Generation PV partners with Apricus Solar to bring you high efficiency,
low cost Solar Thermal Systems that work ALL YEAR ROUND. Unlike
flat-plate collectors, or vacuum tube with flat copper foil, the
Apricus System utilizes a cylindrical collector which takes advantage
of the sun all day, not just when over head.
Solar tubes are the absorber of the solar water
heater. They absorb solar energy converting it into heat for use
in water heating. Solar tubes are also referred to as evacuated
tubes, as the space between the two glass layers is evacuated to
form a vacuum. Solar tubes have already been used for years in Germany,
Canada, China and the UK. There are several types of solar tubes
in use in the solar industry. Apricus collectors use the most common
"twin-glass tube". This type of tube is chosen for its
reliability, performance and low manufacturing cost.
Each solar tube consists of two glass tubes made
from extremely strong borosilicate glass. The outer tube is transparent
allowing light rays to pass through with minimal reflection. The
inner tube is coated with a special selective coating (Al-N/Al)
which features excellent solar radiation absorption and minimal
reflection properties. The top of the two tubes are fused together
and the air contained in the space between the two layers of glass
is pumped out while exposing the tube to high temperatures. This
"evacuation" of the gasses forms a vacuum, which is an
important factor in the performance of the solar tubes.
Why a vacuum? As you would know if you have used a glass lined thermos
flask, a vacuum is an excellent insulator. This is important because
once the solar tube absorbs the radiation from the sun and converts
it to heat, we don't won't to loose it!! The vacuum helps to achieve
this. The insulation properties are so good that while the inside
of the tube may be 150oC / 304oF , the outer
tube is cold to touch. This means that solar tube water heaters
can perform well even in cold weather when flat plate collectors
perform poorly due to heat loss (during high Delta-T conditions).
In order to maintain the vacuum between the two glass layers, a
barium getter is used (the same as in television tubes). During
manufacture of the solar tube this getter is exposed to high temperatures
which causes the bottom of the evacuated tube to be coated with
a pure layer of barium. This barium layer actively absorbs any CO,
CO2, N2, O2, H2O and
H2 out-gassed from the solar tube during storage and
operation, thus helping to maintaining the vacuum. The barium layer
also provides a clear visual indicator of the vacuum status. The
silver coloured barium layer will turn white if the vacuum is ever
lost. This makes it easy to determine whether or not a tube is in
good condition. See picture below.
The Getter is located at the bottom of the solar tube.
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Left Tube = Vacuum Present
Right Tube = Faulty
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Solar tubes are aligned in parallel, the angle of mounting depends
upon the latitude of your location. In a North South orientation
the tubes can passively track heat from the sun all day. In an East
West orientation they can track the sun all year round.
The efficiency of a solar water heater is dependent upon a number
of factors, one important one being the level of solar radiation
(insolation) in your region. To learn more about insolation and
the average values for your area click
here.
Solar Tube Basic Specifications
| Length (nominal) |
1500mm /1800mm |
| Outer tube diameter |
58mm |
| Inner tube diameter |
47mm |
| Glass thickness |
1.6mm |
| Thermal expansion |
3.3x10-6
oC |
| Material |
Borosilicate Glass 3.3 |
| Absorptive Coating |
Graded Al-N/Al |
| Absorptance |
>92% (AM1.5) |
| Emittance |
<8% (80oC) |
| Vacuum |
P<5x10-3
Pa |
| Stagnation Temperature |
>200oC |
| Heat Loss |
<0.8W/ ( m2oC
) |
| Maximum Strength |
0.8MPa |
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
va pour rapidly rises to the top of the heat pipe transferring
heat. As the heat is lost at the condenser (top), the va
pour 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 Generation PV 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
Below is an AP-20 solar collector being tested for SRCC OG100
certification.
The performance curves the AP solar collector,
based on the SPF test results are shown below.
The performance results provided by these testing
bodies allow calculation of energy output for the AP solar collector
in various enviornmental conditions. SPF and SRCC heat output
values do not take into consideration IAM performance effects.
Don't
have Acrobat Reader installed? Click here to download a free copy.
Each Generation PV Solar Thermal System comes in heavy duty packaging
and is generally shipped on skids. We make sure your system gets
to you safely and promptly. We charge a small crating fee and
don't skimp on packing material.
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