SOLAR INNOVATIONS

Solar panels have many pathways to continued improvement

THE SOLAR POTENTIAL

·       Solar panels don’t like heat.

·       Agrivoltaics saves space to grow crops and keep them cool and also cools the surrounding area which helps the solar panel become about 3% more efficient. Over time that’s a lot.

·       Flotovoltaics, on bodies of water that are typically cooler and therefore help more efficiency.

·       Direction of panel. Best head on so a fixed angle that gives the most sunlight has been common. Trackers are in two axes to follow the sun’s arc. The systems use 5 – 10T% of energy but gain 45% depending on geographical location. IN locations where seasonal variations in summer and winter are large these are very useful.

·       Aesthetics. Silicon atom is convenient to lose an electron but sunlight through windows would be  good too. Organic compounds using polymers or dyes can turn light into electricity too and be very small amounts of material. Can use infra-red light. Nice tinted window that lets through 43% of light. Even phone screens could generate power. Only convert about 13%. Easier than silicon to make. Easy to apply.

·       Solar fabrics, integrating solar cells into the fibres to generate power just by walking outside.

·       3mm x 1.5 mm solar panels, embedded into yarn and woven into clothing. 200 cells in a prototype charged a fitbit. Awnings and canopies textiles are good.

·       Solar thermal power. Solar thermal fuels absorb suns energy as chemical bonds and release it as heat later. It stores it well. Can store for up to two decades. Molecules release the stored energy as heat. You can heat a home on this. Emissions free and reusable.

 This solar blog talks about improvements to solar panels each of which can significantly add to power production. They cover two different technological domains and we believe they are additive and can operate together to obtain a 40% addition or more to the power output of a solar panel. Such a huge power addition would make it possible to have fewer panels for a given output or have a profitable experience with a lower PPA which are already in the unbelievably low $0.02+/kWh region in sunny areas. The consequences would be profound and so, important to discuss. This is why our discussion also goes over some brief history and characteristics of solar so that the innovations can then be seen in the context of the technology they are addressing.

 Solar History and Falling Costs

Solar panels have evolved rapidly since the phenomenon was first observed in 1839, 181 years ago, when Alexandre Becquerel saw the photovoltaic effect exhibited by sunlight on the element selenium. In 1888 Aleksandr Stoletov built the first actual solar cell, recognizing correctly that outer shell valency electrons were emitted when sunlight hit the surface of a photoelectric material. In 1904, just prior to the year of his famous paper on Relativity, Einstein published a paper that won him a Nobel Prize for Physics, concerning the photoelectric effect. Building on this intellectual progress, Bell Laboratories produced the first modern photovoltaic cell in 1954 which cost $250 per Watt at a time when coal fired electricity plants could produce a Watt of power for just $2.00. While solar was really expensive it was a boon to the space satellite industry, where a source of electric power to operate increasingly complex satellites in the cold war was essential and the relative cost was small.

Vanguard 1 was launched in 1958 using solar cells to back up its main power source. In 1967, Soyuz 1, a Soviet manned spacecraft, also used basic, but still expensive, solar cells to complement batteries as a power source. In 1973, the US launched the Skylab space station but, on the launch, some of the solar panels needed to power it were damaged. This unfortunately limited the duration of the mission to just 6 years before the craft was de-orbited in 1979. In the same year, Lockheed Martin installed a total of 700-kW basic silicon in two solar arrays in two Saudi villages near Riyadh, the largest solar array at the time, for $46 million, or $65 per watt. In 1979 President Jimmy Carter fitted 32 water-heating solar panels to the Whitehouse, which were taken down by a dismissive President Reagan in 1986.

Advances in the technology in the 70’s dropped prices from $100/Watt to just $20/Watt. This was accompanied by an increasing production volume. In 1983, 21.3 megawatts of solar panels were produced. In 1991 President George H.W. Bush created the National Renewable Energy Laboratory (NREL) responsible for accelerating the growing renewable energy industry. In 1999 there were 1,000 megawatts of solar cells produced. George W. Bush installed 9 kW of electric solar panels and some hot water ones in the Whitehouse grounds and President Obama had the state-of-the-art electrical solar panels installed on the White House roof in 2013, which will have paid for themselves by 2021.

