3.1. Feasibility Assessment of a DIP Technology for the Scroll Expander
The DIP technology is a novel design solution for positive displacement expanders consisting of the introduction of a further intake port after the main one. The second intake port is in an angular position corresponding to the expansion phase of the machine, positioned just after the first one. In this way, the machine can elaborate a higher mass flow rate without producing an increase of the expander inlet pressure, and in the process keeps the pressure ratio across the expander constant. This produces an enhancement of the indicated power of the machine, as can be observed in
Figure 8, where the indicated cycle of an original manufacturer machine (SIP) and of a DIP machine is reported. It has been calculated thanks to the model previously validated. In the DIP machine, the suction produced by the second port starts immediately at the end of the main one, thus the decrease in pressure due to the expansion is prevented by the extra mass flow rate entering the machine through the additional intake port.
This provides an increase in the indicated power and consequently of the mechanical power provided by the machine. In other words, the DIP technology increases the permeability of the machine α expressed as the ratio between the mass flow rate elaborated by the expander and the pressure ratio at the intake and the exhaust side of the machine Δ
pexp (Equation (3)):
The extra mass flow rate entering the machine leads to an enhancement of the mechanical power produced by the expander, enhancing the energy recovery of the unit and avoiding an increase in inlet pressure which would be produced by an extra working fluid mass flow rate (when compared to the designed one). It is also evident how the fluid entering through the second port increases the pressure at the opening of the discharge port, producing an isochoric expansion which represents a loss with respect to an eventual residual adiabatic expansion.
The capability to aspirate a greater mass flow rate allows for a greater thermal energy recovery from the exhaust gases and could be suitable when the ICE works at full load. This means that the HRVG would be able to operate in off design conditions, enabling a greater working fluid flow rate. Nevertheless, in a conventional expander, the intake expander pressure grows quite linearly with the mass flow rate [
32], which means that a high increase of it could lead to an excessive intake pressure value, compromising the integrity and operability of the machine’s components (e.g., sealing systems). Therefore, the DIP technology allows for an increase in α, thereby maintaining the intake pressure and allowing for a significant increase in the working fluid mass flow rate.
The feasibility of DIP technology has been widely assessed by the authors in the context of sliding rotary vane expanders, where an increase of 40–50% of the expander power was observed with respect to the SIP version [
31]. DIP technology is expected to be similarly beneficial from a conceptual point of view when it comes to scroll expanders, as they are positive displacement devices. Nevertheless, due to their more complex geometry, the introduction of a DIP technology is less straightforward in this context than in the case of SVREs. In fact, in SVREs it is sufficient to introduce a further port on the stator (after the main intake port), while for scroll expanders, two symmetric orifices are needed (
Figure 9). This is because, during the operation, the scroll intake chamber (1a + 1b in
Figure 9) branches off in two parts (2a and 2b); therefore, to feed them during the dual intake phase, two additional ports are needed (blue orifices in
Figure 9, which simultaneously feed the scroll chambers 3a and 3b).
In this way, the two orifices (which behave like a second intake port) allow for the a and b chambers (
Figure 9) to be synchronously fed, thus preventing imbalances between the mass flow rates of the two intake branches during the cycle.
Moreover, despite this consideration of the DIP position, this technology could be easily implemented also for the scroll, as the orifices can be placed on the front casing of the machine, which in general presents a quite flat surface. Another aspect to consider is that the orifice should have a diameter which avoids the communication channel between adjacent chambers (3a and 3b in
Figure 9). This situation should be avoided because it breaks the symmetry of the intake phase; thus, the diameter of the second port realized by means of two orifices cannot exceed the thickness of the orbiting and fixed spirals.
Considering all these aspects, a DIP scroll machine was designed for the test case (with orifice diameters equal to spiral thickness, 2.25 mm). Several positions of the orifices were considered and their effects on the scroll machine were evaluated thanks to the proposed previously validated theoretical model.
