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The search for a parameter quantifying technological sustainability : Development of a sustainability coefficient

Department of Organic Chemistry, University of Gent, Coupure Links 653, B-9000 Gent, Belgium.
Laboratory for Applied Thermodynamics and Phase Equilibria, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft, The Netherlands.

Summary

This paper describes the development of a parameter, which enables to quantify the sustainability of technological processes. Technology is one of the key processes in society: it delivers goods to people starting from resources out of the ecosphere; however simultaneously it emits waste products into the environment. The parameter quantifying the sustainability of technology proposed in this work is based on thermodynamics; energy carriers and materials (products, waste, …) are expressed in the same unit: exergy. The developed parameter includes three aspects. It contains a factor reflecting to which extent renewable resources are used. Next, the technological efficiency affects the sustainability coefficient. Finally, it takes into account the generation of waste products and the energy required for converting the waste into products harmless for or assimilable in the ecosphere. The developed parameter was applied for two types of processes. In a first application, ethanol production was studied. Two typical production routes were investigated, i.e. starting from fossil oil and from agricultural products. Additionally, a route based on the synthesis from CO2 and H2 was examined, in which H2 was generated by hydrolysis powered by photovoltaic solar energy conversion. Next to ethanol production, electricity generation by natural gas and by photovoltaics were compared.

The developed parameter and its applications show several challenges to enhance the level of sustainability of technologies. The first challenge is that technology should be more solar driven: it is the only 100% renewable resource. Next, each process innovation enhancing the exergetic efficiency and decreasing the amount of 'lost work' or irreversibility of the production process, contributes to sustainability. In this view, the chain of subsequent process stages starting from the raw material must be limited as much as possible. So development of technologies with high efficient operations and with a minimal number of operations is a second challenge. As a third challenge, attention has to be paid to the generation of non-products or waste. In this sense disciplines such as e.g. clean production strategies or green chemistry contribute to a more sustainable technosphere. Finally, waste treatment technologies will remain an important technological issue. Abatement technology must be able to reduce the emissions so that the impact of the pollutants does not affect the ecosystem. In conclusion, the exergy based sustainability index provides a quantitative tool to assess technologies on their sustainability, including the three main steps of technology: resource extraction from the ecosphere, generation of products for the society and delivery of outputs to the ecosphere.

1. Introduction

Natural sciences generate the technology to manufacture the goods for society, starting from resources delivered by the ecosphere. Although these ‘applied sciences’ or technology are essential for the delivery of goods, they can simultaneously endanger the possibilities of current and future generations by short- and long-term effects, and hence affect the sustainability. The threat can arise from direct threats (acute danger or toxicity). On a longer time scale, the technosphere interacts with the ecosphere via two mechanisms. It extracts material and energy resources from the ecosphere. So, a first threat of the technology for sustainability lays in the rate of consumption of resources. If the consumption rate of these resources in technological applications is higher than the rate of resource production in the ecosphere, then the fulfilment of the needs of mankind in the future may be endangered. Secondly, the technosphere emits waste products into the ecosphere. This means that these emitted products may damage the ecological mechanisms and hence the resource production capacity of the ecosphere.

The first sustainability condition requires an insight into the different character of resources available in the ecosystem. First, mineral resources are available, e.g. metal ores, mineral oils, which need at least millions of years to be generated and hence being non-renewable. These stocks can be seen as natural deposits. Secondly, there are so-called renewable resources, which are delivered by the ecosphere on a time scale of years and can be considered as funds.

The second condition for a sustainable technosphere is the prevention of emissions interfering with processes in the ecosphere. Emissions of damaging products, like chlorofluorocarbons and dioxins, must be avoided. Indeed the assimilation capacity of the ecosphere for these types of compounds is very limited. However, even this prerequisite can be unsatisfactory to meet the second condition: too high emission rates of 'harmless' compounds might disturb the ecosphere (e.g. CO2). The second condition for sustainability implies also that the production of 'harmless' (waste) products may not exceed the assimilative uptake rate of these compounds in the ecosphere. High quality products delivered by the ecosphere as wood and wheat, are consumed by the technosphere and subsequently delivered again to the ecosphere as low quality products, carbon dioxide and water. Thanks to the solar power, the ecosphere is able to assimilate these waste products and to deliver high quality products.

