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solat-hot-water-systems

VIEWS: 14 PAGES: 10

									 Solar hot water systems
               By Jeffrey Yago, P.E., CEM
                Photos by Patrick Redgate




With continued shortages of wood for home heating, the Greeks built the North Hill section of
Olympus in the 5th century B.C. This planned community had all streets running east and west,
with each parallel street at a higher elevation as you went north. In this way every home had a
large south-facing window which allowed the sun to heat up the uninsulated heavy masonry
walls during the day, while heavy drapes were drawn at night to seal in this solar heat. If a
neighbor later built an addition that blocked the warming sun, the offended homeowner could
sue in court to remove the shading construction.

Around the 1st century A.D., the Romans improved on this design by splitting mica into thin
sheets or using hand-formed sheet glass to cover open skylights and south-facing windows.
This made it more efficient to warm their many public baths using the sun's rays, while holding
in the heat. By the 1800s many upscale London homes included solar greenhouses or
conservatories, and glass-covered insulated wood boxes were being used to heat water with
the sun.

                                                By the early 1900s, many upscale homes in the
                                                United States were able to buy solar water
                                                heaters for the huge price of $25 including
                                                installation. These insulated glass-covered
                                                boxes contained multiple metal water tanks
                                                which were installed on a south-facing roof.
                                                Valves were located next to the bathtub so the
                                                homeowner could route the cold water supply up
                                                through the solar-heated tanks then back into
                                                the bathtub. Through most of the 1930s many
                                                homes in California and Florida had
                                                commercially-made solar water heaters to heat
                                                hot water where utility natural gas was not
    Thermo-siphon solar hot-water heater
                         
                                                available.

These early solar water heaters were very simple to operate and did not require pumps or
controls. A solar panel was installed at the lowest part of a south-facing roof just above the roof
eave. An insulated storage tank was mounted as high up in the attic as possible. Since heat
rises, the heated water in the lower solar panel pushed its way up to the top of the upper tank,
and the colder water at the bottom returned to the solar panel to be reheated as a result of this
thermo-siphon effect.
When an attic was too small to house the tank, or the roof slope was too shallow to provide
enough tank elevation, the installer would cut a hole in the roof peak and the hot-water tank
would be mounted upright sticking up out of the roof. A fake chimney was then built around the
tank to provide insulation and camouflage. Once low-cost oil and gas was available and piped
throughout the towns and cities, solar hot water heaters were gradually replaced with cheap
gas-fired hot water heaters and the solar industry in the United States disappeared until a
revival in the late 1970s after an energy crisis brought on by two oil embargos.

When I was receiving my solar training in Florida back in the early 1970s, we were asked to
replace an aging solar hot water heater with a new system. When we removed the old solar
thermo-siphon system, the nameplate on the commercially-made solar panel was dated 1933.
As you see, solar hot water heating is not new, but today's systems are much more efficient and
are made with longer-lasting materials.

System types
There are actually endless varieties and types of solar hot water heaters, but I am going to only
address the most popular system types used in the United States today:

Solar batch heater:

The solar batch heater is actually just an updated version of the early 1800s batch heaters, and
is the most popular type of homemade solar water heater since they are so easy to build. The
basic design consists of a large insulated box with a glass cover, containing one or more bare-
metal water-storage tanks laying side-by-side. Homemade units have external enclosures made
from heat-resistant redwood or concrete blocks, while commercially made units are fabricated
from galvanized steel or aluminum.

Batch solar heaters can be fairly heavy since the roof-mounted tanks are full of water, and there
is a high risk of damage when temperatures drop below freezing. Therefore, solar batch heaters
are usually only found in southern climates. Since the heater's black painted-metal tanks quickly
heat up inside the insulated boxes after the sun's rays pass through the glass cover, these
batch heaters are still the simplest and lowest cost forms of solar water heaters. Batch solar
water heaters are used extensively in developing and tropical countries.

