Category Archives: BACKGROUND

Selection of vegetal species

The selection of the most appropiated plants is an important question. The aspects that are being considered in the project are : preferent growing directions, speed of growing, maximum size and structural loads, selection of suitable decidous/evergreen species (the periods without leaves must be selected according to the heating/cooling demands of the
buildings), volume of earth substrate, attraction of animals (birds, insects), low toxicity, main­tenance works needed and their frequency and commercial availability.

Conclusion and lookout

It has been shown before that microstructures are suitable for the use as light-shading devices. Also the technical feasibility was proved and prototypes with structured surfaces with periods of approx. 20 pm were generated and replicated in transparent plastic up to a size of 375 x 375 mm[28] [29] [30] [31] [32]. The structures have been coated successfully, both all over and face-selectively. It has been shown that they add a new free design parameter to the conception of microstructured sun shading devices.

Switchable coatings have been attached successfully, too. This offers a wide range of concepts for new controllable light guiding devices like the example shown above. In further research, several of these concepts will be investigated and optimised. Also other ways to combine microstructured surfaces with an optical switching e. g. the combination with thermotropic layers will be analysed

Acknowledgements

The authors wish to acknowledge the contributions of C. Buhler, B. Blasi (Fraunhofer ISE) and of J. Mick (of IMTEK, Univ. of Freiburg, Germany) to the work presented here.

The samples build with a ultra precision milling cutter (see figure 8) where manufactured at the Institut for Microsystem Technology (IMTEK), A.-L.-University Freiburg by J. Mick and C. Muller

The work presented here was funded by the German "Bundesministerium fur Wirtschaft und Arbeit BMWA" under reference number 0327312 A and B. The authors assume the responsibility for the contents of this paper.

Experimental and Simulation Results

The design of the two double fagade sections is based on an earlier simulation study by Charron and Athienitis [4]. The double fagades in the outdoor test-room were fully instrumented with thermocouples attached at several points on the cavity surfaces, hot wire anemometers for velocity measurement, pressure sensors and a weather station. Note that the back panel in each fagade consists of polystyrene enclosed on both sides with 1 cm thick plywood panels; the total RSI value is equal to 1 and it has negligible heat storage. The heat transfer through the panel to the room was negligible compared to other energy flows.

Figure 2. Results from section with PV on outer skin of double fagade for January 26; quasi-steady state conditions near solar noon (gap width Li=91 mm; height = 0.99 m, width = 0.91m; two photowatt panels are connected in series; S = total incident solar radiation; V = average velocity in cavity).

Figures 2 and 3 below compare results from the two fagades for January 26, 2004. This day was cold and clear with negligible wind. The data were collected every minute and averaged for about half hour from 11:20 am to 11:50 am. Quasi-steady state conditions existed with no major (not more than 5%) change of any of the parameters measured.

Tmix-4.4°C

Figure 3. Results from section with PV as middle layer for January 26, 2004; quasi-steady state conditions near solar noon (Lo=35mm, Li=55 mm).

As can be seen from Figure 3, much higher thermal efficiencies are obtained when we have air flow on both sides of the PV than when the PV panel is exposed. For the case of Figure 2, the electricity generated was 87 W and the thermal energy (heating of air) 337 W without taking into account the significant heating at the inlet; as can be seen from Fig. 2 the air at the inlet is heated about 3 °C and this effect may also be due to some air leakage from the room into the cavity. The resulting electrical efficiency was about 10% and the thermal efficiency 37% for a total efficiency of 47%.

By comparison, in the configuration of Figure 3, the electrical efficiency of the PV was only 6% (but it was not at its maximum power point) and its thermal efficiency was 65% for a total of 71%. One disadvantage of the double cavity configuration of Fig.3 is that the PV operates at a higher temperature — a maximum temperature of 40.7°C when the outside temperature is -17 °C. The temperature of the air exiting the cavities may be controlled by varying the flow rate and mixing with indoor air.

Studies of the temperature and velocity profiles across the horizontal (air gap) were also performed. Figure 4 shows a typical inlet and outlet temperature profile corresponding to the measurements in Fig. 2. The velocity profiles that were used to compute the average velocity (equal to 0.6 m/s) showed a buoyancy-induced peak near the PV followed by flat region in the middle. The flow is complex, definitely a mix of natural and forced (fan — induced) convection, laminar at the inlet and turbulent near the top of the cavity.

