Final Report Summary - SPACE TRIPS (Space Thermoacoustic Radio-Isotopic Power System)
Executive Summary:
Summary
The project concerns an advanced thermal to electric conversion for radio-isotopic power systems (RPS). Indeed RPSs are a key for space exploration as the solar power is very low in deep space, notably in Jupiter orbit and beyond. These systems will be also useful for Mars and even Moon exploration, where solar power is subject to nights and dust storms.
So Europe aims to get its independence for such missions, and ESA have initiated development of RPS. If thermoelectricity fits well with small RPS (e.g.20We) for 100We range or more, high efficiency conversion is desirable. Indeed this leads to save between 2/3 up to 3/4 of the radioisotope mass. This is of real importance in term of cost and safety. Stirling converters under development in USA have low reliability due to pistons (sensitive to launch vibrations and shocks, subject to wear).
Thermo acoustic (TAc), coupled with magneto hydrodynamic (MHD) generator is innovating technology free of moving parts. Unfortunately, the Technological Readiness Level of such an association is low and the priority is still currently given to Stirling in ESA’s programme, even if ESA has supported the first studies of TAc-MHD systems. This project is complementary with ESA’s approach.
So, the objective of this Project is to raise the TRL of this technology from 2 to 3-4 and show that this option is realistic for European RPSs. The approach is based on 3 axes:
− Theoretical modeling, which has been already developed but needs to be validated,
− Experimentation of a thermo acoustic engine coupled with a MHD generator,
− Design of the space RPS, equipped with this conversion system, to check if the technology is suitable for space mission.
The targets are:
− to validate the process efficiency (close to 20% or above),
− to justify the compatibility of the technology with space missions,
In Europe, there is a strong expertise in thermo acoustic and MHD. But these technologies have never been coupled. The consortium, coordinated by the start-up company HEKYOM, associates three research organizations: CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) in France, IPUL in Latvia and Rossendorf in Germany. The space and nuclear industries are represented by THALES ALENIA Space-Italy and AREVA. THALES ALENIA Space represents also the end-user in term of space systems.
Project Context and Objectives:
Summary description of the project context and the main objectives
1- Objectives: Summary description
Objective: Energy is always a key, for space systems as well as for all terrestrial systems. For space systems, solar arrays well suit in Earth orbit and near space missions. Unfortunately, this technology is less suitable for deep space missions and for planets exploration. Indeed, distance and orientation from the Sun for deep space missions or nights and atmosphere for planet exploration could make solar power irrelevant. Consequently other energy sources must be used, and the only recognized candidate is nuclear power, and typically radio-isotopic power systems for low power range (from Watts to hundreds Watts).
Radio-isotopic power systems (RPSs) have been developed mainly by USA, and the question is raised for Europe to get its own technology, allowing higher programmatic independence.
Beyond the nuclear source, the conversion system is a key technology which must be reliable but also efficient. Indeed, a high efficiency leads to save a significant amount of nuclear material, i.e. to lower the cost of the mission and also to lower the radiological risk at launch.
So both reliability and efficiency are essential for RPS but existing devices match either reliability or efficiency. Thermoelectric (Seebeck based) conversion is highly reliable, but efficiency is low, less than 5%. High efficiency requires dynamic conversion and USA have developed a Stirling converter, but this is a piston machine and the reliability is questionable, intrinsically.
This project relates to the conversion of thermal energy to electrical proposing a technology which is reliable (free of moving parts) while highly efficient (dynamic cycle). The proposed conversion system is based on thermo acoustic effect coupled with a magneto hydrodynamic generator.
Thermo acoustic is the object of extensive research works in Europe and in the world. The targeted applications are electricity generation and cooling devices. At the moment there is no industrial product, only prototypes have been developed and tested.
The interest of magneto hydrodynamic is to allow electricity generation without piston, or other mechanical device, that would be subject to wear. Coupling thermo acoustic with hydrodynamic generation is a new concept that needs to be validated.
2- Concept – Thermo acoustic and MHD short presentation
The concept is based on the coupling of a thermo acoustic engine and magneto hydrodynamic (MHD) generator. The thermo acoustic engine converts thermal energy into mechanical energy, and then the MHD generator converts the mechanical energy into electricity.
The descriptions given bellow are summarised, the objective is only to remind the physical phenomena.
THERMO ACOUSTIC
The thermo-acoustic effect is a thermodynamic process allowing transforming heat into mechanical energy. The engine based on this principle is extremely simple, even if the mathematical model is not so easy, and the physic difficult to explain
The main element of the engine is a kind of heat exchanger with an extremely high heat exchange surface. As the heat exchange takes place only into the boundary layer (interface between solid and gas), this element is designed for getting a great number of boundary layers as for instance with a “stack” of parallel plates located between the two heat exchanger connected to external heat sources. This “stack” can be placed in either a linear tube or a loop. When a temperature difference greater than a critical value is imposed on the both ends of the stack by means of the two heat exchangers, a pressure wave is spontaneously generated which propagates in the tube at the speed of sound. The amplitude of this pressure wave depends of many factors such as: geometry of the engine and physical properties of the fluid and the stack, temperature gradient along the stack and mean pressure of the thermodynamic fluid. It is admitted that, in good conditions, the amplitude of the pressure oscillation can reach up to 10% of the imposed mean pressure. The best fluid generally invoked in the literature seems to be helium due to its relatively high value of the heat transfer coefficient. However, in the space TRIPS project Aragon was chosen for thermos dynamics reason. Based on this principle two types of engine can be considered:
1- A standing wave engine using a linear tube. In this case the wave is oscillating in a tube and the mechanical power is proportional to the scalar product of the oscillating pressure by the oscillating velocity. if a proper system of energy transfer is applied acoustic energy could be transferred. Note that a standing wave field is such that acoustic pressure and displacement are in phase and lead to induce into the gas a Brayton cycle.
2- The travelling wave concept uses a close loop in which the thermo-acoustic wave propagates continuously. In this case, a new element, the thermo-buffer tube must be added to adjust the temperature at the hot and cold sources. This case seems to be better on the thermodynamics point of view because it is more reversible than the standing wave concept. Indeed, acoustic pressure and displacement are that time out of phase and this configuration allows performing a Stirling thermodynamic cycle. A thermo-acoustic engine is then a thermodynamic engine which converts thermal energy into mechanical energy.
In both cases, the process is quasi static in the sense that the mechanical energy is generated on the form of a fluid vibration without any global circulation. The thermodynamic cycle is thus close to the Stirling cycle. This is why its efficiency is attractive: at least 50% of Carnot efficiency is expected. Like the Stirling engine, the thermo-acoustic engine is a reversible process able to produce cold. It is much simpler than the Stirling engine and does not involve any mechanical moving part.
Thermoacoustic design of a suitable TAC was in fact a huge work.
It must be emphazised that TAC has to drive a quite specific acoustic impedance, that is the MHD generator that is strongly complex (nearly 80% imaginary).
4 acoustic configrations were computed. Although everyone compatible with MHD generator, it was decided to choose the one for which no starting difficulties could eventually occurred. (see other deliverables)
The adopted configuration for the coupling between TAC engine and MHD generator is such as: The MHD generator is located in “a diameter” of a toroidal TAC loop. The two active elements (TAc engines 1 and TAc engine 2) works in push pull conditions imposing a Thermo acoustic wave travel around the toroidal structure. So when the pressure wave has maximum amplitude one one side the amplitude is minimum on the other side and reversibely according the frequency evolution of the thermo acoustic wave. Theoretical models of the phenomena have been developed, and experimentations have been carried out.
MHD GENERATOR
The problem now is to convert the mechanical energy supplied in the form of a vibration into electrical energy. An MHD generator could be used to realize this function. The process uses the induction mechanism.
When a pulsating magnetic field is applied through a closed coil, the magnetic flux variation induces an electric current in the electrical circuit of the coil able to supply the load represented here by an electrical resistance.
