Feasibility study of the high‐temperature organic Rankine cycle in combined heat and power state from energy, exergy, and economic point of view

The organic Rankine cycle (ORC) has received a lot of attention in recent years due to its wide application in energy recovery and the use of low‐temperature energy sources. In this article, the energy, exergy, and economic analyses of a high‐temperature ORC (HTORC) in combined heat and power production mode have been performed. In this system, the heating water at 90°C for domestic or industrial purposes is provided in the HTORC condenser. Two working fluids, hexamethyldisiloxane (MM) and siloxane mixture (MDM), have been evaluated and compared in HTORC. The system has been modeled in engineering equation solver software and key parameters such as energy efficiency and exergy of the system, output power, heat‐to‐power ratio, and levelized cost of electricity (LCOE) have been calculated. The energy and exergy efficiency of the system for the two working fluids MM and MDM are equal to 40.8%, 45.6%, 22.45%, and 19.3%, respectively. From the point of view of energy and exergy, the working fluid MM performs better. The LCOE of the system with MM working fluid is equal to 0.5946 US$/kWh, which is slightly higher than MDM working fluid (0.5702 US$/kWh).


| INTRODUCTION
Biogas is a renewable natural energy source that has practical effects on nature and industries.This gas is produced from the decomposition of organic materials, including animal manure, food waste, and sewage.Fertilizers and waste produce biogas through anaerobic digestion (i.e., without the presence of oxygen). 1,2iogas is a combination of methane and carbon dioxide gases and some other gases including hydrogen sulfide, nitrogen and hydrogen, methyl mercaptan, and oxygen.This combination is an ideal option for making renewable energy. 3epending on the input materials about 50%-70% of biogas is methane.Therefore, it is flammable.Fuel is produced from the combination of methane, hydrogen, carbon monoxide, and oxygen gases.Biogas is the most economical renewable fuel that is used in many countries.Potential applications are cooking, cooling and heating, electricity, methanol, and steam generation, waste management, and mechanical power generation. 2,3iogas has many advantages over other fuels, the most important of which are as follows 4 : 1. Energy production (including heat, light, and electricity).2. Reducing the volume of disposed waste.3. Reduction of pathogens (flies, worm eggs).4. Converting waste containing organic matter into high-quality fertilizer. 5. Increasing productivity in the field of livestock and agriculture.6. Reducing soil and water pollution.7. Organic fertilizer production.8. Economic efficiency production process.9. Reducing reliance on fossil fuels.10.Easy cooking.11.Compatibility with nature.
Electricity can be produced from biomass in different ways.For example, biomass can be used as fuel in an internal combustion engine (IC engine) and micro gas turbine or it can be used to heat the intermediate fluid in the boiler to generate electricity in cycles with a lowtemperature source such as organic Rankine cycle (ORC), Goswami cycle, or Kalina cycle. 3With the electricity produced by the mentioned systems with biomass fuel, the electricity needs of a residential building, residential tower, or residential area can be met.Also, in remote rural areas where it is not possible to transfer and distribute electricity to them or it is very difficult, using these systems with biomass fuel is a suitable option. 5lso, biomass fuel can be used as a renewable energy source along with other renewable energy sources such as solar or geothermal to produce electricity as a hybrid system. 6everal types of research have been done about the ORC powered by biomass.
Świerzewski et al. 7 have investigated the economic effects and optimization of ORC with biomass fuel in combined heat and power production (CHP) mode.In this research, the effects of rated power and operating fluid of ORC were investigated.The investigated parameters include power generation efficiency, financial indicators, net present value (NPV) and internal rate of return (IRR).The results of this research showed that the best NPV and IRR can be achieved in the nominal power range of 1000-2600 and 750-1400 kW.In general, hydrocarbon-based working fluids are economically more affordable than silicon oils.Braimakis et al. 8 have done the technoeconomic evaluation of ORC with biomass fuel in CHP mode.In the investigated system, ORC is coupled with district heating (DH) to meet the electricity and heat needs of residential areas.The working fluid of ORC in this research is R1233zd.To estimate the electricity and heat needs, the weather conditions of three cities, such as Athens, Berlin, and Helsinki, with a peak load of 10,200 kWh th have been investigated.The discount payback period of this system for the three investigated cities is calculated to be around 5-7 years.
Mascuch et al. 9 have investigated the commercial development of an ORC pilot to produce 2 kW of electricity and 50 kW of heat.The fuel used by the ORC pilot in this research is biomass.The discussed system is installed in a village to meet the electricity and heat needs of the municipal building.With 3000 h of operation, this system can produce 125 MWh of heat and 2.34 MWh of electrical energy.The results of this research showed that only the economic discussion is not enough to investigate this system and other benefits of this system in terms of environment and ability to adapt to the needs of consumers should also be investigated.
Carraro et al. 10 investigated the optimal sizing of the ORC (CHP) system for a DH system with coal fuel.In this research, the effects of boundary conditions on the optimal features of ORC have been investigated.The results of this research showed that the optimal size of ORC for DH with heating is 30 MW in the range of 1-2 MW.
Feng et al. 11 investigated the problems of optimal sizing of ORC (CHP) systems for DH systems with coal fuel.The fuel used by ORC is biomass.In this research, the effects of boundary conditions on the optimal design characteristics of ORC have been investigated.The results of this research showed that the optimal size of ORC for DH with heating is 30 MW in the power generation range of 1-2 MW.Similar research has been done by Cruz et al. 12 de Mena et al. 13 investigated the problems of biomass combustion.For this investigation, they modeled a new CHP system including an updraft gasifier, ORC with toluene working fluid and an external combustion chamber.The biomass used in this research is olive leaves.The investigated system produces 93.8 kW of electricity and 412 kW of heat.The CHP efficiency of this system is equal to 58.4% and 10.8% with the consumption of 240 kg/h biomass.The gasification efficiency is equal to 88.1%.The ORC modeling results showed that the ORC efficiency reaches 18.7% at a pressure of 25 bar and a temperature of 300°C at the expander inlet.

