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Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP
Journal Pre-proof Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP
Erkinjon Matjanov PII:
S0360-5442(19)32320-5
DOI:
https://doi.org/10.1016/j.energy.2019.116625
Reference:
EGY 116625
To appear in:
Energy
Received Date:
07 January 2019
Accepted Date:
23 November 2019
Please cite this article as: Erkinjon Matjanov, Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP, Energy (2019), https://doi.org/10.1016/j.energy.2019.116625
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Journal Pre-proof Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP PhD Erkinjon Matjanov, Scientific-Technical Center with Constructional Bureau and Experimental Production of Academy of Sciences of Uzbekistan republic. Durmon yuli str. 33, Tashkent 100125,Uzbekistan Abstract Ambient conditions affect the performance of the gas turbine cycle. Calculations of 28.1 MW gas turbine in Tashkent CHP show, the gas turbine power output is decreased down to 24.1 MW while ambient temperature is +45 ºC, simultaneously electrical efficiency is reduced from 34.2 % to 32.0 %. An absorption chiller is proposed to cool the inlet air. To drive the cooling process the absorption chiller is analyzed to use three types of a heat source: gas turbine waste gases, HRSG waste gases, solar energy. 4260 kW heat is required to cool the inlet air from +45 ºC to +15 ºC. Obtained results show, that using the heat of gas turbine waste gases in absorption chiller could not be economical profitable, because CHP efficiency is reduced from 81.4 % to 74.4 % during ambient temperature +45 ºC. Technical-economical attractive is the scheme of using HRSG waste gases in absorption chiller. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal values. In order to provide the absorption chiller with solar energy heat, parabolic trough collectors with total net aperture area 8064 m2 are required. Analyses show, when HRSG waste gases have enough heat to provide cooling process, so no need in additional solar field. But however, the solar field can be economical profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than +120 ºC. Keywords Gas turbine efficiency; Inlet air cooling; Absorption chiller integrated gas turbine; Inlet air cooling by solar energy; Solar powered absorption chiller. Highlights - Increase in the ambient air temperature reduces the gas turbine output and efficiency; - Absorption chiller is proposed to cool the inlet air for gas turbine CHP; - Waste gases of a heat recovery steam generator drive cooling process in the absorption chiller; - Solar energy powered absorption chiller is used for gas turbine inlet air cooling. Abbreviations CCPP
Combined cycle power plant
CHP
Combined heat and power
CSP
Concentrated solar power
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Journal Pre-proof DNI
Direct normal irradiation
GHI
Global horizontal irradiation
GT
Gas turbine
HRSG
Heat recovery steam generator
JSC
Joint stock company
TPP
Thermal electric power plant
ETA
Efficiency of appropriate parts.
1. Introduction In Uzbekistan “Uzbekenergo” JSC performs centralized energy supply of the economy and population, as well as heat supply to industrial and household consumers of the country. Up to date, the total installed power output of Uzbekistan power plants exceeds 14100 MW, 85.8% of which belongs to thermal electric power plants (TPP) [1]. Main part of the TPPs are based on Rankin steam cycle and were built during 60th–80th of XX century, they are morally and physically aged. Government of Uzbekistan is paying a great attention to upgrading energy sector of the country. Particularly, almost all of the existing TPPs are being equipped with new CCPPs as well as new Turakurgan CCPP (900 MW) is under construction [2]. During last years one gas turbine CHP and four CCPPs were entered into operation: - 28.1 MW gas turbine with HRSG in Tashkent CHP; - 478 MW CCPP in Navoiy TPP; - Two CCPPs of 450 MW each in Talimarjan TPP; - 370 MW CCPP in Tashkent TPP. But unfortunately, new CCPPs can’t achieve design parameters: their power output and electrical efficiency are lower than design values. Indeed, the gas turbine in Tashkent CHP is being operated with power output of 22 MW instead of 28.1 MW, its efficiency is also lower. CCPP in Navoiy TPP in summer periods reaches 370 MW power output instead of 478 MW. After analyses of state of the art of CCPPs in Uzbekistan following problems were determined: -
High ambient temperature leads to decrease gas turbine performance and electrical efficiency;
-
Dust pollutions in content of air lead to clogging gas turbine air filters that also decreases the gas turbine performance and electrical efficiency.
In order to eliminate the mentioned problems, caused owing to high ambient temperature, we are proposing to equip gas turbines with absorption chillers. Three types of heat sources are being analyzed to power the absorption chiller: -
Heat of gas turbine waste gases;
-
Heat of HRSG waste gases;
-
Heat of solar energy.
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Journal Pre-proof 2. State of the art of the problem Ambient conditions affect the performance of the Brayton cycle. Especially, air temperature and pressure are the most crucial ambient conditions. Relative humidity has only a minor influence, but becomes more important if the cooling concept is equipped with an evaporative cooling tower. There are three reasons why ambient air temperature has a significant influence on the power output and electrical efficiency of the gas turbine [3, 4]: -
Increasing the ambient air temperature reduces the density of the air and consequently reduces the air mass flow into the compressor as constant volume engine. This is the main reason for changes in the gas turbine power output.
-
The specific power consumed by the compressor increases proportionally to the air inlet temperature without a corresponding increase in the output from the turbine part.
-
As the air temperature rises and the mass flow decreases, the pressure ratio within the turbine is reduced. Due to the swallowing capacity of the turbine section and the reduced mass flow, the pressure at the turbine inlet is reduced. This leads to a lower pressure ratio within the turbine, and applies inversely, of course, to the compressor; however, because its output is less than that of the turbine, the total balance is negative.
The exhaust gas temperature rises as the ambient air temperature increases because the turbine pressure ratio is reduced, although the gas turbine inlet temperature remains constant. The result is a decrease in the gas turbine electrical efficiency and power output while the ambient air temperature rises. However, the effect on the performance of the steam part of the CHP is less because a higher exhaust gas temperature improves HRSG performance. At the moment a variety of options have been developed to particularly account for the inlet air cooling, such as: -
Evaporative cooling.
-
Fogging.
-
Evaporation compressor cooling (over/high fogging).
-
Chiller.
Results of studies in field of the gas turbine inlet air cooling were announced by multiple researchers. As well as several authors announced results of case studies on gas turbine performance improvement using absorption chillers. Hall et al. [5] documented the performance of a 36 MW gas turbine plant in which a chilled water-based storage refrigeration system was tasked with cooling inlet air. The cooling system was able to reduce the inlet air temperature from +35 C down to +7 C, enhancing plant performance by 10%. Kumar at all [6], Edware at all [7] had investigated cooling the inlet air by spray cooler. Ehyaei et all [8] had investigated the effects of inlet fogging system on the first and second law efficiencies in sample of Shahid Rajaee power plant (Iran). New function was proposed for system optimization that includes the social cost of air pollution for power generating systems. It is concluded that using inlet fogging system, increases the average output power production, the first and the second law efficiencies during three months of year (June, July and August) by 7%, 5.5% and 6% respectively. Ehyaei et all [9] have conducted a comprehensive thermodynamic modeling of CCPP and have investigated effects of gas turbine inlet fogging system on the first and second law efficiencies and net
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Journal Pre-proof power outputs of CCPP. A new objective function based on the first law efficiency and the energy cost which includes the external social cost of air pollution was considered for the optimization purpose. As well the effect of variation in inlet temperature had been investigated. It was concluded that using inlet air cooling system for the CCPP system and its optimization results in an increase in the average power output, first and second law efficiencies by 17.24%, 3.6% and 3.5%, respectively, in summer months of year. Absorption refrigeration also had been paid more attention as it can utilize low grade energy of the exhaust gas instead of electrical energy to provide the necessary cooling. The first law analysis of compressor inlet air cooling using lithium bromide-water absorption refrigeration had been done by Karakas et al. [10], Salvi and Pierpaoli [11], Bassily [12], Amery and Hejazi [13] and Boonnasa et al. [14], Dawoud et al. [15] and Hosseini et al. [16]. Hosseini et al. [16] have reported the effect of inlet air cooling using other methods such as evaporative cooling, compression refrigeration and aqua-ammonia absorption refrigeration. All these studies show that decreasing inlet air temperature results in an increase of the power output and electrical efficiency of the gas turbine. However, more attractive papers are shortly described below. Kakaras et al. [10] had carried out a computer simulation on integration of absorption chiller to reduce the inlet air temperature in gas turbine. IPSEPRO software had been used for computer simulations. A simple gas turbine cycle and a CCPP were studied. The absorption chiller cooling system had shown a higher gain in power output and electrical efficiency than evaporative cooling for the simple gas turbine cycle. As well as the results for CCPP also had shown that the absorption chiller considerably increases the power output. Ameri and Hejazi [13] had reported about carried out feasibility studies on installing an absorption chiller for cooling the inlet air for gas turbines of Chabahar power plant. According to results of the feasibility studies, an average power output can be increased by 11.3% while a maximum power augmentation 2.4 MW is attainable. The electric energy generation is increased by 14 000 MWh/year. The payback period of capital investments is equal to 4.2 years. Boonnasa et al. [14] analyzed possibilities of improving the power output of CCPP in Bangkok by reducing intake air temperature down to +15 C. They proposed an absorption chiller to cool the inlet air. Authors reported, that it is possible to improve power output of the gas turbine by 10.6% and the CCPP by 6.24%. Payback period of capital investments – 3.81 years. Mohapatra and Sanjay [17] made a comparison of two different methods of inlet air cooling (evaporative cooing and vapor compression cooling) integrated to a cooled gas turbine power plant. A parametric study of the effect of pressure ratio, compressor inlet temperature, turbine inlet temperature, inlet temperature ratio, ambient relative humidity and ambient temperature on performance parameters of plant had been carried out. The inclusion of inlet air cooling systems had improved specific work and efficiency of the gas turbine. This improvement is higher at higher ambient temperature and lower ambient relative humidity. Ehyaei et all [18] had made an exergy, economic and environmental analyses for a 4900 kW absorption chiller integrated 159 MW gas turbine unit. The effect of using absorption chiller in gas turbine power plants for Tabas and Bushehr (Iran) climate conditions is conducted. Results had shown that using
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Journal Pre-proof absorption chiller in gas turbines leads to increase the power output on 11.5% and 10.3% in Tabas and Bushehr respectively. Moreover, the second law efficiency was increased to 22.9 and 29.4% for Tabas and Bushehr respectively. The gas turbine power increases from 137 MW to 153 MW during the hottest month (August) in Bushehr when the inlet air was cooled from +37 C down to +15 C. Moreover, efficiency rose from 33.4 % to 43.2 %. Announced results of researches and feasibility studies on solar powered absorption chillers in several branches of industry [19, 20, 21] testify about attainability of realization and reliability of the solar powered absorption chillers. Although there are many results were announced in field of inlet air cooling using absorption chiller, in this paper we investigate the possibilities of cooling the inlet air using an absorption chiller, where gas turbine waste gases, HRSG waste gases and a solar energy are compared with each other to be used as a heat source. As well as we consider performance data of the gas turbine and the gas turbine CHP. It is proposed to lower the inlet air temperature from +45 C down to + 15 C as the ambient temperature in Tashkent reaches up to 45 C in summer. 3. Modelling of governing equations 3.1. Gas turbine CHP thermodynamic model Tashkent gas turbine CHP consists mainly from Hitachi H27 gas turbine and HRSG. Consequently, we consider thermodynamic model for such system. Energy flow diagram of the gas turbine CHP without supplementary firing is presented in Fig. 1.
