July 18

uGrid: Critical circuits home grid backup (part 1)

“A microgrid is a localized grouping of electricity sources and loads that normally operates connected to and synchronous with the traditional centralized grid (macrogrid), but can disconnect and function autonomously as physical and/or economic conditions dictate.” – Microgrids at Berkly Lab

Boom, crash, the lightning flashed, and out went the power.

This is something that happens from time to time, though far less so where I live now than in my youth (where we kept a generator in the shed for such occasions).  However, several recent outages started me thinking: how would I bridge the occasional and sometimes relatively long power gaps?

Phase 0: What happened, and initial parameters

Major component(s): Data, data, data! (I cannot make bricks without clay)

So the most recent outage was over 24 hours.  Long enough that we familiarized ourselves with food safety during a power outage (the refrigerated and frozen food briefs at FoodSafety.gov were quite useful, as would have been the USDA guidelines for emergency food safety).  As we did not take any supplemental precautions (such as icing the freezer) most everything was lost.  But in the time and uncertainty between the work reports (we have power; no, we don’t though some of our neighborhood does; we will have power soon – I can’t fault the response teams, they busted butt to get the most people restored as quickly as possible), I kept started running through ways to save the contents of the freezer, to charge cell phones, to run fans.  Why not power my home myself?  Or just a small, critical part of it.  After the power returned and the damage was assessed, I shared my musing with my spouse, who supported it rather enthusiastically1.  So, now to form a plan!

Broad estimates (not quite this broad, but definitely more guiding than accurate) are the place to start for a sense what is and is not within the scale of what I am thinking of.  My brain dredges up the idea of a 100A service connection being within the range of home standards, so start there.  Noting that lead-acid nominal voltage is 12V and US household voltage is 120 means I can assume that they are actually 10 and 100 respectively when doing power comparisons.  Power law then gives us about 10,000W to keep the house going at full load, which is about 1,000A from batteries.  A quick search for batteries I can purchase listing their Ah rating in large numbers (100Ah is not unreasonable to find) tells me I will need ten per hour, or 40 total.  Already well beyond the scale I wanted to deal with.  This is not surprising, but rather a reference point: where is the top and how far am I from it?

Now to the more reasoned consideration – powering a small, critical part of the house.  I started identifying what might be “critical” and some other initial parameters:

– Run the refrigerator, and a few small appliances or devices as needed (fan, coffee maker, phone charger, TV, lights, modem and router2)
– Run time ~4 hours at full capacity
– Fast (relatively) automatic switching between locally provided and grid power
– Automatic maintenance of battery bank
– Somewhat cheap and somewhat easy to set up
– Expandable in capacity and capability
– Dedicated ‘fridge outlet, uGrid outlets upstairs and accessible, and uGrid outlets downstairs near storm shelter

This seems like a modest list, with provisions to expand.  So start filling in some of the lines with more defining data.  First the instantaneous power requirements. There are several estimators available for how much power to budget for appliances, including the listing on the appliances itself (which may be a requirement for a UL listing), or you can do perform a direct measurement.

Measured relevant device power consumption (specific items mentioned to better extrapolate to general cases)
Device Nominal Power Peak Power avg kWh / hours measured Source
Refrigerator  300W nom 600W
GE single-serving coffee maker(Keurig-style coffee maker) 1W standby23W prefill 890W heat (~2 min)2.5W dispense n/a (impulse use) Kill-a-watt
Small flat-screen TV  60W <random web>
Entertainment center
Modem, wifi, and network stack  36W nom 42W peak 0.352 kWh / 4 hrs Kill-a-watt
Phone / Tablet Charger
CFL worklight
George Foreman-style grill
Amplified set-top off-air TV antenna 1W nom 1.1W peak 0.001kWh / calculated Kill-a-watt
Old CRT TV 3W standby88W nom 80W startup 0.090kWh / calculated Kill-a-watt
Digital-analog TV converter 0.8W nom 2W startup 0.001kWh / calculated Kill-a-watt
Mini fridge 95W nom 218W peak 0.055kWh / 2.75 hours Kill-a-watt

There are two simple (well, readily accessible) commercial ways to store power: electrochemically in batteries and oxidizer fuel for a heat engine, which in turn produces electricity by inverter or generator respectively.  I chose the inverter route over an auto-start generator for a variety of reasons.  Generators do have advantages, including quick recharge time (top off the tank), higher capacity per dollar, and a larger section of the overall system pre-assembled.  However, a generator requires ventilation and a large up front cost for capacity.  It is noisy and requires fuel storage; scalability and flexibility is limited and fully automatic operation is impossible (someone has to fill the fuel tank).  An inverter/battery based system maximizes flexibility at all stages of construction, and provides the minimum cost to meet minimum parameters (save the ‘fridge!), while reusing components in later, more advanced stages.  It more easily allows improving capacity, capability, and function to suit and along the way.  Largely, too, because an inverter based system drew on my more developed skill sets and allowed me to fiddle in a domain I wanted to experiment with.

Phase 1: Your car powers your house

Major component(s): Inverter

Notable material(s): Heavy-duty extension cord

So how to quickly deal with failures?  At the most basic level, you need a source of energy (kilowatts) to power any notable section of your home.  It also needs to be about 120Vrms and approximately AC.  Later phases call for a battery bank to provide electrochemical storage of energy, to be converted on demand to DC watts, then to AC watts.  The latter conversion is performed by an inverter, the other half of the electrochemically-based storage system.

