Natural Lighting

The primary goal of the following daylight analysis is the evaluation of the potential of the design to provide appropriate levels of natural illumination. Two methods of calculation are applied:

Method 1. Artificial sky Method 2. Computer modelling

Method 1. Artificial sky

The artificial sky at Bath University School of Architecture was utilized to carry out a number of daylight tests on building sections. The artificial sky at the university conforms to the standard CIE overcast sky scaled to produce a variable 10,000 lux sky at model roof level. The tests were

Plan of building indicating two areas for analysis

1. Street daylighting test area

2. Office daylighting test area

Sections of the three roof options for the street carried out to establish the daylight effects on a number of building forms and roof configurations.

CIBSE Guidelines recommend an average daylight factor of 5 per cent and a minimum of 2.5 per cent within general office areas with manual computer usage. Entrance areas and reception have a lower CIBSE recommendation of 2 per cent average and 0.6 per cent minimum. An improved level is anticipated due to the use of the entrance hall as a natural light well for the rear of the offices adjacent to the entrance hall.

Three options for the street were selected from a number which were considered, and a full daylight study was considered for each option.

Of these three, Option 1 was considered to be the most appropriate for the following reasons:

• Greater uniformity of lighting levels

• Reduction of glare into offices adjacent to the street

• The other two options tended to lead to 'overlighting' of the space

• Excessive solar gain into the street, creating overheating problems

• Excessive heat loss in summer.

Results from the model below the artificial sky:

Street Option 1, lower level

1. A minimum daylight factor of 3 per cent was measured with a maximum of 14 per cent for the lower office area; the average being 6 per cent.

2. The analysis indicated an average daylight factor of 7 per cent for the street at floor level, ensuring that a good daylight quality would be achieved.

Street Option 1, upper level

A minimum daylight factor of 4 per cent was measured for the upper office area with a maximum of 14 per cent at the perimeter, with an average of 7.4 per cent.

The model used for the analysis

Conclusions

The results are very encouraging, and meet the requirements of the CIBSE Guidelines. As expected the results for the upper level are slightly higher. Whilst measurements were not taken directly up to the glazing line, it is expected that the figure would be higher at this point.

The comparison of the model at the street lower level with the subsequent computer results, shows acceptable agreement; 7 per cent average below the North lights and 4 to 12 per cent over the office place.

Plans of the grid layouts for the upper and lower levels. Natural lighting contours measured by computer

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Plans of the grid layouts for the upper and lower levels. Natural lighting contours measured by computer

Option 1

Measured natural light !evel3 street north end upper level

Grid layouts for the position of photocell measurements with readings as follows: Street North End. Natural light levels. Lower and upper levels. Office Floor. Natural light levels. Lower and upper levels

Typical Office Wing results from the model, below the artifical sky: Office Wing, lower level

A minimum daylight factor of 2.3 per cent was measured for the lower office area with a maximum of 6.8 per cent and average of 4 per cent.

Office Wing, upper level

A minimum daylight factor of 1.9 per cent was measured for the upper office, with a maximum of 6.8 per cent and average 3.2 per cent.

Conclusions

The minimum and average figures are slightly lower than the CIBSE recommendations, the prime reason being the shading elements. The model has solid south side shading elements at roof level, which is to be revised to an open louvred system to improve the conditions.

The effect of the shading system at roof level can be seen by examining the results between the lower and upper levels. The lower level shows a small improvement in the figures.

Wessex Water staff predominantly use PCs; as a result, the control of daylight is an important issue. Too much light would result in blinds being drawn.

Comparing the model studies with the computer results, the computer results are higher by the perimeter. This is to be expected as the computer readings cover the full floor plate, whilst the model studies are spot measurements only. The maximum computer level of 28 per cent is directly by the window. In addition, the measured model results have a built in reduction of 50 per cent to allow for the interior design, with fitted office space, furniture, colour schemes, changes to height, maintenance and light fittings.

