Principles of Energy Flashcards
What is energy?
A measurable quantity of heat, work, or light
What are the only two significant sources of energy?
Sun, nuclear material
What’s potential energy?
Stored energy
What’s kinetic energy?
Energy in transition or motion changing from one type of energy to another
Doing work requires two things:
Converting potential energy to kinetic, Transporting the energy to where its needed
What’s the easiest form of energy to move?
Electricity
What’s the most difficult form of energy to move?
Mechanical energy because of huge losses due to friction
What’s the first law of thermodynamics?
Energy is neither created nor destroyed
Energy transfers and changes forms
What’s the second law of thermodynamics?
In any system, heat always moves from high temperature to low temperature, unless there is some other external source of energy
What’s temperature?
A measure of the kinetic energy, or molecular movement, within an object
What is heat?
The flow of thermal energy between two objects whenever there is a temperature difference between them
What is enthalpy?
The internal heat of an object, measured in BTUs per pound
What’s a BTU?
A measurement of heat. 1 BTU = the heat required to raise 1 pound of water 1 degree F.
What’s specific heat?
Number of BTU’s that one pound of any material will absorb for each degree F of temperature change.
What’s the specific heat of water?
1 BTU/lb?/degreeF
What is sensible heat?
Heat that causes a temperature change
What is latent heat?
Heat that causes a change of state
What type of heat can transfer more energy?
Latent heat
Describe steam heating.
Water is vaporized (absorbs latent heat)
Steam is condensed (releases latent heat)
Describe air conditioning.
Refrigerant is vaporized (absorbs LH)
Refrigerant is condensed (releases latent heat)
What is power?
The rate at which work is done or energy is released, measuring the capacity to do work. Measured in Watts, HP, BTUH
What is energy?
The amount of work that gets done in a given timeframe. Energy has a time component.
Energy =
Power x Time
Heat, electricity, and fluids flow when 2 things occur:
An open path allows their flow
There is a pressure difference.
Heat: path = molecules, pressure = temp difference
Electricity: path = wires, pressure = voltage
Water: path = pipe, pressure = height difference
Fluids flow from…
high pressure to low pressure
When pressure equalizes, flow will ______.
stop
What are three types of flow paths?
Conductor - allows rapid flow
Resistor - flow occurs slowly
Barrier - flow is stopped or negligible
Heat flows in what three ways?
Conduction, convection, radiation
What is conduction?
Heat flow through solids, between adjacent molecules of objects in contact with one another
Conductance, or K-value, measures what?
The rate of heat transfer through materials
What is convection?
The transfer of heat through moving fluid (typically air or water)
What is radiation?
The transfer of heat through space from one object to another
All types of radiant energy are categorized by what?
The electromagnetic spectrum
Solar energy consists of…
UV (5%) Visible light (46%) Infrared heat (49%)
When solar radiation strikes an object, 1 of 3 things occurs:
Reflectance, absorbance, or transmittance
What are the only common substances that reflect both solar and infrared radiation?
Polished aluminum, steel, tin, silver, nickel
What’s the only common material that transmits most solar radiation?
Glass
Glass creates a greenhouse effect because it…
transmits solar radiation but absorbs earthly infrared
Emittance or Emissivity measures…
a material’s characteristic ability to emit radiant energy. Emittance of 1 is a theoretically perfect emitter. Most materials have emittances of 85% to 95%.
What are Low-E coatings?
Metallic coatings that are placed on glass in order to reduce solar transmittance by reflecting most radiation and to reduce the flow of radiant heat from outside to inside (or opposite depending on climate) by decreasing the emissivity of the glass.
What is comfort?
Being in thermal equilibrium with the environment without having to sweat or shiver.
What conditions determine outdoor comfort?
Temperature, humidity, solar radiation, precipitation, wind
ASHRAE 55 defines the factors of indoor comfort:
Air temp, radiant temp, relative humidity, air movement (metabolic rate, clothing - uncontrolled)
What’s the primary factor in indoor comfort?
Air temperature
What are ASHRAE’s indoor design temperatures?
70 degrees F in winter
75 in summer
What is ASHRAE’s heating balance point temperature?
65 degrees F - the outdoor temp above which no heating is needed to maintain 70 indoors. Sun and heat sources make up difference. Heat gain and heat loss are in balance.
What’s a Heating degree-day?
A unit to describe how long the temp is below 65 degrees and are thus used to determine the intensity of a heating season.
How do you calculate HDDs?
- Calculate avg temp
- Subtract it from balance point (65)
Answer is the HDD for that day. The HDD From every day are added together to obtain the yearly HDD total.
What’s the cooling balance point?
The outdoor temp below which no indoor cooling is needed to maintain 75 indoors. The default is 65 degrees.
What’s a cooling degree-day?
A unit to describe how long the temp is above 65 and are thus used to determine the intensity of a cooling season.
How are CDDs calculated?
- Calculate avg temp
- Subtract it from the balance point and remove negative
Number gives CDD for that day.
Why are CDDs less reliable than HDDs as a predictor of cooling costs?
Cooling is more complicated due to other factors of summer comfort: relative humidity, solar gain or shade, internal loads
Homes with poor insulation, air leaks, and single-pane windows will have ______ interior surfaces.
colder
Well-insulated and air-sealed homes maintain _____ interior surfaces, contributing to better comfort.
warmer
Relative humidity is determined by 2 things:
Absolute amount of water vapor in the air
Temperature of the air
_____ air can hold more moisture than _______ air.
Warmer, colder
As air cools, its relative humidity _______.
increases
100% relative humidity indicates that the air is at its ________, or is completely saturated with water. Rain (or condensation) will occur.
dew point
Dew points above ___ degrees are considered uncomfortable.
68 degrees F
Does humidity affect heat flow?
No, there is no energy advantage to inside humidity in winter.
Indoor relative humidity guidelines:
In summer, ceiling fans are only effective if:
You can feel the moving air
The thermostat is turned up considerably
What is psychrometrics?
The science of air and its properties related to buildings and comfort
A psychrometric chart is used to understand human comfort and the energy required to produce it. Each state point on the chart has values for:
Dry bulb temp Wet bulb or dew point temp Enthalpy Relative humidity Absolute humidity
What is enthalpy?
The total energy of the air measured in BTU/lb. Enthalpy is sensible heat + latent heat.
What is relative humidity?
The ratio, in percent, between the actual water vapor in the air vs what the air can hold at that temperature.
What is absolute humidity?
The actual amount of water in air, measured in grains of water vapor per pound of dry air.
