By Rob Dixon
The year 2011 has been dubbed the year for power optimization for the solar industry. Distributed electronics such as Maximum Power Point Tracking (MPPT) DC to DC converters and micro-inverters are growing in popularity as a result of their ability to improve the energy generating capacity, or energy harvest of solar arrays. In a way, these computerized electronics are bringing a level of ¡®smarts¡¯ to solar generation never before experienced. In total, IMS Research expects this space to represent US$700 million in yearly business and a total of US$1.5 billion in global revenues over the next four years, for the suppliers of these products.
Power optimized electronics result in a faster return on investment for system owners because power that would otherwise have been lost is recovered and used. As an example, Azuray Technologies DC to DC Converters harvest, or recoup, up to 99.2% of available power from shaded panels. This results in an overall increase in total system energy harvest of up to 25 to 30%. Greater energy harvest means less energy drawn from the grid and higher energy savings for system owners over the long term.
The Challenge for Solar Arrays
Any system is subject to a variety of real-world factors that limit functionality and PV technology is no different. The most obvious obstacle to efficiency and yield for PV arrays is from objects, dirt, debris and obstructions that cause shading or partial shading on panels. Less obvious and more complicated factors include panel mismatch, incorrect orientation and/or misalignment of panels.
Due to the nature of solar array configurations, small amounts of shade, such as less than 10% of the surface area of a PV system can lead to a 50% decline in efficiency. Simply stated, power optimization aims to increase performance efficiency and energy harvest from new and existing panels by correcting these discreet imbalances and disproportionate power losses.
An example of how power electronics recoup lost power might go something like this; a 10 panel array with shade covering the lower fourth of five panels produces 1741.46 W on its own. If these five panels are then equipped with MPPT DC to DC converters, the array produces 1979.42 W. By employing power optimization electronics on the panels most likely to experience shading, the array produces an additional 237.96 W of energy. This is an energy harvest increase of 13.66%.
In addition to this electronic adaptivity to shading and obstructions, power optimization electronics provide system owners with communication and monitoring capabilities to ensure systems are producing the power that is expected, as well as to warn of system problems. With these abilities power optimization electronics are in many respects making solar arrays ¡®smarter¡¯.
Considerations in Power Optimization
With the growing variety of power optimization solutions on the market today understanding which key factors play critical roles in system performance and reliability is crucial. Well-constructed PV arrays are those which have been designed from a holistic approach. These are systems in which each product has been carefully selected not only for its individual features, but also for the benefits it brings to the whole system. High quality components, high temperature ranges and inverter compatibility are all key factors in selecting products that work well on their own, as well as within the structure of new and existing systems. These products have been designed with inherent long term reliability as a primary requirement and can be depended on to last as long as the solar panels they serve.
The following are six elements that should be considered when making power optimization decisions.
1. DC to DC Converter vs. Micro-Inverter
The basic difference between these two product offerings is that micro-inverters relocate the DC to AC function to the panel level and thus eliminate the central inverter. MPPT DC to DC converters however support the role of the traditional central inverter to optimize the solar power generated by the panels. DC to DC converters can be less expensive to the overall cost of a system compared to micro-inverters.
The argument for replacing the central inverter with multiple micro-inverters is that the central inverter is traditionally the most prone to failure and breakdowns. When the central inverter goes down the whole solar array stops producing power until the inverter can be repaired or replaced. By contrast, if one micro-inverter on a panel fails, the whole system still continues to produce power, minus the failed element. However, repairs to a central inverter can be less dangerous than repairs to multiple micro-inverters on a sweltering hot roof several feet above ground.
Ultimately, the choice to use DC to DC converters or micro-inverters is one of preference. This is why companies like Azuray Technologies are bringing to market both types of power optimization electronics. The company is offering the customer the choice that matches their system needs and individual preferences.
2. Temperature Range
An accepted guideline for the thermal acceleration of failure rate states that the failure rate for a given component doubles for every ten degrees of temperature increase. In other words, for every ten degrees of operational rating in a given environment, the failure rate for a given part at a specific temperature is cut in half. For example, a +105¡ÆC rated part will have four times the reliability of a +85¡ÆC rated part, at a given temperature. Likewise, a +125¡ÆC rated part will have sixteen times the reliability of a +85¡ÆC rated part.
The basic temperature range for companies offering MPPT or micro-inverter technologies are -40¡ÆC to +70¡ÆC. For commercial/industrial grade components +85¡ÆC is the industry standard. However, it is not uncommon for dark colored roofs in the sweltering sun to exceed temperatures of +85¡ÆC. Heating an electronic device above its maximum temperature rating significantly stresses and in some cases over-stresses the power optimization components and puts the PV array¡¯s ability to increase energy harvest at risk. Thus, a component¡¯s temperature rating can fundamentally affect the failure, not only of a particular component but of the entire product.
