Energy and economic performance parison of heat pump and power cycle inlow grade waste heat recovery
Zhimin Tan° Xiao Fenga* Minbo Yang° Yufei Wang
XFonJing Uesiy N'on Shoxt 710049 Chse Chne Uty f Pm Beg 102249 Chs
A B S T R A C T
A R TI C L E 1 N F 0
For the sake ofmaing an aropriate chi f the reus w gae ate hat tmchica hat u steam turbine (ST) amd organic Rankine cyele (ORC) are modeled by Aspen Plus. The working conditions of100-150 C waste heat empentures as well as 10-30 C hat pump temperature lits are simulated and calculathicio frity tht sn amrtt icril economic performance f the thre technologies in low grade waste heat recovery are pared and analyed bsed on the eergy effieny and l reeue.The reults inicate that in tems of t exrgy ffcieny themechanal hat p s ays th bt che Frm th ie f th a vnu emechnal heat pump is the best choice in mot cases. The chamce that the anmual revemue of the ORC and ST is greater thanthat of the mechanical heat pump arises oly when the price ratio of electricity to hat is greter than 5. As fr the payback period allthe cycles can meet therequirements of enterprises. A gusde of the selection of waste heatrecovery technology based on economics is drawn.
Kegande Waste heat recoveryMechasial hst pump Organic rankine cydeEsrg effkcincy Soeam tarbine Annual revenue
etc.
appropriate waste heat utilization technology is very important. This When the waste heat has many acceptable consumers identifying anstep needs to pare different types of waste heat recovery methods under the same waste heat conditions. There are researches paringdifferent technologies. Tan et al. [10] pared the energy and eco- nomic performance of using AHP to output 130 °C saturated steam andAC to generate 7/10 °C cold water when the waste heat temperature is 95125 °C. The results show that from the coefficient of performance(COP) exergy effcieney and nit exergy operating cost the AH is better than the AC. Brtickner et al. [11] analyzed the economics of theAHP AC and mechanical heat pump (MHP) in various application scenarios as well as the change in maximum acceptable investmentthe AC MHP AHP should be greater than 6500 h 4000 h and 3000 h costs with operation time. The results showed that the operation time ofrespectively to be economically feasible in the three scenarios. Furthermore the investment cost of the existing MHP has been les thanor equal to the maximum acceptable investment cost for two types ofconsumers demonstrating that the technology has significant mer- cial application value. Wang et al. [12] investigated the optimal con-100200 °C and pared the energy performance of the two cycles ditions for the AC using LiBr/HyO as working fluid and KC at
1. Introduetion
Fossil fuels have bee an important resource ffeting social andeconomic development since the industrial revolution. However it is dificult for the reserves of fossil energy to meet the growing demand ofhuman society so carrying out energy conservation is of vital impor- tance Under the data released by theIntemational Energy Agency [1] ansnpu Suμnoeynueu aq q pounsuo s X8uau [eqo8 aq po puu and 50% of such energy is discharged as industrial waste heat. 60% ofthe waste heat can be utilized while the utilization rate in China is only 0 upea ape mo jo nq nb u ae s eu asm a [] %0the dificulty of direct utilization. Converting such waste heat into high ap mo u em uouo s 3om Jo Aoedes Suoo e ape18waste heat recovery. These methods are realized by hest pumps refrigeration cycles and power cycles respectively. Research on these technologies currently focuses on the seleetion of working fluids [3] new system developments such as absorption chiller (AC) with two dd g s-raes ] ss asthe integration of LiBr/H2O AC and Kalina cycle (KC) [6] and the bination of organic Rankine cyele (ORC) and heat pump [7] and‘[6] 
‘[s] (dHv) dund eaq uogdaosqe uado aog uogezudo mas(s
Nomenclature w h power consumed or generated kW specific enthalpy lkJ/kgAcroryms AHP absorption heat pump m 5 specific entropy kJ/(kg K) mass flow rate kg/sMHP IS mechanical heat pump steam turbine T Kelvin temperature KORC organic Rankine cycle Subscripts el electricityGreek letters T temperature lift C he 0 reference state 298.15 K 1 atm heatr exergy efficiency cwin cold water input pressorParameters AR annual revenue $/α pump out pump steam/work outputC E physical exergy kw investment cost S whin tur turbine waste heat inputPP d price s/kWh paybeck period a x pump) exergy input (work consumed by the pressor andthe price ratio of electricity to heat
using mechanical work as an indicator. The results suggest that when the waste heat temperature is more than 175 °C the KC works better;otherwise the AC is a superior option. Oluleye et al. [13] pared the waste heat recovery performance of heat pumps (AHP absorption heattransformer (AHT) MHP) ORC AC and direct heat utilization based on the minimum exergy degradation when the waste heat temperature islower than 265 °C and gave the application scope of different waste heat recovery technologies which has guiding significance for industrialwaste heat recovery. This study just addressed the energy factor but when industries select waste heat recovery technology the economics isequally essential. The integration of two sites (which have waste heat resources) with boiler feed water (BFW) heating ORC AC and MHP was zation objective. The results reveal that under 140 ~C the ORC costs the considered by Kwak et al. [14] with economic benefits as the optimi-. 00 ad q am u m sam] optimized BFW heating has the lowest cost in winter and summer fol.lowed by ORC. The cost of the MHP is close to that of the ORC and thatof the AC is the highest. However for different sites it requires distinct modelling and optimization and the findings are not universl.
