Organic Rankine cycle ORC is one of the best options available for small scale efficient energy production systems. In Organic Rankine Cycle an organic fluid is used in the Rankine cycle instead of water as the organic fluid has a lower boiling point as compared to water so required less heat input. Waste heat from any source, when combined with organic Rankine cycle ORC using a heat exchanger like shell and tube heat exchanger, make it the best and most environmentally friendly energy solution. Optimize parameters allows the minimum mass flow rate and less area required for the heat exchanger saving power and cost required at the heat exchanger system without compromising the efficiency of the system.
With a continuous increase in the world population, the problems associated with human needs are also increasing. One such problem is the need for energy to meet certain needs like electricity. In today’s industrial world where there is a complete working industry for every human need, every industry itself has a need and the biggest of them is electricity. The world current setup to meet its need for electricity heavily depends on the use of natural resources which has increased to its maximum in the present time (Energy.gov, 2019). Natural resources like coal, gas, and fossil fuel on which humanity depends to meet their need for electricity have limited known stock due to which researchers and engineers are forces to developed new resources to produce electricity(Energy matter 2019). The use of natural resources like coal is also harmful to the earth's environment and the current use of this type of natural resource to produce electricity has polluted every land, river, sea, and even oceans. So there is a desire need to develop some renewable and alternative resources which can produce clean electricity.
Figure 1 global energy consumption [(energy.matter, 2019).
Figure 2 energy consumption by resources (energy.gov, 2019).
There are some methods already developed like solar farms, wind turbine farms, geothermal sources, and hydroelectricity which use renewable natural resources like sunlight, wind, geothermal heat sources, and flowing water for electricity production (Energy matter 2019). All these resources do not harm the environment as coal or natural gas do and they all are also renewable but these resources have limitations of their now. The hydroelectric method is only possible in those locations that the huge amount of flowing water with the all necessary setup to store and control the flowing water. Solar farms need continuous sunlight for production which limits their use to an area with getting sunlight 12 months a year but still they only operate during the day timing. The wind farm covers a lot of surface area and they are also limited to geographic regions that have potential wind energy 12 months a year.
Contrary to this the conventional resources do not have regional limitations and product electricity 24/7 for 365 days a year but they pollute the environment and with limited natural resources world cannot rely on them for long. So there is a need to integrate conventional resources with renewable ones to increase electricity production without affecting the environment. One such way is to run steam power plants with renewable resources as one of the most commonly used and high energy production methods having quite good overall efficiency as compared to other methods of electricity production. Steam turbines operate on the Rankine cycle uses conventional methods to heat water and produce steam at the required temperature and pressure. An improved form of regular Rankine cycle is the Organic Rankine Cycle which uses an organic fluid in the place steam as its main working fluid. This research work optimizing the organic Rankine cycle with the help of renewable and alternative resources.
Hybrid Organic Rankine Cycle
In the hybrid organic Rankine cycle, a renewable and alternative source of heat is used in the place of a conventional boiler and fuel-burning systems. In one of the systems, a solar heater is used to heat the organic fluid and convert it into steam during the day time and biofuel is used to heat organic fluid at night. The solar heater collects solar heat at its collector and transfers it to a fluid that fluid carries that heat to an organic fluid of the Rankine cycle and heat the fluid until it converted into steam. Conversion of heat happens in a heat exchanger which also works as a biofuel burner during the night time to heat the organic fluid (S. Quoilin 2011).
Figure 3 Hybrid Rankine Cycle (S. Quoilin 2011)
Heat Source for Hybrid ORC
A conventional organic Rankine cycle makes use of conventional resources for the heat they required to vaporize the organic fluid inside the boiler stage of the Rankine cycle. The conventional resource of heating in the organic Rankine cycle is the burning of fossil fuel. To transfer the dependence from fossil fuel, the use of alternative and renewable fuel is needed for the organic Rankine cycle.
There are a number of alternative resources available for the heating of organic fluid of organic Rankine cycle and they are explained as follows.
1. Waste Heat Recovery
One of the best, biggest, and most efficient sources of the heat for the organic Rankine cycle is the waste heat coming out of any heating source. This source of heat or organic Rankine cycle is considered best as it does not require any running cost of the organic Rankine cycle. The waste heating coming out of any industrial unit or domestic unit can be used as a heat source for the organic Rankine cycle and the fact that organic fluid of organic Rankine cycle needless amount of heat as compared to water for vaporization makes this waste heat source quite effective for organic Rankine cycle. Using waste heat recovery for the organic Rankine cycle also requires less initial investment as only a heat exchanger is required for extracting waste heat and transferring it to the organic fluid of the organic Rankine cycle.
Biomass as a heat source for the organic Rankine cycle is one of the simplest, cost-effective, and environmentally friendly sources. This limitation of biomass as a fuel is the low quantity of temperature obtains from it does not apply here as the organic fluid of the organic Rankine cycle requires less temperature or heat to get vaporize as compared to water. The biomass as a source of heating does not require the high cost of running the organic Rankine cycle as biomass is usually a waste product of a process that will decompose in nature if not utilized. Some of the advantages of utilizing the biomass as fuel in organic Rankine cycle are, it does not require any specific or expensive machine or set for heating, it is not restricted to any specific area or conditions, it does not have high operation cost and it is completely environment friendly.
The geothermal source of heat for the organic Rankine cycle is considered effective and efficient as it clean, alternative, and renewable source of the heat of the organic Rankine cycle. This source of heat does not require any boiler cost of machinery but the setup required for the geothermal source of heat involves high initial cost. Geothermal source of heat in the organic Rankine cycle does not have high operation costs but has a high limit of the geographic location of the geothermal source. Geothermal source of heat in the organic Rankine cycle is very economical and is highly environment friendly.
4. Solar Heating
Using heat available in the sunlight to heat the organic fluid of the organic Rankine cycle is also an effective, efficient, renewable, and cost-effective solution. This alternative and renewable resource of heating require only a single solar collector which can focus sun rays onto a pipe containing specific fluid usually a refrigerant. The solar heating methods also required a heat exchanger to transfer solar heat absorbed by the refrigerant to the organic fluid of the organic Rankine cycle. This method of heating in the organic Rankine cycle does not require any fuel to operate, does not have a hard geographic limitation, and is super environment friendly but is only applicable during the day timing. Due to this limitation, an extra source of heat is always required in this method which also increases its cost of installation and operation.
The different energy sources which can be used to provide the heat required for the organic Rankine cycle include waste heat energy from industrial units, solar heat energy, geothermal energy, and biomass. From these available energy resources, the waste heat from the industrial unit is most favorable as it does not require any running cost of the organic Rankine cycle. Using waste heat recovery for the organic Rankine cycle also requires less initial investment as only a heat exchanger is required for extracting waste heat and transferring it to the organic fluid of the organic Rankine cycle. For this study waste heat coming out of the kiln of the cement factory where a lot of heat is wasted into the environment as the end of the process will be used. The fluid which contains heat moving out of the kiln and into the environment is simple heated air. Shell and tube heat exchanger with the optimum design parameters of the organic Rankine cycle was designed in such a method its required minimum mass flow rate of air which allows energy saving at the pump and cost-saving at the heat exchanger installation and maintenance without compromising the efficiency of the system.