Final Text
Part I
Administration
13VAC5-200-10. Application.
Application for solar equipment tax exemption must be made to
the local building department on forms provided by the Department of Housing
and Community Development.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-40. Approval.
The applicant for tax exemption must demonstrate to the local
building official or to the Department of Housing and Community Development
that the proposed or existing solar system performs its intended function.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-50. Certification.
If, after examination of such equipment, facility or device
the local building department determines that the unit is designed and used
primarily for the purpose of providing for the collection and use of incident
solar energy for water heating, space heating or cooling or other application
which would otherwise require a conventional source of energy, and conforms to
the criteria set forth in this document, the local building department shall approve
and certify such application. The local department shall forthwith
transmit the application form to the Department of Housing and Community
Development, which shall certify to the local assessing officer those applicants
applications properly approved and certified by the local
building department as meeting all the requirements qualifying such
equipment, facility or device for exemption from taxation.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-60. Appeals.
Any person aggrieved by a decision of the local building
department may appeal such decision to the State Technical Review Board local
board of building code appeals, which may affirm or reverse such decision.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-70. Assessment.
Upon receipt of the certificate from the Department of
Housing and Community Development, local building department the
local assessing officer shall, if such local ordinance be in effect, proceed to
determine the value of such qualifying solar energy equipment, facilities or
devices. The value of such qualifying solar energy equipment, facilities or
devices shall not be less than the normal cost of purchasing and installing
such equipment, facilities or devices.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-80. Exemption.
The tax exemption shall be determined by applying the local
tax rate to the value of such equipment, facilities or devices, and subtracting
such amount, wholly or partially, from the total real property tax due on the real
property to which such equipment, facilities or devices are attached. The
exemption shall be effective beginning in the next succeeding tax year and
shall be permitted for a term of not less than five years; provided, however,
in the event the locality assesses real estate pursuant to §55- 811.1, of the
Code of Virginia effective when such real estate is first assessed, but not
prior to the date of such application for exemption in accordance with
§58.1-3661 D of the Code of Virginia.
Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.
13VAC5-200-100. Functional description.
The following section has been reprinted from Appendix C of
Solar heating and hot water system functional description is contained in
HUD Intermediate Minimum Property Standards for Solar Heating and Domestic Hot Water
Systems, NBSIR #77-1226.
Solar Heating and Hot Water Systems: Functional Description
The basic function of a solar heating and domestic hot
water system is the collection and conversion of solar radiation into usable
energy. This is accomplished--in general terms--in the following manner. Solar
radiation is absorbed by a collector, placed in storage as required, with or
without the use of a transport medium, and distributed to point of use. The
performance of each operation is maintained by automatic or manual controls. An
auxiliary energy system is usually available for operation, both to supplement
the output provided by the solar system and to provide for the total energy
demand should the solar system become inoperable.
The conversion of solar radiation to thermal energy and the
use of this energy to meet all or part of a dwelling's heating and domestic hot
water requirements has been the primary application of solar energy buildings.
The parts of a solar system--collector, storage,
distribution, transport, controls and auxiliary energy--may vary widely in
design, operation, and performance. They may, in fact, be one and the same
element (a south-facing masonry wall can be seen as a collector, although a
relatively inefficient one, which stores and then radiates or
"distributes" heat directly to the building interior). They may also
be arranged in numerous combinations dependent on function, component
compatibility, climatic conditions, required performance, site characteristics,
and architectural requirements.
Of the numerous concepts presently being developed for the
collection of solar radiation, the relatively simple flat-plate collector has
the widest application. It consists first of an absorber plate, usually made of
metal coated black to increase absorption of the sun's energy. The plate is
then insulated on its underside and covered with a transparent cover plate to
trap heat within the collector and reduce convective losses from the absorber.
The captured heat is removed from the absorber by means of a working fluid,
generally air or water. The fluid is heated as it passes through or near the
absorber plate and then transported to points of use, or to storage, depending
on energy demand.
The storage of thermal energy is the second item of
importance since there will be an energy demand during the evening, or on
sunless days when solar collection cannot occur. Heat is stored when the energy
delivered by the sun and captured by the collector exceeds the demand at the
point of use. The storage element may be as simple as a masonry floor that
stores and then re-radiates captured heat, or as relatively complex as a latent
heat storage. In some cases, it is necessary to transfer heat from the
collector to storage by means of a heat exchanger (primarily in systems with a
liquid working fluid). In other cases, transfer is made by direct contact of
the working fluid with the storage medium (i.e., heated air passing through a
rock pile).
The distribution component receives energy from the
collector or storage, and dispenses it at points of use. Within a building,
heat is usually distributed in the form of warm air or warm water.
The controls of a solar system perform the sensing,
evaluation and response functions required to operate the system in the desired
mode. For example, if the collector temperature is sufficiently higher than
storage temperature, the controls can cause the working fluid in storage to
circulate in the collector and accumulate solar heat.
An auxiliary energy system provides the supply of energy
when stored energy is depleted due to severe weather or clouds. The auxiliary
system, using conventional fuels such as oil, gas, electricity, or wood
provides the required heat until solar energy is available again.
The organization of components into solar heating and
domestic hot water systems has led to two general characterizations of solar
systems: active and passive. The terms active and passive solar systems have
not yet developed universally accepted meanings. However, each classification
possesses characteristics that are distinctively different from each other.
