Detailed Description of Components and Materials
THE SOLAR COLLECTOR (LIQUID TYPE)
"A good solar collector... should last the life of the building."
The function of the solar collector is to allow solar energy to enter the collector,
then to turn the solar energy into heat energy, then to trap the heat so
that it cannot get out, and finally to transfer the heat to a liquid (a
heat transfer fluid) so that we can take the heat out and put it to use.
A good solar collector is similar in construction to a commercial storefront
window, and should last the life of the building.
A solar collector recommended by Radiantec will be simple, durable and reasonable
in cost. It will be highly efficient at low to moderate temperatures, but
will lose efficiency at very high temperatures. It will be free to high
tech gizmos which raise high temperature performance but lower low temperature
performance. It will not cause high temperature problems that need expensive
solutions. A solar collector that can go up to 400 degrees F is not really
your friend.
The important components of a liquid type solar collector are:
- The cover sheet.
- Absorber plate
- Rubber gasket.
- Frame material
- Insulating material.
THE COVER SHEET
Recall that whenever solar
energy comes in contact with a material, the energy is either, transmitted
through the material, or reflected off of the material, or absorbed by
the material.
When we select the cover
material, we will be looking for a material which transmits the maximum
amount of solar energy, so that the sun's energy can get into the solar
collector, and yet does not transmit heat energy, so that the energy is
trapped. The material of choice is glass. Glass transmits most of the energy,
but unfortunately, ten percent is still reflected off of the surface and
about five percent is absorbed.
The glass cover sheet should
be "low iron". Iron is a contaminant of glass that absorbs solar energy
and gives the glass a greenish tint. Low iron glass will be vodka white
when viewed on edge.
The cover sheet should be
"tempered glass". Tempering greatly strengthens glass and if it does break,
it will break into small pieces instead of large dangerous shards. It is
just too dangerous to work with untempered glass and usually not responsible
to those underneath.
THE ABSORBER SHEET
Here, we will be looking
for a material which absorbs the maximum amount of solar energy and also
is able to conduct and transfer the heat to a liquid so that the heat can
be pumped out of the collector and then put to use. The material of choice
will be a copper sheet with a black coating and with copper tubing attached.
We need to select the absorber
coating material with care. Not only must the material do its job of absorbing
the sun's energy well, but it must also last a long time without peeling
or discoloring. Some coating materials have so called "low emissivity"
properties. All warm materials emit energy in the form of low wavelength
infared waves (like a woodstove or a red-hot stovetop element). If we can
reduce this emissivity, less energy will be radiated towards the cover
sheet where some of it will be lost to the outside air. The result is a
solar collector that conserves energy better at very high temperatures,
but the collector will also stagnate at higher temperatures.
A solar collector is said
to be in stagnation when bright sunlight is upon it, but the heat is not
being taken away for some reason (a rolling blackout perhaps). Solar collectors
must be able to withstand their stagnation temperatures in direct sunlight
without damage and the rest of the system must be designed such that stagnation
causes no harm or safety problems. The system must not boil over and should
not damage other materials when the power comes back on and very hot antifreeze
is pumped through they system. The system should not steam trap, so that
the system will not flow because steam within the panels is acting like
a large air bubble.
For these reasons, a solar
heating collector provided by Radiantec will have an absorber plate with
a "moderately selective" coating (It will absorb radiant energy well, but
emit it only moderately). This absorber coating material will last a very
long time because it is ceramic rather than a high-tech, multi-metal,
coating process. It will give some added performance at higher temperatures,
but not so much that it will cause additional problems upon stagnation.
RUBBER GASKET
It is important to keep water
out of the solar collector. Water within the collector will turn to vapor
under high temperature and then condense upon the inside of the cover material,
causing appearance problems and some performance compromise. The only way
to remedy the situation is to remove the cover sheet and clean it. Water
can also cause corrosion problems.
Accordingly, collectors provided
by Radiantec will have high quality EPDM gaskets around the cover sheet
and at any plumbing connection. Also, any cracks or seams in the collector
will be sealed with silicone rubber sealant.
