Category Archives: Sonar-Collecttors

Energy Consumption

The most obvious improvement is the comfort level for those working and visiting the building. In a region which has 50 frosts and below freezing temperatures in winter, over 40 degree C heat in summer, and a diurnal range in spring and autumn of 20 degrees C plus, the benefit of a pleasant work environment goes without saying.

Previously the staff were cold in winter, often supplying their own small electric blow heaters at each desk, whilst in summer only a few offices had individual air conditioning units, and most of the staff sweltered in the heat. A comparison of power consumption over similar periods between the new completed building and the previous two buildings combined, indicates that although floor area has increased, the energy use per m2 is only marginally greater, and at times slightly less — refer Graph 1. Thus, despite the addition of full heating and cooling to the larger building, it has not significantly increased its impact on the environment, producing an average of 109.5 kg CO2/m2 (gross area) annually.

NOTE: Vacation of the main building to enable construction commenced in April, 1999, and the completed building was not fully back in operation until April 2000; Whilst consumption records for the new building have been provided monthly, the old building

Graph 1 —

Series 1 — Old Buildings, May 1998 to Jan 1999 Series 2 — New Building, May 2000 to Jan 2001 Series 3 — New Building, May 2003 to Jan 2004

Further, using the SEDA benchmark for office buildings with full HVAC systems3, rated for NSW at 135 kWh/m2/pa or 486 Mj/m2/pa, this building potentially performs extremely well. Although the figures above are based on gross floor area, it is expected that when a thorough AGBR rating (Australia Building Greenhouse Rating) is carried out, the building will still score extremely well, most likely above industry best practice.

Taking into account the increase in both floor area and services, the increased energy use and therefore emissions generation is in effect less than that generated by the old buildings — refer Table 3. And this is in a climate of extremes and heavy heating and cooling loadings.

SERVICES & ENERGY

Old Buildings

New Building

Facilities Provided —

HVAC system

Individual as hoc heaters & fans

Lift

Auto doors

Networked computer systems

Electronic security system

Extensive communication system

Controlled thermal comfort

Total Energy Consumption (av., based on gross area

KWh/m2/pa

94.9

109.5

Mi/m2/pa

341.64

394.2

Kg CO2/m2/pa

94.9

109.5

Table 3 — Energy & Services Comparison between Old and New Buildings

Design Parameters Influence on Flux Distribution through Prismatic Channels of Volumetric Absorbers

Marcelino Sanchez, Maria J. Marcos, Manuel Romero, CIEMAT, High Solar Concentration Systems, Avenida Complutense 22, 28040 Madrid, Spain

Claudio Estrada, CIE-UNAM, Aptdo. Postal 34, CP 62580 Temixco, Morelos, Mexico

The application of monolithic volumetric receivers to solar processes is increasing due to their optimal optical-thermal properties. At high temperature, the volumetric absorber design means that the effective area for solar absorption is many times larger than the receiver aperture area, this effect leads to a minimization of radiation losses. For thermal and photochemical applications monolithic catalysts have significant advantages in lowering pressure drop and improving both chemical and photon contact surfaces. For those and other applications the precise calculation of photons penetrability as a function of monolithic parameters such as pitch length, wall thickness, and material reflectance, has to be performed in order to choose the optimal configuration. Photon trap efficiency and thermal performance is not at all trivial for this 3D structures where contradictory effects may appear regarding “volumetricity”. The authors analyze the influence and the relationship between that parameters. A sensitivity analysis of photons penetrability is performed by means of ray tracing techniques and discussed in connection to three-dimensional CFD simulation of heat transfer efficiency and thermal losses for a single channel. The results obtained suggest that many experimental experiences with volumetric receivers reported until now are noticeable away from a theoretical highly volumetric structure and that operational limits still remain unexplored offering enough room for design improvements.