Fig. 1. Energy flow diagram of the gas turbine CHP without supplementary firing: 1 – Compressor; 2 – Combustion chamber; 3 – Gas turbine; 4 – HRSG; 5 – Superheater; 6 – Evaporating surfaces; 7 – Economizer; 𝑊𝐺𝑇 – Gas turbine power output; 𝑄𝐴 – Ambient air heat; 𝑄𝐹 – Heat input into the gas
𝑄𝐿𝐺𝑇 – Gas turbine heat losses; 𝑄𝐺𝑇 – Gas turbine waste gases heat; 𝑄0 – HRSG steam heat; 𝑄𝐹𝑊 – Feed water heat; 𝑄𝐿𝐻𝑅𝑆𝐺 – HRSG heat losses; 𝑄𝑊 – HRSG waste gases heat.
turbine;
3.1.1. Gas turbine In order to evaluate the gas turbine electrical efficiency, we have to consider each elements of the gas turbine. The consumed work by the gas turbine compressor is equal to (fig. 1):
𝐻𝐶 = 𝑐𝑝.𝑎 × (𝑇2 ― 𝑇1), where:
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Journal Pre-proof 𝑐𝑝.𝑎 – Isobaric specific heat capacity of air (kJ/kgK); 𝑇1– Ambient air temperature (K); 𝑇2– Compressed air temperature (K). The generated work while extraction of waste gases in the gas turbine is determined by equation:
𝐻𝐺𝑇 = 𝑐𝑝.𝑔 × (𝑇3 ― 𝑇4) , where:
𝑐𝑝.𝑔 – Isobaric specific heat capacity of waste gases (kJ/kgK); 𝑇3– Temperature at gas turbine inlet (K); 𝑇4– Temperature at gas turbine outlet (K). The internal power consumed by the compressor is determined by equation:
𝑊𝐶.𝑖 = 𝐺𝐴 × 𝐻𝐶 where:
𝐺𝐴 – The air mass flow through compressor (kg/s). Generated internal power while extraction of waste gases in the gas turbine:
𝑊𝐺𝑇.𝑖 = 𝐺𝐺𝑇 × 𝐻𝐺𝑇 where:
𝐺𝐺𝑇 – The waste gases mass flow through gas turbine (kg/s). Internal efficiency of the gas turbine can be evaluated by equation:
𝜂𝑖 =
𝑊𝐺𝑇.𝑖 ― 𝑊𝐶.𝑖 𝑄𝐹
=
𝑊𝐺𝑇.𝑖 ― 𝑊𝐶.𝑖 𝐺𝐹 × 𝐿𝐻𝑉
where:
𝑄𝐹 – Heat input to the gas turbine (kW); 𝐺𝐹 – Fuel mass flow (kg/s); LHV – Lower heating value of the fuel (kJ/kg). Electrical efficiency of the gas turbine can be determined by equation:
𝜂𝑒 =
𝑊𝐺𝑇 𝑄𝐹
𝑊𝐺𝑇
= 𝐺𝐹 × 𝐿𝐻𝑉
where:
𝑊𝐺𝑇 – Gas turbine power output (kW). 3.1.2. Heat recovery steam generator Heat output of the HRSG, i.e. accepted heat by water and steam in HRSG, is equal to (fig. 1):
𝑄𝐻𝑅𝑆𝐺 = 𝑄0 ― 𝑄𝐹𝑊, It is also possible to determine the heat output of the HRSG by equation:
𝑄𝐻𝑅𝑆𝐺 = 𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝑊) = 𝐷0 × (ℎ0 ― ℎ𝐹𝑊), where:
𝐼𝐺𝑇 – Enthalpy at gas turbine outlet (kJ/kg); 𝐼𝑊 – Enthalpy of HRSG waste gases (kJ/kg); 6
Journal Pre-proof 𝐷0 – HRSG steam output (kg/s); ℎ0 – HRSG superheated steam enthalpy (kJ/kg); ℎ𝐹𝑊 – HRSG feed water enthalpy (kJ/kg). It is possible to write the heat balance equation for each heating surfaces of HRSG. For common heating surfaces of superheater and evaporating surfaces the heat balance equation is written as following:
𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝐸) = 𝐷0 × (ℎ0 ― ℎ𝐸), where:
𝐼𝐸 – Waste gases enthalpy at evaporating surfaces inlet (kJ/kg); ℎ𝐸– Water enthalpy at evaporating surfaces inlet (kJ/kg); Waste gases enthalpy at evaporating surfaces inlet 𝐼𝐸 is defined by equation:
𝐼𝐸 = 𝑐𝑝.𝑔 × (𝑡𝑠 + 𝛿𝑡𝐸), where:
𝑡𝑠– Saturated steam temperature in appropriate pressure (ºC); 𝛿𝑡𝐸 – Temperature difference between waste gases and water at evaporating surfaces inlet (ºC). The HRSG steam mass flow is defined by equation:
𝐷0 =
𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝐸)
(ℎ0 ― ℎ𝐸)
.
Heat balance equation for economizer:
𝐺𝐺𝑇 × (𝐼𝐸 ― 𝐼𝑊) = 𝐷0 × (ℎ𝐸 ― ℎ𝐹𝑊), It is possible to define the HRSG waste gases temperature:
𝑡𝑤 = 𝑡𝐸 ―
𝐷0(ℎ𝐸 ― ℎ𝐹𝑊) 𝑐𝑝.𝑔 × 𝐺𝐺𝑇
,
where:
𝑡𝐸 - Waste gases temperature at evaporating surfaces inlet (ºC). Waste gases temperature at evaporating surfaces inlet 𝑡𝐸 is defined according to the enthalpy Now it is possible to define the HRSG efficiency (utilization coefficient):
𝜂𝐻𝑅𝑆𝐺 =
(𝐼𝐸 ― 𝐼𝑊) (𝐼𝐸 ― 𝐼𝐴) ,
where:
𝐼𝐴 – The waste gases enthalpy at ambient temperature (kJ/kg). 3.1.3. Gas turbine CHP The CHP electrical efficiency is defined by equation:
𝑊𝐺𝑇
𝜂𝑒 = 𝐺𝐸 × 𝐿𝐻𝑉 , 𝐹
where:
𝐺𝐸𝐹– The fuel mass flow for electric energy generation (kg/s). The CHP heat production efficiency is defined by equation:
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𝐼𝐸 .
Journal Pre-proof 𝜂𝑒 =
𝑄𝐻𝑅𝑆𝐺 𝐺𝐻 𝐹 × 𝐿𝐻𝑉
,
where:
𝐺𝐻 𝐹 – The fuel mass flow for heat generation (kg/s). The CHP heat utilization factor is defined by equation:
𝜂𝑒 =
𝑊𝐺𝑇 + 𝑄𝐻𝑅𝑆𝐺 𝐺𝐸𝐹 × 𝐿𝐻𝑉
.
3.2. Absorption chiller Single-effect absorption cooling system is used in the study. The single-effect cooling system is based on the basic absorption cycle, which contains a single absorber and generator (Fig. 2). In the generator 1 a heat source produces ammonia vapor from a strong ammonia solution. As noted above, in this paper three types of heat sources are considered for providing the generator: gas turbine waste gases, HRSG waste gases and solar energy. The vapor (refrigerant) enters the condenser 2 where it is condensed. After cooling, it goes through a valve 3 and pressure and temperature are reduced, then evaporated in the evaporator 4 at low temperature and pressure. The cooled refrigerant is absorbed in absorber 5 by a weak solution, which returns from generator 1 through valve 7. A rich mixture created in absorber is pumped by pump 6 and returned into the generator 1. The absorption process is exothermic, the absorber is chilled with water.
