Solar Domestic Hot Water Heating Systems Design

Solar Domestic Hot Water Heating Systems
Design, Installation and Maintenance
Presented by:
Christopher A. Homola, PE
A Brief History of Solar Water Heating
Solar water heating has been around for many years because it is the
easiest way to use the sun to save energy and money. One of the earliest
documented cases of solar energy use involved pioneers moving west
after the Civil War. They would place a cooking pot filled with cold water in
the sun all day to have heated water in the evening.
The first solar water heater that resembles the concept still in use today
was a metal tank that was painted black and placed on the roof where it
was tilted toward the sun. The concept worked, but it usually took all day
for the water to heat, then, as soon as the sun went down, it cooled off
quickly because the tank was not insulated.
A Brief History of the American Solar Water Heating Industry
1890 to 1930's - the California Era
The first commercial solar water heater was introduced by Clarence Kemp in the
1890's in California. For a $25 investment, people could save about $9 a year in coal
costs. It was a simple batch type solar water heater that combined storage and
collector in one box.
The first thermosyphon systems with the tank on the roof and the collector below
were invented, patented, and marketed in California in the 1920's by William Bailey.
One of the largest commercial systems in California was installed for a resort in
Death Valley.
Natural gas was discovered in Southern California and cheap natural gas,
aggressively marketed by utility companies, ended the solar water heating market.
Patents were sold to a Florida company, owned by HM Carruthers in 1923 and the
solar hot water industry began in the coastal cities of central Florida and southern
Florida.
1930's to 1973 - the South Florida Era
Floridians purchased or shipped to the Caribbean more than 100,000
thermosyphon water heaters between 1930 and 1954 when the industry
collapsed. During the second World War (1942 to 1945) copper was
reserved for the military and the solar industry was not able to make solar
collectors.
After the war, the Florida industry boomed again for about six years. Half of
Miami homes had solar water heaters with over 80% of new homes having
them installed. In the early 1950's electricity became cheap in Florida and
utility companies gave away electric water heaters in an effort to eliminate
the solar water heating industry.
By 1973, there were only two full-time solar water heating companies left in
the United States both operating out of Miami, Florida.
1973 to 1986 - Oil Embargo and Tax Credits
The oil embargo of 1973 resulted in a rise in fuel prices. A few companies
started experimenting with solar water heaters and designing systems but there
were really no national solar collector manufacturers with widespread
distribution until the late seventies.
The federal government sponsored a few HUD Grants for domestic solar water
heaters in the period just before the start of the 40% Federal tax rebate in 1979.
The tax credit era, 1979 to 1986, started a nationwide boon in solar hot water
systems that resulted in hundreds of manufacturers and thousands of
contractors and distributors starting new businesses.
Equipment has improved since the 1980’s. Improvements were
precipitated by both certification design review and experienced
installers.
Today, more than 1.2 million buildings have solar water heating
systems in the United States. Japan has nearly 1.5 million buildings
with solar water heating. In Israel, 30 percent of the buildings use solarheated water. Greece and Australia are also leading users of solar
energy.
There is still a lot of room for expansion in the solar energy industry.
There are no geographical constraints.
For colder climates,
manufacturers have designed systems that protect components from
freezing conditions. Wherever the sun shines, solar water heating
systems can work. The designs may be different from the early solar
pioneers, but the concept is the same.
Environmental Benefits
 Solar water heaters do not pollute.  Solar water heaters help to avoid carbon dioxide, nitrogen oxides, sulfur
dioxide, and the other air pollution and wastes created when the local utility
generates power or fuel is burned to heat domestic water.  When a solar water heater replaces an electric water heater, the electricity
displaced over 20 years represents more than 50 tons of avoided carbon
dioxide emissions alone. Long‐Term Benefits
 Solar water heaters offer long‐term benefits that go beyond simple
economics. In addition to having free hot water after the system has paid for itself in
reduced utility bills, owners could be cushioned from future fuel
shortages and price increases.  Solar water heaters can assist in reducing this country's dependence on
foreign oil.  It is estimated that adding a solar water heater to an existing home raises
the resale value of the home by the entire cost of the system.
Homeowners may be able to recoup their entire investment they sell
their home.
Economic Benefits
Many home builders choose electric water heaters because they are easy to install and relatively inexpensive to purchase. However, research shows that an average household with an electric water heater spends about 25% of its home energy costs on heating water.
It makes economic sense to think beyond the initial purchase price and consider lifetime energy costs, or how much you will spend on energy to use the appliance over its lifetime. The Florida Solar Energy Center studied the potential savings to Florida homeowners of common water‐heating systems compared with electric water heaters. It found that solar water heaters offered the largest potential savings, with solar water‐heater owners saving as much as 50% to 85% annually on their utility bills over the cost of electric water heating.
Economic Benefits Continued
A solar hot water heater heats the same amount of water for a fraction of the
cost. A solar hot water heating system’s performance is dependent on the
intensity of the sun in its location. The initial expense of installing a solar hot
water heater ($3500 to $5500) tends to be greater than installing an electric ($450
to $650) or gas ($750 to $1000) water heater.
The costs vary from region to region. Depending on the price of fuel sources, the
solar water heater can be more economical over the lifetime of the system than
heating water with electricity, fuel oil, propane, or even natural gas because the
fuel (sunshine) is free.
Economic Benefits Continued
However, at the current low prices of natural gas, solar water heaters cannot
compete with natural gas water heaters in most parts of the country except
in new home construction. Although you will still save energy costs with a
solar water heater because you won't be buying natural gas, it won't be
economical on a dollar‐for‐dollar basis.
