Self-Sustaining Street Light Introduction
Paper 131-192
Reid A. Berdanier, Karen E. Hernandez, Charles P. Raye, Christopher P. Horvath, Laura M. Graham, Timothy P. Hatlee, Nhan H. Phan, P. Michael Pelken & Thong Q. Dang
•Fully integrated, off-the-grid street lighting • Utilize a vertical-axis wind turbine (VAWT) and photovoltaic (PV) solar cells to gather energy to power a light-emitting diode (LED) street light • Decrease money spent on lighting • Significantly decrease CO2 emissions
•Use a housing with funneling to amplify the wind velocity entering the VAWT
Design Highly-integrated and aesthetic design of a self-sustainable street light. LED lights, PV solar cells work synergistically with the VAWT. Housing of the light post provides wind amplification effect and a surface on which to mount the LED light and PV solar panel.
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Self-sufficient as a goal of sustainability Creating products that produce the power that is required for operation. a) existing self-sustaining streetlight product with add-on wind turbine and PVs. b) fully integrated self-sustaining streetlight design. c) and d) buildings implementing a similar funneling effect to increase wind speed for integrated wind turbines
Rotor Design Vertical Axis Wind Turbine: •Omni-directional wind acceptance Two phase-offset stages Savonius Bach-type Rotor: •Low cut-in speed
CFD Analysis and Results
a) b) a) Rendering of prototype design b) Rendering of future production model Computational fluid dynamics tests were performed to quantify the wind amplification effect’s influence on rotor power coefficient Converging housing accelerates the wind speed prior to engagement with the wind turbine. Converging angle is limited by potential flow separation.
Initial Test Results Post-production prototype tests showed successful power production and self cut-in of wind turbine in low wind. Future tests will include measurement of power production capability as a function of wind speed and solar radiation.
Integrating Vertical-Axis Wind Turbines and Photovoltaic Solar Cells to Power a Self-sustaining Outdoor Light Source Reid A. Berdanier1,*, Karen E. Hernandez1, Charles P. Raye1, Christopher P. Horvath1, Laura M. Graham1, Timothy P. Hatlee1, Nhan H. Phan2, P. Michael Pelken3 and Thong Q. Dang4 1
Undergraduate Student, Dept. of Mech. & Aerospace Eng., Syracuse University, New York Ph.D. Candidate, Dept. of Mech. & Aerospace Eng., Syracuse University, New York 3 Assistant Professor, School of Architecture, Syracuse University, New York 4 Professor, Dept. of Mech. & Aerospace Eng., Syracuse University, New York 2
*Corresponding email: rberdani@syr.edu
ABSTRACT This paper investigates the development of a new and innovative self-sustaining outdoor light source through the implementation of vertical-axis wind turbines (VAWTs) and photovoltaic solar cells. This fully-integrated design improves upon existing designs which already address the combination of these efficient energy-producing technologies. Currently existing devices are characterized by an additive combination and the assembly of readily available system parts. The introduction of a VAWT provides the ability for wind energy to be harnessed from any wind direction. This omni-directionality of the VAWT is further increased by the introduction of multiple, phase-offset rotor stages. Moreover, this design incorporates a unique feature enabling the amplification of wind velocities through a converging housing section, allowing for an increase in rotor power coefficient and a low cut-in wind speed. This wind funneling effect is implemented via synergistic integration of the three main product components of the lighting system: wind turbine, solar panel, and light-emitting diode (LED) lighting. Computational fluid dynamics results were incorporated to assist in the development of the rotor sizing parameters and the turbine housing. An LED light product was selected to reduce the amount of power required from the energy production systems. Given the required power of an LED light for an average ten hour night, individual components of the light source, including the wind-swept area of the rotor and the effective area of the PV cells were appropriately defined for sufficient power production. Development and testing of a functioning prototype is also discussed. KEYWORDS Wind turbine, VAWT, Solar, Self-sustaining, Light source INTRODUCTION For the last half century, communities such as New York City have utilized both high pressure sodium (HPS) and metal halide fixtures for their street lights, which are considerably inefficient by modern-day standards (Galgano et al. 2009). Despite efforts to install more efficient luminaires, the New York City Department of Transportation (NYCDOT) continues to purchases 259.2 million kWh (approx. $31.1 million) annually solely to power street lights (Bloomberg et al. 2008). Specifically, the standard luminaires used for street lighting by NYCDOT are the 100W and 150W HPS cobra head and the standard luminaires used for pedestrian lighting are 100W and 70W HPS. It is estimated that reducing the energy consumed by streetlights in NYC by 50% through the use of light-emitting diode (LED) luminaires would considerably reduce CO2 emissions. However, these emissions could be
further reduced to zero if self-sustaining street lights were implemented. The main challenge which remains is to determine balanced sizing parameters for such a light source design in order to obtain sufficient solar and wind energy.
