Remote Communities Rely on Microgrids With Energy Storage
By Jim McDowall, Saft
Microgrid operators have traditionally relied on diesel generators for electric power, but some are turning to renewable energy, especially solar photovoltaic (PV) installations, to reduce their reliance on diesel fuel. This switch offers considerable savings in terms of the fuel purchase, transport and handling costs, as well as maintenance. In addition, no greenhouse gases are emitted from renewable generation sources.
Adding solar and wind power to microgrids lowers per kilowatt-hour (kWh) costs significantly below that of using only diesel generators. Because renewables provide intermittent energy, however, diesel gensets are still needed to maintain a stable supply.
Standard power electronics make it is feasible for solar PV to contribute up to 25 percent of power traditionally generated by the diesel genset. By adding dedicated software, however, it is possible to increase PV penetration to around 50 percent.
To maximize renewable energy’s contribution to microgrids, an energy storage system (ESS) must be incorporated. This can ultimately enable diesel-off operation and create fuel savings of 50 to 75 percent.
Lithium-ion (Li-ion) battery systems are generally regarded as the main option for ESS applications. This is because they have a high energy density that enables megawatt-scale power and megawatt-hour levels of storage capacity to be deployed within a relatively compact footprint.
|Li-ion Energy Storage Takes Microgrids To The Next Level|
A typical 12 MW microgrid might be supplied by six gensets, each rated at 2 MW. An ESS could support either power smoothing or time-shifting and would be sized accordingly.
A relatively small ESS could be used for power smoothing for about 20 minutes to compensate for changing weather conditions. The ESS helps ensure a smooth power output and delivers both operating and maintenance savings by avoiding the need to ramp the diesel gensets up and down. The gensets are still required to provide spinning reserves and recharge the battery when PV production is low.
Additional PV will allow an operator to achieve incremental savings. For power smoothing, however, a medium sized battery is a better option. In fact, an ESS sized at about 40 percent of the system power (4.6 MW) rated for a 20-minute discharge will deliver a 50 percent reduction in fuel consumption. That is about 4 million liters of diesel per year.
If the aim is to achieve time-shifting, then two hours of energy storage is needed so that PV energy generated during the peak hours of daylight is used during the morning and evening demand peaks. A larger battery enables more PV integration-at up to 18 MW peak this could represent as much as 150 percent of the diesel genset’s capacity.
In this case, the aim is to run the microgrid using only PV and energy storage, creating potential fuel savings of almost 10 million liters of diesel per year. For this approach to be successful, rigorous system sizing and sophisticated control electronics are required because spinning reserve is not immediately available.
As a rule, when a microgrid does not incorporate an ESS it can achieve optimized fuel savings only at levels of PV penetration below 50 percent. A medium-sized ESS increases this to between 50 and 100 percent, while a large ESS is required when PV penetration rises above 100 percent.
Every Microgrid has its own Needs
The potential for increased PV penetration and fuel savings depends on many variables. Typically, these include: the load profile; the PV generation profile; the environmental and economic conditions; the nature of the load; and the reliability of the connection to the main grid, if one exists.
That means that no single ESS will meet the needs of all microgrid sites. Expertise is required to identify the optimum ESS and PV system, as well as implement a control regime adapted to maximize fuel savings, integrate PV energy and minimize costs. An important element in this process is high resolution MATLAB-based modelling of the Li-ion battery system’s electrical and thermal characteristics. The modelling aims to mimic the actual behaviour of the battery systems, including the evolution of battery power, state of charge and aging over time in a given application with dynamic charge and discharge operation.
Practical Applications From Arctic to Equator
The 150-strong community of Colville Lake is located 50 miles inside the Arctic Circle in Northern Canada. It was served by a small microgrid with 150 kW peak and 30 kW baseload met by two 100 kW diesel generators that were becoming old and unreliable. The community also faced a major challenge because diesel deliveries could be made only once a year via an ice road.
In 2015, Northwest Territories Power Corp. (NTPC), the distribution network operator, implemented a microgrid combining 136 kW peak of PV with an additional 150 kW of diesel generators and an ESS. A key aim was to reduce the runtime of the diesel generators, especially in the summer when the sunlight is available for virtually 24 hours per day.
NTPC needed an ESS that would withstand harsh temperature variations from -50 C to 35 C. Furthermore, to ensure maximum value it was essential that NTPC balance the ESS capacity and cost against the size of the PV panels and potential fuel savings. Saft’s response was to provide an Intensium Max 20M Medium Power containerized system with 232 kWh energy storage capacity to work with a 200 kW power conditioning system.
The Colville Lake ESS is a special cold temperature package that combines layers of high-tech insulation with a hydronic heating coil using the same hot glycol that maintains the diesel gensets at operating temperature. This minimizes the cost of keeping the battery in its optimum temperature range.
The ESS supports the network frequency and voltage. It also allows the diesel generators to operate at the point of maximum efficiency and to shut down whenever possible. The runtime has thus been reduced to around 50 percent, providing significant fuel savings.
Kotzebue Electric Association
Also in the Arctic, Saft delivered an Intensium Max+ 20M containerized battery system with the same cold temperature package to Kotzebue Electric Association Inc. (KEA), an electric cooperative based in Kotzebue, Alaska. The town is not connected to the grid or to any road system and has traditionally relied on diesel generators for electric power. These generators were recently augmented with a variety of wind turbines.
With 950 kWh of energy storage, the existing hybrid wind-diesel power system can now achieve its full potential, providing cleaner, more reliable and less expensive power to the local community. The key benefits are that the KEA microgrid can ride through fluctuations in wind output and time-shift excess wind energy, providing significant reductions in diesel consumption. Currently, KEA is running trials using the ESS grid-forming capability, allowing diesel-off operation.
In Bolivia, the remote province of Pando is benefiting from an energy storage system (ESS) at a hybrid power plant that combines a 5 MW solar PV array with a 16 MW diesel generator, the largest plant of its type in the world.
Pando is in the Amazonian rainforest near the border with Brazil and Peru. It is not connected to the country’s national grid, resulting in electricity coverage of just 65 percent, with the annual 37 GWh demand previously being met exclusively by diesel generation.
To both reduce diesel consumption and increase electricity coverage, the Bolivian government funded the PV plant and energy storage facility, which was constructed by Isotron SAU, a subsidiary of Spain’s Isastur Group. The hybrid power plant coordinates PV and diesel generation to maximise the use of clean solar power to meet around half the energy demand in Pando’s capital city Cobija and nearby towns-a total population of around 50,000 people.
The ESS comprises two Saft Intensium Max 20 M Medium Power containers, each with 580 kWh storage and 1.1 MW peak power output.
Effective energy storage plays a critical role in the plant by ensuring system stability and smoothing out short-term variations in output from the PV array, both of which are essential to achieve the highest possible contribution of PV to the energy mix. Integration of PV with energy storage and diesel generation is reducing annual fuel consumption by an estimated 2 million liters, saving the utility millions of dollars and reducing CO2 emissions.
The growing number of practical, commercial installations, like those mentioned here, now demonstrates that deploying energy storage in combination with diesel generators and a solar PV plant offers important advantages for remote off-grid microgrids.
Jim McDowall has worked in the battery industry since 1977 and is business development manager with Saft, primarily associated with grid systems. Involved in the energy storage market since 1998, McDowall was director of the Energy Storage Association for 14 years and is a past chair of the organization. Jim is a senior member of IEEE and is standards coordinator of the IEEE Stationary Battery Committee, the chair of two of its working groups, and a past chair of the main committee. Jim is a frequent speaker at energy storage conferences and related events.