To produce the pulsating magnetic field, the process uses the interaction of an imposed DC magnetic field, with a pulsating liquid metal flow . This pulsating liquid metal flow is produced by the interaction with the pulsating gas created by thermos acoustic effect, so the gas imposes its vibration at the interface between the two fluids, i.e. gas/liquid metal. The liquid metal, here the sodium due to its remarkable properties, is located inside a tube submitted to its two extremity to the pulsating force imposed by the gas. A toroidal permanent magnet surrounding the sodium tube produces a radial DC magnetic field which interacts with the liquid metal to generating an induced toroidal AC electrical current. This AC current generates itself an induced AC magnetic field which produces a magnetic flux variation in a coil connected with the load represented in spaceTRIPS by a resistanc
The main transfert of energies are recalled. The radio isotopic elements are the hot source of the system and supply a thermo acoustic loop by conduction process. The thermo acoustic loop converts thermal energy in mechanical one and this mechanical energy is introduced inside the MHD generator to produce electricity. The residual heat that have not been transfromed in mechanical energy is transferred at the cold source and evacuated in space by radiation.
3-Goal of the project
The space TRIPS project has consisted in:
To study, built and test a earth prototype of a thermo acoustic/MHD generator for space application at a level of electric power around 200 W.
In space the hot source will be constituted by radio isotopic element (around 1000 W) thermal power being introduced at the hot source of the system by conduction.
Cold source will be a radiative cooler evacuating heat in space by radiation. The surface of exchange of this cold source will be around 1 m2. The heat to be evacuated will be transferred from the thermo acoustic engine to the radiator also by conducytion.
On the earth prototype.
The hot source will be constituted by electrical resistances, which transmit the thermal power by the same elements than that for the space prototype.
The cold source will be only a circulation of cold water in a classical cold exchanger.
Project Results:
Description of the main S & T results/foregrounds
The main description and main results of the work realised are reported below, they concerns:
- The design, thermo acoustic loop, MHD generator and the assembly of both.
- The construction, thermos acoustic loop, MHD generator and the assembly of both
- The tests
- And a draft of design of an electrical generator for space.
1-The design: Thermo acoustic loop.
The detailed design is reported on the deliverable D2.1: Tac MHD design file. The two elements that composes the electrical generator, i.e.; the thermos acoustic loop and the MHD generator was studied separately, but they was studied to be connected. The global and detailed designs was done by the SERAS (CNRS design office in Grenoble France).
It is important to notice that this two engine have been studied for the first time. For the thermos acoustic loop the adopted geometry is completely original and newer been realised and tested, and the concept of MHD generator is completely new concept never imagined and newer studied in the past.
From the main difficulties encountered for the study of the concept have been concerned with three types of elements.
Firstly: to realise the heat transfer by minimising the thermal losses, and in parallel to take into account the deformation due to the temperature level and the mechanical constraint generated by this level of temperature and the temperature gradient.
Secondly: In parallel with the temperature problem the thermos acoustic loop and MHD generator have to resist against a high level of mean pressure, around 40 b which is the working condition of then thermos acoustic loop and to take into account the oscillation of pressure that can reach several bar at frequency around 50 Hz.
Thirdly, the combination of both the temperature level and high-pressure level make difficult to solve the tightness problem.
Additively the problem of vibration able to produce resonance have been also studied.
2-The construction:
TAc engine manufacture.
The construction of the loop was done by IPUL (Institute of Physics of the University of Latvia. But an helps was obtained from the SERAS offices (CNRS Grenoble) during the construction. In particular, special tools were designed and constructed to arrange the magnets in their housing. This tool is indispensable because the strong forces exerted by the magnets do not permit to displace them with the hands. Main parts of the thermo acoustic engine was realised at the IPUL by the technician men of this laboratory, excepted important elements that was subcontracting, zirconia flanges importation from china, Titanium flange that was operated by 3D prototyping by the society (SOREMET). The cold exchanger and the regenerator was realised by the same French company.
Some difficulties appeared along the construction. The pieces ordered in china, and using zirconia material, which has good properties, it is a good isolating material, with some relatively good mechanical properties, have brooken during the assembly. The problem comes probably from the quality of the material realised in china , this is one hypothesis the other one being that the mechanical properties of this material is not well known at high temperature and the estimation done from the properties at low temperature level was not sufficiently precise. So these pieces that are mainly four flanges, have been rebuilt by machining, using Inconel instead off zirconia. In parallel new seals was ordered for adaptation with the new flanges.
To realise the tests it was necessary to fix the thermos acoustic loop on a special chassis able to absorb the vibration generated by the thermos acoustic effect.
Last thing for the tests of the thermos acoustic loop alone, that means without the connection with the MHD generator, a special element was realised, under the specification done by Hekyom company specialised in thermos acoustic, with the objective to dissipate the mechanical energy furnished by the thermos acoustic loop. This element was a simple guillotine walve, dissipating the mechanical energy by viscous friction.
MHD machine manufacture.
Main of the pieces were realised by IPUL, by classical machines, under the direction of of Janis Freibergs dorector of IPUL. Some parts was subcontracted.
For the assembly some very specific tools was machined, having been designed by the SERAS office (CNRS Grenoble).This specific tool were realised for the assembling of the magnets on the generator. This is necessary by the by the fact that is impossible to maintain the magnets with the hand due to the intense magnetic forces between the Ferro magnetic material and the magnets. Therefore, the magnets are mounted with glue on a special device and approached slowly, by screw up to the contact with the MHD generator. When the contact is established, the screw is removed that is possible because the magnetic force is higher than the glue is able to support. So step by step, reproaching two magnets located on a diameter alternatively, all magnets was placed on the MHD generator.
After this operation the coil was wound on the magnets, (and the Ferro magnetic material used for the closure of the magnetic field, assembled externally to the coil. Then all these elements was introduced in the external envelope surrounding the generator.
No special difficulties excepted the magnet arrangement was observed during the realisation of the MHD generator.
Just a last remark, a new material was used for the realisation of the magnetic magnetic circuit. It is called SOMALOY. SOMALOY is a Ferro magnetic material realised by sintering which has a very good magnetic permeability being constituted by a mixing of Ferro magnetic powder in an isolating matrix. The interest of this new material is due to the fact that it can be machined and so it is accessible to complicated shape not possible with the classical Ferro magnetic sheet used for the realisation of electrical transformer by example.
3-Tests
Test of the electrical generator.
The tests were realised at first of the two engine separately. For that a specific installation was studied and built by IPUL, specifically to test the MHD generator.a system able to simulate the oscillation imposed by the thermo acoustic effect was realised. The used system is constituted by two compressor and a rotating walve imposing the oscillation of pressure inside the MHD generator. The first series of tests was not satisfactory due to the instabilities at the free at the interface between the sodium and the gas. This difficultiy was anticipated and was studied in the frame of the project under two possibilities.
- The first one was studied by Thales Alenia space Italy consisting in a. sliding piston. TAS-I was in charge of investigating the use of a physical (solid) separation between the fluids to counteract phase mixing due both to launch mechanical loads and microgravity environment.
TAS-I performed a first investigation of materials and layouts. As a result, it became clear that the most promising solution was the usage of a metallic sliding piston. The ensuing study had an extremely specific nature involving metallurgy and chemical compatibility of materials, physics and chemistry of liquid metals, and mechanical design of small scale pulsating interfaces. For these specific aspects TAS-I had no past heritage nor dedicated niche expertise. Therefore, the decision was taken to subcontract that part of the activity to a partner university, the Politecnico di Torino
- The second one was based on the use of Surface tension possibilities: HZDR activities. Many experiments were performed about the stability of the interface between gas and liquid metal under high frequency high amplitude pulsating motion. The facility was mainly what was presented in the previous reports but the tests were focused on the use of honey combs. Using classical honey comb do not permit to reach the high frequencies because the level of power required over pass the possibility of the generator. So the solution was to reduce to one unique element, one tube of 1 mm of diameter; the number of mesh. In this case it was possible to reach the full frequency even if the amplitude of the motion were less than expected on the TAC/MHD generator. But any perturbation of the free surface was observed in this condition confirming the theoretical approach.
