Urban photovoltaics: polygeneration with solar energy

The cost of the electricity produced by photovoltaic technology is higher than that from conventional fuels. This is a major obstacle for sustained long-term growth of solar technologies, and currently requires massive governmental support to create artificial markets for solar electricity. Two reasons contribute to this high cost: the need to use large amounts of expensive semiconductor material, and the low conversion efficiency.

A combination of two innovative approaches can achieve a synergy that addresses both
the abovementioned issues: Concentrating Photovoltaics (CPV) and Cogeneration. The concentration reduces the area of expensive cells. Cogeneration collects the thermal energy generated in the cells in addition to the electrical energy, resulting in overall efficiency that can reach 75 %. The combination of both approaches is a CPV/Thermal (CPVT) solar collector
system. Such unique collectors will be demonstrated for the first time in a commercial scale installation. An important requirement for CPVT systems is the location of the solar collectors as collectors have to be close to the end-user. The Di.S.P. and SHAP collectors are small enough to integrate into an urban environment, for example on a building rooftop.

CPVT systems are suitable for a wide range of thermal applications, including absorption cooling and air-conditioning, steam production, desalination, and industrial process heat. Such systems can satisfy the energy needs of many users in the urban areas of sunbelt countries, greatly reducing the peak load of the electrical grid during summer, and displacing large amounts of conventional fuels.
Another advantage is the fact that the solar energy replaces conventional energy bought at retail cost, which is much higher than the production cost to the utility. Therefore, the same solar technology may be non-competitive at the utility’s power station, but competitive at the end-user site. If the end-user enjoys government incentives given to renewable energy systems, the system will be even more competitive, accelerating the pace of public adoption of renewable energy.

Approach

Project participants have developed CPVT collector technology for urban applications under national and bilateral programmes. The collectors are small to permit easy integration and installation in buildings. Concentration is by a factor of a few hundred. Triple-junction cells with nominal efficiency of 35 % are used, to obtain the highest possible electrical conversion efficiency, and also since this type of cell is capable of operating under higher temperatures relative to silicon cells. Two collector versions with different geometries have been developed
for applications in various types of building – a roof-integrated stationary collector, and a freestanding tracking parabolic dish collector.

In UPP-Sol, the innovative CPVT collector technology will be integrated for the first time into a fully functional system at a commercial scale. Two complete demonstration systems will be built and operated, in typical buildings located at Colleferro (Italy) and Valladolid (Spain). The two plants will include conversion of the thermal energy into air conditioning by absorption chillers to demonstrate the use of the thermal energy throughout the year, for cooling and
heating as needed. Plant integration and control including all auxiliary equipment for the electrical and thermal parts, is developed by project partners. The project will also include testing of triple junction cells with comparison with current state of the art cells manufactured in the USA.

Results

Work has been conducted on improvement of collector technology, resulting in enhanced collector design that is easier and less expensive to manufacture, while improving performance relative to the prototype versions. The design and optimization of the complete plants is under
way. Simulation and optimization software was developed in order to predict and analyze the plant’s performance, and to serve as a design tool for future commercial projects.

Future prospects

The project expects to deliver a first demonstration of a full CPVT plant operating under realistic field conditions, showing the capability of producing simultaneously electricity, air conditioning and heat. Such a demonstration will allow the assessment of this new technology and the validation of its benefits, from both the economics and energy efficiency point of view. Following a successful demonstration and operation of the two demonstration plants, the project partners will be able to widely disseminate and implement the CPVT technology. Italy and Spain are two relevant markets for CPVT systems and offer a wide range of possible applications and market niches where such technology can offer superior benefits relative to other solar technologies,
and even compete against conventional energy supplies. Following the home markets, the CPVT systems can also be exported and implemented in many other sunbelt countries.

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