1 Introduction

Renewable energies are steadily accounting for a greater part of the global energy mix (Moncada Lo Giudice et al. 2013); renewables met 13 % of the world primary energy demand in 2011 (IEA 2013), and among them, concentrated solar power (CSP) is considered one of the most interesting alternatives to traditional fossil fuel-fired power plants. CSP is expected to grow fast internationally, with projects under construction or development in several countries, and its energy potential production is estimated to cover the 2.5 % of EU energy demand by 2020 (European Commission 2008). In order to complete the Rankine cycle, i.e., to condense back the steam exiting the turbine to liquid water, current CSP systems use water-cooled condensers (WCCs) or air-cooled condensers (ACCs). WCCs represent an effective cooling solution, but for their operation, large water quantities must be available near the plant location. Otherwise, the use of ACCs allows to eliminate more than 90 % of CSP plant water consumption, but it causes a lower CSP plant global efficiency. The inability of ACCs to maintain optimum values of pressure and temperature at the condenser, coupled with their difficulty to respond to daily variations of ambient temperature, may generate a higher fan power consumption and, therefore, a lower net power output of the CSP plant, with an estimated reduction up to 25 % on warm days (Poullikkas et al. 2011).

The MACCSol research project provides an innovative dry cooling approach to reduce the inefficiencies of conventional ACCs, thus aiming at a decisive deployment of CSP plants in desert areas. During the design phase, a typical modular air-cooled condenser (MACC) module 2 × 2 m was defined. This module uses small diameter fans (1 m), and it could be outfitted with three different kinds of tube bundle geometries: circular finned, plate finned, and continuous finned. The full MACC condenser will consist of a certain number of modules, which can vary, being anyway related to some characteristics of the CSP plant (e.g., the plant power output).

In the present work, a first MACC condenser prototype (300 modules, circular finned tube bundle) is compared with conventional condensers in a reference 20-MW CSP plant through a life cycle assessment (LCA) approach. LCA is widely used to evaluate the environmental impact of renewable energies and, in particular, of solar technologies (Jungbluth et al. 2005; Suwanit and Gheewala 2011; Davidsson et al. 2012; Traverso et al. 2012; Bayer et al. 2013; Garrett et al. 2013; Koroneos and Stylos 2014), but it is not common for the analysis of renewable energy power plants components. In this paper, a complete environmental profile of each one of the three condensers considered is evaluated taking into account the main environmental issue of CSP plants (i.e., freshwater consumption), and the differences among the systems are therefore investigated using different methodologies.

2 Overview of the existing cooling technologies and description of the innovative ACC

Common water cooling technologies used in CSP plants are represented by two typologies of condensers: once-through WCC (Fig. 1a) and circulating evaporative WCC (Fig. 1b). In once-through WCCs, the inlet water required to remove the heat from the condenser is taken from a source (lake, river, or sea) and then is completely discharged back into the same source. This type of WCC is conceptually and constructively quite simple, but it requires the availability of a large quantity of “water to lose.” In circulating evaporative WCCs, instead, the cooling water flows from the condenser to an evaporative cooling tower and the collected heat energy is rejected to the environment via evaporation. Most of the cooling water is lost as clouds of water vapor to the atmosphere, as it comes in contact with the air moving upstream in the tower (naturally or by fans), and it is necessary a make-up water flow to compensate the evaporation.

Fig. 1
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a Once through WCC. b Circulating evaporative WCC. c Indirect ACC system. d Direct ACC system

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WCC systems are the most used in CSP plants and, generally, in traditional power plants. A CSP plant with this type of condenser installed requires approximately the same water quantity in cubic meters per megawatt hour of a nuclear or coal-fired power plant (Poullikkas et al. 2011).

