Tiny House Chronicles: Off Grid Electrics

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Published on the Doomstead Diner on July 22, 2016

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Tiny House offgrid electrics: further insights and modifications

ELECTRICAL 20uly2016

As I mentioned previously we must alter our plans if the original items we intended to source turn out to be unavailable or unaffordable to us. My further research showed that although lithium batteries have fallen in price, the models available to me still remain significantly more expensive than lead acid batteries, watt hour for watt hour, even taking into account the greater depth of discharge and longevity of lithium. Furthermore due to the specific electronics required for lithium systems (battery management systems, specific chargers) and the still small market for them, the costs of these additional essential electronic components remain high.

I will delay sourcing the batteries and solar PV panels for as long as possible because the prices seem to be constantly falling.

If lithium remains too expensive by crunch time, I wish to keep open the option of staying with good old, tried and true lead acid batteries, however one of the most essential features of a lead acid system must be a low voltage cutoff device located at the battery bank to prevent excessive discharge (>50%) and hence damage to the batteries.

My original plan was to run most of the tiny house appliances directly on 24V DC which should lose less energy over the transmission distance than a 12V system. There are a number of 24V DC appliances available and most DC fridges can run on both 12V and 24V, however the market is vastly bigger for 12V DC appliances. For example I was able to find 12V DC but not 24V DC models for the ceiling fan and kitchen rangehood (and hence would need to obtain 24V DC to 12V DC converters to run those).

LVcutoffThe nail in the coffin against me using a 24V household system was my complete inability to source a low voltage cutoff device for a 24V lead acid battery system. Nominal "24V" lead acid battery systems may actually deliver around 29V when fully charged, but when half depleted may deliver around 23V and should be automatically disconnected then to protect the batteries.1

The market for 12V DC appliances is massively larger than 24V, because 12V is the standard for the automotive industry and for RVs and boats. Hence it is easy to obtain a low voltage cutoff device for a "12V" lead acid system which will cut off around 11V or 11.5V depending on your preference.

Hence for DIY tiny house electricians using lead acid batteries, it may be best to stick with a 12V DC system, not 24V, and to use extra thick copper cables to minimise voltage losses over distance, especially the cable to the fridge. You can use a pure sine wave inverter intermittently for the few items where DC appliances are unavailable eg washing machine.

DIY builders must not do their own high voltage AC internal household wiring unless they are suicidal. Market pressures these days are forcing people to use AC appliances (even for RVs) and it must be admitted that the efficiency of AC appliances has vastly improved over the years, whether they be fridges or computers or TVs (which all seem to be LED with no CRT or even plasma displays being sold nowadays). Furthermore the market for and hence availability of AC appliances is magnitudes larger than that for DC appliances.

My main previous reasons to avoid 100% AC in the household and use DC as much as possible were:

  1. Everything being completely dependent on one single device, namely the DC to AC inverter, represents a potential "choke point" for total system failure. (The same can be said for the MPPT charger, however that particular item cannot be avoided no matter what system you choose).

  2. Excessive complexity – DC current from the batteries being inverted to AC, then going to individual appliances and being rectified to DC again. Much simpler for the DC current from the battery to directly power DC appliances which minimises potential points of failure and hence enhance reliability and durability.

  3. Efficiency losses (as heat) from inverter and rectifiers. In particular an inverter which is constantly on, even when no appliances are in use, represents a parasitic current drain.

  4. An inverter may be rated as highly efficient eg >90%, however that depends on the load. At optimal load eg a 3kW rated inverter running a 2kW load, it may well be >90% efficient, however at a low load eg running only a 30W laptop computer, it may only be 50% efficient, depending on the efficiency curve.

The new arguments to adopt 100% household AC wiring are:

  1. I understand that AC to DC rectifiers in just about all modern household appliances are extremely reliable. For example, many LED light manufacturers guarantee their AC bulbs (which incorporate rectifiers) for 10 years.

  2. I was informed that modern inverters do not need to be fully "on" constantly. They can automatically go into sleep mode when no appliances are on, with miniscule current consumption, and can be woken instantly when there is a load sensed.

  3. Modern inverters incorporate programmable low voltage cutoff devices. The commonest offgrid lead acid battery arrays are nominally rated "24V" DC and I understand that it is best to build up the battery system using numerous 2V cells rather than just a few 12V high capacity (eg 260Ah) batteries, because the former confer lower internal resistance. If, despite string protection, one of big 12V batteries fails, that entire costly battery will have to be replaced and until then, the whole system will run at much reduced capacity. If however a string of 2V cells fail, they can be removed and the whole system will run at only slightly lower capacity with the inverter reprogrammed to accept the lower 22V DC battery output and also to a lower cutoff voltage eg 21V (rather than cutoff at 23V for a 24V system).

