Solar heat
exchanger
Solar hot water heaters and heat exchangers
A solar heat exchanger can be a required component
of a solar hot water heater. Heat exchangers are used to transfer heat
from the system's heat
transfer
fluid to the domestic water. Without the assistance of a heat exchanger, solar hot water heating systems in harsh climates would be
highly
susceptible to freeze. There are several design and performance considerations to make when
selecting a heat exchanger. In some instances, the solar water heating
design and the local building codes can determine application.
Application
Heat exchangers are required in all closed loop solar water heater designs, also called indirect circulation systems. Whether your considering purchasing the major components and
tailoring a design, or your going with a reputable solar supplier;
understanding how heat exchangers function within whole systems is
important.
External application
In all external applications, the solar heat
exchanger is located in the plumbing loop between the hot water panel
and the storage tank. An external application can allow for the
additional flexibility of incorporating a used (and slightly modified)
conventional storage tank to serve as the
system's solar storage tank.
Solar hot water heaters that have
external heat exchangers generally require two solar hot water pumps,
one to circulate the domestic water from the storage tank and another
to distribute the heat transfer fluid to the panel (or collector) and
the solar heat exchanger.
Internal
or immersed application
In all internal applications, the solar heat
exchanger is immersed in the storage tank. Internal applications place
the stored domestic water in direct contact with the outer wall of the
heat exchanger.
In fact many manufacturers have introduced integrated
solar hot
water tanks that already come equipped
with heat
exchangers. Many industry experts consider an internal heat exchanger
as the more energy efficient application.
Single wall and double wall
With both applications (external
applications and internal applications),
solar heat exchangers come both single walled and double walled.
Single
Wall
Heat Exchangers
Single
wall models have one layer of material between the domestic
water and the heat transfer fluid. These models can be more efficient,
simply because there is less material between the fluids.
Double
Wall Heat
Exchangers
Double wall models have two layers of material
between the two fluids. These models also provide an extra air space
between
the two fluids. In the event one of the inner walls is compromised,
this extra air space will ensure
that the two fluids will never mix.
Many double wall models allow for easy
leak detection as well. In these models small exit holes are pre
manufactured in the vented air spaces, so if a leak were to occur, the
fluid will
noticeably leak outward from the holes in the air space.
It's
important to note the International
Association of Plumbing and
Mechanical Officials, or IAPMO, has developed the 2006
Uniform Solar
Energy Code. This uniform or code recommends that only double wall
solar heat exchangers be used in residential water heating
systems.
Some
U.S states, and local municipalities, have since adopted this code
while other states have approved the use of single wall heat
exchangers as long as non toxic heat transfer fluids are used.
It
would be strongly advised to check with a local solar installer,
or
with your local or state government about such potential building
codes.
Solar heat exchanger designs
All
internal or external and single wall or double wall solar heat
exchangers come in one, or more, of three different design types; coil
(which is sometimes referred to as wraparound), tube in shell (or shell
in tube) and plate design.
Coil or
Wraparound
Coil and wraparound heat exchangers often come
both single
walled and double walled. Coil and wraparound models are
either internally immersed or wrapped around the solar storage
tank.
This design type is
essentially a twirled pipe that's wrapped around the the tank
between the outer lining and the internal insulation or immersed deep
inside
the tank in direct contact with the domestic water.
Tube
in Tube, or Shell in Tube
This design is essentially made up of a layer of
pipe (or tube) within a separate layer of pipe (or tube). The heat
transfer fluid circulates through one tube (or pipe) and the indoor
domestic water simultaneously circulates in the opposite direction
through the other tube (or
pipe).
Tube in tube, or shell in tube, solar heat
exchangers are generally used in external applications, and more times
than not come in double wall designs.
Plate Design
The plate design is essentially a number of plates
spaced apart and enclosed, or capped, around the ends and/or sides.
This design provides separate directions and passage ways for the two
fluids to simultaneously circulate through.
More times than not plate solar heat exchangers
offer a relatively large amount of heat
transfer surface area, and the heat transfer surface area is
effectively and efficiently
packed
into a relatively small space.
Plate heat
exchangers are
used in external applications, and more times than not, come in single
wall designs.
Important design factors
The material used to construct the heat exchanger,
the heat transfer surface area and the flow rates of the
fluids circulating through the heat
exchanger will all have a large impact on the efficiency and
performance of the solar hot water heater.
Material
Used
The
material used to construct both the inner and the outer
surface of
the
solar heat exchanger is important. The thermal conductivity, or simply
put the amount of heat that the material absorbs and transfers, will be
key.
Most quality heat
exchangers in solar hot water heaters are made up of copper, because
copper has a high thermal conductivity as well as a high
resistance to
rust or oxidization.
Heat
Transfer Surface Area
The
heat transfer surface area is the where the all important heat
transfer takes place. More specifically, it's the area where the
outside portion of the tube, coil or plate comes in direct contact with
the circulating or stored domestic
water.
The heat transfer
surface area must be large enough to enable the solar fluid to
transfer
enough heat to all of the circulating or stored domestic water.
Fluid Flow Rate and Volume
The volume or flow rate of the fluids
circulating through the solar heat exchanger will be important as well.
Simply put, the higher the volume or flow rate of fluids generally the
better and more efficient the transfer of heat.
It's important to note, a properly sized and
installed solar hot
water pump will ensure an acceptable flow rate of fluids
throughout the solar hot water heater.
Keep in
mind
At the end of the day a high surface area
to flow rate ratio is
what's most ideal. But the heat transfer
surface area is generally the single most important design factor,
because
more heat transfer area for a sizable amount of volume or flow rate
will always produces
a quality heat transfer.
More on solar
hot water heaters
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