Antifreeze systems:

Larger families, with increased hot water demands for multiple showers, clothes washing, and
dishwashing, usually require more hot water per day than most solar batch heaters can provide.
Since homeowners expect hot water in early mornings or late evenings when the sky is dark,
larger and heavier solar storage tanks had to be moved from the attic into interior basements or
utility rooms. Since the solar storage tank would now be below the elevation of the roof-
mounted solar panels, a circulating pump was required to circulate the solar-heated water.
A simple temperature control turns this pump on
when the solar panel is hot, and off when the
solar panel is colder than the water in the tank.
The simplest designs circulated domestic water
instead of antifreeze solutions from the tank
through the solar panels. These are normally
only installed in southern climates due to the
danger of the water freezing in the above-roof
piping and solar panels.

To reduce the risk of freezing, a heat-exchanger
coil can be added inside near the bottom of the
insulated hot water tank, or wrapped around the
perimeter shell of the tank before covering with
insulation and outer shell. Now an antifreeze         Non-antifreeze solar hot water system in
solution can be circulated through the roof-          extreme southern climate where freezing
mounted solar panels then back through the                          is not a concern
heat exchange coil to heat the domestic water
stored in the insulated tank without mixing with the water in the tank.

Since ethylene-glycol automobile antifreeze is highly toxic, building codes required design
modifications to insure a leak in the antifreeze piping loop could not contaminate the domestic
water supply. Non-toxic propylene-glycol antifreeze designed specifically for solar hot water
systems are now available that are much safer. Care must be used when sizing an antifreeze-
based solar hot water heater since each time the tank reaches its high-temperature limit, the
control turns off the circulating pump, but the sun may still be blazing. This means the antifreeze
and water remaining in the above-roof piping can easily reach boiling point and turn to steam.

Relief valves and a small expansion tank can be installed in the system to relieve this pressure,
but venting steam will result in reducing the fluid level in the solar piping loop.

Most systems include an automatic valve that will add domestic cold water to repressurize the
solar-piping loop when this happens. However, after repeated cycles of overheating and refilling
with tap water, this can result in the solar fluid becoming mostly water, which could freeze if
freeze prevention chemicals are not manually refilled periodically by a service technician.

Many building codes will not allow a direct piping connecting between the chemically-treated
solar piping loop and the home's domestic water supply. Refilling a solar-piping loop requires a
portable pumping system and chemical feed tank which refills the system while removing any
air trapped at all high points in the solar-piping loop. Trapped air can cause the solar pump to
cavitate (form bubbles) and stop pumping when this air is allowed to move through the piping
then into the pump. Higher-cost antifreeze systems usually include a heat rejecting fan-coil or
other device to waste this excess heat which avoids making steam by keeping the fluid
circulating through the solar panels.

Drain-back systems:

Although the drain-back system is about as fail-safe as you can get, it does require extra effort
during system sizing and installation. The drain-back system normally uses flat-plate solar
collections having all-copper water passages, since water is used as the heat transfer fluid.
Untreated water would quickly damage solar panels containing aluminum tubing which require
special heat transfer fluids, so copper-tube water passages are used in most brands of solar
panels.

A drain-back system does not require an expansion tank because a separate holding tank is
used to store all water circulating through the solar loop when the pump is off. This small
insulated tank must always have an air space remaining above the water level. This allows air
to flow back through the solar loop piping, since this water drains back into the holding tank
when the pump is turned off.

                                                Since this draining of all water from the above
                                                roof piping and solar panels occurs due to
                                                gravity alone, a power outage will also cause the
                                                system to drain back, which makes it fail-safe.
                                                All piping in the solar loop and all solar panels
                                                must be installed with a slight slope towards this
                                                holding tank so no liquid remains in the panels to
                                                freeze when the pump turns off.

                                                Since this piping and the solar panels fully drain
                                                when the pump turns off, there will be no liquid in
                                                the very hot solar panels to flash to steam and
                                                be discharged out the pressure relief valve.
   Residential-size propylene-glycol solar      When there is a significant difference in
                hot-water system
                        
                                                elevation between the hot water storage tank
                                                and the solar panels, some drain-back system
designers will locate the drain-back holding tank on an upper floor level and not next to the hot
water tank which may be located in a lower basement or ground floor.

