Precipitation and buildings: estimation of the natural potential of...

R.H. Crawford and A. Stephan (eds.), Living and Learning: Research for a Better Built Environment: 49th

International Conference of the Architectural Science Association 2015, pp.45–54. ©2015, The Architectural Science Association and The University of Melbourne.

Precipitation and buildings: estimation of the natural potential of locations to sustain indirect evaporative cooling strategies

through hot seasons

Claudio Aurelio Diaz, Paul Osmond and Steve King University of New South Wales, Sydney, Australia

[email protected], [email protected], [email protected]

Abstract: Precipitation is a relevant climatic variable for building and urban design in hot climates, because of its potential to naturally mitigate heat excess in buildings and cities by evaporative cooling; and as a primary source of water to artificially reproduce this cooling mechanism, particularly in the humid tropics and subtropics. However, precipitation is commonly neglected in the analysis and development of climate responsive architecture and is rather seen as a cause of problems. This paper proposes a practical graphical method for building designers and planners which facilitates the meaningful “reading” of a climate, to reveal the potential use of precipitation in architecture. This method supplements existing climate analysis tools by defining a scale and benchmarks that easily link potential water requirements of buildings with water availability from precipitation. To complement this method, the concept of Urban Precipitation Surplus is also proposed, a measure of the excess of precipitation that is usually discarded which could be exploited for building cooling and contribute to regenerate the water cycle and improve microclimates in cities. Finally, a brief discussion is given about the analogy between buildings and vegetation, and the importance of enriching architecture with concepts from fields like agriculture and climatology.

Keywords: Precipitation; indirect evaporative cooling; hot climates; climate analysis.

1. Introduction

Precipitation is an important factor for planning of buildings and cities in regions with seasonally or permanently hot climates. Research in locations with these climates has recognized the potential of rainfall as a natural way to mitigate heat excess in buildings and urban heat island effects through evaporative cooling (EC) (Rao, 1990; Saneinejad et al, 2011; Saneinejad et al., 2014). Other research examines building cooling and energy saving strategies based on EC principles, like water spraying (He and Hoyano, 2008; Kitano et al., 2011), cyclic water film (Qahtan et al., 2014), etc.; which do not make direct use of the rain but require adequate water. The necessary amount of water could be readily provided by precipitation in various places in the humid tropics and subtropics.

46 C.A. Diaz, P. Osmond and S. King

Despite all the above, precipitation is an overlooked variable in the analysis and development of climate responsive architecture and its value as a primary source of water is usually neglected (Kinkade-Levario, 2007). Additionally, most research gives little attention to water availability and the substantial water surplus in locations of the Hot Humid Tropics is largely ignored, even in cases where the potential of EC is recognized (Figure 1). All this shows the need for a better understanding of the potential of precipitation and its importance architecture.

Figure 1: Relation of water availability from precipitation and regions with most research on building and urban EC. a) By zonal means (base graph source: NASA, 2010), b) By annual patterns.

2. Precipitation and buildings

The relation between precipitation and buildings is not commonly perceived as constructive. On the one hand precipitation is seen as an element from which buildings must be protected (e.g. Harriman and Lstiburek, 2009). On the other, buildings are seldom considered as active factors among the urban strategies for control and reuse of water from precipitation, even though they cover a significant portion of the total area of a city. Even so, a positive relation between precipitation and architecture is possible, which can be identified in two directions:

From precipitation to architecture:

Precipitation can provide water required for human use as well as for other needs of the building.

Precipitation has a natural cooling effect over the building fabric, which is favourable for buildings in hot environments. This is useful to balance the thermal performance of buildings and reduce the impact of extreme heat waves (Figure 2).

From architecture to precipitation:

Buildings can contribute to the control and management of the water from precipitation, to minimize the impact of extreme events and to reduce the load on stormwater infrastructure.

Buildings can be an integral part of systems to recover and maintain natural water cycles.

