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 The Solar Car Book A complete kit for making a cool solar racecar. Everything is included: wheels, axles, motors, wires and a genuine one-volt solar cell. |
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Build a Solar Tracker
A solar tracker is a device for orienting a solar photovoltaic panel or concentrating solar reflector or lens toward the sun. Concentrators, especially in solar cell applications, require a high degree of accuracy to ensure that the concentrated sunlight is directed precisely to the powered device, which is at (or near) the focal point of the reflector or lens. Non-concentrating applications require less accuracy, and a tracker is not necessary, but can substantially improve the amount of power produced by a system by enhancing morning and afternoon performance. Strong afternoon performance is particularly desirable for grid-tied photovoltaic systems, as production at this time will match the peak demand time for summer season air-conditioning. A fixed system oriented to optimize this limited time performance will have a relatively low annual production. Sun path refers to the apparent significant seasonal-and-hourly positional changes of the sun (and length of daylight) as the Earth rotates, and orbits around the sun. To effectively gather solar energy, a solar collector (glass, solar panel, etc.) should be within about twenty degrees either side of perpendicular to the sun. The farther from perpendicular, the lower the solar gain. More than thirty-five degrees from perpendicular results in a significant portion of sunlight being reflected off of the solar collector surface. For low-temperature solar thermal applications, trackers are not usually used, owing to the relatively high expense of trackers compared to adding more collector area and the more restricted solar angles required for winter performance, which influence the average year-round system capacity. Compared to photovoltaics, trackers can be relatively inexpensive. This makes them especially effective for photovoltaic systems using high-efficiency panels. Some solar trackers may operate most effectively with seasonal position adjustment and most will need inspection and lubrication on an annual basis.
Tracker mount typesSolar trackers may be active or passive and may be single axis or dual axis. Single axis trackers usually use a polar mount for maximum solar efficiency. Single axis trackers will usually have a manual elevation (axis tilt) adjustment on a second axis which is adjusted on regular intervals throughout the year. A single axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 6%.[1] There are two types of dual axis trackers, polar and altitude-azimuth. Polar
Polar trackers have one axis aligned close to the axis of rotation of the earth, hence the name polar. By this definition, only high accuracy astronomical telescope mounts rotate on an axis parallel to the earth's axis. For solar trackers, so called "polar" trackers have their axis aligned perpendicular to the "ecliptic" (an imaginary disc containing the apparent path of the sun). Simple solar trackers are manually adjusted to compensate for the shift of the ecliptic through the seasons. Adjustment is usually at least twice a year at the equinoxes; once to establish a position for autumn and winter, and a second adjustment for spring and summer. Such trackers are also referred to as "single axis", because only one drive mechanism is needed for daily operation. This reduces the cost and allows the use of passive tracking methods (described below). Horizontal axle
Several manufactures can deliver single axis horizontal axis trackers which may be oriented by either passive or active mechanisms, depending upon manufacturer. In these, a long horizontal tube is supported on bearings mounted upon pylons or frames. The axis of the tube is on a North-South line. Panels are mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the sun through the day. Since these do not tilt toward the equator they are not especially effective during winter mid day (unless located near the equator), but add a substantial amount of productivity during the spring and summer seasons when the solar path is high in the sky. These devices are less effective at higher latitudes. The principal advantage is the inherent robustness of the supporting structure and the simplicity of the mechanism. Since the panels are horizontal, they can be compactly placed on the axle tube without danger of self-shading and are also readily accessible for cleaning. For active mechanisms, a single control and motor may be used to actuate multiple rows of panels. Manufacturers include Array Technologies, Inc. Wattsun Solar Trackers (gear driven active), Zomeworks (passive) and Powerlight (active). Vertical axle
A single axis tracker may be constructed that pivots only about a vertical axle, with the panels either vertical or at a fixed elevation angle. Such trackers are suitable for high latitudes, where the apparent solar path is not especially high, but which leads to long days in Summer, with the sun traveling through a long arc. This method has been used in the construction of a cylindrical house in Austria (latitude above 45 degrees north) that rotates in its entirety to track the sun, with vertical panels mounted on one side of the building[1]. Altitude-azimuthTwo-axis mount
Restricted to active trackers, this mount is also becoming popular as a large telescope mount owing to its structural simplicity and compact dimensions. One axis is a vertical pivot shaft or horizontal ring mount, that allows the device to be swung to a compass point. The second axis is a horizontal elevation pivot mounted upon the azimuth platform. By using combinations of the two axis, any location in the upward hemisphere may be pointed. Such systems may be operated under computer control according to the expected solar orientation, or may use a tracking sensor to control motor drives that orient the panels toward the sun. This type of mount is also used to orient parabolic reflectors that mount a Stirling engine to produce electricity at the device.[2] Multi-mirror reflective unit
A recent development, this device uses multiple mirrors in a
horizontal plane to reflect sunlight upward to a high temperature
photovoltaic or other system requiring concentrated solar power.
Structural problems and expense are greatly reduced since the mirrors
are not significantly exposed to wind loads. Through the employment of
a patented mechanism, only two drive systems are required for each
device. Because of the configuration of the device it is especially
suited for use on flat roofs and at lower latitudes. While imited
commercial availability was expected in 2007 the company has removed
the descriptive web page from their site and is now promoting a
two-axis clustered fresnel lens device. The units illustrated each
produce approximately 200 peak DC watts. Drive typesActive trackersActive trackers use motors and gear trains to direct the tracker as commanded by a controller responding to the solar direction. Active two-axis trackers are also used to orient heliostats - movable mirrors that reflect sunlight toward the absorber of a central power station.
