Energy Efficiency in Developments
There are many different energy efficiency options. Their applicability depends on the type of project, in particular whether it is a new building or a refurbishment. The following section describes low energy solutions and renewable energy options, drawing specific attention to considerations relevant to different sectors.
DESIGN
In the case of new buildings, energy issues should be given consideration at the early stages of design (ideally at project inception) to enable the best technical and economic solutions to be achieved. At an early design stage there are many opportunities to develop an integrated approach to energy use and savings, including the determination of the physical form and characteristics of the building. Consideration should be given to the following:
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orientation - sun path around the site, prevailing wind direction and the need for clear views
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reducing the amount of exposed external area compared with enclosed internal volume
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incorporation of atria, courtyards or sunspaces can reduce energy consumption in deep plan buildings
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thermal mass should be fully considered and included or excluded as appropriate, to correspond with activity and occupancy patterns
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landscape design can influence the microclimate (and reduce energy demand) by providing shelter from driving rain and wind
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the ratio of glazing to solid material in the external walls can be optimised to provide benefits from natural light and useful solar gains, whilst avoiding excessive heat losses or gains
These measures are influenced by decisions made throughout the design process. Cost implications are variable. In many cases, these design choices can lower capital costs through reducing building service requirements. In addition, these energy saving measures typically require little or no maintenance and last throughout the lifespan of the building with no energy input.
Where appropriate, these issues should also be applied to refurbishment projects, although it is recognised that the majority of these factors will be predetermined.
Energy Efficent Design
A - Warm air exists at high level due to natural 'stack' effect
B - Well insulated building fabric
C - Thermal mass in exposed concrete floors/ceiling
D - Warm air rises naturally up the atrium. drawing cooler air in from outside
E - Trees can provide shade in the summer and shelter form the wind and rain in exposed areas
F - Overhanging eves to provide solar shade
G - Integration of renewable energy, eg solar panels
MATERIALS SPECIFICATION
The materials specified for use in the construction of a building heavily influence energy performance, in both new build and refurbishment projects.
Insulating the building fabric to a high standard is an effective way of reducing energy use and is also a cost effective solution. U-Values listed in Building Regulations should be considered as a minimum standard and should always be improved on as far as is technically and commercially practicable.
U-Values
The rate of heat loss through a building element, such as a wall, roof or floor, depends on a number of factors. It is industry convention to combine all the effects of these factors into a single measurement, termed a “U-Value”.
The U-Value is the rate of heat transfer through a building element, subject to the difference in internal and external temperature. The lower the U-Value, the less significant the heat losses are, which results in a more energy efficient building.
Careful detailing of the building fabric and junctions between building elements such as windows and external walls also reduces heat losses from air leakage. Targets for air leakage rates should be specified at the design stage and verified during construction through pressure testing, which gives the opportunity to address any problem areas.
The specification of glazing should be carefully considered. Low U-Values can be achieved through the selection of low ‘e’ coated glass, the use of argon rather than air in the cavity between the panes, or through the use of triple glazing. Although these specifications can be more expensive than standard glazing options, they can offset costs for building services through reducing heat gains and/or losses.
Energy efficent glazing
Commonly available energy efficient windows include the following:
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low ‘e’ coating – this is a thin film of metal oxides or semiconductors placed directly onto a pane of glass to reduce heat transfer. In a double glazed unit, the film is applied to the outer surface of the inner pane to reflect heat back into the room. There is virtually no associated visual impact and solar energy still passes into the building.
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argon filled cavity – it is possible to use gases other than air in the cavity of double glazed units. Argon is a typical alternative, which reduces heat transfer due to the properties of the gas.
INTERNAL ENVIRONMENT – ventilation, heating, cooling
Energy efficiency can also be achieved through the incorporation of passive environmental control features. In particular, the use of daylighting, natural ventilation and passive heating and cooling can create a low energy building with reduced environmental impacts, whilst still achieving comfortable internal conditions for occupants.
Passive systems need to respond to and be appropriate for the local environment and include the following:
Atria and courtyards
Enclosed glass volume, can be used to bring daylight and natural ventilation into the centre of deep plan buildings.
Void in the middle of a building, or group of buildings, which is open to the elements.