- The third one,Floating piston..But these two solutions that are very relevant for space are relatively difficult and particularely expensive. Therefore After the last review meeting that taken place at the REA offices in Brussels, January 15, it was decided to test another possibility of control of the interface. In close collaboration with the SERAS offices of the CNRS Grenoble, the society Hekyom together with Aster society, and with the participation of AREVA TA, a concept of floating piston was propose designed and tested in a specific loop using water instead of sodium and Plexiglas tube to visualise the interface evolution. The principle is to ask a hollow floating piston, 0.2 mm thick, and very lightweight about 16 g on the free surface between the argon of thermo acoustic loop and the liquid sodium. The piston then penetrates about 1 cm in the sodium. Therefore it perfectly follows the evolution of the free surface since all abruption in relation to this surface created a vacuum that effectively pick the piston on the interface. So the system reduces almost completely the free surface perturbations. On the other hand to equilibrate the pressure inside and outside the floating piston, a tube was welded on the top of the piston allowing this necessary equilibrium. Of course because the system presents two identical interfaces (free surfaces) two identical pistons were realised .
This last solution, floating piston was used with success and was addpted for the global tests the two engine after assembly.
Test of the thermo acoustic loop.
The thermo acoustic loop has been tested separately from the MHD generator, with the help of two specialised company (the alones in Europe to work specifically on thermo acoustic applications), Hekyom leader of spaceTRIPS project and the society ASTER. The first tests done at low level of temperature a little less than 400°C at the hot source and mean level of pressure about 25 b was sufficiently convincing to decide to test the complete electrical generator resulting from the assembly of the two engine, Thermo acoustic loop and MHD generator. The signal was obtained at 58 Hz instead of the 45 that was initially calculated. This can be corrected by adjusting the impedance of the thermo acoustic loop that can be obtained by modification of the length of the loop. But due to the delay already accepted from the E.U. to finish the program it was decided to conserve this frequency that diminish a little the efficiency.
The test of the electrical generator after assembly of the two parts, Thermo acoustic loop and MHD generator.
After assembly of the two part of the electrical generator a first series of global tests were performed at low level of power, introduced only 300 W of thermal power at the level of the hot exchanger, and a level of temperature of order of 350 to 400 °C. Three level of mean pressure was tested corresponding to
25 b
30 b
40 b.
As explained above the mean temperature imposes the amplitude of the oscillating pressure. So increasing the mean level allows increasing the level of power produced.
The main results are reported in the following table for different value of the load resistance. As it can be seen on the three tables corresponding to three different levels of the mean pressure, 25, 30 and 40 b, the best results in term of efficiency of the MHD generator is obtained for the lowest level of pressure but the level of power is the highest for high level of pressure.
A second series of tests is planned at higher level of temperature, and pressure but are not reported in the present report. So the new results will be reported latter.
Mean pressure: 25 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Thermal energy 300 W 300 W 300 W 300 W 300 W
Oscillating pressure (b) 0.486 0,498 0,503 0,5198 0,476
Flow rate (l/s) 0,293 0,302 0,296 0,3023 0,281
cos 0,761 0,753 0,748 0,735 0,769
Current (A) 0,331 0,214 0,156 0,076 0,63
Voltage (V) 16,729 21,224 23,415 26,1 4,82
Electrical energy (W) 2,77 2,266 1,82 0,997 3,724
Mechanical energy (W) 10,84 11,32 11,16 11,55 10,289
Coil energy (R~9.2 ) 1,006 0,42 0,22 0,054 1,534
Total electrical efficiency 0,348 0,237 0,183 0,091 0,511
Main results for mean pressure 25 b
Mean pressure: 30 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Oscillating pressure (b) 0,863 0,876 0,908 0,999 0,821
Flow rate (l/s) 0,538 0,536 0,55 0,55 0,517
cos 0,638 0,637 0,634 0,626 0,641
Current (A) 0,507 0,33 0,24 0,119 0,989
Voltage (V) 25,77 32,63 36,06 40,47 7,48
Electrical energy (W) 6,54 5,38 4,32 2,404 3,701
Mechanical energy (W) 29,65 29,897 31,747 34,48 27,26
Coil energy (R~9.2) 2,37 1,002 0,529 0,13 9,01
Total electrical efficiency 0,3 0,214 0,153 0,077 0,474
Main results for mean pressure 30 b.
Mean pressure 40 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Oscillating pressure (b) 0,845 0,85 0,85 0,85 0,825
Flow rate (l/s) 0,493 0,499 0,472 0,503 0,482
cos 0,69 0,68 0,675 0,65 0,719
Current (A) 0,458 0,3 0,22 0,11 0,846
Voltage (V) 23,26 30,055 32,36 37,51 6,44
Electrical energy (W) 5,33 4,55 3,48 2,07 2,726
Mechanical energy (W) 28,72 28,84 27,085 27,78 28,62
Coil energy (R~9.2 ) 1,93 0,84 0,43 0,11 6,596
Total electrical efficiency 0,25 0,19 0,144 0,079 0,33
Main results for mean pressure 40 b
4-A draft of design of an electrical generator for space.
Cold source definition
This task aims to propose a design for the cold source. The cold source must operate with atmosphere (e.g. on Earth before launch or on Mars) and in vacuum (e.g. during space transit). Operating temperature will be driven by the inputs issued from modelling. Technological options will be analysed in order to select the most attractive option in terms of performance, mass and volume, feasibility. The cold source consists of:
- The cold heat exchanger
-The heat transport device (likely heat pipes)
-The radiator itself which is a radiatice cooler of surface of exchange of about 1 m2.
Moreover, an additional cold source should be necessary for the phases of the mission where radiator cannot be used (e.g. during the launch).The cold source will be design, this design being integrated in the reference design. The task leader is THALES ALENIA SPACE ITALIA, Schedule: To+14 - To+20, with the benefit of collaborations with AREVA TA and CNRS Grenoble.
Overall, these tasks aim to draw up architecture for the SRPS, propose a design for the cold source, and refine the definition according to iterations performed in the overall Space TRIPS project.
Hot source Definition.
This task aims to propose a design for the hot source of the thermo acoustic loop. The power is provided by a radio isotopic source, heating permanently. The hot source is supposed to be one or several modules, according to works performed under ESA’s contract. Operating temperature will be driven by the inputs issued from modelling. The main issue is the thermal and mechanical interface of the modules with the thermo acoustic loop. The thermal interface must be efficient, in order to transfer the heat into the loop, minimizing the loss. The source must also be thermally insulated in order to force the heat into the loop. The mechanical interface will be driven by mechanical loads (thermo acoustic vibrations in operation, loads at launch) but also by the integration of the source in the system. Indeed the source is radioactive and the possibility of a late access is of importance. The task leader is AREVA, working in collaboration with CNRS and Thales
Draft of design of the space prototype.
A draft design of the electrical generator for space was performed by AREVA TA. The objective is to regroup all the element of the engine in a very small volume. All the elements mean hot source cold source, thermo acoustic loop and MHD generator. One of the most important requirements from a nuclear safety point of view is that the Heat Source modules must be freely released in case of inadvertent re-entry. That is why the heat spreader is not a closed box. In order to maintain each Heat Source module in its housing, a rod attached to the main housing and pushing the Heat Source module is foreseen. This rod includes a spring to insure that the pressure will remain even in case of small bending. In case of inadvertent re-entry, the structure quickly burns, the rods are released and consequently the heat source modules are released in turn. Another important consequence of this choice is that the insulation material is not supporting the weight of the Hot Source.
This last design was compared to the MMRTG (RTG currently used on Mars) regarding the electrical power delivered, the efficiency, the volume and the mass. We can see, as shown on the figure below, that:
• The two volumes are quite similar,
• The electrical power delivered is a little higher
• The expected efficiency is more than twice the efficiency of the MMRTG,
•
The mass is significantly higher than the one of the MMRTG, but this is mainly due to the choice of the radioisotope (241Am instead of 238Pu whose mass at equivalent power delivered is 5 times higher
Potential Impact:
Dissemination and social impact of the project.
1-The dissemination of the results.
It was an important activity of the partners of the project. The social impact of the dissemination was very efficient.
The dissemination. An international conference was organised in Latvia Riga and have hosted about 200 participants coming from about 25 countries. The proceeding have been reported in the participant portal of space TRIPS. In parallel with this conference, a summer school devoted to the space technology was organised in parallel at the same location and same period. About 40 students attended this summer school from about 10 European countries.
The summary of the dissemination that comprises also workshop, participation in international conference and seminar is reported below.
