In general, most heat losses in industrial dryers arise due to the discharge of humid air. By using heat pump drying (HPD) systems, heat from the exhaust humid air can be recovered, thus improving the energy efficiency substantially. In this study, the performance of thermally driven HP integration in an animal food and a blood dryer were examined. Computer simulation models of the original high temperature dryers and the proposed system with HP integration and auxiliary heating were developed. It is found that, when using a gas engine, the maximum energy cost saving is limited by the temperature of the coolant fluid. The maximum energy cost saving when using a gas turbine is a bit higher, however at a much higher operating temperature

Logie, Jamy

Tran, Minh Cuong

van den Broek, Martijn

Vanslambrouck, Bruno

English

Ghent University Academic Bibliography

CASE STUDIES OF THERMALLY DRIVEN HEAT PUMP ASSISTED DRYING  Minh Cuong Tran, Jamy Logie, Bruno Vanslambrouck, Martijn van den Broek  Department of Flow, Heat and Combustion Mechanics, Ghent University – UGent, Ghent, Belgium email: minhcuong.tran@ugent.be, jamy.logie@ugent.be, bruno.vanslambrouck@ugent.be, martijn.vandenbroek@ugent.be  ABSTRACT  In general, most heat losses in industrial dryers arise due to the discharge of humid air. By using heat pump drying (HPD) systems, heat from the exhaust humid air can be recovered, thus improving the energy efficiency substantially. In this study, the performance of thermally driven HP integration in an animal food and a blood dryer were examined. Computer simulation models of the original high temperature dryers and the proposed system with HP integration and auxiliary heating were developed. It is found that, when using a gas engine, the maximum energy cost saving is limited by the temperature of the coolant fluid. The maximum energy cost saving when using a gas turbine is a bit higher, however at a much higher operating temperature.  Keywords: heat pump drying; simulation; energy analysis; economic benefit; hp4drying  INTRODUCTION  Drying is one of the most energy intensive processes for many industrial sectors. Generally, heat losses in industrial dryers are mostly due to the discharge of moist air and conduction through the drying chamber walls [Minea, 2015]. By using HPD systems, which have the ability to recover partially this heat loss, up to 50% or more primary energy used can be saved [Minea, 2015]. In order to evaluate the performance of the HPD, various studies, from numerical simulation to experimental tests, have been carried out and published. For example, Minea, 2012, carried out experimental studies on wood drying and compared the performance of HPD with the conventional dryer. It was shown that the HP dryer saves up to 48% of the total equivalent (electrical and fossil) energy use. A batch-type HP dryer for medicinal plants was investigated by Ziegler et al., 2013. It is found that the energy efficiency of the HPD system operated in partially open mode is higher in comparison with a closed cycle system. While comparing with a conventional dryer, the energy efficiency of a partially open HPD can be higher or lower dependent on the ambient temperature. In the study of Prasertsan et al., 1996, 1997a, b, two open and two partially closed HPD systems using R22 as working fluid were investigated. The influences of the ambient air conditions, the recirculation air ratio (RAR) as well as bypass air ratio (BAR) on the performance of these systems were determined. Dryers using complex HP systems such as two-stage evaporator systems [Hawlader et al., 2001; Chua et al., 2005] and two cycle HPs [Lee et al., 2010] were also investigated.   However, when comparing to a conventional dryer using primary energy as the heat source, the operating energy cost of an electrical driven HP dryer significantly depends on the ratio of electricity to gas price. To avoid this dependence, employing a thermally driven HP such as a gas engine/turbine driven compressor HP could be a good solution. The performance of these HPD systems integrated in two drying processes is investigated and presented in this study.    SIMULATIONS  Models of the original high temperature dryers and of the proposed system with HP integration and auxiliary heating are implemented in Engineering Equation Solver (EES).  Simulation of the original dryers  The layouts of the two original dryers are represented in figure 1. In these drying processes, the wet product enters into the upper part and exits from the bottom part of the drying chamber. However, there are some differences in the air path of the two drying processes. In the animal food drying process, the fresh air is directly heated up to 140oC by a heater before it comes to the drying chamber and the exhaust air temperature exits the cyclone at around 50oC. Meanwhile, in the blood drying process, a water/air heat exchanger is used to recover heat from the exhaust air after the cyclone and pre-heats the fresh air to around 50oC before it is heated up to the required temperature of 180oC by the heater. The temperature of the exhaust air after the water/air heat exchanger is around 50oC.   