EX 2: 11- Solar Energy
air-type collector
a collector that uses air as the heat transfer fluid
Liquid type collector
a collector using liquid as the heat transfer fluid
concentrating collector
a device which concentrated the sun's rays on an absorber surface which is significantly smaller than the overall collector area
cover plate
a sheet of glass or transparent plastic placed above the absorber in a flat plate collector.
Passive System
a solar heating or cooling system that uses no external mechanical power to move the collected solar heat.
Architectural devices which shield vertical surfaces
are a common feature of vernacular architecture and a natural part of building design. (Ex. Frank Lloyd Wright houses)
Large areas of skylight
are usually a very bad idea for energy usage and daylighting.
Shading devices (light shelves)
benefit control of solar gain. Deciduous vines shade in summer, but not in winter. Some trees, like Black Locust do not leaf until late in spring.
Table C.2
is used to determine heat gain through windows for various times of day. SHGF--- (Solar Heeat Gain Factor)
domestic hot water
(DHW) useful through the year beneficial in areas with good partial-year sunshine
Forced Convection
The transfer of heat by the flow of fluids (such as air or water) driven by fans, blowers or pumps.
Sun Path Diagram
A circular projection of the sky vault, similar to a map, that can be used to determine solar positions and to calculate shading.
Liquid Type Collector
A collector using a liquid as the heat transfer fluid.
Earth Axis Tilt
23.45°
Earth's degree of Variation
46.90° of angle of incidence - between summer and winter Solstice
One-Tank Closed-Loop System
A conventional DHW tank, usually electrically heated, is converted to a solar DHW storage tank by installing an external heat exchanger coil. The lower electrical element is removed, leaving the uppermost of the usual two elements to provide auxiliary water heating and to achieve good stratification (layering of hotter water over progressively colder water).
Heat Storage
A device or medium that absorbs collected solar heat and stores it for use during periods of inclement or cold weather.
Heat Exchanger
A device, such as a coiled copper tube immersed in a tank of water, that is used to transfer heat from one fluid to another through a separating wall.
Eutectic Salts
A group of materials that melt at low temperatures, absorbing large quantities of heat.
Solar Rights
A legal issue concerning the right of access to sunlight.
Langley
A measure of solar radiation ; equal to one calorie per square centimeter
Heat Sink
A medium or container to which heat flows.
Flat Plate Collector
A solar collection device in which sunlight is converted into heat on a plane surface without the aid of reflecting surfaces to concentrate the rays.
Indirect System
A solar heating or cooling system in which the solar heat is collected exterior to the building and transferred inside using ducts or piping and, usually fans or ducts.
Active System
A solar heating or cooling system that requires external mechanical power to moe the collected heat
Active System
A solar heating or cooling system that requires external mechanical power to move the collected heat.
Passive System
A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.
Selective Surface
A surface that absorbs radiation of one wavelength (for example, sunlight) but emits little radiation of another wavelength (for example, infrared); used as a coating for absorber plates.
Hybrid Solar Energy System
A system that uses both active and passive methods in its operation.
Collectors
AVAILABLE SOLAR ENERGY IS SO LARGE ONLY A SAMaLL AREA OF EARTH WOULD BE NEEDED WITH SOLAR COLLECTORS TO REPLACE ALL PRIMARY ENERGY SOURCES.
Pyranometer
An instrument for measuring solar radiation
Seasonal Change
Horizontal surfaces heat more in Summer. Vertical Surfaces heat more in Winter.
Energy Intensity
Btu per hour per square foot. SI Units: W/m2
Solar Radiation (Solar Energy)
Electromagnetic radiation emitted by the sun
Energy Intensity
Falling on a surface E (Btu/ft2) = L Btu/h/ft2 x cos θ where L = incident intensity θ= angle of incidence
Standby Heat Loss
Heat lost though storage tank and piping walls.
Daylighting
IT SHOULD BE MENTIONEd THAT THE DESIRED AMOUNT OF CLEAR SKY ROOF FOR DAYLIGHTING IS 5% (think Pantheon)
Defuse Radiation
Indirect Sunlight that is scattered from air molecules, dust and water vapor.
