TECHNICAL PAPER # 49
 
                        UNDERSTANDING SOLAR
                           WATER HEATERS
 
 
 
                      Illustrated & Written By
                           Trinidad Martinez
 
                          Technical Reviewer
                           James K. Pringle
 
 
                               Published
 
                                  VITA
                    1600 Wilson Boulevard, Suite 500
                      Arlington, Virginia 22209 USA
                 Tel:  703/276-1800 * Fax:   703/243-1865
                       Internet:  pr-info@vita.org
 
 
                    Understanding Solar Water Heaters
                           ISBN:  0-86619-266-2
           [C] 1986, Volunteers in Technical Assistance
 
 
 
                              PREFACE
 
This paper is one of a series published by volunteers in
Technical Assistance to provide an introduction to specific
state-of-the-art technologies of interest to people in developing
countries.  The papers are intended to be used as
guidelines to help people choose technologies that are suitable
to their situations.  They are not intended to provide
construction or implementation details.   People are urged to
contact VITA or a similar organization for further information
and technical assistance if they find that a particular
technology seems to meet their needs.
 
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on
a purely voluntary basis.  Some 500 Volunteers were involved
in the production of the first 100 titles issued, contributing
approximately 5,000 hours of their time.   VITA staff included
Suzanne Brooks handling typesetting and layout, and
Margaret Crouch as project manager.
 
The author of this paper, VITA Volunteer Trinidad Martinez,
has extensive experience in the design and construction of
greenhouses and solar-wall air heaters, and constructed his
own 1,200 square foot adobe home in Tres Ritos, Mexico.   Mr.
Martinez was also a technical reviewer for "Understanding
Adobe."  The technical reviewer for this paper, VITA Volunteer
James Pringle, is an Associate Editor/Analyst with the Datapro
Research Corporation in Delran, New Jersey.
 
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.
VITA offers information and assistance aimed at helping individuals
and groups to select and implement technologies
appropriate to their situations.   VITA maintains an international
inquiry Service, a specialized documentation center,
and a computerized roster of volunteer technical consultants;
manages long-term field projects; and publishes a variety of
technical manuals and papers.
 
            UNDERSTANDING SOLAR WATER HEATERS
             by VITA Volunteer Trinidad Martinez
 
I.  INTRODUCTION
 
The easiest and most practical application of solar energy is
for heating water.  It has been technically feasible to heat
household water using solar energy since the 1930's.   Solar
water heaters for homes and industry have been employed
extensively in Israel, Australia, and Japan, and were quite
popular in Florida and California prior, to World War II.
 
A solar water heater consists of a solar collector, a storage
tank, and, in most cases, a system of pipes to transfer water
between them.  The solar collector is to the solar heater what
a boiler is to a conventional heater:   it heats the water or
fluid.  Depending on the technology used, solar water heating
systems can make use of pumps or natural circulation and can
use water or other fluids to conduct heat.
 
Heat from the sun's rays is easily captured by a solar
collector.  A flat piece of metal that conducts well, such as
copper, steel, or aluminum, is painted flatblack and faced
toward the sun.  The surface that heats up when exposed to the
sun is called an absorber.  As the absorber heats, it
transfers the heat to the fluid within the collector but also
loses heat to its surroundings.   To minimize this loss of
heat, the bottom and sides of this flat plate are insulated
and a glass or plastic cover is placed above the absorber
with an airspace between the two.   This glazing over a well-sealed
collector box will produce the familiar "greenhouse
effect"-allowing sunlight to enter while preventing heat from
escaping (Figure 1).

30p02.gif (540x540)


 
For the maximum collection of sunlight, a collector must face
south in the northern hemisphere or north in the southern
hemisphere.  Any variation within 20 degrees east or west is
acceptable, although a slightly westward orientation is
better since the high afternoon temperatures make the solar
collector more efficient.  Maximum efficiency will occur three
hours after solar noon.  During the winter, 90 percent of the
sun's output occurs during these times.
 
The angle that the sun's rays makes with a perpendicular
surface is called the angle of incidence.   This angle
determines the percentage of direct sunshine intercepted by a
surface.  The sun's rays that are perpendicular to the surface
of a collector are not reflected back into the atmosphere.
The collector should ideally be tilted so that it is
perpendicular to the sun's rays.   With some frames and mounts,
the collector tilt can be easily adjusted; when a fixed
position is desired, the angle of tilt can be adjusted to
latitude plus 10 degrees.
 