Method 2. Computer modelling

A computer study was undertaken to predict daylighting levels within the street and the central office wing.

The daylight studies assumed a CIE Standard overcast sky. The daylight study of the open plan offices forming the central wing was undertaken with a daylight study of the street area indicated in the figure above.

In both cases the results were analysed at the working plane level. The street lower level

The minimum daylight level was calculated to be 2.5 per cent.

The average daylight factor has been calculated to be 6.3 per cent. As with the central office area, this is a good value to achieve for a place predominantly lit by natural light.

The maximum daylight factor was found to be 22 per cent close to the windows.

Conclusions

The computer results compare well with the model studies

This is an important area of the building, where visitors gain a first impression of the building. The space should be well lit, welcoming, and feel natural in lighting terms.

Natural lighting contours as measured by computer in the North Street and Lower office level, associated with location plan.

The Office Wing

The minimum daylight factor was calculated to be 1.78 per cent. This falls below the recommended levels, but only applies to 2 per cent of the floor area.

The average daylight factor was found to be 5.53 per cent. The recommended level of daylight in an office is between 2.5 per cent and 5 per cent. Bearing in mind that the daylight will be linked to the artificial lighting, the average level is adequate.

The maximum level of 28 per cent will cause problems of discomfort glare, but this occurs only in the immediate vicinity of the windows.

Natural lighting contours calculated by computer for a typical office floor

ARTIFICIAL LIGHTING

An analysis was carried out to consider the most energy efficient lighting system for the open plan spaces for Wessex Water New Operations Centre.

The design criteria is as follows:

• Lighting level 300-400 Lux

• Window to window dimensions 15 m.

The design should take account of the following regulations which directly affect the lighting:

The display screen equipment shall be free from disturbing glare and reflections

There shall be an appropriate contrast between the screen and its background

Adjustable coverings shall be provided for windows.

Several combinations of lamps and luminaires were investigated; the final recommendation was for the use of twin 35 w T5 luminaires, installed at a grid of 3 m by 2.4 m, suspended 400mm from the ceiling soffit.

To provide a design output of 400 lux, the system proposed provides 11.43 W/m.2 (for comparison the 'Good practice benchmark for offices would be 12 W/m. 2).

North side

Luminaire choice

Lamp - Fluorescent

Reduced energy use, less than the conventional T8 lamp Reduced glare with the use of three-dimensional glare control system The T5 lamp uses less glass (40 per cent) and less mercury (80 per cent) than conventional lamps

Extended lamp life minimizes maintenance and reduces disposal and recycling costs

Luminaire body /louvre

Louvre high gloss mirror optic

Small lamp body size reduces materials of manufacture

Computer simulation of artificial lighting

Indirect/direct louvre with 25 per cent indirect and 75 per cent direct, this configuration produces high performance with high efficiency.

The artificial lighting was simulated by computer on the assumption that no natural lighting was available (a night time scenario) allowing for cleaning and lamp degradation. The results indicate a minimum light level of 235 Lux with a maximum of 919 Lux with an average of 636 Lux.

Conclusions

It can be concluded that the proposed artificial lighting solution will produce the correct lighting levels within the office spaces. At the design development stage an energy consumption figure of 100 Wh/m2/annum for normal working hours (excluding special energy requirements for computer suites control room and kitchen equipment).

There is no reason to amend this figure, but its success will depend upon the control regimes applied by the BEMS. This would no doubt take into account any savings of energy available due to daylight linking.

EXAMPLE 2

Whilst the first example is an extensive study, leading to a successsful conclusion in the subsequent built form, where model studies were compared with computer studies.

The following example is a preliminary study made at an early stage in the design of a building to check whether the daylight available would be satisfactory, or whether changes needed to be made to the daylight strategy.

DPA (Lighting Consultants) were commissioned to investigate the daylighting for a proposed extension to the Leeds College of Art and Design, Blenheim Walk Campus, providing new studio space over four floors. The building is referred to as the 'New Design and Communication Building'. The building designed by architects Aedas Architects Ltd contains large open-plan teaching studios, a studio/theatre

photography area, and ancillary accommodation including WCs and offices.