Paybacks vary widely, but you can expect a simple payback of 4 to 8 years on
a well‐designed and properly installed solar water heater. You can expect
shorter paybacks in areas with higher energy costs. After the payback
period, you accrue the savings over the life of the system, which ranges from
15 to 40 years, depending on the system and how well it is maintained.
Economic Benefits Continued
You can determine the simple payback of a solar water heater by first
determining the net cost of the system. Net costs include the total installed
cost less any tax incentives or utility rebates. After you calculate the net
cost of the system, calculate the annual fuel savings and divide the net
investment by this number to determine the simple payback.
An example: Your total utility bill averages $160 per month and your water
heating costs are average (25% of your total utility costs) at $40 per month.
If you purchase a solar water heater for $2,000 that provides an average of
60% of your hot water each year, that system will save you $24 per month
($40 x 0.60 = $24) or $288 per year (12 x $24 = $288). This system has a
simple payback of less than 7 years ($2,000 ÷ $288 = 6.9).
For the remainder of the life of the solar water heater, 60% of the hot water will be free, saving $288 each year. You will need to account for some operation and maintenance costs, which are estimated at $25 to $30 a year. This is primarily to have the system checked every 3 years.
If you are building a new home or refinancing your present home to do a major renovation, the economics are even more attractive. The cost of including the price of a solar water heater in a new 30‐year mortgage is usually between $13 and $20 per month. The portion of the federal income tax deduction for mortgage interest attributable to the solar system reduces that amount by about $3 to $5 per month. If your fuel savings are more than $15 per month, the investment in the solar water heater is profitable immediately.
Peak Power Benefit
A typical residential solar water heating system (SWHS) for a family of
four delivers 4 kilowatts of electrical equivalent thermal power when
under full sun and when the temperature of the water in the storage
tank is about the same as the air temperature. Such a system
typically has about 64 square feet of solar collector surface area and
produces approximately the same peak power as 400 square feet of
photovoltaic panels.
Production Capacity Benefit
Ratings of collectors and systems, along with other information
specific to the local area, can be used to calculate the specific
reduction in a utility’s peak demand. On average, for every solar
water heating system that is installed, 0.5 kilowatts of peak
demand is deferred from a utility’s load.
Energy Production Benefit
Because peak performance occurs infrequently, a more realistic
indication of solar thermal system performance is the rated daily
energy output of the collectors or system.
Using this method, a typical solar water heating system contributes
7 to 10 kilowatt-hours per day, depending on the solar resource and
type of collector.
Electric water heating for residential applications typically
consumes about 12 kilowatt-hours per day, depending on ground
water temperature.
Annual site-specific energy savings for domestic water heating
systems are available at www.solar-rating.org for all systems
certified by the Solar Rating and Certification Corporation (SRCC).
Using this data, a typical solar water heating system produces
about 3,400 kilowatt-hours per year, depending on local conditions
and type of collector.
What Influences the Amount of Solar Radiation?
•Atmosphere
•Angle of Incidence
•Geography
•Latitude and Season
•Air Pollution and Natural Haze
Atmosphere
The atmosphere absorbs certain wavelengths of light more than others. The exact spectral
distribution of light reaching the earth's surface depends on how much atmosphere the light
passes through, as well as the humidity of the atmosphere. In the morning and evening, the
sun is low in the sky and light waves pass through more atmosphere than at noon. The
winter sunlight also passes through more atmosphere versus summer. In addition, different
latitudes on the earth have different average “thicknesses” of atmosphere that sunlight must
penetrate. The figure below illustrates the atmospheric effects on solar energy reaching the
earth. Clouds, smoke and dust reflect some solar insolation back up into the atmosphere,
allowing less solar energy to fall on a terrestrial object. These conditions also diffuse or
scatter the amount of solar energy that does pass through.
Angle of Incidence
The sun’s electromagnetic energy travels in a straight line. The angle
at which these rays fall on an object is called the angle of incidence. A
flat surface receives more solar energy when the angle of incidence is
closer to zero (i.e. perpendicular) and therefore receives significantly
less in early morning and late evening. Because the angle of incidence
is so large in the morning and evening on earth, about six hours of
“usable” solar energy is available daily. This is called the “solar
window.”
Absorptance vs. Reflectance
Certain materials absorb more insolation than others. More absorptive
materials are generally dark with a matte finish, while more-reflective
materials are generally lighter colored with a smooth or shiny finish.
The materials used to absorb the sun's energy are selected for their
ability to absorb a high percentage of energy and to reflect a minimum
amount of energy. The solar collector's absorber and absorber coating
efficiency are determined by the rate of absorption versus the rate of
reflectance. This in turn, affects the absorber and absorber coating's
ability to retain heat and minimize emissivity and reradiation. High
absorptivity and low reflectivity improves the potential for collecting
solar energy.
Collecting and Converting Solar Energy
Solar collectors capture the sun’s electromagnetic energy and
convert it to heat energy. The efficiency of a solar collector
depends not only on its materials and design but also on its
size, orientation and tilt.
Available solar energy is at its maximum at noon, when the sun
is at its highest point in its daily arc across the sky. The sun's
daily motion across the sky has an impact on any solar
collector's efficiency and performance in the following ways.
1.Since the angle of incidence of the solar energy – measured
from the normal (right angle) surface of the receiving surface –
changes throughout the day, solar power is lower at dawn and
dusk. In reality, there are only about 6 hours of maximum
energy available daily.
2.The total energy received by a fixed surface during a given
period depends on its orientation and tilt and varies with weather
conditions, time of day and season.
Insolation
Insolation is the amount of the sun’s electromagnetic energy that
“falls” on any given object.
