Adapting buildings and cities to heat waves: are green solutions the best?

Transcription

1 Adapting buildings and cities to heat waves: are green solutions the best? [published on Linked In: Authors (with current affiliations): - Prof.dr.ir. Bert Blocken, Eindhoven University of Technology, NL & KU Leuven, Belgium Prof.dr.ir. Jan Hensen, Eindhoven University of Technology, NL Prof.dr. Harry Timmermans, Eindhoven University of Technology, NL Dr.ir. Twan van Hooff, KU Leuven, Belgium & Eindhoven University of Technology, NL Dr. Christof Gromke, Karlsruhe Institute of Technology, Germany Highlights: Green roofs have almost no effect in reducing overheating in buildings. Most effective measures against indoor overheating are also the cheapest: peak ventilation and exterior solar shading. Green roofs generally provide very limited extra thermal insulation in winter when substrate is wet. Well-irrigated avenue trees can reduce outdoor air temperature in a studied street canyon by up to 1.6 C. Adding green roofs and green facades for all buildings in the studied street canyon only has a very small effect on outdoor air temperature in this street. Making uildings and ities green is undoubtedly a valuable ambition but only in the metaphorical sense of the word green, i.e. energy-efficiency and sustainability. A green color in itself does not imply that the measure is energy-efficient and/or sustainable. [NOTE 1: Linked In posts evidently only provide a summary of the research with some main findings. Full details of the work can be found in the scientific publications (downloads) at the bottom of the post] [NOTE 2: The analysis was performed for buildings and cities in the Netherlands (moderately cold oceanic climate - Cfb in the Köppen climate classification). Extrapolation to other countries in other climates should be done with care, and will be allowed to a larger extent for the results pertaining to the building level (indoor overheating) rather than for the outdoor environment]

2 Figure 1. Left: Green roof (CC-BY-SA DASonnenfeld). Right: Green facade (CC-BY-SA Lamiot). Climate change is happening and at least on the short term it is irreversible. It is therefore essential that national and international governments take immediate and effective actions to reduce climate change and the impacts of climate change. A distinction is made between climate change mitigation and adaptation. In short, mitigation refers to addressing the problem at the source by reducing emission of greenhouse gases, hile adaptatio refers to a epti g that a ertai degree of limate change is unavoidable and trying to limit its effects by adapting to the consequences. The remainder of this post deals with climate adaptation. In particular, we focus on the adaptation of buildings and cities to heat waves, which are expected to continue to increase in frequency and intensity as a result of climate change. Major heat waves, such as the European heat waves of 2003 and 2006, could become common events by 2040 [1,2]. Increased heat waves and heat stress will cause increased heat-related morbidity and mortality, as illustrated for the hot summers of 2003 and 2006 [3,4]. During the summer of 2003, more than 70,000 heat-related deaths were reported in Europe [5]. Due to increased intensity and frequency of heat waves, cooling energy demand in summer is expected to increase by 72% worldwide by 2100 [6]. These problems are potentially aggravated by the urban heat island effect, which simply means that temperatures in cities are higher than in their rural surroundings. Recently, a large research project on climate adaptation, called Climate Proof Cities, was finalized. It included the evaluation of climate change adaptation measures for heat waves [7]. The research reported below was part of this project. When addressing heat waves in cities and the resulting overheating, a distinction has to be made between overheating in the outdoor and overheating in the indoor environment, i.e. outside and inside buildings. Outdoor environment To assess the potential of green measures such as avenue trees, green roofs and green facades (Figure 1) in reducing the air temperature in an actual street during a heat wave, a case study was set up for a main street in the Dutch city of Arnhem (Figure 2). The study was a computational study based on high-resolution simulations with Computational Fluid Dynamics (CFD) with ANSYS Fluent, carefully validated based on measurements in different situations (see article [8]), following an

3 earlier CFD heat wave study with validation based on satellite imagery in the city of Rotterdam [9]. Figure 3 shows part of the computational grid for the Arnhem study. Figure 2: J.P. van Muijlwijk street in Arnhem, the Netherlands. Figure 3: Part of computational grid for J.P. van Muijlwijk street in Arnhem and surroundings [8].

