by Makari Krause
While fossil fuels will continue to meet the vast majority of the planets energy needs for the foreseeable future, renewable energy is the fastest growing source of electrical power in the world, increasing at a rate of 2.8 percent annually.[i] With the effects of green house gas emissions becoming more apparent on the planet, renewables will become more and more important in global energy generation. Solar energy is one of the most affordable, cleanest, and most secure of the renewables, and is at the forefront of renewable power. This post will give a brief analysis of the different types of solar power and their prospects for the future.
At the end of the day, almost all energy comes directly or indirectly from the sun with the exception of geothermal power. The sun’s energy evaporates water, which falls as rain and drives our hydropower plants. The sun’s energy also produces the wind that drives our wind turbines. Plants use the sun’s energy to grow thereby producing biofuels or decomposing and eventually forming fossil fuels. In these posts the two main types of direct solar power generation, solar photovoltaic and concentrating solar, will be discussed.
Solar Photovoltaic systems (PV) convert photons from the sun directly into electricity. First generation PV solar panels are made from silicon, which naturally creates an electrical charge when exposed to sunlight. A silicon solar panel, or module, is made up of a series of connected cells that can be one of three types: Monocrystalline, polycrystalline, or amorphous. Monocrystalline cells are cut from a single crystal of silicone and are the most expensive but also most efficient solar cells. Polycrystalline cells are cut from a block of silicone and consist of a multitude of different crystals, they are less efficient and less expensive. The cheapest and least efficient cells are amorphous cells, these are manufactured from a film of non crystalline silicone and unlike the first two forms, can be flexible.[ii]
Second-generation solar cells or thin film solar cells use much less material than first generation solar cells and can be produced at a much lower cost. These include amorphous silicon cells and cells made from non-silicon material such as cadmium telluride and copper iridium gallium diselenide. Their low production costs have made these types of cells popular recently but there are concerns about the toxicity and availability of some of the materials used in production.
Third generation solar cells are mostly in the development stage and are made from non-silicon materials.[iii] They have the potential of reaching much higher efficiencies than many of the cells currently being used.
Solar PV technology has skyrocketed over the past 50 years. Energy crises and the development of the space program have led to a lot of research and development, which accelerated advancements in PV. For a long time it was thought that the maximum conversion efficiency for solar PV was approximately 31%. With the development of third generation cells, however, it has become apparent that the attainable efficiency may be much greater. Other techniques can be combined with these new cells to further increase their efficiency. One example is the use of multi junction devices. These devices stack cells according to their bandgap, the photon energy that they obsorb. This allows cells to be positioned so that they are only absorbing photons of the energy that they can most efficiently process. This technique greatly broadens the spectrum of energy that can be converted. This and many other techniques are in development and have the potential to increase efficiency to the upper 60% mark. The combination of these techniques makes production of these advanced types of modules expensive but by concentrating the suns rays using a lens or mirror onto a small amount of PV material, the amount of energy that would normally be absorbed by a large panel can be absorbed by a much smaller panel thereby decreasing costs. Currently over 80% of the PV industry is based on silicon globally. Because of a predicted shortage in silicon this will have to change and the more efficient second and third generation cells will need to penetrate the market.
In traditional fossil fuel power plants, heat from combustion of the fuel is used to boil water and the produced steam is used to spin a turbine and produce electricity. The second form of solar power generation, concentrating solar power (CSP), uses a similar process to a fossil fuel plant but rather than burning fossil fuels, utilizes the suns energy to either directly or indirectly produce steam. There are four main CSP technologies:
Parabolic Trough Systems
A parabolic trough system is made up of a series of long concave mirrors that are angled towards the sun. The mirrors focus the suns rays onto a tube that sits at the focal point and runs the entire length of the mirror. Some type of liquid or salt flows through this tube, is superheated by the sun’s energy, and is then used to boil water. The produced steam is used to spin a turbine and generate electricity. Parabolic troughs have been in operation since the 1980s and are the most common CSP systems.
Fresnel Reflector System
The Fresnel reflector system has the same power generation process as the parabolic trough. It differs in the way it concentrates solar energy. Fresnel systems use a tube that runs above a series of long flat mirrors that are each angled to direct sunlight at the tube. Each of the mirrors can move independently which allows for better tracking of the sun throughout the day.
Dish Engine System
A dish engine system uses a mirrored dish to direct light at a thermal receiver located at the focal point. The thermal receiver converts the solar energy into thermal energy and, though the use of a device such as a sterling engine, uses it to turn a generator and produce electricity. Dish systems are the most efficient of the CSP technologies but are smaller scale than the other technologies and can be expensive at the utility level. The sterling engine directly generates electricity from thermal energy, thereby avoiding the process of producing steam to spin a turbine. As you will see later in the post, this turns out to be a downside of the dish systems.
The last form of concentrating solar systems is the power tower. The system utilizes a field of flat, sun-tracking mirrors to concentrate the sun’s energy on a thermal receiver at the top of a tower. The thermal receiver on the power tower converts solar energy into thermal energy, eventually boiling water to turn a generator and produce electricity.
