# Techno-Economic Analysis of a Seasonal Thermal Energy Storage System with 3-Dimensional Horizontally Directed Boreholes

^{*}

## Abstract

**:**

## 1. Introduction

_{th}/a and 4.5 GWh

_{th}/a. A test site was constructed to conduct a thermal response test and validate the numerical model developed in this work. The experimentally validated model was expanded to allow a more comprehensive techno-economic analysis for larger configurations and different charging temperatures. Boreholes were spaced between 1.5 and 3 m in a pattern consistent with former vertical installations [14,15]. Five different configurations with 4, 7, 12, 24 and 42 boreholes were modeled with COMSOL Multiphysics [16]. The focus of this work is outlined by the following points.

- Decarbonization of building heat using renewable energy and seasonal thermal energy storage.
- The use of the underground below established surface structures as a storage medium.
- Determine the economic feasibility of a new application for storing heat underground.

## 2. Materials and Methods

#### 2.1. 3D-HDD Applied to BTES Systems

#### 2.2. Experimental Test Site

#### 2.3. Modeling a BTES System

_{sys}) were assessed (Section 2.4.). The flow rate maximizing BTES system thermal output was modeled for a 10-year period to assess a steady-state operating cycle representative of the average performance for a 50-year lifespan. The quantity of heat extracted was evaluated for the number of Swiss multifamily homes (MFH) supplied by a representative district heating network (Section 2.4.1). A basis of comparison between the number of boreholes installed, capacity of each configuration, and the influence of charging temperature was achieved through this line of analysis.

#### 2.4. BTES System

- the difference between the ambient temperature $\left({T}_{amb}\right)$ and storage temperature
- the difference between the return temperature from the heat sink $\left({T}_{r,con}\right)$ and ${T}_{amb}$
- the minimum and maximum outlet temperatures for a heat and cold storage, respectively
- the quantity of stored thermal energy $\left({Q}_{in}\right)$
- the installed depth of the storage system
- the ratio between the length and cross-sectional width of the energy storage system
- the thermal properties of the underground, e.g., groundwater, permeability, anisotropic properties of different soil layers, belowground infrastructure, surface covering, etc.

#### 2.4.1. BTES System with a DHN

#### 2.5. BTES/DHN Equipment, Investment, and Operating and Maintenance Costs

#### 2.5.1. Electrical Energy Costs

_{el}[38].

#### 2.5.2. Borehole Costs

#### 2.5.3. Heat Pump Costs

_{th}, with a second tier from 10 to 70 kW

_{th}, as calculated with cost Functions (3) and (4), respectively. An absence of data between 70 and 500 kW

_{th}existed where linear interpolation bridged the price gap in this range. This was done by taking ${I}_{AW70}$. as the lower-bound and ${\overline{I}}_{AW500}$. as the upper-bound using cost Functions (4) and (5), respectively [40]. The resulting cost correlation between 70 and 500 kW

_{th}is given in cost Function (6). AWs larger than 500 kW

_{th}used a second linear interpolation for average prices between 500 and 10,000 kW

_{th}. This was calculated with cost Function (7) [40]. O&M costs of AWs were calculated with cost Function (8) [41].

_{cap}) in cost Function (9). O&M costs were assumed to be calculated the same way as air-water heat pumps with cost Function (10). Heat pump investment and O&M costs are listed below in Appendix A, Table A2, for each configuration and charging temperature.

#### 2.5.4. Hydraulic Pump Costs

#### 2.5.5. DHN Costs

## 3. Results

#### 3.1. Experimental Results

#### 3.2. Model Validation

#### 3.3. Multi-Borehole Configurations

#### 3.4. Economic Evaluation

## 4. Discussion

#### 4.1. Experimental Results

#### 4.2. Model Output and Economic Analysis

_{th}with scale. The trend in LCOS in Figure 32 shows decreasing cost per kWh with increasing configuration size and charging temperature with a minimum of 0.025 CHF/kWh

_{th}. The trend in Figure 33 shows a decreasing LCOH for heat delivered to the building, including distribution costs and losses with size and charging temperature. Larger configurations with higher charging temperatures incurred more investment costs to commission a system as shown in Figure 34. Results from multi-bore numerical models suggest the combination of a charging temperature ≥ 60 °C and storage with >42 boreholes has the potential to break below an LCOH of 0.10 CHF/kWh

_{th}. Equipment costs can be further reduced by raising the storage temperature to a level where extracted heat from the BTES system renders the discharging heat pump unnecessary. Additional savings are possible in a district with higher linear head demand density reducing the length of the district heating network.

