Thin Film Solar Cells

Space-based solar power (SBSP) systems involve the placement of large (multi-megawatt scale) photovoltaic (PV) arrays in geosynchronous (GEO) orbit where they can generate power and subsequently transmit this it back to the surface of the earth via microwaves. In contrast to terrestrial-based solar power generation, SBSP has its fundamental utility in having a capacity factor nearing 100% and relatively unlimited “real estate” for deployment. However, there are clearly significant challenges faced in deploying large PV arrays 22,000 miles in orbit – and this is where the design requirements for space-based PV technology diverge from that of conventional land-based systems.

PV technology was originally developed to power satellites over 50 years ago. Significant progress has been made over that timeframe in re-applying the technology to terrestrial power generation, initially in niche markets such as remote or off-grid power, and over the past decade to mainstream utility markets. The industry has now reached economies of volume in crystalline Si (c-Si) based PV, resulting in over 20 gigawatt/year of global manufacturing capacity and significant cost reductions at both the module and system level. Thin-film PV technologies, initially developed as a lower-cost alternative the c-Si technology, have also made significant strides with global manufacturing capacities over 3 GW/yr and cost at the module level of less than $1.00 per watt. The advances in manufacturing, and hence cost, performance and reliability, are largely due to the modular form-factor of PV and the ability to combine individual module units, crystalline or thin-film, into power delivery systems ranging from hundreds of watts to, potentially, hundreds of megawatts in single installations. Unfortunately, much of this legacy infrastructure and technology advancement cannot be applied to SBSP.

The economics of SBSP – i.e. the potential to reach a few cents per kilowatt-hour with nearly 100% availability – has four main components:

1. Getting the solar arrays into space (“Launch” costs)
2. Economically generating electricity from the Sun (“Solar” costs)
3. Transmitting the electricity back to the earth’s surface (“Transmission” costs)
4. Receiving, conversion, and re-distribution of the power to end-users (“Distribution” costs)

While today’s PV industry is dialed in to cost minimization strategies, achieving this within the construct of space deployment offers a whole new range of challenges. A few of the design and environmental criteria for the PV component of SSP systems include:

1. 

   ASTM G173-03 Reference Spectra

   Solar Insolation: In space, the magnitude (in W/m²) and spectral distribution (# of photons of a specific energy) of the Sun’s energy, referred to as Air Mass Zero (AM0), is different than here on the earth’s surface, referred to as Air Mass 1.5 (AM1.5). AM0 intensity is 1,366 W/m² vs 1,000 W/m² for AM1.5, and the AM0 spectral distribution is shifted to the higher energy (blue and ultra-violet) part of the spectrum. As such, solar cells may be designed differently for AM0 operation and will register different levels of performance (or “efficiency”) when deployed in each of these environments. }
2. 24 Hour Illumination: In GEO orbit, solar arrays will see the sun all but a few hours twice a year. This leads to a nearly 100% capacity factor – hence the attractiveness of SBSP - and very little thermal variation over the course of a day or year (though, they will ultimately reach higher operating temperatures). This is obviously different than terrestrial based PV which is considered to have, on average, a 15-25% capacity factor. The GEO orbit is also very different than the low-Earth orbit (LEO) at about 200 miles altitude employed by solar arrays used in conventional satellites. In these orbits, satellites will move in and out of the Earth’s shadow, relative to the Sun, 16 times a day. This produces extreme temperature variations as well as variable capacity factors in whatever service is being provided.
3. Space Environment: The space environment offers both advantages and disadvantages relative to terrestrial deployment. On the one hand, there is no weather – wind, rain, salt, hail – that requires protective packaging or gravity that requires substantial mechanical support structures. On the other hand, the PV arrays and other structures will be subject to significant amounts of radiation exposure in the form of electrons, protons, other charged particles and cosmic rays. Each of these forms of radiation will have a different effect on PV materials, their packaging and their ancillary electronic components. Protecting the active PV materials from radiation, if necessary, will add both cost and weight.
4. Mass / Weight: For terrestrial applications, we are most concerned with either $/Wp or ¢/kwh without primary regard to the weight of the system (note: the economic benefits of lighter weight systems for terrestrial PV will eventually come to bear, but it is presently of secondary concern). In SBSP, weight becomes a primary factor as it will directly impact Launch costs. For this consideration, the term Specific Power, specified in Watts per kilogram (W/kg), is brought into play. The higher the specific power, the better.
5. Power Density (aka – efficiency): PV technologies are typically rated by efficiency – power out (electrical) / power in (sunlight). Alternatively, performance can be rated as Watts per square meter of panel area (W/m²). In space, the output of a PV panel will not vary significantly over a day or years time due to a constant level of illumination, in contrast to day/night and Sun/cloud/seasonal variations for land-based PV. The additional advantage, as mentioned above, of 24 hr/day, 365 day/yr illumination is a roughly 5X improvement in the economics of delivered electricity (in ¢/kwh) relative to the deployed cost of the PV (in $/W). The improvement in kwh/kw for SBSP over terrestrial PV leads to some relaxation of cost, performance and/or lifetime requirements in order to achieve competitive electricity pricing.

Thin Film Photovoltaic Cells

With these environmental and design considerations in mind, we can draw a few preliminary conclusions:

1. Crystalline forms of PV, based on either c-Si or III-V compound semiconductors requiring radiation protective coverings, or thin-films deposited on glass substrates, will not achieve the specific power thresholds of ~1000W/kg required by SSP. Today’s traditional c-Si or thin-film module, based on glass packaging, deliver a specific power of 5-15 W/kg and would result in exorbitant Launch costs. Even the crystalline technologies that have been optimized for satellite power applications fall short at 300-400 W/kg at end-of-life.
2. Thin-film PV technologies, including amorphous Si (a-Si) and CIGS, fabricated on lightweight flexible substrates can achieve, and have demonstrated, specific powers at the cell level well above 1000 W/kg. They have also demonstrated excellent radiation tolerance with minimal or no protective coverings.
3. While plastic (organic) substrates provide the lightest weight option, their tolerance to radiation over 10-20 years may be a weakness.
4. Very thin (10-25µm) metallic substrates of radiation-hard metals such as titanium can achieve and exceed the required specific power targets.
5. Emerging alternative thin-film technologies may prove to be an effective alternative, but will require substantial development work and testing in the space environment.
6. Thin-film based arrays can achieve 1000 W/kg with sufficient optimization of both cell performance and materials utilization.

In summary, the technology exists today to provide photovoltaic power for SBSP at price, power, and weight performance factors that enable SBSP to effectively compete with terrestrial generation modalities.

Last Updated: July 9, 2011, 3:40 p.m.
Author: Dr. John R. Tuttle   Credit: Skypoint Solar, Inc.