These differences significantly influence solar dwelling and system design.
An active solar system can be characterized as one in which
an energy resource--in addition to solar--is used for the transfer of thermal
energy. This additional energy, generated on or off the site, is required for
pumps, blowers, or other heat transfer medium moving devices for system
operation. Generally, the collection, storage, and distribution of thermal
energy is achieved by moving a transfer medium throughout the system with the
assistance of pumping power.
A passive solar system, on the other hand, can be
characterized as one where solar energy alone is used for the transfer of
thermal energy. Energy other than solar is not required for pumps, blowers, or
other heat transfer medium moving devices for system operation. The major
component in a passive solar system generally utilizes some form of thermal
capacitance, where heat is collected, stored, and distributed to the building
without additional pumping power. Collection, storage, and distribution is
achieved by natural heat transfer phenomena employing convection, radiation,
conduction, in conjunction with the use of thermal capacitance as a heat flow
control mechanism.
Solar Heating and Hot Water Systems: Operational Description
A. Solar Heating System
Solar systems may be designed to operate in a number of
different ways depending on function, required performance, climatic
conditions, component and system design, and architectural requirements.
Usually, however, solar systems are designed to operate in four modes. In a
very basic manner, the four modes of solar system operation for both active and
passive systems are described and illustrated below.
1. HEATING HOUSE FROM COLLECTOR. Solar radiation captured by
the collectors and converted to thermal energy can be used to directly heat the
house.
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2. HEATING STORAGE FROM COLLECTOR. If the house does not
require heat, the captured (collected) thermal energy can be placed in storage
for later use.
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3. HEATING HOUSE FROM STORAGE. Heat from storage can be
removed to heat the house when the sun is not shining--at night or on
consecutive sunless days.
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4. HEATING HOUSE FROM AUXILIARY. If heat from the collector
and storage is not sufficient to totally heat the house, an auxiliary system
supplies all or part of the house's heating requirement.
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B. Domestic Hot Water System.
The solar system may also be designed to preheat water from
the incoming water supply prior to passage through a conventional water heater.
The domestic hot water preheat system can be combined with the solar heating
system or designed as a separate system. Both situations are illustrated below.
1. DOMESTIC HOT WATER PREHEATING - SEPARATE SYSTEM. Domestic
hot water preheating may be the only solar system included in some designs. A
passive thermosyphoning arrangement is shown above.
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2. DOMESTIC HOT WATER PREHEATING - COMBINED SYSTEM. Domestic
hot water is preheated as it passes through heat storage enroute to the
conventional water heater.
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Basics of Solar Utilization
A. Climate
Solar radiation, wind, temperature, humidity and many other
factors shape the climate of the United States. Basic to using solar energy for
space heating and domestic hot water heating is understanding the relationship
of solar radiation, climate and dwelling design.
The amount and type of solar radiation varies between and
within climatic regions: from hot-dry climates where clear skies enable a large
percentage of direct radiation to reach the ground, to temperate and humid
climates where up to 40 percent of the total radiation received may be diffuse
sky radiation, reflected from clouds and atmospheric dust, to cool climates
where snow reflection from the low winter sun may result in a greater amount of
incident radiation than in warmer but cloudier climates.
As a result of these differences in the amount and type of
radiation reaching a building site, as well as in climate, season and
application - heating or domestic hot water - the need for and the design of
solar system components will vary in each locale.
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B. Solar Radiation.
The sun provides almost all of the earth's energy in the
form of radiation. Solar energy, also known as solar radiation reaches the
earth's surface in two ways: by direct (parallel) rays, and by diffuse
(nonparallel) sky radiation. The solar radiation reaching a building includes
not only direct and diffuse but also radiation reflected from adjacent ground
and building surfaces. It is these three sources of solar radiation that may be
used for space and domestic hot water heating.
1. THE SOLAR CONSTANT. A nearly constant amount of solar
energy strikes the outer atmosphere = 429.2 BTU per square foot per hour. This
quantity is known as the solar constant. A large amount of this energy,
however, is lost in the earth's atmosphere, and cannot be regained regardless
of collector design.
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2. ABSORPTION AND REFLECTION. On the average, almost half of
the solar radiation reaching the earth's outer atmosphere is lost by absorption
in the atmosphere and by reflection from clouds, as it passes through the
atmosphere to the earth's surface. The radiation lost actually varies between
60% in Seattle, Washington to only 30% in Albuquerque, New Mexico.
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3. EARTH'S ATMOSPHERE. As already stated, the radiation
reaching the earth's surface is diminished by the condition of the earth's
atmosphere; its vapor, dust and smoke content. At lower sun angles, the length
of travel through the atmosphere is greatly increased, so the relative amount
of radiation received is further diminished.
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4. DIFFUSE RADIATION. clouds and particles in the atmosphere
not only absorb solar energy, but scatter it in all directions. As a result, a
part of the solar radiation reaching the earth's surface is diffused, and
received from all parts of the sky. Diffuse radiation, as opposed to direct
radiation, is more predominant on hazy days than clear ones. At most, however,
diffuse radiation can only be about one quarter of the solar constant, or about
100 BTU/hr./sq. ft.
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5. DIRECT RADIATION ON A HORIZONTAL SURFACE. although the amount
of radiation remains constant, less radiation strikes a given horizontal area
as the sun gets lower in the sky.