technologies with substantial energy-saving potential. The heat pump Heat pumps [15] and power cycles [16] are two waste heat recoveryutilizes waste heat below 150 °C while the power cycle recoveries waste ja o aeado a qov [11 . 0g on . oot wonj eprinciples there is an overlap between the two cycles’ waste heat uti- lization ranges. If waste heat is available for both cycles at such tem-perature range it is critical to pare the two cycles so that the rau smq asem 1noqe wopspp poumgu ue aeu Keu Asnputechnology. There are some researches focus on the parison of the two types of technologies. Tan et al. [18] pared the exergy efficiencywaste heat temperature is 100150 °C and the results indicate that the of the power cycle and MHP under different temperature lifts when theexergy effciency of the power cyele is always lower than that of the MHP. However this study only pares the energy performance whilejo qsa ua p p pn[1] enterprises often give priority to economy in the decision-making.using the ORC and steam MHP to recover low grade waste heat in 19refineries based on the natural gas and electricity prices in five coun- tries. The results show that for China and India when the waste heat isabove 110 °C and 135 °C and the waste heat is greater than 1.5 MW and 2.5 MW respectively the annual revenue of using the MHP is higher..e .o s respectively and the waste heat is greater than 2 MW the paybackperiod of the MHP is shorter than that of the ORC. For regions with low
2. Methodology
2.1. Selection of systems to be pared
natural gas prices/high electricity prices (Saudi Arabia the United States etc.) the MHP is always uneconomic. This study only analyzesthe specific natural gas and electricity prices of these countries and ad o psad ab e and energy price on econmy. Van derboret al. [19 studied the nergy and economic benefits of the heat pumps (pression-absorption steam pression and transcritical heat pumps) and power cycles (ORC KC and trilateral cycle) at waste beat temperature of 4560 °C.The results show that the economic benefits of the heat pumps are 2.511 times of power cycles in this temperature range. Since powerjo adai ea se a moq e s a dustries [20] the parison within this temperature range is of littletechnologies is insuffcient for nly considering enegy performan r referential value. It is clear that the current parison of these twoeconomics. At the same time while paring economies researchersare constrained to certain regions which limits the application of the conclusions.
analyzed at the waste heat temperature of 100150C. The heat pmps In this paper heat pumps and power cycles are pared andtakes heat as the utput while the power ycles take lectric energy as the output The electricity and heat prices jointly affect the economy ofdifferent waste heat recovery cycles. Therefore the parameter the price ratio of electricity to heat (R) is innovatively taken as a decision var-able. The energy and economic performanoes ofeachcycle are evaluated based on exergy effciency investment cost annual revenue andpayback period and the best seltionofwasteheat rcovery teh ogies under diffrent working conditions is explored. This work canprovide a quick selection chart for industrial waste heat recovery.
The procedure in this paper is shown in Fig 1. The first step is toidentify the systems to be pared in this article then key parameters are selected to explore the effects of different working conditions/re-gions on the systems’ performances. Next the models of the selected systems are established and the evaluation indexes applicable toindex is calculated. Finally the selection suggestion is put forward. The different technologies are chose. After the simulation each evaluation detailed description of each step is shown in sections 2.1 ~ 2.5.
