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The
table below is primarily a guide for Building
Surveying and Construction Students but
readers of course are more than welcome.
EPCs, THERMAL MASS AND ENERGY EFFICIENCY IN
BUILDINGS
Abstract:
The built environment uses 50% of the
UK’s energy, 90% of which comes from fossil
fuels.
The reduction of emissions
from buildings can provide high
proportionate savings and legislation has been
enacted to this effect.
The physics of thermal energy is
explained and how these physical properties can
be harnessed as tools by the designer to
conserve energy utilising Thermal mass.
Glazing is explained as a modulator of
thermal mass. Savings
from the use of thermal mass is reviewed.
Key words:
Thermal Mass, sustainable construction, U
Values, Renewables, specific heat capacity,
emissivity, thermal conductivity, thermal
resistance, surface resistance, Phase change
products.
The earth is warming because of energy
production by burning fossil fuels.
If this continued it is predicted that
the concentration of greenhouse gasses will
double by 2035.
(Stern, /Blundell/King).
The world summits commenced in Stockholm in
1972.
The Kyoto protocol agreed methods to
tackle the problem in 2005.
The UK is committed to a 12.5% reduction of
greenhouse gasses from 2008-2012.
It has set itself a voluntary reduction
target of 20% by 2010 and a long term aim of 60%
by 2050.
Each sector of the UK economy will be
required to reduce their CO2 emissions.
50% of the UK’s energy is used in the built
environment; 29% in dwellings and 53% of that is
used to heat the dwellings.
The emissions of the built environment will be
reduced by regulation.
Part L of the building regulations has
been framed to reduce current emission by 20%
within new builds.
This is achieved by a notional dwelling
emission rate (DER) being reduced by 20% which
is the Target emission rate (TER) the new
building must meet.
SAP and SAP2005 are the procedures and
methodologies for rating and demonstrating
compliance. (BREDEM
(Anderson et al 1985).
These standards require:
-
High efficiency
boilers with modern controls
-
Insulation of cavity
walls, lofts and cylinders
-
Modern Compact
flourescent lights
-
Accredited
Construction details
-
Draught Proofing
-
Maximum glazing
dependent on floor area
These standards reward the use of renewable
energies:
Legislation will encourage their10% growth as a
proportion of supply by 2010 and to twice this
by 2020.
Within the standards freedom is given to
designers to reduce co2 emissions utilising the
physical characteristics of heat.
The EEC made EPCs mandatory via Directive
202/91/EC - Energy Performance of Buildings
subsequently clarified by Directive 2010/31/EU
The Physical Characteristics of Heat:
-
The force known as cohesion holds materials
together.
Brownian motion describes the
molecular movement within materials.
This movement is Kinetic energy.
-
As energy is applied to a material potential
energy will be added.
These total energies, kinetic and
potential are known as heat or thermal
energy.
-
The quantity of thermal energy alters the
position of the molecules and their distance
apart and determines the phase of their
mass: solid, liquid, vapour or gas.
-
Temperature is the measurement of heat.
The Celsius or Centigrade scale is
gradated on the melting point of water (0)
and its boiling point (100) – the Kelvin
scale uses absolute zero as its zero and
uses the same Celsius unit.
Thermal energy is transferred by conduction,
convection and radiation.
Conduction is the transference of heat from
hot to cold until equilibrium is reached.
Convection is the movement of heat (thermal
energy) within a material which is free to
travel such as a liquid or a gas.
Radiation is heat transference by
electromagnetic waves.
This does not require a transfer
medium and is effective over long distances.
The sun radiates heat which is transmitted,
reflected or absorbed by terrestial materials.
Some of this absorbed heat is radiated
out.
The radiated heat of the sun is in the
spectrum of ultra-violet,
visible and infra-red which
known as
short wave radiant energy.
The radiated heat of the terrestial
materials is in the far infra red and is known
as longwave radiant energy. A wavelength is
measured in namometers which is .000000001
Meter.
Materials have different capacities to absorb or
release heat which is known as their specific
heat capacity (j/Kg).
The release of this energy is known as
emissivity.
The themal
conductivity (λ)of a
material is the amount of energy in watts which
is conducted through 1M2 with a temperature
difference of 1°K
A
material’s thermal resistance (R) is its
resistance to the passage of heat.
