Voltage Swell Mitigation Matlab

Robert M. Hamwey†

Cen2eco: Centre for Economic and Ecological Studies, Geneva, Switzerland Abstract: To date, international efforts to mitigate climate change have focussed on reducing emissions of greenhouse gases in the energy, transportation and agriculture sectors, and on sequestering atmospheric carbon dioxide in forests. Here, the potential to complement these efforts by actions to enhance the reflectance of solar insolation by the human settlement and grassland components of the Earth’s terrestrial surface is explored. Preliminary estimates derived using a static two dimensional radiative transfer model indicate that such efforts could amplify the overall planetary albedo enough to offset the current global annual average level of radiative forcing caused by anthropogenic greenhouse gases by as much as 30% or 0.76 Wm-2.

Terrestrial albedo amplification may thus extend, by about 25 years, the time available to advance the development and use of low-emission energy conversion technologies which ultimately remain essential to mitigate long-term climate change. While a scoping analysis indicates the technical feasibility of sufficiently enhancing human settlement and grassland albedos to levels needed to achieve reductions in radiative forcing projected here, additional study is required on two fronts. Firstly, the modelled radiative forcing reductions are static estimates. As they would generate climate feedbacks, more detailed dynamic climate modelling would be needed to confirm the stationary value of the radiative forcing reduction that would result from land surface albedo amplification. Secondly, land surface albedo amplification schemes may have important economic and environmental impacts. Accurate ex ante impact assessments would be required to validate global implementation of related measures as a viable mitigation strategy.

Keywords: albedo, atmosphere, bioengineering, climate modelling, climate change mitigation, geoengineering, grasslands, human settlements, land use.

1. Introduction

The negative environmental and economic impacts of anthropogenic climate change, and the inherent difficulties of reducing causative greenhouse gas accumulations in the atmosphere, are now widely acknowledged. Greenhouse gas emissions – an unavoidable by-product of oil, gas and coal energy conversion – occur principally in the form of carbon dioxide (CO2). Since the onset of industrialisation in 1750, energy-related CO2 emissions have accumulated in the atmosphere, raising the CO2 concentration level from 278 ppmv in 1750 to 378 ppmv at the end of 2004 (Keeling and Whorf 2005). Rising atmospheric concentrations of CO2 and other greenhouse gases increase the radiative forcing of the climate system (Hansen et al.

1997; Ramaswamy et al. 2001) that leads to climate change. The Intergovernmental Panel on Climate Change (IPCC) has estimated that the total radiative forcing of the climate system due to anthropogenic emissions of long- ∗ Accepted for publication in Mitigation and Adaptation Strategies for Global Change, Springer, NL on 10 November 2005.

Arxiv:Physics/0512170 V1 19 Dec 2005

R Hamwey: Active amplification of the terrestrial albedo to mitigate climate change

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lived and globally mixed greenhouse gases was 2.43 Wm-2 ± 10% in 1998 (Houghton et al. 2001). CO2 emissions resulting from fossil fuel combustion were estimated to account for 60%, or 1.46 Wm-2, of this anthropogenic forcing, making it the dominant human-influenced greenhouse gas. More recent studies have indicated that total anthropogenic radiative forcing has risen to a current value of over 2.50 Wm-2 while the relative contribution of CO2 to this total has increased to 62% (Hofmann et al. 2005).

In view of their dominant role in precipitating adverse climate changes, international mitigation efforts have largely focussed on reducing emissions of CO2 through energy efficiency and conservation measures, and on sequestering atmospheric CO2 through afforestation programmes (UNFCCC 2003). These activities continue to be motivated by national policies pursuant to the United Nations Framework Convention on Climate Change (UNFCCC), and more recently, by the Convention’s Kyoto Protocol. However, analyses suggest that achievement of current Kyoto Protocol reduction targets is at best consistent with a trajectory to future stabilisation of CO2 concentrations at 550 ppmv, a level substantially higher than current values and one that falls short of avoiding significant adverse climate change during the current century (Nakicenovic and Swart 2000). This points to the need for stronger mitigation efforts, not only through higher emissions reduction targets and wider international participation in emissions reduction activities, but also through novel approaches to climate change mitigation. Within this context, the IPCC has recognised that, in addition to greenhouse gas emissions reductions, geoengineering offers a potential approach for mitigating changes in the global climate (Apps et al. 2001).

Geoengineering involves large scale and purposeful efforts to circumvent the anthropogenic greenhouse effect by actively managing the energy balance of the Earth (NAS 1992; Flannery 1997; Keith 2000). A common element of many geoengineering schemes involves reducing the incident radiative flux of solar energy in the lower atmosphere and at the Earth’s surface to offset the warming effect of greenhouse gases.

In a recent study, it was shown that the geoengineering schemes that reduce incident solar radiation uniformly by 1.8% could largely mitigate global and annual mean climate change resulting from a doubling of atmospheric CO2 concentrations from pre- industrial levels (Govindasamy and Caldeira 2000).

