 7 10 88 8 4 5,9 85 Optifloat Green 46 5 49 75 7 8 5,9 59 Arctic Blue 38 5 57 56 6 38 5,9 52 Sun Protection 9 4 87 10 5 85 5,9 31 Electrochromic Bleached 65 14 21 73 11 16 5,9 70 Electrochromic Dark 17 9 74 21 8 71 5,9 36

The results in table 3 have been calculated with “Window Calc”. The program is available at the solid state physics department, Uppsala University.

4.2 Heat Leakage of the Cab

The purpose of experiment 2 is to find the U-value of the cabin. The heat leakage is obtained by placing a heat source with constant power output inside the cab, while measuring the temperature inside and outside the cab. The heat source provides as much heat as is transferred from the cab to the outside when the cab reaches balance temperature. The U-value is calculated by using equation 15. Experiment 2 was used as reference measurements for Annika Karlsson’s degree project.

The results for the first test in experiment 2, with the heat source set at 600 W is presented in table 4. The outer surface area of the truck is A = 20.3 m2. The temperature in the garage was constant around Tout = 21.3 °C. The heat fan output varied between 686-709 W.

Table 4: The temperature of a L2H2 cab was measured while being heated with a 600 W heat source, under a period of 12 hours.

(34)

To investigate if there were any differences with the higher power setting of the heat fan, it was turned up to 1200 W. The result for this test is presented in table 5. The temperature in the garage was constant around Tout=20.1°C. The fan output varied between 1296-1303 W. Table 5: The temperature of a L2H2 cab was measured while being heated by a 1200 W heat source, over a period of 12 hours.

Time Temperature (min) (°C) 0 21,8 130 27,8 270 40,8 395 42,3 720 42,5

The balance temperature is calculated with equation 15. In this case Ucab2 = 58 W/K, which is in agreement with the lower power result.

The final test was to investigate the cool-down rate of the cab. The cab was heated for a 1 000 min and reached a temperature of 42.8 °C, with an outside temperature 18.5 °C and an average power of 1340 W. This corresponds to U = 55 W/K. Table 6 shows the measurements of the cool-down test.

Table 6: The cool-down test was run for 12 hours.

Time Temperature (min) (°C) 0 42,8 141 27,2 286 22,9 380 21,6 720 20,1

The cool-down test acts in a similar way as the heating tests, both have a quick temperature change in the beginning, which evens out with time. The average U-value for the cab is approximately U = 58 W/K.

4.3 Heat Leakage of the Test Panes

(35)

4.3.1 Heat Leakage of the Insulated box

In experiment 3a the U-value of the polystyrene box was tested. The test was completed in 3 hours and the results are shown in table 7. The insulated box was shut with a polystyrene lid to determine the U-value of the box without the sunroof. The temperature outside the box was Tout = 22 ° C. The heat source output was approximately 103 W.

Table 7: The rise in temperature inside the box was measured, while being heated by a lamp.

Time Temperature

(min) (°C)

0 21,9 142 104,9 186 105

The temperature difference between the outside air and inside the box is ΔT = 83 K. The power input is 103 W. Equation 15 gives Upolybox = 0,83 (W/m2,K).

4.3.2 Heat Leakage of the Insulated box Exposed to Convection,

In experiment 3b the U-value of the box was tested with a forced convection to simulate wind speed. The test was completed in 4 hours and the results are shown in table 8. The insulated box was shut with a polystyrene lid to determine the U-value of the box without the sunroof. The temperature outside the box was Tout = 21.8 ° C. The heat source output was approximately 103 W. The wind speed was approximately 18 m/s.

Table 8: The rise in temperature inside the box was measured, while being heated by a lamp and exposed to simulated convection.

Time Temperature

(min) (°C)

0 21,9 200 104 240 103,6

The temperature difference between the outside air and inside the box was ΔT = 82 K. The power input was 104 W. Equation 15 gives Upolybox,conv = 0,85 (W/m2,K). The heat leakage from the polystyrene part of the box was later deduced from the heat leakage calculated in experiment 3c and 3d.

