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Hydrodynamic behavior of a new wave energy convertor: The Blow-Jet
Ocean Engineering 106 (2015) 252–260
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Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Hydrodynamic behavior of a new wave energy convertor: The Blow-Jet Edgar Mendoza n, Xavier Chávez, Juan Carlos Alcérreca-Huerta, Rodolfo Silva Engineering Institute, National University of Mexico, Circuito Escolar s/n, Ciudad Universitaria, Coyoacán, 04510 México, DF, México
art ic l e i nf o
a b s t r a c t
Article history: Received 23 December 2013 Accepted 24 June 2015 Available online 28 July 2015
The work presented here focuses on evaluating the interaction between ocean waves and a device designed to harness their energy. The device, called Blow-Jet, consists of a narrowing structure that concentrates waves and transforms their energy from potential and kinetic to kinetic, turning the oscillatory flow into an intermittent jet that can be easily guided to an impulse turbine. The new device is designed to feed low energy demands such as secondary fuel generators (e.g. hydrogen production). Several experimental tests were carried out to investigate the hydraulic behavior of the Blow-Jet. Results indicate that even for low energetic wave trains, the device is able to capture energy and amplify the water velocity, which is indeed energy conversion capacity. In order to evaluate the flow field inside the device, a high resolution 3D numerical model OpenFOAM is utilized. Good agreement was found between the pressures exerted to the walls of the device with that computed with the numerical tool. The combination of numerical and experimental results confirmed the optimal geometry, location and position of the Blow-Jet, while the need for some geometric changes was detected after the analysis of numerical model results. The Blow-Jet has shown itself to be a potential WEC with acceptable efficiency. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Wave energy convertor Tapchan Blow-Jet OpenFOAM Blowhole Renewable energy
1. Introduction The search for a means of producing clean and renewable energy is underway worldwide and efforts focused on generating energy from almost every natural source available are being investigated. The marine environment is not an exception for researchers, as it provides many different possibilities including marine current power, tidal power, ocean thermal energy, osmotic power, salinity gradient power and wave energy. It has been stated that the combination of these sources may give enough energy to fulfill the entire needs of the planet one day. However, considering only wave energy, by 2030, an estimated of 150 GW of wave energy production would contribute 11–14% of total electricity consumption in Europe IEA/OES (2010); this statement is, partially, the motivation of the research work presented here. The generation of energy from waves has been a common challenge for many countries over the years, probably because of the proximity humankind has with waves. As capturing the energy of ocean waves in offshore locations has been demonstrated as technically feasible, there are a huge number of devices, systems and concepts designed to capitalize on wave energy, e.g. pelamis (Yemm et al., 2012), wave dragon (Kofoed et al., 2006), pontoon power (www.pontoon.no). Additionally, research in developing n
Corresponding author. E-mail addresses: [email protected] (E. Mendoza), [email protected] (X. Chávez), [email protected] (J.C. Alcérreca-Huerta), [email protected] (R. Silva). http://dx.doi.org/10.1016/j.oceaneng.2015.06.036 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
designs for wave energy conversion (WEC) devices is being conducted in countries such as USA (Rhinefrank, 2005), UK (Wavegen, 2002), Portugal (IEA/OES, 2010) and many others. Several reviews, e.g. (Falcão, 2010; Pérez-Collazo et al., 2015), can be consulted by the reader to get an updated view of the available technologies. Compared with other forms of offshore renewable energy, such as solar photovoltaic (PV), wind, or ocean current energy, wave energy is a continuous but highly variable source. Many of these devices have their implementation limited to high energy sea states, but this cannot be an obstacle to continued development in this field. In turn, a great advantage of waves is that they naturally travel towards the coast so there is almost no need to find a specific place to set the WEC devices. Every nation has different needs and socioeconomic conditions which affect research into wave generated energy production. In the case of Mexico, according to recent investigations, the density of wave energy in offshore areas has been quantified as not very high (Pontes and Falcão, 2001). However, due to the length of the Mexican coastline and environmental pressures (IPCC, 2007), it is reflected that the exploration of technology for wave energy generation is much needed. In this scenario, the National University of Mexico (UNAM) has been working on harnessing wave energy by means of wave amplifiers (Ruiz et al., 1994 and Alatorre, 2003) based on the fact that, with a few exceptions, the yearly mean sea states all around the country hardly reach 1–2 m of significant wave height (Silva et al., 2008), which means that any WEC device placed on the Mexican coast should work almost all year long. This paper
E. Mendoza et al. / Ocean Engineering 106 (2015) 252–260
presents a novel concept that combines various natural and artificial fluid structures to produce a wave energy catching device which covers the needs and conditions found on the Mexican coast. The investigation includes an exhaustive experimental program aimed at evaluating the efficiency of the device and its geometry. Furthermore, a recently developed toolbox by Jacobsen et al., 2012 is used for the examination of the pressure field within the device and compared to that observed in the laboratory during the operation of the device. It is anticipated that the combination of the experimental results with those from the numerical tool will enable optimization of the device. The aim of this paper is to analyze the response of the Blow-Jet to different wave conditions and to develop a numerical model from which the device can be further improved.
