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.

For Functional description, see Virginia Administrative Code print product.

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.

V0130004.JPG, SIZE-17 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130005.JPG, SIZE-17 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130006.JPG, SIZE-17 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130007.JPG, SIZE-17 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

2. DOMESTIC HOT WATER PREHEATING - COMBINED SYSTEM. Domestic hot water is preheated as it passes through heat storage enroute to the conventional water heater.

V0130008.JPG, SIZE-17 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130009.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130010.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130011.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130012.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130013.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130014.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130015.JPG, SIZE-8 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

C. Solar Window.

V0130016.JPG, SIZE-25 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130017.JPG, SIZE-10 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130018.JPG, SIZE-12 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130019.JPG, SIZE-16 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130020.JPG, SIZE-19 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130021.JPG, SIZE-13 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130022.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130023.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130024.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130025.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130026.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130027.JPG, SIZE-14 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130028.JPG, SIZE-13 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130029.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130030.JPG, SIZE-6 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130031.JPG, SIZE-6 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130032.JPG, SIZE-6 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130033.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130034.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.)

V0130035.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130036.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130037.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130038.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130039.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130040.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130041.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130042.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130043.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130044.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130045.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130046.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130047.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130048.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130049.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130050.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130051.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130052.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130053.JPG, SIZE-15 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130054.JPG, SIZE-35 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130055.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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].

V0130056.JPG, SIZE-28 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130057.JPG, SIZE-7 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130058.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130059.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130060.JPG, SIZE-9 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130061.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130062.JPG, SIZE-10 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130063.JPG, SIZE-10 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130064.JPG, SIZE-7 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130065.JPG, SIZE-13 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.

V0130066.JPG, SIZE-11 PICAS, TYPE-DPI

For Functional description, see Virginia Administrative Code print product.

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.