TURBO CHILLERS AND VACUUM ICE GENERATION TWO APPLICATIONS OF A NEW GENERATION OF HIGH SPEED, HIGH CAPACITY CENTRIFUGAL COMPRESSORS Marcus HONKE, Mathias SAFARIK, Ralf HERZOG Institute of Air Handling and Refrigeration (ILK Dresden), Bertolt-Brecht-Allee 20, Dresden, 01309, Germany mathias.safarik@ilkdresden.de ABSTRACT Some of the physical properties of water (especially the low vapour density and operation under sub atmospheric pressure) cause major challenges for the design and application of water as refrigerant in vapour compression chillers. On the other hand water is the only natural, non-toxic and non-flammable refrigerant that reaches an efficiency level comparable to other refrigerants used in chillers today. Since 1993 ILK has been working in the field of vapour compression refrigeration with water as refrigerant, for high capacity water chillers. During the past two decades significant advancement in scientific research and product development has been made. Based on the self developed compressor technology the world s first vapour compression chillers using were introduced in 1999 by a German start up company. Several installations have been completed until 2003. Some of them are still in operation today proofing the applicability of water as refrigerant in vapour compression chillers. In an attempt to reduce the investment cost of technology a new compressor design was developed. So far this semi-hermetic compressor was deployed in an 800 kw chiller and a 50 kw vacuum ice generation and storage system. The design of both the chiller and the vacuum ice plant, experiences made during installation and operation as well as experimental results are discussed within this contribution. 1. INTRODUCTION ID:789 Environmental concerns and derived trends in regional and international legislation have renewed the interest in natural refrigerants. The choice of natural refrigerants is limited. For the use in large chillers which contain a considerable amount of refrigerant the refrigerant should ideally be non-toxic and non-flammable. Furthermore it should allow an energy efficient operation. Water () is the only natural refrigerant that fulfils all of these requirements. Given the facts that cooling loads are typically uneven over time and that future electricity supply will more and more rely on fluctuating renewable sources another advantage of water moves into the focus: the possibility to generate ice by direct evaporation of water at an evaporation temperature of -1 C. This is only slightly lower than the evaporation temperature used to produce chiller water of 6 or 7 C. And ice is the ultimate latent storage material with its high energy density of 333.5 kj/kg. 2. Physical properties of in comparison with HFC and its consequences 2.1. Comparison of physical properties of and However, the evaporation process in both chillers and vacuum ice generators occurs under low absolute pressure conditions (< 9 mbar or 900 Pa). This sets outstanding requirements to the water vapour turbo compressor needed to realise these processes. Under these conditions the volume flow rate of vapour that needs to be compressed is large in relation to the produced cooling capacity, see Figure 1.
1,E+06 t E = 4 C Volume flow rate [m³/h] 1,E+05 1,E+04 1,E+03 1,E+02 0 200 400 600 800 Refrigerating capacity [kw] Figure 1: Comparison of the volume flow rate at evaporation condition of refrigeration processes with and Moreover, the pressure ratio of a refrigeration process is much higher than with HFCs. Figure 2 shows a comparison between and regarding the pressure ratio. Depending on the external conditions and the design and dimensioning of the evaporator and condenser the pressure ratio of a chiller is in the range of 6 to 7. For limiting the size of the whole machine no more than two compressor stages should be used in a chiller with radial turbo compressors. This means the turbo compressor must be able to achieve a pressure ratio above 2.5 per stage for the realisation of a double stage vapour compression process under typical external conditions. 16 14 t c = 36 C Pressure ratio p 12 10 8 6 4 2 0-3 0 3 6 9 Figure 2: Pressure ratio of refrigeration processes with and 2.2. Consequences for the compressor design The pressure ratio of a turbo compressor relates proportional to the circumferential speed of the impeller s trailing edge. However, this circumferential speed needs to be well below the speed of sound which limits the ambition to choose higher speeds. The overall size of the compressor is also determined by the suction volume flow rate of the refrigerant vapour. As shown in Figure 1 and Table 2 the volume flow rate of is more than two orders of magnitude larger than with for the same refrigerating capacity. A calculation example using the parameters shown in Table 1 will compare and. Table 1: Process parameters of the calculation example Refrigerating capacity Evaporation temperature t E [ C] 1000 kw Evaporation temperature t E 4 C