ACCs can be classified in indirect dry cooling systems or Heller systems (Fig. 1c) and direct dry systems (Fig. 1d). In indirect dry-cooled systems, a condenser is included as an intermediate step between the turbine exhaust and the cooling tower. Therefore, heat is firstly transferred to the circulating water in the condenser, and then, it is discharged to the environment through a fin tube array placed in a tower. This kind of operation is similar to the operation of a wet cooling tower, but the condenser circulating water flows in a continuous closed loop from the turbine to the tower and it is not exposed to the ambient air. In direct dry systems, normally called ACC, the turbine exhaust steam flows through sealed ducts to a fin tube array and the heat is rejected to the surrounding atmosphere by air that is blown by fans. The tube array is typically set in an A-frame configuration, in order to maximize the heat exchange surface maintaining limited the footprint area of the condenser. This A-frame is equipped with fans from 10 to 12 m in diameter blowing air from the lower part through the tube array. The condensate is collected in a sump, returned to the boiler, and thus reused for steam generation (Serth and Lestina 2014).

Another cooling option is the use of hybrid systems. Among these systems can be identified two typologies: one that aims at plume abatement and another one that aims at reducing water consumption. Since CSP plants are naturally located in dry and remote areas, plume abatement is generally not an issue; therefore, hybrid designs aimed at reducing water consumption compared to wet-cooled plants result more interesting. These hybrid systems, which can led to a 20–30 % water consumption reduction (Rezaei et al. 2010), typically use water evaporation to increase the heat rejected by the ACC (Fig. 2a) or, alternatively, couple dry and wet units that operate in parallel (Fig. 2b).

Fig. 2
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Hybrid cooling systems. a Dry-cooled condenser with wet section. b Parallel cooling system (PCS)

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The MACC, i.e., the innovative modular dry-cooled system developed within the MACCSol project, consists of modules with a length of 2 m and a width of 2 m, which use 1-m-diameter fans to blow air—both in forced draft or induced mode—across the tube bundle where the steam exiting the turbine flows. The tube bundle could be equipped with circular finned, plate finned, or continuous finned tubes (Fig. 3), and an optimization analysis (Asdrubali et al. 2013) demonstrated that a circular finned configuration represents the best solution in terms of thermodynamic performance, costs, and impacts on the environment. The fan speed is continuously varied on the basis of a key parameter: the water temperature at the end of the expansion, which is measured by sensors placed in the condensate steam. Thus, for low values of ambient temperature, fan rotational speed is optimized and MACC power consumption results minimal. The reduction of fan power consumption, coupled with the retention of optimum outlet conditions at the turbine, allows to obtain a significant plant power output increase and also reduces parasitic power losses that typically affects current ACC designs (Moore and Grimes 2011; Walsh and Griffin 2011; O’Donovan et al. 2014; O’Donovan and Grimes 2014).

Fig. 3
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MACC module layout and tube bundle geometries. a Circular finned. b Continuous finned. c Plate finned

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3 Methodology

3.1 LCA analysis

The goal of this LCA study consists of comparing the first MACC prototype with conventional condensers (a WCC and an ACC), all installed in a central tower (CT) CSP plant. The construction phase of materials, the transports (both to manufacturing company and to CSP site, implementing two different scenarios), the operation, and the end of life phase of the condensers are considered. Compared to usually considered life cycle system boundaries in LCA of CSP plants (Lechón et al. 2008; Burkhardt et al. 2011; Desideri et al. 2013; Corona et al. 2014), some exclusions were done. In particular, condenser erection, maintenance, and dismantling activities were omitted. The maintenance activities on the condensers could be considered negligible during the life cycle of the CSP plant, which is supposed to be equal to 15 years. Regarding each condenser, in fact, during the period of 15 years is predictable a marginal routine maintenance (e.g., lubricating oil reintegration/replacement in the gearbox), which does not has significant consequences on the life cycle impacts. Data concerning erection and dismantling activities, instead, are closely related to the technology used and to the characteristics of the construction site; thus, as specific data were lacking, it was decided to overlook them in order to limit the uncertainty affecting the results (Fig. 4). Anyway, the waste management activities at the end of life (recycling, landfilling, incineration) were considered. The functional unit chosen is 1 kWh of electricity produced by the plant, and the results evaluated through the life cycle were then referred to the operating period of 1 year (chosen as the normalization period for the comparison).