Simply put, when we are talking about solar radiation, we are
referring to insolation.
In Florida (at about sea level), an object will receive a maximum of
around 300 Btu/ft2hr (about 90 watts/ft2 or 950 watts/meter2) at high
noon on a horizontal surface under clear skies on June 21 (the day
of the summer equinox).
PV Solar Radiation (Flat Plate, Facing South,
Latitude Tilt)—Static Maps
These maps provide monthly average daily total
solar resource information on grid cells of
approximately 40 km by 40 km in size. The
insolation values represent the resource available
to a flat plate collector, such as a photovoltaic
panel, oriented due south at an angle from
horizontal to equal to the latitude of the collector
location.
Resource:
National Renewable Energy Laboratory
www.nrel.gov/gis/solar.html
Optimum Performance Considerations Optimum Tilt:
• To latitude for greatest performance or up to latitude minus 5 degrees.
• Optimum Summer Load: Latitude minus 15 degrees (e.g. solar air conditioning).
• Optimum Winter Load: Latitude plus 15 degrees (e.g. solar space heating).
Optimum Azimuth:
• Toward the equator (e.g. Facing south in northern hemisphere).
Figure 1. Sun Path Diagrams for 28º N. Latitude
Seasonal Variations
The dome of the sky and the sun’s path at various times of
the year are shown in Figure 1.
Figure 2a And 2b. Collected Energy Varies with Time of Year And Tilt
For many solar applications, we want maximum annual energy harvest. For others, maximum
winter energy (or summer energy) collection is important. To orient the flat-plate collector
properly, the application must be considered, since different angles will be “best” for each
different application.
Actual Collector Orientation Possibilities
Collector Orientation
Collectors work best when facing due south. If roof lines or other factors dictate
different orientations, a small penalty will be paid, as shown in Figure 3. As an
example: for an orientation 20 degrees east or west of due south, we must increase
the collector area to 1.06 times the size needed with due south orientation (dashed
line on Figure 3) to achieve the same energy output. The orientation angle away
from due south is called the azimuth and, in the Northern Hemisphere, is plus if the
collector faces toward the east and minus if toward the west.
Figure 3. Glazed Collector Orientations
Tilt Angle
The best tilt angle will vary not only with the collector’s
geographical location but also with seasonal function. Solar
water heating systems are designed to provide heat year-round.
In general:
A)Mounting at an angle equal to the latitude works best for yearround energy use.
B)Latitude minus 15 degrees mounting is best for summer energy
collection.
C)Latitude plus 15 degrees mounting is best for winter energy
collection.
Various Collector Tilt Angles
Solar Water Heating System Basics
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: Active, which have circulating pumps and controls, and Passive, which don’t.
Most solar water heaters require a well‐insulated storage tank.
 Solar storage tanks have an additional outlet and inlet connected to and from the solar collector. In two‐tank systems, the solar water heater preheats water before it enters the conventional water heater. In one‐tank systems, the back‐up heater is combined with the solar storage in one tank.
Electric Back-Up
Solar systems with single tanks are designed to encourage
temperature stratification so that when water is drawn for service, it is
supplied from the hottest stratum in the tank (i.e. top of tank).
While a solar system tank in the United States normally contains a
heating element, the element is deliberately located in the upper third
of the tank.
The electric element functions as back-up when solar energy is not
available or when hot water demand exceeds the solar-heated supply.
Natural Gas Back-Up
Natural gas back-up systems may use passive (thermosyphon or
integral collector system) solar preheating plumbed in series for
proper operation.
Or two separate tanks may be used for active solar systems with
natural gas back-up heating systems.
The solar storage tank is piped in series to the auxiliary tank sending
the hottest solar preheated water to the gas back-up tank.
Solar Collectors
Four types of solar collectors are used for residential applications:
 Flat‐plate collector
Integral collector‐storage systems
 Batch system
Evacuated‐tube solar collectors
Flat‐Plate Collector
Flat plate collectors are designed to heat water to medium temperatures (approximately 140 degrees Fahrenheit). Flat plate collectors typically include the following components:
1.Enclosure: A box or frame that holds all the components together.
2.Glazing: A transparent cover over the enclosure that allows the sun’s rays to
pass through to the absorber. Most glazing is glass but some designs use clear
plastic.
3.Glazing Frame: Attaches the glazing to the enclosure. Glazing gaskets prevent
leakage around the glazing frame and allow for contraction and expansion.
4.Insulation: Material between the absorber and the surfaces it touches that
blocks heat loss by conduction thereby reducing the heat loss from the collector
enclosure.
5.Absorber: A flat, usually metal surface inside the enclosure that, because of its
physical properties, can absorb and transfer high levels of solar energy.
6.Flow Tubes: Highly conductive metal tubes across the absorber through which
fluid flows, transferring heat from the absorber to the fluid.
Integral Collector Storage (ICS) Systems
In other solar water heating systems the collector and storage
tank are separate components. In an integral collector storage
(ICS) system, both collection and solar storage are combined
within a single unit. Most ICS systems store potable water
inside several tanks within the collector unit. The entire unit is
exposed to solar energy throughout the day. The resulting
water is drawn off either directly to the service location or as
replacement hot water to an auxiliary storage tank as water is
drawn for use.
Cutaway of an ICS system
Batch System
Batch solar water heater
The simplest of all solar water heating systems is a
batch system.
It is simply one or several storage tanks coated with
black, solar-absorbing material in an enclosure with
glazing across the top and insulation around the other
sides.
It is the simplest solar system to make. When exposed
to sun during the day, the tank transfers the heat it
absorbs to the water it holds.
