GRC Transactions, Vol. 46, 2022

Joint Development Between Eavor and Turboden
Arianna Bonzanini(1), Christian Besoiu(2), Michael Holmes(2), Joseph Bonafin(1)(1) Turboden S.p.A(2) Eavor Technologies Inc.

Keywords
Geothermal, closed-loop, multilateral, Eavor-Loop, organic rankine cycle, baseload power
ABSTRACT
Eavor’s proprietary geothermal power technology consists of two vertical wells connected by one
or more passes designed to circulate a working fluid in a closed loop system, with no interaction
with the underground formation water, to supply heat at the surface.
Turboden Organic Rankine Cycle (ORC) Technology converts the heat available at the surface
into electricity by means of a closed thermodynamic process involving a suitable working fluidwhich is heated up in a heat exchanger and expanded in a turbine to generate electricity.
Eavor and Turboden performed a joint optimization of a surface heat-to-power facility for
integration with an Eavor-Loop™. Because Eavor employs a closed-loop design, there are several
optimization levers not applicable to traditional ORC applications that require an integrated systemview to properly optimize.
The joint scope of work consisted of the creation of an integrated Eavor-Loop™ + Turboden ORC
techno-economic optimization workflow for baseload power operation of the system. The
optimization targeted minimizing the levelized cost of electricity of the Eavor-Loop™ + Turboden
ORC system and analyzed each piece of surface equipment, choice of working fluid, and any otherdesign considerations to determine if any modifications to traditional ORC design could be madeto improve the economics.
The paper summarizes the joint optimization work, including key findings from the joint
optimization effort, design considerations for an Eavor-Loop™ + Turboden ORC integrated
system, and final recommendations of fluids and cycle configurations.

Bonzanini et al.
1. Introduction
Eavor-Loop™ is a disruptive geothermal system which eliminates or mitigates many of the issues
with traditional geothermal. The key technical differentiator relative to conventional geothermalis that it is a subsurface closed-loop radiator, or heat exchanger, that relies only on conductive heattransfer, rather than convection or reservoir fluid flow. This design removes the need for a rarehydrothermal source, has no fracking or continuous water use, eliminates the complex resource
characterization cost and time associated with geothermal reservoirs (natural or man-made),
provides extremely predictable output, and is truly dispatchable without a reduced capacity factor.
These features allow the system to be financed and scaled massively with a standardized,
repetitive, manufacturing approach similar to wind or solar – without being held back by a scarce
resource and high-risk exploration.
Turboden S.p.A. is an Italian company founded in 1980 by Prof. Mario Gaia of Politecnico di
Milano. Today it is a world leader in designing, manufacturing and supplying Organic Rankine
Cycle (ORC) systems, with more than 400 references worldwide. Since 2014, Turboden is part of
the Japanese industrial giant Mitsubishi Heavy Industries, world leader for installed power fromgeothermal sources.
Due to the differences between an Eavor-Loop™ and traditional geothermal, an integrated systemlevel approach is needed to optimize the operation of an Eavor-Loop™ with the design of the
ORC. In an Eavor-Loop™ system, a feedback effect between the Eavor-Loop™ inlet temperatureand the outlet temperature (a higher inlet temperature corresponds to a higher outlet temperature)
impacts the selection of an Eavor-Loop™ operating point and the ORC design. Another uniquefeature of the Eavor-Loop™ is that the flowrate of working fluid (driven by thermosiphon) can becontrolled to achieve a desired outlet temperature. The joint optimization effort between Eavor
and Turboden aimed to identify the optimal operational conditions for the Eavor-Loop™, and the
optimal ORC configuration for baseload operation with the objective of minimizing the levelized
cost of electricity production over a 30-year project lifetime.
2.0 System Description
2.1 Eavor-LoopIn the Eavor-Loop™ 2.0 configuration considered for the analysis in this paper, two vertical wells
are drilled from the same surface location and 12 passes (1 pass = 2 legs) propagate from thevertical wells at a slight off-vertical angle (10-30°), with the intersection occurring at the toes ofthe passes. Figure 1 illustrates the progression of the Eavor-Loop™ design. The circulation rate
through the loop is maintained through the thermosiphon effect between the inlet and outlet wellbores. This effect arises from the fluid density difference between the cold inlet and hot outlet
wells and generates the required pressure driving force eliminating the need for a parasitic
circulating pump.