- For the publications the partners will take advantage of the international journal Magneto Hydro Dynamics which is under the responsibility of the LATVIJAS UNIVERSITATES AGENTURA LATVIJAS UNIVERSITATES FIZIKAS INSTITUTS (partner 3).
- Two main events was organised this year under the responsibility of Thales Alenia space and AREVA TA They concern the organisation of a workshop that taken place in Torino on November 19 in the frame of aerospace symposium. The other one was organised at the ESA offices in December 8.
- Other contributions in the dissemination were achieved by the partners of spaceTRIPS consisting in oral presentation in international conferences. In particular two papers were accepted for oral presentation at the”3th International workshop on Thermo acoustics Enschede, October 26-28, 2015”.
- Also some seminars were given especially at the CNES office in Paris focused on the main principles of thermo acoustic and its coupling with MHD generators. another one in Italy in the CIRA offices where the program was explained in the frame of a proposal of cooperation for a new project focused on the solar sail submitted to the EU program H2020, FETOPEN.
Mains results
The selection of the best papers presented at the pamir 2014 conference have been published in international Latvian journal Magneto Hydrodynamics in August 2015. This important works, especially in the selection of papers and printing, was realised by the partnership of spaceTRIPS, the edition of the two special (number 52 and 53) issues having been done by the Latvian partner.
An other very important results obtained by this effort about the dissemination concerns the submission of a new project devoted to the use of a combination of thermo acoustic and MHD, “Prometheus”, acronym of Plasma-based Research On Magnetogasdynamics Effected by THermoacoustics for Electrical Unmoving generatorS, based on the use of an ionised gas that allows, if the feasibility will be verified, to suppress the interface liquid and gas that could be an important improvement for space application; This project was submitted in the frame of H2020, FET OPEN.
2-Impact of the project.
The oral presentation of the space TRIPS project, at the international conference organised by the European Union in Roma for the H2020 presentation (September 2014) was followed by many discussion with the other participants. It results from this discussion a new project Prometheus which concerns the use of a coupling between thermo acoustic loop and MHD generator. The project is based on the realisation of a loop at high level of electrical power for energy sources at various applications including space for power supply of MHD thruster.
The main elements of this new project called Prometheus are reported here after.
This type of generator is also particularly suitable for the exploitation of renewable energies (solar energy for example) as well as for waste recovery.
Following the Horizon 2020 program presented in September 2014 in Roma a new project devoted to high electrical power able for example to supply MHD thrusters in space, based on the use of a combination of Thermo acoustic loop and MHD generator have been presented in collaboration with many partners from 4 countries, French, Italy, Poland and Spain. In the new system the MHD generator is based on the use of ionised gas instead off liquid metal, supressing the problem of interface between liquid and gas.
This new project enjoys a demonstration of interest expressed by the CNES. It is called Prometheus for:
Plasma-based Research On Magnetogasdynamics Effected by Thermoacoustics for Electrical Unmoving generatorS and benefit of the participation of 8 partners from 4 european countries.
No Participant organisation name Short name: Country
1 University of Cagliari (Coordinator) UniCA Italy
2 HEKYOM Hekyom France
3 Centre National de la Recherche Scientifique CNRS France
4 Added Value Solutions AVS Spain
5 Polish Academy of Science PAS Poland
6 Ingegneria Sistemi Impianti Servizi ISIS Italy
7 Consorzio RFX RFX Italy
8 Commisariat à l'énergie atomique et aux énergies alternatives CEA France
Novelty, level of ambition and foundational character
The technology proposed in this Project (Patent No. 21348EP, owned by AREVA-TA, CNRS, Univ. Cagliari) offers a solution to all the limits of space TRIPS and in particular it allows for suppressing the gas/liquid interface. The working principle of the proposed technology is depicted. The stack, submitted to a high temperature gradient imposed by the hot source and cold source of the system, generates the thermoacoustic vibrations. In the same volume, an electrical discharge ionizes the gas, and then the charge carriers are separated by a stationary external electrical field. The ions are captured in a first stack and the electrons in a second one. The electrical field is applied to the gas by means of a capacitor. The electrical field maintains the charge carriers in a static equilibrium. When the thermo acoustic vibrations occur, the charges participates to the oscillating motion, giving rise to an alternate electrical current. Such current induces an alternated electromotive force into a coil surrounding the duct, wrapped all around a toroidal ferromagnetic core. Finally, this coil is connected to the electric load. It is possible to adjust voltage and intensity of the induced current by properly choosing the number of turn of the coil.
In the same manner than for the SpaceTRIPS PROJECT, The thermoacoustic effect is obtained by means of two stacks located into a toroidal loop, and working in push pull . Concerning the hot and cold sources, they can be based on the use of heat pipe (sodium heat pipe at the hot source, around 800 °C) that maintains the advantage of static system, which is important for space application using small nuclear reactor at hot source and radiative cooler at the cold source.
The described system exploits some physical phenomena whose technological aspects are well established, such as thermo acoustics and waves propagation, while other ones, such as charge generation and confinement and magnetic induction of ionic currents, need to be further investigated. The gas, which constitutes the working fluid, has to fulfil both fluidodynamic and electrical properties, because it has to promote the occurrence of vibration, but at the same time the charge carriers should have low electrical mobility, because the electrical forces hinder their vibration. A second aspect concerns the charge confinement. The conversion process is strongly affected by the quantity of involved charge carrier and, consequently the density of charge carrier should be maximized. To this end, in order to maintain the applied voltage within reasonable values, the best solution is maximizing the capacitance, by increasing the surface of the plates. The thermoacoustic stack having the same requirement, therefore it is worth evaluating the possibility to use a unique element as capacitor plate and thermoacoustic stack. The stacks could be represented by a great number of parallel sheets, electrically connected among them by a sleeve. The sheets could be very thin, while the gap among them has to be optimized in order to maximize the value of the ionic current. This way of charge confinement has important analogies with technology of membranes, which are used in electrochemical energy conversion and storage devices (P.N. Pintauro, Polymer Reviews, 55:201–207, 2015). That analogy could be usefully exploited in developing the charge confinement system in the Project. In particular, it is important that the charge carriers stay far from the walls, where the velocity of particles is null. This can be obtained by properly setting the external voltage applied to the capacitor. Indeed, the clouds of charge are surrounded by the plates (see Fig. 4) then they are drawn by opposite forces which allows the charges do not stack on the wall, as usually happens in the capacitors. The ionic alternate current works like a primary winding of a transformer, while the toroidal coils are the secondary winding.
The conversion rate of the process depends on many parameters, such as charge density, cross section of the duct, frequency and amplitude of the vibration, output power, and range of temperatures. On their turn, such parameters depend on several design parameters, which have to be designed for each specific application.
Thermo acoustic and solar energy.
In space: Another very important project which is currently studied concerns a “space” project. This new possible application of thermo acoustic was a result of the summer school organized in Latvia (Riga September 2014) in which have participated Xavier Genest from ESA. The new project concerns the use of a parabolic solar collector in space as hot source and classically a radiative cooler as cold source. The thermal energy is transmitted to the thermo acoustic loop by a heat pipe (sodium Heat pipe) working at high level of temperature. The thermal energy is converted into mechanical energy by thermo acoustic and mechanical energy is transformed in electricity by a specific alternator.
This project is supported also by the research and development departement of AIRBUS Industry.
On the earth: Many program are in course of development by Hekyom Company, resulting of the of space TRIPS project. They are reported below.
Thermo acoustic electrical generator supplied by stoked solar energy. Three concepts are under study:
1- Thermo acoustic engine at high efficiency (>50%) without any moving part and very low maintenance
2- heat solar harnessing by parabolic concentrator, air gas transfering , solid particle for heat stocking : 2 cases +:
a. high temperature (concentration) 600°C
b. middle température (Fresnel) 300°C
c. possible complementary heat source by means of gas or biomass combustion for a 24/24-7/7 available power delivery
3- Conversion acoustic to electric by means of high reliability bidirectional turbine + alternator.
