The Passive Zone

The LT Method uses the concept of passive and non-passive zones. Passive zones can be daylit and naturally ventilated and may make use of solar gains for heating in winter but may also suffer overheating by solar gains in summer. They are defined by orientation. Non-passive zones have to be artificially lit and ventilated and in many cases cooled.

The first step in the use of the LT Method is the designation of the passive zones by orientation, and non-passive zones as shown in the floor plan diagram opposite (for 3 m floor to ceiling). The depth of the passive zone should be limited to twice the ceiling height, but a default value of 6m for passive zone depth can be used. All of the top floor can be a passive zone if rooflit. The zone areas are worked out and then entered into the LT Worksheet.

When defining the orientation of a passive zone in a corner always choose the best performer (the zone most influenced by solar gain) - for example, south in preference to east and west, unless this is precluded by a blank wall. The zone behind an inside corner is both poorly lit and naturally ventilated and it is best to designate this as non-passive.Top floor zones are potentially all passive zones, since they can be daylit and ventilated over their whole area.

The LT Worksheet

The LT Curves

An example of an LT curve is shown opposite. The vertical axis represents the annual primary energy consumption in MWh/m², and the horizontal axis is the glazing area as a percentage of total facade area. Curves are provided for vertical glazing orientated south, east/west, and north, and for horizontal glazing (roof-lights).

Each subset of curves has three cooling curves corresponding to no shading, 70% transmission (light shading) and 35% transmission (heavy shading). There are also three total curves corresponding to the three shading levels.

The cooling curve includes an allowance for fan power as well as refrigeration. The total without cooling can be used of a non air-conditioned, naturally ventilated building. However, a fixed allowance for fresh air mechanical ventilation must be added for all non passive zones. This value is given at the bottom of each set of four graphs. For buildings with high internal gains, these non passive areas would be air-conditioned and the cooling and fan power should be read off from the LT Curves at the zero % glazing intercept.

Choose the set of curves corresponding to the appropriate climate zone and building type. If monthly temperatures do not comply with either zone, use the lighting and cooling curve from the appropriate zone for summer, and the heating curve from the appropriate zone for winter. There may be some sites where the winter temperature is considerably below 6 ^oC or the summer temperature is several degrees above 24 ^oC. For example, the january temperature for Milano is 0.6 ^oC and the July temperature for Athens is 27.6 ^oC. There is no formal procedure for dealing with this, but the user can make allowances by increasing the heating and cooling consumption by up to 20%.

It may also be interesting to look at other building types to get a feel for the effect of occupancy. Note how steeply the lighting curve drops as daylight becomes available.

The total curve shows a fairly distinct optimum for glazing ratio. Note that this is much smaller for the horizontal glazing due to the higher illuminance of horizontal surfaces compared with vertical surfaces.

For vertical glazing most curves show only a slow increase in energy consumption after the minimum. However, the overheating risk will increase significantly with large glazing areas. So too will other comfort problems such as glare and 'cold radiation'. Thus glazing ratios well away from the optimum in either direction should be avoided.

For top floor areas daylit by rooflights, conductive heat gains and losses due to differences in air temperatures, through the opaque roof envelope are accounted for correctly. If the top floor is sidelit, the LT Curve for sidelighting assumes no losses through the ceiling and a small error results. This can be accounted for by glazing ratio, and adding it to the heating loads from the appropriately orientated sidelit curve. If a top floor is both sidelit and toplit, the floor can be divide into rooflit and sidelit zones. If the top floor is lit with monitor or other rooflight configurations with glazing tilted to more than 45^o to horizontal, use the vertical glazing curve for the appropriate orientation, and applied to the whole floor area, adding the heating load to zero % glazing ration on the rooflight curve as described above.

The effect of solar gains through the opaque roof is not accounted for. This is equivalent to the assumption that the roof is either well insulated and of high reflectance, or if not of high reflectance is also well insulated and includes a ventilated void. Solar gains through poorly designed roofs can create considerable cooling loads.

