In the challenging environment of Sakha (Yakutia), power supply systems based on RES technology have spread throughout the entire region. That's because the power utility system is decentralized. Diesel generator units ensure round-the-clock power supply in remote localities of Sakha. So, large share of electricity costs is related to logistically challenging delivery of diesel fuel to areas of storage and consumption. Limited vehicle access to power plants and seasonality of transport routes are associated with climate pattern of the region. All these factors make people look into further ways of saving on diesel fuel and consider using renewable energy sources.

Power supply systems based on solar (photovoltaic) panels are the most used renewable energy sources in Sakha. Photovoltaic panels are used in combination with grid-tie inverters, which convert the DC output of a solar panel into single-phase or three-phase AC in phase with the bulk electrical system and provide some of capacity sufficient to supply consumers. This solution reduces the load applied to a diesel-generator unit and, accordingly, reduces the cost of diesel fuel and its delivery. However, this solution has several shortcomings and limitations (Fig. 1).

One of disadvantages is that the use of this solution makes it impossible to stop a diesel generator even at the increased solar activity and significant capacity of the solar power plant. Grid-tie inverters need a reference voltage to generate electricity. Another related disadvantage is capacity constraints in the use of an on-grid solar power system. In the event that the solar power generation is high and the solar power is sufficient to fill the needs of power consumers, the diesel generator remains mostly unloaded and is low- performing. Should the capacity of solar power plant exceed the capacity of consumers, generator will act as a load for the inverter and start running in the electric motor mode. All this can lead to malfunction of power plant, then the staff will have to shutdown inverters, and so the surplus solar energy is plain out lost (Fig. 2).

ENELT Group Co. Ltd. suggested using electrical energy accumulators based on storage batteries. Accumulators comprised a set of bi-directional battery inverters, input and synchronizing cabinets, protective devices, monitoring and control modules, and batteries as such. Battery inverters supplied by an AC current can become battery chargers and generate three-phase alternating current by means of energy accumulated in the batteries (Fig. 3).

This solution has helped retain and store the extra energy of the solar power plant for later use and also meet the following targets:

• Since battery inverters generate reference voltage, it is now possible to fully shut down the diesel- generator unit and work along with solar power plant as an extra power (battery discharge);
• In addition to fuel savings, you economize on the diesel generator service hours and, accordingly, reduce maintenance costs;
• Optimizing the DGU fuel consumption rate by increasing the load to its optimal capacity. Free energy of the diesel generator is used to charge the storage batteries;
• Leveling the DGU load by means of inverters and energy accumulated in the batteries.

The sites chosen for tests were hybrid solar- diesel power systems in Batamay, Kobyaysky District and Yuchyugey, Oymyakonsky District, Sakha Republic. The entire accumulator equipment system was placed in weathertight containers with the heating and air conditioning functions. This solution allows you to conveniently expand the modular configuration of existing automated diesel engine power plants.

The electrical energy accumulators used in Batamay were Li-ion batteries using LiFePO4 as cathode materials. The battery voltage is 48 V, total accumulator capacity is 1800 A/h. Li-ion batteries have the advantage of a high charge rate, a wide operating temperature range and an extended service life in terms of the number of discharge/charge cycles. Li-ion batteries can only be used in combination with a BMS (Battery Management System) charge/discharge controller. ENELT Group Co. Ltd. uses proprietary BMS.

The BMS charge/discharge controller has the following main functions:

• Protecting batteries against overcurrent, overcharge, deep discharge, and overheating;
• Performing cell balancing;
• Local and remote monitoring of battery status.

ENELT Group Co. Ltd. produces autonomous electrical energy accumulators based on Li-ion batteries. The Company has implemented projects involving the use of such systems in the energy and telecom sectors.

In Yuchyugey, sealed lead-acid storage batteries that use gel electrolyte (OPzV type) were used as an electric power holder. Battery voltage is 48 V; the total capacity is 3000 A/h. The advantage of using lead-acid batteries is the simplicity and robustness of the solution, and lower original value. However, lead-acid batteries are more demanding of the operating temperature range and the charging rates. They have less discharge/charge cycles.

In Batamay, Kobyaysky District, where a 60 kW solar power plant was built, the anticipated direct fuel economy achieved by non-use of diesel generators during significant sun's activity is 8617 l per year. Additional fuel savings from optimized performance of diesel generator will be 702 l. The total anticipated savings at the current rate of consumption of about 250 l per day will be 9319 l per year (approximately 35 l per day).

Electrical energy storage systems combined with grid-tie inverters (solar power plants) are an easily scalable solution and enable the construction of large systems up to fully autonomous solar power plants. This makes it possible to completely stop using diesel generators in spring and summer resulting in significant diesel fuel savings.

ENELT Group Co. Ltd. cooperates with Melentiev Energy Systems Institute of the Siberian Branch of the RAS. The optimization package developed by the Institute allows for the integrated optimization of decentralized power supply systems that use RES technology and storage batteries, that is:

• Identifying technologies for the most efficient power production;
• Selecting auxiliaries;
• Finding the optimal unit size for the equipment;
• Selecting the optimal ratio of generating capacities.

 

Aleksandr GORYANSKY
Head of Design and Engineering Department, ENELT Group Co. Ltd.

 

"ELECTRIC POWER. Transmission and Distribution", issue no. 4, July - August 2015

Article published in "ELECTRIC POWER.
Transmission and Distribution"