CASE STUDY: ANALYSIS OF SOLAR CONTROL & HEAT GAIN AT Bedzed sustainable development
COURSE-PRINCIPLES OF ENVIRONMENTAL DESIGN(AR-827) 2017-2018
TUTOR-Dr. RICHARD WATKINS
It is often articulated that about 50% atmospheric carbon emissions are from buildings (Twinn, 2003). It was found that ‘Counter-urbanization’ was one of the many factors to have contributed to 20% increase in energy consumption by households in England over the last 25 years. (Dean Hawkes, 2002) In the year 1999, a report ‘Towards an Urban Renaissance’ was published by UK government which outlined the practical solutions to the designs of towns, cities and urban neighborhood. The report was aiming towards a ‘new vision for urban regeneration on the principles of design excellence, social well-being and environmental responsibility within a viable economic and legislative framework’ (F.Spon, 1999) The conclusions of this report were that an increased mixed building types in more compact urban forms close to existing and new transportation interchanges were essential. The Beddington Zero Energy Development (BedZED) was one of the first large-scale mixed-use development project which was taken up as a challenge to meet the above demand.
Designing the buildings to integrate the passive environmental strategies are proven to be less energy intensive. At BedZED, amongst many environmental strategies, the passive solar strategy has been one of the identified renewable sources of energy and has been utilized for daylighting and to bring in the Sun during winter from the exemplary South glazed façade of the houses. The Study concentrates on the analysis of Solar control and heat gains and potential concerns connected to the dependency on Solar gains from the South facade.
Bill Dunster architects
18 work/live units
1,600 cubic.ft of workspace
BedZED combines 82 homes, 1600 cubic meter of workspace on a 3.5-acre site which indicates substantial dense packing to carefully utilize the limited land resources to the maximum (Twinn, 2003). It also has other facilities like sports hub, football pitch, nursey, health center and an organic food shop.
LOCATION & THE SITE
BedZED is located in South London, borough of Sutton. Building the sustainable development on an Agricultural land was not sensible enough for Bioregional. This led them to choose a site which was a former sewage work location on the southern edge of London. This is about 15 min walk from the nearest tram line promoting public transport.
Figure 1 (Below): Google map image of BedZED
Figure 2 (Right): The master plan (Kucharek, 2010)
The master plan has 5 rows of terraces running east to west. The houses are oriented on the South side of the terrace to promote maximum solar potential and the workspaces are on the North side, shaded by dwellings and North-lit. (Dean Hawkes, 2002)Every house has a private garden relieving the scheme of early generation one-dimensional layout of the passive solar housing schemes. (Dean Hawkes, 2002)
· Temperate Climate (Koppan classification)
· Mean annual temperature of 9.6 deg C.
· Rainfall is considerably below England’s average (1971–2000) level of 838 mm and every month is drier overall than the England average.
Figure 3: Climate data for London Heathrow and London weather center (Anon., n.d.)
FACTORS WHICH DETERMINED THE PASSIVE SOLAR DESIGN
The BedZED strategy for sustainable design had a holistic approach to tackle the environmental issues from economic and ecological objectives. In ordinary scenarios, sustainable technologies are considered as ‘ add-ons’ to the building cost and hence unwelcomed by most funding agencies. (Twinn, 2003) After making the shift from the conventional approach, ‘Advanced analytical techniques explored how passive systems could be enhanced enough to allow active systems to be completely omitted.’ (Twinn, 2003) As a result, BedZED amalgamated some of the site-driven factors and self-set sustainability targets which led them to choose Passive Solar Technique.
ORIENTATION- Of the variety of structures, houses and work/live units consume the maximum amount of space at the site. These were occupied by people and had to be placed carefully after consideration for orientation. The Workspaces have high internal gains due to the presence of computers and other equipment releasing heat. (Dean Hawkes, 2002)This was one of the driving factors for them to position the work/live units away from southern Sun and be shaded by another structure. Dwelling units normally have low internal gains which include gains from occupants, appliances and by certain activities like cooking. This motivated them to position the dwelling units with large glazing area on the Southern facade and adding a sunspace to all the dwelling units facing South for Solar heat gains during winter to maintain the comfortable temperatures inside.
ENERGY CONSIDERATIONS- The objective of the BedZED design team was to balance the energy demand with available renewable energy sources leaving zero carbon footprint. (Dean Hawkes, 2002) In the UK, balancing the energy demand and supply is considered difficult and hence it was key to reduce the energy demand. Consequently, a high-performance building fabric was used and passive strategies were adopted. Low-grade energy for space heating through passive solar means combined with internal gains. The Larger area of exposed thermal mass could store the passive heat gains in the walls, slabs, floors and release it when needed. Passive stack ventilation for dwellings.
