Power for a century

In the list of primary power sources in one of the previous entries, I dismissed temperature as a non-viable source except if the target device is placed in the vicinity of a regular source of high temperature (fireplace, furnace, etc.)

Turns out, I am wrong (as usually).

This clock is said to run without being manually wound up for more than one-and-a-half century, by now; and ticking. It uses an enclosed "bellows" (which could be filled by gas/air/vacuum/liquid+its vapours), which expands and collapses as temperature and atmospheric pressure changes. These movements then wind up the conventional weight-and-chain "accumulator".

However, this still does not mean this power source is practical. As noted already, the mechanical clock got optimized to minimal power consumption during the past centuries up to a very impressive point, which may be impossible to surmount by a practical electrical/electromechanical device. Also, the "commercially produced" version of this clock is a superluxury item, and this may indicate that  developing and constructing the "bellows" and associated mechanisms may be not entirely trivial...

This is an electrical device which runs uninterrupted (mostly) from 1840. It contains its own power source - a couple of high-voltage battery "pile" - and does not produce any useful work, but it does this for an impressively long time.

While this may appear to defy my assertion that devices based on electrochemistry can't last for a century. This again has to be viewed in context: that the "Oxford Bell" performed so well in long term is almost an accident, rather than result of directed research. The materials used - mainly the sulphur insulation - is not very viable for a practical battery. Modern batteries rely on polymers and other organic materials used as electrical insulation and sealants, and that's those materials which are the most likely to degrade/change in the long term. Also, the processes in primary cells are much more simpler and less prone to various side effects resulting in gradual degradation of performance, than accumulators and supercaps. Also, note, that these "piles" provide high voltage and miniscule current (and even that in short pulses and very low fill factor); while our electronics requires relatively low voltage and comparatively much higher and steady current. Also, we can't even copy the design, as we don't know for sure what's inside...

To develop a battery - yet even accumulator or supercap - guaranteed to last a century, would undoubtedly take decades of work. Given little to no demand, there is no way this will happen in any foreseeable future...

To prove the concept of century-long-lasting device, the proposed purely-solar-powered clock with no electrochemical primary or secondary battery should be built.

As commercial energy harvesting ICs probably won't support complex switching between various parts of the application with different power consumption, the following simplified solution is proposed:

Here, the low-consumption part has its own storage (capacitor), which is "switched" (decoupled) by a simple diode. Of course, the diode has to be carefully chosen for reverse leakage - 10s of nA may be a showstopper - a 1N4148 leaks just around 10nA at 3V reverse voltage, but we may try to find some better diode. The external leakages must be reduced, too; here an absolutely clean PCB is a must.

If extra fun is desired, the mechanical storage/primary source can be attempted.

Back to power, from the analysis above (or below, given the order how log entries are rendered) there is only one instantly viable combination for the low-powered-wireless class of gadgets: photovoltaic (solar) primary source and capacitor as backup storage. (The mechanical solution for primary source and backup/storage sounds very attractive, but is something which needs more thinking, investigation and work).

Using photovoltaics as primary source somewhat limits the placement of the gadgets, but we knew it won't be all that easy. On the other hand, this is something which could be considered at an early stage, designing appropriate locations for gadgets with respect to windows and their orientation, when designing the house itself.

Part of the survival strategies is also RECONSIDERING the SCHEDULING and NEEDS of the particular gadgets so that it matches the varying nature of energy supply. The simplest thing is to schedule the power-demanding tasks, if this is possible. However, this may involve certain "intelligence", so instead of some simplistic logic a microcontroller and developing algorithms may be involved.

Let's have a look at some examples. Say, we want to log the temperature every half a hour even during the night, so that we can optimize heating in variously used rooms; but we need the data only the next day, so that the thermometer does not need to transmit them during the night, it's enough to store them locally and transmit them later.

In case of clock, the clock may stop displaying or move the indicators when dark - nobody looks at it anyway. It's enough to keep the time. If a radio receiver is employed to adjust the clock periodically, it may be run only when there's enough power. The longwave receivers such as the european DCF77 are usually lost in the "digital/switching" noise in areas remote from the transmitter (such as my location around Bratislava, Slovakia) so nighttime reception is a must; but common clock crystals are precise enough to be able to run autonomously with acceptable precision for days, so it's enough to try to get synchronized say once a night, and only if there was enough sunlight the previous day so that a bigger capacitor could be charged fully. 

