The demand for bringing more renewable energy power-generation capability online is enormous, both in the U.S. and internationally. Replacing the need for foreign oil imports is a growing national defense need; the U.S. Department of Defense has directed base commanders to reduce, and eventually eliminate, their dependence on foreign oil imports and to use renewable energy. At the same time, most utilities across the U.S. are required to meet state-mandated, renewable energy, electric power generation goals. In Arizona, for example, the renewable energy portfolio must be 15 percent of all energy supplied by 2025.
Today, the technology is available to allow solar energy to compete with coal, natural gas and nuclear power. Research is underway at Raytheon to develop a highly efficient, cost-competitive solar power conversion unit (PCU) for military and commercial applications — one that utilizes "photon recycling" within a photovoltaic cavity converter (PVCC) (Figure 1).
The development of a high concentration photovoltaic (HCPV) solar energy power conversion system shows great promise. By collocating 2,304 state-of-the-art triple junction photovoltaic (PV) cells within an enclosure, and recycling reflected photons within the enclosure, we can increase the direct conversion efficiency of sunlight to electricity at the system level by more than 25 percent over existing triple junction PV cell designs. The compact arrangement of all the triple junction cells on a flat surface, which measures less than 0.5 square meters, enables the use of highly automated fabrication and assembly techniques to manufacture the PVCC, resulting in a design that is very cost-competitive with fossil fuel-generated electricity. Locating all 2,304 PV cells on a small, compact flat panel array, compared with distributing the same number of cells over the entire solar collection surface (in this case 120 square meters), will significantly reduce the manufacturing and maintenance costs of the system.
Raytheon, working with United Innovations (UI), a small business located in San Diego, Calif., has developed an innovative solar energy PCU, using a kaleidoscope PVCC receiver. The University of Arizona joined the Raytheon-UI team to design, develop and fabricate the required large parabolic dish reflector, a critical component of the PCU. This partnership also includes the California Energy Commission and Science Foundation Arizona.
The PCU for both military and utility applications will consist of a large parabolic dish reflector, 12.5 meters in diameter (120 square-meter dish planar area), with two-axis sun tracking. This dish will collect approximately 120 kilowatts of sunlight, with the PCU converting that input solar flux to approximately 40 kilowatts of electric output (approximately 33 percent system efficiency). By concentrating the sunlight at 500 suns (500 times the normal intensity of the earth's sunlight) on 36 identical sub-panels that contain a total of 2,304 cells within the PVCC, only one five hundredth of the number of PV cells will be required, compared with more conventional flat panel applications using these same PV cells operating at one sun.
The system efficiencies of today's HCPV and thermal-concentrating solar power systems are approximately 25 percent and 22 percent, respectively. Raytheon's HCPV system with photon recycling is expected to yield system efficiencies greater than 33 percent, which will represent approximately 30 percent and 50 percent relative efficiency improvements, respectively, over current HCPV and solar thermal systems.
Combining Raytheon's highly reliable defense technology with high-rate production processes from the automotive and commercial electronics industries will enable the PCU to meet the demanding goals set by the U.S. Department of Energy. The primary goal of DOE is to achieve a levelized cost of electricity (LCOE) of 5 to 7 cents per kilowatt hour in fiscal year 2005 dollars by developing power systems that can be manufactured and installed for less than $3 per watt. Working with the potential supply base, design to unit production cost (DTUPC) goals were set, resulting in a unit PCU cost of less than $2 per watt installed. The DOE validated Raytheon's LCOE forecast of less than 6 cents per kilowatt hour using its Solar Advisory Model for the established DTUPC cost targets.
The basic approach Raytheon selected to convert solar energy into electric power was HCPV, which utilizes Raytheon's patent-pending, kaleidoscope photon recycling concept, where sunlight is concentrated using a large parabolic dish reflector (12.5 meters in diameter) and guided into a closed photon-to-electricity cavity converter. The photons trapped in this converter, initially reflected from the PV cell array, are provided more than one opportunity to strike the active portion of the multi-junction cells as they are reflected around the inside of the PVCC. Other flat panel array designs offer collected photons only one opportunity to be absorbed by the multi-junction PV cells, since reflected photons from these other arrays are immediately lost. Commercially available Emcore triple-junction cells were used in making the PVCC. These off-the-shelf cells consist of indium gallium phosphide (InGaP with a 300-650 nanometers wavelength absorption band), indium gallium arsenide (InGaAs with a 650-850 nanometers absorption band), and germanium (Ge with an 850-1,800 nanometers absorption band), which convert the absorbed photons in each layer to electrons, collectively covering the solar spectrum from 300 to 1,800 nanometers. The kaleidoscope design was carefully sized using ray tracing models to achieve a plus-or-minus 5 percent variation in solar flux density across the PV cell array located on the back wall of the PVCC. A uniform flux density across the cells is required since these PV cells are connected in series and the power output from the array is limited by the cell with the smallest output.
