Solar water heater
performance is often presented as a graph,
or set of three performance variables.
Values may be provided based on gross area,
aperture area or absorber area. In Europe,
aperture or absorber is often used, in the
US, gross area is often used. It doesn't
really matter which values is used, as long
as you use the correct value. ie. Don't use
absorber area when using performance values
based on gross area.
To adjust from one to the other, multiply by
the size difference.
ie. Absorber area = 0.6m2, gross area =
1.1m2. If performance variables are provided
for gross area, multiply by 1.83 (1.1/0.6 =
1.83) to obtain absorber area values. The
smaller the area used, the higher the
performance variable values.
The three performance variables for the AP
solar collector as provided by the SPF
testing laboratory in Switzerland (SPF
report C632LPEN) are as follows (for metric
calculations - absorber area):
Conversion Factor:
h0 = 0.717
Loss Coefficient: a1 = 1.52 W/(m2K)
Loss Coefficient: a2 = 0.0085 W/(m2K2)
As well as the three performance variables
shown above, insolation level (G) in Watts/m2,
ambient temperatures (Ta) and average
manifold temperature (Tm) must be know.
These values give the value x,
also sometimes presented as T*m, used in the
formula below.
(other slightly different
forms of this formula are used, but provide
the same result)
How to use the formula?
Based on the ambient temperature, average
manifold temperature and insolation level
firstly calculate the value for x.
Eg. At 2:00pm, the ambient temperature is 25oC
(77oF), and the average water
temp [(Tin+Tex)/2]
is 50oC (122oF). The
insolation level is 800Watts/m2
(252Btu/ft2).
x = (50-25)/800 = 0.03125
Now enter all the values
into the formula:
h(x) = 0.717 - (1.52*0.03125) -
(0.0085*800*0.031252)
h(x) = 0.717 - 0.0475 - 0.0066 = 0.663
The solar conversion efficiency for that
specific point in time and set of
environmental conditions is 66.3%. That is:
66.3% of the energy provided by the sun is
actually used to heat the water.
Based on the assumption that those three
environmental factors (G, Tm and Ta) are
stable for a period of one hour, then 800 x
0.663 = 530.4 Watts of energy per m2
of absorber area will be used to heat the
water (168Btu/ft2)
530.4Watts is equivalent to 456kcal, which
could heat 100L of water by 4.56oC
(20 Gallons by 10.9oF)
Below is a graph showing the performance
curves for the AP solar collector at three
different insolation levels, from 0 to 80oC
Delta-T. In most cases the Delta-T values
will be in the range of 20-50oC,
with higher values present for high
temperature heating such a for absorption
cooling applications, or during very cold
weather. As can be seen conversion
efficiency is highly dependent on solar
insolation levels, with higher insolation
yielding greater levels of solar conversion.
In reality ambient temperature will
fluctuate, and the manifold temperature will
gradually increase as the water is heated.
Furthermore insolation levels may fluctuate
with intermittent cloud cover. In order to
more accurately calculate energy output per
day/month/year a more complete set of
environmental data must be considered and many
(hourly) performance calculations throughout
the day taken.
Heat pipes might
seem like a new concept, but you are
probably using them everyday and don't even
know it. Laptop computers often using small
heat pipes to conduct heat away from the CPU,
and air-conditioning system commonly use
heat pipes for heat conduction.
The
principle behind heat pipe's operation is
actually very simple.
Structure and Principle
The heat pipe is hollow with the space
inside evacuated, much the same as the solar
tube. In this case insulation is not the
goal, but rather to alter the state of the
liquid inside. Inside the heat pipe is a
small quantity of purified water and some
special additives. At sea level water boils
at 100oC (212oF), but
if you climb to the top of a mountain the
boiling temperature will be less that 100oC
(212oF). This is due to the
difference in air pressure.
Based on this principle of water boiling at
a lower temperature with decreased air
pressure, by evacuating the heat pipe, we
can achieve the same result. The heat pipes
used in AP solar collectors have a boiling
point of only 30oC (86oF).
So when the heat pipe is heated above 30oC
(86oF) the water vaporizes. This
vapour rapidly rises to the top of the heat
pipe transferring heat. As the heat is lost
at the condenser (top), the vapour
condenses to form a liquid (water) and
returns to the bottom of the heat pipe to
once again repeat the process.
At room temperature the water forms a small
ball, much like mercury does when poured out
on a flat surface at room temperature. When
the heat pipe is shaken, the ball of water
can be heard rattling inside. Although it is
just water, it sounds like a piece of metal
rattling inside.