EFFECT OF MULTIPLICATION OF THE AMOUNT OF DESORBED HYDROGEN MOLECULES IN RESPECT TO THE MAGNITUDE OF THE MOIST AIR INFLOWING MOLECULES (MADHM-EFFECT)

In the course of experiments, it has been obtained that the pressure of desorbed hydrogen molecules is approximately by 21 times as much as the calculated pressure of moist air being supplied through the microleak.

So a multiplicative effect [6,7] is realised on the semiconductor surface of the superinsulation. It should be noted that similar multiplicative phenomena have been observed
in the work [41] where the argon desorption stimulation has been investigated by means of the addition of oxygen. The relative enhancement of the desorption in the presence of oxygen has been increased approximately by 20 times as compared with pure argon crystals.

Production of Heat and Electricity

A block diagram of both heat and electricity production in the pilot project "Solar roof Spansko-Zagreb" is presented in Figure 3. Solar thermal collector with surface area of 10 m2 converts solar energy to heat. The difference between absorbed heat and heat looses is denoted as a useful heat. Working fluid transfers heat into a well-insolated 750-liter storage tank. In this way the useful heat is stored for needs of space heating and hot domestic water preparation. City gas as environmental acceptable fuel is used as a backup source of energy.

Figure 4 presents distribution of measured units in solar thermal — gas system described here for space heating and hot domestic water preparation. The measurement line consists of five transducers:

1. Consumed hot water transducer

2. Solar collectors transducer

3. Backup gas heater transducer

4. Space heating transducer

5. Solar collector used for the space heating transducer

The transducers are integrated with a flow sensor, two temperature sensors and a display. The distribution of the thermal transducers measuring points in the system is shown in Figure 3. Remote supervision and measurements is provided using a PC with
dedicated software on the Windows platform via serial interface and a modem connection. Some parameters are displayed on the outside panel mounted in front of the house (Figure 2).

Thermal energy balance of consumed hot water:

sol cas

Qchw Qchw Qchw

Thermal energy balance of space heating:

Qsh = QT + Qh,

Qchw Consumed hot water thermal energy Qcshowl Solar collector thermal energy QgZ Backup gas heater thermal energy Qsh Space heating thermal energy

Qsghas Backup gas heater used for space heating thermal energy

Є

sol

sh Solar collectors used for space heating thermal energy

According to the power purchase agreement with the local utility, the counters of electric energy register the amount of electric energy delivered to the network and electric energy supplied from the network. There are three additional counters of electric energy connected to the PC. These are counters of produced electricity, consumed electricity and surplus energy delivered to the network when the needs of the loads in the house are satisfied (Figure 2).

Required safety measures are systematically applied, according to Croatian and European regulation for this type of system. When one or more phases in the distribution network switches off, the additional circuit breaker installed in the switchboard disconnects the PV generator from the utility.

Figure 2 External visualization board


Figure 4 Block diagram of solar — gas system for space heating and domestic hot water
preparation in the pilot project “Solar roof Spansko-Zagreb”

Dynamic simulations

By means of the calibrated model the system operation was simulated from June to August using climatic data for a typical year in Milano. The aim was to verify the importance of the progressive temperature increase in the perturbed ground on the system performance.

An on/off control acting on the pumps was introduced in the simulation model, with an air temperature set point T set = 24.5 °C. The mass flow rates had the same values as in the experimental campaign. Simulation results, in terms of discomfort and COP, are reported in Table 1 for each month and for the whole summer. The minimum discomfort is obtained in June but even in July and August it assumes very low values: discomfort values in the cooled room are about ten times lower than in the reference room.

discomfort|free floating

(h)

discomfort|sys

(h)

relative discomfort index

COP

June

230

11

0.05

3.1

July

797

93

0.12

2.6

August

584

68

0.12

2.1

Whole

summer

1611

172

0.11

2.4

Table 1: dynamic simulation results for the system in its current configuration

Assuming again typical weather conditions for Milano, a parametric study was carried out investigating the influence of the following parameters:

— the ground loop mass flow rate mg and the room loop mass flow rate mr

— the area of the radiant ceiling surface Ac

— the number of pipes N, the length of each pipe L and the distance between the axes of adjacent pipes d

Variation of any of the above-mentioned parameters affects the pressure drop in the loops. Then for each configuration the power demand of the pumps was calculated by considering the characteristic curve (pressure drop vs mass flow rate) of the system and average values of mechanical efficiencies for commercial pumps.