In the past CSP systems have been cheaper than PV systems at the utility scale. There are many operational CSP systems in the US and Spain and the technology is gaining popularity globally even though it is still relatively unfamiliar to other countries. A number of new plants are currently being tested around the world and the success of these programs will likely determine their global desirability.
Due to decreasing costs, solar PV has recently become increasingly popular at the utility scale. PV is attractive because it can be deployed at a smaller scale and can produce electricity over a wider range of solar conditions. While CSP can only utilize direct sunlight, PV systems can use the sun’s diffuse energy and thereby produce electricity even when the sun is not directly shining. This becomes a major advantage in regions that experience periodic cloud cover. Due to this limitation the placement of a CSP plant is more important than that of a PV plant and can render some regions totally unsuitable. While PV is convenient in some situations, CSP remains popular because of its ability to combine energy storage with energy generation, a merit that will be addressed later on in this post.
While solar power generation still only makes up 1.13% of US electricity production, solar is growing at a blistering pace. In 2013 solar accounted for 29% of all new electricity generation in the US, making it the second largest source of new electricity behind natural gas.[iv] Half of that increase is from home and business owners who are installing small solar PV systems that are linked to the grid. The rest of the growth is in utility scale PV and CSP systems.
While utility scale solar is still not competitive with fossil fuel power generation, a study by the Stanford Graduate School of Business found that we are now at the point where commercial and residential scale PV power production has reached a price competitive level in certain conditions. The reason for this difference is that the comparative price of electricity in dispersed solar generation is the retail price while the comparative price at the utility level is the wholesale price. In order for solar to be competitive at the business and residential level, however, the business must have available roof space, it must be located in a sunny part of the country such as the Southwest, and it must be able to take advantage of federal tax breaks. While a subsidy is, unfortunately, still necessary to make solar competitive on this level, solar module costs have been steadily declining and if this trend continues over the next 10 years solar may be able to reach the point where it is cost competitive without subsidies. It is very important at this point that subsidies continue because the advancement of solar technology is largely driven by the high production volume that is currently being experienced. If subsidies were to disappear the market would shrink and with it research, development, and cost improvements.[v]
A Fully Renewable Energy Portfolio
It has never been thought that our energy portfolio could consist entirely of renewables because of the problem of intermittent supply and lack of storage. With recent technological advancements and an increased desire to limit GHG emissions the option of a 100% renewable energy portfolio is receiving more attention and seems to be more attainable. Fossil fuels are an amazing way to store energy; you can burn them when needed and quickly and easily meet the power load at any given time. With our modern technology and weather forecasting, however, power generation that relies on weather or the sun can be combined to meet load just as effectively as fossil fuel combustion. There are four main ways in which this can be achieved: Fossil fuel backup, storage, a diversified portfolio of renewables, and geographical expansion. [vi]
Because renewable energy such as wind and solar depend on weather, geographical expansion can result in a more consistent power supply and alleviate some of the intermittency. While localized weather patterns may inhibit solar production in a certain area, if plants are widely dispersed they will not all be shut down at one time. Diversification of the renewable portfolio can also help to provide a consistent power supply because different renewables can provide power at different times of the day, at different times of the year, and in different weather conditions. Storage is an easy way to quickly fill in the gaps resulting from intermittent power supply but is very expensive and, at least with currently available technology, you can’t store all of the power that you will need. Despite these downsides storage remains essential to quickly meet shortfalls in energy supply when other options are unavailable. The last option is to use fossil fuel power generation to back up the renewable power supply. Like storage, fossil fuel backup can quickly be used to alleviate shortages and has the added benefit of being cheap. This option reintroduces pollutants, however, and demands a nonrenewable resource. While it is not ideal, using fossil fuels for backup is extremely convenient and will be important in the transition. It is clear that none of these fixes are sufficient on their own but a combination of the four might be able to substantially level power production from renewables and allow demand to be met 24/7.[vii]
CSP, while still not competitive with fossil fuels on the utility scale, offers and easy way to incorporate a number of the abovementioned fixes. First and foremost is that many of these plants can be built to store generated electricity. Salt is becoming an increasingly popular capture mechanism for thermal energy in CSP and is beneficial because it maintains its liquid state at extremely high temperatures and can be used to store thermal energy until it is needed. Storage of superheated salt allows solar thermal plants to continue producing electricity even when there is no solar input. Absent major breakthroughs in energy storage technology, this method of storing energy gives solar thermal plants such as power towers, parabolic trough systems, and Fresnel reflector systems an advantage that Solar PV and dish systems lack. Additionally, solar thermal systems can incorporate other forms of electricity generation to augment the solar. A concentrating solar power system that produces steam to turn a turbine can easily be combined with fossil fuel power generation as they both act in the same way. In times of low solar power production, fossil fuel combustion can provide the thermal energy needed to operate the system.[viii]
The use of these technologies along with a geographically and technologically diversified portfolio of renewables may allow us to produce almost all of the energy that we need from renewables and, most importantly, have it be available to us when we need it. The system would have to be designed to meet demands even when power generation is at its lowest and therefore when generation is high there would be large excesses of power. This seems wasteful but almost all of the costs of renewable energy are capital costs and the fuel is completely free so producing excess power is not really a problem. This excess power could be put to other uses such as heating, which is currently not done with electricity because fossil fuels are so much cheaper and more convenient. [ix]
Negatives of Solar Power
While the benefits of solar power generation are not debatable, there are many environmental concerns associated with the wide scale use of solar. Because the sun’s energy is not highly concentrated when it reaches the earths surface, solar facilities must span large areas of land to produce the energy they require. Large utility scale PV facilities use 3.5-10 acres per megawatt hour and CSP facilities use 4-16.5 acres per megawatt depending on the intensity of the sun in the region, the technology used, and the topography of the site. This large land area coupled with the fact that solar facilities are high impact on the land raises concerns about land degradation and habitat loss. Land used for solar generation is also incompatible with other land uses such as agriculture or recreation, which may be able to share land with activities such as wind power generation.