2. Concept of the Blow-Jet device The device presented in this paper takes some ideas from the narrowing structure for wave amplification called Tapchan (tapered channel) (Mehlum, 1985). The Tapchan was used to convert wave energy in Norway with relatively good results from 1985 to 1990, but its main disadvantage was that it needed a large reservoir to store the water that overtopped the structure. If this reservoir is not naturally found, its construction may make the system unaffordable; the same problem is found for almost any device working under the principle of turning kinetic to potential energy. On the other hand, an advantage of the Tapchan was the absence of movable parts which is very convenient, in terms of maintenance, for devices expected to work as much as possible throughout the year. The use of narrowing, energy harnessing structures enhances the energy of the devices as shown by Deng et al. (2014). The second idea behind the Blow-Jet is that of reproducing the hydraulic behavior of natural structures known as blowholes. Blowholes are relatively common coastal features and are usually given local, colloquial names; the most famous example in Mexico is that found on the Pacific, in Ensenada, Baja California, called the “Bufadora”, which is an onomatopoeic term. In terms of energy conversion, a blowhole turns relatively small waves into very strong air-water jets. The great advantage of this natural structure is that kinetic energy is amplified and the resulting jet can be easily directed to an impulse turbine. Also, once this water has been used to move the turbine, it can be returned to the sea and, unlike the Tapchan, no storage is needed. Unfortunately very few studies focused on the internal morphology of blowholes are available, but simple observation tells us that the key elements of these formations are a wide entrance, a submerged or semisubmerged compression chamber and a discharge hole. The different sizes, slopes and arrangement of these three components determine the force and air-water mixture of the expelled jet. If these elements and operation are to be used to design wave energy convertors, it is desirable to minimize wave reflection inside the device to avoid energy waste as much as possible and to reduce the amount of air in the jet, to reach higher efficiencies of the impulse turbine (e.g. Pelton turbine) that is to be used. It is important to note that a blowhole can also be seen as a natural
Oscillating Water Column device because the waves that get inside the structure compress the air contained in the chamber. The third idea used to define the Blow-Jet geometry comes from the design of the brass instrument, the tuba. These instruments are built on the basis that the effort needed to generate sound should be minimal; the internal profile of the instrument has to be optimum for sound waves to travel through it. Several tuba profiles can be found in the different brand catalogues; for the Blow-Jet those corresponding to F, C and B keys were selected for the construction and testing of a scale model. The actual size of the tubas is too small for the experimental facilities, so the equations had to be scaled; the design parameters and final profile expressions are shown in Table 1. Fig. 1 shows the fiber glass models used in the experimental tests. The proposed geometry differs from the Tapchan as it does not work by overtopping and the narrowing section of the Blow-Jet is circular. It differs from blowholes as the size of the compression chamber has been reduced and the expelled jet is horizontal instead of vertical. These characteristics turn the Blow-Jet into a novel and simple device with low maintenance needs, capable of converting energy from low energy sea states and suitable for applications that can be fed with variable power, e.g. the production of secondary fuels or energy storage systems. The simplicity of the Blow-Jet makes it attractive for inclusion in several projects and conditions; it can be placed, fixed, nearshore, floating offshore, or even onshore attached to a breakwater or coastal structure. Its construction is conceived as a monolithic block, so the Blow-Jet can easily be constructed “in situ”. It is foreseen that the best results will be found if this device is placed in farms (Renzi et al., 2014; Sarkar et al., 2014), which can also be used as coastal defence as shown by Mendoza et al. (2014).
3. Experimental set-up and program The wave flume at UNAM is 37 m long, 0.80 m wide and 1.20 m high, allowing a maximum submerged depth of 0.8 m. The wave generation system was acquired from HR Wallingford and is capable of generating waves with different shapes and spectra; it is controlled from a PC via the HR WaveMaker software. The flume has an active absorption system designed to absorb rereflected waves and a passive absorber has been placed at the other end of the flume. The experimental set up consisted of three wave gauges placed in front of the device for registering the incident and reflected waves, and one more placed behind the device to measure the transmitted waves. The water velocities at the entrance of the device and at the jet were measured with micro Acoustic Doppler Velocimeters and Particle Image Velocimetry. The distances between wave gauges were set according to Mansard and
Table 1 Design parameters for the three Blow-Jet models Model
Input diameter (m)
Output diameter (m)
Length (m)
1 (F) 2 (C) 3 (B)
0.366 0.443 0.418
0.021 0.021 0.021
0.630 0.800 0.860
253
Fig. 1. Idealization of the least resistance profile.
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Fig. 2. Blow-Jet models.
Funke's (1980) methodology in order to determine incident and reflected wave energy (see Fig. 2). Two experiments were conducted with a fixed Blow-Jet to cover as many design aspects as possible. The first experiment focused on verifying whether the proposed geometry really could take waves and turn them into an expelled jet; i.e. to usable energy. For these tests, the efficiency of the device was determined in terms of measured velocities at the entrance and at the discharge hole. This first case required more than 300 tests in order to get a wide view of the hydraulic behavior of the device. Regular wave trains with different heights; H, (8-20 cm) and periods; T, (0.8-2.0 s) were generated. Four axis angles; θ, from the still water level to the device central axis were tested (0, 10, 20 and 301) for three submergences; S, (1/3, 1/2 and 2/3 of the BlowJet entrance diameter). The device was not tested fully submerged because the operation hypothesis considered having breaking wave conditions inside the device in order to get a stronger expelled jet; the right panel of Fig. 2 shows how S and θ are defined. Some conditions were eliminated from the analysis because waves were breaking before reaching the device and for some arrangements no jet was generated. With the purpose of investigating in greater detail the momentum generated by the waves inside the device chamber, a second experiment was carried out to register pressure inside the BlowJet. For this, ten pressure transducers were placed inside the device; five at the bottom, 15 cm apart, three at the horizontal symmetry plane, 30 cm apart and two more at the top, 15 cm apart. The pressure transducers measured up to 1.0 m of water column (see pictures in Fig. 3).
