Understanding IAM

Performance measurements are normally taken with the solar insolation level measured perpendicular to the collector panel (ie. facing the same direction as the collector). When the light shines on the collector from an angle the performance changes, and this is what the IAM (Incidence Angle Modifier) values provide us, an angular performance factor. A value of 1 is achieved when the collector is perpendicular to the suns rays, and therefore receiving maximum radiation. For flat plate collectors, 1 is the maximum value, dropping off in both morning and afternoon. Evacuated tube collectors, however, often provide values in excess of 1 during these periods, as factors such as reflective panels and reflection off neighbouring tubes can influence the relative performance.

A solar collector that is mounted on a device to track the sun from sunrise to sunset (as sometimes used in PV applications) will maintain a IAM value of 1 throughout the day, as the collector is always facing the sun, and therefore providing performance output in line with the standard performance curve.

For most solar collectors currently on the market, IAM is not an important consideration when comparing performance. This is because flat plate collectors, evacuated tube collectors with a flat absorber, or those that using reflective panels usually have a fairly similar set of transversal and longitudinal IAM values. The value of most concern for fixed position collectors is transversal IAM, as this reflects the change in performance throughout the day. Longitudinal IAM is useful when looking at installation angle, and the changes in heat output throughout the year as angle of the sun above the horizon changes between winter and summer.

The longitudinal and transversal IAM values for the SunSurfs solar collector are as follows:

IAM \ Angle	0o	10o	20o	30o	40o	50o	60o	70o	80o	90o
Kq (longitudinal)	1.0	1.0	0.99	0.98	0.96	0.93	0.87	0.74	0.38	0
Kq (transversal)	1.0	1.02	1.08	1.18	1.37	1.4	1.34	1.24	0.95	0

(SPF report C632LPEN)

For longitudinal insolation, a value for 50o is generally only provided, however the curve is standard for most collectors, very similar to the transversal curve for flat plate collectors (see graph below).

The following graph displays the transversal IAM values for the SunSurfs collector, a leading flat-plate and leading evacuated tube reflective panel solar collector.

chart

As can be seen, the SunSurfs solar collector has a curve which is quite different to the other two collectors . This is due to the cylindrical absorber area, which passively tracks the sun throughout the day. At 40-50o there is no light lost between the tubes, and no tube overlap, and reflection of neighbouring tube, hence a peak in relative performance. This is ideal, as during this period (mid morning through mid afternoon) solar isolation levels are quite high. The peak at 70o provided by the ET-reflect is of little benefit as this angle corresponds to early morning or late afternoon when solar insolation levels are very low. The flat plate collector's IAM values drop away from 1 as the angle increases, and as such solar conversion efficiency is only at peak levels at midday.

To understand how the tubes passively track the sun throughout the day, refer to the diagram to the left.

When looking at the tubes from above (0o) each tube's surface is clearly visible, and therefore exposed to the maximum amount of sunlight. At this angle however some light is lost between the tubes, and therefore because this is used as a reference point for the IAM value of 1, when the gaps close up, the IAM value with actually increase (a greater % of light shining on the collector is actually being absorbed).

When the sun reaches an angle of 40o which correlates to 2h 40min before or after midday, the solar tubes are still fully visible with no gaps between, and no overlap. It is at this point that the pure IAM values reach their peak. The tubes are exposed to all the sunlight shining towards them, and all the tubes are still perpendicular to the sun.

As the angle increases, the tubes start to overlap, shading each other. They are still facing the sun, but the actual surface area of absorber exposed to the sunlight is reduced. Only a small amount of sunlight falls beyond 40o (early morning and late afternoon), and so this decrease in surface area has minimal influence on the total daily energy output of the collector.

The IAM effect increases the heat output of the SunSurfs solar collector by as much as 25% when compared to a flat collector surface of the same absorber area and performance variables. It is very important to consider IAM effect when calculating the heat output of SunSurfs solar collectors and indeed when comparing to other solar collectors.

Surprisingly, the SunSurfs solar collector actually provides more heat in a stationary position, than if it physically tracked the sun throughout the day!

Cosine Adjustment

If solar insolation levels that are based on tracking the sun throughout the day are used, a Cosine adjustment is required. SunSurfs recommends using data that is based on equator pointed surface measurements, therefore a cosine adjustment is not required and the IAM factors can be used directly in the performance equation to determine heat output. For more on determining insolation levels, click here.