THE FRAMING MATERIAL
The framing material, (top,
bottom, and sides of the solar collector) should be a higher quality aluminum
extrusion. Any metal that comes in contact with it should be chosen with
care. When one metal comes into contact with another in a moisture environment,
the whole thing becomes a weak battery. The metal most resistant to corrosion
eats away at the other. In order to prevent this corrosion, any metal that
comes in contact with another metal in a moisture environment should be
either exactly the same metal, or should be stainless steel. The framing
material is mitered like a picture frame and fastened with stainless steel
crews. The joints must be carefully sealed with silicone rubber. A back
plate, made of exactly the same grade material, is attached with rivets.
INSULATION
Insulation must be provided
at the sides of the collector and also at the bottom. The insulation material
must be stable at solar collector stagnation temperatures and should contain
no impurities which could vaporize at stagnation temperatures and condense
upon the cover sheet (outgassing). Do not use compressed fiberglass as
this material contains petrochemical impurities which will outgas. Solar
collectors recommended by Radiantec will have insulation made of polyisocyanurate.
THE PLUMBING MECHANICAL PACKAGE
A closed system is one that
is not open to the outside environment. The solar heating loop is a closed
system. Any closed system needs to have certain components for proper operation.
The purpose of the plumbing mechanical package is to provide these components
in a form that is complete, preassembled, pretested, and in the proper
order. If you make the Plumbing Mechanical Package yourself, select the
components with care according to the following criteria. Most problems
with heating systems in general, and with solar heating systems in particular
are caused by improper components or by components installed in the wrong
place. It will help if you can do the work in the shop under ideal conditions
and test it. Then bring it to the site for quick trouble free installation.
Starting from the left to right, in the direct of flow:
DRAIN VALVE - This valve is a
way to get the heat transfer fluid (HTF)
out of the system for flushing or draining the system. It can be hooked
up to a garden hose or washing machine hose. The valve itself should be
Teflon seated ball valve instead of a rubber stopper because some HTFs
will react with rubber and make a leak that will be a nuisance to repair.
SHUT OFF VALVE/METERING VALVE - You can
use this valve to isolate most components in the PMP for easier repair. This valve can
also adjust the flow to the HTF if necessary.
ONE-WAY VALVE - This valve allows flow in
the proper direction, but prevents flow in the opposite direction. Without this valve, the solar
loop would flow in the opposite direction by gravity all night long because
the cold fluid in the collectors will be colder and denser than the fluid
in the building. This action would cause heat loss to the building and
could damage the domestic hot water heat exchanger. On bitterly cold nights,
the HTF could go below the freezing point of water and cause the heat exchanger
to rupture.
FILL VALVE - Allows the system to be flushed
and filled with HTF. It should have the same construction as the drain valve.
AIR ELIMINATOR - It is worthwhile to spend the
extra money on a good air eliminator. It must be installed of a horizontal section of pipe
and should be installed at a point in the system with the lowest pressure
(just before the pump). Air is not good for heating systems. Air interferes
with heat exchange and promotes corrosion. A large enough air bubble will
slow or even stop the flow of the HTF. If a quality air eliminator is installed
at the low pressure point of the system, it can remove dissolved air and
can even eliminate trapped air bubbles in remote locations. Air will come
out of solution under low pressure at the air eliminator (just before the
pump). On the other side of the pump, the HTF will be under higher pressure
and will be able to dissolve air. If any flow at all can be established
past a trapped air bubble, the HTF will dissolve the air bubble and bring
the dissolved air back to the air eliminator for removal.
The air eliminator will have
a screw cap on the top that must be loosened in order to allow air to escape.
The screw cap will likely arrive in a tightened down position to avoid
loss during shipment, and the air eliminator will not work until it is
loosened.
EXPANSION TANK - All fluids will
tend to expand in size when heated and
contract in size when cooled. This expansion and contraction can present
problems in a heating system that is closed (shut off from the environment)
and rigid. Expansion of the HTF could cause a rigid system to burst or
even explode. Contraction can cause air to enter through the air eliminator.