Notes: following projects underway or completed either by HEKYOM at TRL6-7 or by IDHELIO at TRL5-6
List of Websites:
http://space-trips.eu.seventhframework.sal.lv/
Summary
The project concerns an advanced thermal to electric conversion for radio-isotopic power systems (RPS). Indeed RPSs are a key for space exploration as the solar power is very low in deep space, notably in Jupiter orbit and beyond. These systems will be also useful for Mars and even Moon exploration, where solar power is subject to nights and dust storms.
So Europe aims to get its independence for such missions, and ESA have initiated development of RPS. If thermoelectricity fits well with small RPS (e.g.20We) for 100We range or more, high efficiency conversion is desirable. Indeed this leads to save between 2/3 up to 3/4 of the radioisotope mass. This is of real importance in term of cost and safety. Stirling converters under development in USA have low reliability due to pistons (sensitive to launch vibrations and shocks, subject to wear).
Thermo acoustic (TAc), coupled with magneto hydrodynamic (MHD) generator is innovating technology free of moving parts. Unfortunately, the Technological Readiness Level of such an association is low and the priority is still currently given to Stirling in ESA’s programme, even if ESA has supported the first studies of TAc-MHD systems. This project is complementary with ESA’s approach.
So, the objective of this Project is to raise the TRL of this technology from 2 to 3-4 and show that this option is realistic for European RPSs. The approach is based on 3 axes:
− Theoretical modeling, which has been already developed but needs to be validated,
− Experimentation of a thermo acoustic engine coupled with a MHD generator,
− Design of the space RPS, equipped with this conversion system, to check if the technology is suitable for space mission.
The targets are:
− to validate the process efficiency (close to 20% or above),
− to justify the compatibility of the technology with space missions,
In Europe, there is a strong expertise in thermo acoustic and MHD. But these technologies have never been coupled. The consortium, coordinated by the start-up company HEKYOM, associates three research organizations: CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) in France, IPUL in Latvia and Rossendorf in Germany. The space and nuclear industries are represented by THALES ALENIA Space-Italy and AREVA. THALES ALENIA Space represents also the end-user in term of space systems.
Project Context and Objectives:
Summary description of the project context and the main objectives
1- Objectives: Summary description
Objective: Energy is always a key, for space systems as well as for all terrestrial systems. For space systems, solar arrays well suit in Earth orbit and near space missions. Unfortunately, this technology is less suitable for deep space missions and for planets exploration. Indeed, distance and orientation from the Sun for deep space missions or nights and atmosphere for planet exploration could make solar power irrelevant. Consequently other energy sources must be used, and the only recognized candidate is nuclear power, and typically radio-isotopic power systems for low power range (from Watts to hundreds Watts).
Radio-isotopic power systems (RPSs) have been developed mainly by USA, and the question is raised for Europe to get its own technology, allowing higher programmatic independence.
Beyond the nuclear source, the conversion system is a key technology which must be reliable but also efficient. Indeed, a high efficiency leads to save a significant amount of nuclear material, i.e. to lower the cost of the mission and also to lower the radiological risk at launch.
So both reliability and efficiency are essential for RPS but existing devices match either reliability or efficiency. Thermoelectric (Seebeck based) conversion is highly reliable, but efficiency is low, less than 5%. High efficiency requires dynamic conversion and USA have developed a Stirling converter, but this is a piston machine and the reliability is questionable, intrinsically.
This project relates to the conversion of thermal energy to electrical proposing a technology which is reliable (free of moving parts) while highly efficient (dynamic cycle). The proposed conversion system is based on thermo acoustic effect coupled with a magneto hydrodynamic generator.
Thermo acoustic is the object of extensive research works in Europe and in the world. The targeted applications are electricity generation and cooling devices. At the moment there is no industrial product, only prototypes have been developed and tested.
The interest of magneto hydrodynamic is to allow electricity generation without piston, or other mechanical device, that would be subject to wear. Coupling thermo acoustic with hydrodynamic generation is a new concept that needs to be validated.
2- Concept – Thermo acoustic and MHD short presentation
The concept is based on the coupling of a thermo acoustic engine and magneto hydrodynamic (MHD) generator. The thermo acoustic engine converts thermal energy into mechanical energy, and then the MHD generator converts the mechanical energy into electricity.
The descriptions given bellow are summarised, the objective is only to remind the physical phenomena.
THERMO ACOUSTIC
The thermo-acoustic effect is a thermodynamic process allowing transforming heat into mechanical energy. The engine based on this principle is extremely simple, even if the mathematical model is not so easy, and the physic difficult to explain
The main element of the engine is a kind of heat exchanger with an extremely high heat exchange surface. As the heat exchange takes place only into the boundary layer (interface between solid and gas), this element is designed for getting a great number of boundary layers as for instance with a “stack” of parallel plates located between the two heat exchanger connected to external heat sources. This “stack” can be placed in either a linear tube or a loop. When a temperature difference greater than a critical value is imposed on the both ends of the stack by means of the two heat exchangers, a pressure wave is spontaneously generated which propagates in the tube at the speed of sound. The amplitude of this pressure wave depends of many factors such as: geometry of the engine and physical properties of the fluid and the stack, temperature gradient along the stack and mean pressure of the thermodynamic fluid. It is admitted that, in good conditions, the amplitude of the pressure oscillation can reach up to 10% of the imposed mean pressure. The best fluid generally invoked in the literature seems to be helium due to its relatively high value of the heat transfer coefficient. However, in the space TRIPS project Aragon was chosen for thermos dynamics reason. Based on this principle two types of engine can be considered:
1- A standing wave engine using a linear tube. In this case the wave is oscillating in a tube and the mechanical power is proportional to the scalar product of the oscillating pressure by the oscillating velocity. if a proper system of energy transfer is applied acoustic energy could be transferred. Note that a standing wave field is such that acoustic pressure and displacement are in phase and lead to induce into the gas a Brayton cycle.
2- The travelling wave concept uses a close loop in which the thermo-acoustic wave propagates continuously. In this case, a new element, the thermo-buffer tube must be added to adjust the temperature at the hot and cold sources. This case seems to be better on the thermodynamics point of view because it is more reversible than the standing wave concept. Indeed, acoustic pressure and displacement are that time out of phase and this configuration allows performing a Stirling thermodynamic cycle. A thermo-acoustic engine is then a thermodynamic engine which converts thermal energy into mechanical energy.
In both cases, the process is quasi static in the sense that the mechanical energy is generated on the form of a fluid vibration without any global circulation. The thermodynamic cycle is thus close to the Stirling cycle. This is why its efficiency is attractive: at least 50% of Carnot efficiency is expected. Like the Stirling engine, the thermo-acoustic engine is a reversible process able to produce cold. It is much simpler than the Stirling engine and does not involve any mechanical moving part.
Thermoacoustic design of a suitable TAC was in fact a huge work.
It must be emphazised that TAC has to drive a quite specific acoustic impedance, that is the MHD generator that is strongly complex (nearly 80% imaginary).
4 acoustic configrations were computed. Although everyone compatible with MHD generator, it was decided to choose the one for which no starting difficulties could eventually occurred. (see other deliverables)
The adopted configuration for the coupling between TAC engine and MHD generator is such as: The MHD generator is located in “a diameter” of a toroidal TAC loop. The two active elements (TAc engines 1 and TAc engine 2) works in push pull conditions imposing a Thermo acoustic wave travel around the toroidal structure. So when the pressure wave has maximum amplitude one one side the amplitude is minimum on the other side and reversibely according the frequency evolution of the thermo acoustic wave. Theoretical models of the phenomena have been developed, and experimentations have been carried out.
MHD GENERATOR
The problem now is to convert the mechanical energy supplied in the form of a vibration into electrical energy. An MHD generator could be used to realize this function. The process uses the induction mechanism.
When a pulsating magnetic field is applied through a closed coil, the magnetic flux variation induces an electric current in the electrical circuit of the coil able to supply the load represented here by an electrical resistance.
