Organic Rankine Cycles: Low Temperature, High Efficiency

Nowadays the scientific community is strongly concerned about problems of efficiency increase and emissions reduction in power generation, ship, and vehicle drives such as internal combustion engines (ICE). A system utilizing waste heat recovery (WHR) is an effective solution for the aforementioned problems.

ORC (meaning organic Rankine cycle, not the scary monsters from Lord of the Rings) is one WHR solution which delivers additional power from the turbine/engine exhaust gas/steam energy.  ORC systems operate on hydrocarbon-based fluids which effectively avoid the typical disadvantages associated with water-based steam turbine systems while bringing the advantage of better performance at part load and in non-continuous operation. ORC systems, capable of utilizing low temperature heat sources of 100-200°C, can be designed in compact and modular packages which require very little maintenance.

The design criteria of an ORC system and its components includes finding the maximum possible heat recovery from the available high and low temperature waste heat flows of a turbine or ICE to produce the maximum amount of additional power while decreasing the load on the turbine’s cooling system, under certain restrictions like geometry and cost.

The first step is to design the thermodynamic cycle configuration. Figure 1 is a flow diagram of a dual loop supercritical organic Rankine cycle (SORC) with separate turbines and given design parameters of the components, generated with AxCYCLE™ software, developed by SoftInWay. The cycle consists of 6 heat exchangers, 2 turbines (HPT and LPT), 2 pumps (HPP and LPP) and the condenser. Both turbines operate with the same backpressure – 1.3 bars. The flows of the working fluid (R245fa in this case) are mixed at the condenser inlet and split at its outlet. The temperature – entropy diagram for the presented cycle is shown on Figure 2. The process 1-2-3-4-5-1 corresponds to the high pressure loop operation and the process 10-20-30-40-10 is for the low pressure loop operation. All these can be easily manipulated and obtained with user-friendly interface of AxCYCLE™.

Flow Diagram - ORC CYCLE
Figure 1. The flow diagram of the SORC with separate turbines
The T-S Diagram for the SORC with separate turbines
Figure 2. The T-S Diagram for the SORC with separate turbines

In terms of component design, ORC turbines can be of axial, radial inflow and radial outflow configurations. The type of turbine you should select depends on the application. To delve further into the topic, check out  SoftInWay’s webinar on “Radial Inflow versus Outflow Turbines – Comparison, Advantages and Applicability” here –
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Evaluating the Scalability of Different Combined Cycles with Bottoming sCO2 Turbines

Bottoming cycles are generating a real interest in a world where resources are becoming scarcer and the environmental footprint of power plants is becoming more controlled. With this in mind, reduction of flue gas temperature, power generation boost, and even production of heat for cogeneration application is very attractive and it becomes necessary to quantify how much can really be extracted from a simple cycle to be converted to a combined configuration.

Sco2 Power Unit

Supercritical CO2 is becoming an ideal working fluid primarily due to two factors. First, turbomachines are being designed to be significantly more compact. Second, the fluid operates at a high thermal efficiency in the cycles. These two factors create an increased interest in its various applications. Evaluating the option of combined gas and supercritical CO2 cycles for different gas turbine sizes, gas turbine exhaust gas temperatures and configurations of bottoming cycle type becomes an essential step toward creating guidelines for the question, “how much more can I get with what I have?”
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Direct Off-Design Performance Prediction of an Industrial Gas Turbine Engine

The modern gas turbine engine has been used in the power generation industry for almost half a century. Traditionally, gas turbines are designed to operate with the best efficiency during normal operating conditions and at specific operating points. However, the real world is non-optimal and the engine may have to operate at off-design conditions due to load requirements, different ambient temperatures, fuel types, relative humidity and driven equipment speed. Also more and more base-load gas turbines have to work at partial load, which can affect the hot gas path condition and life expectancy.

