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Analysis of combined cooling heating and power generation from organic Rankine cycle and absorption system
Energy 91 (2015) 363e370
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Analysis of combined cooling heating and power generation from organic Rankine cycle and absorption system Nattaporn Chaiyat a, *, Tanongkiat Kiatsiriroat b a b
School of Renewable Energy, Maejo University, Chiang Mai, Thailand Department of Mechanical Engineering, Chiang Mai University, Chiang Mai, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2015 Received in revised form 5 August 2015 Accepted 18 August 2015 Available online xxx
This paper focuses on the feasibilities of energy, economic and environment of a method to enhance an ORC (organic Rankine cycle) efficiency by CCHP (combined cooling heating and power) generation from an absorption system for reducing the ORC condenser temperature. A projection of a 25 kWe R245fa ORC integrated with a 20 kW LiBr-water absorption unit was considered. The experimental data of both units were generated as performance curves and used to find out the suitable operating conditions. It could be found that the ORC with the absorption system gave higher total efficiency compared with the normal ORC. The ORC efficiency could be increased around 7%, with 15 C of cooled water temperature supplied from the absorption system. But, in term of the economic result, a LEC (levelized electricity cost) of the modified system was around 0.0891 USD/kWh, which was higher than that of the normal system at around 0.0669 USD/kWh. In term of the environmental impact, a released carbon dioxide intensity of the new unit was lower than the normal unit at around 0.203 and 0.216 kg CO2 eq/kWh, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Absorption system Organic Rankine cycle Combined cooling heating and power generation Levelized electricity cost Carbon dioxide intensity
1. Introduction ORC (Organic Rankine cycle) is a cycle that uses an organic working fluid as a working fluid instead of water. Since the boiling point of the organic fluid is lower than that of water then it could be applied with various kinds of low temperature heat sources such as geothermal energy, solar energy, biomass energy and waste heat. Today, rising of fossil fuel prices and the environmental aspects on fossil fuel combustion, a significant market for the ORC is open and there is a challenge to develop an appropriate scale to meet both economic and environmental needs. Many studies on the ORC technique were reported. Walraven et al. [1] studied a system optimization of ORCs cooled by air-cooled condensers or wet cooling towers and powered by lowtemperature geothermal heat sources. The results showed that it was economic to use mechanical-draft wet cooling towers instead of air-cooled condensers. Suna and Li [2] presented the ORC heat recovery power plant using R134a as working fluid mathematical models to evaluate and optimize the plant performance. Thawonngamyingsakul and Kiatsiriroat [3] used a solar water heating system with a climate of Thailand to generate and supply heat to * Corresponding author. E-mail address: [email protected] (N. Chaiyat). http://dx.doi.org/10.1016/j.energy.2015.08.057 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
ORC system. Thermo-economic analysis was used to analyze the system performance for CHP (combined heat and power) generation similar to other reports on CCHP (combined cooling heat and power) [4e6]. Selection of ORC working fluid was also an interesting topic and various literatures [7e14] were reported. R-134a and R-245fa were always the recommended working fluids. Moreover, improvement of the ORC efficiency by reducing condensing temperature such as a design of high performance heat exchanger [15] and design of the condensation temperature with respect to the expander characteristics [16,17] were carried out. From the above mentioned literature review, it could be found that many studies reported about the ORC applications. Techniques to enhance the ORC efficiency such as selection the suitable working fluid and the optimal design were represented. It could be noted that technique of reducing the working fluid temperature at the ORC condenser by using an absorption chiller did not represent in the recent literatures. An interesting approach, a method to enhance the ORC efficiency by CCHP (combined cooling heating and power) generation from the absorption system for reducing the ORC condenser temperature is considered. The experimental results of each technology are performed to evaluate the optimal integrating system performance.
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system (point 2 h) at temperature around 90e120 C and 70e90 C, respectively. Released heat temperature at the generator will drop down to be around 60e75 C (point 3 h). A binary liquid mixture consisting of a volatile component (absorbate) and a less volatile component (absorbent) is obtained at the generator. The binary mixture (weak solution) is heated and part of the absorbate boils at a high pressure (PHigh,Ab) and temperature (TG) at point 1a. The vapor absorbate condenses in a condenser1 (TC1) to be liquid at point 2a. After that, the absorbate in liquid phase is throttled to a evaporator at point 3a of which a low pressure (PLow,Ab) is lower than that of the condenser1. The evaporator is heated by cooling water at temperature around 10e20 C (point 3c), after that cooling water will drop down temperature to be around 5e15 C (point 4c). In this study, cooling water comes from a cooling tower of the ORC system (points 1c-2c), which is the cascade connection. This technique could be increased the cooling capacity and decreased the cooling water temperature of the integrating system. The absorbate at the evaporator is boiled to be vapor at point 4a and enters an absorber. Meanwhile, the strong solution from the generator, at point 8a is sent through a heat exchanger and a pressure reduction valve at points 9a and 10a, respectively, into the absorber at the low pressure. In the absorber, the strong solution absorbs the absorbate vapor to be the weak solution again. This liquid mixture leaves the absorber at point 5a at a medium temperature (TA) around 40e50 C, which is similarly the condenser temperature (TC). The weak solution at point 6a is compressed by a solution pump to the high pressure (point 6a) through the heat exchanger (point 7a) into the generator again and new cycle restarts. Fig. 2 also shows the concept of combined cooling heating and power, which waste heat recovery from the ORC boiler is the useful heating mode. In cooling part, cooled water from the evaporator of absorption chiller is combined with power generation to reduce the ORC condenser and the ORC efficiency could be enhanced.
The aims of this study are as follows: 1. To evaluate performance curves of the ORC and absorption systems based on the testing results. 2. To find out systematic determination of optimum design parameters of the integrating unit. 3. To analyze economic result of the normal and new systems in term of a LEC (levelized electricity cost). 4. To analyze environmental impact of the normal and new systems in term of a carbon dioxide intensity.
2. System description The operating principle of the ORC system is manifested in Fig. 1, hot fluid as heat source at temperature around 90e120 C enters a boiler to heat and vaporize the working fluid at point 1 h and the outlet temperature of heat source is dropped at around 15e20 C at point 2 h. The super-heating working fluid exiting the boiler at point 1 is expanded in a screw expander to produce a mechanical work at point 2. An expander-generator set is used in this study. The expander operates at around 8000 rpm for feeding power and driving a reduction gear box. The output of the gear box is around 3000 rpm and directly drives an induction generator. The expander-generator set is constructed to be a semi-hermetic screw type, where the expander is integrated with the generator and installed inside a common casing to avoid from vapor leak-off. A lubrication system is used to reduce friction of a rotor (point 2), which consists of oil and vapor separator (point 3), filter and oil pump (points 4e5). The presence of lubrication system has lower the maintenance costs [18,19]. Lubricant and the vapor working fluid are separated at the oil and vapor separator (point 6). After that, the vapor is then condensed in a condenser by cooling water at temperature around 25e35 C to a low pressure (PLow) as the subcooled working fluid at point 7. The fluid at liquid state is compressed by a refrigerant pump as a multi-stage centrifugal pump to a high pressure (PHigh) at point 8 and the new cycle restarts. Fig. 2 shows the operating procedure of the combining unit, which the absorption system is used to reduce the working fluid temperature at the ORC condenser. Heat source supplies heat to the ORC boiler (point 1 h) and sends to a generator of the absorption
3. Materials and methods 3.1. The ORC test rig In the past, several working fluids were considered with the ORC cycle such as Hydrochlorofluorocarbon (HCFC), Hydrofluorocarbon (HFC) including of mixture refrigerants etc [7e14]. In this study, a
Boiler (QB,TE,PHigh) 1' (THW,o) (THW,i)
2h 1h
Grid line
Expander 1
8'
Generator
5 4
(WExp)
(WORC)
2
8
Oil pump
Oil and vapor separator
(WOP) Refrigerent pump (WP)
(QC,TC,PLow) Condenser
3 6
7
(TCW,i)
(TCW,o)
1c
3c
Cooling tower 2c
Cooling pump Fig. 1. Schematic diagram of organic Rankine cycle.
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Fig. 2. Schematic diagram of absorption system combined with organic Rankine cycle.
