Applied Energy 135 (2014) 165–173
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Enhancing micro gas turbine performance through fogging technique: Experimental analysis M. Renzi a,⇑, F. Caresana b, L. Pelagalli b, G. Comodi b a b
Libera Università di Bolzano, Facoltà di Scienze e Tecnologie, piazza Università, 5 39100 Bolzano, Italy Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A test bench that has been designed
to implement the fogging IAC technique to a MGT. Electric power gain depends on ambient humidity and it ranges from 5% to 13%. Electric power enhancement is 1.03 kW/°C of inlet air temperature reduction. Electric conversion efficiency increases by about 0.41%/°C. Performance gains are the higher, the hotter and drier the climate is.
a r t i c l e
i n f o
Article history: Received 18 April 2014 Received in revised form 2 August 2014 Accepted 21 August 2014
Keywords: Inlet air cooling Fogging Micro gas turbine
a b s t r a c t This paper describes a test bench that has been designed to implement the fogging inlet air cooling technique to a 100 kWe Microturbine (MGT) and reports the power and efficiency increase of the machine. Indeed, one of the main issues of MGTs, which has also been observed and documented in large sized gas turbines, is their strong sensibility to inlet air temperature. One of the most interesting technology in terms of low plant complexity to limit the MGTs performance loss is the high pressure fogging. Although cooling down the inlet air temperature with this technique has already been analyzed for medium/large gas turbines systems, there are very limited reports available on MGTs and few experimental data are documented. Results show that the machine’s electric power gain depends on ambient humidity and it ranges from 5% to 13% (corresponding to an inlet temperature drop between 4 and 10 °C) in the location where the plant is installed. Power enhancement corresponds to 1.03 kW for each Celsius degree of inlet air temperature reduction. As regards the electric conversion efficiency, the increase reaches about 0.41%/°C. Being the inlet air saturation the thermodynamic limit, the absolute power and efficiency gains are the higher, the hotter and drier the climate is. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author. Tel.: +39 0471 017816; fax: +39 0471 017009. E-mail address:
[email protected] (M. Renzi). http://dx.doi.org/10.1016/j.apenergy.2014.08.084 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
Microturbines (MGTs) are a relatively new technology that is currently attracting a lot of interest in the distributed generation (DG) market [1–4]. Their electric output varies from 25 kW to 500 kW, which is a power range particularly well suited for
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Nomenclature _ m n p T x
mass flow rate (kg/s) rotational speed (rpm) pressure (Pa) temperature (K) specific humidity (g/kg)
Subscripts air air H2O water nom nominal sat saturated
cogeneration applications in the service sector, households and small industry. MGTs are generally preferred to ICEs [5,6] even if they are less efficient in generating electricity, thanks to their high power density, low environmental impact in terms of pollutants, low operation and maintenance costs and multi fuel capability [7]. One of the biggest drawbacks of this kind of machines, which is also reported in larger GT, is the strong dependence of their performance, mainly output power and electric efficiency, on the ambient conditions, being the ambient temperature the most affecting parameter. This substantially limits their application in hot climates and does not allow to exploit the full potential of the machine which is normally rated in ISO conditions (15 °C, 1013 hPa, 60% RH) [8]. The influence of atmospheric conditions on medium/large GTs performance is well known and widely documented in literature. As regards large sized GT, the power output loss has been evaluated in several works and it ranges between 0.5 and 0.9%/°C depending on the machine type [9–11]. For GTs of smaller capacity, Mohanty [12] reported that power output loss with hot ambient temperature can be even greater than in larger GTs. This evidence is also reported by Amell and Cadavid [13]: they explained that this behavior of smaller GTs is due not only to the air density reduction but also to the volumetric flow decreases with temperature. Being the effect of ambient temperature on the performance of GTs so significant, several Inlet Air Cooling (IAC) techniques have been studied and applied to reduce its influence. Literature is rich of works that evaluated a series of solutions both theoretically [14–19] and experimentally [20–25]. In particular, Al-Ibrahim et al. [14] reported an interesting review listing the following main technical solutions: (i) wetted media evaporative cooling; (ii) high-pressure fogging; (iii) absorption chiller cooling; (iv) refrigerative cooling and (v) thermal storage together with their key benefits and drawbacks. In particular they highlight that the techniques based on water evaporation in the inlet air stream are simple and reliable in design, have low unit capital and operational costs, do not introduce significant parasitic power consumption but yield limited power gain due to the ambient wet-bulb limitation on inlet air temperature and require large amounts of purified water. The techniques using an heat exchanger placed upstream of the compressor are not sensitive to ambient-air wet-bulb temperature and thus guarantee greater performance increase than evaporative or fogging, but have higher unit capital and operational costs. Refrigerative cooling are relatively simple and reliable in design and operation but require large electric power (that can be reduced using thermal storages to cool the inlet air during peak-power demand). Absorption chiller cooling techniques have minimum parasitic electric power demand as they can recover energy from the GT exhausts but are complex systems requiring expertise in design, operation and maintenance.