Fig. 2. Schematic diagram of the absorption chiller: 1 – Generator; 2 – Condenser; 3 – Valve; 4 – Evaporator; 5 – Absorber; 6 – Pump; 7 – Valve. Thermodynamic models for various types of absorption chillers have been developed by multiple researchers and described in various textbooks. Therefore, as the paper is intended to study the gas turbines CHP behaviors, thermodynamic models of absorption chillers are not described here as well as they are not opened in computer simulation models. Nevertheless, widely used absorption chiller
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Journal Pre-proof thermodynamic models were accepted in conducting the study. The absorption chiller efficiency 60 % had been accepted for computer simulations. 3.3. Parabolic trough Several studies on modelling and simulation of parabolic trough collectors have been carried out up to date. Currently parabolic trough collectors with thermal oil are most widely used concentrating solar power technology. The thermal oil serves as a heat transfer fluid in the parabolic through. The heat transfer fluid runs through the tube to absorb the concentrated sunlight. At the moment it is possible to get a temperature of up to ≈ 395 °C in the parabolic through. The solar field with parabolic trough collectors was modeled to generate power for the absorption chiller. For modeling Ebsilon®Professional software was used. Several factors – field efficiency, optical efficiency, net aperture area, effective heat, incident angle correction, shading and wind were taken into consideration while modelling. The size of the field was determined by the number of collectors and the data of the collector model. The heat input into the fluid is defined by equation: 𝑄𝐸𝑓𝑓 = 𝑚1 × (ℎ2 ― ℎ1). where: 𝑚1 – Mass flow of the fluid (kg/s); ℎ1 – Enthalpy of the inlet fluid (kJ/kg); ℎ2 – Enthalpy of the outlet fluid (kJ/kg). The available heat input depends on the solar heat input and the thermal losses of the receivers and the field piping: 𝑄𝐴𝑣𝑎𝑖𝑙 = 𝑄𝑆𝑜𝑙𝑎𝑟 ― 𝑄𝐿𝑜𝑠𝑠 ― 𝑄𝑃𝑖𝑝𝑒. where: 𝑄𝐴𝑣𝑎𝑖𝑙 – Available heat input (kW); 𝑄𝑆𝑜𝑙𝑎𝑟 - Solar heat input (kW); 𝑄𝐿𝑜𝑠𝑠 – Thermal losses of the receivers (kW); 𝑄𝑃𝑖𝑝𝑒 - Thermal losses of the field piping (kW). Due to limited capacities of heat consumption the solar field will often not use all of the available solar energy. Part of the fields will be defocused. The effective heat generated in the solar field is therefore reduced by the fraction which is lost due to defocused collectors: 𝑄𝐸𝑓𝑓 = 𝑄𝑆𝑜𝑙𝑎𝑟 × 𝑅𝐹𝑜𝑐𝑢𝑠 ― 𝑄𝐿𝑜𝑠𝑠 ― 𝑄𝑃𝑖𝑝𝑒 , where: 𝑅𝐹𝑜𝑐𝑢𝑠 –Actual focus state of the collector field. Value of 𝑅𝐹𝑜𝑐𝑢𝑠 is determined by the selected power limitation method. The solar input 𝑄𝑆𝑜𝑙𝑎𝑟 is determined by the equation: 𝑄𝑆𝑜𝑙𝑎𝑟 = 𝐷𝑁𝐼 × 𝐴𝑁𝑒𝑡 × 𝐹𝑂𝑝𝑡_0 × 𝑘𝐼𝐴 × 𝐹𝑆ℎ.𝐿 × 𝐹𝐸𝑛𝑑.𝐿 × 𝐹𝑊𝑖𝑛𝑑 × 𝐹𝐶𝑙𝑒𝑎𝑛 × 𝐹𝐴𝑣𝑎𝑖𝑙 , where: 𝐷𝑁𝐼 – Direct normal irradiance (W/m2);
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Journal Pre-proof 𝐴𝑁𝑒𝑡– Net aperture area (m2); 𝐹𝑂𝑝𝑡_0– Peak optical efficiency; 𝑘𝐼𝐴– Incident angle correction; 𝐹𝑆ℎ.𝐿–Factor to include shading losses; 𝐹𝐸𝑛𝑑.𝐿–Factor to correct end loss effects determined from model; 𝐹𝑊𝑖𝑛𝑑–Factor to include optical losses due to wind impact; 𝐹𝐶𝑙𝑒𝑎𝑛–Factor to correct for actual mirror cleanliness; 𝐹𝐴𝑣𝑎𝑖𝑙–Field availability. The net aperture area was calculated according to the following equation: 𝐴𝑁𝑒𝑡 = 𝑁𝐶𝑜𝑙𝑙 × 𝐿𝐶𝑜𝑙𝑙 × 𝐴𝐶𝑜𝑙𝑙 × 𝐾𝐴𝑐𝑡𝑖𝑣𝑒, where: 𝑁𝐶𝑜𝑙𝑙 –Number of collectors (pcs); 𝐿𝐶𝑜𝑙𝑙 – Length of a collector array (m); 𝐴𝐶𝑜𝑙𝑙 –Aperture width of a collector array (m); 𝐾𝐴𝑐𝑡𝑖𝑣𝑒–Optical active portion of aperture (coefficient). 4. Analyses of the Tashkent gas turbine CHP Hitachi Gas turbine H27 with HRSG was installed in Tashkent CHP in 2012. The gas turbine has electric power output 28.1 MW and electrical efficiency 34.2 %. Exhaust gases of the gas turbine are entered into HRSG at temperature 552 ºC. HRSG serves to generate a steam with pressure 3.6 MPa and temperature 416 ºC for a heat supply system of Tashkent city. At nominal loadings a mass flow of the generated steam is 47 t/h. HRSG is natural circulation type with drum. HRSG consists of economizer, evaporating surfaces and superheater. Feed water with pressure 4 MPa and temperature 70 ºC is entered into the economizer of the HRSG. Waste gases of HRSG are thrown out into the environment at temperature 146 ºC. Experience of exploitation of the gas turbine in Uzbekistan climate conditions had shown following problems: -
Electric power output of the gas turbine is reduced down to 24.1 MW in summer period owing to high ambient air temperature (up to +45 ºC).
-
Electrical efficiency of the gas turbine falls down to 32 % in hot periods.
-
Electric power output of the gas turbine is decreased down to 17-18 MW during clogging periods of the air filters owing to impurities in content of the inlet air.
In order to analyze the dependency of power output and efficiency from ambient temperature a computer model of the existing H27 gas turbine CHP was developed in Ebsilon®Professional software. The simulation model in ambient temperature +15 ºC is shown in fig. 3. Energy-temperature diagram of the HRSG in ambient temperature +15 ºC is shown in fig. 4.
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Journal Pre-proof
Fig. 3. Simulation model of H27 gas turbine CHP developed in Ebsilon®Professional software. ETA – efficiency of appropriate parts.
Fig. 4. Energy-temperature diagram of the HRSG. Calculations in the developed simulation model were done for ambient temperature range from –15 ºC up to +45 ºC. Received results certify, the power output of the gas turbine reaches 31.8 MW while ambient temperature is –15 ºC, simultaneously electrical efficiency of the gas turbine rises up to 35.9 %. Electric
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Journal Pre-proof power output and electrical efficiency of the gas turbine are decreased down to 24.126 MW and 32.0 % respectively while ambient temperature is +45 ºC (Table 1). Table 1. Gas turbine output and electrical efficiency, CHP efficiency as function of the ambient temperature Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine output, MW
31.8
31.2
30.5
29.9
29.3
28.7
28.1
27.4
26.7
26.1
25.4
24.8
24.1
Gas turbine electrical efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
33.9
33.5
33.1
32.8
32.4
32.0
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
81.6
81.8
82.1
82.3
82.6
82.8
Fig. 5 shows the relative power output and electrical efficiency of the gas turbine and CHP efficiency due to the ambient temperature. The increase in the ambient temperature has a slightly positive effect on the CHP efficiency. Because the increased temperature in the gas turbine exhaust enhances the efficiency of the HRSG, it more than compensates for the reduced efficiency of the gas turbine. Temperature-entropy diagram for H27 gas turbine for ambient temperatures -15C, +15 C and +45 C is presented in fig. 6. It is possible to see in the diagram, that gas turbine performance in low ambient temperatures is bigger than in high ambient temperatures.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 5. Gas turbine output and electrical efficiency, CHP efficiency as function of the ambient temperature
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Fig. 6. Temperature-entropy diagram for H27 gas turbine for ambient air temperatures -15 C, +15 C and +45 C. In order to eliminate the problems of decreasing in the gas turbine power output and electrical efficiency we are proposing to use an absorption chiller. The absorption chiller has to lower the gas turbine inlet air temperature. 5. Simulation model of the gas turbine CHP with absorption chiller and solar field The developed simulation model of the gas turbine CHP was upgraded with including an absorption chiller and a field of solar energy (Fig. 7). As well as heat exchangers for heat extraction on the waste ducts of the gas turbine and HRSG are entered. The absorption chiller serves for cooling the gas turbine inlet air. The absorption chiller consumes a heat to drive the cooling process. Here we consider three heat sources: -
Heat of the gas turbine waste gases;
-
Heat of the HRSG waste gases;
-
Heat of the solar energy.
In the simulation model in order to change the heat sources the absorption chiller can easily be connected to pipelines of each of three heat sources. The absorption chiller is switched off while the ambient air temperature ranges between – 15ºC up to +15 ºC. Main equations, which are used in the computer simulations, were described in 3rd section of the paper.
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Fig. 7. Simulation model of the gas turbine CHP with absorption chiller powered by gas turbine waste gases. Ambient temperature is +45 ºC. ETA – efficiency of appropriate parts. 6. Results 6.1. Gas turbine CHP with absorption chiller powered by gas turbine waste gases The absorption chiller powered by gas turbine waste gases was entered in the simulation model. Calculations were carried out for ambient air temperature from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is shown in Fig. 7. Main data of the simulations are given in table 2.
