History
Of Solar
Energy
A common form of energy utilized
throughout the world is solar energy. Solar energy is radiant energy
that is conveyed from the sun to the earth through electromagnetic
waves. For thousands of years this form of energy has been used by
humans to keep warm and to assist with various activities. "Serious
studies of the sun and its potential began in the seventeenth
century-when Galileo and Lavoisier utilized the sun in their researches.
By 1700 diamonds had been melted and by the early 1 800s heat engines
were operating with energy supplied by the sun. In the early twentieth
century solar energy was used to power distillation plants in Chile and
irrigation pumps in Egypt."[1] By the end of the 1930's, solar
technology was used to heat the first building with converted solar
water heaters at the Massachusetts Institute of Technology. [2] The
conclusions drawn from this project and solar heating experiments in
subsequent years, aided in the development of solar hot water systems.
Today, approximately one million residential homes (one percent of all
residences) and two hundred thousand commercial buildings use solar
power to heat domestic water in the United States.[3] Through the
analysis of different solar hot water systems and their components, it
can be noted that these systems are energy efficient and cost-effective.
How
A Solar Hot Water System Works
Domestic hot water is the second highest
energy cost for the average home in North America. A solar hot water
system can reduce these costs up to sixty-five percent. [4] While there
are a number of different systems, the general operation of solar hot
water units is similar. The process begins with sunlight heating the
absorber surface on a solar collector. This solar energy is then
converted into usable heat, and is delivered through tubes attached to
the absorber to either a heat-transfer fluid or the actual
potable water to be used. The heated water is collected in a separate
pre-heater water tank or a conventional hot water tank (sized for the
number of individuals in the household or business). When additional
heat is required, it is supplied by a conventional system. A solar hot
water heater does not always replace an electric, natural gas, propane,
or an oil water heating system. It often acts as a pre-heater by raising
the temperature of the water before it enters the conventional hot water
tank. Consequently, the solar design reduces the amount of heat the
conventional system has to produce and minimizes the use of fossil fuels
or electricity.
The
Solar Collector
The collector is the main component of
the solar hot water system. Solar collectors are mounted either on the
roof of a building or on the ground close by. These devices are
dependent upon the sun and absorb direct (70 to 80 percent of total
solar energy absorbed), diffuse, and reflected radiation. [5] As a
result, many collectors are installed with reflectors, motors, and
tracking mechanisms to follow the sun as earth revolves on its axis. The
typical residence with a solar system has one to three solar collector
panels, which are commonly 4 feet wide by 6 to 10 feet long. [6] The
size of the collectors is dependent upon on the kind of collector used
and how much hot water is needed daily. In order to absorb the most
amount of the sun's energy, solar collectors have to be installed at a
certain angle (depending on the geographical latitude of the home and
the time of the year), and placed on either the south-side of the roof
in the northern hemi sphere or on north-side of the roof in the southern
hemisphere.
