第 三 能源的應用範例 Examples of Energy Application 3-1 Home Energy Conservation and Heat-Transfer Control 3-2 Air Conditioning and Heat Pumps 3-3 Heat Engines

3-1Home Energy Conservation and Heat-Transfer Control

Building Materials (1) Material that is a good conductor of electricity, such as a metal, is usually a good conductor of heat. An insulator, such as fiberglass or Styrofoam, will retard the flow of heat from one object to another. We say that it provides a high “resistance” to heat transfer. Air can be a good insulator, especially if it is motionless, and so porous materials such as fiberglass have an excellent resistance to heat flow thanks to the air trapped inside.

Building Materials (2) The color of a material also influences heat transfer when radiation is concerned. A black object is both a better emitter and a better absorber of radiant energy than a white or shiny- surfaced object. To be in thermal equilibrium with its surroundings, an object that is a good emitter must also be a good absorber. Conversely, a poor emitter will be a bad absorber.

Building Materials (3) Starting from the same temperature, a hot black object will radiant energy faster than a similar hot light-colored object. Ice water will remain cold longer if it is placed in a silver-colored dish rather a charcoal-colored one. Light colored clothes are preferable on hot, sunny days, as they are poor absorbers of radiant energy. The absorber plates on solar collectors are painted flat black to better absorb the sun’s energy.

Building Materials (4) Add cream immediately or later?

Building Materials (5) The answer is that you should add cream as soon as possible. The cream makes the black coffee lighter in color, which makes it a poorer radiator. Also the temperature will drop when the cream is added, so the rate of heat transfer by radiation and conduction will decrease as a result of the smaller  T.

Building Materials (6) Thermos bottle

House Insulation and Heating Calculations (1) R=  /k

House Insulation and Heating Calculations (2)

House Insulation and Heating Calculations (3)

House Insulation and Heating Calculations (4) To obtain an R-value of 22 ft 2 -h- o F/Btu, you would have to use the indicated thickness of various materials

House Insulation and Heating Calculations (5) Superinsulated office building in Toronto, Canada

House Insulation and Heating Calculations (6)

House Insulation and Heating Calculations (7) Thermal drapes mounted in this arrangement will contribute to heat loss rather than stop them.

House Insulation and Heating Calculations (8) This arrangement is better for reducing convective losses.

House Insulation and Heating Calculations (9) Cold air infiltration can account for 50% of the energy needs of a house.

House Insulation and Heating Calculations (10) Caulking around the foundation of a home reduces air infiltration.

House Insulation and Heating Calculations (11)

House Insulation and Heating Calculations (12)

House Insulation and Heating Calculations (13)

House Insulation and Heating Calculations (14)

A very important factor in convective heat loss is the wind. Heat loss through walls and roofs as well as through window, door, and foundation cracks directly depend on the wind velocity. Heat load calculations indicate that an increase in wind velocity from 10 to 15 to 20 mph will increase the building infiltration by 100% and 200%. Site Selection (1)

Site Selection (2) Planting trees on the leeward side of a hill can substantially reduce the wind velocity over the site. Vegetation or walls can block or deflect natural air flow patterns and so reduce convective heat loss.

Cooling (1) Less expensive solutions to summer cooling loads can be achieved through many types of passive cooling techniques. The main objective here is to control the heat a building gains from its environments. These principles involve: * Site selection--- location, orientation, vegetation * Architectural features--- surface-to-volume ratio, overhangs, window sizing, shades * Building skin features--- insulation, thermal mass, glazing

Cooling (2)

Cooling (3)

Cooling (4)

Cooling (5)

Cooling (6) Radiant barriers can reduce heat gain in the attic of a house.

3-2 Air Conditioning and Heat Pumps

Air Conditioning and Heat Pumps (1)

Air Conditioning and Heat Pumps (2)

Air Conditioning and Heat Pumps (3)

Air Conditioning and Heat Pumps (4)

Air Conditioning and Heat Pumps (5) C.O.P. (Coefficient of Performance)= heat transferred/electricity input

Because a heat pump’s performance varies with the local climate, it is better to speak of a seasonally averaged C.O.P. The seasonal performance factor (SPF) is defined as SPF= total output/total energy consumed Air Conditioning and Heat Pumps (6)

Air Conditioning and Heat Pumps (7) An electrically driven heat pump using Freon as a working fluid. In principle, the system becomes an air conditioner if the fluid flow direction is reversed.

Air Conditioning and Heat Pumps (8) Relative costs of different types of heating fuels.

3-3 Heat Engines

The Energy Content of Fuels (1) The burning of hydrocarbon fuels is merely the combining of the carbon and the hydrocarbon from the fuel with oxygen from the air. C + O 2 -  CO 2 + heat energy H2 + O 2 -  H 2 O + heat energy These two simple reactions are both included in the overall reaction formula for an actual fuel. For example, C 7 H O 2 -  7CO H 2 O x10 6 calories per 100g C 7 H 16

The Energy Content of Fuels (2) The general pathways by which we utilize energy from fossil fuels

The Thermodynamics of Heat Engines (1)  =(1-Q cold /Q hot )x100% Efficiency=work done/energy put into the system =Q hot -Q cold /Q hot

Carnot Cycle: Q cold /Q hot =T cold /T hot  =(1-T cold /T hot )x100% The Thermodynamics of Heat Engines (2)

Generation of Electricity (1) A diagram of a fuel-burning electric power plant. Here a river provides cooling water to the condenser.

Generation of Electricity (2) An elementary alternating current generator. A loop of wire is forced to rotate in a magnetic field.

Generation of Electricity (3) Typical efficiency of an electric power plant for converting chemical energy in the fuel into electric energy. The best new plants now achieve nearly 40%.

Gasoline Engines The four strokes of a four-cycle spark-ignited internal combustion engine. (a) compression, (b) combustion, (c) exhaust, and (d) fuel-air intake.

Diesel Engines Ignition is accomplished by the high temperature produced by the compression of air.

Cogeneration A small cogeneration plant that uses the combustion of natural gas to drive a gas turbine coupled to an electric generator. The hot exhaust gases boil water to steam for use in space heating and cooling.

Multiple Choice Questions 1. The “R-value” of material is not related to its ________. a. composition b. area c. thickness d. resistance to heat flow

Multiple Choice Questions 2. Infiltration in a house will ________. a. increase on windy days b. raise the heating load by about 10% c. reduce the number of air changes d. be increased by caulking around windows

Multiple Choice Questions 3. Hot water in a _______ can will cool off faster than hot water in any other color can. a. white b. redc. blackd. silver

Multiple Choice Questions 4. A heat pump will have an efficiency (heat out to electrical in) of about ________. a. 35% b. 50% c. 75%d. 100% e. 200%

Multiple Choice Questions 5. At a restaurant you are served coffee before you are ready to drink it. In order that the coffee be the hottest when you are ready for it, when would it be best to add cream (at room temperature) to it? a. when you are ready to drink it b. right when you are served the coffee c. it doesn’t matter, as the final temperature will be the same