PrintFocus on power generation from variable renewables
Most information in this section comes directly from World Energy Outlook 2013, pp 208 - 211.
Unlike dispatchable power generation technologies, which may be ramped up or down to match demand, the output from solar PV and wind power is tied to the availability of the resource. (Electricity generation from (non-dispatchable) variable renewables, such as wind and solar, is weather dependent and can only be adjusted to demand within the limits of the resource availability.) Since their availability varies over time, they are often referred to as variable renewables, to distinguish them from the dispatchable power plants (fossil fuel-fired, hydropower with reservoir storage, geothermal and bioenergy). Wind and solar PV power are not the only variable renewables – others include run-of-river hydropower (without reservoir storage) and concentrating solar power (without storage) – but PV and wind power are the focus of this section as they have experienced particularly strong growth in recent years and this is expected to continue. The output of wind and solar can be adjusted, but only if there is sufficient wind or sun available at a given point in time.
The characteristics of variable renewables have direct implications for their integration into power systems (IEA). The relevant properties include:
Variability: Power generation from wind and solar is bound to the variations of the wind speed and levels of solar irradiance.
Resource location: Good wind and solar resources may be located far from load centers. This is particularly true for wind power, both onshore and offshore, but less so for solar PV, as the resource is more evenly distributed.
Modularity: Wind turbines and solar PV systems have capacities that are typically on the order of tens of kilowatts (kW) to megawatts (MW), much smaller than conventional power plants that have capacities on the order of hundreds of MW. This is changing as the industry matures and large utility-scale projects come are constructed.
Uncertainty: The accuracy of forecasting wind speeds and solar irradiance levels diminishes the earlier the prediction is made for a particular period, though forecasting capabilities for the relevant time-frames for power system operation (i.e., next hours today-ahead) are improving.
Low operating costs: Once installed, wind and solar power systems generate electricity at very low operating costs, as no fuel costs are incurred.
Non-synchronous generation: Power systems are run at one synchronous frequency: most generators turn at exactly the same rate (commonly 50 Hz or 60 Hz), synchronized through the power grid. Wind and solar generators are mostly non-synchronous, that is, not operating at the frequency of the system. To correct this, wind and solar electricity are usually run through an inverter, which converts the direct current (DC) to alternating current (AC) and converts the electricity to the proper frequency.
The extent to which these properties of variable renewables pose challenges for system integration largely depends on site-specific factors, such as the correlation between the availability of wind and solar generation with power demand, the flexibility of the other units in the system, available storage and interconnection capacity, and the share of variable renewables in the overall generation mix. The speed at which renewables capacity is introduced is also important, as this influences the ability of the system to adapt through the normal investment cycle. Elective policy and regulatory design for variable renewables needs to co-ordinate the rollout of their capacity with the availability of flexible dispatchable capacity, grid maintenance and upgrades, storage infrastructure, efficient market operation design, as well as public and political acceptance.
Wind power
Generating power from wind turbines varies with the wind speed. Although there are seasonal patterns in some regions, the hourly and daily variations in wind speed have a less predictable, stochastic pattern. Geographically, good wind sites are typically located close to the sea, in flat open spaces and/or on hills or ridgelines, but the suitability of a site also depends on the distance to load centers and site accessibility. (See the Wind Power page in this lesson for an illustration of wind power availability across the U.S.)
For onshore wind turbines, capacity factors – the ratio of the average output over a given time period to maximum output – typically range from 20% to 35% on an annual basis, excellent sites can reach 45% or above. The power output from new installations is increasing, as turbines with larger rotor diameters and higher hub heights (the distance between the ground and the center of the rotor) can take advantage of the increased wind speeds at higher altitudes. Moreover, wind projects are increasingly being tailored to the characteristics of the site by varying the height, rotor diameter, and blade type. Wind turbines that are able to operate at low wind speeds offer the advantage of a steadier generation profile, reducing the variability imposed upon the power system, but likely reducing annual generation. All of this adds up to less expensive electricity; as you may have noted in the Lazard LCOE reading, onshore wind power is one of the least expensive electricity sources available (though this is only in ideal locations, it should be noted).
Wind turbines located offshore can take advantage of stronger and more consistent sea breezes. Wind speeds tend to increase with increasing distance from the shore, but so too does the seafloor depth, requiring more complex foundation structures. Capacity factors are generally higher ranging from 30% to 45% or more, as distance from the shore or hub height increases. However, offshore wind turbines are more expensive to install because of the high costs associated with the foundations and offshore grid connections. Bottlenecks can also occur due to a shortage of specialized installation vessels.
Solar photovoltaics
Power generation from solar PV installations varies with the level of solar irradiation (irradiation is the amount of solar energy hitting a surface over a period of time) they receive. Irradiation is usually measured in kWh/m2/day or kWh/m2/yr. Geographically, solar irradiation over the course of a year increases with proximity to tropical regions and is more uniformly distributed than wind. Seasonal and daily patterns in output from solar PV systems can be fairly well forecast – on a clear day, solar will follow a consistent pattern, based on the path of the sun through the sky. The power received from the sun is called irradiance, generally measured in W/m2. The irradiance from the sun can be predicted with reasonable accuracy for a given location at a given time of year. Of course, local conditions (particularly shading) can significantly impact irradiance levels. A heavily-shaded area can result in near-zero irradiance levels. (See the solar power page in this lesson for an illustration of irradiation levels across the U.S.)