Küçük Bir Güneş Bacasının Sıcaklık Dağılımlarının İncelenmesi

Cevre dostu ve surdurulebilir enerji politikalari gun gectikce onem kazanmaktadir. Elazig'da yapilan prototip bir gunes bacasi, sicaklik dagilimi ve gunes baca etkinligi acisindan deneysel olarak incelenmistir. Elazig, sicakligin -15 ° C ile + 42 ° C arasinda degistigi ucuncu derece gun bolgesindedir. Deneysel sonuclar, gunes baca yuksekliginde bir artisla verimliligin arttigini gostermistir. Verim, baca yuksekligine ve ortam sicakligina baglidir. Baca yuksekligi ne kadar yuksek olursa verimlilik o kadar fazla olur. Bununla birlikte, ortam sicakliginin verimlilik uzerine etkisi cok dusuk gorunmektedir. Gunes baca sisteminin Elazig sartlarinda enerji uretimi icin alternatif bir sistem olabilecegi gorulmustur.


Giriş
Today, about 75% of global energy consumption is provided from fossil fuels that cause climate change and various environmental problems. (Jamali et al., 2019). It is estimated that, the current world energy requirements can be supplied by renewable energy by more than 3000 times with more than 93% from solar energy (Ellabban et al., 2014).
Due to its geographical location, Turkey has a better chance compared to other countries in terms of its solar energy potential. Turkey has an average annual time of 2640 hours (7.2 hours per day) and an average annual solar radiation of 1311 kWh/m²-year (3,6 kWh/m² per day) (EİE, 2006). Since solar energy is an inexhaustible and inexpensive source of energy and does not harm the environment, the use of solar energy is becoming more common.
The idea of the solar chimney was first introduced by Schlaich in the late 1970s. Then, the construction of a pilot plant in Manzanares, Spain was started. This pilot plant with a power capacity of 50 kW has produced electricity for 7 years.
Increasing use of solar energy has been effective in the development of solar chimney technology in recent years. A solar chimney system typically comprises of three important components, a chimney, a solar collector, and a turbine.
A solar chimney power plant was analyzed in the Arabian Gulf region (Hamdan, 2011). The analysis indicated the most essential physical factors for the design of a solar chimney are the height of the chimney and the turbine pressure head.
An improved solar chimney concept to produce electricity with low-grade heat in thermal power plants was performed by Ghorbani et al. (2015). Their results indicate a maximum of 0.538% rise for the thermal efficiency of the power plant with fossil fuel.
A simulation for the full-year power capacity of a conventional and a sloped solar chimney power plant with maximum solar radiation angle and maximum power generation angle were performed by Cao et al. (2018). It was concluded that the two important factors which influence the sloped solar chimney power plant's power capacity and accumulated power generation were the temperature rise of the air in the solar collector and the system height. There are studies investigating the effects of ground materials (Al-Azawie et al.,2014), wind speed and direction (Aja et al. 2013), the usage of phase change materials (Li and Liu, 2014), geometrical effects (Kasaeian A. 2014, Ming et al. 2013) and air humidty (Ninic 2006) on the performance of solar chimneys. An updated review on solar chimneys has been provided by Kasaeian, 2017. In this study, a solar chimney was constructed in Elazig in the third degree day region of Turkey. The temperature distribution and power generation of the system is investigated experimentally.