To date, geoengineering proposals have typically been of high technological content and cost, implemented at the macro-level by governments, and centred on schemes to enhance terrestrial carbon sinks or partially shield the Earth’s surface from sunlight (Flannery et al. 1997; Keith 2001). The latter have included schemes to increase the planetary albedo of the Earth by injecting aerosols into the atmosphere and deploying an array of metallic balloons in the stratosphere or reflective mirrors in Earth orbit.

Terrestrial albedo amplification through land surface modification described here is distinctly different from previous ‘geoengineering’ proposals in two ways. Mechanistically, it aims to increase the amount of solar radiation reflected by the Earth’s surface rather than reducing incident solar radiation flux. Secondly, in practical terms, land surface modification schemes have relatively low technological content and are based on the collective voluntary efforts of local actors rather than a centralised effort of government institutions. These features imply that land-based surface albedo amplification costs could be relatively low and distributed among the participating global population, making the approach attractive from both an economic and implementation perspective.

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2. Land-based Surface Albedo Amplification as a Mitigation Option Over the past decade. a number of studies have demonstrated that land use changes resulting from natural and human activities can significantly modify radiative climate forcings on regional scales by changing surface albedos and the energy budget of the lower atmosphere that they regulate (Charney et al. 1977; Bonan et al. 1992; Henderson-Sellers et al. 1993; Xue and Shukla 1993; Myhre and Myhre 2003). Such studies have examined the effects on climate of regionally specific land-use changes.

For example, it has been shown that during the past two centuries, regional-scale replacement of natural forests by agricultural crops in the continental United States has increased surface albedos and reduced radiative forcing (Bonan 1997). The climatic effects of projected future land use changes have also been examined. One study has shown that increased radiative forcing arising from decreases in surface albedos associated with large-scale boreal and temperate forestation programs, may offset the climate change mitigation effects of carbon sequestration underpinning such programs (Betts 2000).

In addition to albedo related radiative effects, many of these studies have shown that regional changes in surface albedo also trigger important climatic feedbacks, including changes in regional hydrological cycles resulting from modified evapotranspiration patterns of soil and vegetation. Detailed climate models coupling general atmospheric circulation and land surface models have been advanced to examine the climatic effects of different land use patterns (Bonan 1995). These models capture not only the changes in the surface energy budget arising from changes in surface albedo, but also the associated climate feedbacks resulting from induced changes in latent, sensible and sub-surface heat flux (Bonan 1996).

A major finding emerging from research on the climatic effects of land use patterns is that the regional climate forcing caused by modern land use practices can be comparable to other anthropogenic climate forcings including those resulting from increased greenhouse and aerosol emissions (Bonan 1997). This suggests that there may be significant potential to offset greenhouse gas climate forcing through – as yet unexplored – intentional efforts to increase the Earth’s land surface albedo on a global scale by modifying the radiative characteristics of human settlements and vegetation.

3. Methods, Model Description And Results

The terrestrial albedo amplification scheme described here is motivated by the observation that just as greenhouse gas emissions that cause climate change have grown with increasing world population, so too have the surface areas of settled land and human-managed grasslands and their contributions to the value of the overall planetary albedo. Approximations of the current magnitude of these contributions are combined with global demographic and vegetation data, and estimates of the extent to which land surface albedos may be increased, to construct globally amplified terrestrial albedo distributions for postulated amplification scenarios. These distributions are then convolved with solar insolation data to model first order spatially resolved radiative forcing reductions achievable under each albedo amplification scenario. In addition, the potential of these radiative forcing reductions to offset the positive radiative forcing associated with rising concentrations of greenhouse gases in the atmosphere is evaluated. Two amplification scenarios are considered. Scenario 1 involves enhancing the albedo of human settlements (manmade surfaces). Scenario 2 involves enhancing the albedo of the world’s grasslands.

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A first order estimate of the magnitude of a perturbation in the radiative

∆

, of the Earth’s climate due to changes in the shortwave (0.2 – 4.0 µ) surface albedo can be readily determined when the magnitude of the all-sky downward shortwave solar flux at the Earth’s surface,Φ , and the perturbed, a , and unperturbed, a , values of the all-sky shortwave surface albedo are specified for each point and time on the Earth’s surface (Hartmann 1994). In this first order approximation, which ignores feedback perturbations arising from changed surface albedos (see Section 4 for a discussion), the decrease in global radiative forcing at an element s of the Earth’s surface due to albedo amplification at any time t is given by:

(1)

and the annual average decrease in global radiative forcing is:

(2)

where the perturbation contributions from each surface element, s , are integrated over the Earth’s total surface S over an entire year T. A discrete monthly (

; T = 12),

radiative transfer model of 1 x 1 degree latitude-longitude resolution was constructed to

∆

reported here. All data used in the model were obtained from the International Satellite Land- Surface Climatology Project (ISLSCP) Initiative II Data Archive (Hall et al 2005).

ISLSCP data are derived from various sources (the original source of each dataset is noted below). All datasets have a spatial resolution of 1 x 1 degree, and all model results were obtained from computations covering the 64,800 1 x 1 degree cells defining the gridded 180 degree latitude by 360 degree longitude Earth surface. The NASA EOS Land Mask was used to discriminate between land and water cells.

