2011

Improving Microinverter Performance in Photovoltaic Systems

We have known for decades that the sun radiates enough energy to meet the world’s needs for power, both now and in the future. As in any energy conversion from original source to a “usable” form, however, the various stages of adaptation introduce efficiency losses. In the case of the photovoltaic system, this has limited what could be exponential growth. In fact, the best performance available from solar cells of any kind (excluding the concentrated approach) never rose above 15% efficiency. In cascade to the solar cell itself, a classical centralized inverter would add barely more than 90% efficiency to the chain. This “poor” performance and the obvious lack of large successes has also helped to keep the installation costs very high.

altOnly in the last ten years have multicrystalline cells gone beyond the limit, reaching the 20% efficiency dream. We also have seen the introduction of thin film panels, with efficiencies just above 10% at a cost less than one third of multicrystalline panels. Inverter technology, with the best use of new materials, has also gone above 95% efficiency and dramatically reduced system costs.

All of these advances, championed by a population leaning more and more toward green energy sources, and sponsored by national, corporate, and local incentives, has led to worldwide yearly installations in the 10GW range, from humble beginnings of way below 1GW just a few years back.

Regardless of the changes in technology, both in panels and inverters, the basic principles always remain the same: • Solar energy hits the solar cells (any technology) (Figure 1). • Due to the photovoltaic effect, the cells release a DC voltage and a certain amount of current. • Depending on how the cells are arranged, the resulting DC voltage and current will be converted into AC usable electrical energy by the inverter (performing other functions as well to maintain a safe and reliable grid connection) (Figure 2).

The Evolution of Solar Systems

There are basically three main types of solar cell installations: • Residential (on top of single family houses or town houses). • Commercial (larger roofs on commercial buildings). • Ground, also called solar farms.

altTypically, installed power systems are 3kW to 10kW for a residential installation, up to 20kW-100kW for a commercial installation, and practically limitless for solar farms.

Research and development investments in solar technologies have increased together with the worldwide growth in adoption of photovoltaic systems. Furthermore, solar cell technologies and inverter topologies, as well as system architectures themselves, have evolved and consist of more complex aspects than in the past.

Let’s consider the four main architectures: • Until very recently, a typical installation of any kind of power would consist of a certain number of panels, connected through junction boxes in series and parallel, feeding one single inverter. Such an installation was called centralized. This main inverter would use communication technology in order to send data back to data servers about several aspects of the total energy conversion. • In the last ten years, in order to improve system redundancy and efficiency, the string inverter was introduced and achieved success, by means of dividing the solar field into strings, each connected with a smaller inverter. The string inverter communicates with a concentrator that dispatches the cumulative information about the system’s energy flow data.