4 In the Arnhem study, six different scenarios (1 real, 5 virtual), were analyzed for a heat wave day with given meteorological conditions: Status quo: Few, rather small deciduous trees within the street canyon and a patch of deciduous trees No vegetation Avenue-trees in street canyon + trees from status quo Green facades + trees from status quo Green roofs + trees from status quo All measures combined + trees from status quo The locations of these measures are indicated in Figure 4. Figure 4: Position of trees in status quo (black), avenue trees, green roofs and green facades. Four cross-sections of the street canyon are indicated by blue lines and numbers 1 to 4 [8]. The trees had a leaf area density (LAD) of 0.55 m²/m³ and a cooling power (Pc) of W/m³. The green facades had a leaf area index LAI = m²/m² and Pc = W/m³, while the green roofs had LAI = 0.75 m²/m² and P = 375 W/m³. While the LAI values do not seem very large, it should be noted that these values pertain to the entire facade or roof area and that they take into account the presence of doors, windows and rooftop structures, where typically no vegetation is placed. Figure 5 shows the results in terms of air temperature contours in the street at 2 m height for three scenarios, for wind direction along the street. In addition, Table 1 provides the mean and maximum temperature changes for every scenario compared to the status quo, taken from the 4 cross-sections in Figure 4 and from a horizontal plane over the entire street canyon at 2 m height.

5 Figure 5. Air temperature contours in horizontal plane at 2 m above ground for three scenarios [8]. Table 1. Mean and maximum temperature changes compared to status quo. The study showed that: Avenue trees, when well irrigated during heat waves and exhibiting proper cooling power, can reduce the air temperature in the studied street canyon by up to 1.6 C, which is substantial and in accordance with research results by other researchers in other countries and climates [10,11].

6 Green roofs and/or green facades, even when applied to all buildings in the street, had only a limited effect on the air temperature in the street canyon, with maxima up to C but average values below 0.1 C which is considered the accuracy of simulations and measurements. From an urban physics point of view, these results can be explained because a tree in a street can be fully exposed to wind flow through the street and can effectively allow cooling. Wind speed near a wall however is more limited. For many wind directions, roofs and facade will be situated partly or completely in flow separation and recirculation zones with low wind speed. It is therefore not surprising that the effect of measures (green roofs, facades) close to walls on the outdoor air temperature is limited. Indoor environment A range of residential buildings was subjected to a heat wave using the well-validated Building Energy Simulation software EnergyPlus. The buildings included a detached house (Figure 6), a row house and an apartment. They were assumed to be free-running: no active cooling systems were employed. Two categories were distinguished: buildings erected according to common practice in the Netherlands and Belgium in 1970 (rather poorly insulated) and contemporary buildings satisfying building regulations in the Netherlands and Belgium in 2012 (well-insulated). Some main properties of the reference ase ase buildings are given in Tables 2 and 3. The performance of such buildings in winter is well-known: the higher the thermal insulation levels all other parameters being constant the lower the energy consumption in winter. Therefore, we focus on the summer heat wave situation, which according to climate change predictions is going to be the challenge for our buildings in the coming decades and centuries. Figure 6. Facades, floor plans and building dimensions of the detached house (modified from Ref. [13]). Triangles in windows and doors indicate operable windows/doors for the additional ventilation configuration. Dimensions in mm.

7 Table 2 & 3. Overview of construction details for the base case building from the 1970s (left) and 2012 (right). The following climate adaptation measures were contemplated (Figure 7): Higher thermal insulation levels (up to 5 and 6.5 m²k/w) Lower thermal mass (replacing inner limestone walls and concrete floors by wood-frame construction) Higher short-wave reflectivity (higher albedo) for exterior roof and facade surfaces (from 0.3 up to 0.6 and 0.8) Natural peak ventilation (i.e. the ability to open windows when indoor temperature is higher than outdoor temperature, without safety/security issues) o all day long o only during the day, not night Exterior movable solar shading for windows Green roof ideal s e ario ith very high leaf area index of 5 m²/m²) Figure 7: Adaptation measures considered. The full study is reported in [12], only the main results for the detached house are given below. First, the results for the 1970 detached house are provided. Figure 8 illustrates the changes in overheating hours achieved by implementation of each of the above-mentioned measures separately. The results indicate that some measures actually would make things worse. Adding thermal insulation (RC50 and RC65) will increase the number of overheating hours. This is because an important part of the overheating during heat waves is caused by short-wave solar radiation that enters the building through the window glass and is absorbed by internal surfaces and subsequently transmitted as longwave radiation, which cannot pass through the glass anymore. As a result, this heat has to leave the