Bonzanini et al.

Figure 1: Eavor-Loop™ evolution

2.1.1 Baseload Operation of an Eavor-Loop™
An Eavor-Loop™ initially produces fluid at temperatures close to the reservoir temperature (withsome cooling as the fluid is produced up the vertical production well). This temperature declines
steeply during the initial days of production as the rock immediately surrounding the wellbore iscooled. Over time, the temperature decline is diminished, and temperatures stabilize with only aslight decline observed to year 30+. An Eavor-Loop™ has remarkably consistent and predictable
output over long timeframes (productive lifetime of 100+ years) (Holmes et.al, 2021). As heat is
extracted from the earth the radius of the temperature affected area around the wellbore expands;
beyond this radius the reservoir is still at virgin temperature. This radius is a logarithmic functionof time, and the lateral wellbores have minimal interference with each other if spaced properly.
2.2 ORC Technology
The ORC turbogenerator (Macchi, Astolfi, 2017) uses the geothermal fluid to preheat and vaporizea suitable organic working fluid, which is expanded in the turbine, generating electrical power.
Organic fluids have unique thermodynamic characteristics, if optimally selected, to allow for the
cost-effective exploitation of low to high enthalpy geothermal resources.
2.2.1 ORC Binary Geothermal Power Plant
Figure 2 shows the temperature-entropy diagram of a general thermodynamic cycle of the ORC,
and Figure 3 shows schematically the ORC binary loop. The organic working fluid is pumped(1→2) into the recuperator (2→3), where it is heated before entering the evaporator (3→4→5);
here the geothermal source, is cooled through the heat exchangers, providing heat to the working
fluid which flows inside an independent loop. The cooled geothermal source can be reinjected.
The hot working fluid passes through the turbine (5→6), which is directly coupled to the electric
generator, resulting in reliable electric power. The exhaust organic vapour flows through theregenerator (6→7), where it heats the organic liquid (2→3) and then enters the condenser (withBonzanini et al.
water or air) where it is condensed by the cooling circuit (7→8→1), thus completing the closedcycle operation.

Figure 2: Temperature vs Entropy diagram of a general ORC thermodynamic cycle.

Figure 3: Schematic representation of the ORC binary loop.

The ORC conversion efficiency is dependent on the temperature of geothermal resource, as
illustrated in Figure 4. The ORC efficiency is computed as the net power output divided by the
total thermal input. In the case of high enthalpy field, the ORC efficiency can reach significantly

Bonzanini et al.
beyond 20%. ORC technology is the ideal heat-to-power solution for an Eavor-Loop™ across awide range of geological conditions.

Figure 4: ORC efficiency depending on the geothermal source (traditional hydrothermal reservoir, Macchi,
Astolfi, 2017).

3.0 Baseload Eavor-Loop™ + ORC Optimization Workflow
A techno-economic workflow was established to integrate the results of Eavor’s subsurface modeland Turboden’s ORC models to reach an optimized Eavor-Loop™ + ORC configuration. The
workflow consisted of four main steps:
1) The generation of Eavor-Loop™ thermodynamic performances across a range of operating
points;2) Initial scoping to determine the optimal Eavor-Loop™ + ORC operating window;
3) Detailed analysis using commercial software and proprietary cost databases to identify the
optimal ORC specifications;4) Sensitivities on alternative ORC designs and the expected energy production throughout the
lifetime of an Eavor-Loop™ + ORC system.
The focus of this paper is on the ORC design and system integration with the Eavor-Loop™, while
holding the well design and geometry constant.
3.1 Eavor-Loop™ Thermodynamic Performance
Eavor’s propriety subsurface 2-D model calculates the heat conduction from the rock into thewellbore based on radial and axial coordinates and has been validated based on the thermodynamic
and wellbore data from the Eavor-Lite™ demonstration project in Alberta (Toews, Holmes, 2021).
Additionally, Eavor’s subsurface model has been validated by numerous independent third parties
including NREL (Beckers, Johnston, 2022), TNO (van Wees, 2021) and UT Austin (Fallah et. al.
2021). The key input parameters are the inlet well pressure, temperature, flow rate, rockBonzanini et al.
temperature and rock thermal conductivity. Combining these parameters and specifying thewellbore geometry, the outlet temperature and pressure of the system are calculated numericallyby closing the energy and momentum equations for each discretized segment of the Eavor-Loop™
vertical and lateral wellbores. In this paper, a high gradient Eavor-Loop™ 2.0 system, indicativeof the geological conditions in western United States, with the following specifications were
simulated and analyzed in conjunction with an ORC system:
Table 1: Eavor-Loop™ and Geology Specifications
Parameter
Eavor-Loop™ Specifications
Vertical Well Length
Leg Length
Total Vertical Depth
Leg Angle (0=Vertical)
Number of Passes
Lateral Section Roughness
Vertical Section Roughness
Wellbore Diameter
Working Fluid
Geology Specifications
Geothermal Gradient
Bulk Formation Thermal
Conductivity
Bulk Formation Density
Bulk Formation Specific Heat
Capacity

