E L S E V I E R Energy and Buildings 23 (1996) 251-256

EI I[| G't A N D

UILDING-

LT Method 3 . 0 - a strategic energy-design tool for Southern Europe

Nick Baker, Koen Steemers The Martin Centre for ArchitectJtral and Urban Studies, Department of Architecture University of Cambridge, 6 Chaucer Road, Cambridge CB2 2EB, UK

Abstract

The LT Method is an energy-design tool which responds to parameters available early in the design development. It provides an output of annual primary energy for lighting, heating cooling and ventilation. This paper introduces LT 3.0 i a version for Southern Europe which includes a procedure to evaluate the affect of shading devices on cooling loads and on lighting.

Keywords: Energy-design tools; Strategic; Architecture; Southern Europe

1. Introduction

Energy modelling has developed to a high standard in the past decade. The growing power of computers together with rigorous procedures for validation, relying upon high-quality data from test cells, has greatly increased the accuracy with which thermal models can predict temperatures and energy use. However, this is often at the expense of complication - in order to develop the potential accuracy, more detailed building data is required than is likely to be available until the design is fully developed. This means that if the model is to be used as a design tool, the iterative loop of design pro- posal: testing/design modification/re-testing, etc., is a wide one. The conceptual momentum of a design team, and prac- tical considerations, make this iterative process unlikely to occur.

A second problem with these powerful models is that few integrate other energy uses into the analysis; the emphasis has been on thermal models. But energy use for electric light- ing, mechanical ventilation and refrigeration, often consumes a much greater proportion of the total energy. This is espe- cially true for non-domestic buildings in milder climates.

The LT 2 Method has atlempted to address these two prob- lems. This has been at the. expense of modelling precision, but there is clearly no virtue in being able to take account of a parameter, if information of that parameter is not yet avail- able.

t The full data and documentation is published by the Energy Research Group, Department of Architecture, University College Dublin, Dublin 14, originally as supporting material for the EC sponsored 'Zephyr' Architec- tural Competition.

2 LT: Lighting and Thermal energy.

0378-7788/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved SSD10378-7788(95)00950-3

In considering the factors which influence energy use in a building, we can classify them into three broad categories: (i) building; (ii) services, and (iii) occupants. Studies of Baker have shown that all three categories independently have about equal impact on energy use. Within building fac- tors we can identify two sub-categories building-design par- ameters and engineering parameters. For example, the plan depth of a building is a fundamental building-design param- eter, interacting with many other parameters and having an impact on the form and performance of the building in terms of daylight, heating and cooling, and natural ventilation. In contrast, the U-value of the wall, whilst it will have significant effect on the thermal performance, can take on values inde- pendently from other parameters.

More to the point is that at the strategic stage of design the architect is really not very interested in U-values or the lumi- nous efficacy of the lamps; he or she is free to choose them at a later stage. However, the plan, section, facade design, orientation, position on the site in relation to other buildings, organisation of functions within the building are all of great interest during the phase when the 6B-pencil sketches are being created.

In developing the LT Method we have concentrated upon these design parameters. The engineering parameters do influence energy performance, but these have been given 'sensible' default values, or reduced to simple categories such as 'high internal gains' or 'low internal gains'.

It so happens, that to assist in the dissemination of the LT Method, it has been developed predominantly in a manual form. That is to say that the procedure is carried out using scale rule, pocket calculator and pencil. This may seem anach- ronistic in a time when the computer prevails, but in fact the

252 N. Baker, K. Steemers / Energy and Buildings 23 (1996) 251-256

constraint has usefully forced us to simplify data requirc- ments and output options. It also means that the method has to rely upon pre-computed data presented in the form of graphs and tables.

Although we are now developing electronic versions, the presentation ofpre-computed data in this way is still adopted• This maintains the transparency of the method and confronts the user with his or her design proposal set against other performance data. It shifts the emphasis from the number to the trend; from evaluation to comparison.

2. The LT Method

The LT Method relies upon the concept of the passive zone as shown in Fig. 1. This is an area in the building within a maximum distance from a perimeter wall, or an area under a roof. These passive zones can receive the benefit of daylight, natural ventilation and useful solar gains in winter, but also the disbenefit of heat loss through the envelope and solar gains in summer. Non-passive zones are away from the enve- lope, and thus require mechanical ventilation and artificial lighting, but do not suffer from unwanted solar gains or fabric heat loss• In the LT Method the passive zone depth from a side wall is defined as twice the floor to ceiling height, or 6 m as a default. (This is quite a useful way to assess a building even if it is not used as part of the LT procedure.)