Mass flow rate. The influence of the mass flow rates was studied by giving the other parameters the current values in the experimental set up i. e. Ac= 10.8 m2, N = 10, L = 6 m and d = 1.5 m. For simplicity we chose mg — mr. The relative discomfort and the COP

dependence on the mass flow rates are plotted in Figure 4, while Qroom and Epjmps are reported in Figure 5. In the explored mass flow rate range, the flow regime remains laminar and so the convective coefficient in the pipes is the same for all mass flow rates. The pumps energy consumption Epumps rapidly grows with the mass flow rate and drives the COP trend. An optimal operating condition can be found minimising the relative discomfort: choosing a mass flow rate value around 100 kg/h gives the lowest discomfort with a still high energy efficiency.

Geometric parameters. The influence of the four geometric parameters was investigated by setting mg — mr = 100 kg/h and by varying Ac, N, L and d simultaneously, with the values given below:

— Ac: 3.6, 7.2, 10.8 and 14.4 m2 corresponding to respectively 25, 50, 75 and 100 % of the floor surface of the cooled test room

— N: 5, 10, 15, 20 and 30

— L: 3, 6, 10, 15 and 20 m

— d: 0.75, 1.5 and 3 m

From the simulation outputs it results that the dependence of the relative discomfort on the geometric parameters of the ground heat exchanger can be conveniently represented in terms of an effective heat exchange surface Seff. This quantity is defined as Seff = S • d/D,

where S = N.2nRpL is the total heat exchange surface in the ground and d/D is the ratio between the pipe spacing and a suitable linear dimension of the earth surface occupied by the pipes. The factor d/D weights the actual heat exchange surface taking into account the pipes density in the ground. At high density, i. e. low values of d/D and Seff, the tubes will be in contact with a portion of ground where temperatures might be significantly higher than in the undisturbed soil, hence reducing the effectiveness of the heat exchange. As it is shown in Figure 6, for every value of the radiant ceiling surface AC the relative discomfort is an exponentially decreasing function of Seff. The fitting parameters are reported in Table 2.

fitting function: y(Seff) = 1 + A(1 — e Seff/S*c)

Ac (m2)

A

Seff, c (m )

Z2

3.6

0.768

1.561

0.002

7.2

0.959

1.478

0.006

10.8

0.999

1.357

0.001

14.4

1.007

1.251

0.001

Table 2: Exponential fit parameters

The larger AC the lower the horizontal asymptote, i. e. the minimum discomfort achievable. In order to supply the comfort demand the radiant ceiling surface must be larger than 50 % of the ambient surface, but increasing from 75 to 100 % gives marginal advantages.

Design guidelines for the ground heat exchanger can be derived from the decay coefficient Seff. c of the exponential that decreases with AC. Once the effective surface is 4 times Seff, c, any additional increase produces marginal effects on the system performance.

The COP is a decreasing function of the relative discomfort, as shown in Figure 7. In fact the considered variation of the geometric parameters has a significant influence on the delivered thermal energy Qroom but little influence on the system pressure drops and on the electricity consumption Epjmps. Hence the best geometrical configuration from the point of view of comfort is also optimal from the point of view of efficiency.

Conclusions

In this paper the study of a ground cooling system carried out by means of a measurement campaign and dynamic simulations is reported. The experimental results highlight the interesting potentials of the proposed system, looking at the comfort conditions obtained and at the low electricity consumption. A TRNSYS simulation model of the system was developed and calibrated against measured data. A discomfort index and an energy efficiency index were introduced in order to evaluate the system performance. Dynamic simulations on the current system configuration for typical weather conditions in Milano showed that the progressive increase of the ground temperature during summer due to the system operation does not impair the system performance. A parametric study was carried out by changing the water mass flow rates and the main geometric parameters of the radiant ceiling and of the earth-to-water heat exchanger. Increasing the mass flow rate over a certain limit has little effect on the delivered thermal energy but compromises the energy efficiency. An optimal value of about 100 kg/h, giving the lowest discomfort and a rather high COP (around 10), was found. From the point of view of their influence on the relative discomfort index, the parameters of the ground heat exchanger can be grouped together into an effective heat exchange surface in the ground. For every value of the radiant ceiling surface the relative discomfort is an exponentially decreasing function of the effective surface. Design guidelines were extracted from these plots. In order to supply the comfort demand the radiant ceiling surface must be greater than 50 % of the ambient surface and the effective surface must be 3-4 times Seff, c. The COP is a decreasing function of the relative discomfort, so that a geometric configuration which minimises the discomfort maximises the COP at the same time.

