Use of 3D printing for micro ORC unit development

The principle of the ORC energy unit is the cycle of a steam engine, where the working substance is an organic medium that receives heat from an external source into a closed loop. The device converts thermal energy into mechanical work, which can be used to generate electricity or to drive other devices (eg cooling compressor). The working medium undergoes phase changes during the cycle. The most commonly used refrigerants as a working medium are those that allow the use of low-potential heat from any energy source, both from the primary source and the waste heat. Then it is a renewable resource with zero emissions and universal use. The aim of our project is the development of a micro ORC cogeneration unit with an output of 1.5 and 5 kW.

Use of micro ORC units

In the case of micro cogeneration, these are outputs up to 50 kWe. The principle of cogeneration is the production of both heat and electricity, both from primary and secondary sources. In the case of primary sources, it will allow us to cover local electricity consumption, reduce transmission losses and allow us to operate in an independent island system. It is a highly efficient process of using primary energy with an efficiency of up to 95%. In the case of waste heat, it is possible to process residual heat from the flue gases of biomass boilers, process heat from food, woodworking, glass and chemical plants or conventional combustion engines used to drive compressors or power plants.

Specifics of the proposed solution

The main criterion of our approach to the design of the micro cogeneration ORC unit was to achieve the highest possible efficiency of the entire plant and low production costs for productive production. This results from the economic advantage of these low-power energy equipment, where the return on investment is on the verge of profitability. So we needed to design a device that was highly efficient to produce electricity and yet cheap. The main components we focused on were the choice of the expander used and the choice of a suitable working medium. The goal was to develop a micro cogeneration unit with an output of 1.5 and 5 kW. Design and power parameters of the 1.5 kW unit:  
Parameter Nominal value Unit
Inlet pressure 10.4 bar
Inlet temperature 100 °C
Heat output of the source 21 kWt
Electrical power at terminals 2.0 kWe
Net electrical power 1.5 kWe
Inlet temperature of cooling water 30 °C
Outlet temperature of cooling water 50 °C
Usable heat 18 kWt
Total efficiency 85 %

Micro turbine

The greatest attention was paid to the choice and design of the expander. We needed to achieve the highest possible efficiency in the widest possible working range of temperatures and steam flow rates, and at the same time we needed the most compact dimensions so that we could supply the whole ORC unit in combination with biomass boilers of 25 kW, Fig. 2. turbine with axial arrangement, partial spray and action impeller.

A project to implement the ORC unit is currently underway, together with a 25 kW pellet boiler and battery storage. The ORC unit will supply 1.5 kWe of constant electrical power.
We designed the micro-turbine unit in a unique arrangement subject to a utility model. The unit includes the turbine itself with the inlet spiral, the partial spray nozzles, the impeller with compensating holes and the outlet diffuser.

The turbine shaft is connected to the generator or alternator gearbox by an electromagnetic clutch. The electromagnetic clutch contains a baffle that hermetically separates the refrigerant circuit from the environment and predestines the entire unit for automatic and maintenance-free operation. The ball bearings are lubricated with refrigerant and no service is required.

Choosing the right refrigerant

Due to its thermodynamic properties, the refrigerant can strongly affect the efficiency of the entire cycle. Thermodynamic efficiency depends on the interaction of thermodynamic properties of the medium such as critical point, specific heat, density, etc. High steam density provides us with smaller volume and thus equipment dimensions, low viscosity then means better heat transfer and lower friction losses, lower evaporation pressure then facilitate the design of the equipment. An important property of the medium is also stability at higher temperatures (elimination of degradation) as well as safety and toxicity of the medium and flammability. Recently, much attention has also been paid to the environmental impact of the GWP (Kyoto, Montreal Protocol and the Paris Agreement). Last but not least, the availability and price of the media is equally important. For the temperature range we use, we chose the R245fa refrigerant, which we eventually replaced with the ecological version of the R1233 ZD (E) refrigerant.

Description of the ORC unit test circuit

The test equipment of the 1.5 kWe micro cogeneration unit was operated in combination with an electric boiler with a heat output of 18 kW, which supplied water to a steam superheater at 1000 C. The steam, which was subsequently processed in a micro turbine, condensed in an air-cooled condenser and supplied via a recuperator to the steam superheater.
Digital prototype of a micro turbine with an electromagnetic clutch The development of the entire ORC micro turbine unit with a generator was carried out with the consistent application of digital prototyping methods. Input values for the design of the current part of the micro turbine:
Parameter Value Unit
Working medium R1233zd (E)
Input temperature at the turbine casing inlet t0 100 °C
Input pressure at the turbine casing inlet p0 10.42 bar
Outlet pressure behind the turbine p2 1.3 bar
Mass flow rate mT 0.366 kg.s-1
Shaft speed nT 35 000 min-1

The complete flow design was performed using the STAR-CCM+ CFD system and the Simcenter HEEDS optimization engine using the Sherpa assistance system to select the most suitable combination of optimization methods.

Furthermore, a static and dynamic inspection of the impeller and electromagnetic clutch was performed. The design of an electromagnetic clutch for high operating speeds of up to 35,000 rpm proved to be critical during development. It was necessary to use high-strength materials for the design of coupling bodies and special mounting of magnets, due to the high stresses from centrifugal forces. All static calculations were performed using the FEA method in Simcenter 3D.

NX Nastran solvers were used to check the dynamics of the two impeller and clutch mass systems.

Prototype production

The production of the components of the ORC micro turbine unit was carried out with the consistent use of 3D printing methods for the creation of working models of the unit and also the functional parts of the turbine housing molds. The flow parts of the turbine were finally machined by 5-axis machining.

Experimental measurements

The measurements took place in a test room, where a hot water electric boiler with an output of 18 kW was used as a source of waste heat.

ORC micro turbine unit:

Entire ORC unit:

The prototype was each equipped with more than 30 measuring points for monitoring physical quantities such as pressures, temperatures, turbine speeds, working medium flows, cooling water, levels in evaporators, etc. for connection to a central control system (regulation, data acquisition). The ORC unit was completely developed, mostly manufactured in cooperation with MCAE Systems. Circuit assembly, connection of pipes and filling with working substance (refrigerants R1233zd (E)) incl. The supply of the measuring control panel and the connection of the sensors took place in cooperation with an external partner qualified to work with refrigerants.
In order to measure the performance characteristics, MCAE Systems has also developed an electrical load (electricity consumer) with proportional power control, which covers the required power range. 

The test results confirmed that the chosen path of the ORC unit concept (see UV) is correct. In this context, it is necessary to continue the trend and further develop the necessary technical and personnel infrastructure, plan an extensive testing program using existing very valuable knowledge and new opportunities in the field of rapid prototyping, such as metal printing, which we have available, etc.
The tests verified the power parameters, turbine efficiencies, overall circuit efficiencies, etc. at the various operating modes envisaged. Furthermore, it is necessary to implement long-term durability and reliability tests with monitoring of wear of unit components to gain knowledge to further improve the functionality and durability of the unit for full commercial use.
The overall results of the prototype behavior confirm the achievement of the expected parameters in the field of micro cogeneration.

In the future, for the further development of the unit on the principle of ORC, it is also necessary to take into account the diversity of possible applications and thus a number of variants of arrangement and use of the unit and related development work.

The new product will be offered to end customers through TechSim Engineering’s own business activities and in cooperation with business partners.

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