Renewable Energy Electricity Technologies:
Wind:
Wind energy systems convert the kinetic energy of moving air into electricity or mechanical power. They can be used to provide power to central grids or isolated grids, or to serve as a remote power supply or for water pumping. Wind turbines are commercially available in a vast range of sizes. The turbines used to charge batteries and pump water off-grid tend to be small, ranging from as small as 50 W up to 10kW. For isolated grid applications, the turbines are typically larger, ranging from about 10 to 200kW.
As of 2005, the largest turbines are installed on central grids and are generally rated between 1 and 2 MW, but prototypes designed for use in shallow waters offshore have capacities of up to 5 MW.
A good wind resource is critical to the success of a commercial wind energy project. The energy available from the wind increases in proportion to the cube of the wind speed, which typically increases with height above the ground. At minimum, the annual average wind speed for a wind energy project should exceed 4 m/s at a height of 10m above the ground. Certain topographical features tend to accelerate the wind, and wind turbines are often located along these features. These include the crests of long, gradual slopes (but not cliffs), passes between mountains or hills, and valleys that channel winds. In addition, areas that present
few obstructions to winds, such as the sea surface adjacent to coastal regions and flat, grassy plains, may have a good wind resource. Since the early 1990s, wind energy technology has emerged as the fastest growing electricity generation technology in the world. This reflects the steady decline in the cost of wind energy production that has accompanied the maturing of the technology and industry: where a good wind resource and the central grid intersect, wind energy can be among the lowest cost provider of electricity, similar in cost to natural gas combined-cycle electricity generation.
Small Hydro Systems:
Small hydro systems convert the potential and kinetic energy of moving water into electricity, by using a turbine that drives a generator. As water moves from a higher to lower elevation, such as in rivers and waterfalls, it carries energy with it; this energy can be harnessed by small hydro systems. Used for over one hundred years, small hydro systems are a reliable and well-understood technology that can be used to provide power to a central grid, an isolated grid or an off-grid load, and may be either run-of-river systems or include a water storage reservoir.
Most of the world’s hydroelectricity comes from large hydro projects of up to several GW that usually involve storage of vast volumes of water behind a dam. Small hydro projects, while benefiting from the knowledge and experience gleaned from the construction of their larger siblings, are much more modest in scale with installed capacities of less than 50 MW. They seldom require the construction of a large dam, except for some isolated locations where the value of the electricity is very high due to few competing power options. Small hydro projects can even be less than 1 kW in capacity for small off-grid applications.
An appreciable, constant flow of water is critical to the success of a commercial small hydro project. The energy available from a hydro turbine is proportional to the quantity of water passing through the turbine per unit of time (i.e. the flow), and the vertical difference between the turbine and the surface of the water at the water inlet (i.e. the head). Since the majority of the cost of a small hydro project stems from up front expenses in construction and equipment purchase, a hydro project can generate large quantities of electricity with very low operating costs and modest maintenance expenditures for 50 years or longer.
In many parts of the world, the opportunities for further large hydro developments are dwindling and smaller sites are being examined as alternatives giving significant growth potential for the small hydro market (e.g. China).
Solar:
(i) Photo-Voltaic: Photo-voltaic systems convert energy from the sun directly into electricity. They are composed of photo-voltaic cells, usually a thin wafer or strip of semi-conductor material, that generates a small current when sunlight strikes them. Multiple cells can be assembled into modules that can be wired in an array of any size. Small photo-voltaic arrays are found in wristwatches and calculators; the largest arrays have capacities in excess of
5 MW.
Photo-voltaic systems are cost-effective in small off-grid applications, providing power, for example, to rural homes in developing countries, off-grid cottages and motor homes in industrialized countries, and remote tele-communications, monitoring and control systems worldwide. Water pumping is also a notable off-grid application of photo-voltaic systems that are used for domestic water supplies, agriculture and, in developing countries, provision of water to villages. These power systems are
relatively simple, modular, and highly reliable due to the lack of moving parts.
Photo-voltaic systems can be combined with fossil fuel-driven generators in applications having higher energy demands or in climates characterized by extended periods of little sunshine (e.g. winter at high latitudes) to form hybrid systems.
Photo-voltaic systems can also be tied to isolated or central grids via a specially configured inverter. Unfortunately, without subsidies, on-grid (central grid-tied) applications are rarely cost-effective due to the high price of photo-voltaic modules, even if it has declined steadily since 1985. Due to the minimal maintenance of photo-voltaic systems and the absence of real benefits of economies of scale during construction, distributed generation is the path of choice for future cost-effective on-grid applications. In distributed electricity generation, small photo-voltaic systems would be widely scattered around the grid, mounted on buildings and other structures and thus not incurring the costs of land rent or purchase. Such applications have been facilitated by the development of technologies and practices for the integration of
photo-voltaic systems into the building envelope, which offset the cost of conventional material and/or labour costs that would have otherwise been spent.
Photo-voltaic systems have seen the same explosive growth rates as wind turbines, but starting from a much smaller installed base. For example, the worldwide installed photo-voltaic capacity in 2003 was around 3,000 MW, which represents less than one-tenth
that of wind, but yet is growing rapidly and is significant to the photo-voltaic industry.
(ii) Solar-thermal power:
Several large-scale solar thermal power projects, which generate electricity from solar energy via mechanical processes, have been in operation for over two decades. Some of the most successful have been based on arrays of mirrored parabolic troughs. Through the 1980’s, nine such commercial systems were built in the Mohave Desert of California, in the United States. The parabolic troughs focus sunlight on a collector tube, heating the heat transfer fluid in the collector to 390ºC (734ºF). The heated fluid is used to generate steam that drives a turbine.
The combined electric capacity of the nine plants is around 350 MW, and their average output is over 100 MW. The systems have functioned reliably and the most recently constructed plants generate power at a cost of around $0.10/kWh.
Several studies have identified possible cost reductions.
Another approach to solar thermal power is based on a large field of relatively small mirrors that track the sun, focussing its rays on a receiver tower in the centre of the field. The concentrated sunlight heats the receiver to a high temperature (e.g., up to 1,000ºC, or 1,800ºF), which generates steam for a turbine. Prototype plants with electrical capacities of up to 10 MW have been built in the United States, the Ukraine (as part of the former
USSR), Israel, Spain, Italy, and France.
A third solar thermal power technology combines a Stirling cycle heat engine with a parabolic dish. Solar energy, concentrated by the parabolic dish, supplies heat to the engine at temperatures of around 600ºC. Prototype systems have achieved high efficiency.
All three of the above technologies can also be co-fired by natural gas or other fossil fuels, which gives them a firm capacity and permits them to be used as peak power providers. This makes them more attractive to utilities, and gives them an advantage over photo-voltaic, which cannot necessarily provide power whenever it is required.
On the other hand, they utilize only that portion of sunlight that is direct beam and require much dedicated land area. Solar thermal power is still at the development stage: the costs of the technology should be reduced together with the associated risks, and experience under real operating conditions should be a further gain.