2.3.1
Experimental Set-up and Measurements
In order to compare our proposed architecture to existing solutions, we implemented the different existing architectures for laboratory experiments, using breadboards and evaluation boards. In order to minimize the differences due to implementation choices, the same components and circuits were reused as much as possible.
The realized prototype is functional and is able to self-start. Functionality of the controller has also been validated using a simple switch algorithm, which alternatively closes the switches each DSW = 1 s. Our system is used to power a wireless sensor
developed by Wi6labs [144]. A simple power manager is implemented on the MSP430, which measures the main energy storage SOC and commands the sensor node to transmit a packet if the storage is charged. Therefore, the more energy is harvested, the lower the period between two transmissions DT X will be. Voltage generators
are used to emulate sources. The first source is set at 4.2 V, while the second is set between 1.5 V, 3.1 V or 3.7 V. Both sources are limited to 1 mA in order to simulate low power sources. However, these sources are still oversized compared to real energy sources. As the power provided by real sources would be lower, the power consumption overhead of the circuit would have a higher impact on the harvested energy.
Even if these energy sources are oversized, compared to low power provided by real energy sources, the use of deterministic sources allows an accurate characterization of the proposed system. Different measurements can be performed in a consistent set-up, without being affected by the environment. The input energy buffers are arbitrarily chosen and have respectively a 4700 µF and 1000 µF capacitance, while the energy storage is a capacitor array with a 34.7 mF total capacitance. The following cases, illustrated in Fig. 2.13a to Fig. 2.13d are evaluated:
S1 S2 D DM P P T Switch1V 1Switch3V P arallel 0 15 30 45 60 75 90 34.3 84 54.2 34.3 77.2 84 22.3 34 45.4 53.4 33.6 55.5 29.6 15.6 31.3 39.5 67.2 12.3 48.5 21 13.5 DT X (s ) S2 = 1.5 V S2 = 3.1 V S2 = 3.7 V
Figure 2.14: Period in s between two LoRa TX depending on case and S2 voltage. • D: Source 1 and 2 connected to a single PMIC through ideal diode circuit,
without MPPT (Fig. 2.13a)
• DM P P T: Source 1 and 2 connected to a single PMIC through ideal diode circuit,
with MPPT (Fig. 2.13b)
• Switch1 V 1/3V: Periodic switch (DSW = 1s) with PMIC VM P Pset set to 1.1 V/3.0 V
(Fig. 2.13c)
• P arallel: Parallel architecture - two PMIC directly connected to the battery, with MPPT (Fig. 2.13d)
For all situations, the period DT X between two consecutive LoRa packet trans-
missions is measured ten times. The average value DT X is then computed. These
results are shown in Fig. 2.14. When more energy is harvested, the energy storage is charged faster, and the period between two messages is reduced. On the other hand, a less efficient energy harvesting system will induce a higher delay between messages.
2.3.2
Discussion
The situation D performs significantly worse than single-source situations, while DM P P T performs better. This is due to the lack of any MPPT in situation D, and
62 Industrial implementation
Component
Switching Arch.
Parallel Arch.
Name Unit Price Count
Price Count
Cost
SPV1050
$2.21
1
$2.21
N
$N × 2.21
MSP430
$0.70
1
$0.70
0
$0
TPS60210
$1.47
1
$1.47
0
$0
SiP32431
$0.28
N
$N × 0.28
0
$0
Ideal diode
$0.30
N
$N × 0.30
0
$0
Total
1 $(4.38 + N × 0.58)
1 $N × 2.21
Table 2.2: Cost breakdown of the solution.
shows the impact of MPPT. However, DM P P T is a naive implementation, and is not
efficient if the two input voltages are too different. When the MPPT circuit measures VOC, it measures the highest voltage in all sources, and sets its VM P P accordingly.
Thus, a source with a lower voltage operates far from its MPP, or does not provide power even if its voltage is smaller than the measured VM P P.
Our solution is hindered by the lack of MPPT. By setting Vset
M P P too low, as in
situation Switch1 V 1, the sources operate far from their optimal power point, and
provide less energy. Alternatively, rising Vset
M P P too high may render some sources
useless. Indeed, when S1 is set at 4.2 V, rising Vset
M P P to 3 V brings the converter
operating point close to the source MPP. But, if the second source voltage is smaller than Vset
M P P, no current will be drawn, and the source will not be used at all. However,
if Vset
M P P is close to both sources MPP, such as situation Switch3V with S2 set a 3.7 V,
our solution performs correctly. This demonstrates the potential of our solution, when the sources operate close to their MPP. The P arallel architecture does not suffer from this limitation, as a different MPPT circuit is used for each energy source. The energy buffer size has also an impact on the system: an oversized buffer takes more time to charge, and the energy source takes a longer time to reach its VM P P.
In order to maximize harvesting efficiency, decision algorithms should be designed so that the sources operate near their MPP. Possible solutions include setting Vset
M P P
from a DAC and adapt its value according to the selected source, or adding a full- featured MPPT circuit between a source and its energy buffer.
Cost was a primary concern when designing our system. Table 2.2 shows the cost associated with a state-of-the-art parallel architecture and our switching architecture. Only active components are taken in account. Prices are obtained from standard distributors for 100 pieces of each component. Ideal diode cost is estimated. N designates the number of energy sources in the system. Our system becomes cost effective for N ≥ 3, which makes it interesting for systems using a large number of energy sources.