Parabolic Trough Tried and Tested
Parabolic trough solar technology is the most proven and lowest cost large-scale solar power technology available today, primarily because of the nine large commercial-scale solar power plants that are operating in the California Mojave Desert. These plants, developed by Luz International Limited and referred to as Solar Electric Generating Systems (SEGS), range in size from 14–80 MW and represent 354 MW of installed electric generating capacity. More than 2,000,000 m2 of parabolic trough collector technology has been operating daily for up to 18 years, and as of the end of the year 2001, these plants had accumulated 127 years of operational experience.
Build One Yourself
You can easily build one of these collectors. There are open and a closed design trough type and the reciever tubes can be vacuum (generating higher temperatures) or non vacuum. The concentration ratio depends on the apperture size and reciever diameter.
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Basic shape
The following formula describes the critical parameters of a parabolic surface:
Y is the distance from the baseline, X is the distance from the Y axis and P is the focal distance, or the Y coordinate of the focal point or focal length.
We can send you the curve points of any parabola if you send us the focal length and donate a little to our cause.
The concentration ratio of a parabolic mirror is determined by the apperture width (the top width of the trough) and the surface area of the reciever (the tube in the focal point) and factors to do with refelctivity, mirror accuracy and heat losses. The normal range for concentration is 10 to 24 times.
Increasing the yield
| 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. |
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