Let's now go for the nasty details.

PHOTOVOLTAIC panels come in various sizes, and the common variety could produce around . Sure, inside a house the light level will be lower, but that could be compensated by using a larger panel. Let's have an example - this is a relatively small, 6x4 cm panel, delivering cca 3.9V*15mA=50mW at 50klux illumination. The catalogue has a nice table detailing available light at various environments - turns out that moderate electric lighting delivers two orders of magnitude less power (~500lux) than non-direct sunlight outdoor (~50klux); so even taking into account some degradation during the years, it should be able to power a clock (see analysis in the first blog entry - it consumes a few hundreds of uW in average). As there's no sunlight in the night, we need to have roughly twice that much power and store half of it for the night; but it's easy to add another panel or buy a twice-as-big one.

Solar panels are typically constructed (by connecting the individual cells into suitable serial/parallel  connections) so that output voltage of solar panels is around 3-4V. This may simplify the voltage conditioning electronics, possibly reducing it to a simple protection device (Zener diode or similar). On the other hand, using sophisticated energy harvesting ICs (e.g. the ADP5090) can help to squeeze out the maximum available power using MPPT and boost conversion; and may run at as low as 80mV of input voltage, ie. delivering power even at low light level.

CAPACITORS are unfortunately not very good as long-term energy storage. To be able to power the clock at hundreds of uW during night, we need to store say 300 uW * 10 h = cca 10J (yes, that is that daily windup of the pendulum clock!). The energy content of a charged capacitor is (CV^2)/2; considering discharge from 3V to 1V this results in capacitance of around 2.5F. That's way out of what's possible, especially since we ruled out the electrolythics (and supercaps too). But maybe the clock's consumption decreases with the voltage, so let's put it in other way, 300uW at 3V is some 100uA of current, so the clock behaves roughly as a 30 kOhm resistor. If it should take 10 hours to discharge a capacitor from 3V to roughly one third i.e. 1V, that means RC constant is 36000 seconds, resulting in 1F... still no go.

So this is the moment where we need to reconsider the real need for power. We may for example drop the idea of mechanical indication, and go for LCDs; they should consume somewhat less, although the large area ones (and we want these things to be nice, once they will be around us for a long time) may still go as high as tens of a hundred of uA. So let's consider something else. While the clock may in average draw around 100uA, if during night only the timekeeping runs, that can go quite well below one uA, into hundreds of nA. There are RTCs available, running off as little as 40nA. Calculating 300nA instead of 300uA brings down the requirement for capacity down to a few mF, which may be at the brim of possible. The mechanism then may readjust for the current time when the sun comes up, or when somebody switches on the light (maybe for having a look at the clock). 

Unfortunately, it's still not done. Capacitors leak, i.e. self-discharge. Even if the considered ceramic and foil types leak much less than electrolytics, it still may be limiting.

Leakage can be modeled as a resistance parallel to the capacitor, through which current from charged capacitor flows. This current may flow around the dielectric, or across it. The first mechanism results in a parasitic resistance, given by the capacitor's construction, but at a given construction/size independent of  capacity. The second mechanism directly scales with the dielectric's area i.e. with capacity. As with higher capacities the second mechanism is prevalent, no matter how big the capacity is or if we stack capacitances in parallel, they will discharge roughly in the same time. The manufacturers often give the leakage for capacitors of higher values in seconds (which equals to Ohm * Farad, or MegaOhm * microFarad). The typical value for  high-permitivity ceramics (X5R, Y7R/V) is in hundreds of seconds. We need 10 hours, this is some 36000 seconds.

So the solution is to use foil capacitors. The best dielectric material in this regard is polypropylene. Typical high-quality PP film capacitors, such as these, have rated leakage of 30000s - as this is given at 100V, and as the leakage will scale down with voltage (given many leakage mechanisms depend on electric field intensity), it will suffice for our purpose. They also have a guaranteed lifetime of >30000 hrs which is 35 years, at 40 degrees; so it's almost sure they will last 100 years at slightly lower temperature. Unfortunately, the largest model in this line is 10uF and they are also quite costly - the needed 100pcs would cost around 700 Euros... Not even the enlarged budget would hold that.

