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ScienceDirect ScienceDirect Energy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Energy Procedia (2017) 000–000 Energy Procedia 129 (2017) 200–207 Energy Procedia 0000 (2017) 000–000 Energy Procedia 00 (2017) 000–000

IV International Seminar on ORC Power Systems, ORC2017 IV on Systems, 13-15Seminar September 2017,Power Milano, Italy ORC2017 IV International International Seminar on ORC ORC Power Systems, ORC2017 13-15 September 2017, Milano, 13-15 September 2017, Milano, Italy Italy

Closed Loop Organic Wind Tunnel (CLOWT): Closed Organic Wind (CLOWT): Closed Loop Organic Wind Tunnel (CLOWT): The 15th Loop International Symposium on Tunnel District Heating and Cooling Design, Components and Control System Design, Components and Control System Design, Components and Control System a Felix Reinkera,∗the , Eugeny Y. Kenigbof , Max Passmann , Stefan aus der Wieschea Assessing feasibility using the heat demand-outdoor a,∗ b a a Felix Reinker ,, Eugeny Y. Kenig Passmann der Wiesche a,∗Engineering, b , Max a , Stefan aus a Department of Mechanical Muenster University of Applied Sciences, Stegerwaldstrasse 39, 48565 Steinfurt, Germany Felix Reinker Eugeny Y. Kenig , Max Passmann , Stefan aus der Wiesche temperature function for a University long-term district heat demand forecast of Mechanical Paderborn, Pohlweg 55, 33098 Paderborn, Germany DepartmentDepartment of Mechanical Engineering,Engineering, Muenster University ofof Applied Sciences, Stegerwaldstrasse 39, 48565 Steinfurt, Germany a

b a a Department of Mechanical Engineering, Muenster University of Applied Sciences, Stegerwaldstrasse 39, 48565 Steinfurt, b Department of Mechanical Engineering, University of Paderborn, Pohlweg 55, 33098 Paderborn, Germany b Department of Mechanical Engineering, University of Paderborn, Pohlweg 55, 33098 Paderborn, Germany

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*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Correc

Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Abstract Veolia Recherche Innovation, 291 Avenue Dreyfous Daniel,energy 78520 Limay, France However, experimental Abstract Organic Rankine Cycle (ORC) systems offer a&suitable technique to achieve reduced consumption. c Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France work is largely and there is an offer overweight in theoretical work. Forreduced this reason, theconsumption. computation of dense gas flows still Organic Rankinelacking, Cycle (ORC) systems a suitable technique to achieve energy However, experimental Organic Rankine Cycle (ORC) systems offer a suitable technique to achieve reduced energy consumption. However, experimental represents a major difficulty in ORC development, and there is a need for validation studies based on experimental which work is largely lacking, and there is an overweight in theoretical work. For this reason, the computation of dense gasdata, flows still work is largely lacking, and there is an overweight in theoretical work. For this reason, the computation of dense gas flows still is currentlya hardly Closed Loop Organic WindisTunnel is astudies facilitybased to fillon thisexperimental gap. In the data, first part of represents major available. difficulty inThe ORC development, and there a need(CLOWT), for validation which represents a major difficulty in ORC development, and there is a need for validation studies based on experimental data, which thiscurrently contribution, design ofThe the Closed CLOWT is presented. A close look will be takenis ata the maintocomponents, as compressor is hardlythe available. Loop Organic Wind Tunnel (CLOWT), facility fill this gap.such In the first part of isAbstract currently hardly available. The Closed Loop Organic Wind Tunnel (CLOWT), is a facility to fill this gap. In the first part of (especially shaft sealing) andofchiller. Furthermore, typical A wind tunnel asmain diffuser, settling chamber, and nozzle, this contribution, the design the CLOWT is presented. close look components, will be takensuch at the components, such as compressor this contribution, the design of the CLOWT is presented. A close look will be taken at the main components, such as compressor are briefly discussed. Based the test rig design,typical the second part shows the basicsuch operating principle of this closedand gasnozzle, cycle, (especially shaft sealing) and on chiller. Furthermore, wind tunnel components, as diffuser, settling chamber, District heating networks commonly addressed in wind the literature as one of the most effective solutions for decreasing (especially shaft sealing) andare chiller. Furthermore, typical tunnel components, such as diffuser, settling chamber, and nozzle,the focusing an exemplary thermodynamic cycle at maximum compressor power. The third part deals with of thethis control system of the are brieflyondiscussed. Based on the test rig design, the second part shows the basic operating principle closed gas cycle, greenhouse gas emissions thetest building sector.theThese systems requirethehigh investments areofreturned through heat are briefly discussed. Basedfrom on the rig design, second part shows basic operating which principle this closed gas the cycle, CLOWT.on Foranaexemplary closed wind tunnel, the setting operation points forpower. testing The requires attention, somesystem similarity to focusing thermodynamic cycleofatthe maximum compressor thirdspecial part deals with theand control of the focusing on antoexemplary thermodynamic cycle at maximum compressor power. The third part deals with the future controlcould systemdecrease, of the sales. Due the changed climate conditions and building renovation policies, heat demand in the closed cycle turbinewind systems exists. However, in case of a closed tunnel,requires inventory backward control is difficult CLOWT. Forgas a closed tunnel, the setting of the operation pointswind for testing special attention, andapproach some similarity to CLOWT. Forthe a closed wind tunnel, the setting of the operation points for testing requires special attention, and some similarity to investment returnthis period. toprolonging realize directly. To overcome problem, an in inventory control approach is designed forcontrol the CLOWT. Theis findings closed cycle gas turbine systems exists. However, case of aforward closed wind tunnel, inventory backward approach difficult closed cyclescope gas turbine systems However, in case of ausing closed control approach is difficult The main ofTothis paper used isexists. to assess thewind heat tunnel, demandinventory – outdoorbackward temperature function for heat demand presented in this part are being buildthe upfeasibility a distributed to realize directly. overcome thistoproblem, an inventory control forwardsystem. control approach is designed for the CLOWT. The findings to realize directly. To overcome this problem, anLisbon inventory forward control approach is designed for the CLOWT. The findings forecast. The district of Alvalade, located in (Portugal), was used as a case study. The district is consisted of 665 c 2017 The Authors. Published by Elsevier Ltd. presented in this part are being used to build up a distributed control system. presented inthat this vary part are beingconstruction used to buildperiod up a distributed control system. buildings in both and typology. Three weather scenarios (low, medium, high) and three district Peer-review responsibility of the scientific c 2017 The under Authors. Published by Elsevier Ltd. committee of the IV International Seminar on ORC Power Systems. crenovation 2017 The Authors. Published by Elsevier Ltd. intermediate, deep). To estimate the error, obtained heat demand values were © 2017 The scenarios were developed (shallow, Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. Peer-review under responsibility of the scientific committee ofof the on Systems. Peer-review under responsibility ofRig; theFacility; scientific committee theIV IVInternational International Seminar onORC ORC Power Systems. Wind Tunnel; Control System; Keywords: compared ORC; with results from aTest dynamic heat demand model, previously developedSeminar and validated by Power the authors. ORC; Wind Tunnel; Test Rig; Facility; Control System; Keywords: The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Keywords: ORC; Wind Tunnel; Test Rig; Facility; Control System; (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.decrease Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.renovation Introduction scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the 1. Introduction coupled valuesenergy, suggested could be used modify the be function for theenergy scenarios considered, and A hugescenarios). amount ofThe thermal often called wastetoheat, could used toparameters reduce current demand. On the improve the accuracy of heat demand estimations. A huge amount ofRankine thermal energy, often systems called waste couldtechnique be used totoreduce energy demand. On the other hand, Organic Cycle (ORC) offer heat, a suitable achievecurrent reduced energy consumption,

