Hybrid systems: key to managing grid connected PV systems

November 13th, 2015, Published in Articles: Energize

 

Hybrid systems have traditionally been seen as large network connected units consisting of more than one technology. The development of smart inverters has led to the application of hybrid principles to smaller grid connected systems that can optimise the use of renewable energy, conventional standby plant and storage. This arrangement has the potential to make a radical change in small to medium rooftop solar installations.

The range of operation of most small to medium grid connected PV systems is limited by the capability of the inverter. One of the limitations of a conventional system is the fact that the inverter shuts down under mains fail conditions.

Various arrangements have been used to allow off-grid type operation under mains fail conditions, or resynchronisation and re-activation to a standby plant if fitted. All of these arrangements have drawbacks:

  • In the off-grid operation case the solar PV/storage plant must supply all the power required, or be limited physically to supplying essential loads only, which means some form of load separation.
  • Where a standby plant is fitted and the solar PV is used as a fuel saver, there is an inevitable interruption of power during changeover to the standby plant and during re-connection to the mains supply.

Other than the availability of storage in larger units and at ever decreasing prices, the biggest advantage available comes from the development of “smart” inverters. A straight PV system can only deliver the instantaneous amount of power generated by the PV panels. Battery storage combined with a smart inverter makes the power delivery independent of the PV system, and limited only by the battery and the inverter.

Hybrid inverter systems

The hybrid system is a double conversion system that allows the use of multiple sources of electricity in number of different modes. The functional block diagram of the system is shown in Fig. 1.

Fig. 1: Layout of a typical hybrid inverter system.

Fig. 1: Layout of a typical hybrid inverter system.

The following characteristics are noteworthy:

  • The load is supplied through the inverter at all times, and the inverter is rated for full load operation.
  • There is a DC link between the incoming supply (either mains or standby plant) and the inverter. This removes the need for frequency and phase synchronisation between the mains supply and the AC output.
  • The battery is sized to supply the full load for short periods, but may also supply a partial load during times when sources are available.
  • Smaller systems appearing on the market are supplied as integral units, incorporating all modules is a pre-defined configuration. It is possible however to configure each module individually to meet the requirements of any particular application.

Hybrid system components

  • AC/DC converter: This unit converts the incoming AC supply to a suitable DC voltage which can be used to supply the inverter or charge the battery or both. The converter will be sized to supply the full load as well as any battery charging requirements. The capacity will be determined both by the load requirement and the battery charging requirements for the size of battery employed. The unit could be bidirectional if the net-metering function is required.
  • Inverter: This unit will be sized to cater for the full load requirements, including any start-up current needs. The battery makes it possible to supply current in excess of the average current for short periods, and the inverter needs to be sized to cater for this.
  • Charge controller: This unit controls the flow of DC current between the solar PV, the battery and the inverter, and must cater for the maximum charge and discharge currents from the battery, and ensure that the battery is charged at the correct rate.
  • Control unit: Controls the interaction between the different units depending on the mode of operation.

The hybrid unit can also have the facility to operate in net-metering mode when the supply from the solar plant exceeds the combined load and battery charging requirement.

A typical example would be the hybrid 5048E inverter system introduced by Eaton. Although limited to a 5 kVA load at the moment, the system incorporates and demonstrates all the facilities described above, and according to Eaton, the control unit is the heart of the system and the individual components could be configured to meet any load and supply requirement.

Operational modes

The facilities and flexibility of the hybrid are best illustrated by considering the different modes of operation [1]. Examples are taken from the Eaton 5048E system. The modes possible in a particular application will depend on will depend on the load and the amount of PV and storage installed.

Fig. 2 : Battery charging modes.

Fig. 2 : Battery charging modes.

Line-PV mode, line- hybrid mode and line-mode (Fig. 2)

These are all battery charging modes. In line-PV mode the AC load is provided totally by the grid and there is sufficient PV output to charge the batteries, without drawing any charge current from the grid. In line-hybrid mode, the load is supplied from the grid and the battery charging current is firstly supplied by the PV system and supplemented by current from the AC supply. This mode is used when the PV supply is too low to charge the battery on its own. Line mode is used when the PV system is not available to charge the battery and the full charge current is drawn from the AC supply.

Fig. 3: Modes used in the absence of AC power.

Fig. 3: Modes used in the absence of AC power.

Inverter-PV mode, inverter-hybrid mode and inverter mode (Fig. 3)

These are all modes of operation in the absence of AC supply. They would typically be used to bridge the time gap between the failure of the grid and start-up of the standby plant, but depending on configuration may well be used to supply power for a longer period.