Regardless, many power optimization manufacturers employ +85¡ÆC rated parts in their DC to DC converters and micro-inverters. The effect is to end up with products that have very high reliability requirements over a long lifetime that have been designed with parts that will regularly experience over-stress conditions which will significantly reduce their useful lifetime.
At Azuray Technologies we believe the use of +85¡ÆC rated parts puts the extreme reliability of power optimization products at risk. Thus, +85¡ÆC rated components are excluded from the Azuray AP product line in mission critical applications. Instead, Azuray uses the most rugged, highest capability components available. In the AP product line, every mission critical component is rated to greater than +85C. Taking advantage of the highest rated parts available provides Azuray with lower stress ratios on components compared to its competitors. This directly relates to lower failure rates at the product level and a higher durability and safety in over-stress conditions.
3. Component Parts
Capacitors are crucial components in any power conversion device. The reliability of these components can be a critical to the product¡¯s dependability. If a single component has insufficient performance or contains technologies known to have long term reliability risks, that component will limit or reduce the overall reliability of the product to that of the lowest performing element.
In solar power electronics, capacitors must be durable enough to withstand extremes in both temperature and voltage as solar energy is harnessed. The most commonly found types in PV systems include ceramic, electrolytic and film capacitors.
Electrolytic capacitors are widely regarded by industry experts as extremely unreliable and fraught with problems. Despite this, they are still used in some power optimization products.
A step up in reliability from electrolytic capacitors are ceramic capacitors. They are better suited for high temperature environments. However, they are also prone to cracking in applications requiring exposure to high thermal cycling such as found on a PV panel.
By contrast, film capacitors are widely considered to be the best material in extreme temperature environments, because of their durability and reliability. One way to discover if products use high quality parts is to look for mentions of Automotive (SAE) or Military/Aerospace grade components, such as in Azuray¡¯s AP product line. These parts are routinely rated to function in temperatures of +105¡ÆC or +125¡ÆC and rugged enough to handle high stress environments in long mission time end uses.
Using inferior parts may help reduce the initial cost of a product. However, they may also degrade system efficiency and performance. This ultimately costs system owners more in the long-run through replacements and repairs. Total cost over the lifetime of the product should always be considered.
4. Energy Harvest Gains and Efficiency
This is the core function of power optimization, maximizing energy harvest and doing so efficiently. Most companies in the marketplace today are competitively rated, with total system energy harvest increases between 5 and 30%. With regards to efficiency the range is 95.5% on the low end and 98.6% efficiency on the high end, with companies such as EIQ and Azuray Technologies at the top respectively.
5. Inverter Capability
Many DC to DC converters are compatible with a wide range of central inverters. This makes them ideal for retrofits on existing solar power systems that are not performing to the standards originally set for them at installation. However, there are some DC to DC converters on the market today that require the purchase of a proprietary inverter to function properly. If solar system installers and integrators desire options in choosing which central inverter to use, inverter compatibility is an important consideration.
6. Series vs. Parallel Strings
Most DC to DC converters support series string architecture. Micro-inverters by contrast are wired in parallel. There are DC to DC converters that require parallel wiring to function. The premise here is that with parallel strings, each module is connected to the central inverter independently and able to operate without interference from neighboring panels. Proponents of parallel wiring advocate that this type of architecture allows for more design flexibility and lower balance of system costs. However, with so few solar installers familiar with it, the majority of DC to DC converter manufacturers support string architecture over parallel. As string architecture is the traditional way solar panels are installed it continues to be the architecture most widely used and implemented in the solar industry.
Over the next decade, the sector of power optimization is likely to continuously evolve. Companies who are committed to high quality, long life and extreme reliability offer customers the potential for more dependable increases in energy harvest and greater confidence that ROI forecasts will remain consistent over the long haul.
Having this confidence is pivotal to ensuring ROI calculations are accurate. When system owners and integrators feel assured an array will generate solar power to the upmost of its ability, regardless of shading mismatch and other obstructions, that confidence supports the financial models used to fund the array¡¯s installation. Thus, power optimization electronics employed to increase a system¡¯s energy harvest must be highly functional and extremely reliable to encourage their use in new and existing PV installations.
Rob Dixon is the Senior Reliability Engineer at Azuray Technologies (www.azuraytech.com), a Senior Member of the American Society for Quality and Certified Quality Engineer. Rob has over 25 years of reliability engineering experience focused on high reliability applications in medical, telecom, military/aerospace and power industries.
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