Introduction

Porous volumetric absorbers have a large potential for use in receivers of solar thermal power plants, high-temperature catalysis or hot air supply in industrial process heat. Early solar prototypes of several-hundred kW were developed by Sanders in the USA in late seventies [1] and Sulzer in Europe in middle eighties [2]. Since then, the more extensive operational experience in the field has been collected at the Plataforma Solar de Almeria where more than 20 different absorbers and receivers have been tested within the range between 200 to 3,000 kW [3]. The list of materials and porous structures tested includes high-temperature metallic alloys and high thermal conductivity ceramics composing wire mesh absorbers, fibres, foams, fins and monolithic channels [4].

In general the volumetric concept offers experimental absorber efficiencies higher than 80%. In spite of the high temperature at the working fluid, above 700°C, the high efficiency is due to the photon trap effect of the porous medium. Theoretically, the porous material of the absorbing surface and the gradual absorption through the porous matrix acts as a black body and leads to much lower temperature gradients between the wall temperature (solid) and the thermal fluid. Solar flux is high at the cold air inlet (front surface) and it is low at the hot air outlet. This means that both air and wall temperatures are higher at the inner portion of the absorber, leading to lower IR radiation losses.

One of the options offering substantial benefits as volumetric absorbers are monolithic matrixes made of prismatic channels. Indeed monolithic structures are commonly used in photo-catalytic applications because of their outstanding properties as photon/heat exchangers [5]. Monolithic solar absorbers made of metallic foils or ceramic honeycomb

have significant advantages in lowering pressure drop and improving both chemical and photon contact surfaces. These makes them especially useful for receiver designs with large flowrates and treatment capacities, and subsequently for scaling-up the volumetric receiver technology. In addition, prismatic channels allow to compartmentalize air flow patterns in individual ducts so that high degrees of “volumetricity” can be achieved but controlling fluid-dynamics by means of appropriate modular designs [6].

A good design of a monolithic volumetric absorber must necessarily be associated to a balance between photon penetrability, absorber porosity, radiation losses and profile of the heat transfer to the fluid. Even though experimental results have demonstrated the feasibility of producing hot air at temperatures above 700°C with absorber thermal efficiencies higher than 80%, it is also evident that current designs can be significantly improved in terms of solar radiation penetration, heat transfer efficiency a nd minimization of radiation losses. Then, the real operational limits of volumetric receivers still remain unexplored in practice. The present work analyzes the effect of several, sometimes contradictory, parameters like channel dimensions, solar concentrator/receiver aperture view angle, material reflectivity and specularity, on photons penetrability and the corresponding effect in terms of thermal behaviour.

Computational procedure

The 4th order Runge-Kutta method allowed the numerical solution of the system of equations governing the collector loop. The development of a program for computing the thermodynamic properties of the working fluids was necessary for the theoretical analysis of the cooling cycle.

• Results and discussion

It is obvious that the global performances of the studied system depend on the levels of temperature of the heat sources and sinks (Coefficient of Performance of the endoreversible cooling system) and the efficiency of the solar collector.

The theoretical parametric study of the three heat reservoir cooling system showed the influence:

— of the site (Algiers (north algeria), Hassi-Messaoud, Tamanrasset (south algeria))

— of the geometrical parameters of the collector (thickness of the absorber, spacing between tubes, diameter of tubes) and of the characteristics of material of the absorber.

— of the nature of the working fluid (R134-a and R123).

As shown in Figures 5, 7, 8, 9, the performances obtained with the refrigerant R123 are sharply better than those obtained with the R134-a but the coefficient of performance COPF, Q remains relatively low for both fluids compared to conventional cooling systems. For an ambient temperature of 40°C and a cold reservoir temperature of 0°C, the coefficient of performance COP varies from 0.1 to 0.6 in average.

