Solar Technologies

The NREL Chart shows many solar technologies, but when it comes down to what is actually installed, the options boil down to three types.

The chart below is a very rough estimate I calculated based on 2013 Electricity Generation numbers. I did some guestemation, so the data is not precise, but I believe it is close enough to tell the story on the breakdown.

Notice that we have three types of Solar technologies, and that the install base is split 50 / 50 between Residential/Commercial (i.e. Rooftop) and Utility (installation > 1 MW). Also notice the overwhelming majority of Solar is PV (Crystalline Silicon and Thin Film).

Solar Technology Breakdown (Estimates from 2013 Generation #s)^{[1, 2]}
T	c-Si PV	Thin Film PV	CSP	Total
Res/Comm	47%	5%	T	52%
Utility	38%	T	10%	48%
Total	85%	5%	10%	100%

Photovoltaic (PV) Options:

Crystalline Silicon (c-Si)

As we can see in the chart above, Crystalline Silicon makes up the large majority of the Solar energy install base. There are two types of Crystalline: Monocrystalline and Polycrystalline. Monocrystalline has had the larger install base until recently. Over the last 5 years, Polycrystalline is now more popular, and this is because Poly is cheaper to manufacture. In a monocrystalline structure a large crystal must be grown. Mono is a little more efficient, and handles low light situations better. Poly is less expensive. It's about a wash between the two in terms of overall cost when you include all the factors. However, Polycrystalline has become the more popular of the two in recent years.

Crystalline PV

Thin-Film

Thin Film was up to about 20% of the install base, but started to lose ground due to the popularity of Polycrystalline. Thin Film is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate such as glass, plastic, or metal. Thin Film is more flexible (literally) and less expensive than Crystalline Silicon, but is also a little less efficient. There are primarily three materials used for Thin Film: Cadmium Telluride (CdTe), Copper Indium Gallium (CIGS), and Amorphous Thin Film Silicon (a-SI, TF-Si). CdTe has been widely deployed in the recent Utility-Scale PV build-out (more on that later).

Thin Film PV

Solar Thermal Options

Concentrated Solar Power (CSP)

Concentrated Solar Power uses mirrors and optics to grab sunlight from a wide area and concentrate it into a small area. It captures the Sun's energy in the form of heat, and that heat drives a heat engine (usually a steam turbine) connected to an electrical power generator. I was not able to find definitive figures on efficiency, but it appears CSP efficiency ranges from 20% to 30%. CSP is almost exclusively the domain of the utility companies due to the cost and size of the installation.

Concentrated Solar Power

Next: U.S. Energy Consumption and Production >

Sidebar 3: Emerging Solar ^[3]

As the NREL Efficiency chart shows, there are many Solar technologies and several emerging technologies. However, as we've seen there are really only 3 that are mainstream: Crystalline Silicon, Thin Film, and Concentrator Solar Power (thermal).

It always boils down to cost/performance. Each technology has its strengths, and its weaknesses. The marketplace is the decider, and it has selected Crystalline Silicon, Thin Film, and CSP for now. We don't see emerging technologies because they have key limitations preventing them from moving forward in the marketplace, at this time.

For all, the name of the game is to generate the most electricity from the Sun's photons within the same given space. This essentially boils down to gathering more photons into a given space or generating more electricity with a given set number of photons, or both.

Let's look a little closer at some of the emerging technologies.

Thin Film CdTe: PV technology. Benefits are lower costs than Crystalline Silicon, and close in efficiency at 20% in the lab. Disadvantages include rarity of key material Tellurium (roughly as rare as Platinum).

Multi-Junction (MJ)In the How Solar Works section we showed that Silicon is the most widely used material for photovoltaics. We also learned that its band gap-energy limited the amount of photons that could knock off electrons and form an electrical current- limiting its efficiency at 30%.

What if we could stack multiple layers of different materials - each with different band-gap energies? We'd capture more photons or a wider range of the electromagnetic spectrum in a given space. This is the technology behind Multi-Junction.

Crystalline PV

Each layer in the above chart forms a different P-N junction, and each has a different band-gap. In this way, more of the Sun's energy is captured in a given space.

Concentrator Photovoltaic (CPV): A further development in MJ is to pair it with concentrator (optics) technology to increase the amount of sunlight shining on the MJ cells. There is roughly 330 MW of CPV installed worldwide. There are two CPV plants in the U.S.: Hatch (5 MW) and Cogentrix Alamosa (30 MW).

MJ advantages include very good efficiencies, up to 40%. Disadvantages include high manufacturing costs and poor performance in overcast conditions. They must also use a Sun Tracker to get maximum efficiency, which further increases costs.

Quantum Dot Cells:Different than PV. They are very small, close to atomic level, and they can tune band-gaps. Based on the size of the Quantum dot, they can produce different band-gap energies to capture more of the electromagnetic spectrum (similar to MJ). They are being discussed as a good fit in MJ cells for this reason. They could conceivably pass the 30% efficiency level, but they are very early days and not yet practical for the field use.

Dye-Sensitized Solar Cells (DSSC): Thin Film-like. Low-cost production, but expensive materials (platinum and ruthenium) and liquid electrolyte material not practical in all weather conditions. Less efficient than Thin Film, but cost/performance could make it interesting.

Organic Solar Cells: Some organic materials can absorb sunlight and produce electricity. These cells are easy to manufacture due to their solution-based processing. Advantages are low cost production, but disadvantages are degradation in the field.

Perovskite Solar Cell: Organic-Inorganic hybrid. Advantages are low cost to produce due to solution-based processing, lightweight, transparent, but disadvantage is degradation in the field due to water-solubility.

References