A binary cycle is a method for generating electrical power from geothermal resources and employs two separate fluid cycles, hence binary cycle. The primary cycle extracts the geothermal energy from the reservoir, and secondary cycle converts the heat into work to drive the generator and generate electricity.[1]
Binary cycles permit electricity generation even from low temperature geothermal resources (<180°C) that would otherwise produce insufficient quantities of steam to make flash power plants economically viable.[2] However, due to the lower temperatures binary cycles have low overall efficiencies of about 10-13%.
In contrast to conventional geothermal power generation methods like dry-steam or flash, which use a single open cycle, a binary cycle has two separate cycles operating in tandem, hence binary cycle. The primary cycle extracts heat from the geothermal reservoir and provides this to the secondary cycle, which converts heat into work (see Heat Engine) to drive a generator and produce electricity. Thermodynamically, binary cycle power plants are similar to coal-fired or nuclear power plants in that they employ Rankine Power Cycles, the main difference being the heat source and the choice of cycle working fluid.[1]
The geothermal reservoir's hot in-situ fluid (or geofluid) is produced to the surface via a wellbore, if necessary assisted by a pump. On the surface, the hot geofluid transfers some of its heat to the secondary cycle, via a heat exchanger, thus cooling in the process. The cold geofluid is then reinjected into the geothermal reservoir via a separate wellbore, where it is reheated. The primary cycle is considered an "open" cycle.
Cold high-pressure working fluid is heated and vapourised in a heat exchanger by the hot geofluid. The hot high-pressure vapour is expanded in a turbine before being cooled and condensed in a condenser. To close the loop, the cold low-pressure liquid is repressurised via a feed pump. The secondary cycle is a closed cycle.
The two main secondary cycle configurations are Organic Rankine cycles (ORC) or Kalina cycles, the main difference being the choice of working fluid; an organic fluid (commonly a hydrocarbon or refrigerant) or a water-ammonia mixture respectively.
The earliest example of a binary cycle geothermal power plant is thought to have been located on Ischia, Italy, between 1940-1943. The plant is thought to have used Ethyl Chloride as the working fluid at an effective capacity of 250 kW. However, owing to the Second World War taking place at the same time, not much is known about this particular installation.
Another binary cycle geothermal power plant was taken into operation in 1967 near Petropavlovsk on the Kamchatka peninsula, Russia. It was rated at 670 kW and ran for an unknown number of years, proving the concept of binary cycle geothermal power plants.
As of December 2014, there were 203 binary cycle geothermal power plants across 15 countries worldwide, representing 35% of all geothermal power plants, but only generating 10.4% of total geothermal power (about 1250 MW).[1]
The working fluid is evaporated at two different pressure levels, and thus temperatures. This improves efficiency by reducing exergetic losses in the primary heat exchanger by maintaining a closer match between the geofluid cooling curve and the working fluid heating curve.[3]
Two secondary cycles are operated in tandem, each with a separate working fluid and boiling point. This improves efficiency by reducing the exergetic losses of the heat introduction process, by ensuring a closer match between the geofluid cooling curve and the working fluids' heating curves.[4]
The performance of a simple binary cycle and its individual components can be calculated as follows:
| W |
turbine=
| m |
wf*ηturbine*(h1-h2s)
| W |
turbine
| m |
wf
ηturbine
h1
h2s
The equation below can be used to determine the condenser duty and mass flow rate of coolant required.
| Q |
condenser=
| m |
wf*(h2-h3)=
| m |
coolant*(hy-hx)
| Q |
condenser
h2
h3
| m |
coolant
hx
hy
| W |
pump=
| m |
wf*(h4s-h3)/ηpump
| W |
pump
h4s
h3
ηpump
The equation below can be used to determine the primary heat exchanger duty and mass flow rate of geofluid required.
| Q |
PHX=
| m |
wf*(h1-h4)=
| m |
geofluid*(ha-hc)
| Q |
PHX
h4
| m |
geofluid
ha
hc
There are a number of different definitions of efficiency that may be considered; these are discussed below.
The first law efficiency (from the First law of thermodynamics) is a measure of the conversion of the heat provided to the cycle into useful work. When accounting for real life losses and inefficiencies, real binary cycle geothermal plants have a first law efficiency of between 10-13%.
| th | |
| η | |
| I |
=
| |||||
|
=
| |||||||||||
|
See main article: Carnot efficiency. The Carnot efficiency gives the efficiency of an ideal thermodynamic cycle, operating between two reservoirs of different temperatures, as such it provides a theoretical maximum to the efficiency of any heat engine. For this reason, a geothermal power plant producing hot geofluid at 180°C (≈450 K) and rejecting heat at 25°C (≈298 K) has a maximum efficiency of just 34%.
ηCarnot=1-
| TC | |
| TH |
TC
TH
The second law efficiency (from the Second law of thermodynamics) is a measure of the utilisation of the ideally maximum work available and conversion into useful work.
| util | |
| η | |
| II |
=
| |||||
|
=
| |||||||||||
|
| E |
geofluid
h0
s0
T0
See main article: articles and Working fluid selection. The working fluid plays a pivotal role in any binary cycle and must be selected with care. Some criteria for selecting a suitable fluid are given below.[5]
There are numerous binary cycle power stations in commercial production.