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6. DIRECT SOLAR RADIATION ON A TILTED SURFACE. The same
principle applies to a tilted surface such as a collector. By tilting the
collector so that it is nearly perpendicular to the sun's ray, more energy
strikes its surface, undiminished by a cosine factor.
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C. Solar Window.
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THE SOLAR WINDOW. Imagine the sky as a transparent dome with
its center at the solar collector of a house. The path of the sun can be
painted (projected) onto the dome, as can be the outline of surrounding houses
and trees. The morning and afternoon limits of useful solar collection (roughly
9 A.M. and 3 P.M.) and the sun's path between those hours throughout the year
scribes a "solar window" on the dome. Almost all of the useful sun
that reaches the collector must come through this window except for the added
effect of diffuse radiation. If any of the surrounding houses, trees, etc.,
intrude into this "solar window," the intrusion will cast a shadow on
the collector. The isometric drawing above illustrates the "solar
window" for a latitude 40° N. The solar window will change for different
latitudes.
SIDE VIEW OF SKY DOME WITH "SOLAR WINDOW". A side
view of the sky dome from the east illustrates the relative position and angle
of the sun throughout the year that defines the boundaries of the "solar
window."
ANGLE OF INCIDENCE, a term often used in solar collector
design, is the angle measured from the normal of the collector surface to the
line indicating the sun's altitude at a particular time. The diagram
specifically identifies the angle of incidence for June 21.
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PLAN VIEW OF SKY DOME WITH "SOLAR WINDOW". Viewed
from above the sky dome, the seasonal path of the sun can be plotted thus
defining the boundaries of the "solar window." This is easily
accomplished by the use of a standard sun path diagram for the proper latitude.
Sun path diagrams are widely reproduced and used for determining the azimuth
and altitude of the sun at any time during the year, and give the points which
can be plotted to determine the solar window.
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PANORAMA OF THE SKY DOME. As with the spherical earth, the
spherical sky dome with its "solar window" can be mapped using a
Mercator projection, in which all latitude and longitude lines are straight
lines. Such a map is very useful for comparing the site surroundings with the
"solar window" outline, since both can be easily plotted on the map.
Any elements surrounding the site that intrude into the "solar
window" will cast shadows on the collector.
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D. Solar collection and conversion
Basic to the utilization of solar energy for space and
domestic hot water heating is the process by which solar radiation is converted
to thermal energy. This conversion process is the basic link between the energy
supply - the sun - and the energy load - the dwelling. The process is best
understood by briefly explaining solar radiation and then discussing the
characteristics of collection.
1. SOLAR ENERGY CONVERSION. Solar radiation is
electromagnetic radiation generated by the sun, which reaches the earth's
surface with a wavelength distribution of .3 to 2.4 micrometers. Radiation is
perceived as visible light between .36 and .76 micrometers. For most solar
applications, solar radiation in the visible and near infrared range is the
most important.
The drawings to the left [below] show the principle of solar
energy collection and conversion. When incoming solar radiation impinges on the
surface of a body, it is partially absorbed, partially reflected, and, if the
body is transparent, partially transmitted. The relative magnitude of each
varies with the surface characteristics, body geometry, material composition,
and wavelength.
For solar applications, energy must be first absorbed, then
converted into thermal energy and, finally removed by a heat transfer mechanism
in order to be useful. Absorbed radiation heats up the absorbing body, which
then reemits energy in the form of thermal radiation in the infrared (longwave)
part of the spectrum. If the absorbing surface is exposed to the atmosphere,
part of the absorbed energy will be lost by converted of radiation.
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2. THE GREENHOUSE EFFECT. Most glass and some plastics are transparent
in the solar wavelength region and hence are used as windows. At the same time,
this glazing has low transmission in the infrared (longwave) region. By placing
glass or plastic over the absorber in a collector, energy is trapped in two
ways: first, the infrared radiation emitted by the absorbing surface is stopped
by the glazing, with a portion reradiated back toward the absorber, and thereby
trapped. Second, the glazing also traps a layer of still air next to the
absorber and reduces the convective heat loss. This behavior of glazing is
called the "greenhouse effect" and is used in most solar collectors.
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E. Collector Orientation and Tilt
Solar collectors must be oriented and tilted within
prescribed limits to receive the optimum level of solar radiation for system
operation and performance.
1. COLLECTOR TILT FOR HEATING. The optimum collector tilt
for heating is usually equal to the site latitude plus 10 to 15 degrees.
Variations of 10 degrees on either side of this optimum are acceptable.
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2. COLLECTOR TILT FOR HEATING AND COOLING. The optimum
collector tilt for heating and cooling is usually equal to site latitude plus 5
degrees. Variations of 10 degrees on either side of the optimum are acceptable.
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3. COLLECTOR TILT FOR DOMESTIC HOT WATER. The optimum
collector tilt for domestic water heating alone is usually equal to the site
latitude. Again, variations of 10 degrees on either side of the optimum are acceptable.