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Journal Pre-proof Table 2. Main technical-economic data of H27 gas turbine CHP with the absorption chiller powered by gas turbine waste gases Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine power output, MW
31.8
31.2
30.6
29.9
29.3
28.7
28.1
28.1
28.1
28.1
28.1
28.1
28.1
Gas turbine efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
34.2
34.2
34.2
34.2
34.2
34.2
HRSG efficiency, %
74.5
74.5
74.5
74.5
74.5
74.5
74.5
73.6
72.8
71.9
71
70.1
69.1
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
80.2
79.1
77.9
76.7
75.6
74.4
Heat extraction for absorption chiller, MW
0
0
0
0
0
0
0
0.71
1.42
2.12
2.83
3.45
4.26
HRSG steam mass flow, t/h
47.3
47.2
47.2
47.1
47.1
47
47
45.8
44.7
43.5
42.3
41.2
40
HRSG inlet gas temperature, °C
552
552
552
552
552
552
552
545
538
531
523
516
509
HRSG waste gas temperature, °C
146
146
146
146
146
146
146
149
152
154
157
160
163
Dependency graph of gas turbine electrical efficiency, gas turbine power output and gas turbine CHP efficiency from ambient temperature while including an absorption chiller powered by gas turbine waste gases is shown in fig. 8. In this case, HRSG steam mass flow is reduced down to 40 t/h owing to lower temperature of the entered waste gases. As well as temperature of HRSG waste gases are raised up to 162.8 ºC, which leads to decrease CHP efficiency down to 74.4 %. It is seemable in the table 2 and fig. 8, that using the gas turbine waste gases in absorption chiller is not economical profitable, because CHP efficiency is reduced down to 74.4 % during ambient temperature +45 ºC, although gas turbine performance is maintained in nominal value.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 8. Gas turbine output and efficiency, CHP efficiency as function of the ambient temperature while including an absorption chiller powered by gas turbine waste gases. 6.2. Gas turbine CHP with absorption chiller powered by HRSG waste gases In the simulation model an absorption chiller powered by HRSG waste gases was included and calculations were carried out for ambient temperature from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is shown in Fig. 9. Main results are given in table 3, as well as in fig. 10 as a dependency graph.
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Fig. 9. Simulation model of the gas turbine CHP with absorption chiller powered by HRSG waste gases in ambient temperature +45 ºC. ETA – efficiency of appropriate parts.
18
Journal Pre-proof Table 3. Main technical-economic data of H27 gas turbine CHP while including the absorption chiller powered by HRSG waste gases Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine power output, MW
31.8
31.2
30.6
29.9
29.3
28.7
28.1
28.1
28.1
28.1
28.1
28.1
28.1
Gas turbine efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
34.2
34.2
34.2
34.2
34.2
34.2
HRSG efficiency, %
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
81.4
81.4
81.4
81.4
81.4
81.4
Heat extraction for absorption chiller, MW
0
0
0
0
0
0
0
0.71
1.42
2.13
2.84
3.46
4.26
HRSG steam mass flow, t/h
47.3
47.2
47.2
47.1
47.1
47
47
47
47
47
47
47
47
HRSG inlet gas temperature, °C
552
552
552
552
552
552
552
552
552
552
552
552
552
HRSG waste gas temperature, °C
146
146
146
146
146
146
146
146
146
146
146
146
146
Waste gas temperature behind heat extraction, °C
146
146
146
146
146
146
146
139
131
123
115
107
100
According to the data, entered into table 3, extraction of 4260 kW heat from HRSG waste gases doesn’t make any influence to HRSG performance. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal design values. Just temperature of the HRSG waste gases is reduced down to 100 ºC.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 10. Gas turbine output, gas turbine efficiency, CHP efficiency as function of the ambient air temperature while including an absorption chiller powered by HRSG waste gases
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Journal Pre-proof 6.3. Gas turbine CHP with solar powered absorption chiller In the simulation model the H27 gas turbine CHP was upgraded with including a solar powered absorption chiller. The solar field consists of system of parabolic through, circulating pump for the thermal oil, as well as a heat exchanger. The heat exchanger is intended to transfer a heat from the thermal oil to a heat transfer fluid of absorption chiller. Solar data of the Parkent district [22], which is located in 60 km from Tashkent city, was used for simulations: -
Annual direct normal irradiation (DNI) is 1753 kWh/m²a;
-
Mean daily DNI from ground measurement – 5 kWh/m2;
-
Mean daily global horizontal irradiation (GHI) from ground measurement – 4.6 kWh/m2.
Monthly DNI for Parkent district varies from 55 kWh/m2 in December up to 247 kWh/m2 in July. During May-September months it is approx. 200-247 kWh/m2, i.e. daily DNI is approx. 6.5÷7.9 kWh/m2. In April and October months a monthly DNI is approx. 147 kWh/m2 and daily DNI is approx. 4.7 kWh/m2. Although during could seasons monthly DNI is approx. 2 kWh/m2, mean daily DNI 6.5 kWh/m2 was accepted for simulations, because no need in inlet air cooling for gas turbine during could seasons, as ambient air temperature is – 25 ºC …+15 ºC. Parabolic trough collectors with 50 m length, 5.76 m gross aperture width and 1.71 m focal length have been used while simulations. Number of parabolic trough collectors – 28 pcs, total net aperture area – 8064 m2. Parabolic trough collectors are able to produce a heat up to 4260 kW. Calculations were carried out for ambient temperature range from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is presented in Fig. 11.
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Journal Pre-proof
Fig. 11. Simulation model of the gas turbine CHP with solar powered absorption chiller in ambient temperature +45 ºC. ETA – efficiency of appropriate parts. Difference of this scheme from above considered schemes is absence of additional heat exchangers in waste gas ducts for heat extraction as well as presence of the solar field. The heat demand in the absorption chiller from solar energy is the same as in the previous schemes. Comparison of the considered three upgrading schemes notify, that no need in additional field of solar energy, because of HRSG waste gases have enough heat to provide cooling process in the absorption chiller. But however, the solar field for such purpose can be technical achievable and economic profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than approx. +120 ºC.
7. Conclusions The present study deals with evaluation of efficiency improving opportunity in existing gas turbine CHP by means of the inlet air cooling. Analyses of the existing H27 gas turbine CHP had shown, the gas turbine power output is decreased from 28.1 MW down to 24.1 MW and the electrical efficiency is decreased from 34.2 % down to 32.0 % while ambient temperature rises from +15 ºC up to +45 ºC.
22
Journal Pre-proof The absorption chiller was proposed to cool the inlet air. To drive the cooling process the absorption chiller was analyzed to use three types of heat source: the gas turbine waste gases, the HRSG waste gases, the solar energy. According to results of the study, 4260 kW heat is required in absorption chiller in order to reduce the inlet air temperature from +45 ºC down to +15 ºC. In order to provide the absorption chiller with the necessary heat additional heat exchangers were proposed to install on the gas turbine and HRSG waste gas ducts. The results of the study notify, that using the heat of gas turbine waste gases in absorption chiller is not economical profitable, because CHP efficiency is reduced down to 74.4 % during ambient temperature +45 ºC, although gas turbine performance is maintained in nominal value. Technical-economical attractive is the scheme of using HRSG waste gases in the absorption chiller. According to the proposed scheme (fig. 9), extraction of 4260 kW heat from HRSG waste gases doesn’t make any influence to HRSG performance. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal design values. In order to provide the absorption chiller with 4260 kW heat of solar energy, 28 pcs parabolic trough collectors with total net aperture area 8064 m2 are required. Parabolic through collectors with 50 m length, 5.76 m gross aperture width and 1.71 m focal length were used while simulations. As HRSG waste gases have enough heat to provide cooling process in the absorption chiller, so no need in additional solar field. But however, the solar field for such purpose can be economic profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than approx. +120 ºC.
Acknowledgements Author gratefully acknowledges the support of Dresden fellowship program [23] of the Technische Universitaet Dresden (Germany). As well author thanks to Prof. Dr.-Ing. Uwe Gampe, Head of Chair of Thermal Power Machinery and Plant of the Technische Universitaet Dresden, for consultations in carrying out the current research project.
References [1] On the current state and prospects for energy development. Retrieved June 6, 2018, from
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Journal Pre-proof [6] Kumar P. N., A Rama Krishna, G. R. Vidya Sagar Raju. Comparative analysis on performance of a gas turbine power plant with a spray cooler. // International Journal of scientific research and management (IJSRM), 2013, Volume 1, Issue 7, pp. 354-358. [7] Egware H. O., Obanor A. I. Energy cost analysis of incorporating air inlet cooling system in Omotosho phase 1 Thermal power plant. // Journal of Energy Technologies and Policy, ISSN 2224-3232 (Paper), ISSN 2225-0573 (Online), Vol.3, No.7, 2013, pp. 29-33. [8] Ehyaei M.A., Mozafari A., Alibiglou M.H. Exergy, economic & environmental (3E) analysis of inlet fogging for gas turbine power plant. Energy 2011; 36: 6851-6861. [9] Ehyaei M. A., Tahani M., Ahmadi P., Esfandiari M. Optimization of fog inlet air cooling system for combined cycle power plants using genetic algorithm. Applied Thermal Engineering 2015; 76: 449461. [10] Kakaras E, Doukelis S, Karellas S. Compressor intake-air cooling in gas turbine plants. Energy 2004; 29: 2347-2358. [11] Salvi D., P. Pierpaoli. Optimization of inlet air cooling systems for steam injected gas turbines. Int. J. Thermal Sci. 2002; 41: 815-822. [12] Bassily, A.M. Performance improvements of the intercooled reheat recuperated gas-turbine cycle using absorption inlet-cooling and evaporative after-cooling. Applied Energy 2004; 77: 249-272. [13] Ameri M, Hejazi S. H. The study of capacity enhancement of the Chabahar gas turbine installation using an absorption chiller. Applied Thermal Engineering 2004; 24: 59-68. [14] Boonnasa S, Namprakai P, Muangnapoh T. Performance improvement of the combined cycle power plant by intake air cooling using an absorption chiller. Energy 2006; 31: 2036-2046. [15] Dawoud B., Y. H. Zugirat, J. Bortmany. Thermodynamic assessment of power requirements and impact of different gas-turbine inlet air cooling techniques at two different locations in Oman. Applied Therm. Eng., 2005. 25: 1579-1598. [16] Hosseini R., Beshkani A., Soltani M. Performance improvement of gas turbines of Fars (Iran) combined cycle power plant by inlet air cooling using a media evaporative cooler. Energy Convers. Manage.: 2007; 48: 1055-1064. [17] Mohapatra A. K., Sanjay. Comparative analysis of inlet air cooling techniques integrated to cooled gas turbine plant. Journal of the Energy Institute 2015; 88: 344-358. [18] Ehyaei M. A., Hakimzadeh S., Enadi N., Ahmadi P. Exergy, economic and environment (3E) analysis of absorption chiller inlet air cooler used in gas turbine power plants. International Journal of Energy Research 2012; 36: 486–498. [19] Arabkoohsar A., Andresen G.B. A smart combination of a solar assisted absorption chiller and a power productive gas expansion unit for cogeneration of power and cooling. Renewable Energy 115 (2018) 489-500. [20] Agyenim F., Knight I., Rhodes M. Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store. Solar Energy 84 (2010) 735–744. [21] Marc O., Lucas F., Sinama F., Monceyron E. Experimental investigation of a solar cooling absorption system operating without any backup system under tropical climate. Energy and Buildings 42 (2010) 774–782.
