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Daylight: an Energy Saving Resource
Kevan Shaw
Lecture delivered at the Lights of India conference, Goa, India, 2010
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Introduction
Daylight is the light source our eyes have evolved to use. It is available for the majority of the working
day everywhere. Why have architects been designing buildings that do not take full advantage of this
generally available and usually plentiful source of light, particularly in these environmentally aware times
when the potential energy saving aspects of utilising this resource might be seen as a desirable benefit?
Natural daylight was the only practical, generally available source until the mid 19th century when gas
lighting and kerosene fuelled oil lamps with efficient wick designs came to the market, predominantly in
already industrialised countries. These new light sources were able to provide reasonable working levels
of light for many tasks, not the least of which was reading, past sunset. From this point one begins to see
lower floor-to-ceiling heights and reduced window areas in many building types. As we reach the electric
age we also reach the level of building technology that allows tall buildings with large floor plates, and we
see the increasing dependence on electric light in the workplace.
Over the 20th century artificial light became very cheap and land prices more expensive. We have seen
land prices spiral upwards and the resultant pressure to maximise the let-able floor area in every
building. The result has been buildings with large floor plates and relatively low ceiling heights, limited
window area, and with predominantly artificial lighting. Now the symbolism of towers has also grabbed
the attention of both building owners and architects so a building type that originated in the crowded
cities of America and to a lesser extent Europe, has become iconic for developing nations and corporate
wealth. Extending this building type into regions with an overabundance of sunlight results in glazing with
low transmission materials that do little to provide quality daylight to the interior space.
Daylight and Energy Saving
In the past few years the issues around energy use have come to the forefront of everybody’s attention,
and lighting has become a focus for energy saving ideas. Largely these have been attempts to apply
technology to improve the efficiency in the conversion of electrical energy to visible light energy. Where
the uptake of these technologies has not been particularly effective, we are facing legislation to ban
“inefficient” lamp types irrespective of light quality (1). So far little effort has gone into maximisation of the
use of natural light either in building design or legislation; in fact legislation exists in the UK to reduce
window areas to increase the thermal insulation of buildings. In tropical climates architecture has also
sought to exclude direct sunlight for the very opposite reason, that of unwelcome heat gain and to some
extent glare.
Daylight: an Energy Saving Resource
Kevan Shaw, 2010
www.ksld.com
1
The issue connected to daylight ingress is heat, either excessive heat gain in tropical regions or heat loss
in the temperate regions. We have to consider this very carefully when it comes to striking a balance
between natural and artificial light. It is usually very easy to make an argument that on lighting costs alone
natural light is an easy winner; however, we do need to factor in the additional energy requirements for
heating or cooling required to balance out larger glazed areas.
The challenges around designing for effective daylighting also vary according to local climatic conditions.
Where there is a preponderance of direct sunlight and clear skies there are many more options for
separating the light from the heat. Where there is a lower level of available direct sunlight then the only
answer is massive areas of glazing open to overcast skies with some degree of control to moderate the
light when the sun is shining.
Daylight and Building Design
Designing for daylighting has to be considered at the earliest stages of building design. The orientation of
the building and glazing relative to the sun path is the single most important decision. This is followed by
design of the facades and roof, selection of glazing systems, and daylight controls such as blinds and
louvres. Unfortunately many of these things have already been decided by the architect long before the
lighting designer is brought on board the design team. A lot of the time we are asked to “fix” problems
created by the architecture; for example glare from extensive south facing glazing, balance between areas
in an atrium open to the sky and areas with solid ceilings, etc.
More usefully we are able to develop lighting controls that will optimise the effective use of daylight by
dimming down unnecessary electric light when daylight is available, and suggest changes to details such as
light shelves to make them more effective (2). Another area where we can have some influence is in the
use of daylight powered light fittings; by this I mean light gathering and distributing systems such as sun
pipes or other similar devices. These do not need to be very high tech: the principle is very simple, you
are capturing a section of sky and reflecting it through a mirrored tube to where you want it. Obviously
there are limitations to this as it is not very practical to carry light downwards for more than one floor in
a building, and doing so eats into that precious let-able area that the client wants.
Daylight as an Energy Saver
How valuable is daylight as an energy saver? As I mentioned previously this is a complicated question. A
number has come to my attention that does illustrate how efficient artificial lighting has already become.
Normally when we are talking about light source efficiency we discuss this in terms of lumens per watt
(lm/W). Typically we are thinking of watts as a measure of electricity. Watts however are a measure of
energy as well, and lumens are a measure tied into the specific range of the human visual system in
working with the electromagnetic spectrum. If we look at sunlight, for example, we find that it has an
efficiency of 93 lumens per watt of radiated energy. Given that we are pushing efficiencies of 100 lumens
per electrical watt we have to be certain that we are not compromising lighting quality excessively. For
the record the maximum efficiency achievable with a full spectrum light source is 248 lm/W, and with a
source emitting only in the peak of our visual spectrum 648 lm/W (3); there are some absolute limits on
the technical efficiencies we can achieve. Right now we do not have measures that can incorporate the
energy effects of heating and cooling requirements other than looking at the total systems energy use for
a building. Ultimately we do need to think in a more holistic manner about the place of lighting in the
complete energy use of a building as it does not stand alone.
Considering daylight as an energy source we should also look at other methods of using this in the total
building design. Photovoltaic systems are a significant technology in converting the energy of sunlight into
electricity. The economics of photovoltaic system do not really work where there is access to reliable
centrally generated electricity; however off grid they do provide a reasonable way of providing electricity.
Daylight: an Energy Saving Resource
Kevan Shaw, 2010
www.ksld.com
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From a lighting point of view they generate electricity when there is available daylight, not when we need
artificial light at night! To make use of daylight in this way does require storage of electricity and that
means batteries. In seriously off grid situations lanterns that you put outside during the day to charge,
and use indoors or outdoors in the evening are now a very practical proposition and we are seeing
these appear as products. We are also seeing larger scale photovoltaic charging stations that can serve
entire isolated communities. Really large scale solar electricity generation is also being explored in full
scale pilot plants including this one in Spain (refer to figure 1 below); again these have the disadvantage
of not being able to provide power at night when we most need it for lighting!
Figure 1: pilot 300MW solar thermal electricity generation project, Seville, Spain
Typically there is a cost to anything that looks like a free energy source. The best energy conversion rates
practically achievable are around 15% to 20 % with practical technologies having a theoretical maximum
of 29%. However gallium arsenide junctions are showing higher efficiencies in the lab . This is the same
technology as LEDs so we may in the future see a single device that can gather daylight and charge a
battery by day and glow at night! (4)
Conclusion
In conclusion daylight is a major light source that is radically underused in modern architecture. We
believe that a greater consideration of daylight use in the early stages of architectural design would lead
to a significant saving in lighting energy. Due to potentials for heat gain and loss through glazing the
energy saving in lighting needs to be balanced with any energy impact of heat gains or losses. Lighting has
to be considered holistically with all aspects of the building design to achieve the optimum solutions for
energy efficiency and quality of light.
Daylight: an Energy Saving Resource
Kevan Shaw, 2010
www.ksld.com
3
References
1.
http://www.buildingdesign.co.uk/elec-technical/profile-t1/profile-building-regulations-effect-onlighting-design.htm
2.
http://www.designbuilder.co.uk/programhelp/_lighting_control.htm
3.
http://en.wikipedia.org/wiki/Luminosity_function
4.
http://www.azom.com/details.asp?ArticleID=1166
Daylight: an Energy Saving Resource
Kevan Shaw, 2010
www.ksld.com
4