The heat under our feet…a look at geothermal power

The Sonoma Calpine 3 geothermal power plant in The Geysers field in the Mayacamas Mountains of Somona County, Northern California. Photo: Stepheng3, Wikimedia Commons

Soaring oil and gas prices, war in Europe, geopolitical tensions, bottlenecks in supply chains, stricter environmental regulations and climate change…the pressures on the energy industry seem to grow by the week, testing every link in the supply chain. The silver lining is that these factors are also speeding up the energy transition. Geothermal power could play a leading role.

By Joanne McIntyre

Geothermal energy has successfully been exploited by humans through the ages; archaeological evidence suggests that the earliest direct use of geothermal power occurred at least 10,000 years ago in North America1. The first geothermal electric power generation took place in Laderello, Italy in 1904. Since then, countries in areas with high thermal activity such as New Zealand and the US have rapidly advanced this technology (see box ‘Geothermal power technologies’).
In a recent report, the Geosmart consortium stated that geothermal plants collectively have the highest capacity factor of any electricity generation technology – renewable or not. As market incentives evolve, geothermal power plant operators are likely to integrate the value of fl exibility in their business models3.

Great flexibility

Geothermal power plants are increasingly being designed around the capacity to provide flexible services. Plants in Germany have shown that output can be ramped up or down by 70% in a matter of seconds to comply with balancing requirements. Moreover, geothermal technologies provide many different types of benefits to the electricity system beyond the simple operation of a fl exible electricity generator. Geothermal Combined Heat & Power (CHP) plants can optimise the supply of heat or the storage in a district heating network, and thermal storage can also be a solution for dealing with electricity market variability and seasonality4.

Challenging environment

While geothermal production wells are drilled to various depths, the trend in the industry is to go deeper for increased efficiency. As temperature and pressure increase, this puts higher demands on the materials used. Corrosive conditions can be severe, with high chloride and H2S contents and low pH, calling for the use of higher alloyed corrosion-resistant materials4.

Furthermore, geothermal plants are often exposed to problems that affect performance. According to the trans-European PERFORM project, typical problems are mineral scaling, clogging by particles, corrosion and the temperature or stress related effects of geothermal flow and injectivity infrastructure5.

The changes in conditions that geothermal fluid is subjected to travelling from subsurface to the surface and then being re-injected back to the aquifer, lead to changes in the brine properties and the potential risk of mineral deposition and scaling. Scaling precipitation could impact the production and injection by increasing the hydraulic resistance, i.e. the resistance to flow. Additionally, it can reduce the heat transfer rate in the heat exchangers and reduce the overall COP (Coefficient of performance) of the system5.

Geothermal power technologies2

Binary Cycle Power Plants – The vast majority of geothermal plants built in the last few decades. Hydrothermal reservoir water is used to heat another working fluid with a much lower boiling point. The hot water is reinjected back into the reservoir with the heated working fluid becoming vaporized, thereby generating steam that spins a generator turbine.
Enhanced Geothermal Systems (EGS) – Make use of very deep reservoirs which are often not permeable enough for hot water or steam to flow up to the surface. EGS inject fluids into the ground at high speeds to fracture the deep rock, permitting flowing channels to form and water to reach more accessible depths and deposits.
Flash Steam Power Plants – The most common geothermal power plants; More energy-efficient than dry steam plants, the technology relies on highly-pressurised reservoirs where the water exceeds 182°C (360°F). Pressure pushes superheated water to the surface, where it enters a tank at a much lower pressure than that which exists underground. Once in the tank, the lower pressure causes some of the water to be “flashed,” or vaporized, generating steam that turns a turbine. Excess water is injected back into the hydrothermal reservoir for later use Dry Steam Geothermal Plants – The original power plant systems which use the steam provided by a hydrothermal reservoir to directly spin generator turbines.

Table 1. Well materials – CRA approach7 Source: Wood Group - Corrosion Review and Materials Selection for Geothermal Wells, 2017

Selecting the right materials

Selecting the right materials is an important prerequisite to minimise scaling and corrosion. A Wood Group report released in 2017 explained that material selection for geothermal wells is typically based on choosing the casing material and the tubing material first as these are generally exposed to the most critical service environments, and then selecting compatible materials for the remainder of the wellbore equipment and surface facilities7.

“The Geysers (California) is the world’s biggest geothermal complex with 22 geothermal plants drawing steam from 350 wells.

There are two main technically acceptable options; carbon steel with inhibition treatment or corrosion resistant materials. For example, the minimum suitable corrosion resistant material in the Dutch geothermal conditions is 13Cr martensitic stainless steel. If following the carbon steel option, corrosion resistant materials are still necessary for some critical components. Table 1 gives minimum recommended materials and alternate materials for corrosion resistant alloys.7

Figure 1. Top countries producing geothermal power8

References

1. Alberta Culture & Tourism www.history.alberta.ca

2. Conserve Future Energy https://www. conserve-energy-future.com/

3. Geosmart https://www. geosmartproject.eu/

4. Sandvik Materials Technology www. materials.sandvik/en/

5. PERFORM https://www. geothermperform.eu/

6. Corrosion of Materials Used in Geothermal Power Production, A Coleman, 2016

7. Wood Group: Corrosion Review and Materials Selection for Geothermal Wells, 2017

8. NS Energy www.nsenergybusiness.com/

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