The expansion tank is basically
a rubber balloon within a steel tank. This rubber bladder should be made
from a material such as butyl rubber that is compatible with the antifreeze
in the HTF, or the service life will be shortened. An expansion tank with
an intact bladder will make a ringing sound when tapped with a metal object
like a screwdriver. If the bladder within the expansion tank ruptures,
the expansion will no longer perform its function. The system could lose
pressure, or the pressure in the system may become erratic, or the pressure
relief valve might discharge.
PRESSURE GAUGE - The pressure gauge
will indicate the pressure within
the system in pounds per square inch, at the location of the gauge. The
system should have enough pressure in it to raise a column of water to
the highest point in the system. Divide the vertical distance from the
gauge to the highest point in the system by 2.3 to calculate the pressure
required and then add 5 psi. In other words, if the vertical distance from
the gauge to the top of the system is 16 ft., you should have 12 psi in
the system. (16/2.3 = 6.9 psi + 5 psi = 12 psi)
Small variations in pressure
(2 or 3 psi) are to be expected and are most likely due to daily temperature
changes in the HTF, but other changes in pressure may indicate a problem.
If the pressure goes very
high when the sun is out, the system may be flowing too slowly or not at
all. Collector temperatures that are too high is confirmation.
If the system loses a little
pressure every day, you probably have a leak somewhere.
If the system drops a few
pounds but stabilizes at a lower pressure, the air eliminator has probably
removed an air bubble.
If the system pressure falls
to zero at night, but runs high during the day, the system has either lost
HTF, or the expansion tank has failed, or both.
IF THE HEATING SYSTEM
PRESSURE IS THE SAME AS THE BUILDING'S WATER PRESSURE, SUSPECT A BREACH
IN THE WALL OF THE DOMESTIC HOT WATER HEAT EXCHANGER. ALSO SUSPECT CONTAMINATION
OF THE POTABLE HOT WATER. DO NOT DRINK IT OR OTHERWISE USE IT UNTIL YOU
ARE SURE THAT IT IS SAFE. THE PRESSURE RELEIF VALVE SHOULD BE SET LOWER
THAN THE BUILDING'S WATER PRESSURE SO THAT THIS CONTAMINATION CANNOT OCCUR.
THE PUMP - The pump needs to be carefully chosen for a) high quality
material, and b) compatibility of the metals with the other metals in the
system, and c) the ability of the pump to lift a column of HTF to the high
point in the system (called pump head), and d) the ability of the pump
to make the HTF flow at the proper rate, and e) the ability of the pump
to do its work efficiently and with a minimum of electrical consumption.
Readers who have some familiarity
with conventional heating systems know that boilers that are supplied today
usually come with a pump, typically a small cheap one. I do not see how
a pump can be selected without knowing anything about what kind of system
the pump is going into or what the pump is going to have to do. This kind
of sloppiness should not have become standard in the design of conventional
heating systems, but there is no hope of a solar heating system working
properly unless careful selection is paid to the pump.
If there is one thing to
leave to an expert, pump selection might be it, but with the following
material and the pump calculation procedures in the appendix, it can be
done well. With the following information, the customer can at least verify
that all important factors have been considered.
a) MATERIALS - The pump is the only moving part in a Radiantec
type solar heating system and this is not the place to pinch a penney.
It is a reasonable goal to have a solar heating system to work flawlessly
for 20 years or more. Then, replace the pump and the HTF, and have it work
flawlessly for another 20 years or more. This is less maintenance than
any heating system that I know of.
All moving parts should
be stainless steel, but the shaft can be ceramic. If possible, the housing
should be either bronze or stainless steel.
b) PUMP HEAD - It is desirable, but not
absolutely necessary, if
the pump can lift a column of HTF to the high point in the system all by
itself without help. When the system is properly full of fluid, the pump
doesn't really have to do this. There is an equal column of HTF pulling
downward to match the column of fluid going up, and the pump only has to
circulate the fluid.