Figure 4: Schematic cross section showing various environmental strategies involved (Arup) (Anon., 2012)
DAYLIGHT FACTORS- In workspaces, a good uniform daylighting is the essential to minimize the use of electric light and improve the overall visual environment. (Dean Hawkes, 2002) Daylighting in offices is considered especially important as these spaces are used mostly during the day and have higher illumination requirements and glare to be avoided (Dean Hawkes, 2002). This also implied that a more uniformly distributed light was required which does not add on to the increase in internal temperatures and hence the workspaces to avoid exposure to the South facade. This gave chance to the designers to utilize South Sun to light the interiors of the dwelling better along with heating them during winter months.
POST-OCCUPANCY EVALUATION & AREA OF CONCERN:
BioRegional, the environmental consultants along with a person with experience in Post-occupancy evaluation have been monitoring the progress of the set target to assess the efficiency of the designed buildings at BedZED. This feedback system has helped the design team to learn from the past and adopt in the future designs (Jessica Hodge, 2009). Interestingly, BedZED has made this monitoring process interactive to make the occupants more aware of the consumptions. Afterall, for buildings especially like dwellings, occupant’s awareness plays a crucial role in achieving the target numbers. Meters have been placed to monitor the consumption of electricity, water and heat for each dwelling. (Jessica Hodge, 2009)Interviews with questionnaires are done to the occupants and their feedbacks are recorded.
Figure 5: Table showing the occupant’s response to the temperature inside the dwelling (Jessica Hodge, 2009)
In 2007, 71 occupants were interviewed out of 100 on various topics including rating the temperature of their dwellings in both Summer and winter seasons from 1(Too hot) to 7 (Too cold). (Jessica Hodge, 2009) The study shows that nearly half of the dwelling seems to have a comfortable temperature during winter, however, summertime results do not look impressive. Nearly 86% occupants claim ‘not so comfortable’ by giving rating 4 or lower. With this as the area of concern, the study concentrates on critically evaluating South glazed façade of the dwellings for Passive Solar controls, especially in Summer months. This includes evaluating potential factors affecting the increase in internal temperatures during Summer months. The analysis is done for a single unit of dwelling highlighted in Appendix A.
3 bedroom flat: Ground floor+First floor area=155sq.m
1 bedroom flat: Second floor area=67sq.m
Vertical area exposed to Sun in the sunspace on
Figure 6- Floorplans of the dwelling with South facing sunspaces
Note: The areas are calculated based on scaling the plan & redrafting.
POTENTIAL FACTORS AFFECTING INCREASE IN INTERNAL TEMPERATURES:
In line with the temperature standards set by the design team, BedZED dwellings should not below 18deg C at all times when they are occupied. (Jessica Hodge, 2009) Although internal temperature data are unavailable for the study, the above table is enough to start evaluating the case along with certain assumptions.
INTERNAL HEAT GAINS- This includes heat gains from people, lighting and appliances, cooking and domestic hot water. According to the ‘BedZED seven years on report’ random average of a number of people per dwelling was noted to be 2. (Jessica Hodge, 2009) Area of the 3 bedroom house is 155 sq.m and area of 1 bedroom house being 67 sq.m excluding sunspaces( balconies). At BedZED, all the lighting used are compact fluorescent lighting (Mehmet Sinan Senel, 2010).Conventionally, these warm up slowly and electricity consumption is minimal.All the internal appliances like fridge, freezer and washing machine are energy efficient (Mehmet Sinan Senel, 2010)and everything put together will not add up to the unwanted amount of internal heat gain as these are already accounted for in the original design.
SOLAR HEAT GAIN- The dwellings designed with South facing sunspaces have the entire facade with consciously specified double glazed windows on the outside and another layer of double glazed windows on the inside (Dean Hawkes, 2002). In theory, ‘an appropriate horizontal shading device could provide shading in the summer but allow the entry of solar radiation in the winter’ as mentioned by Steven Szokolay in his book ‘Introduction to Architectural Science’. However, let us analyze this in practice by the following methodology.
1. Identifying overheating periods on the solar chart.
2. Determining the altitude of Sun.
3. Comparing the existing roof overhang to ideal roof overhang to protect the interior from overheating.
4. In case of design inefficiencies, suggesting alternatives/improvements.
NOTE: Ideal roof overhang is considered as that shading device which successfully blocks the summer sun during the overeating period. This should allow for low angle winter Sun as well. Even if a part of solar radiation reaches the interior, it should essentially fall on the exposed thermal mass instead of glazing which re-radiates inside making the room uncomfortable during summer,
SOLAR CONTROL ANALYSIS
STEP 1: Hourly temperature data table has been considered as given in Appendix B. Temperature indicating 23 deg C and above has been marked as a potential overheating period.Appendix C indicated the critical time to block the Sun. Between 10:00 am-2:00pm.
