Elsevier

Energy

Volume 177, 15 June 2019, Pages 121-135
Energy

Predicting the performance of a micro gas turbine under solar-hybrid operation

https://doi.org/10.1016/j.energy.2019.04.064Get rights and content

Highlights

  • The MGT system equilibrium running point shifted, reducing the surge margin.

  • Solar-hybrid operation was only possible for solar share of at least 20%.

  • Work output and cycle efficiency drop below standard MGT levels at given solar share.

  • Sudden change in solar irradiation corrected by altering the fuel flow.

  • Solar-hybrid equilibrium running could eliminate the risk of running into surge.

Abstract

There are currently no commercial solar-hybrid gas turbine systems readily available off-the-shelf. Several operation and control challenges still exist, and significant development effort is still required to provide technically proven units. To address this gap, this study modeled the performance of a solar-hybrid micro gas turbine (MGT) system, considering both steady-state and transient operation. Based on the component matching, the equilibrium running point shifted on the compressor characteristic, to counter the additional system pressure losses, and ensure a useful work output, albeit with a reduced surge margin. Solar-hybrid operation was only possible for solar share of at least 20%, while the work output and cycle thermal efficiency drop below standard operation levels beyond certain solar share. In contrast to standard operation, a higher nominal work output of 20 kW, at a lower SFC of 0.0004 kg/kWh and a higher cycle thermal efficiency of 8% was predicted, the latter potentially increasing to 20% with recuperation. Solar-hybrid equilibrium running could eliminate the risk of running into compressor surge. The findings from this study should guide operation and control strategies for the proposed, and future solar-hybrid MGT systems, which should in turn contribute to their development and commercialization.

Introduction

Current state-of-the-art central receiver concentrating solar power (CSP) plants are based on steam turbine (Rankine cycle) technology [1], and only a few gas turbine (Brayton cycle) technology based plants have been demonstrated [[2], [3], [4]]. Gas turbine based central receiver CSP plants combine high concentration ratios of central receiver technology – and the resultant high receiver outlet temperatures – with gas turbine characteristics, such as quick start ups, compactness, high power to weight ratio and multiple fuel applications [5], to ensure high efficiency power generation. In addition, the gas turbine is a low-water-usage technology – since there is no requirement for cooling water [6] – and this makes it suitable for application in the normally arid and water scarce high solar resource regions. The use of gas turbine technology also allows implementation of hybridisation, in the form of combusting backup fuel, which ensures power production whenever demand occurs.

Research relating to solar-hybrid gas turbine systems mainly focuses on the solar components, with little or no focus on the gas turbine systems [4,7,8]. For the few demonstrated solar-hybrid gas turbine systems, the usual approach involved selecting a gas turbine in the desired power range and (physically) adapting it for solar operation [8,9]. Challenges to adapting gas turbines to solar operation mainly relate to integrating the solar receiver and the externally heated air into the gas turbine systems, developing and implementing a control system to switch gas turbine operation modes and, modifying the combustion chamber to allow continuous operation at elevated inlet air temperatures [[10], [11], [12], [13], [14]].

Some of the earliest gas turbine focused solar-hybrid gas turbine system research work involved a study performed by Solar Turbines International (STI), and funded by the Electric Power Research Institute (EPRI) [13]. The study investigated the modifications required to adapt a commercially available STI Centaur recuperative gas turbine-generator set for solar-hybrid operation, with the two identified key technology gaps being in the fields of combustion and engine control. Subsequent studies included Gallup and Kesseli [15] and Kesseli and Wells [16]; who proposed a 30 kWe twin shaft parabolic dish/Brayton solar gas turbine system based on the then commercially available Schwitzer Inc. turbocharger units, as an alternative to parabolic dish/Stirling engine solar systems. The use of turbocharger technology was to ensure low cost and high reliability of the turbomachinery. The proposed parabolic dish/Brayton solar gas turbine system was never commissioned.

The European Commission (EC) previously funded three solar-hybrid gas turbine demonstration projects; SOLar hybrid GAs Turbine Electric (SOLGATE) power system, SOLar-HYbrid power and COgeneration (SOLHYCO) plants and SOLar Up-scale GAs turbine System (SOLUGAS). The main goal of these projects was to demonstrate the technical feasibility and cost reduction potential of solar-hybrid gas turbine systems, with the focus on improvement in receiver design and performance [[2], [3], [4]]. These projects achieved varying degrees of success; several hours of solar-hybrid gas turbine operation were accumulated for the duration of the three projects – including 2 h of the first ever solar-only micro gas turbine operation during the SOLHYCO project – but at the same time, several challenges were faced, including: unsuccessful emergency shutdowns, difficulty controlling the heat supplied by multiple heat sources, an inability to handle the high combustion chamber inlet air temperatures, receiver cavity defects and unsatisfactory combustion during micro-turbine start up. These challenges eventually cut short the testing phases of these projects [[2], [3], [4],7,12,14,17].

