Chena Geothermal Power Plant Project Overview
Carrier Chillers versus UTC Power Plants
Other applications for the technology
Onsite Resource Development for the Power Plant
Chena Geothermal Resource Overview
Download a FACTSHEET on our geothermal power plant
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Alaska has more geothermal resources than any other state in the country, and
yet none of these resources has been developed for power generation prior to
2006. In 2004, Chena Hot Springs Resort entered into a partnership with
United Technologies Corporation (UTC) to demonstrate their moderate temperature
geothermal ORC power plant technology at Chena Hot Springs.
Project partners include:
The Chena geothermal power plant came online in late July 2006, putting Alaska squarely on the map for new geothermal technologies. Chena Hot Springs is the lowest temperature geothermal resource to be used for commercial power production in the world. We hope this will be the first step toward much greater geothermal development in the state. The cost of power production, even in semi-remote locations such as Chena, will be reduced from 30¢ to less than 7¢ per kWh once the UTC plant is installed and operational.
The challenge for moderate temperature small scale geothermal development has been to bring the cost down to a level where it is economical to develop small geothermal fields. UTC has been working toward that goal. In the past, small geothermal power plants have been built to order using tailor made components, which has greatly increased both the expense and the lead time for such units.
UTC’s Research Center has teamed up with their sister divisions, Carrier and UTC Power, to reverse engineer mass produced Carrier chiller components to dramatically reduce the cost of production, and allow for modular construction. UTC has already proven this technology with the release of their PureCycle 225 power plant in 2003, which is designed to operate off waste heat applications.
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Because the geothermal water at Chena Hot Springs never reaches the boiling point of water we cannot use a traditional steam driven turbine. Instead a secondary (hence, "binary") fluid, R-134a, which has a lower boiling point than water passes through a heat exchanger with 165°F water from our geothermal wells. Heat from the geothermal water causes the R-134a to flash to vapor which then drives the turbine. Because this is a closed loop system virtually nothing is emitted to the atmosphere. Moderate temperature is by far the most common geothermal resource and most geothermal power plants in the future will be binary cycle plants. Here are the steps in the cycle:
STEP 1: Hot water enters the evaporator at 165ºF (480gpm). After the hot water runs through the evaporator, it is returned to the geothermal reservoir via our injection pump and injection well system. Some of the water is also used to heat buildings on site before it is reinjected.
STEP 2: The evaporator shell is filled with R-134a, a common refrigerant found in many air conditioning systems. The 165ºF water entering the evaporator is not hot enough to boil water, but it is hot enough to boil the R-134a refrigerant. The evaporator is a giant heat exchanger, with the hot water never actually coming in contact with the refrigerant, but transferring heat energy to it. The R134a begins to boil and vaporize.
STEP 3: On initial system startup, the vapor bypasses the turbine and returns directly to the condenser via a bypass valve. Once there is adequate boiling/evaporation of the refrigerant, the bypass valve closes and the vapor is routed to the turbine.
STEP 4: The vapor is expanded supersonically through the turbine nozzle, causing the turbine blades to turn at 13,500rpm. The turbine is connected to a generator, which it spins at 3600rpm, producing electricity.
STEP 5: Cooling Water enters from our cooling water well which is located 3000ft distant and 33ft higher elevation than the power plant. Cold water (40ºF-45ºF) is siphoned out of this well and supplied to the power plant condenser at a rate of 1500gpm.
STEP 6: The cooling water entering the condenser and recondenses the vapor refrigerant back into a liquid. As in the evaporator, the condenser only allows heat transfer to occur between the refrigerant (in the shell) and the cold water (in the tubes within the condenser). The two liquids never actually come in contact.
STEP 7: The pump pushes the liquid refrigerant back over to the evaporator, so the cycle can start again. By doing so, it also generates the pressure which drives the entire cycle.
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Carrier Chillers versus UTC Power Plant
The concept of running a refrigeration cycle in reverse to generate power has been known for a long time. However, until now the refrigeration industry has never seriously pursued this idea. When Carrier Refrigeration was conducting performance tests on the 19XR centrifugal chiller, a high reverse rotational speed was noted that during simulated power outages. This is typical in a refrigeration unit, as pressures between the condenser and evaporator equalize during shutdown. However, the 19RX chiller was designed using a discrete passage diffuser, rather than the vaneless diffuser used on previous products. The vaneless diffuser was observed to allow reverse rotational speeds up to 75% of normal operating speeds, and this triggered the idea of actually using the compressor as a turbine. Carrier further developed this concept with their sister divisions, United Technologies Research Center and United Technologies Power, ultimately resulting in the 2004 release of the PureCycle™ 200. The PureCycle™ uses waste heat exhaust gases and air cooled condenser equipment. The Chena Power plant takes the PureCycle™ concept one step further, generating power economically off a 120°F temperature differential between the evaporator and condenser temperature. Interestingly enough, this approaches the temperature differential of the air-conditioning unit the power plant was derived from. For this reason, the same refrigerant typically used in Carrier Refrigeration systems can be used for the Chena power plant.
