4.3. Domestic Heating Systems
In this part, different aspects of domestic heating are investigated. Domestic heating is referred to a building, regardless of its use and size. Although geothermal energy is a renewable source, it is not free of GHG emissions, where this factor can be vastly attributed to the construction phase of the plant.
A major aspect is the comparison of different technologies in terms of environmental impacts and economic criteria, targeting the most appropriate. When combined with LCA, the prospective energy, exergetic and environmental performance of three regularly used residential building heating systems was investigated in Turkey. In more detail, a conventional coal boiler, a condensing natural gas boiler and a Ground Source Heat Pump (GSHP) were compared. From a thermodynamic perspective, the GSHP was an efficient heating system for the given application in terms of the coefficient of performance and exergy efficiency. No matter how it is compared with other systems, LCA results showed that the greatest impacts came from GSHP’s environmental effect: (a) borehole drilling, polyethylene pipes and copper pipelines, all of which are used during installation, and (b) the refrigerant top-up in the maintenance stage. According to the study, condensing gas boilers were the most cost-effective and ecologically friendly option for heating applications in Turkish buildings at that time [
4].
Geothermal energy-based heating systems require indispensable connection and utilization of the existing power grid. This is of main concern, since in many countries (e.g., Greece, USA) the leading resource used for electricity production is coal. That means that a geothermal system will not only have GHG emissions in its construction phase but also in its operational phase. An interesting study comparing the life cycle implications of three heating plant systems that differ in their energy source and system type has been implemented. An electric heat pump, an absorption water–water heat pump and a natural gas-fired boiler were studied in further depth using Eco-indicator ′99 as the LCA approach. The Ecoinvent 2.0 LCI database was applied to gather data on the extraction of raw materials and fuels, the fabrication of heating equipment and their transportation. Single score, damage category and effect category indicators were studied by the researchers. All calculations for characterization, normalization and weighting phases were simulated by SimaPro 7.3.2 throughout the complete system’s life cycle. In that investigation, it was obvious that heating plants employing a low temperature geothermal source had a lower eco-indicator than a gas boiler unit did; because of this, the comparison between absorption and electrical heat pumps revealed that the former had a lesser environmental effect. Accordingly, despite a high eco-indicator, it was revealed that the gas boiler was the least harmful to human health as Coefficient of Performance (COP) and power generation profiles dictated the environmental effect of the electrical heat pump. The greater the COP, the lower the power used and the emissions. Human health suffered significantly in Poland, where about 90% of the country’s power is generated from coal [
14].
Research into shallow geothermal systems, such as open and closed Geothermal Heat Pump (GHP) systems, had resulted in an efficient and renewable energy technology for cooling and heating buildings and other structures. By utilizing a cutting-edge LCA, the researchers were able to comprehensively assess the environmental costs and advantages of using shallow geothermal systems, including net energy consumption and GHG reductions due to GHP operation.
Figure 3 shows the relative contributions of such GSHP systems to environmental degradation in terms of resource depletion (34%), human health (43%) and ecosystem quality (23%), as shown by the LCIA technique (ReCiPe 2008). Out of the overall number of environmental damages, 55.4% may be attributed to climate change. Additionally, LCIA found that the heat pump refrigerant, heat pump manufacturing, transport, heat carrier liquid and the borehole and Borehole Heat Exchanger (BHE) were all major contributors to the environmental burden of GSHP systems. When utilizing the continental European power mix of 0.599 kgCO
2eq/kWh, an average life cycle of 20 years was determined to have an average of 63 tCO
2eq. However, the CO2eq reductions for Europe range from 31% to 88 % when compared with traditional heating systems such as oil-fired boilers and gas furnaces [
15].
A new apartment building in Switzerland performed a comparative LCA between a solar thermal system, an Air-Source Heat Pump (ASHP), a natural gas furnace, an oil furnace and a wood-pellet stove. The solar thermal system showed potential benefits over all other systems in terms of reductions in bought primary energy (from 84% to 93%) and reductions in GHG emissions, according to a variety of life cycle scenarios (from 59% to 97%). Due to intensive industrial operations and the specific metals used in production, the solar thermal system was found to have a larger demand for resources, which in proportion to the natural gas system, may be almost 38. Although the heat pump systems had similar potential human health implications, they were more advantageous than the fossil and biomass driven systems in this regard. In
Figure 4, it is evident that most GHG emissions, related to GSHP, were from electricity required for the system operation. Additionally, the GSHP’s infrastructure impacts were lower compared with the solar systems’ and greater than those of the conventional ones. This verifies the electricity mix problem: a cleaner electricity mix means a cleaner operation phase of GSHP systems [
16].