This significantly reduces the size of the solar loop pump since it does not have to pump from a
very low elevation up to a very high elevation. This design issue is not a problem in sealed
antifreeze-based solar loops, since both the supply and return risers are full of liquid and have
no elevation pumping head to overcome.

Solar panel construction
Not counting the solar heaters mentioned earlier, which have the hot-water tank located on the
roof inside or above the solar panel, most solar hot water heating designs use a flat-plate solar
panel. These are typically a shallow box made from aluminum or galvanized sheet steel.
Although panel sizes are increasing, the original flat-plate solar panels made in the United
States were covered with storm door glass. This strong, tempered glass easily handled high
temperatures, thermal shock, and resisted breakage from hail and stray baseballs.

Solar panel manufacturers in the 1970s discovered storm-door glass to be very low-cost and
readily available, but the odd dimension of this glass caused most solar panels to be about 3 by
6½ feet. This size panel was harder to install, since the construction industry has
standardized on four-foot spacing. Now that solar is more mainstream, solar panel
manufacturers are starting to use custom-sized tempered-glass sheets, which allows for making
larger and more standard-sized hot water panels.
A formed lip around the box frame supports the tempered glass, and a stainless-steel rim and
high-temperature rubber gasket supports the glass and seals the front of the fabricated box. All
four sides and back of the box are lined with rigid insulation board. Due to the very high
temperatures involved, standard building insulation cannot be used, as these materials contain
oils and chemicals that will outgas and coat the inside of the glass cover, which will reduce the
efficiency of the solar panel.

Mounted on top of this insulation bed is the
absorber plate, which consists of multiple
parallel fluid passages with a large surface area
to absorb the sun's heat. Many early panel
designs were all aluminum construction, but the
aluminum water passages would quickly corrode
and leak after the antifreeze was diluted. Since
aluminum sheet is still less costly than copper,
many flat-plate solar panels are made with small
copper tubing to carry the heated fluid between
the lower and upper piping headers inside the
solar panels. Wide aluminum "fins" are
mechanically pressed around the small copper
tubes, which provides good heat transfer               Drain-back solar hot water system with
between the large aluminum absorber plates and           solar photovoltaic electric system
the copper tubing without the high cost of a solid
copper absorber plate.

In warmer climates, there will be only one layer of glass covering the solar panel. The heat
absorbing plates and piping will have a high-temperature flat black paint finish. In middle
latitudes, a special black-chrome layer is electroplated onto the surface of these aluminum fins.
This is more costly than paint, but provides better heat absorption and less reflection of the
sun's rays. In extreme northern climates, a double layer of glass covering is used that includes
an insulating airspace between the glass layers to reduce heat loss from the solar panel, but
this will significantly increase system cost.

One of the newest solar heat collector technologies is the evacuated tube collector. This design
requires only an upper piping header with no bottom header.

Sloping down from this upper piping header will be a number of parallel copper tubes which are
sealed at their bottom ends. After this header and support frame have been installed, special
hollow glass tubes are installed and locked into the upper support frame. The smaller copper
tubes fit down through this hollow center of these sealed glass tubes. These glass cylinders
look like the glass canisters inside a thermos bottle, and include a mirrored surface on the rear
back half of the tubes.

This assembly of evacuated glass cylinders provides an extremely good insulator around the
copper tube fluid passages, while focusing the sun's rays directly into the copper tubing with the
mirrored back surface. Although these evacuated tube collectors can provide much higher
temperatures and need less roof area due to their higher thermal efficiency, they also require
more care during installation and better monitoring of the antifreeze fluid to prevent freezing. In
addition, most evacuated-tube solar collectors will require a way to reject excess heat when the
water storage tank has reached its high temperature limit, or these collectors will quickly
produce steam which will lead to high pressure and system damage.
Solar tanks & exchangers
As briefly mentioned earlier, commercially-made solar tanks include a built-in heat exchanger
coil. This can be installed either inside the tank near the bottom where the water is colder, or
wrapped around the outside surface of the inner tank shell and insulated by the outer shell.
System designers want to collect as much solar energy as possible during the relatively short
sun-hours each day, so they want the water as hot as possible to make it last longer during the
hours the sun is not shining. To handle the higher tank temperatures, many solar storage tanks
                                                  are lined with special concrete and are referred
                                                  to as being "stone-lined."