We need to investigate this further. So, maybe THIS is going to be the real challenge!

Having ruled out the power sources and storages based on chemistry, providing power appears to be the major issue for the century-lasting electronics. But let's talk a bit also about those "less important" issues.

• ELECTRONIC PARTS - while this is seldom stressed, the electronics parts industry appears to have a pretty impressive record as far as long-term reliability is concerned; and it also has improved much during the decades it exists, mainly due to pressure from automotive and aerospace industries. While restorators of early electronics (mainly broadcast radio receivers) have to cope with many various failed parts - resistors, capacitors, electromechanical parts (and of course the inherently failure prone electron tubes/valves), in "solid state" electronics usually the electrolythics are the central point of failure (and we've already identified that as a problematic type of part). With careful fully automated manufacturing from pure materials in clean room environment, parts now have typical guaranteed life of 10-20 years at 85-105°C, so it's very likely they will be OK after a century of operation at room temperature.
• PCB, SOLDERING - the glass-epoxy combination appears to last for ever, judging from vintage electronics. It would be better to avoid lead-free solder, even if vendors swear the whiskers and tin plague have been wrestled down, it's better to stay conservative. Anyway, who would dump a gadget to landfill, if it's perfectly OK even after a century and can be continuously used or resold for good price? ;-)
• MECHANICAL MATERIALS (enclosure, PCB support) - while for most gadgets' encapsulation, plastic is the material of choice; I'd rule that out for this particular purpose. There is a very wide variety of polymers, copolymers and similar materials, often enhanced by various additives and fillers. Unfortunately, we've seen too many of them to get discolored, brittle or soften, and break apart after several years. Their internal structure changes upon UV irradiation, exposure to heat or chemicals/gases (sometimes those commonly used in households e.g. for cleaning), or sometimes even just like that, spontaneously. While there are plastic materials which are deeply investigated also for long term durability, there's just too many of them to say safely this is one which would last. So, in this particular case, it's wise to be conservative and use materials which have been proven by centuries of use: metal, ceramics, glass, and - with some care - wood.
• SIZE, MECHANICAL CONSTRUCTION ISSUES - as these gadgets are going to be part of a house, their size does not need to be very constrained. Some of them may be built into walls or "integrated" with the house in some other way, so a couple of extra inches in every dimension does not really count. What does count, though, is their appearance and design, this is something which has to be considered carefully; it's not going to be your average white or black box. There may be one great opportunity in having wireless and maintenance-free electronics, namely that it could be hermetically enclosed e.g. in a glass or combined glass/metal enclosure (with careful choice of sealing method and mainly material - rubbers are out). This would also increase resilience to any adverse outer environmental impact (humidity, gases and chemicals, rapid temperature changes).
• COST - as these are to last and serve for an order of magnitude more time than the common stuff, the budget may be also roughly an order of magnitude higher.

Two less vital things before we proceed.

One is, that some gadgets we mentioned (e.g. the door lock) have two power sources (a primary and a plan B); the "power controller" part then gets inevitably more complex, having to handle both and switch between them as appropriate. Also, some gadgets have several functions, with different power consumption and different priorities for scheduling/permanent or intermittent powering - this is for example the clock, with timekeeping part (RTC), display and control, and radio receiver for precise time setup.

The other thing is an unrelated idea or question: having mentioned mechanical power source, what amount of energy can we talk about? Let's push things a bit to the edge: say having a 100kg weight running all the way from attic to cellar, say 10m, can it power a clock for a century?

The total energy is , let's assume an unrealistic 100% conversion efficiency (or 50% and compensated by having the weight 200kg); that's h.m.g = cca 10kJ. As mentioned previously, an alkaline AA cell which can run the clock roughly one year contains around 3-4Wh of energy, 3600 seconds in a hour... it's just around 10kJ!

The good old pendulum clock needed to be wound up every day, say 1kg weight up 1m, that's 10J - and given the number of days in a year it's around 3.6kJ. Turns out, the mechanical clocks were pretty well optimized per energy consumption.