The technical and environmental performance of a GSHP using LCA was investigated for the Pylaia Town Hall in Thessaloniki, Greece. A ground heat exchanger installation was assessed for its impact on the environment using an LCA study. The researchers focused on the GSHP system throughout its life cycle, from manufacturing and transportation to installation and operation, and recorded energy consumption and air emissions. The manufacturing of raw materials including copper, plastic, steel, aluminum and rubber was part of the system’s border. Heat pumps and pipes were transported as well as the GSHP system was operated, and ultimately the assembly was completed. Moreover, the environmental impact categories considered were those of greenhouse effect, ozone depletion, acidification, eutrophication, carcinogenesis, winter smog and heavy metals. The system analysis indicated that 73% and 14.54% of the emissions were attributed to acidification and greenhouse effect, respectively, while SO
2 was produced by the use of lignite (coal) in the Hellenic electric power production, resulting as the main cause for the acidification (
Figure 5).
In this view, the authors assumed that when increasing the renewable energy fraction in the electricity power mix of Greece, the environmental impacts of the geothermal systems would definitely improve [
17].
The mitigation of the effects of the existing power grid on the environmental efficiency of geothermal systems can be achieved by combining them with other renewable energy sources such as PV panels or solar heating systems. The European Centre for Public Law in Legraina, Greece used a hybrid solar and geothermal heating and cooling system, according to the results of research. A saline groundwater well, a water storage tank for 6 hours’ autonomy, an inverter that regulated geothermal flow, a heat exchanger, two electrical water source heat pumps set in cascade, fan coils, air handling units and solar air collectors for air preheating in winter were considered. Moreover, the building hostel’s hot water supply was achieved by solar water heaters. Solar energy’s ability to contribute to the building’s energy balance was demonstrated during winter measurements, boosting the overall proportion of renewable energy consumption [
18].
By examining small-scale multi-generation systems, CHP, Combined Cooling, Heating, and Power (CCHP) as well as traditional systems with sixteen Heating/Cooling Energy Generation Systems (H/C-EGSs), the case of technological combination was further strengthened. A comparison approach for evaluating the energy performance of buildings under the European Building Performance Directive (EBPD) was utilized. Local and global cost optimums for an office building in Helsinki, Finland were calculated for each of the H/C-EGS. A total of 144 building combinations and 2304 examples of H/C-EGSs were included in the proposed energy-saving measures. According to the findings, the GSHP with free ground cooling was the most cost-effective option available globally. Only with great overall efficiency and a low power-to-heat ratio might biomass-based CHPs be economically viable due to low investment and operational costs. There were no economic or environmental advantages to biomass-based CCHPs over biomass-based CHPs due to the considerable rise in both investment and operational expenses. Using coal-fired CHPs, which had significant operating costs, was the most inefficient and ecologically damaging option. The net zero energy office building was created by extending the cost-optimal solutions with a PV solar panel system [
19].
An alternative way for the reduction of the energy consumption of large public buildings in Beijing by comparing three different air-conditioning systems has been examined. ASHP coupled with GSHP and GSHPs with solar assistance were all considered. Using DeST modeling software, the building load was calculated and economic indicators such as initial investment, LCC, operating cost, payback period, energy saving rate and cooling and heating costs per hour were evaluated. Results implied that a solar-assisted GSHP coupled with an air-source heat pump system had better economic results than the other two, especially the air-source heat pump system and, although the initial investment was higher, it had a payback period of less than 3 years compared with the air-source heat pump system [
20].
On the other hand, Bartolini et al., [
21] presented a techno-economic and environmental analysis of four different weight concentration fluids: propylene glycol at 25% and 33%, calcium chloride at 20% and pure water. The outcomes revealed that the use of pure water as a heat carrier fluid was appropriate for cooling buildings (i.e., in Seville, Lisbon and well-insulated buildings in Bologna), but, for heating-dominated buildings, this choice led to a remarkable increase in the length of needed BHE. However, OpenLCA software calculated the carbon footprint of the BHE during the installation phase, showing an amount 25.61 kgCO
2eq/m of BHE. Regarding the carbon footprint of other fluids: 4.67 and 1.02 kgCO
2eq/kg emitted for the propylene glycol and the calcium chloride, respectively, while the water’s carbon footprint was negligible.