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Figure 1. a) Street light with add-on PVs and wind turbine, b) Integrated self-sustaining light post, c) Bahrain World Trade Center building, d) China Pearl River building. Several companies have developed products that accomplish this goal of sustainability. Each of these products maintains the common thread of being grid-independent; in other words, they are self-sufficient in terms of producing the power they require for operation. Figure 1a shows a typical design of a LED street light powered by photovoltaic (PV) solar cells and a wind turbine. It is clear from the picture that the design is an “add-on” design; in other words, the two sustainable components are simply added to a conventional street light. In this paper, a highly integrated and aesthetic design of a self-sustainable street light is presented and is shown in Figure 1b. Through this design, the LED lights and the PV solar cells work synergistically with a vertical-axis wind turbine (VAWT) to provide the funneling effect of the wind (also referred herein as wind amplification effect). Such a concept is not new and has recently been implemented in large-scale commercial buildings in Bahrain and China (see Figures 1c and 1d). METHODS The main feature of the proposed self-sustaining light post is in the design of the housing for the wind turbine. The housing provides not only the wind amplification effect for the wind turbine, but also means to mount the LED light and the PV solar panel. This investigation looks to improve upon each of the designs discussed earlier through the optimization and inclusion of a few key characteristics. Primarily, the energy harvesting devices – notably, the PV solar cells and the VAWT – and the LED luminaires for the new product design are integrated into the overall housing of the system. This integration allows for components to benefit the functionality of one another. Such functionality is highlighted by the central design feature of the lighting system’s housing. Through the combination of these characteristics, a final product design capable of significant marketability on the fronts of integration and sustainability is expected.
A VAWT was selected over traditional horizontal-axis wind turbines (HAWTs) which are implemented by companies such as Loopwing and Tangarie due to axial symmetry, making it ideal for aesthetically-pleasing inclusion. Moreover, VAWTs provide further benefit through their ability to accept wind from any direction; there is no need for a VAWT to change orientation through the use of a directional vane, as with a HAWT. Within the VAWT subdivision, a selection needs to be made between a lift-based turbine (e.g., Darrieus rotors and H-rotors) and a drag-based turbine (e.g., Savonius rotors). Comparing performance characteristics for these two designs, lift-based rotors prevail in their slightly higher values of power coefficient CP (percentage of power extracted from wind power = 12 ρ V 3 per unit area) and their ability to operate over a wider range of tip-speed ratios λ (ratio of blade-tip speed to wind speed). The Savonius rotors’ maximum efficiency of around 0.30 occurs at a tip-speed ratio near 1 (low RPM at a given wind speed), while the Darrieus rotors’ maximum efficiency of around 0.35 occurs at a much higher tip-speed ratio of around 5 (high RPM at a given wind speed). Despite their lower performance characteristics, drag-based rotors are the focus of this investigation due to their relative simplicity and lower cut-in speed. However, it is important to note that lift-based rotors could also be integrated into the proposed design.