The heated water can be drawn directly from the tank
or it can replace hot water that is drawn from an interior
tank inside the building.
Evacuated Tube Solar Collectors
This type of system features parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for commercial applications.
Evacuated-tube collectors generally have a smaller solar collecting surface
because this surface must be encased by an evacuated glass tube. They
are designed to deliver higher temperatures (approximately 300 degrees
Fahrenheit). The tubes themselves comprise the following elements:
1.Highly tempered glass vacuum tubes, which function as both glazing and
insulation.
2.An absorber surface inside the vacuum tube. The absorber is surrounded
by a vacuum that greatly reduces the heat loss.
Active Solar Water Heating Systems
There are two Solar Water Heating System types: Active and Passive
There are two types of Active Solar Water Heating Systems:
 Direct Circulation Systems
 Indirect Circulation Systems
Direct Circulation Systems
Pump circulates domestic water through the collector(s) and into the building. This type of system works well in climates where it rarely freezes.
Direct Pumped System
Direct System with Photovoltaic‐Powered Pump
Direct System with Automatic Drain-down system configuration
The direct pumped system has one or more solar energy collectors installed on the roof and a storage tank located somewhere within the building. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun’s heat is transferred directly to the potable water circulating through the collector and storage tank. Neither an anti‐freeze nor heat exchanger is involved.
This system has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 15‐20°F warmer than the water in the storage tank, the pump is turned on by the controller. When the temperature difference drops to about 3‐5°F, the pump is turned off.
In this way, the water always gains heat from the collector when the pump operates.
A flush‐type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector.
The collector should always allow for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves.
Automatic recirculation is another means of freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the storage tank.
Direct System Advantages
• Service water used directly from collector loop.
• No heat exchanger – more efficient heat transfer to storage.
• Circulation pump (if needed) needs only to overcome friction
losses – system pressurized.
Direct System Disadvantages
• Quality of service water must be good to prevent corrosion, scale
or deposits in components.
• Freeze protection depends on mechanical valves.
• Recommended in climates with minimal/no freeze potential, and good water quality.
Indirect Circulation Systems
Pump circulates a non‐freezing, heat transfer fluid through the collector(s) and a heat exchanger. This heats the water that then flows into the home. This type of system works well in climates prone to freezing temperatures.
Indirect Pumped System Using Anti‐Freeze Solution
This system design is common in northern climates, where freezing weather occurs more frequently. An anti‐freeze solution circulates through the collector, and a heat exchanger transfers the heat from the anti‐freeze solution to the storage tank water. When toxic heat exchanger fluids are used, a double‐walled exchanger is required. Generally, if the heat exchanger is installed in the storage tank, it should be located in the lower half of the tank.
A heat transfer solution is pumped through the collector in a closed loop. The loop includes the collector, connecting piping, the pump, an expansion tank and a heat exchanger. A heat exchanger coil in the lower half of the storage tank transfers heat from the heat transfer solution to the potable water in the solar storage tank. An alternative of this design is to wrap the heat exchanger around the tank. This keeps it from contact with the potable water.
The differential controller, in conjunction with the collector and tank sensors, determines when the pump should be activated to direct the heat transfer fluid through the collector. The photovoltaic panel located on the roof supplies the power to operate the circulating pump. Indirect Pumped System Using Anti‐Freeze Solution and Wrap Around Heat Exchanger
A fail‐safe method of ensuring that collectors and collector loop piping never freeze is to remove all the water from the collectors and piping when the system is not collecting heat. This is a major feature of the drain back system. Freeze protection is provided when the system is in the drain mode. Water in the collectors and exposed piping drains into the insulated drain‐back reservoir tank each time the circulating pump shuts off. A slight tilt of the collectors is required in order to allow complete drainage. A sight glass attached to the drain‐back reservoir tank shows when the reservoir tank is full and the collector has been drained.
In this particular system, distilled water is recommended to be used as the collector loop fluid‐transfer solution. Using distilled water increases the heat transfer characteristics and prevents possible mineral buildup of the transfer solution.
When the sun shines again, the circulating pump is activated by a differential controller. Water is pumped from the reservoir to the collectors, allowing heat to be collected. The water stored in the reservoir tank circulates in a closed loop through the collectors and a heat exchanger at the bottom of the storage tank.
The heat exchanger transfers heat from the collector loop fluid to the potable water located in the storage tank.
Indirect System Advantages
• Freeze protection provided by antifreeze fluid or drainback.
• Collector/piping protected from aggressive water.
Indirect System Disadvantages
• Must account for reduced heat transfer efficiency through heat exchanger.
• Added materials = added cost.
• If not using water, fluids require maintenance.
• Most designs require added pumping cost.
Passive Solar Water Heaters
Passive solar water heaters rely on gravity and the tendency for water
to naturally circulate as it is heated.
Passive solar water heater systems contain no electrical components,
are generally more reliable, easier to maintain, and possibly have a
longer work life than active solar water heater systems.
The two most popular types of passive solar water heater systems
are: Integral-Collector Storage (ICS) andThermosyphon systems.
Integral Collector Storage System
In an integral collector storage system, the hot water storage system is the collector. Cold water flows progressively through the collector where it is heated by the sun. Hot water is drawn from the top, which is the hottest, and replacement water flows into the bottom. This system is simple because pumps and controllers are not required. On demand, cold water from the building flows into the collector and hot water from the collector flows to a standard hot water auxiliary tank within the building.
A flush‐type freeze protection valve is installed in the top piping near the collector. As temperatures near freezing, this valve opens to allow relatively warm water to flow through the collect to prevent freezing. In areas of the country, the thermal mass of the large water volume within the integral collector storage collector provides a means of freeze protection.
