Geoindex - Shallow geothermal energy

Objectives

The design of a ground source heating and cooling system with geothermal heat pumps (GSHPs) depends on the characteristics of three factors:

• Ground characteristics. Ground thermal conductivity and groundwater flow at the chosen location determine the amount of heat energy that is possible to exchange with the subsoil. Ground characteristics affect the thermal efficiency of the heat exchangers field and influence on their size.
• Geothermal heat pump. The power and operating temperatures of the GSHPs are influenced by the ground characteristics (ground temperature) and at the same time affects the size and the design of the heating and/or cooling system.
• Heating and cooling demand. The heat exchanger size and the GSHP power must consider the demand characteristics (base load and peak loads) of the object to be supplied: the energy in kWh per year and the capacity in kW.

Project status (December 2018)

The Geoíndex - Shallow geothermal energy provides information about ground characteristics and climatic settings for the utilisation of very low temperature (<30 ºC) geothermal resources (<200 m).

The GIS data viewer is designed as an interdisciplinary project that considers the information from diverse disciplines such as geology, geophysics, climatology and hydrogeology and intends to:

1. Introduce shallow low-temperature geothermal energy, its implementation and future status by estimating the shallow low-temperature geothermal potential in Catalonia.
2. Introduce subsoil parameters influencing the design of a ground source heat/cooling system.
3. Provide users with information on the thermal parameters and the estimated temperature distribution in the ground and a preliminary assessment of shallow geothermal potential for vertical closed-loop systems (shortly, an evaluation map of shallow geothermal potential for open loop systems will be published).
4. The Geoíndex - Shallow geothermal energy is published in digital format at 1:50.000 scale and elements are visualised in 2D. Information that is presented have been elaborated using several methodologies and workflows including empirical and analytical models. The results do not take into account details of particular areas, hence deviation from real values due to assumed theoretical models can occur.

For planning, sizing and designing ground source heat/cooling system in a specific location, it is recommended to carry out a detailed study, which includes in-situ data collection and 3D terrain analysis. Specifically, for planning, sizing and designing a closed loop system it is recommended to follow procedures defined in the Spanish norm UNE 100715-1 guidelines: “Guide for the design, implementation and monitoring shallow geothermal systems. Part 1: Vertical Closed Loop Systems” (UNE 100715-1 “Diseño, ejecución y seguimiento de una instalación geotérmica somera. Parte 1: Sistemas de circuito cerrado vertical”. May, 2014).

Structure

Currently, the GIS data viewer contains 29 information layers divided into 10 thematic groups. 

Assessment of shallow geothermal potential

The shallow geothermal potential dataset (for closed loop systems) allows estimating the number and the length of geothermal Borehole Heat Exchangers (BHEs), which makes also possible to estimate the total cost of investment in specific areas.

Two information layers are included:

• The geothermal potential expressed as the amount of energy [MWh/yr] that can be extracted from the ground during the heating season considering a BHE of 100 m deep from ground surface, a diameter of 150 mm and a thermal resistance (Rb) of 0.095 mK/W.

• The geothermal potential expressed as a power [kW].

G.POT (Geothermal POTential) calculation method was used. It allows estimating the maximum quantity of heat that can be sustainably exchanged by a Borehole Heat Exchanger during a heating or cooling season. (Casasso, A., 2016), developed by DIATI (DIATI – Politecnico di Torino, Italy).

Shallow geothermal installations

The ICGC has compiled the available information provided by ICAEN and consulted other sources of public access information on the existence of surface geothermal installations in Catalonia. The information collected has been incorporated into the Surface Geothermal Facilities Database (BdIGSCat).

Update (December 2018). The GIS data viewer shows two layers:

• Map of locations of installations managed, funded and/or operated by the public sector. Most of these installations correspond to buildings and facilities of local or regional administrations. Depending on the available data, it is possible to consult the installed GSHP capacity and length of installed BHEs in each location.

• Map representing the number of installations by municipality corresponding to the private sector. This layer displays the minimum number of installed facilities and total minimum power generated in each municipality, according to the available information.

Difficulty of drilling

The geological characteristics influence the difficulty of drilling, and this determine the overall cost of installation of ground source heat/cooling systems. The GIS data viewer provides information on the complexity to drill a vertical borehole (down to 100 m) at a given location taking into account the geotechnical estimated characteristics and the possible occurrence of karstification.

The following layers of information are included:

• Map of drilling complexity.
• Areas of possible presence of karstification.

Superficial soil information (for horizontal close loop systems)

Information about soil is necessary when a horizontal ground source heat/cooling system is designed. Soil thermal properties and soil thickness influence the overall performance of horizontal ground source systems.

