Smart meters, an Electronics Weekly guide
What is a smart electricity meter, and why have one?
Power grids run most efficiently when power demand is constant from hour to hour and day to day.
Any variation in power consumption means power stations have to be kept running during the power troughs (in ‘spinning reserve’) so that they can be throttled up to meet the peaks. Spinning reserve wastes power.
If consumers can be persuaded to even-out their power demands, for example by moving some of their day time use to hours when the nation sleeps, the UK will emit less CO2.
“A lot of smart meter ideas are to encourage consumers by offering different tariffs at different times – like Economy 7 on steroids,” MD of Cambridge utility meter design firm Sentec Mark England, told EW.
He defines a smart meter like this: “It is a meter with two-way communications, a consumer display, and generally a disconnect as well. It generally displays consumption in lots of different ways.”
Consumers get to watch their energy consumption
In addition to displaying the instantaneous tariff, a smart meter might, for example, list the power consumed and the cost incurred by individual appliances over the last week or the last year.
A consumer will learn that non-time-critical loads like washing machines, dish washers, and tumble dryers are cheaper to run late at night when electricity is less expensive because power stations are idling – Hence Economy 7 and storage heaters.
“If you make the tariffs available, some users will take advantage of them,” said England.
For a detailed breakdown of consumption, the appliances or their plugs need to communicate individually with the meter, or the meter has to deduce which appliance is operating when by other means.
By monitoring their voltage and current waveforms from the meter, University of Oxford spin-out Intelligent Sustainable Energy claims to be able to identify appliances, even to the brand in some cases.
A smart meter can also curb excessive consumption by locally increasing a user’s tariff during periods when their consumption crosses an agreed threshold.
Gas and water?
Battery-powered gas and water meters already exist, some of them with wireless interfaces. Battery life even with a wireless link can be 15 years. As water and gas can be stored easily, there is no need for load-levelling.
The only reasons for making these meters smart are to add remote consumption displays, and to extend meter reading beyond walk and drive-by to a network.
In both cases, most schemes envisage the electricty meter acting as a hub for the gas and water meters.
The smart grid
Economy 7 and smart meters provide some demand-side management to level the grid’s load variation through a day.
However, these do nothing to manage the huge peaks that occur, for example, when kettles are turned on by TV viewers at half time in football matches.
This is where supply-side load-levelling measures such the Dinorwig pumped water storage power station and gas turbine generators exist.
“Meeting peak loads is generally, although not always, more carbon intensive than meeting base load,” Mark England, MD of utility meter firm Sentec, told EW.
The existing system, already peaky on the demand side, also gets peaky on the supply side when renewables like solar and wind power are added in.
A smart grid has mechanisms to reach into user premises and turn off non-essential loads during peak demand.
For example, when all the kettles go on, the nation’s tumble dryers and electric heaters could be turned off for a few minutes to compensate.
There are several ways to implement this kind of system.
One is to make it a condition of sale that every new power-hungry domestic appliance include a device that respond to some form of signalling – a subtle variation in the supply frequency for example, or a broadcast radio signal.
Some Economy 7 systems already work by radio broadcast (‘Teleswitch’) on the BBC’s national 198kHz frequency.
Another method is that a home network would control individual appliances, probably via a ZigBee wireless or powerline signalling, with the smart electricity meter acting as a control node which knows the electricity price at any moment.
“You would sign up to agree to this kind of programme and get a better tariff,” said England, “and you could over-ride any particular request if you were at home, but loose some benefit that time.”
With a smart grid, tariffs could get complex.
“It is going to end up with power tariffs like mobile phone tariffs, deliberately not compatible,” said England.
What about the communication network?
Smart meters report consumption to the utility company regularly: perhaps daily or four times during the day.
This said, they also have to provide enough internal storage to operate through a multi-day breakdown in the communication infrastructure.
The data stream has to be encrypted to prevent fraud.
There are several ways for data to get back and forth from the power company.
In no particular order, these include:
Low-power wireless mesh networks
With a mesh network, each house in an estate will be a node in the network, passing data to a local aggregator – a street box or pole-mounted master that is connected to some form of backbone or cellular network.
These tend to operate in free-to-air bands and therefore need to be robust enough to withstand the householder, for example, sitting a cheap-and-nasty Wi-Fi router right next to the meter.
For reliable operation, each node needs to see plenty of other nodes.
Powerline communications use existing mains wires to communicate to a local aggregator.
These systems in general cannot operate through local distribution transformers, and so need one aggregator per transformer which may not be cost-effective in situations where there are few houses per transformer.
Powerline data transmission is also blamed fro emitting RFI as the local mains network can act as an aerial of uncontrolled size and geometry.
Cellular systems need more power when transmitting, but are an established, and robust, solution. Sentec’s England claims that if each meter can see three or four base station towers, then each base station tower can handle hundreds of thousands of meters.
How does the UK decide how to implement smart metering?
In May 2009, the UK Government announced that it aims to equip every home with a smart meter by 2020.
With the huge number of options available, particularly for the data transfer infrastructure between meter and power generator, and the likely introduction of a smart grid, the Government launched an open consultation at the same time.
The real thing
- Anatomy of a smart meter
- Smart meter plan is potential boost to industry
- Tamper-resistant smart power meters rely on isolated sensors