Solar on the water - and in space

There is now over 580 GW of grid-linked solar photovoltaic (PV) generation capacity in place globally, around 205 GW of it in China, according to IRENA.  Japan has around 62 GW, the USA 60 GW and Germany 49 GW. IRENA has said solar PV could supply 25% of global electricity by 2050, with 8.5TW in place. And, as I noted in my last post, others have come up with even higher figures. The Covid 19 crisis has slowed the growth of most renewables, including PV, so some of the very high estimates may have to be revised, but it seems clear that PV is going to be big.

Will there be space for it? Roof tops are ideal for PV solar, but there may not be enough space there to meet all our energy needs, and there may also be limits to how many arrays we can accept on the land in solar farms and other large ground-mounted projects.  If there’s not enough land space available, one idea has been to put PV arrays on lakes & reservoirs. That has now been done on a wide scale, with over 1GW already installed, much of it in India, Japan & China, some even offshore. The UK also has some schemes, as does France and the USA. A 2018 NREL study suggested that floating solar might supply around 10% of US power.

It is still expanding around the world, in Asia especially, with more planned on a larger scale in India, including a 105 MW scheme. China’s largest project so far is a 70 MW chain of floating arrays, but a 150MW expansion is planned.  South Korea is reportedly looking to a 102 MW system. And moving up scale further, in India there are even plans for a 1GW project.

There are storm damage risks, local eco-system impact and scenic intrusion issues to consider, and installation costs are higher than with land based units, but water-mounting keeps the PV cells cool, so that they work 10-20% more efficiency, and they can also reduce evaporation, both being key issues in hot climates.  Overall, it’s claimed that 400 GW might be installed globally, although some put the total global resource very much higher, if hydro reservoirs are used. 

Off planet

However, if we need more space, and more capacity, some say, rather fancifully, why not go off-planet – into space? It is interesting that over the years there have been proposals for putting very large PV arrays in deep-space geo-stationary orbit, perhaps mounted on thin Mylar film stretched out over a lightweight frame, like a sail, and transmitting the energy to earth receiving stations via microwave beams. The US and Japan have both looked at the idea, with some novel designs emerging and continued interest being shown in ambitious schemes.

The attractions are clear: 24-hour power supply delivered to any point on the globe able to install a receiving station dish, with no further land use. However, given the high cost (especially of launching units into orbit), it seems like a very long shot, with there also being a range of potential safety risks and eco-impacts.  It’s the stuff of science fiction disaster movies.

However the idea resurfaces regularly, and new projects are promoted. Indeed, some even wilder ideas have been proposed, including to so-called Lunar Ring, a 11,000-km belt of solar panels encircling the moon's equator, in a width from just ‘a few kilometers to 400 km,’ as proposed by Japanese engineering and construction firm Shimizu. Power harvested from the sun would be transmitted to earth from giant (20-km diameter) dishes via 20-GHz microwave beams, alongside high-density laser beams which would be concentrated by Fresnel lenses and mirrors to generate solar PV power on earth.

In all, the Luna Ring would, it is claimed, supply up to 13,000 terawatt hours of power, a large part of global energy needs, with a construction start of 2035 being mentioned, using robots and material from the moon. The idea won support from WIRED magazine. Maybe a little far fetched. But more recently, China has announced that it intends to develop solar-from-space technology, with an initial $15 million test programme using PV cells mounted on balloons at 1000 meters to send power to the ground, prior to the possible establishment of orbital systems by 2050. 

Only Connect

While ambitious plans like this capture the imagination of some and may even eventually be tested out in full scale reality, it is hard not be be a little cautious given the major technical challenges and financial problems. There are plenty of desert areas, and also marginal land, on the planet, as well as reservoir and even coastal water surfaces. That could be enough to meet a large part of our energy needs, if fully exploited. With all that included, roof tops and rural solar farms as well, LUT/EWRs latest scenario, has PV supplying 69% of global primary energy by 2050.

PV has its limitations. It’s not just space using, it is also seasonal, less available in winter. It is also not available at night! A mixed system, with PV and wind, helps deal with that, and to go further and make the system even more viable, without requiring a lot of storage, local PV and other renewable resources could be linked up with low energy-loss terrestrial HVDC supergrids. In addition to continuing interest in  EU/North African links for PV/CSP and also wind, there are ideas for pan-Asian links, as in the so-called Golden Ring, and EU-Asia grid links are also being looked at. So has a cross Atlantic undersea link. So it could be possible, eventually, to have a global grid system, with that, crucially, linking up the night and the day sides of the planet. And, alongside its space-solar project, China is reportedly working on that too. We may not need to go into space to get 24-hour global solar energy.

Though wherever we do go, we have to be careful about how it’s done. We cannot plaster PV cells over everything without causing some changes to the ecosystem - and there are always risks of weather related disasters and grid system breakdowns. Things can go wrong on earth, just as they can in space.

PV solar can do a lot of things, but it does have lower annual average load factors than wind, at most, for good conventional unfocused PV systems, 20%, compared to 30-40% of on-shore wind and 50-60% for offshore wind. That’s why IRENA looks to wind supplying 35% of global power from 6GW installed by 2050, compared to only 25% from 8.5GW of PV.  Even so, with some integration, and run alongside wind, it clearly has a key role to play in future. It does have spatial limitations, but then so does on-shore wind, and off-shore wind is not an option for land-locked countries, whereas PV may well be.   Interestingly there are also ideas for using  PV at night, generating power from outward thermal radiation flows into space, albeit less efficiently.   Of course you can also just use solar heat daytime-  the traditional approach to solar energy collection, and that’s still doing well, as I explore in my next post.