Technology is basically a set of processes in which input materials and energy carriers are transformed. The second law of thermodynamics states that all these processes generate entropy, reflecting a quality loss of the input energy. The exergy concept, sometimes called availability, is the thermodynamic tool which expresses both the quantity and quality of energy: it is the amount of the energy which can be used to produce work with respect to an equilibrium environmental state, the so-called dead state.

The exergy approach provides a quantitative base with the same unit (Joule) for all types of energy carriers and materials. The exergy concept has the capability to serve as a basis to quantify the sustainability of the technological aspect of the society. All different types of material steams, including resources, products and non-products (wastes), and technological operation efficiencies can be judged on the same scale.

The aim of this work is to find a measure to quantify sustainability of technology. In a first step, different thermodynamically based factors contributing to sustainability will be treated. Starting from these contributions, a sustainability coefficient is to be defined. Further on, the developed quantitative approach of sustainability will be illustrated for two types of product.

2. Approach of quantification

As a first element in investigating the sustainability of technological processes, the nature of the resources has to be considered1. First, a distinction has to be made between resources that are renewable and those that are non-renewable resources. Renewable resources are defined as resources that are generated at a rate at least as high as the consumption rate; and non-renewables are those with a higher consumption rate than production rate. This is illustrated in equation 1, i.e. for a renewable resource the renewability condition * is equal to or higher than 1:

(1)

Now, based on the renewability condition, consumed resources can be divided in to renewables and non-renewables.

Next, a renewability parameter * can be defined as the fraction of renewable exergy consumption with respect to the total exergy consumption:

(2)

If one wants to calculate the coefficient *, then all resources used in the production processes to manufacture the product have to be considered. All energy carriers brought into the production process have to be traced back to the resources from the ecosphere required to produce them. Main extractables are fossil organic resources (*=0), inorganic ores (*=0) and solar energy (*=1). The first two categories are non-renewable. By consequence, the coefficient * reflects the fraction of direct solar exergy in the exergy consumption of a process.

Sustainable technology has to meet a second condition1: no harmful products are to be emitted by the technosphere. The total exergy required for a production process is:

(3)

with R1 the exergy required for the abatement of emissions during production, R2 the exergy required for the production process and R3 the exergy required for transforming the waste product into harmless products (all in GJ/yr). The emission of harmful waste streams is basically a double loss for a process. Indeed, the waste stream can contain exergy which may be used in the process. Secondly, it can require an abatement exergy.

Now we can define an environmental parameter *1 of the production process:

(4)

where 0 * *1 *1. If we have an environmental parameter *1=1 for the production process, the abatement exergy requirements R1 and R3 become zero. The second condition does not mean that the emitted compounds have no exergy content after treatment: this is not always necessary. The emissions have to be treated using exergy inputs R1 and R3 so that they can be assimilated by the ecosphere or at least that they do not affect the resource production capacities of the ecosphere.

Next to the environmental parameter of the manufacturing, also the efficiency of the production process is important to express sustainability1: the more efficient a process can generate a product out of a fixed number of exergy resources, the higher the sustainability. A production efficiency parameter *2 is defined as

(5)

with 0 * *2 *1 and E + P + IP = R2. E represents the non-useful exergetic emission rate of the production process (in GJ/yr), whereas P is the exergetic production rate of useful products (GJ/yr). The irreversibility of the process is expressed by IP (GJ/yr). By combination of the equations 4 and 5 and considering equation 3, it can be found that the resource consumption is proportional to the exergy content of the useful products generation rate P (GJ/yr) and inversely proportional to the environmental and production efficiency parameters *1 and *2:

(6)
or
(7)
in which *=*1. *2 is the overall exergetic efficiency of the technosphere.

