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PV solar – is that all we need?

By Dave Elliott

PV solar is booming, as I noted in my last post, with over 130 GW in place globally and some see it as overtaking all other renewables, with prices falling dramatically. Indeed a new study “The Economics of Grid Defection” by the US Rocky Mountain Institute (RMI) says that PV solar and new cheap battery technology will soon mean that we won’t need power grids. RMI claims that already in the US Southwest “the conservative base case shows solar-plus-battery systems undercutting utility retail electricity prices for the most expensive one-fifth of load served in the year 2024; under the more aggressive assumptions, off-grid systems prove cheaper than all utility-sold electricity in the region just a decade out from today.”

Off-grid PV may be getting cheap, and operationally that may be fine for remote US homesteads, but does it really make sense in urban areas? Or for countries as a whole? Don’t grids help us to balance out variations in demand and supply in different locations, for a range of renewables at various scales, and even internationally, as with supergrids? For a good study on the problems (and balancing costs) of PV variability see:

On its website blog, RMI says a bit blandly: “Grid defection introduces its own set of considerations, including over-sizing systems to account for individual peak demand, rather than more efficiently sharing distributed resources as part of a connected smart grid”, and says they will be looking at that soon. Maybe at the root of what’s under discussion here is US individualism vs European collectivism! But RMI doesn’t take sides. It says “the future of the grid need not be an either/or between central and distributed generation. It can and should be a network that combines the best of both”. Well yes, then we could link in windfarms, on-land and offshore, wave and tidal projects, hydro and geothermal, as well as community-scale district heating using solar thermal and biomass/biogas.

That’s not to say there isn’t a key role for PV solar – it can deliver power directly to users, without grid losses. In the UK, DECC says we might get 20 GW by 2020 (it’s at 3.8 GW now). And storage and smart grids would certainly help as part of a new decentralized energy system :

At present most storage systems are expensive and the most efficient options are large scale – hydro pumped storage for example. Flow battery, liquid air, compressed air, hydrogen, and heat store options are all also being developed, mostly at large or medium scale. But if cheap small local-scale storage really does materialize, then PV can not only meet day time peak loads, for example in offices and for air conditioning in summer, but also supply power at night for homes. That’s beginning to happen in Germany, with domestic scale PV plus battery storage systems evidently nearing grid price parity. See the presentation by Ammon:

However, even without storage, there are things that can be done to make PV more suited to domestic energy supply in the early evening. Most PV arrays in the Northern Hemisphere are orientated south to maximize solar energy collection. But, a US study has found that the average house in a sample of 14 houses with west-facing solar arrays produced more electricity than the average of 24 houses with south facing arrays in Austin, Texas, during the three months from June 1 to August 31st, 2013, even though over the full year a south-facing orientation produced more total energy than other orientations. But the key result of this Pecan Street study, was that west-facing arrays are much better at reducing peak loads in climates with air-conditioning driven peak demand, such as Austin. As Forbes reported, according to the study, “an equal sized west facing system would have produced 49% more electricity during the peak demand hours of the summer months than a south facing system. Only 58% of electricity from south facing systems was used in the home, with 42% being sent back to the utility grid.  Fully 75% of electricity from west facing systems was used in the home, with only 25% sent back to the grid. Because they help more to reduce peak load, and put less strain on electricity distribution systems, west-facing PV systems may have more value to the grid than do south-facing systems, despite producing less total energy over the course of a year.”

Although the climate is different, that’s an issue that has led Germany to consider westerly orientation so as to maximize early evening generation when demand is often high:  and

While some still tilt at PV as unreliable due to its dependence on sunshine, maybe tilting PV westwards can help at bit while we wait for cheap storage. Even so, we will still need all the other renewables, at a range of scales, and grid links to help balance the system. Unless, that is, you believe that private individual energy self-sufficiency is a realistic goal.

Roof-top power for homes is of course not the only application for PV. In addition to conventional solar farm PV arrays, large-scale grid-linked focused solar, CPV, with concentrating PV using mirrors or lenses, is developing rapidly, with large projects in the US and elsewhere. Some of these track the sun to get maximum energy for PV cells:

And moving on to a completely new area, this idea is also being used to upgrade PV for on-site electric vehicle battery charging. The Ford C-MAX Solar Energi Concept car has a 12 sq. m photovoltaic panel on its roof that charges its onboard 7.6 kWh lithium-ion battery, at a module efficiency of around 20%, generating up to a peak of 300 watts from normal Sun exposure. That won’t give a full typical daily charge, but that can be achieved if the car is sat under a special 12 sq. m solar canopy that is also being offered, which has a Fresnel lens array to focus sunlight by a factor of around 8, assuming the car is slowly moved (automatically under its own power) so that the lens/PV roof alignment is adjusted to track to the Sun’s movement to get optimal energy capture. Clever – pushing it up to 2 kW or more. Though doesn’t that assume the car is at home all day? Or that a canopy is available at work, or wherever it is parked most in the daytime? Mains plug-in charging is available as a back-up, which may be just as well (2.5 hours to full charge) since in electric-only power, a 20 mile range has been mentioned, but that’s boosted to 620 miles in petrol hybrid operation. Ford says that up to 75 % of all trips could be powered by the C-MAX Solar Energi Concept’s PV alone, depending on climate: we wonder about UK winters! And presumably not if it’s run at its top speed of 85 mph too much or with the aircon/heater on.

However, it’s another option, and does illustrate how flexible solar PV can be, at various scales and for various uses. Although it’s only part of the wider energy transformation that’s underway – something I will be looking at in my next post.

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