Designing Energy Harvesting Solar Powered Sensors
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Presented 19 Nov 2014 at Energy Harvesting and Storage USA in Santa Clara California http://www.idtechex.com/events/presentations/designing-energy-harvesting-solar-powered-sensors-005321.asp
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Designing Energy Harvesting Solar Powered Sensors
- 1. Designing Energy Harvesting Solar Powered Sensors Dan Wright, Engineering Manager Leviton Manufacturing Co., Inc. 19 November 2014
- 2. Agenda: Energy Harvesting Design Agenda Why Energy Harvest? Product Landscape Low Power Wireless Controllers Solar Cell Landscape Energy Storage Options Power Management Sensor Technologies Lens for Occupancy Sensors Design and Product Challenges 2 Our BUILDING Blocks
- 3. 3 Why Energy Harvest? Availability of free energy: Light Reduce energy consumption Solar cell (Photovoltaic) technology improvements Low power technology maturity (often wireless) Easy to install Images: Microsoft clipart, Plow & Hearth
- 4. 4 Product Landscape Many Energy Harvesting products leverage EnOcean wireless module Product Characteristics: • Very low power, typically passive sensor: PIR, Humidity, CO2, ALS, Temp, Switch • Solar Powered • Ability to store charge for extended bridge time Images: Leviton, KMC Controls, EchoFlex, AdHoc, EnOcean
- 5. 5 Hardware: Low Power Wireless Controllers EnOcean: TCM300 (8051 Based): 315, 868, and 902MHz ZigBee (2.4GHz): TI, Silicon Labs (Ember), Atmel, NXP, etc. • CEL offers MeshConnect based module Sigma Designs Z-Wave: 868, 908, 921MHz frequencies Bluetooth technology: Various Others (no standard, Sub GHz): Micrel, Silicon Labs, Microchip, etc. Images: EnOcean, CEL, Sigma Designs
- 6. 6 Hardware: General Circuitry Requirements Least power consumption possible (< 1uA) Work under a wide range of voltages (1.8 – 5V) Average current draw (including radio transmit) < 10uA at 3.0V May want to have black-out “no transmit” periods (1-2 minutes) Select low leakage capacitors (Ceramic, Tantalum)
- 7. 7 Hardware: Solar Cell (Photovoltaic) Landscape Amorphous Silicon Solar Cells (Glass Substrate) • Sanyo (Panasonic Eco Solutions) Amorton • Sinonar Solar • Trony (Solars-China.com is reseller) • Wasonlong / Blue Solar Alternatives • IXYS – Thicker, more efficient, requires boost • G24i Power – Flexible, requires boost • AltaDevices – High efficient (24.1%), flexible, new company • SolarPrint – High power density, glass base, requires boost Flexible solar panel efficiencies have reached 18.7% which is on par with silicon (ECN Magazine, 5/2014). Images: Blue Solar, IXYS, and G24i Power
- 8. 8 Hardware: Solar Cell (Photovoltaic) Example Sanyo 55x20mm (AM-1805) I-V Data • At 200 Lux will supply 12uA @ 3.0V (minimum), 15.5uA typical. • At 50 Lux will supply 8-9uA @ 4.5V using two solar cells in parallel. Source: Sanyo AM-1805 Datasheet addendum
- 9. 9 Hardware: Solar Cell (Photovoltaic) Example Sanyo 55x20mm (AM-1805) I, V vs. Lux • Working range up to 1k (~5.5V) • In sunlight will > 6.0V Source: Sanyo AM-1805 Datasheet graph Images: Cooper Industries (Direct, Indirect lighting: Corelite, Class A), Synergy Lighting, 7-11 Goes LED
- 10. 10 Hardware: Energy Storage Options – Bridge Time Bridge Time: Length of time the device will continue to work without light Selecting correct storage device is critical! • The more storage the device has, the long it will take to CHARGE the device. • Increase Cap = Increase Bridge Time and Charge time Example: 0.33F SuperCap charge time: Charge Time (hr) Light Level (Lux) 28 50 14 100 6 200 At around 10uA an hour would drain 0.33F cap: 1V per 24hrs Last somewhere around 36-48hrs at full charge (4.5-5V).