To produce the pulsating magnetic field, the process uses the interaction of an imposed DC magnetic field, with a pulsating liquid metal flow . This pulsating liquid metal flow is produced by the interaction with the pulsating gas created by thermos acoustic effect, so the gas imposes its vibration at the interface between the two fluids, i.e. gas/liquid metal. The liquid metal, here the sodium due to its remarkable properties, is located inside a tube submitted to its two extremity to the pulsating force imposed by the gas. A toroidal permanent magnet surrounding the sodium tube produces a radial DC magnetic field which interacts with the liquid metal to generating an induced toroidal AC electrical current. This AC current generates itself an induced AC magnetic field which produces a magnetic flux variation in a coil connected with the load represented in spaceTRIPS by a resistanc
The main transfert of energies are recalled. The radio isotopic elements are the hot source of the system and supply a thermo acoustic loop by conduction process. The thermo acoustic loop converts thermal energy in mechanical one and this mechanical energy is introduced inside the MHD generator to produce electricity. The residual heat that have not been transfromed in mechanical energy is transferred at the cold source and evacuated in space by radiation.
3-Goal of the project
The space TRIPS project has consisted in:
To study, built and test a earth prototype of a thermo acoustic/MHD generator for space application at a level of electric power around 200 W.
In space the hot source will be constituted by radio isotopic element (around 1000 W) thermal power being introduced at the hot source of the system by conduction.
Cold source will be a radiative cooler evacuating heat in space by radiation. The surface of exchange of this cold source will be around 1 m2. The heat to be evacuated will be transferred from the thermo acoustic engine to the radiator also by conducytion.
On the earth prototype.
The hot source will be constituted by electrical resistances, which transmit the thermal power by the same elements than that for the space prototype.
The cold source will be only a circulation of cold water in a classical cold exchanger.
Project Results:
Description of the main S & T results/foregrounds
The main description and main results of the work realised are reported below, they concerns:
- The design, thermo acoustic loop, MHD generator and the assembly of both.
- The construction, thermos acoustic loop, MHD generator and the assembly of both
- The tests
- And a draft of design of an electrical generator for space.
1-The design: Thermo acoustic loop.
The detailed design is reported on the deliverable D2.1: Tac MHD design file. The two elements that composes the electrical generator, i.e.; the thermos acoustic loop and the MHD generator was studied separately, but they was studied to be connected. The global and detailed designs was done by the SERAS (CNRS design office in Grenoble France).
It is important to notice that this two engine have been studied for the first time. For the thermos acoustic loop the adopted geometry is completely original and newer been realised and tested, and the concept of MHD generator is completely new concept never imagined and newer studied in the past.
From the main difficulties encountered for the study of the concept have been concerned with three types of elements.
Firstly: to realise the heat transfer by minimising the thermal losses, and in parallel to take into account the deformation due to the temperature level and the mechanical constraint generated by this level of temperature and the temperature gradient.
Secondly: In parallel with the temperature problem the thermos acoustic loop and MHD generator have to resist against a high level of mean pressure, around 40 b which is the working condition of then thermos acoustic loop and to take into account the oscillation of pressure that can reach several bar at frequency around 50 Hz.
Thirdly, the combination of both the temperature level and high-pressure level make difficult to solve the tightness problem.
Additively the problem of vibration able to produce resonance have been also studied.
2-The construction:
TAc engine manufacture.
The construction of the loop was done by IPUL (Institute of Physics of the University of Latvia. But an helps was obtained from the SERAS offices (CNRS Grenoble) during the construction. In particular, special tools were designed and constructed to arrange the magnets in their housing. This tool is indispensable because the strong forces exerted by the magnets do not permit to displace them with the hands. Main parts of the thermo acoustic engine was realised at the IPUL by the technician men of this laboratory, excepted important elements that was subcontracting, zirconia flanges importation from china, Titanium flange that was operated by 3D prototyping by the society (SOREMET). The cold exchanger and the regenerator was realised by the same French company.
Some difficulties appeared along the construction. The pieces ordered in china, and using zirconia material, which has good properties, it is a good isolating material, with some relatively good mechanical properties, have brooken during the assembly. The problem comes probably from the quality of the material realised in china , this is one hypothesis the other one being that the mechanical properties of this material is not well known at high temperature and the estimation done from the properties at low temperature level was not sufficiently precise. So these pieces that are mainly four flanges, have been rebuilt by machining, using Inconel instead off zirconia. In parallel new seals was ordered for adaptation with the new flanges.
To realise the tests it was necessary to fix the thermos acoustic loop on a special chassis able to absorb the vibration generated by the thermos acoustic effect.
Last thing for the tests of the thermos acoustic loop alone, that means without the connection with the MHD generator, a special element was realised, under the specification done by Hekyom company specialised in thermos acoustic, with the objective to dissipate the mechanical energy furnished by the thermos acoustic loop. This element was a simple guillotine walve, dissipating the mechanical energy by viscous friction.
MHD machine manufacture.
Main of the pieces were realised by IPUL, by classical machines, under the direction of of Janis Freibergs dorector of IPUL. Some parts was subcontracted.
For the assembly some very specific tools was machined, having been designed by the SERAS office (CNRS Grenoble).This specific tool were realised for the assembling of the magnets on the generator. This is necessary by the by the fact that is impossible to maintain the magnets with the hand due to the intense magnetic forces between the Ferro magnetic material and the magnets. Therefore, the magnets are mounted with glue on a special device and approached slowly, by screw up to the contact with the MHD generator. When the contact is established, the screw is removed that is possible because the magnetic force is higher than the glue is able to support. So step by step, reproaching two magnets located on a diameter alternatively, all magnets was placed on the MHD generator.
After this operation the coil was wound on the magnets, (and the Ferro magnetic material used for the closure of the magnetic field, assembled externally to the coil. Then all these elements was introduced in the external envelope surrounding the generator.
No special difficulties excepted the magnet arrangement was observed during the realisation of the MHD generator.
Just a last remark, a new material was used for the realisation of the magnetic magnetic circuit. It is called SOMALOY. SOMALOY is a Ferro magnetic material realised by sintering which has a very good magnetic permeability being constituted by a mixing of Ferro magnetic powder in an isolating matrix. The interest of this new material is due to the fact that it can be machined and so it is accessible to complicated shape not possible with the classical Ferro magnetic sheet used for the realisation of electrical transformer by example.
3-Tests
Test of the electrical generator.
The tests were realised at first of the two engine separately. For that a specific installation was studied and built by IPUL, specifically to test the MHD generator.a system able to simulate the oscillation imposed by the thermo acoustic effect was realised. The used system is constituted by two compressor and a rotating walve imposing the oscillation of pressure inside the MHD generator. The first series of tests was not satisfactory due to the instabilities at the free at the interface between the sodium and the gas. This difficultiy was anticipated and was studied in the frame of the project under two possibilities.
- The first one was studied by Thales Alenia space Italy consisting in a. sliding piston. TAS-I was in charge of investigating the use of a physical (solid) separation between the fluids to counteract phase mixing due both to launch mechanical loads and microgravity environment.
TAS-I performed a first investigation of materials and layouts. As a result, it became clear that the most promising solution was the usage of a metallic sliding piston. The ensuing study had an extremely specific nature involving metallurgy and chemical compatibility of materials, physics and chemistry of liquid metals, and mechanical design of small scale pulsating interfaces. For these specific aspects TAS-I had no past heritage nor dedicated niche expertise. Therefore, the decision was taken to subcontract that part of the activity to a partner university, the Politecnico di Torino
- The second one was based on the use of Surface tension possibilities: HZDR activities. Many experiments were performed about the stability of the interface between gas and liquid metal under high frequency high amplitude pulsating motion. The facility was mainly what was presented in the previous reports but the tests were focused on the use of honey combs. Using classical honey comb do not permit to reach the high frequencies because the level of power required over pass the possibility of the generator. So the solution was to reduce to one unique element, one tube of 1 mm of diameter; the number of mesh. In this case it was possible to reach the full frequency even if the amplitude of the motion were less than expected on the TAC/MHD generator. But any perturbation of the free surface was observed in this condition confirming the theoretical approach.