But if the system develops
a large air bubble, or if it is trapped by steam, a smaller pump will not
be able to move the fluid and a maintenance call will be needed to flush
the system (but a steam trap will resolve itself when the sun goes down).
If the pump is big enough,
however, all the homeowner need to do is add a little water with a garden
hose, a procedure that takes ten minutes and requires no tools.
I should mention here that
with reasonable workmanship and appropriate design, these problems will
not happen in the first place.
c) SYSTEM FLOW - A solar system should
flow at the proper rate
for best performance. There is some latitude here and flow does not have
to be perfect for good operation, but proper flow can easily make a difference
of 15-20% improvement in system energy harvest, and many owners of a solar
heating system would never know the difference.
So that you will know the difference, here are the issues.
There are two types of flow.
One is laminar flow, where the flow is fairly slow and smooth. Because
of friction of the tubing wall, most of the flow goes through the center
of the tube and the fluid nearest the wall does not circulate as well as
it might. The other type of flow is turbulent flow, where the fluid sloshes
around more because of its increased velocity.
Turbulent flow is good for
heat exchange between the tubing and the HTF. Laminar flow is efficient
as far as pump work is concerned.
What we want, is for the
solar collectors and any heat exchangers to be in slightly turbulent flow.
Too much turbulence wastes pump energy and in extreme cases can wear away
the tubing material prematurely (erosion corrosion).
The tubing that connects
the solar panels to the heat exchanger might as well be in laminar flow
in order to save energy and achieve maximum service life. Use ¾"
tubing up to 8 gallons per minute. Use 1" tubing up to 16 gallons per minute.
Use 1 ¼" tubing up to 26 gallons per minute.
In general, a 4' X 8' solar
collector should flow at the rate of 1-1 ½ gallons per minute. The
temperature of the HTF coming out of the collector will be 10-15 degrees
warmer than it went in under strong sunlight conditions.
ANOTHER SHUT OFF VALVE - This shut off
valve can be used to isolate nearly all mechanical components between two valves for
easier maintenance. It can also be used to adjust (meter) the flow.
PRESSURE RELIEF VALVE - This is the last
line of defense against damage to the heating system from excessive pressure. The first
line of defense is good design and the expansion tank. The pressure relief valve will vent
the system to the atmosphere whenever a set pressure is exceeded. This
valve should be set above any normal pressure situation that may occur,
including stagnation. This is an emergency valve that is not designed to
operate frequently. If it operates frequently because of some design flaw
in the system, it will eventually start to leak and will need replacement.
However, this valve should be tested once in a while to be sure that it
still works and is not compromised by corrosion.
If this valve discharges,
it could be quite hot, and it should be plumbed so that it will discharge
in a safe place.
THIS VALVE MUST NEVER
BE ISOLATED FROM THE HEAT PRODUCING EQUIPMENT (THE SOLAR COLLECTORS). THERE
MUST BE NO SHUT OFF VALVE OR COMBINATION OF SHUT OFF VALVES THAT WOULD
ISOLATE THE COLLECTORS FROM THE PRESSURE RELIEF VALVE.
SERIOUS MECHANICAL DAMAGE
AND POTENTIAL FOR SERIOUS PERSONAL INJURY OR DEATH CAN RESULT FROM IMPROPER
LOCATION OF THE PRESSURE RELIEF VALVE.
I don't mean to be an alarmist,
but this warning applies to any boiler or domestic hot water heater as
well, and solar energy can hurt you if you are not thinking.
WORKMANSHIP - The Plumbing Mechanical
Package is the place where most of
the potential problems with solar heating systems can be avoided. If you
make your own PMP please consider the following. Leaks are not compatible
with long lasting trouble free performance. A solar collector is not going
to leak in its lifetime because it will be carefully braised and pressure
tested. A straight piece of tubing is not likely to leak. A carefully made
soldered fitting is not going to leak. If you make your own PMP with care,
it will never leak either. Here is how.
