Chena 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:

● Chena Hot Springs Resort

● Chena Power

● United Technologies Corp.

● Department of Energy

● Alaska Energy Authority

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.

How Chen Geothermal Power Plant Works?

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:

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.

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.

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.

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.

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.

6. The cooling water entering the condenser and recondensesthe 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.

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.

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.