                                                This thicker masonry lining is substantially
                                                heavier than a standard glass-lined tank, but
                                                glass liners do not like high tank temperatures
                                                and can crack and leak, which will eventually
                                                rust out the outer steel tank. This means
                                                masonry stone-lined solar tanks can store water
                                                in the upper 180 degree range, while lasting
                                                much longer than a glass-lined tank. Obviously,
                                                this is too hot for a shower or hand washing, so
                                                a tempering valve is installed between the tank's
                                                hot water outlet and the cold water inlet. This
                                                device mixes in cold water to the hot water
                                                supply line as needed to lower the hot water
                                                temperature to a safer 120 to 130 degree range.
                                                This mixing of cold water also reduces the flow
                                                of hot water from the tank, which makes the very
                                                hot water in the tank last longer.

                                                 Unfortunately, these stone-lined tanks are very
                                                 costly to ship and are not normally stocked
                                                 locally, so some solar manufactures offer a
                                                 special "kit" to convert a standard electric hot
                                                 water tank into a solar heating tank. Although
                                                 these glass-lined tanks will have a shorter life,
    Separate stone-lined solar storage tank      they cost much less and are stocked in various
  located next to the backup gas-fired water sizes at all builder supply outlets. Most solar
                     heater
                         
                                                 conversion kits include a long slender multi-
                                                 layered heat exchanger tube that fits into the
tank by inserting through one of the extra pipe fitting openings in the tank top. This special
exchanger includes a tube inside a tube, with separate inlets and outlets for the solar-piping
loop. This allows the heat transfer fluid to pass down and back out of the exchanger tube
without mixing with the water stored in the tank.

Most building codes require any heat exchanger that uses an antifreeze solution to have a
double-wall layer between the solar fluid and the domestic water. Double-walled heat
exchangers are more expensive to manufacture and will have a slightly lower heat transfer
efficiency.
Solar controls
Except for batch heaters which have no electronic control devices, any solar system that
includes automatic valves or solar loop pumps will require a differential temperature controller.
More expensive temperature controllers will include a digital display to indicate system
temperatures and alarms, but all are based on a very simple control strategy. One temperature
sensor is mounted inside the solar panel on the roof, and one temperature sensor measures the
water temperature inside the solar storage tank.

The control concept is simple; when the solar panel sensor is hotter than the water in the tank,
a relay inside the controller is activated which turns on the pump. When both sensors read the
same, the relay opens and the pump stops. More sophisticated controllers allow the installer to
adjust these temperature setpoints to fine tune the system.

Most water tanks designed for solar applications
will include one or more separate temperature
wells to insert temperature sensors directly
through the outer tank insulation and tank wall.
Non-solar water tanks usually require strapping
the tank temperature sensor to the hot-water
piping exiting the tank.

Solar pumps
Except for a passive solar batch heater which
does not need a pump, most solar water heating
systems require at least one circulating pump,
and systems having an external heat-exchanger
may require two pumps.

Since heat rises, mounting the solar hot water
panel at an elevation above the storage tank will
require a pump to counter this reversed natural
thermal flow, and to increase system heat
transfer efficiency. If a piping loop is totally
sealed and uses chemically-treated water or
antifreeze, it may be possible to use a lower-cost
cast iron pump typically found in home hot water
boiler heating systems. However, systems using
little or no water treatment, and any system that
allows air to enter the piping each time the flow    Packaged solar conversion kit for standard
is stopped, will require a stainless-steel pump to           electric hot water tank
avoid corrosion of the working parts by the
oxygenated hot water.

