Off Grid Solar Latronics Inverter - by Stand Alone Power

Case Study – 3.43 kW Stand Alone Solar Power System

Date: October 17, 2012

The system shown here typically represents a stand alone solar power system (off grid solar) suitable for a domestic application occupied on a full time basis where their electrical load is relatively constant. The electrical demand during summer use is 16.25 kW/hrs per day and during winter is 12.62 kW/hrs. The main discrepancy for a fluctuation in electrical demand during the seasons is due to refrigeration and cooling loads being greater in summer.

Consisting with a generator, located on the adjacent side of the renewable energy components, excluding the photovoltaic array, located on the roof, the primary function of the generator is to provide a complete back-up of electrical demand, in turn providing a large current to recharge the battery bank in the advent of any failure of the system components i.e. failure of the Off Grid solar inverter, maximum power point tracker (MPPT) or solar array e.g. lightening strike.

The configuration mentioned above allows complete future proofing of an electrical demand as the generator can supply all of the electrical demand to all enabled circuitry, rather than conventional systems which, although may consist of a generator simply still relies on the Off Grid solar inverter for supplying electrical demand and the generator’s function is it to act as a device to recharge the batteries.

The battery bank, comprising of Exide Energystore deep cycle lead acid batteries, which still remain a viable technology is capable of delivering the electrical demand whilst reserving 3.5 days storage in the case of higher than average electrical demand or unseasonal weather. As with any correctly designed Off Grid solar power system, the battery bank remains at the heart of the system and is as instrinsictly important as the solar array or any other current producing device for that matter.

The storage device still can have faults if not designed correctly and be overcame by selecting an appropriately sized capacity relevant to the nominal voltage of the renewable energy system. Typically, if a system consists of 48 Volt system, then the most stringent way to avoid faults is by selecting batteries of the required capacity and nominal voltages where they avoid batteries being connected vertically – a 48 Volt system ultimately consists of 2 volt cells – 24 cells. [An indiscreet designer will connect 12 or 24 volt batteries to reach the nominated capacity, thus if one battery fails then the whole system will fail as a result].

The electrical demand, as discussed is an average of 14.43 kW/hrs per day – this is a summation of all the electrical requirements over a 24 hour period – the instantaneous demand is 3.3 kW – that is the maximum electrical demand at any one time. The selection of the Off Grid solar inverter is paramount to achieving an efficiency which suits the instantaneous demand, in this case the 3.5 kW inverter has a peak efficiency at 94% of its instantaneous supply, at 3.3 kW.

The Off Grid solar inverter must also be capable of supplying an electrical load higher than the instantaneous load due to either a surge from appliances typically found in refrigerators, washing machines, pumps etc or from additional appliances used intermittently i.e. kettles, toasters, microwaves and power tools. In this instance, the Latronics 3.5 kW inverter is capable of delivering 10.5 kW instantaneously or 4.1 kW for a period of hour an hour.

The photovoltaic array (solar) comprises of 14 x 245 Watt REC polycrystalline solar panels, was chosen due to their consistently higher efficiencies including 25 year output warranty and a 10 year product warranty. The modules were installed in 2 rows at a varied angle, due to meet the electrical demand in winter where the azimuth of the sun is particularly low.

REC are a Norwegian solar module manufacturer and have a reputation as a leader in efficiencies and reliability in performance.

The selection of a MPPT was chosen as the relative efficiencies are superior than Switched Regulators (PWM) and the system as a whole is Direct Current coupled system (see Q&A: AC Coupling vs. DC Coupling). The Off Grid solar DC Coupled system was chosen because the domestic application in this instance required the direct use of electricity at night for lighting, tv, computers etc.

As a whole, this off grid solar system produces 5.28 MW/hours of electricity per year, with an annual savings of $1,135 per year and over the lifetime of the batteries of 11.36 years an estimated savings of $16,000 (at current electricity rates 10/2012) if the owners had decided that the grid was a better option.

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