• Energy Research
• 2021-2025close

Abstract Sliding vane rotary expanders (SVREs) are widely used in organic Rankine cycle (ORC)-based power units for low-grade heat recovery because of their capability to deal with severe off-design working conditions. In particular, the speed of SVREs is a very effective operating parameter, together with the speed of the pump, to regulate the recovery unit and to lead the involved components in an acceptable operating behaviour when they are far from the design conditions. In this study, a control strategy based on the variation in revolution speed of a SVRE was developed, where the inlet pressure of the expander is the main controlled property, which must be verified when the flow rate of the working fluid is changed to match the thermal power recovery at the hot source. In fact, pressure level control is a key point of the recovery unit for thermodynamic reasons and for the safety and reliability of the expander and, more generally, of the whole recovery unit. The proposed control strategy is based on an original theoretical procedure that relates the expander speed, inlet pressure, volumetric efficiency, and working fluid mass flow rate in an analytical form. This analytical formulation is widely nonlinear and is simplified for use as a tool for the model-based control of the inlet expander pressure. An experimental activity performed on a SVRE operating in an ORC-based power unit, fed by the exhaust gases of a supercharged diesel engine, was the base of the analytical formulation. This provided the possibility of deriving a simplified model-based control of the expander inlet pressure and assessing its effectiveness and limits during off-design conditions. Higher expander global efficiencies were obtained (up to 45%), allowing a greater mechanical energy recovery (up to 2 kW).

<div class="section abstract"><div class="htmlview paragraph">Within automotive sector, there are several high-performance applications, like, for instance, those referred to racing and motorsport, where cooling needs are usually fulfilled by simple circuits with conventional low-efficiency pumps. The cooling needs in these applications are represented by low flow rates delivered (in the range of 10 - 50 L/min). The operating conditions of these small pumps are usually characterized by very high revolution speeds, which intrinsically cause low efficiency and critical intake phenomena (cavitation) if the design is not specifically optimized to address these concerns.</div><div class="htmlview paragraph">Hence, in this paper a small-size pump operating in the racing sector has been designed using a model-based approach, built and tested having reached both high efficiency (aimed to 50%) and absence of intake operational problems (cavitation). Starting from the specific cooling request (design flow rate equal to 14.0 L/min and pressure rise equal to 2.5 bar), the very limited space available on board oriented the design to an operational revolution speed of 12000 RPM. The interest of this study was to introduce a so high revolution speed in more conventional automotive cooling pumps electrically assisted, keeping high efficiency. In fact, the strong reduction of the size of the pump allows an easy and correct positioning on board.</div><div class="htmlview paragraph">The model-based design was done by a two-steps procedure. The first made use of a 0D model which, catching main physical phenomena of the flow even in simplified form, leads to an optimum geometrical design for the impeller and the volute. A final refinement has been done with a CFD code predicting the off-design performance and limiting cavitation zones. Cavitation, which is one of the most critical issues of high-speed pumps, was completely investigated through a CFD numerical analysis.</div><div class="htmlview paragraph">The pump has been prototyped and tested on a dynamic test bench for pumps, which reproduces homologation cycles and real driving. A good agreement has been reached between theoretical and experimental results, being the mean relative error on pressure rise for all operating point close to 4 %. This model-based procedure opens the way to support the development of electric water pumps for more conventional applications (automotive, light duty engines) in which a redesign will be focused to manage the thermal state of the engine and reduce the energy absorbed during the homologation cycle.</div></div>