HFC-245fa refrigerant was selected because of low-pressure, hightemperature, non-corrosive, non-flammable, low-toxicity and friendly environment. R-245fa was carried in a 25 kWe ORC prototype as shown in Fig. 3. The descriptions of each ORC-component were presented in Table 1. In the testing procedure, water from a hot water generator was heated by a set of diesel burner at heating capacity range of 200e600 kW. The required outlet water temperature could be automatically adjusted with an uncertainty around ±0.5 C. The maximum water temperature could be reached 120 C in the closed loop system. Hot water temperature entering the ORC boiler were considered at 4 operating conditions (around 90, 95, 100 and 105 C), while the cooling water temperature entering the ORC condenser was fixed at around 28 C. The testing results of each operating condition were continually recorded every 1 min for 1 h at steady state condition. After that, the average value of the recorded data was evaluated to carry out the ORC performance curve. The ORC prototype was tested and measured by temperature, pressure and electrical power sensors. Signal data form sensors were transferred to PLC programmed controller (FATED programmed controller, FBs-CB55) and recorded into the PLC memory. T-type thermocouple at a precision range of ±0.1 C [21], pressure
transducer (WIKA, type A-10) at a precision range of ±0.25% [22] and current transformer (Shinohawa Electric, type YAL-1) at a precision range of ±1% [23] were used to measure temperature, pressure and electrical power, respectively. 3.2. The absorption test rig In the present study, a 20 kW water-lithium bromide (LiBr) absorption chiller as shown in Fig. 4 was used to find out the thermal performance. In the testing procedure, a set of electrical heater 30 kWe at accuracy of the controlled temperature ±0.5 C was used to heat water in a 1500 L storage tank. The descriptions of each absorption-component were presented in Table 2. For measurement, temperatures of the each working fluid were monitored by a T-type thermocouple at a precision range of ±0.1 C [21]. The output signal of thermocouple was interfaced to a personal computer through a data logger and software of TSUS Instruments at a precision range of ±0.5% [24]. The electrical power consumptions of the each component were measured and recorded by a power logger of KYORITSU [25] at an accuracy ±0.2%. The recording time of power logger was set matching with the temperature recorder. In the next part, the constructed of the ORC and absorption units are tested its thermal performance. The objective of this experiment is to find out a performance curves of each system, which is Table 1 Descriptions of each component in the ORC unit. Components
Properties
Generator
Induction generator Gross power 25 kWe 3 phase, 380 V, 50 Hz Semi-hermetic twin screw type expander Displacement volume 3000 rpm SUS 316 plate type heat exchanger Shell and tube heat exchanger Shell: carbon steel 12 in 3 m Tube: 3/4 in copper tube Vertical type oil separator Oil tank 18 in diameter 0.7 m Viking heavy duty oil pump (GG4195) Motor 3 hp, 3 phase, 380 V, 50 Hz Vertical multi-stage centrifugal pump VFD drive (BN3-17) Motor 2 hp, 3 phase, 380 V, 50 Hz
Expander Boiler Condenser
Oil and vapor separator Oil pump Refrigerant pump Fig. 3. Prototype of the R-245fa ORC system (Hanbell model: RC2-300 [20]).
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N. Chaiyat, T. Kiatsiriroat / Energy 91 (2015) 363e370 Table 3 Testing results of the R-245fa ORC system at varying inlet hot water temperature. Descriptionsa
Data
Hot water inlet (THW,i) [ C] Hot water outlet (THW,o) [ C] Heat source capacity (QB) [kW] Cool water inlet (TCW,i) [ C] Cool water outlet (TCW,o) [ C] Condenser temperature (TC) [ C] Heat sink capacity (QC) [kW] Expander inlet pressure (PHigh) [kPa] Expander inlet temperature (T1) [ C] Expander outlet pressure (PLow) [kPa] Expander outlet temperature (T2) [ C] Refrigerant pump power (WP) [kWe] Oil pump power (WOP) [kWe] Lift temperature (THW,i e TCW,i) [ C] Cycle powerb (WORC) [kWe] Cycle efficiencyc (hORC) [%]
116 89.8 243.2 28 35 37.1 219.0 1097.1 93.7 227.4 75.0 1.78 1.40 88 21.50 8.73
107.8 81 248.2 28 35 37 215.6 1120.0 94.6 227.4 70.6 1.90 1.40 79.8 21.36 8.49
97 75 203.4 28 35 37 210.9 1074.0 92.8 227.0 70.6 1.19 1.40 69 16.70 8.11
88.9 77.8 188.3 28 35 37 211.0 811.3 85.7 239.3 59.5 1.24 1.40 58.8 9.00 4.71
a
Positions referred Fig. 1. Cycle power (WORC) was gross power (WExp) subtracted by power consumptions of refrigerant pump (WP) and oil pump (WOP). c Cycle efficiency was defined by hORC ¼ WORC 100/(QB þ WP þ WOP). b
the correlation between the input parameters and the system efficiency. Projection of the optimal integrating unit by using the both performance curves is presented. The system performance of the CCHP unit could be predicted under various operating conditions. Moreover, this technique could be decreased complication of the fundamental simulation.
4. Results and discussion 4.1. Thermal performance of the ORC system The impact of hot fluid temperature on the 25 kWe ORC machine and the system performance were investigated by setting the inlet hot fluid temperature varying between 90 and 120 C, while the cooling fluid temperature was fixed at around 28 C. The testing result of the ORC system was shown in Table 3. It could be found that the cycle efficiency (hORC) of the R-245fa ORC Table 2 Descriptions of each component in the absorption unit. Components
Properties
Generator (Flooded shell and tube heat exchanger) Condenser (Shell and tube heat exchanger)
Capacity 26.33 kW Heating area 2.04 m2 Capacity 20.00 kW Heating area 0.84 m2 Capacity 25.20 kW Heating area 2.88 m2 Capacity 20.80 kW Heating area 2.32 m2 Flow rate 0.6e3.0 m3/h Maximum pressure 10 bar Power 56 We
Absorber (Flooded shell and tube heat exchanger) Evaporator (Shell and tube heat exchanger) Solution pump (DAB in-line pump)
Vapor volume (10-3 m3/kg)
Fig. 4. Prototype of the water-LiBr absorption chiller (ATS model: absorption chiller 10 RT [24]).
system was lower than 5%, when hot water temperature was lower than 90 C. While, the efficiency could be increased to be over 8%, when hot water temperature was higher than 97 C. This effect came from a vapor volume of working fluid entering the expander, which was a high value at the low temperature. But its volume was nearly constant at the high temperature as shown in Fig. 5. Table 3 also shows the supporting data of the expander inlet pressure (PHigh) and temperature (T1), which were gradually constant at temperatures of 116, 107.8 and 97 C, respectively, and corresponding with the cycle efficiency. The average cycle efficiency of the 3 operating points was 8.44%, which corresponded with the above mentioned literature review [7e17]. Thus, in this study, 3 sets of testing data at temperature higher than 97 C were used to evaluate the performance curve of the ORC system. The correlation of the cycle efficiency (hORC) and the different temperature between heat source and heat sink (THW,i TCW,i) as shown in equation (1) and Fig. 6. This linear correlation was relationships of the related data between the ORC efficiency and input variables, which were water temperatures of the water entering at the condenser and the evaporator. It could be seen that the cycle efficiency increased, when the temperature difference increased, which followed the Carnot efficiency concept. The cycle efficiency decreased, when the temperature difference of heat source and heat sink decreased. This performance curve was used
30 27 24 21 18 15 12 9 6 3 0 50
60
70 80 THW,E,i - TCW,C,i (oC)
Fig. 5. Vapor volume entering the expander.
90
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Cycle efficiency (%)
12
9 y = 0.0327x + 5.8628 R² = 0.9967
6
3 60
65
70
75 80 THW,E,i - TCW,C,i (oC)
85
90
95 Fig. 7. Performance curve of the absorption unit.
Fig. 6. Simplified model of the ORC system.
to predict the thermal performance, when the ORC system was combined with the absorption system.
hORC ¼ 0:0327 THW;i TCW;i þ 5:8628
(1)
4.2. Thermal performance of the absorption system The impact of heat source temperature on the 20 kW absorption unit and the system performance were investigated by setting the inlet heat source temperature varying between 70 and 90 C. Table 4 shows the average heating capacities of the absorption element. It could be seen that the supplied heat at the generator was around 25 kW. The useful cooling capacity at the evaporator was around 16 kW, while the rejected heats at the condenser and absorber were around 22 and 21 kW, respectively. Table 4 also shows the average temperatures of working fluid, water entering and leaving the main components of the absorption system. The cooled water leaving the evaporator was around 15 C, which converts from the heat source temperature around 80 C at the generator. The water-LiBr concentrations at around 0.53 and 0.45%LiBr as the strong and weak solutions based on %LiBr, respectively, were used to circulate between generator and absorber. The experimental result also shown the average COP (coefficient of performance) of the absorption prototype was around 0.66. This result was nearly the COP of the YAZAKI absorption machine at around 0.7 [27]. When the experimental data at the heat source temperature varying between 70 and 90 C were analyzed the absorption performance curve. The empirical correlation of the COPAb and the different temperature term of (TG,i TC1)/(TA,i TE) was found in linear form as shown in equation (2) and Fig. 7.
COPAb ¼ 0:1613 TG;i TC1
TA;i TE þ 1:3178
(2)
Table 4 Testing results of the absorption chiller. Descriptions a
Working fluid temperature Data ( C) Inlet water temperaturea Data ( C) Outlet water temperaturea Data ( C) Heating capacitya Data (kW) a
Positions referred in Fig. 2.
