Parabolic Trough to Produce Steam, Thermocouple Electricity

November 2004

This month we are going to take a look at another type of solar reflector that is relatively easy to build. It is what some people refer to as the trough or parabolic trough reflector. (See Figure 47a)

Trough reflectors may track the sun using either a single-axis or a dual-axis mount. Generally speaking, a dual axis mount will allow a trough reflector to deliver 10 percent more energy by eliminating end spill-off.

Advantages of the trough reflector include:

Fluid temperatures in the absorber tube typically run in the 300-600 degree Celsius range in a 16-foot reflector in a hot sun. While water may be flashed into steam directly in the absorber tube, typically oil is used as the heat transfer fluid to a heat exchanger.

Trough Reflector Construction

To construct the trough reflector we will use the parabolic curve software covered in the March 2004 PTBC article, "Parabolic Reflector Basics". Once the parabolic coordinates are obtained, simply lay them out on a sheet of 4-foot x 8-foot plywood. (Figure 47b)

After cutting out the first form (Figure 47c), use it as a template to duplicate enough forms to span the length of the trough at about two-foot intervals (Figure 47d). An 8-foot long trough would require five (5) forms -- one every two feet. 

Secure the forms to a square, stiff metal or wood frame and attach strip planking to the forms to follow the curve (Figure 47e). Sand the strip planking smooth, fill with Bondo and finish coat with a resin and fabric. 

Aluminum sheet or foil is bonded to the curved surface as the reflective coat. (Figure 47f)

The two-axis mount may be fabricated from 2-inch to 3-inch round or square steel tubing, and is driven by electric or hydraulic linear actuators. (Figure 47g)

Solar tracking electronics will be covered in future editions of the Phoenix Turbine Builders Club, so stay tuned for many more exciting technical tips coming in the months ahead.

Thermocouple Effect

One last thought: the trough collector is a good candidate for direct heat to electricity production using the dissimilar metal thermocouple effect.

Several 1/2-inch copper tubes filled with a molten metal such as lead will produce an electrical flow when heated sufficiently at the focal point of the trough.

Until next time -- keep those new energy experiments rolling.

Ken Rieli