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Materials and Science
Energy usage in Buildings: Environmental impact and energy
conservation
Energy usage
As identified in the previous lecture, the operation of all buildings within
the UK consumes approximately half of the total primary energy
consumption, with roughly half of this half being used within the domestic
sector.
In 2000, the total energy consumption of the UK was 6695 PJ (1 PJ = 1 x
1015 Joules), of which 3080 PJ was used in buildings. This energy was used
essentially to fuel the building services which provide space and water
heating, lighting, and ventilation and air-conditioning systems.
The breakdown of energy consumption by sector and buildings for 2000 is
set out below:
Percentage of total UK delivered energy consumption
By Sector
By Buildings
Transport
35
Domestic
Buildings
Industrial
18
processes
Industrial
4
Commercial and
buildings
Public buildings
Commercial and
13
public buildings
Domestic
29
Industrial
buildings
Agriculture
1
in 2000
63
28
9
Electricity consumption is rising in many existing buildings, often due to
increased usage of office equipment, and sometimes air-conditioning to
remove internal heat gains emanating from this additional equipment. It is
hoped that the current trend in new-build toward more passive solutions,
improved design integration and more efficient engineering systems may
mitigate this rising trend.
Environmental impact
The combustion of fossil fuels contributes to atmospheric pollution,
resulting in a wide range of damage both to the environment and public
health. The increased atmospheric concentration of carbon dioxide (CO2)
caused by burning fossil fuels is increasing global temperatures, and also
results in the emission of sulphur oxides, mainly sulphur dioxide (SO2),
and nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide
(NO2) which interact with water and sunlight and produce the highly
acidic compounds such as sulphuric and nitric acids. The effects of acid
deposition vary according to the sensitivity of the eco-systems upon
which they fall. Acid rain also causes considerable damage to buildings in
industrialised areas.
The efficiency of electricity production in most thermal power stations is
typically between 30% and 50%. The consumption of electricity therefore
can lead to two to three times the CO2 emissions per delivered unit of
energy than the consumption of fossil fuels.
In response to this environmental impact, the UK Government has
introduced a range of policies which have a direct impact on the energy
consumption of buildings. These include taxation, financial support, and
regulation mainly from Part L of the Building Regulations: Conservation of
fuel and power.
The CO2 emissions associated with the use of various fuels is set out in
the table below:
Energy source
Electricity (grid)
Natural gas
Coal (typical)
Petrol
Propane
CO2 emission per kg or kWh (for
electricity) or litre for liquid
fuels
0.43 (1998 figure adopted as
official standard)
0.19
0.29
2.54 kg/litre
1.75 kg/litre
ENERGY CONSERVATION
The energy bill for most existing buildings, especially commercial and
public buildings, could be reduced by at least 20% using cost effective
measures. There are numerous strategies which can be applied.
New buildings and major refurbishments represent even greater
potential. In broad figures, new low-energy buildings consume 50% less
energy than similar existing buildings and 20% less than typical new
buildings.
General principles of passive energy conservation must first take account
of the climate the building will interact with. Hot dry and humid climates
will clearly generally require energy dissipation and cooling, and cool
climates will generally require energy conservation and heating.
Hot dry climates
A Mediterranean courtyard house meets the problem of how to remain
cool in a hot dry climate with clear skies. The courtyards and their
surrounding buildings radiate to the cold night sky and, during the night, a
pool of cool air is built up in the courtyards and in the ground floor rooms.
During the day, the sun shines but the reservoir of heavy cool air will
remain for some time. The walls of the buildings are thick so that
penetration of the sun’s radiation will take a considerable time. Ideally
the sun will reach the interior during the night, when conditions are cool,
and any excess heat can be dissipated by ventilation. The walls are
painted white to minimise absorption of solar gains. Advantage can also be
taken of evaporative cooling from fountains.
Hot humid climates
In these climates, there is no clear night sky to which excess heat can be
radiated. The only available method for improving thermal comfort is by
increasing the air movement around occupants. A raised verandah type
house with louvred walls and overhanging roof will respond reasonably well
to a hot humid climate.
Cool climates
In higher latitudes, overheating is not usually a main concern and heating
is often required along with energy conservation. A typical traditional
cottage with very small windows, highly insulated roof, small volume due
to the low ceilings and a centrally sited fireplace and flue exemplifies an
appropriately responsive construction.
Main design variables
1. Siting: The effect of moving a house from a sheltered site to one with
severe exposure which consequently increases the ventilation rate by
around 25% will increase the overall rate of heat loss by approximately
20%.
2. Multiple use: If two uses can be accommodated within one building, the
energy required for one will effectively be saved.
3. Volume: Heat losses will inevitably increase with increased volumes in
an approximately linear manner. This is due to increased fabric losses
through the increased surface area of the building envelope, and
increased ventilation losses from the increased internal volume.
4. Shape: Moving from the most economical square plan shape to a
plan:aspect ratio of 1:3 results in an increase in heat loss of around 20%.