Figure 1: Layouts of the original animal food dryer (left) and blood dryer (right)  The power use of the original dryer is the heat rate needed to raise the temperature of the incoming air to the required temperature and is defined as:   [1]  where hheated air is the specific enthalpy of the drying air after the heater.  hincoming air is the specific enthalpy of the fresh air for the animal food case and of the preheated air for the blood drying case.  Simulation of a dryer with a gas engine/turbine driven HP integrated    Figure 2: Layouts of a dryer with a gas engine driven compressor HP integrated in the animal food dryer (left) and with a gas turbine driven compressor HP integrated in the blood dryer (right)  The HP system investigated in this research is a R245fa HP with a subcooler which is proven as the best performing system for these cases [Tran, 2015a,b]. In the HPD systems as shown in Figure 2, the evaporator is employed to recover heat from the exhaust air while the subcooler and the condenser are used to preheat the drying air. For the HP system using a gas engine to drive the compressor (figure 2 – left), the available heat from the coolant fluid and exhaust gases of the gas engine are also used to further heat drying air before it enters the auxiliary heater. Therefore, the performance of a gas engine driven compressor HP is represented by a combined heat and power efficiency (ηCHP) and calculated as the ratio of the useful heat used to heat drying air to the energy consumption of the gas engine:       [2]   gasesexhaustthermal,fluidcoolantthermal,mechfuelexhaustcoolantcondsubcoolerfuelusefulCHPηηηCOP           QQQQQ          QQη)h(hmQ air incomingair heatedairdryer original where COP, ηmech, ηthermal,coolant fluid and ηthermal,exhaust gases are:  compcondsubcoolerWQQCOP        [3] fuelcompmech QWη         [4] fuelcoolantfluidcoolantthermal, QQη        [5] fuelexhaustgasesexhaustthermal, QQη       [6] The operating mechanism of a gas turbine driven compressor HP is similar to a gas engine driven compressor HP (figure 2 - right). However, in this case, the only waste heat stream is exhaust gas of the gas turbine. Thus, the ηCHP of a gas turbine driven compressor HP is defined as:    [7]  The total power consumption of a HPD system is:    [8]  where hpre-heated air is the enthalpy of the drying air preheated by the exhaust gas of the gas engine or gas turbine.  Economic benefit calculation  In this study, the economic benefit of a HPD is investigated based only on the operating energy cost of the drying system and is expressed by the relative energy cost (REC), which is defined as the ratio of the energy cost of the HPD system to the energy cost of the original dryer: [%]100CCcostenergyrelativedryer originaldryer HP     [9] The fixed costs (investment, installation) of the HPD are not considered. For the original dryer, the fuel to heat conversion ratio of the air heater is assumed 100%.  Assumptions and inputs  The calculations in this paper are based on the assumptions that the refrigerant flow rate is constant and heat and pressure losses are neglected. For the gas engine, the mechanical and thermal efficiencies at part load operation are the same as at full load operation. These values are the efficiencies of the MAN E0834-E312 (for the animal food drying case) and MAN E0836-E302 (for the blood drying case) gas engines operating at speed of 1500rpm. It is also assumed that the temperature of the coolant fluid is 90oC and the minimum temperature difference of the coolant fluid/air heat exchanger is 10oC. This means that the maximum temperature of the drying air (heated by the coolant fluid/air heat exchanger) that can be reached is 80oC (Ta3, figure 3). For the gas turbine, the mechanical efficiency is also assumed constant while the mass flow rate of the exhaust gas is proportional to the turbine load. The parameters of the gas turbine are obtained from the Capstone C65 and Capstone C200LP gas turbines for the animal food drying and the blood drying systems, respectively. The input parameters of the calculations are presented in Table 1.  