In this step the systems to be pared are selected. This paper aims
in terms of energy performance than the AHP when the temperature liftis lessthan 30 °C and the waste heat temperature is above 40 °℃ it is chosen as the representative of the heat pump cycles in this paper.
steam Rankine cycle (SRC) etc. Wang et al. [26] pared the energy Powercycles used in recovering low grade heat include ORC KC andperformance of the ORC and KC when recovering waste heat from multiple waste heat sources and divided the waste heat into straight convex and concave types. The results show that the KC is better for straight and concave waste heat with Rp (the ratio of the heat above andbelow the most salient/concave point) greater than 0.2 and the ORC is preferred for convex waste heat. Milewski and Krasuski [27] paredthe system efficiency of the KCs and ORCs in the steel industry when they worked with different working fluids. The efficiency of the ORCoperating with butylbenzene is the highest when the waste heat is below 200 C. The KC bees petitive as the temperature rise to 200 C.Varga and Palotai [28] investigated the performance of the KC and ORCunder waste heat temperatures of 130 ~C. The findings suggest that the KC has a greater capability for electricity generation higher efficiency and more CO emission reduetions However due to the need foe higher working pressure and more plex structure than the ORC its in-vestment cost and payback time are higherthan those of the ORC. At the same time the hydrocarbons used in the ORC are safer than theammonia pounds used in the KC. The research of Walraven et al. [29] shows that when the waste heat temperature is 100150 °C theresults it can be seen that the ORC shows stronger petitiveness at the ORC has better energy performance than KC in most cases. From thesethe representative of power cycles Further more since the waste heat waste heat temperature of 100150 °C Therefore the ORC is selected assteam is used as the waste heat source in this paper and the steam turbine (ST) is a device that can directly use steam considering thesimple structure of the steam turbine when recovering the waste heat steam it is also included in the parison.
Fig 1. Procedure of the methodology.
from several mercial cycles. to select one represented heat pump and one represented power cycle
and AHT etc. It is recognized that the AHP and MHP are more widely Heat pumps used in recovering low grade heat include AHP MHP used than other types of heat pumps in industries. Wang et al. [21] showed that when the waste heat temperature is 4095 °C the COP ofthe temperature lift (the difference between condensation temperature the MHP is better than those of the two types of AHPs. However whenand evaporation temperature of a heat pump) is higher than 30 °C the exergy los per total capital investment of the AHPs is smaller than thatof the MHP. That is to say the economy of the MHP is better than that of the AHP at low temperature lift. Xu et al. [22] analyzed and paredthe energy performance of the MHP and three AHPs in recoveringand the output is 60 °C the COP of the MHP is higher but its exergy ratethe MHP are greatly mproved r even exeed those of the AHPsFashi heat temperature rises to 45 °C the exergy rate and exergy efficiency of and exergy efficiency are lower than those of the AHPs. When the wasteet al. [23] analyzed and pared the thermodynamic performance ofthe MHP AHP cascade and hybrid pression-absorption heat pumps (CCAHP and HCAHP) The conclusion is given that when the waste heattemperature is 4060 *C the pression ratio of the CCAHP is the smallest the pressor pressure is the highest and the outlet temper-ature is lw while the peimary energy mtioand eegy ffcieney of the MHP are invariably higher than those of the AHP. Razmi et al. [24]analyzed the COP exergy loss and irreversibility of the MHP AHP and absorption-pression heat pump (ACHP) under variable generation and evaporation temperatures. Itis found that the COP of the MHP is the highest and that of the AHP is the lowest under all the working condi-tions. When the evaporation temperature is very low and the generation temperature is below 60 °C the exergy eficiency of the ACHP canexced that f th MHP Othewise the xergy effcieny of the MHPis the highest and the MHP is always more efficient than the AHP whenparison of the AHP MHP and steam jet pump based n the COP exergy considering exergy efficiency as an index Tan et al. [25] made -efficiency and annual standard coal savings under different user sidedemand when the waste heat temperature varies from 100 to 120 °C. The results show that when there is a high demand on the user side that are the best According to these results as the MHP has more advantages is the waste heat can be fully utilized these three indexes of the MHP
2.2. Setection of key parumeters
the heat pump have a great mpact on the performance of the waste heat Since the waste heat temperature (T ) and the output temperature ofrecovery system this paper considers the heat pump temperature lift (47) and the temperature of the waste heat steam (T) as independentvariables. The two parameters change in the range of 1030 °C and
variable and its expression is shown in Equation (1). The price ratio of electricity to heat is taken as the third independent
(1)
the unit is S/kWh. where Pa and Ps are electricity price and heat price respectively and
the world. Table 1 shows the electricity and beat prices in eight coun- Electricity and heat prices vary in different countries and regions intries which are obtained from online information [30].