Presently the on-the-road transportation sector is responsible of the 21% of the whole CO2 amount emitted into atmosphere. This pushes the International Governments and Organizations to provide strict limitations in terms of ICEs emissions, also introducing fees payment for the car manufacturers. The vehicle electrification allows certainly to meet these requirements, but the higher cost and the need of a green electricity still limit a widespread diffusion among all social classes. Thus, the technological improvement of internal combustion engine plays a key role in the transition period. Among these technologies, the engine thermal management allows to achieve a good compromise between the CO2 emission reduction and related costs. It was demonstrated that replacing the conventional centrifugal pump of engine cooling system with a sliding vane rotary pump (SVRP), important benefits in terms of CO2 emission reduction can be achieved as centrifugal pump efficiency decreases significantly when the engine works far from the maximum load (i.e. design point of the pump). Nevertheless, the complex thermo-fluid-dynamic phenomena taking place inside a SVRP make its design not immediate, particularly if heavy duty ICE cooling systems are considered. These applications indeed are challenging due to the wide operating range and the huge flow rates which pump must deliver. These operating requirements make difficult the choice of the main design parameters: among the different ones, the pump revolution speed and displaced volume. In the present paper a design strategy is developed for this type of pumps based on a comprehensive mathematical model of the processes occurring, predicting volumetric, indicated and mechanical efficiencies. The model was validated with a wide experimental activity so acting as virtual development platform. The results show how the best global efficiency (0.59) is achieved adopting a dual axial intake port configuration, with a suitable choice result of a trade-off between displaced volume and revolution speed. The analysis also show that the pump keeps its efficiency close to the design one for a wide operating range which is particularly suitable for the cooling of an ICE.

A promising solution for the Combined Heat and Power (CHP) micro production is certainly represented by Organic Rankine Cycle (ORC)-based power units. In the domestic appliances with electrical power range of the units below 1 kW, the reduced dimensions of the components represent a critical aspect as well as the need to guarantee a high reliability. When the hot source is represented by solar energy, the optimization of the electricity production keeping insured the thermal energy availability represents an aspect which invites to a proper management of the unit. Solar-based ORC-recovery units frequently work in off-design conditions due to the variability of the hot source and to the Domestic Hot Water (DHW) requirements. For this reason, the design and the selection of the components should be carefully performed. The expander is commonly retained the key component of the unit being the one that mainly affects the behaviour. For the mentioned power ranges, the volumetric expander is the best technological option and, among those available, Sliding Rotary Vane Expander (SVRE) are gaining a sensible interest. At off design conditions, according to permeability theory, the expander intake pressure linearly varies with mass flow rate of the Working Fluid (WF) which is the most suitable and easiest parameter to be changed. This modifies the performances of the unit, both from a thermodynamic and technological point of view. In this paper, the speed variation of the expander is considered as control parameter to restore design expander intake pressure. In order to assess a strategy for the speed variation of the expander, in this paper a comprehensive model of the SVRE is presented when it operates in a solar-driven ORC-based unit. The model is physically based and recovers and widens the permeability theory developed by the authors in previous works. An experimental ORC-based unit was fully instrumented and operated, coupled with a reservoir, usually present when flat plate solar collectors are used, which store the thermal energy which fulfils thermal energy requests and feeds the generating unit. The model was widely validated with the experimental data properly conceived for the purpose. In the unit the expander speed was varied and, thanks to the permeability theory, the relationships between WF flowrate variations, inlet expander pressure and expander speed variation were investigated. The potentiality of a control strategy of the expander revolution speed of the expander was fixed as well as a deeper understanding of the SVRE behaviour and relationships between operating variables. In particular, it was observed that varying the speed from 1000 RPM up to 2000 RPM, the expander behaviour was optimized ensuring proper working condition matching with a (30–100 g/s) flowrate range.

ORC power units represent a promising technology for the recovery of waste heat in Internal Combustion Engines (ICEs), allowing to reduce emissions while keeping ICE performance close to expectations. However, the intrinsic transient nature of exhaust gases represents a challenge since it leads ORCs to often work in off-design conditions. It then becomes relevant to study their transient response to optimize performance and prevent main components from operating at inadequate conditions. To assess this aspect, an experimental dynamic analysis was carried out on an ORC-based power unit bottomed to a 3 L Diesel ICE. The adoption of a scroll expander and the control of the pump revolution speed allow a wide operability of the ORC. Indeed, the refrigerant mass flow rate can be adapted according to the exhaust gas thermal power availability in order to increase thermal power recovery from exhaust gases. The experimental data confirmed that when the expander speed is not regulated, it is possible to control the cycle maximum pressure by acting on the refrigerant flow rate. The experimental data have also been used to validate a model developed to extend the analysis beyond the experimental operating limits. It was seen that a 30% mass flow rate increase allowed to raise the plant power from 750 W to 830 W.