It is important to ensure that shadows from neighboring
buildings, trees, and the general surroundings do not shade
the collector.  No more than five percent of the collector
area should be shaded between 9:00 a.m. and 3:00 p.m.,
standard time, throughout the year.   One of the major sources
of shading is trees, so the installer should be aware of
future growth.  Chimneys, new construction, and even fences
may shade the collector, especially in the winter when the
sun makes a low arc in the sky and shadows are long.
 
II.  DESIGN VARIATIONS
 
The Solar Collector
 
There are two general types of solar collectors:   those that
make use of a large flat black plate to collect heat and
those that concentrate the sun's rays to heat a small area.
The first type is easier and cheaper to construct but offers
less efficiency.  Examples of this type include trickle-type
and flat plate collectors.  The concentrating collector is
more efficient but also more expensive.   Many varieties of
this type of collector can be built.
 
The simplest and cheapest type of collector is the trickle-type
collector.  It consists of three components:  the
absorber, the header, and the trough.   The absorber, which
stops the sunlight, converts it to heat, and transfers this
heat to the passing liquid.  As shown in Figure 2, a water

30p03.gif (540x540)


feed pipe is placed along the top edge of a sloping
corrugated sheet.  Small holes (1/32 inch in diameter) are
drilled into this header at each valley or trough.   A gutter
at the bottom of the sheet gathers warm water and returns it
to the storage tank.
 
The simplicity and low cost of the trickle-type collector
outweigh its poor performance.   With this type of collector,
it is difficult to achieve temperatures greater than 10
degrees Centigrade (18 degrees Fahrenheit) above the
surrounding air temperature unless very small flow rates are
used.  Some applications are the pre-heating of water before a
second heating stage and the heating of water for fish
culture.  The trickle-type collector is self-draining and
needs no protection against corrosion or freezing.   It is
relatively maintenance-free.
 
The most commonly used type of solar collector for solar
water heating is the flat plate collector.   The basic
components of a liquid-type flat plate collector are outlined
in Figure 1.  The absorber stops the sunlight, converts it to
heat, and transfers the heat to the passing liquid in tubing.
As the water is heated, it rises through the tubing.   A
continuous serpentine loop can be seen in some commercial
panels, but its use is limited to systems with a circulating
PUMP.
 
Tubes can be attached to the absorber plate in many ways.  The
closer the contact between the tubing and the sheet the
better the heat transfer.  As liquid is channeled through or
across a plate, heat must be conducted to these channels from
the parts of the absorber that are not touching the fluid.  If
the conductivity (the ability of a material to permit the
flow of heat) is too low, the temperatures of those parts
will rise and more heat will escape from the collector,
making it less efficient.  To reduce heat loss, the absorber
plate will have to be thicker or the tubing more closely
spaced.
 
With a  metal of high conductivity such as copper, the plate
can be thinner and the tubing spaced farther apart.   To get
the same performance an aluminum plate would have to be twice
as thick and a steel sheet nine times as thick as a copper
sheet.
 
Copper is difficult to paint, although it can be done.
Soldering copper to copper is easy.   Aluminum, on the other
hand, is very difficult to weld or solder to any metal.
Aluminum is a good choice for thermal performance, but it is
highly susceptible to corrosion.   Copper is the next choice.
Aluminum and steel are easy to obtain but even aluminum is
becoming scarce.  The best choice of metals will have to be
determined by location, availability, cost, and durability.
 
The coating of an absorber keeps the heat in the collector.
To maximize the percentage of sunlght obtained by the
absorber plate, the absorber coating must be flat black.
High-temperature black paints can be used.   The absorber
surface must be cleaned thoroughly before applying the
coating.  An acid bath assures maximum adhesion.
 
The cover plate or glazing also preserves heat.   Cover plates
are transparent sheets that sit above the absorber.   Short-wave
sunlight penetrates the glazing, becomes trapped, and is
converted to heat in the absorber.   The glazing must provide
many years of service in a wide variety of weather
conditions.  Commonly used transparent materials include
tempered  glass, fiberglass-reinforced polyester (lascolite),
and thin plastic films.  Glass is the preferred choice.  It has
good solar transmittance, allowing penetration of between 85-92
percent of sunlight striking a glass surface at vertical
incidence.
 