A huge amount of thermal energy, often called waste heat, could be used to reduce current energy demand. On the otherseveral hand, Organic Cyclealready (ORC)demonstrated systems offer good a suitable technique achieve consumption, and researchRankine groups have potential of thetoORC in areduced numberenergy of publications [1– other hand, Rankine Cycle (ORC)Ltd. systems offer a suitable technique to achieve reduced energy consumption, © TheOrganic Authors. Published by Elsevier and several research groups have already demonstrated of the ORC a numberwork. of publications [1– 3]. 2017 However, experimental work is largely lacking, andgood therepotential is an overweight in in theoretical In particular, and several research groups have already demonstrated of the ORC in a number of publications Peer-review responsibility of the Scientific Committee good Thepotential 15th International Symposium on District Heating and [1– 3]. However,under experimental work is largely lacking, andofthere is an in overweight in theoretical In particular, Computational Fluid Dynamics (CFD) has made enormous progress the past years, based onwork. the rapid growth in 3].Cooling. However, experimental work is largely lacking, and there is an overweight in theoretical work. In particular, Computational Fluid has made progress in the years, on thea major rapid growth in computing power andDynamics memory. (CFD) Nevertheless, theenormous computation of dense gaspast flows still based represents difficulty Computational Fluid Dynamics (CFD) has made enormous progress in the past years, based on the rapid growth in computing power and memory. Nevertheless, the computation of dense gas flows still represents a major difficulty Keywords: Heat demand; Climate change computing power and Forecast; memory. Nevertheless, the computation of dense gas flows still represents a major difficulty ∗

Felix Reinker. Tel.: +49-2551-962339. E-mailReinker. address:Tel.: [email protected] Felix +49-2551-962339. Felix Reinker. Tel.: +49-2551-962339. E-mail address: [email protected] E-mail address: [email protected] c ©2017 1876-6102 1876-6102 2017The TheAuthors. Authors.Published PublishedbybyElsevier ElsevierLtd. Ltd. Peer-review under responsibility thethe scientific committee IV International Seminar on ORC Power Systems. c 2017 1876-6102 The Authors.of Published by Elsevier Ltd.of the Peer-review under responsibility of Scientific Committee c 2017 The Authors. Published by Elsevier Ltd. of The 15th International Symposium on District Heating and Cooling. 1876-6102 Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 1876-6102 © 2017responsibility The Authors. Published ElsevierofLtd. Peer-review under of the scientificby committee the IV International Seminar on ORC Power Systems. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 10.1016/j.egypro.2017.09.158 ∗ ∗

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in ORC development, and there is a need for validation studies based on experimental data [4,5]. Considering these facts, it is surprising that only few groups are active in the experimental sector. The main reason is the use of an organic working fluid which makes experimental setups very challenging. Experimental setups with air as working fluid, as they have been commonly used for steam turbine development, are not suitable in case of organic fluids [6–8]. Therefore, experimental studies in the field of ORC systems require special test rigs. These facilities are rather challenging, both from the technical point of view and in terms of investment costs [9]. This might be the main reason why experimental data are currently hardly available.

1.1. Experimental Setups for ORC Flow Investigations Experimental setups for the investigation of dense gas flows have to be built as closed systems, to separate the working fluid from the atmosphere. Pressure vessel systems have to be utilized, to achieve the thermodynamic state of the fluid, which is needed for the test case. Sealing these high pressure reservoirs is always a crucial task, especially for dangerous fluids which are toxic or flammable. Many organic fluids also tend to act corrosive, making the choice of material an important part in the pressure vessel design procedure, in terms of both resistance and costs. Based on these facts, working fluid selection criteria for experimental setups differ from selection criteria for real ORC systems [8]. On the one hand, local safety regulations complicate the design and construction of these facilities, but, on the TM other hand, safety should always be ranked first. For this reason Novec 649, a harmless fluorketone, was chosen as working fluid for the Closed Loop Organic Wind Tunnel (CLOWT) [10]. Prior to building up an experimental setup for ORC flow investigations, one has to choose between a facility of intermittent mode or of continuously running mode. Both these setups have their advantages and justification. Final choice should mainly depend on experimental results while economic constraints often dominate. Option 1: Intermittent facility Intermittent facilities, like the Test Rig for Organic Vapors (TROVA) [11], the Ludwieg Tube [12], or the Flexible Asymmetric Shock Tube (FAST) [13], all consist of a single high and a single low pressure reservoir, separated by a fast-opening valve or diaphragm. After charging the high pressure reservoir and evacuating the low pressure reservoir, the fast-opening valve is opened and the fluid is released to the low pressure reservoir. This technique allows for very high pressure ratios, leading to the possibility of highly supersonic flow conditions. In general, expansion processes of blowdown facilities are unsteady, due to the continuously shrinking pressure ratio, which drives the flow. The time constant of this expansion process strongly depends on the reservoirs capacity, which is mostly limited due to economic constraints. To overcome this issue, the fast-opening valve can be controlled, to achieve constant stagnation pressure upstream of the test section [11]. However, the change in total enthalpy of the high pressure reservoir can only be lowered by increasing its capacity. Option 2: Continuous running facility The main reason and advantage to build a facility with continuous running mode is long test durations (theoretically unlimited) at stationary flow conditions. There are different concepts suitable, to reach these test conditions [9]. The Organic Rankine Cycle Hybrid Integrated Device (ORCHID), currently built up at Delft University of Technology, is the so-called phase transition cycle [14]. ORCHID is a kind of a real ORC, consisting of the basis components, such as feed pump, primary heat exchanger, and condenser. However, in contrast to a real ORC, this facility follows a Balance of Plant design, leading to a modular test section design. The first test section is a nozzle, used to perform fundamental investigations. The second one is a test-bench for the investigation of mini-ORC expanders. The hybrid design makes the ORCHID very flexible. But, the operation of phase transition cycles at suitable mass flow rates implies high economic efforts, especially due to high thermal power inputs and corresponding cooling power, needed for this cycle. Nevertheless, the phase transition cycle might be the most elegant and flexible configuration despite that it is also the most cost intensive configuration. In contrast, the CLOWT, which is in focus of this contribution, pursuits an alternative concept, namely the closed gas cycle. It shows some technical similarities to closed-cycle gas turbine systems, which have already been investigated in the 1970s (see also Section 4). More details on the design and the built-in components of the CLOWT is given in the next section.