It is determined by dividing its
thickness in meters by its thermal conductivity.
Its
thermal capacity is
its capacity to hold heat and is the product of
its density and specific heat capacity.
A
u value is the reciprocal of the amalgam of the
thermal resistances within a structure.
It is used as an insulant measurement
scale.
The
surface resistance is
dependent on emissivity, orientation, exposure
etc.
Standard
values for surface
resistances are
-
Rs1
(inside) .12 m2
K/W
-
Rso
(Outside) .06
-
Ra -
Air space of cavity
Evaporation occurs when a liquid releases
particles (affected by Brownian motion) as a
vapour.
Evaporation is speeded by an increase of
the surface area or temperature, wind across the
surface or reduced humidity.
Evaporation removes thermal energy and
thus has a cooling effect on the material,
Condensation is the release of water vapour by
air cooled below the dew point.
The use of the
physical characteristics of heat:
The thermal mass of materials can be used to
conserve thermal energy:
Excess energy from solar heat and internal
energy sources can be stored by selecting and
distributing materials with the highest Thermal
capacity to promote an even distribution of
heat.
·
This stored energy can harmonize external
temperature fluctuations.
·
This stored energy can be retained and released
as required reducing heating and cooling
requirements.
·
Utilising Phase Change Materials (PCMs) to:
“store heat energy in a latent, as well as sensible
fashion, leading to greater heat storage
capacity per unit volume than that of
conventional materials” ( Kelly)
·
Utilising PCMs
with a high thermal capacity such as
Dupont`s energain (Parafin wax) when
refurbishing low mass structures
·
Orientation of the dwelling to maximise solar
gain
·
proper sizing and locating of building overhangs
and shading elements to avoid overheating in the
summer
·
Correct detailing of mass and insulation, i.e.
do not insulate the thermal mass from the
interior of the dwelling.
This heat storage, light and air may be
modulated by:
·
Insulation of the external structure
·
Mechanical and manual ventilation
·
use of energy-efficient glazing and careful
window sizing to reduce heating costs in the
winter, avoid overheating in summer and promote
thermal comfort year round
Glazing:
Windows are the modulator of heat, light and
air.
The window can be used to modulate heat gain and
heat loss dependent on the climatic
requirements.
Ordinary glass is a good transmitter of solar
radiation (84% of the short wave) and a poor
transmitter of terrestrial radiation (long
wave).
This property is ideal for cold climates.
Short wave and long wave radiation may be
selectively controlled by treating the glass so
that it absorbs, reflects or transmits the short
or long wave radiation.
Absorption and reflection is achieved by
tinting the glass utilising the spectrum,
reflection is achieved by applying coatings
known as low emissivity -
high-solar-gain low-E
coatings for cold climates, and for
low-solar-gain low-E coatings for hot climates.
Absorption can act as a solar store.
Glass has a thermal conductivity of 1.01 (CBSE
Guide).
It is used in thicknesses generally of
4 – 6mm in dwellings.
At 6mm
its TC is .0057 which would translate into a u
value of
168.
It is its surface resistance which makes
it a useful construction material – this gives a
6mm pane of glass a u value of 5.51.
There is no appreciable added insulation
value in thickening the glass.
Utilising the resistance of air and
suitable gasses, such as
Argon, Krypton and Xenon
as a filler between
panes of
glass can make double, triple etc
glazing units
excellent insulators.
For example a triple glazed xenon unit
(6mm glass) will give a u value of .118.
Further advances in technology are allowing the,
“smart” control of the window as modulator of
the internal climate by the use of chromogenic
materials.
Thermal energy can be swiftly removed and
modulated by creating through ventilation by
opening windows on opposite external faces of
the building.
CONCLUSION
A reduction of CO2 emissions is not the same as
a reduction in energy use it is minimising the
use of Carbon based energy forms, maximising
energy from renewables and minimising heat loss
by insulation and thermal storage within the
mass of the building and within Phase Change
Materials.
Research shows that utilising thermal mass can
reduce C02 emissions from 5 – 20%.
Data from NAHB suggest energy savings of 11%
were possible utilising thermal mass as opposed
to timber framed light weight construction.
(Kosny et al).
Utilising PCMs can increase these energy savings
by 15% in a thermally moderate environment
(Kelly 1999).
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