The following layers of information are included:

• Map of the superficial soil depth (m).
• Map of the superficial soil thermal conductivity (W/mK).

Hydrogeological information

The GIS data viewer shows the location of special (saline, carbonated or sulphuric) and/or thermal spring waters or other shallow hydrothermal manifestations. Act 22/1973 of July 21st of Mines, water is considered ‘thermal’ when presents a temperature 4 ºC higher than the mean annual air temperature. In many locations where thermal/special water occurs, water is utilised for balneology. The existence of special waters near installed ground source heating and cooling systems can influences its efficiency.

The following layer of information is included:

• Map of special and/or thermal waters.

Thermal properties of the ground

Thermal properties influence how heat is extracted from the ground. The following parameters have to be taken into account:

- Thermal conductivity (W/mK). The capacity of a material to transmit heat. The value of thermal conductivity depends on lithology, porosity, water saturation and mineralogical composition of the bedrock.

- Thermal diffusivity (mm²/s) defines the rate of thermal conductivity divided by density and specific heat capacity of the material at constant pressure.

- Volumetric heat capacity (MJ/m³K) is defined as the amount of heat that is needed to rise temperature of rock/soil unit by 1 ºK.

The following layers of information are included:

• Surface thermal conductivity map (W/mK).
• Surface thermal diffusivity map (mm²/s).
• Surface volumetric thermal capacity map (MJ/m³K).
• Vertical distribution of thermal parameters.

Surface temperatures and intra-seasonal oscillations

Thermal stability of the subsoil is one of the key aspects of the shallow very low-temperature geothermal resources. In general, ground temperature between 5 and 15 m depth equals to average air temperature at the particular location. The GIS viewer contains information about average air temperature (annual, the coldest and the warmest months).

The following information layers are displayed:

• Surface annual average temperature (°C).
• Average surface temperature of the coldest month (°C).
• Average surface temperature of the warmest month (°C).
• Surface thermal semi-amplitude (°C).

These layers were prepared by the GRUMETS Research Group belonging to the Centre de Recerca Ecològica i Aplicacions Forestals (CREAF) and in the Universitat Autònoma de Barcelona (UAB); in collaboration with the ICGC.

Climate severity

To define energy demands of a building and the shallow geothermal potential in a specific location, it is necessary to know climatic setting in the area. Following parameters are taken into account when energy demand is defined:

• Heating Degree Days - HDD: a parameter that defines how much energy is needed to heat up the building. It is expressed by the number of degrees when day´s average temperature is below the reference temperature (15 ºC were used as a reference). It is assumed that when average temperature is below 15 ºC buildings require heating. HDD is expressed as ºC*day/year.

• Cooling Degree Days – CDD: a parameter that defines how much energy is needed to cool down the building. It is expressed by the number of degrees when day´s average temperature is above the reference temperature (23 ºC were used as a reference). It is assumed that when average temperature is above 23ºC buildings require cooling. CDD is expressed as ºC*day/year.

• Duration of heating season: average total number of days in a year when the average air temperature has been lower than the threshold temperature established by the interior of the building from which it is planned to use heating. It is expressed in days and the prefixed threshold temperature to calculate this layer was 15 ºC.

• Duration of cooling season: average total number of days in a year when the average air temperature has been higher than the threshold temperature established by the interior of the building from which it is planned to use cooling. It is expressed in days and the prefixed threshold temperature to calculate this layer was 23 ºC.

The information was generated by the Meteorological Service of Catalonia with the collaboration of the ICGC, based on data collected between 2013 and 2018.

Shallow ground temperature

In the shallow ground, the temperature fluctuates daily and seasonally due to influence of air temperature changes. If horizontal ground source heat/cooling system is planned to be installed, it is necessary to measure temperature changes in the shallow ground in order to determine a correct size of the heat exchangers.

The following information layers are displayed:

• Minimum temperature at 1.5 m depth (°C).
• Maximum temperature at 1.5 m depth (°C).
• Thermal amplitude at 1.5 m depth (°C).
• Depth where the thermal amplitude tend is 0 °C (m).

Ground temperature (at various depths)

Heat exchange capacity between the ground and BHEs depends on thermal properties and its temperature. The GIS viewer shows data of vertical temperature profiles in the ground collected from different monitoring water wells. Three maps with theoretical ground temperate at different depths (50, 100 and 150 m) are displayed as well. These layers were calculated based on an empirical approach considering the theoretical thermal gradient in Catalonia based on heat flow, the thermal bedrock conductivity and in-situ measurements. The GIS viewer provides following vertical subsoil temperature profiles:

• Ground temperature at 50 m depth (°C).
• Ground temperature at 100 m depth (°C).
• Ground temperature at 150 m depth (°C).

References

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