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The paper analyses the thermal performance of the 'show house' in BedZED situated in Beddington, South London. This is achieved using two different models made in TAS software, one with sunspace and one without. The thermal performance is analysed qualitatively using site observations and questionnaire and quantitatively using on-site measurements and simulations using TAS. In this way, it wishes to explore 'how well the Sunspace performs in making the internal condition of the dwelling comfortable in BedZED'.
Keywords: Passive solar design, thermal analysis, Sunspace
'In 2002 around 25,757 million tonnes of coâ‚‚ were emitted from burning fossil fuel. In an average year 250 million people are killed, face starvation, require urgent medical attention or are made homeless because of climate-related disaster - this is five times the number recorded in 1980â€¦â€¦â€¦ The energy demands of modern, urban lifestyles need to be reduced by at least 80 per cent by 2050 to ensure that the residual demand can be met from renewable energy sources'. Dunster, B. et al. 2008. The ZED Book.
Beddington Zero Energy Development is the most well documented Zero Carbon Development in Hackbridge, London Borough of Sutton. The project demonstrates that zero carbon community can be built which is financially feasible for the developers. Completed in 2002, it is arguably the most successful example of sustainable living in UK.
The goal of the project was to create 'net-zero fossil energy development'. One that produce at least as much energy from renewable sources as it consumes. [www.greenlinblog.com]
It was designed to create a prosperous community in which ordinary people could enjoy a high quality of life, while living within their fair share of Earth's resources.
It is a mixed use scheme live and work concept with total 82 units, 271 habitable rooms plus over 2500 mÂ² of office space, studios, shops and community facilities.[BedZED best practice report]
81% reduction in energy use for heating at 5.2kWh/person/day.
45% reduction in electricity use 3.4kWh/person/day.
64% reduction in car mileage 2,318 Km/year.
58% reduction in water use 72litres/person/day
In comparison to the Borough average. [BioRegional-solutions for sustainability]
Materials used in construction were carefully selected for low environmental impact, wherever possible sourcing locally reclaimed and recycled materials. For achieving energy efficient carbon neutral design BedZED invested in more construction materials than standard UK housing, still there embodied environmental impact is 675kg/mÂ² compared to 600-800kg/mÂ² for standard UK housing.
SAP rating for BedZED is equivalent to 150. [bedzed Total Energy Strategy] It achieves density of 50 dwellings/ hectare, 120 workspaces/hectare and over 4000mÂ² of green open space/hectare.
[Bedzed best practice report]
THERMAL BEHAVIOUR OF BUILDINGS
A building can be considered as a thermal system, with a series of heat inputs and outputs similar to human body.
Qi - internal heat gain
Qc - conduction heat gain or loss
Qs - solar heat gain
Qv - ventilation heat gain or loss
Qe - evaporative heat loss.
The system can be depicted by the following equation:
Qi + Qc + Qs + Qv + Qe = âˆ†S
where âˆ† S is a change in heat stored in the building.
Thermal balance exists when the sum of all heat flow terms, thus âˆ†S , is zero: If the sum is greater than zero, the temperature inside the building is increasing, or if it is less than zero, the building is cooling down.
Site is located in London borough of Sutton, Beddington, in South London.
Latitude: 51.21N, Longitude: 00.11W
London has temperate marine climate i.e. cold and wet for most part of the year. Average relative humidity is more than 79% for most of the year. The average dry bulb temperature is 10.63Â°C with maximum DBT 28.6Â°C in May and minimum -5.4Â°C in January. Predominant wind direction is from South West direction, with maximum wind speed of 13.6m/s in March, and minimum wind speed of 0m/s in September.
Sky is overcast for 68.9% time of the year, and an average direct solar radiation of 109.43W/mÂ².
The thermal comfort zone is 17Â°C - 24Â°C, for most part of the year temperature is below comfort zone
SITE MICROCLIMATE ANALYSIS
BedZED is built on a brownfield site (As per www.sustainablebuild.co.uk: In UK a brownfield site is defined as "previously developed land" that has the potential for being redeveloped. It is often (but not always) land that has been used for industrial and commercial purposes and is now derelict and possibly contaminated.) previously used for sewage sludge spreading.
On-site observation and spot measurement of temperature and illuminance using multimeter helped in determining building performance under real condition.
The measurements were taken on an overcast sky day on 19th March at around 2PM. For the spot measurement building was divided in the grid of 1m c/c.
External Temp: 15.6Â°C.
Overall internal temperature was constant at around 19-20Â°C on both ground and first floor. Temperature was well within comfort level without the heating.