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

Term | Definition | Unit |

3D-HDD | 3-dimensional horizontally-directed drilling | - |

AW | air-water heat pump | - |

BH | building heat | - |

BOHRX | horizontal borehole X | - |

BTES | borehole thermal energy storage | - |

COP_{i} | coefficient of performance of i | - |

DHN | district heating network | - |

DHW | domestic hot water | - |

GPS | global positioning system | - |

HDPE | high density polyethylene | - |

HTF | heat transfer fluid | - |

IEA | International Energy Agency | - |

IGE | Institute für Gebäudetechnik und Energie; HSLU | - |

KERNX | vertical borehole X | - |

MFH | Swiss multi-family house | - |

N_{i} | number of component i | - |

PV | photovoltaic | - |

SFOE | Swiss Federal Office of Energy | - |

WW | water-water heat pump | - |

# | number | - |

bh | borehole | - |

calc | calculated | - |

dhn | district heating network | - |

el | electric | - |

hyd | hydraulic | - |

lat | latitude | - |

long | longitude | - |

r | discount rate | - |

recom | recommended | - |

th | thermal | - |

yr | year | - |

A_{i} | area of component i | m^{2} |

C_{p,v} | volumetric specific heat capacity | MJ/m^{3} |

C_{i} | cost of component i | CHF/kWh |

Ø_{i} | diameter of component i | m |

E_{i} | energy of component i | kWh |

HD | linear heat demand density | kWh/m |

I_{i} | average investment cost of component i | CHF |

I_{i} | investment cost of component i | CHF |

λ | thermal conductivity | W/(m·K) |

LCOH | levelized cost of heat | CHF/kWh_{th} |

LCOS | levelized cost of storage | CHF/kWh_{th} |

L_{i} | length of component i | m |

O&M_{i} | operating and maintenance cost of component i | CHF/year |

Q_{i} | energy quantity of i | kWh |

ρ | density | kg/m^{3} |

sp_{i} | spacing between component i | m |

t | operating years | years |

XX_{#} | X-X heat pump capacity | kW |

## Appendix A

Term | Equation | Unit | Index |
---|---|---|---|

${I}_{bh}$ | $220\xb7{L}_{bh}\xb7{N}_{bh}+3000$ | CHF | (1) |

$O\&{M}_{bh}$ | $0.01\cdot {I}_{bh}+{Q}_{el,hyd}\cdot {C}_{el}$ | CHF/year | (2) |

${I}_{AW<10}$. | $1167\cdot A{W}_{<10}+19618$ | CHF | (3) |

${I}_{10\le AW\le 70}$. | $1167\cdot A{W}_{10\le |\le 70}+19948$ | CHF | (4) |

${\overline{I}}_{AW500}$. | $615\cdot A{W}_{500}$ | CHF | (5) |

${I}_{70<AW<500}$ | ${I}_{AW70}+\frac{\left({\overline{I}}_{AW500}-{I}_{AW70}\right)}{\left(A{W}_{500}-A{W}_{70}\right)}\cdot \left(A{W}_{70<|<500}-A{W}_{70}\right)$. | CHF | (6) |

${I}_{AW>500}$ | ${\overline{I}}_{AW500}+\left[\frac{{\overline{I}}_{AW500}}{A{W}_{500}}-\frac{\left(\frac{{\overline{I}}_{AW500}}{A{W}_{500}}-\frac{{\overline{I}}_{AW10000}}{A{W}_{10000}}\right)}{\left(A{W}_{10000}-A{W}_{500}\right)}\cdot A{W}_{\ge 500}\right]\cdot \left(A{W}_{\ge 500}-A{W}_{500}\right)$ | CHF | (7) |

$O\&{M}_{AW}$ | $0.01\cdot {I}_{AW}+{Q}_{el,AW}\cdot {C}_{el}$ | CHF/year | (8) |

${I}_{WW}$ | $W{W}_{cap}\cdot {\left[-0.229+0.355\cdot W{W}_{cap}^{0.192}\right]}^{\left(-1/0.192\right)}$ | CHF | (9) |

$O\&{M}_{WW}$ | $0.01\cdot {I}_{WW}+{Q}_{el,WW}\cdot {C}_{el}$ | CHF/year | (10) |