One big advantage of using refrigeration or air-conditioning equipment for power generation is that the hardware used for these applications has a cost structure substantially lower than that of traditional power generating equipment. By keeping as many components the same as possible, the UTC Power Plant can substantially reduce construction costs by taking advantage of Carrier's mass production line. In fact, if a Carrier Refrigeration mechanic were to come to Chena Hot Springs, they would not be able to tell the different between our turbine/generator assembly and a Carrier compressor/motor.
There are three main changes made to transform a Carrier chiller into a power plant. The turbine/generator assembly has 13 of 171 parts uniquely manufactured for power production versus its corresponding chiller/compressor assembly. A pump has been added to circulate the liquid refrigerant and maintain operating pressure. Also, the heat exchangers, while standard Carrier manufactured units, are specifically sized for power plant operation at design temperature.
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Other
Applications For This ORC Technology
The technology developed by UTC can operate off any heat source, with a minimum of 100°F temperature differential between the heat source and sink. Geothermal energy is only one potential application. Similar systems are already in operation off heat generated from landfill flares and gas turbine exhaust. Another excellent potential application is using biomass as a fuel.
The oil and gas also provides another possible application for UTC's power plant. Because most oil and gas wells are quite deep, they are warmed by the natural thermal gradient of the earth. In 2004 the U.S. produced over 5x1010 bbl (that's 2,100,000,000,000,000,000,000 gallons!) of “waste” water along with the oil and gas production, primarily from the Gulf States with temperatures high enough to produce electricity. This hot water could be used to generate power directly, without impacting oil and gas production. Some estimates suggest up to 5000MW of additional power could be generated in Texas alone -- that's more than 10 times the amount of power used by the entire State of Alaska!
For more information on applications for the oil and gas industry, visit Southern Methodist University's Geothermal Home page at www.smu.edu/geothermal.
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Onsite Resource Development for the Power Plant
The power plant itself is the heart of a much larger
project. In order to operate the plant, we need to pump 500+gpm of
geothermal water from our production well, which is located 3/4 of a mile from
the power plant. We do this via an insulated and buried 8inch HDPE line.
The production well itself took almost a month to complete. It is a 10in
diameter 700ft well, cemented and cased to 450ft.
After the water goes through the power plant, we need to reinject the water back into the reservoir in order to maintain the resource. Fortunately, the injection wells are located within 300ft of the power plant building, and our wells are highly permeable. That means they are capable of 'drinking' a lot of water. Injecting this water in the right place at the right depth is the most critical component of the project, to assure long-term viability of the project. We are constantly monitoring the effects of our injection program at various test well locations throughout the property.
The cooling water we are using to supply the power plant condensers is also extremely critical. In fact, without the good cold water resource we have available the power generation project might not be viable for our low geothermal temperatures. The 1500gpm of cold water is supplied by a well 2700ft distant and 33ft uphill from the power plant. A 16in diameter insulated steel pipeline delivers the cooling water to the power plant. Because of the natural 'drop' of the land along this pipeline route, we are able to siphon the water out of the well and deliver it to the power plant without using a pump. This is very important to Chena, because we want to get every available kW out of the power plant and are trying to minimize 'parasitic' loads which would reduce our net power output wherever possible.
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Chena
Geothermal Resource Overview
The Chena Hot Springs Geothermal Resource, like all interior Alaskan hot springs, is located along the margins of a granite pluton. These plutons are ancient (cooled) magmatic bodies that have pushed up into the surrounding rock at some time in the distance past (at least 80 million years ago). These intrusions cooled below the surface to become huge granite geologic formation, called plutons. Plutons can host geothermal systems in two ways. Granitic rock is very brittle and fractures easily. These deep, often steeply dipping fractures can sometimes act as conduits for water which has circulated deep into the earth's crust (picking up heat along the way) to rapidly short circuit' back to the surface. In the case of the Chena system, this short circuit is probably caused by the intersection of two small faults, the primary one located parallel to Spring Creek and identifiable by the string of natural hot springs and seeps along one section of it.
Granite rock is also frequently high in Uranium and Thorium. When these elements decay, heat is generated which is trapped in the host rock, which in this case is the granite pluton. This radioactive decay generates an abnormally high geothermal gradient in the pluton, which means the water does not need to circulate to extreme depths to pick up heat. In the case of Chena Hot Springs, it appears the water is circulating to a depth of approximately 3000-5000ft and reaching a maximum temperature of 250ºF. Chena is working on an exploration project under the Department of Energy to identify and quantify the deep geothermal resource at Chena Hot Springs. This project will culminate in the drilling and testing of a 4000ft hole sited to intersect the geothermal reservoir at depth. For more information on Chena's GRED III project, click here.