Abbreviations/acronyms DG distributed generation GT gas turbine HP heat and power IAC inlet air cooling ICE internal combustion engine MGT micro gas turbine RH relative humidity RHE recovery heat exchanger
Al-Ansary et al. [15] proposed a hybrid solution using a combination of vapor compression cooling and fogging showing that it can meet the requirements of both dry and humid climates and maximize the effectiveness of the IAC technique. On the other hand, the initial cost and the complexity of the plant are high. Najjar [16] evaluated the chance of adopting an absorption chiller fed by the GT exhausts to treat the inlet air of the gas compressor. Khaliq [17] reported the energetic and exergetic analysis of a cogeneration plant with absorption inlet air cooling combined with evaporative aftercooling showing significant advantages with respect to a traditional basic cycle. Chacartegui [18] described the energetic and economic advantages of applying different kinds of IAC solutions to a commercial cogeneration GT. Yang [19] developed an analytical method to figure the influence of two IAC techniques (fogging and absorption cooling) on the performance of a combined cycle plant and he suggests a range of ambient parameters (namely air temperature and humidity) where the IAC technologies can be favorably applied. Kitchen et al. [20] examined several commercial GTs and evaluated the potential capacity increase applying inlet air cooling techniques; a detailed discussion of the available cooling techniques and the main advantages and drawbacks of each of them were discussed by Giourof [21], De Lucia et al. [22], ASHRAE [23], and Anderpont [24]; finally, a design guide was proposed by Stewart [25]. Literature on high pressure fogging systems applied to medium/ large GTs is rich. Chacartegui et al. [18] analyze the use of this technique in a cogeneration power plant evidencing that it represents a good compromise in terms or effectiveness, pay-back period and application simplicity and it is particularly suitable for hot and dry climates where it is possible to exploit maximally the advantage of the adiabatic saturation. An analytical approach in the evaluation of the compressor map working point with fogging IAC has been reported by [26] for GTs and combined cycle plants: the results of this analysis indicate a significant increase in the net power output of the studied plants and a general trend of the compressor operating point towards the surge line. Roumeliotis [27] investigated the effect of both inlet fogging upstream the compressor and water/steam injection in the combustor applied to several commercial GTs showing results on both performance augmentation and engine operability. A paper by Kim [28] reports on the chance of adopting cycle regeneration and fogging to enhance the performance of GTs. In particular, the thermodynamics of the droplets evaporation is described and the effects on the cycle efficiency as a function of the compression ratio is evaluated. Practical considerations and experimental aspects have been discussed in [29,30] with particular reference to droplet thermodynamics and heat transfer issues.
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Fig. 1. Turbec T100 HP plant scheme.