In inverter-PV mode, the PV system supplies the load as well as the battery charging current. This mode is only possible where the PV system is sized to exceed the full load requirements of the essential load circuits, and there is sufficient output from the PV system at the time. Non-essential loads are not supplied in this configuration.

In inverter-hybrid mode the load is supplied by both the PV system and the battery. This would be a typical mode of operation used to bridge the time gap between the failure of the grid and start-up of standby power in an installation where the PV is sized to provide only part of the total load, or is operating below its capacity.

In inverter mode the battery supplies the full load, and would be used when the PV system is inoperable. The system can also be configured so that the battery supplies part of the load when the AC supply is present and the PV is inoperable. This is a useful feature which allows reduction of the AC load for when PV is not available, for instance during evening peak periods. The length of time that the battery can provide this peak shaving will depend on the configuration of the system. Some systems may be specifically configured for this function.

Charge mode

This mode is a battery charge mode similar to the line-PV mode, with the difference being that the load is not supplied with power.

Fig. 4: Solar/battery hybrid mode.

Fig. 4: Solar/battery hybrid mode.

Solar/battery hybrid supplement mode (Fig. 4)

This would be the normal operating mode for a rooftop or ground mount own-generation PV system. Output from the solar PV system is used to charge the battery and to reduce the power drawn by the load from the AC supply. When the battery is in its fully charged condition the full output of the PV system is available to supplement the supply. In the case of short interruptions or reductions of the PV system output, the battery supplements the output of the PV system and avoids sudden demand changes on the grid. The system can also be configured so that the battery supplies part of the load (peak shaving) during periods of high demand and high tariffs.

The PV supplemental function has proven to be a useful fuel saving feature when the system is operating on standby power, and for systems that experience frequent outages, hydrid PV/battery systems can provide significant savings, without any interruption of supply.

Hybrid inverter applications

The modes of operation possible with hybrid inverters, and the flexibility in configuring the system, particularly in adding storage, opens a door to a range of new applications and ways of managing power and fuel consumption for consumers who are considering installing PV systems. This expands the advantages of PV beyond those previously available.

Arbitrage of energy

Storage of solar energy allows flexibility in load management. In a basic situation, the output of the inverter is limited to the output of the solar array, and no load may exceed this value. A combination of storage and smart inverter however, allows the output of the inverter to exceed the capacity of the solar array and supply higher loads for short periods either in conjunction with the array or using storage alone.

For example, a high domestic load could exist in evening peak period and the smart storage system could be used to limit the amount of power drawn from the grid during this period of several hours, provided that the storage has built up sufficient capacity during the day, thus avoiding peak period charges.

Weekend wasted energy

Many businesses run a five day week operation where the weekend load is very low. This means that two days of solar energy are lost. Tariffs are generally low over weekends, so that using a feed in or net-metering scheme does not provide much of an advantage. Where storage is already installed, the surplus energy from the weekend can be stored, and can be used to supplement the daily generation to reduce energy drawn from the grid. It can also be used for peak lopping during the high rate peaks of the week under a controlled discharge scheme. For example, assume that 50% of the daily consumption can be stored every non-working day, that allows 100% of the daily consumption to be stored, this gives an additional packet of 20% per day that can be used. Again this can be configured to reduce power during peak periods only as an extra advantage.

Supplemental energy

Almost all privately owned rooftop solar PV systems are focused on saving energy and reducing peak demand, but there is an as yet unexplored possibility, and that is using solar PV to increase the energy available rather than substitute it. This is particularly important in the case of an energy limited environment.

In a typical commercial setting, a rooftop solar system provides in the region of 25% of the total energy requirement. This means that with the same installation the energy available could be increased by 25%.

Say you have a grid connection of 100 kVA and you are running out of capacity. Solar could increase your available energy to 125 kVA, but some form of storage would be necessary to level the load. There is no information on whether anyone is doing this.

If the cost of grid electricity becomes so high that it is cheaper to use own generation then we have a new scenario developing – the partial off-grid factory or office. The main challenge here would be technical one – how to integrate the PV system into the supply. The first obvious option is dedicated plant running completely off-grid while having connection to the grid for frequency signals and energy storage.

The increasing use of rooftop solar reduces the maximum demand, so demand could actually be increased using storage to compensate for days of lower solar, and also arbitrage power over low tariff periods.

Maximum demand (MD) reduction using storage to supplement solar

Solar on its own can reduce the maximum demand, depending on when this occurs. If the maximum demand point and the solar maximum do not coincide, storage can be used to add to the solar output during MD time.

References

[1] Eaton: “Hybrid 5048E hybrid inverter”
[2] P Bronski, et al: “The economics of load defection”, Rocky Mountain Institute, April 2015

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