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 July 2007
Feature
PAUL N. HIRTZ is president of Thermochem, Inc., and has worked as a chemical engineer for the last 25 years in the geothermal energy industry. He performs research for the U.S. Department of Energy and the California Energy Commission, and has published over 25 technical articles. He is an associate editor for the international journal Geothermics. Hirtz is chairman of ASTM Subcommittee E44.15.
GEOTHERMAL ENERGY has the potential to be the world’s primary source of baseload renewable power. Unlike most other renewables, geothermal energy is available 24 hours a day and has a capacity factor on the order of 95 percent (1,000 MWe installed = 950 MWe average generation), compared to a non-baseload source such as wind energy with a capacity factor of 20 percent or less (1,000 MWe installed ≤ 200 MWe average generation).

A recent report by the Massachusetts Institute of Technology (MIT) and sponsored by the U.S. Department of Energy has concluded that with a reasonable R&D investment in EGS technology, geothermal energy from EGS alone could provide 100,000 MWe of cost-competitive power for the United States within the next 50 years.4 This is equivalent to our total capacity of nuclear power generation now. Another 11,000 MWe could be generated from low-temperature (≤100 ºC) water that is co-produced from oil and gas wells in the United States using “off-the-shelf” binary-cycle modular power plants.

The Hot Renewable: Geothermal Energy

Renewable geothermal energy is currently used to generate electric power in 24 countries, for a total of 9,000 MWe.1 Geothermal energy is produced and utilized in many forms. The definition of geothermal energy, according to ASTM E 957, Standard Terminology Relating to Geothermal Energy, is quite broad: “the thermal energy contained in the rocks and fluids of the earth.”

This energy is produced in the form of naturally occurring hot water and steam found in hydrothermal reservoirs that drive electric power plants. The fluids are also used directly in industrial processes and to heat buildings. Geothermal energy can be extracted from deep man-made reservoirs using technology known as enhanced geothermal systems. Ubiquitous low-grade geothermal energy is used by geothermal heat pumps to heat and cool buildings (7,300 MWt in the United States alone2).

Hydrothermal systems are responsible for all geothermal electric power generated today, as they are the easiest to develop, and even still only account for a small fraction of the total potential for this clean energy source. A hydrothermal system is a subterranean reservoir that transfers heat energy upward by the vertical circulation of fluids through convection (Figure 1). The surface manifestations of these natural systems are the familiar hot springs and fumaroles (think Yellowstone National Park for an example of perhaps the world’s largest — but closed to development).

The power plants driven by hydrothermal systems typically resemble conventional thermal power plants where steam turbines are used to generate electricity (Figure 2). Remember that even a nuclear plant just boils water to make steam, which then drives the turbine. Most geothermal plants can use steam directly derived from the resource. Binary cycle plants use a secondary fluid to extract heat and generate power from lower-temperature hydrothermal resources (<150 °C), and combined cycle steam turbine/binary plants are now being used to efficiently generate power from higher-temperature resources (~200 – 350 °C) in a cascading extraction process.

Enhanced geothermal systems, or EGS, are defined as engineered reservoirs created to extract heat from hot, dry rock. The energy is extracted using water pumped through a man-made subsurface fracture system, where it is heated by contact with rock and returned to the surface through production wells. Hydrofracturing, used widely in the oil and gas industry to enhance production, creates the EGS fracture network. Figure 3 shows the extent of domestic geothermal resources at a depth of just 6 km, the nominal drilling depth for oil and gas wells. At depths of 10 km, the practical upper limit of current drilling technology, the temperatures are much higher across the entire country.

On the less extreme side of geothermal engineering, geothermal heat pump systems, or GHP, use the year-round stable temperature of the earth at depths of a few metres to the depth of a typical water well. Here the temperature will be a moderate 10 to 20 °C, even with heavy snow on the ground above. This heat is extracted and condensed using a closed-loop heat pump to deliver balmy temperatures to a building. During the summer, the ground serves as the heat sink for the GHP to provide air conditioning.

