In the following table, we compare the efficiency of our small closed parabolic trough with the classical big open parabolic trough found in actual solar power plants. The table shows that the former is 40% more efficient than the latter.
We assume that both collectors use technologies of the same quality as long as possible: same nominal concentration ratio, same mirror reflectivity and glass transmissivity, same selective coating, same operating temperature.
| Loss | Open trough | Closed trough | Comments |
|---|---|---|---|
| Cover reflection loss | 1 | 0.95 | Open trough has no cover. Closed trough has a cover with anti-reflection treatment. |
| Mirror reflectivity | 0.93 | 0.93 | Equal quality mirrors. |
| Glass tube reflection loss | 0.95 | 0.95 | Equal quality tubes, treated anti-reflection. |
| Intercept factor | 0.98 | 0.98 | Equal optical precision supposed. |
| Receiver absorptivity | 0.95 | 0.95 | Equal quality receiver surface. |
| Incidence angle cos effect | 0.82 | 0.99 | Open trough is horizontally installed, latitude 40°. Closed trough is optimally tilted north-south axis, with tilting angle adjusted 2 or 4 times a year. |
| End and join loss | 0.9 | 1 | Open trough has end loss and loss on receiver supporting structure. No such losses for closed trough. |
| Glass tube multiple travel | 0.995 | 0.99 | A small amount of light travels several times through the glass tube. This is slightly more important for the closed trough due to a glass tube of larger diameter. |
| Dust loss | 0.94 | 0.98 | Light to an open trough travels 3 times through dust-coverable surfaces. Only once for closed trough. |
| Row-to-row shading | 0.98 | 0.95 | Closed trough deliberately adopts a more condensed row-to-row distance,
prompted by its lower cost, in order to reduce land use and piping cost. The data result from a computer simulation taking into account the atmospheric attenuation and the Sun's angle. |
| Thermal capacity | 0.95 | 0.99 | The big open trough has a thicker receiver, hence a higher thermal capacity
per unit aperture area. Heat corresponding to the thermal capacity is lost
after sunset or cloud coverage. The thermal capacity is 0.36Wh/m2·K,
or 126Wh/m2 for a temperature elevation of 350°C. Assuming an average
collection of 2.5kWh/m2 per period of sunshine, the loss represents 5%. This loss is 6 times less for the smaller closed trough. |
| Efficiency before thermal loss | 52.8% | 70.6% | Efficiencies above are multiplied; loss below is subtracted. |
| Thermal loss | 10% | 10% | Assume 800W average incoming light intensity and 80W/m2 thermal loss for both cases. |
| Final efficiency | 42.8% | 60.6% | This is the efficiency with respect to direct normal insolation. |
Remark. The loss estimations for open parabolic trough can be found in Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, Appendix D. Test data of closed parabolic trough will come later.
| Back to home | Gang Xiao |