In the spirit of economic and environmental efficiency, Huang and Mauerhofer stated that, apart from the energy saving measures adopted by governments worldwide due to the greenhouse effect, environmental and social impacts should also be considered, ensuring that these measures can also meet sustainable development requirements. An advanced sustainability evaluation method is based on the life cycle theory designed in that study. Case studies were used to evaluate this concept, since GSHP is a renewable technology widely used in China’s building sector. The energy usage of the GSHP cases studied was found to be 40.2% lower than that of a conventional air conditioning system. Global warming, acidification and eutrophication in the manufacturing process and soil temperature change in the operation phase were shown to be the primary environmental consequences of GSHP [
22]. Aiming at the public buildings sector, the environmental impacts of a GHP application in a university building were studied. A process-based hybrid LCI modeling technique was utilized to provide a full system boundary for footprint accounting, offering unique insights into the design and functioning of the researched technology [
23].
However, Heating, Ventilation and Air Conditioning (HVAC) systems were examined in the Winnebago Reservation in northeastern Nebraska as part of an LCC investigation. Rooftop gas heat and direct expansion (DX) cooling units (air-cooled condensers) were one option, as were air-source heat pumps and geothermal heat pumps (GHPs). Building energy modeling software was implemented to evaluate the heating and cooling demands. An estimated 264,000 Btu/h of cooling capacity and 178,000 Btu/h of heating capacity were calculated. Heat demand for the building was 246 kBtu and cooling demand for the building was 479 kBtu, both all year long. The NPV of 30 years of an LCC was calculated for each option in order to compare them. There were no significant differences in LCCs between the GHP and the traditional systems in terms of their NPV, which was determined to be around 18% lower. Installing the GHP system was a little more expensive, but the running and maintenance expenses were far cheaper than with traditional systems. GHG emissions may be reduced by 15 tCO
2eq and 33 tCO
2eq per year by using a GHP system instead of a rooftop gas heat unit or an air-source heat pump, according to their GHG study [
24].
GHPS economic viability was further affected by the Seasonal Coefficient of Performance (SCOP), as described by Junghans. Air-to-air GHPSs were studied on their economic and environmental viability, and the author established the importance of the envelope’s insulation level in determining whether heat pump systems were economically and environmentally viable. A geothermal water-to-air heat pump and an exterior air-to-air heat pump were evaluated for their economic and environmental viability in the context of their local climate and building insulation. Increased insulation levels were shown to have a significant impact on the SCOP, which in turn affects the heat pump system’s economic and environmental viability. SCOP values for heat pump systems were shown to be climatic and building insulation dependent [
25].
4.4. Electricity Generation Systems
Electricity production is one of the most important applications of geothermal energy. Coal power plants that form the majority of electricity generation contribute mainly to the GHG effect worldwide. In this view, attention is paid to more environmentally friendly and resource-independent energy generation technologies. Geothermal energy is a promising candidate as a renewable form of energy. In this part of the paper, the potential of clean electricity production applying geothermal energy is investigated.
Eight important variables have been used to evaluate the long-term viability of power generating. Price, GHG, efficiency, land usage, water consumption and social implications on a per kWh basis were examined for eight alternative ways of energy generation: solar, wind, hydro, geothermal, biomass, natural gas and nuclear power. Coal and nuclear power had the lowest average prices, whereas hydro and geothermal power had the lowest feasible prices, according to the authors. The average and total costs of PVs were the highest of all. The most efficient sources of energy were hydropower and PV, with hydropower coming out on top. Coal, as predicted, emitted the most GHGs of any fossil fuel. Biomass energy crops had the largest water needs, even if in hydropower the vast majority of water was not used but rather recycled back into the stream. Instead of biomass, nuclear, solar and wind power used the least amount of land. In reference to social impacts, wind and PV were the most sustainable, while on the contrary all thermal technologies were the least sustainable [
26].
Several sustainability indicators have been used to evaluate renewable electricity generation technologies (PV, wind, hydro and geothermal), including the cost of generated electricity, GHG emissions over the course of the technology’s entire life cycle, the availability of renewable energy sources, the efficiency of energy conversion, land requirements, water consumption and social implications. Wind power was shown to be the most sustainable energy source overall, followed by hydropower, solar power and finally geothermal energy. On the one hand, wind power contributed the lowest GHG emissions, having the most favorable social impacts compared with other technologies, but on the other hand required bigger land and capital costs. Indicators were examined separately, leading to remarkable statements:
As far as the price of electricity generation is concerned, geothermal energy and wind energy had the same average cost with geothermal energy exhibiting a lower range in price variations.
Geothermal power plants’ average emissions were found to be reasonable at 170 g/kWh by the authors’ calculations, although the range covered all potential values for gas emissions and could be as high as a low-emitting coal-fueled power station. However, technological decisions had the greatest effect on geothermal emissions. Emissions would increase if the waste gases, which included more than 90% CO2 by weight, were discharged directly into the atmosphere. However, most contemporary plants either reinject the CO2 or trap it to make dry ice.
