Hybrid hydro29 January 2021
Recent research has focused on the potential of hybrid hydropower systems to incorporate both wind and solar power.
Technological advances and falling costs of solar photovoltaics (PV) have “considerably” improved the competitiveness of solar power, according to new research by Lee et al published in Renewable Energy. Such competitiveness is helping to pave the way for the emergence of floating solar (FPV) as an increasingly viable form of power generation, where PV systems are sited directly on water bodies such as lakes, ponds and reservoirs.
Competing land-use pressures, climate goals, energy security and resilience motivations are reported to have prompted an increasing interest in FPV, and global deployment is accelerating. In 2007, global installed capacity was below 1MW, by 2018 it was 1314MW and is expected to reach approximately 13,000MW by 2022.
However, the authors claim that despite growing market interest: “FPV system deployment is nascent, and potential adopters remain concerned about the technology, the benefits it offers, the advantages to pairing it in hybrid systems (such as with hydropower), and how to analyse technical potential.”
Aside from electricity generation, the additional benefits of FPV are described as being abundant. These include avoiding land-energy conflicts, and lower land acquisition and site preparation costs. When co-located with hydropower the benefits also include increased PV efficiency due to the temperature regulating effect of water, reduced algae growth, reduced evaporation, reduced shading effects on modules, and reduced capital cost.
“Despite these claims, a gap remains in consolidated evidence to corroborate these co-benefits,” the authors say. “This gap in evidence prevents potential adopters from determining and quantifying the actual co-benefits derived from FPV - potentially hindering investment.”
Previous studies estimate the global potential of FPV is around 400-1000GW and the global potential of FPV paired with hydropower is 4400-5700GW. However, the authors warn that the existing knowledge gap about the benefits of FPV in hybrid systems will hinder data-driven decision making.
In an effort to support decision making, the authors (Nathan Lee, Ursula Grunwald, Evan Rosenlieb, Heather Mirletz, Alexandra Aznar, Robert Spencer and Sadie Cox, from the National Renewable Energy Laboratory and the Advanced Energy Systems Programme in the US), looked at the benefits of hybrid floating solar photovoltaics-hydropower systems and assessed their global technical potential.
FPV systems used in hybrid systems are identical to stand alone ones but are coupled with other generation (ie hydropower). In their research, Lee et al considered full hybrid FPV-hydropower systems with FPV and hydropower coupled at a common substation. This allows for operations to be co-optimised and dispatched in concert. In this research hydropower refers to reservoir impoundments and pumped storage systems only as they are more often highly controllable. Run-of-river schemes were excluded as the authors claim that the lack of reservoir area and presence of water currents may be problematic for FPV deployment.
According to this research, the value added from hybrid FPV-hydropower system operation include:
Improved system operation at different time scales - Solar PV generation is variable and less predictable due to weather conditions, spatial resource qualities, and daily patterns. In contrast, hydropower systems (with sufficient resources) can offer high degrees of generation control and can provide for shortfalls to balance intermittent solar PV generation. The authors also discussed the advantages of these benefits at different timescales on a seasonal, monthly, daily, hourly and even sub hourly scale.
Additional energy storage opportunities - FPV-hydropower hybrids could provide energy storage opportunities through different configurations. These include coupling FPV with pumped storage hydropower to use excess solar generation to pump water into an upper reservoir to store for later use; or the full hybrid in which water resources can be conserved during peak solar production hours - utilising the reservoir as storage for the non dispatchable solar PV.
Improved transmission utilisation - Connecting FPV to existing transmission infrastructure may increase the utilisation rates of transmission lines where additional transfer capacity exists.
Reduced solar PV curtailment - As the system operator views the hybrid FPV-hydropower system as a single generator able to provide predictable, controllable, dispatchable generation (not separate generators), the hybrid system may receive a higher capacity credit - allowing for reduced or no curtailment.
Reduced transmission interconnection costs - Co-locating FPV systems with hydropower allows for connection to existing transmission infrastructure. This may reduce the additional costs of transmission extensions, substations, and other infrastructure requirements as well as reducing time and siting constraints (such as land acquisition).
Resource conservation - Installation of FPV on hydropower reservoirs may help to decrease water evaporation through decreased air flow and absorption of solar radiance - increasing resources for hydropower generation.
With regards to hybrid hydro-FPV capacity, Lee et al state that North America has the largest estimated technical potential (1785GW). Other regions, such as South America (739GW), Eastern (473GW) and Southern Asia (362GW), and Northern Europe (404GW), also have significant potential.
The authors note that there are potential biases in the underlying data sets for these results as North and South America may have more complete hydropower datasets than other regions. However, the results obtained in this global assessment of hybrid FPV hydropower systems are consistent with other recent global estimates. Furthermore, the results show that they could significantly contribute to global energy demands. According to the authors, their estimated annual generation results range from 4251 to 10,616TWh/year for the scenarios considered.
Lee et al acknowledge that their research does not consider the project-siting constraints that developers will eventually face, and that a deep knowledge of local ground-verified data and development regulations is needed to assess the feasibility of a project.
“Therefore, the technical potential results presented here are an optimistic, upper bound for global FPV-hydropower system capacity and generation. Additionally, technical potential does not capture the economic or market potential for floating solar PV or potential future technology improvements. Future, studies at the regional, national, or subnational scale are required,” they add, “to improve the resolution of real-world hybrid FPV-hydropower system deployment potential.”
The authors also consider the various challenges that could hinder further investment and deployment of hybrid FPV-hydropower systems. These include:
- Hydropower operating range may limit the full dispatchability of the hybrid system - reservoir levels are often regulated to avoid erosion on reservoir banks, conserve water for other uses and maintain dam safety etc.
- Hydropower turbine ramping rates may limit hybrid operation as variations in FPV may occur too quickly for some turbines without high degrees of generation control.
- Insulation failures can lead to frequent unplanned downtime.
- Long-term exposure to humidity may expedite PV module degradation. Corrosion of floating structures, waterbody contamination, and/or material fatigue of joints may result in debris that interferes with the local ecosystem or hydropower systems.
- The required electrical and anchoring systems and mooring costs vary dramatically and could be higher than land-based systems.
- Concerns related to aesthetics, intrusion on recreational waterbodies, and environmental impacts, such as temperature fluctuations that could affect local ecosystems.
Lee et al state that some of the above challenges can be overcome in new systems designed for hybrid operation or retrofitted systems. They also say that as hydropower development has historically faced significant attention due to social and environmental impacts, “it is reasonable to assume further development of FPV on reservoir surfaces could also require significant dialogue and analysis to ensure alignment with the broader values of societies”.