Thermal radiation exchange between the sun and sky, and the earth
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Radiation exchange is an important factor in the thermal environment of building and ground surfaces. During daytime these surfaces receive a substantial amount of energy from the Sun in the form of solar radiation. In addition to this, another component of energy input is also received, of smaller amount but continuously day and night, from the sky and nearby objects around these surfaces, in the form of long wave-length radiation. At night-time, this source of input is normally not enough to compensate for the outgoing radiation from building and ground surfaces, resulting a net radiation loss to the sky. In the first part of this investigation, the aims were to study the long wave radiation from Sky and devise a simple instrument to measure this component of heat exchange with some trial measurements under Christchurch conditions. The study was carried out in two ways: 1. To study the analytical approach which leads to the theoretical estimation of sky radiation by Elsasser's and others' charts. 2. To review the experimental approach which introduces the methods of sky radiation measurement and the empirical formulae proposed by some observers to estimate this radiation input from meteorological soundings. A simple radiometer was made which enables the sky radiation and hence the heat loss by radiation from any surface of known temperature to be estimated. A series of trial measurement at night and daytime under Christchurch weather conditions were carried out with this radiometer. The results gave good examples of the variation of sky radiation corresponding to air temperature and sky cloudiness conditions. Quantitatively, the amounts of sky radiation estimated from some measurements were 25%to 30% higher than the values calculated by the empirical formulae at the same conditions due to systematic error. For some other measurements of different method the results are close to Swinbank's empirical formula RL = 1.195 σT4a - 17.09. Analysis showed that this formula may be applied for all cases if the systematic error is eliminated. Modification for the radiometer and precautions necessary to improve the accuracy of measurement are also proposed. The second part of this investigation is concerned with the solar radiation component. It consists of: 1. A brief investigation of solar radiation data for Christchurch through literature and records of Meteorological Offices. 2. A study on the performance of a common type flat plate solar water heater under Christchurch weather conditions. Mean radiation data for Christchurch in the last 5 years are presented and compared with the standard year solar radiation values proposed earlier. Sources of information for these data are located and the method to estimate radiation intensity on inclined surfaces is also presented. Tests were carried out during the last 3 months of 1973 to measure the energy collection and efficiency of operation for a solar water heater under Christchurch conditions. Results are presented in forms of tables and graphs to show some examples of the variation in energy collection, efficiency of operation and water temperature with weather conditions. Estimation for whole year contribution potential based on the test results was also given with some recommendations of angles of inclination for more possible collection.