Geothermal Power Plant Potential Rico, Colorado

Transcription

1 Geothermal Power Plant Potential Rico, Colorado Christopher Tipple, Amy Richards, Micah Jakulewicz Table of Contents 1.0 Introduction Geology Power Demand Motivation Geothermal Power Plant Basics Economics Conclusion..8

2 1.0 Introduction The town of Rico, Colorado is located in the southwest corner of Colorado, between Telluride and Cortez in the Dolores River Valley. The regional location of Rico is represented by balloon A in Figure 1. The population of Rico is estimated to be 250 people. Recent picture of Rico is shown in Figure 2. With a rich heritage in mining and natural resource exploration, the town of Rico is continuing its natural resources development. The town is doing so via the proposed exploration and development of geothermal energy. More specifically, the town is very interested in producing electricity using its geothermal resource. 2.0 Geology The town of Rico, Colorado lies upon the Rico Dome, which is a faulted anticline containing mainly Precambrian greenstone and quartzite at its core. The Rico Dome is approximately 5 km long and contains the following stratigraphy, from youngest to oldest: Proterozoic greenstone, Precambrian Uncompahgre Quartzite, Mississippian Leadville Limestone, Pennsylvania Hermosa Formation and Rico Formation, and Permian Cutler Formation. Stratographic contact between the Proterozoic greenstone and the Precambrian quartzite and between the Uncompahgre and Hermosa formations provides geologic evidence of conduits for fluid flow. [1] In addition to fluid flow channels, Rico is also an epicenter for magmatic activity. Both the Cretaceous age and Pliocene-aged events show magmatic activity including precious metal mineralization. The Pliocene-aged event is called the Rico Paleothermal Anomaly, and is roughly 3 km wide and 2 km high. The Rico Paleothermal Anomaly is centered where the Blackhawk Fault intersects the east-west trending faults. [1] The combination of faulting and magmatic activity suggests that Rico could potentially contain a geothermal energy source, suitable for electricity generation. The complex network of faults provides a channel for fluid transport. The local magmatic activity provides a heat source. Finally, the geology near Rico provides a confining formation that is indicative of a geothermal resource. 3.0 Power The energy demands in Rico and the immediate surrounding area primarily consist of small residential-type loads except for the winter-dominated load from the Telluride ski

3 resort. Currently, these energy demands are being met via pulverized coal and hydroelectric power generation within the region. 4.0 Motivation The citizens of Rico possess, among others, three primary motivations for producing geothermal electricity locally. These include providing local energy to the town and region, providing a zero carbon alternative to pulverize coal electricity generation, and rebuilding the infrastructure of Rico. First, Rico primarily relies on pulverized coal and hydroelectric power generation. Subsidizing both coal and hydroelectric electricity production with geothermal energy production would diversify the energy portfolio of Rico as well as the region, while foregoing the costs associated with costly transmission equipment (including power lines, transformers etc.) to bring additional power capacity to the area from elsewhere. Second, the introduction of electricity produced using geothermal would offset coal electricity production with a cleaner form of electricity production. Final, a geothermal electricity source in Rico would provide long-term economic benefits in the form of stable local jobs, tourism and provide a means for attracting new investors to Rico. In the 1970 s and the early 1980 s the Rico Argentine Mining Company began investigating the geothermal resources near Rico. Multiple exploration boreholes were drilled north of the town. Many of the holes proved to be artesian in nature, and produced hot water at nearly 40 C. In addition, geothermal gradients were observed to approach 114 C/km. [1]

4 Rico exhibits evidence of anomalous heat through warm springs in Rico and the surrounding area. Also, maximum temperatures were measured at wells SC-1, SC- 2, SC-7A, as C at 1650 m, C at 880 m, and C at 1540 m, respectively. A graphical representation of this data is shown in Figure 3. [2] 5.0 Geothermal Power Plant Basics [3] On the most basic level, an operational geothermal power plant consists of a geothermal resource well, power generation equipment, and an injection well. Hot geothermal fluid is pumped up from the production well and processed for energy conversion and expanded through a turbine, thereby powering an electric generator. The utilized geo-fluid is then re-injection into the geothermal reservoir. 5.1 Geothermal Power Plant Types Dry Steam Power Plants Dry steam geothermal resources are rare but provide for very straightforward and efficient power plants. A dry steam resource is typically above 200 C. Figure 4 contains an illustration of this type of plant. Here, steam is fed directly from the production well into the turbine for power generation. The geothermal fluid need not undergo any additional preparation processes prior to introduction into the turbine save for purification from particulate matter. The steam exits the turbine and enters a cooling tower for condensation into liquid and subsequent re-injection. Environmental concerns for plants of this type are few, especially since there is no waste brine to deal with. Very few known resources suitable for a dry steam system exist, so it is very unlikely to be implemented at Rico.

5 Figure 4: Diagram of dry steam plant [4]. Single-Flash Most geothermal sources produce temperatures that are no where near the critical point of water. A single flash resource is typically between 150 C and 200 C [4]. This translates to a steam source that is saturated with vapor and when pressurized will readily condense to the liquid phase. In traditional Rankine cycle turbines the presence of the liquid phase causes dramatic efficiency losses. Preventing vapor from entering a turbine while utilizing a lower temperature resource is generally done by using a flash process before the steam is sent to the turbine. Due to the higher frequency of liquid-dominated geothermal fields, single-flash geothermal power plants are the most commonly installed plants at geothermal fields. A simplified illustration of the single flash power plant is provided in Figure 5.

