Thursday, October 5, 2017

Smart grids- and their limits

Smart grids are integrated power systems designed to allow energy supply and demand to be balanced efficiently, in part by the use of electronic control and management systems. Some of this involves new hardware: energy stores that can be called on when supply is low and demand is high, or back-up plants queued up to meet demand peaks and green energy supply lulls. But some may be just software. For example, variable energy pricing signals sent to consumers may depress demand when supply is low or demand high. This is sometimes called ‘demand response’, and one variant relies on the ability of some domestic and retail  systems (eg freezers) to cope happily for a few hours without power, so that they can be set up to go offline when energy demand is high or supply low. 
The smart grid concept is seen as offering a new more flexible approach to energy supply and use, enabling the wider use of variable renewables and reducing the need for ‘always on’ base-load power. 
There are many practical problems facing the development and integration of such systems, as has already been found with the deployment of the relatively simple smart meters in the UK. The much more complex smart grid will no doubt require even more effort and adjustments, particularly in relation to consumer reactions. For example, consumers may not be happy with interactive load management systems that automatically isolate loads when demand is high- even if that saves them money. It may be better to offer them the choice of pre-setting cut off price levels, and also an over-ride option, so that they can, for example, continue to charge their electric car or run a washing machine even if the price of doing so at that time will be high. There are also issues with the integration of storage into smart grid systems: ideally they should store power when its cheap and use it when its expensive, but, in the case of Vehicle to Grid storage systems, will electric car owners be happy to find that power has been drained out of their batteries if they want to drive somewhere at around peak demand time.  Contractual limits have to be set and consumer friendly interactive control systems developed. Unsurprisingly then, at both the technical and social level, smart grid and demand response systems are currently a big area of research:  http://www.sciencedirect.com/science/article/pii/S1364032113007211
However, there is a larger problem. This sort of system can only deal with relatively small and short-term supply shortfalls.  As a review of smart grids posted on the Energy Matters web site concluded ‘no smart grid is smart enough to generate electricity when the wind doesn’t blow and the sun doesn’t shine’. That seems clear if there are long lulls, but its actually a bit of a simplification: it may not be the case if the concept of smart grids is expanded. 
It is true that demand management can only shift demand peaks by a few hours or in the extreme (with some industrial-scale interruptible contracts) a day or so. Similarly, storage system like batteries can only meet lulls for a few hours or at best a day or so, whereas there can be long lulls in wind and solar inputs, for several days or even a week or more, sometimes across wide areas.  However, studies have suggested that it will be rare for whole continents to be entirely becalmed and cloudy for long periods, so if they are spread widely enough, long distance HVDC supergrid interconnectors can often deal with lulls in some parts by trading power from where there is surplus to where it is needed. For example, the weather systems differ across Eastern and Western, and Northern and Southern Europe and North Africa. Of course there may occasionally be times, depending on local supply and demand, when there will be little current green energy surplus to trade across the supergrid- and certainly, there can be low wind inputs at times: http://tinypic.com/view.php?pic=dgnse0&s=8
That’s where large storage options like hydro reservoir pumped storage and compressed air cavern storage can help (for a few days), pumped up previously using surplus green power. They can supply power locally or via the supergrid to where it is needed. Power can also be generated from stored hydrogen or syngas, these stores being topped up ready for this using gas produced in P2G mode using surplus electricity from renewables previously and run into gas turbines when power is needed.   It’s easier to store gas than electricity (and it can be stored for long times) and even better/more efficient to store heat- so flexible CHP /DH plants with heat stores can also help: their power to heat output ratio can be increased to meet power shortfalls and any heat still needed supplied from the heat store, assuming it has been charged earlier when power demand was low. For the moment, most CHP plants run on fossil gas, but gradually they can be converted to run on biogas and low carbon synfuels/P2G, and solar and geothermal heat can also be used to feed the heat stores. And heat stores can store heat for long periods if necessary, even months. 
This flexible combination of power, heat and gas storage, linked up by supergrids and balanced as far as possible by demand management, expands the smart grid concept, so that, (along maybe with inputs from non-weather dependent renewable sources (hydro, tidal, geothermal, biomass) it should be able to deal with most if not all eventualities. Though that will depend on the economics: some say it will be expensive, others that it will avoid waste, improve balancing and cut costs. The current system is certainly wasteful, and adding variable renewables to it without system changes could be even more so- with surplus output at periods of low demand having to be curtailed. Smart electrical system flexibility can limit that and also some of the need for backup capacity.  Going further, to include heat and gas systems and stores in the mix, along with supergrid trading, should help even more, though as yet the optimal mix is unclear.   
For a review of advanced balancing options see:http://iopscience.iop.org/book/978-0-7503-1230-1 The Energy Matters post: http://euanmearns.com/how-smart-is-a-smart-grid/