- 11. 11 Hardware: Energy Storage Options Supercapacitors (aka PowerStor, Gold Cap, Dynacap, EDLC) • Cooper Bussmann, Panasonic, Elna, Cap-xx, Kemet • 0.33F cost: $0.70/1k • Time (0.33F) = (0.33 * 2V) / 5uA = 132000s = 36.7 hours Solid State/Flat Battery Alternatives: • Cymbet Enerchip (CBC050): 50uA (> 5000 discharge cycles) • ST EnFilm (EEL700A39): 3.9V at 0.7mAh • Rocket Electric Korea (LIP292240): 3.6V, 5mAh, Lithium Ion Polymer Battery Traditional Rechargeable Battery: • Coin cell, AA, AAA, etc. Images: Cap-XX, Panasonic, and Cymbet
- 12. 12 Hardware: Power Management Circuits Boost: Necessary for low voltage solar cells Buck: Good for higher voltage solar cells Specialized Energy Harversting • BQ25504/05/70: Boost, Iq=1.4uA, use with solar cells, battery/supercap storage • LTC3107: Has internal shunt, works with battery and optional capacitor storage • LTC3106: Buck-Boost. Works with solar-cell and battery/supercap. Iq=1.5uA • LTC3129: Buck-Boost DC/DC converter with Iq=1.3 • LTC3330: Nanopower Buck-Boost • MAX17710: Can be used with solar cells and rechargeable batteries • MB39C811, MB39C831 (Spansion): Buck (811) and Boost (831) for energy harvesting • SPV1050 (ST): Buck-Boost converter with built in LDO. Has battery charger • ADP5090 (AD): Ultra low power boost regulator with charge management • TPS82740A/40B (TI): Step down micro SiP module, Iq=260nA
- 13. 13 Hardware: Sensor Technologies Passive Infrared (Motion) • PYD1096, PYQ1098 (Excelitas): Smart DigiPyro, 2/4 element, 15uA • LHI1128, LHI944 (Excelitas): Analog sensors • PaPIR (Panasonic): 1, 2, 6uA Digital PIRs with lens • DigiPyro (Nicera): Similar to Excelitas, 15uA Humidity / Temperature / CO2 Door/Window • SM351LT/SM353LT (Honeywell): Magnetoresistive Sensor IC, Average current 360nA Ambient Light Sensor • MAX44009 (Maxim): I2C controlled with 1uA operating current (lowest in industry) • ISL29020 (Intersil): I2C controlled with 65uA operating current, 0.5uA sleep • BH1730FVC (Rohm): I2C controlled with 200uA operational and 1.5uA sleep Images: Panasonic PaPIR, Excelitas
- 14. 14 Mechanical: Lens Optics for PIR Sensors Optics: • Panasonic – Included with sensors • Fresnel Technologies – www.fresneltech.com • Carclo Optics – www.carclo-optics.com/pir-sensor-lenses Images: Carclo optics
- 15. 15 Lessons Learned: Design Challenges Making product cost effective Minimizing current usage (< 10uA), Black-out periods Startup time at a given light level Bridge time (48 hours with 0.33/0.47F super cap) Analog versus Digital PIR Quite Supply Battery vs. Supercap vs. Rechargeable battery Operational Voltage / Shunt Voltage Wireless communication range / Antenna Position FCC Certification Leaky capacitors (Electrolytic, Super Cap) @ Temp
- 16. 16 Lessons Learned: Product Challenges Customer Expectations vs. Energy Harvesting trade-offs Value Add cost of Energy Harvesting + + + + =
- 17. 17 Questions: 5 Minutes Thanks for Attending Any Questions? Contact Information: danwright@leviton.com, d.m.wright@ieee.org
Editor's Notes
- 27-Oct-2014 (Dan Wright): Begin slide creation on based on Energy Harvesting “white paper” document started in Jan 2014. Link to bio/presentation detail: http://www.idtechex.com/events/presentations/designing-energy-harvesting-solar-powered-sensors-005321.asp 31-Oct-2014: Final Changes Introduction: Thank you for attending this conference session. I’m Dan Wright, an engineering manager for Leviton. Today I’m going to talk about “Designing Energy Harvesting Solar Powered Sensors”