This explanation makes heat pipes sound very
simple. A hollow copper pipe with a little
bit of water inside, and the air sucked out!
Correct, but in order to achieve this result
more than 20 manufacturing procedures are
required and with strict quality control.
Quality Control
Material quality and cleaning is
extremely important to the creation of a
good quality heat pipe. If there are any
impurities inside the heat pipe it will
effect the performance. The purity of the
copper itself must also be very high,
containing only trace amounts of oxygen and
other elements. If the copper contains too
much oxygen or other elements, they will
leach out into the vacuum forming a pocket
of air in the top of the heat pipe. This has
the effect of moving the heat pipe's hottest
point (of the heat condenser end) downward
away from the condenser. This is obviously
detrimental to performance, hence the need
to use only very high purity copper.
Often heat pipes use a wick or capillary
system to aid the flow of the liquid, but
for the heat pipes used in AP solar
collectors no such system is required as the
interior surface of the copper is extremely
smooth, allowing efficient flow of the
liquid back to the bottom. Also AP heat
pipes are not installed horizontally. Heat
pipes can be designed to transfer heat
horizontally, but the cost is much higher.
The heat pipe used in AP solar collectors
comprises two copper components, the shaft and
the condenser. Prior to evacuation, the
condenser is brazed to the shaft. Note that
the condenser has a much larger diameter than
the shaft, this is to provide a large surface
area over which heat transfer to the header
can occur. The copper used is oxygen free
copper, thus ensuring excellent life span and
performance.
Each heat pipe is tested for heat transfer
performance and exposed to 250oC
(482oF) temperatures prior to being
approved for use. For this reason the copper
heat pipes are relatively soft. Heat pipes
that are very stiff have not been exposed to
such stringent quality testing. Given this
strict quality control and high copper purity,
the life expectancy of the heat pipe is even
longer than that of the solar tube.
The operation of the
AP solar collector is very simple!
Solar Absorption: Solar
radiation is absorbed by the solar tubes and
converted into heat.
Solar Heat Transfer: Heat
pipes conduct the heat from within the solar
tube up to the header pipe.
Solar Energy Storage: Water
is ciruclated through the header, via
intermittent pump cycling. Each time the water
circulates through the header the temperatures
is raised by 5-10oC
/ 9-18oF.
Throughout the day, the water in the storage
tank is gradually heated.
Freeze
Protection
Even though the heat pipe is a vacuum and the
boiling point has been reduced to only 25-30oC
(86oF), the freezing point is still the same
as water at sea level, 0oC (32oF). Because the
heat pipe is located within the evacuated
glass tube, brief overnight temperatures as
low as -10oC (14oF) will not cause the heat
pipe to freeze. If the heat pipe does freeze
once or twice the heat pipe will not burst as
the copper can expand, but repetitive freezing
will result in the bottom of the heat pipe to
swell and eventually rupture. In order to
protect the heat pipe from this occurrence, in
areas that regularly experience temperatures
below -5oC (22oF), freeze protected heat pipes
are recommended. The bottom end of the heat
pipe has a stainless steel cover which
strengthens the pipe, forcing the ice to
expand upwards instead of outwards. This
method effectively protects the heat pipe
against damage from repetitive freezing in
cold regions.
All AP Solar Thermal Systems that Distribution
Jean-Pierre Paquette supply are manufactured to our
specifications including Freeze Protection for
our harsh winters in Canada and the Northern
United States.
A
Aperture: The part of the collector
through which light enters. For evacuated
tubes this refers to the cross-sectional
surface area of the outer clear glass tube
measured using the internal diameter, not the
outside diameter.
(Eg. 0.0548m x 1.72m = 0.094m2).
1.72m is the exposed length of the evacuated
tube.
Absorber: The part of the collector
that actively absorbs the light rays. For
solar tubes this is defined as the
cross-sectional area of the inner tube (selective
coated) measured using the outside diameter. (Eg.
0.047 x 1.72m = 0.08m2)
This value is used when calculating efficiency
values. For solar tube collectors with
reflective panels, the entire circumferential
surface area of the inner tube is often used
when calculating absorber area, as the
reflective panel
is supposed to reflect light onto underside of
the evacuated tube. The Apricus AP solar
collector does not use reflective panels.
B
BTU - Stands for British Thermal Units.
This is an imperial unit of measurement for
heat widely used in the US and also in the UK.
