Most electricity is currently produced by thermal power plants, operating based on the principles of thermodynamics. These plants produce power using turbines, which run on various conventional fuels, such as methane, crude oil, liquid natural gas, and so on.
However, the use of such fuels often results in a number of socio-economic and environmental impacts. The cost of operating this type of power plant is also unstable and can be affected by the fluctuating prices of fuel.
Furthermore, there is a real possibility of a fossil fuel shortage in the near future. And with the pressure of global warming, some countries are implementing steps to reduce their carbon footprint.
These are the reasons why photovoltaics has been integrated into the energy production chain, giving rise to a new generation of turbines.
What are solar turbines?
There are actually two references when it comes to the term ‘solar turbine’.
The first is a solar turbine that depends on solar energy as the sole fuel source and photovoltaic technology as the working mechanism of the turbine.
The second is a solar gas turbine, also referred to as solar-integrated gas turbine or solar-assisted gas turbine. Both references have the same purpose, which is to generate power, usually in the form of electricity.
Other types of turbines are steam turbines, hydroelectric, wind turbines and gas turbines.
How do solar turbines work?
To understand how solar turbines work, one has to understand how turbines work. Turbines are rotating devices that generate energy mechanically or electrically. For example, the engine of a jet aircraft is powered by a gas turbine.
In a power plant, the turbine operates a rotating electrical generator to produce electricity. Take the gas turbine as an example.
A gas turbine involves three main sections: compressor, combustion chamber and the turbine itself. The compressor draws air in, pressurizes the air, then feeds it to the combustion chamber at the speed of hundreds of miles per hour.
The combustion chamber is made up of a ring of fuel injectors. These injectors pump a steady stream of fuel into the combustion chamber where the fuel mixes with the pressurized air.
The mixture is then burned at temperatures of over 2,000°F. This high temperature creates a high pressure gas stream that enters and expands through the turbine section.
Within the turbine is an intricate array of alternate stationary and rotating aerofoil-section blades. Hot gas spins the rotating blades.
These blades perform two functions:
- Drive the compressor to draw more pressurized air into the combustion section
- Spin a generator to produce electricity.
A solar turbine works in a similar way but using sunlight as its fuel. Large solar arrays capture the heat of solar energy. This is then used to heat a gas or transmission fluid.
This heated medium then boils water to produce steam which powers turbines. No conventional fuel is employed in a solar turbine.
Solar power plants
These employ solar turbines and Concentrated Solar Power (CSP). CSP technology is thermodynamically comparable with conventional power plants.
It generates electricity by using thermal energy from solar radiation. Using solar arrays, solar radiation is focused on a small area (receiver) where solar energy is converted to heat. Heat is then collected by a working fluid which then drives a gas or steam engine cycle for electricity generation.
Currently, four CSP technologies are frequently deployed in the power sector: linear Fresnel reflector (LFR), parabolic dish concentrators (PDC), parabolic trough concentrators (PTC), and solar tower (ST). PTC and LFR are also applied in gas turbine technology.
Advantages of solar turbines
- Turn vacant, unproductive land into productive land
Massive arrays of PV panels are required for solar turbines to produce enough power. The arrays need a large amount of land with unobstructed sunlight. This is usually in barren areas such as deserts.
- Provide clean energy
No fuels need to be burned for solar turbines to work. The only energy is sunlight, which leaves no carbon footprint.
- No cost to fuel
Sunlight is free of charge. This helps to lower the cost of solar-powered plants and negate the influence of fluctuating conventional fuel costs.
- Improve efficacy of conventional power plants
Solar turbines or CSP can be used to boost the performance of power plants that conventionally run on fossil fuels. Solar thermal energy can aid in increasing the temperature of the working fluid in these plants.
Disadvantages of solar turbines
- Dependent on weather
The nature of solar radiation is intermittent. As such, solar turbines must carry out regular starts and re-heat options for maximum performance.
- Lower heating point
The transmission fluids of current solar turbines cannot be heated above 752°F. This can be overcome by placing the turbine on a high tower with the mirrors focusing sunlight directly on the steam boiler instead of using transmission fluid in the pipes.
