The role of mass flow meters in experiments to simulate the geothermal energy generation process

November 14, 2017 Samuel Neeser
Earthquake Seismograph

Finding new energy sources is a hot topic in today’s society. In Switzerland, I love the scenery we have with beautiful mountains everywhere you look. Wouldn’t it be great if we could use these mountains in a way they can help us creating our own clean energy?

In fact, the Swiss Grimsel Rock Laboratory (owned by NAGRA, ETH Zürich) investigates the possibilities to use the earth’s heat as a energy source, Geothermal energy.

Using Geothermal energy to warm up your power plant?

Electricity production in Switzerland today can be divided in 3 main sources:

  • Hydropower plants
  • Nuclear power plants
  • Conventional thermal power plants and other plants

About 30% of the electricity production comes from nuclear power plants. How nice would it be to reduce this even more into a more clean energy?

Geothermal energy seems to be a good replacement for a part of the currently used nuclear energy here. The idea itself is quite simple: pump cold water into the earth’s crust, allow it to be heated by earth heat, and pump up the hot water to exploit its heat, for example in a power plant.

However, there is catch. At a depth of 4 to 5 kilometres, when the injected water is heated up to 200 °C, it will expand inside the porous rocks. The permeability of the rock is low and need to be enhanced through high pressure fluid injections. This pressure increase can cause induced seismic events.
At the Grimsel Rock Laboratory it is investigated under which conditions such induced earthquakes occur and how the magnitude of such earthquakes can be reduced to be not felt at the earth surface.

grimsel test site

Why use Mass flow controllers?

Bronkhorst mass flow controllers are used in their experiments to simulate the geothermal energy generation process by controlled supply of water flows into the subterranean rocks. Mass flow controllers are required to accurately inject the desired amounts of water into the rocks, at the right pressure.

To investigate which water flow rate will induce a certain activity inside the rocks, the devices should be able to cover a large range of water flow rates.

A series of hydraulic tests such as:

  • pulse injection
  • constant rate
  • constant headcyclic water injections
are conducted to determine the hydraulic properties of the rock mass and to monitor its influence (i.e. the pressure response) within boreholes in close vicinity to the injection point. As rocks with low permeability are part of the investigation, very small amounts of water at very small rates have to be injected over very long times.

Which mass flow controllers were used?

The Bronkhorst solution consists of three different Coriolis mass flow controllers mounted on a flow board, including control and monitoring equipment. The mass flow controllers (MFC’s) were used to control pure water.

To investigate the influence of the low flow rate, many different flow rates of pure water have to be used as input parameter, with only a small number of devices.

Grimsel test site

With the used flow board, each of the three devices can be selected for the appropriate flow rate. Coriolis instruments are used here because of their high accuracy, and because they are able to directly supply a certain mass of water regardless of process conditions, such as ambient temperature and pressure.

Furthermore, water properties such as its temperature and density can be read in real-time. The maximum temperature of the water used at the Grimsel Rock Laboratory, which is located at a depth of 400 to 500 meters, is 40°C (only during the thermal tests).

This setup is a robust, reliable, flexible, compact and easy-to-use way to control the water supply. To track their experiments, the researchers from ETH Zürich use Bronkhorst software, including FlowPlot to make a plot of the entire experiment.

Furthermore, they have the possibility using TeamViewer to control, view and monitor the setup at Grimsel from a remote location, so they do not have to be at the test site the entire period of time.

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