5. Grouping: The effect of moving from a mid-terraced house to an end
terrace and to a detached house demonstrates a linear increase in heat
loss of 50% . Flats demonstrate the most energy economy.
6. Internal planning: heat savings of approximately 5% can be realised
through the central (or useful) location of boiler and flue.
7. Thermal insulation: Adequate thermal insulation is a major determinant
of energy efficiency and is normally addressed through the incorporation
of adequately insulated elements of construction through the
specification of maximum U-Values in the Building Regulations.
8. Ventilation: The ventilation heat loss in a typical office building will
increase from approximately 30% at one air change per hour (ACH) to
60% at two ACH to 70% at 3 ACH, levelling off at around 80% at around
5 ACH.
9. Fenestration and orientation: The glazing of a building is a major factor
in its overall thermal performance, and the heat transfer dynamics can be
complex. A south facing window will tend to show a small gain during the
heating season, but care should be used when increasing the size of south
facing windows as considerable heat losses will arise in cold periods.
10. Personal factors: Activity and clothing thermal resistance can often
usefully be taken into account.
ENVIRONMENTAL IMPACTS OF SOME GASEOUS POLLUTANTS
Sulphur Oxides (SOx)
The oxides of sulphur are probably the most widespread. Although there
are six different gaseous compounds: sulphur monoxide (SO), sulphur
dioxide (SO2), sulphur trioxide (SO3), sulphur tetroxide SO4), sulphur
sesquioxide (S2O3) and sulphur heptoxide (S2O7), the two main oxides of
concern are sulphur dioxide and sulphur trioxide.
Sulphur dioxide has a pungent suffocating odour, with an odour threshold
of 0.5 ppm. It is colourless, non-flammable and highly soluble in water
with which it reacts to form sulphurous acid. It has a density of about
twice that of air. It is relatively stable in air and can be transported up
to around 1000 km. The problem can therefore be an international one.
Reacting photo-chemically or catalytically with other components in the
atmosphere SO2 can oxidise to SO3 which readily reacts with water to
produce sulphuric acid.
In a dusty atmosphere, SO2 and SO3 are particularly harmful because the
acid molecules paralyze the hair-like cilia which line the respiratory tract.
Without their proper function particulates are able to penetrate the
lungs and settle there. The effects of acid deposition on plant life are
well known. Plants are particularly sensitive to SO2 during periods of
intense light, high relative humidity and moderate temperatures.
Acid deposition will also attack building materials, especially those
containing carbonates such as marble and limestone. The insoluble calcium
carbonate reacts with sulphuric acid to form soluble calcium sulphate
which is washed away in the rain, leaving a pitted surface. Exposure to
acid mists also accelerates the corrosion of metals.
Nitrogen oxides (NOx)
As for sulphur, there exist six known gaseous oxides, nitric oxide (NO),
nitrogen dioxide (NO2), nitrous oxide (N20), and the less well known
nitrogen sesquioxide (N2O3), nitrogen tetroxide (N2O4) and nitrogen
pentoxide (N2O5). The two oxides of main concern are nitric oxide and
nitrogen dioxide.
Nitric oxide is emitted to the atmosphere in larger quantities than
nitrogen dioxide. It is formed in high temperature combustion process
when the nitrogen and oxygen combine. It is moderately toxic and like
carbon monoxide can combine with haemoglobin to reduce the oxygen
carrying capacity of the blood. Of concern is that NO is readily oxidised
to NO2 which has serious environmental significance.
Nitrogen dioxide is readily soluble in water forming nitric acid (as well as
some nitrous acid). The gas is highly toxic. At 5 ppm it causes respiratory
problems with irreversible damage to lungs at 150 ppm within 3 hours of
exposure. As for sulphur dioxide, nitrogen dioxide contributes to the
formation of acid rain with the associated damaging effects on both plant
and animal life, and building materials.
Sulphur dioxide and nitrogen dioxide are commonly produced from the
combustion of coal and natural gas respectively. The table below sets out
typical emissions:
Emission factor
Coal
Sulphur dioxide (SO2)
35 kg/tonne
Nitrogen dioxide (NO2) 10.5 kg/tonne
Natural gas
9.5 kg/106 m3
8800 kg/106 m3
Carbon monoxide (CO)
Carbon monoxide arises mainly from the incomplete combustion of fossil
fuels. It is produced from internal combustion engines. Its environmental
impact must be distinguished from carbon dioxide, which is relatively
inert, and acts as a greenhouse gas. Carbon monoxide is colourless,
odourless and toxic. It binds irreversibly with haemoglobin in red blood
cells, impairing the oxygen carrying capacity. The affinity of haemoglobin
is approximately 250 times greater than oxygen. The presence of
carboxy-haemoglobin also makes the remaining oxygen bind more tightly
to the haemoglobin. Once released to the atmosphere it can last for
several weeks before being eventually oxidised to carbon dioxide.