Generator
Evaporator
Absorber
Condenser
TG 64.84 THW,G,i 80.61 THW,G,o 64.92 QG 24.99
TE 9.68 TCW,E,i 20.30 TCW,E,o 14.68 QE 16.46
TA 38.97 TCW,A,i 20.68 TCW,A,o 31.70 QA 22.18
TC1 35.97 TCW,C,i 20.68 TCW,C,o 30.00 QC1 20.47
In this study, the performance curves of the ORC and absorption systems were used to evaluate the optimal integrating unit by the projection method in the next part. 4.3. Projection of the optimal integrating system In the projection process of the optimal integrating condition, the operating parameters and the cycle efficiency were predicted. The initial conditions of the projection method were shown in Table 5. Table 6 shows the projection results. It could be seen that the cycle efficiency of the normal ORC system (hORC,normal) was around 8.08% at the cooling water (T1C) around 32.22 C, which was the typical water temperature of cooling tower under the climate of Thailand. The normal ORC system obtained heat source (QB) around 247.55 kW and converted heat to power (WORC,normal) of 20 kWe. For the modified system, after the same among of heat source (QB) was used to generate electricity, low grade heat leaving the ORC boiler was supplied to the absorption system to produce the cooled water temperature (T4c) around 15 C. The modified system could be decreased the cooling water (T1C) to around 24.87 C, which the cooling tower was cascade connection with the absorption unit. In addition, the electrical power (WORC,CCHP) of 21.39 kWe was generated at the cycle efficiency (hORC,CCHP) around 8.64%. From this result, it could be found that when the ORC condenser temperature decreased, the ORC cycle efficiency increased. Therefore the cycle efficiency of the modified ORC system could be increased around 6.97% compared with the normal ORC system. This result was corresponded with the literature studies of
Table 5 Initial conditions of the projection process. Descriptions The ORC system Cycle power (WORC,normal) [kWe] Extracting capacity of the condensera (QC) [kW] Hot water entering the boilera (T1h) [ C] Hot water leaving the boilera (T2h) [ C] Cooling water leaving the condenserb (T1c) [ C] Cooling water entering the condenserb (T3c) [ C] Performance curve of the ORC system The absorption system Cooled water leaving the evaporatorc (T4c) [ C] Cooling water entering the absorberc (TCW,A,i) [ C] Working fluid temperature at the evaporatorc (TE) [ C] Working fluid temperature at the condenser1c (TC1) [ C] Performance curve of the absorption system a b c
Referred data in Table 3 and positions in Fig. 1. The typical conditions of cooling tower in Thailand. Referred data in Table 4 and positions in Fig. 2.
Data 20 215 100 80 37.78 32.22 Equation 1 15 21 10 36 Equation 2
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Table 6 Projection result of the optimal integrating system.
Table 7 Present commercial cost of the ORC power plant.
Descriptions The normal ORC system Cycle efficiency (hORC,normal) [%] Heat source of the ORC system (QB) [kW] The absorption system The COP of the absorption system Heat source of the absorption system (QG) [kW] The suitable cooling capacity of the absorption system (QE) [kW] The new system Total cooling capacity of the new system (QE þ QC) [kW] Cooling water entering the cooling tower (T1c) [ C] Cycle efficiency (hORC,CCHP) [%] Cycle power (WORC,CCHP) [kWe] Increasing of the ORC efficiency [%]
Data
Manufacturer
ORC capacity (kWe)
Cost (USD/kWe)
8.08 247.55
Infinity turbine [29,30] Turboden [31] Electratherm [32,33]
2e3000 200e3000 50
2500 3000 2530
0.67 247.55 166.50 381.50 24.87 8.64 21.39 6.97
[1,15e17], when the ORC condenser temperature was reduced, the ORC efficiency was enhanced. Moreover, when the evaporator of the absorption unit was installed with the ORC condenser in cascaded connection, the cooling capacity of the integrating unit could be increased around 70%. Thus the cooling water temperature of the integrating unit could be decreased lower than the normal ORC unit, which was the main advantage point of this technique.
4.4. Economic result In economic assessment, the commercial absorption machine cost [26,28] as shown in Fig. 8 was used to evaluate the capital cost of the absorption system at capacity (QE) around 166.50 kW (around 48 TR) as shown in Table 6. For the ORC cost, the present cost of the ORC power plant varies between 2500e3000 USD/kWe [29e33] as given in Table 7. From the table, it could be stated that the capital cost of a microscale ORC power plant at capacity lower than 50 kWe is around 2500 USD/kWe. Therefore this investment cost was used to consider the economical assessment. From the abovementioned literature review, payback period was selected to represent the economic result of the ORC power plant. Because electricity rate of heat source was specified. But in this study, heat source of the ORC system did not fixe. Thus, the LEC (levelized electricity cost) as presented in the studies of Thawonngamyingsakul and Kiatsiriroat [3] and our former work [34] was selected to analyze in this study. Moreover, the criteria for considering electricity price of renewable power in Thailand at payback period 10 y [35] was used to analyze the LEC. The initial conditions of economic evaluation were shown in Table 8. Table 9 shows comparison of economic results in term of the LEC factor, which based from the operating condition of an only one geothermal power plant of Fang District, Chiang Mai Province,
6,000
Thailand. Thus the both LEC costs of this present work should be compared with the electricity cost (Feed-in Tariff, FiT) of renewable power plant in Thailand [37], which unfortunate for the geothermal FiT does not promote. Because the only one geothermal-ORC power plant in Thailand was managed by the government (Electricity Generating Authority of Thailand, EGAT). From the simulation results, it could found that net electrical power production of the normal and modified ORC system were around 16.68 and 17.07 kWe, respectively. The ORC net power (WORC,net) of the normal ORC unit came from the ORC cycle power (WORC) subtracted by the electrical power consumptions of the cooling pump (Ebora model CMB/E 3 T, flow rate 100e280 L/min) and the fan motor of cooling tower (Model BKC 80 RT, fan motor 1.5 hp) at 2.2 and 1.12 kWe, respectively. In case of the new unit, the electrical power consumptions of the solution pump was considered in the absorption cycle at 78 We (DAB in-line pump, model VSA 65/130, flow rate 0.6e3.7 m3/h) [25]. When, the both ORC units operated at 24 h/d and 350 d/y [36], the new unit could generate electricity higher than the normal unit around 8600 kWh/y. For the LEC results, the CCHP unit was around 0.0891 USD/kWh, which was higher than the normal unit around 0.0222 USD/kWh. When these costs were compared with the FiTs of renewable power plant in Thailand [37]. It was found that the both LECs in Table 9 were lower than the FiTs of biomass and waste (integrated waste management) at 0.096 USD/kWh, waste (land fill) at 0.171 USD/kWh and biogas (waste water/waste material) at 0.115 USD/kWh. While the both LECs were similarly with the FiT of biogas (energy plants) at 0.085 USD/kWh. 4.5. Environmental assessment Energy, economic and environment impacts were considered in this study. For environmental assessment, the electrical power consumption of the ORC process was investigated, which consisted of refrigerant pump, oil pump, cooling pump and fan motor of cooling tower. In addition, the power of solution pump was considered in the modified unit. Carbon dioxide intensity of electricity of Thailand [38] at 0.6093 kg CO2 eq/kWh was used to estimate the released CO2 from operating process of the ORC system. It could be found the normal unit consumed the electrical power of 49,644 kWh/y and released the carbon dioxide intensity at around 30,248 kg CO2 eq. While, the new unit could generate electricity higher than that of the normal unit. Thus the carbon dioxide
Absorption cost (USD/TR)
4974.3 5,000
Table 8 Initial conditions of economic assessment.
y = 12522x-0.693 R² = 0.9961
4,000
Descriptions
3,000
a
2,000
1454.0 826.5
1,000
631.4
0 0
10
20
30 40 50 Cooling capacity (TR)
Fig. 8. Cost of absorption unit.
60
70
80
90
Operation time (h/d) Operation daya (d/y) Cycle power (WORC) [kWe] Cooling pump [kWe] Cooling tower [kWe] Cost of the ORC unit [USD/kWe] Capacity of the ORC system [kWe] Payback period [y]
Normal system
Modified system
24 350 20 2.2 1.12 2500 25 10
24 350 21.39 2.2 1.12 2500 25 10
a Referred the operating time of only one geothermal-ORC power plant in Thailand at Fang District, Chiang Mai Province [30].