Monthly average all-sky downward shortwave solar flux at the Earth’s

A S T , From The Wcrp/Gewex

surface radiation budget (SRB) project release 2 (Stackhouse et al. 2001) were used as inputs in the model. Both of these SRB parameters are measured over the 0.2 – 4.0 µ solar spectrum. Monthly SRB data covering the entire January through December 1986 were used in the model. Surface albedo perturbations were applied only to snow-free cells. The presence of snow in a cell during any given month was determined from monthly snow cover data for 1986 from NASA (Armstrong et al. 2003). Since only SRB all-sky radiation parameters were used in the model, and because the derivation of these data within the SRB project implicitly accounts for the effect of monthly cloud transmittance, there was no need to monitor each cell’s monthly configuration of clear- sky and overcast conditions for independent clear-sky and overcast surface radiation flux calculations in the model.

The SRB data were used to compute monthly values of the unperturbed (i.e., baseline state) net radiative shortwave flux at the Earth’s surface due to surface absorption of incident shortwave solar radiation for each cell. To simulate active albedo amplification, positive perturbations in the surface albedo of land cells were introduced in the model (as described below). The difference in net radiative flux between the baseline and perturbed surface albedo states was calculated monthly for each cell (using Eq. 1 above) to estimate monthly perturbations in radiative forcing for each cell, and by integrating these, for each 1 degree zonal band from -90 to 90 degrees latitude. From these data, spatially resolved and (surface area weighted) zonal global annual average decreases in radiative forcing were derived.

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Surface albedo perturbations introduced in the model were set for each cell based on values of human population and grassland fractions characterising each cell and specifications set in the albedo amplification scenarios on how each cell’s surface albedo changes in proportion to these values. Accurate and current spatially resolved population and land cover data thus formed essential inputs to the model.

Global data on population in 1995 from the Gridded Population of the World (GPW) version 2 dataset provided population counts in each land cell over the Earth’s surface for a total world population of ~ 5.67 billion in 1995 (Balk et al. 2004, CIESEN 2000). To simulate the current 2005 global population distribution, the 1995 GPW population data where uniformly scaled up by a factor of 1.14; the ratio of estimated 1995 to 2005 global population (USCB 2005). Land cover data from the United States Geological Survey’s (USGS) Global Land Cover Characterization (GLCC) classifies the fraction of International Geosphere Biosphere Programme (IGBP) vegetation zones in each cell based on 1 km resolution satellite observations covering the 12-month April 1992 to March 1993 period (Loveland et al. 2000). Both the 1 degree and 1 km resolution data were analysed to confirm consistency, however, only the 1 degree resolution data was used in the model. The current 2005 distribution of world vegetation zones was assumed not to have changed significantly from the 1992-93 distribution used to derive the GLCC dataset.

3.1) Scenario 1: Amplification of human settlement albedos Under scenario 1, in order to compute the total surface area of human settlements capable of being ‘whitened’ in an albedo amplification effort, an estimate of the average per capita artificial surface area that makes up human settlements must be made. The area of the terrestrial surface occupied by human infrastructure is not accurately known.

Although the resolution of currently characterised global land surface imagery (~1km2) is capable of capturing urban areas, it does not capture dispersed human infrastructure outside of urban agglomerations. A world total of approximately 260,000 km2 of urban and built-up area under the IGBP classification scheme was derived from 1 km GLCC analyses undertaken here, indicating a global average of 46 m2 of urban surface per capita in 1995.

The magnitude of total (urban and dispersed) artificial surface area per capita significantly exceeds the value of urban surface area per capita. Artificial surface area is generally considered to include all residential, recreational, industrial, commercial, transportation-related and institutional land (occupied by man-made physical structures and adjoining landscaped areas) but excludes agricultural land (USDA 2003). Based on this definition, estimates point to a global average value of 440 m2 to 500 m2 of artificial surface per capita (UNEP/RIVM 2004; Wackernagel et al 2002). Moreover, estimated values of artificial surface per capita vary considerably among regions. The highest regional estimate of artificial surface per capita is for United States where it is estimated to exceed 1,500 m2 per capita (USDA 2003), whereas for East and South Asia, regional estimates are the lowest at about 300 m2 per capita (UNEP/RIVM 2004).

Based on these estimates, a global average per capita reflective surface area for human settlements, σ , of 500 m2 was adopted in this study.

A = 0.15, A Typical Urban

value (Jin et al. 2005; Taha 2005), was assumed globally as the unperturbed surface albedo of human settlements. To simulate the perturbed state, the surface albedo of human settlements was enhanced globally by 100% to a higher value of

A = 0.3,

resulting from an intentional ‘whitening’ of human structures as a climate change R Hamwey: Active amplification of the terrestrial albedo to mitigate climate change

A Were Assumed To Be Invariant With

respect to solar zenith angle and season.

