Solar micro-inverter

A solar photovoltaic micro-inverter is a device that converts direct current (DC) from a single solar module (panel) to alternating current (AC).

Unlike a central or string inverter that aggregates and converts the power generated by the entire array of solar modules, a micro-inverter converts the DC power from a single solar module to AC. When connected to a central or string inverter the modules are all connected in series; when they have micro-inverters, the modules are all connected in parallel.

A grid-tie photovoltaic micro-inverter ensures that the power supplied will be compliant with the grid power. In geographic locations where buyback agreements are in place, this allows installations with surplus power to sell the power back to the utility. In net metering environments the meter turns forward during normal consumption, such as at night or in the day when local loads demand more than the PV system can supply, and backwards when the PV production is greater that the load consumption. The concept of panels delivering AC power has appeal for small-scale home project applications at lower voltage levels.

Central Inverter Description

Centralized PV inverters perform two major functions: power conversion from DC to AC; and Maximum Power Point Tracking (MPPT):

Power Conversion

Traditional solar energy central and string inverters convert current by ‘chopping’ the 200 to 480 volt DC voltage from the source solar strings, typically using local controls and a power conversion bridge. By filtering and tuning the frequency of the supply, the inverter ensures the current is in phase with the grid to ensure the current is grid-compliant and can be used for standard residential or commercial loads or sold back to the utility.

Maximum Power Point Tracking (MPPT)

The goal of the MPPT algorithm is to extract the greatest power available from the solar array. The better the MPPT algorithm, the greater the power output. With a central inverter, the MPPT is performed on the solar array as an aggregate. However, changes in temperature, irradiance, and shading create complex current-voltage curves, thereby making it difficult or impossible for the MPPT algorithm to find a power point that is optimal for all modules. This results in a compromise in operating conditions and results in less than optimal energy harvest.

In addition, varying roof orientations, and module mismatch (often resulting from typical manufacturing tolerances) can also have an impact on energy harvest. This is due to the “Christmas Light Effect” which is a well-known problem in the solar industry. Just as with Christmas lights, solar modules are traditionally installed in series strings, thereby an impact on one module will cause the rest of the array to be affected. Other challenges associated with centralized inverters include the space required to locate the device, as well as heat dissipation requirements. Large central inverters are typically actively cooled. These cooling fans can make a tremendous amount of noise, so location of the inverter relative to offices and occupied areas must be considered.

Micro-inverter description

Micro-inverters were invented to address some of the challenges associated with standard central inverters. A distributed approach to the inverter technology reduces the effect of dust, debris, and shade on the array. Modules are installed in parallel via the AC connection only, so issues with any one module will no longer affect the rest of the array. The solar array is no longer subject to the Christmas light effect by removing the single point of system failure.

History

The micro-inverter as a concept has been in the solar industry since it’s inception. Micro-inverters are also becoming increasingly popular and are commonly built into wind energy turbines. These wind energy machines are often referred to as micro wind turbines and follow a similar growing trend within the solar industry.

Companies like OK4U, Ascension, AES, ExcelTech and others, as well as a number of university design projects pursued developing micro-inverters over the years, but to little success. These early attempts did not have the efficiency, reliability or economy to solve mainstream solar energy needs, where higher voltages, higher efficiency and lower costs are essential.

In practice, crystalline silicon solar panels deliver relatively low voltages (about 30Vdc). They deliver even lower voltages when impaired due to bypass diodes (10-20Vdc). The higher cost, lower reliability and lower efficiency of the electronics necessary to raise those voltages, to deliver even low voltage residential 120-240Vac, have historically limited the use micro-inverters to very small scale installations and academic research projects.

However this changed in Mid 2008 when Enphase Energy released and started shipping their first micro-inverter and has continue to ship new versions since for small or large systems.

Advantages

Proponents of micro-inverters believe the technology makes the system more reliable, smarter, and efficient by increasing energy harvesting through optimal MPPT at the module level. Lower DC voltage, compared to central inverters, could also be less prone to arcing and therefore safer.