Value

Units

4000m
3000m
6819m
20
12
1.5E-4
5.0E-5
0.216
Water

m
m
m
°
#
m
m
m

60
2.5

°C/km
W/m-K

2750
800

kg/m3
J/kg -K-

Different operating points were simulated with the above Eavor-Loop™ and geology
specifications using Eavor’s proprietary subsurface 2-D model, for inlet Eavor-Loop™
temperatures varying from 40°C - 180°C and Eavor-Loop™ flowrates from 20kg/s – 180kg/s.
Figure 5, on the left, illustrates the relationship between Eavor-Loop™ outlet temperature, thermal
duty and mass flowrate of the circulating working fluid for a fixed Eavor-Loop™ inlet temperatureof 110°C at year 5 of the project with the well and geology specifications of Table 1. As flowrate
is decreased, the Eavor-Loop™ circulating fluid approaches rock temperature, and thermal duty
decreases. The figure on the right outlines the feedback effect of Eavor-Loop™ inlet temperatureon outlet temperature, and the corresponding impact on thermal duty for flowrates utilizing all theavailable thermosiphon pressure. The subsurface model outputs were used in the subsequent stepsof analysis to determine the optimal Eavor-Loop™ operating point in combination with
Turboden’s ORC models.

Bonzanini et al.
300

300

50

280

30

220
200

20

180
160

10

140
120

0
20 40 60 80 100 120 140 160 180
Eavor-Loop™ Mass Flowrate [kg/s]

Eavor-Loop™ Outlet Temperature

Thermal Duty

40

260
240

30

220
200

20

180
160

Thermal Duty [MWth]240

Thermal Duty (MWth)

40

260

Eavor-Loop™ Outlet Temperature [°C]280

Eavor-Loop™ Outlet Temperature [°C]

50

10

140
120

0
40

60

80 100 120 140 160 180

Eavor-Loop™ Inlet Temperature [°C]
Eavor-Loop™ Outlet Temperature

Thermal Duty

Figure 5: Thermal duty and Eavor-Loop™ outlet temperature as a function of Eavor-Loop™ flowrate andinlet temperature based on the specifications of Table 1

3.2 Scoping of the Optimal Eavor-Loop™ + ORC Operating Window
The initial assessment of the ORC power output as a function of the Eavor-Loop™ operating
conditions was computed using the Lorenz cycle efficiency (Bertani et al., 2017), rather than the
Carnot efficiency, since the sources of the cycle present temperature glides. Assuming a 55%
conversion factor between the net real thermodynamic cycle efficiency from the ideal Lorenz
efficiency, the net power output of each Eavor-Loop™ operating point was determined for the
purposes of identifying the optimal operating window.
3.3 Detailed ORC Analysis
The detailed analysis and modelling of the ORC loop were performed using Aspen Plus software.
The selection of the ORC design capacity was based on the Eavor-Loop™ fluid properties at year
5, representing the relatively flat thermal output of the Eavor-Loop™ over the project lifetime.
A single-pressure level cycle with four organic fluids (normal-butane, isopentane, normal-pentane,
and cyclopentane) was investigated. The applicability of dual-pressure cycles was also evaluated.
The pinch points of both the hot and cold heat exchangers were fixed for all the analyzed operating
points. An air-cooled condenser (ACC) was considered for the cold heat exchangers with anambient air dry-bulb temperature of 15°C. The turbine efficiency has been described by asimplified proprietary model to account for the differences of the fluids and of the ORC
thermodynamic conditions at turbine inlet and outlet with a range in turbine efficiency between
85% - 91%. The ORC power output was calculated net of the internal ORC power consumption(working fluid pumps and ACC fans consumption).