R 134a

(XY) I 31 M ar 2004 T

TSV

Fig.4 The cooling load vs the solar time for different sites (R134a)

TSV

R123

03

R123

TSV

Fig.7 The cooling power ys the solar time for different sites (R123, mean day)

R134a

(XY) I 31 Mar 20 04T

TSV

Fig.6 The cooling power ys the solar time for different sites (R134a, mean day)

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(XY) I 31 Mar2004 l’

TAMANRASSET 24 AOUT (jour moyen) Latitude 22 47′ N

TSV

TAMANRASSET 24 AOUT (jour clair) Latitude 22 47′ N

TSV

Fig.8′ The cooling power vs the solar time for different working fluids

• Conclusion

The main objective of this paper is to present the results of theoretical performances of a solar powered ejector cooling system using R123 or R134a as the working fluid. We have developed a program that allows calculation of the cooling power. It is maily composed of four subroutines computing the solar radiation, the thermodynamic properties of working fluids, the vapour driving massflowrate and the cooling power respectively.

The parametric study, based on the cycle analysis, shows the influence of the working fluid, the site, the day, the ejector geometry, the solar collector type, the system configuration, etc…

In the comparative study of the cooling performances for the two working fluids, it comes out to be the R123 which leads to better results. The cooling power is about 4 kW in average. However, due to the low massflowrate at the collector exit, the coefficient of performance is relatively low for both fluids compared to conventional cooling systems. That makes this cycle more suitable for air conditioning applications than refrigeration but one can for example, use it with hybrid cycles to improve the efficiency of conventional [4] and non conventional refrigeration systems.

PERFORMANCE OF A NO-VALVE SOLAR ADSORPTION ICE MAKER

K. Sumathy

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

Introduction

Refrigeration is an attractive application of solar energy, because, the supply of sunshine and the need for refrigeration reach maximum levels in the same season. One of the very effective form of solar refrigeration is the production of ice, because ice accumulates much latent heat in it, so that the volume of ice maker can be small. In 1981, Pons and Grenier [1] worked on a solid adsorption pair of zeolite and water, to produce the refrigerating effect and the coefficient of performance was about 0.1. Lately, they had successfully experimented with the adsorption pair of activated carbon and methanol. Sakoda[2], had presented the advantages and limitations of the simultaneous transport of heat and adsorbate in closed type adsorption cooling system, utilizing solar heat. This paper focus on a solar-powered ice-maker with solid adsorption pair of activated carbon + methanol. A simple flat-plate collector having an exposed area of 0.92 m2 is employed to produce ice of about 4-5 kg/day.

water

b) Nighttime(evaporation/adsorption)

Fig. 1 Schematic diagram of the solar-powered solid adsorption ice-maker

System Description

The system consists of a flat-plate collector, a fin-type condenser/heat exchanger and an evaporator which acts as a refrigerator, as shown in Fig. 1. The collector is supplied with activated carbon which is adsorbed with methanol. During the daytime, when solar energy is available on the collector, methanol gets heated up and evaporates from the activated carbon. The vapour is then condensed through the heat exchanger(condenser), and the liquid methanol is stored in the evaporator, which is said to be the regenerating course of the activated carbon. During the nighttime, the collector temperature begins to decrease
due to the heavy heat loss to the atmosphere. In this period, methanol begins to evaporate by absorbing heat from the liquid (water) and gets adsorbed by the activated carbon inside the collector. Since adsorption is a process of releasing heat, the collector must be cooled efficiently at night. As mentioned above, the ice maker operates in an intermittent way to produce the refrigerating effect.

The principle of the solid-adsorption ice-maker is explained using a P-T-X diagram as shown in Fig.2. To begin with, the adsorption bed along with the refrigerant gets heated up, and when it reaches the required desorption temperature(Td1), the methanol gets desorbed. In the evening, the flat-plate collector (adsorption bed) looses its heat to the surroundings and hence the temperature of the adsorbent bed is reduced rapidly (Td2 ? Tai), and the pressure in the adsorber drops to a value below evaporation pressure (Pe). During this period, evaporation could happen, and ice will be made in the refrigeration box.

Fig. 2 Thermodynamic cycle for adsorption

As explained above, the system works in an intermittent way. This system can be made to operate continuously to produce ice, by incorporating two absorbers such that while one works in desorption mode, the other would be set in for absorption.

Line A-B : Represents the heating of AC along with methanol.

