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The role of plants in the reduction of heat flux through green roofs: laboratory experiments.

INTRODUCTION

Green roofs are an emerging sustainable technology that is becoming more popular in North America (Miller et al., 2005). As a definition, green roofs are "specialized roofing systems that support plant growth on rooftops" (Liu et al., 2004). From top to bottom, a typical green roof consists of several layers: (1) vegetation, (2) substrate, (3) filter membrane, and (4) drainage layer. Plants used for extensive green roofs are typically drought tolerant, and selected from the group of native or Sedum plants. Substrate is the soil-like layer where plants grow, and it has to be porous, retain moisture and nutrients, and support plant growth (Snodgrass et al., 2006). The filter membrane prevents drainage clogging by containing the substrate and roots. The drainage layer transports the rainfall water runoff to the roof drainage (Peck, 2002; Snodgrass et al., 2006).

There are basically two types of green roofs: extensive and intensive green roofs. Extensive green roofs have lower weight, lower capital cost, minimal maintenance, and a substrate depth between 2 and 6 inches (5 and 15 cm). Intensive green roofs have greater weight, higher capital costs, wider planting selection, higher maintenance requirements, and a substrate depth between 8 and 24 inches (20 and 60 cm). However, intensive green roofs are less cost-effective than extensive and required more structural support (Peck et al., 1999; Tanner, 2004). Moreover, extensive green roofs represent about 2/3 of the total green roof square footage installed in North America (Johnston, 2007). Therefore, this research project focuses on summer thermal performance of extensive green roofs as a more economically viable solution to be adopted in the building industry.

The popularity of green roofs is increasing due to their potential benefits. In general, green roofs have a potential to (Liu et al., 2004):

* reduce energy demand on space conditioning

* reduce storm water runoff

* improve air quality, and

* reduce the urban heat island effect in cities.

Therefore, green roofs can help address three of the four top problems facing the society in the next 50 years: energy, water, and environment (Smalley, 2005).

GREEN ROOF ENERGY BALANCE

The main challenge of accurately modeling and measuring the thermal performance of green roofs is due to the complex heat and mass transfer processes through the roof by the means of (1) shading, (2) insulation, (3) evapotranspiration, and (4) thermal mass (Liu, 2004). Evapotranspiration represents a combined process of water lost from the soil (evaporation) and plants (transpiration). Transpiration occurs when water from the plant leaf surface goes into the air by diffusion or convection. Most of the water is lost by transpiration through plant stomata, which are adjustable small pores in the leaf that allow the entry of gases needed for photosynthesis such as [CO.sub.2], and the release of [O.sub.2] and water vapor. Thus, plants can control their transpiration rate by opening and closing their stomata (Nobel, 1983; Allen et al., 1998; Hillel, 1998).

All of the heat transfer processes taking place on a green roof are combined in an energy balance equation as following (Hillel, 1998; Jones, 1992):

[R.sub.n] = ET + [Q.sub.sensible] + [Q.sub.condution] + [S.sub.thermal] + M (1)

where

[R.sub.n] = net radiation, equal to solar gain minus infrared heat losses, Btu/h*[ft.sup.2] (W/[m.sup.2])

ET = evapotranspiration, or latent heat flux, Btu/h*[ft.sup.2] (W/[m.sup.2])

[Q.sub.sensible] = convective or sensible heat flux, Btu/h*[ft.sup.2] (W/[m.sup.2])

[Q.sub.condution] = heat flux trough roof, Btu/h*[ft.sup.2] (W/[m.sup.2])

[S.sub.thermal] = thermal storage for substrate, plants, Btu/h*[ft.sup.2] (W/[m.sup.2])

M = metabolic storage (photosynthesis and respiration), Btu/h*[ft.sup.2] (W/[m.sup.2])

In Equation 1, the metabolic storage is often neglected as its contribution to the total energy budget is around 1% to 2% of the net radiation (Jones, 1992; Gates, 1980). All fluxes are dependent greatly on the capacity of the plants/substrate to evaporate water as latent heat flux uses energy from the environment to evaporate water, thus cooling down the plants' surface and roof temperature. Consequently, the latent heat flux partially controls the heat flux going trough the substrate/roof that eventually converts to building cooling loads. Evapotranspiration is also a phenomenon that enables green roofs to decrease the urban heat island effect by lowering the temperature of the roofs by evaporation of the rain water.

