1 Introduction
Water is one of the most important elements in our daily life and basis of existence for almost every creature on our planet. The “blue planet” is named by the colour of the surface water, which covers 71% of the earth surface. The majority of existing water (97%) occurs as salted water in oceans. The remaining 3% are fresh water e.g. in lakes and rivers or locked up as arctic ice (Perlman 2016). Therefore, fresh water, useable for living purposes, is a very rare resource. Due to climate change, growing population and industrialization, water stress in the year of 2040 is predicted by the World Resource Institute (Luo et al. 2015) for majorly industrialized countries, e.g. USA, China, Australia and European countries such as Spain and Italy. Therefore, the use of water is more and more in focus when companies develop their factory related sustainability strategies (Dehning et al. 2016; Kurle et al. 2015).
In industrial context, water is widely used as operational resource, e.g. for washing processes and heat transfer. The usage of water is one main drivers for total energy consumption in industrial process chains (Thiede et al. 2016). The so-called water-energy-nexus describes the relation of water and energy: energy for water, such as water treatment, pumping, heating etc., water for energy, which is used for e.g. extraction, purification, processing of natural gas, coal and electricity generation (Thiede et al. 2017; Walsh et al. 2015). For LCA in production environment, water is usually only considered as resource for the product with rather neutral impact in case of energy flows. Although, the main share of water is not directly used in products, but for energy and mass transfer in the discrete manufacturing (Kurle et al. 2017). Thus, it is necessary to consider water and water-energy-nexus related devices in factories for LCA studies as well as several studies demonstrated (Lévová and Hauschild 2011; Mousavi et al. 2015). Industrial cooling towers (CT) are an example for the water-energy nexus in manufacturing. Several studies have researched Key Performance Indicators (KPI) for operation and control during the usage stage of CT (Thiede et al. 2017; Cortinovis et al. 2009; Schlei-Peters et al. 2017; Guo et al. 2017; Kurle 2018). The studies prove high environmental impacts in green house gas emissions as well as fresh water demand depending on the operation mode as well as the climatic condition of the location (Schulze et al. 2018). Nevertheless, for a holistic assessment of the environmental impacts of CTs all life cycle stages should be taken into account. For that reason, this study presents an LCA for industrial CTs.
2 Background
2.1 Industrial Cooling Tower Systems
Elements of industrial CTs (Schulze et al. 2018)
For industrial purposes, different types of re-cooling systems are broadly differentiated by the type of coolant: dry or wet as well as the type of air supply natural-draft ventilation or forced-draft ventilation. Hence, different construction types are developed for the special field of application. Dry coolers, using only the ambient air as coolant, are used for low cooling requirements or flexible locations without large-scale infrastructure use. Thereby ventilators are used to ventilate the objects to be cooled, e.g. for slow cooling of products. Due to the low heat capacity of air, the cooling capacity of dry CT is limited (Schodorf and Geiger 2012). If larger waste heat streams need to be cooled, coolants with higher heat capacity are used. As a technical medium with one of the highest heat capacities, fresh water is often used as a coolant. In the so-called wet CT types, the warm water is cooled in the ambient air while partly evaporate the water. The evaporation increases the cooling capacity and therefore wet CT can be used also for higher ambient heat temperatures (24–26 °C) during the summer time (Schodorf and Geiger 2012). Furthermore, a particular distinction is made between the types of air supply: natural-draft by using the so-called chimney effect and force-draft ventilation by fan. CTs with forced-draft ventilation are usually used at industrial sites as described above. In contrast to other types, the design can be significantly more compact and requires less space at factories’ sites. In addition, the formation of visible vapour can be avoided. Vapours can be carriers of legionellae, which may be a danger for humans in the workplace.
2.2 Life Cycle Stages of Industrial Cooling Tower
Generic life cycle stages of industrial CTs, inputs and outputs
Raw material and manufacturing stage
Usage stage
End of life stage
2.3 Approaches for Life Cycle Assessments of Industrial Cooling Towers
Different approaches and studies looking at environmental impacts from CT have been identified. Several authors assess the usage of CTs as part of a building or energy generation system. Many approaches particularly focus on water losses through evaporation as well as resulting fresh water demand (Burkhardt et al. 2011; Koroneos and Tsarouhis 2012; Guerra et al. 2014; Ali and Kumar 2016). In industrial context, several studies have been published focusing on the energy and water efficiency of industrial CTs during the use stage (Thiede et al. 2017; Kurle et al. 2017; Schlei-Peters et al. 2017). In Schodorf and Geiger (2012) the usage stage of different types of CT are compared regarding energy demand, CO2 footprint and costs to give a broad overview of the pro’s and con’s of each type. For the assessment of the economic efficiency and CO2 footprint of evaporation and hybrid CT, the Mechanical Engineering Industry Association (VDMA) published a guideline (Markus et al. 2015). It also focuses on the use stage of CT, arguing that CO2 emissions from manufacturing stage would account less than 1% of the total CO2 emissions over the whole life cycle. Schulze et al. present an analysis of the environmental impacts from industrial CTs operation considering individual climate data and energy mixes for different locations (Schulze et al. 2018). Nevertheless, for the holistic assessment of environmental impacts over the whole lifetime a life cycle assessment is necessary.
