1. Energy-efficient buildings
Paul Linden
Department of Mechanical and Aerospace Engineering
University of California, San Diego

2. Outline Wind-driven flow Stack-driven flow Underfloor air distribution
Historical perspective
Environmental perspective
Flow through an orifice
Wind-driven flow through a building
Stack-driven flow
The neutral level
Thermal plumes
Displacement ventilation produced by a single heat source
Mixing ventilation
Underfloor air distribution
Non-uniform cooling
Flow in the plenum

3. Wind-driven flow
Historical perspective
Environmental perspective
Wind-driven flow through a building

4. Yazd, Iran

5. Traditional wind tower, Iran

6. Al Arish, UAE

7. Jame Mosque Isfahan, Iran

8. Sheik Lotfollaf Mosque, Isfahan, Iran

9. Mai Hong Song, Thailand

10. Namwam banquet hall, Korea

11. Energy usage
Over 10% of total annual energy consumption in the US is used in heating and cooling of buildings – at a cost > $100B per annum
In LA, more energy is used in buildings than in transport
Built environment is responsible for > 30% of GHG emissions in US

12. Traditional buildings Modern buildings
Well shaded
Tall interior spaces
Heavyweight
Loose construction
Highly glazed
Low interior spaces
Lightweight
Tight construction

13. Ventilation requirements
For breathing and general fresh air require about 10 ls-1 per person
For a typical one-person office (5 m X 3 m X 2.5 m) ⇒ 1/6 ACH
This is a very low ventilation rate – to remove the heat (100 W) generated by 1 person this flow rate would require an interior temperature about 10 K above the ambient.

14. Ventilation strategies
Natural ventilation
flow driven by wind and temperature
Forced air – mechanical ventilation
fan-driven through ducts
Traditional HVAC
mechanical cooling, overhead distribution
Unconventional HVAC
mechanical cooling, unconventional distribution
Hybrid ventilation
combinations of the above systems

15. Low-energy strategies
Low-energy ventilation
Night cooling
Thermal storage
These have implications for the building forms and structure – need to be considered
at an early stage in the design

16. Natural Ventilation Ventilation driven by natural pressure forces wind
buoyancy - due to temperature differences; the ‘stack effect’
A temperature difference of 50C across a doorway 2m high will give a flow of 0.1ms-1

17. Wind-driven ventilation single-sided ventilation
cross ventilation
single-sided ventilation
Positive pressures on windward side
Negative pressures on leeward side and roof

18. Cross ventilation rules of thumb
Codes allow a zone to be considered “naturally ventilated” if within 6m of an operable window

19. Thermal zoning rules of thumb
6m glazed perimeter
zone is affected by
external environment
Stable interior zone
always requires
cooling

20. ASHRAE field research: Brager & deDear
Occupants in controllable naturally ventilated offices accept a wider range of comfort as acceptable

21. San Francisco Federal Building

22. Building geometry in the naturally ventilated floors
The building will be naturally cross-ventilated (C-V) in most of the floor plan in floors: 6-18.
The building volume with C-V measures: 107x19x52 m and starts at an elevation of 20 m.

23. Windward side
normal full open

24. Leeward side
normal full open:

25. 2- BMS + Informed Users
3- BMS + No Night Cooling
4- BMS + Uninformed Users
5- No BMS + Uninformed users

26. Stack-driven ventilation
The neutral level
Thermal plumes
Displacement ventilation produced by a single heat source
Mixing ventilation

27. Ionica, Cambridge

28. Portland Building, UK

29. BRE low energy office building

30. Inland Revenue Building, UK Architect: Michael Hopkins & Partners
Naturally ventilated office block – control at towers and fans at each vent opening allow outdoor air to cool the indoor space. Exposed concrete ceiling, daylighting

31. Hydrostatic pressure gradient
In a fluid at rest the weight of the fluid produces an increase in pressure with depth
Air is well represented as a perfect
gas

32. The neutral level
Pressure in air at rest is hydrostatic, so pressure gradient is
Thus pressure increases downwards and the gradient is larger when the air is cooler
For a warm building the pressure gradient inside is larger than outside

33. The neutral level height warm neutral level
Neutral level is the height where internal and external pressures are same
pressure

34. The neutral level height p1 p1 p2 warm p2 neutral level p3 p3 p4 p4
pressure
p4 > p3 - pressure difference drives inflow
p2 > p1 - pressure difference drives outflow

35. To stratify or not to stratify …
Displacement ventilation
Mixing ventilation
Minimum flow rate
Maximum outlet temperature
Maximum flow rate
Minimum outlet temperature

36. Displacement Mixing T+DT QDT T Q Filling box – Baines & Turner (1969)
Caulfield & Woods (2001)

37. Mixing flow – draining a hot space
1 window and 1 skylight

38. Mixing flow – draining a hot space
2 skylights

39. Displacement flow – draining a hot space
inflow

40. Single plume with displacement ventilation
Linden, Lane-Serff & Smeed (1990)
outflow
inflow

41. Single source of buoyancy with displacement ventilation
QDT Q
T+DT T
Upper layer has a uniform temperature
Temperature of upper layer is temperature of plume at level of interface
Flow through space is volume flux in plume at level of the interface

42. Flow rate →
local control

43. Entrainment constant α ≈ 0.1
Turbulent plume
Morton, Taylor & Turner (1956) z b
Plume width grows by entrainment
Entrainment constant α ≈ 0.1 B
buoyancy flux
volume flux
reduced gravity

44. Steady state
Match draining flow with MTT plume
buoyancy flux
volume flux
reduced gravity
- volume fluxes
At z = h equate
- densities

45. Children’s Museum, San Diego

46. Underfloor air distribution (UFAD)
Cooling part of the space
Effect on IAQ
Plenum flow

47. Technology Overview - UFAD Concept UFAD – the conceptual design
heat transfer from room into plenum causes supply air to warm up

48. Market Trends- USA

49. Under Floor Air Distribution
UFAD
stratification
layer

50. Initial case 1 heat source and 1 cooling vent

51. Flow in the plume
Heat source

52. The diffuser flow
diffuser

53. UFAD
To be used in the new HQ building for the New York Times in Manhattan

55. Measurements in plenum
75 temperature loggers installed in underfloor plenum
Produced color contour plots of hourly plenum temperature distributions
September 2 – hot day, night flushing
September 25 – cooler day, no night flushing

56. Temperatures in plenum
Movie
Temperature [F]

57. Temperatures in plenum
Temperature [F]