Each type of solar collector is designed
to prevent heat from escaping and deliver radiant energy either
directly or indirectly to a hot water storage tank. Glass or plastic
glazing on the collectors aids in this process by utilizing the
greenhouse effect. This kind of material "readily passes the
shorter wavelengths of light which are received from the sun (0.3 to 2
microns in length) but is almost opaque to reradiated heat wavelengths
(2 to 10 microns in length)."[7]
The absorber plate is crucial to the
operation of the solar collector. Absorber plates have a black surface
that is constructed of aluminum or copper (more reliable and longer life
span than aluminum). 'The temperature differential between one side of
this metal surface and the other is proportional to the amount of heat
that is transferred. When the temperature of the absorber plate
increases, radiant heat moves from the plate to a hot water storage tank
through a system of copper tubing containing a fluid (either water,
potable water, or a glycol solution). The transfer of heat from the
collector to the fluid decreases the temperature of the absorber and
allows this process to continue.'[8]
The solar collector also requires a heat
resistant material or insulation. This is used to keep radiant energy
from escaping through the back of the collector. The common types of fiber
insulation used are PU foaming and glass fiber.[9]
Types
Of Solar Collectors
The most popular type of solar collector
is the flat-plate design (see Appendix A). This form of collector
contains a shallow rectangular box with one or two layers of a
transparent glass or plastic window covering a flat black plate. The
black plate is attached to a series of tubes through which air, water,
or a different transfer fluid may pass. With its relatively inexpensive
design, the flat-plate collectors or "mid-temperature"
collectors use either diffused (on cloudy days) and/or direct
sunlight to increase the water temperature by approximately 160 degrees
Fahrenheit. These solar collectors are typically used for residential
and commercial water heating applications. [10]
Evacuated-tube is another
kind of solar collector that is utilized in hot water heating. It is
"made up of rows of parallel, transparent glass tubes. Each tube
consists of a glass outer tube and an inner tube, or absorber, covered
with a selective coating that absorbs solar energy well but inhibits
heat loss. The air is withdrawn ("evacuated") from the space
between the tubes to form a vacuum, which eliminates conductive heat
loss".[1 1] The evacuated-tube collector uses direct sunlight and
is a "high temperature" collector that raises water/steam
temperatures to a maximum of 350 degrees Fahrenheit. Since this type of
collector can produce such high temperatures, it is ideal for larger
commercial and industrial domestic hot water systems.
Concentrating or
parabolic collectors can be used for residential, commercial, and
industrial hot water applications. This type of collector requires a
tracking mechanism for constant repositioning towards the sun for direct
radiant energy. The concentrating collector is designed with parabolic
troughs that use reflective surfaces to focus solar energy on an
absorber tube or receiver containing a heat-transfer fluid. With these
"high temperature" collectors, the water/steam temperatures
can range from 300 to 600 degrees Fahrenheit.
Types
Of Solar Hot Water
Systems
Solar hot water systems are either
active or passive. Active solar water heaters are forced circulation
systems that use a pump to move the heat-transfer fluid, while passive
hot water systems rely on gravity and do not utilize a pump for
circulating the heat-transfer liquid.
For both designs, hard water used as the heat-transfer fluid can cause
various problems within the
piping. Selecting the most appropriate system depends upon the structure
of the building, geographical location of the site, the amount of
available sunlight, and the daily requirements of domestic hot water.
Active
Solar Hot Water Systems
Active solar hot water heaters are more
complex and expensive than passive systems. The active design requires a
wider range of parts including pumps (usually automatic type sensitive
to temperature differential), valves (including globe, check, drain and
pressure relief valves), heat exchangers (in some designs), expansion
tanks (for expansion of heat-transfer liquid), and controllers
(connecting the pump, collector sensor, and the discharge line). 'With
the number of components required, active water heaters cost between two
and four thousand dollars for the average home. However, this cost can
be recovered through a tax credit, and fossil fuel or electricity
savings less than ten years after installation. '[12]
The active system utilizes pumps which
require a small amount of power. "Some companies now include direct
current (DC) pumps powered by small solar-electric (photovoltaic, or PV)
panels. PV panels convert sunlight into DC electricity. Such systems
cost nothing to operate and continue to function during power
outages."[13]
Active water heaters can be divided into
two main categories: open loop (or direct) systems and closed loop (or
indirect) systems. Open loop systems use potable or household water as
the heat-transfer liquid (see Appendix B). In this design, cold
household water is moved from the bottom of the storage tank to the
solar collectors using a pump. Once the water has been directly heated
by the sun, the water is then sent back to the storage tank
inside the building where is distributed to the rest of home or business
for usage. Most open loop systems should only be used in mild
climates. If these solar hot water heaters experience cold temperatures,
loss of power (pump can not operate) or a lack of sunlight for a long
period of time, the exposed piping for the system could freeze and
potentially break. In order to minimize these effects, open loop systems
either have a valve to completely drain the water from the loop (drain
back or drain down system) or have a pump to recirculate warm water
through the system (recirculating system). While these components
alleviate the problems with mild freezing conditions, they do not
eliminate the effects of severe freezing. Consequently, another type of
solar water heater or a conventional system would have to be used.