The research will prosecute by studying the system behaviour under different climatic conditions and in connection with buildings of different characteristics.

[1]

Figure 2: Operative temperature in test room 1 (cooled) and in test room 2 (free floating),
radiant ceiling surface temperature in test room 1 and outdoor air temperature

SHAPE * MERGEFORMAT

mass flow rate (kg/h)

Daylight Illumination with Fuzzy&Conventional Approach — Experimentation on Real Model

Mateja Trobec Lah*, Dr. Asist., Ales Krainer*, Dr. Prof.,

University of Ljubljana, Faculty of Civil and Geodetic Engineering, Chair for Buildings and Constructional Complexes

Jamova cesta 2, 1000Ljubljana, P. O. BOX 3422 Slovenia

Tel: +386 01 4768 609 fax: +386 01 4250 688 e-mail: MLah@fag. uni-li. si.

* ISES member

Abstract. The main aim of the paper is to present a progressive strategy in automatic control for a shading device — roller blind, based on the fuzzy logic, to obtain the desired inside illumination according to the available momentary solar radiation. To implement the suitable control system for the movable shading device, the combination of the fuzzy and conventional control algorithm was developed. For this purpose we studied the luminous efficacy of the solar radiant energy in all weather conditions. The developed control algorithm contains a cascade control with fuzzy controller as the main and conventional PID — proportional-integral — derivative controller as the auxiliary controller. The illumination fuzzy controller, which is essential for proper roller blind position, was designed progressively during the experimental procedure. It enables us to use the non-linear knowledge about the optical process and to transfer it to an appropriate control action (roller blind alternation) in a way that is close to human thinking. It was optimized iteratively. A well-designed illumination fuzzy controller means moderate continuous movement of the roller blind. It also assures the inside daylight illumination in the area, where the desired value deviates up to ± 50 lx. Automatic active response of the shading device to the outside weather conditions enables the optimization of internal visual performance together with efficient use of energy, also in the sense of the inside thermal performance.

1 Introduction

Using daylighting in buildings is an important and useful strategy in replacing the need for high-grade conventional energy for inside illumination. Energy flows through the building envelope are present all the time. The properties of the building envelope have significant influence on the interaction between the inner and the outer energy conditions in the sense of thermal and lighting flows. The available solar radiation conditioned the response of the building as naturally lit space. The positive aspect of the controlled luminous energy flow through the building envelope is enhanced with the development of the technology, i. e. with the possibility of the automatic active response of the shading elements to the outside conditions. In our case this is realized with a roller blind adaptable to the outside weather conditions. In daylighted spaces the psychological benefit is increased. Also, the lighting energy consumption and the heat gains associated with the electrical lighting are reduced. The main aim of the paper is to present the automatic adaptable roller blind, based on the fuzzy logic control system to obtain the desired inside illumination according to the momentary available solar radiation. A real model of a building, test chamber properly equipped, was built for the development of the fuzzy control system for changeable window geometry. The test chamber allows the investigation and experimentation of illumination control and enables us to study the influences of the movable shade on the luminous efficacy in the interior.

Control system design is a complex procedure usually based on modeling and simulations in the overall iterative approach [1]. In our case the design approach was based only on experimentation. The main focus of the study was on the design and development of the
fuzzy illumination controller. In order to develop an effective illumination control system, sets of preliminary measurements were done, where the daylighting response in the sense of the inside illumination dependent on different sizes of the transparent part of the window and the luminous efficacy was studied. The available daylight inside the building depends on the solar radiation and the building’s geometry. Weather conditions and the level of cloudiness determine the terrestrial total solar radiation and the ratio of diffuse/direct radiation. Actual daylight illuminance in a room is related to the luminance pattern of the sky and also to the window geometry with regard to the room’s dimensions.