3 Life Cycle Assessment of Industrial Cooling Towers
In order to perform an LCA study, the regarded CT system with its system boundaries is described and goal and scope are defined. Subsequently, the underlying data for the Life Cycle Inventory (LCI) is presented followed by the results of the Life Cycle Impact Assessment (LCIA). The model for LCIA was built in the software environment UMBERTO.
3.1 Goal and Scope Definition
The object of this case study is an industrial force-draft CT as part of the TBS of an automotive production plant located in Germany. The CT system includes pump and pipe system, transporting the warm water to and back from the production. Furthermore, the air-draft is soken by the fan located on the top of the cooling tower in counter flow direction to the water. In order to increase the time for energy and mass transfer, the water rinses down over filler built into the cooling tower. As the outcome of CT operation is cooled water, the cooling of 1 kg water from 35 to 28 °C in Germany for the overall usage time is chosen as the functional unit for the LCA. The impact assessment has been conducted using CML 2001 methodology.
As it is assumed that all processes of the life cycle take place in Germany, data for this area have been used. For raw materials and manufacturing of industrial CT, mainly stainless steel coils for the body and polyvinylchloride films (PVC) for the filler packages are considered. The fillers are replaced completely every 5 years as maintenance and prevention for clogging. For the peripheral aggregates, electric motors for pumps and a fan are taken into account. Due to the individual CT design, the pipe system is neglected in this use case. The usage time of this CT accounts for 20 years. During this time, the electricity demand of pumps and fan are considered as energy input flows. Furthermore, regular exchange of cooling water causes fresh water demand as well as waste water generation. As in this case study the CT is located in Germany, data for the energy and fresh water demand for the location of Berlin are taken from Schulze et al. (2018). In end-of-life, a treatment of the scrap is considered for the fractions steel, plastics and electrical components. The details and data for LCI are listed in Table 1.
3.2 Life Cycle Inventory Analysis of Industrial Cooling Towers
In order to analyse the LCI, the flows of energy and material in each life cycle stage have to be determined. Adequate datasets have been selected from Ecoinvent 3 database for modelling the resource demand in UMBERTO (Ecoinvent 2016).
3.3 Results of the Life Cycle Impact Assessment (LCIA)
Total impact share of life cycle stages on the impact categories GWP, HTP, FAETP, TAETP exemplarily, results in percent [%]
The results coincide that the environmental impacts primarily occur during the usage stage of the industrial CT. The life cycle stages of raw material and manufacturing as well as end of life only play minor roles here. These findings correlate with findings of previous studies (Markus et al. 2015). The high environmental impact during the usage stage results from the long operation time (20 years and continuously demand of electrical energy and fresh water).
Results for climate change, GWP 100a
LCI of the industrial CT (Schulze et al. 2018)
Life cycle stage | Objective | Amount | Unit |
|---|---|---|---|
Raw materials and manufacturing | Stainless steel (body) | 850.0 | kg |
PVC (fillers) | 192.3 | kg | |
2 electrical motors 18 kW (pumps) | 600.0 | kg | |
Electrical motor 15 kW (fan) | 135.0 | kg | |
Usage | Operating hours | 8500 | h/a |
Electrical energy demand | 864.0 | MWh | |
Fresh water demand | 63740000.0 | kg | |
Waste water generation | 21246000.7 | kg | |
End of life | Electric scrap | 735.0 | kg |
Scrap steel | 850.0 | kg | |
Waste PVC | 192.3 | kg |
LCIA results for climate change, GWP 100a [kg CO2-eq]
Life cycle stage | Processes of CT life cycle | [kg CO2-eq] | Total share (%) |
|---|---|---|---|
Raw material and manufacturing | 2 electric motors (pump) | 2350.92 | 3.75 |
Electric motor (fan) | 528.96 | ||
Stainless steel (body) | 3036.88 | ||
PVC (fillers) | 11,821.11 | ||
Usage | Wastewater treatment | 8157.32 | 96.04 |
Electrical energy demand | 427,639.13 | ||
Fresh water demand | 18,766.55 | ||
End of life | Waste PVC | 682.85 | 0.14 |
Electronics scrap | 314.01 | ||
Scrap steel | 5.73 | ||
Results | 473,303.46 | 100.00 |
4 Conclusion and Outlook
This paper presents the first LCA of industrial CTs, identifying the hot spots regarding environmental impacts along the entire life cycle. Confirming previous studies reviewed before, the results show that the usage stage has been revealed as the main driver for environmental impacts. This conclusion can be drawn for the selected impact categories GWP, HTP, FAETP and TAETP respectively. This can be explained by the long usage time of 20 years, which is not unusual for TBS devices. With a zoom in for GWP, the electrical energy demand for pump and fan operation is the main driver of environmental impact by far. Furthermore, water-based objectives as fresh water demand and water treatment are at the next positions for environmental impact. As mentioned before, cooling tower is an example of the water-energy nexus in production, it is necessary to consider water in LCA studies. As CT operation is highly impacted by the environmental conditions, different locations should be considered for future LCA work as well. As the environmental footprint of countries’ specific electricity generation mix is typically in continuous change, in particular due to a significant increase in installations of renewable energies, further work should consider future electricity mixes as well. A first study considering different locations and energy mixes is given by Schulze et al. (2018), focussing the usage stage only. Furthermore, there are various types of CT which entail different environmental impacts (Schodorf and Geiger 2012). This study has been applied for an open circuit draft-forced CT only, which is one of the main applied types for industrial purposes. For further studies, the authors propose the extension of the objective to a broaden range of CT types.