From the two previous paragraphs expliciting the two boundary conditions for sustainability in quantitative terms, it is deduced that the maximal level of sustainability of the technosphere is attained if it does not extract resources faster than produced by the ecosphere (*=1) and if all extracted resources are fully transformed into products delivered to the society, without damaging the ecosphere (*=1). From this point of view, a sustainability coefficient (S) can be defined1:

(8)

The higher the coefficients * and *, the closer the technosphere is to overall sustainability. If both coefficients are equal to 1, then S=1. The case with the largest distance to sustainability (*=*=0) gives rise to S=0.

3. Illustrations

The measures of sustainability as proposed in the previous section, are applied now on two products: ethanol and electricity. Ethanol is a widely used chemical produced by industry as well from mineral resources via cracking to ethylene and hydration to ethanol, as by a combination of agriculture and fermentation. It can serve as an energy carrier for e.g. vehicles (Brazil, United States), and as a solvent or chemical reacting agent in industry. Moreover, ethanol produced from agricultural products is consumed by man in several types of beverages. Mulder et al.2 have evaluated the exergetic efficiency of several technological pathways for the production of ethanol. First, they examined the ethanol production from fossil resources and the pathway via agriculture and fermentation. Next, they examined the production via hydrogen by use of photovoltaic cells and electrolysis. In this route ethanol is synthesised in a second step, starting from hydrogen and carbon dioxide, the latter captured from flue gases from power plants. The basic idea was to enhance the content of renewable resources (solar energy) in the final product and to close the material cycle as much as possible. As in agriculture, which can be seen as a technology based on the same biological mechanisms as occurring in the ecosphere, the basic resources are water and carbon dioxide, converted by means of solar energy into high quality products (photosynthesis). These products are degraded in nature into the initial low quality products, CO2 and H2O, as also after use as fuel in furnaces to produce steam and/or electricity, closing the material cycles.

Next, electricity can be generated by combustion of fossil fuels as gas, coal and oil. E.g. Tsatsaronis and Moran3 evaluated an electric power cogeneration system, fueled by natural gas, delivering electricity and steam. Next to this cogeneration power system, the production of electricity by means of multicrystalline silicium photovoltaic solar cells can be considered based upon already available production facilities2.

3.1. Evaluation of the sustainability of ethanol production routes

The required inputs for the production of ethanol (29.5MJ/kg) from fossil resources is 60.13 MJ/kg EtOH1. The input requirements for the ethylene intermediate are those for an averaged weighted situation over 19 European production facilities, where the process consists of two main steps, i.e. oil refining and nafta cracking 4. Next to this input, ethanol production without damaging the environment requires that all 'harmful' waste products from the production process and from the waste product are treated. The input of non-renewable fuels, which are transformed into CO2 and H2O, gives rise to environmental problems by increasing the atmospheric CO2 concentration. Therefore, CO2 has to be recovered from flue gases and subsequently pumped underground. Based on the data on CO2 recovery via ethanolamine absorption and stripping, and subsequent pumping underground to 80 atm5, it can be calculated that 5.86MJ exergy is required per kg CO2 produced from non-renewables. So, given that 0.97 and 0.65 kg CO2 are produced per kg ethanol at the ethylene production and hydration stage respectively, an abatement exergy of R1=9.5MJ/kg EtOH is required1. Next, ethanol is converted to CO2 and water, e.g. when it is used as a fuel or by combustion after its use as a solvent. With a production of 1.91 kg CO2/kg EtOH, an abatement exergy R3 of 11.2MJ/kg EtOH must be delivered.

