Parabolic trough collectors usually track the sun with one degree of freedom using east-west or north-south axis. Solar tracking by these modes maintain the plane of solar beam always normal to collector aperture. Thus solar beam from different points of the parabolic trough reflecting surface is collected on the focal line receiver as shown in Fig. 1. The selection of tracking axis configuration is based on load profile and site latitude. To find a proper tracking axis configuration for Jordan, long term evaluation of collected solar energy on different tracking modes was conducted using TRNSYS package. Hourly weather radiation file was generated by averaging the measured data for number of years in Amman (latitude 32.02°). The results of TRNSYS simulation are presented in Fig.2. It is shown clearly that for the given latitude (32.02°) N-S tracking mode is more efficient than the E-W one.

Additional improvement of the system annual performance could be obtained by inclining the tracking axis. An optimum inclination (adopted in this paper) was found equal to 30° which produces 6% increase in the collected solar energy as compared with the horizontal N-S axis (see Fig.2).

Parabolic Collector System Design

An inclined single axis tracking parabolic trough collector was designed to generate hot water or steam at a pressure close to ambient. The parabolic collector system consists of four main parts: parabolic trough reflector, solar receiver, power supply, tracking control and

mechanism. A brief description of each component is as follows:

Parabolic Trough Reflector

A parabolic steel frame of aperture width 1.8m, rim angle of 74°, focal line 2m, and focal point length of 0.6m was considered.

The type of reflecting surface used in this study is stainless steel sheet (2mx1m) of a reflectivity close to (0.92). The sheet has enough flexibility to adopt the shape of the parabolic steel frame. This frame was constructed from steel trusses (as shown in Fig.3) to withstand the effect of wind force and any deformation in the shape of parabola that may occur. Two stainless steel sheets were connected to the frame from its edges by sliding them inside 3cm grooves at the edge of the frame. Total frame mass including the reflecting surface was found approximately equal to 70 kg. This frame configuration allows the testing of different reflecting surfaces.

Solar Receiver

A flat type solar receiver located in the focal length of the parabolic trough was used. The receiver consists of a 2cm diameter copper tube fixed on a 6cm wide flat plate. The receiver is insulated and inserted inside a rectangular casing with single glazed opening in the side facing the reflector as shown in Fig. 4. Heat loss from this type of receiver is affected by thermal resistance between flat plate-glass cover (convection and radiation), glass cover — surroundings (convection and radiation), polytheren insulation (conductivity), and metal casing-surroundings (convection and radiation). Solar energy concentrated on the receiver is delivered to a thermal storage tank by circulating distilled water in a closed loop. The rim angle of the parabola was designed to be small enough to concentrate reflected beam inside the solar receiver. The receiver was designed with two supports arrangements: tracking the sun with the parabola, or fixed in the plane normal to the tracking shaft. In the tracking arrangement the shadow of the receiver is projected continuously on the center of the parabola, while in the fixed arrangement the projection of the shadow of the receiver moves with sun position form east to west.

Solar flux pattern in the receiver is estimated by evaluating the width of the image reflected by the different reflector segments. Total flux (W/m2) on focal line is evaluated by segmenting the parabolic reflector and evaluating the reflected solar energy of each segment:

n

Total flux = flux(Y) + flux(2) + flux(S) +……………….. + flux(n) = ^ flux(i) (1)

i=l

V у

Where, n is number of reflector segments, i is segment number, Ib solar energy collected from each segment (W), and Lr receiver length (m). Image width (Wmg) depends on segment number or local rim angle (0ri) of the parabola and evaluated by the method given in [12]. To evaluate flux pattern (Watts) in the focal point of the parabola the accumulation of flux from all parabola segments have to be considered. Therefore at any distance from the focal point:

Solar flux pattern is generated at different beam incident angles using Equation (4) and the results are shown in Fig.5. Solar flux reduces as we move away from the receiver center. The deviation of beam incident angle from normal causes significant reduction in solar flux and increase in maximum width of solar image. Based on Fig.5, it can be concluded that receiver width must be around 6cm to intercept all solar image during low solar elevation angle hours.