THE "PVACTM" - MELDING TOGETHER A CPV GENERATOR AND AN MPPT INVERTER INTO ONE ROBUST PIECE OF EQUIPMENT

Mihai Grumazescu (mihaLgrumazescu@futurenpv.com) Futuren PV Inc., Winnipeg, MB, Canada

ABSTRACT

The "PVAC .. 1 (photovoltaic [PV] alternating current [AC]) is a photovoltaic alternator that generates true­sine, grid-grade AC with no inverters. A high-intensity collimated beam is reflected by a 45° spinning mirror onto several high concentration photovoltaic (CPV) receivers directly connected to the primary windings of a transformer. To extract maximum electric power from CPV cells, MPPT algorithms are replaced with the concept of electrical resonance. All PVAC units are simultaneously and continuously synchronized with the grid via fiber optic or radio links. A key advantage of the PVAC is direct photovoltaic conversion into true-sine AC power. The PVAC reduces costs, increases efficiency and improves reliability of commercial- and industrial­scale CPV systems. Most important, this patent-pending and disruptive technology enables system integrators and CPV manufacturers to build direct AC photovoltaic generators that do not require inverters.

PVAC TECHNOLOGY: GOALS

PV systems are well known for their smooth and quiet operation. However, many CPV systems have sub-systems with moving parts, such as sun-trackers and circulating pumps or fans for active cooling. So, why not introduce an extra electric motor having ratings and a lifecycle similar to hard disc drives in computers?

The goal is to significantly reduce costs, while simultaneously improving reliability and efficiency of CPV systems. These goals are achieved by melding together the CPV generator and the inverter into one robust piece of equipment - the PVAC. The PVAC can be defined as a photovoltaic alternator that generates true-sine, grid-grade AC with no inverters.

The PVAC is designed to optimize heat management by reducing the heat buildup in CPV cells. In order to extract the maximum electric power from CPV cells, complicated MPPT algorithms are replaced with a more natural concept - electrical resonance.

Another important goal is to eliminate the DC wiring, DC disconnects and DC grounding in a CPV system by directly generating AC at the CPV cell level. Copper savings, diminished power loss and weight reduction are direct consequences.

All of the above features result in reduced costs, increased efficiency and improved reliability of commercial- and industrial-scale CPV systems.

Finally, this patent-pending and disruptive technology enables system integrators and CPV manufacturers to build direct AC photovoltaic generators that do not require inverters.

978-1-4244-1641-7/08/$25.00 ©2008 IEEE

PVAC TECHNOLOGY: DESIGN AND PRINCIPLES OF OPERATION

Photonic transmission of power in free space or through optical fibers significantly surpasses that of electrical conductors and semiconductors. Silica, commonly used in optical fibers, can theoretically handle up to 100 kW of optical power in a 1 DO-micron diameter fiber. Therefore, instead of electronically switching the equivalent DC power, the PVAC uses a highly concentrated and collimated optical beam distributed by a spinning mirror to a number of solar cells or arrays. An important bonus is the absence of harmonics.

Fig. 1 below is a pictorial representation of a PVAC power unit. Each part or sub-assembly can be purchased off-the-shelf and is presently used in different applications.

Rg. 1: One Bnbodimert of PlfACFbwer Unit

If an intense light beam is directed axially to a spinning mirror having a plane-reflecting surface cut at 45° with respect to the axis of rotation, then it can be shared with a number of CPV cells mounted on a cylinder having the same axis. In this embodiment, each of the six CPV cells is mounted on a receiver together with a blocking diode, and the fan motor is used as the mirror mover.

As shown in Fig. 2, all negative leads of the receivers are connected to the center tap of the primary winding of a transformer and the cathodes of the blocking diodes are grouped as follows: the odd numbered are connected to one end of the primary winding and the even numbered are connected to the other end. Diodes are numbered by the solar cells they are connected to, in the sequence of their illumination.