Closed loop DHW
Less efficient, requires antifreeze agent, lower pressure collector (has both a water heater tank and a heat exchange tank)
LCR
Load/Collector Ratio
Drain back DHW
Moderately efficient, no antifreeze, low pressure collector (has water heater tank and Heat exchange tank)
two reflections rule
Most of the infrared radiation is removed with two reflections (monitor roof)
Collector Tilt
Optimum solar collector tilt angles (as measured from face to ground toward the North) for active collection: Year-round domestic hot water- value of latitude, --winter space space heating- Latitude + 10-20 degrees, --Summer swimming pool- Latitude 10 degree
Radiant Panels:
Panels with integral passages for the flow of warm fluids, either air or liquids. Heat from the fluid is conducted through the metal and transferred to the rooms by thermal radiation.
Drain down System
Potable water is circulated from the storage tank through the collector loop. Freeze protection is provided by solenoid valves opening and dumping the water at a preset low temperature. Collectors and piping must be pitched so that the system can drain down, and must be assembled carefully to withstand 100 psi. city water line pressures. Pressure reducing valves are recommended when city water pressure is greater than the working pressure of the system.
Natural Convection
See Gravity Convection
Photovoltaic Cells
Semi conductor devices that convert solar energy into electricity
Thermistor
Sensing device which changes its electrical resistance according to temperature. Used in the control system to generate input data on collector and storage temperatures.
Flat plate water system
Simple collectors that heat liquid or air at temperatures less than 180 degrees F
Glaubers Salt
Sodium sulfate a eutectic salt that melts at 90°F. and absorbs about 104 Btu per pound as it does so.
SSF
Solar Savings Fraction
Air system
Solar domestic hot water systems employing air-type collectors are available. Hot air generated by these collectors is fan forced through an air-to-liquid heat exchanger with the potable water being pumped through the liquid section of the exchanger. The heated water is then circulated through the storage tank in a similar fashion to the liquid collector system. Air does not need to be protected from freezing or boiling, is non-corrosive, and is free. However, air ducts and air handling units require greater space than piping, and air leaks are difficult to detect.
Direct Radiation
Solar radiation that comes straight from the sun, casting shadows on a clear day.
Open System
Some part of the System is open to the atmosphere, or system contains fresh or changeable water.
Solar data
Table c.15 lists basic information for a variety factors during selected months: --Horizontal Surface Btu/day, --Vertical Surface (South (Btu/day), --Temperature Average (daily) --Heating Degree days.
Angle of Incidence
The Angle light or sound approaches a flat surface measured from a line normal (90 degrees) to that surface.
Solar Constant
The average intensity of solar radiation reaching the earth outside the atmosphere; accounting to two Langleys or 1.94 gram-calories per square centimeter, equal to 442.4 BTU/hr/ft.², or 1395 watts/m².
Absorber
The blackened surface in a collector that absorbs the solar radiation and converts it to heat energy
See Gravity Convection
The cooling of a building or heat storage device by the radiation of excess heat into the night sky.
Risers
The flow channels or pipes that distribute the heat transfer liquid across the face of an absorber.
Insolation
The total amount of solar radiation direct, diffused and reflected-tricking a surface exposed to the sky.
Gravity Convection
The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity
Gravity Convection
The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity.
Percentage of Possible Sunshine
The percentage of daytime hours during which there is enough direct solar radiation to cast a shadow.
Headers
The pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors. This insures that equal flow rates and pressure are maintained
Shading Coefficient
The ratio of the solar heat gain through a specific glazing system to the total solar heat gain through a single layer of clear double-strength glass.
