Energy Harvesting Feasibility
Energy harvesting parameters based on measurements of TRV1.X
- 90000 hr product lifetime (10 years).
- Top area ~20 cm^2 (based on a wild guess that the top is 5 cm in diameter).
- Room lit to 50 lux.
- Room lit 20% of the time.
- PV cell efficiency ~10%.
http://www.earth.org.uk/OpenTRV-archive.html#energyharvesting
- Motor.
- Radio.
- Sensors:
- Pot.
- Temp sensor.
- Light sensor.
- LED: Ignored
- MCU: Ignored
- Active.
- Sleep.
- Leakage currents: Ignored
- 2.5 V.
- Daily consumption.
- All values calculated from max values in datasheet, except where noted.
- 1000% valve movement per day.
P_motor = (10 * 7.1 J) / ((60 s)^2 * 24)
P_motor = 830 uW
- RFM23B
- 1 Tx per 240 s
- Estimate of 15 ms per Tx
I_20dBm = 85000 uA
I_13dBm = 30000 uA
I_1dBm = 18000 uA
P_active = (2.5 V)^2 * 30000 = 190000 uW (No maximum Tx current provided)
P_sleep = (2.5 V)^2 * 0.8 uA = 5.0 uW
duty_cycle = (15 ms) / (240 * 1000 ms) = 6.3e-5
P_radio = (P_active * duty_cycle) + (P_sleep * (1 - duty_cycle))
P_radio = (190000 * 6.3e-5) + (5.0 * (1 - 6.3e-5))
P_radio = 17 uW
- 47 K(+-20%) pot (could use higher values).
- On IO_POWER_UP (Estimate active for 10 ms every 2 s).
- Ignore MCU ADC conversion current.
P_active = (2.5 V)^2 / (47000 * 0.8) R = 170 uW
duty_cycle = (10 ms) / (2 * 1000 ms) = 0.005
P_pot = P_active * duty_cycle
P_pot = 170 * 0.005
P_pot = 0.85 uW
- TEPT4400 w/ 100 K(+-20%)
- Assume saturated ie. PHT is short circuit.
- On IO_POWER_UP (Estimate active for 10 ms every 2 s).
- Ignore MCU ADC conversion current.
P_active = (2.5 V)^2 / (100000 * 0.8) R = 78 uW
duty_cycle = (10 ms) / (2 * 1000 ms) = 0.005
P_light = P_active * duty_cycle
P_light = 78 * 0.005
P_light = 0.39 uW
- SHT21.
- Sample every 60 seconds.
P_active = 1000 uW
P_sleep = 1.2 uW
duty_cycle = (22 ms) / (60 * 1000 ms) = 3.66e-4 (12 bit conversion. 14 bit takes 85 ms)
P_temp = (P_active * duty_cycle) + (P_sleep * (1 - duty_cycle))
P_temp = (1000 * 3.66e-4) + (1.2 * (1 - 3.66e-4))
P_temp = 1.6 uW
- Vf = 2 V
P_active = (2.5-2.0 V) / 470 R
P_active = 1000 uW
duty_cycle = ???
P_led = ???
P_tot = P_motor + P_radio + P_pot + P_light + P_temp
P_tot = 830 + 17 + 0.85 + 0.39 + 1.6
P_tot = 850 uW
Estimates based on http://uk.farnell.com/bussmann-by-eaton/hv1030-2r7106-r/cap-super-10f-2-7v-rad/dp/2148486
Using energy storage equation
W = 0.5 * C * (V0^2-V1^2)
where:
- W = energy in J
- C = capacitance in F
- V0 = initial voltage
- V1 = final voltage after discharging
(2 * 7) / (2.7^2 - 2.2^2) = 5.83 F
Using "10-degrees rule" from https://en.wikipedia.org/wiki/Supercapacitor#Lifetime
L = L0 * 2^((t0 - tx) / 10)
where:
- L = estimated lifetime in hours
- L0 = manufacturer quoted lifetime in hours
- t0 = manufacturer quoted temperature in C
- tx = design temperature in C
1000 * 2^((65 - 25) / 10) = 16000
Not enough but supercaps with a longer life do exist.
- Capacitor charged to 2.7 V using benchtop PSU.
- Capacitor removed from PSU and hooked up to REV7 with:
- Valve attached.
- DMM measuring voltage across power rails.
- Motor consumption test firmware (Constantly runs motor from endstop to endstop while powered).
- REV7 run until DMM measured 2.2 V, counting the number of strokes.