- Agenda (Need to be able to complete in 20 minutes. Somewhere between 15-20 slides) Why is this important? (To me, you, everyone – convince audience of importance) Unique Contribution (Design). Don’t over explain. Present a problem, describe search for solution! Building a sensor to support “retrofit” and hard to reach areas with no wires but available energy harvesting (light) aspects. Retrofit, no wires, dangerous. Save you time. Product Landscape (products on the market) 3) Low power Wireless microcontrollers 4) Solar Cell landscape 5) Supercap vs Batteries and Charging circuits landscape 6) Power Management IC’s 7) PIR, ambient light detector and other sensor landscape 8) Lenses for Occupancy sensors 9) Challenges My agenda is to give you an overview on WHY we want to Energy Harvest, give a brief overview of some products, including one I worked on and then dive into the building blocks needed to build an Energy Harvesting Sensor. I’ll conclude by talking about some of the challenges and lessons learned in doing low power energy harvesting designs. It is a lot to cover in twenty minutes so I’m going to go quick and touch on aspects I’ve found important. I’ll answer questions and provide my my contact details at the end of the presentation.
- Energy Harvesting: Obtaining power via light. Specifically using solar power energy harvesting. The Sun and Indoor light is free energy! It can provide us with all the energy we need if we have a medium to tap that potential energy source. (Image from Microsoft catalog) Public push and conscious choice to not waste our energy sources. To be more conservative. Solar Cell technology has taken off. More an more people seeing the advantage of installing them on their homes, also for those lights to light up walk-ways. (Image from Plow & Hearth website) The technology has advance significantly in the past 5-10 years to allow us to take advantage of this opportunity. There are a lot more solar cell and energy harvesting IC options. Companies like Linear Tech, Intersil, Maxim, TI and others are investing more into technology and getting cost down. With no added wires much of this technology can be finding a place to install and sticking it into place. Great for retro fit applications. Wireless sensor monitoring is gaining traction in many commercial and industrial areas especially in cases where no or minimal power is available. These devices are especially conducive to retrofit applications where it is difficult if not impossible to run additional low voltage wiring used in traditional +24VDC occupancy sensor installs. GFX Cites: MS Office and Plow & Hearth
- BRIEF: A LOT of low power devices out there exist, including many battery powered (Lutron’s Radio Powr Savr occupancy sensors), NYCE, and Philips “OccuSwitch”. On the small, low powered side with solar the product landscape is limited. There are many products which use the EnOcean chipset, much less so with other vendors products. I am not aware of any other wireless chip vendor used in energy harvesting products. From Right to Left (All EnOcean Based): Leviton, Ambient Light Sensor KMC Controls, Wireless Temperature Sensor EchoFlex, RTS Resonate Temperature and Humidity Sensor Ad Hoc, Door and Window (HVAC) sensor EnOcean, Wall Mount Occupancy Sensor Leviton, Ceiling Mount Occupancy Sensor For us, Leviton, it is about lighting controls. Specifically the occupancy sensor and associated lighting relay/switch control. We don’t want you to drive by your work place late at night, after a dinner, concert, sporting event, whatever, and see the lights ON at night. And what if that place you wanted to install had no wiring to where you wanted to place your sensors or you didn’t have the funds to run the wiring. This is where the wireless sensor comes in. More and more schools are beginning to use this technology. Part of it is ease of install and part of it is to avoid running new wires due to cost and disturbance of old buildings: architecture, lead, asbestos. Great retrofit application. GFX Cites: All images from Leviton website (www.leviton.com) and EnOcean Alliance (https://www.enocean-alliance.org/en/home/) System: Would consist of picking several devices to work in an environment. For example we would need an OCCUPANCY SENSOR to work with a RELAY or WALL SWITCH to turn on the lights.