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Journal Pre-proof [22]
UZB TA-8008. Republic of Uzbekistan: Solar Energy Development. Roadmap to Solar Energy
Development.
April
2014.
Retrieved
June
30,
2019
from
[23]
Dresden
Fellowship
Program.
Retrieved
June
30,
2019
from
dresden/profil/exzellenz/zukunftskonzept/tud-people/dresden-fellowship-programm>
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Journal Pre-proof Highlights - Increase in the ambient air temperature reduces the gas turbine output and efficiency; - Absorption chiller is proposed to cool the inlet air for gas turbine CHP; - Waste gases of a heat recovery steam generator drive cooling process in the absorption chiller; - Solar energy powered absorption chiller is used for gas turbine inlet air cooling.
Erkinjon Matjanov PII:
S0360-5442(19)32320-5
DOI:
https://doi.org/10.1016/j.energy.2019.116625
Reference:
EGY 116625
To appear in:
Energy
Received Date:
07 January 2019
Accepted Date:
23 November 2019
Please cite this article as: Erkinjon Matjanov, Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP, Energy (2019), https://doi.org/10.1016/j.energy.2019.116625
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Gas turbine efficiency enhancement using absorption chiller. Case study for Tashkent CHP PhD Erkinjon Matjanov, Scientific-Technical Center with Constructional Bureau and Experimental Production of Academy of Sciences of Uzbekistan republic. Durmon yuli str. 33, Tashkent 100125,Uzbekistan Abstract Ambient conditions affect the performance of the gas turbine cycle. Calculations of 28.1 MW gas turbine in Tashkent CHP show, the gas turbine power output is decreased down to 24.1 MW while ambient temperature is +45 ºC, simultaneously electrical efficiency is reduced from 34.2 % to 32.0 %. An absorption chiller is proposed to cool the inlet air. To drive the cooling process the absorption chiller is analyzed to use three types of a heat source: gas turbine waste gases, HRSG waste gases, solar energy. 4260 kW heat is required to cool the inlet air from +45 ºC to +15 ºC. Obtained results show, that using the heat of gas turbine waste gases in absorption chiller could not be economical profitable, because CHP efficiency is reduced from 81.4 % to 74.4 % during ambient temperature +45 ºC. Technical-economical attractive is the scheme of using HRSG waste gases in absorption chiller. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal values. In order to provide the absorption chiller with solar energy heat, parabolic trough collectors with total net aperture area 8064 m2 are required. Analyses show, when HRSG waste gases have enough heat to provide cooling process, so no need in additional solar field. But however, the solar field can be economical profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than +120 ºC. Keywords Gas turbine efficiency; Inlet air cooling; Absorption chiller integrated gas turbine; Inlet air cooling by solar energy; Solar powered absorption chiller. Highlights - Increase in the ambient air temperature reduces the gas turbine output and efficiency; - Absorption chiller is proposed to cool the inlet air for gas turbine CHP; - Waste gases of a heat recovery steam generator drive cooling process in the absorption chiller; - Solar energy powered absorption chiller is used for gas turbine inlet air cooling. Abbreviations CCPP
Combined cycle power plant
CHP
Combined heat and power
CSP
Concentrated solar power
1
Journal Pre-proof DNI
Direct normal irradiation
GHI
Global horizontal irradiation
GT
Gas turbine
HRSG
Heat recovery steam generator
JSC
Joint stock company
TPP
Thermal electric power plant
ETA
Efficiency of appropriate parts.
1. Introduction In Uzbekistan “Uzbekenergo” JSC performs centralized energy supply of the economy and population, as well as heat supply to industrial and household consumers of the country. Up to date, the total installed power output of Uzbekistan power plants exceeds 14100 MW, 85.8% of which belongs to thermal electric power plants (TPP) [1]. Main part of the TPPs are based on Rankin steam cycle and were built during 60th–80th of XX century, they are morally and physically aged. Government of Uzbekistan is paying a great attention to upgrading energy sector of the country. Particularly, almost all of the existing TPPs are being equipped with new CCPPs as well as new Turakurgan CCPP (900 MW) is under construction [2]. During last years one gas turbine CHP and four CCPPs were entered into operation: - 28.1 MW gas turbine with HRSG in Tashkent CHP; - 478 MW CCPP in Navoiy TPP; - Two CCPPs of 450 MW each in Talimarjan TPP; - 370 MW CCPP in Tashkent TPP. But unfortunately, new CCPPs can’t achieve design parameters: their power output and electrical efficiency are lower than design values. Indeed, the gas turbine in Tashkent CHP is being operated with power output of 22 MW instead of 28.1 MW, its efficiency is also lower. CCPP in Navoiy TPP in summer periods reaches 370 MW power output instead of 478 MW. After analyses of state of the art of CCPPs in Uzbekistan following problems were determined: -
High ambient temperature leads to decrease gas turbine performance and electrical efficiency;
-
Dust pollutions in content of air lead to clogging gas turbine air filters that also decreases the gas turbine performance and electrical efficiency.
In order to eliminate the mentioned problems, caused owing to high ambient temperature, we are proposing to equip gas turbines with absorption chillers. Three types of heat sources are being analyzed to power the absorption chiller: -
Heat of gas turbine waste gases;
-
Heat of HRSG waste gases;
-
Heat of solar energy.
2
Journal Pre-proof 2. State of the art of the problem Ambient conditions affect the performance of the Brayton cycle. Especially, air temperature and pressure are the most crucial ambient conditions. Relative humidity has only a minor influence, but becomes more important if the cooling concept is equipped with an evaporative cooling tower. There are three reasons why ambient air temperature has a significant influence on the power output and electrical efficiency of the gas turbine [3, 4]: -
Increasing the ambient air temperature reduces the density of the air and consequently reduces the air mass flow into the compressor as constant volume engine. This is the main reason for changes in the gas turbine power output.
-
The specific power consumed by the compressor increases proportionally to the air inlet temperature without a corresponding increase in the output from the turbine part.
-
As the air temperature rises and the mass flow decreases, the pressure ratio within the turbine is reduced. Due to the swallowing capacity of the turbine section and the reduced mass flow, the pressure at the turbine inlet is reduced. This leads to a lower pressure ratio within the turbine, and applies inversely, of course, to the compressor; however, because its output is less than that of the turbine, the total balance is negative.
The exhaust gas temperature rises as the ambient air temperature increases because the turbine pressure ratio is reduced, although the gas turbine inlet temperature remains constant. The result is a decrease in the gas turbine electrical efficiency and power output while the ambient air temperature rises. However, the effect on the performance of the steam part of the CHP is less because a higher exhaust gas temperature improves HRSG performance. At the moment a variety of options have been developed to particularly account for the inlet air cooling, such as: -
Evaporative cooling.
-
Fogging.
-
Evaporation compressor cooling (over/high fogging).
-
Chiller.