To find the best position for the orifices, five cases were considered, which involved varying their angular positions while always keeping their diameters constant (2.25 mm). The position of the orifices is expressed as the function of the angular delay φ with respect to the main intake port: in the considered cases, φ is equal to 360° (
Figure 10a), 540° (
Figure 10b), 720° (
Figure 10c), 900° (
Figure 10d) and 1080° (
Figure 10e), respectively. Thus, the angular delay φ expresses the angular rotations which the orbiting spiral of the scroll should perform to feed two symmetric chambers through the second intake ports.
For instance, considering
Figure 10a, where φ = 360°, a complete rotation should be completed by the scroll orbiting spiral prior to the chamber (1a + 1b) branching off into two symmetric chambers, which are simultaneously filled through the second intake ports. The higher is φ, the larger the angular distance between the main and the second intake phases. It is worth to noticing that, for the convention adopted, a reference of the angular rotation is taken at the start of the main intake phase. For all the cases, expander intake and exhaust pressures are equal to 7 bar and 1.4 bar, respectively, while the expander intake temperature is equal to 145.6 °C. The working fluid considered is always R123.
The performance of the scroll with a DIP technology was compared to that of the SIP case and the results are reported in
Table 4 and
Figure 11 in terms of indicated cycles.
The results show that the higher the φ (and hence the distance between the opening of the second port and the closure of the main one), the larger the increase of working fluid mass flow rate aspirated. Indeed, the mass flow rate increases from 0.059 kg/s of the SIP to 0.089 kg/s in the case with φ = 1080° (
Figure 11e): in this case, the maximum angular distance between the main intake port and second one is observed. An increase in machine permeability is evident, with the growth of mass flow rate equal to almost 50% for the same in/out scroll pressure difference. Nevertheless, considering the produced mechanical power, it does not increase continuously with the increase in mass flow rate. Indeed,
Pmech grows until the case with φ = 540° (
Figure 11b), in which the mechanical power increase Δ
Pmech (with respect the SIP) reaches 25%. After this best power case in terms of larger φ values,
Pmech decreases until it is lower (2.9%) than the value for the SIP technology (
Figure 11e).
This means that the second intake port should not be placed beyond a given volume increase when the pressure inside the chamber is too low. In this situation, the further mass flow rate entering the vane does not produce a sensible increase of the chamber pressure, as can be observed in the indicated p-V diagrams reported in
Figure 11. Therefore, this further mass flow rate is useless, as it does not produce a benefit on indicated power. For this reason, the best position for the second port is in correspondence with the middle of the expansion phase, where the best effect on the indicated diagram growth is observed (
Figure 11b).
Considering the expander global efficiency, in all cases the DIP technology presents a lower efficiency with respect the SIP technology, as it can be observed from
Figure 12a, where Δ
ηexp is reported.
From
Figure 12a, it can be seen that after φ = 360° (where the DIP and SIP technologies have nearly the same efficiency (Δ
ηexp = 0)) a linear decrease is observed.
To understand the reason behind such behavior, it is useful to consider the expander efficiency chain, which can be obtained as follows.
In Equation (2), the global efficiency was expressed as the ratio between the mechanical power and the power produced by the expander in case of an adiabatic iso-entropic transformation and in the absence of volumetric losses (leakages). In reality, the global efficiency can be expressed as in Equation (4), which outlines the role of volumetric, indicated and mechanical efficiencies:
The DIP technology modifies all of these three efficiencies, and for a direct comparison with the conventional machine (SIP) it is important to fix the conditions of this comparison. In this case, two situations are possible: either the two machines (a) have the same intake pressure, or (b) are crossed by the same mass flow rate.