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4. MODIFICATION OF OPTIMUM COLLECTOR TILT. A greater gain in
solar radiation collection sometimes may be achieved by tilting the collector
away from the optimum in order to capture radiation reflected from adjacent
ground or building surfaces. The corresponding reduction of radiation directly
striking the collector, due to non-optimum tilt, should be recognized when
considering this option.
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5. SNOWFALL CONSIDERATION. The snowfall characteristics of
an area may influence the appropriateness of these optimum collector tilts.
Snow buildup on the collector, or drifting in front of the collector, should be
avoided.
COLLECTOR ORIENTATION. A collector orientation of 20
degrees to either side of true South is acceptable. However, local climate and
collector type may influence the choice between East or West deviations.
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F. Shading of Collector.
Another issue related to both collector orientation and
tilt is shading. Solar collectors should be located on the building or site so
that unwanted shading of the collectors by adjacent structures, landscaping or
building elements does not occur. In addition, considerations for avoiding
shading of the collector by other collectors should also be made. Collector
shading by elements surrounding the site may be addressed by considering the
"solar window" concept.
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1. SELF-SHADING OF COLLECTOR. Avoiding all shelf-shading for
a bank of parallel collectors during useful collection hours (9 AM and 3 PM)
results in designing for the lowest angle of incidence with large spaces
between collectors. It may be desirable therefor to allow some self-shading at
the end of solar collection hours, in order to increase collector size or to
design a closer spacing of collectors, thus increasing solar collection area.
By making the collector's back slope reflective, one could increase the amount
of solar radiation striking the adjacent collector, thus negating some of the
shading loss.
2. SHADING OF COLLECTOR BY BUILDING ELEMENTS. Chimneys,
parapets, fire walls, dormers, and other building elements can cast shadows on
adjacent roof-mounted solar collectors, as well as on vertical wall collectors.
The drawing to the right [below] shows a house with a 45 degrees North. By
mid-afternoon portions of the collector are shaded by the chimney, dormer, and
the offset between the collector on the garage. Careful attention to the
placement of building elements and to floor plan arrangement is required to
assure that unwanted collector shading does not occur.
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SOLAR HEATING AND HOT WATER SYSTEMS:
Active Systems
Active solar systems are characterized by collectors, thermal
storage units and transfer media, in an assembly which requires additional
mechanical energy to convert and transfer the solar energy into thermal energy.
The following discussion of active solar systems serves as an introduction to a
range of active concepts which have been constructed.
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A. Heating and Domestic Hot Water Diagrams
In common use today is the combined solar heating and domestic
hot water system. The system operates as follows: solar radiation is absorbed
by a collector or series of collectors, and removed to storage in the form of
thermal energy by a heat transfer medium. The heat is later removed from
storage and distributed to the living spaces, again by a heat transfer medium,
which may or may not be the same medium as that flowing through the collector.
Circulation throughout the system is aided by pumps, blowers, or other medium
moving devices. An auxiliary heating system should be available both to
supplement the output supplied by the solar system and to provide for the total
energy demand should the solar system become inoperative. Manual or automatic
controls monitor both the solar and auxiliary system operation. In a solar
heating and hot water combined system, the domestic water supply is preheated
in the heat storage, and then passed through the conventional water heater
before distribution.
1. SOLAR HEATING SYSTEM: PROCESS DIAGRAM. A space heating
system alone can be developed by simply removing the domestic hot water
preheating unit from the heat storage. The operation of the solar heating
system is then the same as described above.
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2. SOLAR DOMESTIC HOT WATER SYSTEM: PROCESS DIAGRAM 1. The
combined system diagram can be modified into a domestic hot water system alone
by eliminating the heating distribution and the auxiliary heating unit, and
also reducing the size of the storage tank. Only the domestic water supply
would then pass through the heat storage, preheating the hot water supply,
enroute to a conventional water heater.
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3. SOLAR DOMESTIC HOT WATER SYSTEM: PROCESS DIAGRAM 2.
Another method of preheating the domestic hot water involves passing the
potable water supply itself through the collectors. The heated water is stored
in the water storage tank until a demand is initiated. An auxiliary heat source
is usually present to boost the water temperature when preheat has been
inadequate. The preheated water is either pumped from storage, or flows by
supply pressure to the house.
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B. Collector--Storage. The removal of heat from the
collector and its placement in heat storage involves the circulation of a heat
transfer medium in a transport loop. Several collector--storage conditions are
shown below.
1. OPEN CIRCUIT LIQUID COLLECTOR. In this system, storage
water itself, treated as necessary to prevent corrosion, is drawn from the
bottom of storage, pumped through the collector and then returned to the top of
storage. The circulating water, which runs through, on top of or under the
absorber plate, is distributed to the absorber by a manifold at the top of the
collector, or pumped up from below the collector through tubes attached to or
integral with the absorber plate. When the system is not running, air is
allowed to enter into the collector and piping, and the water drains into
storage. In open circuit collectors, storage is at atmospheric pressure, a
condition that should be considered in the design of the distribution system.
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2. CLOSED CIRCUIT LIQUID COLLECTOR. In this system, a heat
transfer liquid -- such as treated water, anti--freeze solution or another
liquid -- is pumped through the collector and then through a heat exchanger in
storage and back to the collector, in a closed loop. In this system of separate
transfer and storage mediums, the storage may be pressurized. The loop may
remain filled with fluid, and therefore must be protected from freezing, or may
be drained and replaced with pressurized inert gas.