At these off-design conditions, the gas turbine efficiency and life deterioration rate can significantly deviate from the design specifications. During a gas turbine’s life, power generation providers may need to perform several overhauls or upgrades for their engines. Thus, the off-design performance after the overhaul can also change. Prediction of gas turbine off-design performance is essential to economic operation of power generation equipment. In the following post, such a system for complex design and off-design performance prediction (AxSTREAM®) is presented. It enables users to predict the gas turbine engine design and off-design performance almost automatically. Each component’s performance such as the turbine, compressor, combustor and secondary flow (cooling) system is directly and simultaneously calculated for every off-design performance request, making it possible to build an off-design performance map including the cooling system. The presented approach provides a wide range of capabilities for optimization of operation modes of industrial gas turbine engines and other complex turbomachinery systems for specific operation conditions (environment, grid demands more).

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Different Criteria Options in Turbomachinery Design Software

When people design turbomachines, whether it be a turbine, compressor, blower or fan, they need to find the optimal design based on their criterion under certain constraints.

With AxSTREAM®, people are given several options for their design criteria, which provides flexibility. With that being said, we often get asked what their differences are and here is a brief explanation addressing just that.

The design criteria menu includes power, internal total-to-static efficiency, internal total-to-total efficiency, polytropic efficiency, diagram total-to-static efficiency, and diagram total-to-total efficiency as shown in Figure 1

Figure 1 - Design Criteria Options in AxSTREAM Preliminary Design
Figure 1 – Design Criteria Options in AxSTREAM® Preliminary Design

Power and efficiency are related, but not always the same thing, especially when the boundary conditions are not fixed as design parameters. In AxSTREAM’s Preliminary Design Module, the user can set boundary conditions such as pressure at inlet and outlet, inlet total temperature, etc., as a range instead of a specific value. Along with other parameters, the solver generates hundreds or even thousands of solutions within the range.

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Design of Inlet Guide Vane (IGV) for Centrifugal Compressors

All centrifugal compressor designers want to achieve the highest efficiency as well as wide operating range. With this in mind, the inlet guide vane (IGV) is a convenient and economic option for various applications.

IGVs are a series of blades circumferentially arranged at the inlet of compressor, driven electronically or pneumatically.By adjusting the orientation of IGVs, the air flow enters the impeller at a different direction therefore changing the flow behavior while affecting the passing mass flow rate (throttling). This can effectively reduce the power consumption to increase the compressor’s overall efficiency while avoiding surge to provide a better off design working range.

The designer needs to optimize blade profile and positioning of the IGV for efficient operation of a compressor, which can be a tedious job if one does not have a handy tool. Figure 1 shows an example of IGV working on different angles.

Example IGV Characteristic Curve
Figure 1. Example IGV Characteristic Curve

In AxSTREAM, people are able to add IGV component before the centrifugal compressor impeller which can provide different ways to edit its profile such as: Read More

Gas Turbine Cooling Technology

Turbine Cooling Scheme Designed in AxSTREAM NET
Figure 1. Turbine Cooling Scheme Designed in AxSTREAM NET

People are pushing turbine inlet temperature to extremes to achieve higher power and efficiency. Material scientists have contributed a lot to developing the most durable material under high temperatures such as special steels, titanium alloys and superalloys. However, turbine inlet temperature can be as high as 1700˚C [1] and cooling has to be integrated to the system to prolong blade life, secure operation and achieve economic viability.

A high pressure turbine can use up to 30% of the compressor air for cooling, purge, and leakage flows, which is a huge loss for efficiency. It is worth it only if the gain of turbine inlet temperature can outweigh the loss of cooling. This applies to both aviation engines and land based gas turbines.

The history of turbine cooling goes back 50 years and has evolved to fit different environments. The diversity of turbine cooling technology we see today is just the tip of the iceberg. As time goes on and technology advances, people are able to achieve higher cooling efficiency at lower coolant usages. For different goals and needs, different constructs can be applied but the detailed cooling design must balance with the whole system and make the most of technological advances in the areas. For example, if the flow path is optimized, mechanical design is modified, or if new material is employed, the cooling design needs to change accordingly. One thing worth mentioning is that manufacturing of hot section components and turbine cooling design have an interdependent cause and effect, outpacing and leading each other to new levels. Merging of disciplines and additive manufacturing will, in the future, bring more flexibility to turbine cooling design.

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