Hot

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Tropic of Capricorn

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Saneinejad et al , 2014 Saneinejad et al , 2011 He and Hoyano, 2008

Rao, 1990Hot

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Temperature Range Precipitation Mean Temperature

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of

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10°S 10°N 52°N

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Saneinejad et al2014

Zurich

Saneinejad et al2011

Nicosia

He and Hoyano 2008Tokyo

Rao, 1990Singapore

a b

47

Precipitation and buildings: estimation of the natural potential of locations to sustain indirect evaporative cooling strategies through hot seasons

Figure 2: Reduction of building surface temperatures after incidence of light showers (30 min difference).

However, this positive relation is broken when precipitation is only seen as affecting the integrity of the building, or if the building is seen as a passive element which does not influence its surroundings. Thus, a circle of negative impacts is usually developed: buildings alter natural water cycles and convey the problems to other environments, which in turn contributes (to some extent) to extreme events of precipitation with adverse effects over the built environment. This could be avoided if there is more concern for precipitation in the processes of building and urban design, particularly in those areas with abundant precipitation such as the humid tropics and subtropics.

3. Precipitation analysis for building design

In climate analysis for architecture there is a bias to the four climatic variables which directly influence human thermal comfort, especially to temperature and humidity. Analysis tools for these variables allow intuitive interpretation and direct association to the configuration of the building (e.g. wind rose, sunpath diagram, etc.), which is very practical for building design. However, this is not the case for precipitation because its influence is not usually associated with building function and performance.

Although charts of annual precipitation patterns are very common in climate analysis for architecture, little tangible information is extracted from these for building design purposes. The diagrams usually seen in architecture are limited to describing precipitation profiles and patterns without providing elements which could enrich their interpretation (Figure 3).

`

Figure 3: Typical precipitation analysis found in architecture literature. a) Annual precipitation histogram (source: Szokolay, 2014), b) Minimal description of precipitation (source: Lechner, 2014).

Sp1 1.0Sp2 1.8Sp3 1.5Sp4 1.5Sp5 6.5Sp6 4.9

ΔT

WETDRY

a bThe annual precipitation is quite high at 60 in. (150 cm)and occurs fairly uniformly throughout the year.


Deeper interpretation of precipitation data is important to discern its influence over the function and performance of buildings and their surroundings. For this reason there is a need for methods that help to enable meaningful use of the precipitation data for architecture applications as it happens in the cases of solar radiation, wind, etc.

4. Proposed practical graphical method

This paper proposes a graphical method which emphasizes the analysis of precipitation, with the purpose of helping building designers and planners to carry out meaningful “readings” of climate data and to assess applications of EC for buildings. This method supplements existing climate analysis tools by defining benchmarks and a scale that easily links potential water requirements of buildings with water availability from precipitation. It observes three climate variables at once: Temperature, Humidity and Precipitation, and defines one benchmark for each variable. For instance, the ones used in this paper:

Hot: Dry Bulb temperature (DB) of 23 °C, the lowest temperature of the 0.5 clo ASHRAE Comfort Zone (this could vary according to the sensitivity of the population).

Humid: Dew Point temperature (DP) of 16.8 °C, based on the upper boundary of the ASHRAE comfort zone (widely applied for building design and thermal comfort).

Precipitation Surplus: This benchmark is dynamic, according to the water requirements of buildings. This is defined through a basic calculation which is explained in Section 4.4.

The values of these benchmarks are not fixed. They serve as references for contrasting and analysis, and can be defined according to the needs of the analysis and the user, as long as they are supported on reasonable criteria. Once the benchmarks are defined, is possible to look for relations and patterns in the climate data which are useful to estimate if EC strategies are suitable for buildings in the location that is analyzed; for instance, rainy seasons in phase with hot seasons, days with low humidity, etc.

The method is based on monthly precipitation data, which is readily available, sufficient for preliminary analyses and remains simple and practical for designers and planners (analysis based on daily or weekly precipitation require long historical records and sophisticated calculations). It is applied through adapting diagrams and charts which are conventionally used for the analysis of climates: Ombrothermic Diagrams, Thermohyet Diagrams and Psychrometric Charts.

4.1. Ombrothermic diagrams (climate diagrams)

This diagram combines monthly averages of temperature and precipitation over a year to determine the length of dry, wet, and extremely wet periods. It can reflect how the amount and distribution of precipitation determines native vegetation in an area.