4. Experimental results In this section, the results obtained from the three experiments are presented. The most relevant result is the verification of the conversion of waves into a jet, together with the kinetic energy amplification. This can be translated as finding an increase in the velocity at the entrance compared to that at the output of the device (the ratio of the output to entrance velocities is called hereafter velocity amplification factor). The velocity amplification factor is also a measure of the efficiency of the device, as the power of the jet is directly proportional to the cube of the velocity: Nevertheless it should be pointed out that the over-all power output and efficiency of the device need to be affected by the
cross-sectional area of the device (i.e. the length of the wave front that is being harnessed). In this sense, the efficiency of the BlowJet can be further explained, in terms of converted energy, by the ratio of the available power at the entrance and at the output of the device. The power at the entrance can be expressed as P E ¼ 18 ρgH 2 DE V E
ð1Þ
where PE is the power at the entrance, ρ the water density, H the wave height, DE the entrance diameter of the Blow-Jet (i.e. capture width) and VE the velocity at the entrance of the device. In turn, the power available at the output (jet power) can be estimated as P J ¼ 18 πρD2o V 3o
ð2Þ
where PJ is the power at the output, Do the output diameter and Vo the velocity at the output. The energy balance equation for adiabatic steady flows states that the sum of kinetic energy and enthalpy (i.e. internal energy plus the flow work, given by the product for pressure and velocity) remains constant even in nonequilibrium conditions (e.g. in the presence of breaking waves in the duct). Therefore kinetic, potential and flow work at the inlet and at the outlet must be equal. However, at the duct entrance, the energy that can be converted is the potential fraction, whereas at the outlet the energy is mainly kinetic. Therefore, we define the efficiency as the ratio of the output to the input powers, which yields:
η¼
π V 2o Do g H2
D U VAF
ð3Þ
where D is the ratio of the entrance to the output diameters and VAF the velocity amplification ratio. Results in all tests indicate a velocity amplification factor greater than one, which in turn shows the positive performance of the WEC. The velocity amplification factor for all the tests in experiment one is shown in Fig. 4; it is important to note that a velocity amplification factor equal to zero may indicate that no jet was generated or that the test was not taken into account (mainly because of wave breaking before reaching the Blow-Jet). From Eq. (3) it is noted that the efficiency is directly proportional to the VAF, so the response of the Blow-Jet can be analyzed in terms of it, given that the flow velocities were the variables directly recorded during the experiments. Fig. 4 reveals interesting details about the hydraulic behavior of the Blow-Jet. Firstly, it shows that the central axis inclination of 301 has to be removed from any analysis, as the performance of the device is
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Fig. 3. Blow-Jet experiments, measuring (1) the hydraulic efficiency (left) and (2) the pressures inside the device (right).
Fig. 4. Velocity amplification factor.
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Fig. 5. (a) Tests found giving a linear trend and (b) tests found giving a parabolic trend.
very poor. Another observation is that although the maximum values of velocity amplification occur when S¼1/2, the best overall performance is found when S¼2/3. In general terms the Blow-Jet seems to be a good means of catching wave energy and the selected shape of the internal profile favors the velocity increase. To achieve a better understanding of the response of the device to local wave conditions, the dependency on wave period was investigated. It was almost impossible to define any clear trend with the available data, but analyzing the different tests separately, two features of the velocity at the expelling hole, Vo, as a function of kh were found (k being the wave number and h the still water level); quasi-linear (Fig. 5a) and parabolic (Fig. 5b). It is important to point out the differences between them; the quasi-linear trend means that for larger periods the device is giving unlimitedly higher jet velocities whereas if it is parabolic, it means that an optimal wave condition (kh 2) exists that makes the Blow-Jet operate at its best. In Fig. 5a and b, the trend is clearer if each group of data (symbols) is seen separately. The existence of a maximum value of Vo proves that the BlowJet has acceptable hydraulic efficiency and that its geometry can be determined as a function of the local wave climate (kh). As mentioned before, the second experiment gave results for the pressure distribution inside the Blow-Jet. In this paper some representative examples of all the information gathered are presented. The measured values of root mean square pressure for the top and bottom parts of the device have been drawn in Fig. 6 where the tests with S ¼1/2, T ¼2 s and θ ¼0 and 101 are shown in the left and right panels, respectively. In the left panel of Fig. 6 the pressure recorded close to the output hole is very similar at the top and at the bottom, indicating that for these wave trains the pressure is equally distributed in all directions. This result indicates that the end part of the Blow-Jet is working as a fully filled pipe, it was observed that the expelled jet had low air content which is desirable. When the device is inclined, the pressure envelope registered at both locations also shows a symmetric signature; but with the difference that for higher wave heights a saturation value for the registered pressure is identified (see right panel Fig. 6). This suggests that a maximum wave height exists, at which the Blow-Jet produces its best performance. This is in agreement with the parabolic behavior shown in Fig. 5b and confirms that the design of the Blow-Jet can
easily be expressed as a function of the local wave climate. Fig. 7 shows the pressure distributions for S ¼1/3, T¼ 2 s, and θ ¼0 and 101 in the left and right panels, respectively. A relevant difference that can be seen in Fig. 7 is that the degree of submergence slightly increases the bottom pressure. Nevertheless the pressure distributions are almost symmetric, so the behavior of the device in terms of energy catching should be quite similar to that with S ¼1/2. In contrast it is very interesting to see the results in the right panel, where the symmetry is broken, probably due to wave breaking or air trapped inside the Blow-Jet. In any case, the loss of pressure inside the device seems to be partially responsible for the poor performance in velocity amplification, as can be seen in Fig. 4 (tests 35, 42, 49 and 56). The analysis of all the pressure tests showed that when the device is horizontal it has, on average, better performance than when it is inclined, but when the central axis is inclined, it is possible to reach high peaks. This fact, together with the design size as a function of local wave climate, may give the optimum Blow-Jet for a specific location.