Concerning the agriculture/fermentation pathway, at a first glance one could expect a coefficient *=1, since the main source input is solar energy. However, industrial agriculture and fermentation require mineral resources and fossil fuels for the production of nutrients and pesticides, and power for the use of machinery in agriculture and fermentation. Further more, this way of production delivers also wheat straw (from agriculture), gluten and cake (both from fermentation), being all useful products. Therefore the resources in the agricultural step must be attributed in a proportional way to ethanol and the other useful products. So it is found that the production of ethanol requires an input of R2=13243 GJ/ha.yr, with 13220 GJ/ha.yr delivered by solar power, for 92GJ ethanol/ha.yr1.

Next to this input, to produce ethanol without damaging the environment, all 'harmful' waste products from the production process and from the waste product must be treated. The abatement exergy R3 of the product is zero, since it is converted back into CO2 and H2O, closing the material cycle. Also the CO2 emitted by converting starch into ethanol and CO2 does not have to be treated since it closes the carbon cycle. However, at the agricultural production step and during the fermentation, non-renewable resources are used. For the production of wheat, diesel fuel for mechanical power is required, next to fossil resources for the production of nutrients and pesticides2, resulting in an emission of 1064kg CO2/ha. Next, use of natural gas for electricity and steam production2 for fermentation results in an emission of 1179 kg CO2/ha. This means that an abatement exergy to be attributed to ethanol of R1=6.94GJ/ha.yr is required.

As a third option, Mulder et al.2 evaluated the ethanol production via solar cells based on currently available production techniques. It was found that 5047GJ/ha.yr is required for delivering 409GJ ethanol/ha.yr, where 4681GJ/ha.yr are delivered by solar energy. Similar to the agricultural/fermentation route, all exergetic inputs must be attributed in a proportional way to the outputs. Next, all CO2 emissions caused by input of non-renewables have to be treated. So, with a production of 106.2 x 103 kg CO2/ha.yr from non-renewables, a total abatement exergy of 733GJ/ha.yr is needed, resulting in an emission abatement exergy of R1=114GJ/ha.yr for ethanol. Similar as for the agricultural/fermentation pathway, the abatement exergy R3 is zero.

Based on the theoretical development and data presented in previous paragraphs, the renewability coefficient *, the environmental efficiency coefficient *1, the production efficiency coefficient *2 and the overall efficiency * can be calculated, for the production of ethanol from fossil fuels, via the agriculture/fermentation route and the hydrogen/CO2 synthesis process, based on solar driven hydrogen production.

The ethanol production starting from mineral oil cannot be fully sustainable, because of its large input of non-renewables. The coefficient * for this production pathway is almost zero (*=0.0002, reflecting water input). The process efficiency *2 is high (*2=0.49), but due to its low environmental efficiency (high CO2 production) (*1=0.74), the overall efficiency decreases to *=0.365. These figures result in the smallest sustainability S of all ethanol synthesis pathways (S=0.18). Agriculture and fermentation show a higher ratio of renewables (*=0.998). Also the coefficient *1 (=0.9995) is higher for the photosynthesis based pathway, due to the fact that less non-renewables have to be treated.

On the other hand, the production efficiency of the biologically based route is the lowest one of all routes (*2=0.007): less ethanol can be produced out of a fixed amount of resources. This low efficiency for the bio-route is due to the agriculture step. Indeed, the conversion of solar exergy into wheat grain and straw is low. The production efficiency factor *2 is higher for the solar cell/hydrolysis route than for the agricultural/fermentation route (*2=0.08). The combination of *2 with both parameters * and *1 results in a better sustainability S for the agriculture/fermentation route. So in conclusion, it can be stated that the agriculture route is more sustainable than the solar driven CO2/H2 synthesis route, due to its higher renewable resources coefficient * and its higher environmental efficiency *1, although it has a lower production efficiency factor *2.