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Fig. 1. CAD model of the Closed Loop Organic Wind Tunnel (CLOWT)

2. Design and Components of the CLOWT CLOWT, as its title suggests, follows the concept of a closed loop continuous running wind tunnel, which is suitable for an organic fluid, to be driven at higher temperatures and higher internal pressures. As mentioned before, this concept is also called “gas cycle” [9]. At the time of preparation of this document, the CLOWT is still under construction at the Muenster University of Applied Sciences. The design of the CLOWT is shown in Figure 1. Table 1 shows corresponding technical data of the facility and TM thermodynamic properties of the working fluid Novec 649, a harmless fluorketone. In general, the CLOWT is not TM restricted to Novec 649, but other fluids would require detailed examinations in terms of material compatibility. In TM addition, Novec 649 was already used in a real ORC system [15]. The operating principle is as follows: The flow is driven by a radial compressor unit, which is the key component of this facility. The flow is decelerated in a wide angle diffuser to enter the settling chamber. To avoid flow separation inside of the wide angle diffuser, several turbulence screens are used [8]. The flow enters the settling chamber with an integrated finned-chiller. Due to pressure-vessel restrictions, the piping of the chiller is made from stainless steel, but aluminum fins are used to improve fin-efficiency, resulting in a heat transfer area of 16 m2 . The chiller cooling power is needed to close the thermodynamic cycle, being one of the main variables to control the thermodynamic cycle (see Section 3 and Section 4.2). After passing a honeycomb (flow straightener) and several turbulence screens, the flow is slightly accelerated in the first contraction, which has to be built from three conical pieces, before entering the test section [16]. For fundamental testing of the facility (leakage, heating and cooling, pressure drops, performance of flow conditioners and compressor), a simple pipe (basic test section) is used. The return of the wind tunnel is equipped with a throttle valve to determine the performance of the compressor at several loads. Only after determination of the real curves, it is possible to start the final design procedure of the test section (limited driving power). To measure the real curves, the wind tunnel is equipped with several pressure transducers, measuring head, and an averaging Pitot tube to determine mass flow rate in the return (see Figure 1 and Section 4.3). But, the usage of the throttle valve is not only limited to the determination of the real curves. It can be also used to set the back pressure just after the test section,

Table 1. Technical data of CLOWT and Fluid properties of Novec System pressure range Maximum temperature level Basic test section diameter Basic test section length Total length wind tunnel Flow rate (at 55 kg/m3 ) Power (compressor unit)

0 − 0.6 MPa 423 K (150 ◦ C) 250 mm 2500 mm 7.5 m up to 0.5 m3 /s up to 45 kW

TM

649. Molecular formula Molecular weight Boiling point (at 0.1 MPa) Critical temperature Critical pressure Heat of vaporization (at boiling point) Thermal decomposition

