DogStar Solar's Alternative Energy Updates

Instantaneous Thermal Efficiency
December 23, 2010, 10:45 am
Filed under: Solar Thermal

Ref: Caleffi iDronics July 2009

The performance of any solar combisystem is implicitly

linked to the performance of its solar collector array.

The best solar combisystems are designed to enhance

collector efficiency. Doing so requires a fundamental

knowledge of what collector efficiency is and how it is

affected by operating conditions imposed by the balance

of the system.

In an “ideal” solar thermal system, none of the heat

produced by the auxiliary heat source would enter the

solar storage tank. This prevents the auxiliary heat source

from increasing the temperature of the storage tank above

what it would be based solely on solar energy input. Such

heating, if allowed to occur, delays the startup of the solar

collection cycle, and thus reduces the energy collected

during that cycle.

All solar tanks rely on temperature stratification to direct

heat added by the auxiliary heat source to the upper portion

of the storage tank. This minimizes heating of the lower

portion of the tank, and thus reduces interference with the solar collection control process.

The instantaneous thermal efficiency of a solar collector

is defined as the ratio of the heat transferred to the

fluid passing through the collector divided by the solar

radiation incident on the gross area of the collector, as

shown in figure 1.

Instantaneous collector efficiency can be measured by

recording the flow rate through the collector along with

simultaneous measurement of the collector’s inlet and

outlet temperature. The intensity of the solar radiation

striking the collector must also be measured. The Formula

can then be used to calculate the instantaneous

thermal efficiency of the collector.

Formula :


c = specific heat of fluid (Btu/lb/ºF)

D = density of fluid (lb/ft3)

f = flow rate (gallons per minute)

Tin = collector inlet temperature (ºF)

Tout = collector outlet temperature (ºF)

I = instantaneous solar radiation intensity (Btu/hr/ft2)

Agross = gross collector area (ft2)

8.01 = a unit conversion factor.

The phrase instantaneous collector efficiency can vary

from moment to moment depending on the operating

conditions. Do not assume that a given set of operating

conditions is “average” or “typical,” and

thus could be used to determine the

collector’s efficiency over a longer period

of time.

Instantaneous collector efficiency is very

dependent on the fluid temperature entering

the collector, as well as the temperature

surrounding it. It also depends on the

intensity of the solar radiation incident

upon the collector. This relationship is

shown in figure 2 for a typical flat plate

and evacuated tube collector.

The thermal efficiency of each collector is

plotted against the inlet fluid parameter. This

parameter combines the effects of inlet fluid

temperature, ambient air temperature and

solar radiation intensity into a single number.


1 Comment so far
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wow this is very cool those graphs are amazing. in everything I’ve seen on this site this is the best post

Comment by franklin2004

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