Qa-B = (C pa + CpmWA )(TB — TA) (3)

Qb-d

Cpa + Cpm

WA + WD 2

(2)

+

(Wa — WD)Hdes

Line B-D : Shows that, the collector is connected with the condenser and progressive heating of the adsorbent from B to D causes some adsorbate to be desorbed and its vapour to be condensed.

Line D-F : The collector is closed and cooled. The decrease in temperature from D to F induces the decrease in pressure from PC to PE.

Line F-A : The collector is connected with the evaporator during when the adsorption as well as evaporation occur while the adsorbent is cooled from F to A.

The total energy input (QT) to the system is given by,

TOC o "1-5" h z Qt = Qa-b + Qb-d (3)

The cooling effect (for ice production) is given by,

Qc =(Wa — Wd )[l — Cpm (Ta — TE )J (4)

Therefore, the system efficiency is given by,

COPTH = (5)

TH Qt ( )

Results. Luminance ratios between the window and the wall

Figure 2: Luminance distribution on the wall for the various variants given in Table 2, measured in the black VCE set-up

The luminance value at the window is about 5500 cd /m2. The luminance values for the various variants are given in figure 2. The dark line gives the measurements for the empty window, thus the reference values. With a luminance value of 40 cd/m2 on the wall, this amounts to a luminance ratio of 5500 / 40 = 140, which is much too high. From the light grey lines in figure 2 one can conclude that absorption is not a good way to reduce the contrast ratio between the window and the wall. Luckily, however, from the dark grey lines in figure 2 it can be concluded that reflection and the combination of reflection and absorption is a way to increase the amount of light on the wall next to the window opening, thus decreasing the luminance ratio between the window and the wall to a minimum luminance ratio of 40 for the translucent foil 1 cm in front of the window.

Window properties

The Solar Window is evaluated for its properties as a building component. From this perspective, it can be regarded as a normal window with added features, such as solar shading and internal insulation by the reflector screens. The window consists of

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a double-pane insulating glass unit (IGU). The panes are proposed to have anti­reflective coatings in order to increase the active thermal and PV performance for a vertically oriented element. The insulating and sun-shading properties of the reflectors and the anti-reflective coatings are objects for evaluation.

A 1 m2 prototype of the solar window has been constructed for evaluation of the thermal properties. Five hybrid absorbers with reflectors were mounted in a wooden frame with a double pane IGU attached in the front. The U value of the window together with the closed reflectors has been calculated from measurements in a guarded hot-box, according to ISO 8990 (Johansson 2004, ISO8990 1994). The Solar Window was placed in a square shaped hole between a cold and a hot space of 21.6 m3 each. The hot space contained a guarded measuring box, covering the hole and the heating device. The U value was calculated according to Eq. (1):

U = —

A — AT

[Eq. (1)]

U is the U-value of the construction (W/m2K), Q is the power input for heating the guarded measuring box, A is the area of the window, and ATn is the environmental temperature difference between the hot and cold space.

Tests were made for the window separately and with the solar window components attached with the reflectors in six different positions, with four intermediate opening angles between the fixed open or closed modes. The window separately represents a U value of 2.80 W/m2K. The U value of the whole Solar Window differs between 2.42 in the fully open mode, to 1.33 for the fully closed mode, see figure 4.

The effect on the U value with the reflectors opened derives from the reduced convection due to interruption of cold downdraught. Hence, the effect of the open reflectors could be regarded as an added internal surface resistance, which varies by opening angle.

The prototype construction was not made sufficiently airtight, why some compensation was made for this by sealing the gaps in its closed position. One measurement was made with the reflectors closed and sealed towards the absorber insulation, and another one also with added sealing between absorber insulation and window, in order to reduce the channel of cold downdraught to one individual module. These two steps made the U value drop from 1.33 to 1.22 and 1.17 W/m2K respectively. Air tightness is hence an important criterion for further design studies.

