Solar Thermal Information

4
GROSS COLLECTOR AREA
T
in
T
out
collector efficiency=
thermal output from collector (Btu/hr)
solar radiation striking GROSS collector area (Btu/hr)
• 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.
Some “single tank” combisystems discussed in this
issue do not adhere to this principle. However, they all
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 collector array and any piping outside of the heated
space must be protected against freezing.
• Almost every solar combisystem gathers more energy
on a sunny summer day than can be used by the load
(which is typically just domestic water heating). All solar
combisystems must have a means of dealing with this
surplus energy so it doesn’t damage the system.
INSTANTANEOUS COLLECTOR EFFICIENCY:
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.
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 2-1.
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. Formula
2-1 can then be used to calculate the instantaneous
thermal efficiency of the collector.
Formula 2-1:
Where:
c = specific heat of fluid (Btu/lb/ºF)
D = density of fluid (lb/ft
3
)
f = flow rate (gallons per minute)
T
in
= collector inlet temperature (ºF)
T
out
= collector outlet temperature (ºF)
I = instantaneous solar radiation intensity (Btu/hr/ft
2
)
A
gross
= gross collector area (ft
2
)
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 2-2 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.
figure 2-1
[KJ1]
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. Formula 2-1 can then be used to calculate
the instantaneous thermal efficiency of the collector.
Formula 2-1:
η
collector
=
(8.01× c × D) × f × T
out
− T
in
( )
I × A
gross
Where:
c = specific heat of fluid (Btu/lb/ºF)
D = density of fluid (lb/ft
3
)
f = flow rate (gallons per minute)
T
in = collector inlet temperature (ºF)
T
out = collector outlet temperature (ºF)
I = instantaneous solar radiation intensity (Btu/hr/ft
2
)
A
gross = gross collector area (ft
2
)
8.01 = a unit conversion factor
The word
instantaneous implies that the collector’s efficiency can vary from
moment to moment depending on the operating conditions. Do not assume