Night Operation of a Solar Chimney Integrated with Spiral Heat Exchanger

Solar power is one of the most raising and encouraging renewable source of energy generation. Solar plants are playing an important role in power supplies worldwide. Nowadays, the electrical energy demand is increasing rapidly due to fast-growing daily requirements. In the last few decades, scientific researchers have focused on a novel technology called the solar chimney power plant, sometimes recognized as solar updraft tower. The solar chimney is principally composed of three main constituents, namely, a solar collector, a chimney and a wind turbine. This promising technology addresses a very challenging idea of generating electricity from free solar energy. It is categorized as a viable resource of clean energy for many non developed countries. The world’s first solar chimney prototype was designed and constructed at Manzanares in Spain, as a result of a joint project between the Schlaich Bergermann partner and the Spanish government. The plant is characterized by a tower high 195 m with a radius of 5 m. The radius and height of the collector encircling the tower are respectively 120 m and 1.85 m. The spanish prototype built by the engineer Jorg Schlaich of Schlaich Bergermann operated without significant problems for seven years. Several research projects have been conducted all over the world to design and introduce different solar towers based on experience gained from operating the 50 kW Spanish prototype. To ensure that the energy conversion is maintained at satisfactory levels to guarantee considerable power generation, an unprecedentedly high tower and an immense collector area are needed. This plant is then based on a thermal updraft movement of hot air resulting from natural convection. In this chapter, the simulations were conducted using cylindrical coordinate system. The inner fluid flow is considered turbulent and simulated with the k-ε turbulent model, by means of the CFD commercial software ANSYS Fluent. Numerical data were validated by comparing them with those from experiments. The agreement between simulation results and the measurements taken from the experimental plant in Manzanares is fairly good. A set of mathematical models of the solar updraft power plant have been developed where a model considering the kinetic energy difference within the solar collector was proposed. The operation of such a plant is strongly dependent on the amount of solar radiation. The main disadvantage of this system is the inability to operate constantly at night. A geothermal heating device is suggested to guarantee a continuous operation during night hours. In this chapter, an auxiliary heating system constructed of in-plane spiral coil tubes is proposed to be placed above the ground under the collector. Thus, the computational model is afterward combined with a mathematical model for a geothermal heat exchanger to evaluate the effect of combining both solar and geothermal energy on the plant performance. A parametric study of the hybrid plant is carried out. The study focus essentially on the impact of the collector size, the meteorological conditions as well as the effectiveness of the heat exchanger on the air flow rate, the temperature increase within the collector and the global performance of the solar-geothermal hybrid system.