In the reported papers, the electric power gain achieved by using the fogging technology ranges between 5% and 15% depending on the inlet ambient humidity and, thus, on the capacity of reducing the inlet air temperature. As for MGTs, in literature there are limited works available regarding the effect of inlet air temperature on their performance [31–35]. These studies on the effect of inlet air regard only global parameters without entering in details in the plant components’ behavior. In particular, only Basrawi et al. [31] investigated the effect of the inlet air temperature on the performance of a MGT with cogeneration system arrangement. Employing a model based on experimental results, they found that when ambient temperature increased, electrical efficiency decreased but exhaust heat recovery increased. They also found that when ambient temperature increased exhaust heat recovery to mass flow rate and exhaust heat to power ratio increased. The authors of the present paper have already analyzed the effect of inlet air temperature experimentally by means of a test bench [36], by means of the artificial neural networks (ANNs) methodology [37] and using analytical models [38]. Results showed that the electrical performance is not significantly affected by air pressure and relative humidity, but it is strongly air temperature-dependent; the higher the ambient temperature the lower the electric power and the efficiency. Precisely, a reduction of about 1.22%/°C for the electrical power and a reduction of about 0.51%/°C for the electrical efficiency was assessed for Turbec 100 kWe MGT with respect to the nominal ISO conditions. Even if the influence of ambient conditions on MGTs is significant there is a lack of relevant literature on the application of IAC techniques to these machines. A US patent by Skowronski [39] proposes the injection of water upstream the compressor of a MGT with the aim to increase the density of the elaborated air. Anyhow, there are no experimental data reporting the effective performance increase. Williamson et al. [40] reported on a test bed that uses evaporative cooling devices applied in the inlet duct of a 60 kWe MGT to reduce the inlet air temperature. This cooling devices have the advantage of the low cost, but they are less effective than fogging systems in achieving the saturation point. The authors reported a maximum final relative humidity after the air treatment lower than 90%, but good power enhancements could be all the same performed thanks to the very low humidity of the location where the plant was installed (Southern California). A power output increase between 10% and 25% could be recorded when the air temperature exceeded 25 °C, but no indication on the relative humidity is reported. The aims of the present work are to: (i) describe the test bench that has been developed to apply the fogging IAC technology to a 100 kWe MGT; (ii) report the experimental results on the performance of the machine when operating in hot ambient temperature
with the fogging IAC; (iii) highlight the advantages of this technology in terms of electric power and electric efficiency gain and thus fulfill the lack of relevant scientific literature in the application of the fogging technology to MGTs. The paper is organized as follows: in Section 2 the MGT under analysis is presented and the effect of temperature on its performance is discussed; Section 3 reports the design of the fogging system and the description of the experimental setup; Section 4 illustrates the results of the test campaign; finally Section 5 reports the concluding remarks. 2. The MGT plant 2.1. The Turbec T100 HP microturbine The study of the performance enhancement of MGTs using IAC techniques is done by taking as reference the Turbec T100 HP microturbine. The machine operates at variable rotational speed according to a regenerative Bryton cycle with a compression ratio of about 4.5. In combined heat and power configuration, downstream the regenerator the machine is equipped with a heat exchanger for the recovery of the thermal power of the exhaust gases, as reported in Fig. 1. The turbine and the compressor, both of radial type, are mounted on a single shaft with the electric generator. The generator produces electricity at a voltage and at a frequency that are different from the standards of the grid. The electrical power conditioning is then carried out by means of an electronic conversion system. The MGT generates 100 kW of electrical power with an efficiency of 30% in ISO 2314 [8] conditions; downstream the turbine, combustion products exit at a temperature of 645 °C and cross the regenerator to pre-heat the fresh air at the discharge of the compressor. After the regenerator, exhaust flue gas has a temperature of 270 °C and its thermal power content can be further recovered for cogeneration appliances. In nominal conditions, thermal power production is 155 kW, with an efficiency of 47%. 2.2. Effects of ambient temperature on the MGT working conditions To evidence the effects of ambient temperature on the MGT performances, the running lines at maximum speed (i.e. maximum reachable power for given ambient conditions) are qualitatively represented on the characteristic maps of the compressor and the turbine, in Fig. 2a and b respectively.1 The effects on the entropy-temperature thermodynamic cycle are represented in Fig. 3. As ambient temperature, i.e. compressor inlet temperature T1, increases, the compressor working point moves towards lower 1
Detailed data can be found in [38].
