Combined Solar and Wind Energy Systems

Combined Solar and Wind Energy Systems Y. Tripanagnostopoulos1, M. Souliotis2, Th. Makris3 Department of Physics, University of Patras, Patras 26500, Greece, e-maill: [email protected], e-mail2: [email protected] , e-mail3: [email protected] Abstract. In this paper we present the new concept of combined solar and wind energy systems for buildings applications. Photovoltaics (PV) and small wind turbines (WTs) can be install on buildings, in case of sufficient wind potential, providing the building with electricity. PVs can be combined with thermal collectors to form the hybrid photovoltaic/thermal (PV/T) systems. The PVs (or the PV/Ts) and WT subsystems can supplement each other to cover building electrical load. In case of using PV/T collectors, the surplus of electricity, if not used or stored in batteries, can increase the temperature of the thermal storage tank of the solar thermal unit. The description of the experimental set-up of the suggested PV/T/WT system and experimental results are presented. In PV/T/WT systems the output from the solar part depends on the sunshine time and the output of the wind turbine part depends on the wind speed and is obtained any time of day or night. The use of the three subsystems can cover a great part of building energy load, contributing to conventional energy saving and environment protection. The PV/T/WT systems are considered suitable in rural and remote areas with electricity supply from stand-alone units or mini-grid connection. PV/T/WT systems can also be used in typical grid connected applications. Keywords: Solar energy, Wind energy, Thermal energy, wind turbines, photovoltaics, solar themal collectors

1. INTRODUCTION Energy demand of buildings in electricity and heat corresponds to about one third of total energy consumption in many countries. Renewable energy sources can be used to cover a great part of energy needs in the built sector, contributing to the conventional energy saving and also to the protection of the environment. Solar and wind energy systems appear to be the most interesting among renewable energy sources (RES) for the built environment. Solar energy systems as thermal collectors and photovoltaics are installed on buildings to cover hot water and space heating/cooling needs and provide electricity for lighting and operation of several electric devices. PVs convert a small part of the incoming solar radiation to electricity, with the greater part being into heat. This effect increases their temperature, resulting to its efficiency drop. The combination of the PV module with a water heat extraction unit constitutes the hybrid photovoltaic/thermal (PV/T) system, by which electrical and thermal output is simultaneously provided [1]. In hybrid PV/T systems the extract heat from the PV cells increases the temperature of the removal fluid, which can be stored in a thermal storage tank. The facades and the horizontal or inclined roofs of houses, hotels, athletic centers and buildings of other types are appropriate surfaces for the application of photovoltaic panels and thermal collectors, for electricity and heat demand respectively. At the same time, wind energy is a very promising renewable energy source, estimating to cover the 20% of the global electrical energy demand by 2020. Wind turbines (WTs), especially small size can be mounted on building roofs, mainly at locations with satisfactory wind potential and are effective to both stand-alone and grid connected applications. In this paper, we present the new concept of combined solar and wind energy systems for building application and is an extension of previous works [2-5]. Photovoltaics (PVs) and small wind turbines (WTs) can be install on buildings, in case of sufficient wind potential, providing the building with electricity. The PVs (or the PV/Ts) and WT subsystems can supplement each other to cover building electrical load. In case of using PV/T collectors, the surplus of electricity, if not used or stored in batteries, can increase the temperature of the thermal storage tank of the solar thermal unit. Experimental results from the operation of a small wind turbine, a PV array and a thermosiphonic water heater are given and discussed.

