Hybrid systems

In concentrator photovoltaic (CPV) systems, sunlight is focussed using optics such as mirrors and lenses to form a point or a line. The photovoltaic (PV) cells are placed at the focus of the optics where they receive concentrated light. These systems offer several advantages over flat plate photovoltaic systems, including the replacement of costly solar cells with inexpensive optics (hence reducing overall system costs). Due to the increased concentration of light onto the cells, cells must be either passively or actively cooled to ensure optimum performance. However, this heat introduces the option of creating a hybrid CPV-thermal (CPV-T) system which can generate both electrical and thermal energy from a single integrated system. At CSES a range of linear, single-axis tracking hybrid systems (and their components) are studied.

Microconcentrator

The microconcentrator (MCT) system is a linear, single-axis tracking CPV-T system being developed in partnership with Chromasun Inc., based in the U.S.A.. The system operates at concentration ratios of up to 30X.

Most CPV and CPV-T systems are significant structural installations. Hence, unlike conventional flat plate PV systems, they are generally unsuitable for domestic and commercial rooftop applications. The microconcentrator system reduces the size and weight of all components, resulting in a system which is suitable for rooftop installation. In addition, hybrid receivers allow the option of producing both electrical and thermal power from a single unit. The target performance of each box is to simultaneously produce 2 kW of thermal power and 500 Wp of electrical power.

Three MCT units mounted on a demonstration rooftop. The MCT system was mounted on the same supporting frame as the surrounding flat-plate PV panels.

Fresnel array of lightweight reflectors tensioned at their mounting points.

One of the key design features of the MCT is the sealed enclosure. This enclosure is 3.0m long, 1.2m wide, and 0.3m deep and isolates all the functional components of the system from external environmental influences such as wind loading, humidity, and soiling. The removal of the effect of wind loading on the optics allows the use of a Fresnel array of ultra lightweight reflectors which require no structural support other than tensioning at mounting points at each end of the enclosure. By almost entirely eliminating internal supporting structures, the material costs of the system are significantly reduced. The dramatic simplification of the optics, and also the tracking system, enables the entire system to have a low weight with an area loading of less than 30kg/m2, making it suitable for rooftop installation. The sealed enclosure is also aesthetically designed for consumer appeal. The isolation of the system components from the external environment increases the lifetime of the system, and reduces or eliminates maintenance costs. The system construction can occur off site, with installation carried out in a similar way to installing flat plate PV and solar hot water systems.

One of the fundamental problems hampering the commercial development of low to medium concentration CPV systems is the lack of commercially available cells suitable for these concentrator applications. Hence, a strategic decision was taken that rather than using expensive cells specifically designed for operation under concentration, commercially available high efficiency one sun solar cells would be modified for use under concentration. A significant amount of development work has now enabled the delivery of modified one-sun cells that are suitably efficient at the expected concentration ratio of 15 to 30 suns.

These modified cells are integrated into a sub-receiver using a single substrate which incorporates structural support, heat sinking, electrical interconnection, and bypass protection. Hybrid receivers consist of a series of sub-receivers thermally bonded to the base of a channel in an aluminium extrusion which incorporates a cooling fluid channel along the rear of the extrusion. The cooling fluid is used to cool the cells, extracting the thermal energy for applications such as domestic water heating. The hybrid receivers are mounted at the line focus in the enclosure.

Spectral splitting

A limitation of many conventional CPV-T systems is that the desire to have the circulating fluid cool the cells conflicts with the desire to achieve a high temperature fluid. CSES is now working with UNSW, CSIRO, Chromasun, and NEP to develop a hybrid CPV-T system that can very efficiently deliver both 150˚C heat and solar electricity. The key technical goal of the project is to use spectral splitting to thermally decouple the solar cells from the 150˚C circulating fluid.

Silicon solar cells waste most of the energy of the solar spectrum. When illuminated by the whole solar spectrum, silicon linear concentrator cells have a solar conversion efficiency of 20%. However, when illuminated by monochromatic light of wavelength around 1100nm, the conversion efficiency of a silicon concentrator cell approaches 50%.

Schematic of spectral splitting.

The linear hybrid CPV-T receivers being developed will split sunlight into several bands based on wavelength. The near infrared light (700-1120nm) will be directed to PV cells. The cell efficiency when illuminated by this wavelength range is 35-40%. The balance of the solar power is converted to heat, and is used to pre-heat the thermal fluid. Since the conversion efficiency for this spectral range is high, the cells have substantially reduced cooling requirements. UV and visible light (<700nm) and FIR (far infrared) light (>1120nm) will be absorbed by a thermally insulated absorber. About two thirds of the solar power is in this wavelength range, allowing heating of the thermal fluid to 150 ˚C. Since the thermal and the PV absorbers are decoupled, the thermal fluid can reach high temperatures while the solar cells remain cool.

There are many possible spectral split options, utilizing selective absorption, reflection and refraction. The most promising configurations and designs are being selected based on a cost benefit analysis.

Combined heat and power solar (CHAPS) systems

High performance silicon solar cells designed for concentration applications.

Receivers are mounted at the line focus of the mirrors.

CSES, in collaboration with the Solar Thermal Group, have developed large-scale, linear CPV-T systems based on parabolic trough mirrors. Demonstration systems have been constructed at Rockingham in Perth and at ANU’s Bruce Hall.

High-performance silicon solar cells were designed and manufactured for systems operating in the range 10-60X. These cells operate at high efficiencies (20-24%) and can be obtained at moderate cost using an elegant process sequence. Cells are integrated into concentrator receivers which feature thermal cycling stress relief, bypass diodes, and encapsulation. The receiver can have either a light weight aluminium fin heatsink, or water cooling.

Parabolic trough mirrors.

Mirrors are constructed using an elegant glass-on-metal-laminate technology in which thin rear-silvered glass mirrors are bonded to a coated steel sheet. Stamped tab ribs are fitted to the end of the sheet to produce the correct parabolic profile. This construction produces a lightweight, durable mirror.