5. Numerical tests The 3D numerical model OpenFOAM was used to simulate the hydrodynamic patterns of the Blow-Jet experiments. The numerical toolbox used for these tests is that presented by Jacobsen et al. (2012) which is based on the Reynolds averaged Navier–Stokes equation coupled with a volume of fluid method and extended with a generic wave generation and absorption method. It is worth pointing out that the conical shape of the Blow-Jet makes the problem fully 3D because of wave reflection, pressure distribution and wave diffraction inside the device; so the optimization of the efficiency of the device as a WEC (numerical estimation of the expelled jet velocity) required 3D modelling. The setup selected for the numerical modelling is that of the second experiment, i.e. the Blow-Jet inside the acrylic box, as it is the one that gives information about all the processes of interest. In terms of the model input, the acrylic box and the device itself were considered as zero gradient boundaries, the inlet and outlet boundaries were considered relaxation zones of 1.5 m and 0.5 m long, respectively. The side walls of the flume were set to slip boundaries and free surface condition was left outside the device.
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Fig. 6. Pressure distribution inside the blow-jet for a 2 s wave with S ¼1/2, (left) panel axis angle ¼ 01 and (right) panel with axis angle ¼ 101.
Fig. 7. Pressure distribution inside the blow-jet, S¼ 1/3, axis angle ¼ 01 (left) and 10° (right), T¼ 2 s.
Several regular trains were numerically modelled, all with S ¼2/3 and θ ¼0. Only some representative results are shown in this paper. All the tests carried out have two main common features: almost no wave transmission after the acrylic box, which is a valuable hydraulic property as the device can be used to offer some coastal protection; and secondly, a vortex generated by the
submerged discharge from the output hole, which is not important as the jet would be conducted through a pipe to reach the turbine in a real case. This pipe was connected to the Blow-Jet in the physical experiments but was not included in the numerical grid. The comparison between the measured and computed pressures exerted at the bottom profile of the Blow-Jet is shown in
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Fig. 8. Time series of computed versus measured pressures at the bottom profile of the Blow-Jet (see P1–P4 in Fig. 6).
Fig. 8 for H ¼0.16 m, T ¼1.6 s, S ¼2/3 and θ ¼ 0. Four positions along the bottom profile have been selected to show the agreement found (see positions P1, P2, P3 and P4 in Fig. 6). In Fig. 8 it is clear that the 3D model results are in good agreement with the measured pressure values. The four panels in Fig. 8 show time series covering a little more than three waves; it can be seen that the first and third wave peak values are quite well reproduced while the peak of the second wave is always underestimated, probably because of the time step variability of the model. In Position 1, the peak value of the measured pressure is very similar for the three waves while the model shows a decrease from the first to the third wave, it is noticeable that the value of the peak of the third wave is very close to that estimated by the model. The comparison in Position 2 is similar to Position 1 with the difference that the peaks are a little higher. In Position 3 the measured peak pressure values show an increasing tendency from the first to the third wave and so does the numerical model, slightly overestimating the peak values; in Position 4 the increasing trend is also seen in the measured and computed values but in this case the computed to measured agreement is better. From the latter comments, it is seen that the model accurately reproduces the hydrodynamic behaviour inside the device. A full view of the numerical domain and some representative instants of velocity magnitude, toghether with isometric views of the numerical results are shown in the top panels of Fig. 9. The bottom panels of Fig. 9 show the velocitiy time series at the Blow-Jet output (Position A) and below the device (Position B). The model also estimates quite well the pressure transformation from a quasi-sinusoidal shape to the sawtooth shape, when it is highly influenced by wave breaking, that is, in the position closest to the entrance of the device (Position 1 in Fig. 8). The shape of the troughs is similar to sinusoidal ones; as the wave travels through the device, the troughs tend to “linearize”. This
behaviour is reproduced well by the model and, surprisingly, the agreement is better as the troughs are more deformed. In this sense, the OpenFOAM model is validated as a good tool for reproducing the dynamics inside the device.
6. Conclusions The work presented here describes physical and numerical tests carried out to evaluate the operation of an original WEC which has no moving parts and is capable of converting energy from low energetic wave trains. The device takes advantage of three main ideas: forcing the waves into a narrowing structure, the hydraulic operation of blowholes and the low resistance of the internal profile of a tuba to let a fluid flow. The experimental tests were conducted to record data regarding the velocity, wave and pressure fields generated during the operation of the device. In terms of velocity amplification (which is proportional to the efficiency) a submergence of 2/3 of the entrance diameter, with the device fully horizontal, seems to be the optimal position. On the other hand, an inclination of the central axis of the device, greater than 101, is not recommended. A saturation value was identified for the pressure distribution inside the device, which demonstrates that the design of the Blow-Jet depends on the wave height and period in order of relevance. A high-resolution 3D numerical model, OpenFOAM, was used to evaluate the flow field within the Blow-Jet. It was shown that the numerical results closely reproduced the dynamics within the device, confirming the usefulness of the model as a design tool for optimising the geometry of the device. Notably, two factors are identified as important with regards to the hydraulic operation of the Blow-Jet. Firstly, the possibility of constructing a support structure for the device, which can be
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Fig. 9. Overview of the modelled results of the Blow-Jet.
accommodated as shore protection (e.g. within a breakwater). Secondly, due to the oscillatory nature of the energy generated by the device, it is thought that its combination with conversion systems (e.g. hydrogen cells) could make it a strong candidate to supply electricity for small settlements near the coast or on islands. It is envisaged that the multi-purpose characteristic of the device may be positive in terms of the investment attractiveness of this development and from the results presented it should be stated that the Blow-Jet is a promising device that has proven good operation.
Aknowledgements The authors appreciate the partial funding of this research by DGAPA-UNAM under Contract PAPIIT IT101113 130254 – “Evaluación y optimización experimental de dispositivos de conversion de energía del oleaje”. We would like to thank Rodolfo Peters, Jesus Pinedo and Alejandro Bustos for their assistance with the laboratory work.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.oceaneng.2015.06.036.