Although the use of geothermal energy is constrained to areas where the necessary geothermal resource is already in place, there are many such areas in the globe (24 countries, with a total operational potential of 57 TWh/year). The attraction of geothermal energy is that it can be used around-the-clock to supply reliable “base load” electricity. Even though the extraction rates of the power generation will always be higher than refresh rates, the latter may be made up for reinjection, which greatly increases the lifespan of geothermal installations. If someone wants to avoid a short circuit, then they need to be selective about where they perform the reinjection. Seismic activity was improved by reinjection, but only in terms of its frequency; its intensity remained the same.
Geothermal power had the lowest efficiency, far less than other technologies.
The surface area occupied by geothermal power plants was little, since the bulk of the infrastructure was buried beneath the earth. The entire geothermal field was factored into the footprint analysis to account for the possibility of ground subsidence above the field. The average footprint of geothermal energy was between 18 km²/TWh and 74 km²/TWh.
Geothermal energy plants use a lot of water for cooling purposes. Non-evaporative cooling, pressure management, closed-loop recirculating cycles as well as the complete reinjection of filthy and offensive-smelling wastewater are all methods that might be used to reduce water usage. When compared with thermal power plants, geothermal facilities’ wastewater output was higher, at up to 300 kg/kWh.
Geothermal adversely affects communities when wastes were not properly managed as geothermal process waters are offensive smelling from hydrogen sulfide and are contaminated with ammonia, mercury, radon, arsenic and boron. These issues may be reduced if geothermal fluids were treated in a closed-loop system before being re-injected.
From the above results mentioned it is easily concluded that geothermal energy may not be as environmentally friendly as one would think, but it has certain advantages as compared with others, such as relatively small land use, the ability to provide base load power on a 24-h basis and its independence from weather conditions [
27].
The combined LCA and EMA analysis of a 20 MW dry steam geothermal power plant in the Tuscany region, Italy highlighted the environmental implications of geothermal power generation. The plant relied mostly on renewable resources found in the area, with some support from nonrenewable resources. However, carbon dioxide, hydrogen sulfide, mercury, arsenic and other pollutants were produced during direct consumption of the geothermal fluid, greatly contributing to climate change, acidification potential, eutrophication potential, human toxicity and photochemical oxidation. Despite the thoughts of some locals, the study found that geothermal power plants are generally safe for the environment. However, there are some parts and processes that might use some modifications [
6].
By stressing the direct and indirect contribution in terms of natural capital and ecosystem services to the power plant construction and operation, Emergy Synthesis offers a supplementary perspective to LCA. The geothermal power plant’s environmental effects were also compared with those of other types of power plants, such as those that use renewable energy and fossil fuels. The geothermal plant had a release of 248 gCO
2eq/kWh, which was lower than fossil-fuel-based power plants, but still higher than renewable technologies as solar PV and hydropower facilities. Furthermore, the amount of SO
2eq emitted (3.37 g/kWh) was similar to that of power plants that used fossil fuels. According to the findings, further research into other geothermal solutions (such as binary systems) is required in order to minimize negative environmental effects without sacrificing productivity gains [
6].
In the spirit of comparing different renewable energy options, more than a hundred distinct case studies, including solar (concentrated solar power, PV), wind, hydro and geothermal energy, were evaluated by Asdrubali et al., [
28]. A more accurate comparison of the available renewable technologies was possible, supported by the extensive data collecting, normalization and harmonization. Wind power was shown to have the least CO
2eq emissions and the least embodied energy, whereas geothermal and PV power had the greatest overall environmental effect values and the largest ranges of variability. Concentrating Solar Power (CSP) was rated as having a moderate environmental effect, ranking higher than PV, geothermal and hydropower facilities in nearly all impact categories. However, when the harmonized results were compared with those from traditional power systems (such as hard coal or a natural gas power station), the examination of all effect categories showed that renewable energy technologies provided considerable environmental advantages. However, it was evident that geothermal energy was not as environmentally beneficial as other renewable energy options, but it had a great variability and results cleaner than fossil-fuel-based energy options.
On the other hand, Stoppato and Benato [
10] showed that in the studied 150 kW ORC system attached with a biomass boiler, the corresponding electricity production was 11,160 MWh during the entire life of the plant. For GWP, a noticeably lower amount of 85.2 gCO
2eq/kWh was emitted compared with approximately 500 gCO
2eq/kWh coming from the production of fossil fuels for the Italian fossil mix. Similarly, the CED method resulted that the unit used approximately 7.3 kWh and 0.24 kWh of biomass and fossil fuels, respectively, for each kWh of electricity, mostly due to the requirement of diesel for biomass transportation, chipping and harvesting.
