**4.1. Dry test rig**

480 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

to reduce energy losses.

generator production.

If the load is composed by a combination of continuous small absorptions with occasional high requests of energy, it can be useful to combine the above storage technologies in order

The first prototype was not developed using an independent control system and energy converter, but using an integrated solution were all the three elements are included into one single chip. Two different boards were developed using two chips from Linear Technologies. The first solution used the LTC3108, a buck-boost converter without MPPT algorithm but capable of converting input voltages lower than 200mV. This solution was adopted due to the extremely low power coming out from the generator in the first prototype developed. Increasing the generator production, it was possible to move to the LTC3105 regulator, that is capable of converting input voltages higher than 500mV till to 5V, including an internal MPPT control in order to adapt the load absorption according to the

**Figure 11.** DC/DC converter: LTC3108 controller (1), output voltage selectors (2), input terminal, switching inductance and filters (3), regulated output 2.3V÷5V (4), 5.25V storage battery or capacitor (5)

(< 3V) during switch-off period to prevent irreversible damage to the cells.

**4. Experimental validation** 

In the test board two different topologies were tested. The first one used an output voltage set to 2.2÷2.3V connected to a 1F 2.3V super-capacitor (see Fig. 11). This solution was adopted to supply a very light load like a microcontroller (PIC16F886) that run a simple code to only switch on and off one led. The second solution used an output voltage of 4V to supply directly the load and to recharge the 3.6V 220mA battery; specifically, when the converter was on, the remaining current that was not used by the load was employed to recharge the battery, whereas when the input generator was off, the OUT pin was disconnected from the converter so that all the energy required was provided by the battery. A custom external controller is necessary to ensure battery protection from deep discharges

The system described in the previous sections was implemented and experimental tests were carried out to verify the correspondence between the design model and the real To characterize the electric generator, a "dry" test rig was developed as shown in Fig. 12. It is constituted of a DC electric motor (2), which is used to drive the generator's rotor, and the generator's stator (1). A DC power supply is used to energize the drive motor and to put the generator rotor into rotation at different rotational speeds. The values of induced EMF measured during the tests are compared to the FE model in Fig. 13.

Different configurations of the generator have been implemented and investigated with several values of the air gap *t*: 0.5 mm for the nominal configuration as reported in Tab. 7, 1 mm for the preliminary supply of claw poles generators tested in the dry test rig, and 1.25 mm for claw poles generators compliant with the wet test rig. The experimental values represent the RMS value of the voltage measured between the two ends of the stator coil, whereas the FE model results were obtained by using Eq. 6 and Eq. 7 with the data corresponding to the above mentioned configuration. The obtained experimental results show good correlations with the numerical values resulting from the model simulation.

In Fig. 14 the generated power is plotted as function of the rotating speed of the generator for the different air gaps. It is clear that the rated generated power is significantly less than the 100 mW required. This is due to the fact that it was not possible to find a ferrite with a value of the permanent magnet induction equal to 0.42T but only 0.27T, with a consequent loss in performance. It must also be emphasized the drastic lack of performance in terms of power generated by the increasing of the air gap from the nominal value of 0.5 mm up to the value of the wet test rig equal to 1.25 mm.

**Figure 13.** Induced emf for different values of the air gap : *t*=0.5mm (dashed line) vs exp. (star markers); *t*=1mm (dotted line) vs exp. (circular markers); *t*=1.25mm (solid line) vs exp. (square markers).

**Figure 14.** Air gap Sensitivity Analysis: generated power with external load equal to generator resistance. *t*=0.5 mm (dashed line), *t*=1 mm (dotted line), *t*=1.25mm (solid line).

**Figure 15.** Number of coils Sensitivity Analysis: generated power with external load equal to generator resistance. Nominal *N*=1200 (dashed line), *N*=800 (dash dotted line), *N*=620 (dotted line) and *N*=490 (solid line).

A lower influence has been found with the variation of the number of coils of the generator stator *N* as shown in Fig. 15, leaving a certain degree of freedom from this point of view.

#### **4.2. Wet test rig**

482 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

value of the wet test rig equal to 1.25 mm.

power generated by the increasing of the air gap from the nominal value of 0.5 mm up to the

**Figure 13.** Induced emf for different values of the air gap : *t*=0.5mm (dashed line) vs exp. (star markers);

*t*=1mm (dotted line) vs exp. (circular markers); *t*=1.25mm (solid line) vs exp. (square markers).

