Until recently, microgrids have relied on power from diesel generators. But by integrating solar photovoltaic (PV) panels, operators can reduce the cost of diesel consumption, delivery and storage. In addition, integrating PV will also improve environmental performance and save maintenance costs by reducing the running hours and load on the gensets.
However, the intermittent nature of PV means that its penetration so far has been limited – but by adopting a lithium-ion (Li-ion) Energy Storage System (ESS), the operator can make the most of their PV production and further reduce reliance on diesel generators.
Compared with other technologies, Li-ion batteries have high energy density. This makes them compact and lightweight, which lends itself to straightforward delivery and installation. In addition, they are suitable for scaling to multi megawatt and megawatt-hour levels of power and energy storage capacity.
As an example, consider a 12MW microgrid with six gensets, each rated at 2MW. These could be complemented with solar PV and an ESS, which could be sized to deliver either power smoothing or time-shifting.
The role of power smoothing would require a medium sized ESS that would charge and discharge to overcome short-term changes in cloud cover. Without an ESS, the gensets would need to ramp up and down to compensate for sudden changes in power output, putting additional load on them. Gensets will still be required to charge the battery when PV production is low and provide spinning reserves. The operator will save running costs as they will be able to use fewer gensets and run them at peak efficiency.
Rather than adding an ESS, the operator could opt to increase PV capacity. This will achieve incremental savings but the battery system is a better option. An ESS rated at 40% of the system power over a 20-minute discharge period will typically save 50% of fuel consumption. In this particular case, a 4.6MW ESS would save four million liters of diesel per year.
An alternative role for the ESS is time-shifting, which describes the practice of storing energy during the hours of peak production and drawing on it when it is needed most in the evening and morning peak demand periods. This calls for two hours of energy storage rather than the 20 minutes required for smoothing.
This larger ESS enables integration of more PV power- up to 18MWp. This is up to 150% of the capacity of the gensets, giving the ability to run the microgrid on PV and energy storage alone and switching off the gensets to save almost 10 million liters of fuel per year. However, a note of caution is that this approach need rigorous care when sizing the ESS and installation of a sophisticated control system.
As a rule of thumb, a microgrid without an ESS can optimize fuel savings while integrating PV at up to 50% of genset capacity. A medium-sized ESS increases this penetration to up to 100%, whereas a large ESS is required when PV exceed the genset production.
Unique operating conditions
Each site has its own set of variables that influence the commercial and technical case for energy storage. These include the load profile, PV generation profile, environmental and economic conditions, the nature of the load and the reliability of the connection to the grid, if there is one.
As a result, there is no single ESS and control system that will suit every microgrid. Instead, each site needs an ESS and PV system that is carefully sized to maximize fuel savings, integrate PV energy and minimize costs.
Typically, the best approach is to size ESS units using modelling that is based on operational experience and knowledge of the electrical and thermal performance of the Li-ion battery technology. This modelling mimics the behaviour of successful battery systems in dynamic operation.
Integrating PV in the Arctic
Until recently, the community of Colville Lake in Northern Canada relied on two 100kW aging diesel generators to meet the demand of 150kW peak and 30kW base load. Not only were these generators becoming unreliable, the community is extremely remote. It is located 50 miles inside the Arctic Circle and is only accessible by road via an ice road, making diesel delivery difficult and costly.
The utility, Northwest Territories Power Corporation (NTPC), wanted to reduce reliance on diesel by adopting PV to make the most of the long daylight hours during summer months. It has implemented a microgrid integrating 136kWp of PV panels together with new diesel generation rated at 150kW and an ESS.
Because of the Arctic location, NTPC needed the ESS to withstand temperatures ranging from -50ËC to 35ËC. It also wanted to ensure maximum value for money by balancing the ESS capacity and cost against the size of PV panels and potential fuel savings.
In response, Saft supplied an Intensium Max 20M Medium Power ESS as a special cold temperature package. The containerized system features 232kWh energy storage and a 200kW power conditioning system, along with high-tech insulation and a hydronic heating coil that uses the glycol cooling fluid from the diesel genset to keep the battery at its optimum temperature range when the temperature drops.
The system was commissioning in 2015 and supports network frequency and voltage while allowing the gensets to operate at their point of peak efficiency. NTPC has also been able to reduce runtime to around 50%, with significant fuel savings.
Saft delivered a similar cold temperature package to the Kotzebue Electric Association in Alaska, which uses the system to integrate wind energy.
Energy storage is also supporting a remote community in Bolivia’s Amazonian rainforest, near the border of Brazil and Peru. The province of Pando is not connected to the grid and until recently relied completely on a diesel generating plant in the city of Cobija to meet its anuual 37GWh demand.
A recent government-funded scheme had the aim of reducing diesel consumption and increasing electricity coverage. Under the scheme, a new hybrid power plant was constructed by Isotron SAU, a subsidiary of the Spanish Isastur Group. It features a 5MW solar photovoltaic (PV) array and a 16MW diesel generator, together with an ESS comprising two Saft Intensium® Max 20 M Medium Power containers, each with 580kWh storage and 1.1MW peak power output.
The ESS smoothes out short-term variations in the PV array and now enables solar power to meet around half the energy demand of 50,000 people in the city and nearby towns. As a result, the city is saving around 2 million liters of diesel fuel annually.
Written by Jim McDowall, Saft’s Business Development Manager for energy storage