The effectiveness of precipitation (P) is related to the temperature (T) by a simple aridity index where P=2T, an expression of the limit between the dry season (P<2T) and the wet season (P>2T) (Bagnouls and Gaussen, 1953). The relation is valid for units in C° and mm. (it does not work for F° and/or inches). This index is good enough for architecture applications due to simplicity and because a more accurate definition is not as critical as for other fields (e.g. agriculture, stock farming, etc.).

In the original application of the diagrams, an interval where P>100 mm is considered a surplus period. In the adaptation proposed here for architecture applications, this benchmark would change according to type of buildings and their water demands (See Figure 4). The calculation of this benchmark is explained in section 4.4.

49


Figure 4: Ombrothermic Diagram: a) Original (source: Walter and Lieth, 1967), b) Proposed.

At the bottom of this diagram, the DB line (red) defines the levels where P=2T. Conditions of precipitation below this line would define arid periods and those above the line define wet periods (in purple). When wet periods surpass the precipitation surplus benchmark these are defined as hyper wet (surplus) periods, where water availability exceeds the demand and can be stored for future needs.

Below the DB line is the DP line (blue). The area filled with vertical stripes shows the Dew Point Depression (DPD), which indicates the levels of moisture saturation in the location. Up to 8 °C, each degree of difference approximates a reduction of 5% in Relative Humidity (RH) from 100% (e.g. DPD=7°C equals about 35% reduction, then RH=65%). From 8 °C to 30°C, each degree of difference approximates a 3% to 1% RH reduction.

The precipitation scale above the 100mm line is adjusted by a factor of 10, to control the height of the vertical axis when there are major levels of precipitation.

4.2. Thermohyets diagrams (hythergraphs)

This diagram is plotted in a Cartesian system using monthly rainfall data in the abscissas and temperature in the ordinates. Its primary use was intended for agriculture.

This sort of graph can identify patterns of climate regimes and trends which indicate the dominant air mass influences of the location. Although its use is not seen in architecture it provides a very convenient way to combine temperature, humidity and precipitation in one diagram with practical meaning for architecture applications (Figure 5).

The diagram proposed here has two loops of monthly average points combining temperature and precipitation: a) The Dry Bulb (DB) loop (red) and b) the Dew Point (DP) loop (blue). The plane is divided in quadrants by the Hot and the Surplus benchmarks. All points of the DB loop located in the upper right quadrant (Hot-Surplus) indicate the months in which the hot season is in phase with abundant rain. Months located in the Hot-Surplus quadrant have a significant potential for EC strategies.

Like in the case of ombrothermic diagrams, the vertical distance between the points of the DB loop and the DP loop (that is, the DPD) depicts the level of moisture saturation in the air.

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Figure 5: Thermohyet diagram: a) Original (source: Taylor, 1950), b) Proposed.

4.3. Psychrometric chart

The psychrometric chart is based on air temperature and humidity, and from these two variables other properties of moist air can be determined. In our proposal the chart is adapted for combined analysis of temperature, humidity and precipitation in order to assess the feasibility of EC strategies.

The adapted chart plots hourly conditions of the air at the same time with monthly averages of temperature, humidity and precipitation (connected in an annual loop), and adds concentric circles indicating the precipitation levels (Figure 6-a). It differs from the previous diagrams because it allows contrasting water availability with detailed conditions of the air, both relevant to weigh the feasibility of EC for buildings. This provides a complete picture of the water source, the actual influences over evaporation and potential EC performance (Figure 6).

Figure 6: Proposed psychrometric chart: a) Annual loop of monthly averages (zoom), b) Complete chart.

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EXTENDEDCOMFORT

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a b

51


The possibility of filtering data (e.g. individual month) and to see extreme conditions (e.g. heat wave) is particularly valuable for the analysis of hot humid climates, which are traditionally considered not suitable for EC techniques. As seen in Figure 6-b, even a permanent hot humid climate with annual RH of 67% can have hours with RHs between 30% to 40%. These low saturation conditions together with the significant water surplus signal the good potential for EC in this location. Therefore, for the particular case of Figure 6, EC deserves consideration as a key cooling strategy for buildings in the location.