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3. AIR COLLECTOR. Although many arrangements of air
collector--rock storage and warm- air distribution systems are possible, the
one diagrammed is typical of the most popular system is use. Air from the cold
end of the rock storage bin is pumped through the collector, gaining, heat, and
returned to the hot end of storage.
Warm air distribution systems are usually used with air
collectors to enable direct heating from the collector. In this case, the
dampers must be adjusted to supply heat directly to the house, returning air to
the collector thereby bypassing storage. (See diagram page C3.)
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C. Storage-Distribution Diagrams.
Heat is removed from storage and circulated to the house by
the distribution component. There are numerous ways this storage-distribution
function can be performed, and in numerous combinations with the preceding
collector-storage circuits. Six typical storage-distribution methods are
diagrammed.
1. WARM AIR DISTRIBUTION - HOT WATER COIL IN DUCT. A
warm-air distribution system can be used with liquid heat storage, by pumping
the heated storage medium through a suitably sized heat exchange coil in the
main supply duct of the distribution system.
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2. HYDRONIC DISTRIBUTION. In a hydronic system, with a
pressurized storage, liquid from storage is pumped directly through standard
baseboard convector units. Because of the relatively low temperatures that
usually occur in solar systems during winter conditions, the size of baseboard
units, and possibly the piping may change from ordinary hydronic systems.
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3. INDIVIDUAL FAN-COIL UNIT DISTRIBUTION. When storage is not
pressurized, in a fan coil distribution system (as well as hydronic system), a
secondary, heat transfer fluid is often circulated in a closed loop to prevent
air binding. This fluid is pumped through storage to individual fan-coil units
located throughout the dwelling for heat distribution. The design and sizing
considerations are similar to those for ordinary hydronic distribution.
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4. RADIANT HEAT DISTRIBUTION. In a radiant heating system,
with a non-pressurized storage, a secondary heat transfer fluid is circulated
in a closed loop from heat storage to coils or panels located in the floor,
walls and or ceiling of the living space. Besides the liquid temperature, the
size and spacing of the coils is critical for effective radiant heat
distribution.
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5. WARM AIR DISTRIBUTION FROM ROCK STORAGE. For an
air-collector system employing rock storage, it is advantageous to employ the
natural high level of temperature stratification in storage and distribute air
to the living space from hottest section of storage. As diagrammed, this will
require reversing the flow of air through storage relative to the collection
cycle. The most common method for doing this is diagrammed. Using the same fan
that supplies the collector along with two automatic dampers, the direction of
air flow is reversed from storage, forcing air in a house loop to return,
thereby bypassing the collector ducts.
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6. HEAT PUMP ASSISTED DISTRIBUTION. Either air or liquid
collector-storage systems can be used as the source of thermal energy for a
heat pump distribution system. As diagrammed, liquid from storage is circulated
through a heat exchanger in the pump unit, and heat is transferred to the heat
pump's working fluid. By means of its compression cycle, the heat pump further
elevates the working fluid temperature and it functions as the auxiliary heat
source. This high temperature fluid then transfers heat through another
exchanger to either an air or hydronic distribution system. The heat pump may
also be used in parallel with thermal energy storage to remove heat from the
outside air when storage is depleted.
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D. Domestic Hot Water Preheating.
Domestic hot water can be preheated either by circulating
the potable water supply itself through the collector, or by passing the supply
line through storage enroute to a conventional water heater. Three storage
related preheat systems are shown below.
1. PREHEAT COIL IN STORAGE. Water is passed through a
suitably sized coil placed in storage enroute to the conventional water heater.
Unless the preheat coil has a protective double wall construction, this method
can only be use for solar systems employing non-toxic storage media.
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2. PREHEAT TANK IN STORAGE. In this system, the domestic hot
water preheat tank is located within the heat storage. The water supply passes
through storage to the preheat tank where it is heated and stored, and later
piped to a conventional water heater as needed. A protective double-wall
construction again will be necessary unless a non-toxic storage medium is used.
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3. PREHEAT OUTSIDE OF STORAGE. In this preheat method, the heat
transfer liquid in storage is pumped through a separate heat exchanger to be
used for domestic hot water preheating. This separate heat exchanger could be
the conventional water heater itself. However, if the liquid from storage is
toxic, the required separation of liquids is achieved by the use of a
double-wall exchanger, as diagrammed, in which the water supply simply passes
through enroute to the conventional water heater.
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E. Auxiliary Energy Diagrams.
The provision of auxiliary energy to the dwelling is needed
when the solar heating system becomes inoperative or cannot meet the dwelling's
total energy demand. The auxiliary heating component may operate independently
or in conjunction with the solar storage and distribution systems. The control
of solar and auxiliary system operation becomes an important consideration for
the effectiveness of both. Four possible combinations are shown below.
1. AUXILIARY HEAT COILS IN AIR DISTRIBUTION SUPPLY DUCT. Two
heat exchange coils -- one from solar storage and one from the auxiliary unit
-- are located in the primary distribution supply duct. Depending on the
temperature in storage, the auxiliary energy system may provide a full or
partial temperature boost to supply of air. The need for auxiliary energy is
determined typically by a two contact room thermostat.