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Figure 2. Examples of rotor cross-sections. a) Traditional Savonius rotor, b) Bach rotor. Over the last century, significant research has been conducted on the performance of Savonius rotors, with particular consideration given to methods of improving upon the low CP mentioned earlier. An evolution of traditional Savonius blade cross-sections occurred under the leadership of Bach to create a slightly more efficient cross-section shown in Figure 2b (Bach, 1931). However, further parametric optimization has been conducted, in particular by Ushiyama et al. (1986) and Menet & Bourabaa (2004), to extract as much performance from these rotors as possible. Of particular interest for improving upon efficiency for this design is the implementation of phase-offset rotor stages, as proposed by Hayashi et al. (2005). By incorporating this design alteration in combination with a drag-type rotor, it is possible to mitigate large fluctuation patterns of static torque. Each of these design modifications have been given careful consideration for the rotor design used with this light source. For the final design, it is assumed that the VAWT cross-section will be similar to the Bach model in Figure 2b and that two stages with orthogonal orientation on a common shaft will be utilized. The proposed design implements a wind turbine housing which has the effect of accelerating the wind speed prior to engagement with the wind turbine. Figure 3a shows the housing design where the turbine is placed in between two plates with a converging angle. Since wind power is proportional to wind velocity cubed, a 25% increase in wind speed can effectively double the amount of power harvested from the wind energy. For a given geometry, however, the magnitude of the converging angle is limited by potential flow separation occurring as the airflow exits the turbine (see Figure 3a). Another important advantage of the increased air
speed is the ability of the wind turbine to operate at a lower cut-in wind speed. As mentioned earlier, the two plates also serve as platforms to install a PV solar panel (top plate) and the LED lights (bottom plate). The synergistic integration of these components is further improved with an aesthetic design shown in Figure 1b.
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Figure 3. Rotor and housing design concepts. a) Housing for VAWT, b) 3D CFD model In order to quantify the performance characteristics of a VAWT placed in a housing design with converging plates, computational fluid dynamics (CFD) simulations were used. As the flow is three-dimensional (3D) and unsteady, the most accurate method to simulate the flow around the turbine and its housing is the three-dimensional unsteady “sliding mesh” CFD method. In this technique, the computational domain is divided into two types of zones. For the turbine, a circular zone containing the turbine blades rotates at the turbine rotational speed, while the remaining zones exterior to the turbine blades are stationary. During the calculation, the turbine-blade zone “slides” relative to stationary zones along the grid interface in discrete steps (see Figure 4a), and the unsteady Reynolds Averaged Navier-Stokes equations are solved in the moving coordinate system. At the sliding mesh interface between the rotating zone and the stationary zone, flow variables and their gradients are carefully interpolated so that mass conservation and accuracy of the numerical scheme are preserved. A summary of the sliding mesh technique developed specifically for cross-flow fan analysis can be found in Moon et al. (2003). Kummer & Dang (2006) and Dygert & Dang (2009) have successfully applied the sliding-mesh method to simulate a propulsive airfoil concept with the cross-flow fan, including experimental validations.
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Figure 4. Mesh Construction. a) Unsteady “sliding mesh” method, b) Geometry of Savonius rotor.
The CFD simulations were performed using FLUENT as a flow solver with models and meshes created in GAMBIT (2D) and GRIDGEN (2D and 3D). The flow was assumed fully turbulent, and the k-ε turbulence model with a standard wall function was chosen. Although this assumption is inaccurate for the present application (as the flow is rather laminar/transitional due to its small size and low wind speed), this should not affect the prediction of the torque due to the fact that the turbine employed is of the pressure drag type (i.e. Savonius type – the torque produced by the turbine is a result of the difference in pressure drag between the retreating and advancing blades). Note that this approximation would not be adequate to simulate the lift-based VAWT, where the torque prediction is highly dependent on the lift/drag over the airfoil-type rotors, which in turn are highly dependent on the airfoil’s Reynolds number. Typically, 20,000 cells were used for a two-dimensional simulation, while a 3D simulation required around 1.6 million cells. A typical 3D surface grid is shown in Figure 3b. Mesh cells are clustered near the surfaces to give y+ values in the range of 50-300. In the unsteady sliding-mesh calculation, the time step taken was 1/30th of the turbine rotor period (i.e. 30 time steps per revolution), and a typical unsteady simulation required about five revolutions to converge to a periodic state. In terms of computation time, a 3D unsteady simulation with 1.6 million cells required about 24 hours using a 4-core Intel Core-i7 CPU. RESULTS CFD simulations The CFD code was first