Conventional solar systems are generally designed and operated to produce maximum electricity at all times. But when solar energy is used on a large scale, it becomes necessary for it to contribute to the grid flexibility needed to maintain the supply-demand balance. The combination of photovoltaics with storage allows the control of the energy supplied to the grid, making solar power distributable on demand. What are the best practices regarding the control of these plants and the use of forecasts in this context?
It is not new that network services are provided by renewable energy plants, but the emergence of new technologies and standards allows the supply of energy in an increasingly controlled manner. The Springbok Solar Cluster, a 443 MWp photovoltaic power plant developed and built by 8minute Solar Energy in California, is an example of a pioneering project that provides an unparalleled level of grid services to the Los Angeles Department of Water and Power (LADWP). The project, jointly undertaken by 8minute Solar Energy, EPRI, LADWP and Doosan, provides solar energy on demand through its Springbok 3 PV + storage pilot project with a 1.5 MWh Li-ion battery. Its control algorithms include accurate short and long term forecasting. Compared to a conventional solar power plant in California, forecasting solar power generation up to 30 minutes ahead of time allows advanced grid services and reduces the curtailment of solar energy.
The use of a forecasting system based on a sky imager allows the power plant’s control system to anticipate and react to power ramps which occur in the event of cloud passages. In this article, we examine how such forecasts provide the basis for achieving operational flexibility at the PV plant level and ensure the ability of the grid and the energy market to reliably achieve high solar energy penetration rates.
What is useful to make solar energy dispatchable?
Dispatchable solar power is based on the use of an interoperable specification, i.e. shared by the different operators and suppliers of power plant components. This specification contributes to the robustness of the control algorithm and allows photovoltaic installations to react automatically to changes in the power grid. The objective is to be able to fully shape the production profile, i.e. to control the production of the PV plant + storage to match the load profile of the utility. While the daily load profile provided by the utility defines the required production curve of the plant, the final electricity output can additionally be adjusted using different control modes. These are based on open standards and allow operators to create a certain form of power in a flexible, well-defined and interoperable way. In particular, the open standard MESA-ESS, adapted to control the Springbok 3 project, allows a reactive power support mode, i.e. it generates or absorbs reactive power accordingly. The mode can be activated by the utility and corresponds to a fixed curve. Another reactive power control mode allows a variable power factor correction.
The specifications, based on the ModBus protocol, define three types of control modes, which can be flexibly combined to achieve several service objectives at the same time, and therefore manage any expected deviation between the output power and the load profile. These three modes are active power mode, emergency mode and reactive power mode.
At any given time, the service is composed of a programmed combination of control modes of these three groups, with a flexible schedule that can be adapted or repeated as required. For the Springbok site, there is usually a weekday schedule and a weekend schedule.
The importance of solar production forecasts for the management of a dispatchable solar power plant
A dispatchable solar + storage installation must integrate accurate short-term production forecasts allowing a range of ancillary network services:
- Time shift/energy arbitrage: energy is supplied at peak times;
- Frequency regulation with a one-second response time to maintain grid frequency, and 15 minutes backup power available 24/7 ;
- Power smoothing: the plant’s efficiency is increased or decreased according to the expected solar energy;
- Response to setpoints and guaranteed minimum production steps in 60-minute periods;
- Avoid curtailment;
- Emergency Modes: Voltage and frequency control modes.
The control algorithms providing these services to Springbok are based on one-minute interval forecasts and ensure the economic competitiveness of the solar-battery model for storage durations of up to a few hours. The satellite forecast services used for intraday forecasts and the numerical weather models used for forecasts of up to several days ahead are not sufficient to provide the flexibility required to optimally operate the distributable system according to local conditions. For example, the albedo – i.e. ground reflectivity – can make it more difficult to detect clouds from satellites, which can lead to higher forecast errors. This is one of the reasons why for the Springbok plant located in the Mojave Desert an infrared sky imager and the associated forecast service are being used.
How to use all-sky imagers for dispatchable solar projects?
While the use of sky imagers was primarily to reduce fuel costs in diesel-PV hybrid installations, they are now also a key element to enabling solar power on demand for utility-scale photovoltaic projects worldwide, which will be producing a significant share of electricity in the coming decades. The provision of ancillary grid services from dispatchable solar power plants requires that the system operator has a sufficient degree of confidence in the forecasted solar generation. Highly precise short-term forecasts derived from all-sky cameras allow operators to be prepared for the natural variability of solar power generation.
Sky cameras capture images of the sky from the ground, typically in the visible spectrum. The use of infrared technology provides additional information about clouds, such as their altitude and optical thickness. In addition, infrared observations are not affected by the effects of glare or other problems related to variations in optical brightness and contrast. The installation of a camera on site requires a clear field of view, a power supply and an Ethernet connection. The equipment then operates either in connected mode or within the local network, depending on the availability of an internet connection. An experienced supplier can advise you on the most appropriate hardware configuration according to the project requirements.
Dispatchable renewable energy, its standardization and interoperability are the basis for a successful energy transition. Solar energy on demand paves the way for the grid of the future, and the added value of all-sky imagers make them an essential for the success of dispatchable solar power.
The article was originally published on pv magazine France (01/21/2021).
