Performance Modeling and Simulation of a Hybrid Solar-Microturbine

International Marine and Offshore Engineering Conference (IMOC 2013)

Performance Modeling and Simulation of a Hybrid Solar-Microturbine Mohammed Anany, Sohair F. Rezeka, Mohamed A. Teamah, Iham F. Zidane Department of Mechanical Engineering, Arab Academy for Science and Technology and Maritime Transport. Copyright © 2013 Arab Academy for Science, Technology and Maritime Transport – All Rights Reserved.

Abstract – This paper devoted to study the conversion of solar energy to a mechanical energy in a stand-alone hybrid power generation system. Modeling and simulation using Matlab-Simulink provides expert help in understanding hybrid system design. The dynamic behavior and simulation of an integrated solar-microturbine model is developed. The hybrid system consists of a 30 KW micro-turbine and solar heaters of double-parallel flow. Solar heaters are being used to partially preheat the air entering the combustion chamber of the micro-turbine in order to decrease the amount of fuel consumption. Analytical model is developed to describe the thermal behavior of the solar heaters and integrated with the controlled model of the micro-turbine. The hybrid model has been simulated under several PU speed conditions. Results are obtained for PU torque, PU fuel demand and the amount of fuel saved annually. It was found that solar heater has saved 132300 U.S. Dollars annually according to the data provided in the year 2013.

Keywords: Hybrid Model, Micro-turbine, Solar Air Heater, Analytical Modeling

Nomenclature T: Micro-Turbine Torque (N m) Tex: Exhaust Gas Temperature (Cº) TR: Reference Temperature of the Micro-Turbine (Cº) Ti: Inlet Temperature to the Collector (Cº) Ta: Environment Temperature (Cº) To: Output Air Temperature from the Collector (Cº) N: Per Unit Speed Wf: Per Unit Fuel Demand Signal Qu: The Useful Energy Gained (kW) S: Solar Irradiance Absorbed by the Collector (W/m2) Fr: Collector Heat Removal F’: Collector Efficiency Factor UL: overall Heat Loss Coefficient of the Collector (W/ m² K) Ac: Collector Dimensions (m2) m: Mass Flow Rate of Air (kg/s) Ŋ: The instantaneous efficiency solar heater P1: Atmospheric Pressure (bar) P2: Air Compression Pressure (bar) Qs: Thermal Power saved by the Solar Heater (kW) mf1: Fuel Mass Flow Rate before Using Solar Heater (kg/s) mf2: Fuel Mass Flow Rate after Using Solar Heater (kg/s)

HVMethane: Lower Heating Value of Methane (kJ/kg) Cp: Specific Heat of Air (J/kg K)

1.

Introduction

Renewable energy resources are the primary contributors to achieve sustainable energy production. Energy crisis, climate changes such as atmosphere temperature rise due to the increase of greenhouse gases emission and the Kyoto Protocol restrictions in generation of these gases, coupled with high oil prices, limitation and depletion of fossil fuels reserves make renewable energies more noticeable [21]. Among the renewable energy resources, solar power has had the fastest growth in the world. There has been an increasing interest in using solar air collectors because of their simple designs, cheap construction and maintenance costs, their operational simplicity and their availability for local production. Moreover, they are ecologically friendly. On the other hand, solar air heaters are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small heat conductivity of air. Hence, several types of solar air collectors have been proposed over the recent years in order to improve their performance [1, 5]. Thermal performance of the solar air collectors depends on the material, shape, dimension and layout of the collector. Performance improvement can be

Copyright © 2013 Arab Academy for Science, Technology and Maritime Transport – All Rights Reserved.

