Concentrating Photovoltaic (CPV) systems produce large amounts of waste heat in addition to electricity. This heat could be collected and delivered as an additional and valuable energy product (cogeneration). It could be used for water or space heating, for steam or process heat for industry, or converted to air conditioning using an absorption chiller. The overall efficiency of such a cogeneration plant could be more than 70% (compared to up to 25% when using only the electricity), and the value of the energy to the end user in this cogeneration mode could be approximately double that of the electricity alone. We have developed this concept and analysed its thermodynamic performance, and also its cost effectiveness. We have shown that using cogenerated heat for air conditioning leads to the lowest cost option for solar air conditioning. When using the heat for thermal desalination of seawater, the cost of the water could be lower than desalinating with reverse osmosis, and in some cases the desalinated water could actually cost nothing. We have also developed a technical demonstration of a small cogeneration CPV unit, designed for urban rooftops, providing distributed cogeneration to larger buildings with suitable rooftops.

Collaboration with partners from Italy, Germany and Spain

Support: European Commission, FP6

Solar thermal electricity generation involves the conversion of solar radiation into heat, and then the heat into electricity via a turbine. Conversion efficiency increases with temperature, provided that the high-temperature absorber does not lose the absorbed energy before it reaches the turbine. We are developing selective coatings for solar absorbers capable of efficiently absorbing solar radiation, while at the same time minimising the emission of thermal radiation to the environment. Such selective coatings exist for moderate temperatures of up to 500ºC, but advanced solar thermal plants with higher efficiencies will require similar capabilities at temperatures exceeding 600°C. These coatings are based on a “cermet” composite structure, a ceramic layer with embedded metallic nano-particles. Our research includes selecting the materials for both components of the composite, defining the size, density and arrangement of the particles, characterising the resulting optical, thermal and mechanical properties, and validating the stability of the coating at high operating temperatures.

Collaboration: D. Mandler, S. Magdassi (HU), D. Feuermann (BGU), B. Steinberg (TAU)

Support: Israel Ministry of Science

Contaminated drinking water is one of the main causes of disease and mortality in developing countries. Former Porter Fellow Dr Hadas Mamane, in collaboration with Professor Kribus, is developing a method for disinfecting large amounts of water using solar energy. This exciting new research has the potential of improving the health and quality of life for people in poor communities in third world countries who have little or no access to clean drinking water, and cannot afford modern water treatment technologies.

The ultra-violet (UV) part of solar radiation is known for its ability to disinfect water by destroying or inactivating microbial cells, but solar UV by itself is a very weak disinfectant. Disinfection by solar heating can be effective but requires high temperatures. A combination of both heat and UV can produce higher rates of disinfection than the sum of the two processes operating separately. However, the solar disinfection technology presently available, known as SODIS (solar water disinfection), can treat only small amounts of water at a time, and requires a long time of exposure to solar radiation (typically 6 hours or more).

A recent exciting discovery in Mamane and Kribus’ research is that certain additives can accelerate the disinfection process, reducing the time and lowering the needed temperature. These additives are abundant, inexpensive, and safe materials. The use of these additives, together with knowledge obtained in ongoing studies of the various process parameters, will enable the definition of optimum low-cost but highly effective solar disinfection technology. These advances in solar disinfection can be used in small scale (similar to SODIS) or in large community-scale water treatment systems.

The technology developed by Mamane and Kribus involves what Mamane calls a “synergy” of heat, solar radiation and a natural water additive. “The idea,” explains Mamane, “is to find a method of disinfecting water that is cheap and that takes advantage of the natural properties of solar heating.”

She adds, “Developing countries don’t have much money – but they do have sunlight!”

The common structure of PV cells produces low voltage and high current. This leads to high losses due both to current mismatch when cells are connected together in panels, and to electrical resistance, especially in concentrator systems where currents are high. The Vertical Multi-Junction (VMJ) approach on the other hand produces cells with high voltage and low current, thereby reducing electrical resistance losses (which depend on the current), and enabling a different configuration for connecting the cells (parallel instead of series) that minimises mismatch effects. We have shown that a VMJ cell in silicon can achieve significantly higher efficiencies than those previously demonstrated (close to 30%, compared to around 20% in previous work). The essential difference lies in the  design of the basic unit in the cell, the single junction, which needs to be much narrower than in previous designs in order to achieve optimal conversion efficiency. The optimised VMJ cells also operate efficiently at much higher concentrations than conventional cells made from the same material, for example at more than 1000 suns for silicon cells, while the performance of conventional silicon cells degrades at concentrations exceeding about 300. We are currently investigating different cell structure options, together with corresponding methods of cell manufacture, to find designs that would be both inexpensive and practical.

Collaboration: R Sarfaty (OBC)

Support: Israel Ministry of National Infrastructure

The optimal design of large solar fields (thermal and photovoltaic) will soon become a relevant and important issue for the penetration of renewable energy forms into the energy market. The optimal design is a non-linear multivariable, partial integer, optimization problem with an objective function, equality and inequality constraints and a set of variable limits. The design takes into account shadowing and masking effects. The field design parameters are the length and breadth of the solar field, the number of collector rows, the distance between the rows and the collector heights and inclination angles. The deployment of stationary and tracking collectors in a solar field were studied with the aim of obtaining maximum energy and minimum cost from the solar plant.