Several forms of renewable power production must be implemented into the current electrical power grid; however, the grid has variable demand which must be met by variable power production. Simply adding various renewable technologies is not very simple. For example, adding both solar and wind power to the existing grid has several advantages and disadvantages related to both limited power production and associated costs. The following information provides insight into the problems associated with the transition from our current grid to the future sustainable grid.
Overview
Traditional Unit Commitment and Dispatch
Our electrical grid has a demand profile that must be met with supply. This demand profile has both hourly and seasonal fluctuation. To show this fluctuation, the electrical demand for different seasons in Colorado is shown over the course of three weeks in figure 1.
Figure 1
The electrical demand over the course of a year is ordered and plotted in a Load Duration Curve as shown in figure 2. In this curve, each hour of the year and its respective demanded power is show with the maximum peak load shown at hour 1 on the left. The minimum load or base load is shown at the right.
Figure 2: Load Duration Curve
To consistently meet this load, power production is broken into three categories: base load, load following, and ancillary services. Base load is defined as the minimum power load that is needed throughout the year. In Colorado, it is generally a constant between 2500 and 3000 MW (based on previous figure). Load Following is the power that is needed to meet the variabilty in the power demand. There is often a daily cycling between 1000 and 3000 MW with ramp rates up to 634 MW/hour. Ancillary Services are power provided to meet random fluxuation and unforseen changes in either demand or supply. It is broken into regulation reserve, spinning reserve and operating reserve. When a failure occurs in a power generating system, the operating reserve, comprised of spinning and non-spinning reserve, kicks in to replace the missing power generation. Spinning reserve makes up the bulk of operating reserve and consists of increasing the power output of generators already connected to the power generating system. The regulation reserve (also known as the frequency-response reserve) is an automated response to a loss in supply. Generators slow down due to the increased load following a failure and the regulation reserve is essentially a governor that helps to speed up the generators. Generally, the regulation reserve is small, so it isn't considered part of the operating reserve. Figure 3, shown below, displays the operating reserve as a function of time. Notice how little the frequency-response reserve when compared to the other two.
Figure 3: Operating Reserve (Wikipedia)
A unit commitment curve, or stack, (shown below in figure 4) illustrates how the desired production will be met. When creating a stack, price of generation and ramp rates are often considered first. As an example, a cheap source with a low ramp rate such as coal will normally be used for base load while an expensive source with a fast ramp rate will be used to meet the peak power demand.
Figure 4: Unit Commitment Curve
Characteristics of Solar and Wind Power Generation
The addition of solar and wind technologies into the electrical energy system has both cost benefits and drawbacks. While there are installation, O&M, integration, and miscellaneous costs associated with each technology, they save on fuel costs and remove the uncertainty of fluctuating fuel prices. In addition, neither emits pollution or any greenhouse gases. However, the primary drawback of solar and wind power generation is their limited predictability in the availability of power due to the irregularity of their energy source. This uncertainty adds uncontrollable fluctuations to the energy grid which presents a problem when considered with variable power demand.
As can be seen below in figure 5, the addition of wind into the power grid will decrease the required base load, but will also increase the amount of flowing power, rates to ramp power, and the uncertainty of net load.
Figure 5 - Integrating Wind Power Impacts Net Load More than Peak Load
Integration of Solar and Wind into Electric Systems and Associated Costs
Challenges
The main challenges associated with integrating solar and wind energy into existing utility systems are:
1) increased short term fluctuations
2) increased ramp rates
3) change in forecasted load
4) decreased minimum load
How do utility companies deal with the challenges of integrating solar and wind energy into their system you ask? Part of the answer is by relying on wind integration studies.
Wind Integration Studies - These studies simulate the existing utility system with and without wind using an expensive commercial software package that includes the existing generation mix and transmission system. When the utility system is modeled with wind, the wind data can come from any meso-scale meteorological modeling data as shown in figure 6.
Figure 6 - Meso-scale Meteorological Wind Data (Minnesota & Colorado from Xcel)
When wind data is integrated into the electricity generation simulation, one must first consider the spatial diversity of the wind resources. Increasing the spatial diversity of the wind resource smoothes the wind farm output and decreases ramp rates, as can be seen below in figure 7. Essentially, the more wind turbines, the better (highly dependent on location).
Figure 7 - Wind Spatial Diversity Impacts - Smoothes Output & Decreases Ramp Rates
Ultimately, the wind integration studies are used to assess implementation costs. Typical results are shown in figure 8 below. The graph on the left shows the impact of wind energy penetration on integration cost (via hour-ahead uncertainty) while the table on the right shows the wind capacity penetration impacts on operating costs.
Figure 8 - Wind Integration Studies - Cost Comparison
"Ultimate" Limits of Solar and Wind Technologies and the Role of Storage
The ultimate limit to solar and wind integration is primarily determined by economics, and whether integration costs exceed some set threshold. One possible upper limit is determined by generator (solar or wind) curtailment.
Figure 9, shown below, displays a scenario pertaining to solar power integration and figure 10 shows a wind integration example. The figure on the left shows a case where all of the available solar power from the photovoltaic panels is utilized and offsets the normal load demand while the figure on the right shows a case where all of the power from the solar pv system cannot be utilized because it exceeds the demand profile. This example illustrates the need for integrating energy storage when deploying wind and solar energy systems so that any generation may be used. For more information regarding storage, please see Energy Storage.
Figure 9 - Solar PV Examples - Fully Utilized and "Spilled" PV Generation
Figure 10 - Wind Integration - Potential Storage Need
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