Critical Review on Research Status on Water-Energy Nexus
Sanders 2014 Critical Review: Uncharted Waters? The Future of the Electricity-Water Nexus
This review contains a huge amount of information.
It starts with background, where the water use of different fuel, prime mover, and cooling system is summarized. The content is not limited to the general argument based on the thermal efficiency of generation technology and known ranking among once-through, recirculating, and dry cooling systems due to the inclusion of rich details (numbers and citations). It touches upon the potential impact of various policies and environmental regulations, and comprehensively summarized the existing consensus on how the thermoelectric generation sector is going to change. It concludes with some suggestions on how research can move forward.
EIA Form 2012 shows that about 87% of U.S. generation needed water for cooling. About 62% of the electricity generation were from steam turbine facilities, and about 24% from combined cycle facilities.
1. Fuel and prime mover considerations
Fuel: Coal; Natural Gas; Nuclear; Biomass, CSP and geothermal systems; Hydroelectric; Wind and Solar PV
Prime mover: Steam; Combined cycle; Combustion turbine
---- [All Cons] Coal are relatively inefficient and water-inefficient compared to natural gas because coal contains moisture and contaminants, and needs more pollution controls. According to literature review, the most water-efficient IGCC power plants consumes more than 300 Gal/MWh, while average Natural Gas Combined Cycle (NGCC) consumes 205 Gal/MWh.
---- [All Pros] NGCC power plants has low water requirement per unit electricity generation because of their high efficiency. They also have "efficiency, cost, and operational benefits over other forms of thermoelectric generation". From a life-cycle perspective, replacing coal-fired generation by NGCC (using shale gas) in Texas result in 60% reduction in water consumption per unit electricity generation, though the detailed spatial distribution of the consumption is not always decrease (citation: Grubert et al. 2012; Pasci et al. 2014).
---- [Only peak load] Natural gas combustion turbines has little water requirement, but they are usually peaking facilities because they are smaller and more expensive to build, but ramp fast. Some natural gas combustion turbine units pre-chill the air with small amount of water to increase efficiency. the majority of the natural gas combustion turbine and NGCC units in California are equipped with this technology.
---- [Pro: CO2; Con: water] Nuclear facilities represent ~19% of the electricity generation in the United States. They generally operate with lower thermal efficiency than coal and natural gas power plants, and no heat is brought away in flue gas, resulting in higher water use. They also cannot use dry cooling due to safety reasons, unless the experimental small modular reactors (SMRs) become commercially widely deployed.
---- [Neutral] Biomass, CSP, and geothermal power plants require water for cooling.
---- [Difficult] Hydroelectric power plants consume water via evaporation, but the consumption varies several magnitudes (from 10.6 Gal/MWh to 55200 Gal/MWh) depending on climate conditions and design. They also have water quality and ecosystem impacts.
---- [All Pros] Wind turbines and solar photovoltaics (PV) use no water and thus are good for arid regions. Example study on water savings conducted on Germany - 80% renewable penetration by 2050 translates to 70% decrease in annual water consumption (citation: Johst et al. 2014)
2. Cooling system considerations
Cooling system: Once-through, Recirculating, Dry, Hybrid
---- Based on the USGS model-estimated water use for 2010 (citation: Maupin et. al. 2014)
-------- Once-through cooling represents 30% of the water-cooled generation capacity, 70% of the water withdrawal, with 75% freshwater and 25% saline, and 20% of the freshwater consumption.
-------- Recirculating pond and tower cooled systems represented 50% of the water-cooled generation capacity, 2% of the water withdrawal, and 56% of the freshwater consumption.
-------- Hybrid power plants (multiple cooling system types, generation tech., or fuels) represent 20% of the water-cooled generation capacity, 28% of the water withdrawal, and 24% of the freshwater consumption.
---- Once-through cooling systems have declined significantly in the past. The water withdrawal per unit electricity generation decreased from 63 Gal/KWh in 1950 to 21 Gal/KWh in 2000, after the Clean Water Act requirements on thermal discharge in the 1970s (citation: Abrams & Hall 2010).
---- The shift from once-through will result in increase in consumption, which is not likely to negatively impact water supply in the water-abundant east, but may impact the west (citation: Ackerman & Fischer 2013; USGS 2005; Stewart et al. 2013; Tidwell et al. 2012). The shift can increase the power plants' resilience to drought (citation: Stillwell et al. 2011; Stillwell et al. 2013). (Note to myself: these studies are relevant; how have they quantified the consumption change? need follow-up)
---- Dry cooling systems are limited in deployment because the are more expensive, may reduce the electricity generation efficiency by 8%-25% during hot summer days and by 2% on annual average (citation: Badr et al. 2012). An evaluation suggested that large-scale conversion to dry-cooling is not a cost-effective way to reduce water consumption in the western United States (citation: Ackerman & Fischer 2013) (Note to myself: what is the water consumption reduction estimated by Ackerman & Fischer? need follow-up)
----- To use alternative cooling supplies from wastewater is another option to reduce water use, apart from dry cooling. This can be cost-effective when the price difference between freshwater and treated municipal wastewater is more than $0.14 per kL and resources are widely available, though additional water treatment is needed for the cooling water, and the equipment can be negatively impacted by scaling, stress cracking, and biological growth (citation: Zemlick et al. 2013; Walker et al. 2013; Tidwell et al. 2014; Hightower & Pierce 2008). Wastewater reuse in the United States is growing by about 15% per year (citation: Hightower & Pierce 2008).
