Energy-efficient buildings

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

2. Outline • Wind-driven flow • 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. Traditionalwind tower, Iran

6. Al Arish, UAE

7. Jame Mosque Isfahan, Iran

8. Sheik LotfollafMosque, 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. Well shaded Tall interior spaces Heavyweight Loose construction Highly glazed Low interior spaces Lightweight Tight construction Traditional buildings Modern buildings

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

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, UKArchitect: 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. QDT QDT T+DT T+DT QDT T Q Displacement Mixing 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 T+DT QDT T Q • 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. 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 Atz = hequate - 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 case1 heat source and 1 cooling vent Cooling vent Heat source