Designing Virtual Power Plant Systems

The Internet of Power (#IoP) is almost upon us. Slowly we are undergoing an energy generation and storage revolution, which is transforming how we generate, distribute, store and use electrical energy to power our everyday lives. The key to this revolution is not just the electronics and control systems required but also the energy storage technologies available and our own thinking when it comes to electricity.

In our current world, we are all used to turning a switch and having electricity available. Whilst this will not change how the electricity is provided and used will. Slowly we are turning to renewables as a source of generation be it solar PV or wind turbine installations. These large-scale power plants generate electricity into the National Grid but only when the conditions are right in terms of solar radiation and wind speed.

On a large scale, these types of renewable power installation are classed as ‘farms’ – solar farms or wind farms and the do require large areas. We are used to seeing offshore wind farms as we look out to sea and these lie in the property of the Crown Estate. On-shore solar PV farms and wind turbine installations are becoming more common sites placed on land rented or leased from their owners.

The connection to the national grid of these large-scale installations is via some form of MV/LV switchgear and more often than not when on-shore MV/LV packaged substations. These can now also include some form of energy storage connection, whereby the generated electricity can be stored for later use by the National Grid to balance out supplies (both frequency and voltage) as part of one of their balancing programmes.

The Role of UPS Systems and Generating Sets

At a more local level we are currently reliant on uninterruptible power supplies and standby power generators to provide an emergency source of electrical power when there is a break in electricity supply. Large generator owners know that their systems provide no protection when the mains power supply is present. The generators sit in an idle state, with their electronics ‘live’ and in some instances kept warm to ensure they start-up when required. Once a month a generator is usually fired-up to keep the oils moving and fuel stirred and to test breakers and connections. Known as a ‘black start test’ a mains power failure is simulated.

Whether mains power is present or not, under a generator test or not, your local uninterruptible power supply will continue to protect your critical loads. An uninterruptible power supply draws power for its inverter from two sources. Firstly a rectified input AC (alternating current) supply that can be electricity supplied by the national grid or a local standby generating set and secondly from a connected standby DC supply.

The DC supply for a UPS has traditionally been a battery set and one made up from strings of Valve Regulated Lead Acid (VRLA) batteries. In a UPS system application the battery set is in a standby mode for most of its working life and is only called upon occasionally to power the inverter i.e. during a brief power cut or prolonged mains power outage.

Lead acid batteries have a design life of typically 5 or 10 years and will require replacement within years 3-4 or 7-8 respectively. Lead acid batteries have limited cycle bands (300-400 charge discharge cycles) and are not tolerant of temperatures outside 20-25degreesC. Their design life can halve for every 1degreeC rise above 30. Lead crystal batteries are sometimes used as an alternative as they are less temperature sensitive and can have a longer design life. Lead crystal may be more suited to UPS applications in marine, renewable power, traffic and rail applications.

Alternative DC sources can include flywheels and fuel cells and even super capacitors. The most common alternative to a VRLA battery set is a Lithium-ion (Li-ion) type battery. This is the type of battery used by electric vehicles (EVs) smart phones and tablets and there are several types including Lithium Cobalt Oxide (LiCoO2), Lithium Manganese Oxide (LiMn2O4), Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC), Lithium Iron Phosphate (LiFePO4), Lithium Nickel Cobalt Aluminium Oxide (LiNiCoAlO2) and Lithium Titanate (Li4Ti5O12).

What’s important to note is that compared to Lead Acid or Lead Crystal, Lithium-ion batteries are more suited to multiple and rapid charge/discharge cycles. This makes them not only more suitable for electric cars and smart devices but also some UPS system and energy storage applications where a more rapid and frequent charge/discharge cycle is required.