| 1029
After reviewing the previous works, it can be concluded that until now there is no research has been done in the field of ORC systems with high-temperature working fluids, hexamethyldisiloxane (MM) and siloxane mixture (MDM).In this proposed ORC system due to the high temperature of the condenser, the water used to cool the condenser can be used for heating purposes (CHP ORC).The temperature of the cooling water reaches 90°C at the outlet of the condenser.
In this article, an ORC system in CHP mode with MM and MDM working fluids has been investigated from the perspective of energy, exergy, and economy (3E analyses).ORC's energy source is biomass fuel (bean straw).In short, the innovations of this article are as follows: 1. Introducing ORC in CHP mode so that condenser cooling water can be used as a heat source.2. Energy, exergy, and economic analyses of ORC in CHP mode with two high-temperature working fluids.3. Parametric study of ORC in CHP mode.

| System description
Figure 1 shows the schematic diagram of the hightemperature ORC (HTORC) powered by bean straw.In the digester, solid biomass reacts with air, and syngas is produced (Points 1, 2, and 3) The generated syngas is coupled to the heat transfer fluid (HTF: Therminol VP-1) in the heater (Points 3, 4, 5, and 6).In the second step, the HTF is heated up further in the boiler (Points 4, 6, 7, and 15).
The energy source of the boiler is the outlet syngas from the heater (Point 4).The HTF transfers heat to the ORC working fluid in the evaporator (Points 7, 8, 10, and 11), and superheated vapor is provided at the expander inlet.The gaseous working fluid rotates the expander and generator to generate electricity (Points 11 and 12).The outlet working fluid of the expander is in the form of saturated vapor, which turns into a saturated liquid after exchanging heat with the cooling fluid of the condenser (Points 9, 12, 13, and 14).
F I G U R E 1 Schematic diagram of the studied system.ORC, organic Rankine cycle.
After passing through pump II, the saturated liquid of the working fluid becomes a compressed liquid.(Points 9 and 10).The hot HTF at the outlet of the evaporator is turned into a compressed liquid in Pump I and thus the cycle is completed.Table 1 shows the physical specifications of working fluids MM and MDM.Table 2 shows the physical specification of Therminol VP-I. 14he biomass of the ORC is bean straw (a = 1.5611; b = 0.7842; c = 0.0166).The moisture content, lower heating value, and molecular mass are 20%, 13,645 kJ/kg, and 26.64 kg/kmol, respectively.
For 3E analyses of the proposed system, the following assumptions can be noticed: 1.The polytropic efficiency of the pump and expander is 80%.2. The system operation is steady state.
3. The pressure drop can be neglected.4. The pressure and temperature of the environment are assumed to be 1 atm and 25°C.5.The heat losses are ignored.6. Potential and kinetic exergy can be ignored.7. The heat exchanger effectiveness factor is 70%.8.The working fluids in the ORC are MM and MDM. 9.The highest and lowest pressures in the ORC are 8 and 1 bar.10.The condensing pressure is 1 bar.

