Archive for Category Power Conditioning Unit

1. Inverters and BOS in Roof TOp PV Systems

BOS, Balance of Systems and Inverters are integral parts of any Solar Power Generating unit. It is imperative that specified BOS and inverters are used for any given configuration of a solar power system for optimal energy efficiency. BOS includes the wires connectors, AJB’s etc. For Energy Efficiency, it is important to use the wires with correct thickness to avoid heat loss and voltage drop. Ohm’s law ( V=IR ) quantifies the voltage drop as a function of current and resistance. The figure below broadly indicates the wire thickness v/s current function to minimize the voltage drop.

The MPPT Charge Controller

MPPT (Maximum Power Point Tracking) Charge Controller tries keeping the panel at its maximum voltage and simultaneously produces the voltage required by the battery. A basic charge controller simply prevents damage of batteries by over-charging, by effectively cutting off the current from the solar panels (or by reducing it to a pulse) when the battery voltage reaches a certain level. On the other hand, a Maximum Power Point Tracker (MPPT) controller performs an extra function to improve your system efficiency.

What does the MPPT Controller Do ?

Besides performing the function of a basic controller, an MPPT controller also includes a DC to DC voltage converter, converting the voltage of the panels to that required by the batteries, with very little loss of power. In other words, it attempts to keep the panel voltage near its Maximum Power Point, while supplying the varying voltage requirements of the battery. Thus, it essentially decouples the panel and battery voltages so that there can be a 24 volt system on one side of the MPPT charge controller and panels wired in series to produce 48 volts on the other. Thus, offering the ability to provide some charging current even in dull conditions when a simple controller would not help much.

Inverter Efficiency

Solar PV system produces DC current, which is converted to AC using an inverter. Although inverters come with wide ranging efficiencies but typically affordable solar inverters are between 80% to 90% efficient.

Battery Efficiency

Batteries are needed for charge storage. Lead acid batteries are most commonly used.

Since lead acid batteries are usually charged at the float voltage of about 13.5 V and the discharge voltage is about 12 V, the voltage efficiency is about 0.88. In average the coulomb efficiency is about 0.92. Hence, the net energy efficiency is around 0.80

A lead-acid battery has an efficiency of only 75-85% (this includes both the charging loss and the discharging loss). From zero State of Charge (SOC) to 85% SOC the average overall battery charging efficiency is 91%- the balance is losses during discharge. The energy lost appears as heat which warms the battery. It can be minimized by keeping the charge and discharge rates low. It helps keep the battery cool and improves its life.

2. Why Have DC Priority on Solar Inverters

Solar Inverters are the back bone of any Solar Power Generating Unit. They perform the following tasks -

  • harnessing the DC Current produced by the panels and converting it to PURE SINE WAVE AC.
  • Solar Inverters are bi-directional and have an inbuilt MPPT charge controller, to charge the batteries, either directly from solar panels ( Dc to DC), or Grid to Batteries ( AC to DC), or a mixture of both, using a synchronized DC and AC Bus mechanism. Thus, during the day, the charger/inverter unit will first attempt to direct all solar energy to load. Any excess will be sent to the battery for charging. If the current being sent to the battery bank is still insufficient, it will harness some from the grid, convert it to DC, synchronize it with solar DC and then send it to the battery bank to compensate the shortfall.
  • During Peak Sunlight hours, the inverter/charger will automatically cut off the grid and batter-bank, and only use solar. During hours of diffused light, when solar alone is not sufficient to cater to the load, it will harness the shortfall from the grid, thus saving your battery from a discharge cycle. Remember, Battery has limited no of charge-discharge cycles, so its best to keep it charged. In the absence of sunlight, it will switch over to grid, and in the absence of grid, it will switch over to the battery bank as a last resort.
  • In the above context, let us take an example of a 0.75 kw panel source connected to a 1 KVA Pure Sine Inverter. During the day time say 9 AM to 5 PM, it will run on Solar Energy, so during this segment running cost is practically zero. However, usage of battery incurs a cost, since batteries need to be replaced. Let us assume that we use batteries to a DOD of 40 to 50%, wherein, the No. of life cycles are 4000 or so for a 150AH, C-10 Battery, which costs around 16000, and produces ( gives a back up) equivalent to 0.900 to 1 KW (40-50% DOD) . So the running cost on battery is 16000 divided by (4000 cycles x 1 kw on each cycle), i.e. about Rs 4 per KWH, which is slightly higher than the grid. So it is in our interest to run the load on SOLAR, WHICH IS ABSOLUTELY FREE, followed by GRID and as a last resort, on BATTERIES (MOST EXPENSIVE)
  • The above priority is in-built into our inverters- fuzzy logic algorithm.
  • Since our Solar inverters condition the output power to better that what is received, in terms of quality ( Voltage, sine curve, frequency etc.), they are also called PCU’s or POWER CONDITIONING UNITS

3. Solar Micro-Inverter, Power Optimizer Market Forecast

  • The global market for PV solar micro-inverters and power optimizers ( also called PCU), is predicted to triple by 2018. Market revenue for solar micro-inverters and PCU's, also called module-level power electronics is likely to touch $1.1 billion in 2018, up from $329 million in 2013.
  • Micro-inverters are small inverters which connect to each PV Panel, converting the DC to AC current. Although they are more expensive, micro-inverters are said to be capable of harvesting up to 25 percent more electricity than central inverters.
  • "Demand for MLPE has been driven by key markets in the New world countries like USA, UK, and Australia. Future demand for micro-inverters and power optimizers is expected to be spurred by continued acceptance in mature European PV markets, such as Germany and France. Some of the major Asian markets, like Japan and China, will generate huge opportunity in the next few years as MLPE technology begins to penetrate these markets in larger volumes.
  • These findings can be found in the report, "PV Micro-inverters and Power Optimizer- 2014," from the Solar service of IHS Technology.
  • The United States is the largest consumer of micro-inverters- with residential systems representing some of the largest markets.
  • Although Enphase and SolarEdge continue to be the leaders in the micro-inverter and power optimizer market, this has not stopped new suppliers from entering the space. For example, leading inverter suppliers, such as Kaco and Delta, have released new micro-inverter models recently as they continue to expand and diversify their inverter portfolio.
  • "Traditional inverter suppliers have been cautious to date in entering the micro-inverter market. But as the market has matured, an increasing number have moved in by acquiring a pure-play micro-inverter supplier or by designing in-house," Gilligan said.
  • As a result of new micro-inverter and power optimizer suppliers entering the market and existing suppliers entering new markets, revenues of MLPE are forecast to increase 28 percent per year to reach more than $1.1 billion in 2018. This represents a huge opportunity for new and existing suppliers alike to grow their business, with the market likely to attract new players in the next few years.