Fig. 4
figure 4

Life cycle system boundaries

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3.2 Life cycle impact assessment

Among the evaluation methods implemented in the SimaPro software, the following were used: the single issue method International Panel on Climate Change (IPCC) 2007, which classifies the emissions according to their contribution to the greenhouse effect and expresses the impact in terms of CO2 equivalent emissions through the global warming potential (GWP) of each gas, and the scoring method Ecoindicator 99, which considers a total of 11 impact categories (carcinogens, respiratory diseases, climate changes, ozone depletion, radiation that causes ionization, acidification/eutrophication, ecotoxicity, land use, mineral resource depletion, and fossil fuels) regarding the damage to human health, ecosystem quality, and resources. Secondly, to evaluate the severity of the local impact due to freshwater consumption and water stress conditions, the Ecological Scarcity 2006 method was used. Freshwater consumption is one of the major environmental issues of power generation (Pfister et al. 2011) and even more of CSP plants. Its shortness can be expressed by the scarcity ratio (SR), defined as the share of gross consumption in the available renewable water resource (i.e., as the ratio between water withdrawal and water availability); the water stress can be classified as follows (Alcamo et al. 2000; Vörösmarty et al. 2000; OECD 2004; Oki and Kanae 2006):

  • Low, if SR is <10 %

  • Moderate, if SR is between 10 and 20 %

  • Medium, if SR is between 20 and 40 %

  • High, if SR is >40 %

The Ecological Scarcity 2006 method defines two further water stress categories (very high stress, SR between 60 and 100 %, and extreme stress, SR ≥100 %), and for each category, it weights the environmental impact by applying an “eco-factor” (FOEN 2009) that depends on SR (Eqs. (1)–(3)Footnote 1).

$$ \mathrm{E}\mathrm{c}\mathrm{o}-\mathrm{factor}=K\cdot \frac{1\cdot \mathrm{E}\mathrm{P}}{F_{\mathrm{n}}}\cdot W\cdot c $$
(1)
K :

Characterization factor of a pollutant or of a resource

F n :

Normalization flow: current annual flow in Switzerland

c :

Constant (1012/a) to obtain readily numerical quantities

EP:

Eco-points: unit of environmental impact assessed

W :

Weighting factor

$$ W={\left(\frac{F}{F_{\mathrm{k}}}\right)}^2={\left(\frac{\mathrm{current}\ \mathrm{annual}\ \mathrm{flow}\ \mathrm{in}\ \mathrm{the}\ \mathrm{reference}\kern0.5em \mathrm{area}}{\mathrm{critical}\ \mathrm{annual}\ \mathrm{flow}\ \mathrm{in}\ \mathrm{the}\ \mathrm{reference}\kern0.5em \mathrm{area}}\right)}^2 $$
(2)
$$ W={\left(\frac{\mathrm{Water}\kern0.24em \mathrm{Consumption}}{\mathrm{Available}\kern0.24em \mathrm{Resource}\cdot 20\%}\right)}^2={\mathrm{SR}}^2\cdot {\left(\frac{1}{20\%}\right)}^2 $$
(3)

3.3 Reference CSP plant and condenser design

The reference plant for the comparison is the Gemasolar power plant, a 20-MW CT CSP plant located in the south of Spain. The solar field is made up of 2650 heliostats for a global footprint area of about 185 ha. Molten salts are both used as heat transfer fluid and storage medium. The storage capacity is 15 h, and the mean annual electricity production is 110,000 MWh. The water-steam cycle of the plant is constituted by a regenerative Rankine cycle (4 + 1 extractions) with intermediate reheating. The installed steam condenser is a WCC, whose main design data are reported in Table 1. The ACC and the MACC were designed considering a maximum backpressure accepted by the steam turbine of 300 mbar and, respectively, a minimum backpressure in operation of 60 mbar and 30 mbar (Tables 2 and 3) (MACCSol 2013).

Table 1 WCC main design data
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Table 2 ACC main design data
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Table 3 MACC main design data
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3.4 Inventory data collection and calculation

Regarding the construction materials of the WCC and the MACC, the inventory data were collected from the design documents, referring to a single WCC cell and to a single MACC module. Then, the inventory of each condenser was evaluated multiplying the collected data by the total number of elements:

$$ {I}_i=n\cdot {I}_{i,\mathrm{d}} $$
(4)
I i :

Inventory data of the i component

n :