For this very first step, we are going to (relatively) quickly verify our initial parameters, and have a plan in place should The Wrath of Mother Nature strike again.  The battery will be something we probably have available right now, sitting in our driveway:

If you can't see it, imagine a car here

In a pinch, an idling car is a generator-backed battery with cupholders.  A car battery is not well-suited by itself for use as a backup power source (more in Phase 2), but if the car is running the battery is supported by the engine / alternator.  Not efficient or elegant, but workable, leaving the inverter and some miscellany to complete a rough off-grid solution.

There are a multitude of off-the-shelf inverter options with various features and price points.  Looking at the complexity, development time, and cost of parts, I found it better to buy than build the inverter. So with a sense of the scope in mind (kW continuous power required, approximate budget), shopping commenced (shopping is something both I and the engineers I know tend to do a lot of).  This Power Bright PW1100-12 seemed fit my parameters well (~1kW capacity with peak capacity >1.5kW, $100 range, available from known dealers), so a few clicks later it was wending its way to my door.

Testing is important; I wanted to verify that the inverter could run the ‘fridge with room to spare.

SUCCESS!  Phase 1 is verified*: I can clamp on to the battery posts of the car with the short cables provided and run an extension cord indoors. Keep the wires from the battery to the inverter as short and thick as possible – this is where a lot of loss can occur. This highlights another good reason for Phase 1: one of the (potentially) most expensive and least scalable pieces can be tested before proceeding.

* I haven’t actually done the battery to car as generator setup.  This is direct battery feed: it works, and I was satisfied that the rest was demonstrated enough in other cases to work without testing.  I plan to try it later, but the next phase is calling…

Finding and specifying the inverter

Inverters can be found in auto parts stores, big box retailers, camping goods stores, hardware stores, and all sorts of places online, and have a good many specifications.  A few key specs are input voltage, continuous output power, peak capacity, output type, standby power/current, efficiency, thermal shutoff, and low battery shut off.

Input voltage is fairly straightforward: does it require an input voltage of 12V dc, or some other DC voltage?  Most consumer inverters will be 12V dc; anything designed to plug into or hook up to your car is most likely 12V dc.

Continuous and peak power will be major winnowing parameters.  Continuous power capability translates to how many devices you can run at once.  Some devices, such as refrigerators, fans, and devices with motors or warm-up periods, will initially demand a great deal more power at startup than they do running.  The peak power capacity of the inverter allows you to exceed the stated power capacity for some short time.  Read the manufacturer’s user manual to determine what they define as a peak capacity (e.g. duration and frequency of demand), and do not depend on surge capability for continuous use options.

Sine, modified/stepped sine, square wave

A notable differentiation between inverters is output type: square wave, modified sine wave, and true/pure sine wave.  A true sine wave inverter most closely approximates the shape of the power coming from the grid; a perfect sine wave with little harmonic energy.  This is what the electronics you plug in are designed to run on, but it is rather difficult to create from a DC source, much moreso at high power.  Square wave output is fairly easy to create.  However, a great deal of the energy is spread along the harmonics of the source, often resulting in problems starting certain types of motor (or they buzz when they run),  radio-frequency emissions that play havoc with electronics, lower efficiency / higher power demand, and possible strange behaviors.  To address this, most inverters now use a modified or stepped square wave signal that more closely approximates the sine wave.  Their advantage is they can provide more comparable power at literally a fraction of the cost (1/2 to 1/5) of an equivalent true-sine converter.

Standby power or current and efficiency are a couple of parameters to keep an eye on, and can help with a decision between two otherwise similar devices.  All inverters will consume some power when on, whether idle or running full tilt.  A lower idle current is more desirable, as most of the time, through all the Phases, the inverter will be idle.  Efficiency is fairly straightforward: higher efficiency will mean that the battery bank capacity will provide power longer.  I would probably rate lower idle current as slightly more important, but the idle current may become a non-issue if a longer failover time is allowed (by disconnecting the inverter when not in use, Phase 4 below); efficiency is always relevant.

Thermal shutoff and low battery shut off are both small but essential factors.  Thermal shutoff or shutdown of the inverter will potentially save your inverter if you overload it or if its ventilation gets clogged.  Not having that feature is an equipment hazard, and a fire hazard in a permanent installation.  During 1kW of continuous output operation, an 85% efficient converter will dissipate ~175W of heat.  If it can’t reliably vent that to the environment, the insides of that inverter will heat up quick.  Low battery shutoff is a bank-saver; the battery bank is going to be full of expensive and dangerous components (lead-acid batteries).  Discharging them too deeply will damage them.  Beware that this feature can be an annoyance if the conductor loss is too great: reporting a dead battery due to voltage drop in the cables.  For this reason we will be specifying and designing to that parameter (among others) in later Phases

This is a Project in Progress.  Edits, changes, updates, and even radical changes on this page are par for the course.


1 Always helpful.  Actually, they do suffer most of my projects kindly, but buy-in is always helpful.  I think having to triage the ‘fridge with me helped.

2 What?  We don’t want to live like savages, now do we?

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Posted 2015-07-18 by JourneymanWizard in category "Projects and Tech

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