For higher water temperatures and a greater range of possible
applications, THE CONCENTRATING COLLECTOR IS THE BEST
POSSIBLE CHOICE.  These collectors use one or more
reflecting surfaces to concentrate sunlight onto a small
absorber area.  This multiplies the amount of energy per unit
area and makes it hotter faster.   The small absorber area
limits the heat loss.  A curved reflecting surface can reflect
incoming sunlight onto an even smaller area, such as a
blackened pipe with water running through it.   Such a focusing
collector will perform extremely well in direct sunlight but
will not work at all during cloudy or hazy skies because only
a few rays will be captured and reflected onto the blackened
pipe.  To be efficient, such a collector requires that light
hit the reflector or lens at a certain angle.   One technique
for accomplishing this is an automatic sun tracking system
linked to an electronic motor.   Simpler concentrating
collectors can be adjusted manually.   For the most part,
concentrating collectors require sophisticated design and
manufacturing techniques and are, therefore, difficult to
make.
 
The compound parabolic collector is the most simple to
construct and use.  This collector uses an array of parallel
reflecting troughs to concentrate solar radiation onto a
blackened copper tube running along the base of each trough.
with good conditions, a collector efficiency of three to
eightfold concentration is possible.   The collector operates
at 50 percent efficiency while generating 150 percent above
that of the outside air.  On cloudy or hazy days all rays
entering the trough are funneled to the absorber bottom.  With
an east-west orientation the collector need not track the
sun.  A monthly adjustment in tilt angle is sufficient.
 
Most solar water heating systems fall into four broad
categories:
 
     1)   Direct natural thermosiphon systems,
     2)   Pumped or direct systems,
     3)   Secondary fluid or heat exchange systems, or
     4)  Integral or Breadbox systems.
 
THE SOLAR HEATER
 
Most solar water heating systems fall into one of four broad
categories:  natural thermosiphon systems, pumped or "direct"
systems, secondary fluid or "heat exchange" systems, and
integral or "breadbox" systems.   A description of each of
these systems follows.
 
Natural Thermosiphon Systems
 
The oldest and most reliable method of heating water using
solar energy is the direct thermosiphon system.   This system
takes advantage of the fact that water heated in the
collector expands and becomes less dense.   The weight of the
cooler, heavier water from the storage tank displaces the
warmed water in the collector, forcing it to flow uphill into
the top of the storage tank, which is the warmest part.   The
supply pipe to the bottom of the collector should feed into
the coolest part of the tank-the bottom.   Since the weight of
the cool column of water causes the flow, the storage tank
must be located above the collector.
 
There are three basic components of a thermosiphon system and
the way in which natural circulation works.   Water flows in a
continuous system from the water source to the water outlets.
Cold water is delivered to the building under pressure from a
well or central water supply.   Gravity flow or a mechanical
pump can also provide pressure.   Transfer pipes convey the
heated water from the absorber to the storage tank.
 
The movement of water in this system is not very rapid, since
the driving force (the difference in water densities) is not
great.  The designer/builder should make sure that the system
provides unrestricted flow.  The plumbing should not have any
bends, jogs, or double backs, as these configurations
decrease the natural flow of water.   Circulation occurs only
when solar energy is available, so the system is self-controlling.
The higher the solar radiation, the greater the
heating and the more rapid the circulation.
 
The pipes must slope upward from the collector to the storage
tank so that air cannot become trapped and stop the flow.  The
rising column of hot water in the transfer pipe leaves the
collector at opening B and enters the tank at opening A.
 
Water in the collector must have a continuous upward path to
the storage tank.  Therefore, all pipes entering the tank from
the collector must have a continuous upper slope along their
full length.  This will prevent air bubbles from forming in
the cold or hot water pipes.   Air will pass through the
sloping pipes into the hot water storage tank.   A continuous
rise at an angle is better than a vertical rise followed by a
horizontal run.  Long horizontal runs should be avoided.  A
continuous upward slope (1-3 centimeters) is needed in the
system from the bottom of the storage into the top of the
tank.  This method will not work because any air in the system
will collect in the horizontal pipes and will impair the
water circulation.
 