It can be seen from Table 1 that the price ratio of electricity to heat in
Table 1Prices of electricity and heat in different countres (2009).
Elecricity priceHeat price (3.53.9 MPag steam) S/kWh $/kWhChine Cemany Sinppore 8100-5100 0.077-0.089 0.037-0.069 0.063 0.305 0.154 3.514.09 3.4443.948 2.2254.161America Bnel 0.043-0.057 00-200 0.065 0.133 1.1451.508 3.1005.715South Africa 0.014-0.017 0.058 3.4654.257Saadi Arabia 0.006240.00627 0.032 4.9425.124
2.3. Model conszruction
2.4. Parameters taking in the model
all these regions varies from I to 6. Therefore R is taken as 1 to 6 in thispaper.
For the MHP the open cycle is considered in this paper as shown in The waste heat recovery cycle built by Aspen Plus is shown in Fig. 2.Fig. 2 (a) and the ST model is shown in Fig. 2 (b). Only water is used in both cycles so steamNBS is selected as the physical property methodthe working fluid of the ORC so the physical property method is [31]. The process of the ORC is shown in Fig. 2 (c). R123 is selected asREFPROP [32]. The modules selected for each ponent of the three cycles in Aspen Plus are summarized in Table 2
In the simulation some parameters are taken based on the followingassumptions.
1. The pump’s power of the MHP can be ignored for itis far les thanthat of the pressor. Therefore the efficiency seting of the pump2. The pressor's effciency of the MHP is taken as 0.7 [33] is 1.3. The turbine's efficiency of the ORC is set to 0.75 [34] and the pump’s is set to 0.8 [35].4. The efficiency of the ST is set to 0.6 [36 37]. 5. The pressure drop of the evaporator and condenser in the ORC isignored in the simlation and the outle oftheevaporator is asumed to be saturated steam.
The eficiency of each ponent is summarized in Table 3.
The input of the MHP is 100150 °C saturated steam and the outputis 110180 °C steam according to different temperature lifts. The outlet pressure of the pressor is the saturation pressure corresponding tothe product steam. In the ORC teheat released by the waste heatsteam is absorbed by R123 in the evaporator. Set the temperature difference ofthe evaporator and waste heat to 10 °C and the inlet pressure of theturbine is the saturation presure corresponding to the evaporation temperature The temperature of cooling water is 25 °C the temperatureturbine is the saturation pressure corresponding to the condensation diference of the condenser is taken as 9°℃ and the utlet pressure f thetemperature. Since the device in this study is small it is difficult to reduce the outlet pressure of the steam turbine to a very low level so it isspecified that the outlet pressure of the steam turbine is 0.02 MPa. The main parameters of the MHP and ORC are summarized in Tables 4 and 5.
Table 2Modules of different
Table 3 Efficieney of each ponent.
Parameters of the MHP Table 4
Table 5 Parameters of the ORC.
2.5. Evaluation index
Fig. 2. Models of different waste heat recovery systems [18].
Cyele ModuleMeclusicl bt pump pump peesor COMPRESSOR PUMPOrganic Rankine cyde mier turbine MIXER TURBINEevuporator condenser HEATX HEATXStears turbine pump turbine PUMP
Qyde Pump Emdency Compresoc/TurtineMechaticl het gump 0.7Organic fuaskine cycle Steam turlbine 0.8 0.75 0.6
puauoduexg/usaas Waste heat Parameter Temperature C 100-150 ValueProduct Wate Temperature °C Temperature °C 110-180 100Waste heat Conpeesar Outlet presure MPa Flow rate kg/s 0.14331.003 1.1
ValoeWiate bat Evaporator Cooling water Temperate C Temperature °C Temperntrs ℃C 051-001 90-140Turtene Turtine Outlet pr MPa Inlet pressure MPa 25 0.6281.750 0.126Wodking fud Flow rate kg/s 12.5-135
s sa an neaq asem jo sad aa a ueduo uapaper cosiers using exergy eficieney to evaluate energy performance and annual revenue to evaluate economy At the same time the equip-ment investment cost and payback period are also calculated in order to
provide suggestions for the selection of waste heat reovery techologies. The calculation methods of these indexes are introduced in this seetion.
where Ccam and Ca are the investment cost of pressor and turbinerespectively in $.