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2. SYSTEMS DESCRIPTION The PV-WT-Themal Collector-Hybrid system has been installed on the roof of the Physics Department‘s main building in Patras (38o 24’ N, 21o 73’ E), Greece. The electrical subsystem was design as a 12 V DC system supplying AC load. As wind turbine a SouthWest Windpower Air-X Land with a rated power of 400 W at 12.5 m/s was chosen. The cut-in wind speed of the turbine is according the producer’s manual 2.5 m/s and the start-up wind speed is 3.0 m/s. The PV array consists of four p-Si modules in parallel connection with a rated power of 54 W each manufactured by Kyocera (KC 50T model). The modules are installed with a tilt of 42o towards south. The output of PV and wind turbine utilized for charging battery bank, which consists of one gel lead-acid battery with total capacity 180 Ah at 12 V (~2 kWh). The power station consists of a charge controller for regulation of voltage and an inverter in order to convert electrical power from DC into AC form at the desire frequency of the load. The total load consists of a variable resistor which can altered in a range of 3 W to 200W. As thermal subsystem a closed loop solar thermal collector with a horizontal cylindrical tank was used. The collector, with dimensions 1.82 m x 1 m x 2.01 m and aperture surface area 1.785 m2 was made of electrolitically anodized aluminum with rockwool sheet insulation 40mm thick. The volume of the storage tank was 90 lt and inside of tank a stainless steel resistance of 3 kW was used in order to heat up the water when there is no sun light. The temperatures at various locations in the storage tank and collector were measured using thermocouples.

3. MEASUREMENT SYSTEM Experiments were conducted to compare the electrical and thermal performance of the system. The measurement system consists of two parts; the meteorological measurements and the measurements of system system parameters. The solar radiation is measured with a pyranometer CMP 3 from Kipp & Zonen. The wind set from Campbell Scientific consists of an anemometer and a wind vane to measure the wind speed and wind direction. A number of thermocouples were used in order to measure the ambient and PV cells temperature. The small WT and the PVs provide a load with electricity through controller, battery and inverter. The solar thermal operates without water drain for 3-4 days to reach the maximum temperature and then, after the draw off, it starts heating again with cold water. All experimental measurements were monitored and recorded using a data acquisition system linked to a PC. Each set of measurements were taken every 15 s, recording the instantaneous current and voltage of PV modules and wind turbine, the voltage of battery, the solar radiation, the wind speed and direction and various temperatures.

4. SYSTEM AND SUBSYSTEM PERFOMANCE ANALYSIS 4.1 Meteorological data The meteorological data that are included in this paper (irradiance, wind speed, wind direction and air temperature) were obtained over a period of one year. Fig. 1a shows the monthly mean variation of wind speed, ambient temperature and solar radiation taken from the data acquisition system installed at the test site. It is observed that during summer months the solar mean daily radiation and ambient temperature reach the maximum values as expected, Gmax= 555.06 W m-2 and Tamb-max=32.69 oC respectively. From the other hand, monthly wind speeds are higher during winter, vmax= 4.44 m s-1, as opposed to low wind speeds during other seasons. It is also seen from Fig. 1a that solar and wind power sources are complementary over a year. The summer provides good solar irradiance but poor wind conditions, while a relatively effective wind speeds but poor solar radiation occurs in the winter. The above shows that the combination of two energy sources can provide a better utilization factor for the energy demand.


Irradiance

Temperature

Wind speed 5

500

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-2

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400 3 300 2 200

NW 1%

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SW 17% E 33%

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S 35%

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SE 7%

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FIGURE 1. (a) Monthly mean wind speed, ambient temperature and irradiance and (b) Annual frequencies for each cardinal coordinate.

The annual frequencies for the eight cardinal coordinates calculated from the hourly meteorological data are presented in Fig. 1b. As seen from graph above, the most frequents wind directions are south (S) and east (E) with a percentage 35% and 33% respectively.

4.2 Wind turbine performance The recorded 30-min mean power output from the turbine is plotted against the wind speed in Fig.2a. The output was measured for an electric load 15W. The results showed that the turbine provides less energy than expected. Furthermore, the start-up wind speed is 3 m s-1, which is a little higher than specified by the manufacture. In addition, as showed in Fig. 2, there are two separated concentrations regions of data points; the upper part corresponding to the turbine’s normal operation and the lower part when the WT is in furling position. The reduced power generation of the wind turbine is caused by a number of possible reasons. The majors reasons are the furling position of the wind turbine, the atmospheric turbulence, yaw adjustment and possibly the time space when the data acquisition systems was adjusted to collect the data. 70

30

W ind turbine efficiency (% )

E lectric outpu t of W T (W )

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Wind speed (m s )

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FIGURE 2 (a) Measured power output of wind turbine and (b) wind turbine efficiency versus wind speed

The efficiency of the turbine against the wind speed is plotted in Fig 2b. As showed in the graph, the peak performance is 14.59% and occurs at a tip speed of 5.6 m s-1. Furthermore, the efficiency of WT begins to decrease when the wind speed reaches 6 m s-1. The reasons for this behavior are the furling of wind turbine and the high turbulence of air mass in front of the rotor of the WT.