Number of WCC cells/MACC modules constituting the condenser

I i.d :

inventory data of the i component collected from the design documents

With regard to the ACC, instead, the design documents of the condensers installed in two multi-MW power plants were available. Therefore, it was necessary to adapt the inventory data to the condenser as designed for the comparative analysis (Table 2). Hence, the data collected referring to a single A-frame were multiplied by the total number of A-frames and by a scaling factor, calculated as the ratio between the dimensional characteristics of the components:

$$ {I}_i=\frac{D_i}{D_{i,\mathrm{d}}}\cdot n\cdot {I}_{i,\mathrm{d}} $$
(5)
I i :

Inventory data of the i component

n :

Number of A-frames constituting the condenser

I i.d :

Inventory data of the i component collected from the design documents

D i :

Dimensional characteristic of the i component

D i,d :

Dimensional characteristic of the i component from the design documents

The inventory data regarding the transport were calculated with the following hypotheses: (i) mean distance from the semifinished manufacturing companies to the component producer companies equal to 30 km and (ii) distance from the component producer companies to the plant location evaluated through specific GIS software. Two transportation scenarios were implemented; transportation scenario 1 refers to truck and container ship as means of transport, while scenario 2 refers to truck and aircraft.

Regarding the condenser operation, the power consumption of fans and auxiliary components was estimated through simulations (MACCSol 2014) and used to obtain the annual net energy output of the plant. The natural resource consumptions (i.e., cooling water) were evaluated on the basis of design documents available.

As stated before, condenser erection, maintenance, and dismantling activities were omitted for all considered condensers (WCC, ACC, MACC).

The hypothesized waste management scenario is shown in Table 4 (European Commission 2014).

Table 4 End of life scenario
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Inventory data regarding all the processes included in the system boundaries were implemented in SimaPro using the Ecoinvent 2.2 datasets. Regarding the power consumptions, since the electricity consumed by the condensers is produced by the CSP plant, it was created a specific process taking into account the life cycle of the CSP plant itself. Obviously, the condenser was not included in this process, in order to avoid double counting.

Detailed data on the life cycle inventory are provided as Electronic Supplementary Material.

4 Results

Results presented in the following figures refer to transportation scenario 1 (scenario 2 shows the same trend, with a slight increase in the life cycle impacts). In terms of gCO2/kWh × year (Fig. 5), the overall impact of the dry-cooled condensers results significantly higher than the impact of the WCC, due essentially to the nature of materials used for the construction. The components used in the ACC and in the MACC, in fact, are characterized by a greater impact per unit of mass and by a higher specific weight, which also generates a significant increase of the impact associated with the transport activity. An interesting evidence, anyway, is that the overall impact of the MACC results appreciably lower than the impact of the ACC (about 21 % less), despite the minor positive contribution of recycling at the end of life (about 28 % less); in fact, the impacts of the construction and operation are considerably lower (respectively, −21.5 and −41 %). It is also worth noting that, differently from the dry-cooled condensers, the impact of WCC operation results higher than the impact of its construction, and the impact reduction due to material recycling is negligible.

Fig. 5
figure 5

Life cycle impact assessment—IPCC 2007 method (GWP 100 years)

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The Ecoindicator 99 method confirms the environmental benefits achievable through the use of the innovative dry-cooled system, compared to conventional ACCs (Fig. 6). Results, in fact, are characterized by the same global trend and show a more appreciable difference in the impact between the MACC and the ACC. The overall impact reduction results equal to 27.5 %; in detail, the impact of the construction is 13.2 % less, the impact of the operation 41 % less, and the impact reduction due to the recycling at the end of life results very similar. Regarding the WCC, all the life cycle phases give a positive contribution to the overall impact. Looking at the breakdown of impacts by category (Fig. 7), it emerges that the main contributors are “carcinogens,” “respiratory inorganics,” and “fossil fuels.” These categories account together for the 76.8 % of the WCC overall impact, the 76 % of the ACC overall impact, and the 71.6 % of the MACC overall impact. The main share of the remaining part of the overall impact of each condenser is due to the categories “minerals,” “ecotoxicity,” and “climate change,” with a joint contribution of 20.4 % for the WCC, 22.2 % for the ACC, and 26.1 % for the MACC. The other categories (acidification/eutrophication, land use, ozone layer, radiation, respiratory organics) contribute negligibly with a combined share included in the range of 1.8–2.9 %.