As an example of energy balance, Figure 1 shows the percentages of each heat transfer component divided by the incoming shortwave radiation for a particular experimental setup with a green roof. Experiments are described in more details later in the paper, while these results are presented here for the illustration of typical heat fluxes and heat balance on a green roof. Table 1 provides a summary of the total heat flux components for the two different sets of experiments, one with the plants and another without plants. Because all fluxes were divided by the incoming shortwave radiation having the same units, the results are dimensionless. The sum of all heat fluxes shown in Table 1 was very close to 100% for the experiments without plants. In contrast, the sum of heat fluxes for the experiments with plants was systematically lower than 100% by 10-15% due to the assumption of a horizontal flat plate for the convective heat transfer and simplified infrared radiation model. For proper comparison, the sums of all heat fluxes shown in Table 1 were normalized to 100% for both cases. As shown in the table, the latent heat flux played an important role in the heat transfer process by diverting from the roof about 55% to 80% of the incoming shortwave radiation to the process of evapotranspiration. It is important to mention that these percentages were obtained for a wet green roof, while for a dry green roof these percentages are substantially lower.

[FIGURE 1 OMITTED]
Table 1. Percentage of Heat Fluxes Relative to Incoming to Shortwave
Radiation Obtained in Current Experiments, Day Two

                                                   No Plants  Plants

Incoming Shortwave Radiation                           -100%   -100%

Infrared Radiation/Incoming Shortwave Radiation          -3%    -24%

Reflected Shortwave Radiation/Incoming Shortwave          5%     11%
Radiation

Latent Heat Flux/ Incoming Shortwave Radiation           56%     82%

Sensible Heat Flux/ Incoming Shortwave Radiation         20%     16%

Conductive Heat Flux/Incoming Shortwave Radiation        22%     15%


The thermal performance of green roofs has been studied worldwide using three different approaches: (1) field or laboratory experimentation, (2) theoretical studies, and (3) a combination of laboratory or field experiments with numerical models. From these three approaches, only the field and laboratory experiments have focused on comparing energy fluxes from bare soil surface to planted surfaces.

Most of the green roof field studies have focused on heat flux reduction through the roof. Interestingly, an on-site study found a significant reduction of heat flux from a green roof compared to a bare soil roof (Wong et al., 2003). The study concludes the difference was due to the shading of plants because the heat flux at night was mainly the same for both roofs. Another study came to similar conclusions by analyzing an irrigated bare soil roof, and then adding a shading device over the roof (Pearlmutter et al., 2008). Our present study finds that it is not just the shading, but also the evapotranspiration (latent heat flux) that gets alerted by the presence of plants. This finding was possible because we conducted tightly controlled laboratory experiments with laboratory graded instrumentation.

A laboratory study inside a greenhouse compared the latent heat flux of green roof samples with other samples without plants (Rezaei, 2005; Berghage et al., 2007). The latent fluxes were measured based on gravimetric method that continuously weigh the samples after initial watering. The highest latent heat flux, which corresponds to the highest weight loss, was during the first day of measurements with a value around 111 Btu/h [ft.sup.2] (350 W/[m.sup.2]). This peak latent heat flux coincides with the peak solar radiation, while the soil was the wettest during that day. The latent heat flux for the sample with no plants was about half the value compared to the sample with plants (Berghage et al., 2007). For the sample with plants, Figure 2 shows latent heat fluxes during the first four days of measurements for summer conditions in Pennsylvania (Rezaei, 2005). It is important to notice that the latent heat flux decrease substantially from day to day as the available water content decreased.

[FIGURE 2 OMITTED]

LABORATORY EXPERIMENTS

Controlled laboratory experiments are proposed as a solution to understand non-steady state heat transfer phenomena through a green roof. After gaining insight from previous on-site and laboratory studies, our research team designed and built a new experimental apparatus called "Cold Plate" to test the thermal performance of green roofs. The design of the Cold Plate was inspired by ASTM standards C177 (ASTM 1997a) and C1363 (ASTM 1997b), and later modified based on experiments with non-homogeneous samples (Tabares-Velasco et al., 2007). A detailed description of the experimental apparatus is available in the literature (Tabares-Velasco et al., 2007). Most importantly, tests performed with the Cold Plate are conducted under tightly controlled conditions inside a state-of-the-art environmental chamber. This chamber contains data acquisition systems that measure and/or control the energy consumption, air quality, and thermal comfort of different heating, ventilating and air conditioning systems (HVAC). Figure 3 shows locations of several data acquisition sensors installed in green roof samples to measure heat fluxes, irradiance, volumetric water content (VWC) in substrate, air and substrate temperatures, and air speed.