<FIGURE 3>

30p06.gif (486x486)


One way to overcome this problem is to weld a 3/4 inch iron
pipe coupling onto the upper 1/3 portion of the tank.   The
tank drain opening can be used for the cold water inlet or
another 3/4 inch iron pipe coupling can be welded about 3
inches from the bottom of the tank.
 
If it is not possible to weld an opening to prevent trapped
air, a bleeder tube can be used to bleed air from the system.
The air bleeder line must be of small diameter to prevent a
siphoning problem which would occur with larger diameter
tubing.
 
<FIGURE 4>

30p07.gif (540x540)


 
Galvanized iron, copper, or even plastic pipe can be used to
construct the transfer pipes.   For a collector 3 square feet,
two transfer pipes one inch in diameter are needed.   If the
pipes are smaller in diameter, thermosiphoning will start
later in the morning and earlier in the afternoon because
higher collection temperatures will be needed to drive the
flow.
 
 
Transfer pipes must be well insulated in order to maintain
the high temperature of the water coming from the collector.
By locating the collector close to the tank and reducing the
length of the transfer pipes, the total surface area through
which heat can be lost is reduced.   The pipes must also be
able to withstand continuous pressure from a central water
system.
 
The storage tank is the reservoir that holds the heated water
from the collector, making it available when needed.   The tank
should be glass-lined or plastic-lined to stop rust.   The
storage tank must be located above the collector.   The minimum
effective distance between the storage tank and the collector
is 24 inches.  Below this distance, natural circulation will
not work.
 
The actual size of the tank depends on hot water consumption.
The tank should be made large enough to provide a day's hot
water needs for each person.   A sample formula for determining
tank size is as follows:
 
     1)   Hot water demands per person per day = _______gallons.
     2)   Number of people in household = ______gallons.
     3)   size of storage needed (1x2+20% (1x2))=______gallons.
 
Note:  For an all-solar system, this capacity should be
doubled.
 
Collector size is proportional to tank size.   A good general
rule is one square meter (39 1/2 square inches) of collector
area for 41 1/2 liters (11 gallons) of hot water desired.
 
The tank can be placed either horizontally or vertically,
although an upright position is best since more hot water can
be drawn before the colder water begins to flow out.   The
higher the tank is from the collector, the further the tank
can be moved horizontally away from the collector.   A
convenient formula is two horizontal feet for every foot of
vertical pipe from the collector to storage tank.
 
The storage tank must be well insulated to reduce heat
loss.  It should be, if possible, within a building.  Before
mounting the storage tank, the installer should consider that
each gallon of water weighs 8.33 pounds, and the structure on
which the tank rests should be capable of supporting the
required weight of both water and tank.
 
Whether one or two tanks are used, solar energy serves to
preheat the household water.   At night and on cloudy days a
conventional back-up heater can bring the storage tank up to
the desired temperature.  On sunny days, the back-up heater
should remain off.
 
Pumped Systems
 
The next most common solar water heating system is called a
pumped system, also called an open system or a direct system
because the system circulates potable water under utility
pressure directly through the solar collectors and into
storage.  A pumped system is used in cases where piping runs
are too long or there is no position for an elevated tank.
The storage tank is placed below the collector and the
pump-1/10 horsepower or less-is used to move the water from
the collector to the storage tank and back again.   The
collector has a higher efficiency with an assured steady flow
of water.
 
<FIGURE 5>

30p09.gif (540x540)


 
Freedom in the system layout allows for greater flexibility
in the placement of the storage tank and collector.   The
collector and its piping form a single circulation loop tied
into the host water storage tank.   In this system, for
instance, the collector can be mounted on the roof, the
storage tank in the basement, and the water faucets on the
first floor.
 
This system operates by sensing the temperature difference
between the collector and the storage tank.   A differential
thermostat that has two sensors-one near the collector outlet
and one near the tank outlet-senses when the collector is a
certain amount hotter than the storage tank (normally 8-11
degrees Fahrenheit).  This automatic controller senses the
temperature difference, turns on the pump, and circulates
water from the cold storage tank to the warm collector.   This
process continues until the temperature at the bottom of the
tank gets to within 3-5 degrees Fahrenheit of the collector
temperature.  Then the pump shuts off and circulation stops.  A
much less expensive alternative is a thermostat set to
activate the pump when the collector reaches a preset
temperature, about 130 degrees Fahrenheit, or to turn off the
pump when storage tank reaches a predetermined temperature of
about 160 degrees Fahrenheit.
 