N. Chaiyat, T. Kiatsiriroat / Energy 91 (2015) 363e370 Table 9 Economic results of the integrating system. Descriptions
Normal system
Modified system
Net electrical power production (WORC) [kWe] Net electricity production [kWh/y] Cost of the ORC unit [USD] Cost of piping, housing and cooling systemsa [USD] Cost of the absorption system [USD] The ORC power plant cost [USD] LEC [USD/kWh]
16.68 140,112 62,500 31,250 e 93,750 0.0669
18.00 151,165 62,500 31,250 40,924 134,674 0.0891
a
Calculated at 50% of the ORC cost.
intensity from the output product of electricity 1 unit of the modified unit was lower than the normal unit of 0.203 and 0.216 kg CO2 eq/kWh, respectively. 5. Conclusions and recommendations From the study result, it could be concluded that the method to enhance the ORC efficiency by combined cooling heating and power generation from the absorption system for reducing the ORC condenser temperature was effective. The performance curves of each technology were performed by using the experimental results for predicting the system performance. Projection of the integrating system represented the better efficiency, when the ORC condenser temperature was decreased. The ORC efficiency could be increased around 7%, when the cooled water temperature at 15 C was supplied from the absorption system. But, in the economic result, it could be found that electricity cost in term of LEC of the modified system was higher than the normal system of 0.0891 and 0.0669 USD/kWh, respectively. While, the environmental impact, the released carbon dioxide intensity of the new unit was lower than the normal unit at around 0.203 and 0.216 kg CO2 eq/kWh, respectively. For the future study, varying the ORC condenser temperature should be studied to find out the suitable cooled water temperature from the absorption system. The comparison effect of the ammonia-water and water-LiBr solutions of the absorption system should be investigated. Moreover, different types of refrigeration system such as adsorption system and ejector refrigerator should be studied. Acknowledgments The author would like to thank the School of Renewable Energy, Maejo University and Center of Excellent for Renewable Energy, Chiang Mai University and the Office of the Higher Education Commission, Thailand under the National Research University Program, Chiang Mai University for supporting testing facilities. Abbreviations and symbols
Nomenclature P pressure, (bar) Q heat rate, (kW) T temperature, ( C) W work, (kW) Subscript A absorber Ab absorption system B boiler
C CW e E Ex G HS HW i o OP P
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condenser cooling water electrical power evaporator expander generator heat source hot water inlet outlet oil pump refrigerant pump
References [1] Walraven D, Laenenb B, D'haeselee W. Minimizing the levelized cost of electricity production from low-temperature geothermal heat sources with ORCs: water or air cooled. Appl Energy 2015;142:144e53. [2] Suna J, Li W. Operation optimization of an organic Rankine cycle (ORC) heat recovery power plant. Appl Therm Eng 2011;31(11e12):2032e41. [3] Thawonnagamyingsakul C, Kiatsiriroat T. Performance and economic analysis of a solar organic rankine cycle with flat-plate and evacuated-tube solar collectors as heat source in the northern part of Thailand. In: The 1st ASEAN Plus three graduate research congress. Chiang Mai, Thailand; 2012. [4] Imran M, Park BS, Kim HJ, Lee DH, Usman M, Heo M. Thermo-economic optimization of regenerative organic Rankine cycle for waste heat recovery applications. Energy Convers Manag 2014;87:107e18. [5] Esen H, Inalli M, Esen M. Technoeconomic appraisal of a ground source heat pump system for a heating season in eastern Turkey. Energy Convers Manag 2006;47(9e10):1281e97. [6] Esen H, Inalli M, Esen M. A techno-economic comparison of ground-coupled and air-coupled heat pump system for space cooling. Build Environ 2007;42(5):1955e65. [7] Hettiarachchi HDM, Golubovica M, Worek WM, Ikegamib Y. Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources. Energy 2007;32:1698e706. [8] Schuster A, Karellas S, Karl J. Simulation of an innovative stand-alone solar desalination system with an organic Rankine cycle. In: The 46th Conference on simulation and modeling, Trondheim, Norway; October 13e14, 2005. [9] Guo T, Wang HX, Zhang SJ. Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources. Energy 2011;36:2639e49. [10] Sauret E, Rowlands AS. Candidate radial-inflow turbines and high-density working fluids for geothermal power systems. Energy 2011;36:4460e7. [11] Liu B, Riviore P, Coquelet C, Gicquel R, David F. Investigation of a two stage Rankine cycle for electric power plants. Appl Energy 2012;100:285e94. [12] Edrisi BH, Michaelides EE. Effect of the working fluid on the optimum work of binary-flashing geothermal power plants. Energy 2013;50:389e94. [13] Li T, Zhu J, Zhang W. Comparative analysis of series and parallel geothermal systems combined power, heat and oil recovery in oilfield. Appl Therm Eng 2013;50:1132e41. [14] Rodriguez CEC, Palacio JCE, Venturini OJ, Silva LEE, Cobas VMD, Santos DM, et al. Exergetic and economic comparison of ORC and kalina cycle for low temperature Antonio Yock. In: Proceeding of seminar on short course on surface exploration for geothermal resources, El Salvador; October 17e30, 2009. [15] Wang J, Wang M, Li M, Xia J, Dai Y. Multi-objective optimization design of condenser in an organic Rankine cycle for low grade waste heat recovery using evolution aryalgorithm. Int Commun Heat Mass Transf 2013;45:47e54. [16] Li J, Pei G, Ji J, Bai X, Li P, Xia L. Design of the ORC (organic Rankine cycle) condensation temperature with respect to the expander characteristics for domestic CHP (combined heat and power) applications. Energy 2014;77: 579e90. [17] Liu Q, Duan Y, Yang Z. Effect of condensation temperature glide on the performance of organic Rankine cycles with zeotropic mixture working fluids. Appl Energy 2014;115:394e404. [18] Smith IK, Stosic N, Kovacevic A. Screw expanders increase output and decrease the cost of geothermal binary system. GRC Trans 2005;29:787e94. [19] Hung TC, Shai TY, Wang SK. A review of organic Rankine cycle for the recovery of low-grade waste heat. Energy 1997;22(7):661e7. [20] HanPower Energy Technology Co.,LTD. Cost of organic Rankine cycle set, TaoYuan Hsien, Taiwan. 2014. [21] OMEGA Engineering inc. Thermocouples. http://www.omega.com/prodinfo/ thermocouples.html [accessed 15.04.15]. [22] WIKA Instrument Corporation. General purpose pressure transmitters type A10. http://www.wika.us/upload/DS_PE_A_10_en_us_16479.pdf [accessed 15.04.15]. [23] Shinohawa Electric Co.,LTD. Shinohawa product Catalog. http://www. kingelectric.cn/products.htm [accessed 15.04.15]. [24] TSUS Instruments. Model: DTL 16/24/32 CH. 2012.
370
N. Chaiyat, T. Kiatsiriroat / Energy 91 (2015) 363e370
[25] KYORITSU, KEW 6310. http://www.kew-ltd.co.jp/en/ [accessed 15.04.15]. [26] Advance Thermo Solution Co.,LTD. Cost and specification of absorption chiller unit, Samut Prakan, Thailand. 2014. [27] YAZAKI, Our environmental products. http://www.yazaki-circonditioning. com/products.html [accessed 18.06.15]. [28] Voltas Technologies. Heat to cooling absorption. http://www. voltastechnologies.co.za/heat-to-cooling/heat-to-cooling-absorption/36voltas/login-knowledgement/79-quick-budgeting-tool [accessed 08.08.2015]. [29] Rowshanzadeh Reza. Performance and cost evaluation of organic Rankine cycle at different technologies. http://kth.diva-portal.org/smash/get/diva2: 410363/FULLTEXT01 [accessed 08.08.15]. [30] Infinity turbine, Introducing the radial outflow turbine generator e organic Rankine cycle e ORC turbine. http://www.infinityturbine.com/ORC/ORC_ Waste_Heat_Turbine.html [accessed 08.08.15]. [31] Turboden. Turboden ORC: recent developments and new applications in organic rankine cycle technology. http://www.all-energy.co.uk/__ novadocuments/54368?v¼635376693736400000 [accessed 08.08.15].
[32] Arvay P, Muller M R, Ramdeen V, Cunningham G. Economic implementation of the organic Rankine cycle in industry. http://aceee.org/files/proceedings/ 2011/data/papers/0085-000077.pdf [accessed 08.08.15]. [33] Electratherm. https://electratherm.com/ [accessed 08.08.15]. [34] Chaiyat N. Assessment alternative energy for organic Rankine cycle power plant in Thailand. Int J Eng Technol 2015;7(1):317e26. [35] Tongsopit S, Greacen C. Thailand's renewable energy policy: FiTs and opportunities for international support. In: WRI-ADB Workshop on Feed-in Tariffs. Manila: Philippines; 2012. [36] Electricity Generating Authority of Thailand (EGAT). Project of geothermal energy for multipurpose, Amphoe Fang, Chiang Mai Province. 1988. [37] Energy Policy and Planning Office, Ministry of Energy, Thailand, Feed-in tariff of renewable energy in Thailand. http://www.eppo.go.th/power/fit-seminar/ Fit_2558.pdf [accessed 06.05.15]. [38] Carbon dioxide intensities of fuels and electricity for regions and countries. https://www.ipcc.ch/pdf/special-reports/sroc/Tables/t0305.pdf [accessed 20.10.14].
