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In the last decades, the search for an economical and workable system, that efficiently turns solar radiation into electric energy, is leading the activities in the field of solar energy. Sunlight can be converted into electric energy using standard photovoltaic modules (PV modules) and concentrating solar power system (CSP system), which have been merged in the so-called concentrated photovoltaic system (CPV). |
However the issue about the integration of photovoltaic cells in concentrated solar power systems has not been overcome and the issue about the high temperatures reached is still pending. The E²PHEST²US project starts from this background and aims to design and realize innovative and scalable components for CSP system that:
- efficiently generate at the same time electric energy and heat power;
- reliably work at high temperatures (800-1000°C);
- recover and exploit heat at intermediate temperature.
An original conversion module for the production of electric and heat energy
will be developed based on direct thermionic and thermoelectric direct
converters, thermally combined in series to increase the efficiency
(thermal-to-electrical efficiency estimated at 35%). A heat recovery system will
be designed to collect the residual wasted heat (standard efficiency of 65%) and
provide additional energy product (co-generation). Innovative hybrid cable, able
to carry at the same time high-temperature fluids and electricity will be
designed and tested within the project lifetime.
The use of new materials will enable the exploitation of thermionic effect at
temperatures around 800-1000°C, enhancing the conversion performance of
traditional thermionic systems which work at high temperatures (> 1000-1500 °C).
On the other hand, at low temperatures (<500°C), thermoelectric devices perform
better than thermionic (h~8%, hot electrode at 500-600 °C). These complementary
properties will allow the development of an integrated system making the best
use of thermal energy generated at different temperatures and achieving a better
value of total efficiency. This complex multi-tasking module could be integrated
in different types of concentrating solar systems, since it shows high
flexibility and handling characteristics.
At the end of the project the realization of a small-scale prototype system will
allow the comparison with similar plants in terms of output power, production
cost, duration and reliability.
The main activities scheduled in the first period have been performed
satisfactorily, with good results. More specifically, the planned activities
implemented were:
- Study and basic research for the definition and treatments of specific materials, making up the conversion module and the process of thermionic emission.
- Study and modeling of the solar tracking and concentrating system, designed specifically for the construction of the solar platform
- Modeling and design of the hybrid cable for the simultaneous transportation of electricity and hot fluid.
At the end of the period, the construction of the solar test platform (STP), in
which the conversion module will then be housed, has been kicked off and
significant results have been achieved in the development of the conversion
module.
Concerning the Solar Test Platform (STP), the main activities carried out
focused on the study and the optimization of the parameters of the different
systems involved (optical, mechanical and tracking) directed to realize an
efficient concentrating solar system able to provide the conversion module with
a high amount of solar radiation power. The different components have been
designed according to the property features of the conversion module. The
different subsystems (mechanical, optical and electrical systems) as well as the
thermal circuit have been designed while the control system is close to its
completion.
The Solar Test Platform was completed by the end of August 2011.
Concerning the Conversion module (CM), the activities, that are nearly
completed, were mainly directed to the design and specification of it and to the
development and assembling of specific materials of which it is composed.
Specifically, the absorber materials (complex high temperature ceramic carbides
and nitrides) were selected as a function of mechanical stability, operating
temperatures, oxidation resistance, light absorbance, black-body emission,
electrical and thermal conductivity, compatibility with thermal and/or
electrical conversion stage. A special procedure for surface nano-structuring of
the solar receiver material has enabled a sensible increase of radiation
absorption. Moreover the definition and production of the proper materials, for
the thermionic conversion stage (polycrystalline n-doped diamond films Fig.6),
has been made as a function of electron emission efficiency, thermal insulation
capability and electrical properties of collector.
Commercial thermoelectric modules will be placed thermally in series with
thermionic stage. The development of new complex thermoelectric materials,
operating efficiently at high temperatures (~ 500 °C), to improve conversion
efficiency, is scheduled at the beginning of September.
The project final result will be an innovative-concept small-size CSP that can be used for distributed solar generation in urban areas, directly located at the end-user sites. This technology will include a combination of new concentrating solar system components, including optical reflecting lenses, solar receiver, solar energy converter and electrical connections. Simultaneous generation of electric energy and heat power can be obtained and exploited. The performance improvements compared to present CPV systems are obtained by extending the upper limit of temperature operating range, up to 800-1000 °C (prohibitive for PV technology) by the introduction of a new concept thermionic-thermoelectric module, able to convert directly energy into electricity.
From a technological point of view, the main impacts of em>E²PHEST²US project will be:
- development of innovative high temperature solar-radiation absorbing, thermionic and thermoelectric materials with possible benefits in other R&D fields, mainly high-temperature industrial applications;
- electrical conversion efficiency of the module potentially higher than standard PV semiconductors or equal to the multi-junction-based PV modules;
- high working temperature values and high conversion performance, unlike in semiconductor PV systems, severely damaged above 400 °C;
- easy-to-handle system, totally scalable in dimensions, modular, and integrated in different high efficiency solar concentrating apparatuses (dishes, Fresnel lenses, parabolic mirrors, etc.);
- a system easy to be installed on roof-tops or facades of rural isolated houses or on city buildings;
- a technology that can be potentially transferred to solar space applications, or to other thermal energy recovery applications for high temperature process industry, automotive, aerospace (furnaces, engines, etc.).
Generally, E²PHEST²US project is expected to generate also the following impacts:
- energy saving benefit from the reduction in consumption of fossil fuel, directly translated into monetary benefits;
- reduction of environmental pollution. This technology replaces oil consumption and contributes to reducing greenhouse gases and pollutant emissions, according to the EU policy towards the Kyoto protocols. Conventional energy generation and transmission methods can damage air, climate, water, land and wildlife landscape, as well as raise the levels of harmful radiation;
- creation of new jobs, mainly related to the construction and installation of the plants, in every building;
- at the end-user site, an additional benefit of reducing fuel transportation and electricity transmission losses;
- no land use will be required since the system will be added to the roof of the existing buildings. The solar elements can be used as architectural elements and are designed to fit closely to the existing roofline.