tailieunhanh - solar collectors and panels theory and applications Part 14

Tham khảo tài liệu 'solar collectors and panels theory and applications part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Applications Oriented Research on Solar Collectors at the Politehnica University of Timiậoara 381 Some regions of the absorbing surface are shadowed by the window supports and by the walls of the chassis. The first effect has a daily variation while the second one may be considered to have a hourly variation. A cross section through the lateral window support is represented in Fig. 3. The fluid crosses n times the shadows created by the central and lateral supports with n 5 the number of pipes. The total length of the shadow may be expressed as L1 n ft tan 0 b - d 1 where h is the overheight Fig. 1 b is the width of the central support Fig. 2 d is width of the insulation Fig. 3 and 0 is the angle between the incoming sun ray and the normal to the absorber. For example at equinox 0 rnĂT with a - the apparent angular speed of the Sun and At - time from noon. The length of the pipe that is irradiated allowing for the heat to be absorbed is L Lcd - L1. 2 The surface of the fluid that is irradiated Ac aL results Ac a IL - n h tan 0 -d b 3 so that the fraction of surface that is effectively used is n tan 0 b - d I f 1 - L a n 0. ----- 4 The variation of the fraction f with the hour is represented in Fig. 4. It may be seen that f X for a time interval of 4 - 6 hours centred at noon. 382 Solar Collectors and Panels Theory and Applications In order to find the equations that characterize the system we note that the heat obtained by thermal conversion is transferred to the working agent. The fluid enters the collector at a temperature Tf and exits at a temperature Tfe. The energy balance for the fluid that flows through a small segment of pipe of length Ay is m Cpf -m cJi y q Ay 0 5 where m is the mass flow rate Cp is the isobar specific heat of the fluid qU is the heat flux absorbed by the unit length of a current tube and Tf is the temperature of the fluid. f -------------1 ---------r-------------t----------- 8 10 12 14 16 Hour Fig. 4. Irradiated fraction of