STEP 2: The marked temperature range is shaded on the Solar chart as shown in figure 7. It is important to note that the Southern facade is 26 deg East of South.(This measurement has been taken on the basis of the floorplan image traced and scaled in AutoCAD) The required forward projection angle to block the Sun during this overheating period from May to September is measured using the protractor. The forward projection angle is 48 deg as measured in Appendix D.
Figure 7-Solar chart indicating overheating period(Author’s own drawing)
STEP 3-To justify the use the chosen months, a window heat gain tool was used to determine the months with high window heat gains. (Gronbeck, 2009) The tool essentially gives the average output window heat gain of every month from 4:00 am to 8:00 pm along with total window heat gains in Kilowatt-hours/sq.m. The following input data was given to the tool. The climate conditions are considered for the worst case scenario and Ground reflectance and Window SHGC are assumed data.
Latitude: 51.3 deg. North
CLIMATEclearness: 1Sunshine (Jan. to Dec.): 100%
Window type: double-glazed low-E 0.20 (wood/vinyl)Window SHGC: 0.55Window orientation: 30 deg. E of S
GROUNDSurface: bright green grassGround reflectance: 0.25
See Appendix E for the detailed output table. The summary of the output is, after consideration of temperature input for every month, temperature between May to September being highest, the window heat gains output beyond 10 Kilowatt-hours/sq.m is undesirable and hence heat gains during this period should be avoided.
STEP 4- The existing building part section towards South façade needs to be considered and the incident Sun rays determined from the forward projection angle ought to be overlaid on the drawing to analyze the existing situation of Solar control. In the plan, we have two different cases due to variations in the specifications.
Figure 8-South elevation of 1 unit (Twinn, 2003)
Figure 9-Second floo plan
Typical section through Case 1.
(Author’s on drawing)
Figure 11- Typical section through Case 2.
(Author’s on drawing)
OVERLAYING THE INCIDENT RAYS
Figure 12-Case 1 section with overlaying incident rays on Aug 30th at 10:00 am (Author’s own dwg)
Figure 13- Case 2 section with overlaying incident rays on Aug 30th at 10:00 am (Author’s own dwg)
The Sunrays with a forward projection angle of 48 degrees are indicated on the Southern glazed facade as shown in figure 12 showing the case 1.In this case, most of the Solar radiations on both ground and first-floor sunspaces are mostly incident on thermal mass like exposed wall or floor. This is not much of a concern as the designed thermal mass for the building is performing well (Jessica Hodge, 2009) However the sunspace on the second floor seems to be getting a lot of radiation from both vertical side and from rooftop solar PV panel. Here it is important to note that on the second floor there exists a top hung window just below the roof level. During Summer season, the movement of air from inside to outside just through the top hung window is not going to be significant due to lack of greater pressure difference.
In figure 13 indicating case 2, due to the presence of door at the center of the plan, the larger interior area is exposed to the Sun compared to case 1. Ground floor sunspace seems to be protected due to the presence of balcony at first-floor level. First floor and second-floor sunspaces are significantly exposed to the Summer Sun. A combination of a glass door at the center with a clear glass roof on top causes a significant heat gain during summer. The sides of each dwelling unit are closed and hence there is no scope of ventilation in E-W direction.
Possible causes for overheating area:
· Lack of provision for Solar shading on the Southern side during the overeating period.
· Lack of provision for sufficient ventilation.
Due to this overheating especially on the second floor, it could adversely affect the performance of Solar PV panels as they require good ventilation for their efficiency.
Figure 14-Arial view of South façade of dwellings.Visual evidence of occupants covering the Sunspaces with curtains. (Anon., 2012)
After studying the existing condition, the proposed improvements are in two different strategies. The first one is a necessity to create better ventilation through sunspace.The second solution is to shade the overheated solar radiation by retrofitting a horizontal overhang wherever necessary.Let us look at the proposed improvements case-wise and floor-wise.