By focusing on DTUPC objectives from the outset, a PCU design concept was developed that will result in solar electric power costs that meet DOE's solar energy LCOE goals and state-defined renewable energy portfolio standards for the next five to 15 years. A related DOE objective is to increase the capacity of photovoltaic solar power generating equipment in the U.S. to 5 to 10 gigawatts by 2015 — an aggressive but achievable goal.
The collaborative project test objectives were to demonstrate:
- 1. The workability of photon recycling.
- 2. A dramatic increase in photon to electricity conversion via recycling.
- 3. The ability of University of Arizona mirror technology to focus the sunlight collected by a 2.5-meter diameter reflective dish into a focal area less than 1.5 inches in diameter.
- 4. The ability of a closed-loop cooling system to maintain the array of PV cells at less than 50 degrees Celsius.
A demonstration unit was built and tested with support provided by Tucson Embedded Systems, a Raytheon small-business partner. The sub-scale PCU consisted of a single sub-panel containing an array of 64 triple junction PV cells (an 8 x 8 array) and a parabolic dish reflector 2.5 meters in diameter with a focal length to diameter ratio (f/D) of 0.6. (See Figure 2.)
The ongoing test program has been highly successful; the first three objectives have been fully met. Photon recycling worked and resulted in a 33 to 54 percent relative improvement in conversion efficiency (recycling versus no recycling for the same flat panel array) for tests conducted, ranging from 10 to 500 suns. Figure 3 shows the demonstration PVCC containing the 64 triple junction PV cell array. The 2.5-meter University of Arizona mirror, which consists of 21 mirror segments (Figure 4), concentrated the sunlight into a focal area of less than 1 inch in diameter. Although objective No. 4 has not been fully met, temperatures were controlled to less than 50 degrees Celsius at 400 suns concentration, and temperatures less than 60 degrees Celsius have been demonstrated for steady-state operation at 500 suns concentration. Work continues on refining the heat transfer design to fully meet the objective of maintaining the PV cell array less than 50 degrees Celsius during full sun concentration.
The next steps in the team's development effort are:
- 1. Demonstrate that by reducing the resistivity of the electrical grid fingers on the surface of the PV cells (by increasing their number by approximately 40 percent), system efficiency can be increased by an additional few percent absolute (even though photon reflections from the surface of the PV cells will be increased), which is achievable with this design, given the unique ability to recycle reflected photons.
- 2. Develop a full-size, 40-kilowatt prototype design that meets the DTUPC goal of less than $2 per watt installed with an LCOE of less than 6 cents per kilowatt hour.
- 3. Develop a smaller scale, mobile PCU that could be used for U.S. Dept. of Defense Forward Operating Base Field applications.
As more advanced multi-junction PV cells are developed — progressing from today's 39 percent efficient triple junction cells to a target of 58 percent efficient six junction cells — system efficiencies using our photon recycling concept should increase from 33 to 40 percent, with the successful development of 45 percent efficient four- and five-junction cells. The ultimate goal is 50 percent system efficiency using six-junction PV cells, as they become commercially available. These advanced PV cells can easily be integrated into Raytheon's PVCC, enabling achievement of these increased system efficiencies. But even with these breakthroughs, over 50 percent of the energy will be lost, mainly through dissipated heat. To convert more of the available solar energy to electricity, Raytheon is investigating the use of thermoelectric devices to complement the PVCC PCU concept. In addition, the University of Arizona is developing concepts that can use the low-energy content, warm water (possibly in the range of 50 degrees Celsius) from our closed-loop cooling system to purify brackish water and even desalinize sea water into drinking water, which would further increase the cost effectiveness of this solar energy concept.
The major benefits of this unique solar energy power conversion system are that it:
- Results in higher overall system conversion efficiency, compared with non-photon recycling HCPV systems.
- Eliminates the use of boilers to generate steam and large turbines to generate electricity, resulting in far fewer moving parts than solar thermal systems, significantly reducing maintenance costs and increasing system reliability.
- Dramatically reduces the use of water for cooling and cleaning of mirrors, which is critical when operating in the arid desert Southwest.
- Eliminates emissions of carbon dioxide from the power generation process.
Solar energy is rapidly becoming cost-competitive with fossil fuel power plants, in particular coal-burning plants. In 1990, solar energy power generation costs were in the 55 to 65 cents per kilowatt hour range, and today the cost has dropped below 11 to 13 cents per kilowatt hour — the price range for many of today's typical utility power purchase agreement contracts. An HCPV solar electric power plant of 240 megawatts (typical power plant size) would consist of approximately 6,000 solar electric PCUs of 40 kilowatts each. Today such a power plant could support a minimum of 60,000 homes. At rate production and a price of $2 per watt installed, a contract to supply and install the PCUs for a 240-megawatt plant would be about $500 million.
John P. Waszczak, Steven L. Allen