Thermosyphon System
As the sun shines on the collector, the water inside the collector flow‐
tubes is heated. As it heats, this water expands slightly and becomes lighter than the cold water in the solar storage tank mounted above the collector. Gravity then pulls heavier, cold water down from the tank and into the collector inlet. The cold water pushes the heated water through the collector outlet and into the top of the tank, thus heating the water in the tank.
In a thermosiphon system there is no need for a circulating pump and controller. Potable water flows directly to the tank on the roof. Solar heated water flows from the rooftop tank to the auxiliary tank installed at ground level whenever water is used with the building.
The thermosiphon system features a thermally operated valve that
protects the collector from freezing. It also includes isolation valves, which allow the solar system to be manually drained in case of freezing conditions, or to be bypassed completely.
Typical Components of a Direct Flat Plate Collector System
AIR VENT
Allows air that has entered the system to escape, and in turn prevents air locks that would
restrict flow of the heat-transfer fluid. An air vent must be positioned vertically and is usually
installed at the uppermost part of the system. In active direct systems supplied by pressurized
water, an air vent should be installed anywhere air could be trapped in pipes or collectors.
Indirect systems that use glycol as the heat-transfer fluid use air vents to remove any dissolved
air left in the system after it has been pressurized or charged with the heat-transfer fluid. Once
the air has been purged in these indirect systems, the air vent mechanism is manually closed.
TEMPERATURE-PRESSURE RELIEF VALVE
Protects system components from excessive pressures and temperatures. A pressuretemperature relief valve is always plumbed to the solar storage (as well as auxiliary) tank. In
thermosiphon and ICS systems, where the solar tanks are located on a roof, these tanks may
also be equipped with a temperature-pressure relief valve since they are in some jurisdictions
considered storage vessels. These valves are usually set by the manufacturer at 150 psi and
210° F. Since temperature pressure relief valves open at temperatures below typical collector
loop operating conditions, they are not commonly installed in collector loops.
PRESSURE RELIEF VALVE
Protects components from excessive pressures that may build up in system plumbing. In any
system where the collector loop can be isolated from the storage tank, a pressure relief valve
must be installed on the collector loop. The pressure rating of the valve (typically 125 psi) must
be lower than the pressure rating of all other system components, which it is installed to protect.
The pressure relief valve is usually installed at the collector.
PRESSURE GAUGE
Is used in indirect systems to monitor pressure within the fluid loop. In both direct and
indirect systems, such gauges can readily indicate if a leak has occurred in the system
plumbing.
VACUUM BREAKER
Admits atmospheric pressure into system piping, which allows the system to drain. This
valve is usually located at the collector outlet plumbing but also may be installed anywhere
on the collector return line. The vacuum breaker ensures proper drainage of the collector
loop plumbing when it is either manually or automatically drained. A valve that incorporates
both air vent and vacuum breaker capabilities is also available.
ISOLATION VALVES
These valves are used to manually isolate various subsystems. Their primary use is to
isolate the collectors or other components before servicing.
DRAIN VALVES
Used to drain the collector loop, the storage tank and, in some systems, the heat exchanger
or drain-back reservoir. In indirect systems, they are also used as fill valves. The most
common drain valve is the standard boiler drain or hose bib.
CHECK VALVES
Allow fluid to flow in only one direction. In solar systems, these valves prevent
thermosiphoning action in the system plumbing. Without a check valve, water that cools in the
elevated (roof-mounted) collector at night will fall by gravity to the storage tank, displacing
lighter, warmer water out of the storage tank and up to the collector. Once begun, this
thermosiphoning action can continue all night, continuously cooling all the water in the tank. In
many cases, it may lead to the activation of the back-up-heating element, thereby causing the
system to lose even more energy.
FREEZE-PROTECTION VALVES
Are set to open at near freezing temperatures, and are installed on the collector return line in
a location close to where the line penetrates the roof.
Warm water bleeds through the collector and out this valve to protect the collector and pipes
from freezing. A spring-loaded thermostat or a bimetallic switch may control the valve.
TEMPERATURE GAUGES
Provide an indication of system fluid temperatures.
A temperature gauge at the top of the storage tank indicates the temperature of the hottest
water available for use.
Temperature wells installed at several points in the system will allow the use of a single
gauge in evaluating system operation.
Selecting a Solar Water Heating System
Investigate local codes, covenants, and regulations. Consider the economics of a solar water heating system.
 Evaluate the site’s solar resource.
Determine the correct system size.
Estimate and compare system costs.
Building Codes, Covenants, and Regulations for
Solar Water Heating Systems
 Before installing a solar water heating system, you should investigate local building
codes, zoning ordinances, and subdivision covenants, as well as any special regulations
pertaining to the site. A building permit will probably be required to install a solar energy
system onto an existing building.
 Not every community or municipality initially welcomes renewable energy installations.
Although this is often due to ignorance or the comparative novelty of renewable energy
systems, compliance with existing building and permit procedures to install a system is
unavoidable.
 The matter of building code and zoning compliance for a solar system installation is
typically a local issue. Even if a statewide building code is in effect, it's usually enforced
locally by the city, county, or parish. Common problems owners have encountered with
building codes include the following:
Exceeding roof load
Unacceptable heat exchangers
Improper wiring
Unlawful tampering with potable water supplies.
Building Codes, Covenants, and Regulations for
Solar Water Heating Systems Continued
 Potential zoning issues include the following:
Obstructing sideyards
Erecting unlawful protrusions on roofs
Siting the system too close to streets or lot boundaries.