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2. AUXILIARY WITH SEPARATE DISTRIBUTION. The auxiliary
energy system may be a totally separate component not integrated with solar
storage or distribution. This may involve a totally separate distribution
network, such as individual electric baseboard units placed in the dwelling in
locations and numbers as required. The two separate heating systems, however,
are linked by temperature controls.
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3. AUXILIARY HEATING WITH COMBINED DISTRIBUTION. In this
system, the auxiliary energy source is located between the storage and
distribution components. In this way, an integrated control component monitors
whether heat from storage or heat from the auxiliary source is in use. Pumps
and valves located at the connection points between the systems regulate the
auxiliary energy supply use, and prevents the auxiliary from heating storage.
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4. AUXILIARY HEATING WITH AIR COLLECTION-DISTRIBUTION. In
this system, the auxiliary heat unit is located within the distribution air
ducts downstream from the system's fan or blower. In this way, the auxiliary
subsystem provides an energy boost to the heated air coming either: 1) from
storage, or 2) directly from the collector. The auxiliary, unit may be a coil
in the duct, containing boiler heated water, or an electric resistance element,
or it may be a furnace. The auxiliary and solar system operation is maintained
and monitored by an integrated control component.
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Solar Heating and Hot Water Systems:
Passive Systems
Passive solar systems are characterized by the use of the
sun's energy alone for the transfer of thermal energy throughout the system.
Four passive systems are discussed below -- three space heating and one
domestic hot water preheating system. There are innumerable other concepts, but
the following will serve as an introduction to passive solar systems.
SPACE AND BUILDING-SURFACE HEATING. This concept relies on
a large transparent surface for the southern exposure, to increase heat gain
directly into the building -- thus heating the space. To avoid daytime
overheating, and adequate area and thickness of a thermal mass, such as heavy
masonry, should be used on the floors or walls to absorb heat during the day
and release it to the space after the sun has set. Insulation devices should
also be available to regulate daytime solar exposure and to minimize nighttime
heat loss.
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LIQUID ROOF MASS. This concept is similar to the previous
passive system except that the thermal mass -- water -- is now located in
containers above the living space. In some climates, both heating and cooling
can be provided by this system. Like the previous concept, proper control must
be maintained over the heat exchange process. This can be accomplished by the
use of movable insulating panels to expose or cover the containers, or by
filling and draining them according to heating or cooling demand.
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COMBINED COLLECTOR- STORAGE-DISTRIBUTION WALL This passive
concept relies on the solar exposure of a south facing thermal mass
(containerized water, masonry or concrete) located behind a transparent surface
and a separating air space. The thermal mass acts as the collector, storage,
and distribution components. Solar radiation collected and stored in the
thermal mass is distributed to the space by: 1) radiation, 2) convection, and
3) conduction.
When collection ceases due to lack of solar radiation, it
is advantageous to prevent heat loss through the transparent surface to the
outside, by an insulating device. In this example air valves or dampers allow
air to circulate across the hot face of the storage mass for convective heat
transfer.
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THERMOSYPHONING SYSTEM: DOMESTIC HOT WATER PREHEATING. This
passive concept utilizes the natural upward movement of heated fluids for the
collection and storage of domestic hot water. The cold water supply is pressure
fed to the bottom of a storage tank located above a solar collector. Exposure
of the collector to solar radiation allows the cold water to circulate by
convection -- through the collector-- from bottom to top -- and, once heated
back into storage. The heated water is stored in the tank until a demand is
initiated; then water is drawn off the top and fed directly to the dwelling or
to a conventional water heater.
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Solar heating and Hot Water Systems:
Component Description
A solar heating and domestic hot water system is composed
of numerous individual parts and pieces including: collectors; storage; a
distribution network with ducts and/or pipes, pumps and/or blowers, valves
and/or dampers; fixed or movable insulation; a system of manual or automatic
controls; and possibly heat exchangers, expansion tanks and filters. These
parts are assembled in a variety of combinations depending on functions,
component compatibility, climatic conditions, required performance, site
characteristics and architectural requirements, to form a solar heating and/or
domestic hot water system. Some components that are unique to the collector
system or that are used in an unconventional manner are briefly illustrated and
discussed in the next few pages.
A. Flat-Plate Collectors: An Exploded View.
The flat-plate collector is a common solar collection
device used for space heating and domestic water heating. The collector may be
designed to use either gas (generally air) or liquid (usually treated water) as
the heat transfer medium. Regardless of the medium used, most flat-plate
collectors consist of the same general components, as illustrated below.
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1. BATTEN. Battens serve to hold down the cover plate(s) and
provide a weather tight seal between the enclosure and the cover.
2. COVER PLATE. The cover plate usually consists of one or
more layers of glass or plastic film or combinations thereof. The cover plate
is separated from the absorber plate to reduce reradiation and to create an air
space, which traps heat by reducing convective losses. This space between the
cover and absorber can be evacuated to further reduce convective losses.
3. HEAT TRANSFER FLUID PASSAGE. Tubes or fins are attached
above, below or integral with an absorber plate for the purpose of transferring
thermal energy from the absorber plate to a heat transfer medium. The largest
variations in flat- plate collector design occurs with this component and its
combination with the absorber plate. Tube on plate, integral tube and sheet,
open channel flow, corrugated sheets, deformed sheets, extruded sheets and
finned tubes are some of the techniques used for liquid collectors. Air
collectors employ such configurations as gauze or screens, overlapping plates,
corrugated sheets, and finned plates and tubes.
