CPV laminated interconnection system.
One common kind of concentration photovoltaic (CPV) system is based on the combination of primary refractive optics with high-efficiency photovoltaic cells arranged in a matrix pattern.
A CPV module is usually constituted by a front lens panel and a back panel, each lens in the front panel concentrating the solar radiation on a corresponding photovoltaic cell on the back panel. The back panel must dissipate any excess heat to the environment, acting as a heat sink.
In order to constitute the back panel, it is a common practice to first mount the cells on CPV receivers and the receivers on a base plate. We name this a CPV receiver panel.
A CPV receiver is a micro-electronic assembly including the photovoltaic cell and usually also secondary optics and a bypass diode. The cell is mounted on a suitable substrate providing good electrical insulation and low thermal resistance, as a substantial amount of heat must be dissipated from the cell. Examples of substrates used are IMS, DBC, screen-printed alumina or insulated lead frame. The cell is usually soldered to the substrate using vapour phase or vacuum reflow ovens. The substrate includes an electric circuit to which the cell and optionally the bypass diode are electrically connected. The receiver assembly must include some interconnection means.
These CPV receivers are then mounted on a base plate and series connected in order to maximize module voltage. The first and last receivers in the module string are then connected to cables or sockets allowing for module interconnection. Receivers are attached to the base plate using techniques which provide good thermal contact, like mechanical fastening and thermal grease or graphite pads, heat conductive epoxy or heat conductive adhesive tape.
As said before, it is required that the back panel promotes heat dissipation to the environment. As the receivers are placed at a significant distance from each other, it is desirable first to help heat flow expand to the whole back surface area and then to use convection to the environment and radiative cooling mechanisms. So, the common technique is to use a relatively thick aluminium base plate, which will at the same time provide good structural stiffness and strength to the CPV module.
CPV modules are also series connected between them, in order to maximize system voltage. It is common to aim at maximum allowed DC voltages, as this minimizes Joule losses and usage of copper. A high system DC voltage also helps increase inverter (DC/AC conversion) efficiency because a voltage boost stage is avoided, and thus also decreases its cost. It is therefore a common practice to bring system DC voltages up to 600 V in the US and 1000 V in Europe, the maximum allowed.
This combination of high DC voltages and a grounded metal back plate is challenging, because receiver interconnections are very close to the aluminium heat sink. Dielectric break-down resistance in the order of 3000 V must be assured.
It is a common feature of other CPV systems that they are properly insulated under dry conditions, but not under wet conditions, assuming that the module inside is always water-free. As CPV modules are physically like boxes, containing a significant amount of air, condensations can never be completely discarded, and also accidents may happen which would imply some rain water filtering inside the module. So, in case of wet conditions inside the module, most CPV designs fail to fulfil the electrical insulation requirements, therefore posing a significant safety risk. Some systems use active drying systems in order to avoid or get rid of condensations, but these systems may also eventually fail.
Besides losing insulation, for systems relying exclusively on module water-tightness and active drying systems, inside materials would degrade after a condensation or water leakage event, and so long term performance and reliability would be hurt. Any accident, including for instance an active drying system malfunction, could therefore derivate into safety risks and/or system degradation.
Also, it is an obvious objective of CPV systems to become the most competitive solar conversion technology. This means that a high production volume and low-cost technique is needed. Also, very accurate positioning for receivers is required in order to assure that the photovoltaic cells are kept in focus. Therefore an automated receiver mounting and interconnection process is highly desirable.
Finally, an added problem lies in the fact that any materials used inside the module, if exposed to the inside surface must be able to withstand highly concentrated radiation. Under normal operation, when solar tracking works properly, all the solar radiation will be focused inside the receiver solar aperture, but it may eventually happen that the solar tracking system is stopped, and it is an obvious requirement that the CPV modules can withstand this off-focus situation without any long term damage.
Summary of the Invention
Our invention objective is to obtain a CPV receiver panel which provides long term high electrical insulation degree (even under wet conditions), has good thermal conductivity, is easy and cheap to manufacture, and yields very accurate positioning of receivers to base plate and base plate to module structure.
We name this invention the CPV Laminated Interconnection System (CPVLIS).
Our invention essentially consists in sandwiching the receiver electrical interconnection system between two dielectric insulation layers, with a top off-focus radiation protection layer and a bottom aluminium structural base plate acting also as heat sink (see section view, figure 9), yielding a completely protected laminated receiver panel (see overall view of CPVLIS receiver panel, figure 1).
Main advantages of our invention are:
- The design assures that insulation requirements are fulfilled both under dry and wet conditions. This means that the system would not pose any safety risk in case of internal condensation or water leakages.
- Also, the receiver panel would not deteriorate under wet conditions, being water-tight and all surfaces water-proof. Water would not be able to reach the receivers inside nor the interconnectors, the cells remaining completely encapsulated. Therefore, in case of accidental water leaks or occasional condensations, the module performance would not suffer any long term degradation, thus providing a strong foundation for a highly reliable CPV system.
- The CPVLIS design yields a very compact, reliable, environment protected and mechanically strong product, facilitating its logistics and factory manipulation.
- The receiver assembly and interconnection processes are accurate, repeatable and fast.
- CPVLIS receiver panels will be very easy to mechanically integrate at module and system level.
- The possibility to integrate secondary options during the lamination process.
The key elements of the CPVLIS are:
- Using an aluminium base plate as heat sink, on which mounting holes are made with high positioning accuracy, like CNC punching, in such a way that these mounting holes can be used as fiducials for subsequent assembly stages.
- CPV receivers (DBS, IMS, lead frame or hybrid circuits) including or not secondary optics.
- End connection plates, including an insulated end connector and an interconnection pad.
- Robotized mounting system to attach the CPV receivers to the base plate, with high positioning accuracy and repeatability thanks to reference holes on the base plate. With two options:
- Using insulated receivers and conducting thermal interface materials.
- Using non insulated receivers and an insulated interface.
- Laying a first insulation layer on top of the base plate, with windows already made on it, in order to allow receivers and end connection plates to protrude through the insulation material.
- Interconnecting receivers using copper ribbon and a resistance soldering method.
- Laying a second insulation layer and a final cover sheet protecting the insulation layers from off-focus radiation.
- Using surface adhesives or heat under vacuum lamination to join together the several layers.
- Having the option to assemble secondary optics during the lamination process.