Mohamed Anany, Sohair F. Rezeka, Mohamed A. Teamah, Iham F. Zidane

achieved using diverse materials, various shapes and different dimensions and layouts. The modifications to improve the heat transfer coefficient between the absorber plate and air include the use of an absorber with fins attached, corrugated absorber and matrix type absorber [16, 10, 12 and13]. For high solar gains, an efficient thermal coupling between absorber and fluid is required. Increasing the absorber area or fluid flow heat transfer area will increase the heat transfer to the flowing air. On the other hand, it will increase the pressure drop in the collector, thereby increasing the required power consumption to pump the air flow crossing the collector [14]. Since the 1970’s, several prototypes of solar air heaters were designed and tested. However, there are only four basic configurations, differentiated by the way in which the air flows in contact with the absorber plate (1) air flows between the absorber plate and the transparent cover, (2) air flows between the absorber plate and the bottom of the collector, (3) double flow: an air stream between the absorber plate and the transparent cover and another stream between the absorber plate and the bottom of the collector, in parallel flow or counter flow arrangements and (4) air flows through a porous matrix or a perforated plate. Each of these configurations has got different conversion efficiencies of solar energy into heat, increasing their values from first to last [8]. Microturbines are small electricity generators that burn gaseous and liquid fuels to create high-speed rotation that turns an electrical generator. The size range for microturbines available and in development is from 20 to 500 kilowatts (kW). It consists of four components: compressor, combustor, turbine and generators [11]. They are able to operate on a variety of fuels, including natural gas, sour gases, and liquid fuels such as gasoline, kerosene, and diesel fuel/distillate heating oil. In resource recovery applications, they burn waste gases that would otherwise be flared or released directly into the atmosphere.The distributed generation systems based on the microturbine technology is gaining more potential and becoming a viable distributed energy source in the recent years. This is due to their salient features such as high operating efficiency, ultra low emission levels, low initial cost and small size [20]. The microturbine generation system (MTG) can be operated in stationary or mobile, remote or interconnected with the utility grid. Once connected to a power distribution system, these generators will affect the dynamics of the system. Hence dynamic models are necessary to deal with issues in system planning, interconnected operation and management. There is lack of adequate information on the performance of MTG system when connected to distribution network, even though microturbine is based on gas turbine technology, which is well established [4]. The microturbine can be either one of two types, namely single- and split-shaft. In the single-shaft arrangement, the turbine and generator are mounted on the same shaft. Output frequency of the microturbine is from about 400 Hz up to several kilo Hertz. It must be converted to 50

Hz using electronic power converters. In the spilt-shaft microturbine, the shaft is connected to the generator by a gearbox and the converters are not needed [15, 18 and 9]. The GAST model, developed by General Electric, is one of the most commonly used models to simulate a gas turbines [15]. This paper will discuss the performance of the hybrid solar micro-turbine model and the amount of fuel consumption saved.

2.

Proposed System Description

The hybrid system proposed in this paper consists of a microturbine which produces a certain amount of torque to drive a high-speed permanent magnet synchronous machine, and solar heaters which are implemented in order to partially preheat the air entering the microturbine to decrease the amount of fuel consumption, as shown in the schematic in Figure 1.

2.1. Micro-Turbine Model The microturbines are a smaller version of heavy-duty gas turbines which are compact in size and components. A block diagram of the simplified single shaft microturbine along with its control is shown in Figure 2. This consists of fuel, speed, acceleration and temperature control along with the combustor and turbine dynamics. The turbine torque is expressed as T= 1.3 (Wf -0.23) + 0.5 (1-N)

(1)

The exhaust gas temperature is expressed as Tex=TR-700 (1-Wf) + 550 (1-N)

(2)

The simplified single shaft gas turbine including all its control systems which is implemented in MATLAB /SIMULINK is shown in Figure 3.This turbine model was proposed by W I Rowen [19]. 1. Speed Control: It operates on the speed error formed between a reference speed and the rotor speed of the MTG system. It is the primary means of control for the microturbine under different load conditions. It is a leadlag transfer function [7]. 2. Acceleration Control: It is used primarily during gas turbine startup to limit the rate of rotor acceleration prior to reaching governor speed. Acceleration controller is an integrator and acts on the error between the derivative of p. u. speed of generator and a constant reference signal. 3. Temperature Control: It limits the gas turbine output at a predetermined firing temperature, independent of variation in ambient temperature or fuel characteristics. 4. Fuel Control: The output of low value selector represents the least amount of fuel required for that particular operating point. The output of the ‘Vce’ limiter is multiplied by 0.77 and offset by no load fuel flow value to ensure the continuous combustion process. The



fuel flow controls are represented by a series of blocks including the valve position and flow dynamics. 5. Combustor and Exhaust Delay: combustor delay is the time lag associated with the compressor discharge volume and the exhaust delay is due to the transport of gas from the combustion system through the turbine.

