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Solar hot water systems use basic principles and components to
capture incoming solar radiation and heat water for domestic and
other uses. Over the years, a variety of system designs have
been developed and tested to meet the specific consumer needs
and environmental conditions. Sometimes, particularly for
a new solar customer, the vocabulary and options can seem a bit
overwhelming. This paper provides background information
on the common uses for solar hot water systems, system types,
sizing and system components.
The
DayStar Thermal System Solar Hot Water Systems are available for
residential, commercial and RV applications. These systems
can furnish solar energy heating for domestic hot water, radiant
heating and pool/spa requirements. A unique modular
manifold system is used minimizing installation and service
issues. All system valving, pumps monitoring as well as
service fittings are installed in this manifold.
Domestic Hot Water
Solar hot water systems are most commonly used to heat water for
basic household needs such as laundry, bathing, dishwashing and
cooking. These systems are commonly referred to as
"domestic hot water systems". Domestic hot water systems
typically use solar energy to pre-heat the water that is
incoming to a conventionally fueled heating tank. The
warmer the water from the solar heater, the less conventional
fuel will be needed to provide the household's hot water needs.
A properly sized solar hot water system in Central Oregon will
provide up to 80% of a household's hot water needs.
The
size (and total area) of the solar collectors suitable for your
site will depend upon your hot water usage and location,
typically they will require between 40 and 80 square feet of
collection area. The most common mounting technique used
today is to install the collectors flush with the roof on a
south facing exposure. Alternatives, such as ground
mounting, or rack mounting on gable ends are also sometimes
possible.
Space Heating
Solar hot water heating can also be used to provide space heating.
The same set of solar collectors can be used to provide hot
water for both domestic hot water and space heating needs,
although space heating will generally require a much greater
collector area and storage capacity. Additional controls
and heat exchangers are also needed. Due to these extra
costs, and because sunshine is relatively scarce when heating
loads are highest (for example at night and during the winter)
solar energy should be looked at augmenting the total heating
loads. You should also make sure to carefully consider passive
solar and other building efficiency measures that will reduce
your heating loads to help you take maximum advantage of the
available solar resource.
Spa and Pool Heating
Solar heaters are often the most economical way to heat a swimming
pool. Compared to conventional pool heaters using propane,
electricity or oil, solar pool heating systems can pay for
themselves in four years or less. If you currently don't
heat your pool, a solar heating system can provide an economical
way to extend your pool season, starting earlier in the spring
and extending later into the fall. Solar pool heaters work
by circulating pool water directly through collectors and then
rerouting the warmed water to the pool. System controllers
sense when collectors are warmer than the pool water, and open
valves diverting water from the pool circulator through the
collectors and then back into the pool. The controller can be
set to automatically keep the pool temperature anywhere between
65 and 100 degree Fahrenheit. The collectors used for pool
heating systems are often less expensive than those used for
domestic hot water systems, providing significant economic
advantages.
Commercial
Applications
Commercial facilities with high hot water demands and access to a
good southern exposure can be great candidates for solar hot
water. Restaurants, bakeries, beauty salons, health clubs,
and hotels are all potentially good sites. A commercial
installation generally makes use of the same system design and
components as residential systems, including a conventional
back-up for hot water heating during high load and low sun
periods.
Solar Hot
Water System Types
There are
numerous solar hot water systems, some good and some bad.
We are only targeting those systems that are efficient and
economical to operate in this market.
Closed
Loop - Glycol System
Closed loop systems use a heat-transfer fluid to collect heat and
a heat exchanger to transfer the heat to household water.
These systems use electric pumps, valves, and controllers to
circulate the heat-transfer fluid, usually a glycol-water
antifreeze mixture, through the collectors. This
glycol-water antifreeze mixture makes closed-loop glycol systems
effective in areas subject to freezing weather. For this
reason, closed loop systems are preferred for year round use in
Central Oregon
Closed Loop - Drainback
System
Drainback systems use a heat transfer fluid within the collector
loop similar to the Closed Loop System. The water is forced
through the collectors by a pump and then is drained by gravity
to a storage tank when the system shuts down when the water in
the storage tank becomes too hot. These systems are
typically used for systems that have large collector areas such
as systems used to augment the heating loads.
Closed Circuit -
Thermal Siphon
Thermal siphon for freezing climates use a a closed circuit
system, utilizing a jacketed tank and heat transfer fluid; water
is only held in the storage tank. The fluid in the collectors
and jacket forms a closed circuit, and is a glycol-water
antifreeze mixture. The fluid uses the Thermal Siphon
Principle to transfer its energy into the storage vessel to heat
the water.
Components of
a Solar Hot Water System
Collectors
Solar collectors transform solar radiation into heat and
transfer that heat to a medium (water, solar fluid, or air).
Then solar heat can be used for heating water, to back up
heating systems or for heating swimming pools. Collectors
must face a southerly direction with performance maximized by
placing the collectors perpendicular to the sun. Optimum
collector angle and roof angle are not always compatible.
When making these compromises, care must be taken to understand
all of the consequences.
Flat Plate Collectors are the most common method of
converting the energy from sunlight into heat. A
flat-plate collector consists of an absorber, a transparent
cover, a frame, and insulation. Usually an iron-poor solar
safety glass is used as a transparent cover, as it transmits a
great amount of the short-wave light spectrum. Flat Plate
Collectors demonstrate a good price-performance ratio, as well
as a broad range of mounting possibilities (on the roof, in the
roof itself, or unattached).
Evacuated-tube collectors use an absorber strip located in
an evacuated and pressure proof glass tube to collect the heat.
The heat transfer fluid flows through the absorber directly in a
U-tube or in countercurrent in a tube-in-tube system.
Several single tubes, serially interconnected, or tubes
connected to each other via a manifold, make up the solar
collector. A heat pipe collector incorporates a special
fluid which begins to vaporize even at low temperatures.
The steam rises in the individual heat pipes and warms up the
carrier fluid in the main pipe by means of a heat exchanger.
The condensed liquid then flows back into the base of the heat
pipe. Evacuated tubes offer the advantage that they work
efficiently with high absorber temperatures and with low
radiation. Higher temperatures also may be obtained for
applications such as hot water heating, steam production, and
air conditioning.
Storage Tanks and
Heat Exchangers
The purpose of the hot water storage tank is to stockpile energy
for days with poor solar radiation. Its volume capacity should
be 1.5 to 2 times more than the square footage of the collector
(flat plate collector). Additional storage is usually
always a benefit. In most cases the heat exchanger and
pumps are part of the domestic hot water solar system storage
tank. The heat exchanger can be internal or external, both
system produce about the same amount of heat. A control
system is required to operate the solar system in a safe and
efficient mode. Temperatures in a solar hot water system
can reach boiling points very quickly if not controlled.
In
most cases the solar hot water system will be feeding into an
existing domestic hot water tank. Additional controls can
be added to the solar system increasing the storage capacity as
well as allowing the standard tank to remain off for extended
periods of the year. |