The visual shading effect of the reflectors as sunshades in a closed position is total, which means that effective solar shading, visible shading and effective insulation is obtained simultaneously. However, the shading effect of the reflectors in an open position needs to be evaluated. Especially the risk of glare due to the concentration

37°

1,73

47°

1,86

56°

2,07

66°

2,25

a: 0°

U: 1,33

95°

2,43

Figure 4: U values for different opening angles, a

Figure 5: Ray-tracing illustrating the distribution of direct radiation with the reflectors opened at a solar height of 30° (left) and 20° (right).

of daylight from the reflectors needs to be observed and reduced to a minimum. A two-dimensional ray-tracing analysis, made by hand in a CAD program (see figure 5), shows that most of the radiation reflected will be distributed upwards to the next element above and then spread again. For solar angles at 20° and lower, there is a small risk of glare from the concentrated daylight. This problem is likely solved by reducing the rotation angle from 95° (which was set for minimizing the horizontal obstruction of view) to 90°.

POWER LAW vs. CONSTANT PRESSURE POTENTIAL MODEL

A key question is how the value of m affects the prediction of plant power. As a reference condition use the case where V = Vmax/(n+1)1/n and pt/pp = n/(n+1), and denote it with an asterisk (*). When pt/pp = n/(n+1) in the power law model, then from Eqs. (1), (2) and (3):

к

= (n + 1)V. n-m (19)

-L

Substitute Eq. (19) into Eq. (6) to get the volume flow at MFP condition:

Vmfp = (m + 1)V. n-m )

Using Eqs. (1), (8) and (20), the turbine pressure drop at the MFP condition is:

ptMFP

m /(n_m)

pt*

(21)

= (m + 1)1/(n-m)V. (20)

By substituting Eqs. (20) and (21) into (4), the fluid power at the MFP condition follows:

Pmfp = [1-m)(1 + m)(1+m)/(n-m)P* (22)

m

— 0.2

— 0.3

— 0.4

— 0.5

— 0.6

— 0.7

— 0.8

— 0.9

Vmfp /V*

0.904

0.856

0.808

0.758

0.703

0.640

0.564

0.452

ptMFP /pt*

1.123

1.205

1.307

1.436

1.606

1.845

2.217

2.963

Pmfp /P*

1.014

1.032

1.056

1.088

1.129

1.181

1.248

1.339

Table 1: Power law MFP values as a fraction of constant pressure potential MFP values

In the table below, volume flow, pressure potential and power, all at MFP condition, are listed as a fraction of the respective reference value over a range of collector transfer efficiencies at n = 2. At a collector transfer efficiency of 70 % (m = -0.70) we can see that the MFP volume flow may be as low as 64 %, the MFP turbine pressure drop may be as high as 185 % and the power production may be 118 % compared to the reference value. Even at moderate collector floor-to-exit efficiencies of around 50 % the optimal turbine pressure drop may be seriously underestimated by using the 2/3 rule.

Schlaich (1995) gives typical values of ncoll around 0.55 and a around 0.80, leading to ncfe = 0.69. Applying the above model we get m = -0.69 and from Eq. 8 and assuming n = 2 the optimal ratio is pt/pp = (n-m)/(n+1) = 2.69/3 = 0.90. This value is much higher than the value of 2/3, and also higher than the value of 0.82 derived from values in a table of data given by Schlaich (1995). On the other hand, a value of pt/pp = 0.82 corresponds to ncfe = 0.46, and if a = 0.8, to ncoll = 0.8×0.46 = 0.37, which is very low. The value of 0.9 agrees with the value recommended by Bernardes (2003). In his analysis he found optimum values of as high as 0.97, resulting in m = -0.91. This would, in combination with his value of collector floor absorption coefficient, a = 0.9, imply a collector transfer efficiency of 0.91/0.9, which exceeds 100 %. If the value of 0.8 for a given by Schlaich is replaced by 0.9 in his data set, then ncfe = 0.61, with optimal ratio of pt/pp = 0.87.