 Special area regulations—such as local community, subdivision, or
homeowner's association covenants—also demand compliance. These
covenants, historic district regulations, and flood-plain provisions can
easily be overlooked.
Renewable Energy Funding Sources
The Database of State Incentives for Renewables & Efficiency (DSIRE) is
a comprehensive source of information on state, local, utility, and federal
incentives that promote renewable energy and energy efficiency. The
website is http://www.dsireusa.org.
Federal Level Funding
Federal Incentives for Renewable Energy
 U.S. Department of Treasury - Renewable Energy Grants
Eligible Renewable Technologies:
Solar Water Heating, Solar Space Heating, & Photovoltaic Systems
 Energy Efficient Mortgages
Federal Housing Authority (FHA) & Veterans Affairs (VA) programs
Eligible Renewable Technologies:
Solar Water Heating, Solar Space Heating, & Photovoltaic Systems
State Level Funding
State of Ohio Incentives for Renewable Energy
 Ohio Department of Development - Advanced Energy Program Grants
- Multi-Family Residential Solar Thermal Incentive
Eligible Renewable Technologies:
Solar Water Heating & Solar Space Heating Systems
Applicable Sectors: Multi-Family Residential, Low-Income Residential
 Ohio Department of Development - Advanced Energy Program Grants
- Non-Residential Renewable Energy
Eligible Renewable Technologies:
Solar Water Heating, Wind, & Photovoltaic Systems
Applicable Sectors: Commercial, Industrial, Nonprofit, Schools, Local
Government, State Government, Agricultural, Institutional
Site Assessment
Solar Path Finder
http://www.solarpathfinder.com
Collector Positioning
Flat-plate collectors for solar water heating are generally mounted on a building or the ground in a fixed
position at prescribed angles. The angle will vary according to geographic location, collector type and use of
the absorbed heat.
Since residential hot water demand is generally greater in the winter than in the summer, the collector
ideally should be positioned to maximize wintertime energy collection, receiving sunshine during the middle
six to eight daylight hours of each day. Minimize shading from other buildings, trees or other collectors. Plan
for lengthening winter shadows, as the sun's path changes significantly with the seasons.
Ideally, the collector should face directly south in the northern
hemisphere and directly north in the southern hemisphere.
However, facing the collector within 30° to 45° either east or west of due
south or north reduces performance by only about 10 percent.
A compass may be used to determine true south or north.
The closer to the equator, the less the need to maintain the orientation
and direction of the collector, but be aware of the seasonal position of
the sun in the sky and how it may affect the seasonal performance of
the system.
The optimum tilt angle for the collector is about the same as the site's
latitude plus or minus 15°. An inexpensive inclinometer will aid in
determining tilt angles. If collectors will be mounted on a sloped roof,
check the roof's inclination to determine whether the collectors should be
mounted parallel to the roof or at a different tilt. In general, collectors
should be mounted parallel to the plane of a sloped roof unless the
performance penalty is more than 30 percent. The mounted collector
should not detract from the appearance of the roof.
Total length of piping from collector to storage should not exceed 100
feet. The longer the pipe run, the greater the heat loss. If a greater length
is necessary, an increase in piping diameter or pump size may be
required.
If the collectors will be roof-mounted, they should not block drainage or
keep the roof surface from properly shedding rain. Water should not
gather or pool around roof penetrations. Roof curbs may be require.
To Estimate Shading of a Rooftop/Pole Mount on the Future Site
To Estimate Needed Pole Height to Avoid Shading
To Estimate How Much to Crop Tree to Avoid Shading
During the site visit, the assessor should provide:
 A basic analysis of the project’s energy needs.
 Recommendations for energy efficiency in order to reduce the size and cost of the proposed renewable energy system.
 Provide an evaluation of the renewable energy resource at the site.
 Information regarding the best place to site the solar system.
 Additionally, the assessor should follow‐up with a written report detailing the site assessment information.
Site Assessment Benefits
A renewable energy site assessment conducted by a certified assessor provides an opportunity to discuss with an experienced,
objective third party about the characteristics of the property and learn about a variety of equipment and options. A site assessment is essential when considering a solar project.
The site assessors report can be used to present a summary of information and options to decision makers for their approval.
Cost of a Renewable Energy Site Assessment
Certified assessors establish their own fees for their services.
On average, the full cost of an assessment is between $300 and $500. The cost varies depending on the number of technologies being assessed and the complexity of the system, as well as the assessor’s travel costs. When arranging for a site assessment, discuss with the assessor your expectations so that you can receive an accurate cost estimate.
Sizing the Solar Hot Water Heating System
Just as you have to choose a 30‐, 40‐, or 50‐gallon conventional water heater, you need to determine the right size solar water heater to install. Sizing a solar water heater involves determining the total collector area and the storage volume required to provide 100% of your household's hot water during the summer. Solar‐
equipment experts use worksheets or special computer programs to assist you in determining how large a system you need.
Solar storage tanks are usually 50‐, 60‐, 80‐, or 120‐gallon capacity. A small (50 to 60 gallon) system is sufficient for 1 to 3 people, a medium (80‐gallon) system is adequate for a 3‐ or 4‐person household, and a large (120‐gallon) system is appropriate for 4 to 6 people.
A rule of thumb for sizing collectors: allow about 20 square feet of collector area for each of the first two family members and 8 square feet for each additional family member if you live in the Sun Belt. Allow 12 to 14 additional square feet per person if you live in the northern United States.