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Contents lists available at ScienceDirect
Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Hydrodynamic behavior of a new wave energy convertor: The Blow-Jet Edgar Mendoza n, Xavier Chávez, Juan Carlos Alcérreca-Huerta, Rodolfo Silva Engineering Institute, National University of Mexico, Circuito Escolar s/n, Ciudad Universitaria, Coyoacán, 04510 México, DF, México
art ic l e i nf o
a b s t r a c t
Article history: Received 23 December 2013 Accepted 24 June 2015 Available online 28 July 2015
The work presented here focuses on evaluating the interaction between ocean waves and a device designed to harness their energy. The device, called Blow-Jet, consists of a narrowing structure that concentrates waves and transforms their energy from potential and kinetic to kinetic, turning the oscillatory flow into an intermittent jet that can be easily guided to an impulse turbine. The new device is designed to feed low energy demands such as secondary fuel generators (e.g. hydrogen production). Several experimental tests were carried out to investigate the hydraulic behavior of the Blow-Jet. Results indicate that even for low energetic wave trains, the device is able to capture energy and amplify the water velocity, which is indeed energy conversion capacity. In order to evaluate the flow field inside the device, a high resolution 3D numerical model OpenFOAM is utilized. Good agreement was found between the pressures exerted to the walls of the device with that computed with the numerical tool. The combination of numerical and experimental results confirmed the optimal geometry, location and position of the Blow-Jet, while the need for some geometric changes was detected after the analysis of numerical model results. The Blow-Jet has shown itself to be a potential WEC with acceptable efficiency. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Wave energy convertor Tapchan Blow-Jet OpenFOAM Blowhole Renewable energy
1. Introduction The search for a means of producing clean and renewable energy is underway worldwide and efforts focused on generating energy from almost every natural source available are being investigated. The marine environment is not an exception for researchers, as it provides many different possibilities including marine current power, tidal power, ocean thermal energy, osmotic power, salinity gradient power and wave energy. It has been stated that the combination of these sources may give enough energy to fulfill the entire needs of the planet one day. However, considering only wave energy, by 2030, an estimated of 150 GW of wave energy production would contribute 11–14% of total electricity consumption in Europe IEA/OES (2010); this statement is, partially, the motivation of the research work presented here. The generation of energy from waves has been a common challenge for many countries over the years, probably because of the proximity humankind has with waves. As capturing the energy of ocean waves in offshore locations has been demonstrated as technically feasible, there are a huge number of devices, systems and concepts designed to capitalize on wave energy, e.g. pelamis (Yemm et al., 2012), wave dragon (Kofoed et al., 2006), pontoon power (www.pontoon.no). Additionally, research in developing n
Corresponding author. E-mail addresses: [email protected] (E. Mendoza), [email protected] (X. Chávez), [email protected] (J.C. Alcérreca-Huerta), [email protected] (R. Silva). http://dx.doi.org/10.1016/j.oceaneng.2015.06.036 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
designs for wave energy conversion (WEC) devices is being conducted in countries such as USA (Rhinefrank, 2005), UK (Wavegen, 2002), Portugal (IEA/OES, 2010) and many others. Several reviews, e.g. (Falcão, 2010; Pérez-Collazo et al., 2015), can be consulted by the reader to get an updated view of the available technologies. Compared with other forms of offshore renewable energy, such as solar photovoltaic (PV), wind, or ocean current energy, wave energy is a continuous but highly variable source. Many of these devices have their implementation limited to high energy sea states, but this cannot be an obstacle to continued development in this field. In turn, a great advantage of waves is that they naturally travel towards the coast so there is almost no need to find a specific place to set the WEC devices. Every nation has different needs and socioeconomic conditions which affect research into wave generated energy production. In the case of Mexico, according to recent investigations, the density of wave energy in offshore areas has been quantified as not very high (Pontes and Falcão, 2001). However, due to the length of the Mexican coastline and environmental pressures (IPCC, 2007), it is reflected that the exploration of technology for wave energy generation is much needed. In this scenario, the National University of Mexico (UNAM) has been working on harnessing wave energy by means of wave amplifiers (Ruiz et al., 1994 and Alatorre, 2003) based on the fact that, with a few exceptions, the yearly mean sea states all around the country hardly reach 1–2 m of significant wave height (Silva et al., 2008), which means that any WEC device placed on the Mexican coast should work almost all year long. This paper
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presents a novel concept that combines various natural and artificial fluid structures to produce a wave energy catching device which covers the needs and conditions found on the Mexican coast. The investigation includes an exhaustive experimental program aimed at evaluating the efficiency of the device and its geometry. Furthermore, a recently developed toolbox by Jacobsen et al., 2012 is used for the examination of the pressure field within the device and compared to that observed in the laboratory during the operation of the device. It is anticipated that the combination of the experimental results with those from the numerical tool will enable optimization of the device. The aim of this paper is to analyze the response of the Blow-Jet to different wave conditions and to develop a numerical model from which the device can be further improved.
