Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle coupled with ice storage

Transcription

1 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle coupled with ice storage Jorge A. J. Caeiro The Bartlett School of Graduate Studies, Faculty of the Built Environment, University College London, (Torrington Place Site), Gower Street, London WC1E 6BT, UK Abstract The experimental findings are reported from testing an innovative hybrid refrigeration cycle bringing together a single effect Lithium Bromide-Water refrigerator and a steam ejector and aimed at vacuum freezing an ice store. An experimental set-up was designed and fabricated in order to study the operational performance of the novel arrangement under different conditions. The novel system delivered desired results with the ice store frozen in useful time using solely low grade heat to power the cycle. The average COP of the freezing process is Keywords ice storage; absorption refrigeration; steam ejector 1 Introduction It is now recognised that the combination of CHP with absorption refrigeration for District Heating and Cooling schemes can lead to reductions in electricity demand when waste heat from the CHP unit is used to provide space cooling. It is believed that the addition of ice storage systems to such schemes could provide a means of smoothing loads on district cooling networks. This could further reduce the installed electricity generating capacity of CHP plant by removing the need for electrically powered air chillers, whilst reducing the installed cooling capacity of the absorption plant and therefore reducing capital cost whilst saving energy. Among the different solutions used to store cold, ice storage presents some significant advantages namely the density of the ice packing that can almost be 10 times below the equivalent cooling capacity of chilled water [1]. In current ice storage/load shifting systems, ice is by and large produced by electrically powered refrigeration facilities. Lithium-Bromide-Water chillers became standard use in comfort cooling applications because of the toxic nature and the fire risk of ammonia chillers should leaks occur in the piping or evaporator systems. Ammonia refrigeration systems are therefore mostly confined to industrial refrigeration applications. In this paper is reported the experimental investigation of an innovative cycle combining Lithium- Bromide-Water absorption and steam ejector refrigeration. The main objective of this arrangement is to create an entirely heat powered system capable of producing ice for thermal comfort. The absorption refrigerator can this way benefit from all the technical and economical advantages associated to coupling with ice storage. In addition, the novel cycle is an environmentally friendly and economically competitive alternative to conventional chillers that can be powered by low grade heat. Sources of low grade heat are plentiful, for example, solar energy, low pressure

2 60 Jorge A. J. Caeiro steam, flue and exhaust gases, etc. These sources of energy are available; at least some of them, at minimal cost and their utilization have obvious advantages. Adding to these environmental benefits the novel system uses water as a refrigerant, an inexpensive and innocuous refrigerant. 2 Description of the experimental set up A schematic diagram of the prototype system tested in laboratory is shown in Figure 1. The facility equipment consists mainly of a steam generator, an ejector, an ice store, an evaporator, an absorber, a condenser and a solution heat exchanger. The piping system, valves and pumps connect these components. The experimental setup is completed with measuring and control devices (Figure 2). The system can operate in two distinct cycles: ice store charging and discharging. The charging cycle is by far the most complex because it involves balancing the capacities of three dissimilar technologies: absorption refrigeration, steam jet refrigeration and thermal ice storage (TIS). The main aim of the charging cycle is to store cold as latent heat of fusion at times of the day when cooling is not required so that it can be used to top up the cooling effect provided by the absorption system during peak hours. Whilst charging the TIS the system operates under three different pressure levels. In the high pressure level the condenser saturation temperature determines the pressure in the generator. At the intermediate level the temperature and concentration of the weak solution determines the pressure in the absorber. At the low level the pressure is determined by the freezing point of water. When operating in the discharge mode Figure 1. Schematic of the experimental system.

3 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle 61 Figure 2. Picture of the experimental set up.

4 62 Jorge A. J. Caeiro the system resumes to a single effect absorption refrigerator coupled with ice storage. 2.1 The ice store main features In traditional encapsulated ice storage systems the storage media can be deionised water or a eutectic salt. The ice storage elements are immersed in chilled water glycol or other antifreeze solution. The solution, usually at a temperature lower than 5 C, is pumped through the vessel causing the water in the elements to freeze. To discharge the cooling from the storage, warm fluid carrying heat from the load is circulated through the vessel, melting the encapsulated media. The cooled solution is then used to meet the load either directly or through a heat exchanger. A common design for the ice containers is dimpled high density polyethylene spheres designed to accommodate the expansion and resulting pressure of the freezing ice. The containers are filled with deionised water and nucleating agent. Other configurations are also available like rectangular and cylindrical enclosures. Despite these basic similarities there are significant differences between the conventional and the vacuum freezing processes. In conventional encapsulated ice storage systems the heat is drawn from the elements by mixed conduction and convection. The storage media never gets in contact with the cooling solution. Vacuum freezing is an evaporative cooling process and therefore the media not only stores cold but also acts as the refrigerant. The experimental TIS consists of a cylindrical glass vessel packed inside with several layers of cylindrical ice storage elements. The singularity of the media containing stainless steel capsules is that these being punctured allow water freely flashing into vapour (Figure 3). An agglomerating material is added to water to turn the media into a semi solid gel. The gel is contained in sachets made of permeable paper that allowed the easy diffusion of moisture and vapour. The material added to water is a powdered superabsorbent polymer with the chemical denomination of partially neutralised poly acrylic acid. Superabsorbent polymers are a special group of polymers because of their ability to absorb many hundreds of times their own mass of water. In their dry crystalline form they are specially formulated to swell very rapidly in water but they do not dissolve. Their Figure 3. Diagram of an ice storage element.