- Perhaps the best place to start when designing a system is with the “heart” of the system and that is often the MAIN CPU. These days most companies integrate that CPU with their wireless radio. A lot of early developments were 8 or 16 bit based on 8051. These days both TI and Ember (and others) are offering ARM Cortex M3 microcontroller based solutions. Often with 128k to 512k. Sufficient to do OVER THE AIR. EnOcean (www.enocean.com): TCM300U Image. Original developed chip used a TI Chipcon IC and Microchip PIC processor and was AM based wireless signal. Newest devices are ASIC, 8051 based with Frequency modulation. Currently limited to 32k. For compatibility purposes it may worth considering buying a chip that is supported by an alliance. EnOcean, Z-Wave or ZigBee alliance. ZigBee: Has many suppliers. Tier 1 suppliers are: TI and Silicon Labs. Tier 2: NXP, Atmel, Tier 3: Microchip, ST, Marvell, etc. ZigBee: Module Makers: Anaren (TI), Digi International (Silicon Labs), Atmel, California Easter Labs (Silicon Labs), Microchip, Embit, Radiocrafts Z-Wave: Sigma Designs based and chipset: http://z-wave.sigmadesigns.com/ Bluetooth: www.bluetooth.com is a rising technology in the low power area. Initially devices were mainly limited to about 30ft but that can be extended to 100 meters depending on class/technology used. Others: Micrel has sub GHz based FSK chips. So does Silicon Labs which also competes with its ZigBee line COSTS: $2-5 for chipset and $8-$20 for modules.
- As we pick our microcontroller we need to be thinking about our power budget. We need to select components to be power misers. To keep all current draw to a minimum. Want every component used to draw less than 1uA of current. If no voltage regulation, will need components to support a wide range of voltages The < 10uA includes radio wake up and transmission currents, which for EnOcean is around 30-40mA for 15mS. When not including radio transmission Leviton occupancy sensor was < 5uA. Leviton occupancy sensors have 1, 2 minute periods of NO transmission to keep excessive transmissions from occurring. Capacitors and other components with limited leakage.
- Solar Cell suppliers: Various sizes http://panasonic.net/energy/amorton/en/products/ These are all the various options for solar products. You can go traditional and save a lot of money with China sourced parts. Or go with newer technology, unproven, and perhaps more expensive. Images from: Blue Solar cell producer, IXYS and G24i power Many of these are produced for INDOOR light sources. Units requiring BOOST circuits can output in the 1-2Volt range. Others will produce upto 6 Volts depending on light level up to 1000 LUX is often shown in datasheet. The traditional glass substrate products can cost anywhere from 20-30 cents for China sourced to $1 for the higher end Sanyo product. We used Sanyo, part of that was it is the best product as our test proved and they also provided a value add service of attaching wires for us. The alternatives look promising and warrant a look.
- Graph from Sanyo Datasheet. WHAT is your POWER REQUIREMENT? In order for us to meet operation we need >5uA. Full operation at 3V within 5 minutes, charging up 400-600uF capacitors. Characteristics do vary over time and temperatures. Often it is fairly linear over light level. Sinonar, Trony and Blue solar would all be similar but not as efficient and not work as well at the lower light levels.