In order to highlight the impacts associated with electricity generation, a comparison between renewable and conventional power generating technologies from an LCA perspective was conducted. To this end, the GREET model was used to conduct a life cycle energy and GHG emissions study for several geothermal power producing systems (
Table 1), taking into account Argonne National Laboratory’s expanded GHGs, regulated emissions and energy consumption in transportation. The researchers extended the GREET model to include power plant building for coal, natural gas combined cycle, nuclear, hydropower, wind, solar and biomass, and performed an identical study for these systems. It was found that steel and concrete were used less in traditional power plants than in renewable energy systems (see
Figure 6). Enhanced geothermal and hydrothermal binaries needed more of these resources per MW than other renewable power generating technologies, with the exception of the concrete requirements for gravity dam hydropower. When considering both plant capacity and lifetime, energy and GHG ratios per kWh of power generation have been determined. In general, the infrastructure costs for renewable energy plants were greater per unit of energy produced than those for conventional plants. Construction plants followed a pattern with similar increases in GHG emissions per kWh of energy generation. Although certain renewable systems might produce GHG emissions during plant operation, these emissions were far lower than those produced by fossil fuel thermoelectric systems. The GHG emissions from binary geothermal systems were negligible in comparison to those from fossil fuels. The GREET model found that fossil thermal plants used nearly an order of magnitude more fossil energy and produced about twice as many GHG emissions per kWh of electricity as renewable power sources, including geothermal power [
29].
Concentrated solar power, integrated gasification combined cycle and fossil/renewable (termed hybrid) geothermal technology, in the form of co-produced gas and electric power plants from Geo Pressured Gas and Electric (GPGE) sites, were all introduced by the previous authors in a later article. In the latter example, they examined two scenarios: gas and electricity export and solely electricity export. Additionally analyzed as a function of well depth were the cement, steel and diesel fuel needs for drilling geothermal wells. The impact of construction activities on new plant construction was also calculated. The research findings were consistent with those of the prior study. Construction and components of fossil combustion-based power plants needed the fewest raw materials. Hydrothermal flash power and biomass-based combustion power were found to have the lowest GHG emissions, whereas traditional fossil-based power systems had the highest [
30].
An LCA study on GHG emissions and fossil-energy use associated with geothermal electricity production was accomplished [
31]. Hydrothermal flash and dry steam facilities operating GHG emissions were the subject of this study. Focusing on understanding GHG emissions caused by geothermal power plant operations, the analysis included findings for both the plant and the fuel cycle components of the overall life cycle. Only flash and dry-steam geothermal power facilities produced significantly high levels of such pollutants (zero values for binary plants). It was possible that the latter plants’ GHG emissions would be anywhere from nearly null to more than 400 g/kWh. Values for fossil energy consumption and GHG emissions during the whole life cycle were calculated and then compared across a variety of fossil, nuclear and renewable power sources. GHG emissions of geothermal power plants were comparable with other renewable energy options and much lower than those of fossil-fuel-based options, except nuclear power plants. It can be obtained that geothermal energy had the potential to bring better environmental results if certain requirements are met.
EGS, hydrothermal binary systems, hydrothermal flash systems and geo-pressured geothermal systems were all compared in a separate research, with their possible implications and influencing variables highlighted. A 20 MW EGS plant, a 50 MW EGS plant, a 10 MW binary plant, a 50 MW flash plant and a 3.6 MW geo-pressured plant that co-produces natural gas were all considered and analyzed. Finally, the impacts associated with these power plant scenarios were compared with those from other electricity generating technologies. The results displayed that geothermal energy was capable of low carbon emissions, which were primarily attributed to the construction phase, similar to most renewable energy technologies [
32].
Producing power from geothermal sources is constrained by the need for a constant supply of hot water or steam. High-enthalpy reservoir locations, where power plants can operate efficiently, are rather uncommon. Low-temperature resources, present over extensive geological regions, constitute a massive as-yet-untapped geothermal potential. Therefore, in the recent past, efforts have been made to investigate and develop suitable techniques for capturing this energy and transforming it into electrical power, resulting in the EGS. The basic idea was to use hydraulic stimulation at great depth (more than 2.5 km) in very hot crystalline rocks (about 150–200 °C) to improve and/or generate a geothermal resource. In this view, it was very important to understand the opportunities that this new technology offered and to explore possible ways that it could be advantageous [
33].