| Energy, mass balance, and exergy analyses
In general, the mass and energy balance equations can be written as follows 15 : (2) where Z m , ̇, h, W , ̇V, g, and Q ̇are the height, mass flow rate, enthalpy, power, velocity, gravitational acceleration, and heat transfer rate, respectively.The reaction in the gasifier can be written as follows 16,17 : where CH a O b N c and w denote the chemical formula for biomass and the moisture content.m is the amount of | 1031 inlet air.Considering Equation ( 3), the mass balance equations can be written as follows 16,17 : (5) Furthermore, the following chemical reaction should be noticed 16,17 : For the chemical equilibrium constants Equations ( 8) and ( 9) can be written as follows 16,17 : The combustion reaction that takes place in the boiler can be written as follows: (13) a Air Fuel (16)   The mass and energy balance relations for each component are shown in Table 3.
The net output power of the ORC can be calculated by the following relation: The ORC energy efficiency (ENE) is calculated by 18 The system ENE is calculated by 18 Specific exergy has four types: chemical, physical, potential, and kinetic denoted below [19][20][21]   e x e V gz h h T s s T A B L E 3 The mass and energy equations for each component.

Mass and concentration balance
Energy balance Abbreviation: EDR, exergy destruction rate.
where e, x, z, g, V, h, s, T, and y depict specific exergy, mass fraction, height, gravitational acceleration, velocity, specific enthalpy, specific entropy, temperature, and mole fraction.i, 0, and ch denote species, dead states, and chemicals.The exergy destruction rate (EDR) for each component of the system is shown in Table 4.
The biomass chemical exergy can be calculated by the following equation 22 : where h fg is the vaporization enthalpy.
The ORC EXE can be calculated by the following equations: (23 The system EXE can be calculated by the following equations:  (24)

| Economic analysis
The annual income for this system can be calculated by 23,24 CF Y k = .
elec elec (25)   In the above equation, k denotes the specific cost for electricity (0.61 US$/kWh). 25he capital cost of the system can be written as follows 23,24 : where C represents the installation and investment costs.It should be noted that the cost in Equation ( 26) only includes the purchasing equipment costs.To calculate (total investment cost) TCI, factor 1.18 should be considered for other costs such as electricity, piping, and instrument and control. 26Furthermore, the operation and maintenance cost should be considered at 3% of the initial cost. 23,24he effect of the inflation rate is considered by the following relation 27 : where n and i denote the number of years and inflation rate (3.1%). 28he levelized cost of electricity (LCOE) can be calculated by the following relation 23,24,29 : The cost function of components is shown in Table 5.
For the calculation of the heat exchanger surface effective area, the following equation is noticed 35 : where T Δ In , F t , U, A, and Q ̇denote logarithmic mean temperature difference, correction factor, overall heat transfer coefficient, effective area, and heat transfer rate.The U for the boiler and heat exchanger can be 500 and 700 W/m 2 K. 36

| RESULTS AND DISCUSSION
For modeling the system, equation engineering solver (EES) software has been used.For the thermodynamic properties of each point of the system, the library of the EES software has also been considered.