6 Figure 5: Illustration of single-flash steam plant [4]. The steam and liquid are separated into two distinct phases for processing. The steam is sent to the turbine and the liquid is sent back to the injection wells. After the steam is used to generate power, it is condensed back to a liquid in a cooling tower before being re-injected into the reservoir. Because of interaction between the geo-fluid and machinery or piping, care must be taken in material selection so as to minimize scaling and corrosion. This added design complexity increases the capital and maintenance costs of the system. Flashing in the pipe carrying the separated liquid to the injection well due to a pressure drop is also something that must be avoided as is chemical precipitation from a drop in temperature. An environmental concern for this type of plant is the water vapor plume that will be visible from the cooling tower. Harmful gases present in the geo-fluid must be contained in a closed loop system utilizing re-injection or isolated and treated before the geo-fluid can be released into the surrounding environment. Double-Flash Double-flash plants may produce 15-25% more power than a single-flash system for the same geothermal fluid conditions. The increase in efficiency, however, comes at a higher initial capital cost since the system is far more complicated. A double flash resource is typically between 150 C and +200 C [4]. It operates in much the same way as a singleflash plant, but instead of sending the separated liquid directly to the re-injection well it is sent to a second separator to generate additional steam at a lower pressure. The turbine for this type of system must be able to incorporate the lower pressure steam at an appropriate stage for smooth incorporation. Another option is to use two separate turbines. Due to increased complexity over the single-flash plant, capital and maintenance costs are significantly higher. As with a single-flash plant, scaling and corrosion concerns exist.

7 Binary Power Plants A binary power plant flows moderate temperature geo-fluid ( C) through a heat exchanger heating a secondary working fluid that generally has a lower boiling point than water. The geothermal fluid is then re-injected into the geothermal reservoir. The heated working fluid is then expanded through a turbine, which powers an electric generator. A simplified illustration of the binary power plant is provided below in Figure 6. Figure 6: Diagram of simplified binary system [4]. Since it is generally understood that each geothermal source is unique in its temperature, pressure and chemistry, it is advantageous to have flexibility in the process design of a power plant s energy conversion system. This is where a binary power plant excels. The specific working fluid used in a particular binary system may be matched to the unique geothermal fluid temperature. Primarily the working fluid is selected to maximize the thermodynamic efficiencies of a particular application, while minimizing the degradation of the system s materials and minimizing costs by reducing the need for exotic materials. The chemical compatibility of the working fluid and the wetted metal surfaces of the binary power plant effectively extends the system s lifetime by reducing the amount of maintenance over the plant s lifetime. Also the lower boiling points of many of the applicable working fluids allow for lower geothermal source temperatures. Spring water in the area of Rico is known to contain calcium bicarbonate, iron, manganese, and calcium bicarbonate-sulfate. The thermal water has also been found to contain 38 picocuries per liter of Radium 226, which is the highest known amount of any of the thermal waters in Colorado [5]. The total dissolved solids in these springs were approximately 2,700 mg/l. Waters of these types pose difficulties for a geothermal

8 system. As water temperature increases, the solubility of minerals also increases. When the temperature drops, water that was once unsaturated with a mineral now becomes saturated and particulates precipitate out of the water. These particulates can result in scaling on piping and machinery. Therefore the less equipment that a geo-fluid comes into contact with will minimize the amount of equipment that will be damage by scaling, corrosion and abrasion over the lifetime of a geothermal power plant. A binary power plant addresses the equipment degradation issues posed by geothermal water by limiting the temperature drop seen by the geo-fluid in the primary heat exchanger. The high levels of radium content of the water in the Rico area poses environmental and safety issues if it where to be discharged to the above ground environment. Another advantage of the binary power plant system is that it discharges the used geo-fluid through a re-injection well. This helps to maintain a closed loop geo-fluid reservoir and mitigates any health and environmental risks posed by the geo-fluid s chemistry. 6.0 Economics Megamoly, Inc. conducted a preliminary economic analysis of the Rico geothermal project using the Renewable Energy Technology Finance Model developed by the National Renewable Energy Laboratory (NREL). In this analysis the following assumptions were made. First, a 10 MW binary geothermal power plant would be established. Second, each production well would cost $3 Million and produce 3 MW of electricity. Third, Resource temperatures would be classified as moderate (not exceeding 200 C). Fourth, the power plant would have a 20-year project life, and capital debt would be re-pated in 15 years. Fifth, Capital would be depreciated over 5 years. Sixth, a 10% investment tax credit would offset other income. Finally, the plant would be operating 90% of the time. The economic analysis concluded that the cost of geothermal energy would be 7.62 c/kwh and the equity investor internal rate of return would be 37%. In their business plan, Megamoly, Inc. noted that these numbers were preliminary and were subject to change based upon the available resources, tax credits and other capital costs. [1] 7.0 Conclusion In this document we have touched on the possible geothermal resource that is present in Rico, Colorado. Given the presented information, a binary geothermal power plant would bust suit the geothermal resource in Rico for the following reasons: the geo-fluid temperature falls within the moderate range, the high levels of radium in the geo-fluid and the local desire to produce clean electricity economically.

9 References [1] Megamolly, Incorporated, Business Plan for Geothermal Power Production near Rico, Colorado, March 6 th, 2009, Prepared by Ausburn Geoscience, Incorporated. [2] Medlin, E. W., Modeling Local Thermal Anomalies: Constrains From Conductivity, Gravity and Heat Flow, Mater of Science Thesis, Department of Geology and Geophysics, University of Wyoming, December, [3] DiPippo, R., Geothermal Power Plants, Second Edition, Elsevier Ltd., 2008 [4] Nakagawa, M., Lectures on Geothermal Energy, MNGN 498/MNGN598, Mining Engineering Department, Colorado School of Mines, Spring, [5] Pearl, R. H., Colorado s Hydrothermal Resource Base An Assessment: Colorado Geologic Survey, Resource Series 6, Colorado Department of Natural Resources, 1979.