Results of studies in field of the gas turbine inlet air cooling were announced by multiple researchers. As well as several authors announced results of case studies on gas turbine performance improvement using absorption chillers. Hall et al. [5] documented the performance of a 36 MW gas turbine plant in which a chilled water-based storage refrigeration system was tasked with cooling inlet air. The cooling system was able to reduce the inlet air temperature from +35 C down to +7 C, enhancing plant performance by 10%. Kumar at all [6], Edware at all [7] had investigated cooling the inlet air by spray cooler. Ehyaei et all [8] had investigated the effects of inlet fogging system on the first and second law efficiencies in sample of Shahid Rajaee power plant (Iran). New function was proposed for system optimization that includes the social cost of air pollution for power generating systems. It is concluded that using inlet fogging system, increases the average output power production, the first and the second law efficiencies during three months of year (June, July and August) by 7%, 5.5% and 6% respectively. Ehyaei et all [9] have conducted a comprehensive thermodynamic modeling of CCPP and have investigated effects of gas turbine inlet fogging system on the first and second law efficiencies and net
3
Journal Pre-proof power outputs of CCPP. A new objective function based on the first law efficiency and the energy cost which includes the external social cost of air pollution was considered for the optimization purpose. As well the effect of variation in inlet temperature had been investigated. It was concluded that using inlet air cooling system for the CCPP system and its optimization results in an increase in the average power output, first and second law efficiencies by 17.24%, 3.6% and 3.5%, respectively, in summer months of year. Absorption refrigeration also had been paid more attention as it can utilize low grade energy of the exhaust gas instead of electrical energy to provide the necessary cooling. The first law analysis of compressor inlet air cooling using lithium bromide-water absorption refrigeration had been done by Karakas et al. [10], Salvi and Pierpaoli [11], Bassily [12], Amery and Hejazi [13] and Boonnasa et al. [14], Dawoud et al. [15] and Hosseini et al. [16]. Hosseini et al. [16] have reported the effect of inlet air cooling using other methods such as evaporative cooling, compression refrigeration and aqua-ammonia absorption refrigeration. All these studies show that decreasing inlet air temperature results in an increase of the power output and electrical efficiency of the gas turbine. However, more attractive papers are shortly described below. Kakaras et al. [10] had carried out a computer simulation on integration of absorption chiller to reduce the inlet air temperature in gas turbine. IPSEPRO software had been used for computer simulations. A simple gas turbine cycle and a CCPP were studied. The absorption chiller cooling system had shown a higher gain in power output and electrical efficiency than evaporative cooling for the simple gas turbine cycle. As well as the results for CCPP also had shown that the absorption chiller considerably increases the power output. Ameri and Hejazi [13] had reported about carried out feasibility studies on installing an absorption chiller for cooling the inlet air for gas turbines of Chabahar power plant. According to results of the feasibility studies, an average power output can be increased by 11.3% while a maximum power augmentation 2.4 MW is attainable. The electric energy generation is increased by 14 000 MWh/year. The payback period of capital investments is equal to 4.2 years. Boonnasa et al. [14] analyzed possibilities of improving the power output of CCPP in Bangkok by reducing intake air temperature down to +15 C. They proposed an absorption chiller to cool the inlet air. Authors reported, that it is possible to improve power output of the gas turbine by 10.6% and the CCPP by 6.24%. Payback period of capital investments – 3.81 years. Mohapatra and Sanjay [17] made a comparison of two different methods of inlet air cooling (evaporative cooing and vapor compression cooling) integrated to a cooled gas turbine power plant. A parametric study of the effect of pressure ratio, compressor inlet temperature, turbine inlet temperature, inlet temperature ratio, ambient relative humidity and ambient temperature on performance parameters of plant had been carried out. The inclusion of inlet air cooling systems had improved specific work and efficiency of the gas turbine. This improvement is higher at higher ambient temperature and lower ambient relative humidity. Ehyaei et all [18] had made an exergy, economic and environmental analyses for a 4900 kW absorption chiller integrated 159 MW gas turbine unit. The effect of using absorption chiller in gas turbine power plants for Tabas and Bushehr (Iran) climate conditions is conducted. Results had shown that using
4
Journal Pre-proof absorption chiller in gas turbines leads to increase the power output on 11.5% and 10.3% in Tabas and Bushehr respectively. Moreover, the second law efficiency was increased to 22.9 and 29.4% for Tabas and Bushehr respectively. The gas turbine power increases from 137 MW to 153 MW during the hottest month (August) in Bushehr when the inlet air was cooled from +37 C down to +15 C. Moreover, efficiency rose from 33.4 % to 43.2 %. Announced results of researches and feasibility studies on solar powered absorption chillers in several branches of industry [19, 20, 21] testify about attainability of realization and reliability of the solar powered absorption chillers. Although there are many results were announced in field of inlet air cooling using absorption chiller, in this paper we investigate the possibilities of cooling the inlet air using an absorption chiller, where gas turbine waste gases, HRSG waste gases and a solar energy are compared with each other to be used as a heat source. As well as we consider performance data of the gas turbine and the gas turbine CHP. It is proposed to lower the inlet air temperature from +45 C down to + 15 C as the ambient temperature in Tashkent reaches up to 45 C in summer. 3. Modelling of governing equations 3.1. Gas turbine CHP thermodynamic model Tashkent gas turbine CHP consists mainly from Hitachi H27 gas turbine and HRSG. Consequently, we consider thermodynamic model for such system. Energy flow diagram of the gas turbine CHP without supplementary firing is presented in Fig. 1.
Fig. 1. Energy flow diagram of the gas turbine CHP without supplementary firing: 1 – Compressor; 2 – Combustion chamber; 3 – Gas turbine; 4 – HRSG; 5 – Superheater; 6 – Evaporating surfaces; 7 – Economizer; 𝑊𝐺𝑇 – Gas turbine power output; 𝑄𝐴 – Ambient air heat; 𝑄𝐹 – Heat input into the gas
𝑄𝐿𝐺𝑇 – Gas turbine heat losses; 𝑄𝐺𝑇 – Gas turbine waste gases heat; 𝑄0 – HRSG steam heat; 𝑄𝐹𝑊 – Feed water heat; 𝑄𝐿𝐻𝑅𝑆𝐺 – HRSG heat losses; 𝑄𝑊 – HRSG waste gases heat.
turbine;
3.1.1. Gas turbine In order to evaluate the gas turbine electrical efficiency, we have to consider each elements of the gas turbine. The consumed work by the gas turbine compressor is equal to (fig. 1):
𝐻𝐶 = 𝑐𝑝.𝑎 × (𝑇2 ― 𝑇1), where:
5
Journal Pre-proof 𝑐𝑝.𝑎 – Isobaric specific heat capacity of air (kJ/kgK); 𝑇1– Ambient air temperature (K); 𝑇2– Compressed air temperature (K). The generated work while extraction of waste gases in the gas turbine is determined by equation:
𝐻𝐺𝑇 = 𝑐𝑝.𝑔 × (𝑇3 ― 𝑇4) , where:
𝑐𝑝.𝑔 – Isobaric specific heat capacity of waste gases (kJ/kgK); 𝑇3– Temperature at gas turbine inlet (K); 𝑇4– Temperature at gas turbine outlet (K). The internal power consumed by the compressor is determined by equation:
𝑊𝐶.𝑖 = 𝐺𝐴 × 𝐻𝐶 where:
𝐺𝐴 – The air mass flow through compressor (kg/s). Generated internal power while extraction of waste gases in the gas turbine:
𝑊𝐺𝑇.𝑖 = 𝐺𝐺𝑇 × 𝐻𝐺𝑇 where:
𝐺𝐺𝑇 – The waste gases mass flow through gas turbine (kg/s). Internal efficiency of the gas turbine can be evaluated by equation:
𝜂𝑖 =
𝑊𝐺𝑇.𝑖 ― 𝑊𝐶.𝑖 𝑄𝐹
=
𝑊𝐺𝑇.𝑖 ― 𝑊𝐶.𝑖 𝐺𝐹 × 𝐿𝐻𝑉
where:
𝑄𝐹 – Heat input to the gas turbine (kW); 𝐺𝐹 – Fuel mass flow (kg/s); LHV – Lower heating value of the fuel (kJ/kg). Electrical efficiency of the gas turbine can be determined by equation:
𝜂𝑒 =
𝑊𝐺𝑇 𝑄𝐹
𝑊𝐺𝑇
= 𝐺𝐹 × 𝐿𝐻𝑉
where:
𝑊𝐺𝑇 – Gas turbine power output (kW). 3.1.2. Heat recovery steam generator Heat output of the HRSG, i.e. accepted heat by water and steam in HRSG, is equal to (fig. 1):
𝑄𝐻𝑅𝑆𝐺 = 𝑄0 ― 𝑄𝐹𝑊, It is also possible to determine the heat output of the HRSG by equation:
𝑄𝐻𝑅𝑆𝐺 = 𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝑊) = 𝐷0 × (ℎ0 ― ℎ𝐹𝑊), where:
𝐼𝐺𝑇 – Enthalpy at gas turbine outlet (kJ/kg); 𝐼𝑊 – Enthalpy of HRSG waste gases (kJ/kg); 6
Journal Pre-proof 𝐷0 – HRSG steam output (kg/s); ℎ0 – HRSG superheated steam enthalpy (kJ/kg); ℎ𝐹𝑊 – HRSG feed water enthalpy (kJ/kg). It is possible to write the heat balance equation for each heating surfaces of HRSG. For common heating surfaces of superheater and evaporating surfaces the heat balance equation is written as following:
𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝐸) = 𝐷0 × (ℎ0 ― ℎ𝐸), where:
𝐼𝐸 – Waste gases enthalpy at evaporating surfaces inlet (kJ/kg); ℎ𝐸– Water enthalpy at evaporating surfaces inlet (kJ/kg); Waste gases enthalpy at evaporating surfaces inlet 𝐼𝐸 is defined by equation:
𝐼𝐸 = 𝑐𝑝.𝑔 × (𝑡𝑠 + 𝛿𝑡𝐸), where:
𝑡𝑠– Saturated steam temperature in appropriate pressure (ºC); 𝛿𝑡𝐸 – Temperature difference between waste gases and water at evaporating surfaces inlet (ºC). The HRSG steam mass flow is defined by equation:
𝐷0 =
𝐺𝐺𝑇 × (𝐼𝐺𝑇 ― 𝐼𝐸)
(ℎ0 ― ℎ𝐸)
.
Heat balance equation for economizer:
𝐺𝐺𝑇 × (𝐼𝐸 ― 𝐼𝑊) = 𝐷0 × (ℎ𝐸 ― ℎ𝐹𝑊), It is possible to define the HRSG waste gases temperature:
𝑡𝑤 = 𝑡𝐸 ―
𝐷0(ℎ𝐸 ― ℎ𝐹𝑊) 𝑐𝑝.𝑔 × 𝐺𝐺𝑇
,
where:
𝑡𝐸 - Waste gases temperature at evaporating surfaces inlet (ºC). Waste gases temperature at evaporating surfaces inlet 𝑡𝐸 is defined according to the enthalpy Now it is possible to define the HRSG efficiency (utilization coefficient):
𝜂𝐻𝑅𝑆𝐺 =
(𝐼𝐸 ― 𝐼𝑊) (𝐼𝐸 ― 𝐼𝐴) ,
where:
𝐼𝐴 – The waste gases enthalpy at ambient temperature (kJ/kg). 3.1.3. Gas turbine CHP The CHP electrical efficiency is defined by equation:
𝑊𝐺𝑇
𝜂𝑒 = 𝐺𝐸 × 𝐿𝐻𝑉 , 𝐹
where:
𝐺𝐸𝐹– The fuel mass flow for electric energy generation (kg/s). The CHP heat production efficiency is defined by equation:
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𝐼𝐸 .
Journal Pre-proof 𝜂𝑒 =
𝑄𝐻𝑅𝑆𝐺 𝐺𝐻 𝐹 × 𝐿𝐻𝑉
,
where:
𝐺𝐻 𝐹 – The fuel mass flow for heat generation (kg/s). The CHP heat utilization factor is defined by equation:
𝜂𝑒 =
𝑊𝐺𝑇 + 𝑄𝐻𝑅𝑆𝐺 𝐺𝐸𝐹 × 𝐿𝐻𝑉
.