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Fig. 2. (a) MGT’s compressor performance map; (b) MGT’s turbine performance map.
and their performance are thus strongly dependent on the ambient conditions, mainly relative humidity. Indeed, once 100% relative humidity is reached, these systems are unable to evaporate more water into the air stream. Therefore, the nominal machine’s working temperature may not be reachable in wet climates [14]. As already mentioned, the authors of the present paper have described the design and the experimental results of a test bed that allows to evaluate the performance of the MGT in several working conditions [36] with particular reference to its electric and thermal output and the corresponding efficiency. This test bench has been used and modified also to study the effect of the fogging system on the performance of the MGT as it will be described in the next paragraph. 3. The fogging system and the test bench Fig. 3. Effect of ambient temperature on the MGT thermodynamic cycle.
compression ratios and lower corrected mass flows (in Fig. 2a this means moving from point A to B). This means that the compressor delivers less air at a lower pressure. The turbine working point moves accordingly towards lower expansion ratio and lower corrected mass flow (in Fig. 2b from point A to B). Considering that the machine control system keeps the turbine outlet temperature, T5, constant, the decrease in the expansion ratio implies also a decrease in turbine inlet temperature, T4. As a consequence the net specific work decreases. This effect, together with the decrease in mass flow, causes a significant reduction of net power and also of plant’s efficiency. The IAC techniques aim at reestablishing the inlet temperature conditions to reduce the power loss due to ambient temperature increase. Considering point A of Fig. 2a and b as the nominal working condition, an ideal IAC system would bring back the machines from B to A. The techniques using an heat exchanger placed upstream of the compressor in the inlet duct can theoretically reestablish in any condition the original air inlet temperature (or even lower); the limitations are the availability of the cooling fluid (mass flow and temperature level) and how much additional energy consumption the cooling system requires [14]. The techniques based on water evaporation in the inlet air stream can reduce temperature only till wet bulb temperature,
The fogging technique can be easily applied to existing plants without requiring significant modifications; no heat exchangers must be placed in the inlet and in the discharge of the machine, thus no significant pressure losses are introduced. Also the moving parts are limited and the electric power consumption for its operation is minimal if compared to refrigerative chilling. With respect to wetted media solutions it has the advantage of granting a faster and more reliable achievement of the saturation condition. The operation of fogging systems is based on a pumping unit that pressurizes water at high pressure (usually in the range between 70 bar and 200 bar) [41] and sends it to a set of spray nozzles that pulverize water into extremely small droplets. The evaporation rate of the droplets is proportional to the contact surface between the water and the air: the smaller the diameter of the droplets, the faster the evaporation. The nozzles currently used in fogging systems allow to obtain 90% of the drops with a diameter less than 20 lm and more than 70% with a size of less than 10 lm. Thanks to the very small dimension of the droplets it is possible to realize an almost complete adiabatic saturation process, bringing the air to a relative humidity close to 100%. In addition, small droplets avoid impacts on the walls of the channels and, therefore, also on the blades of the compressor, as reported in a study by Alkhedhair et al. [42] and Chaker et al. [43].
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The major limitation of this technology consists in the impossibility to regulate the temperature of the cooled air. Air temperature reaches the wet bulb temperature, which strongly depends on the initial ambient conditions, mainly ambient temperature and relative humidity. In the following paragraphs, the technical characteristic, the dimensioning and the application of the fogging technique are presented for the case of the Turbec T100 PH MGT. 3.1. Separation of the air flow The air supply in the T100 PH is realized via a single duct as reported in Fig. 4. Inside the cabin the whole air flow (about 1.6 kg/s) is used for two purposes (see Fig. 5): a certain amount is sucked by the compressor and it represents the working fluid of the gas turbine group; the remaining part performs the function of cooling air for the auxiliary systems. This characteristic occurs only in those MGTs in which the intake air, both process working fluid and auxiliaries’ cooling, is realized through a single duct, as in the case of the T100 PH unit. Therefore, in order to apply the fogging technique to this MGT, it is convenient to split the two air flows. In fact, lowering the temperature of the cooling air does not bring benefits to the performance of the MGT and cooling must be operated only on the air that is sucked by the compressor in order to avoid unnecessary water and energy consumption. This aim can be carried out in the T100 MGT only by physically separating the two air flows both inside and outside of the cabin. Based on the current configuration of the MGT air supply system, the flow separation can be achieved by making some modifications to the cabin, without altering the architecture of the system. In our experimental system a horizontal partition wall was inserted in order to separate the upper area, that faces to the inlet cone of the compressor, and the lower one, in which heat exchangers from the auxiliaries’ cooling are installed (upper part and lower part of Fig. 5). The air separation wall allows the passage of the pipes and the cables, but air leakage is avoided; in addition, a thermal insulation between the two zones is granted to reduce heat transfer to the treated intake air. Finally, a new opening for auxiliaries’ cooling air (comprising the power electronics) was realized in the lower part of the casing. After the application of this limited modifications to the system, the two air streams arrive to the MGT unit via two separate air ducts, thus allowing to operate the fogging air cooling only in the duct that brings the working air to the compressor.