Client: The Peabody Trust (One of the largest and oldest housing associations in London)
Architect: Bill Dunster Architects (ZEDfactory)
Environmental Consultants: BioRegional Development Group
Energy and engineering consultants: Arup
Gross Site area: 1.7 hectares
Number of Dwellings: 82
Area of public green space: 4873sqm
Area for Car Parking: 986sqm
The housing is mixed one, two and three bedroom flats and maisonettes. All the buildings are placed in such a way that it benefits from passive solar gain, day-lighting and private garden or outdoor space. The residential units terraces are facing south to benefit from passive solar gain and each residential units are backed by North facing offices, where minimal solar gain is required, too avoid overheating in office, lit by large triple-glazed north lights.
300mm thick 'jacket' super-insulation is applied to the entire building, fully glazed on the South with sunspace and minimum openings on the rest of the sides, by doing this building heat losses are reduced to such an extent that they can be compensated by internal incident heat gains. Everyday activity such as cooking, cleaning, occupants own body heat and electric appliance generates enough heat to keep the building at a comfortable temperature. This eradicates the use of space heating for a major part of the year.
Concrete floor slab and dense concrete blockwork provide good thermal mass, which absorbs heat during warmer period and releases during cooler times, which helps in keeping the building temperature constant. Internal partition walls and doors are not insulated so that they help in spreading the heat gained during day time in south facing rooms and kitchen to the rest of the house. Internals doors have undercuts for the same reason.
If the temperature falls below 18Â°C in any of the rooms when the house is not occupied, a back-up trickle heat source is activated. Special care was taken care to avoid any thermal bridge in the building while construction.
Heat loss in building fabric is so much reduced that heat loss through air flow becomes very significant to avoid it air tightness of 2 air change per hour (at 50kPa) keeps building fabric air leakage to minimum this is much more rigorous in comparison to standard practice in UK of 15-30 air change per hour.[bedzed toolkit part2]
The most noticeable feature of BedZED the Wind Cowl placed on the roof provides well ventilated healthy environment in the building by passive ventilation. These cowls are placed on bearing, which aligns itself with the wind direction, and helps in removing stale air from kitchen and toilet and providing fresh air in. The heat exchanger in the wind cowl takes the warmth of the outgoing stale air and heats the incoming fresh air, 70% of the heat that would have lost through ventilation is recovered using this system and this works totally on wind speed and the buoyancy forces of hot and cold air doesn't require electricity for its functioning.[ZED products 'From A to ZED'
PASSIVE SOLAR DESIGN
Passive solar design features, for reducing heat load in winters include:
Large amount of South facing windows to capture low winter sun.
Sun space on the South to trap heat during winters and minimising heat loss through conduction.
High thermal mass to soak excess heat gain during the day and releasing it during the night and on cloudy days when its needed, without the need of additional heating.
Minimum openings on the North to avoid any heat losses.
Passive solar design features, for the need of cooling during the warm months include:
Minimal east/west glazing. West glazing can cause lots of overheating in summer because of low afternoon sun.
Weather sheds on the south to prevent summer sun to penetrate into the building.
High thermal mass absorbs excess heat during the day and releases during the night which can be exhausted via night cooling ventilation.
Sun space is well ventilated to exhaust any excess solar gain.
Main windows are oriented on the south and the smaller on the North. Weather sheds on the south blocks high summer sun to enter in the building which prevents overheating and allows the lower winter sun for solar passive heating.
The graph shows the results of simulation of BedZED sunspace in between January and May. It compares percentage of time during the day (10AM to 8PM) the temperature of the sun space, to that of external temperature. There is no heating in the sunspace. The graph shows for more than 24% of time, in between Jan to May the temperature is above 20Â°C and for 50% of the time temperature is above 16Â°C. Overall throughout sunspace is 6Â°C warmer than external temperature.
In winters the outer windows are closed and the inner doors are opened to allow sunlight to penetrate and the heat to pass through, from sun space to the building, since almost 25% time during the first five months temperature is above 20Â°C. Sun space can be used as an outdoor terrace in most of the winters and can be part of living room in the summers. During the summers the outer windows are kept open and the inner windows are kept shut, doing this warm air is exhausted out externally without actually reaching the habitable room areas, since during summer days external temperature is higher than comfort level. Sun Spaces can also be used for drying clothes and growing plants.
Windows facing south (i.e. sun space) both the skins are double glazed. The building effectively has quadruple glazing with a very large number of three airspaces. This configuration means overall flow of heat in winters is from outside to inside, with south facing windows harvesting more heat in the winter than it losses. Specifications for the south facing windows are double glazed, soft low E coated, argon filled sealed units with a minimum overall U-value of 1.4 W/mÂ²K, high performance softwood for window and door frames.
As per Living Planet report, 2000, WWF. In UK the ecological footprint of each person is 6.29 hectares. This means that we need 3 planets to sustain with the current UK lifestyle.