4.3.3 Heat Leakage of the Test Panes

(36)

Table 9: The rise in temperature inside the box was measured, while being heated by a lamp.

Clear Float Arctic Blue

Time (min) Temp (°C) Time (min) Temp (°C)

0 22 0 22,3 255 52 207 52,6 287 51,9 288 52,7

Optifloat Green Sun Protection

0 22,2 0 22,4 310 52,4 253 52,2 366 52,1 372 52,2

The total inside area of the box is 1.2 m2, and the area of sunroof is 0.3 m2, according to figure 10a and 11. The average temperature difference between the box and the outside was ΔT = 30.1 K. The average input power is 103 W. The heat leakage from the test panes was calculated from equation 15 with the heat leakage from the box deducted. The U-value was approximately the same for all the test panes Up = 8.1 (W/m2K). Compared to the measured values for vertical panes ranging from 5.7-5.9 (W/m2K) the result for a horizontal pane is not unreasonable. The difference that might occur lies in the fitting of the sunroof to the cab and the insulated box.

4.3.4 Heat Leakage of the Test Panes Exposed to Convection,

In experiment 3d the U-value of the sunroof was tested with a simulated speed wind corresponding to a truck driving at 65 km/h. The measured temperatures are presented in table 10.

Table 10: The rise in temperature inside the box was measured, while being heated by a lamp and exposed to simulated convection.

Clear Float Arctic Blue

0 2,19 0 22,1 260 47,7 235 47,2 300 47,7 281 47,4

Optifloat Green Sun Protection

0 22,2 0 22 290 47,5 260 47,6 310 48,1 315 47,8

(37)

5.4 Heat Leakage of the Test Panes in a Simulated Environment

In experiment 4 the temperature difference between the inside of the insulated box and the outside of the box when exposed to simulated solar radiation was determined. Experiment 4a simulates a parked truck and experiment 4b simulates a truck driven at 65 km/h. These experiments were performed to find the difference in heat flow through the different test panes, and the difference between a stationary and a moving truck.

5.4.1 Sun Simulation

In experiment 4a the temperature difference between the inside of the box and the outside of the box was measured. The lamps heated the inside of the box until it reached a maximum temperature. The experiment was carried out for 4-5 h per pane. The experiment was concluded when the temperature increase did not supersede 0.1 °C per 30 minutes. The result of the measurements is presented in table 11. The difference between T1 and T2 is that T1 is covered by a sheet of cardboard paper so that the radiation does not hit it directly. The positions of the measurement points are shown in figure 14. The average temperature outside the box was 40 °C.

Table 11: The temperature T1 is the measurement point inside the insulated box, not exposed to direct radiation. The temperature T2 is the temperature inside the box, exposed to direct radiation.

Clearfloat Arctic Blue

Time (min) T1 (°C) T2 (°C) Time (min) T1 (°C) T2 (°C)

0 30,6 31,5 0 33,9 33,6

15 54,4 58,3 15 47,5 56,7

180 93,5 92 175 80,2 92,2

225 94,2 93,7 210 80,6 92,4

240 94 92,1 240 80,5 92,5

Optifloat Green Sun protection

0 25,2 25,7 0 23 22,7 15 43 50 15 33,4 45,1 260 58,3 90,6 190 77,4 90,2 275 85,7 90,8 215 77,7 90,8 300 85,4 90 250 77,5 90,6 Electrochromic Time (min) T1 (°C) T2 (°C) 0 28,9 29,2 5 31,3 36,8 180 81,3 87,6 210 82 87,6 240 82,3 87,6

(38)

experiment could be found. With the result of experiment 1 presented in table 3 and equation 10 the power transmitted, reflected and absorbed was determined and is presented in table 12. The average temperature Ta is found by adding the two most recent values of T1 and T2 respectively and dividing by 4. The average temperature difference was also calculated by subtracting the average temperature outside the box, which was 40 °C.