Sizing the Solar Hot Water Heating System Continued
A ratio of at least 1.5 gallons of storage capacity to 1 square foot of collector area prevents the system from overheating when the demand for hot water is low. In very warm, sunny climates, experts suggest that the ratio should be at least 2 gallons of storage to 1 square foot of collector area. For example, a family of four in a northern climate would need between 64 and 68 square feet of collector area and a 96‐ to 102‐gallon storage tank.
(This assumes 20 square feet of collector area for the first person, 20 for the second person, 12 to 14 for the third person, and 12 to 14 for the fourth person. This equals 64 to 68 square feet, multiplied by 1.5 gallons of storage capacity, which equals 96 to 102 gallons of storage.) Because you might not be able to find a 96‐gallon tank, you may want to get a 120‐
gallon tank to be sure to meet your hot water needs.
Resources
Analysis Tools
Preliminary Screening: To determine if a project is a possible candidate for solar hot water heating, try using the Federal Renewable Energy Screening Assistant (FRESA) software. This is a windows based software tool which screens projects for economic feasibility. It is able to evaluate many renewable technologies including solar hot water, photovoltaics, and wind.
Another and somewhat more detailed screening tool, Retscreen, is
provided by Natural Resources Canada. Go to http://www.retscreen.net/ to download the simulation software.
Resources Continued
 Analysis Tools
Detailed Performance: Once preliminary viability has been established, it will eventually be necessary to evaluate system performance to generate more precise engineering data and economic analysis. This can be accomplished based upon hourly simulation software or by hand correlation methods based on the results of hourly simulations. Two software programs which are available include:
FCHART, a correlation method available from the University of Wisconsin. Go to http://www.fchart.com/ to download the simulation software.
TRNSYS, software available from the University of Wisconsin. Go to http://sel.me.wisc.edu/trnsys/ to download the simulation software.
FCHART can be used with the following:
Collector Types
Flat-Plates
Evacuated Types
Integral Collectors
System Types
Water Storage Heating
Building Storage Heating
Domestic Water Heating
Integral Collector-Storage DHW
Indoor and Outdoor Pool Heating
Features
Life-cycle economics with cash flow
Weather data for over 300 locations
Weather data can be added
Monthly parameter variation
2-D incidence angle modifiers
English and SI units
Approved for use in California
Versions for Mac, DOS, and Windows
F-Chart
Example Input
Parameter Input Screen for Flat-Plate Collector
F-Chart
Example Input
Parameter Input Screen for General Solar Heating System
F-Chart
Example Output
F-Chart
Example Output
Graphical Output Screen showing Solar vs. Month
Installation
Installation of the Solar Hot Water System
The proper installation of solar water heating systems depends on many factors. These factors include solar resource, climate, local building code requirements, and safety issues. Wind Loading
A mounted collector is exposed not only to sunlight and the rigors of ultraviolet light
but also to wind forces. For example, in parts of the world that are vulnerable to
hurricanes or extreme wind storms, the collector and its mounting structure need to
be able to withstand intermittent wind loads up to 146 miles per hour. This
corresponds to a pressure of about 75 pounds per square foot. Winds, and thermal
contraction and expansion may cause improperly installed bolts and roof seals to
loosen over time. As always, follow local code requirements for wind loading.
Roof Mounting Considerations
Do not mount collectors near the ridge of a roof or other places where the wind
load may be unusually high. The figure below shows a desirable location for a
roof-mounted collector. Mounting collectors parallel to the roof plane helps
reduce wind loads and heat loss.
Example of a Collector mounted down from
roof ridge to reduce wind loading and heat losses
Ground Mounting
In an alternative to roof mounting, the collector for a solar water
heating system may be mounted at ground level. The lower edge of
the collector should be at least one foot above the ground so it will
not be obstructed by vegetation or soaked by standing water.
Roof Mounted Collectors
There are four ways to mount flat-plate collectors on roofs:
1. Rack Mounting. This method is used on homes with flat roofs. Collectors are
mounted at the prescribed angle on a structural frame. The structural connection
between the collector and frame and between the frame and building, or site must
be adequate to resist maximum potential wind loads.
Example of a Rack-mounted collector
2. Standoff Mounting. Standoffs separate the collector from the finished roof
surface; they allow air and rainwater to pass under the collector and minimize
problems of mildew and water retention. Standoffs must have adequate
structural properties. They are sometimes used to support collectors at slopes
that differ from that of the roof angle. This is the most common mounting
method used.
Example of a Standoff-mounted collector
3. Direct Mounting. Collectors can be mounted directly on the roof
surface. Generally, they are placed on a waterproof membrane covering
the roof sheathing. Then the finished roof surface, the collector's structural
attachments, and waterproof flashing are built up around the collector. A
weatherproof seal must be maintained between the collector and the roof
to avoid leaks, mildew and rotting.
Example of a Direct- or flush-mounted collector
4. Integral Mounting. Integral mounting places the collector within the roof
construction itself. The collector is attached to and supported by the structural
framing members. The top of the collector serves as the finished roof surface.
Weather tightness is crucial in avoiding water damage and mildew. Only collectors
designed by the manufacturer to be integrated into the roof should be installed as the
water/moisture barrier of buildings. The roofing materials and solar collectors expand
and contract at different rates and have the potential for leaks. A well sealed flashing
material allows the expansion and contraction of the materials to maintain a water
seal.
Example of an Integral-mounted collector
Roof Work Considerations
The most demanding aspects of installing roof-mounted collectors are
the actual mounting and roof penetrations. Standards and codes are
sometimes ambiguous about what can and cannot be done to a roof.
Always follow accepted roofing practices, be familiar with local building
codes, and communicate with the local building inspector. These are
prime roof work considerations:
1. Perform the installation in a safe manner.
2. Take precautions to avoid (or minimize) damage to the roof area.
3. Position collectors for the maximum performance compatible with
acceptable mounting practices.