An analysis on the environmental performances from an LCA perspective of the above-mentioned systems (i.e., EGS) of ten significant design options located in central Europe has been presented [
33]. Each of these configurations was assigned a unique set of technical criteria, one of which was the potential for induced seismicity. Compared with conventional power plants, the results suggested that the consequences of EGS were on a par with those of other renewable energy sources. In addition, they could provide affordable base load electricity, making them an attractive choice for the energy systems of the future. Recommendations on the 10 scenarios’ environmental appropriateness were produced by comparing them. Additionally, the risk of induced seismicity was shown to be a crucial differentiating factor, with its importance growing in direct proportion to the environmental gain. The five-impact-category model was helpful for getting an overview of the environmental restrictions of EGS installations, and it might be used again to assess similar installations using alternative design approaches. One of the most important findings, corroborated by several studies, was that drilling had the greatest environmental impact of any step in the production of geothermal energy. Connecting to the national grid or some alternative energy source during this stage might significantly enhance their environmental performance [
33].
An intriguing study has been given on the topic of EGSs used for both power generating and district heating. The examined topics were the public’s adoption of geothermal energy, along with its parameters of economic viability, the thermodynamic efficiency in resource utilization and its life cycle environmental impacts. Utilizing a multi-period approach, it accounted for seasonal changes in district heating demand through the use of an LCA and multi-objective optimization approaches, in addition to process design and process integration. Single- and double-flash systems, as well as ORCs and Kalina cycles, were among the several conversion methodologies studied. The optimal configuration for the EGS was calculated for a range of depths, from 3000 to 10,000 m, and for a range of district heating network installed capacities, from 0 to 60 MWth. All optimal economic configurations were shown to have a beneficial environmental balance, measured in terms of avoided CO
2eq emissions and avoided impacts across the life cycle. However, there were substantial differences in the best possible configurations, which depended on factors such as the EGS construction depth, the size of the district heating design and the technology selected. EGS with depths between 5500 and 6000 m with a Kalina cycle for cogeneration and a district heating network with an installed capacity between 20 and 35 MWth were found to be the optimal configurations for all studied performance metrics in the shallowest depth range (3500–6000 m). When comparing the economic and exergetic benefits of cogeneration of district heating with those of single electricity production at the deepest depths (7500–9500 m), cogeneration of district heating was found to be less advantageous from both perspectives (11% and 17% relative penalty, respectively, for a district heating network with an installed capacity of 60 MWth). Nevertheless, it was more advantageous in terms of environmental performance (37% of relative improvement for avoided CO
2 emissions) [
34].
The question raised is the application ability of the geothermal binary power plants from a cradle-to-grave point of view, as they have gained increasing interest in reducing GHG and consume less finite energy resources. To this end, a complete LCA of geothermal power generation from EGS low-temperature reservoirs has been carried out, with results showing that the environmental consequences are considerably impacted by the geological parameters at a given location (
Figure 7). Binary geothermal power generation could greatly contribute to a more sustainable power supply at places with ordinary and above average geological characteristics. However, only a selected few plant layouts were capable of compensating for the energy and materials needed to seal the geothermal reservoir at less-than-ideal locations. However, geothermal binary power plants could have significant environmental impacts due to the extensive resources needed for their construction, particularly the underground portion of the plant. Consideration must also be given to the substantial impact that the auxiliary power needed to transport the geothermal fluid from the reservoir had on the net power production.
Enhancing reservoir productivity, designing deep wells reliably and making effective use of geothermal fluid for net power and district heat generation were essential components of ecologically friendly plants. The authors argued that low-temperature geothermal resources may be used to generate heat and electricity in the near and far future, resulting in a more sustainable energy system [
35].
In the discussion above, a different perspective of geothermal energy is raised, since EGS power plants are economically and environmentally beneficial compared both with thermal based power plants and with renewable energy power plants. In the following paragraphs, two very important factors of GHG-related emissions on geothermal power plants, the refrigerant used in the cooling stages and the diesel fuel consumed during the construction, especially drilling, are highlighted.
An effort has been conducted to assess the environmental impacts of electricity generation, as it is deemed fundamental for designing a low-carbon future. Methods for evaluating geothermal plants’ impact on the environment, based on physical and/or monetary data, were compared. As part of that research, a hybrid LCA was carried out for the Wairakei Geothermal Project, which involved taking stock of both material needs and financial resources. The ISO 14040 series standard was utilized for the evaluation [
36]. Some hybrid (mass-monetary) inventories were found to produce considerably different findings across effect categories. However, for specific geothermal systems studied, direct emissions of geothermal fluids dominated the few impact categories to which they contributed [
37].
