Home Power Magazine - Solar Hot ...

Home Power Magazine - Solar Hot Water Basics, Home Power Magazine

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Issue 107
June & July 2005
Standard of Living
High Quality & Low Impact
p. 12
Fuel
Prices
Make Your Own
Biodiesel
p. 52
Log Home
Secrets of Cordwood
Construction
p. 36
The
Ultimate
Beginners’ Guide
to Solar Hot Water
p. 18
homepower.com
$6.95 US $10.95 CAN
06
Drought-Proof Your Summer
Collect Rainwater for Home & Garden
p. 46
7
2033678082
6
Choose the
New
Beat High
Build a Better
Solar
Hot Water
John Patterson
©2005 John Patterson
I was dazzled on a cold November morning in 1979 to see
my new solar water heater turn on. The gauges showed 50°F
(10°C) water going to the collectors and 60°F (16°C) water
coming back. At that moment, I became a believer.
Even in the cloudiest climates, the sun can provide 50 to 60
percent of a household’s annual water heating, and in sunnier
places, 80 percent or more. How does it work? Here is a simple
breakdown of the most common solar water heating systems
and their main components.
Systems vary—not all equipment is necessary for every
system type. For the sake of simplicity, some lesser yet
necessary, components have been omitted. Equipment
such as drain and fill valves, temperature and pressure
relief valves, air vents, check valves, and temperature and
flow gauges are important to the safety and function of
these systems. See past
Home Power
articles for detailed
descriptions of the importance, placement, and use of these
components.
System Types
Five main types of solar water heating systems are sold
today. These five are a distillation of dozens of types sold
over the past 25 years. They are:
•
Batch
•
Thermosyphon
•
Open-loop direct
•
Pressurized glycol
•
Closed-loop drainback
The proven winners are simple, reliable, and long
lasting. Some systems are “open loop” (the domestic water
itself is directly heated) and some are “closed loop” (a
heat transfer fluid is heated by the collector and the heat
is passed on to the domestic hot water by means of a heat
exchanger). Some systems are “active,” using moving parts
such as pumps and valves, and others are “passive,” using
no mechanical or moving parts.
SDHW System Characteristics
Characteristic
Batch Thermosyphon
Open-Loop
Direct Glycol Drainback
Low profile—unobtrusive in appearance
Lightweight
Freeze tolerant
Easy installation & infrequent service
Passive operation —no pumps or controls
Space saving—storage tank unnecessary
18
home power 107
/ june & july 2005
hot
water
1
Solar Collectors
AKA: Solar thermal panels
A solar collector consists of a network of pipes through
which water (or in colder climates, antifreeze) is heated.
Collectors come in various sizes, with 4 by 8 feet (1.2 x
2.4 m) the most common.
In evacuated tube collectors, a glass tube surrounds each
individual pipe in a va cuum. This nearly eli minates the
influence of ambient air temperature. Evacuated tubes
perform better than flat plate
collectors in cloudy weather,
and can achieve higher
temperatures compared to
other collector types, but are
typically more expensive. All
active systems and some
thermosyphon systems
may use either flat plate
collectors or evacuated
tube collectors.
On a typical summer day (sunny and warm), the fluid in
the collectors reaches 140 to 180°F (60–80°C). On a clear
winter day (sunny and cold), it can reach 120 to 150°F (50–
65°C). When it’s cloudy and warm,
it can reach 70 to 90°F (20–30°C),
and when it’s cloudy and cold,
50 to 60°F (10–15°C). As long as
the temperature in the collector
is greater than that of your
incoming cold water (usually
about 50°F; 10°C), your
solar hot water system is
saving you energy.
collectors in cloudy weather,
and can achieve higher
temperatures compared to
other collector types, but are
typically more expensive. All
active systems and some
thermosyphon systems
may use either flat plate
A third type, called
integra ted collector
storage (ICS) or
batch, combines the
solar collector and storage
tank into one unit. An ICS panel can resemble a flat plate
collector with greater depth (6 inches; 15 cm). A simple
batch heater can be a tank within a
glazed box.
Several types of solar
collectors are on the
market. Flat plate
collectors are often
Evacuated tube collector
Flat plate collector
compared to skylights.
They are thin (3–4 in.;
7–10 cm), black, and
covered with glass to hold
in the sun’s energy.
compared to skylights.
They are thin (3–4 in.;
7–10 cm), black, and
covered with glass to hold
Collection and storage
in one unit:
thermosyphon (left)
and batch (right)
2
Collector Mounting System
AKA: Mounts, racks
The three most common mounting systems for solar
collectors are the roof mount, ground mount, and awning
mount. Roof mounted collectors are held by brackets,
usually parallel to and a few inches above the roof. Ground
mount systems can be as simple as four or more posts in
the ground, lengths adjusted to affect optimal tilt. An
awning mount attaches the collectors to a vertical wall.
Horizontal supports push the bottoms of the collectors out
to achieve the desired tilt.
are within acceptable parameters. If weight is an issue,
ground mounts can be a good choice. Wall mounts are
another solution that can work well in some situations.
Find the sunniest spot for your collectors. Generally,
you want no shading between 9 AM and 3 PM. Facing
collectors up to 30 degrees east or west of true south,
and at your site’s latitude plus 15 degrees tilt, generally
will still yield results within 15 percent of optimum. Any
nominal losses from tilt, orientation, or even shading can
usually be overcome by adding more collector area.