- Repeat a few times to get a rough average.
| Capacity (F) | % stroke | Notes |
|---|---|---|
| 5 | 150 - 200 | Depending on whether starting on up or down stroke + motor can be driven below cutoff V |
| 15 | 300 - 500 | Accidentally shorted the cap so may have been damaged. |
- The results fit the above calculation.
- Self discharge seems high (0.01 V drop took > 30 secs with DMM attached). Worth investigating further.
- TRV1.0 body does not seem stiff enough. This probably has an effect on motor efficiency as well.
A rough feasibility test for running a TRV1.0 off a photo-voltaic cell.
| Assumption | Symbol | Value | Unit | Comments |
|---|---|---|---|---|
| PV-cell area | A | 1.9e-3 | m^2 | Based on TRV1.0 cap size (5cm diameter circle) |
| Room lit duty-cycle | D | 0.2 | n/a | Based on hallway. Ask DHD |
| Required energy | W | 20 | J | 400% valve travel + some extra |
| Room Illuminance | Ev | 50 | lx | Estimate from DHD |
| Luminous Efficiency | ne | 0.5 | n/a | Pessimistic estimate. Should be between about 0.02 and 0.2 for most home lighting. |
| PV-cell Efficiency | npv | 0.1 | n/a | Estimate from DHD |
| MPPT Efficiency | nb | 0.7 | n/a | Estimate based on ADP5090 datasheet |
| Property | Symbol | Unit |
|---|---|---|
| Irradiance | Ee | W/m^2 |
| Irradiant Power | Pr | W |
| Usable power after losses | P | W |
| Average power consumption | Pw | W |
| Seconds in a day | T | s |
Calculation of the power we can get from ambient lighting.
Estimate of the irradiance from the illuminance:
Ee = Ev / ne (1)
Power irradiated onto a given area:
Pr = Ee * A
therefore by substituting Ee:
Pr = (Ev * A) / ne (2)
Power we can extract is therefore irradiated power multiplied by PV-cell and MPPT efficiencies, and the lights on duty cycle:
P = Pr * npv * nb
Substituting (2)
P = (Ev * A) * ((npv * nb) / ne) * D (3)
Calculation of the average power required over a day.
W = Pw * T
where T is the number of seconds in a day (60^2 * 24 = 86400)
Therefore:
Pw = W / T (4)
In order for the device to run all day, P must be greater than Pw
Therefore, using (3) and (4):
(Ev * A) * ((npv * nb) / ne) * D > W / T
Using equation (3) with above assumptions
P = (50 * 1.9e-3) * ((0.1 * 0.7) / 0.5) * 0.2
P = 95e-3 * 0.14 * 0.2
P = 2.7e-3 W or 2.7 mW
Using equation (4) with above assumptions:
Pw = 20 / 86400
Pw = 232e-6 W or 232 uW
Simple, not optimal/efficient (non-MPPT, discarding power above 2.7V, significant forward loss and reverse leakage with the blocking diode, use of multiple cells in the PV makes partial shading problematic):
Ki-CAD: http://www.earth.org.uk/OpenTRV/circuitFragments/201610-V0p2-SimplePVPower/
DHD201612: was not generating and maintaining sufficiently high voltage even without load, probably too much leakage in various of the diodes.
All the numbers are based on estimates, but it seems worth further investigation.
https://en.wikipedia.org/wiki/Supercapacitor https://www.researchgate.net/publication/261028996_Study_of_Supercapacitor_Aging_and_Lifetime_Estimation_According_to_Voltage_Temperature_and_RMS_Current
Useful life at 20C >10Y: http://uk.rs-online.com/web/p/products/1135876/
http://www.analog.com/media/en/technical-documentation/data-sheets/ADP5090.pdf
Photovoltaic Sample-and-Hold Circuit Enabling MPPT Indoors for Low-Power Systems http://eprints.soton.ac.uk/272832/1/PV%20Journal.pdf
Small PV speced at 200lx (Sanyo AM-1454CA) http://uk.rs-online.com/web/p/photovoltaic-solar-panels/6646762/
http://www.linear.com/solutions/1786
RF, heat, water flow
Flow: http://www.iop.org/resources/energy/index.html#.WCsXwwuekeo.email
Combined PV and storage: http://www.nature.com/articles/srep45585
Lowest-power MCU Operates At 0.25V In Energy Harvesting Apps http://chipsnwafers.electronicsforu.com/2017/04/15/lowest-power-mcu-operates-at-0-25v/