For ages now humans have been using fossil fuels as primary energy source. High dependence on this poses threat to the environment. In addition concerns about energy security arises because depletion in fuel reserves, regional conflicts over resources, price fluctuations. Apart from having high economic impact, usage of fossil fuels for power generation increase the carbon content of the atmosphere, which results in global warming. Transition towards renewables for meeting energy requirements in turn increases dependence on fossil fuels for backup power supply. Mostly gas turbine power plants are being used for such purposes. Solar energy is the most popular candidate among the renewables, sun is the source of limitless free energy for our planet. Talking about solar energy the problem before us is the energy storage technologies, batteries are not so efficient and are not easily affordable.
But we already have a proven technology - Gas turbines, that exhibits higher efficiency than steam turbines (thermal power plants), more environment friendly with low carbon and NOx emission. We can rely on gas turbines for power generation until more efficient solar cells are innovated. In this vein, international and national policies are being reviewed to increase the share of renewable energy in the energy mix. Gas turbine is such a technology that can be operated by renewable sources and bio-fuels.
Solarisation of Gas-turbines
Existing gas turbines run on conventional fuels, oil and natural gas. In all gas turbine power plants we use a working fluid (air or gas) to drive a turbo-machinery for power generation purposes. It is a type of internal combustion engine which works on the principle of Brayton Cycle with air as working fluid. It uses high-temperature pressurized gas from combustion of fuels to produce mechanical work. This shaft work can be used to generate electricity for numerous purposes. It is a type of internal combustion engine which works on the principle of Brayton Cycle with air or any suitable gas as working fluid. Modifications are required before integrating the use of solar radiations in Gas turbines. Solar radiation can be converted to high-grade heat (up to 1773 K) using concentrating solar power (CSP) technology. These ranges of temperature are suitable for solarisation of the gas turbine system.
Sun produces radiations at an equivalent black body temperature of about 6,000 K with a constant intensity, but is attenuated by the atmosphere. The sunlight consists of direct and diffused radiations but for solar power technologies only direct radiation can be used. Absorption of solar radiation by a receiving surface is maximum when the angle of incidence is zero. The recommended minimum annual sum of direct normal irradiation for CSP technology to be economically viable is 2,000 kWh/m^2, and many locations within the sunbelt meet this threshold. It has been found that the worldwide potential of CSP is estimated at 3,000,000 TWh/year which significantly exceeds the world electricity consumption of about 29,000 TWh/year in 2015. This makes solar energy as an attractive candidate for advancement in gas turbine technology. Solar heating capabilities can be used to improve power plants based on fossil fuels.
The main components of a solar gas turbine (SGT) are compressor, solar field, combustor, turbine and generator. The solar field comprises concentrators and receivers. Four widely investigated concentrating solar power (CSP) technologies are the parabolic trough concentrator (PTC), linear Fresnel reflector (LFR), parabolic dish concentrator (PDC) and solar tower (ST). The solar tower method has shown highest potential for use in SGTs. In this a heliostat field is configured for direct radiations and concentrate these radiations at the top of a centrally positioned tower unit where the high pressure air from the compressor is heated, increasing the temperature before it enters the combustor.
(A heliostat is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a target, according to the sun's apparent motions in the sky)
Conventionally reheat has been in use for increasing the mean temperature of heat addition in any power plant for achieving higher efficiency, this is done by heating the compressed air by the hot exhaust gases from combustion. But there are still some drawback to it, like the maximum temperature to which we can heat the compressed fluid is fixed, that is of the exhaust gas temperature. But if we use Solar energy for reheat process may be we can achieve even higher temperature, therefore increasing the efficiency. Recent R&D on power conversion cycles focus on high temperature outlet from the heat source such as gas combustors or nuclear reactors. Hence, utilizing solar energy for reheat process creates more possibilities for such high temperature power plant design which uses Brayton cycles.
Figure:- Concentrating Solar tower
Thermodynamic cycles play a vital in power plant operation. The Brayton cycle (gas turbine) has higher prospects for solarisation because it has higher efficiency than the Rankine cycle. As it uses air as the working fluid which shows higher thermodynamic mobility than water or steam used in Rankine cycles. Pollution control is vital for power generating system. Solar‐assisted gas turbines are appropriate technology in power generation due to their some advantages such as lower greenhouse gas emission compared with conventional fossil fuels plants. Based on studies, using solar energy to preheat the compressed air before combustion can be employed to increase the efficiency of the cycle (as per a study 60.9% as compared to reference condition with 51.4%).
In turn fuel utilization is reduced for generating same amount of power and hence, results in lower carbon emission and levelised cost of power generation. In addition, the generated power can be increased. In another configuration, solar energy is applied to heat‐up air or other working fluids to run a turbine in a Brayton cycles.
Utilizing supercritical fluids in the solar‐assisted gas turbines is another idea proposed by researchers. Integrating Brayton and Rankine cycles for SGTs leads to higher efficiency. Selection of appropriate fluids for top and bottom cycles for combined cycles is significant for cost of production. Further studies mainly focus on optimizing working conditions, fluid type and materials for the turbines blades etc.
Solar energy is apparently unlimited free source of energy for planet earth, exploiting it by integrating it with such robust mechanical power generating systems can speed-up sustainable development.
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