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Analysis of combined cooling heating and power generation from organic Rankine cycle and absorption system Nattaporn Chaiyat a, *, Tanongkiat Kiatsiriroat b a b
School of Renewable Energy, Maejo University, Chiang Mai, Thailand Department of Mechanical Engineering, Chiang Mai University, Chiang Mai, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2015 Received in revised form 5 August 2015 Accepted 18 August 2015 Available online xxx
This paper focuses on the feasibilities of energy, economic and environment of a method to enhance an ORC (organic Rankine cycle) efficiency by CCHP (combined cooling heating and power) generation from an absorption system for reducing the ORC condenser temperature. A projection of a 25 kWe R245fa ORC integrated with a 20 kW LiBr-water absorption unit was considered. The experimental data of both units were generated as performance curves and used to find out the suitable operating conditions. It could be found that the ORC with the absorption system gave higher total efficiency compared with the normal ORC. The ORC efficiency could be increased around 7%, with 15 C of cooled water temperature supplied from the absorption system. But, in term of the economic result, a LEC (levelized electricity cost) of the modified system was around 0.0891 USD/kWh, which was higher than that of the normal system at around 0.0669 USD/kWh. In term of the environmental impact, a released carbon dioxide intensity of the new unit was lower than the normal unit at around 0.203 and 0.216 kg CO2 eq/kWh, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Absorption system Organic Rankine cycle Combined cooling heating and power generation Levelized electricity cost Carbon dioxide intensity
1. Introduction ORC (Organic Rankine cycle) is a cycle that uses an organic working fluid as a working fluid instead of water. Since the boiling point of the organic fluid is lower than that of water then it could be applied with various kinds of low temperature heat sources such as geothermal energy, solar energy, biomass energy and waste heat. Today, rising of fossil fuel prices and the environmental aspects on fossil fuel combustion, a significant market for the ORC is open and there is a challenge to develop an appropriate scale to meet both economic and environmental needs. Many studies on the ORC technique were reported. Walraven et al. [1] studied a system optimization of ORCs cooled by air-cooled condensers or wet cooling towers and powered by lowtemperature geothermal heat sources. The results showed that it was economic to use mechanical-draft wet cooling towers instead of air-cooled condensers. Suna and Li [2] presented the ORC heat recovery power plant using R134a as working fluid mathematical models to evaluate and optimize the plant performance. Thawonngamyingsakul and Kiatsiriroat [3] used a solar water heating system with a climate of Thailand to generate and supply heat to * Corresponding author. E-mail address: [email protected] (N. Chaiyat). http://dx.doi.org/10.1016/j.energy.2015.08.057 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
ORC system. Thermo-economic analysis was used to analyze the system performance for CHP (combined heat and power) generation similar to other reports on CCHP (combined cooling heat and power) [4e6]. Selection of ORC working fluid was also an interesting topic and various literatures [7e14] were reported. R-134a and R-245fa were always the recommended working fluids. Moreover, improvement of the ORC efficiency by reducing condensing temperature such as a design of high performance heat exchanger [15] and design of the condensation temperature with respect to the expander characteristics [16,17] were carried out. From the above mentioned literature review, it could be found that many studies reported about the ORC applications. Techniques to enhance the ORC efficiency such as selection the suitable working fluid and the optimal design were represented. It could be noted that technique of reducing the working fluid temperature at the ORC condenser by using an absorption chiller did not represent in the recent literatures. An interesting approach, a method to enhance the ORC efficiency by CCHP (combined cooling heating and power) generation from the absorption system for reducing the ORC condenser temperature is considered. The experimental results of each technology are performed to evaluate the optimal integrating system performance.
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system (point 2 h) at temperature around 90e120 C and 70e90 C, respectively. Released heat temperature at the generator will drop down to be around 60e75 C (point 3 h). A binary liquid mixture consisting of a volatile component (absorbate) and a less volatile component (absorbent) is obtained at the generator. The binary mixture (weak solution) is heated and part of the absorbate boils at a high pressure (PHigh,Ab) and temperature (TG) at point 1a. The vapor absorbate condenses in a condenser1 (TC1) to be liquid at point 2a. After that, the absorbate in liquid phase is throttled to a evaporator at point 3a of which a low pressure (PLow,Ab) is lower than that of the condenser1. The evaporator is heated by cooling water at temperature around 10e20 C (point 3c), after that cooling water will drop down temperature to be around 5e15 C (point 4c). In this study, cooling water comes from a cooling tower of the ORC system (points 1c-2c), which is the cascade connection. This technique could be increased the cooling capacity and decreased the cooling water temperature of the integrating system. The absorbate at the evaporator is boiled to be vapor at point 4a and enters an absorber. Meanwhile, the strong solution from the generator, at point 8a is sent through a heat exchanger and a pressure reduction valve at points 9a and 10a, respectively, into the absorber at the low pressure. In the absorber, the strong solution absorbs the absorbate vapor to be the weak solution again. This liquid mixture leaves the absorber at point 5a at a medium temperature (TA) around 40e50 C, which is similarly the condenser temperature (TC). The weak solution at point 6a is compressed by a solution pump to the high pressure (point 6a) through the heat exchanger (point 7a) into the generator again and new cycle restarts. Fig. 2 also shows the concept of combined cooling heating and power, which waste heat recovery from the ORC boiler is the useful heating mode. In cooling part, cooled water from the evaporator of absorption chiller is combined with power generation to reduce the ORC condenser and the ORC efficiency could be enhanced.
The aims of this study are as follows: 1. To evaluate performance curves of the ORC and absorption systems based on the testing results. 2. To find out systematic determination of optimum design parameters of the integrating unit. 3. To analyze economic result of the normal and new systems in term of a LEC (levelized electricity cost). 4. To analyze environmental impact of the normal and new systems in term of a carbon dioxide intensity.
2. System description The operating principle of the ORC system is manifested in Fig. 1, hot fluid as heat source at temperature around 90e120 C enters a boiler to heat and vaporize the working fluid at point 1 h and the outlet temperature of heat source is dropped at around 15e20 C at point 2 h. The super-heating working fluid exiting the boiler at point 1 is expanded in a screw expander to produce a mechanical work at point 2. An expander-generator set is used in this study. The expander operates at around 8000 rpm for feeding power and driving a reduction gear box. The output of the gear box is around 3000 rpm and directly drives an induction generator. The expander-generator set is constructed to be a semi-hermetic screw type, where the expander is integrated with the generator and installed inside a common casing to avoid from vapor leak-off. A lubrication system is used to reduce friction of a rotor (point 2), which consists of oil and vapor separator (point 3), filter and oil pump (points 4e5). The presence of lubrication system has lower the maintenance costs [18,19]. Lubricant and the vapor working fluid are separated at the oil and vapor separator (point 6). After that, the vapor is then condensed in a condenser by cooling water at temperature around 25e35 C to a low pressure (PLow) as the subcooled working fluid at point 7. The fluid at liquid state is compressed by a refrigerant pump as a multi-stage centrifugal pump to a high pressure (PHigh) at point 8 and the new cycle restarts. Fig. 2 shows the operating procedure of the combining unit, which the absorption system is used to reduce the working fluid temperature at the ORC condenser. Heat source supplies heat to the ORC boiler (point 1 h) and sends to a generator of the absorption
3. Materials and methods 3.1. The ORC test rig In the past, several working fluids were considered with the ORC cycle such as Hydrochlorofluorocarbon (HCFC), Hydrofluorocarbon (HFC) including of mixture refrigerants etc [7e14]. In this study, a
Boiler (QB,TE,PHigh) 1' (THW,o) (THW,i)
2h 1h
Grid line
Expander 1
8'
Generator
5 4
(WExp)
(WORC)
2
8
Oil pump
Oil and vapor separator
(WOP) Refrigerent pump (WP)
(QC,TC,PLow) Condenser
3 6
7
(TCW,i)
(TCW,o)
1c
3c
Cooling tower 2c
Cooling pump Fig. 1. Schematic diagram of organic Rankine cycle.
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Fig. 2. Schematic diagram of absorption system combined with organic Rankine cycle.
