Researchers at DNV-GL did a fine report for the New York Independent System Operator a few years ago. Titled A Review of Distributed Energy Resources, it offered this definition of the various distributed energy resources (DERs) examined in the report:
“… DER technologies are defined as ‘behind-the-meter’ power generation and storage resources typically located on an end-use customer’s premises and operated for the purpose of supplying all or a portion of the customer’s electric load. Such resources may also be capable of injecting power into the transmission and/or distribution system or into a non-utility local network in parallel with the utility grid. These DERs include such technologies as solar photovoltaic (PV), combined heat and power (CHP) or cogeneration systems, microgrids, wind turbines, micro turbines, back-up generators and energy storage.”
Granted, the research team did acknowledge that some sources – including the New York Public Service Commission – included customer load in its list of DERs, but load wasn’t one of the DERs covered in the report. That’s too bad because load can hold its own against other DERs for a variety of grid-supportive purposes.
The more, the mightier
The versatility of load as a DER is what makes it so powerful. Unlike most behind-the-meter generation, which also is variable in nature, load is always available somewhere, provided there is a big enough collection of assets connected to the load-control network.
More important, there is plenty of process storage available. Scientists at the U.S. Department of Energy’s Oak Ridge National Lab found that processes performed within the top 30 industries in the U.S. alone could deliver flexibility equal to some 26 gigawatts of storage.
That’s why load can be used for things like renewables firming. For example, Enbala provides this service for a utility in the Maritimes region of Canada, using some 30 customer sites with approximately 2,000 connected loads to provide a reliable and dispatchable three megawatts of capacity in response to utility signals.
This real-time, dynamic response solution operates non-stop, and it meets energy dispatch requests that call for a specified increase or decrease in energy consumption over 15-minute intervals. Load is also being used today for many other grid balancing purposes:
Regulation service – Multiple independent system operators are using flexible load to support frequency regulation in their service territories. In the case of the ISOs with whom Enbala is working, the DER control and optimization platform dispatches optimal power set points to connected load assets every two seconds, with the set points considering the status of the load, customer-defined constraints, process storage and current power consumption of each asset in the network, as well as other site-specific processes that may affect the operation of these assets. Each load asset responds to delivered set points, and the aggregated response tracks the target signal requested by the grid operator.
Fast ramping capacity or contingency reserve service – Load curtailment has long been used for contingency reserve, but now the fast-response that grid operators can get with loads as distributed energy resources is all the more important. That’s because increasing penetration of solar PV is creating the need for fast-ramping resources at day-end, when solar production is subsiding and residents are just getting home, ready to fire up their air conditioners, clothes dryers, dishwashers and other electricity-using appliances. Connected loads, as part of a connected network that may well also include distributed generation, storage and other DERs, can deliver capacity to the grid within minutes of notification.
Peak-demand management – Constraint-based load management systems let customers define their own site- and asset-specific parameters for determining how much load flexibility they can offer and when they’ll offer it. They also allow customer-defined peak-demand periods as system constraints to avoid peak-demand charges or high-peak, time-based rates. This application also can shift the energy consumption for demand-charge mitigation. Using flexible load for peak demand management helps utilities and energy service providers support the grid and simultaneously deliver immediate value to their end customers.
Voltage management – One of the biggest problems high levels of solar PV penetration can bring is voltage excursions on the distribution system. That’s because injecting real power on a circuit causes voltage to rise at the point of common coupling.. But, when that injection of real power goes away or subsides – as it does when clouds cover a solar PV panel – voltage drops instantly. Not surprisingly, such PV impacts have already caused voltage trouble in areas with high PV penetration, including Hawaii, California and Germany.
One way grid operators control voltage is with volt amperes reactive or VARs, which don’t travel well down power lines without creating line loss. This is why volt/VAR balance should be maintained locally.
Synchronous motors and synchronous condensers can create or absorb VARs, and distributed energy resource management systems can – and should be able to - bring those reactive devices into play to support distribution system voltage. Power electronics also can be used to supply or absorb VARs, managing volt-VAR balance using algorithms that calculate and control grid assets to optimal voltage targets along the length of a feeder. The result is a predictable voltage profile between controlled nodes along the entire length of a feeder with line losses significantly reduced.
This allows for a dramatic increase in the penetration of intermittent DERs without negatively impacting the power quality for neighboring customers. It also minimizes the impacts of intermittency on the utility by reducing or eliminating voltage excursions and reducing tap changer operations.
Despite the benefit, this approach to voltage management must be done with a DER-management system that is drawing on DERs – including loads – from several customer sites and devices.
Why? Because if a large percentage of loads on a single feeder suddenly stopped drawing current at the same time, more voltage excursions might result. Care must be taken to ensure that the DER control software used is designed to optimize power flows across the entire grid system.
When it comes to keeping our power grids in balance, distributed energy resources offer loads of opportunities (pun intended!). Much can be accomplished by leveraging the power of flexible loads in and of themselves. When combined with storage, demand response, smart inverters, CHP, virtual power plants and other distributed energy resources, flexible loads will help enable a future where supply and demand can respond to each other dynamically, benefitting customers, utilities and grid operators alike.