Solar Engineering Applications

Silicon Solar Cells

Solar cells are two-terminal photovoltaic (PV) devices that convert sunlight directly into electricity.  The majority of solar cells used in presently deployed solar energy conversion systems are silicon cells, with the basic cell material being either thin-film amorphous silicon, polycrystalline silicon, or monocrystalline silicon.  A number of factors are considered in choosing which type of material to use in any particular application. Monocrystalline silicon cells are the most efficient, they have the longest productive lifetime, they have the best performance at high temperatures, and they have the lowest power degradation rates with respect to time. However, in many applications, amorphous or polycrystalline cells are chosen over monocrystalline cells because of their lower cost. In SEA systems, the cost of solar cells is a minor factor in determining overall system cost because the concentrating power of the parabolic trough reflectors reduces by a factor of 25 to 30 the area of cells needed to generate a given amount of electrical power. This reduces the cost of the cells to only 3 to 4 percent of what it would be in non-focusing systems with the same output power.  In SEA systems, where solar cell cost is not an important system consideration, monocrystalline cells are used because they are more efficient, because they have considerably longer life, because their power output degrades relatively slowly during their operational lifetime, and because their power output is greater at elevated temperatures.

Most solar cells in use today have one electrode attached to the front (sun-facing) surface and one electrode attached to the rear surface that faces away from the sun.  The front-surface electrodes for these cells are made in the form of a very thin metallic mesh in order to allow more incoming sunlight to reach the surface of the semiconductor.  However, cell efficiency is decreased by the high electrical resistance of the metalization comprising the mesh. So the size and spacing of the conductors comprising the mesh in the front-surface electrode are always an engineering compromise between the desire to make the wires large, which decreases electrode resistance, and the desire to make the wires small, which allows more sunlight to reach the semiconductor surface.  

Present solar cell designs have front surface electrodes that block about 10% of incoming light. These types of cells have electrodes with series resistance values that are acceptable for moderate levels of incident solar radiation, but when the cells are exposed to concentrated solar radiation—as in an SEA system—the front-surface-electrode resistance values are unacceptable because resistive losses are proportional to the square of the intensity of the incident radiation.  For instance, if a solar cell were exposed to solar intensity equivalent to 30 suns (a concentration ratio of 30) the current and power generated by the cell would both be increased by roughly a factor of 30 relative to one sun irradiance. However, the losses in the cell’s series resistance (I-squared-R losses) would increase by a factor of 30-squared, or by a factor of 900.

In order to avoid the large energy losses which occur when concentrated sunlight is incident on cells with front surface electrodes, SEA systems use monocrystalline silicon cells which have both electrodes on the rear surface (away from the incident sunlight). Back-contact cells were disclosed in US Patent 2780765 (February, 1957) but practical development came much later, with important contributions being made by Dr. Richard Swanson, a researcher at Stanford University who founded SunPower Corporation in 1985. Since its founding, SunPower has been a leader in developing practical, high-efficiency silicon solar cells. Of particular interest is their development of high efficiency, back-contact single-crystal cells (Swanson’s US Patent 7,468,485 and other related SunPower patents). Since the doping geometry in these cells is such that both electrodes can be located away from incoming sunlight, there is no need for a compromise between series resistance and shadowing losses.  In practice, SunPower’s back-contact electrodes are relatively wide and thick and this decreases the series resistance. Back contact cells have much lower series resistance, they are physically more robust, and 100% of the incident sunlight still reaches the front surface of the cells. That’s why back-contact cells are ideal for receiving concentrated solar radiation and converting it to electricity, and that’s why they’re an integral part of the SEA solar energy conversion system design.

Over the years, SunPower and other companies have made many innovative developments in the design of their cells and in the manufacturing processes used in fabricating them.  As of a few months ago, SunPower back-contact, single-crystal cells held the record for single-junction silicon cell efficiency. Because of the heavy back surface electrodes, SunPower cells are extremely rugged and they exhibit excellent heat transfer through the mounting surfaces at the rear of the cells.  Also, the special doping and passivation techniques used in manufacturing the cells provide long service life and very slow power degradation rates. SunPower cells have exhibited efficiencies of more than 25% in tests conducted by the National Renewable Energy Lab—and that’s getting pretty close to the theoretic maximum of 29% for single junction silicon cells.

The way SunPower cells are used in an SEA system eliminates many other problems that exist in other solar-cell-based systems.  One important advantage offered by the SEA system is that the solar cells always face in a downward direction to receive sunlight reflected from the system’s upward-facing parabolic trough reflectors.  The cells are mounted on the bottom surface of the cooling channels, which means that the channels protect the cells from bird droppings, dust, rain, hail, and wind-borne debris. These are all serious, even life-limiting problems for flat panel solar arrays that face in an upward direction.  

Also, since the cells in an SEA cogeneration system are mounted directly to cooling channels, the range of temperatures they experience is much less than for cells in systems with passive cooling. Repeated cycling between temperature extremes can significantly shorten the lifetime of solar cells and seriously degrade efficiency over time.  In an SEA system, the temperatures of the solar cells are always held within a specific range. The cells don’t get too hot or too cold. On warm sunny days, the coolant flow rate through the cooling channels is controlled to limit the maximum temperature of the cells. On cold winter nights, a slow flow of coolant through the cooling channels keeps the solar cell temperature above a specified value.

If you’re interested in learning more about photovoltaics, a great introduction to the field can be found at pveducation.org.  The site has material on subjects ranging from the properties of solar radiation to the fabrication of PN-junction devices.  There is particular emphasis on silicon solar cells, which, as mentioned above, are used almost exclusively in PV installations all around the world.