Reading off values from the LT Curves is made easier by the use of an LT Curve reading aid. On a small piece of paper, about 100 mm x 150 mm, draw two narrow lines about 20 mm from the top and the right hand edge and accurately at right angles to each other. Place the point of intersection of the lines on a curve so that the vertical line intersects the chosen glazing ratio. Then, by eye, make this line parallel to the vertical gridlines, and read off the energy value where the horizontal line cuts the vertical axis.

Envelope glazing and orientation

By looking at the curves and considering other design constraints, decide the ratio of glazed area to total external wall (or roof) area as shown in the figure below. Note that it is not the ratio of the glazing to the opaque wall area. Note further that it is the nominal glazing area, i.e. the structural opening and allows for obstruction to daylighting by framing and glazing bar of 20%.

If the actual value of framing obstruction is known to be significantly different from this, compensation should be made when reading off the lighting energy curve. For example, if the glazing design results in on 5% obstruction, which is possible for very large panes of glass, then a 30% nominal glazing area read off the lighting energy curve at 30% + (30 x (20-5)) % = 34.5% .

The maximum value of glazing ratio on the graph is 90%. This is because even when a facade is nominally all glass, about 10% is obstructed by the structural floor zones. However, 90% external glazing would then correspond to 100% (nominal) floor to ceiling as` seen from inside.

From the appropriate curve read off the annual primary energy consumption per square metre for each passive zone and enter the values on the worksheet.

The quick method is to take total energy (the top curve) and enter only these values on the worksheet. If, however, a breakdown of energy use (or delivered energy type) is required, then you can read off and enter lighting, heating and cooling separately.

The energy for non-passive zones can be calculated by reading from the curve at zero glazing area. The non-passive zones must always have a fan power allowance, at least for fresh air supply, (found at the bottom of the page). For non-passive zones in buildings with high internal gains and high lighting levels, a cooling energy obtained from the zero glazing intercept of the cooling curve should always be added.

LT Curves have been produced for four orientations only, in order to minimise the total number of curves. This means that curves have to be used when they are within 45^o of the orientation of the facade. The overall energy consumption is not highly sensitive to orientation and this procedure is compatible with the overall accuracy of the method.

Shading

The unshaded cooling curves clearly show that large areas of unshaded glazing lead to high energy consumption, or in the absence of mechanical cooling, by implication, serious overheating.

The shaded curves are drawn for two transmission coefficients (sometimes erroneously referred to as shading coefficients), 70% and 35%. These could be regarded as light shading and heavy shading respectively.

Two further categories must be defined:

Shading Type A

1. Movable shading which is only in place when there is a cooling load; or
2. Fixed shading which is so designed that it does not reduce the daylighting at the back of the room and still permits useful solar gains in winter.

Shading Type B

1. Fixed shading which reduces solar gain in summer, daylight, and solar gain in winter, by an equal and fixed fraction.

Shading Type A

Shading types A(1) and A(2) only reduce cooling load. In the case of type A(1), this is based upon the assumptions that when the shading is deployed the daylight is sufficiently bright to maintain daylighting levels above the switching datum, even if the shading does not improve the daylight distribution. This results in no increase in either lighting or heating energy.

In the case of type A(2), the geometry of the shading assists in the re-distribution of light towards the back of the room, but rejects a large fraction of the direct radiation from the high summer sun. This will also result in no increase in lighting energy, but may lead to a slight increase in heating energy due to some reduction of useful solar gains in winter. Shading devices of this type include light shelves, reflective louvres and louvres combined with prismatic or holographic glasses. Indeed some systems claim that daylight penetration is improved. This could be evaluated using the LT Method by designating deeper passive zones when calculating lighting energy.

Shading Type B

Shading type B has a seriously detrimental effect on daylight, since it reduces daylight by the same amount as solar gain. Type B shading with a transmission coefficient of 35% has the same effect on the daylight as if the glazing area is also reduced to 35% of its original value.

This can be evaluated on the LT Curves by reading off the lighting energy from an effective glazing ratio. For example, if fixed shading type B of 70% is applied to a facade with 45% glazing ratio, read off the lighting energy from an effective glazing ratio of 45% x 70% = 34%. The increased lighting energy may more than compensate for the reduction in cooling energy.

