![Sample our Built Environment journals, sign in here to start your access, Latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5926/TOC-Subject-Sample-2021-Built-Environment.jpg"})
ABSTRACT
In parabolic trough collector, evacuated or non-evacuated glass tubes are more valuable for medium and high-temperature applications to reduce the heat losses from the absorber tube. Use of such glass tubes is not convenient under lower temperature applications because of lower surface temperature and high initial cost. In this work, an effort has been made to improve the performance of the absorber tube by reducing the heat losses without using a glass cover tube. A new technique of cut tube absorber was designed, developed and tested experimentally in the conditioned room by optimizing area with 45% cut on the upper portion of the tube, which does not receive concentrated sunrays and further covered with flat plate. Also, the presence of insulation and coating over cut tube absorber was studied. Water and sigma therm-K was used as a heat transfer medium to examine the heat losses for the working fluid having a temperature range below and above 100°C. Experimental results reveal that reduction in overall surface area of the tube and coating leads to a decrease in thermal heat losses by 16.13% in water and 10.13% in sigma therm-K. Adding insulation would reduce heat losses more by 7.33% in water and 33.43% in sigma therm-K as compared to uncut tube with coating.
Nomenclature
| A | = | Area (m2) |
| BT | = | Bare tube |
| C | = | Specific heat of fluid (J/kg K) |
| CR | = | Concentration ratio |
| CTBCC | = | Cut tube with black chrome coating |
| CTBCCI | = | Cut tube with black chrome coating and insulation |
| di | = | Inside diameter of absorber tube (m) |
| do | = | Outer diameter of absorber tube (m) |
| DST | = | Down surface temperature |
| FVM | = | Finite volume method |
| g | = | Gravitational constant (m/s2) |
| h | = | Outside convective heat transfer coefficient (W/m2 K) |
| HTF | = | Heat transfer fluid |
| IPH | = | Industrial process heating |
| K | = | Thermal conductivity (W/m K) |
| L | = | Length of the absorber tube (m) |
| Nu | = | Nusselt number |
| PTC | = | Parabolic trough collector |
| Pr | = | Prandtl number |
| UST | = | Upper surface temperature |
| Ra | = | Rayleigh number |
| TBCC | = | Tube with black chrome coating |
| Ta | = | Ambient air temperature (°C) |
| Ts | = | Average surface temperature of the absorber tube (°C) |
| Qcond | = | Conduction heat loss (W) |
| Qconv | = | Convection heat loss (W) |
| Qf | = | Heat loss by fluid (W) |
| Qrad | = | Radiation heat loss (W) |
| Qt | = | Total heat losses (W) |
| V | = | Volume of fluid (m3) |
Greek symbols
| = | Stefan Boltzmann constant (W/m2 K4) | |
| = | Emissivity | |
| β | = | Coefficient of thermal expansion (K−1) |
| = | Density of fluid (kg/m3) | |
| = | Kinematic viscosity (m2/s) |
Subscripts
| a | = | Ambient |
| b | = | Bare tube |
| bs | = | Both sides |
| c | = | Cut |
| cond | = | Conduction |
| conv | = | Convection |
| f | = | Fluid |
| l | = | Lower |
| u | = | Upper |
| rad | = | Radiation |
| s | = | Surface |