Performance Analysis of a Savonius Wind Turbine in the Solar Integrated Rotor House

Rooftop, building integrated and building augmented micro wind systems have the potential for small scale power generation in the built environment. Nevertheless, the expansion of micro wind technology is very slow and its market is strongly affected by the low efficiency of conventional wind generators. WAG-RH (Wind Accelerating and Guiding Rotor House) which is a new technique introduced to enhance the efficiency of vertical axis rotor. The present study utilizes other green energy element by integrating the WAG-RH with a solar heating system. In this effort roof of the WAG-RH has been utilized to heat air through micro solar chimney for creating buoyancy effect in the air flow channel at rotor zone in the WAG-RH. The integration is capable of improving the performance of rotor setup in the WAG-RH as well as providing hot air with sufficient air mass flow rate for space heating. The WAG-RH had brought about 138% increase in the performance coefficient(Cp) of conventional three bladed Savonius rotor, whereas solar integrated WAG-RH has contributed 162% increase in the Cp of the same rotor.

Both are freely available, reliable and clean energy sources.
The ascending trend of power generation from renewable energy sources shows good share of wind energy and plays a major role in replacement of fossil fuels [4,5]. The estimates are that, wind energy is expected to provide about 9% of global electricity by 2030 and about 12% by 2050 [6,7].
Though, building-integrated micro-wind turbines are the potentially low-cost wind power generating systems but the reasons behind their limited installation in urban areas are the low mean wind speeds, and high levels of turbulence [8,9].
The systems designs which incorporate such wind conditions and have ability to increase flow velocity in the rotor zone [10][11][12][13][14] were being suggested for the built environment. In this connection the literature prefers the use of wind turbine enclosures with wind accelerating characteristics [15][16][17][18].
It is also notable that the socio-economic development of any nation in the world depends on the provision of reliable electricity supply systems. However, a significant drawback of the two major renewable energy sources, such as, solar and wind is that their output power depends largely on the unpredictable weather or climatic changes. The renewable energy systems working in hybrid techniques have been preferred to cover such drawbacks. One source of energy can cover the weakness of the other, hence, improves the generation, economy and reliability of system. One of the advantages of a wind and solar hybrid system is that its reliability is enhanced compared with their simple working systems [19][20][21]. Solar air heating is a renewable energy heating technology employed to heat or condition air for buildings or process heat applications. A solar air collector integrated with a central updraft tower which has been employed, since long, for generating solar induced air flow to ventilate buildings and the power of this air flow to drive turbines for generating electricity [22][23][24]. In the ventilation process air contained in the solar collector, is heated by solar heat flux so that its density is reduced. The central updraft tower being opened at upper end allows the less density lighter air to be pulled by buoyancy to flow upward through the tower or a chimney-liked structure. A solar collector intake, opens in the structure to be ventilated, sucks impure air, which, is manipulated to induce the surrounding air to ventilate the building. Solar chimney system which, can make full use of solar energy to produce upward momentum to a mass of air thereby converting thermal energy into kinetic energy.
Here, the literature concludes with recommendations that clean and renewable energy sources must be preferred.
Building-integrated micro-wind turbines are better choice but their limitations should be addressed by using efficient technologies. Combination of two green energy elements is mostly suggested for power generation enhancement and achieves reliability.

MATERIALS AND METHOD
The WAG-RH is a novel technique proposed by the author to improve performance of vertical axis wind turbine. The WAG-RH improved the performance coefficient of a three bladed Savonius rotor up to 138%.
The rotor house is an omni-directional structure which

Methodology
Changing temperature at the hot space with the help of electric heater affects the velocity of air in the HV-channel.
Temperature and air velocity at various points of importance were measured using thermocouples and hot wire anemometer respectively.

RESULTS AND DISCUSSION
The

Air Flow in the HV-Channel
Air flow velocity, mass flow rate and air flow temperature at different locations in the HV-channel have been investigated for different hot space temperatures, with and without any support from external wind flow.

