The balance of the HRVG is presented in Figure 11: the major loss is the enthalpy owned by the exhaust gases leaving the evaporator (25-30%), while irradiation accounts for about 1.5% in every cases studied.
HRVG efficiency, defined as the ratio between the power recovered and absorbed by the working fluid and the entire enthalpy of the exhaust gases (eq.
bypassing part of the gas from the HRVG is an effective way to manage the transients generated by the engine working conditions.
HRVG efficiency must be considered to express the overall plant efficiency of the ORC based power unit according to eq.
Figure 13 shows the efficiencies considered in the working conditions tested: HRVG efficiency is in the order of 0.6 to 0.75, while thermodynamic efficiency is about 10%.
The temperature of the gas exiting the HRVG is, therefore, higher and, so, HRVG efficiency is lower.
It is found that out of 100% fuel exergy which is the difference of exergy input at HRVG and exergy of industrial waste heat at generator output, around 5.61% is produced as exergy of refrigeration due to all the cooling effects.
The component like ejector, HRVG, and condenser ejector where maximum exergy is destroyed needs special attention from second law point of view.
This is due to the fact that increase in turbine inlet pressure which lowers the mass flow rate of refrigerant vapor will result in a reduced absorption of heat from the exhaust gases passes through the HRVG which leads to the higher value of HRVG exit temperature.
Maximum exergy is destroyed in the ejector, HRVG, and condenser of the ejector.