**Figure 14.** Air gap Sensitivity Analysis: generated power with external load equal to generator

resistance. *t*=0.5 mm (dashed line), *t*=1 mm (dotted line), *t*=1.25mm (solid line).

A second test rig has been developed in order to test the electrical generator coupled with the hydraulic turbine, which makes it possible to characterize both the generator and the hydraulic machine as shown in Fig. 16. This wet test rig is composed by the scavenger (1), connected in series to a flow meter (2), which are both supplied by a domestic water pipeline, where the water flow is adjustable by a tap (3). The measuring system consists of a flow meter (2), a multimeter (4) and an oscilloscope (5) in order to correlate the generated power with the available flow rate. In the lower left box of Fig. 16 an enlarged view of the rapid prototyping realization of the scavenger (1) is also shown.

In Fig. 17 a cross-section of the scavenger is reported; the device incorporates a eighteen blades Banki turbine (1) (see Tab. 5 for other specifications) and a claw-poles voltage generator (2) (see Tab. 7 for nominal specifications). The design of the integration between the two parts required special attention, in particular to ensure the sealing between rotating and fixed parts and to prevent the direct contact between the main water flow and the electrical generator. The presence of the magnet in the generator rotor and ferromagnetic residues in the water may lead to choking risk. To solve the problem of sealing an O-ring (4) has been introduced; to limit as much as possible the choking risk labyrinth seals have been used among the rotor housing and the runner.

**Figure 16.** Wet Test Rig: scavenger (1), flow meter (2), domestic water supply and tap (3), multimeter (4) and oscilloscope (5).

**Figure 17.** Cross-section view of the scavenger: Banki turbine (1), claw pole generator (2), permanent magnet rotor (3) and O-ring (4).

Owing to manufacturing problems related to the choice of the rapid prototyping process, the implementation of the first scavenger prototype presented an air gap equal to 1.25 mm which is greater than the nominal value. This fact has a significant impact on the system performances as shown in Fig. 18. Comparing the data of the power generated by the wet test rig with those of the dry test rig, it is noticed a further drop of performance of the generated power. It goes from 9.1 mW produced at 1000 rpm on the dry test rig to 5.2 mW generated by the wet test rig. This decay can be explained with stick-slip phenomena present among rotating and non rotating parts of the turbine, and it will be addressed and fixed in future studies.

**Figure 18.** Wet Test Rig: generated power with external load equal to generator resistance. Nominal *N*=1200 (dashed line), *N*=800 (dash dotted line), *N*=620 (dotted line) and *N*=490 (solid line).

### **5. Conclusions**

484 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

used among the rotor housing and the runner.

(4) and oscilloscope (5).

magnet rotor (3) and O-ring (4).

residues in the water may lead to choking risk. To solve the problem of sealing an O-ring (4) has been introduced; to limit as much as possible the choking risk labyrinth seals have been

**Figure 16.** Wet Test Rig: scavenger (1), flow meter (2), domestic water supply and tap (3), multimeter

**Figure 17.** Cross-section view of the scavenger: Banki turbine (1), claw pole generator (2), permanent

Owing to manufacturing problems related to the choice of the rapid prototyping process, the implementation of the first scavenger prototype presented an air gap equal to 1.25 mm which is greater than the nominal value. This fact has a significant impact on the system performances as shown in Fig. 18. Comparing the data of the power generated by the wet test rig with those of the dry test rig, it is noticed a further drop of performance of the generated power. It goes from 9.1 mW produced at 1000 rpm on the dry test rig to 5.2 mW This study presents the trade-off analysis, the design, and the experimental validation of an Hydraulic Energy Scavenger applied to a motorized valve for domestic heating systems. The trade-off analysis conducted on the hydraulic and electric machines has identified the Banki turbine coupled to a claw poles generator as the solution to investigate and design. In this configuration the axis of rotation of the machine results to be perpendicular to the flow of the water thereby limiting problems of choking. The models underlying the design are validated from the electrical perspective by the dry test rig, and for the whole system by the wet test rig. The investigation was performed for different values of the air gap t, of the number of coils N, and of the resistive load of the device. The comparison between model and experiments show a good correlation, even though the power generated by the device resulted to be lower than the desired design value. This lower power production is essentially related to manufacturing issues, namely an higher value of the air gap and a lower value of the permanent magnet induction. However, the good correlation between the experimental and theoretical data makes it possible to predict the achievement of the desired performance in case the indicated designed parameters are respected.