8 building through the combination of radiation, convection and conduction, which is much less effective (i.e. the so-called greenhouse effect). The result is indoor overheating. Increasing insulation levels will have little effect on the incoming solar radiation but will slow down its removal even further, hence the negative effect. Reducing thermal mass (TM00) can have a positive or a negative effect. While thermal mass could provide some heat buffering and reduction of indoor overheating for short hot periods, during heat waves the buffering capacity is often exhausted, in which case it can yield a net negative effect. Increasing the short-wave reflectivity of roofs and facades (ABL06 and ALB08) can also have a positive effect, but the most effective measures are the cheapest ones: peak ventilation (NV_all and NV_day) and exterior solar shading (SH). Green roofs (VR), even if optimally irrigated, have a negligible effect. Figure 8: Overheating hours for the detached house with 1970s construction (modified from [12]). The results for the 2012 detached building (or older buildings having undergone thermal renovation) are shown in Figure 9. Adding thermal insulation leads to more overheating while reducing thermal mass can have a negative or positive effect. Higher short-wave reflectivity helps a bit, green roofs have a negligible influence. The most effective measures, which outperform all others by far, are again the two cheapest ones: peak ventilation and exterior solar shading.

9 Figure 9: Overheating hours for the detached house with 2012 construction (or renovated) (modified from [12]). The fact that modern buildings but also renovation projects in North-West Europe generally do not even consider allowing peak ventilation or exterior solar shading anymore is a clear indication that the above-mentioned effects are insufficiently known in the building design and construction community. What about interior solar shading? While some might argue that interior solar shading is equally good, it is well known in the building physics community that this is absolutely not true. With interior solar shading, the short-wave solar radiation has already entered the building and after absorption by internal building surfaces (often the internal solar screen itself), it is (apart from some potential reflection) emitted as long-wave radiation that cannot pass through glass anymore. What about winter? As opposed to many claims, a green roof also does not provide a strong thermal insulation benefit which could be relevant for winter situations - if the roof already had a minimum level of insulation. A persistent misconception is that the thickness of the substrate would provide a large increase in insulation. At least in North-western Europe, in a regular winter, characterized by low temperature, high relative humidity, low evaporation rates, rain and even snow, the substrate will remain wet

10 probably throughout the entire winter. Moisture has a very destructive effect on the thermal resistance of porous materials/substrates. Designing for energy-efficiency in moderately cold climates for both the winter and the summer situation does not imply that thermal insulation levels should be lowered, on the contrary. In residential buildings in these climates, the heating loads will generally be much larger than the cooling loads. High thermal insulation levels (for winter) and proper heat wave measures such as peak ventilation and exterior solar shading (for summer) should be sufficient to limit the energy needed for heating, limit overheating and avoid the use of active cooling systems. Conclusions Addressing the question in the title of this blog or post: Adapting buildings and cities to heat waves: are green solutions the best?. The answer depends on which connotation is attached to the word green. In far too many policy documents, it seems that the distinction between its literal meaning (the color) and its metaphorical use for energy-efficient and/or sustainable (whatever the latter word may mean) is no longer made. This is an important misconception, where the incorrect use of the ord gree is for i g expensive and potentially inefficient solutions into new design and redevelopment projects. While several of the above-mentioned adaptatio easures ould orre tly e gi e a gree la el because they can actually yield energy savings and/or reduce the effects of heat waves, this does not hold for all of them. At least concerning the studied street canyon, green roofs and green facades have only a very small positive effect. Concerning the indoor environment, green roofs have no significant effect. The two most effective measures are the cheapest ones: exterior solar shading and peak ventilation (i.e. the ability to open windows without safety/security issues). These are surprisingly and unfortunately exactly those that have been at least in many countries in North-West Europe partly or completely abandoned by many contemporary designers. On the contrary, there is a rapidly increasing trend to install active cooling systems in residential buildings. Green roofs and green facades also come at a much higher cost than traditional systems such as external solar shading and peak ventilation (both of which, as opposed to green roofs, have used extensively throughout the past centuries for good reasons). Also in winter, a green roof might not live up to expectations. For poorly insulated buildings, a green roofs might have a small benefit in terms of reduced energy demand for heating, but this comes at a cost that does not justify the application of a green roof, certainly not when compared to regular thermal insulation. The above indicates that green roofs and green facades ost likely do ot deser e a gree The green color in itself is not enough to justify this label. la el. Research team This research work was performed by a team consisting of the following core members (present affiliations given): - Prof.dr.ir. Bert Blocken, Eindhoven University of Technology, NL & KU Leuven, Belgium Prof.dr.ir. Jan Hensen, Eindhoven University of Technology, NL