Based on typical geothermal conditions in Germany, an LCA was performed on binary power plants that generate electricity using geothermal energy. Working fluid losses and environmental effects were included in an LCA of several power plant ideas (subcritical one-stage and two-stage ORC power systems and supercritical cycles). Since fluorinated refrigerants are prohibited by EU law, research into alternative working fluids with a low GWP for ORC systems is a priority. In particular, a second law analysis was performed on the concept of replacing R245fa and R134a with other working fluids such as R1233zd and R1234yf or natural hydrocarbons. Additionally, the ecological footprint of each potential power plant design was determined. The collected findings showed that the low GWP fluids tested guided to an equivalence of the second law efficiency and vastly reduced environmental effects compared with typical fluorinated working fluids. Using R1233zd as the working fluid instead of R245fa lowered the ORC’s global warming impact by 78% and caused a 2% loss in second law efficiency when dealing with a low-temperature heat source. The efficiency of the supercritical cycle operating with R1234yf raised by 37%, while the produced amount of CO
2eq remarkably decreased. The studied optimization options boost efficiency by as much as 7% in geothermal circumstances with higher temperatures of the geothermal fluid and a limitation of the reinjection temperature, such as in the Upper Rhine Rift Valley. The idea of a two-stage ORC seemed promising in this setting. The two-stage ORC with R1233zd resulted in 2% greater exergetic efficiency and a reduction in global warming impact (CO
2 emissions) from 78 to 13 g/kWhe when compared with a subcritical one-stage system using R245fa as the working fluid [
38].
Geothermal power generation has been the subject of an updated evaluation of life cycle environmental studies. The findings have been organized according to the following technologies for energy conversion: dry steam, binary cycle, single flash and double flash. The development of pilot projects for improved geothermal systems is also mentioned. The research concluded that the primary factor responsible for the associated impact on global warming was the consumption of diesel fuel, which was required for the construction stages (well drilling and completion, drilling fluid and cement pumping, casing due to steel production and well and fluid transport piping). Additionally, data availability dependent LCA hot areas for each effect category were identified, together with their accompanying information on global warming, eutrophication, acidification, resource consumption and land use. Similarly, a conclusion could be drawn that the life cycle environmental impacts varied depending on two factors: local geological characteristics and other methodological choices inherent to LCA methodology, such as the definition of the functional unit, the system boundaries, the lifespan, the impact assessment method and the allocation procedure [
8].
The environmental implications of various energy producing systems have been evaluated and compared. The ReCiPe midpoint technique was applied to a standardized collection of LCIs representing a broad variety of methods for generating power. The LCI analysis took into account the manufacturing and rollout of the technologies over nine geographical areas. Based on the data collected, it was determined that even low carbon power required more metals than traditional fossil power, that renewable and nuclear power reduced several environmental consequences and that CO
2 collection and storage raised the number of non GHG impacts. The production of low-carbon technologies was crucial and could serve as an early indicator of the most desirable technology. The geothermal power plant used in this analysis was expected to last for a long lifetime and had a high load factor. This resulted in less pollution throughout manufacturing. When comparing GHG, toxicity, particulate matter emissions, photochemical ozone production and acidification, direct emissions were at least an order of magnitude greater than indirect emissions. The high geogenic emissions were the cause of this situation: 83 gCO
2/kWh, 0.1587 gSO
2/kWh, 0.75 gCH
4/kWh, 0.06 gNH
3/kWh and 4 gHg/MWh. As most environmental impacts were caused by direct site-specific emissions from the geothermal fluid during the plant operation, these assumptions could be considered conservative, especially for human toxicity and freshwater ecotoxicity, for which the characterization factor of Hg was one of the highest across all substances [
39].
The LCA of a binary-cycle power plant that used high-enthalpy geothermal resources and a closed-loop GHP system that used low-enthalpy resources has been considered. Geothermal electricity is suitable enough to replace fossil-derived electricity, according to the LCA of binary-cycle power plants that use high-enthalpy geothermal resources.