Shading type B will also have a considerable effect heating energy. This is because it reduces useful solar gains, but does not reduce the high conductive losses through the glass. To allow for this, read the heating energy for a shaded (type B) south facade from the north curve (heavy shading), or the east/west curve (light shading) and always read type B shaded east/west facade from the north curve.

Shading devices of this type include tinted and reflective glass, fitted glass, fixed grids and meshes. Overhangs and fins lie somewhere between the type A and B.

Summary table - Shading procedure / Effect of shading types

type		light		heat		cool
A(1) & A(2)		no effect		no effect		read from shaded curve
B		read off curve at reduced glazing ratio*		for S facade read from E/W curve(light Shading) and from N curve(heavy shading)		read from shaded curve
				for E/W facade always read from N curve

^*reduced glazing ratio = (actual glazing ratio)x(transmission coefficient)

Assessing the shading characteristics and performance of specific detailed shading designs is beyond the scope of this course. The LT Method should be used to establish performance criteria for shading devices (types A, B and transmission coefficient). Other analytical procedures should be used to check that the proposed shading device meets these criteria.

Shading due to adjacent buildings

The effect of adjacent buildings obstructing the sky is similar but not identical to the effect of shading devices. Obstructions to the east and west will shade the low angle sun, bringing some useful reduction in cooling load, and may remove the need for shading altogether, but it will also obstruct daylight. Obstructions to the south give less benefit, for unless the obstruction is very high, it will not shade the high angle summer sun, but will reduce daylight. Obstructions to the north carry no benefit, only disbenefit by obstructing daylight.

The overshadowing of a particular site for a proposed building should be investigated with other analytical tools such as sun charts or the heliodon.

A more detailed treatment of the effect of overshadowing is give in LT Method 1.2, published in Energy in Architecture, The passive Solar Handbook, Batsford for the EC.

To air condition or not?

Although the LT Method gives values for cooling energy consumption, giving the required primary energy (delivered as electricity) to power the fans and the refrigeration equipment. It should not be taken as recommendation for mechanical cooling, Indeed, the whole concept of energy tools for passive design has the avoidance of air-conditioning as a main objective.

Rather, the cooling load should be used as an indication of the potential energy demand if the building were air-conditioned, and thus used to support the case for passive measures to reduce the cooling load. If the cooling load can be reduced sufficiently, then further passive measures, (which cannot be evaluated by the LT Method), will be able to meet this cooling load and the need for air-conditioning will be eliminated.

These passive cooling methods, such as thermal mass combined with night (`purge') ventilation, buried pipes, evaporative cooling, must be evaluated with other energy design tools.

From a different viewpoint, if it is known that the building will not be air-conditioned, then the cooling load can indicate the probability of overheating. The actual incidence of overheating will be dependent upon other detailed factors which are not explicitly dealt with by LT, such as air movement, radiant temperature and thermal mass.

It is also important to note that LT uses many default values which represent good practice in terms of energy performance. For example, lighting of high efficiency and modest levels of internal gains are assumed for the office curves. Also, the lighting curve always assumes that artificial lighting is off when the datum value of illuminance is achieved with daylight.

Carbon dioxide emissions

The reason for expressing energy in primary energy has been discussed earlier. Primary energy can be related to CO₂ production as shown in the table below. When completing the final box in the LT Worksheet, an overall conversion can be used, which is biased to the conversion factor for electricity, the largest user. Or individual factors can be applied to particular fuel types if they are known. Lighting and cooling (inc. fan power) will both use the conversion factor for electricity - the other factors will apply only to heating.

These conversion factors are based on the mix of electricity generation - fossil fuel, nuclear, hydro etc. over Europe, and may not represent the CO₂ emissions on a national basis.

Table - CO₂ production for various forms of primary energy

tonnes CO2 /MWh primary or Kg CO2 /KWh primary
All energy types		0.24
electricity		0.22
gas		0.19
coal		0.31
oil		0.28