- Graph from Sanyo Datasheet: AM-1805 When we look at the Voltage or Current versus LIGHT we’ll see they are fairly linear. WHAT IS IMPORTANT? It is a good idea to have ability to shunt off any excess voltage that will exceed the circuits max tolerance range. Use a Zener like BZT52 series from Vishay has 100nA leakage. Make sure it will turn ON at those lower currents to clamp more than your MAX circuit requirements. This maybe 5 or 5.5V. Most indoor lighting doesn’t go above 500 LUX unless maybe you use a ceiling mount sensor on indirect lighting source then we might run into over voltage challenges. Image for Direct/Indirect lighting http://www.cooperindustries.com/content/dam/public/lighting/products/documents/corelite/brochures/class_a_final.pdf http://synergylightingusa.com/7-11-goes-led-with-synergy-lighting/
- So, we know our power requirements, we have selected a wireless controller and our solar panels. Now we need a way to store some of that excess capacity we have, especially if we find our in a bright environment. Tradeoffs are larger supercap takes longer to charge and cost more money. But it can get you a longer bridge time.
- Page 6: Super Capacitors – EDLC = Electric Double Layer Capacitor As with everything else, using a supercap has trade-offs: Cost An increase in bridge time also meets an increase in charge time If the sensor doesn’t have enough light per day the supercap may never fully charge Capacitance can vary by 20% or more making bridge time vary Leakage Highly affected by temperature Q = C*V = Capacitance * (Voltage drop over time) Iavg = Current Leakage + Current Usage Bridge Time = Q / Iavg Cymbet: Nice technology but would need more than one 50uA/h unit to carry decent bridge time. Infinite Power Solutions – out of business ST EnFilm – new Rocket Electric – pretty much a Lithium Ion battery The most economical solution is a battery, however these will need to be replaced someday and that will add some expense 5-10 years down the road. After that there is the question of what is the product trying to accomplish? Maybe a simple rechargeable battery solution, over a supercap, is viable for solution. Or perhaps going with a solid state battery which is permanently mounted (unless on a replaceable module) is an option. I suppose this can be considered a pay now or pay later dilemma. Currently all the toilets, soap dispensers, and faucets use batteries and those need replaced every couple of years.
- Several semiconductor companies have created innovative energy harvesting solutions as more and more designers seek out energy harvesting solutions. A few of these companies are Linear Technology, Maxim integrated, Intersil, and TI. These new products are often found under their Power Management IC product catalog. Here are a few products that can help with building an energy harvesting sensor: Linear Tech is the company definitely leading the forefront on this technology. We’ve been trying to work with them to develop something we can use. Finally the LTC3106 is due out soon. This offers a Buck and Boost Solution. Works also to manage the battery or supercap voltage. There are a lot of trade-offs in this area. Using these devices may result in inefficiencies during bucking and boosting and extra current loads to run these circuits. Extra Discussion on Linear Tech: LTC3106 Vstore = battery backup, Vin = Energy harvesting (operate from 300mV) Has ability to regulate Vout (1.8V, 3V, 3.3V 5V) RUN pin can establish a user programmable turn on and off threshold. (Vin undervoltage threshold) Vstore / Vcap has undervolatage and overvoltage thresholds that can be set. Both Vin and Vstore can operate in the Buck-Boost mode. Has a PGOOD output pin which can be monitored to enable circuit, or Enable the wireless radio processor.
- Photocell and Occupancy sensor are likely the most common type of energy harvesting device Honeywell devices could be used in place of door/window sensor reed switch type devices. - Digital seems to be the way to go to build complete system.