In case (a), DIP technology leads to an improvement of volumetric efficiency, with
being increased and
being comparable with the value in the case of SIP technology. The increase of
causes a more pronounced isochoric expansion when the exhaust port opens, so the indicated efficiency decreases (
Figure 12b). This is because the remaining adiabatic isentropic expansion becomes significant until the discharge pressure. Moreover, the mechanical efficiency sees two opposite effects when DIP technology is considered: an increase in indicated power, but also an increase in friction. The overall effect therefore depends on the balance of these two contributions. The overall effect is demonstrated in
Figure 12b.
In case (b), the DIP expander is characterized by a lower intake pressure when compared to the SIP. This has a positive impact on volumetric efficiency, being the pressure difference the main driver of the volumetric losses which decrease. Concerning the indicated efficiency, even though the angular position of the second port is properly designed, the benefit among different contributions regarding its definition is not guaranteed. On the contrary, the mechanical efficiency of DIP technology increases because the friction losses diminish, meaning that the average mean pressure is lower. In most cases, the net effect is an increase in the global efficiency when compared to the SIP technology.
In addition the impact of the DIP on expander performance, it is also important to observe the benefits introduced by such technology in terms of plant efficiency.
In fact, the use of the DIP technology provides an increase of the whole ORC-based plant efficiency if the position of the second port is optimized. The comparison should be carried out considering the same working fluid flow rate entering the expander, a condition which means that a fixed thermal power from the hot source is recovered. For a flow rate equal to 0.071 kg/s, the SIP expander operates at 7.9 bar with a pressure drop inside the machine equal to 6.1 bar (see
Table 2), while the DIP expander operates at 7.0 bar with a pressure drop equal to 5.6 bar (see
Table 4). The power produced in the first case is 1269 W and 1352 W in the second case. Considering that the power absorbed by the pump is lower in the DIP case, the plant efficiency is therefore greater when a double intake port is considered.
3.2. Comparison with DIP Technology for Sliding Rotary Vane Expander SVRE
It has been already observed that volumetric expanders have intrinsic advantages with respect to dynamic machines for small size ORC-based power units. A comparison between scroll and sliding vane rotary expanders is more complex and does not lead to a clear and definitive conclusion. In the following, the two machines were compared in terms of performance due to the adoption of a DIP technology.
The DIP technology was originally conceived to be applied to sliding rotary vane expander [
31]. As reported by the authors in their previous works [
31,
32], the benefits introduced by such novel technology to SVREs are multiple and noteworthy, since DIP technology allows for the improved performance of the expander performance in many respects. Firstly, the dual intake technology ensures that the operability of the machine can be widened. Indeed, the introduction of a further suction port makes the expander more permeable in such a way it can increase the mass flow rate for a given pressure difference between expander intake and exhaust sides. Consequently, the dual intake (or supercharged) expander produces a higher mechanical power, thus increasing the energy recovery. These benefits were also observed in terms of the SVRE design. As matter of fact, the adoption of DIP technology opens the way to expander downsizing with respect to the case of the single port. This is made possible by another aspect of permeability growth, which can be observed when a comparison with the SIP expander is conducted while keeping the mass flow rate constant. In this case, the DIP expander, being more permeable, presents a lower intake pressure with a slight reduction in the power produced. Nevertheless, by reducing the dimensionsof the DIP expander, the intake pressure grows, producing a comparable power more efficiently. In fact, if the dimensions of the expander diminish, the power loss due to friction decreases too, as the mass of the elements in relative motion are lower. Considering these important benefits, confirmed both experimentally and numerically [
31,
32], the DIP SVRE was considered as a reference for the evaluation of the suitability of that novel technology. Therefore, the performance of the DIP Scroll was compared to that of the DIP SVRE to analyze similarities and differences in the behavior of the two supercharged machines.
For this reason, a single intake port (SIP) SVRE was designed to operate in the same conditions in which the scroll expander were analyzed. The conditions were:
= 0.059 kg/s,
pexp,in = 7 bar and
pexp,out = 1.4 bar (first column of
Table 4). This was done thanks to the experimentally validated numerical model developed in [
31]. The main dimensions of the SVRE are reported in
Table 5. In this design, a further intake port was introduced with different angular positions, defined by the angle φ due to the difference between the end of the main intake port and the start of the dual intake port (
Figure 13). Therefore, the performances of the SVRE were calculated when the DIP technology is considered and compared to those of the scroll expander previously discussed.
