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EDNE MAY 2015

Battery , battery , on the wa ll... edn. comment Elon Musk, of Tesla Motors fame, is nothing if not adept at capturing headlines; this time he has done it with his Powerwall concept, of battery-based energy storage for the home. Tesla is proposing to pair local (in-home) energy storage with local renewable-energy capture resources – mainly photovoltaics. Here’s the introductory paragraph from Tesla’s statement; “Tesla Powerwall is a rechargeable lithium-ion battery designed to store energy at a residential level for load shifting, backup power and self-consumption of solar power generation. Powerwall consists of Tesla’s lithium-ion battery pack, liquid thermal control system and software that receives dispatch commands from a solar inverter. The unit mounts seamlessly on a wall and is integrated with the local grid to harness excess power and give customers the flexibility to draw energy from their own reserve.” The unit will come as a nicely-packaged and styled box to hang on a wall (hence the name) either singly or in multiples. Tesla proposes two basic units, at 7kWh (at around $3000), for load-shifting (store energy when the sun shines, power your stuff by night) and 10kWh ($3500) for continuity through grid outages – in effect, a domestic UPS. These would use the output of the company’s “gigafactory”, planned, largescale Li-ion battery production facility. Tesla has repeated the approach it took with its cars. We know that we simply do not have any electrical energy storage technology that remotely approaches the energy-density of hydrocarbon fuels. Rather than lament the fact that there is no way of replicating the 1000 km-per-tank of a conventional car, Tesla said (in effect) – take what we do have, and maximise its capabilities into a product that people might actually buy. So the Tesla sports car contents itself with modest range, but optimises performance, “kerb appeal” and green appeal.. So with the Powerwall; some quick arithmetic on power densities will show that there’s no radically new technology here, but a repackaging of available Li-ion cells into a format that will appeal to consumers already interested in, say, a solar cell installation. EDN Europe readers don’t need any help with the maths to show that 7kWh (or even two or three such units) doesn’t go all that far. As most of our European readers live on power grids that are less fragile than is the case in some parts of the USA, the attraction of a domestic UPS is probably less relevant. In fact, any examination of battery capacity tables will show that there is not a great deal that could not be done with traditional lead-acid batteries – in kWh/m³ terms if not in kWh/kg, at least. But as leadacid is of little use for deep-discharge cyclic operation, the advent of large-scale Li-ion does have something new to offer in daily-cycling. The hype about capturing energy during the day and running your home from the Powerwall by night may be, more than a little, informed by coming from Southern California/Nevada mindset. If all you have to run to get through the evening is LED lighting, your 60-inch flat screen, a few iPads and a broadband modem, you might be OK; throw any sort of resistive heating or cooking into the equation and the picture becomes rather less favourable. Nor will it “kill the grid”, as some of the wilder speculation around the product launch has speculated. In fact, Tesla’s proposals depend on charging from the grid, making use of differential (peak/off-peak) tariffs to spread the load for the utility companies, and reduce the cost to the consumer. The Powerwall is unlikely to be a route to large numbers of consumers going “off the grid”. It is more about Tesla saying to us, “We’ve got this largecapacity manufacturing plant due to come on-line that will make rechargeable cells on a new scale and a new price point: and I’ll package that in whatever way will each a mass market.” (My interpretation, not a quote from Elon Musk himself.) If that leads us, in turn, to conclude that while it’s not the battery/energy storage technology of our dreams, we might nevertheless do interesting and useful things with it, that will be no bad outcome. 4 EDN Europe | MAY 2015 www.edn-europe.com


EDNE MAY 2015
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