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Rg. 2: BasIc S:hamatlc of PlfACAlwer Unit

AC LOAD

Once per cycle, the footprint of the parallel-ray light beam generates an electrical pulse in each CPV cell. The shape of the pulse is very close to a half-sine. The transformer assembles the bi-directional half-sine pulses into a full-sine wave that can be extracted at the secondary winding to be used by an AC load. The transformer can be a variable step-up type with flux control, to maintain the output AC voltage at a constant value. The resonant circuit formed with the capacitor ("C") and the primary winding of the transformer extracts the maximum power from the solar cells. The dynamic characteristics of the CPV cells (mainly their high capacitance) also play an important role in this process. MPPT algorithms, such as "perturbation and observation" or "incremental conductance", are no longer needed. The transformer is part of the resonator and provides galvanic isolation at the same time.

A Fourier analysis of this circuit's response, while varying the spinning mirror's speed and the transformer's load, reveals the maximum power transfer from solar cells to the load under given illumination conditions. One major advantage of the PVAC is direct production of true-sine AC power in a simple and robust setup. Power semiconductors for switching and large capacitors are eliminated, together with the associated switching losses and harmonics. Therefore, suppressing filters are not required.

The idea of piping concentrated light through optical fibers and directing it to stationary CPV cells is not new. Tests done using sunlight are well documented. Photonic transmission of power to remote sensors based on this concept is commercially available. Even direct AC generated by exploiting the negative differential resistance exhibited by certain CPV cells in the presence of high-flux illumination (where the cell is connected to a resonator), is laboratory proven.

978-1-4244-1641-7/08/$25.00 ©2008 IEEE

However, using the PVAC's optical distributor, and simple and robust circuitry, leads to interesting results. The AC wave generated by the PVAC can be treated as a wild frequency (similar to that used to power avionics) or it can be synchronized and connected to the grid. Depending on the transformer design, a wide range of output voltages can be achieved in one stage.

Fig. 3 shows a PVAC-based CPV system structure, which can be developed for medium to large-scale CPV power plants using multiple PVACs.

"'-__ .... '- ~ACn -i , 1 _______ ..

Rg. 3: PlfAc.Based fPII ~em f1ruc:ture

Solar collectors and concentrators can be of any type currently used in CPV systems. One important feature ofthe PVAC power unit is that it accepts a round footprint of concentrated light, which means that there is no need for secondary or tertiary optics such as optical rods. Another advantage is that hot spot formation does not occur and the intermittence of illumination of each solar cell leads to less heat buildup.

Fig. 3 highlights how all PVAC units in a field can be simultaneously and continuously synchronized with the grid. The synchro interface collects the frequency and phase information from the grid transformer and sends it (at the speed of light) either via a fiber optic (FO) or radio link to all generators, in order to perfectly synchronize all motors moving spinning mirrors. Embodiments with only two CPV cells or arrays per PVAC power unit are also envisaged. In this case, the mirror is not spinning but reciprocating, being moved by a galvano scanner or similar device.

PVAC technology is scalable, modular and easy to manufacture with off-the-shelf components. For grid connected systems, a feasible and recommended size is 5-7 kW. The elimination of any DC wiring together with all associated drawbacks, including compliance with more onerous codes, is a very attractive technical and economic feature.

Further research suggests that a technique derived from the PVAC concept may be an important tool in solar cell characterization, along with I-V tracers, open circuit voltage decay (OCVD), and time domain and impedance spectroscopy for all kinds of solar cells. Instead of flashing a xenon lamp simulator (usually

used), the lamp can be operated in continuous mode, which is a better way to control light intensity and spectral composition. Automated PVAC production-flow testers are also envisaged.

PVAC TECHNOLOGY: OTHER APPLICATIONS

The PVAC is a scalable and modular integrated technology with many promising space and terrestrial applications, as well as civilian and military applications. Together with hybrid lighting and remote FO lighting, the PVAC concept can provide new opportunities for these niche industries, such as using the optical distributor to develop side-emitting FO luminaries that function like fluorescent bulbs. In addition, remote sensors, underwater remotely-operated vehicles, space elevators and other robotics can be powered through photonic transmission (FO or free space) and PVAC converters.

The PVAC can also work in combination with hybrid and remote lighting by splitting the main light-carrying optical cable into bundles, part for lighting and part for electric power generation.

Notes 1 PVAC is a trademark of Futuren PV Inc. Copyright 2008 Futuren PV Inc. All rights reserved.

978-1-4244-1641-7/08/$25.00 ©2008 IEEE