4.3 Photovoltaic modules performance As mentioned earlier, the 220 W PV generators consist of four p-Si photovoltaic modules in parallel series. The output power generated by the entire PV subsystem was measured over a period of one year with an electric load 40 W and it is shown in Fig. 3a. The results show that the output power generated is linearly dependent on the irradiance. Fig 3b shows the trend of experimental efficiency of the photovoltaic field for August 2008. The experimental efficiency values range between 4.20 % and 5.68 %, with the mean monthly value of 5.16 %.

7

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E ffic ie n cy o f P V s u b sy s tem (% )

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Irradiance in plane of PV modules (W m )

FIGURE 3 (a) The output power versus irradiance and (b) Experimental efficiency of the PV field for August 2008.

4.4 Thermal performance of solar collector The testing procedure of the experimental models includes system operation without water drain for 3-4 days, starting with low water temperature in the first day up to stagnation in the end of the experiment. The mean daily efficiency nth is related to the ratio (Ti-Ta)/G by the equation:

nth = FR (τα ) − FRU L (ΔΤ / G)

(1)

where FR : Removal factor, U L : Heat loss coefficient and τα : Optical efficiency TANK Low Ta

TANK Middle G

TANK Up Vw

TANK Mean

100 0.8

1000

n th = 0.7142 - 4.9219 (/G)

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/G (K W

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m )

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(a)

x 10

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-1)

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Irradiance (W m )

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700 70

wind speed (m s

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Mean daily efficiency nth

0.7

100

9:30

12:30

15:30 18:30 21:30 Time (hours)

0:30

3:30

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(b)

FIGURE 4 (a) Mean daily thermal efficiency of collector and (b) Variation of mean water temperature of storage tank for one day.


In Fig. 4a we present the experimental results of the mean daily efficiency nth as a liner function of (Ti-Ta)/G. Using equation (1) test data give line’s intercept FR (τα) = 0.714, and the line’s slope, FRUL = 4.922 oCW-1m2. The variation of the water temperature inside the storage tank corresponds to the effective water heating by the absorbed solar radiation. Fig. 4b presents the variation of water temperatures in different positions inside the storage tank, the solar radiation, the wind speed and the ambient temperature for one day.

4.5 System performance The performance of the system was evaluated for a time period of one year (January-December 2008). The highest value of irradiance was in July with 555.06 Wm-2 and the lower was in December was 262.79 Wm-2. The total yearly generated power from PV modules was 194.60 kWh, while the total amount of solar energy in-plane of photovoltaic modules was 3187.87 KWh and the mean ambient temperature was 21.55 oC. In addition, the overall efficiency of the PV sub system was 6.1%. The amount of electrical energy generation from the wind turbine for same time period, was 37.08 kWh while the total amount of wind energy that flowed through the rotor of the WT was 620.82 kWh. The mean conversion efficiency was 5.7%. In Fig. 5 we present the monthly generated energy from PV modules (Fig. 5a) and wind turbine (Fig. 5b) versus the available solar and wind energy. Incident energy from sun

Wind turbine

PV modules

Energy of wind through rotor

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350

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70 60

Energy (kWh)

Energy (kWh)

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FIGURE 5 (a)-(b) Total generated energy from PV module and wind turbine

The calculation of the generated energy of the solar collector is a complicated procedure due the dependence of performance of collector from the mean water temperature, the ambient temperature and the solar radiation. For the calculation of useful energy of collector we assume that the mean daily temperature of thermal system operation is Tmean=40oC. From the equation (1) and taking into account the measured meteorological data, the energy produced was 1686.04 kWh, while the energy input to the aperture of solar collector was 3120.58 kWh, resulting to a mean yearly thermal efficiency of 54 %.