- Lens image from Carclo optics. PIR with Lens will allow detection. Devices work best in walk around rather than walking towards (less sensitive)
- Cost Effective: SuperCap, Solar Cell, Low power IC’s all cost extra money. Our design requirements: 40 LUX (4 FC). Using solar cells. Be operational within 5 minutes. Sustain operational for at least 30s after lights removed in order to turn lights back on (Vacancy requirement – Title 24 California) Range of 50-150ft Bridge time greater than 2 days. EnOcean has a couple great application notes on designing Wireless Solar powered occupancy sensors. AN207 – Solar Panel Design Considerations AN306 – Motion Sensor AN311 – Motion Sensor based on STM300C Leaky capacitors (AlEl). We tested at 25C and saw 600nA leakage with 440uF Capacitance. At 60C we saw 9uA! Article: http://www.embedded.com/electronics-blogs/break-points/4430050/Using-a-capacitor-to-sustain-battery-life To minimize losses in low power systems all capacitors should have low leakage. < 0.01 (CV). Much lower. Ceramic caps work best then low leakage tantalums. Avoid most Electrolytic and any POS caps. Use X7R or X5R Ceramic caps. A noisy supply can Making the product cost effective: Low power IC’s cost more $ Keeping the current usage to a minimum: IC selection and wireless transmission rate, detection rate of the sensor (1 minute or 2 minute blackout/sleep period for microcontroller). Startup Time: Performance upon first working device with no light and 0V. Designing working time with no light: The bridge time needs of the device will affect design. A larger super capacitors will require longer charge time. Analog versus Digital: Adjustable PIR versus non-adjustable PIR, RF immunity, false tripping. Quiet Supply: No noise on transmission of radio—want to not impact sensor performance. Battery vs supercap vs recharable solution: Need to think about bridge time and cost for these. How many energy sources are needed? Operational Voltage: Will the device operate at fixed voltage or vary based on input power source? What will be the cutoff working voltage? Will over voltage be shunt? Shunting Voltage: Use low current zener to shunt potential over voltage conditions. Make sure it does not draw significant current when not shunting. TI suggested BZT52 series. Antenna Position: Can make a huge difference in wireless transmission range. Very important with respect to the radio design. FCC Certification: If the wireless radio device does not have modular certification then $5k-$15k FCC visit for FCC Part 15.321 and IC RSS-210 will be required. Leaky Capacitors: Poor choices will result in more current leakage. Also at higher temps it gets worse.
- Technology is here! EH costs are high! I’ve got a $40 cost target. I’ve met that but our sale cost is $80. Will customers pay that? Is it worth it to them over a battery operated version they can get for $60? Do they see the value in energy harvesting. Or, do I have to have the battery in there so it can always work and when light is available use the energy harvesting capabilities? We’ve found we get a lot of complaints so having the battery backup is important to keep customers satisfied. Perception! Lessons Learned: We do have product installed all over including NYC Public Schools. Also, in 2012 (January), University of California Santa Cruz’s (UCSC) Science and Engineering Library. Will customers accept the device NOT working for duration it is energy harvesting. Will customer pay to have energy harvesting only to turn their back when it doesn’t meet expectations? In the end, it may still need a battery to meet the customers requirements (handle bridge time). BUT we added a battery and that devalues the energy harvesting aspect. Wrap-up discussion. - Wireless option saves money. But wireless energy harvesting option will cost more money (solar cells and super capacitor).
- Final Slide: 5 minutes, Comic drawn by Dan Wright ~1990 Note on Functionality Basics of Leviton Occupancy Sensor: The basic occupancy sensor startup functionality from Time 0 with no charge on any capacitor follows: Apply light to solar cells. A minimum of 40 lux (4FC) is required. These bulk capacitors will begin charging and at around 2.4V all the circuits will become functional. Within 5 minutes the device will reach 3.0V (40 lux); within 3 minutes at 60 lux and 1.5 minutes at 120 lux. After the device reaches 3.1V any excess charge will trickle charge the supercap. Any time after the circuit is functional it will respond to occupancy detection and transmit wireless signal: A wireless transmission last 15mS A red LED blinks during the 15mS transmission After a wireless transmission of occupancy, circuit will ignore any occupancy for 60s (blackout period). A POT can be used to adjust the circuit (OpAmp) gain. A LEARN button can be pressed to force a wireless transmission anytime the circuit is functional. With sufficient light the super capacitor will charge up to around 5.1V. Once fully charged the bridge time in total darkness will be approximately 48 hours If sensor does not receive sufficient light the voltage can decay back down to 0V. At 0V, the maximum startup time for operation will again be 5 minutes in low light conditions.