As the tabulated data in Schlaich (1995) were not obtained with a pt/pp ratio of 2/3, we cannot apply Eqs. (20-22) directly to come up with the values for volume flow, turbine pressure drop and power at MFP condition. We first have to find the equivalent * condition, by using Eqs. (1) and (2) to get values for Kp and KL. Assuming these coefficients as well as the exponents m and n to remain constant over a restricted range of flow rate and using the * condition together with Eqs. (1) and (2) from Eq. (19) V* is:

1

f 3KL л V Kp У

m-n

V* =

(23)

The value of m follows from ^cfe, and n is taken as 2. We can then find V* and evaluate pp*, pt*, pL*, and P*, and find values for the same variables at the MFP condition with Eqs. (20-22).

Since n is numerically about three times as large as m, V* is rather insensitive to the exact value of m. Taking m = -0.66 and working with the 100 MW plant data we find that PMFP is 3.7 % higher than PSchlaich (for the 30 MW plant it is 3.5 % and for the 5 MW it is 3.0 %). Changing m by 20 % to -0.79 leads to a PMFP that is 8.4 % higher than PSchlaich, a change of only 4.5 %.

Solar Chill — a solar PV refrigerator without battery

Per Henrik Pedersen, Seren Poulsen & Ivan Katic

Danish Technological Institute

P. O.Box 141

2630 Taastrup

Denmark

Introduction

A solar powered refrigerator (SolarChill) has been developed in an international project involving Greenpeace International, GTZ, UNICEF, UNEP, WHO, industrial partners and Danish Technological Institute. The refrigerator is able to operate directly on solar PV panels, without battery or additional electronics, and is therefore suitable for locations where little maintenance and reliable operation is mandatory. The main objective of the SolarChill Project is to help deliver vaccines and refrigeration to the rural poor. To achieve this objective, the SolarChill Project developed — and plans to make freely available a versatile refrigeration technology that is environmentally sound, technologically reliable, and affordable. SolarChill does not use any fluorocarbons in its cooling system or in the insulation.

For domestic and small business applications, another type of solar refrigerator is under development. This is an upright type, suitable for cool storage of food and beverages in areas where grid power is non-existent or unstable. The market potential for this type is thus present in industrialised countries as well as in countries under development.

The unique feature of SolarChill is that energy is stored in ice instead of in batteries. An ice compartment keeps the cabinet at desired temperatures during the night. SolarChill is made from mass produced standard components, which results in a favourable cost compared with other vaccine solar refrigerators.

The SolarChill has undergone intensive laboratory tests in Denmark, proving that it fulfils the objectives set for the project. In addition, a field test programme in three different developing countries is ongoing with the aim to gather practical experience from health clinics.

The paper describes the product development, possible SolarChill applications and experience with the two types of solar refrigerators, as well as results from the laboratory and field test.

Background

A developing project funded by the Danish Energy Agency and conducted by the Danish Technological Institute started in 1999 in co-operation with Danfoss Compressors, Vestfrost and other Danish companies. The aim was to develop a photovoltaic powered vaccine cooler without battery back-up. Instead energy storage of ice should keep the temperature stable during nights and periods without sunshine.

In parallel to that discussions were held at various times (starting in 1998-99) between UNEP, WHO, Greenpeace and GTZ with the objective to promote environmentally sound refrigerators. The idea to bring all these interested parties together arose at a refrigeration summit in Chicago in November 2000, which then led to a common meeting at GTZ headquarters in 2001. This resulted in an international project with the aim to develop, test, and use environmental sound, affordable and reliable photovoltaic powered vaccine cooler.

The Solar Chill project is a unique partnership among key international agencies, research

and industry bodies. The Project Partners and their main respective roles are:

□ Greenpeace International provides project coordination and fundraising;

□ GTZ Proklima provides technology advice and assessment and fund raising;

□ United Nations Children’s Fund provides need analysis and technology advice and assessment;

□ United Nations Environment Programme provides overall technology assessment and policy advice;

□ World Health Organization provides equipment specifications and technology advice and assessment

□ Program for Appropriate Technology in Health provides technology advice and conducts field test

□ Industry partners: Vestfrost, Vibocold, Danfoss, Gaia Solar provide hardware

□ Danish Technological Institute coordinates the technology development;

Experimental outline

The experiment was conducted on the rooftop of the Main Research Building at the Building Research Institute. The building used as a subject was a 7-story RC-made building; the rooftop finish was asphalt roofing. Table 1 shows the outline of the test pieces. Two cases were assumed; one with the test section of asphalt roofing (AR, hereafter) only, and another with that of water-permeable tile (ceramic), perforated bricks (red bricks) and artificial turf (polyethylene resin). The test piece was installed directly on the rooftop slab. Based on the unit of 300x300mm, a 300mm interval was placed between individual test pieces to prevent any influence of the adjacent section.