4. ABSORBER PLATE. Since the absorber plate must have a good
thermal bond with the fluid passages, an absorber plate integral with the heat
transfer media passages is common. The absorber plate is usually metallic, and
normally treated with a surface coating which improves absorptivity. Black or
dark paints or selective coatings are used for this purpose. The design of this
passage and plate combination is of significance in a solar system's
effectiveness.
5. INSULATION. Insulation is employed to reduce heat loss
through the back of the collector. The insulation must be suitable for the high
temperature that may occur under no-flow or dry-plate conditions, or even
normal collection operation. Thermal decomposition and outgassing of the
insulation must be considered.
6. ENCLOSURE. The enclosure is a container for all the above
components. The assembly is usually weatherproof. Preventing dust, wind and
water, from coming in contact with the cover plate and insulation, is essential
to maintaining collector performance.
B. Flat-plate collectors.
A flat-plate collector generally consist of an absorbing
plate, often metallic; which may be flat, corrugated or grooved; coated black
to increase absorption of solar radiation insulated on its backside to minimize
heat loss from the plate; and covered with a transparent cover plate to trap heat
within the collector and reduce cooling of the absorber plate. The captured
solar heat is then removed from the absorber by means of a working fluid,
generally air or treated water, which is heated as it passes through or over
the absorbing plate. Although there are innumerable variants, three type of
flat-plate collectors will be discussed here as an introductory classification.
1. FLUID TUBE AND PLATE COLLECTOR. Most flat-plate
collectors in use today employ water, oil or an antifreeze solution as the heat
transfer medium. The liquid is pumped through fluid passage ways attached to or
integral with the absorber plate. There it is solar heated before being
circulated through storage in either a closed or open circuit. Freeze
protection and prevention of corrosion and leaks require special consideration.
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2. TRICKLING WATER COLLECTOR. This type of collector uses
corrugated metal panels for the exposed circulation of the heat transfer
medium. The transfer medium "trickles" down the channels from a
manifold or spray distribution at the top to a trough to the bottom of the
collector. The heated water then flows by gravity to the storage tank. Because
of the heat transfer fluid's exposure to the atmosphere in this collector, it
is always used with the open circuit collector-storage system. Therefore, when
collection is not occurring, the transfer medium drains back into storage.
Efficient operation of this collector is limited to low temperatures because of
evaporation effects.
3. FLAT-PLATE AIR COLLECTOR. Air collectors circulate air or
other gases through or over the absorber plate, returning heated air through
the ducts to storage or the living space. Compared with liquid collectors,
leakage, maintenance, and freeze protection problems are minimal. However, air
collectors do require relatively large ducts for their heat transfer medium and
often require more mechanical transfer energy per unit of solar energy
delivered.
C. High Temperature Collectors.
For heating and cooling systems requiring higher operating
temperatures, evacuated tube or concentrating collectors are available.
Depending upon the optical and thermal insulation design, the performance of
these systems is influenced by the ratio of the diffuse to total available
solar radiation.
EVACUATED TUBE COLLECTOR. These collectors employ a vacuum
to contain the absorber. The vacuum serves to reduce convective heat losses
allowing higher working temperatures and efficiencies. The absorber consists of
metal or glass tubes or fins which transfers captured thermal energy to the
heat transfer medium (which may be a liquid or gas). The basic modes of heat
transfer within the collector are analogous to those illustrated for flat-plate
collection. No insulation is required for the tubular collector itself;
however, the manifold and connecting piping require insulation similar to
flat-plate units. Both direct and diffuse radiation can be collected.
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CONCENTRATING COLLECTORS. Concentrating collectors (also
known as focusing collectors) employ curved and multiple point target reflectors
to focus radiation on a small area. The area where solar radiation is absorbed
can be a point -- the focal point -- or a line-- the focal axis.
A concentrating collector consists of three basic
components: the reflector and/ or lens, the absorber, and the housing which
maintains alignment and contains insulation for the absorber and connecting
piping. Often a mechanism is required to allow the collector/reflector or the
absorber to follow or track the sun's movement across the sky. Maintenance of
the reflective surface, particularly in dusty or air polluted areas, and of the
tracking mechanism are important considerations for collector performance.
Concentrating collectors are usually best suited for areas
with clear skies where most solar radiation is direct. The high temperatures
generated may make concentrating collectors particularly viable with solar
cooling systems.
As with flat-plate collectors, numerous variations of
concentrating collectors have been developed including linear and circular
concentrators, lens focusing collectors, collectors with directional and
non-directional focusing and tube concentrators. A number of concentrating
configurations are shown to the left [below].
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D. Collector Mounting.
Flat-plate collectors are generally mounted on the ground
or on a building in a fixed position at prescribed angles of solar
exposure--angles which vary according to the geographic location, collector
type, and the use of the absorbed heat. Flat-plate collectors may be mounted in
four general ways as illustrated below.
1. RACK MOUNTING. Collectors can be mounted at the
prescribed angle on a structural frame located on the ground or attached to the
building. The structural connection between the collector and the frame and the
frame and the building or site must be adequate to resist any impact loads such
as wind.
