This is the second article in a two-parter on the intriguing idea of putting float solar on top of pumped hydro storage reservoirs. There are several pros and cons to putting floating solar on pumped hydro reservoirs: evaporation control (covered in the first article), panel efficiency, reuse of transmission connections, volatile water levels, shadowed reservoirs, water movement, and relative cost.
Solar panels work better when they are cooler, and the evaporative cooling and heat sink of the reservoir cool the panels. Connor’s numbers show that the combination means that in hotter climates the panels operate about 10% more efficiently. Given that hotter climates are also where there’s greater evaporation, that’s a good combination. It also bears on raft design. A raft that’s fully decked reduces this value proposition quite a bit, so you end up with more of a lattice-work of flotation.
The third factor in favor of floating solar in conjunction with pumped hydro storage is also straightforward. Both require MW-scale grid transmission connections. The economics suggest that it’s more useful to push electricity from the solar panels to the grid instead of using it to push water uphill as time arbitrage of cost of electricity both makes it cheaper to buy electricity at night, and more profitable to sell electricity during the day. But both need power management, transformers, and grid connections. It doesn’t add much to cost or complexity to provide for the additional requirements compared to building them in separate locations.
The fourth pertinent factor, and a disadvantage, is volatile water levels.
There are two approaches to building reservoirs for closed loop pumped hydro storage: dry gulch and turkey’s nest. In the dry gulch approach, a gully up in the hills is dammed at its lower end, providing a very efficient ratio of fill to reservoir by using the walls of the gully. The turkey’s nest, on the other hand, is built on flat land by digging out a reservoir and using the fill to build dams all the way around the reservoir. Both have advantages, but in general it’s cheaper to use a dry gully if one is available. A common pairing is a dry gully in the hills paired with a turkey’s nest at the base of the hills.
However, what’s relevant for floating solar is that the water levels drop and rise. While the floating solar is easy to align to the sun on the level water of the reservoir, when the reservoir is empty, alignment is in many cases lost. This suggests that more often than not, a turkey’s nest reservoir is more suitable as making the bottom flatter can be done during the already necessary excavation, something unlikely to be done in a dry gully reservoir.
But there’s an added wrinkle: mooring. Floating solar rafts have to be solidly moored and they have to be moored in segments for the simple reason that wind load on the raised panels is a physical force that must be countered. However, that mooring means that with rapid and regular changes in water level, potentially of 30 or more meters, the mooring cables have to adapt to that. This isn’t an insurmountable concern, but it does have an impact.
But this does lead to the next factor: shadowed reservoirs. Dry gulch reservoirs are much more likely to have significant shadowing during the day simply because they are gullies among hills. Turkey’s nest reservoirs on flatter land are much less likely to be shadowed, but this is still a concern for the simple reason that the reservoirs are dug into the ground and surrounded by up to 20-meter-high earthen and concrete dams. Given the likelihood of being empty in the morning, this would shadow the already less aligned panels in the bed of the empty reservoir. It’s unclear if this would eliminate the 10% efficiency advantage from cooler panels or not, but it would at least diminish capacity factor.
The next pertinent factor is that a great deal of the time the panels will be in rapidly moving water. As a gigaliter of water rushes from the upper reservoir to the lower reservoir over a few hours, it doesn’t simply gently rise, but gushes with tremendous force from the outlet. The mooring system has to not only accommodate the wind load, the slack of the cables when the reservoir is empty, but also the much greater physical load of moving water. This can be engineered around with outlet placement and some other obvious things, but it does increase the engineering demand as well.
The final factor is relative cost. Connor and his team have created very simple extruded PVC rafts that are easy to assemble with unskilled labor and launch. But it’s still not as cheap as just building a utility-scale solar farm on level ground. In the discussion with the US developer, the pumped hydro region under consideration has very cheap land costs and is sparsely populated. It’s much cheaper just to put a hundred hectares of flat land somewhere nearby under solar than to build it on top of the reservoirs.
The combination of these factors suggest that floating solar is limited in terms of the number of pumped hydro sites it would be suitable for. In general, it comes down to a hot, arid climate where the evaporation control and improved efficiency are stronger positives and sites where a broader turkey’s nest reservoir is suitable.
In discussions with the developers in Scotland and the US, neither meets the criteria. The Scottish developer, Mark Wilson with his firm Intelligent Land Investments, is working on three sites, all of which use an existing loch for the lower reservoir, per my understanding. The upper reservoir of the one I’ve looked at, on storied Loch Ness, is a turkey’s nest, but there’s no evaporation concern at all given the prevalence of fresh water in the region and the constantly refreshing loch that’s the lower reservoir. The US site uses dry gullies for both upper and lower reservoirs, and the annual cost of getting new water pumped into the closed loop system to account for evaporation is inexpensive compared to the economics of adding solar.
In other words, floating solar isn’t a slam dunk for pumped hydro storage. That doesn’t mean that it doesn’t have niches within the space of pumped storage hydro however, and it’s worth keeping in the back pocket as a potential solution. Certainly in the southeastern US, if a turkey’s nest site were to be developed in that arid country, the social license value might be sufficiently of value to make it worthwhile. A future article will deal more with social license for pumped storage hydro, but for now, the floating solar series is closed.
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