tailieunhanh - Energy Management Systems 2012 Part 9

Tham khảo tài liệu 'energy management systems 2012 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 148 Energy Management Systems For the diffuse irradiation coming from the ground is beside the geometry important also the ground reflectivity characterised by the albedo reflectivity of the surrounding surfaces cig -. A constant albedo of typical grassland was used in most previous studies. Some other approaches as Gueymard 1993 also suggest a seasonal albedo model. Such an albedo changes over the year according to the latitude and land cover of the observed area or to anisotropic approaches Arnfield 1975 . These models are mainly appropriate for areas where a direct reflection is possible. Tilted solar irradiance Etilt of a tilted plane is written by many authors as the sum of the abovementioned contributions. The diffuse component coming from the sky decreases with the tilt angle while at the same time the ground diffuse part of the irradiance increases Etilt Etilt dir Etilt dif f 71 dir s It Egl dif sfv ag Egl dir Egl dif gfv. 5 cosSs Just recently the first two authors of this contribution have elaborated a more exact and conceptually proper approach based on the integration of isotropic radiance of sky Lsky that gives when integrated over the whole hemisphere the diffuse irradiance of horizontal receiving surface Egkitif ĩĩLsky. Integration over the hemisphere for which part of it has the radiance of sky Lsky and the other part has a different radiance of ground Lgr results in irradiation of the tilted surface Etilt. If the albedo Ugr and the coefficient k describing the contribution of the diffuse irradiation to the global irradiation Eglytif k-Egi are also considered an alternative more accurate estimate of irradiation of the tilted receiving surface is obtained T7 _ T7 I r Egl dir r 1 I -1 . .2 _ ttgr -2 _ I r- Etilt Etilt dir Etilt dif s n s E l dif 1 cos T T sin T 5 cos s s 1 2 k J Expressions 4 and 5 differ only as regards diffuse irradiation the difference depends on reflectivity Ugr and on contribution k of the diffuse irradiance to the global .

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