How a Solar Cell Works

In an atomic structure, electrons circle the atomic nucleus in orbits. The outer orbit is called the conduction band while the inner orbits are called valency bands. Electrons can be freed from the atom or even change bands if they are pressured to do so. Even thermal energy can knock an electron from the conduction band of silicon. The 4 electrons in silicon normally don’t have much freedom of movement. If phosphorus, which has 5 valency electrons, is doped into the silicon, it is termed N-type silicon and this gives it free electrons. In sunlight these electrons will move randomly and can’t be collected. If you dope a lower layer of the silicon in the cell with boron atoms which have only 3 valency electrons, you get P-type silicon, which has too few electrons, also called holes. If you join N and P types together, electrons from the upper N type will migrate to the lower P side to fill the holes there. This leaves this connection region, termed the depletion region (DR), with no electrons This leaves the N side slightly negative and the P side slightly positive which sets up an electric field. Now, if light strikes the N side and penetrates to the DR it will generate electron/hole pairs in the DR. The electric field will drive the electrons into the N region and the holes into the P region resulting in a voltage between the two layers. Now these electrons can be collected by metal bus bars on the surface, and taken to an electrical load, and then they return to the P side and fill the holes. Over time, solar cell designers thinned out the upper N side of the cell to allow the sunlight to easily penetrate down to an also much wider DR zone. Performance is further enhanced if the P zone is only lightly doped and is much thicker. This means the electron/hole pairs are generated in a much wider DR, more current collected and more voltage generated.

Efficiency

The first Bell Labs solar cell was probably about 6% efficient and in 1959, Hoffman Electronics made a solar cell that was 10% efficient. Another way to look at this is that, at the time, solar panels lost 90% of the available energy. There was clearly room for improvement! Inefficiency had been noticed in systems before. Thermodynamic heat engines like steam engines had inefficiencies that were itemized by Nicolas Leonard Sadi Carnot in 1824 when he was just 27 years old! These included friction, heat loss, combustion heat loss, entropic irreversibility and design inefficiency. This is what prevented a coal fired power station from ever being more than 56% efficient and hitting an actual high efficiency level of 47%. In the same way, there are forces that mitigate against a 100% efficient solar cell. In 1960, William Shockley and Hans-Joachim Queisser produced a paper that showed how to assess the maximum theoretical efficiency of a solar cell known as the Shockley Queisser Limit (SQL). turns out, solar inefficiencies also include heat loss, termed black body radiation. Solar also suffers from recombination or the tendency of electrons to go back to the positive hole they came from and reflection and spectral losses which refers to the particular spectrum sensitivity of the light sensitive material, most commonly silicon. S & Q explained that, in unconcentrated sunlight with a single positive/negative junction cell, silicon had a theoretical maximum efficiency of 32% because its sensitivity to the electromagnetic spectrum, its bandgap, is limited. and the highest efficiency of any material possible was 33.7%, or 337 Watts/m2.

For a long time, visible light and heat were the only things known to come from the sun. Infra-red, ultra-violet, microwaves, radio waves and X rays were also discovered to be part of the continuum of wavelengths known as the electromagnetic spectrum when James Clark Maxwell combined them all with his amazing mathematical analysis of electric fields. It turned out that the observed photoelectric effect was shown to be different with different semiconductor materials. Silicon, the material most commonly used today, is sensitive to only red light. This is known as a band-gap. Other active photoelectric effect materials are sensitive to different parts of the electromagnetic spectrum (have different band-gaps) and include aluminum, phosphorus, indium, arsenic, gallium and antimony. Sometimes these elements can be placed next to each other using techniques such as organometallic vapor phase epitaxy (OMVPE). Recently one such “multi-junction” solar cell reached a single-sun record efficiency of 39.2% because it was able to release electrons from 5 different sunbathed elements, each of which has a different band-gap sensitivity. These solar cells are much more expensive however and earlier versions are the space solar panels that you see on the International Space Station (ISS).

The size of a solar panel for industrial or rooftop installation has evolved to just over 5 feet by 3 feet for a panel total of 17.3 square feet. Today (June 1st 2020), Sunpower, a leading manufacturer of top of the line, tier 1, monocrystalline silicon panels, are selling their Equinox 400 Watt solar panel for $2.90/Watt and if you fit the panels yourself, you can buy second hand panels at less than $0.50 per Watt.

