Solar PV Store

The (hopefully) long hours of summer sunshine are great if you have solar panels. But what about when the sun goes down?

This project is an enhancement to an existing solar PV installation. Using micro-controllers to measure the surplus energy being generated by the micro generation system, the surplus energy is then diverted into a battery bank. The stored energy can then be used during the hours of darkness to power the house electrical system via pure sine wave inverters.

Here in the UK, most owners of a solar PV system benefit from the 'feed in tariff' - that is, they earn money for the electricity that they feed into the national grid. I don't know if it is the same in other countries, but if so, check your small print first! The small print attached to the UK feed in tariff is long and complicated, but in the end, it all comes down to the rule: "You can use as much of the energy that your solar PV system generates, as long as any surplus is fed back into the service grid." Therefore, this means that any or all of the generated power can be used for domestic purposes in the home, including charging electric cars or even storage batteries as long as any unused power passes to the grid. With this fact in mind, it would seem to me that storage of surplus generated power for use during the hours of darkness (when the solar PV system is in shutdown) is allowed and permitted. But I am not a lawyer, and in any case have no idea what is permitted in other countries, and therefore any installation you might undertake after reading this instructable is at your own risk!!!

Step 1: Microcontroller Hardware

The difficult part of this project is determining the quantity of surplus energy being generated. This is where a microcontroller comes into its own. With current sensors, one on the solar PV cable and one on the house distribution board supply cable, the difference between the power being used and the power being generated can be calculated.

The signals from these sensors are fed into an Arduino / Nanode micro controller where these values can be analysed and evaluated. The results from these processed results can then be used to charge batteries or heat water etc.

Micro controllers have digital and analog inputs and outputs. The current sensors input their signals via analog inputs. If the surplus energy is to be used for water heating, an analog output using pulse width modulation is used to power a solid state switch which then powers a standard immersion heater. If the surplus power is to be used fo battery storage, digital outputs can be used to power battery chargers via a solid state switch.

The software used to control micro controllers is written in C code. The control code is written in Arduino IDE (integrated development environment). Connection is made between the microcontroller and the programming computer via the serial interface.

The Arduino sketches I used can be downloaded here:

Step 2: Battery Discharge Controller

If the surplus energy is used to charge batteries, charge controllers must be used to ensure that the batteries arecharged to the correct level otherwise over charging or over discharging will damage the batteries. The charge controllers used in this project were sourced via Ebay. Control circuitry also needs to be employed with respects to the battery discharge. Again the micro controller is more than capable of managing this operation. Battery voltages are monitored via the analog inputs of a microcontroller and household loads can be connected or disconnected according to the power level of the battery bank.

Step 3: Household Loads and 12v Systems

Many household appliances and services need a 240 volt/50 Hz supply (at least, here in the UK!) To achieve this, pure sine wave inverters are needed. It is imperative that the output from any inverter is never connected directly to the mains. The only inverters that can be connected to the mains are of synchronized type.

Many household appliances can be powered directly from the 12 volt storage batteries - such items as routers, alarm systems, door bells, cameras, low voltage (LED) lighting, etc. can be powered directly from the storage batteries. This is a more efficient way of using the stored energy.