Installation manual

5.12. Preventing excessively hot water with PWM & MPPT regulation

Overheating your water tank can be a problem on larger hydro installs or when the home

owners are away on holiday. Low pressure vented copper water tanks that are fitted with

tempering valves can generally be allowed to boil.

Other systems may have an upper temperature limit, as may the connected pipe work.

In such systems the simplest method to prevent overheating damage to your system is to fit

a temperature display with audible upper temperature warning. Just use more hot water

when you have too much of it.

A simple way to automate this is to fit a radiator via a small AC circulation pump that will turn

on when the water tank reaches say 70 degrees Celsius and off at say 60 degrees Celsius.

Low cost thermostat switches commonly used in dishwashers can be employed to switch on

a small AC hot water circulation pump to dump heat to a radiator.

Never try to switch DC in this manner as the thermostat will arc and

burn out. If you need to switch DC current, then use a slave relay.

Also remember all AC wiring has to be completed by a registered

electrician.

5.13. Battery Bank (see also Section 7)

In off-grid systems a battery bank is required to store power. The

nominal voltage of the battery bank dictates the nominal voltage of the

system (12 V, 24 V or 48 V DC) with 48 V being the most common.

The quantity of batteries in the bank is dependent on the power

requirements and the intermittency of power generation at your site. It

is typical to have a number of batteries arranged in parallel and series

to provide the desired voltage and capacity. Lead-acid batteries are

most commonly used, although most other types are also suitable.

So for example, in a very small renewable energy system we could

have a 12-volt battery and a turbine that charges it at around 12 volts

DC, and we take our "load" power from the battery. Most people then use a battery-based

inverter to convert the DC battery power to AC power (just like the grid) for using their loads.

In reality a 12-volt battery will have a slightly higher than nominal voltage when on charge,

the optimum being around 14 volts or so. If the battery is overcharged, then the voltage will

rise above 15 volts and the battery is in danger of being overheated, dried out and damaged.

We consider 12 volts as a classic example, but in modern renewable energy systems the

nominal battery is more often 24 or 48 volts. The system will actually work at 28 or 56 volts,

charging the battery most of the time and using it on demand.

PLT14 (12 V DC) cannot be used at sites where more than 50m of cable is needed due to

high cable losses, unless MPPT type controllers are used. Cable losses are much less of an

issue if you install a PLT40 or PLT80, and the MPPT controller then converts to 12/24/48 V

DC to suit your battery bank.

Batteries can also be used in on-grid systems to provide power when the grid is down. If

there is a grid power outage and your PowerSpout is connected via an on-grid inverter

(without battery), then it will disconnect itself from the grid, so your home will also lose

power. The extra cost to install a backup battery bank is difficult to justify though, unless you

have frequent grid outages.