The closed loop active system is more
diverse than the open loop design (see Appendix
C). "A closed loop
is a piping system that is both closed to the atmosphere and separated
from the storage water."[14] In this system, the domestic water is
indirectly heated by radiant solar energy. The heat transfer liquid in
the closed loop water heater is commonly a glycol-water antifreeze
mixture. This mixture is pumped up from the storage tank to the solar
collector where it is heated by sunlight. The heat-transfer fluid then
flows down to a heat exchanger in the building and delivers heat from
the glycol-water solution to the potable water collected in the storage
tank. In the heat exchanger, the antifreeze mixture uses the heat
transfer method of convection and does not directly contact or
contaminate household water.
The closed loop design with the
glycol-water solution is ideal for climates that experience cold
temperatures over a prolonged period of time. While the antifreeze
mixture is more expensive than using water as a heat-transfer
fluid, it provides the solar water heater with excellent freeze
protection.
In drain back or drain down closed loop
systems, water is used as the heat-transfer liquid in the collector
loop. This design allows for the water to be drained by gravity back to
the heat exchanger and storage tank. As a result, there is no water in
the collectors when the system is not activated. This provides freeze
protection for the drain down system when the pumps are not circulating
the water.
Passive
Solar Hot Water Systems
Passive water heaters have an
uncomplicated and inexpensive design. "The term passive is widely
used in the industry to define a heating or cooling system in which the
energy flow occurs entirely through natural means."[ 15] The
passive water heater does not require a pump to circulate the
heat-transfer fluid. Since the solar hot water tank is commonly located
on the roof with these systems, the building structure has to be strong
enough to support its weight. In addition, the passive design does not
have a number of solar, electrical, or piping components. It requires
less maintenance and is cheaper than the active design. Typically,
passive water heaters cost between one to three thousand dollars for a
residential system.
There are basically two types of passive
solar hot water systems: integral collector storage systems ("bread
box" systems or batch heaters) and thermo siphon systems.
The integral collector storage solar hot
water heater combines the storage tank and the solar collector into a
single unit (see Appendix D). In this design, a black coloured water
storage tank is placed inside the collector (an insulated box with a
transparent cover) on the roof or on the ground of the building. Cold
household or potable water is directly heated within the collector
storage system and then flows back to the building by gravity for
consumption. Integral collector storage (I.C.S.) water heaters are more
suitable for mild climates or for use during certain parts of the year.
While insulated pipes help this system withstand occasional freezing,
the design is susceptible to pipes bursting in extremely cold weather.
Consequently, I.C.S. solar water heaters can be drained and a back-up
conventional domestic hot water can be utilized under these conditions.
Thermo siphon passive systems rely on
natural convection to move the heat-transfer fluid of water through the
solar collectors to the storage tank (see Appendix
E). With this water
heater design, the storage tank is above the solar collector on the
roof. In a direct thermo siphon system, the sunlight warms up the water
directly in the collector and the water rises to the top. Once this has
occurred, the water is transferred from the top of the collector to the
top of the storage tank through a piping system. "Denser, cooler
water from the bottom of the tank sinks to the bottom of the collector
to be warmed. The warming process continues as long as there is a
difference in temperature. The water in the system just gets hotter and
hotter as it recirculates."[1 6]
In order to withstand freezing
temperatures, an indirect thermo siphon solar hot water system can be
used. A glycol-water antifreeze mixture is heated in the solar collector
and rises up to the storage tank in a closed loop. Within the storage
tank, heat is transferred from the antifreeze mixture to the household
water for usage in the building.
Benefits
Of Solar Hot
Water Heaters
Solar hot water heaters have many
economic advantages. The various types of solar water heaters permit the
consumer to select the most appropriate system to be used with their
conventional system. Although the initial cost of solar water heaters is
higher than conventional water heaters, solar energy is free. A
well-installed solar hot water heater in a home can supply sixty to
seventy percent of the energy needed to heat water for a year. [17] In
addition, the original cost of the solar heating system (providing it
has been installed correctly) will be paid off within four to eight
years following installation. [18] The price of solar water heaters is
competitive with electric water heaters, and it also economically
advantageous as a pre-heater for oil, natural gas, or propane water
heaters.
