Energy Efficiency Optimization of Low Grade Waste Heat Recovery via a Carbon Dioxide Rankine Cycle

Global energy consumption is continually increasing, and intelligent energy supply is
a critical concern for our generation. Rising concerns about climate change, as well
as rising greenhouse gas emissions from the usage of fossil fuels, highlight the need of
clean energy generation. Waste heat recovery might be one solution to this problem.
Waste heat is generated as a byproduct of various processes and is discharged into
the environment and the majority being generated at temperatures below 250°C. This
lost energy has the potential to be absorbed and turned into useable energy, notably
electricity. A Rankine cycle is capable of converting heat energy into electricity. Organic
Rankine cycles employ working fluids with low evaporation temperatures (41-78°C),
but are harmful to the environment due to their high global warming potential and
greenhouse gas factor. CO2, on the other hand, has become a popular refrigerant in
recent years due to its environmental friendliness and strong heat transfer capabilities.
For medium to high temperatures about 250°C, converting waste heat to power works
effectively. However, the efficiency is quite poor at temperatures below 100°C.
This work seeks to improve the energy efficiency of waste heat recovery at low temperatures.
Different techniques from the literature on how to increase the thermal cycle
efficiency were studied in order to evaluate the optimization potential. Cycle adjustments
such as reheated expansion, intercooled compression, and recovery were reported.
However, the majority of the existing work is limited to medium to high temperatures.
They were examined under equal operational settings to see if these cycle adjustments
can also be used at low temperatures. A thermodynamic simulation using Matlab and
EBSILON Professional was created for this purpose. For waste heat temperatures ranging
from 60 to 100°C and a heat sink temperature of 20°C, the evaluated power cycles
produced cycle efficiencies ranging from 2.35 to 8.16 percent. The aforementioned cycle
adjustments only had a minor impact on efficiency at low temperatures. As a result,
the basic cycle arrangement with no layout changes was determined to be the optimum
for the examined temperature range. Another significant discovery was that fluid compression
is the primary cause of poor efficiency. Because the compression of the fluid
consumes a considerable portion of the energy provided by the turbine, the net power
output is poor. Consequently, lowering the compression energy increases the net power production and thus the cycle efficiency.
A thorough examination of how to compress a fluid with less (electric) energy input
was carried out. Based on this, a thermal compression device (TCD) was designed,
which uses heat rather than electricity to increase the pressure of a fluid. FLOWNEX
and Matlab were used to simulate the TCD. A fluid flow due to gravity and pressure
differential was developed in this model. The isochoric heat addition (IHA), a batch
process, was modelled along with two buffer vessels upstream and downstream. For the
thermodynamic cycle, the buffer vessels should allow for a steady mass flow. The results
demonstrated that the pressure difference between the vessels has a significant influence
on system performance and must not be neglected. Furthermore, the simulation revealed
that the mass flow is declining during operation, indicating that the containers have not
been completely emptied. As a result, the TCD’s throughput is reduced. This issue
may be solved by adding a piston in the vessel, which allows for exact adjustment of
the vessel volume and fluid flow control. The enhanced TCD, which incorporates the
changes was also simulated in FLOWNEX. The main advantage of the TCD is that it
uses less electrical energy to pressurize a fluid by using waste heat, which is abundant.
Another key advantage is that the constraint of a conventional pump or compressor to a
distinct fluid phase - liquid or gaseous - is removed. The TCD can manage both of these
fluid phases, as well as a phase shift during the pressurization process. The economic
analysis contrasts the traditional ORC with the ORC using the TCD, revealing that the
cost of electricity production is cut in half and the investment payback time is cut by 11
years.