Energy efficiency & green technology

It would only take 0.04% of the solar energy from the Sahara desert to cover the electricity demand of Europe.

Greenpeace International

The global raising of awareness regarding CO2 emissions following the COP21 agreements in Paris has created a strong push for “clean” or “green” sources of energy. Consequently, there has been a surge in implementing concentrator solar photovoltaic systems to generate free power from the sun by converting sunlight into electricity with zero emissions and no moving parts.

Concentrated photovoltaic (CPV) systems allow sunlight to be converted into electricity more efficiently than conventional flat-plate collectors can achieve. This is mainly due to the use of highly efficient multi-junction PV cells and to the increasing conversion yield of chips as a function of irradiation. In addition, CPV systems offer cost advantages over flat-plate collectors because the semiconductor area is reduced by the concentration factor of the lens, which is typically 500. However, the packaging of current commercial CPV systems is not yet mature. For example, current systems only collect electrical power and dissipate thermal power to the ambient surroundings, which causes a general problem of reduced efficiency due to high chip temperatures.

The use of optical concentrators to obtain high optical intensities means that the solar cells located at the focal point must be cooled actively

High-concentration photovoltaic (HCPV) systems based on triple-junction (3J) solar cells yield the highest light conversion efficiency. These cells are typically used in a configuration of a passively cooled cells with an area of <1 cm² and concentrations between 500x and 1000x. Higher concentrations can reduce the cost of CPV systems, but require active cooling unless the cell area is very small.

Despite the high electrical yield of 3J cells, about 70% of the collected energy is dissipated as waste heat. Using a low-thermal resistance-receiver allows the cell to be cooled with 80°C “hot” water while the cell temperature is maintained below 110°C. With such a system it is highly beneficial to collect low-grade heat and use this recovered heat in applications such as space heating, domestic hot water, air-conditioning, refrigeration, desalination, and Rankine cycle-mediated conversion to electrical energy [8]. This allows investments in active cooling technology to be exploited fully.

Other cost drivers for high-concentration, point-focus systems are the structure, tracking system, and optics. By carefully selecting processes and designs, we can manage these cost drivers to achieve competitive systems compared with non-concentrating PV in terms of electrical output and that also have a second energy output in the form of heat.

The goal of the “Sunflower” system is to achieve poly-generation of electricity together with process heat, desalinated water, or cooling, thus improving its competitiveness

A high-concentration photovoltaic thermal (HCPVT) system developed jointly with Airlight Energy is based on low-cost optical concentrator materials, a high-efficiency, densely packed multicell receiver array and a hierarchically stacked hot-water-cooling structure embedded in the receiver body.

The PV cells were cooled efficiently by minimizing the distance and number of interfaces between the photovoltaic cells and the cooling channels. Besides stabilizing PV-cell operation at higher temperatures, the low-grade heat generated by the hot-water cooling approach can be used further for such applications as desalination and thermally driven cooling.


 

HCPVT system with parabolic concentrator 3JPV cell array


HCPVT system with parabolic concentrator 3JPV cell array mounted on high-performance cooler for hot-water cooling. This allows the system to provide electrical power at high yield (>30%) as well as thermal energy (~50%) to drive an adsorption cooler or a thermal desalination process.


 

HCPVT receiver module

HCPVT receiver module with 3JPV cell array mounted on high performance cooler for hot water cooling. From [3].

HCPVT receiver module

Sunflower primary concentrator system with 40 m² collector area; 12 kW electrical and 22 kW thermal capacities expected. From [3].


 

Sunflower


Three stages of development for a high-concentration photovoltaic thermal (HCPVT) system.
A. Stage-1 prototype system with 4.3 m2 collector area and reduced active area due to shadowing by the mounting beams (yellow transparent area overlaying the dish mirrors).
B. Stage-2 early adopter with 40 m2 collector area and no shadowing thanks to central mounting of the receiver.
C. Stage-3 product-class system with 40 m2 collector area; 12 kW electrical and 22 kW thermal nameplate capacities expected. From [3].


Publications

[1] S. Paredes, P. Ruch, E. Lörtscher, F. Malnati, A. Pedretti, and B. Michel, “First Outdoor Results of the ‘Sunflower’ HCPVT System”, CPV 12, Frankfurt, Germany, April 25-27 (2016).

[2] S. Zimmermann, H. Helmers, M.K. Tiwari, S. Paredes, B. Michel, M. Wiesenfarth, A.W. Bett, D. Poulikakos,
A high-efficiency hybrid high-concentration photovoltaic system,” Int'l J. Heat and Mass Transf. 89, 514–521 (2015).

[3] S. Paredes, B.R. Burg, P. Ruch, E. Loertscher, F. Malnati, M. Schmitz, T. Cooper, M. Cucinelli, D. Bonfrate, A. Mocker, A. Bernard, A. Steinfeld, G. Ambrosetti, A. Pedretti, and B. Michel,
Receiver-Module-Integrated Thermal Management of High-Concentration Photovoltaic Thermal Systems,” IEEE PV Specialist Conference, New Orleans (2015).

[4] B. Burg, P. Ruch, S. Paredes, and B. Michel,
Placement and Efficiency Effects on Radiative Forcing of Solar Installations,” AIP Conf. Proc. 1679, 090001-9 (2015).

[5] B.R. Burg, A. Selviaridis, S. Paredes, and B. Michel,
Ecological and economical advantages of efficient solar systems,” AIP Conference Proceedings 1616, 317-320 (2014).

[6] V. Garcia-Heller, S. Paredes, C.L. Ong, P. Ruch, and B. Michel,
Exergoeconomic Analysis of High Concentration Photovoltaic Thermal Co-generation System for Space Cooling,” Renewable & Sustainable Energy Reviews 34, 8-19 (2014).

[7] S. Zimmermann, H. Helmers, M.K. Tiwari, S. Paredes, P. Neves, B. Michel, and D. Poulikakos,
HCPVT Receiver Package Using Advanced Liquid Cooling for High Energy Efficiencies,” AIP Conf. Proc. 1556, 248-254 (2013).

[8] C.L. Ong, W. Escher, S. Paredes, A.S.G. Khalil, and B. Michel,
A novel concept of energy reuse from high concentration photovoltaic thermal (HCPVT) system for desalination,” Desalination 295, 70-81 (2012).

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CTI project

High Concentration Photovoltaic Thermal System using Low-Cost Innovative Materials

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Scientists for A Smarter Planet: Bruno Michel