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.