As hot water is drawn from the back-up tank it will be
replaced with cold water, thereby reducing the temperature of
the storage tank.  When the temperature of the storage tank
drops enough to trigger the controller, the controller will
turn on the pump and start water circulating through the
collector again.  Once the tank is brought up to temperature
the system will turn off.
 
Freeze-protection for this system involves two methods.   The
first method is to drain the collector when a freeze
condition occurs and is called "drain down."   The "drain down"
method automatically drains down the collectors and any
exposed piping by operating a differential thermostat that
activates two two-way selonoid valves.   An added feature
includes a maual "drain-down" in case of a power failure.
 
The second method is called "circulating" and simply starts
the pump to circulate warm water from the storage tank to the
collector when the collector temperature reaches 38 degrees
Fahrenheit and to continue pumping until the collector
reaches about 50 degrees Fahrenheit.   In real cold climates it
is possible that all the energy in the storage tank will be
depleted in trying to keep the collector from freezing under
cold conditions and thereby cause a system failure.
 
To prevent reverse circulation or thermosiphoning at night or
on a cold day, a check valve is installed in the collector
return line to the storage tank to prevent flow from the hot
tank to the cold collector.
 
The pumped system like the thermosiphoning system can be used
with an electric or gas back-up system.   Both tank and
collector must be designed to withstand twice municipal water
pressure.
 
The pumped system is also called an active system because it
involves complex and interdependent components.   The
collectors, fluid transport systems and heat storage
containers require a network of controls, valves, pumps,
fans, and heat exchangers.  They are generally more
appropriate for apartment buildings, schools, hospitals, and
office buildings than for single-family dwellings.   In some
cases, the active system can consume more energy in running
the pumps and controls than the solar system saves.
 
Unlike the thermosiphon system, the problem of trapped air is
eliminated as the pump can force the air through the lines
into the tank where it can then be released at the faucets.
 
This type of system is generally more expensive than the
thermosiphoning system.  However, the longer life, lower risk,
and performance make this system marketable, and therefore
more appealing.
 
The most effective method of protection against freezing the
collector is the "closed system" or the use of an antifreeze
and water solution in the collector piping.   This heat
exchange fluid delivers heat from the solar collector to the
storage tank.  One form of heat exchanger is a coil of copper
tubing immersed in the storage tank.   The coil is connected to
the storage tank to form a single "closed" flow loop.
 
The major advantage of this system is its durability as a
result of reduced corrosion.   The efficiency of this system
depends on heat exchanger design, surface area, and type of
fluid used.  A glycol-based antifreeze water mixture is most
commonly used.  It is similar to that used in automobiles.
Corrosion inhibitors in the fluid protect the pipes.   However,
in time glycol-based antifreezes become corrosive and must be
replaced.
 
Operation of this system is relatively trouble-free.   However,
the designer must be careful that the fluid does not leak
into the storage tank.  Antifreeze is a very toxic substance
and must never enter the water supply.   A common way to
overcome this problem is to wrap the heat exchanger around
the storage tank.  The entire tank and heat exchanger is then
well insulated with adequate blanket insulation.
 
To control collector pressure and prevent vapor locks, some
special and expensive accessories not needed on open systems
are required on closed systems.   These include:
 
     1)   an expansion tank,
     2)   a special low-pressure relief valve,
     3)   a purge valve for use during filling,
     4)   a pressure gauge for filling,
     5)   expensive oil or antifreeze filling, and
     6)   an installer possessing special equipment for filling
         the system.
 
"Breadbox Systems"
 
An efficient type of water heater that can be used in most
areas is the "breadbox" solar water heater.   The breadbox
solar water heater combines solar collection and water
storage functions in one unit.   It is usually used to preheat
water for a conventional water heater.   Placing it as close as
possible to the back-up heater keeps pipe runs short,
reducing heat loss.  As with all solar hot water systems,
pipes carrying hot water must be well insulated.   To allow for
repairs and regular maintenance, the plumbing should be
arrranged so the breadbox can be bypassed if necessary.   The
breadbox should also be equipped with a drain.
 
