Understanding Solar Radiation Components and Internal Surface Energy Balance

1. Lecture Objectives: • Finish with Solar Radiation Components • Introduce Internal Surface Energy Balance

2. Next week: Intro to the energy simulation software (Project 1) Solar Decathlon 2015

3. Solar Angles qz • - Solar altitude angle • – Angle of incidence

4. Solar components • Global horizontal radiation IGHR • Direct normal radiation IDNR Direct component of solar radiation on considered surface: Diffuse components of solar radiation on considered surface: qz Total diffuse solar radiation on considered surface:

5. Global horizontal radiation IGHRand Diffusehorizontal radiation measurements qz

6. Measurement of Direct Solar Radiation

7. Ground and sky temperatures Sky temperature Swinbank (1963, Cole 1976) model • Cloudiness CC [0-1] 0 – for clear sky , 1 for totally cloud sky • Air temperature Tair [K] Tsky = (9. 365574 · 10−6(1 − CC) Tair6+ Tair4CC·eclouds)0.25 Emissivity of clouds: eclouds = (1 − 0. 84CC)(0. 527 + 0. 161exp[8.45(1 − 273/ Tair)]) + 0. 84CC For modeled T sky theesky =1 (Modeled T sky is for black body)

8. Ground and sky temperatures Sky temperature Berdahl and Martin (1984) model - Cloudiness CC [0-1] 0 – for clear sky , 1 for totally cloud sky • Air temperature Tair [K] • Dew point temperature Tdp [C] !!! Tclear_sky = Tair (eClear0.25) eClear = 0.711 + 0.56(Tdp/100) + 0.73 (Tdp/100)2 - emissivity of clear sky Ca = 1.00 +0.0224*CC + 0.0035*CC2 + 0.00028*CC3 – effect of cloudiness Tsky = (Ca)0.25* Tclear_sky esky =1

9. Ground and sky temperatures For ground temperature: - We often assume: Tground=Tair • or we calculate Solar-air temperature • Solar-air temperature – imaginary temperature • Combined effect of solar radiation and air temperature Tsolar = f (Tair , Isolar , ground conductivity resistance)

10. External convective heat fluxPresented model is based on experimental data, Ito (1972) Primarily forced convection (wind): Velocity at surfaces that are windward: Velocity at surfaces that are leeward: U -wind velocity Convection coefficient: u surface u windward leeward

11. Boundary Conditions at External Surfaces 1. External convective heat flux Required parameters: - wind velocity • wind direction • surface orientation N leeward Consequence: U Energy Simulation (ES) program treatsevery surface with different orientation as separate object. windward

12. Wind Direction Wind direction is defined in TMY database: “Value: 0 – 360o Wind direction in degrees at the hou indicated. ( N = 0 or 360, E = 90,   S = 180,W = 270 ). For calm winds, wind direction equals zero.” N http://rredc.nrel.gov/solar/pubs/tmy2/ http://rredc.nrel.gov/solar/pubs/tmy2/tab3-2.html leeward U windward Wind direction: ~225o

13. 2.5 m Internal surfaces 10 m 10 m HW1 Problem Solar angles and Solar radiation components calculation You will need Austin weather data: http://www.caee.utexas.edu/prof/Novoselac/classes/ARE383/handouts.html

14. Boundary Conditions at Internal Surfaces

15. Internal Boundaries Internal sources Window Transmitted Solar radiation

16. Surface to surface radiation Exact equations for closed envelope Tj Ti Fi,j - View factors ψi,j - Radiative heat exchange factor Closed system of equations

17. Internal Heat sourcesOccupants, Lighting, Equipment • Typically - Defined by heat flux • Convective • Affects the air temperature • Radiative • Radiative heat flux “distributed” to surrounding surfaces according to the surface area and emissivity

18. Surface Balance For each surface – external or internal : All radiation components Conduction Convection Convection + Conduction + Radiation = 0