validated against test data reported in Le Gouriérès (1982) and Menet & Bourabaa (2004); this data is available for the baseline VAWT without the housing. The radius of the rotor used was d = 0.5 meter, and the geometrical parameters e = d/6 and e’ = 0 (see Figure 4b); the height of the rotor was taken to be 1 meter. Due to symmetry, only half of the rotor height was simulated. Figure 5a shows a comparison of the power coefficient CP as a function of tip speed ratio λ between test data and CFD results, showing very good agreement over a wide operating range. For the baseline VAWT, it is seen that the maximum power coefficient is 30% (or 30% of the wind power is harvested) at a tip speed ratio of around 1 (blade tip speed equal to wind speed). It is noted that in the range of tip speed ratio of 0.6 to 1.3, the corresponding power coefficient is in the range of 0.25 to 0.30. These results show that the CFD model employed here can predict CP very accurately. This validation study gives confidence that the CFD model can accurately predict the VAWT performance with the housing. Next, the VAWT was placed in a housing and the wind amplification effect (or a converging flow path for the inlet side) was simulated, with the converging angle varying from 0 degrees to 30 degrees (refer to Figure 3a for the definition of converging angle). Note that no experimental data is available for these coupled VAWT/housing geometries. The results of power coefficient CP versus tip-speed ratio λ are shown in Figure 5b. The CFD results show that, with the proposed housing, the performance of the turbine is greatly enhanced. The power coefficient CP gradually increases with increasing converging angle, and it is nearly doubled to 0.60 with a converging angle of 30 degrees. Note that the tip-speed ratio at the maximum power coefficient changes only slightly, from 1 to 1.1. Moreover, steady increases in performance are observed as the converging angle is increased – even up to an angle of 30 degrees – signifying that concerns of flow separation discussed earlier appear to be unwarranted.
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Figure 5. CFD simulation results. a) Validation with test data for Savonius rotor data without housing, b) Effect of housing for various converging angles. Physical model design As one might expect, the LED luminaire was the initial focus, due to the nature of the project and the fact that the light is the primary basis for this light source development. The power requirements of the luminaire were determined by first selecting a product which provides adequate spread of illumination (as per supplier isofootcandle plots) at the determined pole height – in this case, 7.62 meters. Given this required amount of system power production, a parametric investigation into the sizing requirements of other parts of the system began. By varying the housing diameter, a corresponding size constraint for the PV panel was adjusted, thus changing the available solar power output. Moreover, by changing the effective wind-swept area of the rotor, the power changed accordingly. At this stage, initial wind power calculations were based on experimental power coefficient data acquired by Modi & Fernando (1989) for a rotor cross section similar to the one shown in Figure 2b; the power values at this stage were calculated according to a wind velocity design point of 4.5 m/s – an average value corresponding to local wind data acquired at Hancock International Airport in Syracuse, New York. Table 1. Light source sizing parameters. Pole Height (m)
Solar Area (m2)
Outer Housing Diameter (m)
Wind-Swept Area (m2)
Single Rotor Diameter (m)
Single Rotor Height (m)
7.62
0.48
1.52
1.25
0.91
0.69
This parametric investigation yielded a series of possible size combinations for the PV panel, outer housing diameter, and wind-swept rotor area (defined by the product of the rotor diameter and height). Finally, the rotor area was decomposed into diameter and height according to aspect ratio, AR, observations made by Modi and Fernando (1989) showing ideal conditions occurring for the case of AR = 0.77 (see Table 1). The following information should be noted for the data from Table 1: values for rotor area correspond to a two-stage rotor design explained earlier, therefore providing for an effective doubling of wind-swept area; solar area was calculated assuming a PV efficiency factor of 18%; and calculations for these initial sizes were performed according to an assumption of an average illumination requirement of a ten-hour night for the previously stated average wind velocity of 4.5 m/s.
Finally, the evolution to the final design of the self-sustaining light post is shown in Figure 6, including the engineering prototype and the foreseen production model.
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Figure 6. Evolution of design. a) Prototype model under construction, b) Rendering of future production model. DISCUSSIONS From the results of Figure 5b, it is apparent over the investigated range of angles that increases in converging angle lead to corresponding increases in power coefficient. These data are particularly promising for the design of this light source because it shows that large converging angles are acceptable, thus allowing for a decrease in rotor area without compromising power output capabilities. Such a decrease in rotor area is beneficial because it then allows for a decrease in overall light source size – a definite advantage considering the calculated size required for appropriate operation in Table 1.