When choosing a mounting system, roof mounts are
usually the cheapest option, provided tilt and orientation
19
it can reach 70 to 90°F (20–30°C),
and when it’s cloudy and cold,
50 to 60°F (10–15°C). As long as
the temperature in the collector
is greater than that of your
incoming cold water (usually
about 50°F; 10°C), your
solar hot water system is
hot
water
3
Solar Storage Tank
AKA: Solar water tank, solar tank
4
Water Pump
AKA: Circulating pump, circulator
A solar water tank is an insulated water storage
tank. Cold water that used to go directly to your
conventional water heater enters the solar tank and
solar-heated water exits. In closed-loop systems, the
water is heated by contact with
a coil of pipe containing
the water or antifreeze
that circulates through the
collectors. In open-loop
systems, the potable water
is directly circulated up
through the collectors and
back.
Pumps are used in active systems, but are not required
in batch or thermosyphon systems. They
circulate water or antifreeze between
the solar collector and the storage
tank. The right pump for the
job depends on the size of the
system and the distance and
height between the collector(s)
and the storage tank. AC pumps
plug into a wall outlet while DC
pumps are powered from a DC
source, such as a photovoltaic
panel. Good pumps can last as
long as 20 years with heavy use.
circulate water or antifreeze between
The preheated solar water
is then plumbed back to the
cold side of your existing
heater, which now functions
as a backup. Whenever hot
water is turned on in the
house, preheated solar hot
water is moved from the
solar tank to the backup
heater.
Integrated Collector
Storage:
Batch heater
1
3
Solar
Storage
Tank:
In collector
Solar Batch Heaters
For a hundred years, simple solar batch heaters have been used
in the United States. The term ICS (integrated collector storage)
tells us that the collector and storage tank are combined into
one unit. A tank of water, enclosed in an insulated box covered
with glass, is placed in the sun facing south. Cold water is
piped to the bottom of the tank; hot water is taken off the top.
Whenever there’s a call for hot water, water pressure from the
home moves hot water from the top of the solar batch heater
as cold water is pushed into the bottom.
Since the potable water is heated directly, this system
is open loop. And since no pump is used to move the
water from collector to end use, it is passive. The batch
heater is a popular choice for homes in moderate climates
where freezing is not much of an issue. Commercially
manufactured batch heaters are relatively low cost. Crude
batch heaters can even be homemade. If batch heaters are
installed on the roof, weight has to be taken into account.
Commercial batch heaters can weigh 200 pounds (90 kg)
dry, and when filled with 40 gallons (150 l) of water, more
than 320 pounds (145 kg) is added.
Because of their relatively low cost and simplicity,
for those living in moderate climates with good sunshine
available, the batch heater is probably the best value for
heating domestic water.
2
Collector
Mounting
System
8
Cold In:
Potable
Isolation
Valves
Hot Out:
Potable
10
Tempering Valve
9
Backup
Water Heater
20
home power 107
/ june & july 2005
hot
water
5
Heat Exchanger
6
Expansion Tank
Heat exchangers are used in closed-
loop solar hot water systems. They
enable the transfer of heat from
one fluid to another without the two
mixing. Internal heat exchangers are
inside the tank and not visible. They
can be as simple as a coil of pipe
resting in the bottom of the tank, or
wrapped around the outside beneath
the insulation and cover. As the
heated fluid from the solar collector
travels through the coil, the heat is
passed from the hotter fluid to the
cooler potable water.
Closed-loop systems require an expansion tank. An
expansion tank has a chamber in which air is locked
inside a bladder or diaphragm. It screws into standard
1
/
2
-inch or
3
/
4
-inch threaded plumbing fittings. When
pipes are filled with heat transfer fluid (water and
glycol), and the operating pressure of the system is
set, the fluid will occupy a given volume based on
the temperature. As the fluid is heated by the sun, it
expands. This is where the expansion tank is critical.
Without it, something would blow!
The expansion tank allows the fluid to safely expand
by compressing the air in the chamber. The size of the
expansion tank needed depends on the total volume
of fluid, which is determined by the number and size
of collectors, and the length and diameter of the pipes
in the solar loop.
An external heat exchanger is
usually a pipe within a pipe. The
solar fluid and potable water
flow counter to one another, and
heat is transferred within the heat
exchanger pipe. Fluid may be moved
with pumps, thermosyphoning, or a
In most cases, a total of 3 to 6 gallons (11–23 l) of fluid
is in a solar loop. A #15 (2 gal; 7.6 l) expansion tank
is usually adequate. It never hurts
to go larger, especially for
systems with more than
60 square feet (5.6 m
2
)
of collectors. A #30 has
twice the expansion
capability. With the
proper expansion tank
in place, the fluid can
go from 0 to 200°F (-18–
93°C) with the pressure
in the solar loop
remaining the same.
Thermosyphon Systems
Another relatively simple, passive
system, and the most popular solar
water heater worldwide is the
thermosyphon. Common in Japan,
Australia, India, and Israel, they are
easily recognizable by the fact that the
tank must be located directly above
the collector.
Thermosyphon systems work on
the principal of heat rising. In an open-
loop system (for nonfreezing climates
only), the potable water enters the
bottom of the collector and rises to the
tank as it warms. In colder climates, an
antifreeze solution, such as propylene
glycol, is used in the closed solar loop,
and freeze-tolerant piping, such as
cross-linked polyethylene (PEX), is
used for the potable water lines in the
attic and on the roof.
Several international manu-
facturers make thermosyphon systems.
The advantage of this system over the
batch heater is that solar heat is stored
in a well-insulated tank, so hot water
can be used any time, without the
penalty of overnight losses.
Solar Storage
Tank
3
10
Cold In:
Potable
8
Tempering
Valve
1
Hot Out:
Potable
Isolation
Valves
Solar
Collector
Solar
Collector
Collector
Collector
Mounting System
2
9
Backup
Water Heater
21
Solar
Solar
Solar
Solar
Collector
Collector
Collector
Collector
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