The model described in 3.3 can be used for a parametric simulation of the system, to evaluate the main constraints affecting the ORC performance. The ORC electric efficiency is mainly related to three main aspects: the choice of the working fluid, the evaporation temperature and the condensation temperature. These three aspects are somewhat correlated, as the operation temperatures are determined by the primary heat source and the heat sink, but also by the characteristics of the working fluid.
The commercial units available show a wide range of electric efficiencies, depending on size, fluid, application, etc. The nominal gross efficiencies of some commercial units are reported in Figure 3.4 with respect to nominal output power.
The chart is based on an extract of available commercial units (data from technical papers, see [29], [30], [31] and [37]), in the range 50 kWel ÷ 3 MWel. Larger units are available for some tailored geothermal applications, and therefore will not be considered in this study. The units are quite heterogeneous in terms of working fluids, applications, system layouts and reference temperatures. However, the value of the gross electric efficiency for CHP systems is in the range of 18% ÷ 20% for units larger than 300 kWel of output power. The full electric systems reach higher levels, up to 25%, thanks to the possibility of taking advantage from a lower condensation temperature. Smaller systems (< 300 kWel) have efficiencies down to 7%, due to the lower evaporation temperatures and the size of components resulting in some technical constraints.
Supply side: Organic Rankine Cycle simulation
Figure 3.4 Gross electric efficiency over gross output power for some commercial units.
The simulation model has been used to evaluate the effect of the different parameters from a thermodynamic point of view. As discussed in 3.2, the working fluid choice is usually an iterative process that have to be performed for each application. The aim of this paragraph is to provide some general trends and considerations, as it is not possible to perform a general analysis that can be valid for each application. The model has been applied considering an ORC layout with regenerator (layout B of Figure 3.1), and considering a sub-critical cycle for the working fluids reported in Table 3.2. The R134a has been excluded, as with its critical temperature of around 100°C is not suitable for the temperature range of interest.
Figure 3.5 reports the results of the ORC simulations performed with different fluids, showing a comparison of the maximum gross electric efficiency that can be reached in an ORC unit. The efficiency increase associated with increasing turbine inlet temperature is evident, with some little differences between working fluids, depending on their characteristics and saturation curves. Real efficiencies are usually lower, due to the pressure drops and energy losses that have not been considered in the thermodynamic simulation. The temperature range of each curve is related to the typical application of each organic fluid. The results of Figure 3.5 have been obtained with an isentropic efficiency of 0.8 and a condensing temperature of 30°C. The toluene shows higher performances on a wider range. However, it has to be observed that MM and MDM plants in wood biomass applications are usually designed in "Split layout", obtaining higher conversion efficiencies.
0% 5% 10% 15% 20% 25% 30% 0 500 1000 1500 2000 2500 3000 Gro ss elec tric efficien cy
Gross Output Power [KW]
Supply side: Organic Rankine Cycle simulation
Figure 3.5 Simulation of ORC maximum electric efficiency w.r.t. turbine inlet temperature.
The evaporation temperature is usually fixed by the available heat source, and it is generally one of the most important design parameters for the choice of the working fluid and system layout. For this reason, the evaporation temperature should be kept as constant as possible during the operation of the system, although in some cases the primary heat source can have a variable temperature profile.
On the other hand, the condensation temperature is hardly constant during the operation of the system, and these variations can affect significantly the average performance of the ORC system. This issue can be related to multiple causes, depending on the type of condenser that is used. Full electric ORC systems have lower condensing temperatures, the lower limit being generally associated with ambient temperature for air-cooled systems or water temperature for wet-cooled condensers. For this reason the electric efficiency varies throughout the year, and can be significantly depending on climate conditions. The availability of a water reservoir for condenser cooling gives generally interesting advantages, as it happens with steam cycles. However, even if the systems are generally smaller than steam cycles, potential environmental impacts on water need to be taken into account.
CHP systems use the available heat at the condenser to supply a heat user, which can be a district heating network, an industrial facility or another kind of user. The nominal temperature of heat supply is a design factor (usually in the range 60°C ÷ 90°C), but this value can vary over the year
0% 5% 10% 15% 20% 25% 30% 0 50 100 150 200 250 300 350 Gr o ss e le ctr ic e ff ic ie n cy
Turbine Inlet Temperature [°C]
Supply side: Organic Rankine Cycle simulation
depending on the kind of load control of the system. A detailed analysis of the electric efficiency variation with respect to DH temperature is reported in [38].
The effect of the condensing temperature on the cycle efficiency has been investigated through some additional simulations. Figure 3.6 shows the results of the simulation, with a comparison between gross electric efficiency and condenser inlet temperature for two different fluids commonly used in biomass systems, MDM and R245fa. The evaporation temperature has been set accordingly to the maximum obtainable for each fluid, and it is the main cause for the huge difference between the performances of the two fluids. The condenser pinch point has been set to 10°C, whereas the working fluid entering the condenser has generally a little degree of superheating.
Figure 3.6 Relation between ORC electric efficiency and condenser temperature.
It is clear that the use of R245fa for CHP applications appears to provide a low electric efficiency, unless the heat is supplied at very low temperature (e.g. for low temperature space heating). The choice of MDM, which can benefit from the higher evaporation temperature, allows to obtain an suitable electric efficiency even with relatively high temperatures at the condenser. However, this analysis is a simplified thermodynamic evaluation of the performance: other aspects need to be taken into account when designing a real system, such as the minimum available size, the need of an additional thermal loop, etc.
Another aspect to be considered is the consumption of the feeding pump, which can in some cases become a significant issue in ORC systems. Figure 3.7 shows a simulation of the auxiliary consumption of the pump, for the same cases of Figure 3.5. The pumping consumption has been calculated considering an isentropic efficiency of 0.7 and a pump efficiency of 0.9. The worst fluid appears to be R245fa, while toluene and MDM show the best performance. The pumping consumptions are generally the larger part of the energy required for the auxiliary system components. 0% 5% 10% 15% 20% 25% 30% 0 20 40 60 80 100 G ro ss e le ctric ef fici en cy
Condenser Inlet Temperature [°C]
Supply side: Organic Rankine Cycle simulation
Figure 3.7 Simulated auxiliary consumption of the pump.
0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 0 50 100 150 200 250 300 350 Au xiliary co n su m p tio n (s h ar e o f gro ss o u tp u t p o w er)
Turbine inlet temperature [°C]
Operation analysis of real ORC
systems
A part of the study has been devoted to the operation analysis of two real ORC systems currently in operation. This choice has been done in order to apply the simulation model to real systems, considering real operation data where multiple external factors can affect the performances and the operation conditions. The analysis of real plants allows to deal with multiple aspects that are usually not considered in standard simulations.
Two different case studies have been included in the analysis. A critical point has been the availability of operation data with a narrow time step, together with the plant owner's willingness of performing operation analyses. The case studies considered represent two typical applications of the ORC technology to biomass to energy conversion systems.
The first case study is composed by two small size ORC units (125 kWel each) connected to a superheated water boiler running on biomass residuals from pruning activities and green waste management. The type of ORC installed in this plant is one of the first installed in Italy in this range of power output for biomass systems.
The second case study is related to a more standardized ORC unit, with a nominal output power of about 1 MWel. The ORC unit is part of a biomass CHP system installed in an industrial facility. In the following paragraphs these two case studies will be described in detail, providing the main data available from the operation of the system, together with the results of the analysis.