11 - Prof.dr. Harry Timmermans, Eindhoven University of Technology, NL Dr.ir. Twan van Hooff, KU Leuven Belgium & Eindhoven University of Technology, NL Dr. Christof Gromke, Karlsruhe Institute of Technology, Germany The team acknowledges the support of Knowledge for Climate, the collaboration and support from the governmental organizations involved and the collaboration in the Climate Proof Cities consortium, with: TNO (consortium leader), Wageningen University & Alterra, Delft University of Technology, Utrecht University, Radboud University Nijmegen, Deltares, KWR, Unesco-IHE. The team also acknowledges the research collaboration and partnership with ANSYS. References [1] Kovats, R.S., Hajat, S., Heat stress and public health: a critical review. Annu Rev Public Health 29, [2] Stott, P.A., Stone, D.A., Allen, M.R., Human contribution to the European heatwave of Nature 432, [3] Fischer, P.H., Brunekreef, B., Lebret, E., Air pollution related deaths during the 2003 heat wave in the Netherlands. Atmos. Environ. 38, [4] Haines, A., Kovats, R.S., Campbell-Lendrum, D., Corvalán, C., Climate change and human health: Impacts, vulnerability and public health. Public Health 120, [5] Robine, J.-M., Cheung, S.L.K., Le Roy, S., Van Oyen, H., Griffiths, C., Michel, J.-P., Herrmann, F.R., Death toll exceeded 70,000 in Europe during the summer of C. R. Biol. 331, [6] Isaac, M., Van Vuuren, D.P., Modeling global residential sector energy demand for heating and air conditioning in the context of climate change. Energy Policy 37, [7] Albers, R.A.W., Bosch, P.R., Blocken, B., Van Den Dobbelsteen, A., Van Hove, L.W.A., Spit, T.J.M., van de Ven, F., van Hooff, T., Rovers, V., Overview of challenges and achievements in the Climate Adaptation of Cities and in the Climate Proof Cities program. Build. Environ. 83, [8] Gromke, C., Blocken, B., Janssen, W., Merema, B., van Hooff, T., Timmermans, H., CFD analysis of transpirational cooling by vegetation: Case study for specific meteorological conditions during a heat wave in Arnhem, Netherlands. Build. Environ. 83, [9] Toparlar, Y., Blocken, B., Vos, P., van Heijst, G.J.F., Janssen, W.D., van Hooff, T., Montazeri, H., Timmermans, H.J.P., CFD simulation and validation of urban microclimate: A case study for Bergpolder Zuid, Rotterdam. Build. Environ. 83, [10] Alexandri, E., Jones, P., Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Build. Environ. 43, [11] Shashua-Bar, L., Pearlmutter, D., Erell, E., The influence of trees and grass on outdoor thermal comfort in a hot arid environment. Int. J. Climatol. 31, [12] Van Hooff, T., Blocken, B., Hensen, J.L.M., Timmermans, H.J.P., On the predicted effectiveness of climate adaptation measures for residential buildings. Build. Environ. 82, [13] Agentschap NL. Referentiewoningen nieuwbouw Sittard, the Netherlands: Agentschap NL; 2013.

12 Five related publications by members of research team Blocken, B., Computational Fluid Dynamics for Urban Physics: Importance, scales, possibilities, limitations and ten tips and tricks towards accurate and reliable simulations. Building and Environment 91: doi: /j.buildenv Blocken, B., years of Computational Wind Engineering: Past, present and future. Journal of Wind Engineering and Industrial Aerodynamics 129: doi: /j.jweia Gromke, C., Blocken, B., Janssen, W., Merema, B., van Hooff, T., Timmermans, H., CFD analysis of transpirational cooling by vegetation: Case study for specific meteorological conditions during a heat wave in Arnhem, Netherlands. Building and Environment 83: doi: /j.buildenv Toparlar, Y., Blocken, B., Vos, P., van Heijst, G.J.F., Janssen, W.D., van Hooff, T., Montazeri, H., Timmermans, H.J.P., CFD simulation and validation of urban microclimate: A case study for Bergpolder Zuid, Rotterdam. Building and Environment 83: doi: /j.buildenv Van Hooff, T., Blocken, B., Hensen, J.L.M., Timmermans, H.J.P., On the predicted effectiveness of climate adaptation measures for residential buildings. Building and Environment 82: doi: /j.buildenv