As each mirror in a large field will have an individual orientation
these are controlled programmatically through a central computer
system, which also allows the system to be shut down when necessary. Passive trackers
Passive trackers use a low boiling point compressed gas fluid that is driven to one side or the other (by solar heat creating gas pressure) to cause the tracker to move in response to an imbalance. As this is a non-precision orientation it is unsuitable for certain types of concentrating photovoltaic collectors but works fine for common PV panel types. These will have viscous dampers to prevent excessive motion in response to wind gusts. Shader/reflectors are used to reflect early morning sunlight to "wake up" the panel and tilt it toward the sun, which can take nearly an hour. The time to do this can be greatly reduced by adding a self-releasing tiedown that positions the panel slightly past the zenith (so that the fluid does not have to overcome gravity) and using the tiedown in the evening. (A slack-pulling spring will prevent release in windy overnight conditions.) See alsoReferencesExternal links
Sun path refers to the apparent significant seasonal-and-hourly positional changes of the sun (and length of daylight) as the Earth rotates, and orbits around the sun. To effectively gather solar energy, a solar collector (glass, solar panel, etc.) should be within about twenty degrees either side of perpendicular to the sun. The farther from perpendicular, the lower the solar gain. More than thirty-five degrees from perpendicular results in a significant portion of sunlight being reflected off of the solar collector surface. An effective solar energy system (passive solar, active solar, building, equipment, etc.), takes into account the significant seasonal 47-degree solar altitude angle difference above the horizon, and the sunrise/sunset solar azimuth angle from summer to winter. Accurate location-specific knowledge of sun path and climatic conditions is essential for economic decisions about solar collector area, orientation, landscaping, summer shading, and the cost-effective use of solar trackers. Precise knowledge of the path of the sun is essential to accurately model, and mathematically predict, annualized solar system performance - To explain, for example, why vertical equator-facing glass is cost-effective, the benefit of solar energy reflectivity off of winter snow when the sun is low, and why roof-angled glass (in greenhouses, skylights and conservatories) can be a solar furnace during the summer, (when the sun is nearly perpendicular to the glass), and then lose more energy in the winter than it collects, (when the sun is 47-degrees lower on the horizon, and warm interior air rises to convect (convective heat transfer), conduct (heat conduction), and radiate (thermal radiation) heat transfer out of the building on cold winter nights).[1]
Tilt of the EarthEarth's rotation tilts about 23.5 degrees on its pole-to-pole axis, relative to the plane of Earth's solar system orbit around our sun. As the Earth orbits the sun, this creates the 47-degree peak solar altitude angle difference, and the hemisphere-specific difference between summer and winter. In the northern hemisphere, the winter sun rises in the southeast, peaks out at a low angle above the southern horizon, and then sets in the southwest. It is on the south (equator) side of the house all day long. Vertical south-facing (equator side) glass is excellent for capturing solar thermal energy. In the northern hemisphere in summer, the sun rises in the northeast, peaks out nearly straight overhead (depending on latitude), and then sets in the northwest. A simple latitude-dependant equator-side overhang can easily be designed to block 100% of the direct solar gain from entering vertical south-facing windows on the hottest days of the year. Roll-down exterior shade screens, interior translucent-or-opaque window quilts, drapes, shutters, movable trellises, etc. can be used for hourly, daily or seasonal sun and heat transfer control (without any active electrical air conditioning). The latitude (and hemisphere)-specific solar path differences are critical to effective passive solar building design. They are essential data for optimal window and overhang seasonal design. Solar designers must know the precise solar path angles for each location they design for, and how they compare to place-based seasonal heating and cooling requirements. In the U.S., the precise location-specific altitude-and-azimuth seasonal solar path numbers are available from NOAA - The "equator side" of a building is south in the Northern hemisphere, and north in the Southern hemisphere, where the peak summer solstice solar altitude occurs on december 21st. The sun rises in the east and sets in the west everywhere on Earth. On the Equator, the sun will be straight overhead and a vertical stick will cast no shadow at noon (solar time) on March 21 and September 21, the equinox. 23.5 degrees north of the equator on the Tropic of Cancer, a vertical stick will cast no shadow on June 21, the summer solstice. The rest of the year, the noon shadow will point to the North pole. 23.5 degrees south of the equator on the Tropic of Capricorn, a vertical stick will cast no shadow on December 21, the summer solstice, and the rest of the year its noon shadow will point to the South pole. North of the Tropic of Cancer, the noon shadow will always point north, and conversely, south of the Tropic of Capricorn, the noon shadow will always point south. North of the Arctic circle, and south of the Antarctic circle there will be at least one day a year when the sun is not above the horizon for 24 hours, and at least one day (six months later) when the sun is above the horizon for 24-hours. In the moderate latitudes (between the circles and tropics, where most humans live), the length of the day, solar altitude and azimuth vary from one day to the next, and from season to season. The difference between the length of a long summer day, versus a short winter day increases as you move farther away from the equator. Solar path building design simulationBefore the days of modern, inexpensive, 3D computer graphics, a heliodon (precisely-movable light source) was used to show the angle of the sun on a physical model of a proposed building. Today, mathematical computer models calculate location-specific solar gain (shading) and seasonal thermal performance, with the ability to rotate and animate a 3D color graphic model of a proposed building design. Passive solar building design heating and cooling issues can be counterintuitive (like roof-angled glass). Precise performance calculations and simulations are essential to help avoid reinventing the wheel and duplicating previously-made expensive experimental construction errors (like a summer solar furnace). See alsoExternal links
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Solar Tracker" |