| Model validation
For the model validation of the HTORC, Agromayor and Nord 37 is considered.Thus, the specification of the HTORC is inserted into the EES program developed for this research.The EXE of the HTORC for the two working fluids (MM and MDM) are compared.According to fig. 2 in Agromayor and Nord 37 , the EXE of the HTORC with MM and MDM working fluids are 47% and 67%, respectively, while the developed program for this research calculated those values at 45.4% and 60%, respectively.The percentage of error is equal to 3.4% and 3.2% for MM and MDM working fluids in HTORC, which is acceptable in engineering calculation.
For the gasifier validation, see Kanagarajan. 38The biomass type is similar to this research.The biomass feed and intake air molar are equal to 0.1294 kg/s and 0.0006 kmol/kmol biomass, respectively.The root mean square error is around 3.4%.

| Results
Tables 6 and 7 present the thermodynamic properties of the CHP/ORC for two working fluids named MM and MDM.
Table 8 shows the amount of output power, ENE, and exergy efficiency (EXE) and the heat-to-power ratio of the ORC for two working fluids MM and MDM.
For both working fluids, the outlet temperature of the evaporator is considered to be 20°C higher than the saturation temperature of the working fluid.The highest and lowest pressures in ORC are considered equal to 8 and 1 bar.
The output power of ORC with the MM working fluid is about 32.1% higher than the output power of ORC with the MDM working fluid, which also applies to the output power of the system.ORC ENEs are equal to 6.1% and 5% for the MM and MDM operating fluids, this efficiency in CHP mode for the whole system is equal to 40.8% and 45.6% for the MM and MDM working fluids, respectively.ORC EXE is equal to 26.3% and 16.7% for the MM and MDM working fluids, respectively, and for the whole system and in CHP mode, these values reach 22.4% and 19.3%.The heat-to-power ratio is also higher for MDM working fluid than MM.In general, in terms of output power production, ENE, and EXE, MM working fluid for ORC has a better performance than MDM working fluid.
Figure 2 shows the EDR percentage for each system component relative to the whole system for MM and MDM working fluids.The highest EDR is related to the boiler and condenser, which is due to the chemical reaction of combustion in the boiler and heat transfer in the condenser.This heat transfer has a significant amount of EDR, which is due to the high-temperature difference between the cooling fluid and the operating fluid of the ORC.
After the boiler and condenser, the gasifier and heater have the highest EDR, which is due to the chemical reaction of gasification in the gasifier and heat transfer in the heater.Pumps I and II have the lowest EDR value.
Figure 3 shows the initial cost percentage for each of the system components.The highest price percentage is related to gasifier and the lowest is related to pump II.The expander also includes a significant percentage of the initial cost.The cost percentage of the evaporator and heater are similar to each other.The LCOE of the system for the MM and MDM working fluids is 0.5946 and 0.5702 US$/kWh, respectively.
The LCOE of the MDM working fluid is slightly lower than that of the MM working fluid, but it is clear that due to other advantages of the MM working fluid, such as higher output power, ENE, and EXE, the priority is related to MM.
Except in the case that more output heat is required, in which case, due to the higher heat-to-power ratio for the working fluid MDM, this working fluid can be considered.
Figure 4 shows the changes in the output power of ORC and the system about the changes in the superheat temperature of the MM working fluid.As the superheat temperature of the working fluid increases, the output power of the ORC and the system increases; in general, with the increase in superheat temperature.The working fluid increases the output power of the expander, which increases the output power of the ORC and the system.

