IAM Adjustments

When calculating the heat output of a collector, the IAM performance adjustment factor (K) should be included in the efficiency formula.

formula

The key difference to that formula presented on performance basic of this section is the inclusion of the factor K in front of the y-intercept value. The K value can also include the longitudinal IAM factor, thus taking into consideration the installed angle of the collector too. This is achieved by simply multiplying the two IAM factors to provide a complete IAM value.

If we continue with the example from performance basic

Eg. At 2:40pm; transversal angle of 40o = IAM of 1.37; ambient temperature of 25oC (77oF); average water temp [(Tinlet+Texit)/2] of 50oC (122oF); insolation level of 800Watts/m2 (252Btu/ft2).

x = (50-25)/800 = 0.03125

Now enter all the values into the efficiency formula:

h(x) = 1.37*0.717 - (1.52*0.03125) - (0.0085*800*0.031252)

h(x) = 0.98 - 0.0475 - 0.0066 = 0.926

The performance value calculated with the above formula can then be used to calculate total heat output:

Heat Output = Performance x Insolation x Collector Surface Area

Example:

Performance @ 40o angle= 92.6%
Insolation = 800 Watts/m2
Absorber Surface Area = 2.4m2

Heat Output = 0.926 x 800 x 2.4 = 1777Watts

So the collector will provide 1.77 kW of heat output.

How can the performance reach 92.6%? Remember that the efficiency variables and therefore this performance value is is expressed for absorber area, if using gross surface area the % efficiency value would be almost half this, but given the larger surface area if using gross, the heat output is the same. It is possible therefore for the % value to exceed 100% - it does not mean that more heat is being produced that the sun is providing, but rather due to reflection off neighbouring tubes etc, the amount of light the absorber is exposed to is greater than when the collector perpendicular to the sunlight rays (midday).


Simplifying IAM Adjustment Calculations

The calculation completed above is only for a specific point in time, and does not give an indication of the the actual performance over an entire day. Using performance modeling software, hour by hour calculations can be made taking into consideration average daily temperature changes, cold water temperatures, hours of sunlight, solar insolation levels in addition to collector performance variables and IAM values. Monthly and annual average performance values may therefore be estimated.

To complete a simple single day calculation for the purpose of comparing collector performance, an average IAM value can be use, along with an average Watt/m2 value. Although this won't give a completely accurate indication of the heat output for the day, it allows a comparison between the two collector to be made.

As the majority of useful solar radiation falls during the middle 6-7 hours of the day, an average of the IAM values during this period can be used. If 1 hour corresponds to 15o then 7 hours corresponds to 50o either side of midday. The average cosine adjusted IAM for the SunSurfs solar collector for this period is 1.2, and a flat plate collector is 0.97. These factors can therefore be used in the performance formula. See the additional links more detail.