5. Thermal Design. Objective: Control heat flow to: Maintain comfortable indoor conditions

Transcription

1 5. Thermal Design Objective: Control heat flow to: 2. Maintain comfortable indoor conditions 3. Reduce heating/cooling loads, which reduces operating costs 4. Control vapor movement/condensation 5. Design to accommodate contraction/expansion of building

2 Heat Flow T Heat flow Warmer area Cooler area T2 T1 Temp gradient Factors affecting thermal energy flow: 2. Solar radiation 3. Air temperature 4. Wind/air movement 5. Humidity

3 Flow of Thermal Energy Conduction : direct transfer by contact of solid, liquid or gas

4 Flow of thermal Energy- Continue Convection: transfer of heat by the movement of air or water

5 Flow of thermal Energy- Continue Radiation: Flow of energy in the form of electromagnatic waves ( light, ultraviolet, infrared heat) Solar radiation is a function of latitude, time of the year, slope/orientation of the surface

6 Solar Effect Absorptance > 0.95 Reflectance > 0.95 Emittance > 0.95 Transmittance (calculated) Note that Reflection + emittance + transmittance is equal to 100%

7 Thermal Performance Factors affecting thermal performance: Air space Thermal mass Thermal resistance

8 Thermal Mass Heat migrates through solid materials from the hot side to the cooler side. The time delay involving absorption of the heat is called thermal lag The amount of energy necessary to raise material temp is proportional to the wt of the material

9 Thermal Mass- Continue Heavy materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies

10 Thermal Resistance Thermal conductance, C, is the time rate of heat flow through ft2 of area at a temp difference of 1 F for a specific thickness of material. Thermal conductivity, k, is the conductance for a standard unit thickness.

11 Thermal Resistance- continued Btu/h.ft2. F Thermal resistance F/Btu/h.ft2 C=k n R=l c Btu/h.ft2. F.in. Thickness in inches 1/R range > 8.0 (per t = 1n) The thermal efficiency of a building s component/assembly are normally judged by its accumulative thermal resistance. RT =? Ri of each layer

12 U-value Overall heat transfer coefficient: U=I thermal transmittance RT Btu/h.ft2. F It is useful in determining the overall thermal performance of a bldg envelope that includes different construction assemblies in parallel heat flow. Some bldg energy codes give allowable uvalue based on a weighted average for the coefficients for opaque walls, windows (fenestration) and doors.

13 Thermal Gradient

14 Effect of Insulation on Thermal Gradient

15 Caculation of Thermal Gradient

16 Effect of Thermal Inertia

17 Effect of Thermal Inertia- Continue

18 Effect of Moisture Content on Thermal resistance

19 Thermal Energy Thermal energy can flow from one object to another by: Conduction - direct transfer by contact of solid, liquid or gas. Convection - transfer of heat by the movement of air or water Radiation - flow of energy in the form of electromagnetic waves (light, ultraviolet, infrared heat) Natural convection - movement of fluid air by difference in temp/density. Forced convection - movement of fluid by a pump or fan. Direct Diffuse Reflected (see figure) Solar radiation is a function of latitude, time of the year, slope/orientation of the surface. Evaporating - function of temp, RH, air movement.

20 Factors affecting thermal performance Air space - affects both conductive and convective heat flow. Wide/long air space ---> reduce conductive HT. Narrow air space ---> reduce convective HT. For LV4 air space ---> diminish convective heat flow and increases conductive heat flow.

21 Factors affecting thermal performance- continued Mass Heat migrates through solid materials from the hot side to the cooler side. The time of delay involving absorption of the heat is called thermal log. The amount of energy necessary to raise material temp is proportional to the wt of the material. Heave materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies.

22 U-value continued U0 = (UwAw)+(UfAf)+(UdAd) Aw+Af+Ad Overall heat gain/loss per ft2 = U0 (t0 - ti) W here t0 - ti is the Temperature difference is between outside and inside. Af Aw Ad

23 Types of Insulation Loose (fibers,chips) - fill insulation (poured, blown) Flexible and semirigid (batt, blanket) Rigid (wood, fiberglass board) Formed-in-place (urethane foam)

24 Effects of Insulation Improve the thermal performances of building walls and roofs by reducing both conductive heat flow through the section and corrective heat flow in air spaces: 2. Results in more comfortable indoor air temp and less fluctuation 3. Reduces cooling/heating loads Thermal efficiency of insulation depends on: 6. Thermal resistance R 7. Stability over time (R value dimensional stability) 8. Resistance to deterioration 9. Securing attachments

25 Effect of Insulation- Examples

26 Effect of Thermal InsulationContinue

27 Effects of Thermal Bridging A thermal bridge occurs when a subject of high thermal conductivity penetrates a material of low thermal conductivity (insulation) increasing the rate of heat flow at the penetration. To account for thermal bridging correction factors (<1.0) should be used. Example 1 Use table 3.9 in text for correction of R value ( > 0.38).

28 Thermal Bridging- Examples

29 Insulation Requirements State and local codes specify requirements for minimum thermal resistance of bldg components to save energy, see map.

30 Recommended Minimum Thermal resistance of BE in the US

31 Loss of Thermal Resistance It is recommended to have TRR be greater than 80%. Less than 80% insulation is considered wet. See table 3.13 in text. Thermal Resistance Ratio: TRR = wet thermal resistivity dry thermal resistivity