Dual-action GHP systems supply both heating and cooling simultaneously, as needed by supermarkets and ice rinks. Geothermal heat pumps are used in all 50 states today, with about 80,000 systems being added each year. Given this rapid growth, the GHP industry is in need of standardizing practices for the installation, design and performance specifications for these systems, as noted recently in an energy journal.3

Contributions by E44.15 to Geothermal Energy

Subcommittee E44.15 on Geothermal Field Development, Utilization and Materials, part of Committee E44 on Solar, Geothermal and Other Alternative Energy Sources, has developed standards to provide consistent terminology, evaluate the quality of geothermal resources, determine material compatibility for geothermal hardware and define the performance of power conversion technologies. The subcommittee’s current standards are:

E 947, Specification for Sampling Single-Phase Geothermal Liquid or Steam for Purposes of Chemical Analysis;
E 957, Terminology Relating to Geothermal Energy;
E 974, Guide for Specifying Thermal Performance of Geothermal Power Systems;
E 1008, Practice for Installation, Inspection, and Maintenance of Valve-Body Pressure-Relief Methods for Geothermal and Other High-Temperature Liquid Applications;
E 1068, Test Method for Testing Nonmetallic Seal Materials by Immersion in a Simulated Geothermal Test Fluid;
E 1069, Test Method for Testing Polymeric Seal Materials for Geothermal and/or High Temperature Service Under Sealing Stress; and
E 1675, Practice for Sampling Two-Phase Geothermal Fluid for Purposes of Chemical Analysis.

Standard Practice E 1675 for Two-Phase Sampling

The most widely used Subcommittee E44.15 standard today is E 1675. This standard is used in 17 countries to obtain representative samples of two-phase fluids (water and steam) produced from hydrothermal wells. The proper collection and preservation of samples are specified for subsequent chemical analysis of the water, brine, condensate and noncondensable gases that may be produced by these wells. The chemical composition data is used in many applications important to geothermal energy exploration, development and resource management. These applications include determining reservoir temperatures and the origin of reservoir fluids, the source of recharge fluids, and the compatibility of fluids with piping and steam turbines (corrosivity and scale deposition).

The heart of E 1675 is the two-phase sampling separator (Figure 4). This cyclone device separates steam from liquid with minimal heat loss and pressure drop (isenthalpic and isobaric process) from the main pipeline through which the two-phase fluids are produced, such as the production pipeline from a geothermal well. Although it is virtually impossible to obtain a representative mixture of the two-phase fluids in the same proportions (same steam-to-liquid ratio) as they exist in the bulk flow through the pipeline, representative samples of each phase can be collected without changing the chemical composition.

View Figure 5 and Figure 6.

To aid in the separation process, two separators are often used on the top and bottom of the pipeline (Figure 7). The two-phase flow regime usually approaches slug or stratified flow in the large horizontal pipelines on the surface, known as the gathering system through which fluids are collected from each well and piped to the power plant.

Using fluid samples collected according to E 1675 and the total fluid enthalpy, the composition of the original reservoir fluid can be reconstructed. In most cases, this means the “pre-flash” fluid composition — the original deep hydrothermal reservoir liquid before it boiled on the way up the production well. The composition of the deep geothermal fluids can be used to evaluate the ability of a reservoir to sustain a large power plant.

The geochemical interpretation includes geothermometry, where the concentrations of species such as silica, or the ratio of ions such as Na+, K+ and Ca++, determine the temperature of the deep fluid. Silica dissolves rapidly from rocks into hot water and equilibrates with the mineral quartz in the temperature range of about 200 to 330 °C, yielding a very precise and “recent” reservoir temperature. This temperature can be used to determine the total heat content (enthalpy) of the produced fluid if the reservoir is single-phase liquid.

Other minerals that leave a signature in the water, in the form of dissolved Na+, K+ and Ca++, have a long-term “memory” and indicate the maximum temperature the fluid had been exposed to. This is important in geothermal exploration where the primary resource may not have been reached yet, and peripheral well chemistry is used to determine if a high-temperature resource exists nearby.

Use of E 1675 in Two-Phase Flow and Enthalpy Measurement

To obtain the full benefit of chemical composition data, physical data related to the two-phase discharge is required, such as the total fluid enthalpy and pressure or temperature at the sample point. The most widely used method to determine the total enthalpy and production flow rate of two-phase geothermal wells, the tracer flow test method, specifies the use of E 1675. The TFT method is based on the precise and constant injection of chemical tracers into two-phase geothermal fluid streams to determine the flow rate of each phase and thus the total enthalpy.

