How cities can minimise energy use

Conventional buildings consume much energy for heating and cooling to protect them from the temperature effects of climate and seasons. But some basic thought and planning, in combination with these 10 passive solar building design techniques, can help to radically reduce these energy costs. Here’s 4 key ideas:

1. Optimize the spatial layout

Inappropriate (left) and appropriate (right) spatial layouts for settlements in hot climates. Grid layouts borrowed from other climates, and wide spacing of buildings, do not provide shade or wind shelter. Organic, non-grid layouts do provide shade and can be designed to block winds, preventing issues with wind funnelling. Credit: author.

Right: Sample layout for housing estate in higher latitudes such that each property has both privacy and an equator-facing aspect and roof to maximise potential use of solar energy. Grey circles are trees, grey lines are hedges (preferably) or fences.Credit: author.

2. Optimize the building form and layout

A low surface area to volume (S/V) ratio is optimal for a passive, low-carbon building. This is the ratio between the external surface area and the internal volume.

Compactness C = Volume / Surface Area

Size is also a factor: a small building with the same form as a larger one will have a higher S/V ratio. Buildings with the same U-values, air-change rates and orientations but differing S/V ratios and/or sizes may have significantly different heating demands. This has the following consequences:

• small, detached buildings should have a very compact form (square is close to the perfect optimum, the circle);
• larger buildings may have more complex geometries;
• high S/V ratios require more insulation to achieve the same U-/R-value.

In temperate zones, aim for an S/V ratio ≤ 0.7m²/m³.

Form factor

The ratio of the usable floor area, F, to above-grade enclosure area E is more useful, because it favours buildings that require less floor-to-floor height.

Form factor = F/E

The more compact the form, the higher the ratio, which is better. Large buildings (e.g., 172,800 ft2 over 12 stories) have a much more efficient form than small buildings or large high-bay buildings for heating load (but not cooling, where the opposite is true).

This metric permits comparisons of the efficiency of the building form relative to the useful floor area. Achieving a heat loss form factor of ≤3 is a useful benchmark guide when designing small Passivhaus buildings. This also reduces the resources required and the cost. Most building uses do not require volume but floor area. This metric also does not include the ground contact area, but does include the roof.

A building with a more complex form is also likely to have a higher proportion of thermal bridges and increased shading factors that will have an additional impact on the annual energy balance.

The effect of form on total energy consumption for a given floor area is reduced as buildings increase in size. Besides permitting greater design flexibility, this lets designers use daylighting and natural ventilation cooling strategies also to reduce energ demand, as these require one dimension of the building to be relatively narrow (between 45 and 60ft (14–18m).

Example:

For a small office of 20,000 ft2 (1800 m2) a narrow two-storey form, ideal for natural ventilation and daylighting, may have a form factor ratio of 0.88, whereas a deep square plan have one of 1.02. For the former to have the same enclosure heat loss coefficient as the latter, its overall average enclosure R-value would need to be 1.02/0.88 = 16% higher. This would require a significant increase in the opaque wall area R-value, a reduction in window area, or a more expensive window.

An increase in the S/V ratio of 10% (the building in the middle) would require 20mm of insulation more than the good form on the left to achieve the same level of insulation. The one on the right (a 20% higher S/V ratio) would require an extra 40mm of insulation.

Optimum room layouts in dwellings according to the climate.

3. Adapt the dwelling forms and room layouts according to latitude

For latitudes above 25°: the sun-facing glazing area should be at least 50% greater than the sum of the glazing area on the east- and west-facing walls. Orientation is long on the east-west axis, which should be within 15 degrees of due east-west. At least 90% of the sun-facing glazing should be completely shaded (by awnings, overhangs, plantings) at solar noon on the summer solstice and unshaded at noon on the winter solstice. The room plan should – if it is a dwelling – incorporate the main living rooms on the equator-facing side, with utility rooms, less used rooms and garage if any on the north side. Morning rooms are typically bedrooms. On the side away from the equator windows should be kept to a minimum and as small as possible for lighting to minimise heat loss. This wall should also have high thermal mass or/and be externally insulated, to retain heat in the building.

For latitudes less than 25° or where topography significantly impacts insolation, the opposite should be the case. Bedrooms, for example, need light in the morning. The whole building needs to be protected from low angle heat.

Around 25° there is some leeway depending on local conditions. In these mid-latitudes different parts of a building may be used in the winter and summer, as equator-facing rooms become too hot and occupancy is switched in summer to rooms on the non-equator-facing side (not shown in the above left plan).

Table: The shape of the building has different requirements according to the local climate:

Climate Elements and requirements Purpose Warm, humid Minimise building depth for ventilation Minimise west-facing wall to reduce heat gain Maximise south and north walls to reduce heat gain Maximise surface area for night cooling Maximise window wall for ventilation Composite Control building depth for thermal capacity Minimise west wall to reduce heat gain Limited equator-facing wall for ventilation and some winter heating Medium area of window wall for controlled ventilation Hot, dry Minimise equator-facing and west walls to reduce heat gain Minimise surface area to reduce heat gain and loss Maximise building depth to increase thermal capacity Minimise window wall/window size to control ventilation, heat gain and light Mediterranean minimise west wall to reduce heat gain in summer Moderate area of equator-facing wall to allow winter heat gain Moderate surface area to control heat gain Small to moderate window size to reduce heat gain but allow winter light  Cool temperate Minimise surface area to reduce heat loss Moderate area of pole-facing and west walls to receive heat gain Minimise roof area to reduce heat loss Large window wall for heat gain and light  Equatorial upland Maximise north and south walls to reduce heat gain Maximise west-facing walls to reduce heat gain Medium building depth to increase thermal capacity Minimise surface area to reduce heat loss and gain

 

4. Optimize the roof shape and orientation

In hot climate zones, vaulted roofs and domes dissipate more heat by natural convection than flat roofs. They give greater thermal stability and lower daytime temperature. The best orientation requires that the vault form receive maximum daily solar radiation in winter and minimum in summer.

A north-south axis orientation for a vaulted roof is better for winter heating, receiving the minimum direct solar radiation in the summer, while an east-west axis orientation will maximise summer heating, receiving the most irradiation in the morning and evening. The results are summarised by example for a 30° latitude site below.

Table: The effect of vault orientation on seasonal direct solar radiation.[i] CSR = Cross Section Ratio. This is the ratio between vertical height of the vault and the horizontal width.

Orientation Season

Loss of direct solar radiation (%)

CSR1 = 0.5 CSR1 = 0.8 CSR1 = 1 CSR1 = 1.25 CSR1 = 2 W-E Summer 12.4 20.1 23.9 29 37.8 Winter 9.8 17 19.6 23.2 30.4 N-S Summer 17 28.6 35.1 42.1 56.4 Winter 6.3 7.1 8 8.9 10.7 NE-SW Summer 14.7 23.9 29.3 34.8 45.6 Winter 8.9 13.4 16 18.8 24.1 NW-SE Summer 14.7 23.9 29.3 34.8 45.6 Winter 8.9 13.4 16 18.8 24.1

The effect of vault orientation on received seasonal direct solar radiation.

This article is published in collaboration with Sustainable Cities Collective. Publication does not imply endorsement of views by the World Economic Forum. 

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Author: David Thorpe is Special Consultant of Sustainable Cities Collective

Image: Vehicles move along New Delhi’s Connaught Place during evening hours, October 28, 2014. REUTERS/Anindito Mukherjee