2. Concept of the Blow-Jet device The device presented in this paper takes some ideas from the narrowing structure for wave amplification called Tapchan (tapered channel) (Mehlum, 1985). The Tapchan was used to convert wave energy in Norway with relatively good results from 1985 to 1990, but its main disadvantage was that it needed a large reservoir to store the water that overtopped the structure. If this reservoir is not naturally found, its construction may make the system unaffordable; the same problem is found for almost any device working under the principle of turning kinetic to potential energy. On the other hand, an advantage of the Tapchan was the absence of movable parts which is very convenient, in terms of maintenance, for devices expected to work as much as possible throughout the year. The use of narrowing, energy harnessing structures enhances the energy of the devices as shown by Deng et al. (2014). The second idea behind the Blow-Jet is that of reproducing the hydraulic behavior of natural structures known as blowholes. Blowholes are relatively common coastal features and are usually given local, colloquial names; the most famous example in Mexico is that found on the Pacific, in Ensenada, Baja California, called the “Bufadora”, which is an onomatopoeic term. In terms of energy conversion, a blowhole turns relatively small waves into very strong air-water jets. The great advantage of this natural structure is that kinetic energy is amplified and the resulting jet can be easily directed to an impulse turbine. Also, once this water has been used to move the turbine, it can be returned to the sea and, unlike the Tapchan, no storage is needed. Unfortunately very few studies focused on the internal morphology of blowholes are available, but simple observation tells us that the key elements of these formations are a wide entrance, a submerged or semisubmerged compression chamber and a discharge hole. The different sizes, slopes and arrangement of these three components determine the force and air-water mixture of the expelled jet. If these elements and operation are to be used to design wave energy convertors, it is desirable to minimize wave reflection inside the device to avoid energy waste as much as possible and to reduce the amount of air in the jet, to reach higher efficiencies of the impulse turbine (e.g. Pelton turbine) that is to be used. It is important to note that a blowhole can also be seen as a natural
Oscillating Water Column device because the waves that get inside the structure compress the air contained in the chamber. The third idea used to define the Blow-Jet geometry comes from the design of the brass instrument, the tuba. These instruments are built on the basis that the effort needed to generate sound should be minimal; the internal profile of the instrument has to be optimum for sound waves to travel through it. Several tuba profiles can be found in the different brand catalogues; for the Blow-Jet those corresponding to F, C and B keys were selected for the construction and testing of a scale model. The actual size of the tubas is too small for the experimental facilities, so the equations had to be scaled; the design parameters and final profile expressions are shown in Table 1. Fig. 1 shows the fiber glass models used in the experimental tests. The proposed geometry differs from the Tapchan as it does not work by overtopping and the narrowing section of the Blow-Jet is circular. It differs from blowholes as the size of the compression chamber has been reduced and the expelled jet is horizontal instead of vertical. These characteristics turn the Blow-Jet into a novel and simple device with low maintenance needs, capable of converting energy from low energy sea states and suitable for applications that can be fed with variable power, e.g. the production of secondary fuels or energy storage systems. The simplicity of the Blow-Jet makes it attractive for inclusion in several projects and conditions; it can be placed, fixed, nearshore, floating offshore, or even onshore attached to a breakwater or coastal structure. Its construction is conceived as a monolithic block, so the Blow-Jet can easily be constructed “in situ”. It is foreseen that the best results will be found if this device is placed in farms (Renzi et al., 2014; Sarkar et al., 2014), which can also be used as coastal defence as shown by Mendoza et al. (2014).
3. Experimental set-up and program The wave flume at UNAM is 37 m long, 0.80 m wide and 1.20 m high, allowing a maximum submerged depth of 0.8 m. The wave generation system was acquired from HR Wallingford and is capable of generating waves with different shapes and spectra; it is controlled from a PC via the HR WaveMaker software. The flume has an active absorption system designed to absorb rereflected waves and a passive absorber has been placed at the other end of the flume. The experimental set up consisted of three wave gauges placed in front of the device for registering the incident and reflected waves, and one more placed behind the device to measure the transmitted waves. The water velocities at the entrance of the device and at the jet were measured with micro Acoustic Doppler Velocimeters and Particle Image Velocimetry. The distances between wave gauges were set according to Mansard and
Table 1 Design parameters for the three Blow-Jet models Model
Input diameter (m)
Output diameter (m)
Length (m)
1 (F) 2 (C) 3 (B)
0.366 0.443 0.418
0.021 0.021 0.021
0.630 0.800 0.860
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Fig. 1. Idealization of the least resistance profile.
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Fig. 2. Blow-Jet models.
Funke's (1980) methodology in order to determine incident and reflected wave energy (see Fig. 2). Two experiments were conducted with a fixed Blow-Jet to cover as many design aspects as possible. The first experiment focused on verifying whether the proposed geometry really could take waves and turn them into an expelled jet; i.e. to usable energy. For these tests, the efficiency of the device was determined in terms of measured velocities at the entrance and at the discharge hole. This first case required more than 300 tests in order to get a wide view of the hydraulic behavior of the device. Regular wave trains with different heights; H, (8-20 cm) and periods; T, (0.8-2.0 s) were generated. Four axis angles; θ, from the still water level to the device central axis were tested (0, 10, 20 and 301) for three submergences; S, (1/3, 1/2 and 2/3 of the BlowJet entrance diameter). The device was not tested fully submerged because the operation hypothesis considered having breaking wave conditions inside the device in order to get a stronger expelled jet; the right panel of Fig. 2 shows how S and θ are defined. Some conditions were eliminated from the analysis because waves were breaking before reaching the device and for some arrangements no jet was generated. With the purpose of investigating in greater detail the momentum generated by the waves inside the device chamber, a second experiment was carried out to register pressure inside the BlowJet. For this, ten pressure transducers were placed inside the device; five at the bottom, 15 cm apart, three at the horizontal symmetry plane, 30 cm apart and two more at the top, 15 cm apart. The pressure transducers measured up to 1.0 m of water column (see pictures in Fig. 3).