From the three standpoints of "latent heat of evaporation,” "ventilation” and "solar reflection”, three types of passive cooling methods were applied: (a) watered AR surface

(Case 2 ~ 5), (b) ceramic tiles with and without water content (Case 6, 7) and (c) perforated bricks with and without a coat of white paint (Case 8, 9). In cases of (a), to compare the cooling effect at the time of sprinkling, we did a 0.5mm sprinkling once a day in the morning, noon or evening (8:00, 12:00 or 16:00) or three times a day (morning, noon and evening). In the preliminary test, when the amount of sprinkling exceeded 0.5mm, the AR surface overflowed. Thus, we set the one-time sprinkling amount to 0.5mm. In cases of (b), after soaking ceramic tiles for two hours, we placed them with a 30% volume water content in the test section in the evening (18:00, September 2). In cases of (c), white paint was painted on the surface. The temperature (Tn) at the upper test piece in (b) and (c) corresponds to the rooftop surface temperature when the test piece is used as a rooftop finish. However, in this study, we expressed Tn as the upper test piece temperature in order to distinguish it from Case 1~5, in which no test piece was installed. We regarded T| in Case 1 as the non­measure rooftop surface temperature. We then regarded the fact that Ti, Tii, and Тш had decreased compared to that surface temperature in other cases of different surface finishes and of similar insulation structure as the cooling effect.

The total measurement period was one week from September 1 to 7, 2003. Main measurement items were the rooftop surface temperature (Tab.1, Ti), upper test piece temperature (Tab.1, Tii), lower test piece temperature (Tab.1, Тш), outside temperature and solar radiation. The outdoor temperature was measured at the Assmann’s aspiration psychrometer (approximately 1.2m in height). For other temperature measurements, we used the Type-E thermocouple and recorded the results at 10-minute intervals. For general meteorological data such as wind direction, wind velocity and rainfall, we utilized the measured values from the nearest local weather station.

SIMULATIONS WITH RADIANCE

Radiance is a lighting simulation program developed by the Lawrence Berkeley National Laboratory [1]. It is a backwards ray-tracing program which can give calculations and a visualisation of illuminance and luminance values. The desktop version of Radiance is a plug-in module that works with computer aided design (CAD) tools.

Calculations in Radiance can be done with a clear sky and with a CIE-overcast sky.

An exposition space is modelled in Radiance. The dimensions of the exposition space are 10 x 10 x 6 m3and of the tube 2 x 2 x 6 m3(fig.5). The materials of the wall, the floor and the ceiling are chosen from the library of Radiance. Calculations are done with a CIE overcast — sky and with a clear sky. The location on earth for the calculations is adjusted to Delft in the Netherlands. Model calculations are done for December, March and June. Figure 6 shows the illuminance values for simulations with a CIE-overcast sky for the 21st of March at 14:00 h. At that moment the illuminance in the free-field situation is 10300 lux, a value which is present during 70 % of the year in the Netherlands. The camera position of figure 6 is placed in a corner of the space, so the picture shows a part of the ceiling, the floor and the four walls. A contour of 150 lux is shown on the wall, which corresponds to a daylight factor of

1.5 %. The contours shown on the floor are of 250 lux, 350 lux and 450 lux, or daylight factors 2.5 %, 3,5 % and 4.5 %. These values are well suited for an exposition space. In summer the illuminance values are most of the time so high that a sunshade will be needed. Figure 7 and 8 show model calculations with a clear sky for 21 March and 21 June. These simulations show the problems of the direct sun: At some places there are lightspots of high illuminance levels, which are in general visually uncomfortable and in any case unsuitable for an exposition space. To avoid the negative effects of direct sunlight we must protect the exposition space against it. Only in winter the direct sunlight is not strong enough to damage the art objects (fig. 9).