Table 12: Pin is the power needed to heat the inside of the box to Ta, which is the average

temperature in the box. The power transmitted through the glazing is represented by Pτ. The power reflected by the glazing is represented Pρ. The power absorbed by the glazing is represented by Pα. Normal Ta (°C) ΔT (°C) Pin (W) Pτ (W) Pρ (W) Pα (W) Clearfloat 93,5 53,5 183,3 182,6 15,4 22,0 Optifloat Green 88,0 48,0 164,4 101,2 11 107,8 Arctic Blue 86,5 46,5 159,3 83,6 11 125,4 Sun Protection 84,2 44,2 151,3 19,8 8,8 191,4 Electrochrom bleached 84,9 44,9 153,7 143 30,8 46,2

Table 12 shows that the difference between having a clear float pane in the sunroof compared to having an Optifloat glazing is approximately 20 W with irradiance of 1000 W/m2 and when the truck is parked. The difference of energy let into the box is presented in figure 23. The differences in temperature in the box between the different test panes are presented in figure 24.

Power flow into the box

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 Clearfloat Optifloat Green

Arctic Blue Sun

Protection

Electrochrom bleached

Power

(W)

Figure 23: The power let into the box by the different test panes.

Temperature difference 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Optifloat Green Arctic Blue Sun Protection Electrochrom bleached

Cent ig ra d e s ( C )

(39)

The temperature difference between the solar control windows and the Clearfloat window shows that it is possible to lower the maximum temperature by 5 °C using the Optifloat Green pane. It would have been very interesting to see the result for the dark electrochromic state. An estimate of the temperature and energy reduction for the dark electrochromic state would be somewhere between the Arctic Blue pane and the Sun Protection pane. That would lead to an approximate power reduction of 30 W for the sunroof, and a maximum temperature decrease of approximately 8 °C compared to the clearfloat pane.

5.4.2 Sun Simulation and Convection Simulation

A fan was mounted in front of the insulated box to simulate the wind speed of a vehicle in motion, the setup of the experiment is illustrated in figure 16a and 16b. The average speed was chosen to around 18 m/s (64.8 km/h). In experiment 4b the temperature difference between the inside of the box and the temperature outside the box was measured. It took approximately 3 h for the system to reach maximum temperature. The result of experiment 4b is presented in table 13. The difference between T1 and T2 is that T1 was covered by a sheet of cardboard paper so that the radiation did not hit it directly. The measurement points positioning are shown in figure 14. The average temperature outside the box was 40 °C.

Table 13: The temperature T1 is the measurement point inside the insulated box, not exposed to direct radiation. The temperature T2 is the temperature inside the box, exposed to direct radiation.

Clearfloat Arctic Blue

0 23,6 23,4 0 35,1 34,4

15 52,2 48,1 40 56,3 53,5

150 75,3 63,5 150 59,5 54,9

160 75,2 63,4 165 59,6 54,6

170 75,4 63,6 180 59,8 54,6

Optifloat Green Sun protection

0 33,2 33 0 31,2 30,7 15 52 50 15 38 43 90 63,7 57 150 49,7 51 120 64,5 57,7 165 49,9 51,1 140 65 57,6 180 50 51 Electrochromic Time (min) T1 (°C) T2 (°C) 0 24 23,3 25 39,1 53,2 150 66,3 56,3 175 66,6 58,2 190 66,7 58,1

(40)

The results are presented in table 14. The average temperature Ta was found by adding the two most recent values of T1 and T2 respectively and dividing by 4. Secondly, the average temperature difference was calculated by subtracting the average temperature outside the box, which was 40 (°C)

Table 14: Pin is the power needed to heat the inside of the box to Ta, which is the average

temperature in the box. The power transmitted through the glazing is represented by Pτ. The power reflected by the glazing is represented Pρ. The power absorbed by the glazing is represented by Pα. The average wind speed is 18 m/s.

