AP Efficiency - Part 2 Understanding IAM

IAM is an acronym for Incidence Angle Modifier and is simply a numeric value with refers to the amount of available solar radiation striking the absorber of the collector - "Daily Light Exposure" may be an appropriate term to use. A value of 1 is achieved when the collector is perpendicular to the suns rays, and therefore receiving maximum radiation.

A solar collector that is mounted on a device to track the sun from sunrise to sunset (as commonly use in PV applications) will maintain a IAM value of 1 throughout the day, as the collector is always facing the sun. A collector that is installed at a fixed angle (generally equal to the location's latitude), will experience decreased radiation levels (IAM value < 1) in the morning and afternoon when the sun is not perpendicular to the absorber surface.

For most solar water heaters 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 angle collectors is transversal IAM, as this reflects the solar radiation 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 AP solar collector are as follows:

0	0^o	10^o	20^o	30^o	40^o	50^o	60^o	70^o	80^o	90^o
Kq (longitudinal)	1.0	0	0	0	0	0.93	0	0	0	0.0
Kq (transversal)	1.0	1.02	1.08	1.18	1.37	1.4	1.34	1.24	0.95	0.0

(SPF report C632LPEN)

Note: 1^o = 4min. So 1hour = 15^o

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

As can be seen, the AP 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 40-50^o there is no light lost between the tubes, and no tube overlap, hence a peak in relative performance. This is ideal, as during this period (mid morning through mid afternoon) solar isolation levels are quite highest. The peak at 70^o 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.

In order to accurately assess these values, they must be cosine adjusted, to account for the change in solar radiation levels on a given surface area as the sun passes across the sky each day.

Cosine Adjusted Transversal IAM Values (IAM Adjustment)

Because of the round shape of the solar tubes, the absorber passively tracks the sun from 40^o either side of midday (9:20am to 2:40pm). The cosine adjusted IAM values confirm this, as the collector maintains a value of close to 1 up until 40^o, beyond which the tubes start to overlap and the relative surface area decreases. Flat plate collectors, and other collectors with flat absorbers display a fairly standard bell curve, only peaking at midday.

	0^o	10^o	20^o	30^o	40^o	50^o	60^o	70^o	80^o	90^o
AP Solar Water Heater	1.0	1.0	1.01	1.02	1.05	0.9	0.67	0.42	0.16	0.0
Flat Plate	1.0	0.97	0.91	0.82	0.71	0.57	0.4	0.19	0.03	0.0

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 (0^o) 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 40^o 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. This is why even at this point the cosine adjusted IAM is still 1.

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 40^o (early morning and late afternoon), and so this decrease in surface area has minimal influence on the total daily energy output of the collector.

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Last modified: 10/19/24