A , The Surface Albedo Values Of Cells

were revised upward in proportion to the human population in each cell. For each cell s and month t the perturbed surface albedo was computed as:

(3)

where SM(s,t) is a no-snow mask equal to 1 (0) if snow is absent (present) in cell s during month t; P(s) is the estimated 2005 population, and SA(s) is the surface area, of

Cell S; And ( , )

a s t is the baseline surface albedo of cell s during month t. The decrease in radiative forcing in each cell s during month t immediately follows from Eq. 1. Monthly changes in surface albedo and radiative forcing were calculated for each cell. The baseline data indicate an annual average global all-sky surface albedo of 0.140804. In response to the increase in the surface albedos of human settlements under scenario 1, the model projects this figure to increase by 0.000875, and the annual globally averaged radiative forcing to decrease by 0.17 Wm-2.

3.2) Scenario 2: Amplification of grassland albedos Under scenario 2, amplification of the planetary albedo is achieved by increasing the surface albedo of the world’s grasslands. The ‘grassland’ area assumed in the model, and referred to hereafter simply as grasslands, includes 3 IGBP classifications: open shrubland, grasslands and savannah. Taken together, about 30% of the Earth’s land area falls under these three IGBP categories.

It is not possible to define a common baseline value of unperturbed grassland surface albedo for all model cells due to latitudinal and seasonal variations in this quantity that result from temporal variations in cloud cover, mean solar zenith angles, precipitation and phase offsets of annual vegetation growth cycles. Therefore, an algorithm was used to dynamically specify grassland baseline surface albedos. Within the model, the mean monthly values of surface albedo for cells in each 1 degree latitude zonal band with a grassland fraction of 80% or more were calculated. These values were used to define the baseline surface albedo of grasslands as a function of latitude and month. For a minority of latitude bands without cells containing a grassland fraction of 80% or more, the baseline value was set to that of the nearest latitude band in which this condition was met. Through this procedure, a set of unperturbed baseline surface albedo

A

s t , was established. The global annual average all-sky grassland surface albedo derived from the SRB dataset was 0.17. Under scenario 2, the baseline surface albedo of grasslands,

(4)

where GF(s) is the grassland fraction of the cell s. The decrease in radiative forcing in each cell s during month t immediately follows from Eq. 1. R Hamwey: Active amplification of the terrestrial albedo to mitigate climate change

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Monthly changes in surface albedo and radiative forcing were calculated for each cell. In response to the increase in the surface albedos of grasslands under scenario 2, the model projects an increase in the annual average global all-sky surface albedo of 0.002626 and a decrease in annual globally averaged radiative forcing of 0.59 Wm-2.

Such a large decrease is not unexpected considering that 30% of the Earth’s land surface is occupied by grasslands, particularly at low and middle latitudes where the value of annual insolation is highest. However, because human intervention to enhance the surface albedo of grasslands may be feasible for only the portion of the world’s grasslands that can be managed by man, the 0.59 Wm-2 decrease in radiative forcing achievable under scenario 2 represents an upper limit for a presumed uniform global grassland surface albedo increase of 25%. If albedo enhancement is feasible only for a fraction, β , of all grasslands, and this subset has the same spatial distribution as the larger set of world grasslands, scenario 2 would result in a decrease in annual globally averaged radiative forcing of 0.59 β Wm-2.

3.3) Detailed Results

Monthly spatial and latitudinal estimates of decreases in radiative forcing were computed by the model for scenarios 1 and 2. Spatial maps of the annual average decrease in radiative forcing resulting from scenario 1, scenario 2, and scenario 1 and 2 combined, are presented in Figure 1. The annual average decreases in radiative forcing resulting from scenario 1 and 2, as a function of latitude, are presented in Figure 2.

These results are discussed further below.

4. Qualification Of Results: Climate Feedbacks

The radiative transfer model used in this study only provides an estimate of changes in the shortwave surface radiation budget. The full surface energy budget includes other parameters that are external to the model. Specifically, under steady-state conditions, the full surface energy balance equation for a cell s is:

−

is net (downward – upward) shortwave radiation flux,

−

is the net (downward – upward) longwave radiation flux, LH and SH are respectively the latent and sensible heat fluxes from the surface to the atmosphere, and HF is the horizontal sub-surface flux of heat absorbed at the surface to adjacent cells. To maintain the energy balance in Eq. 5, the radiative forcing perturbations in

+

+

+

.

(6)

In response to the shortwave radiative forcing perturbation arising from increased surface albedos, Eq. 6 indicates that several parameters will adjust in order to maintain radiative equilibrium at the land surface. However, the radiative transfer model used in this study provides no information on how these adjustments are partitioned; a coupled climate land surface model is required to provide such an indication (Bonan 1997).

