Transcript Document
Natural Ventilation – capabilities and limitations (comfort and energy efficiency in domestic dwellings) ATA Melbourne Branch presentation April 2008 Jim Lambert Ventilation - a few scenarios Summer: • Cooling sensation from airflow • Structural cooling on summer nights General (Winter or summer): • How much ventilation does a “healthy” house need? • How does ventilation affect heating and cooling? • How does ventilation affect the energy needed for heating and cooling? • How do we achieve comfort and energy efficiency together? Cooling sensation from airflow • In a mild summer, natural ventilation can reduce the apparent temperature Cooling sensation deg. C (e.g.up to 80C at an airflow of 2 m/s or so) 10 8 6 4 2 0 0 0.5 1 1.5 2 2.5 3 3.5 Airflow (m/s) Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants Cooling sensation from airflow Question: If you have natural ventilation and no mechanical cooling, what is the hottest summer temperature that allows indoor comfort? • People feel comfortable in still air at about 200C to 230C • Therefore it should be possible to have indoor comfort up to an air temperature of maybe 300C if you can get an indoor airflow of 2 m/s • The reference below has some interesting guidelines for achieving good natural ventilation Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants Some guidelines for good airflow cooling • Maximize air velocities in occupied rooms • Two openings on opposite sides increase airflow. Locate windows on opposite sides of the house. • An inlet window smaller than the outlet creates higher inlet velocity (e.g. 50% smaller) • Horizontal window openings are more effective than square or vertical openings • Vertical air shafts or open staircases or roof ventilation can take advantage of “stack effects” to increase airflow Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants Structural cooling on summer nights • The same kind of ventilation that gives a cooling effect in daytime also helps to cool the structure on summer nights Issues: • Thermal storage (e.g. concrete floors, masonry walls) can use this structural cooling to keep indoor temperature cooler during the next day . • Windows need to offer security while allowing ventilation airflow Temperature (deg C) Summer Example 30 20 10 6am Outside temperature 12noon day 6pm midnight night 6am time Temperature (deg C) Summer Example 30 20 Inside temperature 10 6am Outside temperature 12noon day 6pm midnight night 6am time Summer Example Temperature (deg C) Comfort range with moving air 30 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Summer Example Comfort range with moving air Temperature (deg C) Open all windows 30 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Summer Example Temperature (deg C) Open all windows Close all windows Comfort range with moving air 30 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Summer Example Temperature (deg C) Open all windows Close all windows Start internal fan Comfort range with moving air 30 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Summer Example Temperature (deg C) Open all windows Close all windows Start internal fan Open all windows Comfort range with moving air 30 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Summer Example Temperature (deg C) Open all windows Close all windows Start internal fan 30 Open all windows Gentle forced ventilation overnight Comfort range with moving air 20 Inside temperature 10 6am Normal comfort range Outside temperature 12noon day 6pm midnight night 6am time Comments on Summer Example Good features: • “Natural approach” with minimum energy consumption • Comfort level is fairly reasonable Limitations: • Poor safety margin for warmer days • Must pay attention to outside temperature • Needs lots of “hands-on” actions • Limited parts of the house are comfortable How much ventilation does a “healthy house” need? We need ventilation in these areas: • Humid or smelly places (bathrooms, kitchens) • Where there are people living and breathing (family room, bedrooms, etc.) How much ventilation do we need? • This question does not seem to have a simple answer How much ventilation does a “healthy house” need? (continued) There are Australian Standards about minimum ventilation • e.g. Australian Standard AS1668.2 1991 The use of ventilation and air-conditioning in buildings Part 2 – mechanical ventilation for acceptable indoor air quality (superseded) • e.g. Australian Standard AS1668.2 2002 The use of ventilation and air-conditioning in buildings Part 2 – mechanical ventilation for acceptable indoor air quality (plus 2 amendments and 1 supplement) How much ventilation does a “healthy house” need? (continued) • BUT people say that (for comfort) you really need MORE ventilation than the statutory minimum How does (natural) ventilation affect home heating and cooling? Summer (hotter outside than inside) • Good effects: – Movement of air around people (helps with cooling effect – already covered) – Venting of roof space (removes heating effect of hot air above ceiling) • Bad effects: – When external air is too hot for airflow cooling, fresh air flow for health requires energy for cooling How does (natural) ventilation affect home heating and cooling? (continued) Winter (colder outside than inside) • Good effects: – None (although you do need ventilation for health) • Bad effects: – When external air is cold, fresh air flow for health requires energy for heating – If roof space is ventilated, then potentially useful heat may escape to the atmosphere How does ventilation affect the energy needed for heating and cooling? • Ventilation replaces inside air with outside air • In winter, you need to heat the new air • In summer you need to cool the new air (if outside air temperature is more than say 30oC) • How much energy does this take? Effect of ventilation on energy for heating or cooling Sample calculation • Assumptions: • Size of ventilated space: 250m3 (floor area 100m2, ceiling height 2.5m) • Temperature difference 10oC inside versus outside • Rate of ventilation case 1 (low flow) 0.1 ACH case 2 (high flow) 3.0 ACH (ACH [Air Exchanges per Hour]) • Heat capacity of air 3.4*10-4 kWHr/m3 oC Effect of ventilation on energy for heating or cooling Sample calculation Heat/Cool power required (10oC difference): • Case 1: (0.1 ACH) Power required to maintain indoor temperature 85W (like 1 conventional light globe) • Case 2: (3.0 ACH) Power required to maintain indoor temperature 2.55 kW (like 1 hefty radiator) How do we achieve comfort with energy efficiency? The problem: • Comfort/health needs ventilation • Ventilation introduces outside air • Outside air is often at the “wrong” temperature • Heating or cooling the air needs energy How do we achieve comfort with energy efficiency? A solution – HRV (Heat Recovery Ventilation) (will be covered in Bernard Desormeaux talk) Other Ideas (from Danish experiment - see ref below) • In cold weather, draw incoming air from under PV panel to get solar pre-heating of the air • In hot weather, vent the roof space to atmosphere, draw external air over the ceiling to minimise heat load Guidelines used in experiment: • Efficiency of heat recovery at least 80-90% • Power consumption of ventilation 30-40W for a household • Building completely airtight (natural infiltration 0.1 air exchanges/hour) (assumes optimised insulation, low energy windows) • Noise level less than 25dB (ref “Cost effective PV assisted energy efficient ventilation systems for housing” Pederson, Cenergia Energy Consultants, Denmark) Winter heating example (Solar Venti) Winter heating example Summer cooling example Review Summer: • Cooling sensation from airflow • Structural cooling on summer nights General (Winter or summer): • How much ventilation does a “healthy” house need? • How does ventilation affect heating and cooling? • How do we achieve comfort and energy efficiency together? • These can limit the need for air conditioning • They could benefit from mechanical ventilation Quite a lot (Bernard will give a better answer) Natural ventilation “wastes” energy (Bernard will cover this)