Fig. 6
figure 6

Life cycle impact assessment—Ecoindicator 99/H method

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Fig. 7
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Life cycle impact assessment—Ecoindicator 99/H method (breakdown by category and life cycle phase)

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According to the Ecological Scarcity 2006 method (Fig. 8), the MACC condenser is the best cooling option in terms of environmental impact (total eco-points, evaluated applying the eco-factor described by Eqs. 1-3); in particular, compared to the conventional ACC, the overall impact results 33 % less, and compared to the WCC, the environmental benefit is more significant (impact reduction observed equal to 50.5 %). The main contribution to the impact of the dry-cooled condensers is related to the category “emission into air,” in each life cycle phase (Fig. 9). This category, in particular, accounts for more than a half of the overall impact (55.3 % for the MACC and 63.9 % for the ACC) and for the most of the construction and operation impacts (about 70 % for both the condensers). Regarding WCC, instead, the contribution of emission into air to the overall impact is equal to 8.7 % (76.5 % of the construction impact and 5.5 % of the operation impact).

Fig. 8
figure 8

Life cycle impact assessment—Ecological Scarcity 2006 method

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Fig. 9
figure 9

Life cycle impact assessment—Ecological Scarcity 2006 method (breakdown by category and life cycle phase)

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“Natural resources” is the category that evaluates the impact related to the freshwater consumption; thus, it makes the difference between the dry-cooled condensers and the WCC. While it contributes to the impact of ACC and MACC for about 1 %, its share on the WCC impact is equal to 84.2 %. This is essentially due to the WCC operation, i.e., to the high water consumption in the stress conditions characterizing Spain (which are defined as “medium,” determining an eco-factor equal to 880, as shown in Table 5) and to the treatment applied before its utilization (complete softening). Among the other categories, the contribution of “emission into ground water” (0.002 % for the WCC, 0.02 % both for the ACC and the MACC) and “emission into top soil” (0.05 % for the WCC, 0.7 % for the ACC, 0.8 % for the MACC) results negligible, the category “energy resources” slightly contributes for the ACC and the MACC (respectively, 3.2 and 3.8 %), and it results negligible for the WCC (contribution equal to 0.4 %), while categories “deposited waste” and “emission into surface water” comprise an appreciable share on the overall impact of each condenser (cumulative contribution of 6.7 % for the WCC, 31.0 % for the ACC, and 38.8 % for the MACC).

Table 5 Calculation parameters of the Ecological Scarcity 2006 method
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5 Discussion

The comparison among the condensers through the usually implemented evaluation methods shows that the WCC is the best cooling option in terms of environmental impact. The methods IPCC 2007 and Ecoindicator 99, in fact, show an impact of the WCC significantly lower than the one of the ACC and the MACC, and this is essentially a consequence of the low impact associated to the construction phase. WCC construction materials (mainly plastics and glass fiber reinforced plastic), in fact, have a meaningfully lower impact per unit of mass and a lower specific weight, which also generates a significant reduction of the impact associated with the transport activity. On the contrary, materials used for ACC and MACC construction (mainly steel and aluminum) are characterized by a high impact per unit of mass and by a higher specific weight, thus determining a major contribution of the construction and transport activities. Moving the attention to the operation phase, while it results negligible for the dry-cooled condensers (contribution of about 5–10 %), it assumes a key role on the overall environmental impact of the WCC, being comparable with the impact of its construction and representing a 40–60 % of the total (depending on the evaluation method used). This result is due to the treatment of water, i.e., its complete softening before reintegration in the cooling water circuit, which implies an additional impact to the one associated to fan and auxiliary (circulating pumps) power consumption. According to the Ecoindicator 99 method, the categories that mainly contribute to the impact of each condenser are carcinogens, respiratory inorganics, and fossil fuels; these ones make the difference between the MACC and the ACC, with a reduction equal to 70–80 %. Since the Ecoinvent datasets used for the implementation of the processes in SimaPro were the same (e.g., the dataset “reinforcing steel, at plant” was used for the supporting steel structure both of ACC and MACC), this reduction is a consequence of the material optimization in the MACC prototype arrangement.

