Optimizing CPV Systems
Concentrating Photo-Voltaic (CPV) systems have some competitive advantages, for example,
- CPV uses land efficiently, i.e., MW/acre is excellent for CPV
- CPV does well when ambient temperatures are very high, e.g. in excess of 110 degrees Fahrenheit (43°C).
- CPV needs no water to operate and very minimal water for maintenance work.
- CPV seems to have very minimal ecological impact – better than wind and other forms of solar power generation.
Thus CPV works well in deserts where there is a nearby connection to the grid, and where the regional electric company can take all the power that a CPV farm generates and can put up with no power at night and slightly irregular power due to occasional clouds. (Today CPV systems tend not to have energy storage mechanisms to smooth out the energy delivery to the grid.) This defines the CPV “niche.”
CPV systems have the disadvantage in that they are complex. Starting with three layer multi-junction photovoltaic cells, which are expensive in their own right and somewhat complicated to wire into arrays and panels, they need a Fresnel lens to concentrate sunlight onto (or more precisely into) these cells. They require that the sunlight rays be orthogonal to the plane of the panel, and this means that the panel must track the sun using a two axis system that adjusts both the azimuth and polar angles of the panel. Large panels need to be mounted relatively high so that the panel clears ground objects as it “tracks” during the day. A robust structure is needed to support the system weight and the forces of wind on the system. Depending on the locality, service roads and fencing might have to be installed. Finally each CPV structure needs an inverter to convert DC to AC, and a connection to the grid. The cost of each of these components is, of course, the first point of attack in optimizing CPV systems. These are the capital costs that need to be optimized.
There are of course non-trivial, non-capital installation, calibration, maintenance, and operational costs.
The capital, installation, and operational costs of a CPV system can be mitigated in the near future by a favorable regional government-dictated Feed-in Tariff (FiT), which guarantees a grid connection, a long term contract, and rates that take into account these costs and that guarantee an operational profit. Italy’s FiT is particularly advantageous, since it has a special tariff table for CPV.  Now if competition weren’t enough motivation for CPV vendors to drive costs down, most Feed-in Tariffs reduce the guaranteed rates by a small percentage each year to motivate the vendors to take advantage of technology improvements and reductions in operating costs.
As mentioned in an earlier post on Levelized Cost of Energy (= a system’s lifetime future costs divided by the lifetime energy it will produce) there are many components to lifetime costs, each of which needs to be consciously and systematically driven down by the CPV vendor. What is needed here is a company-specific model for these costs and a continuous improvement program to drive each cost component down. NREL’s System Advisor Model (SAM) is a very nice start (and its earlier versions motivated the definition of LCOE), but a company needs more detail to optimize costs.
For example, a CPV system is particularly sensitive to being aligned precisely (to a small fraction of a degree) to the sun. This necessitates high precision in its two axis tracking system. In addition to its additional capital cost, a tracker has additional installation and maintenance costs. Mounting a tracker on a high pedestal created additional weight and hence wind force factors. Each of these details (accuracy, capital cost, installation cost, maintenance cost, weight, and required structural support to address wind forces) must be optimized by the CPV vendor.
LCOE isn’t the only useful metric for a CPV vendor to consider. Now marketing folks and the press like big numbers, and hence the peak power or Watts-Peak (Wp) makes a lot of headlines. It is the maximum power that can be generated by the system. Sometimes this number is tempered (no pun intended) by NREL’s Standard Test Conditions (STC) to get a “name plate” rating. Probably a little more interesting is the Energy to Peak Power ratio. Define:
SI = Site Irradiation (kWh/m2/yr)
RI = Rating Irradiance (kWp/m2)
PCE = DC to AC power conversion efficiency
ATE = Average temperature efficiency
EPP = Energy to Peak Power Ratio = SI*PCE*ATE/RI
The Site Irradiation, SI, is really the actual energy produced by the CPV system per unit area, say over a year. Since the energy produced varies with time, this needs to be a sum over time-intervals small enough so that the energy produced in each time-interval is approximately constant. Similarly the ATE varies with time, since the device will heat up during the day causing increased degradation. One then uses time-intervals small enough so that the temperature during the time-interval is approximately constant. PCE is just the inverter efficiency. The EPP ratio is a little flawed in that the numerator has the factor of hours per year in it. A mathematician would divide it out, but the industry tends to leave it in. Oh well, …
While LCOE has most of the EPP terms in its ultimate calculation, it is none-the-less instructive to track this ratio as well as to optimize each term. Note that the Energy to Peak Power ratio is a function of locality as is LCOE.
The significant improvement in solar cell efficiencies has driven CPV’s LCOE down in past years due primarily to increased energy production. PV, on the other hand, has had its capital costs driven down dramatically by massive Chinese government investments. This has driven down fixed thin film PV’s LCOE. The net result of these two trends is to make thin film PV more attractive EXCEPT in the niche described earlier for CPV dominance. This race of technology and manufacturing improvements will, of course, continue. Unfortunately, the three major CPV vendors, Amonix, Concentrix, and SolFocus are all going after smaller numbers of large “utility sized” sites. This doesn’t lend itself to cost or manufacturing efficiencies as compared to the goal of putting thin film PV on every rooftop in the world. CPV vendors will need to invest to compensate for this. It will be a challenge.
 “LCOE For Concentrating Photovoltaics (CPV)” by Warren Nishikawa, Steve Horne, Jane Melia, firstname.lastname@example.org, SolFocus Inc., 510 Logue Ave., Mountain View, CA 94043 International Conference on Solar Concentrators for the Generation of Electricity (ICSC – 5), November 16‐19, 2008, Palm Desert, CA USA (www.icsc5.com )