[FIGURE 3 OMITTED]

Both latent and conductive heat fluxes are measured by two independent approaches to add redundancy and check the accuracy of both measurement methods. Incoming shortwave radiation was calculated using measurements from a secondary class pyranometer. Absorbed shortwave radiation was calculated from the measured incoming radiation and variable albedo for the specific wavelength of the lamps obtained from the literature for different soils and plants (Escadafal, 1990; La et al., 2008; Gates, 1980). Convective heat flux was calculated assuming turbulent natural convection on a flat horizontal plane because the Raleigh number was in the order of [10.sup.7]. Infrared heat transfer was calculated assuming two large parallel planes, and used temperature readings at the top layer of substrate and plant leaves, as well as temperature readings for the lamp surface. Lamp temperature was measured by a thermistor located near the light bulbs. Another set of thermistors located under the plant leaves measured the surface temperature of the leaves. Finally, a set of thermistors located under a thin layer of substrate measured the top substrate temperature. Substrate and plants emissivity was set to 0.95 (Pielke, 2005; Gates, 1980; Nobel, 1983). Lamp emissivity was set to 0.90 (Incropera et al., 2002).

Experimental Procedure

Two green roof samples were tested under similar environmental conditions. Both samples had inner dimensions of 48x42x3 1/2 [in.sup.3] (122x107x9 [cm.sup.3]). The samples contained 3 1/2 in. (9 cm) deep green roof substrate, which consists mainly of expanded clay typically used in the green roof industry. As shown in Figure 4a and 4b, the first sample used bare substrate, without any plants, while the second sample was completely covered with Sedum Spurium. Sedums are succulent plants, and have the ability to limit their water loss due to transpiration (Van Woert et al., 2005).

[FIGURE 4 OMITTED]

The sample without plants was tested first, and the sample with plants was tested one week after the first set of experiments. Reflective side panels were added to the Cold Plate for the sample without plants to improve the uniformity of the irradiance. Both samples were watered 48 hours and 24 hours before testing to allow for proper saturation of the substrate and successive drainage of excess water. The sample without plants was tested for 6 days to observe the drying process of substrate to reach very low water content. The sample with plants was tested for 3 1/2 days, and further drying would cause damage to plants, which would eventually start to behave as a bare substrate sample. For each day the lamps were on approximately 14 hours, and were switched off for another 10 hours to allow transition between "day" and "night" environmental conditions. The chamber environmental controls were set to provide the air temperature of 82[degrees]F (28[degrees]C) and relative humidity of 39-44%. The measured return air temperature was 86[degrees]F (30[degrees]C).

RESULTS

Total incoming shortwave irradiance was 57-60 Btu/h [ft.sup.2] (180-190 W/[m.sup.2]) for the sample without plants, and 47-50 Btu/h [ft.sup.2] (150-160 W/[m.sup.2]) for the sample with plants. The 20% difference in irradiance between the two sets of experiments was due to the reflective panels located at the side of the sample without plants. The spectral power distribution of the lamps was mainly in the UVA and visible part of the spectrum. Figures 5a and 5b show the radiative heat fluxes between the lamps and green roof samples as following: absorbed short wave radiation (Q_solar_abs), infrared radiation (Q_IR), and the net radiation (NET_RAD) during the second day of each experiment. Radiative fluxes did not change significantly during the two experiments. Infrared radiation was an incoming flux to the samples during the "day" as the lamps had higher temperature than the surface temperature of the substrate and leaves. This phenomenon is the opposite in clear skies as the sky temperature is typically lower than the observed surface temperature.

[FIGURE 5 OMITTED]

Figure 6 shows the temperature at different vertical green roof layers such as air above the sample, plants (only for the sample with plants), top substrate, bottom substrate and cold plate temperature. Note that the air temperature remained almost the same for both cases, despite the difference in the reflective sides. In contrast, the peak temperature of the top substrate decreases by about 27[degrees]F (15[degrees]C) for the substrate covered by plants.