HFC-245fa refrigerant was selected because of low-pressure, hightemperature, non-corrosive, non-flammable, low-toxicity and friendly environment. R-245fa was carried in a 25 kWe ORC prototype as shown in Fig. 3. The descriptions of each ORC-component were presented in Table 1. In the testing procedure, water from a hot water generator was heated by a set of diesel burner at heating capacity range of 200e600 kW. The required outlet water temperature could be automatically adjusted with an uncertainty around ±0.5 C. The maximum water temperature could be reached 120 C in the closed loop system. Hot water temperature entering the ORC boiler were considered at 4 operating conditions (around 90, 95, 100 and 105 C), while the cooling water temperature entering the ORC condenser was fixed at around 28 C. The testing results of each operating condition were continually recorded every 1 min for 1 h at steady state condition. After that, the average value of the recorded data was evaluated to carry out the ORC performance curve. The ORC prototype was tested and measured by temperature, pressure and electrical power sensors. Signal data form sensors were transferred to PLC programmed controller (FATED programmed controller, FBs-CB55) and recorded into the PLC memory. T-type thermocouple at a precision range of ±0.1 C [21], pressure
transducer (WIKA, type A-10) at a precision range of ±0.25% [22] and current transformer (Shinohawa Electric, type YAL-1) at a precision range of ±1% [23] were used to measure temperature, pressure and electrical power, respectively. 3.2. The absorption test rig In the present study, a 20 kW water-lithium bromide (LiBr) absorption chiller as shown in Fig. 4 was used to find out the thermal performance. In the testing procedure, a set of electrical heater 30 kWe at accuracy of the controlled temperature ±0.5 C was used to heat water in a 1500 L storage tank. The descriptions of each absorption-component were presented in Table 2. For measurement, temperatures of the each working fluid were monitored by a T-type thermocouple at a precision range of ±0.1 C [21]. The output signal of thermocouple was interfaced to a personal computer through a data logger and software of TSUS Instruments at a precision range of ±0.5% [24]. The electrical power consumptions of the each component were measured and recorded by a power logger of KYORITSU [25] at an accuracy ±0.2%. The recording time of power logger was set matching with the temperature recorder. In the next part, the constructed of the ORC and absorption units are tested its thermal performance. The objective of this experiment is to find out a performance curves of each system, which is Table 1 Descriptions of each component in the ORC unit. Components
Properties
Generator
Induction generator Gross power 25 kWe 3 phase, 380 V, 50 Hz Semi-hermetic twin screw type expander Displacement volume 3000 rpm SUS 316 plate type heat exchanger Shell and tube heat exchanger Shell: carbon steel 12 in 3 m Tube: 3/4 in copper tube Vertical type oil separator Oil tank 18 in diameter 0.7 m Viking heavy duty oil pump (GG4195) Motor 3 hp, 3 phase, 380 V, 50 Hz Vertical multi-stage centrifugal pump VFD drive (BN3-17) Motor 2 hp, 3 phase, 380 V, 50 Hz
Expander Boiler Condenser
Oil and vapor separator Oil pump Refrigerant pump Fig. 3. Prototype of the R-245fa ORC system (Hanbell model: RC2-300 [20]).
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Data
Hot water inlet (THW,i) [ C] Hot water outlet (THW,o) [ C] Heat source capacity (QB) [kW] Cool water inlet (TCW,i) [ C] Cool water outlet (TCW,o) [ C] Condenser temperature (TC) [ C] Heat sink capacity (QC) [kW] Expander inlet pressure (PHigh) [kPa] Expander inlet temperature (T1) [ C] Expander outlet pressure (PLow) [kPa] Expander outlet temperature (T2) [ C] Refrigerant pump power (WP) [kWe] Oil pump power (WOP) [kWe] Lift temperature (THW,i e TCW,i) [ C] Cycle powerb (WORC) [kWe] Cycle efficiencyc (hORC) [%]
116 89.8 243.2 28 35 37.1 219.0 1097.1 93.7 227.4 75.0 1.78 1.40 88 21.50 8.73
107.8 81 248.2 28 35 37 215.6 1120.0 94.6 227.4 70.6 1.90 1.40 79.8 21.36 8.49
97 75 203.4 28 35 37 210.9 1074.0 92.8 227.0 70.6 1.19 1.40 69 16.70 8.11
88.9 77.8 188.3 28 35 37 211.0 811.3 85.7 239.3 59.5 1.24 1.40 58.8 9.00 4.71
a
Positions referred Fig. 1. Cycle power (WORC) was gross power (WExp) subtracted by power consumptions of refrigerant pump (WP) and oil pump (WOP). c Cycle efficiency was defined by hORC ¼ WORC 100/(QB þ WP þ WOP). b
the correlation between the input parameters and the system efficiency. Projection of the optimal integrating unit by using the both performance curves is presented. The system performance of the CCHP unit could be predicted under various operating conditions. Moreover, this technique could be decreased complication of the fundamental simulation.
4. Results and discussion 4.1. Thermal performance of the ORC system The impact of hot fluid temperature on the 25 kWe ORC machine and the system performance were investigated by setting the inlet hot fluid temperature varying between 90 and 120 C, while the cooling fluid temperature was fixed at around 28 C. The testing result of the ORC system was shown in Table 3. It could be found that the cycle efficiency (hORC) of the R-245fa ORC Table 2 Descriptions of each component in the absorption unit. Components
Properties
Generator (Flooded shell and tube heat exchanger) Condenser (Shell and tube heat exchanger)
Capacity 26.33 kW Heating area 2.04 m2 Capacity 20.00 kW Heating area 0.84 m2 Capacity 25.20 kW Heating area 2.88 m2 Capacity 20.80 kW Heating area 2.32 m2 Flow rate 0.6e3.0 m3/h Maximum pressure 10 bar Power 56 We
Absorber (Flooded shell and tube heat exchanger) Evaporator (Shell and tube heat exchanger) Solution pump (DAB in-line pump)
Vapor volume (10-3 m3/kg)
Fig. 4. Prototype of the water-LiBr absorption chiller (ATS model: absorption chiller 10 RT [24]).
system was lower than 5%, when hot water temperature was lower than 90 C. While, the efficiency could be increased to be over 8%, when hot water temperature was higher than 97 C. This effect came from a vapor volume of working fluid entering the expander, which was a high value at the low temperature. But its volume was nearly constant at the high temperature as shown in Fig. 5. Table 3 also shows the supporting data of the expander inlet pressure (PHigh) and temperature (T1), which were gradually constant at temperatures of 116, 107.8 and 97 C, respectively, and corresponding with the cycle efficiency. The average cycle efficiency of the 3 operating points was 8.44%, which corresponded with the above mentioned literature review [7e17]. Thus, in this study, 3 sets of testing data at temperature higher than 97 C were used to evaluate the performance curve of the ORC system. The correlation of the cycle efficiency (hORC) and the different temperature between heat source and heat sink (THW,i TCW,i) as shown in equation (1) and Fig. 6. This linear correlation was relationships of the related data between the ORC efficiency and input variables, which were water temperatures of the water entering at the condenser and the evaporator. It could be seen that the cycle efficiency increased, when the temperature difference increased, which followed the Carnot efficiency concept. The cycle efficiency decreased, when the temperature difference of heat source and heat sink decreased. This performance curve was used
30 27 24 21 18 15 12 9 6 3 0 50
60
70 80 THW,E,i - TCW,C,i (oC)
Fig. 5. Vapor volume entering the expander.
90
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Cycle efficiency (%)
12
9 y = 0.0327x + 5.8628 R² = 0.9967
6
3 60
65
70
75 80 THW,E,i - TCW,C,i (oC)
85
90
95 Fig. 7. Performance curve of the absorption unit.
Fig. 6. Simplified model of the ORC system.
to predict the thermal performance, when the ORC system was combined with the absorption system.
hORC ¼ 0:0327 THW;i TCW;i þ 5:8628
(1)
4.2. Thermal performance of the absorption system The impact of heat source temperature on the 20 kW absorption unit and the system performance were investigated by setting the inlet heat source temperature varying between 70 and 90 C. Table 4 shows the average heating capacities of the absorption element. It could be seen that the supplied heat at the generator was around 25 kW. The useful cooling capacity at the evaporator was around 16 kW, while the rejected heats at the condenser and absorber were around 22 and 21 kW, respectively. Table 4 also shows the average temperatures of working fluid, water entering and leaving the main components of the absorption system. The cooled water leaving the evaporator was around 15 C, which converts from the heat source temperature around 80 C at the generator. The water-LiBr concentrations at around 0.53 and 0.45%LiBr as the strong and weak solutions based on %LiBr, respectively, were used to circulate between generator and absorber. The experimental result also shown the average COP (coefficient of performance) of the absorption prototype was around 0.66. This result was nearly the COP of the YAZAKI absorption machine at around 0.7 [27]. When the experimental data at the heat source temperature varying between 70 and 90 C were analyzed the absorption performance curve. The empirical correlation of the COPAb and the different temperature term of (TG,i TC1)/(TA,i TE) was found in linear form as shown in equation (2) and Fig. 7.
COPAb ¼ 0:1613 TG;i TC1
TA;i TE þ 1:3178
(2)
Table 4 Testing results of the absorption chiller. Descriptions a
Working fluid temperature Data ( C) Inlet water temperaturea Data ( C) Outlet water temperaturea Data ( C) Heating capacitya Data (kW) a
Positions referred in Fig. 2.