3.2. Evaluation of the sustainability of electricity production routes

Electricity production by a cogeneration plant as described by Tsatsaronis and Moran3 shows exergetic inputs of natural gas and water of respectively 84.99 and 0.06MW, resulting in two useful outputs: net power of 30.00MW and steam with an exergetic flow of 12.81MW. Next, flue gas is produced (2.77MW). In order to reduce the impact of the emissions onto the eco-system, CO2 in emitted flue gases must be removed. The emission rate of CO2 is 0.577kg/s and with an exergetic requirement of 5.862MJ/kg CO2, a total abatement exergy rate of 3.38MW is needed. This abatement exergy rate is not complete since it doesn’t take into account the abatement of other emissions and waste products of CO2. However, it gives an idea of the required amount of abatement exergy.

The production of electricity by means of multicrystalline silicium photovoltaic solar cells2 delivers 4285GJ/yr.ha net power. An exergetic input to construct the solar cells of 885GJ/ha.yr non-renewable resources is needed, producing 40360kg CO2/ha.yr and requiring an abatement exergy of 236.6GJ/ha.yr.

The electricity generation via solar cells scores better on the use of renewable resources (*=0.9676 vs. *=0.0007), since the only input of renewables in the natural gas fueled system is water for the production of steam. Just because of the extensive use of non-renewables, the gas fueled system requires a higher input of exergy for abatement of emissions resulting in a lower *1-coefficient (*1=0.9932 vs. *1=0.9617). On the other hand, the conversion of inputs into useful outputs is much higher for the gas based power system, showing an overall exergetic efficiency of *2=0.5033. In this sense, the solar cell system is inferior (*2=0.1247). Taking into account all these contributions in order to assess the overall sustainability, it is demonstrated by the sustainability coefficient S that the solar cell driven electricity generation is more sustainable than the gas powered generation (S=0.55 vs. S=0.24). However, also solar photovoltaic conversion cannot achieve 100% sustainability. First, it needs an input of non-renewables to produce the conversion facilities. Secondly, from a theoretical point of view, solar conversion by photovoltaics can attain a maximally possible efficiency of 40.8%, because only a limited range of the irradiated frequencies can be converted into electricity6. Taking into account this efficiency and neglecting exergetic inputs for production of converters and for abatement of outputs harmful for the ecosphere, a maximal sustainability S of 0.70 is found for multicristalline silicon based photovoltaic cells.

4. Conclusions

The proposed judgement of the sustainability of technologies shows that 100% sustainability cannot be attained, because it would require only solar exergy resources (or other nearly 100% renewables as wind energy and hydropower) and reversible processes. On the other hand, the approach demonstrates that technologies used nowadays are always not completely unsustainable (S>0), since they all deliver a product with an exergetic content higher than zero. The developed coefficient S covers the whole life cycle of a product, starting from the resources delivered by the ecosphere, down to the emission of the waste products into the ecosphere. The current approach has taken into account the waste production of the technosphere, not only during the manufacturing of the ‘useful product’, but also during the use and in the disposal phase after the use of the ‘useful product’.

5. References

1. Dewulf J., Van Langenhove H., Mulder J., van den Berg M.M.D., van der Kooi H.J. and de Swaan Arons J. Towards quantifying the sustainability of technology. Illustrations. Submitted to Green Chemistry, 2000.

2. Mulder J., Dewulf J., van der Kooi H.J. and de Swaan Arons J., in preparation, 2000.

3. Tsatsaronis G. and Moran M.J., Exergy-aided cost minimization, Energy Convers. Mgmt., 38, 1535-1542, 1997.

4. Boustead I., Eco-profiles of the European plastics industry. Report 2: olefin feedstock sources. A report for the European Centre for Plastics in the Environment (PWMI), Brussels, May 1993, 23p.

5. Hendriks C., Carbon dioxide removal from coal-fired power plants, PhD thesis, University of Utrecht, 259p., 1994.

6. De Vos A., Endoreversible thermodynamics of solar energy conversion, Oxford University Press, Oxford, 1992, 186p.