So, a basic characteristic is, that gadgets have some primary power source and some means to convert/condition it to voltage needed for their working. To be able to run when the primary source is not available, there is some means to store energy, and the "converter" has to cope with the requirements of the storage, too, convert  voltage back from storage to "application", and switch appropriately between primary/storage source and app/app+storage as power consumer.

Primary power source may be:

• ELECTRIC GRID - from our point of view this is an unlimited source, but the need for wiring and long-term uncertainties mentioned previously make it impractical for many cases. Some gadgets need to have a secondary source for case of outage. VERDICT: ONLY CONDITIONALLY.
• CHEMICAL - omitting exotics like fuel cells and bulky/long-term-wise problematic motor-generators (which are still the probably only viable solution for backing up the whole household, but that's for a different discussion), this leaves us with primary cells/batteries. They have a very high energy content per volume, but are prone to self-discharge and even if some of them (primary lithium) may last a decade or two, it's unlikely they will last a century (for CR2032 manufacturers give self-discharge as 1-2% per year, but at the same time many give useful/shell life as 10 years, all this at room temperature). It's the chemical bonds which contain high energy, but that also means that they sometimes break down without the useful output. There are also other materials in the cells (separator, encapsulation), with which the active materials react chemically and interact physically (e.g. clog up passages), and this all is unavoidable in chemical systems (in button cells the active_material_volume-to-surface ratio is very high, that's why they degrade less than high power batteries where things are arranged to thin films and thin layers and then wound up). Simply put, any chemistry is a big NO long-term-wise. VERDICT: NO.
• MECHANICAL - wind and vibration/sound are popular primary sources in the realm of energy harvesting, but the first is outdoor so out of scope here, and the second delivers too little energy in normal household environment (who would want to live in a house which roars and shakes violently?) While there are more such sources (e.g. water flow from rains) they share one unfortunate characteristics, and that is unpredictability. There is one notable exception though - if the source is unpredictable in time, but its occurrence coincides with the energy requirement. For example, the utility (gas, water) monitors can be powered from a small turbine - when the medium flows thus has to be monitored, power is available. There are already existing piezo-generator-based remote switches - they are actuated by hand, and the command is given by depression, which at the same time provides power for the transmitter. And the electric door lock can have a small generator built into the handle, so in case of power outage it still can be unlocked from a mobile phone or using a passive NFC tag, except that the handle has to be wiggled vigorously a few times.. VERDICT: YES, BUT ONLY WHERE APPROPRIATE.
• THERMAL - there is unusual to have enough temperature difference in a household to have a useful thermal generator providing enough power for enough time. There may be coincidencts, like measuring temperature of a furnace; but that does not sound like something very useful. VERDICT: MOSTLY NO
• NUCLEAR - there exists thermoelectric convertors and even direct beta/alpha convertors or other exotic constructions. These all fall very far from common household, even if they may hold the promise for high output and high life. VERDICT: WE WILL RECONSIDER MAYBE AFTER  A CENTURY
• PHOTOVOLTAIC - scalable, available, safe, in room temperature will have high life expectation. Not cheap though. If - as will be in most cases - primarily solar-irradiated, output is available only roughly half a day. Unusable in some places (e.g. cellar/attic/room with no windows, although for some applications it might be enough to power from electric light). VERDICT: YES, BUT USUALLY ONLY HALF A DAY

Storage may be:

• ELECTRIC - CAPACITORS. This is our main focus. They come in a wide variety of constructions; omitting exotics not applicable here they can be split to three cathegories: electrolythics, ceramic, foil. Elyts are attractive for high power density and low cost, but they leak like crazy and involve chemistry (e.g. they degrade more when not used, which indicates that there are things going on inside one does not really want in the long term)  - this makes them unusable for this project. VERDICT: YES, CERAMIC and FOIL.
• MAGNETIC - supraconductive storage. Out of scope. VERDICT: NO
• CHEMICAL - accumulators and batteries of them. The major drawbacks associated with chemical energy storage has been already discussed above, and they still apply here. Accumulators have high energy density but life limited to years  and thousands of cycles (for 100 years we need at least one-and-a-half order of magnitude more, guaranteed). This is the most usual, but for this particular project unusable option. VERDICT: NO.
• HYBRID CHEMICAL-ELECTRIC - ultracapacitors fall in this cathegory. For some surprisingly, they are *not* capacitors, their high energy density comes from chemical bonds. This is reflected in limited cycle time and for some models surprisingly slow charging and low discharging currents. Again, attractive, but chemistry is a long-term-no. VERDICT: NO.
• MECHANICAL - this sounds funny, but this *may* be a viable option. Think of the good old pendulum clock. They have a long and successful development and deployment history (although only in the purely mechanical side of things), and they apparently can be made to "leak" very little. The electromechanical conversion is certainly lossy and low powered devices won't be easy to construct, but it's worth to pursue this path. Flywheels don't sound like an option; prings sound to be too unstable long-term, given the tensions which are in its essence of functioning; but gas pressure and gravitational (weight-on-a-cord) may be viable, if the mechanics is carefully crafted. VERDICT: YES, MAY NEED RESEARCH

While most of the gadgets may be designed at the time when the house is built, thus power from grid (or from a secondary low-voltage wiring) may be provided, most of the interesting cases is wireless. They feature easy installation, somewhat lower installation cost, and also higher reliability due to no impact of problems of wired power (e.g. there's significantly more HF content in the grid power than it used to be one or two decades ago and who knows  how will this change in the upcoming; houses near modern "renewable" sources such as wind or solar farms are increasingly subject to wild fluctuation of grid voltage; switching power sources are notoriously prone to failure and if such used in low-voltage wiring fails so that it outputs a surge, it may take out the connected devices, etc.). Also, wiring diagrams for a house tend to disappear within decades, and rewiring a failed cable may prove to be exactly as annoying as a failed gadget is.

So, most of the suggested gadgets are wireless, or wireless version of what has a wired version too.

• Clock - clocks are devices with steady but low power demand, so making them completely free from wired power is an attractive option. In this case an appropriate primary power source has to be provided. Clocks typically run out of a single AA cell for roughly a year, given some 4Wh energy content of an alkaline AA cell and slightly below 9000 hours/year, the average consumption is a few hundreds of uW. The current consumtpion in case of LCD display is relatively steady, in hundreds of uA; in case there is a radio receiver for automatic time adjustment there may be periods of higher current consumption, maybe a few mA, for tens of seconds, perhaps once a day. Electromechanically displaying clocks have mA current surges at the moment of  making a tick.
• Thermometer/hygrometer - these usually gadgets with an LCD display, with similar activity thus power profile than the clock, also with a similar consumption. Some of them transmit the measured value wirelessly, so there may be a surge of  power consumption up to a few mA for a couple of seconds, usually once in a few minutes.
• Utility meter (gas, water - electricity is not a problem) check/transmit unit - these usually monitor a dry switch from the locked utility meter, and transmit the consumed amount to some home automation center. Unless thermo/hygro, they don't need a display, but they also have to monitor their input all the time, can't chose when to do that. On the other hand, as the input is a simple switch, their task is relatively simple and the front-end can be designed as of low-consumption asynchronous counter, carefully adding some reasonable means of debouncing. This all may lead to uW static consumption; however data transmission again means an occasional surge of a few mA, with only slightly increase of average consumption.
• Alarm with a dry contact - for example flood alarm, intrusion alarm. They may be almost or completely current-free, until the electromechanical switch is engaged; in which case a one-time or few-times-repeated transmission occurs. A test transmission may be required, say once a day.
• Comfort light - usually a single LED, providing just enough light in the night to move around safely. Requires say ten mW for tens of seconds, maybe a few times during a night. Add a mW continuous consumption if not tripped by a switch, but incorporates a PIR motion sensor.
• Door lock, remote controlled - this is a typically high-powered device, which needs wired power; however, for power outage there must be some plan B, power source for one-time opening (there may be also plan C,  lock having also a mechanical key which is well stored; and/or plan D, a locksmith capable of opening the door while minimizing damage).
• Remote lightswitch - similar power profile than other transmit-upon-switch devices, the difference is that this one is hand-operated.

There are certainly more similar gadgets possible, but for now this will be sufficient as a basic sample.