A profitably disruptive energy strategy

19 August 2019

The basic configuration of electrical power systems for industrial and commercial applications has altered little for decades. A disruptive new technology – hybridisation – is expected to change this, according to Steve Moore.

With energy costs expected to continue to rise for the foreseeable future, businesses need to devote more effort into finding ways of controlling and reducing energy expenditure. An obvious approach is to improve the efficiency of a process so that it consumes less energy. 

While this approach is logical and laudable, particularly as it helps to protect the environment, it is also limited. Inevitably a point will be reached where no further increase in the energy efficiency of a process is realistically possible. 

In this event the only option is to find energy at a lower price and today this requires a creative solution such as hybridisation. 

So, what is hybridisation? In this context, its most general meaning is a power system that has access to two or more independent sources of energy. In this definition, the word independent is important – a power system that can be fed from either of two utility substations, for example, wouldn’t qualify as a hybrid system, because it simply has two connections to the same energy source. However, a power system that can take power from the grid and also from, say, a solar panel installation is definitely a hybrid system.

Adding a renewable energy source to an industrial or commercial power system can lead to useful cost savings but, if energy storage – most often in the form of batteries – is added as well, hybridisation has the potential to save even more money, as well as making the power system much more versatile in operation.

Typically, hybrid power solutions are used in ‘behind the meter’ applications, a good example of which is peak shaving. It’s axiomatic that the power system for a factory has to be designed so that it can safely supply the factory’s maximum load. However, it is highly likely that this maximum power demand will be intermittent – just a few hours a week is typical. Which means that the power transformers feeding the plant are for most of the time working well below their full capacity.

Consider this
Now consider a factory with a hybrid power system incorporating energy storage. The expensive power transformers can now be rated to cope with the average rather than the peak load, with the battery called on to make up the difference during periods of maximum load. This is peak shaving.

Many industrial and commercial supplies are charged on the basis of a ‘maximum demand’ tariff, which means that the utility charges the energy user not only for the amount of energy they consume, but also on the basis of the maximum load they put on the energy supply system. This extra charge is unwelcome but justifiable, as the utility’s plant has to be capable of dealing with the peak demand, however infrequently it occurs. Peak shaving enables energy users to limit their maximum power requirement from the grid by supplying some of their peak energy needs from their batteries. This results directly in a reduced maximum demand charge.

In reality, peak shaving, as described above, is only one of the many benefits offered by hybrid power systems with energy storage. Another option is to use batteries to store energy from the supply system at times when the price is low and release it at those times when energy prices are highest. This can be considered as another form of peak shaving and is capable of significantly reducing energy bills. Yet another capability offered by some hybrid power systems is the ability to generate reactive power on demand. This feature can be used to improve a site’s power factor and thereby save both energy and money.

For power systems that include intermittent energy sources, such as solar panels and wind generators, hybrid power systems offer even more benefits, as they can be used to ‘firm up’ the supply, maintaining the output from the intermittent source at a constant level and avoiding voltage variations on the grid when the energy production of the source changes. 

A hybrid power system with energy storage can reduce capital expenditure on equipment like transformers and save money on energy costs by cutting maximum demand, improving power factor and reducing peak-rate energy usage, but it can also do more. The batteries can continue to supply the plant in the event that the supply from the national grid fails. In such cases, the hybrid system effectively operates as an uninterruptible power supply (UPS) and can, in some cases, eliminate the need for a separate UPS installation to supply critical or sensitive loads. 

Another option offered by hybrid power systems is the ability to feed energy from the batteries, from solar panels or from other local power sources, back into the grid. The payment the utility makes for this energy is an effective way of further reducing energy bills.  And a hybrid power system can generate even more revenue from the grid operator by providing services such as firm frequency response to help balance the grid’s supply and demand.
An available technology
The technologies needed to implement hybrid energy systems is readily available. There have been big developments in the batteries used for energy storage in recent years, with lithium-ion technology being a popular choice, especially for fast-response short-period requirements. 

However, batteries and some renewable energy sources such as solar panels, produce DC power, whereas the national grid and almost all industrial and commercial power systems need AC. Fortunately, once again a solution is available with standard inverters. In their variable speed drive guise, inverters take AC power at supply frequency, convert it to DC (the so-called DC link) and then convert the DC back to AC at the frequency needed to control the motor.

But the DC link doesn’t care where the power comes from, so it can equally be fed from a battery or a solar panel and it will convert the DC to AC at a frequency that can be accurately matched to and synchronised with the power grid. The inverters used in hybrid power system applications are fully bidirectional so can also take power from the grid and use it to top up the batteries.

While at first sight it may seem that the inverter hardware needed for hybrid applications would be somewhat different from that used in variable speed drives, it turns out that this need not be the case. Danfoss inverters for hybrid power systems use exactly the same hardware as variable speed drive applications, though the on-board software is, as would be expected, designed to offer different functionality. 

This should not be taken to mean, however, that designing and implementing a dependable and efficient hybrid power system is merely a matter of buying some standard components off the shelf and following the installation instructions! Expertise is needed to put together an optimised system and anyone thinking about investing in hybrid power is well advised to seek out suppliers with proven expertise in this relatively new area of technology. 

Hybrid power systems are radically different from those that have gone before and they can provide huge benefits in terms of cost savings and reduced environmental impact and it is possible to benefit from this technology today.

Steve Moore is solution sales manager at Danfoss Drives.

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