Determination of steady-state thermal transmission properties of straw bales for construction

Straw is an inevitable product of cereal production and it is available in huge quantities all over the world. In order to use straw bales as a building material, the characteristic values of their performances, including thermal performances, should be determined. To not lose the benefits of the cheapness and sustainability of the material and its “km0” availability, the characteristic values have to be determined with simple, inexpensive and replicable means and procedures, close to the place of use. This research aims to design and to implement tools and methods which have the above-mentioned characteristics, aimed at the determination of the thermal properties of straw bales of various nature and origin. The guidelines dictated by ASTM and ISO have been followed for this study. The test items are the straw bales, which do not have well-defined geometric shape, and which significantly vary for size and density from element to element. A measurement system consisting of a Metering Chamber (MC) has been realized. The front wall of the chamber can be completely closed or provided with a hole for the insertion of the specimen of straw bale. The MC is placed inside a Climate Chamber (CC), maintained at constant temperature by an air-conditioner. A known quantity of energy can be introduced inside the MC. When the "steady state" is reached, temperatures in the MC and CC are constant and then all the energy put into the MC passes through its walls in the CC, where it is absorbed from the air-conditioner. A series of thermopiles, each consisting of several thermocouples, detects the temperature of the various internal and external surfaces of the MC, the specimen-wrapper, the specimen and the interior of the CC. Applying the Fourier’s law, in simplified form, the effective thermal conductivity of the material forming the walls of the MC is determined, and subsequently, using the found value, the amount of energy transmitted by the various parts of the MC and by the specimen is calculated. Finally it is possible to determine the thermal conductivity. The present work is only a first phase of the research: the results so far achieved show that the direction taken is right and that it is justified to continue, with the main purpose to reduce the uncertainty margins, mainly due to some of the adopted simplifications.