5 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle 63 swelling behaviour is characterised mainly by the amount of water they absorb and the rate of absorption. A more detailed description of the physicochemical mechanisms behind the polymer swelling process can be found elsewhere [2, 3, and 4]. The ice store contained a total of 341 elements, stacked in seven batches and capable of retaining 40 kg of water. 2.2 Determination of the steam ejector primary nozzle optimum operating position The steam ejector is the component that most heavily impacts on the overall performance of the freezing process. A singularity of the steam ejector being used is its movable primary nozzle allowing adjusting its position in order to optimize its entrainment ratio. The position of the ejector nozzle relative to the mixing chamber can change the flow pattern and hence the ejector performance. As part of the commissioning tests, the ejector was tested to determine experimentally its primary and secondary flow under the design operating conditions and for different positions of the primary nozzle. The nozzle exit position (NXP) was defined as the distance between the nozzle exit and mixing chamber inlet planes as illustrated in Figure 4. The NXP has a positive value when the nozzle is placed inside the mixing chamber, and is negative when outside the mixing chamber. In order to calibrate the ejector the primary nozzle was moved back and forward from its position by increments of ±10 mm. All positions of the nozzle were precisely measured relatively to NXP = 0 mm. For each position the ejector both primary and secondary flows were measured. From the results recorded the entrainment ratio could be computed in the usual way from the ratio R m = m s / m p. It was found that the primary flow rate did not vary significantly with NXP. The measured maximum and minimum primary flow rates were only 5.73% higher and 3.22% lower respectively than the theoretically estimated flow rate of kgh 1. Conversely the secondary flow rate varied greatly though it was lower on average than predicted by the theoretical model used. It is believed that the deviation from the theoretical predictions may have partially resulted from some of the assumptions made during the design stage of the ejector. Also, the operating conditions varied over a wide range during the tests, e.g. the vapour pressure in the ice store dropped Figure 4. Nozzle exit position relative to the mixing chamber (NXP).

6 64 Jorge A. J. Caeiro quickly down to the triple point and still continued to drop steadily until the test was completed. On the other hand, the actual critical pressure was about 30% greater than the design pressure. The actual critical pressure lift ratio increased therefore, in the same proportion, than the design critical pressure lift ratio, which most likely had a detrimental effect over the entrainment ratio throughout the tests. After the calibration tests were completed the most favourable entrainment ratio was found at NXP = 20 mm. The value found was in close agreement with the recommendations from ESDU [5]. Although this source does not provide any precise advice for the primary nozzle position of an ejector with a constant pressure mixing chamber, it recommends that for ejectors with a constant area mixing chamber the nozzle exit is placed 0.5 to 1.0 times the mixing chamber s throat diameter upstream of the start of the mixing chamber. This would correspond to a range between NXP = 16.5 mm and NXP = 33 mm which validates the option for NXP = 20 mm as the optimum operating position. 3 Assessment of the system s performance from the experimental results The assessment of the working performance of the experimental system is based on two main parameters. These parameters are used to define the energy performance of the whole system and not of individual processes. The parameters used are the Cooling Capacity of the refrigeration system and the Energy Input into the system. The ratio of these two parameters defines the energy transfer effectiveness of the cycle (COP). These parameters were estimated by performing heat and mass balances to individual components of the system. To assess the evolution of the process pressures, temperatures and flow rates of the solution and refrigerant were measured and logged at key points of the system. 3.1 Cooling capacity Depending on which mode the system was operated at e.g. charge or discharge of the ice store, the cooling effect was either produced in the TIS or in the evaporator. Whilst the cooling effect produced in the evaporator during the discharging cycle could be determined by direct measurement of the volume of water evaporated inside of the glass vessel used, the cooling effect produced in the TIS during the charging cycle could only be estimated. As an alternative to a direct measurement procedure the total amount of water evaporated / sublimated was computed using a semi empirical heat and mass transfer model [6, 7]. 3.2 Power input The power input to the generator was measured directly by an electrical power meter. Some heat losses inevitably occurred though despite most of the generator external surface and piping surface being well insulated. These losses were estimated and accounted into the process using heat transfer equations commonly found in text books.