The Ecological Scarcity 2006 method results show a different trend, because of the high influence on the overall impact of the freshwater consumption in water stress conditions classified as medium. The impact of the WCC results about two times higher than the impact of the MACC and 1.35 times higher than the impact of the ACC, due to the operation phase and, in particular, to the category natural resources, which accounts for 92.1 % of its impact. This evidence does not discredit the other evaluation methods, but it shows the major role of water consumption in regions suitable for CSP plant, suggesting that this environmental issue has to be adequately taken into account. Therefore, even if the construction of WCCs results more environmentally-friendly than the dry-cooled condenser one, their operation in CSP plants assumes a significant role in the overall impact, becoming the key issue for choosing the best cooling option in these applications.

In general, all the evaluation methods show that the MACC is better than the ACC: a noteworthy finding, far from being obvious before the comparison since the MACC has characteristics similar to the ACC (same construction materials and similar quantities of materials for analogous components).

Analyzing the impact variation from scenario 1 to scenario 2, a significant increase is noted due to the more influencing transport modes. From the results reported in Table 6, it is evident that the evaluation methods are differently sensitive to the variation of the transport mode and that, among them, the scoring methods (Ecoindicator 99 and Ecological Scarcity 2006) are less affected by the transport mode choice. Disregarding the Ecological Scarcity 2006 method, whose findings are significantly affected by the water consumption, it is also worth noting that the percentage increments observed for the WCC and the ACC result very similar. Data regarding the MACC, instead, result appreciably higher; therefore, it emerges that logistic (e.g., distances involved in the construction phase) and transport mode choices affect the overall impact of MACC more than the overall impact of the other condensers.

Table 6 Overall impact variation from scenario 1 to scenario 2
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6 Conclusions

CSP is widely considered as a competitive and environmentally-friendly technology for power generation. WCCs represent the most used cooling solution in existing CSP plants, even if an increasing use of ACCs is predictable. As a matter of fact, water resources are limited and the cost of cooling water is high; furthermore, several projects aiming at the development of CSP plant in desert areas are planned and undergoing. The MACCSol research project, funded under the EU Seventh Framework Programme, provides an innovative modular dry cooling approach to overcome the typical inefficiencies of ACCs.

A LCA analysis was carried out to compare the first MACC prototype with conventional condensers. The analysis was executed using “conventional” evaluation methods (IPCC 2007, Ecoindicator 99) and also adding the evaluation through the Ecological Scarcity 2006 method, which accounts for the freshwater consumption, one of the main environmental issues of CSP plants. The MACC condenser resulted a better cooling option compared to conventional ACC according to all the evaluation methods used. As the construction material used are basically the same, this result is due to the optimization in the MACC prototype layout, i.e., to a module arrangement that allows a reduction in the quantity of material used for some components (e.g., supporting steel structure). Moreover, taking into account the severity of local impact due to water consumption and availability, the MACC resulted as the best cooling option in regions suitable for CSP plants (the site of operation considered is the south of Spain): its impact resulted halved compared to the WCC and 1.35 times lower compared to the ACC. This high environmental benefit is due to the reduction occurred in the categories natural resources (compared to the WCC), which accounts for the water consumption, and emission into air (compared to ACC).

The findings of this research, therefore, show that dry-cooled condensers cannot compete with WCCs for applications in conventional power plant. These plants, in fact, are generally located where cooling water is available, i.e., in regions characterized by low water stress condition, and the lower impact associated to the operation of ACCs and MACC is not sufficient to justify their use. On the contrary, in CSP plants located in regions where water is scarce, the use of dry-cooled condensers results more suitable, and among them, the innovative MACC condenser results better than conventional ACCs. In this regard, a preliminary estimation (MACCSol 2014-1) shows that the environmental benefits of MACC increase with the increase in water scarcity. In fact, varying the water stress conditions from medium to high, the impact of WCC is expected to increase significantly (up to a +100 %), while the MACC is expected to be characterized by a smaller impact variation (±15–20 %), with its impact that also remains considerably lower than the one of the conventional ACC. Considering that several projects are running to develop CSP plant in desert areas, this finding is noteworthy, suggesting that the use of the MACC could significantly reduce the life cycle impacts of these power plants. The MACC resulted also as the most affected by the choice of transportation mode, and therefore, the transport activity planning represents a key issue to optimize its life cycle impact.