Convection Ta (°C) ΔT( C°) Pin (W) Pτ (W) Pρ (W) Pα (W) Clearfloat 69,4 29,4 119,1 182,6 15,4 22 Optifloat Green 61,2 21,2 85,9 101,2 11 107,8 Arctic Blue 57,2 17,2 69,5 83,6 11 125,4 Sun Protection 50,5 10,5 42,5 19,8 8,8 191,4 Electrochrom bleached 62,4 22,4 90,7 143 30,8 46,2

Table 14 shows that the difference between having a clear float pane in the sunroof compared to having a sun protection glazing is 80 W during a very sunny day when the truck is driven at 65 km/h. The power let into the box by the different test panes is shown in figure 25

Power let into the box by the test panes, when exposed to forced convection

0,0 20,0 40,0 60,0 80,0 100,0 120,0 140,0 Clea rfloa t Opti float G reen Arct ic Bl ue Sun P rotect ion Ele ctro chro m bl eache d Po w e r ( W )

Figure 25: The power flow into the box with the different test panes while exposed to simulated convection, corresponding to the truck driving in 65 km/h.

(41)

5.4.3 Up-scaling of the Results

The power flow into the box in experiment 4a and 4b are compared in figure 26 to evaluate the power input into the box and the differences caused by convection.

Comparison of the power input without and with forced convection 0,0 40,0 80,0 120,0 160,0 200,0 Cle arflo at Opti float Gre en Arcti c B lue Sun P rotec tion Ele ctroc hrom ble ach ed Po w e r ( W ) Normal Convection

Figure 26: The power input through the sunroof of the truck being parked and driven.

Figure 26 shows that the speed increased convection is more important for the panes with high absorption. A solar control window with better reflective abilities would most likely not be affected in the same way.

It is very difficult to scale up the results from the sunroof size to the full cab, due to the complicated nature of heat transfer. The glazed area of the truck is shown in figure 9 and is approximately 2.5 m2. To investigate the difference between the power flow into the cab using the different test panes, the same irradiance as in experiment 4 was assumed. The inflow of power using just the total solar energy transmittance, g-values, from experiment 1 leads to a difference in heat transfer depending on which window is used, corresponding to a stationary truck, presented in table 15.

Table 15: The inflow of power through the truck glazing.

Pin (W) Clearfloat 1700 Optifloat Green 1180 Arctic Blue 1040 Sun Protection 620 Electrochrom bleached 1400

(42)

Table 16: The total solar transmittance adapted to a forced convection, and the corresponding power flow into the cab.

gcon (%) Pin,con (W) Clearfloat 85 1700 Optifloat Green 54 1080 Arctic Blue 47 940 Sun Protection 23 460 Electrochrom bleached 68 1360

If the differences of power transfer from table 12 and 14 are used instead, the results are more reasonable, They are presented in table 17.

Table 17: The Clearfloat power input from table 12 and 14 are used as a reference value Pin* = 183.3 W and Pin,con =119,1*.

Pin Pin*/Pin Pin (W) Pin,con Pincon*/Pincon Pin,con (W)

Clearfloat 183,3* 1,00 1700 119,1* 1,00 1700

Optifloat Green 164,4 0,90 1525 85,9 0,72 1226

Arctic Blue 159,3 0,87 1478 69,5 0,58 992

Sun Protection 151,3 0,83 1403 42,5 0,36 607

Electrochrom bleached 153,7 0,84 1426 90,7 0,76 1295

If a truck was equipped with an Optifloat glazing instead of a Clearfloat glazing the power input into the cab is reduced by 175 W or 88 W/m2 glazing. Driving at 65 km/h would decrease the heat transport significantly by 474 W or 237 W/m2. The interesting aspect with using electrochromic glazing would be that the transmittance could be lowered to 20 % while the car was parked. This would reduce the power flow with approximately 260 W or 130 W/2.