Threaded fittings are much
more prone to leak than a properly soldered fitting. An antifreeze solution
is much thinner (less viscous) than water and is much more prone to leak.
When you make a threaded fitting, screw the fittings together several times.
This will tend to machine the parts together and eliminate any burrs. Use
a good pipe dope and or Teflon tape.
Pressure test at 80-100 psi
for 48 hours. There must be no loss of pressure at all. A solar PMP made
by the Radiantec Company will be made by a senior craftsman who has made
thousands. Nevertheless, a reasonably competent handyman can be successful
if he selects his material with care and pays strict attention to workmanship.
Small variations in pressure
(2 or 3 psi) are to be expected and are most likely due to daily temperature
changes in the HTF, but other changes in pressure may indicate a problem.
If the pressure goes very
high when the sun is out, the system may be flowing too slowly or not at
all. Collector temperatures that are too high is confirmation.
If the system loses a little
pressure every day, you probably have a leak somewhere.
If the system drops a few
pounds but stabilizes at a lower pressure, the air eliminator has probably
removed an air bubble.
If the system pressure falls
to zero at night, but runs high during the day, the system has either lost
HTF, or the expansion tank has failed, or both.
IF THE HEATING SYSTEM
PRESSURE IS THE SAME AS THE BUILDING'S WATER PRESSURE, SUSPECT A BREACH
IN THE WALL OF THE DOMESTIC HOT WATER HEAT EXCHANGER. ALSO SUSPECT CONTAMINATION
OF THE POTABLE HOT WATER. DO NOT DRINK IT OR OTHERWISE USE IT UNTIL YOU
ARE SURE THAT IT IS SAFE. THE PRESSURE RELEIF VALVE SHOULD BE SET LOWER
THAN THE BUILDING'S WATER PRESSURE SO THAT THIS CONTAMINATION CANNOT OCCUR.
THE PUMP - The pump needs to be carefully
chosen for a) high quality
material, and b) compatibility of the metals with the other metals in the
system, and c) the ability of the pump to lift a column of HTF to the high
point in the system (called pump head), and d) the ability of the pump
to make the HTF flow at the proper rate, and e) the ability of the pump
to do its work efficiently and with a minimum of electrical consumption.
Readers who have some familiarity
with conventional heating systems know that boilers that are supplied today
usually come with a pump, typically a small cheap one. I do not see how
a pump can be selected without knowing anything about what kind of system
the pump is going into or what the pump is going to have to do. This kind
of sloppiness should not have become standard in the design of conventional
heating systems, but there is no hope of a solar heating system working
properly unless careful selection is paid to the pump.
If there is one thing to
leave to a expert, pump selection might be it, but with the following material
and the pump calculation procedures in the appendix, it can be done well.
With the following information, the customer can at least verify that all
important factors have been considered.
a-b) MATERIALS - The pump is the
only moving part in a Radiantec
type solar heating system and this is not the place to pinch a penney.
It is a reasonable goal to have a solar heating system to work flawlessly
for 20 years or more. Then, replace the pump and the HTF, and have it work
flawlessly for another 20 years or more. This is less maintenance than
any heating system that I know of.
All moving parts should
be stainless steel, but the shaft can be ceramic. If possible, the housing
should be either bronze or stainless steel.
c) PUMP HEAD - It is desirable,
but not absolutely necessary, if
the pump can lift a column of HTF to the high point in the system all by
itself without help. When the system is properly full of fluid, the pump
doesn't really have to do this. There is an equal column of HTF pulling
downward to match the column of fluid going up, and the pump only has to
circulate the fluid.
But if the system develops
a large air bubble, or if it is trapped by steam, a smaller pump will not
be able to move the fluid and a maintenance call will be needed to flush
the system (but a steam trap will resolve itself when the sun goes down).
If the pump is big enough,
however, all the homeowner need to do is add a little water with a garden
hose, a procedure that takes ten minutes and requires no tools.