Illumination and luminous efficacy correspond to the available solar radiant flux. Spectral distribution of the solar energy is roughly equal to black body spectrum at a temperature of T=5773 K. It is useful to know the impact of the total solar radiation on the optical effect in the interior space. The ratio of the luminous flux Фу [Watt] to the radiant flux Фе [lumen] is defined as luminous efficacy K [lm/W]. Luminous efficacy K of solar radiation is given with the ratio of the luminous flux of the visible light Фу (380 nm < X < 780 nm) and the total solar radiant flux Фу (0 nm <X~ 2500 nm). The maximal value for the solar "overall” luminous efficacy is 93 lm/W. The luminous flux is the part of the radiant power, which is perceived as light by human eye, and the value 683 lm/W is based upon the sensitivity of the eye at 555 nm, which is the peak efficiency of the photopic (daylight) vision curve [2].

In experiments the observed inside luminous efficacy of the solar radiation was the ratio between the measured external available solar radiation (global and reflected) and the measured internal illumination. The observed luminous efficacy K presented in the experiments is:

K [lm/W]: Luminous efficacy = (measured inside illumination)[lx]/(measured external

solar radiation) [W/m2]

Solar luminous efficacy is a quotient, which tells us the real efficacy of the daylight in the sense of internal visual performance taking into account changeable weather conditions. The ratio describes the relationship between the optical and the thermal effect of the available solar energy.

Other sets of experiments were intended to study the relationship between the outside available solar global and reflected radiation and the corresponding inside illumination.

The experiments were done throughout the year using different transparent area sizes to collect and examine the correlation between the solar radiation and the inside illumination. The non-linear mapping between the inside and the outside conditions was used as a basis for defining the fuzzy rules in the illumination fuzzy controllers. For fuzzy rules in the form of if-then statements only the input-output relations are important and not the information about the mechanism, which causes these relations.

The final aim was to create an illumination control algorithm, which enables harmonized variation of the window geometry, i. e. proper positioning of the roller blind in real time conditions. Well designed illumination control algorithm with precise setting of the parameters produces appropriate signals for moderate roller blind alternations according to the outdoor weather conditions and the desired indoor illumination. The result is efficient utilization of the solar energy for the inside harmonized lighting and thermal energy conditions.

Objectives of the project “Sustainable Solar Housing”

Experts from 19 countries (Australia, Austria, Belgium, Brazil, Canada; Czech Republic, Germany; Finland, Indonesia, Italy, Iran, Japan, Netherlands, New Zealand, Norway, Sweden, Switzerland, United Kingdom, USA) work in four topical areas in this project

• Market — Assessment and Communication

• Design and Analysis

• Demonstration

• Monitoring and Evaluation

The target of this project is to examine in detail residential buildings with exceptional performance in the participating countries. Such successes can be used as models for crafting housing to a very high construction quality, efficient energy production and supply systems and unprecedented thermal comfort by at costs level acceptable in the marketplace. To achieve these goals, the experts are exploring ways to achieve:

• less environmental impact over the life cycle of the building,

• greater reliability for the components, systems and the building as whole,

• increased substitution of renewable for non-renewable energy use,

• lower costs than earlier generations of such housing, and

• more market / occupant oriented design, including special features.

New building components are used, such as high performance windows with overall frame and glass combined U-values less than 0.8 W/m2K, reduced thermal bridges in the building shell, high performance insulation systems and very airtight construction. Energy to supply and temper ventilation air is minimized by using mechanical ventilation systems with high efficiency heat recovery, sometimes coupled with an earth-to air heat exchanger or preheating sunspace. The result is a shortening of the heating season and reduction of the needed heating capacity to such an extent that a conventional heating system is no longer needed. Nor, given the minimal heating energy called for, would a conventional heating system make economic sense. By such small absolute energy quantities to amortize the fixed costs of a conventional system, solar systems become more competitive. They require no chimney sweep, exhaust gas control, meter reader, or service subscription and they have a long lifetime. Finally, given the very small demand on non-renewable energy with the resulting drastic decrease in primary energy demand, detrimental effects on the environment are also reduced. Figure 1 illustrates the path of

conversion auxiliary thermal

losses: electricity: losses:

exploration, transport, pumps, fans storage,

power plant, grid,.. controllers, .. distribution, …

Figure 1 Energy flow from the primary energy to the space heating and hot water.

End energy is converted into space heating and hot water by using auxiliary electricity and loss thermal energy in storage and during distribution. If renewable energy is used in a heating system, primary energy for fossil fuels can be reduced.

energy conversion from primary energy to the end uses of space heating and hot water, taking into consideration the conversion losses outside the building and heat losses inside the building.