Bonzanini et al.
3.4 Energy Production
Following the detailed ORC analysis, the electrical energy production of the Eavor-Loop™ + ORC
system over the course of a 30-year project lifetime was calculated. The impact of fluctuating
seasonal ambient temperatures and alternative ORC design capacities were also considered.
4.0 Results
4.1 Optimal Eavor-Loop™ + ORC Operating Window

8000

8000

7000

7000

6000

6000

Net power [kW]Net power [kW]The maximum power production for three representative points (years 1, 5, and 20) in the lifetimeof an Eavor-Loop™ + ORC system across differing Eavor-Loop™ inlet temperatures is illustrated
in Figure 6. In addition to the ORC’s internal power consumption, the pump work required for theflowrates exceeding the Eavor-Loop™ thermosiphon rate is included in the net power figures
below. Figure 6, on the left, illustrates the maximum power computed using Lorentz efficiency
for varying Eavor-Loop™ inlet temperatures. The figure on the right, outlines the system
performance of differing Eavor-Loop™ flowrates, with a fixed Eavor-Loop™ inlet temperatureof 110°C.

5000
4000
3000

5000
4000
3000

2000

2000

1000

1000

0

0
40

60

80

100 120 140 160 180

Eavor-Loop™ inlet temperature [°C]Year 1

Year 5

Year 20

20

40

60

80 100 120 140 160 180

Eavor-Loop™ flowrate [kg/s]
Year 1

Year 5

Year 20

Figure 6: Maximum ORC net power output as function of Eavor-Loop™ inlet temperature and flowrate

The result of this preliminary scoping indicated that the maximum power output of the ORC plant
occurs when the Eavor-Loop™ inlet temperature is relatively high, between 95°C - 115°C, and ator near the maximum flowrate the available thermosiphon pressure can support (120-130kg/s).
Thus, these operating points were the focus of the detailed ORC simulations and subsequent
optimization steps.
4.2 Results of the Detailed ORC Analysis

Bonzanini et al.
The results for two of the five Eavor-Loop™ inlet temperatures analyzed are reported in Figure 7
and the main parameters of the five optimum points of the curves are reported in Table 2.
Eavor-Loop™ inlet temperature 115 °C

7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000

7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000

Net power [kW]Net power[kW]Eavor-Loop™ inlet temperature 95 °C

0

50

100

150

0

200

Eavor Loop mass flow rate [kg/s]
Isopentane

n-pentane

50

100

150

200

Eavor Loop mass flow rate [kg/s]

n-butane

Isopentane

n-pentane

Cyclopentane

Figure 7: ORC net power output as function of the Eavor-Loop™ mass flow rate.

Table 2: Optimum ORC designs as function of the Eavor-Loop™ inlet temperature
EavorEavorLoop™
Loop™ Inlet
OutletTemperature
Temperature
[°C]
[°C]
95
172.8
100
175.8
105
178,8
110
181.8
115
184.9

Optimal
ORC
Working
Fluid

ORC's
Net
Power
[kW]

Indicative
ORC's n°of ACC
Bays

UA
Hot
heat
Cycle netexchangers efficiency[kW/K]