Fig. 5. Detail of the two intake air flows of the T100 MGT: compressor working fluid; auxiliaries’ cooling.
3.2. Dimensioning of the fogging system installed in the test bench The dimensioning and the choice of the pump and the number of fogging nozzles have been realized taking into account the nom_ air required by the MGT in ISO conditions inal air flow rate m (0.8 kg/s) and a reference ambient condition of 35 °C and 30% of relative humidity (the corresponding specific humidity, xnom , is 11.0 g/kg and the adiabatic saturation occurs when water vapor in the air, xsat , is 36.6 g/kg). Therefore the maximum required water _ H2 O , is given by: flow rate, m
_ H2 O ¼ m _ air ðxsat xnom Þ m
ð1Þ
which yields a water flow rate of 20.5 g/s. The pump installed in the fogging system is a volumetric pump (see Fig. 6a) that generates a flow rate of 31.6 g/s with a pressure of 140 bar and it has a maximum electric power absorption of 550 W. The pump sucks water from the water main, with a temperature of about 20 °C, and feeds four impaction-pin nozzles (see Fig. 6b) installed on a specific chassis to suite the intake duct of the MGT. A constant water flow rate is injected in the inlet air. An excess of water is required as not all of the injected liquid water is likely to be vaporized along the available duct length, but a part of it remains in the liquid phase and a part of it drops down on the wall of the duct. Since the manufacturer of the MGT recommends to avoid any presence of liquid water in the inlet of the centrifugal compressor, a specific filtering system, which will be described in detail in Section 3.3, has been installed downstream the fogging system. 3.3. Modified intake duct for the fogging system and the measurement instruments
Fig. 4. Sketch of the standard T100 MGT air flows. 1. Inlet air duct; 2. MGT cabin; 3. Ventilation air discharge duct; 4. CHP heat recovery exchanger; 5. Exhaust gas discharge duct.
In order to perform the experimental measurements, the intake duct, whose inner section is 610 610 mm, was modified and designed to embed the fogging system and the measuring apparatus. Fig. 7a shows a sketch of the intake duct with the main components of the system. The first filter at the inlet of the air duct is required to remove the coarse dust particles present in the intake air. In order to evaluate the condition of the air before and after the cooling treatment, a temperature and a relative humidity probe are installed between the filter and the fogging system; this first set of probes is located 2 m downstream the filter, the sensor of the probe reaches the center of the channel. In between the inlet section of the air duct and the inlet section of the MGT a differential pressure device has also
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Fig. 6. (a) Fogging pumping system; (b) fogging nozzles (red circles) adapted for the MGT intake duct. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. (a) Sketch of the modified intake duct with the fogging system and the measuring apparatus; (b) coalescent filter downstream the fogging system.
Table 1 Measuring instruments used in the test bench. Instrument
Typology
Measurement range
Resolution
Accuracy
Air temperature and humidity Differential pressure sensor Electric power production Fuel flow rate
SIEMENS QFM3171
Temperature: 0–50 °C r. h.: 0–100%
HUBA CONTROL 692
0...0.1–25 bar
Temperature: 0.1 °C r. h.: 0.1% 0.1% f.s.
Temperature: ±0.6 K Humidity: ±0.8% r. h. 0.40% f.s.
CARTEL EMA 14
Voltage: 0–750 V Current: 0–25000 A (with current transformer) 0–30 g/s
1W
