Deploying Effectively Dispatchable PV on Reservoirs Comparing Floating
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Solar Energy 174 (2018) 837–847 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Deploying efectively dispatchable PV on reservoirs: Comparing foating PV T to other renewable technologies ⁎ Marc Pereza, , Richard Perezb,c, Craig R. Fergusonb,c, James Schlemmerb a Clean Power Research, Napa, CA, USA b Atmospheric Sciences Research Center, University at Albany, State University of New York, Albany, NY, USA c Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY, USA ARTICLE INFO ABSTRACT Keywords: In this article, we present a detailed simulation of foating photovoltaic's (PV) energy yield and associated [Floating] photovoltaics evaporation reduction potential for the largest 128 US hydropower reservoirs. A recent article by Cavusoglu Firm power generation et al., published in the journal Nature Communications, outlined a hypothetical evaporation engine that could Hydropower harness the energy of lake water evaporation while simultaneously conserving the water resource. Its authors Evaporation engine technology suggest that evaporation engines deployed across all US lakes and reservoirs could, collectively, yield up to 70% Reservoir evaporation of the total U.S. electricity production. We show that foating PV technology could: (1) deliver considerably more electrical energy than evaporation engines, amounting to 100% of the US production with only a fraction of the lakes; (2) deliver this energy on a frm, efectively dispatchable basis; and (3) conserve as much water as the evaporation engines. 1. Background production potential of the reservoirs. A new study by Cavusoglu et al. in Nature Communications 2. Assumptions and methods (Cavusoglu et al., 2017) claims that deploying evaporation-driven electrical energy generators upon all US lakes could meet a sizeable 2.1. U.S. reservoir characteristics fraction of the US demand for electrical energy. This new technology to turn water evaporation into energy is still at a very early pre-proto- As quantitative support for our investigation, we consider the 128 typing stage (Herkewitz, 2015; Ma et al., 2013; Chen et al., 2014; Zhang largest US reservoirs with a full lake storage capacity of one km3 and et al., 2015; Arazoe et al., 2016; Chen et al., 2015; Kim et al., 2016; Ni above. Many of these reservoirs are exploited for the production of et al., 2016; Xue et al., 2017). Its cost, operational reliability, and en- electricity via hydropower. A table is provided in the Appendix in- vironmental impact have yet to be established. cluding the reservoirs’ physical characteristics and their current hy- We aim to demonstrate that, if one were to seriously consider using dropower generation specifcations. This reservoir sample amounts to lakes for energy technology deployment, PV would be considerably about 30% of the area of the water bodies considered in Cavusoglu et al. more efective than evaporation technology. PV could produce the same (2017) (see Fig. 1). quantity of electricity and cost-optimally deliver this electricity with frm (24/7) production guaranties, while occupying only a small frac- 2.2. Photovoltaic (PV) efciency tion of available lake area – specifcally, only a fraction of managed reservoirs – and provide equivalent water conservation potential. We assume that the highest achieved module efciency of 24% for Through our systematic quantifcation of the electrical power gen- commercial grade crystalline silicon photovoltaic modules (Cousins eration potential and the water conservation potential of foating PV for et al., 2010), while it is cutting-edge today, will represent conservative the largest US reservoirs, we develop clear, new, actionable information mainstream conditions in the near future. This assumption is reasonable for industry and decision-makers to better assess lake deployment op- for the long-term, large scale deployment planning implied in this tions. In addition to a comparison with evaporation engine potential, paper. we also contrast the foating PV potential to the current hydropower ⁎ Corresponding author. E-mail address: [email protected] (M. Perez). https://doi.org/10.1016/j.solener.2018.08.088 Received 9 April 2018; Received in revised form 22 August 2018; Accepted 30 August 2018 Available online 01 October 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved. M. Perez et al. Solar Energy 174 (2018) 837–847 Fig. 1. US lakes and reservoirs – the 128-reservoir sample used in this study is highlighted in red. All lakes and reservoirs larger than 0.1 km2 across the contiguous US (excluding the Great Lakes) were considered in Cavusoglu et al. (2017). (For interpretation of the references to colour in this fgure legend, the reader is referred to the web version of this article.) 