The economics of this are astounding to someone set in their fossil fuel ways. If you drive the US average 13,500 miles a year and you own an electric vehicle which typically obtains 4 miles per kWh, then you only require 9.2 kWh daily for your car from a solar array. A 5-kw array insolated for only 2 hours a day will make this possible. In the UK there are only 1,800 hours of sunshine per year which averages out to just under 5 hours per day. Two brands of European car already are covered in solar panels which results in the battery charging enough to cover this average distance driven without any charging at all! This means you could fuel your vehicle, conveniently from home on solar power for a panel cost of less than $5,000. (including the balance of plant for a solar installation is for the invertor, batteries, labor and grid connection). A typical home consumes about 25 kWh a day, something well within reach for the right solar array and budget.

This is so interesting that scientists have continually tried new methods for capturing the Sun’s energy in one of humanity’s ongoing revolutions in energy technology. Above the Earth’s atmosphere, the sunlight energy, as measured by satellites has 1,361 Watts per square meter. At the Earth’s surface it is absorption by clouds, atmospheric gases and by the surface of the Earth and finally represents an average of 1,000 Watts/m2. Currently the best, but also most expensive, technology therefore can get 470 Watts/m2. The optimal commercial technology will get over 20% efficiency. If the average house roof in the US is 2,400 square feet, then there are

The National Renewable Energy Laboratory (NREL) regularly update a great chart[i] which shows the gains in efficiency over time of various solar technology approaches including concentrated photovoltaic where lenses of different  sorts are used to concentrate the sun on a small portion of photovoltaic material, multijunction PV where the materials sensitive to different parts of the electromagnetic spectrum capture sunlight increasing the energy converted to electricity, silicon the basic version of solar that comes in less efficient multi-crystalline and more efficient mono-crystalline cells, thin film, a term that covers several different types of photosensitive materials deposited in very thin layers and some emerging PV technologies which include Dye sensitized, Perovskite, Organic and Quantum dot approaches. At the top of the range can be seen the 4-junction concentrated PV solar cells which come in at 47.1%. Normally silicon is only sensitive to a narrow section of the electromagnetic spectrum, between

  The two major solar panel improvements that are coming are worthy of a forward-looking blog. One is capturing much of the energy from UV and transferring it to red, the color area on the electromagnetic spectrum that is already absorbed by silicon, so it gets the original power plus a UV bonus. This is pre commercial but works and takes the form of a cheap polymer layer that fits over the panel.

The second innovation, SEC Optics, again pre-commercial, is the use of refraction in optical plastic, again taking the form of a polymer layer with line ridges that refract the sun’s light when it’s low in the sky, in the morning and afternoon. Once the angles to the sun of a given solar array are allowed for, it too causes the bell-shaped power production curve to become an “m” shaped curve, meaning a lot more power during insolation.

These two products could be combined, but for now work as separate layers to augment power output not by just 2%-5% but by considerably more.

Sundensity, Inc., led by Dr. Nish Sonwalkar, (ScD, MIT) is able to take some of the large quantum of solar energy in the ultra-violet bandgap and releases this extra energy in the existing silicon infra-red bandgap. This increases electricity output by 20% making a 300W solar panel into a 360W panel. 8 layers of metal oxides and nano particles are sputtered onto a glass substrate. The layer side is mounted close to the solar cells for protection from the elements. Removing UV also cools the panel, extends its life and confers higher efficiency and reduces costs of installing a given capacity of solar. Enhances energy output of solar panels up to 20%

For a 300 Wdc Panel Creating a Value of $21 (60Wdc) @ 0.35/Wdc at a cost of $3 per panel.

The Photonic Smart Coating (PSCTM) provides more energy at the optimum wavelength for the device.

The layer has metal oxides, nano particles and a third layer with other metal oxides. This layer converts the energy in UV and visible wavelengths to wavelengths that the silicon cell can use, increasing power output by 20%. Tested successfully by the Frauenhofer Institute in Germany. A 60 Watt gain for a $0.35 per Watt cost.

[i] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200406.pdf