In conclusion, new high performance housing shows a way to a new generation of fulfilling domestic needs with drastically reduced primary energy demand and the subsequent impact on the environment. Figure 1 draws, that the primary energy demand can be decrease by extension of renewable energies.

Analysing sources of error in building daylighting. performance assessment by comparison of test. modules and scale models

A. Thanachareonkit, M. Andersen and J.-L. Scartezzini

Solar Energy and Building Physics Laboratory (LESO-PB)
Swiss Federal Institute of Technology in Lausanne (EPFL)
CH — 1015 Lausanne (Switzerland)

ABSTRACT

Scale models represent a standard method for the assessment of daylighting performance in buildings. Recent studies however pointed out their general tendency to overestimate work plane illuminances and daylight factors. The cause of this inaccuracy between real buildings and scale models performance is due to several sources of experimental errors, such as modelling of building details, surfaces reflectance, glazing transmittance, as well as photometers features. To analyse the principal sources of errors, the comparison of a 1 : 1 daylighting test module and its 1 : 10 scale model, placed within identical outdoor conditions, were undertaken. Several scale model parameters were modified in order to determine their impact on the performance assessment, comprising the accurate mocking — up of surfaces reflectance, the model outdoor location, as well as the photometric sensors properties. This experimental study showed that large ranges of discrepancies between daylighting performance figures assessed in both ways can occur, some of them being even caused by slight differences of surfaces reflectance and photometers cosine responses. These discrepancies can be curbed down to a 20 % relative divergence, providing that enough time and effort is spent to mock-up the geometrical and photometrical features of the real building.

Keywords : Daylight factors, work plan illuminance, scale models, experimental errors.

INTRODUCTION

Daylighting contributes in essential way to buildings atmosphere and visual amenity of inhabitants. It is generally preferred to electric lighting, offering simultaneously a visual access to outdoor and contributing to sustainable development through displacement and saving of electricity into buildings (IEA, 2000). Scale models represent a standard method for the assessment of daylighting performance in buildings (Schiler, 1987), over passing computer modelling for certain practical daylighting design (Compagnon, 1993). They are usually used to mock-up real buildings and placed within sun and/or sky simulators to proceed to the assessment of their daylighting performance (Michel et al, 1995). Recent studies however pointed out their general tendency to overestimate work plane illuminances and daylight factors in comparison to the figures observed in real buildings.

The main causes of these inaccuracies were identified by former authors (Love and Navvab, 1991), as due to inappropriate model details construction, such as window frames and surfaces reflectance, leading even to parasitic light into the scale model; the calibration of photometric sensors, as well as their size, levelling and placement in the model, were also mentioned as a source of experimental errors. A more recent study

(Cannon Brookes, 1997), confirmed the former one, pointing out other physical factors, such as building maintenance and dirt, as contributors to these discrepancies.

In order to carry out a detailed analysis of the physical parameters responsible for the overestimation of daylighting performance of buildings in scale models, a comparison of illuminances and daylight factors, monitored within a 20 m2 single office room equipped with side lighting windows (a test module) and its 1:10 scale model placed within identical outdoor daylighting conditions, were undertaken. Moreover, the photometric sensors features, which were used in the test module and in the scale model, were compared. The principal causes of the performance overestimation were identified through this comprehensive experimental study : their understanding should lead to the elaboration of more appropriate roadmaps for architects and lighting designers in their assessment of daylighting performance in buildings.

Absolute errors

The error we are interested in is the error in the maximum power absorbed. Figure 7 shows that for a two pane window where the outer pane is a grey 4 mm float glass and the inner pane is a clear 4 mm float glass, the largest values for absorbed power is in the interval 20-30°. In that interval, we have shown that the Angular Variation Model is more accurate than the other two models.

As with the maximum absorbed power analysis (Figure 2), a selection of three panes serves as examples. The three figures 7-9 show that the largest absolute errors are found in the interval 60-80°, depending on what model is used and what pane is examined.

Figure 7 shows the error in the absorptance in the outer pane of a double pane window where the outer pane has a thin silver layer (Ag+) on the inner side of the outer pane.

Figure 7 Absolute error in absorbed power versus angle of incidence for the outer pane in a double pane window where the outer pane has a thin silver layer (Ag+) on the inner side of the outer pane. The errors are shown for all three models and for four orientations for each model.