To protect against the direct sunlight three possible solutions are investigated: A tube with a shed roof construction (fig. 10), a tube with a light shelf underneath (fig. 11) and a tube with a special ratio of specular and diffuse reflecting materials (fig.12).

Figure 13 shows one of the simulations with a clear sky for a shed roof construction. The window in the shed roof is 2 x 2 m2 and 1 m above ground level. It is possible to exclude the direct sun, but in reallity the tube must be 2 m higher, otherwise people will walk in front of the window. In that case the illuminance levels are too low most of the year.

h

m

fig.5. Model of an underground exposition space.

Materials from Material of the

Material of the

Material of the

Material of the

the Radiance Library: ceiling: White

reflectance 85.77 % specularity 0.00 % walls: Beige Paint 2k216

reflectance 71.00 % specularity 0.00 % floor: Gray

reflectance 21.70 % specularity 0.00 % tube: Luminaire Reflector reflectance 95.00 % specularity 95.00 %

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fig. 6. Model calculations with a CIE sky of the illuminance values for 21 March 14:00 h.

fig. 7. Model calculations with a clear sky of the illuminance values for 21 March 12:00 h.

250

750 650 550 450 350

fig. 8. Model calculations with a clear sky of the illuminance values for 21 June 12:00 h.

fig.9. Model calculations with a clear sky of the illuminance values for 22 December 12:00 h.

Figure 14 shows the simulation of the situation with the light shelf. The illuminance values are largely below 150 lux, only the upper parts of the walls show more than 150 lux. This situation is not suitable, the underground space is much too dark.

Figure 15 shows a simulation with a complete diffuse reflecting tube and figure 16 with a partly diffuse and a partly specular inner tube material. Different possibilities in the ratio specular/diffuse and the ratio diffuse/specular materials in the tube are varied. The best situation to avoid lightspots with high illuminance values is with a diffuse reflecting upper part in the tube and a specular reflecting lower part.

After these model calculations a design concept is developed to vary the ratio diffuse reflecting and specular reflecting material in the tube in order to adapt to the sun situation during the year and to different weather conditions.

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fig.13. Simulaton of the shed roof construction with a clear sky for 21 June at 14:00 h.

fig. 14. Simulation of the underground space with light shelf, clear sky,21 June at 14:00 h.

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fig. 15. Simulation of the space with a tube with complete diffuse reflecting material, with a clear sky for 21 June at 14:00 h.

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fig. 16. Simulation in the case the tube has 2/3 diffuse and 1/3 specular reflecting material, with a clear sky for 21 June at 14:00 h.

2. A DESIGN CHOICE

Different design choices are possible to vary the ratio diffuse reflecting and specular reflecting material in the tube. It is possible to do that, for example, by shifting or rotating mirror panels and diffuse reflecting panels in the tube, but an other solution is chosen in this paper.

fig. 17. The screens inside the tube.

fig. 18. An impression of the tube and the underground space.

The inner material of the tube is supposed to be made of highly specular material. Sunshades or other screens of diffuse reflecting materials are connected to the walls (fig.17). By lowering or lifting the screens it is possible to regulate the ratio diffuse and specular reflecting material in the tube. This concept is worked out by a graduate student. Figure 18 shows an impression of the tube and the underground space.

3. CONCLUSION

Measurements in the daylight chamber and calculations with the lighting simulation program Radiance have shown that in an underground space with one tube in the ceiling the use of daylight is an option. On the walls a daylight factor of about 1.5 % and on the

floor 4.5 % can be reached in case of a diffuse sky. Simulations with a clear sky show that most of the time protection against direct sunlight is necessary to avoid light spots of high illuminances. There are different possibilities to avoid the problems of the direct sunlight. The design concept in this paper is one solution.