4. Seal and flash pipe and sensor penetrations in accordance with good
roofing practices. Use permanent sealants such as silicone, urethane or
butyl rubber.
5. Locate collectors so they are accessible for needed maintenance.
Maintenance
Maintenance
Regular maintenance on simple systems can be as infrequent as every 3‐5 years, preferably by a qualified contractor with experience and knowledge of solar hot water heating systems. Systems with electrical components usually require a replacement part or two after 10 years.
Corrosion and Scaling in Solar Water Heating Systems
The two major factors affecting the performance of properly sited and installed solar water heating systems include scaling and corrosion.
Corrosion
Most well‐designed solar systems experience minimal corrosion. When they do, it is usually galvanic corrosion, an electrolytic process caused by two dissimilar metals coming into contact with each other. One metal has a stronger positive electrical charge and pulls electrons from the other, causing one of the metals to corrode. The heat‐transfer fluid in some solar energy systems sometimes provides the bridge over which this exchange of electrons occurs.
Oxygen entering into an open loop solar system will cause rust in any iron or steel component. Such systems should have copper, bronze, brass, stainless steel, plastic, rubber components in the plumbing loop, and plastic or glass lined storage tanks.
Scaling
Domestic water that is high in mineral content ("hard water") may cause the buildup or scaling of mineral (calcium) deposits in solar heating systems. Scale buildup reduces system performance in a number of ways. If the system uses domestic water as the heat transfer fluid, scaling can occur in the collector, distribution piping, and heat exchanger. In systems that use other types of heat‐transfer fluids (such as glycol), scaling can occur on the surface of the heat exchanger that transfers heat from the solar collector to the domestic water. Scaling may also cause valve and pump failures on the domestic water loop.
Scaling can be avoided by using a water softener(s) or by circulating a mild acidic solution (such as vinegar) through the collector or domestic water loop every 3–5 years, or as necessary depending on water conditions. There may be the need to carefully clean heat exchanger surfaces with medium‐grain sandpaper. A "wrap‐around" external heat exchanger is an alternative to a heat exchanger located inside a storage tank.
Periodic Inspection List
The following are some suggested inspections of solar system components.
Collector shading
Visually check for shading of the collectors during the day (mid‐morning, noon, and mid‐afternoon) on an annual basis. Shading can greatly affect the performance of solar collectors. Vegetation growth over time or new construction on the building or adjacent property may produce shading that wasn't there when the collector(s) were installed. Collector soiling
Dusty or soiled collectors will perform poorly. Periodic cleaning may be necessary in dry, dusty climates. Collector glazing and seals
Look for cracks in the collector glazing, and check to see if seals are in good condition. Plastic glazing, if excessively yellowed, may need to be replaced.
Piping and wiring connections
Look for fluid leaks at pipe connections. All wiring connections should be tight.
Piping and wiring insulation
Look for damage or degradation of insulation covering pipes and wiring.
Roof penetrations
Flashing and sealant around roof penetrations should be in good condition.
Support structures
Check all nuts and bolts attaching the collectors to any support structures for tightness.
Pressure relief valve (on liquid solar heating collectors)
Make sure the valve is not stuck open or closed.
Pumps
Verify that distribution pump(s) are operating. Check to see if they come on when the sun is shining on the collectors after mid‐morning. If the pump is not operating, then either the controller or pump has malfunctioned.
Heat transfer fluids
Antifreeze solutions in solar heating collectors need to be replaced periodically. If water with a high mineral content (i.e., hard water) is circulated in the collectors, mineral buildup in the piping may need to be removed by adding a de‐scaling or mild acidic solution to the water every few years.
Storage systems
Check storage tanks, etc., for cracks, leaks, rust, or other signs of corrosion.
Manufacturers
 ACR Solar International Corporation http://www.solarroofs.com
 FAFCO, Inc.
http://www.fafco.com
 Velux America
http://www.veluxusa.com
 Heliodyne, Inc. http://www.heliodyne.com
 Silicon Solar Inc. http://sunmaxxsolar.com
 Solarhart
http://www.solarhart.com
 SunEarth, Inc.
http://www.sunearthinc.com
 Solene, LLC
http://www.solene‐usa.com
 Thermo Technologies
http://www.thermomax.com
Trade Associations
 American Solar Energy Society (ASES)
http://www.ases.org
 Florida Solar Energy Center (FSEC) http://www.fsec.ucf.edu
 Solar Energy Industries Association (SEIA)
http://www.seia.org
 Solar Rating & Certification Corporation (SRCC) http://www.solar‐rating.org
About the American Solar Energy Society
Established in 1954, the American Solar Energy Society (ASES)
is the nonprofit organization dedicated to increasing the use of
solar energy, energy efficiency, and other sustainable
technologies in the United States
About the Florida Solar Energy Center
The Florida Solar Energy Center (FSEC) was created by the Florida
Legislature in 1975 to serve as the state’s energy research institute.
The main responsibilities of the center are to conduct research, test
and certify solar systems and develop education programs.
About the Solar Energy Industries Association
Founded in 1974, the Solar Energy Industries Association (SEIA) is
the leading national trade association for the solar energy industry.
The mission of the Solar Energy Industries Association is to expand
markets, strengthen research and development, remove market
barriers and improve education and outreach for solar energy
professionals.
About the Solar Rating and Certification Corporation
In 1980 the Solar Rating and Certification Corporation
(SRCC) was incorporated as a non-profit organization
whose primary purpose is the development and
implementation of certification programs and national rating
standards for solar energy equipment.
The End