The conversion to the metric unit kWh is: 1
kWh = 3412Btu, and for surface area values,
1kWh/m2/day
= 314Btu/ft2/day
C
Collector - A solar collector is not
really a solar water heater. A solar water
heater is a system which may include a tank,
pump, controller and solar collector panel. A
solar collector is that part of the system
which absorbs the sun's energy and converts it
into heat. The AP model is separate from the
tank as so is a solar collector.
Celsius - The metric unit for
temperature measurement. Convert as follows:
Fahrenheit = (oC
x 1.8) + 32
Celsius = (oF
- 32)/1.8
For Delta-T measurements the relative
temperature difference is needed.
Eg. Delta-T = 7oC
turn pump on, Delta-T 2oC
turn pump off. How much is that in
oF??
The conversion from Fahrenheit to Celsius is
simple:
Fahrenheit
= oC x 1.8
Celsius = oF
/ 1.8
D
Delta-T Controller:
Delta-T refers to the difference in two
temperatures. This term is often use in
relation to a solar controller. In such case
the Delta-T is the difference between the
solar collector temperature and the
temperature of the water in the solar storage
tank. A Delta-T controller can be configured
to turn on the pump when the Delta-T
difference exceeds a certain level (Eg.7oC
/ 12.7oF) and
off again when the temperature difference
drops below another setting (Eg. 2oC
/ 3.6oF). The
controller turns on the pump when there is
heat potential in the manifold. A Delta-T
controller can also be used to provide freeze
protection by circulating warm water from the
tank through the manifold when the manifold
temperature drops below 5oC.
E
Efficiency:
Solar collector efficiency is usually
expressed as a percentage value, or in a
performance graph. When assessing a
collector's performance make sure it is based
on absorber area. Flat plate collector's
absorber area and gross area is almost the
same, whereas evacuated tube collector
absorber area is usually only around half of
the gross area. When comparing two collectors,
not only the performance graph need be
considered. IAM values have a significant
influence on actual heat output throughout the
day. Looking at just the percentage efficiency
value will not give a true indication of daily
heat output.
Efficiency testing is usually completed by
testing bodies such as SPF, SRCC, FSEC and
other government approved testing bodies.
Tm* is the x axis value on performance graphs
for solar collectors.
Tm* is calculated as:
(water temp - ambient temp)/Insolation
Eg. (44oC
- 20oC)/800Watts
= 0.03
F
Flow Rate: The volume of
water flowing through plumbing in a given
period of time. Usually measured in
volume/minute or volume/hour. 1 Litre/min
= 0.264 US Gallon/min
G
Gravity Feed Tank (GFT): A small tank,
located above the level of the solar collector,
which provides low pressure water supply. The
feed tank is generally fitted with a float
valve, fed by mains pressure cold water. Feed
tanks are generally used for the PO-D solar
collector. but may also be standard in some
residential households.
Gross Area: The total surface
area of the collector including the frame,
manifold and absorber. This area is often used
when comparing collectors, but a better
comparison to use is value for your money.
Roof size is not usually a limiting factor for
domestic solar water heating installations, so
the size of the collector is not really that
important.
H
Heat Pipe: An evacuated rod or pipe
used for heat transfer.
I
Insolation: Don't confuse this with
insulation - the one letter change makes a big
difference. Insolation refers to the amount of
sunlight falling on the earth.
Insulation: The ability to protect
against transfer of heat/cold.
AP solar collectors use compressed glass wool
to insulate the header from heat loss. Glass
wool has excellent insulation properties, is
very light and can withstand high temperatures,
making it an ideal choice for a solar
collector. It is made from a least 80% old
glass bottles and can be recycled so is very
environmentally friendly.
Irridance, Irridation: Basically the
same as Insolation - explained above.
Incidence Angle Modifier (IAM):
refers to the change in performance
as the sun's angle in relation to the
collector surface changes. Perpendicular to
the collector (usually midday) is expressed as
0o, with
negative and positive angles in the morning
and afternoon respectively. Collectors with a
flat absorber surface, which includes some
types of evacuated tubes, only have 100%
efficiency at midday (0o),
whereas Apricus solar tubes provide peak
efficiency mid morning and mid afternoon, at
around 40o
from perpendicular. This results in good
stable heat output for most of the day.
P
Pressure:
Refers to the water pressure in the system.
The conversions for the most commonly used
units are: 1 bar = 1.02kg/cm2
= 14.5psi = 100kPa = 0.1Mpa = 10m water head
