AY Honors/Renewable Energy/Answer Key
Renewable energy is any type of energy which has an abundant and ongoing source, such as the sun, wind, waves, rivers, tides and the heat from radioactive decay in the earth's core as well as biomass. Renewable energy does not include energy sources which are dependent upon a limited resource, such as fossil fuels and nuclear power.
Renewable energy may be used directly (as in solar ovens, geothermal heat pumps, and wind generators) or be used to generate electricity. Only the power of falling water in rivers (hydroelectricity), has been significantly tapped for the generation of electricity so far. Solar energy's main human application has been in agriculture and forestry, via photosynthesis, and increasingly it is harnessed for heat. Biomass (eg sugar cane residue) is burned where it can be utilised. The others are little used today.
Most renewable energy can trace their root to solar energy, perhaps with the exception of geothermal and tidal wave power. For example, wind is caused by the density difference of the air when the sun heats various parts of the earth unevenly. Hydroelectric power can be ultimately traced to the sun too. When the sun evaporates water in the ocean, the vapor forms clouds which later falls on mountains as rain which runs through turbines to generate electrity. The transformation goes from solar energy to potential energy to kinetic energy to electric energy.
Turning to the use of renewable energy sources for electricity, there are immediate challenges in actually harnessing them. Apart from photovoltaic (PV) systems, the question is how to make them turn dynamos to generate the electricity. If it is heat which is harnessed, this is via a steam generating system.
If the fundamental opportunity of renewables is their abundance and relatively widespread occurrence, the fundamental problem, especially for electricity supply, is their variable and diffuse nature (The exception is geothermal, which is not widely accessible). This means either that there must be reliable duplicate sources of electricity, or some means of electricity storage on a large scale. Apart from pumped-storage hydro systems, no such means exist at present and nor are any in sight. For a stand-alone system the energy storage problem remains paramount. If linking to a grid, the question of duplicate sources arises. For large-scale and especially base-load electricity generation there is little scope for harnessing the sun.
Renewable energy sources have a completely different set of environmental costs and benefits than fossil fuel or nuclear power plants. On the positive side they emit no carbon dioxide or other air pollutants (beyond some decay products from new hydroelectric reservoirs), but because they are harnessing relatively low-intensity energy, their 'footprint' - the area taken up by them - is necessarily much larger. Whether large areas near cities dedicated to solar collectors will be acceptable, if such proposals are ever made, remains to be seen. Beyond utilising roofs, 1000 MWe of solar capacity would require at least 20 square kilometres of collectors, shading a lot of country. In Europe, wind turbines have not endeared themselves to neighbours on aesthetic, noise or nature conservation grounds, and this has arrested their deployment in UK. However, European non-fossil fuel obligations have led to increases in offshore wind development. However, much of the environmental impact can be reduced. Fixed solar collectors can double as noise barriers along highways, roof-tops are available already, and there are places where wind turbines would not obtrude unduly.
Types of renewable energy
SOLAR ENERGY
"Solar not nuclear" is a catch-cry of both antinuclear environmental groups and many technological optimists, particularly as advances in direct solar heating continue to be made. Certainly we can expect to see more roof area occupied by some kind of solar collectors in the future, as their price comes down and we adapt our energy usage to utilise better what is available from this source. However, for electricity generation solar power has limited potential, as it is too diffuse and too intermittent. First, solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 15%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (12 - 16 percent), though this has been the subject of much research over several decades.
Two methods of converting the sun's radiant energy to electricity are the focus of attention. The better known method utilises sunlight acting on photovoltaic (PV) cells to produce electricity. This has application on satellites and for certain earthbound signalling and communication equipment, such as remote area telecommunications equipment. Sales of solar PV modules are increasing strongly as their efficiency increases and price. But the high cost per unit of electricity still rules out ordinary use.
For a stand-alone system some means must be employed to store the collected energy during hours of darkness or cloud - either as electricity in batteries, or in some other form such as hydrogen (produced by electrolysis of water) or superconductors. In either case, an extra stage of energy conversion is involved with consequent energy losses, thus lowering overall net efficiency, and greatly increasing capital costs.
Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and USA. Japan has 150 MWe installed. A large solar PV plant was planned for Crete. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day.
A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the energy then being used to drive turbines. The concentrator is usually a parabolic mirror trough oriented north-south, which tracks the sun's path through the day. The absorber is located at the focal point and converts the solar radiation to heat (about 400oC) which is transferred into a fluid such as synthetic oil. The fluid drives a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which generates about a quarter of the overall power output and keeps them warm overnight. Over 350 MWe capacity worldwide has supplied about 80% of the total solar electricity to the mid 1990s.