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2. STAND-OFF MOUNTING. Elements that separate the collector
from the finished roof surface are known as stand-offs. They allow air and
rainwater to pass under the collector thus minimizing problems of mildew and
leakage. The stand- offs must also have adequate structural properties.
Stand-offs are often used to support collectors at an angle other than that of
the roof to optimize collector tilt.
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3. DIRECT MOUNTING. Collectors can be mounted directly on
the roof surface. Generally, the collectors are placed on a water-proof
membrane on top of the roof sheathing. The finished roof surface, together with
the necessary collector structural attachments and flashing, are then built up
around the collector. A weatherproof seal between the collector and the roof
must be maintained, or leakage, mildew, and rotting may occur.
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4. INTEGRAL MOUNTING. Unlike the previous three component
collectors which can be applied or mounted separately, integral mounting places
the collector within the roof construction itself. Thus, the collector is
attached to and supported by the structural framing members. In addition, the
top of the collector serves as the finished roof surface. Weather tightness is
again crucial to avoid problems of water damage and mildew. This method of
mounting is frequently used for site built collectors.
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E. Multiple Collectors.
In active systems, a building's solar collector area is
generally composed of individual collector units or panels arranged to operate
as a single system. The arrangement and relationship of one collector unit to
another, sometimes known as collector ganging, is extremely important for
effective solar collection and efficient system operation. Three basic multiple
collector arrays are shown below.
1. PARALLEL FLOW - DIRECT RETURN. A direct return
distribution circuit circulates the transfer medium from the bottom of the
collector to a return header or manifold at the top. This arrangement may cause
severe operating problems by allowing wide temperature variations from
collector to collector due to flow imbalance. Although the pressure drops
across each collector are essentially the same and at the same flow rate, high
pressure drops occurring along the supply/return header or manifold will cause
flow imbalance. This problem can be reduce by sizing each header for minimum
pressure drop, although this may be prohibitive because of economic and space
limitations. Even manual balancing valves may be difficult to adjust, so
automatic devices or orifices might be required for efficient system
performance. Provisions must also be made to measure the pressure drop in order
to adjust the flow rate to prevent collectors closer to the circulating pump
from exceeding design flow rates and those farther away from receiving less.
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2. PARALLEL FLOW - REVERSE RETURN. Reverse return piping
systems are considered preferable to direct return for their ease of balancing.
Because the total length of supply piping and return piping serving each
collector is the same and the pressure drop across each collector is equal, the
pressure drop across each manifold are also theoretically equal. The major
advantage of reverse return piping is that balancing is seldom required since
flow through each collector is the same. Provisions for flow balancing may
still be required in some reverse return piping systems depending on overall
size of the collector array and type of collector.
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3. SERIES FLOW. Series flow is often used in large planar
arrays, to reduce the amount of piping required, by allowing several collector
assemblies to be served by the same supply return headers or manifolds. Series
flow can also be employed to increase the output temperature of the collector
system or to allow the placement of collectors on non-rectangular surfaces.
Either direct or reverse return distribution circuits can be employed, but
unless each collector branch has the same number of collectors, the reverse
return system has no advantage over direct return -- each would require flow
balancing.
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F. Heat Exchangers.
A heat exchanger is a device for transferring thermal
energy from one fluid to another. In some solar systems, a heat exchanger may
be required between the transfer medium circulated through the collector and
the storage medium or between the storage and the distribution medium. Three
types of heat exchangers that are most commonly used for these purposes are
illustrated below.
1. SHELL AND TUBE. This type of heat exchanger is used to
transfer heat from a circulating transfer medium to another medium used in storage
or in distribution. Shell and tube heat exchangers consist of an outer casing
or shell surrounding a bundle of tubes. The water to be heated is normally
circulated in the tubes and the hot liquid is circulated in the shell. Tubes
are usually metal such as steel, copper or stainless steel. A single shell and
tube heat exchanger cannot be used for heat transfer from a toxic liquid to
potable water because double separation is not provided water supply, in the
case of tube failure.
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2. SHELL AND DOUBLE TUBE. This type of heat exchanger is
similar to the previous one except that a secondary chamber is located within
the shell to surround the potable water tube. The heated toxic liquid then
circulates inside the shell but around this second tube. An intermediary
non-toxic heat transfer liquid is then located between the two tube circuits.
As the toxic heat transfer medium circulates through the shell, the
intermediary liquid is heated, which in turn heats the potable water supply
circulating through the innermost tube. this heat exchanger can be equipped
with a sight glass to detect leaks by a change in color - toxic liquid often
contains a dye - or by a change in the liquid level in the intermediary
chamber, which would indicate a failure in either the outer shell or
intermediary tube lining.
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3. DOUBLE WALL. Another method of providing a double
separation between the transfer medium and the potable water supply consists of
tubing or a plate coil wrapped around and bonded to a tank. The potable water
is heated as it circulates through the tank. When this method is used, the
tubing coil must be adequately insulated to reduce heat losses.
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Statutory Authority
§§36-97 et seq. and 36-137 58.1-3661 of the Code of
Virginia.
Historical Notes
Derived from VR394-01-8 §100.1, eff. October 10, 1978; amended, Virginia Register Volume 24, Issue 26, eff. October 1, 2008.