Explaining the Binary Power Cycle

Geothermal energy is known to be a reliable and sustainable energy source. As the world gives more attention to the state of the environment, people lean towards using more energy sources which have little to no impact on nature. Where it is true that currently no other energy source can outperform fossil fuel due to its energy concentration, geothermal energy is a good prospect as a temporary substitute until a better form of energy supply is found.

There are two types of geothermal power sources; one is known as the steam plant and the other is the Binary cycle. Binary cycles have the conceptual objectives of: high efficiency — minimizing losses; low cost to optimize component design; and critical choice of working fluid. This particular type of cycle allows cooler geothermal supply to be used, which has a huge benefit since lower temperature resources are much more common in nature.

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The way a binary cycle works can be explained using the diagram shown above. Since the temperature of geothermal source is not high enough to produce steam, hot water is fed into a heat exchanger. From there, secondary liquid with lower boiling water than water i.e. isobutane, absorbs the heat generated. As the steam of secondary liquid moves the turbine, electricity will then be produced. This whole process repeats in a cycle since the secondary fluid will then condense back to its liquid state and being used for the same process.

From the process described above, it can be seen that binary cycle is a self-contained cycle — ‘nothing’ goes to waste. This fact leads to the potential of having low producing cost energy source from binary power cycle. That being said, due to the lower temperature, the conversion efficiency of the geothermal heat is also considerably low. Consequently, Carnot efficiency of such process is lower than most power cycles. Large amount of heat is required to operate a binary cycle, leading to a better and larger equipment. Not only that since a bigger amount of heat energy has to be let out to the environment during the cycle, a sufficient cooling system must be installed. Although the production cost is found to be lower, the investment cost for installation would be very expensive. Then, the main question to this particular technology implementation would be how to improve the quality of production and economic feasibility?

First, one of the main aspect of binary power cycle is to overcome water imperfection as a main fluid. Consequently choosing optimal working fluid is a very essential step. Characteristic of optimal working fluids would include a high critical temperature and maximum pressure, lower triple-point temperature, sufficient condenser pressure, high vaporization enthalpy, and other properties.

Second, it was studied on multiple different events that well-optimized ORCs perform better than Kalina cycles. The type of components chosen in the cycle also affect the cycle performance quite substantially, i.e plate heat exchanger was found to perform better in an ORC cycle in the geothermal binary application compared to shell-and-tube. Addition of recuperator or turbine bleeding also have the potency to improve the overall performance of a binary cycle plant. It is important to model multiple thermodynamic cycle to make sure that the chosen one is the most optimized based on the boundary conditions. While designing ranges of thermodynamic cycles, it is common that the cycle is modeled based on ideal assumptions. For binary cycle in geothermal application, plant efficiency would be the most important parameter. In order to achieve a desired plant efficiency, both cycle efficiency and plant effectiveness should be maximized.

Additionally, pinch-point-temperature between condenser and heat exchanger is a substantial aspect to pay attention to, even the smallest change of in temperature is considered a significant change. Thus, including this parameter is a very important aspect.

This particular cycle has many potentials which haven’t been explored. Enhance the advantages of your binary power cycle using our thermodynamic tool, AxCYCLE.

Ref:
https://en.wikipedia.org/wiki/Binary_cycle
http://www.technologystudent.com/energy1/geo3.htm
http://www.researchgate.net/publication/229148932_Optimized_geothermal_binary_power_cycles

Analyzing Thermal Power Generation Efficiency

Figure 1 - Thermal Power Plant Layout
Figure 1 – Thermal Power Plant Layout

Steam turbine power generating plants, also known as Thermal Power stations, are the most conventional type of electricity production today. Most of today’s electricity power is generated though this technology. Naturally, as implied by its name, a thermal power station uses steam power as its prime mover to convert energy in coal, or other fossil fuel, by heating water to steam and utilizing Rankine cycle principles to generate heat and electricity.

The basic theory of thermal power generation is pretty straight forward: in a simple thermodynamic cycle, saturated liquid water is heated to steam. The working fluid will then pass through a steam turbine, where its energy is converted to mechanical work to run the generator and produce the electricity. Then fluid will be condensed to be recycled back in the heater. Just as simple as that, electricity power is generated from the cycle based on Rankine cycle principle.

The utilization of thermal power station comes with the advantage of economical initial and generation cost, easy maintenance and simple cycle operation in practice. That being said, there are also couple major drawbacks associated to the technology, primarily, low overall efficiency –due to the nature of Rankine cycle’s characteristic of thermal efficiency and environmental issues.

There are many scientific reasoning behind thermal power generation’s low efficiency. It is important to know the reasons why to engage in a better technology. These are the primary reasons:

  • During the combustion of carbon, effective conversion more or less is found to be 90%, this happen primarily due to limitation of heat transfer where some heat are lost into the atmosphere. Coal also contains moisture that vaporizes and take the latent heat from combustions.
  • The thermodynamic step, working on Rankine cycle principle, is where 50% (or more) efficiency is consumed. When the steam is condensed for re-use, latent heat of condensation is lost in the cooling water, which decreases the energy input by a very significant magnitude. Losses can also happen in the blades and other components. The Rankine cycle efficiency is determined by the maximum temperature of steam that can be transferred through the turbine, which means the efficiency is also constrained by the temperature associated with the cycle. Two other main factors that affect the thermal efficiency of power plants are the pressure of steam entering the turbine and the pressure in the condenser. That being said, a cycle with supercritical pressure and high temperature usually results to a higher efficiency.
  • During a conversion of mechanical to electrical, some efficiency loss happens in the generator and transformer. A small percentage of energy generated will then be used for internal consumption.

Knowing the causes of low efficiency leads us to the next question: What are the steps to optimize our thermal power plant efficiency?

  • Since thermal efficiency depends on temperature and pressure, it can be improved by using high pressure and temperature steam, though obviously it will be limited based on the boundary conditions of the operating system. A lower pressure can also be set in the condenser.
  • Improvement could also be implemented by the application of reheating steam technology between turbine stages.
  • Waste heat recovery optimization, capture excess heat for reuse, and install insulation to reduce any losses.
  • Upgrading major systems/components of thermodynamic cycles and renewing materials to reduce natural losses in efficiency due to age.
  • Improve efficiency monitoring system to enable instant detection of losses as well as analyzing efficiency based on real data.

These are just some ways that could be utilized to optimize power generation efficiency, indeed each of the steps come with their own specific obstacles of implementation, but there are infinite ways that can be explored to advance the technology.

Learn more about maximizing your power plant productivity through our webinars and explore our tools to help with your efficiency optimization for power generation and component design!

Sources:
http://www.learnengineering.org/2013/01/thermal-power-plant-working.html

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