[FIGURE 6 OMITTED]

Figures 7a and 7b show the volumetric water content for both samples. Volumetric water content is the ratio of volume of water divided by the total volume (water, substrate and void spaces) with units of [m.sub.water.sup.3]/[m.sub.total.sup.3] or [ft.sub.water.sup.3]/[ft.sub.total.sup.3]. Figure 7a shows that after four days of testing the substrate volumetric water content approaches a condition known as the wilting point. For soils with volumetric water content lower than the wilting point, plant physiological functions are affected by the scarce water content in the substrate (Hillel, 1998). Typical volumetric water content values for permanent wilting point vary from 0.39 to 0.17 for soil textures such as peat, clay, and loam soil (Pielke, 2002).

[FIGURE 7 OMITTED]

As shown in Figures 7a and 7b, both samples started the experiment with almost the same volumetric water content. However, the sample without plants evaporates more water as the water content in the substrate decreases more rapidly. A similar trend for the volumetric water content profile has been observed in another green roof study (Takebayashi et al., 2007). In the present study, despite the net radiation differences, potential evapotranspiration for the sample without plants was 0.039 lb/[ft.sup.2]h (0.19 kg/([m.sup.2]h)) or 41 Btu/h [ft.sup.2] (128 W/[m.sup.2]) during the second day of experiments. Similarly, potential evapotranspiration for the sample with plants was 0.041 lb/([ft.sup.2]h) (0.20 kg/([m.sup.2]h)) or 43 Btu/h [ft.sup.2] (135 W/[m.sup.2]) during the second day of experiments. Potential evapotranspiration is defined as the "maximum rate of evapotranspiration from a large area covered completely and uniformly by actively growing vegetation with adequate moisture at all times" (Brutsaert, 1984). For well-watered conditions, potential evapotranspiration depends primarily on solar radiation and secondarily on vapor pressure deficit (Hillel, 1998).

Figure 8 shows the heat flux through the green roof sample calculated from the cold plate apparatus. Average heat flux values were consistent with the calculated values from the cold plate apparatus. There is an approximately 40-50% reduction in the heat flux for the green roof sample with plants compared to the sample with only substrate. This proves that plants have a significant role in reducing the heat flux through a green roof. For each of the samples (plants and no plants), the heat flux through the sample increases at the same time when the latent heat flux decrease, as shown in Figures 9a and 9b. Maximum and minimum heat flux values in Figure 8 do not coincide for the two experiments because each "day" did not last exactly 14 hours.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Figures 9a and 9b show the latent heat flux measured by the gravimetric method using a scale. These latent heat flux values are lower than the previously shown in Figure 2 because the incoming radiation in this laboratory experiments was substantially smaller than the heat flux in the greenhouse experiments (Farzaneh, 2005). Other studies have measured similar latent heat flux values as in the present study for green roof samples in a wind tunnel (Onmura, 2003) and on a roof (Takebayashi et al., 2007).

As shown in Figures 9a and 9b the latent heat flux for both samples decreased from day to day as the water resources in the substrate decreased. During the experiments, plants seem to better manage water as the latent flux does not show as large day-to-day gradients as the bare substrate sample without plants, even though the potential for evapotranspiration is practically the same for both samples. As observed in Figure 9b, heat flux data are missing for the first day because the instrument was not working properly. For the sample without plants, evaporation from the substrate decreased to a minimum of 10 Btu/[ft.sup.2] (31 W/[m.sup.2]) during the 6th day of testing.

Figures 10a and 10b show the sensible (convective) heat flux for both samples. The sample with no plants had significantly higher sensible heat flux due to the high temperature recorded in the top layer of the substrate. The sensible heat flux was around 10 Btu/h [ft.sup.2] (31 W/[m.sup.2]) at the end of the first day, and increased to a value of around 22 Btu/h [ft.sup.2] (69 W/[m.sup.2]) at the end of the 6th day. The decline in sensible heat flux is due to the fact that the amount of heat removed by latent heat was reduced by 80%, while the other heat transfer mechanisms had to increase to maintain a heat balance.

[FIGURE 10 OMITTED]

In contrast, Figure 10b shows that the convective heat transfer remains almost the same. This constant convective heat flux could be caused by our approach to measure the plant surface temperature, which was not completely accurate. An improved measurement of leaf temperatures could lead to lower temperatures. Another reason for almost constant convective flux could be that a different convective heat transfer coefficient is needed for the plant surface. Future experimental procedure will address improvement by addressing thermal contact to leaves and adding thermal imaging.

