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Earth Tubes Trevor Butler The main cause of energy use in commercial buildings is associated with heating, ventilation and air conditioning (HVAC) and artificial lighting. Historically, thermal comfort and fresh air has been provided to buildings through the combustion of wood or fossil fuels (coal, oil) and through openable windows. The need for cooling has evolved through increased use of glass in the façade, electronic equipment and artificial lighting. Some studies, EEBD (2006), have also been carried out that show with improved building fabric (U-values, envelope) beyond a certain limit, there is potential for cooling loads to increase, whilst heating loads will decrease. This is due in part to the increase in electronic - heat giving - equipment, and a building envelope that does not allow the heat gained to leave. Whilst this is fine for heating conditions, the summertime experience has been that overheating is more likely to occur - therefore increased cooling from air conditioning is required to maintain comfort. It is against this backdrop, the author has been working to minimise energy requirements to maintain healthy and comfortable buildings. His interest in earth coupled systems began at Fulcrum in 1994 with the Milton Keynes Future World housing projects, and has grown from there. After having monitored a number of his installations from 1994 to the present date - 2011 - he is now in the process of investigating ideas to create a business to provide earth coupled systems for air-based earth coupled thermal systems for buildings. This project will seek to investigate the potential benefits and identify risks that will be required to see if energy efficient and healthy, comfortable buildings can be delivered using these systems. |
The subject of air-based earth coupled thermal systems have been researched and written about fairly extensively through industry and academia. The basic theory of air-based earth coupled thermal systems has been explored through the fluid dynamics of heat transfer as discussed by Welty et al (2000). In terms of industry application CIBSE (2004) has demonstrated a methodology for calculating the frictional characteristics of different types of pipework. The systems work by drawing fresh (outside air) into buried underground ducts. The temperature of the earth is prone to fewer variations in temperature to the outside air and as such temperature extremes associated with peak summer and winter can be moderated as the fresh air passes through the buried ducts. The volume of fresh air drawn through the ducts needs to be considered in two separate parameters: i. Primary Fresh Air: the minimum fresh air requirements of 20 cubic feet per minute (cfm) per person, ASHRAE (1989) or 10 litres per second (l/s) according to Part F (2006) ii. Thermal cooling load: the pre-cooled air will meet all or part of the cooling loads of the building. The use of buried ducts for tempering fresh air supply has been used for centuries throughout the world. Some of the earliest are recorded in Persia, where they are named 'badgeers' and linked with ventilation chimneys to draw the air through. These are completely passive systems - which is obvious due to the time being pre-industrial revolution. In Germany, with the advent and growth in interest of Passivhaus, Adamson & Feist (1988), the use of earth tubes are required for providing tempered fresh air - mainly for wintertime. |
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