${I}_{dhn}$ | $\left[315+2225\cdot \left(0.0486\cdot \mathrm{ln}(HD\right)+0.0007\right]\cdot {L}_{dhn}$ | CHF | (11) |

${L}_{dhn}$ | $\frac{{N}_{MFH}}{2}\cdot \left(\sqrt{{A}_{MFH}}+s{p}_{MFH}\right)$ | m | (12) |

$O\&{M}_{dhn}$ | $0.002295\cdot {Q}_{dhn}$ | CHF | (13) |

Boreholes T _{in} | ${\mathit{I}}_{\mathit{b}\mathit{h}}$ [CHF] | $\mathit{O}\&{\mathit{M}}_{\mathit{b}\mathit{h}}$ [CHF/yr] | ${\mathit{I}}_{\mathit{A}\mathit{W}}$ [CHF] | $\mathit{O}\&{\mathit{M}}_{\mathit{A}\mathit{W}}$ [CHF/yr] | ${\mathit{I}}_{\mathit{W}\mathit{W}}$ [CHF] | $\mathit{O}\&{\mathit{M}}_{\mathit{W}\mathit{W}}$ [CHF/yr] | ${\mathit{I}}_{\mathit{d}\mathit{h}\mathit{n}}$ [CHF] | $\mathit{O}\&{\mathit{M}}_{\mathit{d}\mathit{h}\mathit{n}}$ [CHF/yr] |
---|---|---|---|---|---|---|---|---|

4 @ 40 °C | 135,000 | 3896 | 120,731 | 32,847 | 49,123 | 103,226 | 40,775 | 245 |

4 @ 50 °C | 135,000 | 4133 | 140,311 | 33,043 | 55,706 | 103,292 | 51,991 | 319 |

4 @ 60 °C | 135,000 | 4262 | 159,930 | 33,239 | 61,788 | 103,353 | 64,860 | 390 |

7 @ 40 °C | 234,000 | 6564 | 139,947 | 33,039 | 73,576 | 103,471 | 88,969 | 536 |

7 @ 50 °C | 234,000 | 7171 | 167,811 | 33,318 | 85,739 | 103,592 | 114,983 | 696 |

7 @ 60 °C | 234,000 | 7643 | 196,622 | 33,606 | 98,368 | 103,719 | 144,599 | 872 |

12 @ 40 °C | 399,000 | 7354 | 172,139 | 33,361 | 121,027 | 103,945 | 200,210 | 1208 |

12 @ 50 °C | 399,000 | 8081 | 214,565 | 33,785 | 150,903 | 104,244 | 278,127 | 1681 |

12 @ 60 °C | 399,000 | 8533 | 256,379 | 34,203 | 181,206 | 104,547 | 361,643 | 2190 |

24 @ 40 °C | 795,000 | 15,660 | 217,936 | 33,819 | 200,223 | 104,737 | 417,209 | 2523 |

24 @ 50 °C | 795,000 | 17,033 | 278,536 | 34,425 | 262,728 | 105,362 | 606,402 | 3670 |

24 @ 60 °C | 795,000 | 17,870 | 345,930 | 35,099 | 333,846 | 106,073 | 834,604 | 5058 |

42 @ 40 °C | 1,389,000 | 27,464 | 257,174 | 34,211 | 296,038 | 105,695 | 712,115 | 4310 |

42 @ 50 °C | 1,389,000 | 32,031 | 355,584 | 35,196 | 416,608 | 106,901 | 1,116,367 | 6756 |

42 @ 60 °C | 1,389,000 | 35,326 | 468,221 | 36,322 | 580,494 | 108,540 | 1,702,572 | 10,316 |

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**Figure 1.**Storage capacity and temperature ranges for different sectors according to the VDI [13].

**Figure 2.**Generalized soil temperature with depth and date with no surface coverage at the test site.

**Figure 24.**Comparison of model data and measurements for experimental borehole inlet and outlet temperatures.

**Figure 31.**Specific heat output (top) and annual heat output (bottom) by configuration and charging temperature.

**Table 1.**Soil and material properties [18].

Rock/Soil Type | $\left(\mathit{\lambda}\right)$ W/(m·K) | $\left({\mathit{C}}_{\mathit{p},\mathit{v}}\right)$ MJ/m^{3} | $\left(\mathit{\rho}\right)$ 10^{3} kg/m^{3} | ||
---|---|---|---|---|---|

