Solar Energy
Solar Photovoltaics (PVs) are arrays of cells containing that contain a material such as silicon that converts incoming solar radiation into electricity. Today, solar PVs are used in a number of applications from mediums scale utility level power generation to small scale residential roof top power generation (Trainer, 2013).
Concentrated Solar Power (CSP) systems use mirrors or reflective lenses to focus sunlight on a fluid to heat it to a high temperature. The heated fluid flows from the collector to a heat engine where a portion of that heat is converted into electricity. Some CSP systems can store heat for a longer period of time so that electricity can be produced at night (Jacobson and Delucchi, 2011).
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| Distribution of solar energy at the Earth's surface (Jacobson, 2010) |
The map above shows the average downward surface solar radiation at the Earth's surface. Globally, there would be 6500 TW of solar energy available from the world's land and ocean surfaces if all sunlight was used to power PVs; however, the deliverable solar power over land in locations where solar PV could practically be developed is about 340 TW.
CSP is estimated as being able to provide about 240 TW of the world's power output, which is less than PV as the land area required for CSP without storage is about one-third greater than is that for PV, however, the figure is still high. With thermal storage, however, the land area for CSP increases more as more solar collectors are needed to provide energy for storage, but energy output does not change and the energy can be used at night.
In cases where water-cooled CSP plants are being used the energy output may differ as plants can require water for cooling during operation (about 8 gal/kWh—much more than PVs and wind (∼0 gal/kWh), but the figure is still much less than nuclear and coal (∼40 gal/kWh) (Sovacool and Sovacool, 2009). Water-cooled plants therefore might be a constraint in some areas and air-cooled plants would have to be used. An advantage of this is that air-cooled CSP plants use >90% less water than water-cooled plants do, at the cost of only about 5% less electric power and with 2–9% higher electricity rates (USDOE, 2008b). As a result, air-cooled plants suggest a viable alternative to water-cooled plants for those constrained regions that may not have access to the water needed for cooling.
CSP is estimated as being able to provide about 240 TW of the world's power output, which is less than PV as the land area required for CSP without storage is about one-third greater than is that for PV, however, the figure is still high. With thermal storage, however, the land area for CSP increases more as more solar collectors are needed to provide energy for storage, but energy output does not change and the energy can be used at night.
In cases where water-cooled CSP plants are being used the energy output may differ as plants can require water for cooling during operation (about 8 gal/kWh—much more than PVs and wind (∼0 gal/kWh), but the figure is still much less than nuclear and coal (∼40 gal/kWh) (Sovacool and Sovacool, 2009). Water-cooled plants therefore might be a constraint in some areas and air-cooled plants would have to be used. An advantage of this is that air-cooled CSP plants use >90% less water than water-cooled plants do, at the cost of only about 5% less electric power and with 2–9% higher electricity rates (USDOE, 2008b). As a result, air-cooled plants suggest a viable alternative to water-cooled plants for those constrained regions that may not have access to the water needed for cooling.
Solar PVs use amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide, and other materials. As for the supply of material, the power production of silicon PV technologies is not limited by crystalline silicon however it is limited by reserves of silver which is used as an electrode (Feltrin and Freundlick, 2008) thus if silver as top electrode can be reduced in the future the supply of materials for solar PV systems will pose no problems for the development of a large global PV system.
Issues
One of the main issues facing the viability of solar (and wind) power today is whether these methods, that produce variable outputs, could provide reliable sources of electric power all day, every day, seasonally and yearly.
An electricity system is therefore expected to be able to respond to changes in demand in a matter of seconds. Addressing this therefore, our current electricity system uses 'automatic generation control' (AGC) to respond to variations in the order of seconds to minutes, it used spinning reserves to respond to variations of minutes to an hour and peak power generation to respond to hourly variations (DeCarolis and Keith, 2005; Kempton and Tornic 2005).
The main challenge facing the electricity system today is the daily and yearly variations in electric demand in comparison the the constant supply of nuclear, coal and geothermal energy. In addition to this, unexpected and extreme events can cause disruptions to plants - unplanned maintenance for example can shut down coal plants while extreme heat could cause nuclear plants to shut down.
The down time of current electric technologies and systems is therefore important - in America coal plants on average were shut down for 6.5% of the year for unscheduled maintenance and for 6% of the year for scheduled maintenance (North American Electric Reliability Corporation, 2009). In comparison to this, solar prove more beneficial as they have an average down time of 1% although some projects have had no downtime, while others have had 10% (Banke, 2010). Moreover, when solar (and wind) project are down only a small portion of electrical production is affected, whereas when a plant is down a large fraction of the electricity grid feels the impacts - if a centralised plant is affected the WHOLE grid can be down!
The main new issue facing solar (and wind) power is that the availability of energy at a single location varies and this variation does not match patterns of demand (North American Electric Reliability Corporation, 2009). As a result there are times when solar power systems can not meet demands and times when they exceed them and are left with a surplus of ultimately wasted energy. Having said that, there are other renewable technologies other than wind and power that do not vary as much. These other methods include tidal power, geothermal energy and hydroelectricity and will be the subject of my blog after solar and wind power have been discussed. In addition to this Delucchi and Jacobson (2010) put forward seven ways to design and operate renewable energy systems so that they can satisfy demands and minimise the production of energy that is not used.
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