Seasonal Efficiency
The ratio, over an entire heating season, of solar energy collected and used to the solar energy striking the collector
Drain back System
The solar heat transfer fluid automatically drains into a tank by gravity. Drain back systems are available in one or two tank configurations. A heat exchanger is necessary, because the city inlet pressure would prevent draining. The heat transfer fluid in the collector loop may be distilled or city water if the loop plumbing is copper. If the plumbing is threaded galvanized pipe, inhibitors may be added to prevent corrosion. Most inhibitors are non-potable and require a double wall heat exchanger. The pump used must be sized to overcome static head.
Insolation
The total amount of solar radiation direct, diffused and reflected-striking a surface exposed to the sky.
High Mass
Thick Wall- Small Window less light comes in. less heat inside
Atmospheric Absorption of solar radiation...
is greater when the angle of incidence to the ground plane is greater
Drain Down DHW
Very heat efficient but wastes water high pressure collector required no need for antifreeze
Evacuated Tube water system
can achieve extremely high temperatures (179-350F) twice as expensive as flat plate
Sun Shades
concrete mass on top of windows, porches galleries (Urban South)
South Facing Vertical Surfaces
fair much better, especially in the 25 degree and 40 degree N latitude curves which have the desired condition of maximal insolation in Winter and minimal in Summer.
Reflection of solar energy
increases dramatically for angles of incidence above 55 degrees. The light leaving the surface is also increasingly polarized.
diffuse radiation
indirect sunlight that is scattered from air molecules, dust and water vapor d
In skylights...
light must be filtered from the beginning (double layer)
trombe wall
non-water collector absorb solar energy and slowly releases it to interior spaces
Earth Axis tilt
on either Equinox the sun will rise due East and set due West. On the Equator, on noon, the sun will be directly overhead. Areas between the Tropics will have the Sun directly overhead ay time(s) during the year. Areas beyond the Arctic or Antarctic will have some days where the Sun does not rise and where the Sun does not set.
swimming pool heat
primarily summer - collector heat loss is low more heat available when pool is in use
Daily Radiation for Horizontal Surfaces
shows the Yearly minimum insolation occurs in the Winter when it is most needed and at a maximum in the Summer when its not. It also shows that the farther from the Equator, the more dramatic this effect becomes, which is exactly the opposite of what is desired.
Thermosiphon DHW
simple and efficient , but collector must withstand high municipal water pressures. it has a freeze danger
direct radiation
solar radiation that comes straight from the sin, casting shadows on a clear day
Winter space heating
sunshine in winter is required COLLECTOR HEAT LOSS TO COLD AIR IS CHALLENGING
Tilt angle
the angle that a flat plate collector surface forms with the horizontal plane.
azimuth
the angular distance between true south an the point on the horizon directly below the sun.
altitude
the angular distance from the horizon to the sun
auxiliary heat
the extra heat provided by a conventional heating system for periods of cloudiness or intense cold when a solar heating system cannot provide enough
Headers
the pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors. This insures that equal flow rates and pressure are maintained.
Thermosyphoning
the process that makes water circulate automatically between a warm collector and a cooler storage tank above it.
Absorptance
the ratio of solar energy absorbed by a surface to the solar energy striking it
collector efficiency
the ratio of usable heat energy extracted from a collector to the solar energy striking the cover
ambient temperature
the temperature of the surrounding air
Low mass-
thin wall- large window Solar shedding needed to control heat. Walls heat rapidly, Walls cool rapidly.
Unglazed water system
uninsulated most widely used collectors in North America Suitable when temperatures below 90 degrees F can be used, as in swimming pool heating.
transpire air collector
use a simple, elegant technology to capture the sun's heat to warm buildings. consists of dark, perforated metal plates installed over a building;s south-facing wall. an air space is created between the old wall and the new facade the dark outer facade absorbs solar energy and rapidly heats up on sunny days--even when the outside air is cold. a fan or blower draws ventilation air into the building through hundreds of tiny holes in the collectors and up through the air space between the collectors and the south wall. the solar energy absorbed by the collectors warms the air lowing through them by as much as 40 degrees F Unlike older space heating technologies, transpired air collectors require no expensive glazing, which caused energy loss due to reflection. The new collectors capture a record-breaking 80% of available solar energy.