Higher angular delays in dual intake port opening were not taken into consideration, because in this case the dual intake phase shares an angular interval with the discharge phase, causing a critical volumetric loss directly from the intake to the exhaust [
31]. In
Table 6, the performance of the SIP SVRE is reported together with that of the DIP by varying the angular position of the auxiliary port.
As
Table 4 and
Table 6 show, despite the SIP SVRE elaborating an equal mass flow rate for the same pressure difference when compared to the SIP Scroll, the power produced by the SVRE is lower. Indeed, in the case of the SVRE, the power is 816 W, while for the scroll machine it is 1131 W. This is due to the lower global expander efficiency of the SVRE (43%) in comparison to the Scroll (58.3%).
Nevertheless, the introduction of DIP on vane expander leads to a boosting of the mass flow rate elaborated by the machine and consequently of the mechanical power produced. The mass flow rate and power gains are higher than those achieved by the DIP Scroll. In fact, in the case of the SVRE, the increase of mass flow rate varies from 95% to 143.2% when φ grows from 43.2° to 83.2°. However, similar to what happens in a DIP Scroll expander, the power increase does not follow the flow rate gain but assumes a maximum in correspondence of 53.2° and then decreases until 70% when φ is equal to 83.2°.
The reason of this behavior can be observed from the indicated cycle of the DIP SVRE reported in
Figure 14. The analysis of the indicated cycle shows that the higher the delay of the dual intake phase, the larger the volume of the chamber when the further mass flow rate enters the expander. Therefore, the extra mass flow rate produces a lower boost effect on chamber pressure, as the density grows less than in the case of the DIP when the chamber presents a lower volume (in the first stage of the expansion phase). These results are in accordance with those reported in [
31], obtained using a different fluid.
The analysis of the indicated cycle of the DIP SVRE also explains the motivation behind the higher impact of the DIP on the SVRE in terms of power in comparison with the DIP scroll. In fact, the DIP technology in the SVRE produces a greater isobaric trend in terms of extended volume of the indicated cycle in correspondence with the dual intake port (
Figure 14) when compared to the scroll machine. The reason behind this is related to the architecture of the two devices.
Indeed, in the SVRE machine, one cycle (intake, expansion and exhaust) is completed within one shaft revolution, whereas the scroll machine needs more shaft revolutions (five in this case) to perform a complete cycle. This means that DIP technology in the SVRE is concentrated in a lower angular interval and, consequently, the extra mass flow rate produces greater benefits on pressure boost and, thus, on power gain.
On the other hand, DIP technology in the SVRE produces a greater decrease in expander efficiency (with respect the SIP version) than in the case of the DIP Scroll. Indeed, as it can be seen in
Figure 15a, the DIP SVRE efficiency decreases from 43% to 30% when the φ rises from 43.2° to 83.2°, whereas the DIP Scroll exhibits a reduction from 60% to 40%. This difference is mainly due by two causes:
The first of these is the higher efficiency of the SIP Scroll (60%) when compared to the SVRE (43%).
The second is that in the SVRE, the isochoric expansion at the end of the auxiliary intake phase is higher than in the case of the DIP Scroll. This is the effect of the higher extra mass flow rate on indicated power in the SVRE.
Nevertheless, for the angular position in which the DIP reaches the best performance (43.2–53.2° for SVRE and 0–540° for Scroll) the efficiency is almost the same as the SIP machines. Indeed, the efficiency reduction of the two machines assumes a maximum value of 10–12% (
Figure 15b). Moreover, this low efficiency reduction is a reasonable price to pay considering the operability gain obtained with the DIP, which leads to a higher recovery efficiency of the unit.