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Heat Energy Flows in Buildings

Heat Energy Flows in Buildings. Building energy fundamentals. Sensible vs. Latent Heat Flow. Sensible Heat Flow – Results in a change in air temp

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Heat Energy Flows in Buildings

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  1. Heat Energy Flows in Buildings Building energy fundamentals

  2. Sensible vs. Latent Heat Flow • Sensible Heat Flow – Results in a change in air temp • Latent Heat Flow – Results in a change in moisture content. Release or storage of heat associated with change in phase of a substance, without a change in substance temp. • Total Heat Flow – sum of latent and sensible heat flows • Heat Energy Flows in Buildings

  3. Sensible vs. Latent Heat Flow • Sensible vs. latent heat: it takes over 5x as much heat to turn water into steam at the same temp than it does to heat liquid water from freezing to boiling temps.

  4. Sensible vs. Latent Heat • Whenever an object is at a temp different from its surrounds, heat flows from hot to cold • In similar fashion moisture flows from areas of greater concentration to areas of lower concentration • Buildings lose sensible heat to the environment (or gain) in 3 principle ways

  5. Conduction, Convection, Radiation • Conduction: transfer of heat between substances which are in direct contact with each other • Convection: movement of gases/liquids caused be heat transfer. As a gas or liquid is heated it warms, expands and rises because it is less dense • Radiation: electromagnetic waves travelling through space. When these waves hit an object they transfer their heat to it

  6. Conduction takes placed through envelope assemblies • Convection is the result of wind or pressure drive air movement • Radiant heat is primarily from the sun

  7. Thermal Effects • Principals are the same, but heat flow under changing conditions is more complex than under static conditions • Heat storage within materials is of greater concern during dynamic conditions • Under static conditions, heat flow is primarily a function of temp difference and thermal resistance

  8. Thermal Effects • Under dynamic conditions, those two factors are still important, but the heat storage in the building envelope is a compounding issue • Heat storage is a function of the density of the material and specific heat; product of these two is the thermal capacity (thermal mass)

  9. Thermal Properties • Every material used for the envelope has properties that determine their energy performance

  10. Thermal Conductivity (k) • Material’s ability to conduct heat • The faster heat flows through a material the more conductive it is • q=Resultant heat flow (watts) • k=thermal conductivity (W/mK) • A=surface area (m2) • T=temp diff between warm and cold sides(K) • L= thickness or length of material (m)

  11. Thermal Conductance • Conductivity per unit area • In basic building materials heat flow is generally measured by conductance (C), not conductivity • Is an object property which relies on the materials and the size

  12. U-factor • In layered assemblies, conductance is combined into a single number called the U-factor or U-value • Lower U factor means worse conduction, which means better insulation • Does not include latent heat (moisture related) • Used only to describe air flow from the outside of the envelope to airflow on the inside of the envelope (ie not for basement walls)

  13. Thermal Resistance (R-value =1/U) • How effective a material is as an insulator • R is measured in the hours needed for 1BTU to flow through 1ft2 of a given thickness of material when the temp diff is 1f • Object property, not material • A 2x6 pine stud has three times the R-value as a 2x2 pine stud • Higher R value indicates better insulating properties

  14. Building Energy Loads • How much energy your building needs • Can be provided by electricity, fuel, or passive means • Lots of terms that can get confusing, next slide has a chart to help with these terms

  15. Energy Loads • Thermal Loads – heating and cooling energy needed to keep people comfortable • Heating loads – energy required to heat the building when to cold • Cooling loads – energy required to cool the building when to hot • Not just about temp, include moisture control (latent heat)

  16. Loads • Heating and cooling loads are met by the HVAC system • Uses energy to add/remove heat and condition the space • Equipment loads – HVAC etc, met by energy or fuel • Plug loads – electricity for computers and appliances • Lighting loads – electricity used for lighting

  17. Thermal Loads • Understanding the heating and cooling loads helps to provide the right sized HVAC system for a space • Reduce the loads as much as possible, and meet them as efficiently as possible

  18. External • Heat transfer through the building envelope from the sun and outside environment • Building envelope includes the roof, walls, floors, windows. Anything that separates inside from outside

  19. External • Common ways heat flows into or out of the building • Heat conduction from the envelope to outside air or ground • Sunlight shining through windows to heat int. • Sunlight warming up ext. of building • Losing inside air to outside, or vice versa, through leaks

  20. How much energy from the sun’s radiation, outside air temp, latent heat in the airs moisture that reaches the inside to affect the comfort depends a lot on the envelope • Materials, design and how well it is sealed • Understanding where heat energy is gained/lost is important for successful passive design strategies

  21. Internal Thermal Loads • Come from heat generated from people, lighting, and equipment (core loads, internal gains) • Thermal loads from lighting and equipment is generally equal to their use • When a light fixture converts a watt-hour of electricity into photons, those photons bounce around until they are absorbed which turns light energy into heat energy • All electrical energy not turned into photons is turned directly into heat energy due to inefficiency

  22. Internal Thermal Loads • Similarly, electrical energy used to move mechanical parts is transformed into heat via friction • Energy used to power this equipment is turned into heat energy via electrical resistance • Thermal loads from people depend on the number of and what activity they are doing • Office buildings are generally dominated by internal loads • Single family residences are typically dominated by external loads

  23. Internal Thermal Loads

  24. Heating and Cooling Loads • How much energy you need to heat and cool the building and control moisture within • Gains that are more than envelope and ventilation losses would cause a net cooling load (the building is too hot) • Losses that are more than the internal gains would cause a net heating loads (the building is too cold). • The heating setpoint is often different than the cooling setpoint, so the distribution of heating and cooling loads is climate dependent

  25. Equipment and Lighting Loads • Lighting loads – energy used to power electric lights, make up nearly 1/3 of commercial building energy use (10-15% in residential) • Look for more efficient lighting • Reduce lighting loads • Reduce cooling loads for the same visible brightness

  26. Plug Loads • Electricity used for other equipment • 20-30% of energy in commercial • 15-20% in residential (these numbers are growing) Equipment Rated Power (watts) Desktop computer 120  Notebook computer  45  17” LCD Display 75  Desktop laser printer 120  Office laser printer  250  Office copier 750  Refrigerator 750  Dishwasher 1,200  Television 100  Commercial refrigerator  1,000  Commercial fryer  10,000  Clothes washer 350  Clothes dryer 2,000

  27. Measuring Energy Use • Energy Use Intensity - Energy intensiveness is simply energy demand per unit area of the building's floorplan, usually in square meters or square feet

  28. EUI based on Building Type

  29. EUI based on Floorspace

  30. Site Energy vs. Source Energy • Energy intensiveness only considers the amount of electricity and heat that is used on-site ("secondary" or "site" energy) • Does not consider the fuel consumed to generate that heat or electricity. This "primary" or "source" energy can be generated on-site or at a power plant far away.

  31. Source Energy • When measure energy used to provide thermal or visual comfort, site energy is the most useful measurement • When measuring total energy usage, source energy is more accurate • Low on-site energy can cause more use upstream • Ex, 2kW of natural gas burned on site is better than 1 kW of electricity used on site. 1kW of site electricity from the average US electrical grid is equal to 3.3kW of source energy due to inefficiencies • Energy Efficiency

  32. Source-Site Ratios

  33. Energy End Use • Commercial and residential use energy differently • Commercial are dominated by internal thermal loads (more people and equipment) • Residential are dominated by external loads, larger percentage of energy use is for heating and cooling to meet those

  34. Sankey Diagrams

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