Recom | Calc | Recom | Calc | ||

clay, dry | 0.4–1.0 | 0.6 | 1.5–1.6 | 1.5 | 1.8–2.0 |

clay, saturated | 0.9–2.3 | 1.4 | 2.0–2.8 | 2.3 | 2.0–2.2 |

sand, dry | 0.3–0.8 | 0.5 | 1.3–1.6 | 1.4 | 1.8–2.2 |

sand, saturated | 1.5–4.0 | 2.3 | 2.2–2.8 | 2.4 | 1.9–2.3 |

gravel/stone, dry | 0.4–0.5 | 0.4 | 1.3–1.6 | 1.4 | 1.8–2.2 |

gravel/stone, saturated | 1.6–2.0 | 1.7 | 2.2–2.6 | 2.3 | 1.9–2.3 |

fixed moraine | 1.7–2.4 | 1.8 | 1.5–2.5 | 2.0 | 1.9–2.5 |

peat | 0.2–0.7 | 0.4 | 0.5–3.8 | 1.6 | 0.5–0.8 |

**Table 2.**Parameter description for heat equation in the underground [19].

Parameter | Description | Unit |
---|---|---|

${T}_{soil}\left(z,{t}_{s}\right)$ | Soil temperature as a function of depth z and time t_{s}. | K |

${T}_{m}\left(z,{t}_{s}\right)$ | Initial temperature at a depth z and time t_{s}. | K |

${T}_{p}\left(\mathrm{lat}/\mathrm{long}\right)$ | Annual amplitude of the monthly average temperature cycle at a given location. | K |

$z$ | Depth under the surface. | m |

$\alpha $ | Thermal diffusivity of soil. | m^{2}/s |

$\omega $ | Frequency of cycle (1 year). | rad/s |

${t}_{s}$ | Time elapsed from Jan 1st. | s |

$\phi $ | Phase shift from Jan 1st for coldest/hottest day of the year. | rad |

Product | Material | ${\mathit{C}}_{\mathit{p}}$ kJ/(kg·K) | $\mathit{\lambda}$ W/(m·K) | $\mathit{\rho}$ kg/m^{3} |
---|---|---|---|---|

pipe | HDPE [20,21] | 1.8 | 0.4 | 960 |

filling material | bentonite [22] | 1.2 | 1.2 | 2000 |

soil | fixed moraine [18] | 0.9 | 2.0 | 2000 |

HTF | water * [23] | 4.195–4.183 | 0.58–0.65 | 998–983 |

Name | Type | Start | End |
---|---|---|---|

H1 | heating | 10 May 2019 | 26 July 2019 |

D1 | drift | 27 July 2019 | 14 October 2019 |

H2 | heating | 15 October 2019 | 24 February 2020 |

D2/R1 | drift/recirculation | 25 February 2020 | 1 June 2020 |

H3 | heating | 2 June 2020 | 18 September 2020 |

**Table 5.**Borehole drilling and installation costs [39].

Geology | up to 100 m | 100–200 m | Installation | Location Equip. |
---|---|---|---|---|

loose rock | 140 CHF/m | 120 CHF/m | 3000 CHF | 30 CHF/m |

rock up to 100 MPa | 180 CHF/m | 170 CHF/m | 3000 CHF | 30 CHF/m |

rock over 100 MPa | 230 CHF/m | 220 CHF/m | 3000 CHF | 30 CHF/m |

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**MDPI and ACS Style**

Beaufait, R.; Villasmil, W.; Ammann, S.; Fischer, L.
Techno-Economic Analysis of a Seasonal Thermal Energy Storage System with 3-Dimensional Horizontally Directed Boreholes. *Thermo* **2022**, *2*, 453-481.
https://doi.org/10.3390/thermo2040030

**AMA Style**

Beaufait R, Villasmil W, Ammann S, Fischer L.
Techno-Economic Analysis of a Seasonal Thermal Energy Storage System with 3-Dimensional Horizontally Directed Boreholes. *Thermo*. 2022; 2(4):453-481.
https://doi.org/10.3390/thermo2040030

**Chicago/Turabian Style**

Beaufait, Robert, Willy Villasmil, Sebastian Ammann, and Ludger Fischer.
2022. "Techno-Economic Analysis of a Seasonal Thermal Energy Storage System with 3-Dimensional Horizontally Directed Boreholes" *Thermo* 2, no. 4: 453-481.
https://doi.org/10.3390/thermo2040030