PhotoVoltaic Systems

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The heart of many IEEE Smart Villages is the solar power micro-grid.

As systems are custom designed for the loads in the small geographic area surrounding the plant, they all take on slightly different characteristics. In our experience, we have found the following solutions and designs to meet the all important cost/benefit curve necessary for sustained operation. .

Photovoltaic Panels

The most cost effective panels are being manufactured in China with a wide variety of yield curves and efficiency ratings. A buyer needs to have confidence in the quality and reliability of the supplier with performance guarantees in the purchase order. We use standard, commercially available panels that are sourced as close to the user’s location as possible. The top world-wide suppliers were identified in an article in PV-Tech

Tracking Systems

Active tracking systems reposition the solar panel so that it has the maximum exposure to the sun’s altitude during the day.   This usually involves robotic gears and microcontrollers for positioning optimization. To date, we have not found these to be cost effective as the increased power yield is quickly offset by the power used for constant positioning and control systems.  Our systems have found fixed positioning to be cost effective, easily maintained, and have long life.

Hybrid AC/DC

An interesting innovation that ISV has pioneered is the use of direct current for in-home/in-business low power uses with a parallel AC distribution between houses and buildings. This requires distributed storage systems and DC rectifiers at each load point.

Control Systems

The “brains” of the PV system is the solar charge controller. Its function is to monitor the output of the solar panels and the batteries charge state and optimize the storage capacity of the battery. Curiously, solar panels that are rated as “12 volt” panels put out about 16 to 20 volts. So if there is no regulation, the batteries will be damaged from overcharging. Most batteries need around 14 to 14.5 volts to get fully charged. A key learning is that panels seldom output their maximum rated power due to clouds, sun angle, or other factors. With the actual design having a higher than needed output voltage, you get enough voltage to keep the batteries at their minimum input level. Conversely, though, there are times that the panels do have peak output. In those conditions, the controller regulates the voltage so that the panels are not damaged.

AC Inverter

The installation of an AC inverter is predicated primarily in the type and power requirements of the load and the distances of the distribution network. In rare circumstances, for example, where the solar panels are on top of the building where the power is to be used, and the appliances in the building all have DC power supplies, then an AC Inverter can be avoided. Otherwise, the DC output of the solar panels must be converted to AC. The recommendation is to produces an output that is commercially compatible with the frequency and voltage of the local macro grid such that any appliances are usable in either environment. It is possible that eventual integration into the macro grid may be desired as well.

Battery Storage

The biggest factor in the long term cost of a PV system is the type and quality of the storage plant that is selected. Modern battery technology has progressed to the point that deep cycle power plants are on the market that will last for many years if maintained properly. However, these are more expensive and increase the up-front cost of starting a micro-grid utility. In worse case examples, some of our system operators have resorted to automotive or marine lead-acid batteries. These quickly reach their charge-discharge capacity and have to be quickly replaced.

The recommended batteries are a true trade-off between initial first costs and life cycle costs. Flooded lead-acid batteries meet the deep cycle requirements and charge/discharge duty cycle requirements for a reasonable cost. However, they require a cool room, constant checking of the fluid levels with top-off using only pure, distilled water. Any ionic impurities accidently introduced in the anodic fluid will reduce its effectiveness. A more costly alternative initially but with lower operating costs is a sealed lead acid battery. It has a little higher tolerance for temperature and does not require the daily fluid checks. The state of the art battery today is a Lithium Ferro Phosphate battery that is about 6 times the cost of the same capacity sealed lead acid battery. However, it has very low maintenance cost, can operate over a broad range of temperatures, and has 10x the charge/discharge duty cycles.

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