CF3 CF2 C(O)CF(CF3 )2 316 g/mol 322 K (49 ◦ C) 442 K (169 ◦ C) 1.88 MPa 88 kJ/kg 573 K (300 ◦ C)

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leading to an additional variable to set the operation point (see Section 4.3). The return of the CLOWT is connected to the suction side of the compressor by an expansion joint to reduce lateral forces induced by thermal expansion of the heated piping. The heating system (heating cables) is separated in several zones, which are thermally insulated to reduce heat losses to the environment. The shaft sealing is one of the most crucial components, as it was already reported in [9]. In case of the CLOWT, the system pressure range is 0−0.6 MPa. The maximum pressure of 0.6 MPa is relatively low, compared to other facilities. In this case, a cartridge multiple dynamic lip seal system can be applied. A pressurization system is needed to set the correct back pressures between the stages (lip seals). Compared to ordinary air flushed sealing systems, contamination of the organic vapor with pressurized air does not occur due to the pressure gradient, which has to be set, depending on system pressure, towards the atmospheric pressure. Deviations from expected compressor characteristics, possibly due to real gas effects, are noted by measuring the real curves prior to final test section design. The concept of a closed gas cycle leads to some technical limitations. Maximum achievable Mach number is strongly limited by the compressor power. The operation range, in terms of thermodynamic regions, is limited by the maximum pressure level of the system (see Table 1). Nevertheless, the CLOWT, with its long test durations, should be considered as an interesting supplement in the small community of organic vapor test rigs, especially for the investigation and validation of organic vapor flows in sub- and transonic regimes.

3. Thermodynamic Cycle Based on the wind tunnel design discussed above, this section presents the thermodynamic cycle of the CLOWT facility, illustrating the basic operating principle and to explain some of the most important features of the design. The wind tunnel works on the principle of a closed gas cycle. A schematic diagram of the components considered in the TM cycle is presented in Fig. 2a). For reference, a temperature versus specific entropy diagram for Novec 649 is shown in Fig. 2 b). An exemplary thermodynamic cycle for the maximum compressor output power (see Table 1) is considered in Fig. 2d). Starting at the discharge side (point 1), we find the maximum total pressure and temperature, i.e. 0.5 MPa and 382 K. The flow enters the wide angle diffuser, where a series of pressure losses occur in the turbulence screens, resulting in slightly lower pressure and temperature at the diffuser outlet (Fig. 2c, point 5). The fluid temperature is reduced in the chiller in an isobaric process (5 → 6), leading to a state of lower entropy. Point 6 brings the process close to the vapor pressure curve and sets the stagnation conditions for the expansion process that takes place in the test section. An isentropic expansion proceeds in a convergent-divergent nozzle (6 → 7), accelerating the flow to a supersonic flow regime. Point 7 represents the state of minimum static pressure and temperature encountered in the system. The isentropic expansion is followed by a normal shock wave (7 → 8), leading to increasing entropy, static pressure and temperature. To accurately estimate the shock losses, the real fluid behavior has to be taken into account. In the TM present case the fundamental equation of state for Novec 649, as implemented in REFPROP, was used. Hence, it is no longer possible to obtain simple analytical expressions, similar to the ideal gas Rankine-Hugoniot equations. For this reason a system of non-linear equations, consisting of the equation of state and the governing equations (continuity, energy, momentum), was solved numerically. This approach allows for highly accurate calculations of the pressure loss across normal shock waves, which are superior to simplified ideal gas calculations. Further information on the calculation procedure and the significant deviations of the predictions, obtained by the real gas- and the perfect gas models, can be found in [17]. Some kinetic energy is recovered in the diffuser (8 → 9), before the compressor raises the pressure level again to point 1. This qualitative assessment of the closed gas cycle process demonstrates that the process is governed mainly by two components, namely, compressor and chiller. The real performance of these components has to be determined experimentally. Once a static operating point is set by the inventory mass of fluid m (see Section 4.2), the exact locations of point 1 and point 6 are controlled by setting the compressor rotational speed, n, and cooling power of the heat exchanger. This aspect is considered in more detail in section 4.1.