Generator
Evaporator
Absorber
Condenser
TG 64.84 THW,G,i 80.61 THW,G,o 64.92 QG 24.99
TE 9.68 TCW,E,i 20.30 TCW,E,o 14.68 QE 16.46
TA 38.97 TCW,A,i 20.68 TCW,A,o 31.70 QA 22.18
TC1 35.97 TCW,C,i 20.68 TCW,C,o 30.00 QC1 20.47
In this study, the performance curves of the ORC and absorption systems were used to evaluate the optimal integrating unit by the projection method in the next part. 4.3. Projection of the optimal integrating system In the projection process of the optimal integrating condition, the operating parameters and the cycle efficiency were predicted. The initial conditions of the projection method were shown in Table 5. Table 6 shows the projection results. It could be seen that the cycle efficiency of the normal ORC system (hORC,normal) was around 8.08% at the cooling water (T1C) around 32.22 C, which was the typical water temperature of cooling tower under the climate of Thailand. The normal ORC system obtained heat source (QB) around 247.55 kW and converted heat to power (WORC,normal) of 20 kWe. For the modified system, after the same among of heat source (QB) was used to generate electricity, low grade heat leaving the ORC boiler was supplied to the absorption system to produce the cooled water temperature (T4c) around 15 C. The modified system could be decreased the cooling water (T1C) to around 24.87 C, which the cooling tower was cascade connection with the absorption unit. In addition, the electrical power (WORC,CCHP) of 21.39 kWe was generated at the cycle efficiency (hORC,CCHP) around 8.64%. From this result, it could be found that when the ORC condenser temperature decreased, the ORC cycle efficiency increased. Therefore the cycle efficiency of the modified ORC system could be increased around 6.97% compared with the normal ORC system. This result was corresponded with the literature studies of
Table 5 Initial conditions of the projection process. Descriptions The ORC system Cycle power (WORC,normal) [kWe] Extracting capacity of the condensera (QC) [kW] Hot water entering the boilera (T1h) [ C] Hot water leaving the boilera (T2h) [ C] Cooling water leaving the condenserb (T1c) [ C] Cooling water entering the condenserb (T3c) [ C] Performance curve of the ORC system The absorption system Cooled water leaving the evaporatorc (T4c) [ C] Cooling water entering the absorberc (TCW,A,i) [ C] Working fluid temperature at the evaporatorc (TE) [ C] Working fluid temperature at the condenser1c (TC1) [ C] Performance curve of the absorption system a b c
Referred data in Table 3 and positions in Fig. 1. The typical conditions of cooling tower in Thailand. Referred data in Table 4 and positions in Fig. 2.
Data 20 215 100 80 37.78 32.22 Equation 1 15 21 10 36 Equation 2
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Table 6 Projection result of the optimal integrating system.
Table 7 Present commercial cost of the ORC power plant.
Descriptions The normal ORC system Cycle efficiency (hORC,normal) [%] Heat source of the ORC system (QB) [kW] The absorption system The COP of the absorption system Heat source of the absorption system (QG) [kW] The suitable cooling capacity of the absorption system (QE) [kW] The new system Total cooling capacity of the new system (QE þ QC) [kW] Cooling water entering the cooling tower (T1c) [ C] Cycle efficiency (hORC,CCHP) [%] Cycle power (WORC,CCHP) [kWe] Increasing of the ORC efficiency [%]
Data
Manufacturer
ORC capacity (kWe)
Cost (USD/kWe)
8.08 247.55
Infinity turbine [29,30] Turboden [31] Electratherm [32,33]
2e3000 200e3000 50
2500 3000 2530
0.67 247.55 166.50 381.50 24.87 8.64 21.39 6.97
[1,15e17], when the ORC condenser temperature was reduced, the ORC efficiency was enhanced. Moreover, when the evaporator of the absorption unit was installed with the ORC condenser in cascaded connection, the cooling capacity of the integrating unit could be increased around 70%. Thus the cooling water temperature of the integrating unit could be decreased lower than the normal ORC unit, which was the main advantage point of this technique.
4.4. Economic result In economic assessment, the commercial absorption machine cost [26,28] as shown in Fig. 8 was used to evaluate the capital cost of the absorption system at capacity (QE) around 166.50 kW (around 48 TR) as shown in Table 6. For the ORC cost, the present cost of the ORC power plant varies between 2500e3000 USD/kWe [29e33] as given in Table 7. From the table, it could be stated that the capital cost of a microscale ORC power plant at capacity lower than 50 kWe is around 2500 USD/kWe. Therefore this investment cost was used to consider the economical assessment. From the abovementioned literature review, payback period was selected to represent the economic result of the ORC power plant. Because electricity rate of heat source was specified. But in this study, heat source of the ORC system did not fixe. Thus, the LEC (levelized electricity cost) as presented in the studies of Thawonngamyingsakul and Kiatsiriroat [3] and our former work [34] was selected to analyze in this study. Moreover, the criteria for considering electricity price of renewable power in Thailand at payback period 10 y [35] was used to analyze the LEC. The initial conditions of economic evaluation were shown in Table 8. Table 9 shows comparison of economic results in term of the LEC factor, which based from the operating condition of an only one geothermal power plant of Fang District, Chiang Mai Province,
6,000
Thailand. Thus the both LEC costs of this present work should be compared with the electricity cost (Feed-in Tariff, FiT) of renewable power plant in Thailand [37], which unfortunate for the geothermal FiT does not promote. Because the only one geothermal-ORC power plant in Thailand was managed by the government (Electricity Generating Authority of Thailand, EGAT). From the simulation results, it could found that net electrical power production of the normal and modified ORC system were around 16.68 and 17.07 kWe, respectively. The ORC net power (WORC,net) of the normal ORC unit came from the ORC cycle power (WORC) subtracted by the electrical power consumptions of the cooling pump (Ebora model CMB/E 3 T, flow rate 100e280 L/min) and the fan motor of cooling tower (Model BKC 80 RT, fan motor 1.5 hp) at 2.2 and 1.12 kWe, respectively. In case of the new unit, the electrical power consumptions of the solution pump was considered in the absorption cycle at 78 We (DAB in-line pump, model VSA 65/130, flow rate 0.6e3.7 m3/h) [25]. When, the both ORC units operated at 24 h/d and 350 d/y [36], the new unit could generate electricity higher than the normal unit around 8600 kWh/y. For the LEC results, the CCHP unit was around 0.0891 USD/kWh, which was higher than the normal unit around 0.0222 USD/kWh. When these costs were compared with the FiTs of renewable power plant in Thailand [37]. It was found that the both LECs in Table 9 were lower than the FiTs of biomass and waste (integrated waste management) at 0.096 USD/kWh, waste (land fill) at 0.171 USD/kWh and biogas (waste water/waste material) at 0.115 USD/kWh. While the both LECs were similarly with the FiT of biogas (energy plants) at 0.085 USD/kWh. 4.5. Environmental assessment Energy, economic and environment impacts were considered in this study. For environmental assessment, the electrical power consumption of the ORC process was investigated, which consisted of refrigerant pump, oil pump, cooling pump and fan motor of cooling tower. In addition, the power of solution pump was considered in the modified unit. Carbon dioxide intensity of electricity of Thailand [38] at 0.6093 kg CO2 eq/kWh was used to estimate the released CO2 from operating process of the ORC system. It could be found the normal unit consumed the electrical power of 49,644 kWh/y and released the carbon dioxide intensity at around 30,248 kg CO2 eq. While, the new unit could generate electricity higher than that of the normal unit. Thus the carbon dioxide
Absorption cost (USD/TR)
4974.3 5,000
Table 8 Initial conditions of economic assessment.
y = 12522x-0.693 R² = 0.9961
4,000
Descriptions
3,000
a
2,000
1454.0 826.5
1,000
631.4
0 0
10
20
30 40 50 Cooling capacity (TR)
Fig. 8. Cost of absorption unit.
60
70
80
90
Operation time (h/d) Operation daya (d/y) Cycle power (WORC) [kWe] Cooling pump [kWe] Cooling tower [kWe] Cost of the ORC unit [USD/kWe] Capacity of the ORC system [kWe] Payback period [y]
Normal system
Modified system
24 350 20 2.2 1.12 2500 25 10
24 350 21.39 2.2 1.12 2500 25 10
a Referred the operating time of only one geothermal-ORC power plant in Thailand at Fang District, Chiang Mai Province [30].