The need for large tracts of land and strong, uninterrupted solar energy often directs solar facility development to deserts and other arid land, which are considered “low value.” Unfortunately many large CSP plants use substantial quantities of water for cooling and these locations are often short on water, causing a supply problem.
Smaller PV arrays are more environmentally favorable because they can be built on existing structures and have minimal impacts on land. Unfortunately there are some concerns about the toxicity of some of the chemicals used in the production of solar PV panels.
Solar prices have never been lower and are starting to reach grid parity in places all over the world, this has driven large increases in solar installation and led to 2013 being the largest year on record for solar PV. Fossil fuel prices are incredibly low at the moment but this may not last. If all of the societal and environmental costs associated with fossil fuel production and use are internalized, solar will become even more cost competitive. Additionally global fossil fuel prices will always fluctuate and with them electricity prices. Solar provides a way to stabilize the price of electricity forever. The need for secure, stable, and emission free power generation has never been clearer and it will become more and more essential over the coming decades. The near future looks bright for all forms of solar with many more planned installations. With the addition of renewable energy quotas and cuts to carbon emissions in the foreseeable future, renewable energy will need to make up a bigger share of our energy portfolio and solar will be largely responsible for that increase.
 “International Energy Outlook 2014.” U.S. Energy Information Administration (EIA). Energy Information Administration, 9 Sept. 2014. Web. 13 Sept. 2014.
 “The Different Types of PhotoVoltaic Panels.” Solar Facts. N.p., n.d. Web. 14 Sept. 2014. <http://www.solar-facts.com/panels/panel-types.php>.
 “3 Generations of Solar Cells: Solar Facts and Advice.” Solar Facts and Advice. Alchemie Limited Inc, n.d. Web. 14 Sept. 2014. <http://www.solar-facts-and-advice.com/solar-cells.html>.
 “Solar Industry Data.” Solar Energy Industries Association. Solar Energy Industries Association, n.d. Web. 14 Sept. 2014. <http://www.seia.org/research-resources/solar-industry-data>.
 “Solar Power’s Bright Future.” Stanford Graduate School of Business. Stanford Graduate School of Business, 6 June 2012. Web. 14 Sept. 2014. <http://www.gsb.stanford.edu/news/headlines/Reichelstein-solar-2012.html>.
 Budischak, C., Sewell, D., Thomson, H., Mach, L., Veron, D. E., & Kempton, W., 2013. Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the time. Journal of Power Sources, 225, 60-74.
[i] “International Energy Outlook 2014.” U.S. Energy Information Administration (EIA). Energy Information Administration, 9 Sept. 2014. Web. 13 Sept. 2014.
[ii] “The Different Types of PhotoVoltaic Panels.” Solar Facts. N.p., n.d. Web. 14 Sept. 2014. <http://www.solar-facts.com/panels/panel-types.php>.
[iii] “3 Generations of Solar Cells: Solar Facts and Advice.” Solar Facts and Advice. Alchemie Limited Inc, n.d. Web. 14 Sept. 2014. <http://www.solar-facts-and-advice.com/solar-cells.html>.
[iv] “Solar Industry Data.” Solar Energy Industries Association. Solar Energy Industries Association, n.d. Web. 14 Sept. 2014. <http://www.seia.org/research-resources/solar-industry-data>.
[v] “Solar Power’s Bright Future.” Stanfortd Graduate School of Business. Stanfortd Graduate School of Business, 6 June 2012. Web. 14 Sept. 2014. <http://www.gsb.stanford.edu/news/headlines/Reichelstein-solar-2012.html>.
[vi] Budischak, C., Sewell, D., Thomson, H., Mach, L., Veron, D. E., & Kempton, W., 2013. Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the time. Journal of Power Sources, 225, 60-74.