One issue is that Lithium-ion batteries require more management functionality than lead acid or lead crystal types. This is because they are a more complex battery that can suffer thermal runaway when over-charged and because they are made from more flammable materials. Whilst a lithium battery is small in a smart device more power hungry devices like electric vehicles require a far larger number of the batteries to be connected to give the power required. A single lithium battery itself is already made up of several smaller battery cells (see how lithium-ion batteries work on YouTube)

Domestic Energy Storage Systems

On a domestic scale new developments include domestic energy storage systems that allow homeowners to store electricity generated from their own solar PV applications. They can choose to use this power to trim down demand at peak times within their own building rather than export this energy into a grid for which they receive a fee for generation known as a ‘Feed-in-Tariff’ or FIT. Domestic energy storage connected to a solar PV installation creates a virtual power plant (VPP) system within the home environment.

A virtual power plant is a system concept that integrates two or more power sources in order to provide a stable supply of power. These sources can include the local electricity grid, microchip, solar PV, wind turbines, small hydro, back-up generators and battery-stored energy.

On a wider scale the virtual power plant concept could be rolled out to power critical IT applications within offices, comms rooms and larger buildings (datacentres) but only with a suitable modified uninterruptible power supply configuration that could be both locally and remotely controlled.

UPS Virtual Power Plants

A UPS virtual power plant (uVPP) offers a wide choice of potential operating modes including power protection, local energy storage, export and and islanding from the local mains power connection. The concept would rely on a suitable battery choice (Lithium-ion based) and a modified uninterruptible power supply able to operate in one of three virtual power plant modes:

1. The UPS system would provide a source of uninterruptible power when the mains power supply fluctuates due to nearby lightning storms or grid switching or during an actual power cut. This is the traditional role required to protect mission critical IT servers, applications and processes.
2. As a revenue generator the battery stored electrical power could be exported back into the grid through a bi-directional inverter in much the same way as a solar inverter operates. The level to which the battery can discharge would be preset to ensure there was always sufficient battery power to protect the load from power quality issues and power cuts. In this mode, the load would most likely have to be powered through it bypass or filtered bypass arrangement and the UPS would need to be set to operate in a smart-mode that automatically decide its priorities (export or load power protection).
3. As functional source of power for the local building and its IT systems, comms rooms and small or micro data centres. In this mode the UPS is a true virtual power plant. The load remains connected to the UPS inverter at all times and is protected but the UPS switches between battery power and an alternative source of power, be this the local mains power supply or a generating set. The concept again relies on the UPS operating in a smart-mode that automatically decides when to disconnect from the local mains power supply and use its lithium-ion battery set and when not to.

So what’s the benefit of UPS systems in virtual power plant applications? The UK electricity grid is changing and many mission critical applications powered by IT require 24/7 power supply and protection. Within the next few years the District Network Operators (DNOs) will be using price as one way to control energy usage at peak demand times. In winter period’s electricity prices could rise 10-20% compared to daylight operation as the former represent peak demand periods. Using a UPS virtual power plant allows an office or building to run on battery between the hours of 16:00 to 18:00 using the energy stored within its UPS battery set. Overnight the battery recharges and rapidly. Alternatively the owner of the UPS system could be paid a Feed-in-Tariff by allowing their battery set to be used as part of a National Grid demand response program.

Whilst the concept of a UPS virtual power plant is in its development and early trial phase there is a growing interest in energy storage and how to connect the thousands of uninterruptible power supplies installed to an ‘Internet of Power’. Demand aggregation like this could be just one way that the UK maintains overall electrical supply stability and availability in the coming decades as new power stations like Hinkley Point are constructed. Whilst UPS manufacturers and suppliers can provide some of the hardware now, demand aggregators will play a key future role, as will developers of the smart internet technologies needed to connect so many devices together into a locally autonomous but remotely managed power system.

For more information on how to design a smart mode UPS system based virtual power plant please contact the Eco Power projects team on 0800 612 7388. We provide a range of suitable uninterruptible power supplies and lithium-ion battery sets suitable for the Internet of Power of tomorrow.