V. Standard
2.4 Potential Work and Expander Recommendations
The success of a small ORC greatly depends on the expander efficiency. An expander that is not suitable for the working fluid or flow conditions will no perform at the desired efficiency.
If the potential work is greater than 1MW a turbine is the best option for the ORC[69]. A project with a potential greater than 1MW can approach large turbine manufactures for their turbine procurement and the following turbine analysis is unnecessary.
A custom designed turbine can achieve high isentropic efficiencies to maximize the power available in the low temperature heat source; however, there will be some development risk that the turbine manufacture and owner must negotiate.
Figure 37 shows the process to the efficiency of the potential expander in an ORC.
0.0 0.3 0.5 0.8 1.0 1.3 1.5 25 50 75 100 125 150 175 200 2x101 5x101 7x101 1x102 1x102 1x102 1x102 2x102
s [kJ/kg-K]
T
[
°C
]
n-Pentane Geothermal BrineWorking fluid cycle
2.4.1 Small Work <250kW
A low temperature resource generally has a lower work output. It has been mentioned that there are two options for a small ORC, either a volumetric expander or a small turbine. A volumetric expander should be used with ORCs that are producing up to 250kW as they are more available and economical[26]. However, if desired the feasibility of a small turbine can be investigated. The market for small turbines for power generation at the time of writing this document was small.
2.4.2 Turbine Feasibility
The best check to understand the efficiency of a turbine in the ORC is the specific speed analysis. Both radial and axial turbines achieve the best performance when the specific speed is between 0.3 and 1[83]. It is preferred that the speed of the turbine is designed to operate at the synchronous speed of the generator – 3000 rpm in New Zealand's case.
This case is for a single stage turbine.
Cycle Design
HX Components Volumetric
Expander Specific Speed Check
(0.3 -1)
Large Turbine Fix Specific Speed – Check
ORC Suitable PPT and PR?
Fix Specific Speed – Change WF – Check PPT and PR YES NO YES NO FAIL PASS FAIL PASS Potential Work 1MW+? Low Work <250kW
Specific speed formula
VB=(/+∆ℎ b -c/i
VB is the specific speed of the turbine is the rotational speed of the turbine rad/s ( is the mass flow rate into the turbine kg/s b is the density of vapour into the turbine m3/kg
∆ℎ is the isentropic enthalpy drop across the turbine stage kJ/kg
The specific speed chart shows the impact on the maximum efficiency.
Figure 38 - Specific Speed Chart – Credit Balje [73] 2.4.3 Desired Specific Speed Check
Because of the low temperature and flow rates in the ORC the turbine is typically not within the best efficiency zone. The speed of the turbine can be changed to overcome this; however, this increases the complexity and cost of the system. Alternatively, the cycle can be adjusted to better match the optimum specific speed. Figure 39 outlines the process to follow to recalculate the specific speed and efficiency of the turbine. If matching the specific speed to the cycle results in a losing more than 30% of the original work, a more complex expander is needed. This will either operate at higher speed with a reduction gearbox or be a multiple stage turbine. This will be covered in the feasibility study and for the rest of this
2.4.3.1 Set Condenser Conditions
The two main variables that impact the specific speed of the turbine are the evaporation temperature and condensing temperature. These two variables impact the mass flow rate, vapour density, and enthalpy drop. The condenser conditions can be less flexible than the evaporator conditions so this is the first variable set before analysing the specific speed.
2.4.3.2 Change Evaporation Temperature
In order to increase the specific speed, normally the issue with small ORCs, it is necessary to decrease the evaporation temperature. This decreased temperature increases the volumetric flow rate of the fluid
Figure 39 - Desired Specific Speed Analysis
Desired Specific speed – Check
ORC Suitable PPT and PR?
Set Condenser Conditions
Change evaporation temperature
Recalculate Specific Speed
Check PPT and PR
and work
Appropriate Specific Speed? Yes Noand decreases the isentropic enthalpy drop. All of these conditions contribute to an increased specific speed.
If the new specific speed is appropriate then the evaporation temperature does not need to be changed again. Once the specific speed is appropriate the pinch point temperatures and pressure ratio should be checked and are not unfeasible.
The results of the ORC should be checked once more to understand the implications of adjusting the cycle for a better specific speed.
If either the PPT is less than 5 or the PR greater than 5 it is unreasonable and the condenser temperature is changed and process redone. Finally if it appears there is no feasible single stage turbine operating at synchronous speed the working fluid can be changed. If there are no solutions this turbine will be explored in more detail in the feasibility study; otherwise, a volumetric expander is a simpler option.
The remainder of the pre-feasibility study can be completed with the new turbine and cycle or use the assumed 85% isentropic efficiency in the original cycle analysis.