7 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle 65 4 Experimental results presentation and discussion 4.1 The freezing process temperature data Figure 5 shows the evolution of central core temperatures in a few selected storage elements. In the first phase, sensible cooling of liquid water took place. Just after liquid water supercooling ended the process of crystallization started (phase II). At this precise moment of the freezing process the recorded vapour dew point temperature inside the TIS was 1.93 C whilst the elements central core temperature was respectively T 1 = 0.6 C, T 5 = 0.93 C and T 8 = 0.90 C. This suggests that a layer of ice had already formed on the outer surface of the elements whilst the central core still remained in a liquid state. The rapid temperature rise recorded was due to supercooled water reverting to dendritic ice, the latent heat released making the temperature of the unfrozen gel rise up to its new equilibrium temperature, approximately 2 C. It was followed by a progressive drop of the central core temperature, taking about two and a half hours, before the freezing process reached completion. Figure 6 shows the formation of ice over the elements surface during the freezing process. After liquid water had totally reverted to ice, the temperature dropped at a much greater rate suggesting that sensible cooling of the ice was taking place (Phase III). In the particular test shown, the temperature of the gel dropped at an approximate rate of 0.81 C / hour during the second phase of the freezing process whilst during the third phase dropped at a much quicker rate of 5.82 C / hour. The entire freezing process took approximately 3 hours to be completed. All other tests performed provided similar results. Figure 5. Evolution of the central core temperature of selected elements.

8 66 Jorge A. J. Caeiro Figure 6. Icicles formation during the freezing process. 4.2 Evaluation of the charging cycle energy performance The COP of the charging cycle was computed from the ratio between the estimated cooling capacity and the average power consumption. Because it was not feasible to measure the ejector vapour flows, a precise value for the cold stored during the charging tests could not be determined experimentally. Alternatively, an estimate was made based on mass and energy balances and for the measured operating conditions. Being known that the initial water content of the bed of elements was around 40.3 kg it was calculated that kg of water would have to be evaporated to chill down to 0 C the ice store. The mass of ice to be sublimated was calculated to be kg. Considering that the average heat of vaporisation and sublimation were calculated as being h fg = kj/kg and h ig = kj/kg, respectively, the average cooling capacity of the combined system was determined to be kw. From there the COP of the charging process was calculated to be Based on the calculations the average secondary flow rate during the chilling period would have to be equal to kg/hour. This value is much higher than the average secondary flow rate found experimentally. This can be explained by the relatively high secondary pressure existing in the TIS during the chilling period. In fact, during the initial 12 minutes the secondary pressure was even greater than the backpressure and for nearly 25 minutes was greater than the design pressure. After supercooled liquid water reverted into the equilibrium freezing temperature the secondary pressure started gradually lowering below the triple point. The average

9 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle 67 secondary flow rate also dropped to kg/hour until sensible cooling of the ice store started where it further dropped to kg/h. The average secondary flow rate for the entire freezing process was kg/hour. This figure is 6.1% lower than the result found experimentally during the ejector calibration process for NXP = 20 mm. The most likely reason for this discrepancy is that the value of the secondary flow found during the ejector calibration is not representative of a complete charging cycle as tests normally lasted just for an hour. After entering the former values in the analytical model it was determined that for the above power consumption the primary flow rate, would have had to equal kg/h, which is 14.53% greater than the flow rate found experimentally. The results of these calculations are consistent with experimental work carried out by other researchers e.g. [8] that showed that for fixed geometry ejectors an increase in motive steam pressure, from the design point, resulted in an increase of the primary flow and in a drop of the entrainment ratio. The COP of the charging process averaged 0.24 which is relatively low when compared with other ice storage technologies. One of the likely contributors towards this low COP was the progressive and continuous pressure drop in the TIS, mostly below the triple point. The steam ejector operated therefore for the most part of the time with a higher than design pressure lift ratio which probably affected its performance [9]. The experimental results show that the average entrainment ratio was lower than initially expected dropping significantly during the sensible cooling phase. Although the average primary pressure was also higher than design, and hence some improvement of the cooling effect could be expected, this was offset by higher energy consumption. As the steam ejector entrainment ratio strongly impacts the cycle s overall performance this may be considered an important area for future improvement of the charge cycle energy efficiency. 5 The discharge cycle testing 5.1 Experimental procedures Following the freezing process completion the system was operated in the discharge mode. The TIS unit can be discharged in series or in parallel with the absorption refrigerator. The parallel method of discharge adopted may be considered singular to the novel system and was only possible because both the absorption refrigerator and ice store operated using the same working fluid. It presented some technical advantages namely its simplicity. Whilst discharging the system resumed to single effect absorption coupled with ice storage. The water contained in the evaporator was circulated through a heat exchanger in order to simulate the building heat load. The volume of water evaporated was periodically measured on a scale marked on the external surface of the evaporator and its temperature recorded by the datalogger. 5.2 Results and discussion Figure 7 shows the progression of the central core temperature of the selected elements in batches 1,2,3,6 and 7, and also the water temperature inside the evaporator (T e ).