Buoyancy Operated Air Flow
The The velocity magnitude at RH inlet was too small to measure for all hot space temperatures, hence, excluded, and for remaining two locations is shown in Fig. 3(a). Fig.   3(a) indicates that the air velocity at tower exit is nearly double than that at the heater inlet. Here, the reason is that, the heater inlet area is double than the tower exit area. Further, it indicates that air velocity in the HV-channel increases with increase in temperature of hot space.
The corresponding air mass flow rates calculated at three locations in the HV-channel are shown in Fig. 3(b), and the measured air flow temperatures are indicated in Fig.   3(c). According to Fig. 3(a-c) the air mass flow rate and air flow temperature increase with increase in the temperature of hot space. These Fig. 3(a-c) conclude that hot space temperature acts as air accelerator in the HV-channel which starts from RH inlet and ends to tower exit. The flowing hot air can be utilized for space heating and momentum can be used to generate power.

Buoyancy-Cum-External Wind Supported Air Flow
In this case, external wind flow at different speeds was allowed to pass through the RH using open circuit subsonic wind tunnel and the velocity of air in the HVchannel was kept under observation. In these tests temperature of hot space was kept same as ambient. Then, in the second step, by changing hot space temperature, combined effect of buoyancy and external wind flow on the air velocity in the HV-channel was investigated. Fig. 4(a-b), indicate change in air velocity at heater inlet and tower exit, and Fig. 5(a-b), describes air mass flow rate at single entry of RH inlet and tower exit, in the HVchannel verses hot space temperature for different external wind flows.
Here, in Figs  The air mass flow rate for the same test case has been described through Table 2 which shows 0.053Kg/s mass flow rate at the tower exit with 5m/s wind speed from wind tunnel at ambient temperature. Further, by setting hot space temperature in three magnitudes, such as, 50, 60, and 70 o C, increase in the air mass flow rate at tower exit of the HV-channel was recorded as 11.32, 28.3, and 41.5%, respectively.
Air flow temperature in the HV-channel has been investigated which is explained through Table 3.
Temperature of air flowing out of the tower exit of the HVchannel depends upon hot space temperature and the air mass flow rate at the tower exit.

TABLE 1. AVERAGE AIR VELOCITY IN THE HV-CHANNEL CAUSED BY BUOYANCY ALONE AND BUOYANCY-CUM-EXTERNAL WIND FLOW
space temperature the temperature of air leaving the tower is different for different air mass flow rates and shows inverse relation with the air mass flow rate.

Effect of Air Flow in the HV-Channel on the Performance of Vertical Rotor in the WAG-RH
The previous section concludes that hot space temperature, showing buoyancy principle, acts as air accelerator in HV-channel. Since, the air flow channel passes from the rotor zone, therefore, the flow must exert force on the rotor blades. Nevertheless, the air flow in the channel changes direction from horizontal to vertical without applying significant force on the vertical rotor in the WAG-RH, therefore, the vertical axis rotor shows no performance with this flow, alone, for all hot space temperatures.
Further, the investigations were carried out to observe the effect of air flow in the HV-channel developed due to combined action of buoyancy and external windon the performance of vertical rotor in the WAG-RH.
In this connection the rotor performance in the WAG-RH without and with integration of air heater was tested and compared at different wind speeds from wind tunnel.
The test results are shown in Fig. 6

Effect of Air Flow in the HV-Channel on the Performance of VRW using Vertical Horizontal Co-Axial Blades Combination Technique
In Further, to investigate performance coefficient for the rotor when working inside solar integrated rotor house, average static torque available on the rotor shaft has been measured at 70 o C HST for different external wind speeds as shown in Fig. 9. Fig. 9 also compares the magnitude of average static torque for the rotor performing in the rotor house (WAG-RH) without and with using VHCBC-technique. The Fig. 9 shows better performance of the rotor in the Performance coefficient of the rotor for three different cases, such as bare rotor, rotor in the WAG-RH, and rotor in the solar integrated WAG-RH using VHCBC-technique, has been compared in Fig. 10(a-b).
The percent rise in the Cp of the rotor contributed by the two techniques has been indicated in Fig. 10 (b).Excellent rise in the Cp of the rotor, particularly at low wind speeds, has been contributed by the introduced techniques.
The Fig. 10 shows that both the techniques contribute well for improving performance coefficient (Cp) of vertical axis rotor at different external wind speeds (iv) Hot air released from air heater can be utilized for space heating or drying purpose.