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Fig. 2. a) Schematic diagram of the cycle configuration, b) planned area of operation in a T-s plane, c) detail of the thermodynamic cycle of the diffuser (1 → 5), d) closed thermodynamic cycle(1 → 9)

4. Control and Operational Performance The wind tunnel can only be operated safely by means of safety devices and suitable control systems which are described briefly in the following. It is important to note that some of the control design challenges and their solutions are not completely new: In the 1970s, substantial efforts were undertaken in this direction in Germany during the development of closed-cycle gas turbine systems [18–20]. These systems show some technical analogies to the closed gas cycle principle which is applied for the CLOWT. Therefore, the findings of closed-cycle gas turbine development can be applied on the CLOWT, to a certain extent, too. 4.1. General Dynamic Behavior and Control Approach As in the case of a closed-cycle gas turbine, the dynamic behavior of the charged closed wind tunnel can be classified and divided into three different groups according to their characteristic time scales. The first group of physical phenomena within the wind tunnel is characterized by small time scales of order 0.1 s or below. Pressure oscillations within the vapor flow belong to this class. The corresponding time scale is of the same magnitude or below the running times of the vapor “particles” in passing the turbo-machine or the ducts. The second group is characterized by medium time constants of order 1 s or slightly greater. Phenomena related to mass storing or energy storing caused by rotor speed changes in connection with rotating masses belong to this class. Finally, a third class exists, characterized by rather long time scales. Thermal processes like heating of the ducts, e.g. during start-up, or cooling down the flow belong to this class. Obviously, all heat conduction phenomena involving great thermal inertias are subject of the third group. If the dynamic behavior of the closed wind tunnel during operation have to be calculated or controlled, it is allowable to consider procedures belonging to the second group only, and to neglect those of the other groups [18]. But, this strategy is not applicable in case of very long measurement times. In this case, to avoid for long time drifts, the characteristic time scales of the thermal processes have to be considered, too. If the start-up procedure should be managed automatically, phenomena of the third group have to be considered as well. In order to meet this objective, the

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characteristic time scales of the CLOWT have to be determined during first operational experiences. For this reason, the control approach of the CLOWT has to be installed step-by-step. The quality of the flow inside of the wind tunnel sections depends on stable flow conditions during operation. It is important to recognize that undesired pressure oscillations caused by flow separation within the contraction zone or the diffusers cannot be suppressed by the control system, because of their small time scales and their local occurrence. These phenomena can only be avoided by a careful design of the entire flow domain. Design procedures of the critical components, the piece-wise conical contraction and the wide-angle diffuser, can be found in [8,16]. 4.2. Filling and Inventory Forward-Control Approach In closed-cycle gas turbine plants, the inventory (also known as pressure-level) control approach is the classical method to set operation points. This method is applicable in case of slow load changes, but it requires huge and expensive gas accumulator systems [21]. In case of a closed wind tunnel, working with an organic vapor, an inventory control system is further complicated due to the presence of evaporation and condensation of the working fluid. It was therefore decided to employ an inventory forward-control approach instead of a full inventory backward-control loop. This means that at the beginning of the operation, the average density ρav = m/V of the wind tunnel flow domain with total volume V is determined by the total mass (inventory) m of the working fluid. Prior to the filling process, the wind tunnel is evacuated (less than 1000 Pa), and dry air is filled in. The wind tunnel is then evacuated again, and filled with dry air. This process step is repeated until the water vapor amount of the evacuated wind tunnel domain is below the tolerable saturation limit level of the working fluid. The filling valve is located at the return of the wind tunnel. The schematics of the wind tunnel system are shown in Figure 3a. Here, V1 denotes the filling valve. After filling the desired mass m of working fluid, the average density ρav is fixed during operation. This is illustrated by means of Figure 3b where two possible operation points (0 and 0’) are shown in the vapor region of the working fluid. The local density ρ at the test section inflow remains constant, in first order, due to the fixed inventory (i.e. constant average density ρav ). The value of the local density depends on the pressure drop and flow characteristics of the wind tunnel including the test section. Different thermodynamic inflow conditions can be achieved by heating or cooling (change of temperature). For doing this, the wind tunnel offers the possibility of adjusting the cooling power of the chiller, placed in the settling chamber, see component GS in Figure 3a. Heating power can be applied continuously. To a certain extent, cooling can be achieved also in the return, see component GR in Figure 3a. Due to the (nearly) fixed local density ρ and temperature T 0 , the local pressure p0 is fixed. The inflow velocity can be controlled by the running speed n of the compressor. During operation, the inflow conditions are controlled by means of a linearized model based on the transfer functions of the diffuser, G D , the settling chamber, GS , the contraction zone, GC , the return, GR , and the compressor. Then, the rotational speed n and the cooling water flow rate of the chiller are the primary variables. The model parameters of the transfer functions have to be determined in the experiment after the start-up of the facility. For safety reasons, an emergency shut-down valve (rupture disc) V2 is placed at the settling chamber of the wind tunnel. In case of critical overpressure the rupture disc bursts and the fluid is released. Since a certain loss of the expensive working fluid occurs in a shut-down, this valve and the corresponding fluid recovery system (not shown in Figure 3a) is not used during regular operation. Instead, the so-called bypass control method by means of a valve V3 could be optionally applied. The bypass control was initially developed regarding fast load changes or load release events for closed-cycle gas turbine plants [22]. Currently, the wind tunnel is not equipped with a bypass control, because the inventory forward control in combination with the running speed and temperature control provides sufficient operation points for fundamental testing. A disadvantage of this cost-efficient control approach is the necessity to pre-calculate the required amount of working fluid mass prior to operation. This calculation cannot be done without an accurate thermodynamic model of the wind tunnel. 4.3. Start-up of the Wind Tunnel After fixing the required amount of working fluid mass and filling, the start-up procedure is initiated. As in case of other thermal power plants, the start-up process has to be performed in accordance with the thermal behavior of the wind tunnel in order to avoid critical thermal stress levels [23]. This issue has been investigated in more detail for the actual wind tunnel in [24]. But, the heating system of the CLOWT is limited to about 10 kW of electrical power,


