What kind of energy storage system is needed for photovoltaic power generation (II)

Three important indicators of chemical energy storage materials and devices, one is energy density, which is related to the continuity of charge and discharge; the second is power density, which is related to the ability to release energy instantaneously; the third is the number of charge and discharge, which determines the energy storage device. life. The figure below shows the energy density, power density and charge and discharge life of lead-acid batteries, lithium batteries, lithium ion capacitors, carbon-based electrochemical double-layer capacitors EDLC, electrolyte capacitors. The extremely demanding application scenarios of photovoltaics have very high requirements on the energy, power and life of energy storage systems. The energy storage capacity, charge and discharge power of the energy storage system directly affects the control of the climbing rate.

Different photovoltaic systems have very different requirements for energy storage systems. The article examines four scenarios of a single component, a 5 kW roof system, a 100 kW small PV plant, and a 7.2 MW large plant. In general, large systems have a large footprint and have a certain "geographical aggregation" power generation output smoothing capability. In Jiang Yu's algorithm, a stuff called cut-off frequency was introduced, and this automatic smoothing phenomenon was considered by empirical formula.

As the system increases, the energy storage system shifts from the energy density limit to the power density limit. In other words, high power density energy storage systems have advantages. In this regard, lithium-ion batteries have significant technical advantages. At present, the cost of lithium-ion batteries is 1 to 3 times higher than that of lead-acid batteries, but there is more room for cost reduction in the future. As for the capacitor series, although the power density is sufficient or even exceeds the requirements, the energy density is far from the power generation buffer of the whole day. The study found that high-energy-density lithium-ion batteries (less than 600 watt-hours per liter) best meet the requirements of photovoltaic output buffering, and can basically achieve 100% climbing rate control requirements of 100%.

The article also proposes a very novel and attractive technology solution - the integration of energy storage systems on micro-inverters and component optimizers. This not only satisfies the requirement of smooth power generation output, but also increases the overall power generation capacity of the system (the energy storage unit makes the photovoltaic component not need to deviate too much from the optimal power point of the component to generate electricity). As a preliminary idea, the author controlled the volume of the energy storage unit within 100 cubic centimeters, which is the revenue junction box. This requires an energy density of at least 400 watt-hours per liter, and a power density of 2,300 watts per liter, which is beyond the reach of today's mass-produced lithium-ion batteries. If measured by today's mass-produced lithium-ion battery performance indicators, 10% of the rate of climb can achieve a compliance rate of 99.5% (ie, every 5,000 occurrences of climb rate exceeds 10%, 995 can be resolved).