Optimisation of glazed areas for balance of daylight against heat gains and losses
Uncontrolled solar gains can result in overheating
The use of solar shading allows diffused light into the space and keeps excessive heat gains out
Narrow floor plates (to facilitate natural ventilation)
narrow floor plates: opening on single side
maximum depth of 7m for effective natural ventilation via an opening on one side of the space.
narrow floor plates: opening on opposite sides
maximum depth of 15m for effective natural ventilation via openings on opposite sides of the space.
thermal mass
Materials such as concrete, brick and stone absorb heat, which can have a cooling effect in a building.
Stored heat is released, subject to a time delay.
Natural ventilation can be used in situations where the external conditions are free from excessive noise or poor air quality and where the intended use of the space allows. In narrow plan buildings, a typical approach is to use openable windows. In larger spaces, additional measures would be required, for example the use of a ‘stack effect’ system (see earlier energy efficient design diagram).
The use of natural light rather than artificial systems can offer significant energy savings. Techniques to bring light further into deep plan buildings include design features such as atria and courtyards. Sun pipes can also be used and can either be included during construction, or can be retrofitted.
Passive solar design is the capture of useful solar gains (heat) to offset heating energy requirements. Atria and sun spaces are typical examples of this approach.
Passive cooling techniques save energy, but also avoid the use of environmentally damaging refrigerants. The most simple and economic approach is the use of night purge ventilation in buildings with exposed thermal mass. However, this is not appropriate for every building and alternatives include the use of absorption cooling from waste heat, surface or ground water cooling, ground coupled air cooling, displacement ventilation, and evaporative cooling.
Night purge ventilation
Daytime:
During the day, heat gains from occupants, activities and solar energy are stored in the thermal mass of exposed concrete, stone or brick in the walls, floor and ceilings. This cools the space during the day without using energy.
Night time:
At night, the heat that has built up during the day is released from the building fabric and leaves the space through an open window or vent. The space is then cool for the start of the next day. In winter, the vents can be closed and the heat retained.
BUILDING SERVICES
Building services equipment, such as boilers, air handling units and lighting systems use energy to provide comfortable internal conditions for building occupants. Where it has been determined that mechanical services are required to provide comfortable internal conditions, efficient plant should always be specified.
Boilers with an operating efficiency of over 90% are readily available and should be specified to ensure efficient use of fuel. Splitting the heating system in larger buildings into sections serving particular areas can reduce energy use by allowing the system to be switched off in unoccupied spaces.
If a mechanical ventilation system is required, then high efficiency fans should be specified to minimise energy use. Heat recovery should also be included, in the form of plate heat exchangers, thermal wheels, or run-around coils. Plate heat exchangers are the most simple and economic option, with no moving parts or energy required for operation.
Plate heat exchangers
Air (or water) moving in opposite directions is separated by thin sheets of metal, which allow heat transfer. In this way, heat can be recovered from the exhaust air and used to pre-heat incoming air.
The use of air conditioning and comfort cooling should be restricted to those areas where it is strictly necessary. Artificial lighting systems for use internally and externally should be as energy efficient as possible. Low energy light bulbs are widely available and use only 20% of the energy of standard bulbs, and can therefore offer significant energy and maintenance/replacement savings.
For refurbishment projects, services should be upgraded wherever possible. If costs are prohibitive, then more economic measures could be implemented, such as insulation of pipes and ductwork.
BUILDING MANAGEMENT SYSTEMS/CONTROLS
The use of an appropriate control system can significantly reduce energy wastage. For example, sensors can be used to detect when there is sufficient daylight in a space and can dim or switch off the artificial lights in response.
A BMS (Building Management System) is a sophisticated network of sensors and controls covering all building services. Systems can be programmed for optimum operation and minimal energy use. These systems are typically installed on larger projects.
Use of a BMS facilitates energy monitoring and can highlight areas of excessive use where savings could be made. In smaller buildings, a simpler approach could be to take regular meter readings.
Variable speed controls on pumps and fans can save a significant amount of energy. This allows building services to use only the energy that is required, rather than continuously operating at full capacity.
Many of these systems can be retrofitted, although this is less cost-effective than their inclusion at the design and construction stage.
Consideration should be given to the maintenance implications of installing controls and sensors. In a few instances, the additional maintenance and replacement costs associated with e.g. additional sensors and motorised controls (such as daylight sensitive lighting) can be greater than the cost savings arising from reduced energy consumption.