Most of the construction materials used in BedZED have very low embodied energy and has very little impact on environment. Entire BedZED construction material were sourced within 65.4 miles by weight compared 106.8 miles for other UK buildings using same materials. By doing this some 120 tonnes of COâ‚‚ emissions were saved. [toolkit part 1] Just by the right choice of materials the total environmental impact of the development was reduced by 20-30%.
Groundworks: Before construction site was used for sewage and sludge spreading. Almost 75% of the contaminated soil was used in the building plinths. Gravel found on site excavation was re-used as sub grade under the road.
Building Envelope: Materials were carefully considered for achieving zero space heating depending on super-insulation, air tight construction, high performance glazing with minimum openings on the east, west and north, passive solar gain, high thermal mass, wind cowl with heat recovery, sunspace on the south, thoughtful placements of the rooms.
Insulation: Entire building is insulated with 300mm thick insulation, walls with rockwool, floors and roofs with extruded polystyrene.
Ground floor: 6mm tile, 50mm screed, 200mm precast hollow core slab, 300mm expanded polystyrene, compressed soil. U value: 0.10 W/mÂ²Â°C
External Walls: Bricks were ordered from local brickworks, just 20 miles from the site. Dense concrete blocks were chosen to provide high thermal mass and effective acoustic insulation which are used on the internal face of the cavity wall.
20mm Plaster, 140mm block, 300mm rockwool Insulation, 120mm brick. U value: 0.11 W/mÂ²Â°C
Ceiling: 50mm soil, 250mm expanded polystyrene insulation, 15mm thick bitumen layer, 200mm precast Hollow core slab. U value: 0.11 W/mÂ²Â°C
Intermediate floor slabs:
200mm thick Pre-stressed concrete hollow core floor slab are used which provides good quality finish and saves time on site in comparison to in-situ and uses less material and has low embodied energy, so it was value for money.
8mm thk tile, 50mm thk screed, 200mm precast hollow core slab.
South façade: Low e Double glazed with argon gas filled. 1.40 W/mÂ²Â°C
Rest all of the glazing including Skylight : triple glazed with two layers of low e glass with argon gas. 0.98 W/mÂ²Â°C
Internal walls: 12mm Plaster, 25mm rockwool Insulation, 12mm plaster.
Door & Window frame: 80mm thk softwood.
Internal doors: 30mm thk softwood.
Internal partition walls and doors are not insulated so that they help in spreading the heat gained during day time in south facing rooms and kitchen to the rest of the house. Internals doors have undercuts for the same reason.
Green Roofs: Small green pockets are given above the workspaces ensuring that the flats on the second floor also get access to private garden. Wherever access is not possible low maintenance sedum roof are given. Roof gardens consist of layer of bitumen membrane on the precast concrete slab, over which polyethylene layer is applied. Then 300mm expanded polystyrene with top soil or mineral fibre is laid for growing medium.
THERMAL ANALYSIS USING SOFTWARE
The thermal analyses were performed using the software EDSL TAS (version 126.96.36.199). Several assumptions were made to for the thermal modelling of the building:
Analysis is done keeping in mind house is occupied by max 3 person at a time.
No heating or cooling considered throughout the year.
Internal gains from the solar radiation, equipment's, lighting & occupants.
During the summer windows starts opening if the temperature in Living Room exceeds 19Â°C. It will be fully open if the temperature reaches 22Â°C & if the external temperature exceeds the internal temperature, windows begins to close.
Wind cowls were not considered in the analysis.
Different schedules and internal conditions were given for each type of room.
Infiltration considered 0.25ach
For the analysis hottest and coldest week of the year is considered.
This short study leads to some interesting conclusions regarding the performance of the thermal behaviour of the sunspace in BedZED.
First of all it proved that for the temperate marine climate of London and most part of the England, the incorporation of Sunspace on the South can significantly improve the thermal performance of the building during winter months, which reduces the need of conventional space heating significantly.
Secondly, the sunlight accumulated in the sunspace not only contributes in preserving the energy for space heating but also it creates amicable thermal comfort conditions in the habitable spaces of the dwelling.
Thirdly, during the hot months sunspace accumulates lot of solar energy which has negative impact on the thermal balance of the house, but it can be eradicated with the use of some external shading devices such as movable louvers or weathersheds can be provided which blocks the summer vertical sun and allows the winter sun or a pergola with deciduous plants.
It should be noted, importance of occupants behaviour is very important regarding overall thermal performance of the house. Although the air temperature goes outside the comfort zone during some part of the year, which may call for heating or cooling, it is energy conscious occupants who minimises energy consumption significantly. Furthermore, during the hottest days, the occupants prefer passive ventilation to remove excess heat from the sunspace during daytime and during the night they use night ventilation to cool off the building mass to prevent overheating the consecutive day.
Overall it can be concluded that building performs quite well thermally throughout the year.
United Kingdom weather is cloudy for most part of the winters, sunspace performance will be improved if the climate is sunnier.