4.4. Precipitation benchmark

Before defining a benchmark for precipitation is necessary to associate the units of precipitation with those of water demand. This makes the interpretation of the diagrams and charts direct and simple.

The relation of precipitation and water demand scales is influenced by the runoff efficiency of the roof or other element from which the water is collected. This is illustrated in Table 1 and is exemplified graphically in Figure 7. Also, the relation can be easily plotted over any of the three diagrams and chart proposed in section 4, and is the base to depict the precipitation benchmark.

Table 1: Relation of collected to supplied water for different runoff efficiencies.

Runoff coefficients

Effective catchment (L/m² day)

mm to supply 1 L/m² day

Material

0.95 0.03167 31.6 Metal 0.9 0.03000 33.3 Metal, concrete, asphalt 0.8 0.02667 37.5 Clay, concrete, asphalt 0.7 0.02333 42.9 Clay, gravel, shingle 0.5 0.01667 60.0 Green roof

Figure 7: Graphical relation of precipitation and water demand for different runoff efficiencies.

The precipitation benchmark (PB) is the minimum level of precipitation required for a building to fully meet its demand of rainwater, and any amount above this level is exceeding precipitation which could be stored by the building for future needs. It can be calculated from the simple formula:

PB = (Dt x 30 days) / ([CRa/HFa] x C) (1)

Where:

1 m³ = 1,000 L1 m

1 m1 m

1 m² (Roof)

1000 mm

0 mm

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1,000 mm of rain = 1,000 L catchment

1 mm = 1 L/m²

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PB = Precipitation benchmark (mm or L/m²); Dt = Total water demand (L/m² day); CRa = Collectable roof Area (m²); HFa = Habitable floor area (m²); C = Runoff Coefficient (unitless).

The denominator in equation (1) is termed the Building Factor (BF), because all its variables depend on the configuration of the building. The minimum precipitation required to satisfy the water demand of the building will vary according to its BF (the requirement increases when BF is <1 and decreases when BF is >1).

If water is required for different purposes (e.g. laundry, toilets, EC, etc.) DT must be calculated previously with the formula:

Dt = De + (Da/HFa) (2)

Where:

De = Water demand for EC (L/m² day); Da = Supplementary water demand (L/day).

This water demand may vary according to the climate and the specific needs of each building. Even very similar buildings may have differences in the amounts of water required (e.g. due to orientation, surrounding influences, etc.). Thus, the particular demand of a building must be individually calculated. Water demands for building cooling used in the examples below are taken from diverse research about EC in buildings. Many of the values found in literature converge in a range between 4 to 10 L/m² day.

Table 2 gives examples of the calculation of PB for three cases:

Case a: 1 story house, 80 m², 2 people, demand for EC, toiles and laundry – hot humid climate. Case b: 2 story house, 120 m², 4 people, demand for EC and toilets – hot dry climate. Case c: 2 story office building, 250 m², 12 people, demand for EC and toilets – hot humid climate

Table 2: Relation of collected to supplied water for different runoff efficiencies.

Case De Da Dt CRa HFa C Roof Type BF PB

L/m² day L/day L/m² day m² m² - - mm or L/m² a 4.11 250.00 8.02 100 64 0.8 concrete 1.25 192.39 b 8.50 280.00 11.30 80 100 0.95 metal 0.76 446.05 c 5.20 220.00 6.77 120 140 0.5 green roof 0.43 474.00

5. Urban precipitation surplus (UPS)

Once we have a practical way to discern the significance of the precipitation amounts, it is possible to estimate if the location has a natural potential to support EC strategies for buildings. For this purpose, this paper proposes the concept of Urban Precipitation Surplus (UPS).

The UPS refers to the excess rainfall of an urban area, usually discarded, which could be exploited for building cooling and could contribute to regenerate the water cycle and improve microclimates in cities. This is limited to the rainfall over buildings which is generally conveyed to the surrounding areas and finally sent to drainage systems without being used. Thus, it does not include the direct rainfall over outdoor areas and streets.

