Renewable energy has immense importance in the energy sector as fossil fuels are vanishing from the Earth due to rapid industrial development. Nowadays, researchers are focused towards the development of alternative energy resources which can reduce dependence on nonrenewable resources. Energy consumption has increased many fold predominantly due to the progress in science and technology with a growing human population. Electricity produced from fossil fuels has low efficiency when equated with the aggregate of fuel and heat sources used. Waste heat from automobiles, industrial processes, and greenhouse gases are released into the atmosphere, which accelerates the degree of global warming. From this perspective, waste heat needs to be used effectively to produce power, which diminishes emissions and also leads to the preservation of fossil fuels.
The organic Rankine cycle (ORC) system has been proposed as an effective technology to recover heat from low-temperature sources; that is, biomass, solar, geothermal, and waste heat from power plants. The organic Rankine cycle works on the same principle of the steam Rankine cycle. The organic fluid is the heat carrier within the cycle, being better adjusted than water to low temperatures. The organic working fluid has a higher molecular mass and lower boiling point compared to water. The type of working fluid and the turbine used in the ORC system governs the efficiency of the overall system. A Rankine cycle with steam as the working fluid and at high temperature and pressure has an efficiency of 35–40%, whereas a Rankine cycle with organic working fluids at low temperature and pressure gives an efficiency of 8–12% [
1]. ORCs have been found to be capable of absorbing the waste heat from various sources such as solar [
1,
2,
3], biogas [
4,
5], geothermal [
6,
7], and other industrial processes. Many researchers have reported on different expanders for ORCs, such as the radial turbine [
8,
9,
10], scroll expander [
11,
12], and screw expander [
13,
14].
A new design method to develop ORC radial turbines was described by [
15], where the method does not deal with the ideal gas equation and average state equations. The results of CFD simulations showed that the incidence angle of rotor blades and speed of the rotor have great impacts on the efficiency and power output of the turbine. A comparative study on the off-design performance of a basic organic Rankine cycle and a dual-pressure organic Rankine cycle was conducted by [
16], and it was found that the efficiency of the radial inflow turbine of both cycles was strongly influenced by the saturated condensing temperature and hot water inlet temperature. Computational design and analysis of a radial inflow turbine for an ORC was conducted by [
17], and it was found that the turbine effectively handled the temperature dissimilarity with an efficiency drop of 1.53%. At higher temperatures and rotational speeds, the efficiency of the system increased, while efficiency progressively decreased with an increase in the pressure ratio. The superheating effect of the evaporator on the solidity of the ORC was studied by [
18], where they found that the system efficiency remained constant and produced more electricity at lower superheat. Hydrofluoro olefins are the fourth-generation refrigerants, which are found to have low ozone depletion potential and global warming potential and can be used as the working fluid in low-temperature ORC systems. Working fluid selection for a combined solar Rankine cycle for RO desalination was done by [
19], and it was found that hexane gave higher first- and second-law efficiencies compared to water and that this depends on the solar collector area, exergy destruction, and specific total cost. Thermodynamic evaluation of the working fluid on the design and performance of an ORC was done by [
20], where they found that fluids having higher critical temperatures will give increased efficiency and also lead to higher expansion ratios. They concluded that fluorocarbons and low-critical-temperature hydrocarbons are the most suitable fluids for low-temperature applications, whereas siloxanes and hydrocarbons with a high critical temperature are promising fluid candidates for high-temperature ORC applications. Thermodynamic modeling and analysis of a solar ORC was done by [
21], and it was found that hexane gave the highest collector efficiency of 59.19% and that cyclohexane showed high thermal efficiency and high net work output. The performance evaluation of a direct vapor generation supercritical ORC driven by a linear Fresnel reflector was done by [
22], and it was found that fluids with high critical temperatures require lower mass flow rates in the system. They found that cyclohexane achieved the highest overall efficiency of 19.65%. A working fluid selection diagram was proposed by [
23] for a parabolic trough and linear Fresnel collector-based Rankine cycle. They found that isohexane achieved a levelized cost of energy near to that of a parabolic trough collector-based steam Rankine cycle. Since the ORC works in a low-pressure range for smaller amounts of power production, a single-stage turbine is mostly used. A two-stage axial turbine was designed and its performance was compared with a single-stage turbine by [
24], and the power output was found to be 16.04 kW for the two-stage turbine, whereas 11.06 kW was the output of the single-stage turbine. The efficiency was also increased from 10.5% for the single-stage turbine to 14.2% for the two-stage turbine. A generalized assessment of both optimal working fluids and radial turbine designs for a small-scale organic Rankine cycle was done by [
25], using the Peng–Robinson equation of state to optimize the working fluid and cycle parameters in the temperature range of 80 to 360 °C. A correlation has been deduced which relates the heat source temperature to the critical temperature of the working fluid, which can be used to find the optimum working fluid for a given temperature. Using zeotropic mixtures of working fluids in an ORC will increase the performance of the ORC. A preliminary design guideline of zeotropic-mixture working fluids in ORCs was proposed by [
26], where they found that the designed mixtures have better performance than pure working fluids. The SIMPLEX algorithm was used as an optimization tool to study the performance of ORCs by [
27], with variable heat source flow rates and inlet temperatures to the evaporator, and it was found that at low strain of the evaporator, the turbocharger produced a variable output of 34.5 to 55.5 kW and this increased with increase in the temperature of the waste stream at the evaporator. A recent publication of the authors [
28] lists the reviewed selection of working fluids and expanders for ORCs along with the applications of ORCs in various topping and bottoming cycles.
The previous research works on ORCs show that the performance of the system is always influenced by the operation parameters, working fluid, and performance of key equipment. Regarding the previous literature, an experimental investigation of an ORC using n-hexane for various operating conditions with a radial turbine expander has not been explored, which marks the novelty in the current work. Hexane is proven to be an effective working fluid for medium- and high-temperature organic Rankine cycles, whereas the usage of hexane in low-temperature ORCs has not gained major importance, and the main advantage of using hexane is that it requires low mass flow rates since the critical temperature is high (224 °C). The main novelty in the current work is varying the process parameters and thus improving the understanding of the influencing parameters on the efficiency of the overall system. So, the present study deals with the experimental study of an ORC with n-hexane as the working fluid and a radial turbine expander. Experiments are conducted for different working parameters of the heat source temperature, mass flow rate of the working fluid, turbine speed, coolant flow rate, and superheating process. The thermal and electrical efficiencies of the system for different working conditions are determined and are reported in the current article.