The building layout is arranged around a central rectangular atrium area with one open-plan studio located either side of the atrium on the first and second floors. The lower ground and ground floor areas contain one open-plan studio per floor, each located on the northern side of the atrium. The atrium is to be provided with daylight via a large rooflight that spans the length of the atrium; daylight being allowed to penetrate downwards through the atrium to illuminate lower circulation areas, by means of voids located in the floor slabs above, at specific locations.

Walls provide valuable pin-up space in the open-plan studios, and as such a compromise was required between providing sufficient areas of glazing for adequate daylighting, whilst maintaining an acceptable amount of wall space for pinning-up.

In order to investigate the effect of increasing the glazed area to the wall abutting the atrium, measurements were undertaken with and without a solid wall inserted in the model.

The aim of the daylight study was to establish whether the level of daylight within the building would be adequate, based on the architects' initial proposals. The study was limited to the open-plan studio areas, a typical office area and the main atrium/circulation space.

The specific aims and objectives of the study included the following:

• Is the level of daylight to the open plan studios adequate?

• Is the level of daylight improved by increasing the amount of glazed area provided within the walls of the studio abutting the atrium area?

• Does the level of daylight achieved within these areas meet with current guidelines and/or legislation?

The approach adopted by the lighting designers was by means of direct measurement in a 1/50 model provided by the architects. The model had been accurately constructed in order to ensure the the window openings related closely to the proposed openings in the 'real' building. It was also important that the internal finishes within the model matched where possible the proposed surface finishes, with regard to the reflection factor, which have a significant effect on the lit environment.

The model was placed under the artificial sky at the Bartlett School of Architecture, which was hired for the study, so that daylight factor measurements might be obtained. The artificial sky consists of a hemispherical array of compact fluorescent luminaires, which can be individually programmed and controlled to provide a luminance distribution which matches that of the CIE overcast sky. Measurements of illuminance at specified locations can be provided by individual sensors or cells positioned within the model, compared with a reference cell located externally to the model allowing daylight factors (DF) to be calculated.

The scale model was placed in the artificial sky, and measurements of illuminance were taken at appropriate locations in each of the spaces under consideration, which, when compared to the reference cell, allowed the daylight factor at each grid point to be calculated.

From these measurements an average daylight factor for each group of measurements was calculated as the arithmetic mean of the individual daylight factor readings for each test session.

In the case of the open-plan studios measurements were taken with and without the solid wall abutting the atrium area. By conducting two

Photographs of the physical 1/50th scale model

comparative measurements, an assessment could be made of increasing the proportion of glazing within this wall.

(Note. Despite the accuracy of the model already described, the size of window openings used in the model did not exactly match those proposed for the building, as window mullions and other framing elements had been omitted for simplicity of construction. Depreciation factors were therefore applied to the measurements of the calculated daylight factors in order to produce more realistic values. These depreciation factors were based on an estimate of the relative areas of the mullions etc. The incorporation of realistic glazing is also difficult within a model and as such glazing is normally excluded; additional depreciation factors based on the light transmission values of the proposed glazing elements, are used to compensate for the reduced light transmission. Other depreciation factors are used to allow for dirt on the glass, the value of the factor depending upon the location of the building and the proposed maintenance regime. CIBSE provided appropriate values for these depreciation factors, based on experimental studies.)

Once the daylight factor measurements had been corrected for depreciation, the results were analysed and the average daylight factors compared with the relevant guidelines.

Finally the artificial sky simulator at the Bartlett is provided with an artificial sun that can be used for solar analysis, and this was used to provide for some images of the solar penetration into the building when subjected to sunny conditions at three-hour intervals for three periods of the year . . . summer, winter solstice, and spring equinox, when the sun reaches its highest, lowest, and mid elevations respectively.

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