I should mention here that
with reasonable workmanship and appropriate design, these problems will
not happen in the first place.
d) SYSTEM FLOW - A solar system should
flow at the proper rate
for best performance. There is some latitude here and flow does not have
to be perfect for good operation, but proper flow can easily make a difference
of 15-20% improvement in system energy harvest, and many owners of a solar
heating system would never know the difference.
So that you will know the difference, here are the issues.
There are two types of flow.
One is laminar flow, where the flow is fairly slow and smooth. Because
of friction of the tubing wall, most of the flow goes through the center
of the tube and the fluid nearest the wall does not circulate as well as
it might. The other type of flow is turbulent flow, where the fluid sloshes
around more because of its increased velocity.
Turbulent flow is good for
heat exchange between the tubing and the HTF. Laminar flow is efficient
as far as pump work is concerned.
What we want, is for the
solar collectors and any heat exchangers to be in slightly turbulent flow.
Too much turbulence wastes pump energy and in extreme cases can wear away
the tubing material prematurely (erosion corrosion).
The tubing that connects
the solar panels to the heat exchanger might as well be in laminar flow
in order to save energy and achieve maximum service life. Use ¾"
tubing up to 8 gallons per minute. Use 1" tubing up to 16 gallons per minute.
Use 1 ¼" tubing up to 26 gallons per minute.
In general, a 4' X 8' solar
collector should flow at the rate of 1-1 ½ gallons per minute. The
temperature of the HTF coming out of the collector will be 10-15 degrees
warmer than it went in under strong sunlight conditions.
ANOTHER SHUT OFF VALVE - This shut off
valve can be used to isolate
nearly all mechanical components between two valves for easier maintenance.
It can also be used to adjust (meter) the flow.
PRESSURE RELIEF VALVE - This is the last
line of defense against damage
to the heating system from excessive pressure. The first line of defense
is good design and the expansion tank. The pressure relief valve will vent
the system to the atmosphere whenever a set pressure is exceeded. This
valve should be set above any normal pressure situation that may occur,
including stagnation. This is an emergency valve that is not designed to
operate frequently. If it operates frequently because of some design flaw
in the system, it will eventually start to leak and will need replacement.
However, this valve should be tested once in a while to be sure that it
still works and is not compromised by corrosion.
If this valve discharges, it could be quite hot, and it should be
plumbed so that it will discharge in a safe place.
THIS VALVE MUST NEVER
BE ISOLATED FROM THE HEAT PRODUCING EQUIPMENT (THE SOLAR COLLECTORS). THERE
MUST BE NO SHUT OFF VALVE OR COMBINATION OF SHUT OFF VALVES THAT WOULD
ISOLATE THE COLLECTORS FROM THE PRESSURE RELIEF VALVE.
SERIOUS MECHANICAL DAMAGE
AND POTENTIAL FOR SERIOUS PERSONAL INJURY OR DEATH CAN RESULT FROM IMPROPER
LOCATION OF THE PRESSURE RELIEF VALVE.
I don't mean to be an alarmist,
but this warning applies to any boiler or domestic hot water heater as
well, and you can get hurt by solar energy if you are not thinking.
WORKMANSHIP - The Plumbing Mechanical
Package is the place where most of
the potential problems with solar heating systems can be avoided. If you
make your own PMP please consider the following. Leaks are not compatible
with long lasting trouble free performance. A solar collector is not going
to leak in its lifetime because it will be carefully braised and pressure
tested. A straight piece of tubing is not likely to leak. A carefully made
soldered fitting is not going to leak. If you make your own PMP with care,
it will never leak either. Here is how.
Threaded fittings are much
more prone to leak than a properly soldered fitting. An antifreeze solution
is much thinner (less viscous) than water and is much more prone to leak.
When you make a threaded fitting, screw the fittings together several times.
This will tend to machine the parts together and eliminate any burrs. Use
a good pipe dope and or Teflon tape.
Pressure test at 80-100 psi
for 48 hours. There must be no loss of pressure at all. A solar PMP made
by the Radiantec Company will be made by a senior craftsman who has made
thousands. Nevertheless, a reasonably competent handyman can be successful
if he selects his material with care and pays strict attention to workmanship.