The main role of solar energy in the future will be that of direct heating. Much of our energy need is for heat below 60 degrees C - eg. in hot water systems. A lot more, particularly in industry, is for heat in the range 60 - 110 degrees C. Together these may account for a significant proportion of primary energy use in industrialised nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.
With adequate insulation, heat pumps utilising the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than from the sun. Eventually, up to ten percent of total primary energy in industrialised countries may be supplied by direct solar thermal techniques, and to some extent this will substitute for base-load electrical energy.
WIND ENERGY
Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Generator units of more than 1 MWe are now functioning in several countries. The power output is a function of the cube of the wind speed, so such turbines require a wind in the range 3 to 25 metres/second (11 - 90 km/hr), and in practice relatively few areas have significant prevailing winds. Like solar, wind power requires alternative power sources to cope with calmer periods.
However, there are now many thousands of wind turbines operating in various parts of the world, with a total capacity of over 20,000 MWe. This has been the most rapidly-growing means of electricity generation at the turn of the century and provides a valuable complement to large-scale base-load power stations. Denmark gets over 10% of its electricity from wind. The most economical and practical size of commercial wind turbines seems to be around 600 kWe to 1 MWe, grouped into wind farms up to 6 MWe. Most turbines operate at about 25% load factor over the course of a year, but some reach 35%.
RIVERS
Hydroelectric power, using the potential energy of rivers, now supplies 19% of world electricity. Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it is so readily stopped and started. It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations.
The chief advantage of hydro systems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilisation of stored water is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.
GEOTHERMAL
Where hot underground steam can be tapped and brought to the surface it may be used to generate electricity. Such geothermal sources have potential in certain parts of the world such as New Zealand, USA, Philippines and Italy. Some 6000 MWe of capacity is operating. There are also prospects in certain other areas for pumping water underground to very hot regions of the earth's crust and using the steam thus produced for electricity generation.
TIDES
Harnessing the tides in a bay or estuary has been achieved in France (since 1966) and Russia, and could be achieved in certain other areas where there is a large tidal range. The trapped water can be used to turn turbines as it is released through the tidal barrage in either direction. Worldwide this technology appears to have little potential, largely due to environmental constraints.
WAVES
Harnessing power from wave motion is a possibility which might yield much more energy than tides. The feasibility of this has been investigated, particularly in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity for delivery to shore. Numerous practical problems have frustrated progress.
Renewable energy needs a reliable and efficient storage system
Sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. In practical terms they are therefore limited to some 20% of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for coal or nuclear power, however important they may become in particular areas with favourable conditions. Nevertheless, such technologies will to some extent contribute to the world's energy future, even if they are unsuitable for carrying the main burden of supply.
If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying base-load energy demand would be much greater. Already in some places pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours this water can be used for hydroelectric generation. Relatively few places have scope for pumped storage dams close to where the power is needed, and overall efficiency is low. Means of storing large amounts of electricity as such in giant batteries or by other means have not been developed.
There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today's peak-load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plant more effectively so that the economic value of that wind contribution is greatly increased.
However, any substantial use of solar or wind for electricity in a grid means that there must be allowance for 100% back-up with hydro or fossil fuel capacity. This gives rise to very high generating costs by present standards, but in some places it may be the shape of the future.
HYDROGEN FUEL CELLS
Hydrogen is widely seen as a possible fuel for transport, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without normal burning, and the pollution that comes with it.
Making hydrogen requires either reforming natural gas (methane) with steam, or the electrolysis of water. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.
With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilised fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.
A quite different rationale applies to using nuclear energy for hydrogen. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean maximum efficiency for the nuclear power plants and that hydrogen was made opportunistically when it suited the grid manager.
About 50kWh is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial.
Statistics and facts
Iceland is a world leader in renewable energy due to its abundant hydro and geothermal energy sources. Over 99% of the country's electricity is from renewable sources and most of its urban household heating is geothermal. Israel is also notably as much of its household hot water is heated by solar means. These countries' successes are at least partly based on their geographical advantages.
References
- The Uranium Information Centre provided much of the original material in this article.
- Energy Information Administration provides lots of statistics and information on the industry - http://eia.doe.gov
- Boyle, G (ed), Renewable Energy - Power for a Sustainable Future. Open University, UK, 1996.