2.2. Solar Air Heater Model

F1 F2 F3 F4 Combustor Delay Exhaust Delay

700 (1-u) 550 (1-u) 1.3 (u-0.23) 0.5 (1-u) 0.01 s 0.04 s

Table 3: Solar Model Parameters

This model represents the solar air heaters of double parallel flow configuration. Figure 4 shows the structure of this configuration. Based on the local energy balances, algebraic expressions for the efficiency factor F’ and the overall heat loss coefficient, UL, as well as air temperature distributions along the collectors were obtained. In addition, the expressions of the mean temperatures of the two air streams and of the absorber plate were determined. The heat transfer coefficients involved in the energy balances were estimated. The useful energy gain is expressed in terms of the air inlet temperature to the collector which is generally a known parameter in the applications [8]. It is expressed by as Qu = Ac Fr [S- UL (Ti – Ta)]

Table 2: Micro-Turbine Model Parameters

(3)

The output air temperature from the collector is expressed as To = Ta + [S/ UL] + [Ti – Ta –(S/UL)] exp [(Ac F’ UL) / (m Cp)] (4)

F’ Fr UL Air Density Air Mass Flow Rate Collector Dimensions

0.82 0.74 5.93 W/ (m² K) 1 kg/ m³ 0.018 kg/s 2.0 x 0.9 m²

The average solar irradiance on the tilted plane of the collector and average environment temperature are being collected during the twelve months in Alexandria, Egypt [2, 3]. They are represented in Figure 7 and Figure 8 respectively. The average inlet temperature to the collector during the twelve months is being calculated according to the following equation: Ti = Ta (P2 / P1)0.4/1.4 (6) Figure 9 represents the average inlet temperature during the twelve months. The average collector efficiency during the twelve months is calculated according to Eq (5) is represented in Figure 10.

The instantaneous efficiency for double flow solar heaters is expressed by as ŋ= 0.54 – [4.56 (Ti – Ta)/S]

(5)

The model simulated in MATLAB /SIMULINK is shown in Figure 5.

2.3. Hybrid Solar Micro-turbine Model The solar heaters model is being integrated with the micro-turbine model to simulate a hybrid solar microturbine model as shown in Figure 6. The Specifications of the micro-turbine are represented in Table 1[6]. The micro-turbine model parameters are represented in Table 2. The Solar model parameters are represented in Table 3[9]. Table 1: Micro-Turbine Specifications

Rated Power Air Fuel Ratio Rated Air Mass Flow Rate Rated Fuel Mass Flow Rate Fuel Type Air Compression Pressure

30 kW 30:1 0.018 kg/s 0.0006 kg/s Methane 5 bar

Figure 1: The Proposed System Schematic

The amount of thermal Power Qs saved by the solar heater is expressed as Qs = [mf2 * HVMethane] (7) Qs = [mf1 * HVMethane] – [Cpair m (To - Ta)] (8)



Figure 2: Micro-Turbine Control Block Diagram

Figure 3: Micro-Turbine Simulink

Figure 4: Solar Heater Configuration

Figure 5: Solar Heater Simulink

Figure 6: Hybrid System Simulink



3.

Figure 7: Average Solar Irredience

Simulation Results and Discussion

MATLAB Simulink™ 7.12.0 is used to evaluate the performance of the proposed hybrid model. This mathematical model is presented in the previous sections. The per unit speed input has been changed during the simulation to study the response of the hybrid solar micro-turbine model. This response was monitored through the per unit torque and per unit fuel demand before using the solar heater. The solar heater model was then integrated to the micro-turbine model to study the amount of fuel saving during the year.