THE HEAT EXCHANGERS
THE DOMESTIC HOT WATER HEAT EXCHANGER - An
external DHW heat exchanger
from Radiantec Company will be stainless steel and will be of the multiple
plate type where solar HTF flows on one side and potable domestic hot water
will flow on the other. Solar heat is transferred across the stainless
steel plate without mixing of the solar HTF and the domestic hot water.
This heat exchanger is the
strongest and longest lasting component of the system because failure of
this part might contaminate the potable water. Accordingly, we can expect
it to outlast the system in general, and thus be discarded before it ever
fails. If over pressure damages the system, something else will be damaged.
DHW heat exchangers are subject
to the accumulation of lime and other minerals on the potable side, which
will lower the exchanger's performance and possible plug it up. Accordingly,
we oversize it somewhat in order to provide a longer interval between maintenance.
Water can hold more minerals
in solution when it is cold than when it is warm. When water is raised
in temperature above about 140°F, the minerals will precipitate upon the
heat exchanger. On the other hand, when temperatures are low, these minerals
will dissolve back into the water. Solar heating systems are benefited
by the fact that they often run at lower temperatures and many systems
have no mineral problems at all, but some problems could be encountered
in areas where there is very hard water.
However, if HX efficiency
decreases significantly, you can dissolve the minerals with an acid solution,
(available at plumbing supply stores). Work with acid at low temperatures
and with the solar system off. Wear rubber gloves and eye protection. Do
not work with an unnecessarily strong acid. Strong acids and high temperatures
are dangerous. The chemical action of acid upon minerals will make heat,
perhaps lots of it.
Do not wait until the system is nearly plugged up or you will have much more difficulty.
HEAT EXCHANGERS FOR SPACE HEATING - These
heat exchangers are for
the purpose of heating the building. Radiantec Company uses the same materials
that it provides for underfloor radiant heat with the exception that extra
attention is paid to high efficiency. If the energy source is oil or gas,
limitations in the effectiveness of the heat exchanger can be made up for
by raising the fluid temperatures.
But high operating temperatures
have a very negative effect on solar collector efficiency, so we specify
a highly effective radiant heating tube and we use lots of it. If we want
the floor within a building to be 75°F, it would be a reasonable goal
to hope the solar collectors would operate at 80-85°F.
Radiantec performed extensive
research into the performance of heat exchanger materials and formed the
opinion that polyethylene is superior to other materials because it is
exceptionally long lasting, resistant to corrosion, has adequate temperature
and pressure ratings, has smooth walls, which reduce pump work, and is
reasonable in cost.
Some may wonder why we do
not use a material that is more conductive such as copper. Copper is indeed
many times more conductive than polyethylene, but this apparent advantage
does not work out in practice. In this heat transfer situation, there are
at least three factors that must be taken as a whole. It is like a chain
that is only as strong as the weakest link. The three important factors
are 1) the rate at which heat is brought to the heat exchanger, 2) the
rate at which heat passes through the wall of the heat exchanger to the
other side and 3) the rate at which the heat is carried away from the heat
exchanger and put to use where it is wanted.
We can pump as much heat
to the heat exchanger as we want, but as far as getting the heat away from
the heat exchanger, we have to rely on the three sisters of heat transfer;
conduction, convection and radiation. This means that our heat exchanger
will need to have a significantly sized surface area no matter how conductive
the tubing material will be.
After considering all of
this, and after careful measurements and testing and research, we are of
the opinion that a 4' X 8' solar collector should have about 200 ft. of
7/8" OD poly tubing (3/4" ID) if the tubing will be placed within a slab,
or earth or other conductive and massive material.
Underfloor heating tubes
can be placed within wooden floor systems by running tubes down the floor
joists. In this case, thermal storage and heat exchanger efficiency will
be reduced, but if we use the domestic hot water heater for storage and
the DHW heat exchanger for heat transfer, we can compensate for to some
extent.
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