3.2. Absorption chiller Single-effect absorption cooling system is used in the study. The single-effect cooling system is based on the basic absorption cycle, which contains a single absorber and generator (Fig. 2). In the generator 1 a heat source produces ammonia vapor from a strong ammonia solution. As noted above, in this paper three types of heat sources are considered for providing the generator: gas turbine waste gases, HRSG waste gases and solar energy. The vapor (refrigerant) enters the condenser 2 where it is condensed. After cooling, it goes through a valve 3 and pressure and temperature are reduced, then evaporated in the evaporator 4 at low temperature and pressure. The cooled refrigerant is absorbed in absorber 5 by a weak solution, which returns from generator 1 through valve 7. A rich mixture created in absorber is pumped by pump 6 and returned into the generator 1. The absorption process is exothermic, the absorber is chilled with water.
Fig. 2. Schematic diagram of the absorption chiller: 1 – Generator; 2 – Condenser; 3 – Valve; 4 – Evaporator; 5 – Absorber; 6 – Pump; 7 – Valve. Thermodynamic models for various types of absorption chillers have been developed by multiple researchers and described in various textbooks. Therefore, as the paper is intended to study the gas turbines CHP behaviors, thermodynamic models of absorption chillers are not described here as well as they are not opened in computer simulation models. Nevertheless, widely used absorption chiller
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Journal Pre-proof thermodynamic models were accepted in conducting the study. The absorption chiller efficiency 60 % had been accepted for computer simulations. 3.3. Parabolic trough Several studies on modelling and simulation of parabolic trough collectors have been carried out up to date. Currently parabolic trough collectors with thermal oil are most widely used concentrating solar power technology. The thermal oil serves as a heat transfer fluid in the parabolic through. The heat transfer fluid runs through the tube to absorb the concentrated sunlight. At the moment it is possible to get a temperature of up to ≈ 395 °C in the parabolic through. The solar field with parabolic trough collectors was modeled to generate power for the absorption chiller. For modeling Ebsilon®Professional software was used. Several factors – field efficiency, optical efficiency, net aperture area, effective heat, incident angle correction, shading and wind were taken into consideration while modelling. The size of the field was determined by the number of collectors and the data of the collector model. The heat input into the fluid is defined by equation: 𝑄𝐸𝑓𝑓 = 𝑚1 × (ℎ2 ― ℎ1). where: 𝑚1 – Mass flow of the fluid (kg/s); ℎ1 – Enthalpy of the inlet fluid (kJ/kg); ℎ2 – Enthalpy of the outlet fluid (kJ/kg). The available heat input depends on the solar heat input and the thermal losses of the receivers and the field piping: 𝑄𝐴𝑣𝑎𝑖𝑙 = 𝑄𝑆𝑜𝑙𝑎𝑟 ― 𝑄𝐿𝑜𝑠𝑠 ― 𝑄𝑃𝑖𝑝𝑒. where: 𝑄𝐴𝑣𝑎𝑖𝑙 – Available heat input (kW); 𝑄𝑆𝑜𝑙𝑎𝑟 - Solar heat input (kW); 𝑄𝐿𝑜𝑠𝑠 – Thermal losses of the receivers (kW); 𝑄𝑃𝑖𝑝𝑒 - Thermal losses of the field piping (kW). Due to limited capacities of heat consumption the solar field will often not use all of the available solar energy. Part of the fields will be defocused. The effective heat generated in the solar field is therefore reduced by the fraction which is lost due to defocused collectors: 𝑄𝐸𝑓𝑓 = 𝑄𝑆𝑜𝑙𝑎𝑟 × 𝑅𝐹𝑜𝑐𝑢𝑠 ― 𝑄𝐿𝑜𝑠𝑠 ― 𝑄𝑃𝑖𝑝𝑒 , where: 𝑅𝐹𝑜𝑐𝑢𝑠 –Actual focus state of the collector field. Value of 𝑅𝐹𝑜𝑐𝑢𝑠 is determined by the selected power limitation method. The solar input 𝑄𝑆𝑜𝑙𝑎𝑟 is determined by the equation: 𝑄𝑆𝑜𝑙𝑎𝑟 = 𝐷𝑁𝐼 × 𝐴𝑁𝑒𝑡 × 𝐹𝑂𝑝𝑡_0 × 𝑘𝐼𝐴 × 𝐹𝑆ℎ.𝐿 × 𝐹𝐸𝑛𝑑.𝐿 × 𝐹𝑊𝑖𝑛𝑑 × 𝐹𝐶𝑙𝑒𝑎𝑛 × 𝐹𝐴𝑣𝑎𝑖𝑙 , where: 𝐷𝑁𝐼 – Direct normal irradiance (W/m2);
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Journal Pre-proof 𝐴𝑁𝑒𝑡– Net aperture area (m2); 𝐹𝑂𝑝𝑡_0– Peak optical efficiency; 𝑘𝐼𝐴– Incident angle correction; 𝐹𝑆ℎ.𝐿–Factor to include shading losses; 𝐹𝐸𝑛𝑑.𝐿–Factor to correct end loss effects determined from model; 𝐹𝑊𝑖𝑛𝑑–Factor to include optical losses due to wind impact; 𝐹𝐶𝑙𝑒𝑎𝑛–Factor to correct for actual mirror cleanliness; 𝐹𝐴𝑣𝑎𝑖𝑙–Field availability. The net aperture area was calculated according to the following equation: 𝐴𝑁𝑒𝑡 = 𝑁𝐶𝑜𝑙𝑙 × 𝐿𝐶𝑜𝑙𝑙 × 𝐴𝐶𝑜𝑙𝑙 × 𝐾𝐴𝑐𝑡𝑖𝑣𝑒, where: 𝑁𝐶𝑜𝑙𝑙 –Number of collectors (pcs); 𝐿𝐶𝑜𝑙𝑙 – Length of a collector array (m); 𝐴𝐶𝑜𝑙𝑙 –Aperture width of a collector array (m); 𝐾𝐴𝑐𝑡𝑖𝑣𝑒–Optical active portion of aperture (coefficient). 4. Analyses of the Tashkent gas turbine CHP Hitachi Gas turbine H27 with HRSG was installed in Tashkent CHP in 2012. The gas turbine has electric power output 28.1 MW and electrical efficiency 34.2 %. Exhaust gases of the gas turbine are entered into HRSG at temperature 552 ºC. HRSG serves to generate a steam with pressure 3.6 MPa and temperature 416 ºC for a heat supply system of Tashkent city. At nominal loadings a mass flow of the generated steam is 47 t/h. HRSG is natural circulation type with drum. HRSG consists of economizer, evaporating surfaces and superheater. Feed water with pressure 4 MPa and temperature 70 ºC is entered into the economizer of the HRSG. Waste gases of HRSG are thrown out into the environment at temperature 146 ºC. Experience of exploitation of the gas turbine in Uzbekistan climate conditions had shown following problems: -
Electric power output of the gas turbine is reduced down to 24.1 MW in summer period owing to high ambient air temperature (up to +45 ºC).
-
Electrical efficiency of the gas turbine falls down to 32 % in hot periods.
-
Electric power output of the gas turbine is decreased down to 17-18 MW during clogging periods of the air filters owing to impurities in content of the inlet air.
In order to analyze the dependency of power output and efficiency from ambient temperature a computer model of the existing H27 gas turbine CHP was developed in Ebsilon®Professional software. The simulation model in ambient temperature +15 ºC is shown in fig. 3. Energy-temperature diagram of the HRSG in ambient temperature +15 ºC is shown in fig. 4.
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Journal Pre-proof
Fig. 3. Simulation model of H27 gas turbine CHP developed in Ebsilon®Professional software. ETA – efficiency of appropriate parts.
Fig. 4. Energy-temperature diagram of the HRSG. Calculations in the developed simulation model were done for ambient temperature range from –15 ºC up to +45 ºC. Received results certify, the power output of the gas turbine reaches 31.8 MW while ambient temperature is –15 ºC, simultaneously electrical efficiency of the gas turbine rises up to 35.9 %. Electric
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Journal Pre-proof power output and electrical efficiency of the gas turbine are decreased down to 24.126 MW and 32.0 % respectively while ambient temperature is +45 ºC (Table 1). Table 1. Gas turbine output and electrical efficiency, CHP efficiency as function of the ambient temperature Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine output, MW
31.8
31.2
30.5
29.9
29.3
28.7
28.1
27.4
26.7
26.1
25.4
24.8
24.1
Gas turbine electrical efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
33.9
33.5
33.1
32.8
32.4
32.0
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
81.6
81.8
82.1
82.3
82.6
82.8
Fig. 5 shows the relative power output and electrical efficiency of the gas turbine and CHP efficiency due to the ambient temperature. The increase in the ambient temperature has a slightly positive effect on the CHP efficiency. Because the increased temperature in the gas turbine exhaust enhances the efficiency of the HRSG, it more than compensates for the reduced efficiency of the gas turbine. Temperature-entropy diagram for H27 gas turbine for ambient temperatures -15C, +15 C and +45 C is presented in fig. 6. It is possible to see in the diagram, that gas turbine performance in low ambient temperatures is bigger than in high ambient temperatures.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 5. Gas turbine output and electrical efficiency, CHP efficiency as function of the ambient temperature
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Fig. 6. Temperature-entropy diagram for H27 gas turbine for ambient air temperatures -15 C, +15 C and +45 C. In order to eliminate the problems of decreasing in the gas turbine power output and electrical efficiency we are proposing to use an absorption chiller. The absorption chiller has to lower the gas turbine inlet air temperature. 5. Simulation model of the gas turbine CHP with absorption chiller and solar field The developed simulation model of the gas turbine CHP was upgraded with including an absorption chiller and a field of solar energy (Fig. 7). As well as heat exchangers for heat extraction on the waste ducts of the gas turbine and HRSG are entered. The absorption chiller serves for cooling the gas turbine inlet air. The absorption chiller consumes a heat to drive the cooling process. Here we consider three heat sources: -
Heat of the gas turbine waste gases;
-
Heat of the HRSG waste gases;
-
Heat of the solar energy.
In the simulation model in order to change the heat sources the absorption chiller can easily be connected to pipelines of each of three heat sources. The absorption chiller is switched off while the ambient air temperature ranges between – 15ºC up to +15 ºC. Main equations, which are used in the computer simulations, were described in 3rd section of the paper.
