This blog is a continuation of general information useful in considering an industrial off-grid solar plant system solution.
Energy storage, Capacity, Mother Nature and Operational expectations
Having established the equipment load demand and the sites geographical, climatic and local variables impacting capture of the sun’s photons, the amount of solar array recharge and energy storage device capacity can now be established to maintain a suitable power capacity, assuring the load equipment’s operational integrity. Sadly many off-grid industrial solar plants are design capacity deficient, “short changing” a system solution to reduce the sales cost and enhance the customer’s purchase acceptance. The old adage rings true “pay me now or pay me later”.
Traditionally off-grid industrial solar plants have used “12 volt building blocks” of solar modules, lead acid batteries and charge controllers as the energy capture, storage and control units. While today’s grid focused solar industry offers other component mediums (ie. higher voltage grid modules and MPPT controllers) that could be used for off-grid solar systems, their availability and cost per watt attractiveness should not be construed as industrial design material suitability. A properly sized and maintained system lead acid battery bank offers a balance of chemical and mechanical capacities adept as an energy deep discharge and quick recharge medium. However, Mother Nature’s unpredictable musings can skew the statistical maintenance and “operational sweet spot” for a lead acid battery bank directly impacting life expectancy, customer value and return on investment (ROI).
Array to Load Ratio (ALR)
The daily solar array energy production (Amps of recharge) for maintaining the energy storage unit (battery bank) draw down by the equipment load is statistically assessed as the system Array to Load Ratio (ALR). Simply put, it’s the ratio of additional solar Amps available to replace the Amps consumed by the load from the battery bank. A minimum design ALR of 1.15 assures the solar array can both “maintain and catch up” for periods of load draw during non solar recharge periods.
Battery State of Charge (BSOC)
A lead acid battery bank “operational sweet spot” referred to as the battery state of charge (BSOC) is a statistical target design balancing a systems solar array recharge capacity, daily expected Insolation exposure and the planned equipment load demand. NV Designs holds to an industry accepted BSOC standard of 80% average battery charge capacity across any month throughout the year. A vital element assuring the 80% BSOC design standard is met begins with a design minimum of five (5) days battery bank operational capacity (autonomy) for the established equipment load.
Loss of Load Probability (LOLP)
The reliability of a solar system design and it’s suitability for providing a constant resource for your load equipment is mathematically assessed as a loss of load probability (LOLP). Quantified as the design expected “Up time”, NV Designs holds to a less than 1% LOLP standard across any operational month. While some customer’s “don’t care if the system goes down”, statistically upholding this LOLP standard assures suitable operational power is available throughout the year for almost all customer project applications while ensuring the best life expectancy for the battery bank.
While industrial off-grid solar power plants have many more design considerations beyond those found in grid tied solar systems, adherence to basic design considerations, standards and quality sub-component selection assure a reliable and justifiable solution for the project investment dollars. When following up on past system deployment, we are often told that the solar plant “always works” and we haven’t been out to that site in years. Like an old Timex, proper engineering, component selection and design standardization assures that the solar plant “takes a licking and keeps on ticking” no matter the daily site conditions.
Next Blog: Off-grid solar plant safety and operational features