The Canadian and U.S. Governments offer
rebates and tax incentives for individuals investing in solar
technology. In Canada, the R.E.D.I. (Renewable Energy Deployment
Initiative) for businesses "will supply 25 percent of the purchase
and installation cost of a qualifying solar hot water system, to a
maximum of$50,000."[19] In the United States, programs such as the
Federal Energy Efficiency Fund of the U.S. Department of Energy
financially assist renewable energy projects. For individuals in both
countries, there are numerous tax incentives through utilities, local
and regional governments for installing solar hot water heaters.
Solar water heaters also provide certain
environmental benefits. By using solar energy for water heating,
families and businesses are not as affected by shortages or price
increases of fossil fuels or electricity. These water heating systems do
not pollute, which is significantly advantageous compared to the air
pollution and waste created by electrical, oil, propane or natural gas
water heaters.
Conclusion
Since the late 1930's, solar energy has
become an important source for heating domestic hot water. All solar hot
water systems have a collector to absorb the sun's energy and convert
that radiant energy into usable heat. However, each of these systems
differ on the type of collector used, the climate that the design is
suitable for, the kind of heat-transfer fluid, how the heat is
transferred to household water, and how this potable water is
circulated. No matter which solar hot water system is used, these
designs are highly beneficial. With sixty to seventy percent energy
savings, tax incentives, and rebates the solar water heater is very
economical. The preservation of the environment by using these systems
is also significant. Thus, the solar hot water heater is a logical
investment for any home or business.
Sources
[1]
[1] Kreider, Jan F. And Kreith, Frank. Solar Heating And
Cooling: En2ineering Practical
Design
And Economics.
p.7.
[2] Kreider, Jan F. And Kreith, Frank. Solar
Heating And Cooling. p.7. [3] Davidson, Jane. And Wood, Byard.
'Solar Hot Water For The Home', Mechanical
En2ineering.
p.1.
[4] 'Solar Boiler', http://www.thermo-dynamics.com/solarboiler.html
p.1. [5] Montgomery, Richard H. And Livingston, Jonathan L. The Solar
Water Heater
Handbook:
A Guide To Residential Solar Water Heaters. p.145. [6]
'Solar Hot Water Heater Consumer Guide', http://www.solarexi,ert.com/Heatdhw.html.
p.1.
[7] Langley, Billy C. Comfort Heatin~.
p.328. [8] Langley, Billy C. Comfort Heatin~. p.330. [9] 'Solar
Heater System Application And Advantage', http ://www. solasia-solarheater.com.tw.
p.3.
[10] 'Solar Water Heating', htti,
://www.eren.doe. gov/femp/prodtech/~w water.html. p.9.
[11] 'Solar Water Heating', http ://www.eren.doe. gov/erec/factsheets/solrwatr.html
p.2. [12] Gay, Larry. Heating The Home Water Supply. p.37.
[13] 'Solar Water Heating', http
://www.eren.doe. gov/erec/factsheets/solrwatr.html p.4.
[14] Montgomery, Richard H. And Livingston, Jonathan L. The Solar
Water Heater Handbook.
p.82.
[15] Havrella, Raymond A. Heating
Ventilating And Air Conditioning Fundamentals. p.303.
[16] Montgomery, Richard H. And
Livingston, Jonathan L. The Solar Water Heater Handbook.
p.11.
[17] Davidson, Jane. And Wood, Byard.
'Solar Hot Water For The Home' http ://memagazine.org/backissues/august96/features/solar/solar.html.
p.2.
[18] 'Solar Water
Heating', http ://www.eren.doe. gov/ere~/factsheets/solrwatr.html
p.6. [2][19] 'Solar Hot Water Systems', http://www.nrcan.gc.ca/es/erb/reed/water
e.htm. p.1.
