Storage-less and converter-less maximum power tracking of photovoltaic cells for a nonvolatile microprocessor

Transcription

1 Seoul National University Storage-less and converter-less maximum power tracking of photovoltaic cells for a nonvolatile microprocessor Cong Wang, Naehyuck Chang, Y. Kim, S. Park, Yongpan Liu, Hyung Gyu Lee, R. Luo, H. Yang 2014/1/22

2 Outline Background Energy harvesting for IoT applications Maximum power point tracking (MPPT) of a photovoltaic module Conventional system architecture and problems Storage-less and converter-less MPPT With a nonvolatile microprocessor System evaluation Conclusion 2

3 Developing IoT applications Internet of things (IoT) on the way Structural health monitoring Smart agriculture Smart transportation Etc 3

4 Energy & maintenance is a big problem Battery powered devices Most widely used Limited capacity Need regular maintenance Volume/weight overheads Potential high cost Battery DC/DC Converter Load Device 4

5 Energy harvesting Power density estimates of different sources Source: Texas Instruments White Paper - ULP meets energy harvesting: A gamechanging combination for design engineers 5

6 Current (ma) Power (mw) Harvesting solar energy Maximum Power Point Tracking (MPPT) Try to extract as much power as possible from the solar panel MPP Voltage (V) 6

7 Traditional system architecture Solar energy is first charged to a energy storage device (supercapacitor/battery) Stored energy is then retrieved and delivered to the load device Solar Panel MPPT/MPTT Charger Energy Storage Voltage Regulator Load Device 7

8 Problems in traditional architecture 2 stage power converters Expensive Significant conversion loss Energy storage Higher cost Weight/volume overhead Limited work cycles (Rechargeable battery) Leakage (Supercapacitor) Source: Y. Kim, N. Chang, Y. Wang, M. Pedram - Maximum power transfer tracking for a photovoltaic-supercapacitor energy system Is there an alternate cheap and efficient way to utilize solar energy? 8

9 Outline Background Energy harvesting for IoT applications Maximum power point tracking (MPPT) of a photovoltaic module Conventional system architecture and problems Storage-less and converter-less MPPT With a nonvolatile microprocessor System evaluation Conclusion 9

10 Storage-less and Converter-less Solar Panel MPPT/MPTT Charger Energy Storage Voltage Regulator Load Device Solar Panel Load Device 10

11 Storage-less and Converter-less Advantages Storage-less No long-term energy storage (battery or super-capacitor) Maintenance free Volume, weight and cost reduction Converter-less Higher power transfer efficiency Lower cost 11

12 Current (ma) Power (mw) Does it work? How to ensure the functionality? V solar collapses if P load > P mpp P load > P mpp P load = P mpp How to perform MPPT? Voltage (V) How to match P load with the varying P solar 12

13 Proposed solution Connect the PV to the load via a load switch Adjust average load current by Dynamic Power Management(DPM) Match the average load current with the MPP current of the solar panel 13

14 MPPT achieved by fine-grained DPM 14

15 Power (mw) MPPT achieved by fine-grained DPM PV Output MPP Set Voltage (V) V 1 V 2 15

16 Need for a nonvolatile microprocessor Transition overheads are NOT negligible Especially when transitions are frequent (C bulk ~1μF, T DPM ~ several ms) Smaller time overhead, more time for task execution Smaller energy overhead, more energy for task execution Transition overheads are significant for conventional microprocessor Typical time overhead Several ms Typical energy overhead 20 ma if write to a Flash 16

17 THU1010N nonvolatile microprocessor Based on standard 8051 micro-controller Fully replace original Flip- Flop with Nonvolatile FeFF Flip-flip Controller Peripherals for embedded applications and online debug 17

18 Transition overheads comparison NV processor is faster in state transitions THU1010N TI-MSP430 with Flash [1] TI-MSP430 with FRAM [2] Backup time 8us 6ms 212us Recovery time 3us 3ms 310us Less energy overhead in state transitions for NV processor THU1010N TI-MSP430 with Flash [1] Ratio Backup energy 23.1nJ 445uJ Recovery energy 8.1nJ 0.6uJ 74 18

19 Storage-less and Converter-less MPPT MPPT Achieved by Dynamic Power Management (DPM) DPM is fine-grained power gating of the node A buck capacitor is used as energy buffer and extend the time constant Nonvolatile microprocessor Minimize transition overheads to improve system efficiency Nonvolatile Microprocessor 19

20 Outline Background Energy harvesting for IoT applications Maximum power point tracking (MPPT) of a photovoltaic module Conventional system architecture and problems Storage-less and converter-less MPPT With a nonvolatile microprocessor System evaluation Conclusion 20

21 Evaluation board Buck Capacitor & Load Switch Nonvolatile Processor Solar Panel Power Management Unit (MSP430 for flexibility) 21

22 Captured waveform Evaluation board 22

23 Simulation Setup Efficiency evaluation P ON = 25mW, P mpp = 14.7mW@200W/m 2 C bulk = 4.7μF, C decoup = 20nF V 1, V 2 = [2.75V, 2.90V] Assume P ON = P ON,OFF = P OFF,ON Transition Time Overhead T ON,OFF = 8μs, T OFF,ON = 3μs (Proposed system with NVMCU) T ON,OFF = 0.3ms, T OFF,ON = 0.2ms (Proposed system with conv. MCU) Omit the power consumption of the power management unit 23

24 Efficiency evaluation 24

25 Efficiency of the proposed system Efficiency up to 95.4% if C bulk = 47μF 25

26 Efficiency of conventional system 26

27 Emerging application JUNE -- a wearable bracelet with UV sensor Sun protection advice SPF, sunglasses Wear a hat 27 Source:

28 Conclusion Storage-less and Converter-less MPPT Provides a very efficient way to power electronic devices with solar panels Low cost and maintenance-free Demonstrates a promising application for nonvolatile microprocessors Extension Combine with traditional system(2 coverters + supercap) to achieve higher efficiency and better QoS simultaneously 28

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