, Eq. 6

indicates that evapotranspiration (LH) and atmospheric convection (SH), and consequently cloud cover and precipitation, may be reduced by increases in regional R Hamwey: Active amplification of the terrestrial albedo to mitigate climate change

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surface albedos. Such reductions have been examined in previous studies of tropical regions (Charney et al. 1977; Henderson-Sellers and Gornitz 1984; Xue and Shukla 1993; Dirmeyer and Shukla 1994). At the same time, these primary feedbacks will themselves induce secondary feedbacks. If reductions in annual average cloud cover relative to the baseline state are substantial, they may raise absorbed shortwave flux, and reduce absorbed longwave flux due to reduced cloud forcing. This could potentially result in a net increase of annual (shortwave + longwave) insolation and an overall heating effect (Charney et al. 1977), or no significant decrease in temperature (Henderson-Sellers and Gornitz 1984), even though surface albedo has increased.

Whereas the above feedbacks may be important in tropical regions, modelling of modern surface albedo changes of order ~ 0.02 – 0.06 occurring in the temperate United States indicate no significant change in seasonally averaged precipitation and a net cooling effect (Bonan 1997). Moreover, a recent global simulation of regional climate responses to land conversion induced surface albedo increases (DeFries et al. 2002) has highlighted that effects are quite different in the tropics, where net warming and drier hydrological conditions are expected, than in temperate regions where net cooling and largely unaltered hydrological conditions result.

The complex nature of the multiple feedbacks above emphasises the need for accurate coupled general atmospheric circulation and land surface modelling of global surface albedo amplification schemes to examine how long-term surface radiation budgets, energy budgets, and climatologies may ultimately be affected in different regions.

5. Potential Mitigation Benefits

The radiative forcing perturbation estimates obtained here suggest that, implemented together, the two albedo amplification scenarios described above could potentially offset 0.76 Wm-2, or about 30% of the approximately 2.50 Wm-2 of radiative forcing caused by anthropogenic emissions of all long-lived and globally mixed greenhouse gases. However, as Figure 1 shows, decreases in radiative forcing vary considerably by region. Expectedly, the spatial and latitudinal distributions of radiative forcing perturbations under scenario 1 and 2 closely trace the underlying physical distribution of population and grasslands. But as these two distributions are dissimilar, the combined effects of scenarios 1 and 2 result in a distribution of radiative forcing perturbations that extends over a fairly large fraction of the terrestrial surface.

Nevertheless, perturbations significantly higher than the global average occur in highly populated and high grassland fraction cells. As Figure 2 shows, the latitudinal distributions of annual average radiative forcing reductions achieved under scenarios 1 and 2 are mostly concentrated in the Northern Hemisphere. Unlike greenhouse gas emissions which result in nearly uniform latitudinal increases in radiative forcing due to mixing in the atmosphere, the decreases in radiative forcing induced by land surface albedo amplification vary in proportion to the underlying latitudinal distribution of population and grasslands. Projections of the effects of latitude dependent albedo amplification on climate and climate change require study in a general circulation model. Yet a recent study of this type (Govindasamy and Caldeira 2000) suggests that it is not necessary for the latitudinal pattern of radiative forcing due to albedo amplification to match that of greenhouse gases to largely negate the effects of the latter.

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The mitigation power of albedo amplification can be compared with that of CO2 emissions reductions by evaluating the decrease in atmospheric CO2 concentration needed to achieve a similar reduction in radiative forcing. A simplified expression for the calculation of radiative forcing induced by CO2 has been validated by the IPCC

(7)

where α = 5.35 and C and C0 are the perturbed and unperturbed CO2 concentrations. This expression indicates that the estimated decreases in radiative forcing of 0.17 and 0.59 Wm-2 under scenarios 1 and 2 respectively, are equivalent to decreases obtained by reductions in atmospheric CO2 concentration of 12 and 40 ppmv, or 50 ppmv when the two scenarios are implemented together (note: Eq. 7 is not linear).

Recent observations in both the Northern and Southern Hemisphere indicating a 2 ppmv annual growth rate of atmospheric CO2 concentrations (NIWA 2004; Keeling and Whorf 2005) would, ceteris paribus, suggest that the albedo amplification measures modelled here could offset as much as 25 years of global CO2 emissions at current levels (~ 6 yrs under scenario 1 and ~ 20 yrs under scenario 2). Although such an offset would not obviate the need for long term emission reductions to reduce future climate change, it could substantially extend the time available to advance the development and use of low-emission energy conversion technologies. Land surface albedo amplification efforts may thus represent potential options to complement mitigation activities focussing on emissions reductions.

6. Implications And Feasibility

6.1) Scenario 1: Amplification of human settlement albedos While the estimates reported here indicate that amplification of human settlement and grassland albedos have the potential to significantly offset radiative forcing caused by anthropogenic greenhouse gases, the scope for, and feasibility of engineering such amplifications on a global scale for climate change mitigation remains largely unexplored. Technologies are readily available to amplify the surface reflectance of human settlements, and these have been the subject of considerable research over the past decade (Rosenfeld et al. 1997) within the context of reducing ambient temperatures and associated energy costs for cooling, and ground-level ozone concentrations, in urban heat islands (UHI). However, the potential of substantially enhancing the albedo of vegetation, particularly grass species, remains unexplored.