Condensation temperature t C 36 C Isentropic efficiency of vapour compression is 0.7 In simple terms it can be said that the specific total supply (effective energy difference between the inlet and the discharge referred to the mass) of a turbo-machine is proportional to the circumferential speed of the impeller s trailing edge. The comparison of the circumferential speed needed and the speed of sound (underlined values in the column in Table 2) illustrates the necessity to use a double stage configuration in chillers with radial turbo compressors. The inlet diameter of the impeller can be determined using the design criterion of a maximal absolute mach number of 0.3 (Grieb, 2009). This results in an inlet diameter of 0.82 m for a 1 MW chiller using. This number illustrates the fact that compressors cannot be as compact as compressors using high-pressure refrigerants. Table 2: Comparison of and within a (theoretical) single-stage process (Refrigerating capacity: 1000 kw; t E = 4 C, t C = 36 C) Volume flow m³/s 66,4 0,403 Evaporation pressure Pa 813 337 750 Pressure ratio - 7.3 2.7 Speed of sound at compressor inlet m/s 412 147 Speed of sound at compressor outlet m/s 563 148 Circumferential speed theoretically needed for a single stage process m/s 684 172 Inlet diameter m 0.82 0.11 For a double stage water vapour process with interstage cooling possible draft dimensions were calculated using the process parameters according to Table 1. The results of these calculations for a compressor with a pressure ratio of 2.7 and a circumferential speed of 554 m/s are shown in Table 3 and Figure 3. Table 3: Draft parameters for a radial compressor with a pressure ratio of 2.7 Impeller Diameter m 0.75 1.0 1.5 Rotational speed RPM 14 100 10 600 7 050 d = 0.75 m Rotational speed [rpm] d = 1 m d = 1.25 m d = 1.5 m Pressure ratio Figure 3: Rotational speed as a function of pressure ratio for different diameters of the impeller
2.3. Cycle configurations for in vapour compression chillers Water () has a higher isentropic exponent than HFC refrigerants. For this reason a classical single-stage vapour compression cycle would lead to a very high discharge temperature (>230 C) besides the problem to design a suitable compressor, see above. Applying a double stage cycle with interstage cooling not only solves these two problems but also significantly increases the efficiency of the process. Figure 4 shows the differences between both cycle configurations. Table 4 shows the results of different cycle calculations for the refrigerants, R1234ze and. The EER for a cycle rises from 5.5 for a single-stage to 6.05 for a double-stage cycle. Regarding the refrigerants and R1234ze a double-stage process does not result in a significant efficiency increase. The comparison of the calculated EER values shown in Figure 5 illustrates that a double stage cycle with reaches the same efficiency level as ammonia and a higher level than cycles using or R1234ze. Table 4: Results of single and double stage cycle calculations single-stage R1234ze single-stage single-stage double-stage Pressure ratio 2.5 2.6 6.4 6.4 Discharge temperature t 4 46 43 232 126 EER 5.65 5.2 5.5 6.05 Ratio: EER Rxxx / EER 1 0.92 0.98 1.08 Boundary conditions: t E = 5.5 C; t C = 35.5 C; superheating: 0.1 K; subcooling: 2 K p qc p qc 5 4 5 4 3 2 6 1 6 1 wt_2 q0 wt q0 wt_1 h Figure 4: Single-stage cycle (left) and double-stage cycle with interstage cooling (right) h 6,5 6,0 5,5 EER 5,0 4,5 4,0 3,5 3,0 R1234yf R1234ze R404A R407C R410A R717 R723 R600a R290 RE170 Figure 5: Calculated EER-Values of double-stage vapour compression cycles with interstage cooling boundary conditions: t E = 5.5 C, t C = 35.5 C, superheating: 0.1 K subcooling: 2 K; first and second stage isentropic efficiency: 0.7