N. Chaiyat, T. Kiatsiriroat / Energy 91 (2015) 363e370 Table 9 Economic results of the integrating system. Descriptions
Normal system
Modified system
Net electrical power production (WORC) [kWe] Net electricity production [kWh/y] Cost of the ORC unit [USD] Cost of piping, housing and cooling systemsa [USD] Cost of the absorption system [USD] The ORC power plant cost [USD] LEC [USD/kWh]
16.68 140,112 62,500 31,250 e 93,750 0.0669
18.00 151,165 62,500 31,250 40,924 134,674 0.0891
a
Calculated at 50% of the ORC cost.
intensity from the output product of electricity 1 unit of the modified unit was lower than the normal unit of 0.203 and 0.216 kg CO2 eq/kWh, respectively. 5. Conclusions and recommendations From the study result, it could be concluded that the method to enhance the ORC efficiency by combined cooling heating and power generation from the absorption system for reducing the ORC condenser temperature was effective. The performance curves of each technology were performed by using the experimental results for predicting the system performance. Projection of the integrating system represented the better efficiency, when the ORC condenser temperature was decreased. The ORC efficiency could be increased around 7%, when the cooled water temperature at 15 C was supplied from the absorption system. But, in the economic result, it could be found that electricity cost in term of LEC of the modified system was higher than the normal system of 0.0891 and 0.0669 USD/kWh, respectively. While, the environmental impact, the released carbon dioxide intensity of the new unit was lower than the normal unit at around 0.203 and 0.216 kg CO2 eq/kWh, respectively. For the future study, varying the ORC condenser temperature should be studied to find out the suitable cooled water temperature from the absorption system. The comparison effect of the ammonia-water and water-LiBr solutions of the absorption system should be investigated. Moreover, different types of refrigeration system such as adsorption system and ejector refrigerator should be studied. Acknowledgments The author would like to thank the School of Renewable Energy, Maejo University and Center of Excellent for Renewable Energy, Chiang Mai University and the Office of the Higher Education Commission, Thailand under the National Research University Program, Chiang Mai University for supporting testing facilities. Abbreviations and symbols
Nomenclature P pressure, (bar) Q heat rate, (kW) T temperature, ( C) W work, (kW) Subscript A absorber Ab absorption system B boiler
C CW e E Ex G HS HW i o OP P
369
condenser cooling water electrical power evaporator expander generator heat source hot water inlet outlet oil pump refrigerant pump
References [1] Walraven D, Laenenb B, D'haeselee W. Minimizing the levelized cost of electricity production from low-temperature geothermal heat sources with ORCs: water or air cooled. Appl Energy 2015;142:144e53. [2] Suna J, Li W. Operation optimization of an organic Rankine cycle (ORC) heat recovery power plant. Appl Therm Eng 2011;31(11e12):2032e41. [3] Thawonnagamyingsakul C, Kiatsiriroat T. Performance and economic analysis of a solar organic rankine cycle with flat-plate and evacuated-tube solar collectors as heat source in the northern part of Thailand. In: The 1st ASEAN Plus three graduate research congress. Chiang Mai, Thailand; 2012. [4] Imran M, Park BS, Kim HJ, Lee DH, Usman M, Heo M. Thermo-economic optimization of regenerative organic Rankine cycle for waste heat recovery applications. Energy Convers Manag 2014;87:107e18. [5] Esen H, Inalli M, Esen M. Technoeconomic appraisal of a ground source heat pump system for a heating season in eastern Turkey. Energy Convers Manag 2006;47(9e10):1281e97. [6] Esen H, Inalli M, Esen M. A techno-economic comparison of ground-coupled and air-coupled heat pump system for space cooling. Build Environ 2007;42(5):1955e65. [7] Hettiarachchi HDM, Golubovica M, Worek WM, Ikegamib Y. Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources. Energy 2007;32:1698e706. [8] Schuster A, Karellas S, Karl J. Simulation of an innovative stand-alone solar desalination system with an organic Rankine cycle. In: The 46th Conference on simulation and modeling, Trondheim, Norway; October 13e14, 2005. [9] Guo T, Wang HX, Zhang SJ. Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources. Energy 2011;36:2639e49. [10] Sauret E, Rowlands AS. Candidate radial-inflow turbines and high-density working fluids for geothermal power systems. Energy 2011;36:4460e7. [11] Liu B, Riviore P, Coquelet C, Gicquel R, David F. Investigation of a two stage Rankine cycle for electric power plants. Appl Energy 2012;100:285e94. [12] Edrisi BH, Michaelides EE. Effect of the working fluid on the optimum work of binary-flashing geothermal power plants. Energy 2013;50:389e94. [13] Li T, Zhu J, Zhang W. Comparative analysis of series and parallel geothermal systems combined power, heat and oil recovery in oilfield. Appl Therm Eng 2013;50:1132e41. [14] Rodriguez CEC, Palacio JCE, Venturini OJ, Silva LEE, Cobas VMD, Santos DM, et al. Exergetic and economic comparison of ORC and kalina cycle for low temperature Antonio Yock. In: Proceeding of seminar on short course on surface exploration for geothermal resources, El Salvador; October 17e30, 2009. [15] Wang J, Wang M, Li M, Xia J, Dai Y. Multi-objective optimization design of condenser in an organic Rankine cycle for low grade waste heat recovery using evolution aryalgorithm. Int Commun Heat Mass Transf 2013;45:47e54. [16] Li J, Pei G, Ji J, Bai X, Li P, Xia L. Design of the ORC (organic Rankine cycle) condensation temperature with respect to the expander characteristics for domestic CHP (combined heat and power) applications. Energy 2014;77: 579e90. [17] Liu Q, Duan Y, Yang Z. Effect of condensation temperature glide on the performance of organic Rankine cycles with zeotropic mixture working fluids. Appl Energy 2014;115:394e404. [18] Smith IK, Stosic N, Kovacevic A. Screw expanders increase output and decrease the cost of geothermal binary system. GRC Trans 2005;29:787e94. [19] Hung TC, Shai TY, Wang SK. A review of organic Rankine cycle for the recovery of low-grade waste heat. Energy 1997;22(7):661e7. [20] HanPower Energy Technology Co.,LTD. Cost of organic Rankine cycle set, TaoYuan Hsien, Taiwan. 2014. [21] OMEGA Engineering inc. Thermocouples. http://www.omega.com/prodinfo/ thermocouples.html [accessed 15.04.15]. [22] WIKA Instrument Corporation. General purpose pressure transmitters type A10. http://www.wika.us/upload/DS_PE_A_10_en_us_16479.pdf [accessed 15.04.15]. [23] Shinohawa Electric Co.,LTD. Shinohawa product Catalog. http://www. kingelectric.cn/products.htm [accessed 15.04.15]. [24] TSUS Instruments. Model: DTL 16/24/32 CH. 2012.
370
N. Chaiyat, T. Kiatsiriroat / Energy 91 (2015) 363e370
[25] KYORITSU, KEW 6310. http://www.kew-ltd.co.jp/en/ [accessed 15.04.15]. [26] Advance Thermo Solution Co.,LTD. Cost and specification of absorption chiller unit, Samut Prakan, Thailand. 2014. [27] YAZAKI, Our environmental products. http://www.yazaki-circonditioning. com/products.html [accessed 18.06.15]. [28] Voltas Technologies. Heat to cooling absorption. http://www. voltastechnologies.co.za/heat-to-cooling/heat-to-cooling-absorption/36voltas/login-knowledgement/79-quick-budgeting-tool [accessed 08.08.2015]. [29] Rowshanzadeh Reza. Performance and cost evaluation of organic Rankine cycle at different technologies. http://kth.diva-portal.org/smash/get/diva2: 410363/FULLTEXT01 [accessed 08.08.15]. [30] Infinity turbine, Introducing the radial outflow turbine generator e organic Rankine cycle e ORC turbine. http://www.infinityturbine.com/ORC/ORC_ Waste_Heat_Turbine.html [accessed 08.08.15]. [31] Turboden. Turboden ORC: recent developments and new applications in organic rankine cycle technology. http://www.all-energy.co.uk/__ novadocuments/54368?v¼635376693736400000 [accessed 08.08.15].
[32] Arvay P, Muller M R, Ramdeen V, Cunningham G. Economic implementation of the organic Rankine cycle in industry. http://aceee.org/files/proceedings/ 2011/data/papers/0085-000077.pdf [accessed 08.08.15]. [33] Electratherm. https://electratherm.com/ [accessed 08.08.15]. [34] Chaiyat N. Assessment alternative energy for organic Rankine cycle power plant in Thailand. Int J Eng Technol 2015;7(1):317e26. [35] Tongsopit S, Greacen C. Thailand's renewable energy policy: FiTs and opportunities for international support. In: WRI-ADB Workshop on Feed-in Tariffs. Manila: Philippines; 2012. [36] Electricity Generating Authority of Thailand (EGAT). Project of geothermal energy for multipurpose, Amphoe Fang, Chiang Mai Province. 1988. [37] Energy Policy and Planning Office, Ministry of Energy, Thailand, Feed-in tariff of renewable energy in Thailand. http://www.eppo.go.th/power/fit-seminar/ Fit_2558.pdf [accessed 06.05.15]. [38] Carbon dioxide intensities of fuels and electricity for regions and countries. https://www.ipcc.ch/pdf/special-reports/sroc/Tables/t0305.pdf [accessed 20.10.14].