Figure 8 shows the error in the absorptance in the outer pane of a double pane window where the outer pane has a thin stainless steel and nitride layer (SsTiN) on the inner side of the outer pane.

Angle of incidence (°)

AVM North

CVM North

CPM North

AVM East

-в-CVM East

— S-CPM East

AVM South

-■Ф-CVM South

— Ф-CPM South

AVM West

-A-CVM West

— Д-CPM West

—— AVM North

-S-AVM East -5-AVM South -*-AVM West

CVM North — O—CVM East -*5—CVM South — i-CVM West

— —CPM North — S-CPM East •■S-CPM South — A-CPM West

Figure 8 Absolute error in absorbed power in the outer pane of a double pane window where the outer pane has a thin stainless steel and nitride layer (SsTiN) on the inner side of the outer pane versus angle of incidence. The errors are shown for all three models and for four orientations for each model. Figure 8 shows the error in the absorptance.

Figure 9 shows the error in the absorptance in the inner pane of a triple pane window where both the outer and the inner pane have a thin silver layer (Ag) on the inner and outer side, respectively.

Angle of incidence (°)

Figure 9 Absolute error in absorbed power absorptance in the inner pane of a triple pane window where both the outer and the inner pane has a thin silver layer (Ag) on the inner and outer side respectively versus angle of incidence. The errors are shown for all three models and for four orientations for each model. Figure 8 shows the error in the absorptance.

Figure 9 shows a pane in a window suited for cold climates, like Stockholm. Most of the power absorbed by the inner pane in a window eventually continues into the room and helps heating it. Figure 9 shows the absolute error in maximum power for different incidence angles in the inner pane of a three pane window where the outer and inner panes have a thin film silver layer and the middle pane is clear. Therefore the absorptance in the inner pane is more interesting for energy balance calculations of the room. Figures 7 and 8 show windows suited for warmer climates, solar control windows. Their coatings are "tuned” so that they not only reflect the radiant heat from the surrounding, but also the near infrared heat in order to minimize the need to use air conditioning [2].

When we went through the 62 tested panes, we noticed that the largest errors in maximum power absorbed generally occurred in the outer glass, a pattern that is also present in Figures 7-9 in this report. When studying the energy balance of the room, though, the amount of irradiation absorbed in the inner pane is more relevant.

Table 2 Ten worst approximations of maximum power absorbed. Column one shows the ten highest absorbed maximum powers found for a pane and a certain angle interval. Column two shows error with Clear Pane Model. Column three shows error with Constant Value Model. Column three shows error with Angular Variation Model.

Pane Category

Maximum power absorbed (W/m2)

CPM error in maximum power absorbed (W/m2)

CVM error in maximum power absorbed (W/m2)

AVM error in maximum power absorbed (W/m2)

Grey Grey Inner

156

8

-2

1

A-Si Clear Outer

183

10

-5

-2

SnO2 Clear Clear Outer

185

10

-7

-2

SnO2 Clear SnO2 Outer

185

10

-7

-2

Clear Grey Inner

236

12

-8

0

Grey

291

15

-12

0

Grey Grey Outer

300

16

-12

0

Grey Clear Outer

303

16

-12

0

Ss Clear Outer

311

7

-1

2

SsTiN Clear Outer

331

8

-1

2

Mean values

248

11

-7

0

Standard deviation

66

3

4

2

All extreme values shown in table 2 occur when the window is facing the west. In eight cases the incidence angle was 30° and in one case 20°.

Conclusions

From the 62 panes in the 27 windows we studied, it can be seen that the largest errors typically occur in the outer panes.

The Angular Variation Model gives smaller errors in absorbed power than with the other models. This is not least the case for the ten worst approximations, as can be seen in Table 1, where the errors with the Angular Variation Model are smaller and more evenly distributed. The errors with the Angular Variation Model is on average 0 for the ten worst cases and the standard deviation of the error for these ten cases is 2 W/m2, whereas the average error for the ten worst cases with the Constant Value Model is -7±4 W/m2 and for the Clear Pane Model 11±3 W/m2.

The main disadvantage with the Angular Variation Model compared to the other two models is that one has to know to which of the 27 window categories the window belongs. When that is known, however, this model is better than the other two presented alternatives.

[1] A. Werner, A. Roos, Trying to find an approximation of the angle dependence of the solar absorptance of a window pane, in proceeding from the conference CIB, Toronto, May 2-5 2004.