4. Experimental results In this section, the results obtained from the three experiments are presented. The most relevant result is the verification of the conversion of waves into a jet, together with the kinetic energy amplification. This can be translated as finding an increase in the velocity at the entrance compared to that at the output of the device (the ratio of the output to entrance velocities is called hereafter velocity amplification factor). The velocity amplification factor is also a measure of the efficiency of the device, as the power of the jet is directly proportional to the cube of the velocity: Nevertheless it should be pointed out that the over-all power output and efficiency of the device need to be affected by the
cross-sectional area of the device (i.e. the length of the wave front that is being harnessed). In this sense, the efficiency of the BlowJet can be further explained, in terms of converted energy, by the ratio of the available power at the entrance and at the output of the device. The power at the entrance can be expressed as P E ¼ 18 ρgH 2 DE V E
ð1Þ
where PE is the power at the entrance, ρ the water density, H the wave height, DE the entrance diameter of the Blow-Jet (i.e. capture width) and VE the velocity at the entrance of the device. In turn, the power available at the output (jet power) can be estimated as P J ¼ 18 πρD2o V 3o
ð2Þ
where PJ is the power at the output, Do the output diameter and Vo the velocity at the output. The energy balance equation for adiabatic steady flows states that the sum of kinetic energy and enthalpy (i.e. internal energy plus the flow work, given by the product for pressure and velocity) remains constant even in nonequilibrium conditions (e.g. in the presence of breaking waves in the duct). Therefore kinetic, potential and flow work at the inlet and at the outlet must be equal. However, at the duct entrance, the energy that can be converted is the potential fraction, whereas at the outlet the energy is mainly kinetic. Therefore, we define the efficiency as the ratio of the output to the input powers, which yields:
η¼
π V 2o Do g H2
D U VAF
ð3Þ
where D is the ratio of the entrance to the output diameters and VAF the velocity amplification ratio. Results in all tests indicate a velocity amplification factor greater than one, which in turn shows the positive performance of the WEC. The velocity amplification factor for all the tests in experiment one is shown in Fig. 4; it is important to note that a velocity amplification factor equal to zero may indicate that no jet was generated or that the test was not taken into account (mainly because of wave breaking before reaching the Blow-Jet). From Eq. (3) it is noted that the efficiency is directly proportional to the VAF, so the response of the Blow-Jet can be analyzed in terms of it, given that the flow velocities were the variables directly recorded during the experiments. Fig. 4 reveals interesting details about the hydraulic behavior of the Blow-Jet. Firstly, it shows that the central axis inclination of 301 has to be removed from any analysis, as the performance of the device is
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Fig. 3. Blow-Jet experiments, measuring (1) the hydraulic efficiency (left) and (2) the pressures inside the device (right).
Fig. 4. Velocity amplification factor.
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Fig. 5. (a) Tests found giving a linear trend and (b) tests found giving a parabolic trend.
very poor. Another observation is that although the maximum values of velocity amplification occur when S¼1/2, the best overall performance is found when S¼2/3. In general terms the Blow-Jet seems to be a good means of catching wave energy and the selected shape of the internal profile favors the velocity increase. To achieve a better understanding of the response of the device to local wave conditions, the dependency on wave period was investigated. It was almost impossible to define any clear trend with the available data, but analyzing the different tests separately, two features of the velocity at the expelling hole, Vo, as a function of kh were found (k being the wave number and h the still water level); quasi-linear (Fig. 5a) and parabolic (Fig. 5b). It is important to point out the differences between them; the quasi-linear trend means that for larger periods the device is giving unlimitedly higher jet velocities whereas if it is parabolic, it means that an optimal wave condition (kh 2) exists that makes the Blow-Jet operate at its best. In Fig. 5a and b, the trend is clearer if each group of data (symbols) is seen separately. The existence of a maximum value of Vo proves that the BlowJet has acceptable hydraulic efficiency and that its geometry can be determined as a function of the local wave climate (kh). As mentioned before, the second experiment gave results for the pressure distribution inside the Blow-Jet. In this paper some representative examples of all the information gathered are presented. The measured values of root mean square pressure for the top and bottom parts of the device have been drawn in Fig. 6 where the tests with S ¼1/2, T ¼2 s and θ ¼0 and 101 are shown in the left and right panels, respectively. In the left panel of Fig. 6 the pressure recorded close to the output hole is very similar at the top and at the bottom, indicating that for these wave trains the pressure is equally distributed in all directions. This result indicates that the end part of the Blow-Jet is working as a fully filled pipe, it was observed that the expelled jet had low air content which is desirable. When the device is inclined, the pressure envelope registered at both locations also shows a symmetric signature; but with the difference that for higher wave heights a saturation value for the registered pressure is identified (see right panel Fig. 6). This suggests that a maximum wave height exists, at which the Blow-Jet produces its best performance. This is in agreement with the parabolic behavior shown in Fig. 5b and confirms that the design of the Blow-Jet can
easily be expressed as a function of the local wave climate. Fig. 7 shows the pressure distributions for S ¼1/3, T¼ 2 s, and θ ¼0 and 101 in the left and right panels, respectively. A relevant difference that can be seen in Fig. 7 is that the degree of submergence slightly increases the bottom pressure. Nevertheless the pressure distributions are almost symmetric, so the behavior of the device in terms of energy catching should be quite similar to that with S ¼1/2. In contrast it is very interesting to see the results in the right panel, where the symmetry is broken, probably due to wave breaking or air trapped inside the Blow-Jet. In any case, the loss of pressure inside the device seems to be partially responsible for the poor performance in velocity amplification, as can be seen in Fig. 4 (tests 35, 42, 49 and 56). The analysis of all the pressure tests showed that when the device is horizontal it has, on average, better performance than when it is inclined, but when the central axis is inclined, it is possible to reach high peaks. This fact, together with the design size as a function of local wave climate, may give the optimum Blow-Jet for a specific location.