Annual Primary Energy consumption per m z is presented for the zones as a function of glazing ratio and orientation. A typical LT Curve is shown in Fig. 2.

In using the LT Method there are three main steps: (i) define passive and non-passive zones and their orien-

tation; measure the areas from a sketch plan and enter on the LT Worksheet (Fig. 3);

(ii) read off the Annual Primary Energy consumption per m 2 from the LT Curves for lighting, heating and cooling applying any correction factors due to the presence of shading devices, and

~ , ~ J ' t / ~ ' ! ' - -

Fig. !. Passive zones (white) and non-passive zones (hatched) on sketch plan.

MWh/m 2 0.30

lotal ~ 0.25 . . . . . . . . . cool 1.0

. . . . . . heat \ . .............

0.20 .7 s.r"

0.15 , , " " " • .,,,0.35

0.10 " " " " * ' " ' " *

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1"o 2o ~o 4"o 5o ~ 7o ~o go S O U T H % glazing rabo

Fig. 2. LT Curve for south-facing office in Southern Europe.

(iii) multiply the areas by the appropriate specific energy consumptions and sum for all zones and uses.

The results can either be presented by the different uses: heating, lighting, cooling, etc., or a total, or on a per m 2 basis• The latter is most useful when comparing the performance of the building with a target.

2.1. The LT Model

The LT Curves are derived by a mathematical model. Fig. 4 shows the energy flows which are modelled. First, the model evaluates the heat conduction through the external envelope, and ventilation heat loss (or gain). Using monthly mean temperatures and a thermal reference point with a correction factor to allow for intermittent heating, a monthly gross heat- ing load is calculated. The model then evaluates the solar gain and applies a utilisation factor to this.

At the same time the monthly hours of available daylight are calculated from the average hourly sky illuminance on the facade, the daylight factor, and an internal lighting datum value. This gives a monthly electrical consumption for arti- ficial lighting, and a monthly heat gain.

The lighting heat gains and useful solar gains, together with a fixed casual gain from occupants and equipment, are then subtracted from the gross heating load to establish the net heating load. When the gains are greater than the gross load, there is a cooling load. The cooling loads contain an allowance for fan and pump energy.

As soon as a cooling load exists, the model eliminates a percentage of the solar gain as defined by the shading device• However, the ventilation remains at the low rate for calculat- ing the cooling load, assuming there are no openable windows and no 'free cooling'. This is to establish what the cooling load would be, if it were mechanically cooled.

Note also that the model assumes 'sensible light-switch- ing', i.e. that lights are only on when the daylighting value drops below the datum value. In practice this would almost certainly require automatic, light-sensing switching. Thus to

N. Baker, K. Steeraers / Energy and Buildings 23 (1996) 251-256 253

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Fig. 3. LT worksheet.

a certain extent the model already assumes a considerable degree of good design.

The monthly energy consumption is calculated for a 'cell' and then reduced to the value of energy consumption per m 2. This is then totalled for the year and plotted as a function of glazing ratio. A 'cell' corresponds to a room surrounded by other rooms. This implies a zero conductive heat loss through all surfaces except the external (window) wail or in the case of rooflighting, the ceiling. Appropriate efficiency factors are applied to reduce all energy to primary energy.

The reason that primary energy is used is that it allows the different 'fuel' inputs for lighting, heating and cooling to be reduced to one common unit. Primary energy is the energy value of the fuel at source. In the case of fuels such as gas or oil when it is to be used for heating, there is an energy over- head required for extracting, refining and distribution, and then the loss of heat due to combustion losses, at the point of use. In the case of electricity, used for lighting and mechanical power for cooling and ventilation, an energy overhead occurs at the power station due to the thermodynamic efficiency of the conversion of heat to raechanical power. This is a large factor: 1 unit of delivered electricity is equivalent to 3.7 units of primary energy.

For fossil fuel, Primary Energy relates well to CO2 and other pollution production, and to cost. Problems arise when a large proportion of electricity is generated by renewable sources such as hydro or nuclear sources. On a national basis, there are wide variations of the fuel used for electricity gen- eration. However, bearing m mind that electricity is exported and imported on the European grid, a single European factor is appropriate.

