Hot water production improves the energy return on investment of geothermal power plants
Highlights
► EROI of geothermal went from 33:1 to 9:1 when excluding hot water. ► EPT (Energy payback time) was approximately 1.2 years. ► Hot water production improved the EROI significantly. ► The plant itself is shown to be the largest energy consumer.
Introduction
Energy Return on Investment or EROI is an approach to calculate the ratio between the energy needed to get energy and the energy delivered to society. The notion relates to ecology [1], where food gathering must leave more energy behind than was used in the gathering process. This is the same for power plants, where society wants the plants to leave more energy behind than was used in the power generation process. The concept has in recent times mostly been used to study energy sources such as oil and natural gas [2]. The methodology has gained much interest in recent years, especially in discussions around peak oil. Less interest has been in the EROI of renewable sources such as geothermal and hydroelectric and there have been very few studies done since the 1980's. The EROI concept is often credited to Charles A. S. Hall, where he used the concept in his PhD dissertation and resulting publications [3], [4], [5]. Similar concept was put forward by Herendeen & Plant [6], which described the term “Energy cost of Energy” which in essence is the same as EROI. Net energy analysis is related to EROI and has been thoroughly investigated in recent times to study various energy sources [7], including geothermal [8]. The methodology has been criticized for its lack of standardization since different studies of the same energy source might produce different results. This is due to the fact that different boundaries can be set, and therefore different parameters can be included between studies. Different timeframes also give different results. Murphy and Hall claimed in 2010 that no data exists on geothermal EROI [9], this is supported by Mansure [10], who states that no studies have been done on the subject of geothermal EROI in recent times. Further review showed that Gilliland [11] calculated the EROI to be 12.6 for a dry steam reservoir and 10.7 for a wet steam reservoir. Herendeen & Plant [6] claim that an EROI of 3 might be a reasonable guess for geothermal energy production, which seems a very low estimate. Mansure concludes that up to date analysis on the EROI of geothermal has yet to be made [10] and at least this field lacks recent and reliable analysis. This is a gap, which this study attempts to fulfill using real data. Efficiency at Nesjavellir geothermal power plant is therefore studied using methods proposed by Murphy et al. [12].
Section snippets
Methodology and study area
When conducting an EROI analysis, the scope must be well defined. One of the biggest discussions around EROI is the lack of standardization, that each study can have different parameters. These parameters include factors such as transportation of goods, refinery of the product etc. In the beginning of the study, a clear set of boundaries regarding the data collection is set. When all boundaries have been set and data gathered, values are converted to energy. The total energy needed to create,
Results
The EROIstnd, EROI3,i, EROIide and the energy payback time for the power plant were calculated using different lifetime scenarios. Scenario excluding hot water was also calculated, to show the EROI of the plant if it produced solely electricity.
Discussions
This study calculated four EROI scenarios and the energy payback time of the Nesjavellir power plant. All the major parts of the power plant have been canvassed, and the embodied energy of these given parts calculated. A comparison of the results after the plants lifetime can be seen in Table 3.
The energy required to operate the machinery in the general construction of the major parts was also calculated. However, it was found that the largest energy-consuming factor on a yearly basis was the
Acknowledgments
This research was supported by the Icelandic Energy Fund (Orkusjodur). Acknowledgments go to Reykjavik Energy hf, Landsnet hf and Efla Engineering hf for providing data.
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