PERFORMANCE ANALYSIS BY SIMULATION  Generally, we would like to recover as much heat as possible from the exhaust air to reduce heat losses. However, more recovered heat means that the HP will operate at a lower evaporation temperature and thus pressure. Moreover, as mentioned above, the heat recovered from the exhaust air is used to preheat the ambient air. Thus, the more heat recovered from the exhaust air, the higher the temperature of the  gasesexhaustthermal,mech ηηCOPCHPη )h(hmQQ air heated-preair heatedairfuelHPD Table 1. The inputs for simulating calculation Parameter Unit Value Animal food drying Blood drying Ambient air temperature oC 20 Ambient air relative humidity % 60 Isentropic efficiency of the compressor - 0.7 Pinch point of the evaporator, condenser oC 8 Pinch point of the subcooler oC 15 Superheating temperature oC 5 Volumetric flow rate of the hot air m3/h 12000 70000 Volumetric flow rate of the drying air flows to the bottom of the drying chamber. m3/h 5000 8000 Volumetric flow rate of the drying air flows to the bottom of the cylinder. m3/h - 2000 Parameters of the gas engine Gas engine power output (full load) kW 47 75 ηthermal, coolant fluid - 0.302 0.308 ηthermal, exhaust gases - 0.186 0.225 ηmech - 0.364 0.367 Parameters of the gas turbine Gas turbine power output (full load) kW 65 190 ηmech - 0.29 0.31 Exhaust gas temperature oC 309 280 Exhaust gas flow (full load) kg/s 0.49 1.3  preheated air. Consequently, an increase in temperature difference between the condensation and evaporation is created. This causes a decrease of the HP system efficiency. Hence, there is a trade-off between the amount of heat recovered and the HP system efficiency.   Gas engine driven compressor HPD systems  Figure 3: REC, Ta3, Ta4 and ηCHP as a function of Ta2 for the system using a gas engine to drive the compressor in the animal food drying case (left) and the blood drying case (right).  Figure 3 presents the REC and ηCHP of the drying system using a gas engine to drive the compressor as a function of the air temperature after the condenser (Ta2) for both drying cases. The temperatures of the air after the coolant liquid/air heat exchanger (Ta3) and after the exhaust gases/air heat exchanger (Ta4) are indicated as well. It can be seen that, for both cases, the energy cost of the systems decreases almost linearly by increasing Ta2 until it reaches the optimum value Topt which depends on the maximum temperature (Ta3max) of the drying air at the exit of the coolant heat exchanger or, in other words, the coolant fluid temperature and the minimum temperature difference of the coolant heat exchanger. Indeed, when Ta2 is lower than Topt and the corresponding Ta3 is lower than this Ta3max, all the heat available from the coolant fluid can be used to heat the drying air, therefore the energy cost decreases when Ta2 increases. Once Ta2 reaches its optimum value and Ta3 reaches Ta3max, the energy used by the gas engine increases with increasing Ta2 in order to supply enough mechanical work to the compressor, whereas the useful heat from the coolant fluid used to heat the drying air decreases. As a result, ηCHP significantly decreases and the energy cost rises. In the animal food drying case, the minimum relative cost is about 67.2% at the Ta2 and ηCHP of 72oC and 2.6, respectively. Meanwhile, these values in the blood drying case are 86.5%, 76oC and 2.1, respectively.  Generally, the exhaust gas temperature is much higher than the coolant fluid temperature and therefore the optimum operating temperature does not depend on the exhaust gases temperature. However, the percentage of heat recovered from exhaust gases affects the performance of the gas engine driven HP according to equation (2).  Gas turbine driven compressor HPD systems  Figure 4: REC, Ta3 and ηCHP as a function of Ta2 for the system using a gas turbine to drive the compressor in the animal food drying case (left) and the blood drying case (right).  The dependence on Ta2 of the REC, temperature of the air after the exhaust gases/air heat exchanger (Ta3) and ηCHP of the HP system using a gas turbine to drive the compressor is shown in figure 4. Since this HP system only has one waste heat stream, being exhaust gases, and its temperature is much higher than the Ta2, the minimum REC is not limited by the waste heat temperature as was the case in the gas engine driven compressor HPD system. In this case, there still is an optimum operating condition at which the REC reaches the minimum value, however this optimum operating condition depends on all of the parameters (Table 1) used for the calculation. Thus, it differs for each drying system. The minimum RECs are 66.2% and 82.2% for animal food drying and blood drying, respectively, which are little smaller in comparison with the gas engine case. However, the optimums Ta2 are much higher being 101oC and 111oC, respectively.   One should also notice that for the animal food drying case, the auxiliary heater could be eliminated if the gas turbine driven HP operates at Ta2 and Ta3 of 112oC and 140oC, respectively.     Comparison of the HPD system performance between two drying cases  In both drying systems, the exhaust air is the heat source for the HP system and its temperature is 50oC. However, the exhaust air relative humidity of the animal food drying case (64%) is higher than that of the blood drying system (50%). In addition, the incoming air temperature of the latter system is already preheated to 50oC, thus it decreases the subcooling. Therefore, the HP COP of the animal food drying system is much higher than that of the blood drying system. As a result, the ηCHP of the animal food drying system is higher than the blood drying system (Figure 3 and 4) even the mechanical and thermal efficiencies of the gas engine/turbine of the former system are a bit lower.  