Isopentane
Isopentane
Isopentane
n-pentane
n-pentane

6550
6625
6656
6643
6630

12
12
12
11
11

2593
2509
2518
2317
2334

15.6%
16.2%
16.7%
17.1%
17.5%

The maximum net power values of Figure 7 and the values in Table 2 coincide with a mass flowratethat utilizes all the available thermosiphon pressure of the Eavor-Loop™ (approximately 126 kg/s
at year 5).
The net power output corresponding to the Eavor-Loop™ inlet temperature 105°C and 110°C iscomparable, however there are equipment and cost differences between the two thermodynamic
cycles. Both the hot heat exchanger surface area (UA hot heat exchangers in Table 2) and thesurface area of the ACC are larger for the isopentane ORC designs without a proportional increasein performance. A larger heat exchanger surface area implies a higher equipment cost. Thus, theBonzanini et al.
normal-pentane thermodynamic cycle with an Eavor-Loop™ inlet temperature of 110°C has been
selected as the optimum cycle.
4.2.1 Evaluation of Dual Pressure Cycles
The optimization analysis was extended to include dual-pressure binary cycles (DiPippo, 2012),
with identical assumptions and operating conditions of the single-pressure cycle analysis.
Figure 8 illustrates the results of the dual pressure ORC optimization for three different working
fluids at varying Eavor-Loop™ inlet temperatures. The optimal dual pressure ORC design
corresponds to an Eavor-Loop™ inlet temperature of 105°C with cyclopentane as working fluid
and an Eavor-Loop™ flowrate of 126 kg/s. However, the net power output gain with respect tothe single-pressure level cycle is 3.2% (6854 kW vs 6643 kW). The ORC capex increase for thedual-pressure cycle was assessed, and the minor increase in net power did not justify the additionalcost. For this example, the dual pressure ORC costs are approximately 30% higher. Note that theadditional capex of a dual-pressure ORC, and its relative impact on the economics of an EavorLoop + ORC system, must be evaluated case by case. A dual-pressure cycle is typically employedwith the objective of dropping the reinjection temperature of the geothermal fluid further, which
is not beneficial with an Eavor-Loop™ 2.0 in high geothermal gradient geologies due to therelatively high optimal inlet temperature.
7500
7000

Net Power [kW]

6500
6000
5500
5000
4500
4000
45

55

65

75

85

95

105

115

Eavor Loop inlet temperature [°C]
Isopentane

N-Pentane

Cyclopentane

Figure 8: Optimum Dual Pressure Level ORC net power as a function of Eavor-Loop™ inlet temperature.

4.3 Baseload Operation Energy Production
With the optimum operating condition of the Eavor-Loop™ and the ORC thermodynamic cycle
defined, the energy production of the Eavor-Loop™ over a 30-year project lifespan was evaluated,
incorporating ambient temperature fluctuations and the expected yearly decline in Eavor-Loop™
thermal output. A generic continental monthly temperature distribution was assumed, with aminimum monthly average of 6°C in January and 25°C as a maximum monthly average in August.
The corresponding annual average temperature is 15°C.

Bonzanini et al.
In the case of traditional liquid dominated geothermal power plants, limitations on the geothermalbrine reinjection temperature may be required to avoid scaling and solid deposition. Typically, an
active control procedure on the ORC plant is used to limit the geothermal resource reinjection
temperature. Since the circulation water of the Eavor-Loop™ does not contain dissolved solids,
there is no need for a constraint on the Eavor-Loop™ inlet temperature. Eliminating this constraint
allows for the maximum utilization of ORC heat exchanger area across a broad range of operating
conditions, increasing system performance in off-design conditions.
4.3.1 Ambient Temperature Off-Design Performance
The performance of the ORC in varying ambient temperature conditions was computed in Aspen
Plus with constraints on the dimensions of the heat exchangers and turbine from the optimized
ORC design in section 4.2. The feedback impact of the adjustable Eavor-Loop™ inlet temperature
was accounted for in the off-design performance calculations of the ORC.
For air cooled ORC systems, as ambient temperatures increase, condensation pressures increase,leading to reduced electrical output. The Eavor-Loop™ + ORC system displays unique behavior
in different ambient conditions as a consequence of the feedback impact within the closed loop
system. In warmer ambient conditions, the optimal Eavor-Loop™ inlet temperature increases fromdesign point, contributing to a slightly higher Eavor-Loop™ outlet temperature, thus, improving
ORC efficiency. The converse effect is prevalent in cooler ambient conditions (reduced EavorLoop™ inlet and outlet temperatures). As shown in Table 3, this interdependence between inlet
and outlet temperatures flattens the net electrical output of an Eavor-Loop™ + ORC system acrossseasonal ambient temperature variations when compared to an equivalent traditional geothermal
system (identical temperatures/flowrates and no limitation on the geothermal reinjection
temperature). However, the balance of these contributions counteracts, with a negligible impact tothe annual energy production. While the effect is relatively minor, it is an advantageous
characteristic of the Eavor-Loop + ORC system, as electricity demand in many regions peaks
during the warmer months of the year.
Table 3: Comparison of the monthly energy production of a traditional geothermal ORC
plant and of an Eavor-Loop™ + ORC system
Jan
Eavor
5.29
Loop
(GWh)
Traditional 5.39
Geothermal(GWh)% Change -1.99