<FIGURE 6>

30p12.gif (540x540)


 
The air venting valve is opened to release air from the
system when filling the tanks and to release trapped air when
in operation.  The vacuum relief valve opens to prevent a
vacuum within the system while draining it.   A temperature
gauge installed on the hot water outlet between the tank and
the house can indicate when to shut down the system to
prevent freezing.  When the temperature consistently
approaches freezing, the system should be drained.
 
Flexible copper hoses are the easiest way to connect tanks in
double tank systems.  When connecting copper tubing to steel
tanks or galvanized metal and copper, the installer should
always use dielectric connections to reduce corrosion.   The
collector storage system should be drained annually and
checked for sediment and leaks.
 
The    tank in the breadbox can be used as the storage
tank       wood fire heat exchanger, making it an ideal
combination of solar and wood water heating.   In frigid areas,
a wood fire during winter can ensure freeze protection.   With
such an auxilliary heating setup, there is a danger of
overheating and consequent steam explosion.   A pressure relief
valve must be included in the system.
 
III.  CHOOSING THE TECHNOLOGY RIGHT FOR YOU
 
The choice of a solar water heater depends upon the resources
available and the application for which it will be used.  The
heating systems can be summarized as follows.
 
The choice among solar collector technologies depends on cost
and on the amount of heat required f or the specific
application.  The trickle collector is a simple, low-cost
option, but it cannot achieve high water temperatures.   The
flat plate collector is more efficient but requires
additional expenses for tubing.   To achieve high water
temperatures, some type of concentrating collector must be
used.
 
The pumped and secondary fluid systems both offer the
advantage of continuous circulation at a more rapid speed
than the direct thermosiphon system.   However, they do require
an external source of power, and they both require pressure
valves.
 
The overall efficiency of the open pumped system is higher
than the closed secondary fluid system, and its cost is
considerably lower.  Fluid in the secondary fluid system
requires more pumping energy than does the same amount of
water in an open system.  The heat exchanger in a closed
system adds resistance to the circulation of the fluid, thus
increasing the energy required for pumping.   Furthermore, the
capacity of the fluids to absorb heat is less than that of
water, so more fluid must be pumped through the system for
the same amount of heat exchange.   On the other hand, the
secondary fluid system is extremely durable.
 
The breadbox system is a highly efficient solar water heater
for use in tandem with a conventional water heater or as the
storage tank for a wood fire heat exchanger.   It does not
require the use of transfer pipes, but does require venting
and valves to releave pressure.
 
In general, solar heating has distinct advantages over the
traditional technologies that are based on coal, oil, or gas.
As the supply of fossil fuels dwindles, environmental damage
further adds to the indiscriminate use of these fuels.
Nuclear power threatens to bring a new round of pollution and
waste.  An answer to energy deprivation is the use of solar
energy to heat and cool our homes as well as to supply a
percentage of the world's electrical and hot water needs.
 
                       BIBLIOGRAPHY
 
Anderson, B. and Riordan, M.   The Solar Home Book, 1976.
 
Fritz, D.  "Solar Water Heater," Village Technology Handbook,
     VITA Inc.   (1970).
 
Hasting, A.  Solar Water Heaters, 1970.
 
Living Alternatives.  Can You Use a Solar Water Heater?  Vol.
     2, No. 4, Jan. 1981.
 
National Center for Appropriate Technology, Breadbox Solar
Hot Water Systems.  023758.  XVIII-DE-2, p.3.
 
New Mexico Solar Energy Institute Solar Fact Sheet.   Breadbox
     Solar Water Heaters.  1980 XVIII-DE-2, p.3.
 
Praudyogiki, G.  Solar Water Heating, Vol 1, No. 2. 1981.
 
Ridenour, S.  Homemade Solar WAter Heaters, ND 221-239, XVIII-DE-2,
     p.3.
 
Schumacher, D., McVeigh, C.  Solar Water Heaters.  007545
     XVIII-DE-2, p.3.
 
Sussman, A., Frazier, R.  Handmade Hot Water Systems, 1978.
 
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