Figure 8 shows the overall findings including Abiotic Depletion Potential (ADP), Global Warming Potential (GWP), Ozone Layer Depletion Potential (ODP), Photochemical Oxidant Formation Potential (POFP), Acidification Potential (ACP), Eutrophication Potential (EP) and Cumulative Energy Demand (CED). Even though geothermal power systems had a positive environmental profile and life cycle energy balance, their performances might be improved by minimizing the material requirements of site operation activities such as drilling and casing using environmentally friendly working fluids. The life cycle assessment of low-enthalpy geothermal resource closed-loop GHP heat generating revealed that high power demand and heat generation usage were the elements that define the environmental performance of geothermal heat systems. The availability of more ecologically friendly electrical networks was a major issue in mitigating the impact of geothermal heat, notwithstanding geothermal heat’s more favorable GWP and lower non-renewable energy consumption than fossil heat. Despite the fact that more efforts must be required to ensure environmental sustainability, the authors believe that geothermal energy systems will play an important part in the future energy systems because of its capacity to deliver energy with low environmental effect [
40].
A critical issue in order to minimize the impact of geothermal heat is a more environmentally friendly electrical grid. To this end, Marriott et al., explored the potential impacts of the energy mix on the results of an LCA case study. The findings showed that regional variations in the local generation mix could significantly affect GHG emission estimates. Similarly, GHG for certain sectors and scenarios could change by more than 100%. Finally, the authors advised practitioners to account for the uncertainties associated with mix choice [
41]. In the spirit of improved results, the following articles investigate new methods for conducting an LCA. Martin Pehnt investigated the potential of a dynamic approach on LCA on the grounds that background system impacts such as supply of materials or the demanded energy for production systems had the potential to be improved over time. The findings showed, therefore, that the inputs of finite energy resources and GHG emissions were significantly lower than in the conventional system. Concerning other environmental effects, the results did not provide a definitive judgement in favor of or against renewable energies [
42].
Onshore and offshore wind, hydropower, marine technologies (wave power and tidal energy), geothermal, PV, solar thermal, biomass, waste and heat pumps have all been the subject of LCA studies, and these studies have been reviewed with remarkable thoroughness. The major focus of that analysis was to show how inconsistent previous LCA studies were in their reporting of GHG emissions from the generation of electricity and heat using Renewable Energy Sources (RES).
Figure 9 and
Figure 10 show that the review found offshore wind to have the lowest GHG emissions (with potential mean life cycle GHG emissions of 5.3–13 gCO
2eq/kWh). Thus, estimates of GHG emissions from the combustion of fossil fuels to generate heat and electricity were compared with the actual GHG emissions, suggesting that conventional sources produced more GHG over the course of their life cycles than renewable ones do, with the exception of nuclear power. However, depending on the feedstock, the chosen limit and the inputs needed to produce it, energy from waste and Dedicated Biomass Technologies (DBTs) were shown to have potentially large GHG emissions, with ranges of 97.2–1000 and 14.4–650.0 gCO
2eq/kWh, respectively. Existing life cycle GHG emission estimates for power and heat generation from renewable energy sources were shown to differ remarkably. Some of these variations might be attributable to changes in real GHG emissions, while others might be related to discrepancies in assumptions and modeling choices. These variations revealed areas for improvement and opportunities for standardization. Future projects in developing renewable energy technology for electricity and heat generation can benefit from the evaluated results by providing appropriate baseline estimations [
43].
The possible environmental impacts of geothermal power plants during their lifetime have been thoroughly investigated. According to the authors, there is a lack of LCA studies on the topic of geothermal power production, and the ones that exist tend to be conducted on a country or even regional scale. Life cycle fugitive emissions, geological hazard risk and the consequences of water and land usage are also very time dependent factors. Emissions and resource consumption ranges for present global geothermal power generation were offered based on their analysis. They did the same thing when they defined a universal case approximating the mean. The data obtained might be used to feed LCIs, however they were not yet fully formed. Local and regional environmental impacts of potential emissions of key harmful compounds such as mercury, boron and arsenic were not sufficiently addressed on a worldwide basis [
44].
Furthermore, a new simplified model based on an LCA study of environmental performance variability of energy pathways deserves attention as a separate but related topic. An EGS power plant life cycle GHG emission estimation model with simplified parameterization has been developed using this technology. The model may be used with a wide variety of plant layouts. The research revealed a two-parameter model to evaluate EGS GHG emissions. In order to characterize a large number of potential EGS power plants in central Europe, a parameterized reference model was built. Using GSA on this baseline model, the impact of changes in installed power capacity, drilling depth and the number of wells as the primary contributors to the observed variation in GHG values were identified. Comparison results of published EGS and LCAs confirmed the representativeness of this new simplified model. Overall, the simplified model allowed for a fast and easy estimation of the environmental performance of an EGS power plant, without resorting to the LCA technique in its entirety. To this end, it provided a straightforward resource for EGS industry stakeholders and decision makers, with the goal of advancing the discussion surrounding the efficacy of this developing technology and the environmental consequences it might have [
45].