3.1. PU Torque and PU Fuel Demand without Solar Heater

Figure 8: Average Environment Temperature

As shown in Figure 11 to 14, at time t =0 s o t = 20 s, the per unit speed input was 0.5. The per unit torque reached to a value of 0.75 at the steady state condition, while the per unit fuel demand reached a value equals to 0.1648 (0.000369 kg/s). From t = 20 s to t = 40 s the per unit speed input has been changed to 1. This means that the rotor speed has reached the rated speed. The rated speed is 96,000 rpm. It was found that the per unit torque dropped suddenly to 0.5, then started to increase till reaching the steady state at a value equals to unity at t = 21.5 s. The per unit fuel demand equals to unity (0.0006 kg/s) at the steady state. Starting from t = 40 s to t = 60 s, the per unit speed has been changed to 0.8. It was found that the per unit torque suddenly jumped to a value equals to 1.1 which is not recommended. It may shorten the life-time of the system components. The per unit torque started to decrease and reached a value of 0.9 at the steady state. The per unit fuel demand was equal to 0.85 (0.000508 kg/s) at the steady state. Finally the per unit speed has been changed to 0.6 from t = 60 s to t = 80 s. The per unit torque suddenly jumped to a value equals to unity which is accepted. The per unit torque decreased and reached a value of 0.8 at the steady state condition. The per unit fuel demand at that period reached 0.7 (0.000415 kg/s).

Figure 9: Average Inlet Temperature

3.2. Enhanced Fuel Consumption Using Hybrid Approach This model has shown a very good impact on saving the amount of fuel per month. As seen in Figure 15 the amount of fuel saved per month in kilograms on average is 12.3 kg. According to ONTARIO [1], the natural gas average price for the year 2013 is 70.5 cents/ liter. This means the average amount saved is 12152 U.S. Dollars per month. Figure 16 shows the amount of money saved per month. From the results shown the amount of money saved per year is 132300 U.S. Dollars on average. Figure 10: Instentanious Solar Heater Effeciency Copyright © 2013 Arab Academy for Science, Technology and Maritime Transport – All Rights Reserved.


Figure 11: Speed PU

Figure 13: Fuel Demand Before Solar Heater PU

Figure 15: Fuel Consumption Saved in Kg

Figure 12: Torque PU

Figure 14: Fuel Demand Before Solar Heater in Kg/s

Figure 16: Amount of Money Saved



4.

Conclusion

In this paper, the integration of a micro-turbine and a solar heater which work together to provide the sufficient load demand required was proposed. A detailed simulation model of a hybrid solar micro-turbine is implemented in MATLAB Simulink™ 7.12.0 using SIMPOWER Systems library. The analytical model described the thermal behavior of the solar heater and its effect on the amount of the fuel consumed. The hybrid model has been simulated under several PU speed conditions. Results showed the performance and the amount of annual fuel savings. According to the data provided in the year 2013, the annual amount saved on average is 132300 U.S. Dollars.

5.

[17] Natural Gas Average Price

http://www.energy.gov.on.ca/en/fuel-prices/fuel-pricedata/?fuel=CNG&yr=2013 [18] ] Nikkhajoei Hassan, Reza Iravani M., 2005, A matrix converter based microturbine distributed generation system. IEEE Trans Power Deliv, 20(3). [19] Rowen, W. I., 1983, simplified mathematical representations of heavy duty gas turbines, ASME Trans. J. Eng. Power, Vol. 105, No. 4, 865–869. [20] Scott, W. G.,1998 , Microturbine generators for distribution systems, IEEE Industry Appl, Vol. 4, No. 3, pp. 57–62,1998. [21] Urban, F., Benders, R. and Moll, H.., 2009, Energy for rural India, Elsevier, Applied Energy, Vol 86, S47-S57.

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