9. Energy resources - Future energy supplies 3: renewable energy resources Flashcards
(51 cards)
what are the properties of renewable energy resources?
- intermittency (not available all the time)
- difficult to predict how much energy can be harnessed
- low energy density
- cannot be stored unless they are converted into other forms of energy
- many modern day technologies have been developed to use fossil fuels and not renewable energy
- usually have low environmental impacts
- requires specific natural processes or geological positions
- size of available resource depends on the natural process that produces the energy
- many renewable technologies are not yet fully developed
- the cost of replacing existing equipment with new equipment for renewable energy is very high
what are the problems with solar power?
- intermittency: the availability and intensity of sunlight depends upon daily and seasonal cycles
- reliability: changes in energy intensity caused by changes in cloud cover cannot be predicted accurately
- energy density: the low energy density of solar power requires very large areas of solar collectors to harness significant amounts of energy. the angle of incidence also changes in daily and seasonal cycles so there is no single optimum position for a solar panel to harness solar power. this can reduce the energy density further
explain the local constraints of solar power
solar power can be used anywhere in the world but it is most viable where light levels are highest such as dry sunny deserts. Systems that concentrate using parabolic reflectors only work where there is no cloud so the rays of light are parallel and reflect onto the absorber. Areas that are very long away from the equator have long summer days which increases the availability of solar power but winter days are short and light levels are low.
What are the different ways to harness solar power?
- Photothermal solar power
- Passive solar architecture
- Photovoltaic (PV) solar power
- Heat pumps
What is photothermal solar power?
Photothermal systems absorb sunlight to produce heat, usually to heat water for low-temperature uses such as space heating or domestic hot water. The heat harnessed by the photothermal panels can be retained in a thermal store for later use. This is usually a well insulated tank containing material such as water, sand or concrete. Molten salt can be used if the energy has been concentrated to produce much higher temperatures.
What is passive solar architecture?
Buildings can be designed to maximise the absorption of sunlight for heating without the use of active working equipment. Overheating in the summer can be reduced with a fixed solar screen (brise soleil) that deflects the sunlight, adjustable screens or by ventilation.
What are heat pumps?
A heat pump uses the change in state of a fluid from liquid to gas to absorb the heat from the environment and releases it within a building when the gas condenses to a liquid. The change in state is caused by changes in pressure using a compressor pump to cause the gas to condense and a pressure relief valve to cause the liquid to boil. The heat source may be the atmosphere or the ground. Heat pumps absorb heat energy from low energy-density sources and produce higher temperatures in the building to be heated. The energy released into the building can be four times as much as the energy to run the heat pump, although less is released if the temperature of the heat source is lower. NB heat pumps can also harness other sources of heat such as geothermal energy.
What is photovoltaic (PV) solar power?
When a PV cell absorbs photons of light, electrons are dislodged from atoms in the upper layer of the PV cell. These will flow along an electrical conductor from this electrically negative layer to the relatively positive lower layer. The moving electrons provide the electric current to power electrical appliances.
What are the different types of PV cells and how efficient are they?
A wide variety of improved PV cells have been developed. Some have lower efficiencies but are cheaper to manufacture, while more expensive ones may have higher efficiencies at converting light to energy.
Multi junction - 46% max efficiency
Single junction gallium arsenide - 29.1% max efficiency
Crystalline silicon - 27.6% max. efficiency
Organic cells - 20.1%
Amorphous silicon - 13.6%
What are the environmental impacts of manufacturing solar panels?
Making solar panels requires the extraction and processing of materials such as metals, plastics, paints, and silicon. Making PV solar panels produces toxic wastes such as silicon tetrachloride and small amounts of cadmium. These can be controlled but this adds to the manufacturing costs.
What are the environmental impacts of solar power during use?
Solar panels do not require much maintenance, although cleaning requires water which may be scarce in the areas best suited for solar power. Large solar farms can occupy land that would have been used for other purposes, although there are very large areas of urban roof space that could have panels installed without any land use conflicts. In desert areas, they can make land productive although there may be environmental impacts on local flora and fauna.
What are the features of multi-junction photovoltaic cells?
Multiple layers made of different materials, each of which absorbs different wavelengths of light. This means that a greater amount of the light available is absorbed and converted into electricity.
What are the features of anti-reflective surfaces?
PV cells with smooth surfaces reflect about 30% of the light that hits them. Having a grooved or textured surface reflects light into the cells rather than away from them. Some designs mimic the structure of corneas of moth eyes which are very efficient at absorbing light.
What are the features of concentrating solar power (CSP) with thermal storage
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What are the features of photovoltaic/thermal hybrid systems (PVT systems)?
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What are the features of transparent PV cells?
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What are the features of heliostats?
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What are the features of self-cleaning panels?
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What is hydroelectric power (HEP)?
The gravitational potential energy of rainwater that lands on upland areas can be harnessed as kinetic energy as it flows downhill. The kinetic energy of flowing water has been used for thousands of years to provide mechanical power and, more recently, to generate electricity. Water wheels have been replaced by turbines which are more efficient.
What does practical exploitation of HEP depend on? (2)
- requires a large enough volume of water available
- suitable topography that creates high water pressures or high flow velocities
What does the ideal site for harnessing HEP include?
- large water catchment area
- high total rainfall, evenly distributed throughout the year
- low water turbidity
- impermeable bedrock beneath the reservoir
- low seismic activity
- suitable topography - narrow exit to a large basin
- no serious land-use conflicts
- close to consumers, or electricity grid, to reduce transport costs
What are the environmental impacts of reservoir creation for HEP?
- Dam construction requires large amounts of material such as rock, sand, gravel and cement. Extraction, processing and transport of the materials requires energy.
- access to roads may also be required
- the reservoir created by the dam will flood land which can cause the loss of wildlife habitats, farmland, and homes.
- the reservoir provides an area of static water in which suspends soils and dead organic matter carried by the river sink. Decomposition of DOM can produce anaerobic conditions and the release of methane gas.
- the reservoir can also have a positive impact on wildlife by providing a new habitat for aquatic wildlife.
What are the environmental impacts of HEP on the river?
- sedimentation in the reservoir can reduce turbidity downstream and the replenishment of nutrients onto the flood plain around the river.
- the natural flow fluctuations of the river are replaced by the water flow that is controlled by the power station operators. This may involve a constant flow rate to produce the maximum power output. The environmental impact of changes in flow rate depend upon the difference between the natural and new flow regimes.
How are HEP stations used to meet peak demands for electricity?
The reservoir stores surplus water when the flow rate of the river upstream is high. The reservoir is used to maintain water flow through the power station when river flow is lower. So, as a result, the water flow rate through the dam can can vary greatly.