10 68 Jorge A. J. Caeiro Figure 7. Progression of the elements' central core temperature for the period of the discharge process. As soon as the discharge process started (minute 10 on the graph) a quick drop of temperature occurred in the evaporator due to the large amount of water evaporated. From that moment on, the pressure measured in the TIS would be close to the saturation pressure in the evaporator. The temperature in the evaporator steadily increased meaning that the heat load slightly exceeded the system s cooling capacity. The temperature measured in the first batch of elements (T 1 ) also rose steadily until it was nearly the same as T e, dropping soon after. This phenomenon repeated itself each time discharges were made from the condenser to the evaporator and slight pressure variations occurred. Eventually it stabilized after 45 minutes when the vapour saturation pressure was reached. The same phenomenon occurred with the next two batches of elements (T 2 and T 3 ) and though the reason for this occurring was not completely identified it is thought that was caused by vapour stratification inside the TIS. Slight pressure changes in the vapour lines caused the movement of the warmer vapour downwards leading to rapid changes of the temperature of the elements along their vertical axis. The bottom side batches of elements temperatures (T 6 and T 7 ) remained nearly unaltered during the whole time of the test. It is believed that the ejector s throat, having a diameter almost a tenth from the pipe driving the vapour to the absorber, restricted somehow the volume of vapour flowing to the TIS and slowed down the rate of discharge. The tests had shown that although the method of discharge tested had the potential to substitute series operation with chilled water circulating the TIS, significant modifications in the piping

11 Experimental testing of an innovative Lithium-Bromide water absorption refrigeration cycle 69 arrangement would be required in order to become effective, namely by passing the steam ejector. An alternative way of discharging the ice store, normally used in conventional series discharge of the bed of elements would be to first run the chilled water from the air conditioning system through a heat exchanger within the evaporator and then feed it through a second heat exchanger on one side of which water from the ice store at 0 C would be used to complete the cooling process. The main advantage of this arrangement is a better heat exchange rate due to the higher density of the circulating fluid and an easier control of the discharge mode. It would however have the disadvantage of requiring additional pumps, heat exchangers and a spare vessel to store the circulating fluid. 6 Conclusions The tests have proven that the novel combined cycle is capable of vacuum freezing the storage media, using almost solely low-grade heat to power the system. The storage media, with an initial mass of water close to 40 kg, was frozen in approximately 3 hours using an average power around 7 kw. This time may be considered suitable for most practical applications considering that the TIS is to be charged overnight. When operating in the charge mode the experimental COP value achieved was in the range of 0.23 to 0.25, with an average temperature in the generator, T g = C (T sat = 57 C) and an average temperature in the absorber, T a = 32 C (T sat = 6 C). Temperatures as low as 10 C were reached in the TIS during the tests. It is believed that there is still scope for further increase in the efficiency of the cycle if a better performance ejector were used. The parallel discharge method tested can also be significantly improved if modifications were made to the piping system allowing less restriction on the vapour flow. References [1] S. M. Hasnain, Review on sustainable thermal energy storage technologies, Part II: Cool thermal storage, Energy Convers. Mgmt, 39 (11), (1998), [2] H. Omidian, S. A. Hashemi, P. G. Sammes and I. Meldrum, A model for the swelling of superabsorbent polymers, Polymer, 39 (26), (1998), [3] R. Wolf, Vieth Diffusion in and Through Polymers, Hanser publishers, [4] D. Vesely, Molecular sorption mechanism of solvent diffusion in polymers, Polymer, 42, (2001), [5] ESDU, Ejectors and jet pumps-design for steam driven flow, Engineering Science data item 86030, Engineering Science Data Unit, London, (1986). [6] M. Worall, An experimental investigation of a jet-pump thermal (ice) storage system powered by low grade heat, Ph.D. thesis, University of Nottingham, United Kingdom, (2001). [7] J. A. J. Caeiro, A Lithium Bromide water absorption refrigeration system combined with steam jet thermal ice storage, Ph.D. thesis, University of Nottingham, United Kingdom, (2004). [8] J. T. Munday and F. D. Bagster, A new ejector theory applied to steam jet refrigeration, Ind. Eng. Chem, Process Des. Dev., 16 (4), (1977), [9] S. Wu, Investigation of ejector re-compression absorption refrigeration cycle, Ph.D. thesis, University of Nottingham, United Kingdom, (1999).