Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.