PowerSwitch

The falling price of grid tie PV systems is starting to affect the economics of battery charging off grid installations.

Traditionally, small scale off grid PV has been a 12 volt battery charged by one or more PV modules sold for the purpose of battery charging.

Grid tied systems usually used modules of higher voltage than would be suited for battery charging, and numerous modules in series to give hundreds of volts into a grid tied inverter.

The higher voltage grid tie modules are often a lot cheaper in £ per watt than are the battery charging modules. The higher voltage modules can be used to charge a 12 volt battery by means of a MPPT controller, these are now much more affordable than was the case a few years ago.

For all but the smallest 12 volt battery charging system, it will now make economic sense to use a large grid tie module and a MPPT

I have seen 250 watt grid tie modules sold for as little as £200, one of those plus a MPPT controller at say £80 would compare favourably to 3 smaller battery charging modules at perhaps £100 each and a simple charge controller at say £30.
A single larger module should also be easier to fit.

For a larger system, two modules can be connected in series to the MPPT.
The higher voltage input allows much longer or thinner cables between module and controller.
In cold weather, the PV module voltage rises, with a simple charge controller this extra voltage is wasted. With a MPPT the extra voltage is converted into greater current into the battery.
Like wise, when the battery voltage is low a MPPT will put say 10% more amps into the battery, a simple charge controller wont.

The first 110w modules I bought about four years ago cost me £440 each. I bought a couple more for £220 and a month ago bought two more for £110 each. I'm up to the 60A maximin for the present controller so it will have to be an MPPT controller before I can put any more modules on the roof. I will need to boost the voltage and lower the current to get the power down the wires that I've got connecting to the battery.

The breakthough for solar is for appliance makers to be forced to allow a wide DC input voltage for supply - say 30-100v perhaps with user setable max current draw to switch the appliance off for priority load use, as well as using normal mains AC. It's easy enough, and most old transformer fed equipment turned it to 12-45 VDC anyway. You then just wire a seperated low voltage supply feeding raw PV power in tandem or replacing your conventional mains, with no stupid conversion losses, or convertors requiring electronics and critical MOSFETs made far away. Obviously the real heater guzzlers showers, clothesiron hairdryers etc will not work well but that applies in all future powerdown scenarios anyway. Presumably the power comanies would not want govs to mandate this.

I have been giving some thought to this problem and the introduction of DC into houses. From what I can see I think 12 VDC is too low as the currents are just too large for anything greater than a USB charger and you will need large cables to avoid a large volt drop.

I would think that 48 VDC would be a good compromise as it can be changed relatively easily to 12 VDC for typical lighting, audio/visual and computing applications and, in Australia at least, voltages less than 50V DC are considered Ultra low voltages and can be worked on by people with fewer qualifications than for higher voltages.

I still think that for larger scale loads such as heaters and stoves and those involving motors greater than 500 Watts will probably have to remain on 240 VAC.

Many things seem to use the ubiquitous USB 5 VDC power supplies these days.

fuzzy wrote:The breakthough for solar is for appliance makers to be forced to allow a wide DC input voltage for supply - say 30-100v perhaps with user setable max current draw to switch the appliance off for priority load use, as well as using normal mains AC. It's easy enough, and most old transformer fed equipment turned it to 12-45 VDC anyway. You then just wire a seperated low voltage supply feeding raw PV power in tandem or replacing your conventional mains, with no stupid conversion losses, or convertors requiring electronics and critical MOSFETs made far away. Obviously the real heater guzzlers showers, clothesiron hairdryers etc will not work well but that applies in all future powerdown scenarios anyway. Presumably the power comanies would not want govs to mandate this.

I don't feel that this would be viable for a couple of reasons.
Firstly although a lot of appliances CAN ALREADY accept a wide range of input voltages there are limits to this.
Many switch mode power supplies will work from 90 volts AC up 250 volts AC, and will usually also accept DC over a slightly higher range from perhaps 110 volts up to about 300 volts DC.
Some DC appliances will accept from about 10 volts up to about 30 volts DC, and some accept AC also over a broadly similar range.

To design an appliance to accept anything from 10 volts (12 volt nominal under adverse conditions) up to 264 volts (240 volt nominal plus 10% tolerance) or even up to 300 volts (USA 277 volt, plus tolerance) would be a costly challenge.
The appliance would have to be designed to the relatively strict standards of construction and earthing or double insulation required for mains voltages, but also be able to handle the significant input current at 10 volts.

In addition, the output directly from a PV module or array, is totally unsuitable for directly powering any common appliance.
NO STORAGE WHATSOEVER is available, so no way will a 2KW PV array work a vacuum cleaner, or a laser printer, or a fridge. The starting or inrush current will result in the voltage falling to virtually nothing and the load never starting.
For any common load, energy storage is required, either in a battery, or by a grid tie inverter that uses the grid as an infinite battery.
For an off grid home a battery is unavoidable.

Specially designed water pumps exist that work directly from a PV array, they use some very sophisticated power electronics (probably incorporating vulnerable MOSFETS from far away) to match the variable input to a variable speed pump.

For those who like doing a bit of DIY there is a useful tool available.

A standard UPS has a 230v to LV DC to charge its small internal battery and then an inverter to produce 230v again.

So you can buy one with worn out batteries very cheaply and at whatever power output you choose. About 1/2 or 1 kW can often be bought for £5 to £15 as they are virtually scrap when their batteries fail.

So using external batteries you can produce a much greater storage capacity and let them be charged by the PV supply but then have a mains supply from the storage batteries available later.

Remember they are NOT locked to the grid like solar inverters so can only be used to power an independent load!

Tony