The UPS is the difference between the average monthly precipitation and the precipitation benchmark for a standard building (UPS=P - PBs). This standard building has to be representative of the majority of buildings of an entire urban area (e.g. city, district, neighbourhood, etc.). In this way, it provides a reference for analysis and design of buildings and urban ensembles.

53


If the value of the UPS is 0 this means that the typical precipitation of the location merely satisfies the demand of water for the function of the standard building and its cooling through evaporation. When the value is >0 the surplus might be sufficient for buildings with low BF (e.g. multistory buildings or buildings with a reduced area to collect water). When the value is <0, the UPS can cover only part of the water demand and its use must be prioritized (e.g. for essential functions or for parts of the building with critical heat gains).

To illustrate the concept of UPS, the city of Cairns in Australia is used as an example. Satellite images and statistics of this city show that a big proportion of its buildings are detached houses, covering between 25 to 30% of the land in urbanized areas; of which 65% have 1 to 3 bedrooms (.id, 2015). From the above, we deduce that the dominant type is the detached single story house, which consequently becomes our standard building to determine the UPS. The BF for this type of dwelling is estimated in 1.44 (based in a floor area of 150 m2), and if we assume a water demand for EC of 10 L/m2 day, the building would require minimum monthly precipitations (PB) of 209 mm.

Table 3: Average monthly precipitation and UPS for the city of Cairns, Australia.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Precip.

396.9 422.1 448.9 224.5 107.1 49.7 25.6 22.3 33.4 43.6 100 190.3

UPS 188.0 213.2 240.0 15.6 -101.8 -159.2 -183.3 -186.6 -175.5 -165.3 -108.9 -18.6

The resulting UPS is positive from January to April (+16 to +241), which means that rainfall in these months can cover the whole demand of the standard building. The UPS is negative between May to December (-19 to -187). However, the period from June to August is not in the hot season, and in December the UPS is close to 0. Then, the critical months are May, September and October, which would require defining priorities for the use of rainwater. Therefore, we confirm that there is a potential for exploiting rainfall during the summer season, when it is most needed.

In brief, the UPS is useful for a preliminary analysis of a location and can help us to identify opportunities for exploiting natural water resources for building cooling, even for cases where the humidity is high (see Section 4.3). It appeals to further investigate and analyse the feasibility of applying EC techniques for buildings, and prevents discarding them due to incorrect assumptions.

6. Conclusions

Throughout this paper we have discussed ideas which suggest an essential analogy between architecture and vegetation. To begin with, all points listed in Section 2, about the positive relations between buildings and precipitation, are equally applicable to plants. The reciprocal influence that exists between buildings and their environment resembles the importance of plants in balance with their natural context. This analogy is confirmed by the suitability of the diagrams in Section 4 for architecture applications, which were originally developed for applications related to vegetation and refer to critical aspects for agriculture and climatology which also have significance for architecture (e.g. aridity, evapotranspiration, rain interception, etc.).

Is also crucial to consider that in locations which are inherently wet, the growth of cities generate a process of “desertification”, where the pre-development vegetation is replaced by impervious materials that repel water and store heat in proportions which are not normal. Dry cities with high levels of runoff are inconsistent with the wet nature of numerous locations in the Humid Tropics and subtropics; in the


same way that cities in dessert areas (e.g. Dubai, Las Vegas) are inconsistent with excessive demands for water.

The issues presented here highlight the need to observe the relation between precipitation and buildings, and to integrate buildings as active elements with positive influence on their environments. One way to achieve this is through a general implementation of EC strategies, like roof and wall spraying, in those locations with significant water surplus. In doing so, buildings could create a significant demand on the stormwater which is usually rejected during the rainy seasons, generate a substantial reduction of sensible heat in urban areas and bring improvements in the local water cycle. Thus, besides improving their own conditions, buildings would contribute to other issues like stormwater management, urban heat island mitigation and adaptation to climate change. All this confirms the need for more research and tools that help to enhance our understanding of the relation between precipitation and buildings and its positive application in architecture.

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Precipitation and buildings: estimation of the natural potential of...

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