5. COMBINATION OF A PV/T SYSTEM WITH A WIND TURBINE In the suggested PV/T/WT systems the output from the solar part depends on the incoming solar radiation and is obtained during sunshine. On the other hand the output of the wind turbine depends on the wind speed at the location of the installation and is obtained any time of the day or night that the wind speed is over a lower limit. Therefore the PV/T and WT sub systems can supplement each other, being primarily used to cover building electrical load and secondary to increase the temperature of the existing thermal storage tank of PV/T system by their surplus electrical energy. The PV/T/WT systems are considered suitable in rural and remote areas with electricity supply from stand- alone units or mini-grid connection. PV/T/WT can also be used in typical grid connected applications [5]. In order to determine the performance of a hybrid PV/T system, we experimented with a system constructed in our laboratory, which was of thermosiphonic type as showed in Fig. 6a. This system includes a hybrid PV/T system with


water heat extraction unit connected with pipes and natural water flow with a water storage tank [6]. The PV/T collector was tested separately to determine its performance in water heating and producing electricity. The PV/T collector was tested as glazed type, to suppress thermal losses and achieve higher water temperatures. In Fig 6b we show the variation of mean water temperature inside of storage tank for a sunny day during spring. Tin Tank Middle Mean Ta

Tout Tank Up Mean Radiation

Tank Low Mean Tank Mean 1000

80

900

70

800

600 500

40

400

30

300

-2

700 50

Irradiance (W m )

Temperature ( °C )

60

20 200 10 0 6:30

100

8:30 10:30 12:30 14:30 16:30 18:30 20:30 22:30 0:30

2:30

4:30

0 6:30

Time ( Hours )

(a)

(b)

FIGURE 6 (a) The experimental thermosiphonic PV/T collector and (b) Variation of mean water temperature of storage tank for one day.

The surplus of electricity from small WT during windy nights, if not used or stored in batteries, can be utilized to increase the water temperature of the thermal storage tank of the solar thermal unit. Doing experiments for this purpose, we found that the energy needed for increasing the water temperature for a quantity ΔΤ= 10 oC is 1.352 kWh. This amount of energy can be produced from the WT if the wind speed is 6 m/s for a period of 22.5 hours.

6. CONCLUSIONS PV, thermal collectors and small wind turbines can be combined to cover energy needs in heating and electricity of buildings. A system with PV modules, wind turbine and solar thermal collector was constructed and tested for a time period of one year. The generated energy from PV modules was 194.6 kWh with overall conversion efficiency 6.1%. At the same time the output energy from the small wind turbine was 37.08 kWh while the conversion efficiency reach the value 5.7%. From the other hand, the maximum thermal efficiency was 71.42% and the energy produced by the thermal subsystem for a operation temperature of collector Tmean=40oC was 1686.04 kWh.

REFERENCES 1. Y. Tripanagnostopoulos, Nousia Th., Souliotis M., Yianoulis P., “Hybrid Photovoltaic/Thermal Solar Systems”, Solar Energy 72, pp 217-234 (2002). 2. Y. Tripanagnostopoulos, Tselepis S., Souliotis M., Tonui J.K., Christodoulou A. “Practical aspects fro small wind turbine applications” Pro. in Int. EWEC 2004 Conf. Track 7; Autonomous and/or Hybrid Systems, London, UK, 22-25 Nov. 2004. 3. Y. Tripanagnostopoulos, M. Souliotis, Th. Makris, “Combined solar and wind energy systems for building application” International Workshop, EPEQUB, 11-12 Sep. 2007, Milos, Greece. 4. Y. Tripanagnostopoulos, M. Souliotis “Photovoltaics, small wind turbines and thermal collectors, an approach for building integrated solar and wind energy technologies” 4th European PV-Hybrid and Mini-Grid Conference, 29-30 May, Glyfada, Athens, Greece, 2008. 5. Y. Tripanagnostopoulos, S. Tselepis, “Hybrid solar/wind (PVT/WT) building integrated systems” 2nd European PV-Hybrid and Mini-Grid Conference, 25-26 Sep., Kassel, Germany, 2003. 6. Y. Tripanagnostopoulos, M. Souliotis, Th. Makris, P. Georgostathis , “Design and performance of hybrid PV/T solar water heaters” European Solar Thermal Energy Conference(ESTEC), June 19-20, Freiburg, Germany, 2007.