5 Truck Driver ISurvey

The purpose of the survey was to get a subjective overview of the improvements made by installing solar control windows and electrochromic windows in trucks. For the development of new technical solutions for thermal comfort and sun protection the customer is a useful resource. The aim of the survey was to investigate the attitude towards the existing climate in the cab, and the attitude toward improvements possible with new solar control technology. The survey was made by Uppsala University in co-operation with Volvo and Chromogenics. The following disposition was the main structure of the survey:

• Starting point • Method

• Implementation • Results

(43)

5.1 Starting point

The participants in the survey were truck drivers, preferably long distance drivers. The aim was to give the survey to between 30 and 40 drivers. The survey concerned the cabin climate and comfort, and how they can be improved through the use of solar control and electrochromic glazing.

The survey consist of a fixed set of questions. The driver was asked to answer question from a set of pre-stated alternatives, or for more complex questions the driver was asked to express their answers freely. The replies from the pre-stated questions are easier to summarize, although the more complex questions contain more information [19]. The answers can also be related to a value scale. This is often done when the respondent is asked to assess something, e.g. a function or the quality of an object. If the driver answers the enquiry freely the responses might be misunderstood [20].

The alternative of making interviews was considered, but was deemed to be too time consuming [19] and to be avoided if possible. However, the advantage of an interview in contrast to an survey is that the questions can be more specific, misunderstandings can be avoided, and follow up questions can be asked to investigate new aspects. The drivers might express themselves differently by speech than writing. Nonetheless interviews were expected to be harder to compile and interpret, due to spin off questions, and were discarded

5.3 Method

The composition of the questions was discussed with our supervisors at Volvo and Chromogenics. The questionnaire was first sent out to a small test group, contacted through Chromogenics, to examine what needed to be clarified and if there were any questions not covered by the survey. The result of the test showed that it would be best to have a brief introduction of the technical aspects of the electrochromic windows. The person conducting the survey should first inform the respondents about the technology of the solar control windows and electrochromic technology, show some glass panes with different transmission so the drivers get a feeling of the different transmissions, and try to have a discussion with the drivers after they have finished with the questionnaire to note new ideas and problems that the drivers wanted to discuss rather than write down. The questionnaire can be found in Swedish in appendix 2 and in English in appendix 3. The Swedish version was given to the drivers.

5.4 Implementation

(44)

5.5 Results

The total number of participants in the survey was 39. It was hard to arrange meetings with long haul drivers because they spend most of their time on the road. The majority of the respondents participating in the survey were distribution drivers. Of the 39 participants 10 were long haul drivers and 29 were distribution drivers.

5.5.1 General Truck and Driver Information

Question 1 to 5 of the questionnaire concerns the brand and age of the truck, and the experience of the drivers, which is presented in table 17.

The two dominating truck brands among Swedish truck drivers are Volvo and Scania. Of the 39 drivers; 19 drove a Volvo, 6 drove a Scania, 7 drove a Mercedes, 3 drove a Renault, 1 drove a MAN and 3 drivers drove lots of different brands. The model year of the trucks was from 1996-2006. The average production year of the trucks is 2002 and the median production year was 2003.

Solar Control Glazing for Trucks

Examensarbete 20 p

Mars 2007

Solar Control Glazing for Trucks

Abstract

Solar Control Glazing for Trucks

Sammanfattning

Contents

1 Introduction

1.1 Background

1.2 Purpose

1.3 Method

2 Theory

2.1 Solar Radiation

2.2 Heat transfer

(

)

2.3 Window Physics

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

(

)

(

)(

)

(

)

[

]

[

]

( ) ( )

( )

∫

∫

( ) ( )

( )

∫

∫

(

)

3 Experiments

3.1 Optical properties of the Test Panes, Experiment 1

3.2 Heat leakage of the Truck Cab, Experiment 2

3.3 Heat leakage of the test panes, Experiment 3

3.4 Heat Leakage of the Test Panes in a Simulated Environment,

Experiment 4

4 Results

4.1 Optical Properties of the Test Panes

4.2 Heat Leakage of the Cab

4.3 Heat Leakage of the Test Panes

5.4 Heat Leakage of the Test Panes in a Simulated Environment

5 Truck Driver ISurvey

5.1 Starting point

5.3 Method

5.4 Implementation

5.5 Results

_{( )}

_{( )}