Climate explained: could electric car batteries feed power back into the grid?



petrmalinak/shutterstock

Alan Brent, Te Herenga Waka — Victoria University of Wellington


CC BY-ND

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz


Why can’t I use the battery from my electric car to export solar power to the grid when I don’t need it?

Technically it is possible. You could charge your electric vehicle (EV) with solar photovoltaic panels (or any other means), and if the EV is not used, the stored energy could be pushed back into the grid, especially during hours of peak demand for electricity when market prices are high.

This is known as vehicle-to-grid technology and is seen as the future as we move towards more electrification of transport and a smart grid.




Read more:
New Zealand wants to build a 100% renewable electricity grid, but massive infrastructure is not the best option


But manufacturers of electric vehicles have been reluctant, at first, to allow the bidirectional flow of power, for two reasons.

First, it could accelerate the degradation of batteries, which means they would need to be replaced more often. Second, the EV has to connect to the grid in the same way a solar photovoltaic system does, complying with standards to protect line operators and maintenance personnel working on the grid.

Such advanced bidirectional charge controllers come at an additional cost. Nevertheless, EV manufacturers such as Audi and Nissan have now taken steps to enable vehicle-to-grid connection with some of their models.

For EV models that do not have onboard inverters (to convert the DC electricity in the electric car to AC electricity we use in our homes), there are now bidirectional inverters available to connect any electric car. But the issue of battery life remains.

The continual charging and discharging through a 90% efficient converter shortens the life of the battery, and depending on brand and model, it may need replacing every five years. At more than NZ$5,000, this is a significant price tag for “energy prosumers” – people who both produce and consume energy.




Read more:
Climate explained: the environmental footprint of electric versus fossil cars


Economic and practical considerations

There are other considerations that are very context-specific. These relate to the additional charges for enabling the export of electricity from households, which vary between lines companies and retailers (or local authorities), as well as the buy-back rate of the electricity, which again depends on the purchaser of the electricity.

At the moment, these specific circumstances are seldom favourable to justify the additional cost of the infrastructure needed to connect an electric car to the grid.

There are also practical considerations. If the EV is used for the morning and evening commute, it is not at the home during the day to be charged with a solar system. And if it is (hopefully) not charged during peak demand hours, but mostly in off-peak hours at night, then the vehicle-to-grid route makes less sense.

It only starts to make sense if an EV is not used daily, or if EVs are available to a larger network than just one household. There are major opportunities for EVs to be used in communities with microgrids that manage their own generation and consumption, independent of the larger grid, or if large smart grid operators can manage distributed EVs remotely and more efficiently.

Investigations are ongoing to make this a more practical reality in the near future.The Conversation

Alan Brent, Professor and Chair in Sustainable Energy Systems, Te Herenga Waka — Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Australia’s electricity grid can easily support electric cars – if we get smart



File 20190411 44790 1myde16.jpg?ixlib=rb 1.1
Smart meters can help share the load of charging electric cars.
Chris Hunkeler/Flickr, CC BY-SA

Marcus Brazil, University of Melbourne

Following opposition leader Bill Shorten’s policy announcement that 50% of new cars will be electric by 2030, questions have been raised about the ability of the electricity grid to cope with the increased demand associated with a substantial increase in the use of electric vehicles.




Read more:
Labor’s plan for transport emissions is long on ambition but short on details


These concerns are not completely unfounded. Modelling and research at the University of Melbourne, conducted as part of a project led by Professor Iven Mareels, has shown that in Victoria even fairly modest rates of electric vehicle uptake could have a major impact on the electricity distribution grid.

However, these problems would be caused by uncoordinated charging, with battery recharging occurring as soon as the driver returns home and plugs in the car. With some simple coordination – perhaps using smart meters – Australia’s grid can easily support far more electric vehicles for decades to come.

The problems

It’s helpful to first understand the challenges to the grid posed by a high number of electric vehicles. The focus here is on the low voltage electricity distribution network, by which we mean the part of the grid “downstream” from local transformers that directly supply electricity to homes and businesses.

This includes most of the grid infrastructure that we see around us every day, such as residential power lines and pole-mounted transformers. Electric vehicle charging can affect this infrastructure in a number of different ways.

Power demand

An electric car with a typical daily commute of 40km requires roughly 6–8 kilowatt hours of energy to recharge, which is equivalent to the daily needs of a small household. In other words, if you purchase an electric vehicle, the impact on the local electricity network is about the same as adding a small house to the neighbourhood.

And in an unregulated environment most electric vehicle owners are likely to plug in and begin charging when they arrive home, around 6 to 7 pm, which is the time residential electricity networks experience peak demand. This can lead to network failures, or component overload where assets such as distribution transformers and the utility lines run beyond their nominal current ratings and capacity limits, substantially shortening their lifetimes.

Voltage drop

Voltage can be thought of as the “electrical pressure” in the network. Each utility line in the distribution network has an associated impedance, meaning that the voltage at each house in the network decreases the further it is from the distribution transformer. As more current is drawn through the lines due to the charging of electric vehicles, this decrease in voltage is exacerbated. If the voltage in some houses falls below regulated limits, household appliances may fail or suffer.

Phase unbalance and power quality

Electricity distribution networks in Australia are generally three-phase, meaning there are three lines carrying the current, each a third of a cycle out of phase with the others. Most houses connect to only one of these phases. If a disproportionate number of households with electric vehicles all happen to be connected to the same phase, then that phase can get out of balance with the others, leading to a significant loss of efficiency in the network. Mass electric vehicle charging could also affect the overall quality of the power in the network, for example by distorting the shape of the 50Hz waveform that carries the current.

Modelling and simulations, based on real Australian data, have shown these negative impacts on the grid can occur at fairly low rates of electric vehicle ownership. For example, in a study based on an area in Melbourne it was shown that an electric vehicle penetration of only 10% can lead to network failures in an unregulated environment.

Getting smart

The good news is that all of these problems can be prevented by implementing a smart charging framework: shifting electric vehicle demand away from peak times.

Electric vehicles are among the most flexible loads in the grid. Unlike showering, cooking and heating our homes, we can shift the demand to other times, such as overnight, when there is more capacity in the network. The trade-off, of course, is that it takes longer until the vehicle is fully charged.

However, most owners are unlikely to notice this, as long as the car is charged and ready to go by the time they need to leave for work. Furthermore a standard commute will generally mean there is enough spare battery capacity to allow the car to be taken out for an emergency late-night run, even if it is not yet fully charged.

Shifting electric vehicle load. If vehicle charging is not controlled, there is a significant increase in peak demand. If the vehicle charging load is shifted to times when there is more capacity, there is no increase in peak load.

Setting up such a charging system would not be particularly difficult or expensive. One suggested scenario is for each residence with an electric vehicle to acquire a home charging terminal that the car plugs into, which receives instructions from the utility operator via the household smart meter. This allows the operator to control vehicle charging across the network based on the current network conditions and demand.

If the charging of electric vehicles can be controlled in this manner, then our existing networks will be able to sustain high uptake rates, without any additional investment into grid infrastructure.




Read more:
Shorten’s climate policy would hit more big polluters harder and set electric car target


Detailed simulations have shown that the same network that started to fail at a 10% uptake with uncontrolled charging is able to sustain more than an 80% uptake when vehicle charging is shifted, using simple optimisation algorithms. Through this sort of demand management, most of our existing networks should be able to handle electric vehicles for decades to come.The Conversation

Marcus Brazil, Associate Professor and Reader in Engineering, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.