May 5, 2009 Raser Technologies has begun delivering 10 megawatts (MW) of clean, renewable electricity to Anaheim, California, from its first low-temperature, binary geothermal plant, the Hatch Geothermal Power Plant in Beaver County, Utah. Traditionally, the lead time on a geothermal plant is three to five years, but the USD$33-million Raser plant has been powered up just five months after ground-breaking. To help meet such rapid construction schedules, the plant uses off-the-shelf modular components, taken from the air conditioning industry, which are essentially running in reverse.

The geothermal plant uses an organic Rankine cycle (ORC) that generates power from ground-sourced geothermal brine, at temperatures ranging from 200ºF to 300ºF (93ºC to 148ºC). At normal atmospheric pressure, water boils at 212ºF (100ºC), but under pressure, typical of a geothermal system, the temperature can increase much higher before flashing to steam. Typical geothermal plants require the temperature to be at least 360ºF (182C) to be feasible. The Raser plant uses an ORC to make use of geothermal resources, previously considered unusable. Depending on the type of organic fluid used, temperatures as low as 158ºF to 176ºF (70-80°C) can be used in an ORC to generate electricity.


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Binary cycle

In a binary cycle closed loop geothermal plant, the hot brine pulled from the earth is passed through a large tank that contains an evaporator. The heat from the brine causes a low-boiling-point working fluid in the evaporator to flash into vapor, much in the same way water is boiled to flash into steam. Raser isn't giving away which working fluid they are using, but ORC systems often use common refrigerants such as monochlorodifluoromethane, normally called R-22, which is nontoxic, noncorrosive, nonflammable, extensively used in household refrigerators and window air conditioners and has a boiling point of -41°F (-44.4ºC), at atmospheric pressure.

The vaporized working fluid, which is now at high pressure, leaves the evaporator and is fed directly into a high efficiency turbine, driving in this case a 250 Kw generator. With most of the energy removed from the vapor, as it drives the turbine, a partially liquefied working fluid is exhausted from the turbine into a second tank that contains a condenser heat exchanger. The exchanger removes any remaining heat and returns the working fluid to a fully liquid state. From there the closed loop is completed by a small pump that takes the refrigerant from the condenser back to the evaporator.

The whole process is similar to the way a conventional coal-fired or nuclear power plant generates electricity, only the working fluid is different. In fact, up to 80 percent of the world's electricity is generated using the Rankine cycle to heat water into high pressure steam, which drives a turbine and, in turn, an electrical generator.

The Raser plant uses 50 binary generator modules supplied by UTC Power, combined in parallel, to give a net output power of 10 MW. The total capacity is about 14 MW, but 3 to 4 MW are required to run the pumps pulling the water up from the hot reservoirs and then back down. In cooler months, when environmental factors will help cool the secondary fluid, the net output will be more like 11 MW.

Running a refrigerator backwards

The concept of running a refrigeration cycle in reverse to generate power is not new. However, until now, the refrigeration industry has never seriously pursued the idea. The power generation modules built by UTC, which own Carrier air conditioners, is a modified version of a Carrier Industrial Chiller. 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, a Carrier refrigeration mechanic couldn't tell the difference between the UTC turbine/generator assembly and a Carrier compressor/motor.

While the Beaver County project brine averages some 260°F (126ºC), the USD$350,000 skid-mounted UTC PureCycle generators will operate at temperatures as low as 165°F (73ºC). The generator is the fruit of six years of research, supported by substantial federal funding. The model is operating only at one geothermal project, at Chena Hot Springs, in Alaska, with water that is just 165ºF (73ºC).

Unlike other renewable power generation systems, such as solar or wind, a geothermal plant can generate base load 24 hours a day. Anaheim is purchasing the 10 MW of electricity that Raser will produce under a 20-year power purchase agreement (PPA), at a rate of USD$78 per megawatt-hour, or USD$13.3 million a year. This should be enough power to supply some 9,000 homes.

Paul Evans

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