More recently, the EC funded the Optimised Microturbine Solar Power (OMSoP) CSP demonstration project. The MGT system was intended to be of a modular design, with the capability of producing 3–10 kW of electricity. This demonstration project began in 2013, with a consortium of 8 partner organisations from 5 European countries, and a total budget of €5.8 million. The solar power system was based on parabolic dish concentrator technology, but with the Stirling engine replaced with a MGT system designed by the City University of London. The project concluded in 2017. Experimental tests showed overheating of the MGT system bearings, which limited the turbine inlet temperature (TIT) to a maximum of 270 °C, well below the design point value of 800 °C. Consequently, a useful work output of only 0.9 kW was realised from the experimental tests [18].

AORA Solar's Tulip™ co-generation system is another solar-hybrid gas turbine demonstration plant referenced in literature [[8], [9], [10]]. The system has a rated electrical and thermal power output of 100 kWe and 170 kWt, respectively. This system is installed at two research and development sites; Kibbutz Samar, in Israel and Plataforma Solar de Almería, in Spain, with the former commissioned in 2009, while the latter was commissioned in 2012 [19]. There is no published literature readily available on the performance of these two demonstration plants.

In contrast to the few demonstrated plants, several thermodynamic studies have been performed on solar-hybrid gas turbine systems. The majority of these studies investigated the steady-state operation of the gas turbine systems, and only a few considered the transient behaviour of the gas turbine systems.

Some of the most recent studies performed on the transient behaviour of solar-hybrid gas turbine systems include Felsmann et al. [20,21] and Kathirgamanathan and Axelsson [22]. Felsmann et al. [21] investigated the effect of both rapid and slow increase/decrease in solar thermal energy input on the dynamic behaviour of the 12 MW THM 1304-12 MAN Diesel & Turbo SE gas turbine, with the variable solar energy resource simulated by a fast/gradual increase/decrease of a 0–29 MW thermal energy input. As a continuation of the initial study, Felsmann et al. [20] investigated the effect of fast load changes in the network and generator load rejection on the transient behaviour of the same gas turbine system. Kathirgamanathan and Axelsson [22] focused on the effect of thermal inertia and additional pressurised air volume on the rotor over-speed of the 1.85 MW OP16 gas turbine system integrated in a hybrid solar thermal power plant, during load shedding/emergency shutdowns. As possible safety measures, these studies all proposed a combination of fast acting flow control valves – air flow control valves, to re-direct the air flow, and allow the switch from solar to non-solar gas turbine operation mode, and fuel control valves, to either regulate or cut-off the fuel supply – and a blow-off valve – to vent the solar heated air. Other safety measures proposed in literature include the use of mechanical brakes and shunt resistors loading the generator [10].

Despite the several thermodynamic studies performed on solar-hybrid gas turbine systems, and the few demonstrated plants, there are currently no commercial solar-hybrid gas turbine systems readily available off-the-shelf. Several operation and control challenges still exist and significant development effort is still required to provide technically proven units. Notably, the available literature does not consider the effect of solar thermal energy input on the complete operating range of speed and power output of the adapted gas turbine systems. In addition, the available literature does not specify at what levels of solar share is solar-hybrid MGT system operation not possible, to dictate when to switch from solar to non-solar gas turbine operation mode. Furthermore, very little attention is paid to the transient behaviour of MGT systems under solar-hybrid operation. The work presented in this study therefore aims to contribute to addressing these gaps.

This study constitutes the theoretical phase of an ongoing research project, whose main objective is to develop a solar-hybrid MGT system for central receiver CSP distributed electricity generation applications in Southern Africa. The aim of this study was to predict the performance of the MGT system under solar-hybrid operation, in order to determine the possible operating range, and develop suitable operation and control strategies for the proposed solar-hybrid MGT system.

Section snippets

System configuration and components

The solar-hybrid MGT system was based on an open-cycle twin-shaft engine (see Fig. 1); where a high pressure turbine drives the compressor, and the combination acts as a gas-generator for the low pressure and mechanically independent (free) power turbine. The power turbine in turn drives an electrical generator, which converts mechanical work into electrical power.

The twin-shaft arrangement allows for flexibility in operation, which is advantageous given the variable nature of the solar energy

Performance prediction

A mathematical model to predict both the steady-state and the transient performance of the MGT system under solar-hybrid operation was created in MATLAB R2017a. The modeling of the steady-state performance of the MGT system was done to, firstly, predict the possible operating range and determine an equilibrium running point for the MGT system under solar-hybrid operation, secondly, predict the nominal rating for the proposed solar-hybrid MGT system, when operating at the determined equilibrium

MGT system equilibrium running points

From the matching of the individual MGT system components, equilibrium running (operating) points for both standard and solar-hybrid operation – denoted by MGT and SHMGT, respectively – were obtained, and plotted on the compressor performance characteristic (see Fig. 8).

The equilibrium running point for the MGT under standard operation lies on the 4891 non-dimensional compressor speed line, while that for the MGT under solar-hybrid operation lies on the 5706 non-dimensional compressor speed

Conclusion and future work

The aim of this study was to predict the performance of a MGT system under solar-hybrid operation, in order to determine the possible operating range, and formulate operation and control strategies for a proposed solar-hybrid MGT system. The modeling considered both steady-state and transient operation, and used meteorological data for four arbitrary selected solar days, one for each climatic season experienced in Southern Africa.

The matching of the individual MGT system components showed that

Funding

This research project is supported by the Centre for Renewable and Sustainable Energy Studies (CRSES), Stellenbosch University.

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