Solar collector

Details Solar collector Solar collector

1983-06-21

1985-01-01

Yeung, James C.

Stoves and furnaces

Tool having fluid fuel burner

Curling iron

126425, 126439, F24J 302

Patent

active

044911254

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to solar collectors and in particular a focussing solar collector.
Focussing collectors can be divided broadly into three systems.
The first two systems can be further subdivided as follows:
Troughs
Dishes
The main problem with troughs is the relatively low maximum temperatures attainable because a line focus rather than a point focus is produced. This applies to both types although the second type is better in this regard than the first. Low temperatures cause heat transfer problems and give low thermodynamic efficiency. In endeavouring to overcome these problems by employing two axes tracking other complications arise namely extracting heat at high temperatures from the focus of a system which is swinging around as the sun is tracked. The hot, exotic heat transfer medium tends to leak past the seals in pipes undergoing relative movement. (The exotic heat transfer fluids are necessary because the pressure of steam at even the relatively low operating temperatures, typically 570.degree. F., is 1,200 p.s.i., which is enough to cause a blow out). This is apart from the actual tracking problem--the rate of swing on the two axes cannot be preset as it varies for each axis during the day, being a maximum for the elevation axis and a minimum for the azimuth axis at dawn and the reverse at noon, then back to the dawn situation at sunset. Furthermore, the changing relationship between the two axial swings changes over successive 24 hour periods during the course of 12 months. The usual tracking method is to adjust the elevation and azimuth at frequent intervals during the day, this being accomplished automatically via input from sensors. However, this results in a jerky movement and a lot of the time an appreciable percentage of the reflected incident radiation misses the target.
Parabolic dishes produce a point focus resulting in high temperatures but they suffer the other disadvantages of two axes troughs, namely a swinging focus, leaky exotic heat transfer fluid lines (exacerbated by the higher temperature) and tracking problems (exacerbated by the generally bigger unit size and the greater precision required). In addition, it is expensive to fabricate and support a large parabolic dish--each panel in the dish is different to every other not on its particular "latitude zone".
Spherical dishes are an improvement in many respects. For instance they can be fabricated out of a large number of a few types of panels and the tracking requirements are simple. A spherical dish has an infinity of optical axis, as opposed to a paraboloid which has one so that even if completely immobile it is, in a sense, always perfectly aligned with the sun. An immobile dish of course would only receive a fraction of the total possible energy intake during the day so in practice the dish is tilted and rotated around a vertical axis through the centre of curvature so that it is facing in the general direction of the sun at all times during the day. The trade off for these advantages is the loss of the point focus--the "non paraxial" incident rays (ie; those parallel but at some distance from the optical axis) are reflected to points on the axis progressively further and further from the paraxial focus as they become more and more "non paraxial". In section the reflected rays form a caustic curve. The result is that in order to collect the reflected radiation a heat transfer fluid pipe must at all times be positioned coincident with the optical axis of the moment. In other words the system reverts to a low temperature line focus with a swinging target. In this instance the target swing is indepedent of the reflector and is relatively straight forward--it rotates around a North-South axis through the centre of curvature inclined at the latitude angle of the site at the same speed as the earth rotates with respect to the sun which is essentially constant at 15.degree. per hour. It also swings on a second axis through the centre of curvature at right angles to the first axis to maintain the

REFERENCES:
patent: 3295512 (1967-01-01), McCusker
patent: 4084581 (1978-04-01), Vigoureux
patent: 4195620 (1980-04-01), Rust
patent: 4211211 (1980-07-01), Toomey

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