5. Numerical tests The 3D numerical model OpenFOAM was used to simulate the hydrodynamic patterns of the Blow-Jet experiments. The numerical toolbox used for these tests is that presented by Jacobsen et al. (2012) which is based on the Reynolds averaged Navier–Stokes equation coupled with a volume of fluid method and extended with a generic wave generation and absorption method. It is worth pointing out that the conical shape of the Blow-Jet makes the problem fully 3D because of wave reflection, pressure distribution and wave diffraction inside the device; so the optimization of the efficiency of the device as a WEC (numerical estimation of the expelled jet velocity) required 3D modelling. The setup selected for the numerical modelling is that of the second experiment, i.e. the Blow-Jet inside the acrylic box, as it is the one that gives information about all the processes of interest. In terms of the model input, the acrylic box and the device itself were considered as zero gradient boundaries, the inlet and outlet boundaries were considered relaxation zones of 1.5 m and 0.5 m long, respectively. The side walls of the flume were set to slip boundaries and free surface condition was left outside the device.
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Fig. 6. Pressure distribution inside the blow-jet for a 2 s wave with S ¼1/2, (left) panel axis angle ¼ 01 and (right) panel with axis angle ¼ 101.
Fig. 7. Pressure distribution inside the blow-jet, S¼ 1/3, axis angle ¼ 01 (left) and 10° (right), T¼ 2 s.
Several regular trains were numerically modelled, all with S ¼2/3 and θ ¼0. Only some representative results are shown in this paper. All the tests carried out have two main common features: almost no wave transmission after the acrylic box, which is a valuable hydraulic property as the device can be used to offer some coastal protection; and secondly, a vortex generated by the
submerged discharge from the output hole, which is not important as the jet would be conducted through a pipe to reach the turbine in a real case. This pipe was connected to the Blow-Jet in the physical experiments but was not included in the numerical grid. The comparison between the measured and computed pressures exerted at the bottom profile of the Blow-Jet is shown in
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Fig. 8. Time series of computed versus measured pressures at the bottom profile of the Blow-Jet (see P1–P4 in Fig. 6).
Fig. 8 for H ¼0.16 m, T ¼1.6 s, S ¼2/3 and θ ¼ 0. Four positions along the bottom profile have been selected to show the agreement found (see positions P1, P2, P3 and P4 in Fig. 6). In Fig. 8 it is clear that the 3D model results are in good agreement with the measured pressure values. The four panels in Fig. 8 show time series covering a little more than three waves; it can be seen that the first and third wave peak values are quite well reproduced while the peak of the second wave is always underestimated, probably because of the time step variability of the model. In Position 1, the peak value of the measured pressure is very similar for the three waves while the model shows a decrease from the first to the third wave, it is noticeable that the value of the peak of the third wave is very close to that estimated by the model. The comparison in Position 2 is similar to Position 1 with the difference that the peaks are a little higher. In Position 3 the measured peak pressure values show an increasing tendency from the first to the third wave and so does the numerical model, slightly overestimating the peak values; in Position 4 the increasing trend is also seen in the measured and computed values but in this case the computed to measured agreement is better. From the latter comments, it is seen that the model accurately reproduces the hydrodynamic behaviour inside the device. A full view of the numerical domain and some representative instants of velocity magnitude, toghether with isometric views of the numerical results are shown in the top panels of Fig. 9. The bottom panels of Fig. 9 show the velocitiy time series at the Blow-Jet output (Position A) and below the device (Position B). The model also estimates quite well the pressure transformation from a quasi-sinusoidal shape to the sawtooth shape, when it is highly influenced by wave breaking, that is, in the position closest to the entrance of the device (Position 1 in Fig. 8). The shape of the troughs is similar to sinusoidal ones; as the wave travels through the device, the troughs tend to “linearize”. This
behaviour is reproduced well by the model and, surprisingly, the agreement is better as the troughs are more deformed. In this sense, the OpenFOAM model is validated as a good tool for reproducing the dynamics inside the device.
6. Conclusions The work presented here describes physical and numerical tests carried out to evaluate the operation of an original WEC which has no moving parts and is capable of converting energy from low energetic wave trains. The device takes advantage of three main ideas: forcing the waves into a narrowing structure, the hydraulic operation of blowholes and the low resistance of the internal profile of a tuba to let a fluid flow. The experimental tests were conducted to record data regarding the velocity, wave and pressure fields generated during the operation of the device. In terms of velocity amplification (which is proportional to the efficiency) a submergence of 2/3 of the entrance diameter, with the device fully horizontal, seems to be the optimal position. On the other hand, an inclination of the central axis of the device, greater than 101, is not recommended. A saturation value was identified for the pressure distribution inside the device, which demonstrates that the design of the Blow-Jet depends on the wave height and period in order of relevance. A high-resolution 3D numerical model, OpenFOAM, was used to evaluate the flow field within the Blow-Jet. It was shown that the numerical results closely reproduced the dynamics within the device, confirming the usefulness of the model as a design tool for optimising the geometry of the device. Notably, two factors are identified as important with regards to the hydraulic operation of the Blow-Jet. Firstly, the possibility of constructing a support structure for the device, which can be
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Fig. 9. Overview of the modelled results of the Blow-Jet.
accommodated as shore protection (e.g. within a breakwater). Secondly, due to the oscillatory nature of the energy generated by the device, it is thought that its combination with conversion systems (e.g. hydrogen cells) could make it a strong candidate to supply electricity for small settlements near the coast or on islands. It is envisaged that the multi-purpose characteristic of the device may be positive in terms of the investment attractiveness of this development and from the results presented it should be stated that the Blow-Jet is a promising device that has proven good operation.
Aknowledgements The authors appreciate the partial funding of this research by DGAPA-UNAM under Contract PAPIIT IT101113 130254 – “Evaluación y optimización experimental de dispositivos de conversion de energía del oleaje”. We would like to thank Rodolfo Peters, Jesus Pinedo and Alejandro Bustos for their assistance with the laboratory work.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.oceaneng.2015.06.036.
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