Figures 11a and 11b show calculated infrared heat transfer between the green roof samples and the lamps for the second day of tests. Both figures show similar order of magnitudes for the radiative heat fluxes for the first day. Overall, the radiative gains for the green roof sample reduced with time as the samples approached steady state surface temperature.

[FIGURE 11 OMITTED]

Figure 12 shows the calculated thermal conductivity for a quasi steady-state condition, which was achieved at the end of each day. Thermal conductivity was calculated after measuring quasi steady-state heat flux through the green roof samples and the respective temperature gradient across the substrate. The calculated data fits within the lower limits of the current literature on green roof substrates (Tabares-Velasco, 2007).

[FIGURE 12 OMITTED]

Figures 9a, 9b, 10a, 10b, and 12 show trade offs between a wet and a dry green roof. Substrate thermal conductivity is the lowest when the soil is dry; in contrast, the latent heat fluxes are the highest when the soil is wet. Overall, the smallest heat flux through the green roof was found when the soil was wet because the latent heat flux diminished any increase in substrate thermal conductivity. Finally, a quasi-state steady R-value for the green roof sample (plants and substrate) ranges from 4.3 [ft.sup.2] h [degrees]F/Btu (0.76 [m.sup.2] [degrees]C/W) to 4.8 [ft.sup.2] h [degrees]F/Btu (0.84 [m.sup.2] [degrees]C/W). The R-value for the plant layer ranges from 2.3 [ft.sup.2] h [degrees]F/Btu (0.40 [m.sup.2] [degrees]C/W) to 2.7 [ft.sup.2] h [degrees]F/Btu (0.48 [m.sup.2] [degrees]C/W). The R-value for the plants is slightly higher than the previously reported for an extensive green roof (Wong et al. 2003). The calculated R-value is preliminary as future research will calculate how the R-value changes depending on different weather conditions and will be considering dynamic behavior of buildings.

CONCLUSION

This paper presents insight into the role of plants in the heat transfer process on a green roof. Two green roof samples were analyzed in a set of laboratory experiments. Overall, plants have an important role in reducing the heat flux through the roof by regulating (1) the latent heat flux through better water management and additional water storage in leaves/roots, and (2) the sensible heat flux with an additional shading layer. In summer cooling conditions, the smallest heat flux through the green roof was found when the soil was wet, despite the increase in substrate thermal conductivity. Plants reduced the conductive heat flux through the green roof sample by 40-50%. The steady state R-value for the green roof sample (plants and substrate) ranged from 4.3 [ft.sup.2] h [degrees]F/Btu (0.76 [m.sup.2] [degrees]C/W) to 4.8 [ft.sup.2] h [degrees]F/Btu (0.84 [m.sup.2] [degrees]C/W). Therefore, any accurate building energy simulation would need to model plants physiological behavior as plants have an important role in the heat flux reduction through the roof. Future experiments will provide specific spectral properties for the Sedum plants, more precise temperature measurement on the plant surface and lamps, as well as further investigate the convective heat flux between the plants and surrounding air. In addition, future research will include a new green roof model for energy building analysis and thermal modeling of green roofs including the physiological behavior of plants.

ACKNOWELDGMENTS

We would like to thank Dr. Robert Berghage, Director of the Penn State Center for Green Roof Research, for his cooperation and assistance with the design and selection of green roof plants and substrate. This study was supported by the Center for Environmental Innovation in Roofing, Washington DC, USA, http://www.roofingcenter.org/, CONACYT (Consejo Nacional de Ciencia y Tecnologia), Mexico, http://www.conacyt.mx/, ASHRAE Graduate Student Grant-In-Aid and ASHRAE Technical Committee TC 4.4 Building Materials and Building Envelope Performance, http://tc44.ashraetcs.org/.

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Paulo Cesar Tabares-Velasco

Student Member ASHRAE

Jelena Srebric, PhD

Member ASHRAE

Paulo Cesar Tabares-Velasco is a graduate student and Jelena Srebric is an associate professor at the Department of Architectural Engineering, The Pennsylvania State University, University Park, PA.
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Author:Tabares-Velasco, Paulo Cesar; Srebric, Jelena
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Date:Jul 1, 2009
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