3. Environmental control considerations (mainly on environmental regulations)
---- There is sometimes synergy and sometimes trade-off between water use reduction and air emission (CCS (CO2), SOx) reduction.
---- The federal Endangered Species Act also limit the water use by thermoelectric generation and limit the development of wind power, solar PV, hydroelectric, and transmission lines.
---- The CWA 316(B) provision's implementation, and the phase-out of once-through cooling systems in California will decrease water withdrawal though the exact impact is not clear.
---- Other environmental regulations that may impact the thermoelectric sector and its water use include the 2011 Mercury and Air Toxics Standards for Utilities, the 2011 CrossState Air Pollution Rule, and the 2010 proposed Coal Combustion Residuals rule. The collective impact of the three, plus CWA 316(B), is that about 234-258 gigawatts of power generation capacity will be retrofitted by 2015, and another 36-59 gigawatts of capacity will be retired or derated by 2018 (citation: NERC 2011). The 2014 Clean Power Plan is expected to result in the retirement of about 30-49 gigawatts by 2020 (citation: NERC 2011).
4. Climate considerations
---- Increase in air and water temperatures can decrease the efficiency of power generation and some research has been conducted, but not enough research has been done at regional level (citation: Maulbetsch & DiFillippo 2006; Santhaye et al. 2013; Li et al. 2014; Miara et al. 2013).
-------- Combustion turbine and NGCC power efficiency loss is because air density is decreased with increase in air temperature. The thermoelectric output is reduced by about 0.3%-0.7% per 1 degree Celsius temperature increase (citation: Li et al. 2014).
-------- Hydropower will be affected by increasing temperature and drought. The hydroelectric power generation decreases by about 3% per 1% decrease in streamflow (citation: Mellilo et al. 2009).
-------- The efficiency of solar PV is decreased by about 0.65% per 1 degree Celsius temperature increase (citation: Li et al. 2014).
---- Other climate change concerns include increasing precipitation and sea-level rise. For example, 25-30 coastal power plants in California are at risk (citation: Sathaye et al. 2013; Heberger et al. 2009).
---- Various anecdotes of how water concerns have impacted electricity generation/resulted in shutdown of power plants (citation: e.g. CAISO 2014)
---- Some system-scale studies on the impacts of water temperature impacts on power generation have been conducted (citation: Flores-López & Yates 2013 (southeastern United States); Forster et al. 2009 (a nuclear power plant in Central Europe); Koch & Voogele 2013 and Koch et al. 2014 (nuclear power plants in in Germany))
---- Increase in air temperature and wild fire frequency can negatively affect electricity transmission and distribution (T&D) infrastructure, and electricity demand increases, adding stress.
5. Grid-scale considerations
---- Future "smart grid", integration of the transportation sector into the grid electricity exchange system, penetration of more intermittent renewable electricity sources, integration of energy storage, ... may have potential impact on water resources.
Future Shifts in the Power-Water Nexus
1. This section first briefly reviewed the status of current projection studies:
---- While the studies agree that future water withdrawal will decline, and that wide deployment of carbon capture and storage and nuclear would increase water consumption, the overall projected trend in consumption varies across studies. Since the result of projection studies depend on the scenarios and modeling techniques, a review table (Table 3) was provided. All except three studies used water use factors from literature, and the different studies covered a range of scenarios (environmental regulation, climate change, mitigation, and business as usual) and electricity projection model.
2. This section then goes on to summarize some consensus on various aspects of the electricity-water nexus:
---- On fuel preferences: coal is expected to decrease, natural gas and renewable energy generation will increase, and the nuclear regulatory environment is unclear
---- On cooling system preferences: once-through cooling system will continue to decline, dry cooling will increase despite the disadvantage in efficiency and cost, and alternative water supply will be increasingly used for cooling.
---- On environmental regulations: increase is expected in greenhouse gas reduction and other pollution control regulations, and the Endangered Species Act is expected to pose increasing constraint on energy development
---- On the impact of climate change: more climate-related disruptions are expected; evaporative water losses will increase, the efficiency of electricity generation will decrease, and hydropower generation will decrease
---- On the expected grid transformation: expanding smart grid, electrification of the transporation sector, increasing renewable energy penetration and electricity storage
Moving forward
- Potential research directions are identified. While the data situation is improving, water-energy planning is yet to be integrated, and innovative policy strategies, for example, dynamic dispatch (citation: Pasci et al. 2013) or water pricing (citation: Sanders et al. 2014), should be devised and implemented.