- The third one,Floating piston..But these two solutions that are very relevant for space are relatively difficult and particularely expensive. Therefore After the last review meeting that taken place at the REA offices in Brussels, January 15, it was decided to test another possibility of control of the interface. In close collaboration with the SERAS offices of the CNRS Grenoble, the society Hekyom together with Aster society, and with the participation of AREVA TA, a concept of floating piston was propose designed and tested in a specific loop using water instead of sodium and Plexiglas tube to visualise the interface evolution. The principle is to ask a hollow floating piston, 0.2 mm thick, and very lightweight about 16 g on the free surface between the argon of thermo acoustic loop and the liquid sodium. The piston then penetrates about 1 cm in the sodium. Therefore it perfectly follows the evolution of the free surface since all abruption in relation to this surface created a vacuum that effectively pick the piston on the interface. So the system reduces almost completely the free surface perturbations. On the other hand to equilibrate the pressure inside and outside the floating piston, a tube was welded on the top of the piston allowing this necessary equilibrium. Of course because the system presents two identical interfaces (free surfaces) two identical pistons were realised .
This last solution, floating piston was used with success and was addpted for the global tests the two engine after assembly.
Test of the thermo acoustic loop.
The thermo acoustic loop has been tested separately from the MHD generator, with the help of two specialised company (the alones in Europe to work specifically on thermo acoustic applications), Hekyom leader of spaceTRIPS project and the society ASTER. The first tests done at low level of temperature a little less than 400°C at the hot source and mean level of pressure about 25 b was sufficiently convincing to decide to test the complete electrical generator resulting from the assembly of the two engine, Thermo acoustic loop and MHD generator. The signal was obtained at 58 Hz instead of the 45 that was initially calculated. This can be corrected by adjusting the impedance of the thermo acoustic loop that can be obtained by modification of the length of the loop. But due to the delay already accepted from the E.U. to finish the program it was decided to conserve this frequency that diminish a little the efficiency.
The test of the electrical generator after assembly of the two parts, Thermo acoustic loop and MHD generator.
After assembly of the two part of the electrical generator a first series of global tests were performed at low level of power, introduced only 300 W of thermal power at the level of the hot exchanger, and a level of temperature of order of 350 to 400 °C. Three level of mean pressure was tested corresponding to
25 b
30 b
40 b.
As explained above the mean temperature imposes the amplitude of the oscillating pressure. So increasing the mean level allows increasing the level of power produced.
The main results are reported in the following table for different value of the load resistance. As it can be seen on the three tables corresponding to three different levels of the mean pressure, 25, 30 and 40 b, the best results in term of efficiency of the MHD generator is obtained for the lowest level of pressure but the level of power is the highest for high level of pressure.
A second series of tests is planned at higher level of temperature, and pressure but are not reported in the present report. So the new results will be reported latter.
Mean pressure: 25 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Thermal energy 300 W 300 W 300 W 300 W 300 W
Oscillating pressure (b) 0.486 0,498 0,503 0,5198 0,476
Flow rate (l/s) 0,293 0,302 0,296 0,3023 0,281
cos 0,761 0,753 0,748 0,735 0,769
Current (A) 0,331 0,214 0,156 0,076 0,63
Voltage (V) 16,729 21,224 23,415 26,1 4,82
Electrical energy (W) 2,77 2,266 1,82 0,997 3,724
Mechanical energy (W) 10,84 11,32 11,16 11,55 10,289
Coil energy (R~9.2 ) 1,006 0,42 0,22 0,054 1,534
Total electrical efficiency 0,348 0,237 0,183 0,091 0,511
Main results for mean pressure 25 b
Mean pressure: 30 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Oscillating pressure (b) 0,863 0,876 0,908 0,999 0,821
Flow rate (l/s) 0,538 0,536 0,55 0,55 0,517
cos 0,638 0,637 0,634 0,626 0,641
Current (A) 0,507 0,33 0,24 0,119 0,989
Voltage (V) 25,77 32,63 36,06 40,47 7,48
Electrical energy (W) 6,54 5,38 4,32 2,404 3,701
Mechanical energy (W) 29,65 29,897 31,747 34,48 27,26
Coil energy (R~9.2) 2,37 1,002 0,529 0,13 9,01
Total electrical efficiency 0,3 0,214 0,153 0,077 0,474
Main results for mean pressure 30 b.
Mean pressure 40 b Resistance 1 Resistance 2 Resistance3 Resistance 4 Resistance 5
50 100 150 340 7,58
Oscillating pressure (b) 0,845 0,85 0,85 0,85 0,825
Flow rate (l/s) 0,493 0,499 0,472 0,503 0,482
cos 0,69 0,68 0,675 0,65 0,719
Current (A) 0,458 0,3 0,22 0,11 0,846
Voltage (V) 23,26 30,055 32,36 37,51 6,44
Electrical energy (W) 5,33 4,55 3,48 2,07 2,726
Mechanical energy (W) 28,72 28,84 27,085 27,78 28,62
Coil energy (R~9.2 ) 1,93 0,84 0,43 0,11 6,596
Total electrical efficiency 0,25 0,19 0,144 0,079 0,33
Main results for mean pressure 40 b
4-A draft of design of an electrical generator for space.
Cold source definition
This task aims to propose a design for the cold source. The cold source must operate with atmosphere (e.g. on Earth before launch or on Mars) and in vacuum (e.g. during space transit). Operating temperature will be driven by the inputs issued from modelling. Technological options will be analysed in order to select the most attractive option in terms of performance, mass and volume, feasibility. The cold source consists of:
- The cold heat exchanger
-The heat transport device (likely heat pipes)
-The radiator itself which is a radiatice cooler of surface of exchange of about 1 m2.
Moreover, an additional cold source should be necessary for the phases of the mission where radiator cannot be used (e.g. during the launch).The cold source will be design, this design being integrated in the reference design. The task leader is THALES ALENIA SPACE ITALIA, Schedule: To+14 - To+20, with the benefit of collaborations with AREVA TA and CNRS Grenoble.
Overall, these tasks aim to draw up architecture for the SRPS, propose a design for the cold source, and refine the definition according to iterations performed in the overall Space TRIPS project.
Hot source Definition.
This task aims to propose a design for the hot source of the thermo acoustic loop. The power is provided by a radio isotopic source, heating permanently. The hot source is supposed to be one or several modules, according to works performed under ESA’s contract. Operating temperature will be driven by the inputs issued from modelling. The main issue is the thermal and mechanical interface of the modules with the thermo acoustic loop. The thermal interface must be efficient, in order to transfer the heat into the loop, minimizing the loss. The source must also be thermally insulated in order to force the heat into the loop. The mechanical interface will be driven by mechanical loads (thermo acoustic vibrations in operation, loads at launch) but also by the integration of the source in the system. Indeed the source is radioactive and the possibility of a late access is of importance. The task leader is AREVA, working in collaboration with CNRS and Thales
Draft of design of the space prototype.
A draft design of the electrical generator for space was performed by AREVA TA. The objective is to regroup all the element of the engine in a very small volume. All the elements mean hot source cold source, thermo acoustic loop and MHD generator. One of the most important requirements from a nuclear safety point of view is that the Heat Source modules must be freely released in case of inadvertent re-entry. That is why the heat spreader is not a closed box. In order to maintain each Heat Source module in its housing, a rod attached to the main housing and pushing the Heat Source module is foreseen. This rod includes a spring to insure that the pressure will remain even in case of small bending. In case of inadvertent re-entry, the structure quickly burns, the rods are released and consequently the heat source modules are released in turn. Another important consequence of this choice is that the insulation material is not supporting the weight of the Hot Source.
This last design was compared to the MMRTG (RTG currently used on Mars) regarding the electrical power delivered, the efficiency, the volume and the mass. We can see, as shown on the figure below, that:
• The two volumes are quite similar,
• The electrical power delivered is a little higher
• The expected efficiency is more than twice the efficiency of the MMRTG,
•
The mass is significantly higher than the one of the MMRTG, but this is mainly due to the choice of the radioisotope (241Am instead of 238Pu whose mass at equivalent power delivered is 5 times higher
Potential Impact:
Dissemination and social impact of the project.
1-The dissemination of the results.
It was an important activity of the partners of the project. The social impact of the dissemination was very efficient.
The dissemination. An international conference was organised in Latvia Riga and have hosted about 200 participants coming from about 25 countries. The proceeding have been reported in the participant portal of space TRIPS. In parallel with this conference, a summer school devoted to the space technology was organised in parallel at the same location and same period. About 40 students attended this summer school from about 10 European countries.
The summary of the dissemination that comprises also workshop, participation in international conference and seminar is reported below.
