Renewable Energy and Environmental Protection in Mexico

Geothermal power is a highly reliable source of clean, renewable power.  This is a feature that utilities such as CFE, looking to increase their share of renewables in the energy mix while maintaining secure supply, already find attractive.  Despite the relatively high installation costs for a direct-cycle geothermal power plant such as Los Humeros in Mexico, the high capacity factor in operation results in a cost of electricity in the region of US$58 to US$93/MWh, even when taking into account the cost of drilling the wells and building the steam collection system.  This makes geothermal comfortably one of the most cost-effective forms of generation available today.[1]

Mexico's Comisión Federal de Electricidad is taking advantage of its geothermal resources.  In a market that stood at around 9.7 GW worldwide in 2007, Mexico is the fourth largest geothermal energy producer after the US, the Philippines and Indonesia. CFE operates a total installed capacity of 58.2 GW, of which 964.5 MW is geothermal; a figure that is likely to increase still further.  In May and December 2009, CFE awarded engineering firm Alstom contracts to build two 25 MW power plants in the geothermal field located in the Puebla state of Mexico, 225 km southeast of Mexico City.

When they begin operation, the two plants-known as Los Humeros II Units 9 and 10-will produce more than 400 GWh annually of reliable, clean electricity, enough to power 100,000 homes in Puebla state.  Under terms of the contract, Alstom will supply the complete engineering, procurement and construction (EPC) services for the power plants including the steam turbine, air cooled turbo-generator, turbine control and distributed control system.

Mexico Starts the Wave

Mexico is an important and growing market for power of all types, including renewables.  In the past, CFE contracted Alstom for several geothermal plants including four 25 MW units at the Los Azufres geothermal power plant in Michoacan state in 2000 and the supply of two 5 MW units to the Las Tres Virgenes geothermal power plant in Baja California Sur in 1998.  The 50 MW from Los Humeros II units 9 and 10 will add to the existing 40 MW produced from eight plants, each of 5 MW, installed at the Los Humeros field 10 years ago.[2]

Los Humeros is one of three geothermal fields under exploitation in Mexico.  It is located in the central-eastern portion of the country, within the Mexican Volcanic Belt, near the Gulf of Mexico.  The other two are Cerro Prieto – the largest, with a capacity of about 700 MW - in the state of Baja California, and Los Azufres in Michoacan state. CFE is currently exploring what further potential there may be in the Los Humeros field.  If there is sufficient reservoir potential, it is expected that the company will extend the capacity in the area up until 2016.  The new power plant is being built at a capital cost of close to US$110 million.

Geothermal resources are concentrated near tectonic plate boundaries where the Earth's crust is thinner.  Power is generated using hot water and steam found in layers of impermeable rock near these tectonic boundaries.  Using drilling technology developed by the oil industry or natural conduits such as geysers, geothermal plants can access this heated water and use it to power a steam turbine to generate electricity. The most common type of geothermal generation in operation today is a flash steam plant, which pulls deep, high-pressure hot water into lower-pressure tanks and uses the resulting flashed steam to drive turbines.  Flash steam plants require fluid temperatures of at least 150 degrees c and are the type used at Los Humeros.

Los Humeros II uses the geothermal steam extracted from wells drilled to a depth of at least 1,500 meters.  CFE has overall responsibility for drilling these wells and installing the pipes that carry the steam of a specified pressure and mass-flow to the boundary of the power plant.  From there, Alstom has full responsibility for supplying all equipment, building and plant commissioning.  Geothermal power plants have their own specific design and construction challenges when compared to conventional steam plant.

Geothermal steam can be aggressive.  It contains around 3 percent gas, about 90 percent of which is carbon dioxide (CO2), 2 to 3 percent hydrogen sulphide (H2S), plus small amounts of ammonia, which is corrosive to the steam turbine and pipework.  This means conventional steel cannot be used in parts of the plant that are exposed to steam, so a large proportion of the steel used in the Los Humeros II plants, particularly for pipework and the condenser, is stainless steel.

Los Humeros II has a hybrid gas extraction system consisting of ejectors and vacuum pumps to remove geothermal gases and prevent them from building up inside the condenser and reducing performance.  The condenser is a direct contact model, specially designed for geothermal applications.  The top exhaust from the turbine has a crossover going into the top of the unit.  Unlike conventional plants, the condenser is almost entirely made from stainless steel.  The condenser, supplied by SPX-Yuba from the United States, uses technology licensed from the US government that was co-developed by Alstom in the late 1990s.  Cooling is provided by standard wooden and fibreglass counterflow cooling towers with PVC packing.[3] The other challenge is that no two geothermal sites are the same.  In a conventional plant it is straightforward to control the size of the boiler, allowing the size of the turbine to be accurately determined.

A significant portion of the components for the Los Humeros II generator were procured from local sub-suppliers.  Approximately 50 percent of all the components and materials for the steam turbine, valves and auxiliaries were sourced from a Mexican supply chain.  In addition, the cooling towers, transformers and switchyard were also all sourced from local companies. With Mother Nature providing the steam, there is no boiler to be commissioned for a geothermal power generator, so the entire commissioning process is shorter than for a conventional steam plant or combined cycle unit.  During operation, the system’s turbine controller guides each steam turbine.  Both units are placed in a single powerhouse and the entire power plant is controlled using Alstom's ALSPA Series 6 distributed control system.

Besides the advances in geothermal technology achieved at Los Humeros II, Mexico is also home to the largest geothermal power station in the world – the Cerro Prieto Geothermal generation plant – with an installed capacity of around 820 MW, across five different units.  This project demonstrates Mexico’s commitment to the technology and its determination to be at the forefront of clean, renewable energy generation.[4]

[1] Luís-Jaime Martinez Toledo, Geothermal Turning Up the Heat at Los Humeros, 16 February 2011,

[2] Luís-Jaime Martinez Toledo, Geothermal Turning Up the Heat at Los Humeros, 16 February 2011,

[3] Luís-Jaime Martinez Toledo, Geothermal Turning Up the Heat at Los Humeros, 16 February 2011,

[4] Luís-Jaime Martinez Toledo, Geothermal Turning Up the Heat at Los Humeros, 16 February 2011,