3. Shading

Shading is recognised as a vital measure to prevent over- heating, in particular in warmer climates. Yet it is clear that most shading devices reduce the daylight, as well as solar gain, within the building. If reduced below a threshold, this then causes an increased lighting energy demand. How can this be taken account of in an integrated energy tool?

Again the LT Method has to make simplifications. Two kinds of shading system are identified: (i) those which only reduce unwanted solar gain (type A), and (ii) those which reduce the daylight by the same proportion that they reduce solar gain (type B). These two extremes may rarely exist in practice, but due to the way LT works, it is more meaningful than it may seem at first.

Take the case of movable shading devices such as fabric roller blinds or venetian blinds (Fig. 5). These need only be deployed when solar radiation is failing onto the window, i.e. when there is a surplus of illuminance. Even if the fabric or louvre system has a diffuse transmission of only 10%, applied to direct solar radiation with an illuminance of around 80 000 lux, this still leaves 8000 lux transmitted through the window plane. Converting this to the equivalent horizontal illumi- nance in order to apply a conventional Daylight Factor (DF), a 2% DF value would give a horizontal illuminance of about 500 lux. Furthermore, in principle the louvre system can be adjusted to give sufficient diffuse illuminance transmission, without admitting direct sunlight. Thus although the system usefully reduces unwanted solar gain, it permits sufficient daylight to exceed the threshold value for artificial lighting.

Now consider a light shelf (Fig. 6), a fixed system. High- angle direct sun is completely prevented from reaching the

254 N. Baker, K. Steemers / Energy and Buildings 23 (1996) 251-256

heaU~

coc~ir~

lighting power

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Fig. 4. Energy flows in the LT Model.

. . . . . . . . . . . . . . . . . . . . . . . . .

0 L _ ~ shading • transmission

500 lux 700 lux 10%

= ,i = / Q

300

0 M m 600 lux SO00 lux

lux oll 1" i

ref poi~ . . . . . . . . "

0 ~ shading transmission

300 lux 600 lux 100%

Fig. 5. Moveable shading is only deployed when there is an excess of illuminance. Type-Al shading.

workplane, most being reflected by the external part of the shelf whereas diffuse radiation is reflected to the back of the room. Even in diffuse sky conditions, although the illumi- nance is reduced near to the window, due to intereflection, it is not reduced at the back of the room. Thus the transmission of the system is geometrically selective.

For the evaluation of artificial lighting energy, LT is only interested in critical conditions. In both these cases LT assumes that the shading device causes no reduction in day- light at the critical reference point in the room (4.5 m from the window wall), and at the critical time (i.e. as the internal daylight level approaches the switch-on threshold value). We will call these two shading types A1 and A2, respectively.

When reading off from the LT Curves, if the shading device has no effect on illuminance at the critical switch-on time, it will not effect lighting energy. Thus for a given glazing ratio, no modification to lighting energy has to be made. For cooling energy, however, the LT Model has been used to produce a family of curves at different shading transmission factor 0.7 and 0.35. For heating energy, it is assumed that low-angle useful winter sun is uninterrupted, either due to the geometry

J ,*,'

:°21 300] ref Point

0 __ . shading ~ ~ = ~ I ~ M transmission

800 lux 1000 lux 35%

300

0 300 lux 450 lux

Fig. 6. A light shelf reduces excess illuminance close to the window but not at the critical reference point. Type-A2 shading.

of the shading device, or because it is not deployed at times of heating demand. So the heating curve is read off with no modification.

In contrast, type B shading devices are really rather dumb, and cannot be recommended. These devices reduce daylight by as much as they reduce unwanted solar gain (Fig. 7). This is the same effect as opaque wall - in other words it is just as good to reduce the glazing area. In fact this is the better solution, because shaded glass still has a high U-value, whereas opaque wall can be well insulated, and at lower cost. If shading devices such as these are to be evaluated using LT, the reduced light transmission must be accounted for. This is

N. Baker, K. Steemers / Energy and Buildings 23 (1996) 251-256 255

lux

: t I re, poi~ .......... . o ' ~

iiim

300 lux 60,3 lux

m

600 t . 300 .... x . . . . . . . . . . . ~..,....e . . . . .