Experimental Set-Up and Procedure
Solar chimney is a thermal system developed to obtain electrical energy from solar energy. There are three basic principles in the solar chimney system. These are greenhouse effect, chimney draft generated by the difference in density and temperature, and the kinetic energy. A solar chimney system typically comprises of three main parts, a solar collector, a chimney and a turbine. Solar radiation collected by the solar collector and the greenhouse effect to warm up the air below the transparent collector roof are used by the system. A large pressure difference occurs between the system and the ambient air because of a temperature and consequently a density difference between the inside and outside of the chimney and this drives the wind turbine settled in the chimney for generating electricity. A prototype solar chimney plant was constructed and mounted on the flat roof of Mechanical Engineering Department of Firat University in Elazig, Turkey. Schematics and the assembly of the solar chimney are shown in Figure 1 and 2, respectively.
The collector, which was made of glass, has a diameter of 3.5 m. The chimney's height is 3 m and diameter is 0.2 m. The dimensions and the measurement points are given in Figure 1

Teorical Model
In general, the output power of the solar chimney is calculated by multiplying the solar energy entering the system with total efficiency (Schlaich et al., 2005): Here; P is the output power of the solar chimney. Solar energy entering the system and the total efficiency are represented by Q and ɳtop, respectively. The total efficiency consists of collector, chimney and turbine efficiencies. If equation (1) is rearranged (Schlaich et al., 2005), Here, ɳcol, ɳch and ɳtr are collector efficiency, flue efficiency and turbine efficiency, respectively. Collector efficiency can be determined as below (Parthasarathy and Pambudi, 2019). where η and m are the collector efficiency and air mass flow rate (kg/s), respectively. cp represents the specific heat capacity of the hot air (kJ/kg K) while the temperature difference between the hot air and ambient air is represented by ΔT (K). Acol and I are the solar collector area (m 2 ) and radiation intensity (W/m 2 ), respectively.
Using the equation below the mass flow rate (m) can be calculated (Parthasarathy and Pambudi, 2019).

̇= (4)
where ρ indicates the density (kg/m 3 ), A and v represent the outlet hot air vent area (m 2 ) and hot air velocity (m/s), respectively. Solar chimney's efficiency can be determined from (Schlaich et al., 2005) Here, the gravitational acceleration is indicated by g. Hch is the chimney height and T0 is temperature of ambient.
Assuming that the air flow in the chimney is uniform, the maximum air velocity in the chimney can be calculated as, where ΔT is the temperature difference between the chimney entrance (collector exit) and the ambient. Fig. 3 shows hourly variations of the solar radiation for Elazığ city. The maximum solar radiation is found at maximum value at 13:00 PM and recorded a value of 539.76 W/m 2 . The temperature variation along the chimney with time is given in Figure 4. As can be seen from the figure, the temperature along the height of the chimney ranges from 29 to 40 ° C and reaches the maximum value at 11:50. The measurements, at the same day at specified hours at the specified points and heights showed that temperature at B1 point reaches the maximum value.   Figure 7 shows the velocity change with time along the chimney. It is seen that the velocity increases with time. The rise of the heated air in the collector is directly proportional to the temperature increase in the collector and the diameter and the chimney height. The air in the collector flows from the collector into the vertical chimney and then discharged to the environment because of the density difference between the heated air in the collector and the ambient air. Therefore, as the solar radiation to the collector increases, the air temperature in the chimney increases because of the temperature increase of the air in the collector. The variation of the solar chimney efficiency with the chimney height and ambient temperature is given in Figure 8. The efficiency depends onthe chimney height and the ambient temperature. The higher the height of the chimney, the more is the efficiency. However, the effect of ambient temperature on the efficiency seems to be very low.

Conclusion
With the increasing sensitivity to environmental pollution in the world, there are constantly new searches to meet the increasing energy needs. Solar chimney power plants make positive environmental and economic contributions to the reduction of fossil fuel use through solar energy. In this study, thermal performance of a prototype solar chimney established in the campus area of Firat University was investigated. The experimental results showed that efficiency increased with an increase in the height of the solar chimney. Besides, the velocity of the air in the chimney increased due to the increase in temperature in the solar collector. Elazig is a city in eastern Anatolian which has an annual average radiation intensity of 1365 kWh/m 2 and has 2664 hours sunshine duration. Therefore, solar chimney system can be an alternative system for energy production under Elazig conditions and a solar chimney power plant can be established in this region.