3. Current state of ILK s chiller technology Since 1993 ILK has been working in the field of vapour compression refrigeration with water as refrigerant in high capacity water chillers. Based on the purpose-built compressor technology the world s first vapour compression chillers using were introduced in 1999 (Burandt, 2003). Several installations have been completed until 2003 (Albring and Honke, 2011). Some of them are still in operation today and have accumulated thousands of operating hours in real life installations proofing the applicability of water as refrigerant in large scale vapour compression chillers. These chillers have a hermetic design as shown on the left side of Figure 6. Both motors are located inside the low-pressure vessel. Direct evaporation and direct condensation are used in combination with external plate heat exchangers which are mainly needed because the external circuits cannot be easily realized as (partly) sub-atmospheric. This design, although advantageous in terms of leak-tightness and effectiveness of the evaporation and condensation, needs special motors that are able to cope with the challenging operating conditions (high speed and low pressure water vapour atmosphere). Such motors are complex and costly. In an attempt to decrease the high investment cost of hermetic chillers the development of a semihermetic chiller was started. Compressor Compressor 2. stage 1. stage Cooling water circuit Compressor Compressor 2 nd stage 1 st stage direct condenser Interstage cooler direct evaporator Interstage cooler chilled water circuit chilled water circuit Cooling water circuit plate heat exchanger plate heat exchanger internal water pumps tube and shell condenser tube and shell evaporator Figure 6: Hermetic (left) and semi-hermetic (right) design of vapour compression chillers The main targets of the semi-hermetic chiller development are: - Simplification of design and construction - Reduction of the number of different components - Reduction of rotational speed - Use of external (ambient pressure) motors - Use of shell and tube heat exchangers as condenser and evaporator - Increase of the overall system efficiency - Improvement of the part-load behaviour The major design differences are shown in Figure 6. External motors and shell and tube heat exchangers are used in the semi-hermetic design. External motors require a sealing system. This challenge could be solved by a new developed and patented sealing system. A new design of the compressor core in combination with a rotating inducer was developed for improving the part-load behaviour and increasing the efficiency. Figure 7 shows the new compressor design. Figure 7: Left new compressor design with rotating inducer; Right inlet of the second stage compressor
The inducer is made of a special fibre-reinforced plastic that can be processed by injection-moulding technique. The impeller blades are produced severally before connected with the hub. The design of the blades allows easy adjustment to different capacity ranges. Figure 8 shows the prototype of the 800 kw semi-hermetic chiller on the test rig. Figure 8: Prototype of the 800 kw semi-hermetic chiller Figure 9 shows some measurement results of the second stage within the semi-hermetic chiller. Figure 9: Compressor map of the second stage semi-hermetic compressor; suction temperature 8 16 C (lines: calculation, cross: measurements) 4. Vacuum ice slurry Air conditioning is a major cause for electrical peak loads and electricity consumption. In Jiangsu, China, air conditioning accounts for 40 % of peak summer load (Lehner, 2010). Measurements in commercial buildings in Hong Kong showed that HVAC caused 30-60 % of the total building electrical demand during office hours (Lam, 2003). Thermal storages can be applied to homogenize the load or to shift it to other times of the day or week or to better integrate fluctuating renewable energies. Sensible heat storages are hardly usable because of the small temperature difference of the chilled water that can be utilised only. Typical cold water air conditioning systems are using a water supply temperature of 6 C and a return temperature of 12 C. That means sensible cold water storage can only use a temperature difference of 6 K which results in a storage density of 7 kwh/m³. Ice storages as latent heat storage (phase change), however, have several advantages (Safarik et al., 2013): - The storage temperature (0 C) is close to common chilled water supply temperatures (6...7 C) - Significantly higher storage density compared to sensible storage. h fus = 333.5 kj/kg (93 kwh/m³). - The storage temperature is constant. No stratification issues arise.
