Seasonal Benefit of a Reflector for a South Facing Vertical Solar Collector

 This study uses the earth-sun simulator to estimate the improvement in energy output that can be achieved using a reflector to provide extra energy input to a vertical south facing solar collector.  For this evaluation, the vertical collector is square, and the reflector is a square of the same size as the collector.  The reflector extends out from the bottom of the collector and is tilted slightly up or down depending on the season.  The evaluation is done for the winter and summer solstice and for the equinox. This test is for latitude 45 degrees -- results will vary some for other latitudes. This study is an example of using the earth/sun simulator is used for the evaluation...

Bottom Line

For this situation of a square, south facing, vertical collector at 45 degrees latitude with a same size square reflector extending southward from the bottom of the collector, the general results are:

- The reflector angle that provides full illumination of the collector is south end down 15 degrees for the winter solstice, up 15 degrees for the summer solstice, and about level for the equinoxes.

- The noon time gain for such a reflector is about 1.8 for the winter solstice, 2.2 for the equinoxes, and 3.0 for the summer solstice.

- For the square reflector the gain falls off on either side of noon because the reflected pattern slides off to the east or west of the collector.  By 2pm, the reflected pattern is down to covering 50 to 70% of the collector.

For more details, see below.  If this does not fit your situation, then build an earth-sun simulator of your own...  its fun :)

Test Setup

The earth/sun simulator is used to estimate and understand the effect and benefits of a reflector mounted at the bottom of a south facing vertical solar collector for each season of the year, and for different times of day.

In this setup, the collector is a square and is mounted vertically.  The reflector is a square of the same size and mounted at the bottom of the collector and extending south from the base of the collector.

In general, for each season, the reflector is angle is set such that at 12 noon, the reflected pattern of light from the reflector covers the full surface of the collector.  This offers the advantage of spreading the light over the full surface of the collector, and the incidence angle of the reflected light on the collector is in the neighborhood of 45 degrees, which is not so large as to have a the collector glazing reflecting off a lot of the light from the reflector.  The result is that the reflector is angled slightly down for the winter, slightly up for the summer, and near horizontal at the equinox.

The sections below show the reflected patterns and benefits for each season of the year.

Winter Solstice

Winter solstice at 12 noon.

 View from position of the sun at solar noon on the winter solstice. Same from the side.

The left picture is taken roughly from the position of the sun looking toward the collector and reflector.
The simulator is set to the to the winter solstice (equator board tipped down in back), and the turntable is set to 12 noon.
The reflector is angled down a bit in front such that the reflected pattern of light pattern just fills the collector but no more.

The tilt down of the mirror is about 15 degrees below horizontal (tilted down toward the south).

Winter solstice at 2 pm

The two pictures just above are at the winter solstice at 2 pm.
Note the pattern of the reflected light on the collector in the right picture -- it is now only covering about half of the collector area.  So, if the reflector is the same width as the collector, the benefit of the reflector falls off in both the morning and afternoon because the reflected light patter is skewed east or west.  This is why it is an advantage to make the reflector wider than the collector if you can easily do this.

The light levels on the surface of the collector, ground, and reflector were measured with a light meter.  The absolute readings don't really mean anything, but the ratios of the readings give an idea how much the reflector is increasing the energy input.

 Ground 135 On reflector surface 218 On collector - no reflector 230 On collector - with reflector 410

So, at noon, the energy gain for the collector is (410/230) = 1.78

This gain will not be as large earlier in the morning or later in the afternoon because (as shown above) the reflected light pattern does not cover the whole collector.

Spring and Fall Equinox

Spring or fall equinox at 12 noon

 View from position of the sun at the equinox. side view at equinox.

The left picture is taken roughly from the position of the sun looking toward the collector and reflector.
The simulator is set to the to the equinox (equator board is horizontal), and the turntable is set to 12 noon.
The reflector is nearly level and with this angle, the reflected pattern of light pattern just fills the collector but no more.

Equinox at 2 pm

The two pictures just above are at the equinox at 2 pm.
Note the pattern of the reflected light on the collector -- it is now only covering perhaps 70% of the collector area.

The light levels on the surface of the collector, ground, and reflector were measured with a light meter.  The absolute readings don't really mean anything, but the ratios of the readings give an idea how much the reflector is increasing the energy input.

 Ground 270 On reflector surface 288 On collector - no reflector 210 On collector - with reflector 458

So, at noon, the energy gain for the collector is (458/210) = 2.18

This gain will not be as large earlier in the morning or later in the afternoon because (as shown above) the reflected light pattern does not cover the whole collector.

Summer Solstice

Summer solstice at 12 noon.

 Reflector and collector at summer solstice as seen from position of sun. . Side view -- summer solstice.

The left picture is taken roughly from the position of the sun looking toward the collector and reflector.
The simulator is set to the to the summer solstice (equator board tipped down in in front), and the turntable is set to 12 noon.
The reflector is angled up a bit in front such that the reflected pattern of light pattern just fills the collector but no more.

The tilt up of the mirror is about 14 degrees above horizontal (the south end is higher than the north end).

Summer solstice at 2 pm

The two pictures just above are at the summer solstice at 2 pm.
Note the pattern of the reflected light on the collector in the right picture -- it is now only covering about 70% of the collector area.  So, if the reflector is the same width as the collector, the benefit of the reflector falls off in both the morning and afternoon because the reflected light patter is skewed east or west.  This is why it is an advantage to make the reflector wider than the collector if you can easily do this.

The light levels on the surface of the collector, ground, and reflector were measured with a light meter.  The absolute readings don't really mean anything, but the ratios of the readings give an idea how much the reflector is increasing the energy input.

 Ground 362 On reflector surface 365 On collector - no reflector 125 On collector - with reflector 404

So, at noon, the energy gain for the collector is (404/125) = 2.98

Kind of an amazing improvement, but it makes sense in that the solar incidence angle on the collector is so large that it is not receiving much useful radiation, whereas the reflector is pointed nearly right at the sun.  Interesting that the radiation level on the collector including the direct and the reflected radiation is almost the same as it is for the winter solstice and the summer solstice.

This gain will not be as large earlier in the morning or later in the afternoon because (as shown above) the reflected light pattern does not cover the whole collector.

Gary June 4, 2011