Software models are an indispensable part of the project development process. There is often an unrealistic tendency to wish for a single model that could serve all purposes, but a model that tried to do that would drown under its own weight. A good metaphor is a map that has all of the detail of the real world. It would be as large as the real world and therefore useless. Building or choosing the right model for a specific purpose is an art.
The goal of software models for large grid-connected projects that sell all of their power on an as-available basis should be as precise as possible about the kWh production in order to obtain the maximum financial leverage. This requires attention to details such as shading, soiling, panel mismatch, and a variety of other factors that individually have a small impact, but collectively can be significant.
PV penetration levels are beginning to reach the point where accommodation of the variability of PV power will become necessary.
Unfortunately, the days of simple “Take-or-Pay” power purchase agreements (PPAs) that sell all of their power on an as-available basis are numbered. In the future, other factors, such as the timing of the power production, and the ability of the purchaser to use the power without violating their ramp rate and minimum loading constraints will become more important. Although the utility industry’s complaints about the variability of power production from PV have not yet been relevant at current penetration levels, penetration levels are beginning to reach the point where accommodation of the variability of PV power will become necessary.
This is particularly true in places with aggressive portfolio standards or where the cost of conventional power is very high, such as in diesel-powered grids.
The win-win solution to increasing renewables while maintaining grid stability involves hybrid systems that contain other components, such as load management, storage, or backup gensets. These hybrid systems can manage the variability of PV power, but they are more complicated and involve more design and operational decisions. This makes these hybrid microgrids harder to model.
The benefit of local generation
The additional components add to project costs, but they also provide additional benefits. Those additional costs and benefits are important factors for evaluating hybrid power projects. The additional reliability that local generation can provide when deployed in a microgrid is the most obvious benefit. With sufficiently flexible regulation, microgrids can also offer stability and power quality benefits to both the end user and the larger grid. In its simplest form, the microgrid becomes an interruptible load. It should get a lower rate on the power that it purchases from the grid in return for dropping off the grid during system emergencies and extreme peak load events and providing grid stability and power quality benefits back to the larger grid. The promise of smart grid technologies would allow even more fine-tuned control.
In its simplest form, the microgrid becomes an interruptible load.
Hybrid microgrids come in a variety of forms. The distinguishing feature is their ability to stand on their own. Although simple diesel microgrids have been around for a long time, both as island grids that stand on their own all of the time and as backup power systems that only stand on their own during grid outages, hybrid microgrids that use renewable power, combined with storage and load management are new. They are finding their first cost-effective applications in island and remote grids that rely on diesel power.
There is also an important entry market niche for them for emergency services such as military bases that need to have extremely reliable power, especially during prolonged outages. The technologies, such as software modeling, but also storage, controls, and power electronics that are now being developed for that market will soon be valuable also for grid connected renewable projects that have to manage their own variability.
The HOMER (Hybrid Optimization Model for Electric Renewables) software was developed at the National Renewable Energy Laboratory specifically to model hybrid powered microgrids. It not only considers renewable technologies, such as solar, wind, hydro, and biomass, but also conventional thermal technologies, storage, load management, and fuel cells. The importance of timing issues, such as the correlation of the loads and resources, and the management of storage and load management requires a fully chronological simulation of the entire power systems. Although this is typically performed at the hourly level, HOMER can be used to model all 525,600 minutes in a year. It also contains an optimization algorithm to identify the least cost combination of components and the trade-offs that occur when varying the capacities of the different components. Its decision analysis algorithm then shows how sensitive these optimal results are to variations in inputs. This is crucially important given how much uncertainty is inherent in future fuel prices and load profiles. Other models can be used to analyze protection schemes and transient stability also play an important role in project development process.
Using software for solar design, this screen capture shows that for this application solar is cost-effective when the cost of diesel fuel (vertical axis) exceeds 60 cents per liter (red area). It also shows that there are many applications (green area with a diesel backup and upper right corner without a diesel backup) where solar and wind complement each other (horizontal axis shows wind speed)
CLICK HERE TO VIEW A LARGER IMAGE
Written by Dr. Peter Lilienthal, Chief Executive Officer of HOMER Energy based in Boulder, Colorado (US)