  4. Even if you run only one 24V DC appliance directly from the 24V DC battery system, if it is inadvertently left constantly on (eg shower exhaust fan), that could overdischarge and damage the lead acid batteries due to the lack of an intermediary low voltage cutoff device. This will not happen if 100% of appliances receive their power from an inverter which incorporates the low voltage cutoff protection.

Hence overall, if you are engaging a certified offgrid electrician to do your household wiring it may be better to go with 100% AC wiring in your tiny house. The system my electrician has suggested to me allows flexibility to accept either lithium or lead acid batteries in the future and it may be simpler to keep a spare inverter on the shelf which can be rapidly swapped if the active inverter fails. He advised me that inverters can usually be repaired, hence the faulty one need not be discarded. If you are building several tiny houses to establish a tiny house community, designing standardised setups allows the possibility of creating a microgrid.

CONCLUSION:

If your system is being wired by a professional offgrid electrician keen to offer you the latest and greatest, and you are too weak to resist the seduction of standard AC appliances (like the author), then you may choose a 100% AC house system which is completely dependent on the inverter and can keep a spare inverter handy.

If you are stronger than the author and better able to adhere to the KISS principle and/or are a DIY electrician who is not intent on suicide, you may prefer a 12V DC system which uses as many household 12V appliances as possible with only one or two items being dependent on an AC inverter. You will use extra thick household copper wires and incorporate a low voltage cutoff device at your 12V battery bank.

If you choose to go with lithium batteries in the first instance, it will be useful to ensure your system can also accept lead acid batteries in the future. This is because if/when industrial society crumbles, replacement high capacity lithium batteries, being uncommon, may be difficult or impossible to obtain. However lead acid batteries, being ubiquitous, should still be easily obtainable for a long time to come.

G. Chia, July 2016.

Many thanks to Lachlan O'Shea of Lockstar energy, specialist offgrid electrician

Any errors in this article are the sole responsibility of the author

 

Footnotes:

1. More precise lead acid battery management is more complex because the voltages mentioned refer to an open circuit without load after the system has "rested" for more than 24 hours. A fully charged "24V" system with an open circuit voltage of, say, 29V, when exposed to high load demand can drop its voltage to 23V, which is not necessarily a trigger for cutting off the system. However those details are beyond the scope of this article.

One Response to Tiny House Chronicles: Off Grid Electrics

  • pdxr13 says:

    There are several solar charge controllers that incorporate "load control" that can be programmed with high-voltage and low-voltage cut-off as well as time. Morningstar TriStar line can do these things up to 60Amps (12v/24/48v) per unit, with PWM or MPPT.

    The most important thing is to know your load and size your battery bank accordingly, then figure how to charge your battery.  In the Pacific North West, we have mid-winter sun of 0-5% of summer, so a genset is critical for a few months a year to keep the battery charged. But, it doesn't have to be a big unit, since it can run any time for a long time.  An inverter/charger can be a good companion for a genset to keep batteries often-charged/not over-discharged/not-over-charged. Remember that lead-acid batteries charge quickly from 40% to 85% (high-current in/high-acceptance rate), then charge slowly from 85% to 100% (low current/low acceptance rate). When discharged deeply, fire up the gennie for a few hours at 4AM, then let the sun take over from your small array for the rest of the day. If you are using lots of current (power tools, blowers, washing machine, etc.) run the genset.

    As you have noted, refrigeration is a relentless off-grid load. I size wire and batteries for 100% refrigeration compressor load (calculate worst-case scenario- hot day with warm beer added to cabinet, frequently opened door, battery voltage at low-limit with 10% voltage drop in the wire if low-voltage DC powered) and find that very-efficent super-insulated chest refer/freezer located within a short wire run of your batteries/controller is essential for DC. With 120v inverted AC, longer wire runs give you more location flexibility at the cost of complexity/cost of adding a good inverter/charger ($2500-5000). You get AC in the whole house with this as a pleasant side-effect, but you can't neglect safely distributing low-current DC everywhere for LED lighting and USB chargers (12v auto chargers to USB 5v/1A) for times when the inverter is off or broken.

    Best wishes!

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