- For the publications the partners will take advantage of the international journal Magneto Hydro Dynamics which is under the responsibility of the LATVIJAS UNIVERSITATES AGENTURA LATVIJAS UNIVERSITATES FIZIKAS INSTITUTS (partner 3).
- Two main events was organised this year under the responsibility of Thales Alenia space and AREVA TA They concern the organisation of a workshop that taken place in Torino on November 19 in the frame of aerospace symposium. The other one was organised at the ESA offices in December 8.
- Other contributions in the dissemination were achieved by the partners of spaceTRIPS consisting in oral presentation in international conferences. In particular two papers were accepted for oral presentation at the”3th International workshop on Thermo acoustics Enschede, October 26-28, 2015”.
- Also some seminars were given especially at the CNES office in Paris focused on the main principles of thermo acoustic and its coupling with MHD generators. another one in Italy in the CIRA offices where the program was explained in the frame of a proposal of cooperation for a new project focused on the solar sail submitted to the EU program H2020, FETOPEN.
Mains results
The selection of the best papers presented at the pamir 2014 conference have been published in international Latvian journal Magneto Hydrodynamics in August 2015. This important works, especially in the selection of papers and printing, was realised by the partnership of spaceTRIPS, the edition of the two special (number 52 and 53) issues having been done by the Latvian partner.
An other very important results obtained by this effort about the dissemination concerns the submission of a new project devoted to the use of a combination of thermo acoustic and MHD, “Prometheus”, acronym of Plasma-based Research On Magnetogasdynamics Effected by THermoacoustics for Electrical Unmoving generatorS, based on the use of an ionised gas that allows, if the feasibility will be verified, to suppress the interface liquid and gas that could be an important improvement for space application; This project was submitted in the frame of H2020, FET OPEN.
2-Impact of the project.
The oral presentation of the space TRIPS project, at the international conference organised by the European Union in Roma for the H2020 presentation (September 2014) was followed by many discussion with the other participants. It results from this discussion a new project Prometheus which concerns the use of a coupling between thermo acoustic loop and MHD generator. The project is based on the realisation of a loop at high level of electrical power for energy sources at various applications including space for power supply of MHD thruster.
The main elements of this new project called Prometheus are reported here after.
This type of generator is also particularly suitable for the exploitation of renewable energies (solar energy for example) as well as for waste recovery.
Following the Horizon 2020 program presented in September 2014 in Roma a new project devoted to high electrical power able for example to supply MHD thrusters in space, based on the use of a combination of Thermo acoustic loop and MHD generator have been presented in collaboration with many partners from 4 countries, French, Italy, Poland and Spain. In the new system the MHD generator is based on the use of ionised gas instead off liquid metal, supressing the problem of interface between liquid and gas.
This new project enjoys a demonstration of interest expressed by the CNES. It is called Prometheus for:
Plasma-based Research On Magnetogasdynamics Effected by Thermoacoustics for Electrical Unmoving generatorS and benefit of the participation of 8 partners from 4 european countries.
No Participant organisation name Short name: Country
1 University of Cagliari (Coordinator) UniCA Italy
2 HEKYOM Hekyom France
3 Centre National de la Recherche Scientifique CNRS France
4 Added Value Solutions AVS Spain
5 Polish Academy of Science PAS Poland
6 Ingegneria Sistemi Impianti Servizi ISIS Italy
7 Consorzio RFX RFX Italy
8 Commisariat à l'énergie atomique et aux énergies alternatives CEA France
Novelty, level of ambition and foundational character
The technology proposed in this Project (Patent No. 21348EP, owned by AREVA-TA, CNRS, Univ. Cagliari) offers a solution to all the limits of space TRIPS and in particular it allows for suppressing the gas/liquid interface. The working principle of the proposed technology is depicted. The stack, submitted to a high temperature gradient imposed by the hot source and cold source of the system, generates the thermoacoustic vibrations. In the same volume, an electrical discharge ionizes the gas, and then the charge carriers are separated by a stationary external electrical field. The ions are captured in a first stack and the electrons in a second one. The electrical field is applied to the gas by means of a capacitor. The electrical field maintains the charge carriers in a static equilibrium. When the thermo acoustic vibrations occur, the charges participates to the oscillating motion, giving rise to an alternate electrical current. Such current induces an alternated electromotive force into a coil surrounding the duct, wrapped all around a toroidal ferromagnetic core. Finally, this coil is connected to the electric load. It is possible to adjust voltage and intensity of the induced current by properly choosing the number of turn of the coil.
In the same manner than for the SpaceTRIPS PROJECT, The thermoacoustic effect is obtained by means of two stacks located into a toroidal loop, and working in push pull . Concerning the hot and cold sources, they can be based on the use of heat pipe (sodium heat pipe at the hot source, around 800 °C) that maintains the advantage of static system, which is important for space application using small nuclear reactor at hot source and radiative cooler at the cold source.
The described system exploits some physical phenomena whose technological aspects are well established, such as thermo acoustics and waves propagation, while other ones, such as charge generation and confinement and magnetic induction of ionic currents, need to be further investigated. The gas, which constitutes the working fluid, has to fulfil both fluidodynamic and electrical properties, because it has to promote the occurrence of vibration, but at the same time the charge carriers should have low electrical mobility, because the electrical forces hinder their vibration. A second aspect concerns the charge confinement. The conversion process is strongly affected by the quantity of involved charge carrier and, consequently the density of charge carrier should be maximized. To this end, in order to maintain the applied voltage within reasonable values, the best solution is maximizing the capacitance, by increasing the surface of the plates. The thermoacoustic stack having the same requirement, therefore it is worth evaluating the possibility to use a unique element as capacitor plate and thermoacoustic stack. The stacks could be represented by a great number of parallel sheets, electrically connected among them by a sleeve. The sheets could be very thin, while the gap among them has to be optimized in order to maximize the value of the ionic current. This way of charge confinement has important analogies with technology of membranes, which are used in electrochemical energy conversion and storage devices (P.N. Pintauro, Polymer Reviews, 55:201–207, 2015). That analogy could be usefully exploited in developing the charge confinement system in the Project. In particular, it is important that the charge carriers stay far from the walls, where the velocity of particles is null. This can be obtained by properly setting the external voltage applied to the capacitor. Indeed, the clouds of charge are surrounded by the plates (see Fig. 4) then they are drawn by opposite forces which allows the charges do not stack on the wall, as usually happens in the capacitors. The ionic alternate current works like a primary winding of a transformer, while the toroidal coils are the secondary winding.
The conversion rate of the process depends on many parameters, such as charge density, cross section of the duct, frequency and amplitude of the vibration, output power, and range of temperatures. On their turn, such parameters depend on several design parameters, which have to be designed for each specific application.
Thermo acoustic and solar energy.
In space: Another very important project which is currently studied concerns a “space” project. This new possible application of thermo acoustic was a result of the summer school organized in Latvia (Riga September 2014) in which have participated Xavier Genest from ESA. The new project concerns the use of a parabolic solar collector in space as hot source and classically a radiative cooler as cold source. The thermal energy is transmitted to the thermo acoustic loop by a heat pipe (sodium Heat pipe) working at high level of temperature. The thermal energy is converted into mechanical energy by thermo acoustic and mechanical energy is transformed in electricity by a specific alternator.
This project is supported also by the research and development departement of AIRBUS Industry.
On the earth: Many program are in course of development by Hekyom Company, resulting of the of space TRIPS project. They are reported below.
Thermo acoustic electrical generator supplied by stoked solar energy. Three concepts are under study:
1- Thermo acoustic engine at high efficiency (>50%) without any moving part and very low maintenance
2- heat solar harnessing by parabolic concentrator, air gas transfering , solid particle for heat stocking : 2 cases +:
a. high temperature (concentration) 600°C
b. middle température (Fresnel) 300°C
c. possible complementary heat source by means of gas or biomass combustion for a 24/24-7/7 available power delivery
3- Conversion acoustic to electric by means of high reliability bidirectional turbine + alternator.
Notes: following projects underway or completed either by HEKYOM at TRL6-7 or by IDHELIO at TRL5-6
List of Websites:
http://space-trips.eu.seventhframework.sal.lv/
