A report from GSES shows the significant relationship between PV arrays and battery days of autonomy in stand-alone power systems.

Battery energy storage is essential to stand-alone PV power systems that rely on intermittent renewable energy as the primary generation source, and the sizing of this battery storage has long been a critical and sometimes contentious issue. Due to the nature of stand-alone power systems, it is always necessary to size the battery according to site-specific loads, customer requirements and customer expectations.

The industry has adopted the term “days of autonomy” as one way of specifying the size of battery storage with reference to the site’s needs; this is defined in AS/NZS4509.2:2010 as “the number of days of operation of the power system without energy input from generators before exceeding the design maximum depth of discharge of the battery”.The recommended minimum days of autonomy is generally between 2 and 5 days, depending on a number of variables.

However, the price of PV solar modules has dropped greatly in recent years and it is now far more economical to purchase additional PV generation capacity than battery storage capacity. Even on cloudy days, some solar irradiation is still available. A PV array that has a rated capacity significantly larger than that required by the site loads may be able to generate enough energy even on overcast days to support the battery. In this scenario, fewer days of autonomy should theoretically be required. Is it reasonable, then, to install an oversized solar PV array and reduce the battery bank’s days of autonomy, while maintaining quality outcomes for a stand-alone power system?

Balance of power

In the early 1990s the industry was having issues with vendors selling under-sized systems and customers not being happy with their stand-alone PV power systems. After the launch of the solar accreditation program in 1993 by Solar Energy Industry Association of Australia (SEIAA), a design guideline was developed and included in SEIAA training courses. It recommended 5 days of autonomy with a maximum depth of discharge of 70%, if appropriate for the equipment.

Five days was a typical figure used at the time, generally because it allowed for a number of cloudy days before the system owner might need to start a generator and it provided a daily depth of discharge of less than 20%. The larger storage capacity also catered for the fact that system designs were often based on the average daily solar irradiation for the worst month of the year. Therefore, on the days when the available irradiation was less than average, the battery would provide the deficit energy to the loads. On sunnier days, excess solar would recharge the battery.

Data in the design

The average monthly irradiation figures for five sites were determined and the worst month was selected; the size of the PV array (in kW) was then calculated based on the design principles of AS/NZS4509.2:2010 using typical assumptions for equipment efficiencies.

The days of autonomy was based on the usable energy of the battery in meeting the load. For lead acid batteries the usable energy is reflected by the maximum allowed depth of discharge of the battery (typically 50-80%). Lithium-Ion battery manufacturers typically describe the usable energy of their product as between 80% and 100% of the product’s rated energy.

A program was developed that used the historical daily irradiation over 28 years to determine whether the available usable load energy in the battery (from the preceding day) plus the available energy from the solar array was sufficient to provide the required daily energy. If not, it was deemed a “blackout” for that day. In doing these calculations the formulae as provided in AS/NZS4509.2 were applied. Hence, when it came to battery efficiency, it was the average battery efficiency being applied and no allowance was provided for the fact that an oversize is required (if there is no generator available) for lead acid batteries as defined in AS/NZS4509.2 to ensure equalisation of the battery is achieved. For no PV array oversizing and various percentages of oversizing, the tables in the original showed: average number of blackouts each month and each year; number of years, out of 28, the system would have had blackouts, and; the highest and lowest number of blackouts determined in a year.

Table 1: Three days autonomy (no oversizing) vs two days autonomy with oversize

Based on decreasing the possibility of a blackout in a year to be less than that for 3 days of autonomy with no oversizing of the array, Alice Springs would need to be oversized by 20%, Bairnsdale by 10%, Cairns by 10%, Darwin by 20% and Parkes by 20%.

Considering the cost of the solar modules and the ageing factor it would be advisable to oversize by more than that. However, at this stage 3 days autonomy is what is recommended in AS/NZS4509 and the user should be advised about the likelihood that they will need to operate a fuel generator or experience a blackout.

Table 2 provides a summary of the analysis for all 5 locations for comparing 5 days autonomy with no oversizing to 2 days autonomy with oversizing.

Table 2: Five days of autonomy (no oversizing) vs two days autonomy with oversize

Based on decreasing the possibility of a blackout in a year to be less than that for 5 days of autonomy with no oversizing of the array then, Alice Springs would need to be oversized by 60%, Bairnsdale by 60%, Cairns by 50%, Darwin by 60% and Parkes by 70%.

Plan for success

The present study shows that when reducing the designed days of autonomy from 3 days to 2 days, the PV array needs to be oversized by 10%-20% to maintain outcomes for the system equivalent to, or better than, 3 days of autonomy. The PV array needs to be oversized by between 50% and 70% to maintain system outcomes equivalent to or better than 5 days of autonomy.

The greater the difference between worst month and other months, the lower the number of blackouts as the array is already oversized for those months.

It is important to consider other design constraints. While lithium-ion batteries are unlikely to experience issues accepting additional charge from an oversized PV array, the maximum charging current of a lead acid battery is limited and directly proportional to its capacity. Depending on manufacturer specifications, increasing the size of the PV array while also decreasing the capacity of the battery bank may not be feasible for lead acid technologies. To ensure customer satisfaction, the customer must be clearly informed of the designed system’s capabilities and constraints, regardless of the days of autonomy designed into a system.