The best pumps for use in residential solar applications will consist of a stainless-steel pump
impeller mounted on ceramic bearings, with the entire assembly sealed inside a stainless-steel
"can." The motor coil and electrical components are mounted outside this sealed can. The
motor coil's magnetic field, needed to rotate the pump's impeller, easily passes through the non-
magnetic stainless-steel enclosure. Since the ceramic bearings are also located inside this
sealed unit and immersed in the circulating water or fluid, this keeps the ceramic bearings
lubricated without needing sealed lubricated bearings or shaft seals that can leak. These solar
circulating pumps are typically supplied in the smaller 1/20 to 1/12 horsepower-size range, but
larger pumps are available for larger commercial applications.

Valves, gauges, and piping
Some manufacturers sell do-it-yourself solar hot water kits that provide everything except the
hot water tank, which you can purchase locally. These kits include pre-insulated flexible soft
copper or high-temperature pex tubing supplied in a twin pipe assembly, making it easy to route
the supply and return lines between the solar tank and the solar panel. Regardless of system
design, it's desirable to have an air venting device located at the highest point in the system,
along with a pressure relief valve. I also recommend installing two easy-to-read dial-type
temperature gauges side-by-side, on both the solar array supply and return piping connections
near the solar tank.

This low-tech dial temperature display makes it easy to see at a glance the temperature of the
water going to and from the solar panels without the need for electronic devices. It's also a good
idea to include isolation valves between all major components, and on each side of the pump,
to make system servicing and refilling easier. A capped pipe "tee" on each side of the pump,
along with several low point drain valves, make system servicing much easier.
Conclusions
As part of the first stimulus package signed into law by George Bush in October 2008, an
energy bill was inserted and passed that allows any homeowner installing a solar system after
January 1, 2009 to take 30% of the solar system's cost as a credit towards their Federal Income
tax. This is not a tax deduction; this is a one-for-one credit to offset any Federal Income taxes
owed. In addition, some states and local utilities offer similar incentives or rebates for the
purchase of a solar hot water system. Hawaii is the first state to require a solar hot water
system to be installed on all new homes constructed after January 1, 2010. No doubt many
other states will soon follow Hawaii's example as one more way to help reduce our nation's
energy demands.

If properly designed and installed, a solar hot water system can be a low-cost way to achieve
long-term utility savings for any homeowner. Heating hot water for showers, clothes washing,
and dishwashing is a significant part of a home's monthly utility cost. However, when similar
Federal Solar Tax Credits were enacted under Jimmy Carter in the late 1970s, many people
purchased solar hot water systems primarily to receive the huge tax credits, so some solar
system buyers didn't care if the solar systems worked or not. This lack of concern for system
performance resulted in many solar systems being poorly installed using shoddy materials.
Most systems never operated long enough to pay back their high initial cost. System prices
were also significantly inflated as another way to maximize the homeowner's tax credits, but
increased prices did not mean higher quality materials were being used.

Since almost all solar hot water systems will also include a backup gas or electric hot water
tank, it was impossible for the homeowner to tell when their solar system failed or needed
service, since there was always hot water coming out of the faucet. Today's solar systems are
    made from much higher-quality materials and components, but it is still important for the
    homeowner considering a system installation to check references of the installer and the quality
    of the components they plan to install. Also make sure the system includes gauges or metering
    equipment that will allow monitoring system performance.

    A properly designed and installed solar hot water system is an excellent investment in reducing
    your home's energy use even without these tax incentives. Solar hot water heating systems will
    no doubt become very common on all homes in the near future.

    All photos courtesy of Patrick Redgate, Ameco Solar Inc., Paramount, California.


    Jeff Yago is a licensed professional engineer and certified energy manager living in Virginia with
    more than 30 years experience in the energy conservation field. He has extensive solar thermal
    and solar photovoltaic system design experience and has authored numerous energy related
    articles and texts.
 

								
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