Major foci of UHI mitigation have been to increase the albedo of building roofs and facades using high reflectivity titanium dioxide (TiO2) paints and films, and roads and other paved surfaces using high albedo white cement (HARC 2004; Taha 2005).

Typically, these technologies raise roof albedos from 0.1 to 0.7, and paved surface albedos from 0.1 to 0.4. Conversion costs range from 15-30 $m-2 for roofs and 15-25 $m-2 for pavements, while the lifecycle of converted surfaces are on the order of 10 years (HARC 2004). Detailed studies of several urban areas in the United States (Rose et al. 2003) have demonstrated, that widely implemented, these measures are able to increase baseline urban albedos, characterised by values ranging from 0.12 – 0.16, by 100% or more, to values ranging from 0.20 to 0.37, depending on urban land cover category (Taha 2005). The 100% enhancement of human settlement surface albedos postulated in scenario 1 thus appears technologically feasible. Moreover, human R Hamwey: Active amplification of the terrestrial albedo to mitigate climate change

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settlement surface albedos could be enhanced still further if high albedo grasses are used to increase the albedo of lawn surfaces (see below). Studies on albedo enhancement abatement costs (i.e., as a climate change mitigation measure in costs per tonne of CO2 equivalent avoided) would need to be completed, and estimated costs compared with other abatement options to gauge the economic efficiency of any globally implemented measure to enhance the albedo of human settlements. Albedo enhancement abatements costs should internalise the economic co-benefits of energy savings and reduced ground level ozone concentrations that surface albedo enhancements generate in urban heat islands.

Measures to increase the reflectance of human settlements may benefit from high levels of public acceptability since, unlike energy conservation measures, they incur only periodic fixed costs and do not reduce energy consumption utility. Moreover, with increasing public awareness of climate change, the perceptible nature of albedo enhancement activities could encourage widespread community engagement by offering local actors a visible way to demonstrate their contribution to climate change mitigation.

Experience with UHI mitigation activities in the United States, where they are most advanced, has been marked by high levels of public support. 6.2) Scenario 2: Amplification of grassland albedos The substantial radiative forcing offset yielded by enhancing the albedo of the world’s grasslands could make such measures particularly attractive. However, one of the principal factors limiting the magnitude of plant albedos, including for grasses, is strong absorption of visible light in the solar spectrum by chlorophyll in plant leaves.

Yet the possibility of increasing grassland surface albedos with grasses having high reflectances in the visible bandpass (0.4 – 0.8 µ) remains considerable. Since the reflectance of near infrared solar radiation (0.8 – 1.4 µ) by plants is relatively high, typically ranging from 0.4 – 0.7 (Asner et al. 1998; Asner 1998; Sims and Gamon 2002), and because the intensity of terrestrial solar radiation drops off substantially in the far infrared ( > 1.4 µ) where reflectance by plants is lower, grasslands populated by natural or bioengineered grasses having increased visible band reflectivities could generate significantly enhanced grassland surface albedos.

There exist a wide variety of naturally occurring light-coloured shrubs and grasses that exhibit high reflectivities over the visible spectrum. The white, light-green and light-yellow colouration of such plants – i.e., increased reflectance of visible light relative to visibly green plants – occurs through: 1) low concentrations of chlorophyll and other pigments in plants’ leaves and stems (Sims and Gamon 2002) and/or 2) the presence of trichomes and waxes (that efficiently reflect visible light) on plants’ leaf surfaces (Bondada and Oosterhuis 2000; Grant et al. 2003). Leaf thickness also plays a role in visible reflectance (Knapp and Carter 1998).

Among the first class of plants are grasses such as Carex hachijoensis and Chlorophytum comosum, and shrubs such as Alpinia zerumbet, Euonymus europaeus, and Ficus aspera. These plants have variegated leaf colours: patches or stripes of their leaves are light-yellow or white in colour (where chlorophyll and other pigments are absent or in low concentrations) and green elsewhere. Up to 60% of their leaf surfaces are light-coloured. Although uncommon in grassland grasses, variegated leaf colours have been observed in Stenotaphrum secundatum (Sud and Dengler 2000).

The second class of plants includes ‘white-coloured’ shrubs such as Cerastium biebersteinii and Senecio cineraria. Dense trichomes on these plants’ surfaces reflect a

Deployment In Out-Of-Position Situations

D. Bendjaballah1, A. Bouchoucha1, M. L. Sahli1,2* and J-C. Gelin2

Abstract

Side-impact collisions represent the second greatest cause of fatality in motor vehicle accidents. Side-impact airbags have been installed in recent model year vehicle due to its effectiveness in reducing passengers’ injuries and fatality rates. In meeting these requirements, simulations of folding and deploying airbags are very useful and are widely used. The paper presents a simulation method for the deploying airbags using three materials in different working conditions. Finite element analysis is primarily used to evaluate this concept. In these simulations, the gas flow is described by the conservation laws of mass, momentum, and energy. The numerical results indicate that the FE method in this paper is capable of capturing airbag deploying process accurately.