About the economic benefits, we can see that the REC of the blood drying case is much higher than that of the animal food drying case. Besides having a lower ηCHP, it is also because the drying temperature of the former (180oC) is higher, or in other words, the proportion of the heat supplied by the HP system to the heat supplied by the auxiliary heater is smaller.    CONCLUSION  This paper presents the results of two case studies about the performance of thermally driven HP integration in an animal food dryer and a blood dryer. In order to study the performance of HP dryers, computer simulation models of the original high temperature dryer and the proposed system with HP integration and auxiliary heating are developed. The results showed that, when using a gas engine driven compressor HP, in which recovered heat from the coolant fluid and exhaust gas of the gas engine is also used to heat drying air, the maximum energy cost saving is limited by the temperature of the coolant fluid. This limitation can be eliminated by using a gas turbine driven compressor HP since a gas turbine only has one waste heat stream being exhaust gases. In addition, the air temperature heated by this system is also higher in comparison with the gas engine driven compressor HP.   NOMENCLATURE  COP  coefficient of performance CHP dryer  total energy cost of the dryer with a HP integrated [Euro/year] Coriginal dryer energy cost of the original dryer [Euro/year] mair  mass flow rate of the drying air [kgs-1] Qcond  heat release rate at the condenser [kW] Qcoolant  heat release rate at the coolant heat exchanger [kW] Qevap  heat delivery rate to the evaporator [kW] Qexhaust  heat release rate at the gas engine exhaust air heat exchanger [kW] Qfuel  power consumption of the gas engine [kW] QHPD  power consumption of the dryer integrated HP [kW] Qoriginal dryer power consumption of the original dryer [kW] Qsubcooler heat release rate at the subcooler [kW] Wcomp  power consumption of the compressor [kW]  ACKNOWLEDGEMENTS  The results presented in this paper have been obtained within the frame of the IWT-CORNET project 130375 "Energetic and environmental optimisation of drying processes by integration of heat pumps - HP4Drying" (hp4drying.eu), funded by the Institute for the Promotion and Innovation by Science and Technology in Flanders. This financial support is gratefully acknowledged.  REFERENCE  Chua, K. J., Chou, S. K. A modular approach to study the performance of a two-stage heat pump system for drying, Applied Thermal Engineering, 2005, 25(8-9), 1363–1379. Tran, C., T'Jollyn, I., Logie, J., van den Broek, M., De Paepe, M., Vanslambrouck, B. Case study of heat pump integration in a blood dryer, 2015a, The 8th Asia-Pacific Drying Conference (ADC 2015), Kuala Lumpur, Malaysia. Tran, C., Logie, J., Vanslambrouck, B., van den Broek, M. Case study of heat pump integration in an animal food dryer, 2015b, The 1st Nordic Baltic Drying Conference (NBDC 2015), Gdansk, Poland. Hawlader, M. N. A., Chou, S. K., Chua, K. J., Ho, J. C., Mujumdar, A. S. On the steady-state modelling of a two-stage evaporator system, International Journal of Energy Research, 2001, 25(10), 859–880. Lee, K. H., Kim, O. J., Kim, J. Performance Simulation of a Two-Cycle Heat Pump Dryer for High-Temperature Drying, Drying Technology, 2010, 28(5), 683–689. Minea, V. Efficient Energy Recovery with Wood Drying Heat Pumps, Drying Technology, 2012, 30(14), 1630-1643. Minea, V. Overview of Heat-Pump–Assisted Drying Systems, Part I: Integration, Control Complexity, and Applicability of New Innovative Concepts, Drying Technology, 2015, 33(5), 515-526. Prasertsan, S., Saen-Saby, P., Ngamsritrakul, P., Prateepchaikul, G. Heat pump dryer Part 1: Simulation of the models, International Journal of Energy Research, 1996, 20(12), 1067–1079. Prasertsan, S., Saen-Saby, P., Ngamsritrakul, P., Prateepchaikul, G. Heat pump dryer Part 2: Results of the Simulation, International Journal of Energy Research, 1997a, 21(1), 1–20. Prasertsan, S., Saen-Saby, P., Ngamsritrakul, P., Prateepchaikul, G. Heat pump dryer Part 3: experimental verification of the simulation, International Journal of Energy Research, 1997b, 21(8), 707–722. Ziegler, T., Jubaer, H., Mellmann, J. Simulation of a Heat Pump Dryer for Medicinal Plants, Chemie Ingenieur Technik, 2013, 85(3), 353–363. 

Case studies of thermally driven heat pump assisted drying

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Case studies of thermally driven heat pump assisted drying

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