The process involves injection of liquid and vapor tracers into a two-phase pipeline, with concurrent sampling of each phase according to E 1675 downstream of the injection point, where the tracers have fully dispersed into their respective phases (Figure 8). The mass flow rate of each phase is calculated based on the measured concentration and injection rate of each tracer.

The mass rate of liquid (QL) and steam (QV) is given by:
QL,V = QT / CT

where:

QL,V = mass rate of fluid (liquid or steam phase);
QT = tracer injection mass rate (liquid or vapor tracer); and
CT = tracer concentration by weight (liquid or vapor tracer).
The total fluid enthalpy is then calculated from a heat and mass balance equation using the known enthalpies of liquid water and steam at the sampling pressure.

The production flow rate and enthalpy of geothermal wells is critical data in itself for geothermal power generation. All of the energy derived from geothermal fluids is sensible and latent heat (enthalpy), as opposed to the chemical energy that is released by burning hydrocarbon fuels. Therefore, the energy per unit mass of geothermal fluid is low compared to oil, and the production rates of geothermal wells must be much higher than oil wells, on the order of 10 to 100 kg/s or more (depending on the steam-to-water ratio), to justify the drilling cost.

The wells must be monitored regularly to ensure their output is sustained and that sufficient capacity is available from all wells to maintain baseload power. The TFT method, in conjunction with E 1675, allows geothermal well output to be measured directly on-line while producing to the power plant, rather than diverting flow to mechanical test apparatus such as full-flow separators and flow meters. This process is currently used in virtually all major geothermal fields in the world, including those in Guatemala, Iceland, Indonesia, Japan, Kenya, New Zealand, Nicaragua, the Philippines and the Western United States, including Hawaii.

Use of E 1675 to Register Geothermal Energy Projects Under the Kyoto Protocol

Another important application of E 1675 is the collection of condensed geothermal steam samples for noncondensable gas analysis. All naturally produced geothermal steam contains some amount of NCG, comprised mostly of CO2, H2S (the familiar rotten egg smell of hot springs), and other trace gases such as N2, H2, CH4 and NH3.

Although most geothermal power plants emit greenhouse gases (primarily CO2), the amounts are very low. The average geothermal power plant in the United States releases 27 kg CO2 per MWh, while the average natural gas and coal plants emit 550 and 1,000 kg CO2 per MWh, respectively.2 Geothermal power generation in the United States alone offsets 22 million metric tons of CO2 annually.

As a result of the broad international acceptance of ASTM standards, E 1675 has been adopted by the United Nations Clean Development Mechanism to register and verify compliance with the CDM program that allows carbon offsets to be traded on the international market. The CDM program provides economic incentives for developing countries to build clean renewable power plants such as geothermal, instead of polluting coal-fired plants. The offsets purchased by participating industrialized countries have become an important strategy in meeting their near-term Kyoto Protocol commitments.

Continuing Activities of E44.15 to Aid Renewable Energy Development

Future activities of Subcommittee E44.15 will focus on standard specifications for two-phase flow measurement by TFT, geothermal steam purity and quality measurement using traversing and fixed multi-nozzle isokinetic sampling probes, and standard methodology for CO2 analysis in geothermal steam.

The geothermal heat pump community will be invited to participate and introduce standards relevant to this important sector of the multidisciplinary geothermal energy industry. Together with all the activities of ASTM Committee E44, renewable energy will continue to advance and displace the energy sources that are not sustainable and contributing to global warming. //

References

1. Lund, J.W.; Koenig, J.; Mertoglu, O.; Stafansson, V. Findings and Recommendations. Antalya, Turkey: 2005 World Geothermal Congress, April 2005.
2. Green, B.D.; Nix, R.G. Geothermal- The Energy Under Our Feet, Geothermal Resource Estimates for the United States. NREL/TP-840-40665. Golden CO.: National Renewable Energy Laboratory, November 2006.
3. Engle, D. Global Warmth: Earth’s Ultimate DE. Santa Barbara, CA.: Distributed Energy, Vol. 5, No. 2, March/April 2007.
4. Tester, J.W.; et al. The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century. Final Report to the U.S. Department of Energy Geothermal Technologies Program. Cambridge, MA.: Massachusetts Institute of Technology, 2006.