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Fig. 7. Simulation model of the gas turbine CHP with absorption chiller powered by gas turbine waste gases. Ambient temperature is +45 ºC. ETA – efficiency of appropriate parts. 6. Results 6.1. Gas turbine CHP with absorption chiller powered by gas turbine waste gases The absorption chiller powered by gas turbine waste gases was entered in the simulation model. Calculations were carried out for ambient air temperature from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is shown in Fig. 7. Main data of the simulations are given in table 2.
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Journal Pre-proof Table 2. Main technical-economic data of H27 gas turbine CHP with the absorption chiller powered by gas turbine waste gases Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine power output, MW
31.8
31.2
30.6
29.9
29.3
28.7
28.1
28.1
28.1
28.1
28.1
28.1
28.1
Gas turbine efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
34.2
34.2
34.2
34.2
34.2
34.2
HRSG efficiency, %
74.5
74.5
74.5
74.5
74.5
74.5
74.5
73.6
72.8
71.9
71
70.1
69.1
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
80.2
79.1
77.9
76.7
75.6
74.4
Heat extraction for absorption chiller, MW
0
0
0
0
0
0
0
0.71
1.42
2.12
2.83
3.45
4.26
HRSG steam mass flow, t/h
47.3
47.2
47.2
47.1
47.1
47
47
45.8
44.7
43.5
42.3
41.2
40
HRSG inlet gas temperature, °C
552
552
552
552
552
552
552
545
538
531
523
516
509
HRSG waste gas temperature, °C
146
146
146
146
146
146
146
149
152
154
157
160
163
Dependency graph of gas turbine electrical efficiency, gas turbine power output and gas turbine CHP efficiency from ambient temperature while including an absorption chiller powered by gas turbine waste gases is shown in fig. 8. In this case, HRSG steam mass flow is reduced down to 40 t/h owing to lower temperature of the entered waste gases. As well as temperature of HRSG waste gases are raised up to 162.8 ºC, which leads to decrease CHP efficiency down to 74.4 %. It is seemable in the table 2 and fig. 8, that using the gas turbine waste gases in absorption chiller is not economical profitable, because CHP efficiency is reduced down to 74.4 % during ambient temperature +45 ºC, although gas turbine performance is maintained in nominal value.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 8. Gas turbine output and efficiency, CHP efficiency as function of the ambient temperature while including an absorption chiller powered by gas turbine waste gases. 6.2. Gas turbine CHP with absorption chiller powered by HRSG waste gases In the simulation model an absorption chiller powered by HRSG waste gases was included and calculations were carried out for ambient temperature from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is shown in Fig. 9. Main results are given in table 3, as well as in fig. 10 as a dependency graph.
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Fig. 9. Simulation model of the gas turbine CHP with absorption chiller powered by HRSG waste gases in ambient temperature +45 ºC. ETA – efficiency of appropriate parts.
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Journal Pre-proof Table 3. Main technical-economic data of H27 gas turbine CHP while including the absorption chiller powered by HRSG waste gases Ambient temperature, ºC
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Gas turbine power output, MW
31.8
31.2
30.6
29.9
29.3
28.7
28.1
28.1
28.1
28.1
28.1
28.1
28.1
Gas turbine efficiency, %
35.9
35.6
35.4
35.1
34.8
34.5
34.2
34.2
34.2
34.2
34.2
34.2
34.2
HRSG efficiency, %
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
74.5
CHP efficiency, %
79.8
80
80.3
80.6
80.8
81.1
81.4
81.4
81.4
81.4
81.4
81.4
81.4
Heat extraction for absorption chiller, MW
0
0
0
0
0
0
0
0.71
1.42
2.13
2.84
3.46
4.26
HRSG steam mass flow, t/h
47.3
47.2
47.2
47.1
47.1
47
47
47
47
47
47
47
47
HRSG inlet gas temperature, °C
552
552
552
552
552
552
552
552
552
552
552
552
552
HRSG waste gas temperature, °C
146
146
146
146
146
146
146
146
146
146
146
146
146
Waste gas temperature behind heat extraction, °C
146
146
146
146
146
146
146
139
131
123
115
107
100
According to the data, entered into table 3, extraction of 4260 kW heat from HRSG waste gases doesn’t make any influence to HRSG performance. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal design values. Just temperature of the HRSG waste gases is reduced down to 100 ºC.
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Journal Pre-proof Gas turbine efficiency (%)
CHP efficiency (%)
38
84
36
82.5
34
81
32
79.5
30
78
28
76.5
26
75
24
73.5
22
72 -15 -10
-5
0
5
10
15
20
25
30
35
40
CHP efficiency (%)
Gas turbine efficiency (%) Gas turbine output (MW)
Gas turbine output (MW)
45
Ambient air temperature (C) Fig. 10. Gas turbine output, gas turbine efficiency, CHP efficiency as function of the ambient air temperature while including an absorption chiller powered by HRSG waste gases
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Journal Pre-proof 6.3. Gas turbine CHP with solar powered absorption chiller In the simulation model the H27 gas turbine CHP was upgraded with including a solar powered absorption chiller. The solar field consists of system of parabolic through, circulating pump for the thermal oil, as well as a heat exchanger. The heat exchanger is intended to transfer a heat from the thermal oil to a heat transfer fluid of absorption chiller. Solar data of the Parkent district [22], which is located in 60 km from Tashkent city, was used for simulations: -
Annual direct normal irradiation (DNI) is 1753 kWh/m²a;
-
Mean daily DNI from ground measurement – 5 kWh/m2;
-
Mean daily global horizontal irradiation (GHI) from ground measurement – 4.6 kWh/m2.
Monthly DNI for Parkent district varies from 55 kWh/m2 in December up to 247 kWh/m2 in July. During May-September months it is approx. 200-247 kWh/m2, i.e. daily DNI is approx. 6.5÷7.9 kWh/m2. In April and October months a monthly DNI is approx. 147 kWh/m2 and daily DNI is approx. 4.7 kWh/m2. Although during could seasons monthly DNI is approx. 2 kWh/m2, mean daily DNI 6.5 kWh/m2 was accepted for simulations, because no need in inlet air cooling for gas turbine during could seasons, as ambient air temperature is – 25 ºC …+15 ºC. Parabolic trough collectors with 50 m length, 5.76 m gross aperture width and 1.71 m focal length have been used while simulations. Number of parabolic trough collectors – 28 pcs, total net aperture area – 8064 m2. Parabolic trough collectors are able to produce a heat up to 4260 kW. Calculations were carried out for ambient temperature range from -15 ºC up to +45 ºC. The simulation model for ambient temperature +45 ºC is presented in Fig. 11.
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Journal Pre-proof
Fig. 11. Simulation model of the gas turbine CHP with solar powered absorption chiller in ambient temperature +45 ºC. ETA – efficiency of appropriate parts. Difference of this scheme from above considered schemes is absence of additional heat exchangers in waste gas ducts for heat extraction as well as presence of the solar field. The heat demand in the absorption chiller from solar energy is the same as in the previous schemes. Comparison of the considered three upgrading schemes notify, that no need in additional field of solar energy, because of HRSG waste gases have enough heat to provide cooling process in the absorption chiller. But however, the solar field for such purpose can be technical achievable and economic profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than approx. +120 ºC.
7. Conclusions The present study deals with evaluation of efficiency improving opportunity in existing gas turbine CHP by means of the inlet air cooling. Analyses of the existing H27 gas turbine CHP had shown, the gas turbine power output is decreased from 28.1 MW down to 24.1 MW and the electrical efficiency is decreased from 34.2 % down to 32.0 % while ambient temperature rises from +15 ºC up to +45 ºC.
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Journal Pre-proof The absorption chiller was proposed to cool the inlet air. To drive the cooling process the absorption chiller was analyzed to use three types of heat source: the gas turbine waste gases, the HRSG waste gases, the solar energy. According to results of the study, 4260 kW heat is required in absorption chiller in order to reduce the inlet air temperature from +45 ºC down to +15 ºC. In order to provide the absorption chiller with the necessary heat additional heat exchangers were proposed to install on the gas turbine and HRSG waste gas ducts. The results of the study notify, that using the heat of gas turbine waste gases in absorption chiller is not economical profitable, because CHP efficiency is reduced down to 74.4 % during ambient temperature +45 ºC, although gas turbine performance is maintained in nominal value. Technical-economical attractive is the scheme of using HRSG waste gases in the absorption chiller. According to the proposed scheme (fig. 9), extraction of 4260 kW heat from HRSG waste gases doesn’t make any influence to HRSG performance. In this case all data, including performance of the gas turbine, the HRSG as well as the CHP, are maintained in nominal design values. In order to provide the absorption chiller with 4260 kW heat of solar energy, 28 pcs parabolic trough collectors with total net aperture area 8064 m2 are required. Parabolic through collectors with 50 m length, 5.76 m gross aperture width and 1.71 m focal length were used while simulations. As HRSG waste gases have enough heat to provide cooling process in the absorption chiller, so no need in additional solar field. But however, the solar field for such purpose can be economic profitable when HRSG waste gases don’t have enough heat, i.e. temperature of HRSG waste gases is lower than approx. +120 ºC.
Acknowledgements Author gratefully acknowledges the support of Dresden fellowship program [23] of the Technische Universitaet Dresden (Germany). As well author thanks to Prof. Dr.-Ing. Uwe Gampe, Head of Chair of Thermal Power Machinery and Plant of the Technische Universitaet Dresden, for consultations in carrying out the current research project.
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Journal Pre-proof [22]
UZB TA-8008. Republic of Uzbekistan: Solar Energy Development. Roadmap to Solar Energy
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from
dresden/profil/exzellenz/zukunftskonzept/tud-people/dresden-fellowship-programm>
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Journal Pre-proof Highlights - Increase in the ambient air temperature reduces the gas turbine output and efficiency; - Absorption chiller is proposed to cool the inlet air for gas turbine CHP; - Waste gases of a heat recovery steam generator drive cooling process in the absorption chiller; - Solar energy powered absorption chiller is used for gas turbine inlet air cooling.