Another advantage of a per module conversion is the ability to track energy produced on a per PV module basis which is very handy for monitoring and diagnosing PV module related issues that are more difficult to find with series / paralleled PV arrays.

Also since each PV module works independently of any other module micro-inverter systems can be expanded on a module by module basis at anytime as manufacture, age and model PV module mismatch, which is problematic to centralized inverters, has no effect on micro-inverter system performance.

Disadvantages

Inverters are generally acknowledged to fail at a higher rate than other components in a photovoltaic system. The warranty of an inverter is typically 10 years, whereas the rest of the system can be warranted for up to 20 years. Thus inverters are most-often rated for shorter warranties than other components. Detractors of micro-inverter technology claim it is not advisable to distribute the least reliable component of the PV system to every module, as the failure rate of an inverter at the module level multiplies failure points. However this is almost completely mitigated by the fact that since the loss in energy produced is only equal to the size of each micro-inverter the impact to power production is minimal as compared to a centralized inverter where typically 100% of energy production is lost.

Also, micro-inverters tend to cost more per watt of power output than a centralized inverter. As of June 2010, a central inverter costs approximately $0.40-$0.60 per watt, whereas a micro-inverter costs approximately $1 per watt.

MPPT

Non-uniform changes in temperature, irradiance, and shading create complex current-voltage curves, making it difficult for an MPPT algorithm to find the optimal power point. Micro-inverters are designed to address this issue by designating an individual MPPT to each solar module.

Shading/Dust/Debris

Solar module installation issues revolve around fixed shade factors and variable shade incidents. Since fixed shade is a predictable, regularly occurring event, its impact on energy harvest is well understood. However, it is the variable shade events that are unpredictable and can significantly impact energy harvest. Variable shading events are a result of dust build-up over time, or debris, such as a leaf or bird droppings that reduce the power harvest. The Power optimizer can solve this problem of disproportional power loss through optimization at the panel level, and Micro-inverters as well.

See also

• Solar inverter
• Enecsys
• Grid tie inverter
• Inverter (electrical)
• MPPT
• Power optimizer

External links

• Model based control of photovoltaic inverter Simulation, description and working VisSim source code diagram

References

General references

K. K. Tse, Member, IEEE, M. T. Ho, Student Member, IEEE, Henry S.-H. Chung, Member, IEEE, and S. Y. (Ron) Hui, Senior Member, IEEE, A Novel Maximum Power Point Tracker for PV Panels Using Switching Frequency Modulation (2002)

Yeong-Chau Kuo, Tsorng-Juu Liang, Member, IEEE, and Jiann-Fuh Chen, Member, IEEE, Novel Maximum-Power-Point-Tracking Controller for Photovoltaic Energy Conversion System (2001)

Toshihiko Noguchi, Member, IEEE, Shigenori Togashi, and Ryo Nakamoto, Short-Current Pulse-Based Maximum-Power-Point Tracking Method for Multiple Photovoltaic-and-Converter Module System (2002)

Yang Chen and Keyue Ma Smedley, Senior Member, IEEE, A Cost-Effective Single-Stage Inverter With Maximum Power Point Tracking (2004)

Soeren Baekhoej Kjaer, John K. Pedersen, Frede Blaabjerg Institute of Energy Technology Aalborg University, Power Inverter Topologies for Photovoltaic Modules – A Review (2002)

Hirotaka Koizumi, Member, IEEE, Tamaki Mizuno, Takashi Kaito, Yukihisa Noda, Norio Goshima, Manabu Kawasaki, Ken Nagasaka, and Kosuke Kurokawa, Member, IEEE, A Novel Microcontroller for Grid-Connected Photovoltaic Systems (2006)

Trishan Esram, Student Member, IEEE, and Patrick L. Chapman, Senior Member, IEEE, Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques (2007)

Nabil A. Ahmed, Masafumi Miyatake, A novel maximum powerpoint tracking for photovoltaic applications under partially shaded insolation conditions (2007)

Tadao ISHIKAWA, GRID-CONNECTED PHOTOVOLTAIC POWER SYSTEMS: SURVEY OF INVERTER AND RELATED PROTECTION EQUIPMENTS (2002)