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Fig. 3. a) Schematic control structure of the CLOWT, b) Schematic operating points at fixed ρav .

leading to start-up times of order 5 h; for this reason thermal stress during start-up is not crucial for this facility. The temperature level of the heating elements is controlled by a separate system. During heating, the working fluid is completely evaporated, and, at this stage, the system pressure increases with the internal temperature in accordance to the saturation curve (see Figure 3b). When the liquid is completely evaporated, the heating results in a superheating of the vapor that can be easily identified by the slower increase of pressure during this stage (see Figure 3b). The compressor is turned on after reaching the operation point region. The mass flow (and hence the velocity level) can be measured accurately by an averaging Pitot tube in the return leg of the wind tunnel where sufficient entrance length is provided (see Figure 1). The mass flow rate is then calculated based on local fluid properties. 5. Conclusion & Outlook An overview on test rigs suitable for the investigation of organic vapor flows was given, focusing on different cycle configurations and their working principles. In this context, the advantages of continuous running mode were pointed out, as well as economic constraints and difficulties for these facilities were shown, too. In view of these circumstances, the Muenster University of Applied Sciences decided to build up a closed gas cycle, namely the Closed Loop Organic Wind Tunnel (CLOWT). In the first part, design and components of the test rig were presented. Especially the shaft sealing of the compressor, the most crucial part, was discussed. Also, the need to determine the real compressor characteristics before designing the final test section was explained. Representing the concept of a closed gas cycle, an exemplary thermodynamic cycle of the CLOWT, at maximum compressor power, was shown in the second part. In the third part a link to closed-cycle gas turbines of the 1970s was created, and similarities to the control strategy of our closed gas cycle were discussed. At the time of preparation of this document the CLOWT is still under construction. Just after security checking, which is mandatory for this facility, fundamental testing of the CLOWT (leakage, heating and cooling, pressure drops, performance of flow conditioners and compressor) can be started (second half of 2017). Inspired by an industrial partner and early findings of Traupel [25], we are going to build up a transonic test section for the investigation and validation of similarity laws for dense gases. Based on these results, a long time goal is the creation of loss correlations. Acknowledgements The CLOWT is the experimental facility of a large research project, aiming at obtaining loss correlations for performance predictions of ORC expanders. It is supported by the German Ministry for Education and Research (BMBF) under Grant No. IN2013-425-204.

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