Figure 15- Case 1 section with indicated improvements (Author’s own dwg)
Figure 16- Case 2 section with indicated improvements (Author’s own dwg)
In both case 1 and 2, top hung window at floor level of second-floor sunspace has been introduced. This will ensure better pressure difference and hence works on the principle of stack ventilation. Given the fact that occupants are not extensively opening the windows as mentioned in the post-occupancy report (Jessica Hodge, 2009), the proposal new windows could be controlled by actuators. A mesh can be introduced at this proposed window to make sure that objects do not fall down through them. In case 2, for the second-floor roof, a louver system called ‘Retroflex-D’ has been proposed to be integrated between the glass panels. In principle, this louver acts as a radiation deflecting system blocking the summer sun. (Koster, 2004)The profile of the louver has been designed in such a way that the bending reduces the height of louver thereby allowing more visibility and transparency. (Koster, 2004) The product has a width of 80mm.
Figure 17-Typical image of Retroflex-D integrated between glass panels on the roof (Straße, n.d.)
The product has a ‘therm’ grade which transmits the energy of flat winter sun and hence is proven to be useful in conservatories and sunspaces. (Koster, 2004) The same roof has been proposed as a horizontal overhang over first-floor extended balcony area in case 2. The horizontal projection is about 800mm.
In case 1, below the PV panels, there is a possibility of adding insulation below it to improve comfort however it will result in decrease reduction in efficiency of the panel and hence may not be advisable.
With the above measures, part of the building will be well protected from the summer sun and part of the building has been designed for better ventilation to reduce the internal temperature. Perhaps there is yet another way to tackle this problem. Reducing the Solar radiation in the first place instead of allowing it to enter the building first and trying to tone it down. This will lead us to our additional retrofitting solution for the first proposal to cut down the summer sun.
Figure 18 and 19 show the extension of the second-floor sun space overhang to further protect the balcony area from overheating. The slope of the extension at both first-floor roof and second-floor roof level are maintained same for aesthetics and it also satisfies the shading requirement. This extension has clearly shaded a significant part of balcony allowing only less than half a depth of sunspace exposed to Sun. The extension is about 680mm from the façade and extension beyond this would practically be very large and could potentially obstruct the viewer’s line of sight.
Figure 18- Case 1 section showing additional improvements (Author’s own dwg)
Figure 19- Case 2 section showing additional improvements (Author’s own dwg)
Designing the buildings to integrate passive environmental strategies are proven to be less energy intensive and hence more favorable. However, it is important to make sure that the passive design strategies work well for all operating conditions and the post-occupancy results are comparable to good building practice results, for all operating conditions.. In the case of BedZED, designing the building with high insulation and allowing for passive solar gain with a large area of South glazing has increased the risk of heating.This implies that when a building is highly insulated, it is important to be able to control certain features especially Passive solar gains in order to achieve comfortable indoor conditions for the occupants. Even if not done in the first place, post-occupancy results and user feedback can always be used and solutions can be found out by either retrofitting or replacing. A combination of suitable projection to block the summer sun and required ventilation in sunspaces will help in a decrease in internal temperature.
Anon., 2012. Geographically. Online Available at: http://geoeuropeansection.blogspot.co.uk/2012/01/Accessed 20 October 2017.
Anon., n.d. Weather Online. Online Available at: http://www.weatheronline.co.uk/weather/maps/cityAccessed 15 October 2017.
Dean Hawkes, W. F., 2002. Architecture, Engineering and Environment. London: Laurence King in association with Arup.
F.Spon, E. &., 1999. Towards an Urban Renaissance:Final Report of the Urban Task Force, s.l.: Department of the Environment, Transport and Regions.
Gronbeck, C., 2009. Sustainable by Design, Seattle Washington. Online Available at: http://susdesign.com/windowheatgain/Accessed 16 November 2017.
Jessica Hodge, J. H., 2009. BedZED seven years on , London: s.n.
Koster, H., 2004. Dynamic Daylighting Architecture-Basics,Systems, Projects. Berlin: Birkhauser.
Kucharek, J.-C., 2010. Building Design. Online Available at: https://www.bdonline.co.uk/revisiting-the-bedzed-community/5002956.articleAccessed 16 November 2017.
Mehmet Sinan Senel, F. N., 2010. Assessment of Sustainable Housing Projects Final Report , s.l.: s.n.
Straße, D., n.d. Retro solar roof glazing. Online Available at: www.retrosolar.de/flash/rf_e_rs.swfAccessed 6 December 2017.
Twinn, C., 2003. BedZED. The Arup Journal, Vol.38(1), pp. 10-16.
Master plan of Bedzed highlighting the part which has been considered for study
Hourly temperature data.
Chart indicates the temperatures for Canterbury
(due to unavailability of the above data for BedZED)
8:00am 10:00 am
12:00 am 2:00 pm
Sun path on 21st August using Sunearthtools.com
Measuring altitude of the Sun during overheating period on Solar chart
Table indicating average hourly window heat gain in Kw-h/sq.m for all months http://susdesign.com/windowheatgain/