One major disadvantage of conventional block ice storage is the considerably lower effective evaporation temperature of the cooling cycle when compared with chilled water generation which reduces the efficiency. Other drawbacks are the need for a glycol system and the fixed relation between charging and discharging performance determined by the size of the heat exchanger (tube or plates with ice on the outside). Vacuum ice describes the process of generating ice by the evaporation of water at triple point conditions. If water of 0 C evaporates the evaporation heat (2.500 kj/kg) is extracted from the remaining water. As this remaining water cannot be sensible cooled down anymore (besides supercooling) a part of it turns into ice providing the heat of fusion (333.5 kj/kg) to the evaporation process. If the created water vapour is constantly removed by a compressor the amount of ice within the remaining water increases creating ice slurry. The compressed water vapour is condensed by using cold water, e.g. from a chilled water network, see Figure 10. The condensate returns to the evaporator. Using an adapted semi-hermetic compressor a vacuum ice energy storage system with an ice generation capacity of 50 kw has been installed within the campus chilled water network of Zwickau University, see Figure 10. In this rather small-scale installation the same tank is used for ice generation and storage. A water volume of 6 m³ and an ice ratio of 50 % result in a storage capacity of 300 kwh. The discharge capacity is determined by the discharge heat exchanger only. In this case it is 100 kw nominal. Some monitoring results of this installation are shown in Figure 11 and Figure 12. For the evaluation of the temperatures and the evaporating pressure in Figure 11 and Figure 12 a NaCl content of about 1 % in the water should be considered which causes freezing temperature and vapour pressure suppression. Evaporator and storage tank Surface area = evaporation and ice generation Chilled water network Suction tube Condenser for water vapour Centrifugal compressor Condenser cooling HX Ice slurry discharge outlet Discharge HX Chilled water network Figure 10: Left Vacuum ice energy storage system at University of Applied Sciences Zwickau, Germany Right Schematic diagram of the Vacuum ice energy storage system Figure 11: Measurement results of a typical charging process of the vacuum ice energy storage system
Figure 12: Measurement results of a typical discharging process of the vacuum ice energy storage system Ice slurry can also be used as secondary refrigerant as it is pumpable. A significant reduction of the energy demand for pumping can be achieved compared to water or water/glycol based distribution systems by using the latent energy of the ice. Also smaller pipes can be used which increases the attractiveness of indirect secondary refrigerant systems compared to direct evaporation systems. The vacuum ice generation technology does not use wearing parts for the production of the slurry which reduces operational costs. Also heat pump applications can be realised with the vacuum ice technology. Water from a natural (lake, river or sea) or artificial (tank) resource can be used as heat source even if its temperature is at 0 C. Heat is removed from the water within the vacuum ice evaporator turning a part of the water into ice. The ice slurry, e.g. with an ice fraction of 25 %, is pumped back to the source. Such a system takes advantage of the constant evaporation temperature of 0 C. It can provide a high efficiency, quiet operation and a constant heating capacity that does not depend on the ambient air temperature. Also seasonal storage is possible by using big tanks. The ice generated during the heating season can be used for cooling in summer. 5. CONCLUSIONS Water () is a viable refrigerant alternative for large scale chillers. It is non-toxic, non-flammable and cheap. A possible leakage causes no hazards to the environment. Chillers using can reach the same efficiency level as chillers using other refrigerants. Some first generation chillers still in operation after 15 years are proofing the applicability to technology. For a broader application chillers need to become less expensive. In an attempt to reduce investment cost a semi-hermetic chiller with a capacity of 800 kw was developed. ILK will continue its efforts to further optimize chillers in the high capacity range to provide a solution to the ongoing refrigerant issue. Low pressure water vapour compressors can also be used to efficiently generate ice slurry by direct evaporation vacuum freezing. A vacuum ice energy storage pilot plant with a charging/ice production capacity of 50 kw has been installed using an adapted compressor. Pumpable vacuum ice slurry has the potentials to be used as secondary refrigerant, to improve batch cooling processes, to better integrate fluctuating renewable energies by storing large amounts of cooling energy using the cheap phase change material water and to enhance district cooling systems. Based on the experiences made ILK will establish vacuum ice systems with higher generation and storage capacities in order to increase the impact and to reduce the specific cost. 6. REFERENCES Albring P., Honke M. 2011, Ice-making and ice-storage with water as refrigerant, Proc. Int. Congress of Refrigeration, IIF/IIR; ID: 508 Burandt B. 2003, Wasser als Kältemittel-Einsatz in Kompressionskälteanlagen, TAB Technik am Bau 34(7/8): 53-54 Grieb H. 2009, Verdichter für Turbo-Flugtriebwerke, Springer, Heidelberg, 696 p. Lam Joseph C, Li Danny H.W, Cheung S.O 2003, An analysis of electricity end-use in air-conditioned office buildings in Hong Kong, Building and Environment 38(3): 493 498 Lehner 2010, In China, Air Conditioning Is Efficient Because of How It's Used, Not Just How It's Built, http://switchboard.nrdc.org/blogs/plehner/in_china_air_conditioning_is_e.html, retrieved: 22.4.2015 Safarik M., Honke M., Heinrich C. 2013, Ice slurry generation by direct evaporation vacuum freezing, Proc. 8th International Renewable Energy Storage Conference and Exhibition (IRES 2013)