Integration of solar in gas turbine systems
Solar gas turbines (SGT) are also called solar-assisted gas turbines. This is because solar energy is integrated into the cycle of the turbine to increase the efficiency and power output.
Solar application can be carried out in altogether four configurations. Two of them belong to the latest category of SGT called hybrid solar gas turbines.
Developments in the field of high-temperature pressurised air receivers have made it possible to integrate solar energy directly into the gas-turbine cycle without the need for intermediary heat exchangers. The two principal hybridisation schemes of hybrid gas-turbine systems are serial and parallel hybridisation.
1. Basic configuration of the SGT
This is the simplest solar gas turbine system. Air from the compressors preheated using Concentrated Solar Power before the air is fed to the turbine.
This configuration is used when the solar receiver outlet temperature is the same as the required turbine inlet temperature. Therefore, solar radiation can be the sole source of heat for driving a gas turbine, as shown by this configuration.
2. Steam-injected gas turbine
In this configuration, solar energy is used to heat water until it becomes steam. The heated steam is then injected into the combustion chamber, which burns fuel.
3. Series hybridization of the SGT
It is applicable for days or regions with low insolation but enough to sustain a CSP.
In a serial scheme, air from the compressor is flowed through the solar heat source to be heated to the desired temperature. This pre-heated air is then sent to the combustion chamber where it is mixed with fuel. The high air temperature results in reduced fuel consumption to reach the desired turbine entry temperature.
This type of hybridization scheme allows both the solar receiver and combustion chamber to operate at maximum temperatures. The final inlet temperature of the turbine is actually the same as the combustor outlet temperature.
The final temperature delivered to the turbine is independent of the degree of solar integration. Greater amounts of solar heat can be integrated into the cycle without adversely affecting the gas turbine’s performance.
4. Parallel hybridization of the SGT
This hybrid configuration is suitable for places or days with high insolation.
It splits the airflow from the compressor into two lines. One airflow is to the solar heat source, and another is the combustion chamber. Hot gas from both flows are mixed together in the mixer before being sent to the turbine.
A parallel hybrid system simplifies the operation of a gas turbine. The combustion and solar sub-systems can be isolated easily in the event that only one heat source needs to be used. This is usually during the start-up of the gas turbine when the fuel-only operation may be preferred. The set-up also makes it simpler to control the operation of the solar sub-system.
Advantages of solar gas turbines
1. Lower greenhouse gas emission
Compared to conventional fossil fuel plants, a power plant using solar-assisted gas turbines would produce less carbon emission. A study in 2018 performed a comprehensive review on various cycles working with solar-assisted gas turbines.
The study found that concentrated solar thermal energy can increase the temperature of the air at the outlet of the compressor before entering the combustion chamber. This would then require less fossil fuel for the turbine to generate the same amount of power.
2. Enhance efficiency
The use of solar thermal energy in turbines can also increase the output of generated power. Research has been done on the effects of solar energy on the Brayton and Rankine cycles.
These studies have shown that using Brayton-Rankine cycles with solar decreases the cost of generated electricity. However, the efficacy and cost of production are also affected by the type of working fluids in both top and bottom cycles.
3. Optimise fuel usage
This is a major advantage of hybrid solar gas turbines. During days of low insolation or at night only fuel can be used while solar energy is reserved for high insolation times.
Disadvantages of solar gas turbines
1. Intermittent solar supply
Solar energy is not constant, and dependent on the weather and season. This affects an SGT’s efficiency significantly.
2. Energy storage and cost
To solve the challenge above, energy storage systems are needed but these are currently very costly. This is even more so for power plants that need massive solar installations.
3. Rare earth materials
Photovoltaic cells are typically made from rare earth minerals such as silicon, and other metals like mercury, lead, and cadmium. These materials are not only expensive but are harmful to the environment.
Both solar turbines and solar gas turbines are undoubtedly greener options for producing power. The former relies only on solar energy while the latter uses a combination of solar with conventional fuels to power turbines. Although the current systems face challenges, the integration of solar in the energy production chain is bound to increase in the near future.