Geothermal Power Generation

Geothermal Power Generation

Geothermal Power Generation

 

Plant Types

 

There are basically three types of geothermal plants used to generate electricity. The type of plant is determined primarily by the nature of the geothermal resource at the site.

 

The so-called direct steam geothermal plant is applied when the geothermal resource produces steam directly from the well. The steam, after passing through separators (which remove small sand and rock particles) is fed to the turbine. These were the earliest types of plants developed in Italy and in the U.S. Unfortunately, steam resources are the rarest of the all geothermal resources and exist in only a few places in the world. Obviously steam plants would not be applied to low-temperature resources.

 

Flash steam plants are employed in cases where the geothermal resource produces high-temperature hot water or a combination of steam and hot water. The fluid from the well is delivered to a flash tank where a portion of the water flashes to steam and is directed to the turbine. The remaining water is directed to disposal (usually injection). Depending on the temperature of the resource it may be possible to use two stages of flash tanks. In this case, the water separated at the first stage tank is directed to a second stage flash tank where more (but lower pressure)steam is separated. Remaining water from the second stage tank is then directed to disposal. The so-called double flash plant delivers steam at two different pressures to the turbine. Again, this type of plant cannot be applied to low-temperature resources.

 

The third type of geothermal power plant is called the binary plant. The name derives from the fact that a second fluid in a closed cycle is used to operate the turbine rather than geothermal steam. Figure 1 presents a simplified diagram of a binary type geothermal plant. Geothermal fluid is passed through a heat exchanger called a boiler or vaporizer (in some plants, two heat exchangers in series the first a preheater and the second a vaporizer) where the heat in the geothermal fluid is transferred to the working fluid causing it to boil. Past working fluids in low temperature binary plants were CFC (Freon type) refrigerants. Current machines use hydrocarbons (isobutane, pentane etc) of HFC type refrigerants with the specific fluid chosen to match the geothermal resource temperature.

 


Figure 1. Binary geothermal power plant


The working fluid vapor is passed to the turbine where its energy content is converted to mechanical energy and delivered, through the shaft to the generator. The vapor exits the turbine to the condenser where it is converted back to a liquid. In most plants, cooling water is circulated between the condenser and a cooling tower to reject this heat to the atmosphere. An alternative is to use so called “dry coolers” or air cooled condensers which reject heat directly to the air without the need for cooling water. This design essentially eliminates any consumptive use of water by the plant for cooling. Dry cooling, because it operates at higher temperatures (especially in the key summer season) than cooling towers does result in lower plant efficiency. Liquid working fluid from the condenser is pumped back to the higher pressure preheater/vaporizer by the feed pump to repeat the cycle.

 

The binary cycle is the type of plant which would be used for low temperature geothermal applications. Currently, off-the-shelf binary equipment is available in modules of 200 to 1,000 kW.

 

POWER PLANT FUNDAMENTALS

 

Power Plant Components

 

The process of generating electricity from a low temperature geothermal heat source (or from steam in a conventional power plant) involves a process engineers refer to as a Rankine Cycle. In a conventional power plant, the cycle, as illustrated in figure 1, includes a boiler, turbine, generator, condenser, feed water pump, cooling tower and cooling water pump. Steam is generated in the boiler by burning a fuel (coal, oil, gas or uranium). The steam is passed to the turbine where, in expanding against the turbine blades, the heat energy in the steam is converted to mechanical energy causing rotation of the turbine. This mechanical motion is transferred, through a shaft to the generator where it is converted to electrical energy. After passing through the turbine the steam is converted back to liquid water in the condenser of the power plant. Through the process of condensation, heat not used by the turbine is released to the cooling water. The cooling water, is delivered to the cooling tower where the “waste heat” from the cycle is rejected to the atmosphere. Steam condensate is delivered to the boiler by the feed pump to repeat the process.

 

In summary, a power plant is simply a cycle that facilitates the conversion of energy from one form to another. In this case the chemical energy in the fuel is converted to heat (at the boiler), and then to mechanical energy (in the turbine) and finally to electrical energy (in the generator). Although the energy content of the final product, electricity, is normally expressed in units of watts-hours or kilowatt-hours (1000 watt-hours or 1kW-hr), calculations of plant performance are often done in units of BTU’s. It is convenient to remember that 1 kilowatt-hour is the energy equivalent of 3413 BTU. One of the most important determinations about a power plant is how much energy input (fuel) is required to produce a given electrical output.