Feb
4.70

Mar
5.17

Apr
4.80

May
4.77

Jun
4.48

Jul
4.42

Aug
4.37

Sep
4.43

Oct
4.91

Nov
5.00

Dec
5.25

4.78

5.24

4.81

4.74

4.41

4.30

4.23

4.35

4.91

5.07

5.34

-1.58 -1.37 -0.24 0.75

1.56

2.73

3.05

1.84

0.00

-1.37 -1.79

4.3.2 Net Energy Production
Figure 9 outlines the electrical production impact of thermal performance decline for the optimized
Eavor-Loop™ + ORC system. The energy production has been calculated with a constant Eavor-

Bonzanini et al.
Loop™ mass flow rate and an assumed overall plant availability of 98%. The total energy
production of the 30 years lifetime of the plant is 1613 GWh.
70

Energy [GWh]

60
50
40
30
20
10
0
0

10

20

30

Year
Figure 9: Annual electrical energy production of the optimized Eavor-Loop™ + ORC system.

5.0 Conclusion
The joint development effort between Eavor and Turboden explored the distinct features of an
Eavor-Loop™ + ORC system as a baseload power plant. A performance and cost optimized single
pressure ORC with n-pentane as a working fluid was designed and simulated in Aspen Plus forheat-to-power conversion of an Eavor-Loop™ thermal output. The transient impact of the EavorLoop™ yearly thermal output decline, changes in ambient temperature, alternative ORC
configurations and capacities were considered to reach an optimized system design that minimized
the levelized cost of electricity over a 30-year period. The optimized system produces 6643 kW ofnet electrical power at the nominal design condition (year 5) and 1613 GWh of electrical energy
over a 30-year period. Continued collaboration between Eavor and Turboden will aim at
identifying additional system level opportunities and further reducing the levelized cost ofelectricity. Further improvement of the present work consists in the study of the dispatchability
mode operation of the Eavor-Loop™ + ORC system: this kind of operation can be coupled withother kind of energy production, such as solar and wind power, to cover different grid power
requirements.

Bonzanini et al.
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Systems as a Zero Emission Load-Following Resource.” Proceedings: World Geothermal
Congress 2021, San Diego, United States, (2021).
Macchi, E., Astolfi, M. “Organic Rankine Cycle (ORC) Power System.” Woodhead Publishing,(2017), 978-0-08-100510-1.
Toews, M., Holmes, M. “Eavor-Lite Performance Update and Extrapolation to Commercial
Projects.” Proceedings: World Geothermal Congress 2021, San Diego, United States, (2021).
Beckers, K., Johnston, H. “Techno-Economic Performance of Eavor-Loop 2.0.” Proceedings:
47th Workshop on Geothermal Reservoir Engineering, Stanford, California (2022).
van Wees, J. “Eavor Loop Audit Report.” TNO, Utrecht, Netherlands, (2021).
Fallah, A., Gu, Q., Chen, D., Ashok, P., van Oort, E., & Holmes, M. “Globally scalable geothermalenergy production through managed pressure operation control of deep closed-loop wellsystems.” Energy Conversion and Management, 236, (2021).
Bertani, R. “Perspective for Geothermal Energy in Europe.” (2017), 9781786342317 1786342316.
DiPippo, R. “Geothermal Power Plants: Principles, Applications.” Case Studies andEnvironmental Impact, (2012), 978-0-08-098206-9.