Keywords: Airbag simulations, Out-of-position, Crash, Modeling, Out-of-position

Background

The passive safety of cars has become a very high prior- ity issue for the automotive industry. Today, there are not only one or two airbags in a car; certain models have ten times more than that. With the increasing usage of airbags, the number of accidents where the airbag itself can cause an injury to the occupant also increases

(Augenstein Et Al. 2003; Gabauer And Gabler 2010;

Audrey et al. 2011). As is well known, safety belts are also now devices designed to provide protection to the users of vehicles during crash events, minimizing the loads necessary to adapt their movement to the move- ment of the car (Freesmeier and Butler 1999; Schmitt et al. 1997). In general, the seat belt is designed to restrain the occupant in the vehicle and prevent the

Occupant From Having Harsh Contacts With Interior

surfaces of the vehicles. The airbag acts to cushion any impact with vehicle structure and has positive internal pressure, which can exert distributed restraining forces over the head and face. As a safety component of auto- mobile, an airbag decreases occupants’ injury likelihood effectively in case of an accident (Ruff et al. 2007). These safety elements can reduce the death rates on the roads, and its protection effects have been widely approved (Crandall et al. 2001; Teru and Ishikawa 2003). With computational tools such as finite element methods designed for dynamic contact problems, crashworthiness simulations can now be used with reliable accuracy to evaluate occupant protection in various collision condi- tions with safety metric/parameters such as acceleration, head injury criteria, intrusion distance, intrusion vel- ocity, and neck forces (neck injury risk or whiplash).

Thus, new types of airbag products are being developed to handle different collision scenarios.

Become Standard Equipment On Most New Passenger

vehicles (Braver and Kyrychenko 2004; Teng et al. 2007; Yoganandan et al. 2007). The airbag cushion is com- posed of a woven fabric which is rapidly inflated during a car crash. The airbag dissipates the passenger’s kinetic energy thereby reducing injury through biaxial stretching of the fabric bag and escaping gas through vents. There- fore, the performance of the airbag is greatly influenced by the mechanical properties of the fabric. Generally, air bags are designed to deploy in a crash that is equivalent to a vehicle crashing into a solid wall at 8 to 14 mph.

Air bags most often deploy when a vehicle collides with another vehicle or with a solid object like a tree. There are various types of airbags: frontal, side-impact, and curtain airbags. In general, the passenger side airbags are usually larger than the driver airbags (see Fig. 1).

Besançon, France

© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Bendjaballah et al. International Journal of Mechanical

Doi 10.1186/S40712-016-0070-2

Extensive studies have shown that the airbag deploy- ment in load cases consists of two occupant loading phases: a punch-out effect where the airbag bursts out of its container with the airbag and airbag module cover accelerating towards the occupant and a second loading phase during which the airbag is taking on its deployed shape and volume (membrane-loading effect). Bankdak et al. (2002) developed an experimental airbag test system to study airbag-occupant interactions during close proximity deployment. The results provided insight for simulating the effect of inflation energy and mass flow on target response. Bedard et al. (2002) found that while left-side (driver-side) impacts accounted for only 13.5% of all crashes, the fatality rate among these

Crashes Was 68.3% In Comparison To Front Impact

(48.3%), right-side impact (31.3%), and rear impact (38.4%). These studies underscore the importance of oc- cupant safety during side-impact collisions. In the last years, the current market requested to reduce the time and cost airbag development. In order to achieve this result, virtual simulations play an important role since they allow to minimize the number of experimental tests (Pei et al. 2013; Cao et al. 2014). Several simulation models of airbag were established (Wang et al. 2007). It is feasible to optimize the parameters of airbag deploy- ment using simulation technology. Experimental and numerical studies have quantified injury risks to close- proximity occupants from deploying side airbags. These studies have focused on the prevention of the most ad- verse effects of airbag deployment (Duma et al. 2003).

Other studies have proposed airbag characteristics to minimize particular biomechanical responses (Haland and Pipkorn 1996). In a more recent study, Marklund and Nilsson (2003) compared deformation patterns with experimental data as well as the computational costs associated with three different airbag deployment simu- lation methods; they concluded that the SPH method is relatively inexpensive and produces incremental deform- ation patterns that compare most closely to the experi- mental results. The process of inflation of an airbag is one of the determining factors in saving lives. The duration from the initial impact of the crash to the full inflation of an airbag is about 40 ms, and during this time, the airbag goes from being in a folded state to a fully inflated state, with a high internal pressure. After achieving this state, the airbag begins to deflate, thus providing a nice cushion for the body impacting it.

Ideally, the person in the crash should come into contact with the airbag at this time. In the present study, a large volume passenger side airbag model is developed to handle different collision scenarios. The main aim is evaluate the performance of deploying of passenger side airbag using finite element methods (FEM).

Materials

The tensile specimens were made in different airbags (P: Peugeot, R: Renault, and VW: Volkswagen) with a length of 200 mm long and a width of 40 mm. Table 1 shows the mechanical properties of the airbag.

