If someone were to heat a building,
there would be dozens of options available. These range from
baseboard convectors to forced air systems and on to radiant floor
slabs. There is no single system type that is always the proper
choice for every installation. Instead, the choice of system is
dependent on the type of building, the climate it is located in, the
size of the load, and budgetary constraints.
Solar systems are simply another type of heating system and
there are several different system designs. Each system design
has it's benefits and drawbacks and it is up to the contractor to
determine the suitability of each design to the site. Basic
P&ID (piping & instrumentation diagrams) schematics are shown
below for all the common commercial solar applications along with a
brief description of the system and highlights of it's capabilities
and limitations. These system types are:
Show P&ID Schematic
Legend
|
| Open-Loop
Freeze Recirculation |
|
.jpg)
click for enlarged diagram
|
The open-loop
freeze recirculation is both the simplest and most efficient type of
solar heating system. In this system, potable water is drawn directly
from the storage tank and circulated through the array to be heated
and returned to a recirculation port in the tank. The
mechanicals for this type of system are almost identical to a boiler
application except that the solar array is the heat source.
The major drawback to this type of system is it's susceptibility
to freeze damage. Although recirculation of fluid by the pump
and thermal bleed valves provide a certain level of protection, these
systems are typically limited to areas that do not freeze during an
average year such as coastal regions of California and Florida
| Advantages |
Disadvantages |
|
- Low cost
- High Efficiency |
- Susceptible to
freeze damage
- Possible scaling of collectors in
hard water areas |
|
| Closed-Loop
Glycol (immersed heat exchanger) |
|
.jpg)
click for enlarged diagram
|
Closed-loop
glycol systems use food grade propylene glycol as the heat transfer
fluid in the solar array to overcome issues of freezing (glycol can
tolerate - 60F in high concentrations) and eliminate scaling.
The glycol is separated from the potable water by means of a heat
exchanger that transfers the energy from the array into the storage
tank. In this particular design, the heat exchanger is a traditional
tube bundle that fits into a ANSI flange in the bottom of the
tank. By using an immersed tube bundle, this design is able to
function with a single pump.
| Advantages |
Disadvantages |
|
- Impervious to
freezing
- Collectors not affected by scaling
in hard water areas |
- Higher cost than
open loop
- Tube bundle causes loss of
efficiency dependent on sizing |
|
| Closed-Loop
Glycol (external heat exchanger) |
|
.jpg)
click for enlarged diagram
|
Another type of
closed-loop system is one with an external double pumped heat
exchanger. In this design, a high efficiency brazed plate or
tube-in-shell heat exchanger is mounted externally and plumbed between
the recirculation ports on the tank. Although using an external
heat exchanger mandates the use of a separate pump, the improvement in
heat transfer more than offsets the extra mechanicals for larger
arrays.
| Advantages |
Disadvantages |
|
- Impervious to
freezing
- Collectors not affected by scaling
in hard water areas
- Very near open-loop performance |
- Higher cost than
open loop
- Requires second pump |
|
| Drainback
(external heat exchanger) |
|
.jpg)
click for enlarged diagram
|
Like glycol
systems, drainback systems are also a closed-loop design that use a
heat exchanger to separate the collector loop from the potable
water. Instead of relying on glycol for freeze protection, this
system allows all of the water in the array to drain back from the
array and into a storage tank in the solar loop when the pump is
turned off. Because the array completely drains, water can be
used in the solar loop, which avoids the need for double-wall heat
exchangers demanded in some localities.
| Advantages |
Disadvantages |
|
- Impervious to
freezing
- Collectors not affected by scaling
in hard water areas
- Very near open-loop performance
- Single-wall HX qualifies in all
localities |
- Higher cost than
open loop
- Requires second pump
- All piping must be sloped at least
1/4" per foot to drain. Can result
in
complex piping runs.
- Requires use of high head pump in
solar loop. |
|
| Drainback
(immersed heat exchanger) |
|
.jpg)
click for enlarged diagram
|
The immersed
heat exchanger variant of the drainback design uses an immersed tube
bundle to transfer the heat into the storage tank instead of the
double pumped external heat exchanger. Like the immersed glycol
design, the tube bundle eliminates the need for a second pump in the
system.
| Advantages |
Disadvantages |
|
- Impervious to
freezing
- Collectors not affected by scaling
in hard water areas
- Single wall HX qualifies in all
localities |
- Higher cost than
open loop
- Tube bundle causes loss of
efficiency dependent on sizing
- All piping must be sloped at least
1/4" per foot to drain. Can result
in
complex piping runs.
- Requires use of high head pump in
solar loop. |
|
| Drainback
(load-side heat exchanger) |
|
.jpg)
click for enlarged diagram
|
The final
variant of the drainback design uses an unpressurized storage tank
coupled with a load-side heat exchanger. Whereas all the
previous closed loop designs used a storage tank filled with potable
water, this design places the potable water in the heat exchanger and
the solar loop fluid fills the tank.
One of the main advantages to placing the pressurized
potable water in the heat exchanger is that the storage tank does not
need to be a pressure vessel and can be fabricated from polyethylene,
EPDM, or any other inert material that can withstand typical operating
temperatures. The single drawback to these designs is that the
heat exchanger must be sized to handle peak water draws, which are
often much larger energy transfers than the near constant charging
that traditional supply side heat exchangers carry.
| Advantages |
Disadvantages |
|
- Impervious to
freezing
- Collectors not affected by scaling
in hard water areas
- Single wall HX qualifies in all
localities
- Can use low cost unpessurized
tank. |
- Higher cost than
open loop
- Load side heat exchanger must be
large to accommodate large draws
- All piping must be sloped at least
1/4" per foot to drain. Can result
in
complex piping runs.
- Requires use of high head pump in
solar loop. |
|
| Thermosiphon |
|
.jpg)
click for enlarged diagram
|
Traditionally
used in smaller residential applications, thermosiphon style systems
may be ganged together for commercial applications. Instead of
using a separate large array and storage tank, these units use
multiple small arrays of 2 or 3 collectors individually coupled to
storage tanks of 120 gallons or less.
| Advantages |
Disadvantages |
|
- Plug & Play
installation
- No pumps required |
- Use of multiple
small tanks
typically increases cost
- Solar supply and return piping
must be large to accommodate
draws
- Weight of tanks on roof may raise
structural concerns |
|