The simle paybeck period of investment can be calculated acord-ing to the investment cost and annual revenue asshown in Equation (9).
2.5.1. Exergy eficiency
the exergy can characterize the quality or grade of energy. Equation (2) The energy consumed and generated by the three cycles are different shows the calculation method of physical exergy E.
(9)
where PP - payback period a; C - investment cost $.
(2)
3. Results and discussion
where E physical exergy kw; m - mass flow rate kg/s; h - specific enthalpy kJ/kg; s - specific entropy kJ/(kg K); T - Kelvin temperature K. Subscript 0 - reference state 298.15 K 1 atm.
According to the simulated data the indexes in Section 2.5 arecaleulated as follows.
Aspen Plus can direetly give the physical exergy of streams
index. Equation (3) shows the caleulation of the of the three cycles. Exergy efficiency is used as the energy performance evaluation
3.1. Exergy eficiency
The exergy efficiency of the MHP ORC and ST under different Tand 7 is calculated and the results are shown in Fig. 3. In Fig 3 the trend of the exergy efficiency of the three cycles with the two variables isconsistent with that in literature [18]. No matter how the two variables change the of the MHP is invariably the highest (greater than 90%) is stable at about 50% The7only has an impact on the of the MHP.If while that of the ST is the lowest (about 30%40%) and the of the ORCthe T remains unchanged increase of the temperature lift will result n decrease of the of the MHP. The variation of the T has an impact on theenergy performane of the MHP ORCandST.When the T rises the of all the three cycdes increases.
(3)
Subscripts cwin-cold water input; out - steam/work output; whin -Jossaudu a fq pomsuoo xpom) 1ndu axo - x μndu q asem and pump).
2.5.2. Ar reveue
Ignoring the value of the waste heat steam the annual revenue of the The annual revenue is determined by the price of heat and electricity.MHP is the diferene beween the benefit bought by prduct and the cost of the power consumed by the pressor as shown in Equation(4). For the ORC and ST the mechanical work output by the systems drive the generator to generate electric energy and create economicbenefits. As the ORC uses pumps to transport the working fluid theelectricity consumed by the pumps should be subtracted as shown in Equation (5). Equation (6) shows that the annual revenue of the ST isworking hours of the three systems are s0 h a ye8r. only determined by the power output of the turbine. Asuming that the
3.2. Anniaal reventae
It can be seen from section 2.5.2 that the annual revenues of the threcycles are linear functions of Pe and the value of P has no effect on theparison of the annual revenues of different cycles. In this paper Pis taken as 0.062 S/kWh.
heat temperature and heat pump temperature lift. Taking the tempera- The annual revenue of the MHP is affected by the price ratio R wasteture lift of 10 °℃ as an example the change of the annual revenue of the MHP withR and waste hest temperature T is shown in Fig. 4 (a) Takingthe waste heat temperature of 100 °C as an example the change of annual revenue of the MHP with R and temperature lift 7 is shown inFig. 4 (b) Under the same R and 7 the ARp increases with rise of the T . The higher the T; is the less work the MHP pressor needs(dependent on th eam peperty) the les electric energy is consumed and the cost is reduced. Under the same T and 7 the ARap reduceswith rise of the price ratio R. When R increases the value of heat is low
(4)
(5)
(9)
Subscripts - pressor; pump - pump; tur turbine. where AR - annual revenue $/a; W - power consumed or generated kW;
2.5.3. Investment cost and payback period
ponent. The pumps of the MHP and ORC have the characteristics of The investment cost of acycle consists of the investment cost of each(water and R123) are not flammable and hazardous So the ANS1 pump low suction pressure duty and temperature and the working fluidsis selected. The evaporator and condenser in the ORC should have theadvantages of high heat exchange efficiency and high pressure resis- tance as well as being able to adapt to a large temperature and pressureture and low cost is selected. The cost of ANSI pumps and shell and tube range. Therefore the shell and tube heat exchanger with simple struc-heat exchangers can be derived directly froem the economics module in Aspen Plus. It is difficult to find a suitable model for the presor andturbine in Aspen plus. In this paper the calculation rules in Ref. [38] are used to evaluate the equipement investment cost. The pressor is madeof carbon steel and the turbine s made of stainless steel Thecalculation rules for the equipment cost of the pressor and turbine are shown inEquations (7) and (8).
()
(8)
Fig. 3. Exergy efficiency of the MHP ST and ORC with 7T under different heat pump temperature lifts.