Tensile Tests

To determine the mechanical properties of the material of airbag used in the test pieces, tensile tests were performed on Lloyd EZ20 universal testing machine in Constantine. These tests were conducted using rect- angular samples. The axial force and axial displacement acquired during a test are converted into stress and the strain in order to be used for the fabric material model.

The continuous recording of the stress-strain data was performed during both the load and unload phases. A minimum of five samples were made in order to check the repeatability of the measurements. All the data was collected by using a PC-based data acquisition system and analyzed by commercial software. The picture frame test device that is made for this study is shown in Fig. 2.

Fig. 1 a Frontal and side airbags. b Oblique view of facet occupant model in sitting posture following airbag deployment (Lim et al. 2014)

0.150

Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

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Figure 3 shows the stress-strain relationship of the airbag sample under axial tensile loads. The results are showing a linear increase in extension with the increas- ing stresses. This is an expected output and it confirms with the theoretical behavior of a sample subjected to tensile stress. The rupture strain values for different airbags (R/P/VW) were 0.322, 0.441, and 0.472, respect- ively. The measured elastic parameters (i.e., Young’s modulus E and initial yield strength) and Poisson’s ratio are summarized in Table 2. The tensile tests of the woven fabrics can show differences on mechanical prop- erties because woven fabrics can resist in-plane shear loads once the yarn lock-up angle has been reached. The differences of material property on material direction can affect the shape of fully deployed bag (see Fig. 3b).

Theoretical Background

Numerical simulations of airbags use very complex and techniques such as an orthotropic model to identify the mechanical behaviors during the airbag inflation and the fluid mechanics (gas flow) to describe the inflator gas flow (pressure gradient) and improve the representation of the pressures within the airbag. To model the airbag as an orthotropic model, three material constants have to be provided. Assuming a plane stress condition, the

Ð1Þ

where σ is the normal stress and τ is the shear stress, the subscript refers to the principal material directions, i.e., the fill and warp directions. Also, ε and γ are the strain components. The material elastic constants Qij are

Ð2Þ

where E1 and E2 are the Young’s modulus in the fill and wrap directions and G12 is the shear modulus of the fabric material. νij is the Poisson ratio of the material.

The gas exerts a pressure load on the airbag causing it to expand. This expansion puts the airbag under tensile stress lowering the expansion rate. In this study, heat conduction and heat transfer is not taken into account.

Fig. 2 A photograph of Lloyd EZ20 universal testing Fig. 3 Stress versus strain using Lloyd EZ20 machine for a three different airbags at 0° and 90° and b VW airbag test specimens at

Different Angles

Table 2 Physical and mechanical properties of the airbag

Page 3 Of 9

In the deployment of an airbag, an inflator supplies high velocity gas into an airbag causing it to expand rapidly. The gas inside the airbag is assumed to be ideal, to be of constant entropy, and to satisfy the equation of state:

Ð3Þ

Here p, ρ, and e are respectively the pressure, density, and specific internal energy, and γ is the ratio of the heat capacities of the gas. The gas flow is described by the conservation laws for mass, momentum, and energy that

Ð4Þ

here, V is a volume, A is the boundary of this volume,

N Is The Normal Vector Along The Surface A, And U

denotes the velocity vector in the volume. Applying Bernoulli’s equation in the case of an ideal gas with

Ð5Þ

Here, the subscript ex denotes quantities at the throat of the tube. Furthermore u, p, and ρ denote the quan- tities inside that part of the tube that is supplying mass.

Materials And Boundary Conditions

The airbag system mainly consists of three parts: the airbag itself, the inflator unit, and the crash sensor or diagnostic unit. Thus, to study the behavior of the airbag using FE simulations, we need to have an FE model of the airbag in the folded position. A FE model of the airbag was used to simulate the test condition as shown in Fig. 5. LS-DYNA® material model FABRIC (MAT_34) is used to simulate the airbag material. It is a variation of the layered orthotropic material model. Additionally, in the LS-DYNA® material model, fabric leakage can be accounted for. However, for this CAB material, the leak- age is almost negligible and therefore no leakage is specified. The mechanical properties can be determined from the physical test. Typical material properties for airbag fabrics are taken as given in Chawla et al. (2004a) (Table 3). These properties are used to simulate inflation process of airbag (see Table 1). The car dashboard is modeled as the rectangular thin plate using a MAT_RI-

Gid Material, And The Degrees Of Freedom Are Con-

strained in all the directions. The similar properties of thermoplastic polymer are assigned for contact purposes. The porosity of the fabric is assumed zero. The nitro- gen gas is taken for inflating the airbag. Properties of nitrogen gas and initial bag conditions are shown in Table 4. The example on which we perform the study is a typical passenger side airbag. The geometric de- tails have been measured from a commercially avail- able airbag. The initial state of the airbag is a closed rectangular whose sides are to be finished to 482 × 635 mm2 and is shown in Fig. 4.

Table 3 Material properties of airbag and rigid plate used in FE

–

Table 4 Initial values used for FE simulation of the swelling of

3.33 × 10−4

Fig. 4 The initial airbag geometry in the form of a rectangular Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

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