Currently, a scaled prototype is in production as a method of validating the results of this study. Primarily to serve as a proof of concept and a test bed for experimental substantiation of the converging section, this prototype model is being built to provide illumination over a five hour period – half of the ten hour period accounted for previously. This decrease in required power provides the opportunity to decrease the size of the wind-swept rotor area by a factor of two or, alternatively, the diameter and height each by a factor of 2 . Moreover, this prototype implements a converging angle of 15 degrees, a conservative value given the data from Figure 5b. The prototype model incorporates a WindMax hybrid charge controller which accepts independent inputs from the PV and from the wind turbine while concurrently monitoring these inputs and sending an appropriate output to the battery (a generic 12V automotive battery for the prototype). Upon completion, this prototype can be experimentally explored and modifications to the design and/or sizing parameters will be made as necessary. Beyond testing of this prototype model, additional future recommendations for inquiry include the incorporation of a lift-type VAWT – ideally achieving higher values of CP – and larger angles of convergence; each of these changes could allow the opportunity for further allowable size parameter decreases. The simulated results are extremely promising for the development of this light source and the authors expect similarly promising results from experimental results and prototype field testing.
CONCLUSIONS Overall, the goal of this design is to provide a means for producing grid-independent area lighting. This light source technology can easily be integrated into any new or existing plans for building walkways, parking lots, and outdoor recreation facilities looking to increase sustainable energy productivity. In addition, further applications of this light source can be applicable to remote areas where grid power is unavailable or where area lighting is needed only for a short period of time (e.g., taking the place of other rental illumination units used in construction areas while avoiding the need for energy consuming generators). The synergistic integration of this light source design makes it superior to its competitors, particularly through its high efficiency (high power coefficient CP), omni-directionality, compact size, low turbine cut-in speed, and grid independence. ACKNOWLEDGEMENTS This project was funded in part by the Mechanical & Aerospace Engineering department at Syracuse University and Prof. Michael Pelken’s Syracuse University School of Architecture / Syracuse Center of Excellence Fellowship. The authors gratefully acknowledge this financial support. Special thanks are also given to Dr. Basman Elhadidi and Dr. Ryan Dygert for their consultations throughout the design process and for their assistance with preparing experimental procedures; and to Prof. Frederick Carranti for his classroom instruction and consultation beyond the classroom. REFERENCES Bach G. 1931. Untersuchungen über Savonius-Rotoren und verwandte Strömungsmaschinen. Forschung im Ingenieurwesen, 2(6), pp. 218-31. Bloomberg M., Skyler E., Kay J. 2008. Mayor’s Management Report, City of New York (USA), 220 pages. Dygert R.K. and Dang T. 2009. Experimental Investigation of Embedded Cross-Flow Fan for Airfoil Propulsion/Circulation Control. AIAA Journal of Propulsion and Power, 25(1), pp. 196-203. Galgano S., Patel G., Newman M., Richardson H., Femminello, A. 2009. Green Light: Sustainable Street Lighting for NYC (USA), 14 pages. Hayashi T., Li Y., Hara Y. 2005. Wind Tunnel Tests on a Different Phase Three-Stage Savonius Rotor. JSME International Journal Series B, 48(1), pp. 9-16. Ishimatsu K., Kage K., Okubayashi T. 2006. Simulation for the Flow around Cross Flow Turbine with End Plates. Japan Society of Computational Fluid Dynamics, 14th JSCFD Symposium, C06-2, 5 pages. Kummer J.D. and Dang T.Q. 2006. High-lift propulsive airfoil with integrated crossflow fan. AIAA Journal of Aircraft. 43(4), pp. 1059-68. Le Gouriérès D. 1982. Wind Power Plants: Theory and Design. Oxford, England. Menet J. and Bourabaa N. 2004. Increase in the Savonius rotors efficiency via a parametric investigation. In: Proceedings of the EWEA - 2004 European Wind Energy Conference, London, 11 pages. Modi V.J. and Fernando M.S.U.K. 1989. On the performance of the Savonius wind turbine. Journal of Solar Energy Engineering, 111, pp. 71-81. Moon YJ, Cho Y, Nam HS. 2003. Computation of unsteady viscous flow and aeroacoustic noise cross flow fans. Computers & Fluids, 32, pp. 995-1015. Ushiyama I., Nagai H., Shinoda, J. 1986. Experimentally determining the optimum design configuration for Savonius rotors. Bulletin of JSME, 29(258), pp. 4130-38.