0 ~ < ~ ~ ~ transmission 105 lux 210 lux 35%

Fig. 7. Fixed screens and grids reduce daylight and solar gain by the same amount. Type-B shading.

done by multiplying the actual glazing ratio, by the shading transmission coefficient, when reading off the lighting energy curve, as shown in Fig. 8.

It is interesting to note that several advanced daylight sys- tems, combining reflective louvres or prismatic glass with shelves, claim to improve the distribution of the daylight. This could be described as having a negative shading factor for daylight, actually increasing the daylight at the critical position, (from the LT viewpoint this would be equivalent to increasing the depth of the lrassive zone) and a positive shad- ing factor for solar gain. This discussion suggests the urgent need for shading devices to be classified in this way - that is a time-integrated ratio of solar shading to critical daylight transmission.

4. Cooling loads and passive buildings

A most important question that designers ask is 'can LT tell me if my building will be OK without air conditioning?' The short answer is no. LT can produce an annual cooling load which responds to solar gains, internal gains, and ambi- ent temperature, but it is quite difficult to translate this into an overheating probability.

This is because overheating in subtropical regions is a result of non-uniformity in conditions, rather than the result of steady or average conditions. This is in contrast to the situation for heating. In most of Europe, the average winter temperature is at least 7 °C below comfort temperature and in many cases 15 °C. This contrasts with the most severe summer condition, e.g. the average temperature in Athens in August of 29 °C which is only 2 °C above an accepted upper comfort limit. Most of Southern Europe's average summer temperature lies well within the comfort zone.

Thus it is difficult to translate annual cooling load direct to overheating frequency. A new LT feature, currently under

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Fig. 8. The effect of fixed shading on lighting energy.

development, are overheating prediction curves. These, how- ever, are based upon the cumulative frequency of tempera- tures above a datum value, and thus are driven by the departure of conditions from the norm, rather than the average conditions. Until this feature is available, we believe that potential cooling loads are of interest, since from a compar- ative viewpoint they can help to optimise design.

5. LT and education

The messages given by the use of LT are quite simple. After having used the procedure a number of times, it would probably be possible to sum up the conclusions in a few lines of good advice. Why then go to the trouble of producing such a complex procedure?

We believe that, quite rightly, designers are sceptical about prescriptive advice particularly when it requires many qual- ifications to make it sufficiently robust. Humans have basi- cally quantitative minds, and advice which is backed by quantities is much more convincing. Furthermore it is more useful because it enables us to rank measures in order of priority. For example, it may be important to know by how much the application of shading devices reduces cooling load compared with, say, reducing the internal gains. Even simple global messages such as 'avoid deep plans' seem to be more convincing when backed up by numbers.

5.1. Validation

So, although strategic energy tools like LT are limited in the number of parameters they can address, they have a role in creating their own advice. This raises the question: is it good advice?

In assessing energy models, validation is regarded as a pre- requisite for their use. This usually consists of comparing

256 N. Baker. K. Steemers / Energy and Buildings 23 (1996) 251-256

measured data from simplified buildings such as test cells, with results from blind simulations using the model. This is by no means as easy as it sounds, but model validation meth- ods have become sophisticated enough to cope with most of the difficulties.

With simplified energy-design tools, the same approach is hardly appropriate. So much of the data has to be given assumed values, that comparing measured and predicted energy-consumption figures would really become a test for the selection of default values. Far more useful is the criterion 'are the general messages given by this tool in accord with the expert view?' This softer criterion does not appear to lend itself to rigorous application. Rather, the continued use of design tools such as LT in education and professional training contributes to a slowly developing confidence.

Bibliography

N. Baker, Low energy strategies for non-domestic build- ings, in S. Roaf and M. Hancock (eds.), Energy Efficient Building, Biackwell, Oxford, 1992.

Other versions of the LT Method are as follows: (i) J. Goulding, J.O. Lewis and T. Steemers, LT 1.2,

(Office buildings in Europe), Energy in Architecture, Bats- ford for the CEC, 1993.

(ii) LT 2.0, (Office buildings in UK), Cambridge Archi- tectural Research Ltd. for Building Research Energy Conser- vation Unit, BRECSU.

(iii) LT 4.0, (Multi-residential Buildings in Eastern Europe), Documentation for architectural competition Liv- ing in the City, Energy Research Group, Department of Architecture, University College Dublin, Dublin 14.