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Fabric Energy Storage - Energy Efficiency
in Commercial Structures
The energy efficiencies offered by concrete based
materials are well known when applied to residential
housing. There has been extensive research to model
the performance of these high mass structures under
New Zealand environmental conditions, and these results
have been well reported in the literature. There have
been fewer examples of high mass construction techniques
being used in the commercial sector in New Zealand.
The most often quoted example is the Maths and Science
Building at Canterbury University. This has been the
subject of ongoing research and several papers. Overseas,
the story is different. There are many examples of
high mass structures being used to produce thermally
efficient, comfortable work environments.
How does fabric energy storage
work?
Concrete walls, columns and floors have a large capacity
to store and release heat. This function has the effect
of regulating the internal environment, by reducing
and delaying the onset of peak temperatures. This
effect is referred to in New Zealand as the thermal
mass advantage. Overseas, the technique of utilizing
the advantages of high mass in commercial structures
is referred to as 'Fabric Energy Storage', or 'FES'.
This technique has been widely used overseas for commercial
offices to create comfortable working environments
for the occupants, and reduced energy consumption
costs for the owner /occupiers.
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Maths & Sciences Building,
University of Canterbury ©ccanz |
Why
use FES?
The operational costs for heating,
ventilation, air-conditioning and lighting are substantial.
The trend in the UK has seen demand for air conditioning increasing
simply to maintain thermal comfort. A developing trend in
offices has been a substantial increase in cooling requirements
brought about by the increasing use of computers, printers,
photocopiers etc, together with uncontrolled solar gain.
UK experience points to
a significant increase in power usage for the refrigeration
plant, pumps and fans necessary to maintain this “comfortable”
environment. The energy consumed by these plants is second
only to the energy used for lighting. Of more concern, is
the fact that this portion of energy consumption is the fastest
growing sector in the commercial / services market.
This growth has been occurring
at a time of increasing awareness of issues such as global
warming, greenhouse gas emissions and climate change.
Not included in any of these costs, are the substantial costs
associated with the purchase and installation of air conditioning
/ heating plants, together with the ongoing maintenance costs
over the life of the structure.
As a consequence of these
environmental pressures, overseas designers are searching
for innovative means of achieving a comfortable working environment.
The thermal storage capacity of concrete comes into play in
this regard, and as more structures are designed and built
using this science, awareness of the advantages of this design
philosophy spreads.
Designers are now looking
for low energy methods of achieving thermal comfort. Here,
the inherent benefits of concrete as a material can come into
play. There have been several notable structures built using
this design philosophy in the UK, and the designs have been
recognised in Industry Awards as a consequence of their performance.
Three such examples are the Lloyd’s Register of Shipping
building in London, the Canon HQ building at Reigate and the
Toyota GB Headquarters at Epsom.
In New Zealand, the use
of FES design principles has been accepted in upper market
residential houses and there are signs that this is a growing
market segment. The commercial market is less developed however,
with one or two notable exceptions i.e. the Maths and Science
Building at Canterbury University.
Which parts of the
structure can be used for FES?
Any section of the building can be
used. The key point to remember is that it is important to
keep the surfaces to be used free from isolating claddings
or other coverings. This means that carpeted floors will not
perform well, but exposed slab soffits will. These elements
represent the largest volume and area within a typical structure
and are usually well distributed throughout the building for
maximum efficiency. Exposed columns and walls also act as
good energy storage media.
Hollow floor slabs can also
be used overnight to reduce the concrete temperature by ducting
cold evening air through the voids, thus removing the slowly
accumulated heat generated by the daytime occupiers.
Ducts cast into the concrete
can also be used to act as enhanced heat sinks by lowering
the temperature of the concrete and increasing the temperature
difference between the air and the concrete. Careful design
will remove the risk of condensation forming on these sections.
What can be expected from a FES structure
by way of thermal performance?
FES can reduce peak internal temperature
by 5°C, shifting the peaks to later in the day often after
the occupants have gone home. A passive FES building can contribute
a cooling effect of 15-20W/m2, which is sufficient
to counteract the effects of computers and printers for an
average office. A greater cooling effect of 25-35 W/m2
can be achieved with an active solution, (some papers have
reported values as high as 40 W/m2) allowing comfort
to be maintained at higher levels of internal heat gain.
If a significant area of
concrete is to be exposed, then consideration should be given
to the attainment of an appropriate surface consistency and
colour consistency for the concrete. To achieve smooth, consistent
surfaces on soffits it may be advisable to cast in custom-made
high quality steel or glass fibre lined moulds.
FES soffits in particular
are most effective (for day lighting and lighting) if they
have a white or pale coloured finish, which helps to reflect
light onto workspaces. Special architectural concrete may
be employed to achieve the desired finish, or normal structural
concrete may be painted.
The designer of a concrete FES building should also address
the issue of coordination and integration of services. At
the concept design stage, the servicing strategies for lighting,
electrics, telecommunications, fire alarm and sprinklers,
wet services and HVAC need to be considered. Early resolution
of servicing runs is desirable so that precasting design work
can be started in earnest early on. Pre-planning is the key
to optimizing both the construction efficiency and the operational
longevity of an FES building.
Indicative performance guidelines
- Peak temperatures can be reduced by 5oC or more
- Temperature peaks offset by up to 6 hours
- A 50% reduction in carbon dioxide emissions if you can
leave concrete walls, floors or ceilings plain or painted
- A 25 W/m2 of passive cooling capacity, which
is more than adequate to cater for heat loading of a typical
commercial building
- Up to 40 W/m2) can be achieved by forced ventilation
through a hollowcore precast concrete floor.
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