2.3. PV array confguration taken into consideration, including aesthetics, loss of recreational use, and water quality issues (e.g., temperature) that could afect ecosys- We assume that foating PV arrays are stationary and tilted south- tems. Nevertheless, in the context of this paper, these potential draw- ward at 10°. This geometry is sub-optimal from an energy production backs are not diferent from the evaporation engine technology’s standpoint per unit of collector area (Dunlap et al., 1994); however, it is against which foating PV is contrasted, and likely, less impactful, nearly ideal from a ground [lake] energy density standpoint. Allowing especially from an aesthetics standpoint (see Fig. 2). 15–20% spacing to account for maintenance and [minimal] row- shading considerations, the footprint conversion efciency of the in- 2.5. Mean PV electrical energy production density stalled arrays thus reaches roughly 20% (i.e., 24% minus the non-PV 2 areas). This amounts to a peak power density of 200 W/m (at 1000 W/ A peak output of 200 W/m2 of ground [lake] surface amounts to a 2 m incoming irradiance) of ground [lake] area under standard test mean output of 30–50 W/m2 in the Contiguous U.S. (CONUS) de- conditions (ASTM E2848). pending on the local solar resource (NREL, 2017). We contrast this number to a recent NREL study reporting a mean density of only 8 2 2.4. Floating PV technology Watts per m for typical utility-scale PV farms (Ong et al., 2013). It is important to note that this NREL study surveyed existing terrestrial The foating PV industry is new but it is fast developing. While it is installations where ground density has not been a premium concern, still experiencing design complexity and cost issues at this stage of its and where module efciency has been historically lower than what is learning curve (Newsletter, 2018), multiple medium-to-large commer- achievable today. There is no physical or technical reason, given cur- cial-scale projects are already currently operational (Trapani and rent and foreseeable PV technology, why [foating] low-tilt PV arrays 2 Redón, 2015; Sahu et al., 2016). These projects totaled several hundred could not yield 30–50 average W/m mean power generation density or MWs as of early 2018. more in the future. The possible benefts of foating PV are multiple and echoed in numerous publications on the subject (Cazzaniga et al., 2018; Rosa-Clot 2.6. Firm PV electrical generation density and Tina, 2017; Ferrer-Gisbert et al., 2013; Liang and Liang, 2017). Floating PV has been implemented in locations with a favorable tra- Firm power generation is the ability to meet a given electrical demand deof compared to land-based deployment, taking advantage of avail- at all times. When considering conventional resources, frmness is able and unexploited areas (wastewater basins, for instance). It can achieved with baseload generation (chiefy nuclear, large hydro and reduce evaporation, algae growth, the formation of waves and coupled coal) plus the dispatching of fexible generation (chiefy natural gas erosion efects. Moreover, it can increase hydropower resource avail- turbines) as needed for production to match demand. ability via evaporation reduction. Furthermore, the foating structural PV generation is inherently intermittent, modulated by day-night elements may serve important secondary functions, such as storage for cycles, weather, and seasonal efects. It cannot be dispatched by grid compressed air. The vast potential for foating PV was perhaps most operators and cannot, by itself, meet demand at all times. Applying recently demonstrated for China. Liu et al. (2017) estimated the po- energy storage can render PV + storage fully dispatchable and trans- tential for foating PV at 160 GW with 2% lake coverage and form PV into a frm power generation resource. However, applying 8000 GW—enough capacity to generate China’s (2015) annual elec- storage alone to deliver frmness would be prohibitively expensive, trical demand—with full lake coverage. even with the most optimistic storage technology cost projections. This Undeniably, foating PV has drawbacks that need to be carefully is because in addition to supplying power at night, storage systems 838 M. Perez et al. Solar Energy 174 (2018) 837–847 Fig. 2. 70 MW Floating PV plant on a clay quarry lake in Anhui Province, China under construction by Ciel & Terre. source: Ciel & Terre. would also have to be large enough to make up for extended cloudy periods and seasonal defcits. However recent work by the authors and others has shown that oversizing PV deployments and proactively curtailing output could deliver frm (365/24/7) electricity to regional power grids (Perez, 2014, 2015; Perez et al., 2018; Minnesota Department of commerce, 2018a) at an acceptable production cost. This counter-intuitive ap- Storage only Firm premium = 18 proach of oversizing PV and dynamically curtailing output signifcantly Storage contributon: 17 reduces storage requirements to the point where total (PV + storage) PV contributon: 1 system costs become considerably lower despite the oversizing of PV. The relationship between the relative cost contributions of PV and Intermitent PV generaton reference storage to meet a baseload production target 100% of the time (i.e., the equivalent of a nuclear generating facility) is illustrated in Fig.