Designing a bespoke off-grid system

Posted Wednesday 20th of March 2019 at 14:03


Designing off-grid power for remote sites

At Leading Edge, our professional and personal service includes a bespoke design service, offering unrivalled expertise. 

We calculate power output for individual months over the year for both solar and wind, making sure both forms of green energy meet your power requirements.  By adding a 20% safety margin should unusual weather patterns occur, we ensure the system continues to function 24/7.    Here's our approach to designing a battery-charging off-grid system.

Brief - To supply off grid power (24 DC) : 24/7 : 365 days (continuous load)

Location - SN 12 27

Example Load requirement - 1x 15W devices + 1x 9.5W device, adding a 20% safety margin - allow for 30W continuous load.

Days Autonomy required - 4 days 

This refers to the number of days the client requires for the system to run purely off the battery bank should there be no wind or solar input.  Once we have the above set of criteria, we can then start to gather the necessary information for sizing the system (wind turbine, solar panels and battery bank). 

Solar and wind data

For our wind & solar calculations we have taken an 18 year annual ‘average’ - data supplied by an international meteorological institute and for a UK based system we cross reference this against a more localised (square kilometre) wind database.

We are aware that many times the speeds will be significantly higher than the average and this has been factored in by using a Weibull distribution curve within our Off-grid Power Matrix.

In this instance we will look at an area within the UK that has an average wind speed of 5.1m/s (11.4 mph).

Once the solar and wind data has been collected, we can input it into our ‘Off-grid Power Matrix’ which assimilates the spread of data over a 12 month period.

The chart below shows an estimate of the potential renewable energy available from our LE-300 Wind Turbine and 220W of solar panels. The red line indicates a continuous load of 30W (specified load of 25W continuous with 20% margin added).

Hybrid power for battery charging

Within our Off-grid Power Matrix we can show the customer the ‘load line’ – we will always factor in a solution that has an element of safety margin during the winter period (often when the solar irradiation levels are at their lowest).

Wind/Solar solution

The proposed solution is 1 x LE-300 24V wind turbine and 2 x 110W industrial grade DC-Solar monocrystalline solar panels. The LE-300 wind turbine outputs 85W at wind speeds of 8m/s and has a rotor diameter of 1m.  Designed for industrial applications, durability and reliability is the corner stone of the LE-300 wind turbine architecture.

The wind turbine has its own charge controller separate from the solar charge controller to avoid over-charging the battery bank and is mounted on a 48.3mm OD pole.  We also supply a Guyed Tower Kit which includes everything required apart from a pole which can be bought locally.

Battery Bank

The battery bank is the ‘heart’ of any off-grid system and we would always recommend using deep-cycle professional batteries. The battery bank quite often represents a substantial part of the budget so in our calculations we always work to a 60% Depth of Discharge as a maximum and add a 1.2 safety margin factor into the calculations – this will help prolong the battery life.

In this instance for a 30W continuous load at 24V the battery bank requires 4x 120Ah deep-cycle 12V AGM batteries connected in series and parallel to create a 24V battery bank for approx. 4 - 5 days autonomy.