F I U R E 2
The exergy destruction rate percentage for each system component relative to the whole system for hexamethyldisiloxane (MM) and siloxane mixture (MDM) working fluids.
Figure 5 shows the changes in ENE and EXE of the ORC and a system for changes in superheat temperature for the MM working fluid until near supercritical temperature.As the superheat temperature increases, the ENE and EXE of the ORC decrease, which is due to the increase in thermooil (HTF) temperature.The EXE of the ORC changes is higher than its ENE.The ENE of the system increases with the increase of the superheat temperature, which is the reason for the increase in the output power of the system and its heat in the CHP mode.
Figure 6 shows the changes in LCOE with the superheat temperature of the MM working fluid.As the superheat temperature increases from 0°C to 53.8°C, the corresponding LCOE of the system increases from 0.56 to 0.65 US$/kWh (about 16%).In general, two changes occur in the system with the increase of superheat temperature of the working fluid: 1. Increasing the output power of the system (positive change).2. Increase in the initial costs of the system due to the increase in the size of the system (negative change).
The second effect is more than the first effect, which has increased LCOE.
Figure 7 shows the variations of the ORC and system output power with superheat temperature and for the MDM working fluid.Similar to the MM working fluid, with the increase of the MDM superheat temperature, the output power of ORC and the system increases.Figure 8 shows the changes of ENE and EXE for the ORC and system with fluid superheat temperature and for the MDM working fluid.Here too, the changes are similar to F I G U R E 3 The initial cost percentage for each of the system components.MM, hexamethyldisiloxane; MDM, siloxane mixture.

F I G U R E 4
The changes in the output power of organic Rankine cycle and the system with the changes in the superheat temperature of the hexamethyldisiloxane working fluid.
F I G U R E 5 The changes in energy and exergy of organic Rankine cycle and system for changes in superheat temperature for hexamethyldisiloxane working fluid.

F I G U R E 6
The changes of levelized cost of electricity with superheat temperature of the hexamethyldisiloxane working fluid.

I G U R E 7
The variations of organic Rankine cycle and system output power with superheat temperature and for the hexamethyldisiloxane working fluid.
the MM working fluid, the physical reasons for which have already been mentioned in this article.
Figure 9 also shows the changes in the LCOE of the system with the superheat temperature of the MDM working fluid and the changes, in this case, are also similar to the MM working fluid.It should be noted that in this case, the percentage of LCOE changes per superheat temperature changed from 0.01°C to 35.01°C by about 20%.

| CONCLUSION
In this paper, a new arrangement of HTORC with two working fluids MM and MDM in CHP mode, whose required energy is provided by biomass, is introduced.Its CHP mode is such that the heat of the condenser is absorbed by the cooling fluid and its temperature has reached about 90°C, which naturally can be used for residential and industrial heating purposes with low temperatures.For the heat transfer media, Therminol VP-I is used.EES software has been used to model the system and the desired system has been examined from the point of view of energy, exergy, and economic.A parametric study has also been done on this system.
The general results of this research are as follows: 1.If the superheat temperature 20°C is considered, the output power of the system with the MM working fluid is about 32.4% higher than the MDM working fluid.2. With the superheat temperature 20°C, the efficiency of energy and exergy of the MM working fluid is around 21.8% and 57.5%, respectively.
F I G U R E 8 The changes of energy efficiency and exergy efficiency for organic Rankine cycle and the system with fluid superheat temperature and for the hexamethyldisiloxane working fluid.en, energy; ex, exergy, Sys, system.

G U R E 9
The variation in the levelized cost of electricity of the system with the superheat temperature of the siloxane mixture working fluid.
3. The system ENE for the MDM working fluid is 45.6%, while for the MM working fluid it is around 40.8%.4. The EXE of the system with the MM working fluid is around 22.4%, while for the MDM working fluid this value decreases to 19.3%. 5.The heat-to-power ratio for the MDM working fluid is equal to 9.1 and for the MM working fluid this value is reduced to 7. 6.The highest percentage of the total price of this system is related to the gasifier, followed by the expander.7. The increase in superheat temperature is desirable from the point of view of energy and exergy, but not desirable from the economic point of view.
Physical specifications of working fluids MM and MDM.
T A B L E 5 The cost function of components.
T A B L E 8 The amount of output power, ENE, EXE, and the heat-to-power ratio of the ORC for two working fluids MM and MDM.