Geothermal energy as an alternative energy for underground transport

The use of underground geothermal energy, as shown through the practical demonstration in the installations at Pacifico station of the Madrid Metro, makes it possible to heat and cool underground spaces, reducing energy consumption, CO2 emissions and machine maintenance by more than 40%.

We can begin by saying that tunnels, by their very configuration, already act as geothermal exchangers; temperatures in tunnels would remain uniform with small variations year round, if they were not altered by the heat load generated inside them by train traffic, electrical and electronic equipment, lighting, and passengers.

This heat can be extracted – and part of it is during certain times of the year – using a system of forced ventilation. However, during periods of cold with temperatures close to 0ºC, or during summer with temperatures above 25ºC, it is not possible to keep the temperature within the minimum comfort ranges for passengers and workers who carry out their activities in these spaces using these systems; the same situation occurs with the operation of the electrical and electronic installations that are required to control the station.

This situation is aggravated in the case of metro stations that contain commercial or office spaces for workers, due to the heat load that they contribute inside the station.

Metro de Madrid is very concerned with matters of energy efficiency and environmental sustainability, so in response to the need to provide heating and cooling for the new offices and commercial spaces to be constructed in the Pacifico station, the possibility was considered whether to use traditional methods or to apply low-enthalpy geothermal energy taking advantage of the excavation of a plot of land for an electrical substation. The latter solution was chosen, correctly in my opinion, and several experts have affirmed that it is the most efficient, ecological, and economically viable system to achieve thermal comfort.

The European Geothermal Energy Council (EGEC) defines geothermal energy as ‘Energy stored in the form of heat below the Earth’s surface’. This definition includes the heat stored in the subsoil and underground water, regardless of their temperature.

Though the purpose of this article is not to quantify or evaluate all of the enormous potential offered by geothermal energy and the different ways in which it can be used depending on the thermal gradient, which increases progressively as a function of depth with reference to the Earth’s surface, Table 1 is helpful in showing the different classifications and most frequent applications.

The variation of temperature with depth is an important parameter in the dimensioning of the earth heat exchanger. This parameter is influenced by different variables such as the physical properties of the ground, the surface covering of the study area, the presence of water-bearing layers, and climate interaction.

Different studies done on the daily and annual variation of the ground show that temperature remains virtually constant all year round below a certain depth. For the purposes of analysis, the ground is divided into four different zones:

• Surface: to a depth of 1m, where the temperature of the ground is very sensitive to environmental changes
• Shallow: which extends down to 15m; this zone is influenced by seasonal climate changes
• Deep zone: which extends down to a depth of 70m, where the variation of temperature with depth remains constant year round
• Depths of more than 70m: where temperature increases with depth with an average gradient of 0.03°C/m.

For the purposes of heating and cooling, what is really of interest is the constant temperature of the subsoil in the ranges described above, with the possibility of reaching depths of 200m thanks to the null variability in relation to external conditions.

It is precisely this fact that makes the use of low-enthalpy geothermal energy interesting. Low-enthalpy geothermal energy can be defined as the energy of a thermodynamic system that can be exchanged with the environment; in other words, the system acts as an energy accumulator that stores or releases depending on the needs of the system. Since the temperature of the ground is virtually constant, the exchange temperature in the subsoil circuit and heat pump is constant as well, which means that the system’s performance is maintained throughout the entire cycle.

The heating and cooling project for the Pacifico metro station, which is located south of the centre of Madrid in the hydrographic river basin of the Tagus River in the Madrid–Talavera hydrological unit, where the hydrogeology consists of porous formations and water tables between 50 and 60m, which indicates ground with thermal conductivity levels that are very favourable for this type of operation1.

As an initial measure, the project development required the analysis of the ground to confirm that the geological data corresponded to the data provided by the Technological GeoMining Institute of Spain (1997). To do this, a test boring was done in the zone to determine the following parameters: soil temperature, variation as a function of depth, thermal conductivity of the ground, heat capacity, water tables, morphology, temperature of fluid to be circulated through pipes, and type of thermal collector material to use. Although some of the obtained values were worse than expected and underground water was not encountered at 145m, the use of a vertical geothermal groundcoupled exchanger or collector was considered to be the best option. This was verified to be a viable option in terms of execution and the energy to be obtained, if the 840m2 of available horizontal surface area were used, would allow the heating and cooling to be expanded to the platforms of the Pacifico station on Line 1 and to heat the station entry accesses and commercial spaces during cold periods by forcing hot air to maintain the desired comfort level.

After the distribution of the thermal demands for the spaces to be heated and cooled were determined, the ground exchanger was sized to maintain the thermal equilibrium of the ground and to be able to supply a cooling power of 120 kW with an energy input of 250 MWht in cooling.

Since cooling demands are greater than heating demands, mainly in the months of July and August and to avoid overheating the ground, the geothermal exchanger was oversized, drilling 32 boreholes 150m deep, which is the equivalent of 4,500m of piping distributed in an earth mass of more than 120,000m3, ensuring the supply of the required energy.

A U-shaped polyethylene pipe 40mm in diameter and 3.7mm thick was inserted into each one of these boreholes, 119mm in diameter. All of the pipes are connected to a common manifold, forming eight groups (four boreholes per group, according to the Tichelmann system). The free space between the borehole and polyethylene pipe was filled with cement-bentonite specially designed for these applications.

The installed exchanger allows the operation of three 40 kW heat pumps that provide a cooling power of 120 kW, of which 90 kW are provided by the ground. In other words, for every kW that is consumed mechanically by the pumps, 4 kW of cooling power are generated.

This performance assessment or Energy Efficiency Rating is equal to four (EER = 4) and refers solely to the heat pump in cooling operations. The expected performance of the entire installation would be significantly greater over the course of the year, mainly in inter – mediate seasons such as spring and autumn, and even on some winter days when the outside temperature is not very low or heat does not have to be added to the system; in these cases, cooling is provided at minimal cost, driving the circulation water through the manifolds without requiring the operation of the heat production machines. This method of operation promotes the thermal equilibrium of the ground over the course of the year.

This installation was put into operation provisionally to provide service to commercial and office spaces at the end of June 2009, obtaining very satisfactory results. In October, the heating installation was completed and put into operation. Since then, the thousands of passengers who pass through the station every day have enjoyed a more comfortable temperature.

The infrastructure for this type of installation requires significant investment, but we feel that it is highly efficient when compared with other heating and cooling systems. We can confirm that energy consumption has been reduced by 75%, with CO2 emissions reduced by the same amount, and machine maintenance costs by 40%. In addition to this quantifiable data, the environmental benefits and elimination of noise generated by traditional systems must also be taken into account.

In our opinion, this is an innovative project that achieves a high level of energy efficiency and can be applied to underground public transportation installations to increase the comfort levels of their different spaces.

Reference

1. http://www.girodgeotermia.com/images/ stories/temp profundidad.jpg

About the author

Manuel Bravo Puente

Manuel Bravo Puente is an Industrial Engineer and has been the Coordinator of Energy Installations in Civil Works for Metro de Madrid since 2004. He has also been in charge of maintenance of high and lowvoltage installations, tunnel ventilation, water pumping and catenary maintenance.